JP4958484B2 - Non-aqueous electrolyte battery and manufacturing method thereof - Google Patents

Description

  The present invention relates to an improvement in a non-aqueous electrolyte battery such as a lithium ion battery or a polymer battery and a method for producing the same, and particularly excellent in cycle characteristics and storage characteristics at high temperatures, and exhibits high reliability even in battery configurations characterized by high capacity. It is related with the battery structure etc. which can be performed.

  In recent years, mobile information terminals such as mobile phones, notebook personal computers, and PDAs have been rapidly reduced in size and weight, and batteries as drive power sources are required to have higher capacities. Lithium ion batteries that charge and discharge when lithium ions move between the positive and negative electrodes along with charge and discharge have high energy density and high capacity. Widely used.

  Here, the mobile information terminal has a tendency to further increase the power consumption with enhancement of functions such as a video playback function and a game function. As a purpose, further increase in capacity and performance are strongly desired.

  Against this background, in order to increase the capacity of lithium ion batteries, battery cans, separators, and positive and negative current collectors (aluminum foil and copper foil) that are not involved in power generation elements are made thinner (for example, the following patent documents) 1) and higher packing of active materials (improvement of electrode packing density) have been researched and developed, but these measures are almost approaching the limit. Essential improvements such as changes are required. However, in the increase in capacity by changing both the positive and negative active materials, although the negative electrode active material is expected to be an alloy-based negative electrode such as Si or Sn, the positive electrode active material has a capacity exceeding the current lithium cobalt oxide, and There is almost no material with the same or better performance.

  In this situation, we have increased the end-of-charge voltage of batteries using lithium cobalt oxide as the positive electrode active material by increasing the depth of use (charging depth) in the region above the current 4.2V. Developed a battery that can The reason why the capacity can be increased by increasing the depth of use in this way will be briefly explained. The theoretical capacity of lithium cobaltate is about 273 mAh / g, but the battery of 4.2V specification (with a charge end voltage of 4.2V). This is because only about 160 mAh / g is used in the battery, and it is possible to use up to about 200 mAh / g by raising the end-of-charge voltage to 4.4V. In this way, by raising the end-of-charge voltage to 4.4 V, a high capacity of about 10% can be achieved as a whole battery.

However, when lithium cobaltate is used at a high voltage as described above, the oxidization power of the charged positive electrode active material is strengthened and the decomposition of the electrolytic solution is accelerated, and the delithiated positive electrode active material itself The stability of the crystal structure was lost, and cycle deterioration and storage deterioration due to crystal collapse were the biggest problems. When we studied, zirconia, lithium cobaltate,
It has been found that by adding aluminum and magnesium, a performance similar to 4.2 V can be obtained under a high-voltage room temperature condition, but as mentioned above, recent start-up terminals consume a large amount of power, and in high temperature environments It is essential to secure performance under high temperature driving conditions such as being able to withstand continuous use, and in that sense, there is an urgent need to develop a technology that can ensure reliability not only at room temperature but also at high temperatures.

JP 2002-141042 A

  As described above, it has been found that in the positive electrode of a battery having an improved end-of-charge voltage, the stability of the crystal structure is lost, and the battery performance is deteriorated particularly at high temperatures. The detailed cause of this phenomenon is unknown, but as far as the analysis results are concerned, elution of the element from the decomposition product of the electrolyte and the positive electrode active material (cobalt elution when lithium cobaltate is used) It is speculated that this is the main factor that deteriorates cycle characteristics and storage characteristics at high temperatures.

  In particular, in a battery system using a positive electrode active material such as lithium cobaltate, lithium manganate, or nickel-cobalt-manganese lithium composite oxide, cobalt and manganese are ionized and eluted from the positive electrode when stored at high temperatures. When these elements are reduced at the negative electrode, they are deposited on the negative electrode and the separator, which causes problems such as an increase in battery internal resistance and a corresponding decrease in capacity. Furthermore, as described above, when the end-of-charge voltage of a lithium ion battery is increased, the instability of the crystal structure increases, and the above problems become more apparent. These phenomena tend to be strengthened even at temperatures around 50 ° C. where there was no slag. In addition, when a separator having a thin film thickness and a low porosity is used, these phenomena tend to become stronger.

  For example, in a battery having a specification of 4.4 V, lithium cobaltate is used as a positive electrode active material, graphite is used as a negative electrode active material, and a storage test (the test conditions are a charge end voltage of 4.4 V, a storage temperature of 60 ° C., a storage period of 5 days). If done, the remaining capacity after storage is significantly reduced, sometimes to nearly zero. Therefore, when this battery was disassembled, a large amount of cobalt was detected from the negative electrode and the separator. Therefore, it is considered that the deterioration mode was accelerated by the cobalt element eluted from the positive electrode. This is because a layered positive electrode active material such as lithium cobaltate increases in valence due to extraction of lithium ions, but tetravalent cobalt is unstable, so the crystal itself is not stable and changes to a stable structure. This is presumed to be due to the fact that cobalt ions are likely to elute from the crystal. Further, when spinel type lithium manganate is used as the positive electrode active material, generally, trivalent manganese is disproportionated and eluted with divalent ions, and is the same as when lithium cobaltate is used as the positive electrode active material. It is known that this problem will occur.

  Thus, when the structure of the charged positive electrode active material is unstable, there is a tendency for storage deterioration and cycle deterioration particularly at high temperatures to become remarkable. Since this tendency has been found to occur more easily as the packing density of the positive electrode active material layer is higher, a problem with a battery having a high capacity design becomes significant. The reason for being involved not only in the negative electrode but also in the physical properties of the separator is that the reaction by-product in the positive and negative electrodes moves to the opposite electrode through the separator and further causes a secondary reaction there. It is presumed that the ease of movement and distance are greatly involved.

As measures for these, attempts are made to suppress elution of cobalt or the like from the positive electrode by physically coating the surface of the positive electrode active material particles with an inorganic substance or chemically coating the surface of the positive electrode active material particles with an organic substance. Has been made. However, since the positive electrode active material repeatedly expands and contracts due to charging and discharging, when physically coated as described above, there is a concern that the inorganic material and the like may fall off and the coating effect may be lost. On the other hand, when chemically coated, it is difficult to control the thickness of the coating film, and when the thickness of the coating layer is large, it is difficult to achieve the original performance due to an increase in the internal resistance of the battery, resulting in a decrease in battery capacity. Moreover, since it is difficult to completely coat the entire particle, there remains a problem that the coating effect is limited. Therefore, an alternative method is necessary.

  Accordingly, an object of the present invention is to provide a non-aqueous electrolyte battery that is excellent in cycle characteristics and storage characteristics at high temperatures, and that can exhibit high reliability even in a battery configuration characterized by high capacity, and a method for manufacturing the same.

To achieve the above object, the present invention provides an electrode body comprising a positive electrode having a positive electrode active material layer containing a positive electrode active material, a negative electrode having a negative electrode active material, and a separator interposed between the two electrodes, and the electrode body. In the non-aqueous electrolyte battery comprising the non-aqueous electrolyte impregnated in, the positive electrode active material contains at least cobalt or manganese, and the separator includes a porous separator body and a negative electrode side of the separator body. The coating layer is formed on the surface, and the coating layer includes filler particles, a water-insoluble binder, and a water -soluble binder.

  If it is the said structure, when the binder contained in the coating layer arrange | positioned at the surface of a separator main body absorbs electrolyte solution and swells, between filler particles will be moderately filled with the swelled binder, filler particle | grains and binder The covering layer containing the material exhibits an appropriate filter function. Therefore, it is possible to suppress the decomposition of the electrolytic solution reacted at the positive electrode and the cobalt ions and manganese ions eluted from the positive electrode active material from being trapped by the coating layer and depositing cobalt and manganese at the separator and / or the negative electrode. Thereby, since the damage which a negative electrode and a separator receive is reduced, deterioration of cycling characteristics at high temperature and deterioration of storage characteristics at high temperature can be suppressed. Moreover, since the filler particles and the coating layer and the separator main body are firmly bonded to each other by the binder, it is possible to prevent the coating layer from dropping from the separator main body, and the above-described effect is maintained for a long period of time.

  By the way, in a non-aqueous electrolyte battery, it is indispensable for ensuring safety that a separator has a current interruption mechanism (so-called shutdown mechanism) due to microporous blockage. ) Melting point. Therefore, when the coating layer is formed, if the separator is heated to a predetermined temperature or higher, the function may be impaired.

  Here, as the binder (binder) for forming the coating layer, it is also possible to use only a water-insoluble binder such as PVDF or an acrylic polymer. However, as a solvent in the case of using only such a water-insoluble binder, generally, NMP (N-methylpyrrolidone) having a boiling point of 200 ° C. or more is often used. This may cause a problem that the separator contracts. Considering this, it is also possible to use alcohol solvents with relatively low boiling points, such as Elsolve, a general-purpose solvent, such as cyclopentanone, but these solvents are flammable and can be stored. There is a problem that handling is difficult and the equipment cost is so high that the amount is limited.

  On the other hand, when the water-soluble binder and the water-insoluble binder are mixed and dispersed in water as a solvent as described above, the water drying temperature is relatively low, so the physical properties of the separator (shrinkage etc. ) Is less affected. In addition, water is extremely easy to handle, and the cost of manufacturing the battery can be reduced because the equipment cost is minimized. In addition, non-water dispersed in water
Since the soluble binder exists as an emulsion, it is not an adhesive form that covers the entire filler particles, but is close to an image like point adhesion (small contact area). Therefore, it is excellent in that moderate flexibility and adhesive strength can be secured in a small amount.

  However, in a battery using a separator having such a coating layer, when the coating layer is disposed on the positive electrode side, when the battery is stored at a high temperature or when the charging depth is increased, the coating layer is formed in a highly oxidized atmosphere by the positive electrode. Decomposition of the binder and the like contained in the battery occurs, and the battery characteristics are remarkably deteriorated. Therefore, in a battery using a separator having the above-described coating layer, it is necessary to dispose the coating layer on the negative electrode side.

  It is preferable to regulate the concentration of the water-insoluble binder with respect to the filler particles to 50% by mass or less, desirably 10% by mass or less, and more desirably 5% by mass or less.
  It is preferable to regulate in this way, when the concentration of the water-insoluble binder becomes too high, the lithium ion permeability is extremely lowered, and the resistance between the electrodes is increased, thereby causing a decrease in charge / discharge capacity. Because.

  The water-insoluble binder is preferably composed of a copolymer containing acrylonitrile units and / or a polyacrylic acid derivative.
  The copolymer containing the acrylonitrile unit can fill the gaps between the filler particles by swelling after absorbing the electrolytic solution, has a strong binding force with the filler particles, and has a dispersibility of the filler particles. This is because it has sufficient properties to prevent re-aggregation of filler particles, and has little elution into the non-aqueous electrolyte, and thus has sufficient functions required as a binder.

The water-insoluble binder is made of a non-fluorine-containing polymer, and the water-soluble binder is at least one selected from the group consisting of cellulosic polymers or ammonium salts, alkali metal salts, polyacrylic acid ammonium salts, and polycarboxylic acid ammonium salts. It is preferably composed of seeds.
If the water-insoluble binder is composed of a non-fluorine-containing polymer, the binding power and flexibility can be exhibited with a small amount of addition, and if the water-soluble binder is composed of a cellulose-based polymer, the dispersibility is improved. Can fully demonstrate.

The coating layer preferably contains a surfactant.
At present, polyethylene (PE) is used as a separator, and this polyethylene repels water. Therefore, it is desirable to add a surfactant that exerts a surface active action to the coating layer. However, when a separator that does not repel water is used, or a separator that exhibits a surface-active action is used, it is not necessary to add a surfactant.

The ratio of the water-insoluble binder to the total amount of the solid content is 10% by mass or less, preferably 5% by mass or less, and more preferably 3% by mass or less.
It is preferable to regulate in this way, when the concentration of the water-insoluble binder becomes too high, the lithium ion permeability is extremely lowered, and the resistance between the electrodes is increased, thereby causing a decrease in charge / discharge capacity. Because. In addition, solid content means a filler particle, a water-insoluble binder, and a water-soluble binder, and when a surfactant is contained, it contains a surfactant.
For the same reason, the total amount of solids excluding filler particles relative to the amount of filler particles is preferably 30% by mass or less.

The thickness of the separator body is x (μm), and the porosity of the separator body is y (%).
In this case, it is desirable that the value obtained by multiplying x and y is regulated to be 1500 (μm ·%) or less.
The reason for this restriction is that the smaller the pore volume of the separator body is, the more easily affected by the precipitates and side reactants, and the characteristic deterioration becomes significant. It is because a remarkable effect can be exhibited by applying the invention.

It is desirable that the value obtained by multiplying x and y is regulated to be 800 (μm ·%) or less.
This is because in such a battery using a separator body, the characteristic deterioration becomes more remarkable, and therefore, by applying the present invention to a battery having a separator body regulated in this way, a more remarkable effect can be exhibited. .
In such a battery, since the separator can be thinned, the energy density of the battery can be improved.

It is desirable that the filler particles are composed of inorganic particles, and particularly composed of rutile type titania and / or alumina.
Thus, the filler particles are limited to inorganic particles, particularly rutile-type titania and / or alumina, which are excellent in stability in the battery (low reactivity with lithium) and cost. This is because is cheap. Also, rutile-structured titania, anatase-structured titania is capable of inserting and removing lithium ions, and depending on the environmental atmosphere and potential, it absorbs lithium and expresses electronic conductivity. This is because there is a risk of short circuit.

  However, since the effect of this kind of filler particles on the effect is very small, in addition to the above-mentioned filler particles, inorganic particles such as zirconia and magnesia, as well as organic substances such as polyimide, polyamide, and polyethylene Submicron particles or the like may be used.

It is desirable that the average particle size of the filler particles be regulated so as to be larger than the average pore size of the separator body.
In this way, if the average particle size of the filler particles is smaller than the average pore size of the separator body, a part of the separator body penetrates when the battery is crushed and the separator body is greatly damaged. In addition, the filler particles may enter the micropores of the separator main body, which may deteriorate various characteristics of the battery, thereby avoiding these disadvantages.
The average particle size of the filler particles is preferably 1 μm or less, and in consideration of the dispersibility of the slurry, those having a surface treatment with aluminum, silicon, or titanium are preferable.

It is desirable that the coating layer has a thickness of 4 μm or less, particularly 2 μm or less.
Although the above-described effects are exhibited as the thickness of the coating layer increases, if the thickness of the coating layer becomes too large, the load characteristics decrease due to an increase in the internal resistance of the battery, or the amount of positive and negative active materials As a result, the battery energy density is reduced due to the decrease in the battery life. Therefore, it is desirable that the thickness of the coating layer is 4 μm or less, particularly 2 μm or less. In addition, since the coating layer is complicated and complicated, the trapping effect is sufficiently exhibited even when the thickness is small. The thickness of the coating layer refers to the thickness when the coating layer is formed on one side of the separator, and the thickness on one side when the coating layer is formed on both sides of the separator. And

The packing density of the positive electrode active material layer is preferably 3.40 g / cc or more.
In this way, when the packing density is less than 3.40 g / cc, the reaction at the positive electrode reacts as a whole rather than a local reaction, so the deterioration at the positive electrode also proceeds uniformly. There is not much influence on the charge / discharge reaction after storage. On the other hand, when the packing density is 3.40 g / cc or more, the reaction at the positive electrode is limited to the local reaction at the outermost surface layer. Deterioration is central. For this reason, since the penetration | invasion of lithium ion in the positive electrode active material at the time of discharge and a spreading | diffusion become rate control, the grade of deterioration becomes large. From this, when the filling density of the positive electrode active material layer is 3.40 g / cc or more, the effects of the present invention are sufficiently exhibited.

It is preferable that the positive electrode be charged until it becomes 4.30 V or higher, preferably 4.40 V or higher, particularly preferably 4.45 V or higher with respect to the lithium reference electrode potential.
This is because, in a battery having a configuration in which the positive electrode is charged at less than 4.30 V with respect to the lithium reference electrode potential, there is not much difference in high-temperature characteristics depending on the presence or absence of the coating layer, but the positive electrode is in relation to the lithium reference electrode potential. This is because, in a battery that is charged at 4.30 V or more, a difference in high-temperature characteristics appears remarkably depending on the presence or absence of a coating layer. In particular, in a battery in which the positive electrode is charged at 4.40 V or more, or 4.45 V or more with respect to the lithium reference electrode potential, this difference appears remarkably.

The positive electrode active material includes at least lithium cobaltate in which aluminum or magnesium is dissolved, and zirconia in electrical contact with the lithium cobaltate is fixed to the surface of the lithium cobaltate. It is desirable that
The reason for such a structure is as follows. That is, when lithium cobaltate is used as the positive electrode active material, the crystal structure becomes unstable as the charging depth increases, and the deterioration is accelerated in a high temperature atmosphere. Thus, aluminum or magnesium is dissolved in the positive electrode active material (inside the crystal) to reduce crystal distortion in the positive electrode. However, although these elements greatly contribute to the stabilization of the crystal structure, the initial charge / discharge efficiency is lowered and the discharge operating voltage is lowered. Therefore, in order to alleviate such a problem, zirconia in electrical contact with the lithium cobaltate is fixed to the lithium cobaltate surface.

Furthermore, it is desirable to apply to a battery that may be used in an atmosphere of 50 ° C. or higher.
This is because the battery is rapidly deteriorated when used in an atmosphere of 50 ° C. or higher, so that the effect of applying the present invention is great.

In order to achieve the above object, the present invention applies a slurry containing filler particles, a water-insoluble binder, a water-soluble binder, and water on one surface of the porous separator body, and dries the slurry. The coating layer is formed between a step of forming a separator by forming a coating layer on one surface, a positive electrode having a positive electrode active material containing at least cobalt or manganese and lithium, and a negative electrode having a negative electrode active material. The electrode body is manufactured by disposing a separator between both electrodes in a state where the electrode body is disposed on the negative electrode side, and the step of impregnating the electrode body with a nonaqueous electrolyte.
By such a manufacturing method, a non-aqueous electrolyte battery using a water-insoluble binder and a water-soluble binder as a binder for forming a separator coating layer can be produced.

It is desirable that a surfactant is further contained in the slurry.
This is for the same reason as described above.

In the step of producing the separator, it is desirable to use a doctor blade method, a gravure coating method, a transfer method or a die coating method as a method for forming the coating layer.
This is because, in the dip coating method, both sides of the separator must be applied, but if the doctor blade method or the like is used, it is possible to easily apply the separator on one side.

  According to the present invention, since the coating layer disposed on the surface of the separator body exhibits an appropriate filter function, cobalt ions and manganese ions eluted from the decomposition product of the electrolytic solution reacted at the positive electrode and the positive electrode active material are covered with the coating layer. It is possible to suppress the precipitation of cobalt and manganese by the negative electrode and the separator. Thereby, since the damage which a negative electrode and a separator receive is reduced, there exists an outstanding effect that the deterioration of the cycling characteristics at high temperature and the deterioration of the storage characteristics at high temperature can be suppressed.

  Hereinafter, the present invention will be described in more detail. However, the present invention is not limited to the following best modes, and can be appropriately modified and implemented without departing from the scope of the present invention.

(First reference form)
In the first reference embodiment, an embodiment in which only a water-insoluble binder is used as a binder for the separator coating layer will be described. If a water-insoluble binder is used as the binder, the dispersibility of the filler particles can be secured only by using the water-insoluble binder, the filler particles, and the organic solvent, so that the coating layer can be easily produced.

[Production of positive electrode]
First, lithium cobalt oxide as a positive electrode active material (Al and Mg are respectively solid-dissolved in 1.0 mol% and Zr is present on the surface of 0.05 mol%), acetylene black as a carbon conductive agent, Then, PVDF as a binder was mixed at a mass ratio of 95: 2.5: 2.5, and then these were stirred using a special machine-made combination mix using NMP as a solvent to prepare a positive electrode mixture slurry. Prepared. Next, this positive electrode mixture slurry was applied to both surfaces of an aluminum foil as a positive electrode current collector, and further dried and rolled to produce a positive electrode in which a positive electrode active material layer was formed on both surfaces of the aluminum foil. . The packing density of the positive electrode active material layer was 3.60 g / cc.

(Production of negative electrode)
A carbon material (artificial graphite), CMC (carboxymethylcellulose sodium), and SBR (styrene butadiene rubber) were mixed in an aqueous solution at a mass ratio of 98: 1: 1 to prepare a negative electrode slurry. A negative electrode slurry was applied to both surfaces of a copper foil as an electric body, and further, dried and rolled to produce a negative electrode. The filling density of the negative electrode active material layer was 1.60 g / cc.

(Preparation of non-aqueous electrolyte)
LiPF ₆ was mainly dissolved at a ratio of 1.0 mol / liter in a mixed solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 3: 7.

[Preparation of separator]
First, a solvent containing TiO ₂ (rutile type, particle size 0.38 μm, KR380 manufactured by Titanium Industry Co., Ltd.), 10% by mass, and an acrylonitrile structure (unit) based on acetone. A polymer (rubber-like polymer) was mixed in an amount of 10% by mass with respect to TiO ₂ and mixed and dispersed using Special Mechanics Films to prepare a slurry in which TiO ₂ was dispersed. Next, the slurry is dip-coated on both surfaces of a separator body made of polyethylene (hereinafter sometimes abbreviated as PE) microporous film (film thickness: 18 μm, average pore diameter 0.6 μm, porosity 45%). The coating layer was formed on both sides of the separator body by drying and removing the solvent of the slurry. The thickness of this coating layer is 2 on both sides.
In addition, since the film thickness of the separator body is 18 μm as described above, the total film thickness of the separator is 20 μm.

[Battery assembly]
A lead terminal is attached to each of the positive and negative electrodes, and a spiral wound electrode is pressed through a separator to produce a flattened electrode body, and then a storage space for an aluminum laminate film as a battery exterior body An electrode body was loaded therein, and a non-aqueous electrolyte was poured into the space, and then an aluminum laminate film was welded and sealed to prepare a battery. In this battery design, the charge end voltage is regulated to 4.4V by adjusting the amount of active material of both positive and negative electrodes, and the capacity ratio of positive and negative electrodes (the initial charge capacity of the negative electrode) at this potential. / The initial charge capacity of the positive electrode) was defined to be 1.08. The design capacity of the battery is 780 mAh.

(First best mode)
In the first best mode, a mode in which a water-insoluble binder and a water-soluble binder are used as the binder of the coating layer of the separator will be described.
A battery was prepared in the same manner as in the first reference embodiment except that the separator was prepared as follows and the coating layer of the following separator was disposed on the negative electrode side.
First, a copolymer containing 10% by mass of filler particles of TiO ₂ [rutile type with a particle size of 0.38 μm, KR380 manufactured by Titanium Industry Co., Ltd.] and an acrylonitrile structure (unit) as a binder ( 1% by mass of water-insoluble polymer), 1% by mass of CMC (sodium carboxymethyl cellulose, water-soluble polymer) as a thickener, 1% by mass of polyalkylene-type nonionic surfactant, solvent Was mixed with 87% by mass of water, and mixed and dispersed using a special machine Films to prepare a slurry in which TiO ₂ was dispersed. Next, the slurry is applied to one surface of a separator body made of a microporous membrane (thickness: 18 μm, average pore diameter 0.6 μm, porosity 45%) made of polyethylene (hereinafter abbreviated as PE). Coating was performed using a blade method, and the solvent of the slurry was dried and removed to form a coating layer on one side of the separator body. In addition, since the thickness of this coating layer is 2 μm and the film thickness of the separator body is 18 μm, the total film thickness of the separator is 20 μm.

[Preliminary Experiment 1-1]
By changing the type of water-insoluble binder (binder) used in the production of the separator coating layer and the dispersion treatment method, what kind of water-insoluble binder and dispersion treatment method can be used to improve the dispersibility of the slurry. Since it was examined whether it is excellent, the result is shown in Table 1. In this experiment, an organic solvent (specifically acetone) was used as a solvent for slurry preparation.

(Water-insoluble binder used and dispersion treatment method)
[1] Water-insoluble binder used PVDF (KF1100 manufactured by Kureha Chemical Industry, which is usually used for a positive electrode for a lithium ion battery. Hereinafter, PVDF for a positive electrode may be abbreviated) and PVDF for a gel polymer electrolyte (PVDF-HFP-PTFE copolymer. Hereinafter, it may be abbreviated as PVDF for gel electrolyte) and three kinds of rubbery polymer containing acrylonitrile units were used.

[2] Dispersion treatment method Disper dispersion treatment method (at 3000 rpm for 30 minutes), special machine Filmics dispersion treatment method (40 m / min for 30 seconds), and bead mill dispersion treatment method (at 1500 rpm for 10 minutes) It was. For reference, untreated samples were also examined.

(Details of the experiment)
While changing the kind of the water-insoluble binder and the addition concentration, the dispersion treatment method was used to determine the state of precipitation of filler particles (here, titanium oxide [TiO ₂ ] particles) after one day.

(Experimental result)
[1] Experimental results on types of water-insoluble binder As is clear from Table 1, both PVDFs (positive electrode PVDF and gel electrolyte PVDF) tend to be more difficult to precipitate as the amount of both PVDFs increases. However, it was recognized that it tends to precipitate more easily than the rubbery polymer containing acrylonitrile units. Therefore, it is preferable to use a rubbery polymer containing an acrylonitrile unit as a water-insoluble binder. The reason for this will be described below.

  In order to exhibit the effects of the present invention, it is preferable to form a coating layer as dense as possible, and in that sense, it is preferable to use filler particles of submicron or less. However, although it depends on the particle size, the filler particles easily aggregate and it is necessary to prevent reaggregation after the particles are crushed (dispersed).

On the other hand, in order to exhibit this effect, the following functions or characteristics are required as a water-insoluble binder.
(I) Function to ensure binding property that can withstand battery manufacturing process (II) Function to fill gaps between filler particles due to swelling after absorbing electrolyte (III) Ensure dispersibility of filler particles (Aggregation prevention) function (IV) Characteristic of less elution into electrolyte

  Here, when inorganic particles made of titania, alumina or the like used as filler particles are used, the water-insoluble binder having high affinity with those having an acrylonitrile molecular structure and having these groups (molecular structures) The dispersibility is higher. Therefore, a copolymer containing an acrylonitrile unit that satisfies the functions of (I) and (II) and has the characteristics of (IV) and can satisfy the function of (III) even when added in a small amount is desirable. . In consideration of flexibility after bonding to the separator body (in order to ensure strength that does not easily break), a rubbery polymer is preferable. From the above, the rubbery polymer containing an acrylonitrile unit is most preferable.

[2] Experimental results on dispersion method As is clear from Table 1, when disintegrating (dispersing) particles in submicron units, precipitation is almost always generated in the disper dispersion method. In the pulverization (dispersion) method (dispersion method generally used in the paint industry) such as the Filmics method and the bead mill method, it is recognized that precipitation does not occur in most cases. In particular, in order to perform uniform coating on the separator body, it is desirable to use a dispersion treatment method such as the Filmics method or the bead mill method in consideration of ensuring the dispersibility of the slurry. Although not shown in Table 1, it was confirmed that sufficient dispersion performance was not obtained when dispersion was performed by the ultrasonic method.

[Preliminary Experiment 1-2]
By changing the type of water-insoluble binder used in the production of the separator coating layer and the dispersion treatment method, we examined what kind of water-insoluble binder and dispersion treatment method can be used to achieve excellent slurry dispersibility. The results are shown in Table 2. This experiment is greatly different from the preliminary experiment 1-1 in that water is used as a solvent during slurry preparation.

(Water-insoluble binder used and dispersion treatment method)
[1] Water-insoluble binder used Water-insoluble binder (specifically, a water-insoluble polymer having a function as a binder) PTFE (polytetrafluoroethylene) and SBR (styrene butadiene rubber) ) And a copolymer containing an acrylonitrile structure (unit).

[2] Dispersion treatment method Disper dispersion treatment method (at 3000 rpm for 30 minutes), special machine Filmics dispersion treatment method (40 m / min for 30 seconds), and bead mill dispersion treatment method (at 1500 rpm for 10 minutes) It was. For reference, untreated samples were also examined.

(Details of the experiment)
While changing the kind of the water-insoluble binder and the addition concentration, the dispersion treatment method was used to determine the state of precipitation of filler particles (here, titanium oxide [TiO ₂ ] particles) after one day. When performing the above dispersion treatment, CMC (sodium carboxymethyl cellulose, the addition ratio is 1% by mass with respect to the total amount of the slurry) as a water-soluble binder (thickener) and polyalkylene as a surfactant Type nonionic surfactant (addition ratio is 1% by mass with respect to the total amount of slurry) was used.

(Experimental result)
[1] Experimental results on the type of water-insoluble binder As is clear from Table 2, unlike the above-described preliminary experiment 1-1, relatively dispersibility is ensured regardless of the type of water-insoluble binder. It is recognized that This is considered to be due to the addition of the water-soluble binder (thickener) as described above in this experiment.

  In particular, it is recognized that excellent dispersibility is exhibited when a copolymer containing an acrylonitrile structure is used. This is presumed to be due to the effect of suppressing reaggregation of the filler particles because the hydrophilic part and the lipophilic part are appropriately present in the polymer molecule due to the structure of acrylonitrile. Therefore, the copolymer containing the acrylonitrile structure satisfies the function of (III) shown in the preliminary experiment 1-1, and also has the functions of (I) and (II) shown in the preliminary experiment 1-1 as described above. This is considered to be due to having the characteristics of (IV). In addition, when the presence or absence of precipitation when the number of days of the slurry was extended was examined, it was also confirmed that the copolymer containing the acrylonitrile structure had less precipitation than SBR and PTFE.

  Although not directly related to dispersibility, a copolymer containing an SBR or acrylonitrile structure is superior in flexibility after drying compared to PTFE, especially on a separator that requires a high degree of freedom in a thin film. In order to ensure the handleability in the coating, the flexibility and strength of the coating layer after coating are important. In that sense, flexibility such as rubber properties is essential, and from this point, a copolymer containing SBR or an acrylonitrile structure is desirable. However, SBR is known to decompose at the potential of the positive electrode, and a coating layer is not disposed on the surface in contact with the positive electrode (that is, the coating layer is disposed on the surface on the negative electrode side of the separator). It is not preferable to use a stable material as the water-insoluble binder. For these reasons, the water-insoluble binder is most preferably a copolymer containing an acrylonitrile structure.

[2] Experimental results on dispersion method As is apparent from Table 1, some precipitation occurs in the disperse dispersion method, whereas crushing (dispersion) methods such as Filmics method and bead mill method (generally in the paint industry). It can be seen that no precipitation has occurred in the dispersion method used). In particular, in the present invention, the particle size of the inorganic particles used is small, and if the dispersion treatment is not performed to some extent mechanically, the slurry settles down and a homogeneous film cannot be produced. Therefore, solutions such as the Filmics method and the bead mill method can be used. A dispersion method capable of crushing particles such as ceramics by applying mechanical stress such as a crushing (dispersing) method is preferred. In addition, it is considered that the above result was obtained because the ability to disintegrate small particle size particles such as ceramics is low in the disperser dispersion method.

[Preliminary experiment 2]
The coating method for forming the coating layer by applying slurry to the separator body was changed, and what kind of coating method could be used was examined. In this experiment, the case where water was used as the solvent during slurry preparation and the case where an organic solvent was used were examined.
(Coating method used)
The slurry was applied to both surfaces of the separator body using a dip coating method, a gravure coating method, a die coating method, a doctor blade method, and a transfer method.

(Experimental result)
(A) When an organic solvent is used as a solvent (when only a water-insoluble binder is used as a binder)
In the method other than the dip coating method, the slurry must be applied to each side of the separator body made of a microporous film. Therefore, when the slurry is applied to one surface, the water-insoluble binder permeates in the back surface direction. For this reason, the water-insoluble binder concentration in the coating layer changes (dilutes), or the water-insoluble binder concentration inside the separator body increases during double-sided coating, resulting in deterioration of air permeability. . In order to avoid such problems, it is desirable to employ a dip coating method.

  In this method, the above problem can be suppressed and double-sided coating is possible at one time, so the coating process can be simplified, and the coating layer can be uniformly applied to both sides by changing the slurry concentration and coating speed. The advantage that can be formed can also be exhibited. In addition, when a uniform coating layer is not formed, it is possible to compress the separator. However, when compressed, there is a high risk of occurrence of pinholes and the like, which is not preferable.

  In addition, in order to sufficiently secure the battery performance, it is preferable that the filler particles are adequately filled, but the dip coating method can be controlled so that the coating density is low. The method is preferably used.

  In addition, when the dip coating method is adopted, it is preferable that the solid content concentration in the slurry (concentration of filler particles and water-insoluble binder) is low in order to apply smoothly, but the solid content concentration in the slurry is Even if it is high to some extent, the coating thickness can be controlled by scraping off or the like. Therefore, the solid content concentration in the slurry can be up to about 60% by mass.

Here, the separator body is often composed of PE (polyethylene) or PP (polypropylene), and considering that shrinkage may occur depending on the temperature applied during drying, depending on the equipment used and the conditions of the slurry In general, however, the drying temperature of the slurry is desirably 60 ° C. or lower. In consideration of the fact that when the separator body is overloaded, if the separator body is not subjected to any load, shrinkage is likely to occur, and it is effective to dry the separator body while applying a certain tension. Further, in view of such a situation, the solvent in which the filler particles are dispersed is preferably highly volatile, and generally has higher volatility and lower boiling point than NMP used in batteries. Examples of such include acetone and cyclohexane.
Examples of methods other than heating include volatilization by dry air and drying by air volume control.

(A) When water is used as a solvent (when a water-insoluble binder and a water-soluble binder are used as a binder)
As described above, in a coating layer using water as a solvent, good battery characteristics can be obtained when the coating layer is disposed on the negative electrode side of the separator. However, when the coating layer is disposed on the positive electrode side of the separator, battery characteristics are obtained. Is extremely reduced. Therefore, when producing a coating layer using water as a solvent, the dip coating method, in which both sides of the separator must be applied, is not desirable, and the doctor blade method, die coating that makes it easy to apply one side of the separator. It is desirable to use a method, a gravure coating method, or a transfer method.

  In addition, when these coating methods are adopted, it is preferable that the solid content concentration in the slurry (concentration of filler particles, water-insoluble binder, and water-soluble binder) is low due to the necessity of forming a thin film. Even if the solid content concentration is high to some extent, the coating thickness can be controlled by scraping off or the like. Therefore, the solid content concentration in the slurry can be up to about 60% by mass.

  In addition, the separator may be compressed when a uniform coating layer is not formed by the above coating method. However, when compressed, there is a high risk of pinholes and the like, which is not preferable.

[Preliminary experiment 3]
When the slurry is applied to the separator body to form a coating layer, the pore size of the separator body is changed, and the filler particles (here, titanium oxide [TiO2] particles) in the slurry may have any particle size. The results are shown in Table 3. For reference, Table 3 also shows the results for the case where no coating layer was formed. In this experiment, an organic solvent was used as a solvent for slurry preparation.
(Separator body used)
Separator bodies having average pore sizes of 0.1 μm, 0.3 μm, and 0.6 μm were used.

(Details of the experiment)
After applying the slurry on both sides of the separator body using the dip coating method, the cross section of the separator was observed with an SEM. The average particle diameter of the titanium oxide particles in the slurry is 0.38 μm.
In addition, a laminate type battery is manufactured using a separator in which a slurry is applied to each separator body to form a coating layer (however, a non-aqueous electrolyte is not injected), and 200 V is applied to each battery to A pressure resistance test was also conducted to confirm the presence or absence of a short circuit.
(Experimental result)

  When the cross section of each separator was observed by SEM, the average particle diameter of the filler particles was larger than the average pore diameter of the separator body (the average pore diameter of the separator body was 0.1 μm and 0.3 μm, respectively). The filler particles have a mean particle size smaller than the average pore size of the separator body (the average pore size of the separator) whereas the filler particles hardly enter the separator body without blocking the micropores of the separator body. Is 0.6 μm), it was confirmed that filler particles invaded considerably from the surface layer of the separator body toward the inside.

Further, as apparent from Table 3, as a result of the pressure resistance test, those having an average particle size of the filler particles smaller than the average pore size of the separator body have a higher defect rate than those having no coating layer formed. On the other hand, it was found that the filler particles having an average particle size larger than the average pore size of the separator have the same defective rate as compared with the case where the coating layer is not formed. This is because, in the former case, the influence of the winding tension and the portion where the resistance is small by partially penetrating the separator body at the time of crushing are partially formed, whereas in the latter case, the separator A uniform coating layer is formed on the surface of the main body, and filler particles hardly penetrate into the inside of the separator main body, so that it is assumed that the penetration of the separator main body is suppressed.

From the above, it can be seen that the average particle size of the filler particles is preferably larger than the average pore size of the separator body.
The average particle size of the filler particles is a value measured by the BET method.
In this experiment, an organic solvent was used as a solvent during slurry preparation, but it is considered that the same effect can be obtained even when water is used as a solvent during slurry preparation.

[Preliminary experiment 4]
In order to investigate how much the air permeability of the separator differs depending on the presence / absence of the coating layer, the thickness of the coating layer, etc., air permeability measurement was performed. In this experiment, an organic solvent was used as a solvent for slurry preparation.
(Separator used)
In carrying out this experiment, a separator made only of a microporous membrane made of PE (separators CS1 to CS6, in which the average pore diameter, film thickness, and porosity are changed), and PE microporous membrane A separator (separators IS1 to IS6, in which the film thickness of the coating layer was changed) in which a coating layer was formed on the surface of a separator body made of a porous film (selected from the separators CS1, CS2, and CS5) was used.

(Details of the experiment)
[1] Measurement of porosity of separator Prior to measuring the air permeability of the following separator, the porosity of the separator (the separator body in separators IS1 to IS6) was measured as follows.
First, a film (separator or separator body) is cut into a square shape having a side length of 10 cm, and mass (Wg) and thickness (Dcm) are measured. Further, by calculating the mass of each material in the sample, dividing the mass [Wi (i = 1 to n)] of each material by the true specific gravity, and assuming the volume of each material, the following (1) The porosity (%) is calculated by the formula.
Porosity (%) = 100-{(W1 / true specific gravity 1) + (W2 / true specific gravity 2) + ... + (Wn / true specific gravity n)} 100 / (100D) (1)

However, since the separator (separator body) in the present invention is composed only of PE, it can be calculated by the following equation (2).
Porosity (%) = 100 − {(PE mass / PE true specific gravity)} 100 / (100D) (2)

[2] Air Permeability Measurement of Separator This measurement was performed according to JIS P8177, and a B-type Gurley Densometer (manufactured by Toyo Seiki Co., Ltd.) was used as a measuring device.
Specifically, a sample piece is fastened to a circular hole (diameter 28.6 mm, area 645 mm ² ) of the inner cylinder (mass 567 g), and air (100 cc) in the outer cylinder is transmitted from the test tube circular hole portion to the outside of the cylinder. The time required for this was measured, and this was taken as the air permeability.
(Experimental result)

As is clear from Table 4, when separators having the same average pore diameter, film thickness, and porosity are compared, the separator having the coating layer has a lower air permeability than the separator having no coating layer. (The comparison between the separator CS1 and the separators IS1 to IS3, the comparison between the separator CS2 and the separator IS4, and the comparison between the separator CS5 and the separator IS5). Moreover, when the separators having the coating layer are compared with each other, it is recognized that the air permeability decreases as the thickness of the coating layer increases (comparison of separators IS1 to IS3).

  Moreover, in the separator which does not have a coating layer, when the average hole diameter of a separator becomes small, it is recognized that the air permeability is falling (for example, separators CS2 and CS4). However, even if the average pore diameter of the separator is small, a decrease in air permeability is suppressed if the porosity is large (comparison between the separator CS2 and the separator CS3). Furthermore, it is recognized that the air permeability decreases as the thickness of the separator increases (comparison between the separator CS5 and the separator CS6).

For a separator having a coating layer using water as a solvent (a separator having a coating layer using a water-insoluble binder and a water-soluble binder as a binder), the air permeability after forming the coating layer is measured. However, in order to facilitate understanding of what separator each battery used in Examples and Reference Examples described later, Table 5 shows the correspondence between each separator and each battery. In addition, the air permeability in Table 5 is the air permeability in the separator main body only (in a state where the coating layer is not formed).

[Preliminary experiment 5]
As described in the background section above, it is preferable to use lithium cobalt oxide as the positive electrode active material in order to increase the capacity of the battery, but there are also problems. Therefore, in order to solve and alleviate the problem, various elements were added to lithium cobalt oxide to investigate what elements are preferable.

(Premises for selecting additive elements)
In selecting the additive element, first, the crystal structure of lithium cobaltate was analyzed, and the results are shown in FIG. Ozuku et. al, J. et al. Electrochem. Soc. Vol. 141, 2972 (1994)].
As is clear from FIG. 1, when the positive electrode is charged to about 4.5 V or higher than the lithium reference electrode potential (4.4 V because the battery voltage is 0.1 V lower than the lithium reference electrode potential), the crystal structure (particularly , The crystal structure in the c-axis) was found to collapse significantly. Therefore, in lithium cobaltate, it was recognized that the crystal structure became unstable as the charging depth increased, and further, it was also found that deterioration is accelerated more when exposed to a high temperature atmosphere.

(Specific contents of additive element selection)
As a result of intensive studies to alleviate the collapse of the crystal structure, it has been found that it is very effective to dissolve Mg or Al in the crystal. The effects of both are substantially the same, but Mg has a smaller influence on the reduction rate of other characteristics described later. Therefore, it is more preferable to dissolve Mg in solid solution.

  However, although these elements greatly contribute to the stabilization of the crystal structure, they may cause a decrease in initial charge / discharge efficiency and a decrease in discharge operating voltage. In order to alleviate these problems, the present inventor conducted extensive experiments and found that the discharge operating voltage was greatly improved by adding tetravalent or pentavalent elements such as Zr, Sn, Ti, and Nb. I found out. Therefore, when lithium cobaltate to which a tetravalent or pentavalent element was added was analyzed, these elements were present on the surface of the lithium cobaltate particles, and were basically not dissolved in lithium cobaltate, The state of direct electrical contact was maintained. Although there are many unclear points in detail, these elements greatly reduce the interfacial charge transfer resistance, which is the resistance at the interface between lithium cobalt oxide and the electrolyte, which contributes to the improvement of the discharge operating voltage. It is guessed.

  However, in order to ensure a state in which lithium cobaltate and the element are in direct electrical contact, it is necessary to fire after adding the element material. In this case, Sn, Ti, Nb, etc. among the above elements usually work to inhibit the crystal growth of lithium cobalt oxide, so that the safety of lithium cobalt oxide itself tends to decrease (if the crystallite is small) Safety is declining). Under these circumstances, it was found that Zr is excellent in that the discharge operating voltage can be improved without inhibiting the crystal growth of lithium cobalt oxide.

From the above, when lithium cobaltate is used at a lithium reference electrode potential of 4.3 V or higher, particularly 4.4 V or higher, Al or Mg is dissolved in the lithium cobaltate crystal so that lithium cobaltate In order to stabilize the crystal structure and supplement the characteristic deterioration caused by dissolving these elements in solid solution, it is preferable that Zr has a structure capable of being in direct electrical contact with the lithium cobaltate particle surface. I understood.
In addition, the addition ratio of Al, Mg, and Zr is not particularly limited.

[Prerequisites for performing the experiment described later (operating environment)]
As described in the background section above, in recent years, portable devices have increased in capacity and output. In particular, mobile phones are required to have high functionality such as color video, video, and use in games, and power consumption tends to increase further. Currently, with the enhancement of the functions of these high-function mobile phones, it is desired to increase the capacity of the battery, which is the power source, but since the battery performance has not improved so far, the user needs to charge it. However, it is often used for watching TV or playing games. Under such circumstances, the battery is always used at full charge, and often has a specification environment of 50 to 60 ° C. due to the effect of increased power consumption.

  In this way, the usage environment has changed greatly from the conventional usage environment for calls and e-mails as mobile devices such as movies and games have become highly functional, so the battery operates in a wide range from room temperature to around 50-60 ° C. It has become necessary to guarantee the temperature range. In particular, higher capacity and higher output generate more heat inside the battery, and the operating environment of the battery is also getting higher, and it is necessary to ensure reliability at high temperatures.

  In consideration of this, we are focusing on improving performance through cycle tests in a 40-60 ° C. environment and storage tests in a 60 ° C. atmosphere. Specifically, the conventional storage test had a strong implication of an accelerated test at room temperature, but with the performance of the battery, the ability to reach the limit level of the material may be brought out. The test implications have gradually faded and are moving to tests that are close to the durability test at the actual use level. In view of these circumstances, this time, the charge storage test (the higher the end-of-charge voltage of the manufactured battery, the more severe the condition of deterioration, so the battery with 4.2V design is at 80 ° C for 4 days, and the battery with more design is not It was decided to study the difference from the prior art with an emphasis on comparison at 60 ° C. for 5 days.

In order to explain the effect of the present invention in a concrete and easy-to-understand manner, it will be described below by dividing it into four reference examples and four examples . In the first reference example to the fourth reference example, when only a water-insoluble binder is used as a binder (when an organic solvent is used as a solvent and corresponds to the first reference embodiment), the first implementation is performed. Examples to the fourth example are cases where a water-insoluble binder and a water-soluble binder are used as binders (when water is used as a solvent and corresponds to the first best mode).

[First Reference Example]
The end-of-charge voltage is 4.40 V, the packing density of the positive electrode active material layer is 3.60 g / cc, and the physical properties of the coating layer formed on the surface of the separator body (water-insoluble binder concentration with respect to titanium oxide and coating layer thickness) On the other hand, the separator ( the separator body in the case of the battery according to the first reference example ) was changed, and the relationship between the physical properties of the separator and the charge storage characteristics was examined. The results are shown below.

( Reference Example 1)
As Reference Example 1, the battery shown in the first reference embodiment was used.
The battery thus produced is hereinafter referred to as reference battery A1.

( Reference Example 2)
A battery was fabricated in the same manner as in Reference Example 1, except that a separator body having an average pore diameter of 0.1 μm, a film thickness of 12 μm, and a porosity of 38% was used. In addition, since the thickness of a coating layer is 2 micrometers on both surfaces, the total film thickness of a separator is 14 micrometers.
The battery thus produced is hereinafter referred to as reference battery A2.

( Reference Example 3)
A battery was fabricated in the same manner as in Reference Example 1, except that a separator body having an average pore diameter of 0.6 μm, a film thickness of 23 μm, and a porosity of 48% was used. In addition, since the thickness of a coating layer is 2 micrometers on both surfaces, the total film thickness of a separator is 25 micrometers.
The battery thus produced is hereinafter referred to as reference battery A3.

(Comparative Example 1)
A battery was fabricated in the same manner as in Reference Example 1 except that the separator was not provided with a coating layer.
The battery thus manufactured is hereinafter referred to as a comparative battery Z1.

(Comparative Example 2)
A battery was fabricated in the same manner as in Comparative Example 1 except that a separator having an average pore diameter of 0.1 μm, a film thickness of 12 μm, and a porosity of 38% was used.
The battery thus produced is hereinafter referred to as a comparative battery Z2.

(Comparative Example 3)
A battery was fabricated in the same manner as in Comparative Example 1 except that a separator having an average pore diameter of 0.1 μm, a film thickness of 16 μm, and a porosity of 47% was used.
The battery thus produced is hereinafter referred to as comparative battery Z3.

(Comparative Example 4)
A battery was fabricated in the same manner as in Comparative Example 1 except that a separator having an average pore diameter of 0.05 μm, a film thickness of 20 μm, and a porosity of 38% was used.
The battery thus produced is hereinafter referred to as comparative battery Z4.

(Comparative Example 5)
A battery was fabricated in the same manner as in Comparative Example 1 except that a separator having an average pore diameter of 0.6 μm, a film thickness of 23 μm, and a porosity of 48% was used.
The battery thus produced is hereinafter referred to as comparative battery Z5.

(Comparative Example 6)
A battery was fabricated in the same manner as in Comparative Example 1 except that a separator having an average pore diameter of 0.6 μm, a film thickness of 27 μm, and a porosity of 52% was used.
The battery thus produced is hereinafter referred to as comparative battery Z6.

(Experiment)
Since the charge storage characteristics (remaining capacity after charge storage) of the reference batteries A1 to A3 and the comparative batteries Z1 to Z6 were examined, the results are shown in Table 6. Further, based on the results obtained here, the correlation between the physical properties of the separator (separator body) and the remaining capacity after storage after charging was examined, and the results are shown in FIG. In addition, charging / discharging conditions and storage conditions are as follows.

[Charging / discharging conditions]
-Charging conditions After a constant current charge at a current of 1.0 It (750 mA) until the battery voltage becomes a set voltage (battery design voltage, 4.40 V in all batteries in this experiment), the set voltage And charging until the current value becomes 1/20 It (37.5 mA).
-Discharge condition The condition that constant current discharge is performed up to a battery voltage of 2.75 V at a current of 1.0 It (750 mA).
The charging / discharging interval is 10 minutes.

[Storage conditions]
The charging / discharging is performed once under the above charging / discharging conditions, and the battery charged to the set voltage by the above charging is left again at 60 ° C. for 5 days.
[Calculation of remaining capacity]
The battery is cooled to room temperature, discharged under the same conditions as the above discharge conditions, the remaining capacity is measured, and using the first discharge capacity after the storage test and the discharge capacity before the storage test, the following (3) The remaining capacity was calculated from the equation.
Remaining capacity (%) = first discharge capacity after storage test / discharge capacity before storage test × 100 (3)

[Discussion]
(1) Consideration regarding advantages of providing coating layer As is clear from the results in Table 6, in all batteries, the design voltage of the battery was 4.40 V, and the packing density of the positive electrode active material layer was 3.60 g / cc. In spite of this, it can be seen that the remaining capacity of the reference batteries A1 to A3 formed with the coating layer is greatly improved compared to the comparative batteries Z1 to Z6. The reason for such experimental results will be described in detail below.

There are several possible causes for the deterioration of the charge storage characteristics, but the positive electrode active material is used up to around 4.5V (the battery voltage is 0.1V lower than this, 4.4V) based on the lithium reference electrode standard. If you consider
The main factors are considered to be (I) decomposition of the electrolyte in a strong oxidizing atmosphere due to an increase in the charging potential of the positive electrode, and (II) deterioration due to destabilization of the structure of the charged positive electrode active material.

  These not only cause the problem of deterioration of the positive electrode and the electrolytic solution, but particularly, due to the decomposition of the electrolytic solution considered to be caused by (I) and (II) and the elution of elements from the positive electrode active material. This is considered to affect the clogging of the separator and the deterioration of the negative electrode active material due to deposition on the negative electrode. Although details will be described later, it is considered that the influence on the latter separator and the negative electrode is particularly large in consideration of this result.

  In particular, in batteries using a separator with a small pore volume (comparative batteries Z2 and Z3), if these by-products are clogged even in a small amount, the performance of the separator is greatly reduced, and the positive electrode is connected to the negative electrode through the separator. It is thought that the rate at which these reactants move is high and increases, and as a result, the degree of deterioration is considered to have increased. Therefore, the degree of deterioration of the battery is considered to depend on the pore volume of the separator.

The reason why the charge storage performance is improved in the reference batteries A1 to A3 having the separator formed with the coating layer is that the electrolytic solution decomposed on the positive electrode and Co eluted from the positive electrode are trapped by the coating layer. This is presumed to be due to the fact that the film moves to the negative electrode, prevents deposition → reaction (deterioration), and clogging, that is, the coating layer exhibits a filter function.

  The water-insoluble binder in the coating layer does not inhibit the air permeability at the time of separator preparation, but many of them swell about twice or more after injecting the electrolyte solution. Filled. This coating layer is intricately complicated, and since the particles are firmly bonded to each other by the water-insoluble binder component, the strength is improved and the filter effect is sufficiently exhibited (even if the thickness is small). It is an intricate structure that increases the trapping effect). The determination index of the electrolyte absorbability is difficult, but it can be roughly grasped by the time required for dropping one drop of PC to disappear.

  In addition, even if the filter layer is formed only with the polymer layer, the charge storage characteristics are improved to some extent. However, in this case, the filter effect depends on the thickness of the polymer layer. In addition, the function of the filter is reduced if the polymer is not completely porous due to swelling of the polymer. Furthermore, since the whole surface of the separator body is covered, the penetrability of the electrolytic solution into the separator body is deteriorated, and adverse effects such as deterioration of load characteristics are increased. Therefore, in order to minimize the influence on other characteristics while exhibiting the filter effect, the filter layer containing filler particles (in this example, titanium oxide) rather than simply forming the filter layer with a polymer alone. It is advantageous to form

In consideration of the above, in a battery provided with a separator having a coating layer formed, the degree of deterioration is almost the same regardless of the type of the separator body. It can be thought of as a result of the damage itself.

-Grounds that the effect of improving the charge storage characteristics is the filter effect After disassembling the battery after the above test was completed and observing the color change of the separator (separator body) and the negative electrode surface, in the comparative battery in which the coating layer was not formed, The separator turned brownish after storage under charge, and deposits were confirmed on the negative electrode as well, whereas in the battery of the present invention in which the coating layer was formed, deposits on the separator body and the negative electrode surface, No discoloration was observed, and discoloration was seen in the coating layer. From this result, it is presumed that damage to the separator body and the negative electrode is reduced by suppressing the movement of the reaction product at the positive electrode in the coating layer.

  In addition, these reactants are reduced by moving to the negative electrode, and are more likely to develop into cyclic side reactions such as self-discharge in which the next reaction proceeds, but by being trapped near the positive electrode, In addition to being able to suppress the circulatory reaction of the reactant, there is a possibility that the reactant itself exhibits an effect as a film forming agent.

(2) Consideration on the separator main body As described above, in the reference batteries A1 to A3 using the separator having the coating layer, the charge storage characteristics are improved, but the improvement rate is determined by the film thickness of the separator (separator main body). Thinner is higher. Furthermore, when the pore volume (film thickness x porosity), which is one of the physical properties of the separator and greatly affects the film thickness, is used as an index, as shown in FIG. 2, about 800 (unit: μm ·%) It has been found that the effect of the present invention appears remarkably at the boundary.

  Here, as a whole, in the comparative batteries Z1 to Z6 using the separator in which the coating layer is not formed, the correlation with the film thickness of the separator does not completely match, but as a tendency, the film thickness of the separator When the thickness is reduced, the degree of storage deterioration becomes very large. In general, the separator needs to be strong enough to withstand the steps in manufacturing the battery, in addition to ensuring insulation inside the battery. When the separator film thickness is reduced, the energy density of the battery is improved, but the membrane strength (tensile strength and piercing strength) is reduced, so that the average pore diameter of the micropores must be reduced. The rate decreases. On the other hand, when the thickness of the separator is large, the strength of the film can be secured to some extent, so that the average pore diameter and porosity of the microporous can be selected relatively freely.

  However, as described above, increasing the film thickness directly leads to a decrease in the energy density of the battery. Therefore, by maintaining a certain thickness (generally around 20 μm) and increasing the average pore diameter, It is generally preferred to increase the porosity. However, when the coating layer is provided while increasing the average pore diameter of the microporous, as described above, the defective rate of the battery tends to increase due to the penetration of the filler particles into the microporous, so that the It is necessary to increase the porosity while reducing the hole diameter.

As a result of intensive investigations in view of such circumstances, the separator on which the coating layer can be installed is (I) a film thickness sufficient to ensure the energy density. (II) A battery by infiltration of filler particles into the microporous interior. The pore volume of the separator body to which the present invention can be applied is based on the fact that it has a microporous average pore diameter capable of reducing defects and (III) has a porosity that can maintain the strength of the separator body. It was found to be 1500 (unit: μm ·%) or less calculated by thickness × porosity.

(3) Summary From the above results, in the 4.4V specification battery, regardless of the material of the separator body, the battery having the separator with the coating layer has greatly improved charge storage characteristics. If the pore volume (film thickness x porosity) is 1500 (unit: μm ·%) or less, and particularly 800 (unit: μm ·%) or less, the effect can be remarkably exhibited.

[Second Reference Example]
Two types of separators (separator body in the case of the battery according to the second reference example ) were used, the packing density of the positive electrode active material layer was 3.60 g / cc, and the physical properties (oxidation) of the coating layer formed on the surface of the separator body While fixing the water-insoluble binder concentration with respect to titanium and the thickness of the coating layer, the end-of-charge voltage was changed and the relationship between the end-of-charge voltage and the charge storage characteristics was examined. The results are shown below.

( Reference Example 1)
The battery was designed so that the end-of-charge voltage was 4.20V, and the positive / negative capacity ratio was designed to be 1.08 at this potential, in the same manner as in Reference Example 1 of the first reference example. A battery was produced.
The battery thus produced is hereinafter referred to as reference battery B1.

( Reference Example 2)
The battery was designed so that the end-of-charge voltage was 4.20 V, and the capacity ratio of positive and negative electrodes was designed to be 1.08 at this potential, as in Reference Example 2 of the first reference example. A battery was produced.
The battery thus produced is hereinafter referred to as reference battery B2.

( Reference Example 3)
The battery was designed so that the end-of-charge voltage was 4.30 V, and the capacity ratio of positive and negative electrodes was designed to be 1.08 at this potential, as in Reference Example 1 of the first reference example. A battery was produced.
The battery thus produced is hereinafter referred to as reference battery B3.

( Reference Example 4)
The battery was designed so that the end-of-charge voltage was 4.30 V, and the capacity ratio of positive and negative electrodes was designed to be 1.08 at this potential, in the same manner as Reference Example 2 of the first reference example. A battery was produced.
The battery thus produced is hereinafter referred to as reference battery B4.

( Reference Example 5)
The battery was designed so that the end-of-charge voltage was 4.35 V, and the capacity ratio of positive and negative electrodes was designed to be 1.08 at this potential, in the same manner as in Reference Example 1 of the first reference example. A battery was produced.
The battery thus produced is hereinafter referred to as reference battery B5.

( Reference Example 6)
The battery was designed such that the end-of-charge voltage was 4.35 V, and the capacity ratio of positive and negative electrodes was designed to be 1.08 at this potential, in the same manner as in Reference Example 2 of the first reference example. A battery was produced.
The battery thus produced is hereinafter referred to as reference battery B6.

(Comparative Examples 1-6)
Batteries were produced in the same manner as in Reference Examples 1 to 6, respectively, except that no coating layer was formed on the separator.
The batteries thus produced are hereinafter referred to as comparative batteries Y1 to Y6, respectively.

(Experiment)
Since the charge storage characteristics (remaining capacity after charge storage) of the reference batteries B1 to B6 and the comparative batteries Y1 to Y6 were examined, the results are shown in Tables 7 and 8. The table also shows the results of the reference batteries A1 and A2 and the comparative batteries Z1 and Z2.
Further, as a representative example, since the charge / discharge characteristics of the comparative battery Z2 and the reference battery A2 were compared, the former characteristic is shown in FIG. 3, and the latter characteristic is shown in FIG.
In addition, charging / discharging conditions and storage conditions are as follows.

[Charging / discharging conditions]
The conditions are the same as in the experiment of the first reference example.
[Storage conditions]
The reference batteries A1, A2, B3 to B6 and the comparative batteries Z1, Z2, Y3 to Y6 have the same conditions as the experiment of the first reference example, and the reference batteries B1 and B2 and the comparative batteries Y1 and Y2 are 80 The condition is that it is left at 4 ° C. for 4 days.

[Calculation of remaining capacity]
Calculation was performed in the same manner as in the experiment of the first reference example.

[Discussion]
As is clear from Tables 7 and 8, in the charge storage test, the reference battery in which the coating layer was formed on the surface of the separator body despite the same separator (the separator body in the case of the battery of the present invention) was It can be seen that the remaining capacity after charging and storage is significantly improved compared to the comparative battery in which the coating layer is not formed (for example, when the reference battery B1 and the comparative battery Y1 are compared or compared with the reference battery B2). When comparing battery Y2). In particular, in the comparative batteries Y4, Y6, and Z2 in which the separator pore volume is smaller than 800 μm ·% and the end-of-charge voltage is 4.30 V or more, the degree of deterioration of charge storage characteristics tends to be very large. On the other hand, in the reference batteries B4, B6, and A2 in which a coating layer is provided on the separator of these batteries, it is recognized that the deterioration of the charge storage characteristics is suppressed.

Further, as is apparent from Table 7, in comparative batteries Y4, Y6, and Z2 having a separator pore volume smaller than 800 μm ·% and a charge end voltage of 4.30 V or more, when recharging after confirming the remaining capacity In addition, it was confirmed that the charging curve meanders and the charge amount greatly increases (see the meandering portion 1 in FIG. 3 showing the charge / discharge characteristics of the comparative battery Z2). On the other hand, in the reference batteries B4, B6, and A2 in which the separator (separator body) of these batteries was provided with a coating layer, the above behavior was not confirmed (see FIG. 4 showing the charge / discharge characteristics of the reference battery A2).

Further, when the pore volume of the separator (separator body) exceeds 800 μm ·%, the above behavior was not confirmed in the comparative batteries Y3 and Y5 with the end-of-charge voltages of 4.30V and 4.35V. In the comparative battery Z1 having a charge end voltage of 4.40V, the above behavior was confirmed. On the other hand, in the reference batteries B3, B5, and A1 in which a coating layer was provided on the separator body of these batteries, the above behavior was not confirmed. When the end-of-charge voltage was 4.20 V, the above behavior was not confirmed regardless of the pore volume of the separator (in the case of not only the comparative battery Y1 but also the comparative battery Y2).

  The above results indicate that the smaller the pore volume of the separator (separator body), the greater the degree of deterioration. Moreover, although it has also shown that the degree of deterioration becomes so remarkable that the charge storage voltage of a battery is high, as long as the behavior with a charge end voltage of 4.20V and a charge end voltage of 4.30V is compared, both of them are shown. It can be seen that the deterioration modes are greatly different, and the degree of deterioration is clearly noticeable when the end-of-charge voltage is 4.30V.

  Although this does not go beyond the scope of estimation, in the storage test where the end-of-charge voltage is 4.20 V, the structure of the positive electrode is not so much loaded, and although there is an influence due to the decomposition of the electrolyte, the positive electrode It is presumed that the influence of elution of Co, etc. from is small. Therefore, the degree of improvement effect due to the presence or absence of the coating layer remains low to some extent. In contrast, the higher the end-of-charge voltage (storage voltage) of the battery, the lower the stability of the crystal structure of the charged positive electrode, and the acid resistance of cyclic carbonates and chain carbonates generally used in lithium ion batteries. Because it approaches the limit of the conversion potential, decomposition of side reactions and electrolytes progressed more than expected at the voltage at which lithium ion batteries have been used so far, resulting in increased damage to the negative electrode and separator It is guessed.

The details of the abnormal charging behavior are unknown, but considering the disappearance of the behavior after several cycles, it is not due to conduction due to precipitation of Li, Co, Mn, etc., or damage to the separator, Presumed to be caused by a kind of shuttle reaction (generation of shuttle material as a side reaction product) caused by a highly oxidized atmosphere or charge / discharge failure due to clogging of the separator (a side reaction generated at a battery voltage of 4.30 V or more) Redox reaction of the product). The basis of this behavior is presumed to be caused by the oxidation-reduction reaction between the positive electrode and the negative electrode, and the coating layer exhibits a filter effect, thereby suppressing the movement of products and the like from the positive electrode to the negative electrode. It can be improved so that it does not occur.

  From the above results, this effect is particularly effective when the pore volume of the separator (separator body) is 800 μm ·% or less, and the charge storage voltage is 4.30 V or more (the positive electrode potential with respect to the lithium reference electrode potential). Is 4.40V or higher), especially 4.35V or higher (positive electrode potential is 4.45V or higher with respect to the lithium reference electrode potential), the discharge operating voltage can be improved, the remaining / recovery rate can be improved, and abnormal charging behavior can be eradicated. Effective in terms.

[Third reference example]
The end-of-charge voltage is 4.40 V, the packing density of the positive electrode active material layer is 3.60 g / cc, and the separator ( the separator body in the case of the battery according to the third reference example ) is fixed to CS1, while the surface of the separator body The physical properties (water-insoluble binder concentration with respect to titanium oxide and the thickness of the coating layer) of the formed coating layer were changed, and the relationship between the physical properties of the coating layer and the charge storage characteristics was examined. The results are shown below.

( Reference Example 1)
As the slurry used for forming the coating layer of the separator, the solid content concentration of titanium oxide with respect to acetone is 10% by mass and the water-insoluble binder concentration with respect to titanium oxide is 2% by mass, and the thickness of the coating layer is 1 μm on both sides. A battery was fabricated in the same manner as Reference Example 1 of the first reference example except that.
The battery thus produced is hereinafter referred to as reference battery C1.

( Reference Example 2)
As the slurry used for forming the coating layer of the separator, a slurry having a solid content concentration of titanium oxide with respect to acetone of 10% by mass and a water-insoluble binder concentration with respect to titanium oxide of 30% by mass is used, and the thickness of the coating layer is 4 μm on both sides. A battery was fabricated in the same manner as Reference Example 1 of the first reference example except that.
The battery thus produced is hereinafter referred to as reference battery C2.

(Experiment)
Since the charge storage characteristics (remaining capacity after charge storage) of the reference batteries C1 and C2 were examined, the results are shown in Table 9. The table also shows the results of the reference battery A1 and the comparative battery Z1.
The charge / discharge conditions, the storage conditions, and the remaining capacity calculation method are the same as those in the experiment of the first reference example.

[Discussion]
As is apparent from Table 9, in the charge storage test, the reference batteries A1, C1, and C2 in which the coating layer was formed on the surface of the separator main body were compared with the comparative battery Z1 in which the coating layer was not formed. It can be seen that the remaining capacity is greatly improved. In addition, when the reference batteries A1, C1, and C2 are compared, the remaining capacity after charge storage varies slightly depending on the amount of the water-insoluble binder contained in the coating layer, but is hardly affected by the thickness of the coating layer. Was recognized.

In consideration of the operational effects of the battery according to the third reference example, it is estimated that the function of the filter increases as the thickness of the coating layer increases and the concentration of the water-insoluble binder increases. It is considered that there is a trade-off relationship with an increase in resistance (due to longer distance and deterioration of lithium ion permeability). Although not shown in Table 9, the concentration of the water-insoluble binder with respect to titanium oxide is When it exceeds 50% by mass, it was found that the battery can be charged / discharged only about half of the design capacity, and the function as a battery is greatly reduced. This is presumably because the water-insoluble binder was filled between the particles of the coating layer, and the lithium ion permeability was extremely lowered. When the amount of the water-insoluble binder is large as described above, it is recognized that the air permeability is greatly lowered even before the electrolyte solution is absorbed and swollen.

Empirically, the water permeability is such that the elapsed time of the air permeability measurement is 2.0 times or less, preferably 1.5 times or less, particularly preferably 1.2 times or less that of the separator having no coating layer. It is preferable to adjust the amount of the conductive binder. Even if the amount of the water-insoluble binder is 1% by mass, the water-insoluble binder is fairly uniformly dispersed in the coating layer by the dispersion treatment method such as the Filmics method described above. In addition to strength, the filter function is very high. The amount of the water-insoluble binder is preferably as small as possible, but considering the physical strength that can withstand the processing during battery production, the effect of the filter, ensuring the dispersibility of the inorganic particles in the slurry, etc. It is preferable to regulate in the range of 1 to 50% by mass, preferably 1 to 10% by mass, particularly preferably 2 to 5% by mass.
On the other hand, the thickness of the coating layer is preferably controlled to 2 μm or less on one side (4 μm or less on both sides) in order to suppress a decrease in load characteristics or energy density of the battery, and in particular, 1 μm or less on one side ( It is desirable to regulate to 2 μm or less on both sides.

[Fourth Reference Example]
The end-of-charge voltage is 4.40 V, the thickness of the coating layer is 2 μm, the separator according to the fourth reference example uses IS4, the comparative battery uses CS2, and the positive electrode active material layer filling density is changed. The relationship between the packing density of the layer and the charge storage characteristics was examined, and the results are shown below.

( Reference Example 1)
A battery was fabricated in the same manner as in Reference Example 2 of the first reference example, except that the packing density of the positive electrode active material layer was 3.20 g / cc.
The battery thus produced is hereinafter referred to as reference battery D1.

( Reference Example 2)
A battery was fabricated in the same manner as in Reference Example 2 of the first reference example except that the packing density of the positive electrode active material layer was 3.40 g / cc.
The battery thus produced is hereinafter referred to as reference battery D2.

(Comparative Example 1)
A battery was fabricated in the same manner as Comparative Example 2 of the first reference example, except that the packing density of the positive electrode active material layer was 3.20 g / cc.
The battery thus produced is hereinafter referred to as comparative battery X1.

(Comparative Example 2)
A battery was fabricated in the same manner as Comparative Example 2 of the first reference example, except that the packing density of the positive electrode active material layer was 3.40 g / cc.
The battery thus produced is hereinafter referred to as comparative battery X2.

(Comparative Example 3)
A battery was fabricated in the same manner as in Comparative Example 2 of the first reference example, except that the packing density of the positive electrode active material layer was 3.80 g / cc.
The battery thus produced is hereinafter referred to as comparative battery X3.

(Experiment)
Since the charge storage characteristics (remaining capacity after charge storage) of the reference batteries D1 and D2 and the comparative batteries X1 to X3 were examined, the results are shown in Table 10. The table also shows the results of the reference battery A2 and the comparative battery Z2.
The charge / discharge conditions, the storage conditions, and the remaining capacity calculation method are the same as those in the experiment of the first reference example.

As is clear from Table 10, when the packing density of the positive electrode active material layer is 3.20 g / cc,
Not only the reference battery D1 but also the comparative battery X1 has a certain remaining capacity. However, when the positive electrode active material layer has a packing density of 3.40 g / cc or more, the reference batteries A2 and D2 have some remaining capacity. It is recognized that the remaining capacity of the comparative batteries Z2, X2, and X3 is extremely low. This is presumed to be a phenomenon caused by the problem of the surface area in contact with the electrolyte and the degree of deterioration at the site where the side reaction occurs.

Specifically, when the packing density of the positive electrode active material layer is low (less than 3.40 g / cc), the deterioration proceeds uniformly, not locally, so the charge after storage is not increased. There is no significant effect on the discharge reaction. Therefore, capacity deterioration is suppressed not only in the reference battery D1 but also in the comparative battery X1. On the other hand, when the packing density is high (in the case of 3.40 g / cc or more), the deterioration at the outermost surface layer is the center, and in the comparative batteries Z2, X2, and X3, While the penetration / diffusion of lithium ions becomes rate limiting and the degree of deterioration increases, in the reference batteries A2 and D2, the deterioration of the outermost surface layer is suppressed due to the presence of the coating layer. It is presumed that the penetration / diffusion of lithium ions into the metal does not become rate limiting, and the degree of deterioration becomes small.

In addition, when the packing density of the positive electrode active material layer was fixed and the packing density of the negative electrode active material layer was changed from 1.30 g / cc to 1.80 g / cc, there was no difference as much as the packing density of the positive electrode active material layer. It was not dependent on the type of separator. Essentially, the side reaction product or dissolved product generated on the positive electrode is trapped by the coating layer and is prevented from moving to the separator or the negative electrode, so that the packing density of the negative electrode active material layer is effective. Do not depend. The negative electrode only contributes to the reduction reaction of by-products and dissolved substances, and is not limited to graphite as long as it is a substance capable of causing a redox reaction.
From the above results, it is particularly effective when the packing density of the positive electrode active material layer is 3.40 g / cc or more. The packing density of the negative electrode active material layer and the type of active material are not particularly limited.

First Embodiment]
The end-of-charge voltage is 4.40 V, the packing density of the positive electrode active material layer is 3.60 g / cc, the physical properties of the coating layer formed on the surface of the separator body (the concentration of titanium oxide, the polymer concentration, the CMC concentration relative to the total amount of slurry) While fixing the surfactant concentration and the thickness of the coating layer, the separator (in the case of the battery of the present invention, the separator body) was changed, and the relationship between the physical properties of the separator and the charge storage characteristics was examined. Is shown below.

Example 1
As Example 1, the battery shown in the first best mode was used.
The battery thus produced is hereinafter referred to as the present invention battery E1.

(Example 2)
A battery was fabricated in the same manner as in Example 1 except that a separator body having an average pore diameter of 0.1 μm, a film thickness of 12 μm, and a porosity of 38% was used. Since the thickness of the coating layer is 2 μm, the total film thickness of the separator is 14 μm.
The battery thus produced is hereinafter referred to as the present invention battery E2.

(Example 3)
A battery was fabricated in the same manner as in Example 1 except that a separator body having an average pore diameter of 0.6 μm, a film thickness of 23 μm, and a porosity of 48% was used. Since the thickness of the coating layer is 2 μm, the total film thickness of the separator is 25 μm.
The battery thus produced is hereinafter referred to as the present invention battery E3.

(Comparative Example 1)
A battery was fabricated in the same manner as in Example 1 except that the separator coating layer was disposed on the positive electrode side.
The battery thus produced is hereinafter referred to as a comparative battery W1.

(Comparative Example 2)
A battery was fabricated in the same manner as in Example 2 except that the separator coating layer was disposed on the positive electrode side.
The battery thus produced is hereinafter referred to as comparative battery W2.

(Experiment)
Table 11 shows the results obtained by examining the charge storage characteristics (remaining capacity after charge storage) of the present invention batteries E1 to E3 and the comparative batteries W1 and W2. The table also shows the results of the comparative batteries Z1 to Z6. Further, based on the results obtained here, the correlation between the physical properties of the separator (separator body) and the remaining capacity after charge storage was examined, and the results are shown in FIG. The charge / discharge conditions and the storage conditions are the same as those in the experiment of the first reference example.

[Discussion]
(1) Consideration regarding advantages of providing coating layer As is clear from the results in Table 11, in all batteries, the design voltage of the battery was 4.40 V, and the packing density of the positive electrode active material layer was 3.60 g / cc. Despite this, it can be seen that the batteries E1 to E3 of the present invention in which the coating layer is formed on the negative electrode side have a significantly improved remaining capacity compared to the comparative batteries Z1 to Z6 in which the coating layer is not formed. The reason for this experimental result is that, as shown in the experiment of the first reference example, the electrolytic solution decomposed on the positive electrode, Co eluted from the positive electrode and the like are trapped by the coating layer, so that the separator body It is presumed that the deposition → reaction (deterioration) and clogging due to movement to the negative electrode are suppressed, that is, the coating layer exhibits the filter function.

  On the other hand, in the comparison batteries W1 and W2 in which the coating layer is arranged on the positive electrode side, not only the invention batteries E1 to E3 in which the coating layer is arranged on the negative electrode side but also the comparison batteries Z1 to Z6 in which the coating layer is not arranged. It can also be seen that the remaining capacity is reduced. This is a phenomenon caused by the electrochemical stability of the surfactant and thickener (CMC) contained in the coating layer, and it is presumed that these materials were decomposed in a highly oxidizing atmosphere by the positive electrode.

  In addition, although the water-insoluble binder used this time has been confirmed to be electrochemically stable with CV characteristics, when water is generally used as a solvent, it tends to be weak against oxidation. When water is used as a solvent, the above binder, thickener and surfactant are often required, but which of the three substances has a strong influence on the cause of oxidation (decomposition). Whether this is done is unknown at this time, and it is highly possible that this is due to the effect of the combination. Further, although the specific decomposition potential is unknown, as long as various materials and conditions are used, the temperature is about 50 ° C., and in the case of potential, the positive electrode potential is 4.40 V or more at the Li reference electrode potential. It became clear that this tendency became stronger.

  Furthermore, in addition to the storage characteristics, the stability by the cycle characteristics at 45 ° C. and 60 ° C. was also evaluated, and the same tendency was shown. In other words, a battery using a separator with a coating layer on the negative electrode side shows performance equal to or better than a battery using a separator without a coating layer, but a battery using a separator with a coating layer on the positive electrode side. In several cycles, gas generation and capacity degradation due to decomposition were observed. However, even when a coating layer is formed on the positive electrode side, there is no abnormality in normal battery performance evaluation (for example, performance evaluation under 25 ° C. condition or performance evaluation with a 4.2 V design battery). It was.

(2) Consideration on the separator main body As described above, in the present invention batteries E1 to E3 using the separator having the coating layer, the charge storage characteristics are improved, but the improvement rate is the film thickness of the separator (separator main body). The thinner is the higher. Furthermore, when the pore volume (film thickness x porosity), which is one of the physical properties of the separator and greatly affects the film thickness, is used as an index, as shown in FIG. 5, about 1500 (unit: μm ·%) In the following, it was found that the effect of the present invention appears sufficiently, and in particular, the effect of the present invention appears remarkably at about 800 (unit: μm ·%). This is considered to be due to the same reason as shown in the experiment of the first reference example.
Therefore, the pore volume (film thickness x porosity) of the separator body is desirably 1500 (unit: μm ·%) or less, and particularly desirably 800 (unit: μm ·%) or less.

[ Second Embodiment]
Using two types of separators (separator body in the case of the battery of the present invention), the packing density of the positive electrode active material layer was 3.60 g / cc, and the physical properties of the coating layer formed on the surface of the separator body (relative to the total amount of slurry) Titanium oxide, copolymer containing acrylonitrile structure (unit), CM
C and each concentration of the surfactant and the thickness of the coating layer were fixed, while the charge end voltage was changed to examine the relationship between the charge end voltage and the charge storage characteristics. The results are shown below.

Example 1
The battery was designed such that the end-of-charge voltage was 4.20 V, and the positive / negative capacity ratio was designed to be 1.08 at this potential, in the same manner as in Example 1 of the first example. A battery was produced.
The battery thus produced is hereinafter referred to as the present invention battery F1.

(Example 2)
The battery was designed such that the end-of-charge voltage was 4.20 V, and the positive / negative capacity ratio was designed to be 1.08 at this potential, in the same manner as in Example 2 of the first example. A battery was produced.
The battery thus produced is hereinafter referred to as the present invention battery F2.

(Example 3)
The battery was designed so that the end-of-charge voltage was 4.30 V, and the capacity ratio of positive and negative electrodes was designed to be 1.08 at this potential, in the same manner as in Example 1 of the first example. A battery was produced.
The battery thus produced is hereinafter referred to as the present invention battery F3.

Example 4
The battery was designed so that the end-of-charge voltage was 4.30 V, and the capacity ratio of positive and negative electrodes was designed to be 1.08 at this potential, as in Example 2 of the first example. A battery was produced.
The battery thus produced is hereinafter referred to as the present invention battery F4.

(Example 5)
The battery was designed so that the end-of-charge voltage was 4.35V, and the capacity ratio of positive and negative electrodes was designed to be 1.08 at this potential, as in Example 2 of the first example. A battery was produced.
The battery thus produced is hereinafter referred to as the present invention battery F5.

(Comparative Example 1)
A battery was fabricated in the same manner as in Example 2 except that the separator coating layer was disposed on the positive electrode side.
The battery thus produced is hereinafter referred to as a comparative battery V1.

(Comparative Example 2)
A battery was fabricated in the same manner as in Example 4 except that the separator coating layer was disposed on the positive electrode side.
The battery thus produced is hereinafter referred to as comparative battery V2.

(Comparative Example 3)
A battery was fabricated in the same manner as in Example 5 except that the separator coating layer was disposed on the positive electrode side.
The battery thus produced is hereinafter referred to as comparative battery V3.

(Experiment)
Since the charge storage characteristics (remaining capacity after charge storage) of the present invention batteries F1 to F5 and the comparative batteries V1 to V3 were examined, the results are shown in Tables 12 and 13. The table also shows the results of the batteries E1 and E2 of the present invention and the comparative batteries Y1 to Y6, W1, W2, Z1 and Z2.
The charge / discharge conditions and the storage conditions are the same as those in the experiment of the second reference example.

[Discussion]
As is clear from Table 12 and Table 13, in the charge storage test, the separators (the separator bodies in the case of the present invention batteries F1 to F5, E1, and E2 and comparative batteries V1 to V3, W1, and W2) are the same. Regardless, the batteries F1 to F5, E1, and E2 of the present invention in which the coating layer is formed on the negative electrode side of the separator main body are after the charge storage compared to the comparative batteries Y1 to Y6, Z1, and Z2 in which the coating layer is not formed. It is recognized that the remaining capacity is greatly improved (for example, when the present invention battery F1 is compared with the comparative battery Y1, or when the present invention battery F2 is compared with the comparative battery Y2). In particular, in the comparative batteries Y4, Y6, and Z2 in which the separator pore volume is smaller than 800 μm ·% and the end-of-charge voltage is 4.30 V or more, the degree of deterioration of charge storage characteristics tends to be very large. On the other hand, in the batteries F4, F5 and E2 of the present invention in which the coating layer is provided on the negative electrode side of the separator body of these batteries, it is recognized that the deterioration of the charge storage characteristics is suppressed.

Further, as is clear from Tables 12 and 13, in comparative batteries Y4, Y6, and Z2, in which the separator pore volume is smaller than 800 μm ·% and the end-of-charge voltage is 4.30 V or more, the remaining capacity is confirmed after confirmation. During charging, the charging curve was meandering, and a behavior (abnormal charging behavior) in which the amount of charge was significantly increased was confirmed (see the meandering portion 1 in FIG. 3 showing the charge / discharge characteristics of the comparative battery Z2). Furthermore, when the pore volume of the separator (separator body) exceeded 800 μm ·% was examined, the above behavior was confirmed in the comparative battery Z1 having a charge end voltage of 4.40V. On the other hand, in the batteries F1 to F5, E1, and E2 of the present invention in which the coating layer was provided on the negative electrode side of the separator body of these batteries, the above behavior was not confirmed. This is considered to be due to the same reason as shown in the experiment of the second reference example.

Further, as apparent from Tables 12 and 13, the batteries F2, F4, F5, E1, and E2 of the present invention in which the coating layer is formed on the negative electrode side of the separator body have the coating layer formed on the positive electrode side of the separator body. It can be seen that the remaining capacity after charge storage is significantly improved compared to the comparative batteries V1 to V3, W1, and W2 (for example, when the present invention battery F2 is compared with the comparison battery V1 or the present invention battery F4 and comparative battery V2 are compared). In particular, in comparison batteries V2, V3, W1, and W2 having a charge end voltage of 4.30 V or more, the degree of deterioration of charge storage characteristics tends to be very large, whereas the batteries F4, F5, and E1 of the present invention. In E2, it is recognized that the deterioration of the charge storage characteristics is suppressed. This is considered to be due to the same reason as shown in the experiment of the second embodiment.

  From the above results, this effect is particularly effective when the pore volume of the separator (separator body) is 800 μm ·% or less, and the charge storage voltage is 4.30 V or more (the positive electrode potential with respect to the lithium reference electrode potential). Is 4.40V or higher), especially 4.35V or higher (positive electrode potential is 4.45V or higher with respect to the lithium reference electrode potential), the discharge operating voltage can be improved, the remaining / recovery rate can be improved, and abnormal charging behavior can be eradicated. Effective in terms.

[ Third embodiment]
The end-of-charge voltage is 4.40 V, the packing density of the positive electrode active material layer is 3.60 g / cc, and the separator (the separator body in the case of the battery of the present invention) is fixed to CS1, while the coating formed on the surface of the separator body The relationship between the physical properties of the coating layer and the charge storage characteristics was examined by changing the physical properties of the layer (concentration of the copolymer containing the acrylonitrile structure relative to the total amount of the slurry), and the results are shown below.

Example 1
The battery was used in the same manner as in Example 1 of the first example except that the slurry used for forming the separator coating layer had a copolymer concentration containing an acrylonitrile structure of 0.5% by mass relative to the total amount of the slurry. Was made.
The battery thus produced is hereinafter referred to as the present invention battery G1.

(Example 2)
A battery was fabricated in the same manner as in Example 1 of the first example except that the slurry used for forming the separator coating layer had a concentration of 2% by mass of a copolymer containing an acrylonitrile structure with respect to the total amount of the slurry. did.
The battery thus produced is hereinafter referred to as the present invention battery G2.

(Example 3)
The slurry used for forming the separator coating layer is the same as that of the first embodiment except that the concentration of the copolymer containing the acrylonitrile structure is 5% by mass with respect to the total amount of the slurry and the thickness of the coating layer is 3 μm. A battery was produced in the same manner as in Example 1.
The battery thus produced is hereinafter referred to as the present invention battery G3.

(Experiment)
Table 14 shows the results obtained by examining the charge storage characteristics (remaining capacity after charge storage) of the batteries G1 to G3 of the present invention. The table also shows the results of the battery E1 of the present invention and the comparative battery Z1.
The charge / discharge conditions, the storage conditions, and the remaining capacity calculation method are the same as those in the experiment of the first reference example.

[Discussion]
As is clear from Table 14, in the charge storage test, the batteries E1, G1 to G3 of the present invention in which the coating layer was formed on the negative electrode side of the separator body were charged and stored compared to the comparative battery Z1 in which the coating layer was not formed. It can be seen that the remaining residual capacity is greatly improved. Further, when the present invention batteries E1, G1 to G3 are compared, the remaining capacity after charge storage is almost equal to the concentration of the copolymer (insoluble binder) containing the acrylonitrile structure relative to the total amount of the slurry and the thickness of the coating layer. It was found that it was not affected.

  Here, the greater the thickness of the coating layer and the higher the concentration of the water-insoluble binder, the more the resistance between the electrodes increases (because the distance becomes longer and the lithium ion permeability deteriorates). The concentration of the copolymer (non-water-soluble binder) containing the acrylonitrile structure with respect to the total amount (total amount of titanium oxide, copolymer containing the acrylonitrile structure, CMC, and surfactant) is preferably 10% by mass or less. Desirably, it is preferably 5% by mass or less, particularly preferably 3% by mass or less.

  On the other hand, the thickness of the coating layer is preferably regulated to 4 μm or less, and more preferably 2 μm or less, in order to suppress a decrease in battery load characteristics and a decrease in energy density. In addition, if the thickness of a coating layer is about 1 micrometer, it has confirmed that the effect of this invention is exhibited.

[ Fourth embodiment]
The end-of-charge voltage is 4.40 V, the thickness of the coating layer is 2 μm, the separator of the present invention is IS15, the comparative battery is CS2, and the packing density of the cathode active material layer is changed. And the relationship between the charge storage characteristics and the results are shown below.

Example 1
A battery was fabricated in the same manner as in Example 2 of the first example except that the packing density of the positive electrode active material layer was 3.20 g / cc.
The battery thus produced is hereinafter referred to as the present invention battery H1.

(Example 2)
A battery was fabricated in the same manner as in Example 2 of the first example except that the packing density of the positive electrode active material layer was 3.40 g / cc.
The battery thus produced is hereinafter referred to as the present invention battery H2.

(Experiment)
Since the storage characteristics (remaining capacity after storage) of the batteries H1 and H2 of the present invention were examined, the results are shown in Table 15. The table also shows the results of the battery E2 of the present invention and the comparative batteries Z2, X1 to X3, and W2.
The charge / discharge conditions, the storage conditions, and the remaining capacity calculation method are the same as those in the experiment of the first reference example.

As apparent from Table 15, when the packing density of the positive electrode active material layer is 3.20 g / cc, it is recognized that the remaining capacity is not only in the present invention battery H1 but also in the comparative battery X1. When the positive electrode active material layer has a packing density of 3.40 g / cc or more, the batteries H2 and E2 of the present invention have a certain remaining capacity, but the comparative batteries Z2, X2 and X3 have a remaining capacity. It can be seen that it is very low. This is presumed to be a phenomenon caused by the problem of the surface area in contact with the electrolyte and the degree of deterioration at the site where the side reaction occurs. The specific reason is considered to be the same as the reason shown in the experiment of the fourth reference example.
From the above results, it is particularly effective when the packing density of the positive electrode active material layer is 3.40 g / cc or more. However, the packing density of the negative electrode active material layer and the type of the active material are not particularly limited.

[Other matters]
[A] Items specific to the case where an organic solvent is used as a solvent (when a water-insoluble binder is used as a binder) (1) The material of the water-insoluble binder is limited to a copolymer containing an acrylonitrile unit. Instead, PTFE (polytetrafluoroethylene), PVDF (polyvinylidene fluoride), PAN (polyacrylonitrile), SBR (styrene butadiene rubber) and the like, modified products and derivatives thereof, polyacrylic acid derivatives, and the like may be used. However, a copolymer or polyacrylic acid derivative containing an acrylonitrile unit is preferable in order to exhibit the effect as a water-insoluble binder with a small amount of addition.

(2) The coating layer is not limited to being formed on both sides of the separator body, and may be formed only on one side. Thus, when it forms only in one side, the thickness of a separator becomes small and it can suppress that a battery capacity falls. Moreover, when forming only on one side, in order to raise a trap effect more, forming in the separator main body by the side of a positive electrode is desirable. That is, if the coating layer is formed on the surface of the separator main body on the positive electrode side, cobalt ions and manganese ions eluted from the electrolytic solution reacted with the positive electrode and the positive electrode active material immediately (before moving to the separator). This is because the above effect can be further exhibited.

[B] Items peculiar when water is used as a solvent (when a water-insoluble binder and a water-soluble binder are used as a binder) (1) The material of the water-insoluble binder is a copolymer containing an acrylonitrile unit. However, other acrylic polymers, nitrile polymers, diene polymers, and non-fluorine-containing polymers such as copolymers thereof are desirable. Fluorine-containing polymers such as PVDF and PTFE can also be used, but it is desirable to use a non-fluorine-containing polymer in order to sufficiently exhibit the function of being able to exert a binding force with a small amount of addition and being flexible. In particular, it is preferable to use an acrylic polymer. The addition amount of the water-insoluble polymer is the total amount of solids (particles forming the porous layer, and in the above examples, the total amount of filler particles, water-insoluble binder, water-soluble binder, and surfactant). 10 mass% or less, preferably 5 mass% or less, more preferably 3 mass% or less. Moreover, in order to fully exhibit binding property, it is desirable that it is 0.5 mass% or more. Moreover, since the coating layer is disposed on the negative electrode side of the separator, it does not come into direct contact with the positive electrode, and the stability with respect to the positive electrode potential does not require special consideration. However, it is not preferable to use a material that is known in advance to decompose at the positive electrode potential, such as electrochemically unstable SBR at about 4.1 V.

(2) Examples of the water-soluble polymer include cellulose-based polymers including CMC, and ammonium salts, alkali metal salts, polyacrylic acid ammonium salts, polycarboxylic acid ammonium salts, and the like thereof. The addition amount of these water-soluble polymers is 10% by mass or less, preferably 0.5% by mass or more and 3% by mass or less, based on the total amount of solids.

(3) Although there is no restriction | limiting in particular in the kind of surfactant, A nonionic surfactant is preferable when the influence on the battery performance inside a lithium ion battery, etc. are considered. Moreover, the addition amount of these surfactants is 3% by mass or less, preferably 0.5% by mass or more and 1% by mass or less, based on the total amount of solids.

(4) The total amount of solids excluding filler particles relative to the total amount of solids (total amount of water-insoluble binder, water-soluble binder, and surfactant in the above examples) is desirably 30% by mass or less.

[C] Items common to both (1) The positive electrode active material is not limited to the lithium cobalt oxide, but is a cobalt-nickel-manganese lithium composite oxide, an aluminum-nickel-manganese lithium composite oxide, A lithium composite oxide containing cobalt or manganese, such as an aluminum-nickel-cobalt composite oxide, or spinel type lithium manganate may be used. Preferably, it is a positive electrode active material whose capacity increases by further charging with respect to a specific capacity of 4.3 V at the lithium reference electrode potential, and preferably has a layered structure. Moreover, these positive electrode active materials may be used independently and may be mixed with the other positive electrode active material.

(2) The method of mixing the positive electrode mixture is not limited to the wet mixing method, and is a method in which the positive electrode active material and the conductive agent are dry mixed in advance, and then PVDF and NMP are mixed and stirred. Also good.

(3) The negative electrode active material is not limited to the above graphite, and any material that can insert and desorb lithium ions, such as graphite, coke, tin oxide, metallic lithium, silicon, and a mixture thereof. Any type.

(4) The lithium salt of the electrolytic solution is not limited to the above LiPF ₆ , but LiBF ₄ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiPF _6-X ( _{C n F 2n + 1) X} [ where, 1 <x <6, n = 1 or 2] may be such, it is also possible to use a mixture of two or more thereof. The concentration of the lithium salt is not particularly limited, but is preferably regulated to 0.8 to 1.5 mol per liter of the electrolyte. Further, the solvent of the electrolytic solution is not limited to ethylene carbonate (EC) or diethyl carbonate (DEC), but propylene carbonate (PC), γ-butyrolactone (GBL), ethyl methyl carbonate (EMC), dimethyl carbonate. A carbonate-based solvent such as (DMC) is preferable, and a combination of a cyclic carbonate and a chain carbonate is more preferable.

(5) The present invention is not limited to a liquid battery, but can be applied to a gel polymer battery. Examples of the polymer material in this case include polyether solid polymer, polycarbonate solid polymer, polyacrylonitrile solid polymer, oxetane polymer, epoxy polymer, a copolymer composed of two or more of these, or a crosslinked polymer. A molecule or PVDF is exemplified, and a solid electrolyte in which this polymer material, a lithium salt, and an electrolyte are combined into a gel can be used.

The present invention can be applied to a drive power source of a mobile information terminal such as a mobile phone, a notebook personal computer, and a PDA, for example, in applications that require a particularly high capacity. In addition, it can be expected to be used in high output applications that require continuous driving at high temperatures and applications where the battery operating environment is severe, such as HEVs and electric tools.

It is a graph which shows the relationship between the change of the crystal structure of lithium cobaltate, and electric potential. It is a graph which shows the relationship between the residual capacity | capacitance after charge storage, and the void | hole volume of a separator. It is a graph which shows the relationship between the charging / discharging capacity | capacitance and battery voltage in the comparison battery Z2. It is a graph which shows the relationship between charging / discharging capacity | capacitance and battery voltage in this invention battery A2. It is a graph which shows the relationship between the residual capacity | capacitance after charge storage, and the void | hole volume of a separator.

1 Meandering part

Claims (21)

An electrode body comprising a positive electrode having a positive electrode active material layer containing a positive electrode active material, a negative electrode having a negative electrode active material, and a separator interposed between the two electrodes, and a nonaqueous electrolyte impregnated in the electrode body In non-aqueous electrolyte batteries,
The positive electrode active material contains at least cobalt or manganese, and the separator includes a porous separator body and a coating layer formed on the negative electrode side surface of the separator body. Contains filler particles, water-insoluble binder and water-soluble binder ,
When the thickness of the separator body is x (μm) and the porosity of the separator body is y (%), the value obtained by multiplying x and y is 1500 (μm ·%) or less. A non-aqueous electrolyte battery characterized by being made .   The nonaqueous electrolyte battery according to claim 1, wherein the concentration of the water-insoluble binder with respect to the filler particles is 50% by mass or less.   The non-aqueous electrolyte battery according to claim 1, wherein the water-insoluble binder comprises a copolymer containing acrylonitrile units and / or a polyacrylic acid derivative.   The water-insoluble binder is made of a non-fluorine-containing polymer, and the water-soluble binder is at least one selected from the group consisting of cellulosic polymers or ammonium salts, alkali metal salts, polyacrylic acid ammonium salts, and polycarboxylic acid ammonium salts. The nonaqueous electrolyte battery according to claim 1, comprising a seed.   The non-aqueous electrolyte battery according to claim 1, wherein the coating layer contains a surfactant.   The nonaqueous electrolyte battery according to claim 1, wherein the ratio of the water-insoluble binder to the total amount of solid content is 10% by mass or less. The total amount of solids excluding filler particles relative to the amount of filler particles is 30% by mass or less,
The nonaqueous electrolyte battery according to claim 1. The x value obtained by multiplying the y is regulated so that 800 (·% μm) or less, the non-aqueous electrolyte battery according to claim 1, wherein. Comprising the filler particles from the inorganic particles, non-aqueous electrolyte battery according to claim 1-8, wherein. The nonaqueous electrolyte battery according to claim 9 , wherein the inorganic particles are made of rutile type titania and / or alumina. The average particle size of the filler particles is regulated to be larger than the average pore diameter of the separator body, a non-aqueous electrolyte battery according to claim 1-10, wherein. The thickness of the coating layer is 4μm or less, the non-aqueous electrolyte battery according to claim 1 to 11 wherein. The filling density of the positive electrode active material layer is 3.40 g / cc or more, the non-aqueous electrolyte battery according to claim 1 to 12 wherein. The positive electrode until the above 4.30V against the lithium reference electrode potential is charged, non-aqueous electrolyte battery according to claim 1 to 13 wherein. The positive electrode until the above 4.40V against the lithium reference electrode potential is charged, non-aqueous electrolyte battery according to claim 1 to 13 wherein. The positive electrode until the above 4.45V against the lithium reference electrode potential is charged, non-aqueous electrolyte battery according to claim 1 to 13 wherein. The positive electrode active material contains at least lithium cobaltate in which aluminum or magnesium is dissolved, and zirconium in electrical contact with the lithium cobaltate is fixed to the lithium cobaltate surface. the non-aqueous electrolyte battery according to claim 1-16, wherein. It may be used in an atmosphere of more than 50 ° C., the non-aqueous electrolyte battery according to claim 1 to 17 wherein. A separator containing filler particles, a water-insoluble binder, a water-soluble binder, and water is applied to one surface of the porous separator body and dried to form a coating layer on one surface of the separator body. Creating a step;
An electrode body in which a separator is disposed between both electrodes between a positive electrode having a positive electrode active material containing at least cobalt or manganese and lithium and a negative electrode having a negative electrode active material with the coating layer disposed on the negative electrode side. Creating a step;
Impregnating the electrode body with a non-aqueous electrolyte;
I have a,
When the thickness of the separator body is x (μm) and the porosity of the separator body is y (%), the value obtained by multiplying x and y is 1500 (μm ·%) or less. A method for producing a nonaqueous electrolyte battery, comprising using a separator . The method for producing a nonaqueous electrolyte battery according to claim 19 , wherein the slurry further contains a surfactant. The method for producing a non-aqueous electrolyte battery according to claim 19 or 20 , wherein, in the step of producing the separator, a doctor blade method, a gravure coating method, a transfer method or a die coating method is used as a method for forming the coating layer.