JP2007280917A - Nonaqueous electrolyte battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte battery having high cycle characteristics and high storage characteristics at high temperature and showing high reliability even in battery constitution providing high capacity. <P>SOLUTION: In the nonaqueous electrolyte battery equipped with an electrode body comprising a positive electrode having a positive active material layer containing a positive active material, a negative electrode, a separator interposed between both electrodes, and an electrolyte comprising a solvent and a lithium salt, which is impregnated into the electrode body, at least cobalt or manganese is contained in the positive active material, the positive electrode is charged to 4.40 V of more vs. a lithium electrode, an inorganic particle layer containing inorganic particles and a binder is formed between the positive electrode and the separator, and LiBF<SB>4</SB>is included in the lithium salt. <P>COPYRIGHT: (C)2008,JPO&INPIT

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 more particularly to a battery structure 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. Is.

  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 by moving between 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.

  Under these circumstances, 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.

  Under these circumstances, we have increased the capacity of the battery using lithium cobaltate as the positive electrode active material by increasing the depth of use (charging depth) in the region above the current 4.2 V. A possible battery was developed. 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. As a result of our investigation, it has been found that by adding zirconia, aluminum, and magnesium to lithium cobaltate, a performance similar to 4.2 V can be obtained under high-voltage room temperature conditions. It is essential to ensure the performance under high-temperature driving conditions such as the power consumption of the start-up terminal and the ability to withstand continuous use in a high-temperature environment. In this sense, technology that can ensure reliability not only at room temperature but also at high temperatures The development of was urgent.

JP 2002-141042 A

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

  In particular, in battery systems 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. In addition, it is presumed that the reason for being involved not only in the negative electrode but also in the physical properties of the separator is that a substance reduced by the negative electrode accumulates and fills the micropores of the separator.

  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 inorganic particle layer is large, it is difficult to achieve the original performance due to the increase in the internal resistance of the battery, and the battery capacity is reduced. In addition, there is a problem that the coating effect is limited because it is difficult to completely coat the entire particle. 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 can exhibit high reliability even in a battery configuration characterized by high capacity.

In order to achieve the above object, the present invention comprises a positive electrode having a positive electrode active material layer containing a positive electrode active material, a negative electrode, a separator interposed between the two electrodes, a solvent, and a lithium salt. A non-aqueous electrolyte battery comprising the non-aqueous electrolyte impregnated in the electrode body, wherein the positive electrode active material contains at least cobalt or manganese, and between the positive electrode and the separator. An inorganic particle layer containing inorganic particles and a binder is formed, and the lithium salt contains LiBF ₄ , and the positive electrode is charged to 4.40 V or more with respect to the lithium reference electrode potential. It is characterized by that.

If LiBF ₄ is added to the electrolytic solution as described above, a film derived from LiBF ₄ is formed on the surface of the positive electrode active material, and a substance (cobalt ion or manganese ion) constituting the positive electrode active material due to the presence of this film. Elution and decomposition of the electrolyte solution on the positive electrode surface can be suppressed. Therefore, precipitation of cobalt ions, manganese ions, or decomposition products of the electrolytic solution on the negative electrode surface can be suppressed.

However, it is difficult to completely cover the positive electrode active material with the LiBF ₄ -derived film, and it is difficult to sufficiently suppress the elution of the material constituting the positive electrode active material and the decomposition of the electrolytic solution on the positive electrode surface. Therefore, when an inorganic particle layer is formed between the positive electrode and the separator, cobalt ions and the decomposition products on the positive electrode are trapped by the inorganic particle layer, and these substances move to the separator and the negative electrode, and deposit → reaction (deterioration). ) Or clogging of the separator is suppressed. That is, the inorganic particle layer exhibits a filter function, and the precipitation of cobalt or the like at the negative electrode or the separator is suppressed. Thereby, the fall of a charge preservation | save characteristic is fully suppressed.

  Here, the inorganic particle layer exhibits the filter function because the binder contained in the inorganic particle layer absorbs the electrolytic solution and swells, so that the inorganic particles are appropriately filled with the swelled binder. Conceivable. Then, a complicated intricate filter layer is formed by forming a layer entangled with a plurality of inorganic particles, and it is considered that the physical trapping effect is also enhanced.

Moreover, there is a limitation that the positive electrode is charged until it becomes 4.40 V or higher with respect to the lithium reference electrode potential for the following reason. That is, as described above, although LiBF ₄ forms a film on the surface of the positive electrode and exhibits the advantage of being able to suppress elution from the positive electrode active material, decomposition of the electrolytic solution, etc., LiBF ₄ is a positive electrode. Therefore, the lithium salt concentration is lowered and the conductivity of the electrolyte solution is lowered. Therefore, when LiBF ₄ is added until the charging of the positive electrode is less than 4.40 V with respect to the lithium reference electrode potential (when the structure of the positive electrode is not so heavily loaded), the above-mentioned effect due to the addition of LiBF ₄ This is because the defects are pushed to the front, and the battery characteristics are deteriorated.

  Furthermore, if it is the said structure, since inorganic particles are firmly adhere | attached with the binder, there also exists an effect that it can suppress over a long period that an inorganic particle falls.

In a battery in which LiBF ₄ is not included in the lithium salt and the inorganic particle layer is not formed, the battery is stored when the positive electrode is charged to 4.40 V or more with respect to the lithium reference electrode potential. During recharging after charging, the behavior of the charging curve meandering and the amount of charge significantly increased was confirmed, but with the configuration of the present invention, it can be said that such abnormal charging behavior can be eliminated. It has also been confirmed that it is effective.
Although the preceding example the addition of LiBF ₄ in the electrolytic solution is disclosed (WO2006 / 54 604 discloses), simply requiring only the addition of the LiBF ₄ in the electrolytic solution be incapable of exerting the effects of the present invention From the above, it is clear.

It is only necessary that the inorganic particle layer is formed on the surface of the positive electrode active material layer and / or the positive electrode side surface of the separator. In these cases, it is desirable that the inorganic particle layer is formed on the entire surface of the positive electrode active material layer and on the entire surface of the separator on the positive electrode side.
The inorganic particle layer may be formed on the surface of the positive electrode active material layer and / or the surface on the positive electrode side of the separator. If the inorganic particle layer is formed on the entire surface of these layers, cobalt ions and manganese in the inorganic particle layer are formed. Since the ion trapping effect is sufficiently exhibited, it is possible to further suppress deterioration of cycle characteristics at high temperatures and deterioration of storage characteristics at high temperatures.

The ratio of the LiBF _{4 to} the total amount of the non-aqueous electrolyte is desirably 0.1% by mass or more and 5.0% by mass or less.
When the ratio of LiBF _{4 to} the total amount of the nonaqueous electrolyte is less than 0.1% by mass, the effect of improving storage characteristics is not sufficiently exhibited because the amount of LiBF ₄ is too small. This is because when the ratio of LiBF _{4 to} the total amount of the non-aqueous electrolyte exceeds 5.0 mass%, the discharge capacity and the discharge load characteristics are significantly reduced due to the side reaction of LiBF ₄ .

The lithium salt contains LiPF ₆ and it is desirable that the concentration of LiPF ₆ is 0.6 mol / liter or more and 2.0 mol / liter or less.
Since LiBF ₄ reacts and is consumed by charging / discharging, when the electrolyte is LiBF ₄ alone, sufficient conductivity cannot be ensured, and the discharge load characteristics are deteriorated. Therefore, it is desirable that LiPF ₆ is contained in the lithium salt. Further, even when LiPF ₆ is contained in the lithium salt, if the concentration of LiPF ₆ is too low, there is a disadvantage similar to the above, so the concentration of LiPF ₆ is 0.6 mol / liter or more. It is preferable. The concentration of LiPF ₆ is preferably 2.0 mol / liter or less. When the concentration of LiPF ₆ exceeds 2.0 mol / liter, the viscosity of the electrolytic solution increases, and the concentration of the liquid in the battery increases. This is due to the reason for the decrease.

The inorganic particles are preferably composed of rutile type titania and / or alumina.
Thus, rutile-type titania and / or alumina are preferable as the inorganic particles because they are excellent in stability in the battery (low reactivity with lithium) and low in cost. This is for a reason. 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 influence of the kind of the inorganic particles on this effect is very small, inorganic particles such as zirconia and magnesia may be used as the inorganic particles in addition to those described above.

It is desirable that the average particle size of the inorganic particles be regulated so as to be larger than the average pore size of the separator.
In this way, if the average particle size of the inorganic particles is smaller than the average pore size of the separator, a part of the separator may penetrate when the battery is crushed, and the separator may be greatly damaged. In addition, the inorganic particles may enter the micropores of the separator to deteriorate various characteristics of the battery, so that these disadvantages are avoided.
The average particle size of the inorganic 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.
In this specification, the average particle diameter means a value measured by the BET method.

The thickness of the inorganic particle layer is desirably 4 μm or less.
Although the above-described effects are exhibited as the thickness of the inorganic particle layer increases, if the thickness of the inorganic particle layer becomes too large, the load characteristics are reduced due to the increase in battery internal resistance, This is because the battery energy density is lowered due to the decrease in the amount of the substance. Considering this, the thickness of the inorganic particle layer is particularly preferably 2 μm or less.

Here, since the inorganic particle layer is complicated, the trapping effect is sufficiently exhibited even when the thickness is small. In addition, LiBF ₄ is added to the electrolyte, and a film derived from this LiBF ₄ is formed on the surface of the positive electrode active material, so that elution of substances constituting the positive electrode active material (cobalt ions and manganese ions) Since the decomposition of the electrolyte solution on the positive electrode surface can be suppressed, there is a problem even if the thickness of the inorganic particle layer is reduced as compared with the case where the inorganic particle layer is formed alone (when LiBF ₄ is not added). Absent. Considering this, the thickness of the inorganic particle layer may be 1 μm or more.
From the above, the thickness of the inorganic particle layer is desirably 1 μm or more and 4 μm or less, and particularly desirably 1 μm or more and 2 μm or less. In addition, the thickness of the said inorganic particle layer shall mean the thickness in one side.

It is desirable that the binder concentration relative to the inorganic particles is regulated to 30% by mass or less.
The upper limit is determined in this way because if the binder concentration is too high, the permeability of lithium ions to the active material layer is extremely reduced, and the resistance between the electrodes is increased, leading to a reduction in charge / discharge capacity. Because. Considering this, the binder concentration relative to the inorganic particles is more preferably 10% by mass or less, and particularly preferably 5% by mass or less.

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 effect of the present invention is more exhibited.

It is preferable that the positive electrode be charged up to 4.45V or more, preferably 4.50V or more with respect to the lithium reference electrode potential.
This is because, in a battery in which the positive electrode is charged at 4.45 V or more with respect to the lithium reference electrode potential, a difference in high-temperature characteristics remarkably appears depending on whether LiPF ₆ is added and whether there is an inorganic particle layer. This difference is particularly noticeable in a battery in which the positive electrode is charged at 4.50 V or more with respect to the lithium reference electrode potential.

The positive electrode active material preferably includes at least lithium cobaltate in which aluminum or magnesium is dissolved, and zirconia is preferably fixed to the surface of the lithium cobaltate.
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 is fixed to the lithium cobalt oxide 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.

When the thickness of the separator is x (μm) and the porosity of the separator is y (%), the value obtained by multiplying x and y is regulated to be 800 (μm ·%) or less. It is preferable to apply to a battery.
The separator pore volume is regulated to 800 (μm ·%) or less because the smaller the pore volume of the separator is, the more easily affected by precipitates and reaction products, and the characteristic deterioration becomes significant. This is because a remarkable effect can be exhibited by applying the present invention to a battery having a separator thus regulated. However, when the pore volume of the separator is 1500 (μm ·%) or less, the above-mentioned effects are sufficiently exerted, and even when the separator has a pore volume of 1500 (μm ·%) or more. The above-mentioned effects may be exhibited.
In addition, since the separator can be thinned in a battery having a small pore volume of the separator, the energy density of the battery can be improved.

According to the present invention, since LiBF ₄ in the electrolytic solution film from LiBF ₄ is formed on the surface of the positive electrode active material by being added, eluted from degradation products and a positive electrode active material was reacted with the positive electrode electrolyte The amount of cobalt ions and manganese ions to be reduced. In addition, since the inorganic particle layer disposed between the positive electrode and the separator exhibits an appropriate filter function, the decomposition products and cobalt ions are trapped in the inorganic particle layer, and cobalt and manganese are trapped in the negative electrode and separator. Precipitation can be sufficiently suppressed. Thereby, since the damage which a negative electrode and a separator receive is reduced remarkably, there exists an outstanding effect that deterioration of the cycling characteristics at high temperature and deterioration of the storage characteristics at high temperature can be suppressed. In addition, since the inorganic particles and the inorganic particle layer and the positive electrode active material layer or separator are firmly bonded to each other by the binder, it is possible to suppress the inorganic particle layer from dropping off from the positive electrode active material layer or the separator. There is also.

  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.

[Production of positive electrode]
First, lithium cobalt oxide as a positive electrode active material (one in which Al and Mg are solid-dissolved in 1.0 mol% and Zr is fixed on the surface of 0.05 mol%) and acetylene black as a carbon conductive agent And PVDF as a binder were mixed at a mass ratio of 95: 2.5: 2.5, and then these were stirred using a special machine combination mix with NMP as a solvent, and a positive electrode mixture slurry Was 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 form positive electrode active material layers on both surfaces of the positive electrode current collector.

Next, acetone as a solvent, inorganic particles of TiO ₂ (rutile type, particle size 0.38 μm, KR380 manufactured by Titanium Industrial Co., Ltd.) are 10% by mass with respect to acetone, and an acrylonitrile structure (unit) ) -Containing copolymer (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, after apply | coating the said slurry to the whole surface of the said positive electrode active material layer using the die-coating method, the solvent was dried and removed and the inorganic particle layer was formed in one side of the positive electrode active material layer. Next, in the same manner, an inorganic particle layer was formed on the entire other surface of the positive electrode active material layer. In addition, the thickness of the said inorganic particle layer was 4 micrometers on both surfaces (one side 2 micrometers), and 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 sides of a copper foil as an electric current body, and further dried and rolled to form negative electrode active material layers on both sides of the negative electrode current collector. The filling density of the negative electrode active material layer was 1.60 g / cc.

(Preparation of non-aqueous electrolyte)
LiPF ₆ is added at a rate of 1.0 mol / liter (M) to a mixed solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed at a volume ratio of 3: 7, and LiBF ₄ is added as an electrolytic solution. It was prepared by dissolving each at a ratio of 1% by mass with respect to the total amount.

[Separator type]
As the separator, a microporous membrane (thickness: 16 μm, average pore diameter 0.1 μm, porosity 47%) made of polyethylene (hereinafter sometimes abbreviated as PE) was used.

[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 disposed therein, and a nonaqueous electrolyte solution was poured into the space, and then an aluminum laminate film was welded and sealed to prepare a battery. In this battery, the battery is designed so that the end-of-charge voltage is 4.40 V (the positive electrode potential is 4.50 V with respect to the lithium reference electrode standard), and the capacity ratio between the positive and negative electrodes (the first negative electrode charge) The amount of active material of both positive and negative electrodes was adjusted so that (capacity / first charge capacity of positive electrode) was 1.08. The design capacity of the battery is 780 mAh.

[First embodiment]
While fixing the end-of-charge voltage and the physical properties of the separator, the presence / absence of the inorganic particle layer and the type of lithium salt are changed, and the presence / absence of the inorganic particle layer and the type / concentration of lithium salt and charge storage characteristics (remaining capacity) Since the relationship was investigated, the result is shown below.
Example 1
As Example 1, the battery shown in the best mode was used.
The battery thus produced is hereinafter referred to as the present invention battery A1.

(Examples 2 and 3)
A battery was fabricated in the same manner as in Example 1, except that the ratio of LiBF _{4 to} the total amount of the electrolytic solution was 3% by mass and 5% by mass, respectively.
The batteries thus produced are hereinafter referred to as present invention batteries A2 and A3, respectively.

(Comparative Example 1)
A battery was fabricated in the same manner as in Example 1 except that LiBF ₄ was not added to the electrolytic solution.
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 no inorganic particle layer was formed on the positive electrode.
The battery thus produced is hereinafter referred to as a comparative battery Z2.

(Comparative Examples 3-5)
A battery was fabricated in the same manner as in Examples 1 to 3 except that the inorganic particle layer was not formed on the positive electrode.
The batteries thus produced are hereinafter referred to as comparative batteries Z3 to Z5, respectively.
(Experiment)
Since the charge storage characteristics (remaining capacity after charge storage) of the present invention batteries A1 to A3 and the comparative batteries Z1 to Z5 were examined, the results are shown in Table 1. In addition, charging / discharging conditions and storage conditions are as follows.

[Charging / discharging conditions]
-Charging condition: 1.0 It (750 mA) current and battery voltage is the set voltage (the charge end voltage described above, in this experiment, 4.40 V for all batteries [4.50 V for the positive electrode potential relative to the lithium reference electrode standard]) The condition is that constant current charging is performed until the current value reaches ½, and then charging is performed until the current value becomes 1/20 It (37.5 mA) at the set voltage.
-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 charging / discharging conditions, and the battery charged to the set voltage under the charging conditions 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, and the remaining capacity is measured. Using the first discharge capacity after the storage test and the discharge capacity before the storage test, the following (1) The remaining capacity was calculated from the equation.
Remaining capacity (%) =
(First discharge capacity after storage test / Discharge capacity before storage test) × 100 (1)

[Discussion]
(1) Overall Consideration As is apparent from the results in Table 1, the inorganic particle layer is formed on the positive electrode (the surface of the positive electrode active material layer) in spite of the fact that the end-of-charge voltage and the physical properties of the separator are the same in all batteries. present battery A1~A3 are comparative battery Z2 of LiBF ₄ and the electrolytic solution without being inorganic particle layer is formed on the positive electrode is not added, the inorganic particle layer on the positive electrode but LiBF ₄ to be formed and the electrolyte is added compared with the comparative battery Z3~Z5 that but Comparative batteries Z1 to have been formed not LiBF ₄ is added to the electrolytic solution, and LiBF ₄ to the electrolyte is added not inorganic particle layer is formed on the positive electrode It can be seen that the remaining capacity is increased (charge storage characteristics are improved). The reason for this will be described separately in consideration of the advantages of adding LiBF ₄ to the following electrolyte and consideration of the advantages of forming the inorganic particle layer.

(2) Study on advantages of adding LiBF ₄ in the electrolytic solution First, when comparing the battery (Comparative Battery Z2～Z5) between which is not an inorganic particle layer is formed on the positive electrode, is LiBF ₄ in the electrolytic solution It is recognized that the added comparative batteries Z3 to Z5 have a larger remaining capacity than the comparative battery Z2 in which LiBF ₄ is not added to the electrolytic solution. On the other hand, even when the batteries (invention batteries A1 to A3, comparative battery Z1) in which the inorganic particle layer is formed on the positive electrode are compared, the invention batteries A1 to A3 in which LiBF ₄ is added to the electrolytic solution are electrolyzed. It can be seen that the remaining capacity is increased as compared with the comparative battery Z1 in which LiBF ₄ is not added to the liquid. This is considered to be due to the following reasons.

First of all, when considering why the charge storage characteristics deteriorate, there are several possible causes, but the positive electrode active material is 4.50 V based on the lithium reference electrode standard (the battery voltage is 0.1 V lower than this, 4.40V) If you consider that it is used near,
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 that the positive electrode and the electrolytic solution deteriorate, but particularly the decomposition products of the electrolytic solution considered to be caused by (I) and (II), the elution of elements from the positive electrode active material, etc. This is considered to affect the deterioration of the negative electrode active material due to clogging of the separator and deposition on the negative electrode.
Therefore, when LiBF ₄ is added to the electrolytic solution as described above, a film derived from LiBF ₄ is formed on the surface of the positive electrode active material. Therefore, the presence of this film can suppress the elution of substances (Co ions and Mn ions) constituting the positive electrode active material and the decomposition of the electrolyte solution on the positive electrode surface, thereby preserving the charge. It is considered that the deterioration of characteristics can be suppressed.

-Grounds that the effect of improving the charge storage characteristics is the effect of adding LiBF _{4 As} a method for simply examining the presence or absence of the eluate and decomposition products from the positive electrode, there is a method of examining the color state of the separator or the like. The reason why it can be investigated by this method is that Co ions eluted from the positive electrode react with the electrolytic solution and adhere to the separator and the like, but the color state of the separator changes according to the reaction at that time.

Therefore, the battery was disassembled after the test was completed and the discoloration of the separator was observed, and the results are also shown in Table 1. As is clear from Table 1, when batteries (comparative batteries Z2 to Z5) in which the inorganic particle layer is not formed on the positive electrode are compared with each other, the comparative batteries Z3 to Z5 in which LiBF ₄ is added to the electrolytic solution are slightly used. In contrast to the degree of coloring, it is recognized that the degree of coloring is increased in the comparative battery Z2 in which LiBF ₄ is not added to the electrolytic solution. On the other hand, even when the batteries (invention batteries A1 to A3, comparative battery Z1) in which the inorganic particle layer is formed on the positive electrode are compared with each other, the invention batteries A1 to A3 in which LiBF ₄ is added to the electrolyte solution are colored. In contrast, it was confirmed that the comparative battery Z1 in which LiBF ₄ was not added to the electrolytic solution was slightly colored. From this result, when LiBF ₄ is added, elution of substances (Co ions and Mn ions) constituting the positive electrode active material and decomposition of the electrolytic solution on the positive electrode surface can be suppressed. It is estimated that the damage of is reduced.

(3) Consideration regarding advantages of forming inorganic particle layer First, when batteries (comparative batteries Z1 and Z2) in which LiBF ₄ is not added to the electrolytic solution are compared, an inorganic particle layer is formed on the positive electrode. Further, it is recognized that the comparative battery Z1 has a larger remaining capacity than the comparative battery Z2 in which the inorganic particle layer is not formed on the positive electrode. On the other hand, when the batteries (invention batteries A1 to A3, comparison batteries Z3 to Z5) in which LiBF ₄ is added to the electrolyte solution are compared, the invention batteries A1 to A3 in which the inorganic particle layer is formed on the positive electrode are It can be seen that the remaining capacity is increased as compared with the comparative batteries Z3 to Z5 in which the inorganic particle layer is not formed on the positive electrode. This is considered to be due to the following reasons.
As described above, when LiBF ₄ is added to the electrolytic solution, a film derived from LiBF ₄ is formed on the surface of the positive electrode active material, but it is difficult to completely cover the positive electrode active material with the film derived from LiBF _4. It was difficult to completely suppress the elution of Co ions and the like and the decomposition of the electrolyte solution.

  Therefore, as described above, when the inorganic particle layer is formed on the positive electrode, the electrolytic solution component decomposed on the positive electrode and Co ions eluted from the positive electrode are trapped by the inorganic particle layer, move to the separator and the negative electrode, and are deposited. Reaction (deterioration) and clogging are suppressed, that is, the inorganic particle layer exhibits a filter function, and Co and the like are suppressed from being deposited on the negative electrode. As a result, it is considered that the battery with the inorganic particle layer is improved in charge storage performance as compared with the battery without the inorganic particle layer.

-Grounds that the effect of improving the charge storage characteristics is the above-mentioned filter effect As is clear from Table 1, when batteries (comparative batteries Z1, Z2) in which LiBF ₄ is not added to the electrolyte are compared, the positive electrode It can be seen that the comparative battery Z1 in which the inorganic particle layer is formed is slightly colored, whereas the comparative battery Z2 in which the inorganic particle layer is not formed on the positive electrode has a higher degree of coloring. On the other hand, when the batteries (invention batteries A1 to A3, comparison batteries Z3 to Z5) in which LiBF ₄ is added to the electrolytic solution are compared, in the invention batteries A1 to A3 in which the inorganic particle layer is formed on the positive electrode, While not colored, it was confirmed that the comparative batteries Z3 to Z5 in which the inorganic particle layer was not formed on the positive electrode were slightly colored. From this result, it is presumed that damage of the separator and the negative electrode is reduced by suppressing the movement of the reaction product at the positive electrode in the inorganic particle layer.

  In addition, the binder of the inorganic particle layer does not inhibit the air permeability at the time of producing the separator, but many of the binder swells about twice or more after the electrolyte solution is injected. Is filled. This inorganic particle layer is intricately complicated, and since the inorganic particles are firmly bonded to each other by the binder component, the strength is improved and the filter effect is sufficiently exhibited (intricate even if the thickness is small). (It is a structure and the trapping effect is increased).

  In addition, even when 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, so the effect is sufficient if the thickness of the polymer layer is not increased. 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 entire surface of the positive electrode is covered, the permeability of the electrolytic solution to the positive electrode is deteriorated, and adverse effects such as a reduction in load characteristics are increased. Therefore, in order to minimize the influence on other properties while exhibiting the filter effect, the inorganic particles containing inorganic particles (titanium oxide in this example) rather than simply forming the filter layer with only the polymer. It is advantageous to form a layer (filter layer).

(4) Summary From the above (2) and (3), by adding LiBF ₄ to the electrolytic solution, elution of substances (Co ions and Mn ions) constituting the positive electrode active material and the electrolytic solution on the positive electrode surface In the present invention batteries A1 to A3, the charge storage characteristics are drastically improved by the synergistic effect that the filter effect is exhibited by forming the inorganic particle layer on the positive electrode. Conceivable.

(5) Other Considerations in the Experiment When the batteries A1 to A3 of the present invention are compared, it is recognized that the effect of improving the charge storage characteristics increases as the concentration of LiBF ₄ added to the electrolytic solution increases. From this, it can be considered that the problem can be solved if the concentration of LiBF ₄ added to the electrolytic solution is increased (extremely, if the concentration of LiBF ₄ is extremely increased, the inorganic particle layer is necessary). It may not be.) However, the present inventors have found that when the concentration of LiBF ₄ added to the electrolyte is excessively increased, battery characteristics (such as initial charge / discharge efficiency) other than the charge storage characteristics are deteriorated. This will be described in the second embodiment below.

[Second Embodiment]
While fixing the end-of-charge voltage and the physical properties of the separator and arranging the inorganic particle layer on the positive electrode surface of all the batteries, the concentration of the lithium salt is fixed at 1.0 M (except for the present invention battery A1), The mixing ratio of LiPF ₆ and LiBF ₄ was changed, and the relationship between the mixing ratio of LiPF ₆ and LiBF ₄ and the charge storage characteristics (residual capacity) and initial charge / discharge characteristics (initial charge / discharge efficiency) were investigated. The results are shown below.

Example 1
A battery was fabricated in the same manner as in Example 1 of the first example except that 0.9M LiPF ₆ and 0.1M LiBF ₄ were used as the lithium salt of the electrolytic solution.
The battery thus produced is hereinafter referred to as the present invention battery B1.

(Example 2)
A battery was fabricated in the same manner as in Example 1 of the first example except that 0.5M LiPF ₆ and 0.5M LiBF ₄ were used as the lithium salt of the electrolytic solution.
The battery thus produced is hereinafter referred to as the present invention battery B2.

(Experiment)
Since the present invention batteries B1, B2, the present invention battery A1 (the concentration of the lithium salt is not 1.0M) and the comparative battery Z1 were examined for charge storage characteristics (remaining capacity) and initial characteristics (initial charge / discharge efficiency) The results are shown in Table 2.
The charge / discharge conditions, the storage conditions, and the remaining capacity calculation method are the same as those in the experiment of the first embodiment.
Further, the initial charge / discharge efficiency was calculated by the following equation (2) by performing charge / discharge under the same conditions as in the experiment of the first example.
Initial charge / discharge efficiency (%) =
(First discharge capacity after battery preparation / First charge capacity after battery preparation) × 100 (2)

[Discussion]
The lithium salt concentration fixed at 1.0 M, and, in the case of forming the inorganic particle layer on the surface of the positive electrode, the present invention cell B1, B2 which LiBF ₄ is added, in Comparative Battery Z1 which LiBF ₄ is not added It can be seen that the remaining capacity is increased (the charge storage characteristics are improved). This is because a LiBF ₄ -derived film is formed on the surface of the positive electrode, and the dissolution of the eluate from the positive electrode active material and the electrolytic solution is fundamentally suppressed, and the dissolved material and decomposition that could not be suppressed even by the effect of LiBF ₄ This is thought to be due to the fact that the product can be trapped by the inorganic particle layer. Further, this is supported by the fact that the separator is not colored in the batteries B1 and B2 of the present invention while the separator is slightly colored in the comparative battery Z1.

Here, the present invention cell B2 proportion of a 0.5M LiBF _4, the proportion of LiBF ₄ in comparison to the present invention cell B1 of 0.1 M, the remaining capacity can be observed that has more often. This is because if the amount of LiBF ₄ added is increased, the film formed on the surface of the positive electrode becomes thicker, so that elution from the positive electrode active material, decomposition of the electrolytic solution, and the like can be further suppressed.

However, the present invention cell B2 proportion of a 0.5M LiBF _4, the proportion of LiBF ₄ in comparison to the present invention cell B1 of 0.1 M, recognized that the initial characteristics (initial charge-discharge efficiency) is reduced It is done. This is presumably because, as the amount of LiBF ₄ added is increased, the film formed on the positive electrode surface becomes thicker as described above, so that Li that can be involved in charge / discharge decreases. Although not carried out in the above experiment, when the ratio of LiBF _{4 in} the lithium salt is large, the LiBF ₄ is highly reactive with the positive electrode, and therefore the conductivity of the electrolytic solution is reduced due to a decrease in the concentration of the lithium salt. There is also a risk that the load characteristics will be lowered.

On the other hand, in the present invention battery B1 having a LiBF ₄ ratio of 0.1M, the initial characteristics are improved, but the effect of improving the charge storage characteristics is reduced. This is because the film derived from LiBF ₄ did not cover the entire positive electrode and could not completely suppress elution from the positive electrode and decomposition of the electrolyte.

From the above, in order to improve the charge storage characteristics without deteriorating the initial characteristics, it is important to control the film thickness on the surface of the positive electrode depending on the lithium salt concentration and the amount of LiBF ₄ added, and the positive electrode that could not be completely suppressed It is important to trap the eluate from the electrolyte and the decomposition product of the electrolytic solution with the inorganic particle layer. In consideration of such a situation, the present inventors have investigated that when the concentration of LiPF _{6 in} the electrolytic solution is 0.6 M or more and 2.0 M or less, the amount of LiBF ₄ with respect to the total amount of the non-aqueous electrolyte. It turned out that it is preferable to regulate a ratio to 0.1 to 5.0 mass%. Thereby, it is possible to greatly improve the charge storage characteristics while suppressing the deterioration of the initial characteristics and load characteristics due to the LiBF ₄ film.

[Third embodiment]
While fixing the physical properties of the separator, the charge end voltage, the presence or absence of an inorganic particle layer, and the presence or absence of addition of LiBF ₄ (the ratio of LiBF _{4 to} the total mass of the electrolyte solution is fixed at 3% by mass) are changed. The relationship between the presence / absence of an inorganic particle layer and the presence / absence of addition of LiBF ₄ and the charge storage characteristics (remaining capacity) was examined, and the results are shown below.

(Examples 1 and 2)
The battery is designed so that the end-of-charge voltage is 4.30V, 4.35V (the positive electrode potential with respect to the lithium reference electrode standard is 4.40V, 4.45V, respectively), and the positive and negative electrode capacities at each potential A battery was fabricated in the same manner as in Example 2 of the first example except that the ratio was designed to be 1.08.
The batteries thus produced are hereinafter referred to as the present invention batteries C1 and C2, respectively.

(Comparative Example 1)
The battery was designed such that the end-of-charge voltage was 4.20 V (the positive electrode potential with respect to the lithium reference electrode standard was 4.30 V), and the capacity ratio between the positive and negative electrodes was 1.08 at that potential. Otherwise, a battery was fabricated in the same manner as in Example 2 of the first example.
The battery thus produced is hereinafter referred to as comparative battery Y1.

(Comparative Examples 2 to 4)
Batteries were produced in the same manner as in Comparative Example 1, Example 1, and Example 2 except that LiBF ₄ was not added to the electrolytic solution.
The batteries thus fabricated are hereinafter referred to as comparative batteries Y2, Y5, and Y8, respectively.

(Comparative Examples 5-7)
Batteries were produced in the same manner as in Comparative Example 1, Example 1 and Example 2 except that the inorganic particle layer was not formed on the surface of the positive electrode.
The batteries thus fabricated are hereinafter referred to as comparative batteries Y3, Y6, and Y9, respectively.

(Comparative Examples 8 to 10)
Batteries were produced in the same manner as in Comparative Example 1, Example 1, and Example 2 except that LiBF ₄ was not added to the electrolytic solution and an inorganic particle layer was not formed on the positive electrode surface.
The batteries thus fabricated are hereinafter referred to as comparative batteries Y4, Y7, and Y10, respectively.

(Experiment)
Table 3 shows the results obtained by examining the charge storage characteristics (remaining capacity after charge storage) of the batteries C1 and C2 of the present invention and the comparative batteries Y1 to Y10. The table also shows the results of the battery A1 of the present invention and the comparative batteries Z1, Z2, and Z4.
The charge / discharge conditions, the storage conditions, and the remaining capacity calculation method are the same as those in the experiment of the first embodiment (however, in the storage conditions, the comparison batteries Y1 to Y1 having a charge end voltage of 4.20V). In Y4, it was set as the conditions of leaving at 80 degreeC for 4 days).

[Discussion]
(1) Consideration when the charge end voltage is 4.20 V (the positive electrode potential is 4.30 V with respect to the lithium reference electrode standard) As is apparent from Table 3, when the charge end voltage is 4.20 V, the inorganic particle layer is the positive electrode. The comparative battery Y1 formed on the surface and added with LiBF ₄ has the inorganic particle layer not formed on the positive electrode surface and the comparative battery Y4 not added with LiBF ₄ and the inorganic particle layer formed on the positive electrode surface. However, it is recognized that the remaining capacity is small (the charge storage characteristics are deteriorated) as compared with the comparative battery Y2 to which LiBF ₄ is not added. This is considered to be due to the following reasons.

When the end-of-charge voltage is 4.20 V, the structure of the positive electrode is not so heavily loaded, so that the elution of Co ions and Mn ions from the positive electrode is small, and the amount of reaction products due to the decomposition of the electrolyte etc. Less. On the other hand, as described above, although LiBF ₄ forms a film on the surface of the positive electrode, although it exhibits the advantage of being able to further suppress the elution from the positive electrode active material and the decomposition of the electrolytic solution, Since LiBF ₄ has high reactivity with the positive electrode, there is also a disadvantage that the concentration of the lithium salt is lowered and the conductivity of the electrolytic solution is lowered. Therefore, the influence of such as elution of Co ions from the positive electrode adding LiBF ₄ to the case where reduced, disadvantage by adding LiBF ₄ than advantage by adding LiBF ₄ is pushed out to the front. For this reason, it is thought that it became the experimental result mentioned above.

In addition, although it is incidental, the comparative battery Y1 in which the inorganic particle layer is formed on the positive electrode surface and LiBF ₄ is added has LiBF ₄ added, but the inorganic particle layer is not formed on the positive electrode surface. When compared with the comparative battery Y2, the charge storage characteristics are hardly changed. Therefore, it can be seen that when the end-of-charge voltage is 4.20 V, it is not very meaningful to form the inorganic particle layer.

(2) Consideration when charge end voltage is 4.30 V or more (positive electrode potential with respect to lithium reference electrode standard is 4.40 V or more) On the other hand, when charge end voltage is 4.30 V or more, the inorganic particle layer has Inventive batteries C1, C2, and A2 formed on the surface of the positive electrode and added with LiBF ₄ are compared with each other at the same end-of-charge voltage (for example, in the case of the inventive battery C1, comparative batteries Y5 to Y5). Comparison with Y7, Y10, Z2 or LiBF _{4 in} which no inorganic particle layer is formed on the surface of the positive electrode and LiBF ₄ is added, but the inorganic particle layer is formed on the surface of the positive electrode. and and Comparative battery Y6, Y9, Z4 not be, an inorganic particle layer is formed on the positive electrode surface in comparison with the comparative batteries Y5, Y8, Z1 that has not been added LiBF _4, making it much remaining capacity (Charge storage characteristics are improved) it is observed. Furthermore, the higher the end-of-charge voltage, the greater the difference in charge storage characteristics between the battery of the present invention and the comparative battery (for example, the present invention is more than the difference between the present battery C1 and the comparative batteries Y5 to Y7). It is recognized that the difference between the battery C2 and the comparative batteries Y8 to Y10 is larger). This is considered to be due to the following reasons.

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, as well as the oxidation resistance potential of cyclic carbonates and chain carbonates generally used in lithium ion batteries. In order to approach the limit, elution of Co ions or the like more than expected from the voltage at which non-aqueous electrolyte secondary batteries have been used so far, or decomposition of the electrolyte proceeds. Therefore, in such a case, there is a significance of adding LiBF ₄ and a significance of forming an inorganic particle layer.

Specifically, the addition of LiBF ₄ in the case described above, since the film derived from LiBF ₄ to positive electrode is formed on the surface, dissolution of Co ions and Mn ions from the positive electrode, the decomposition of the electrolytic solution is suppressed Thus, the effect of suppressing the deterioration of the positive electrode is sufficiently exhibited, that is, the advantage of surpassing the disadvantages of adding LiBF ₄ as described above is exhibited. This is apparent when the comparative batteries Y7, Y10, Z2 and the comparative batteries Y6, Y9, Z4 are compared (compare with batteries having the same charge end voltage).

However, the addition of LiBF ₄ slightly causes Co ions and Mn ions to elute from the positive electrode active material, or causes decomposition of the electrolyte, resulting in a decrease in the remaining capacity after storage. Therefore, by forming an inorganic particle layer on the surface of the positive electrode, the reaction product, etc. that could not be completely suppressed by the LiBF ₄ -derived film is completely trapped by the inorganic particle layer, so that the reaction product, etc. It is possible to suppress the deposition → reaction (deterioration) / clogging, thereby significantly improving the charge storage characteristics. This is apparent when the batteries C1, C2 and A2 of the present invention are compared with the comparative batteries Y6, Y9 and Z4 (comparing between batteries having the same end-of-charge voltage).

[Fourth embodiment]
When the physical properties of the separator, the arrangement position of the inorganic particle layer, and the ratio of LiBF _{4 to} the total mass of the electrolytic solution are changed as compared with the first to third examples, the presence or absence of the inorganic particle layer and Since the relationship between the presence or absence of addition of LiBF ₄ and the charge storage characteristics (remaining capacity) was examined, the results are shown below.

(Example)
A separator having a pore diameter of 0.1 μm, a film thickness of 12 μm, and a porosity of 38% is used, and the inorganic particle layer (positive electrode side surface) is formed on the separator surface (positive electrode side) without forming the inorganic particle layer on the positive electrode surface. A battery was fabricated in the same manner as in Example 2 of the first example except that the ratio of LiBF _{4 to} the total amount of the electrolyte was 1% by mass. The inorganic particle layer was formed on the separator surface in the same manner as the inorganic particle layer was formed on the surface of the positive electrode active material layer.
The battery thus produced is hereinafter referred to as the present invention battery D.

(Comparative Example 1)
A battery was fabricated in the same manner as in the above example except that LiBF ₄ was not added to the electrolytic solution.
The battery thus produced is hereinafter referred to as comparative battery X1.

(Comparative Example 2)
A battery was fabricated in the same manner as in the above example except that the inorganic particle layer was not formed on the separator surface (surface on the positive electrode side).
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 the above example, except that LiBF ₄ was not added to the electrolytic solution and an inorganic particle layer was not formed on the separator surface (positive electrode side surface).
The battery thus produced is hereinafter referred to as comparative battery X3.

(Experiment)
The charge storage characteristics (remaining capacity after charge storage) of the present invention battery D1 and comparative batteries X1 to X3 were examined, and the results are shown in Table 4.
The charge / discharge conditions, the storage conditions, and the remaining capacity calculation method are the same as those in the experiment of the first embodiment.

As is clear from Table 4, the present invention cell D with the addition of LiBF ₄ in and electrolyte to form an inorganic particle layer on the separator surface, the LiBF ₄ in Electrolytic solution to form the inorganic particle layer on the surface of the separator added comparative battery X1 not, comparison batteries LiBF are ₄ by the addition of not form inorganic particle layer on the surface of the separator X2, and the inorganic particle layer is formed on and the surface of the separator without the addition of LiBF ₄ to the electrolyte It can be seen that the remaining capacity is larger than that of the comparative battery X3 (the charge storage characteristics are improved).

This is the same reason as described above, and by adding LiBF ₄ to the electrolytic solution, the elution suppression effect of substances (Co ions and Mn ions) constituting the positive electrode active material and the electrolysis on the positive electrode surface This is considered to be due to the fact that the liquid decomposition inhibiting effect is exhibited and the filter effect is exhibited by forming an inorganic particle layer on the separator surface. Therefore, it is understood that the inorganic particle layer is not limited to being formed on the surface of the positive electrode, but may be formed on the separator surface (surface on the positive electrode side).

[Other matters]
(1) The binder used for the inorganic particle layer is not limited to the copolymer (rubber-like polymer) containing the acrylonitrile structure (unit), but PTFE (polytetrafluoroethylene) or 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.

Here, in order to exhibit this effect, the following functions or characteristics are required as a binder.
(I) Function to ensure binding ability to withstand battery manufacturing process (II) Function to fill gaps between inorganic particles due to swelling after absorbing electrolyte (III) To ensure dispersibility of inorganic particles (reaggregation) Prevention)
(IV) Characteristic of less elution into the electrolyte solution Therefore, on the premise that the function of (II) and the characteristic of (IV) are satisfied, the function of (I) (III) can be satisfied even with a small amount of addition. It is particularly desirable to use a copolymer containing acrylonitrile units as a binder.

(2) The method for dispersing the slurry is not limited to the above Filmics method, and a bead mill method or a disper dispersion method can also be used. However, in the present invention, considering that the inorganic particles to be used have a small particle size and the slurry is vigorously settled without mechanical dispersion treatment, a homogeneous film cannot be produced. A method (wet dispersion method) used for dispersion of paint in the paint industry such as the bead mill method is suitable.
Moreover, as a solvent at the time of slurry preparation, NMP, cyclohexanone, water, etc. other than the said acetone are illustrated, However, It is not limited to these.

(3) The method for forming the inorganic particle layer is not limited to the above-described die coating method, and a gravure coating method, a dip coating method, a curtain coating method, a spray coating method, or the like can also be used. However, taking into account intermittent application to control the decrease in energy density due to coating on the surplus part (unnecessary part) and controlling the accuracy of thickness (implementing thin film coating), gravure coating It is desirable to use a method or a die coating method. Also avoids problems such as reduced adhesive strength due to diffusion of solvents and binders into the electrode (reduced adhesive strength of the positive electrode active material layer due to melting of the existing binder, and increased electrode plate resistance due to binder penetration into the inorganic particle layer). In order to achieve this, it is preferable to use a method with a short coating time or drying time, but also in this respect, it is preferable to use the gravure coating method or the like.

  The concentration of solids (filler particles and binder) in the slurry varies greatly depending on the coating method, but the spray coating method, dip coating method, or curtain coating method is difficult to control the thickness mechanically. When using, it is preferable that solid content concentration is low, and it is desirable that it is about 3-30 mass%. On the other hand, when the die coating method or the gravure coating method is used, it is desirable that the solid content concentration is about 5 to 70% by mass in consideration of the fact that the solid concentration may be high.

(4) The lithium salt to be mixed with LiBF ₄ is not limited to the above LiPF ₆ , but 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] or the like may be used, and a mixture of two or more of these may be used. 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 positive electrode active material is not limited to the above lithium cobaltate, but is a cobalt-nickel-manganese lithium composite oxide, an aluminum-nickel-manganese lithium composite oxide, and an aluminum-nickel-cobalt composite oxide. A lithium composite oxide containing cobalt or manganese such as spinel-type lithium manganate may be used. It is preferable that the positive electrode active material has a layered structure that increases in capacity by charging more than the specific capacity of 4.30 V at the lithium reference electrode potential. This is because, in such a positive electrode active material, the crystal structure becomes unstable and element elution easily occurs from the crystal due to an increase in the charging depth. Moreover, these positive electrode active materials may be used alone or in combination with other positive electrode active materials.

(6) The method for producing the positive electrode slurry is not limited to the wet mixing method, and may be 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. good.

(7) 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 mixtures thereof. Any type.

(8) 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.

Claims (17)

A non-aqueous electrolyte comprising a positive electrode having a positive electrode active material layer including a positive electrode active material, a negative electrode, a separator interposed between the two electrodes, and a non-aqueous electrolyte composed of a solvent and a lithium salt. In the nonaqueous electrolyte battery in which the electrode body is impregnated,
The positive electrode active material contains at least cobalt or manganese, an inorganic particle layer containing inorganic particles and a binder is formed between the positive electrode and the separator, and the lithium salt contains LiBF _4. And the positive electrode is charged until it becomes 4.40 V or higher with respect to the lithium reference electrode potential.   The nonaqueous electrolyte battery according to claim 1, wherein the inorganic particle layer is formed on a surface of the positive electrode active material layer.   The nonaqueous electrolyte battery according to claim 2, wherein the inorganic particle layer is formed on the entire surface of the positive electrode active material layer.   The nonaqueous electrolyte battery according to claim 1, wherein the inorganic particle layer is formed on a surface on the positive electrode side of the separator.   The nonaqueous electrolyte battery according to claim 4, wherein the inorganic particle layer is formed on the entire surface of the separator on the positive electrode side. The nonaqueous electrolyte battery according to claim 1, wherein a ratio of the LiBF _{4 to} the total amount of the nonaqueous electrolyte is 0.1% by mass or more and 5.0% by mass or less. The above lithium salt are included LiPF _6, the concentration of this LiPF ₆ of 2.0 mole / liter or less than 0.6 mol / liter, the non-aqueous electrolyte battery according to claim 6.   The nonaqueous electrolyte battery according to claim 1, wherein the inorganic particles are made of rutile type titania and / or alumina.   The nonaqueous electrolyte battery according to claim 1, wherein the inorganic particles are regulated so that an average particle size of the inorganic particles is larger than an average pore size of the separator.   The nonaqueous electrolyte battery according to claim 1, wherein the inorganic particle layer has a thickness of 4 μm or less.   The nonaqueous electrolyte battery according to claim 1, wherein the binder concentration relative to the filler particles is 30% by mass or less.   The nonaqueous electrolyte battery according to claim 1, wherein a packing density of the positive electrode active material layer is 3.40 g / cc or more.   The nonaqueous electrolyte battery according to claim 1, wherein the positive electrode is charged until it becomes 4.45 V or higher with respect to a lithium reference electrode potential.   The nonaqueous electrolyte battery according to claim 1, wherein the positive electrode is charged until it becomes 4.50 V or more with respect to a lithium reference electrode potential.   The non-aqueous solution according to claim 1, wherein the positive electrode active material contains at least lithium cobaltate in which aluminum or magnesium is dissolved, and zirconia is fixed to the surface of the lithium cobaltate. Electrolyte battery.   The nonaqueous electrolyte battery according to claim 1, which may be used in an atmosphere of 50 ° C. or higher.   When the thickness of the separator is x (μm) and the porosity of the separator is y (%), the value obtained by multiplying x and y is regulated to be 800 (μm ·%) or less. The nonaqueous electrolyte battery according to claim 1.