JP4968578B2 - Positive electrode active material, method for producing the same, and nonaqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a positive electrode active material and a nonaqueous electrolyte secondary battery using the same.

In recent years, non-aqueous electrolyte secondary batteries that are used as main power sources for mobile communication devices and portable electronic devices are characterized by having high electromotive force and energy density. Examples of the positive electrode active material used here include lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), and manganese spinel (LiMn ₂ O ₄ ). These active materials have a voltage of 4 V or more with respect to lithium. On the other hand, a carbon material is generally used for the negative electrode, and this is combined with the positive electrode active material described above to constitute a 4V class lithium ion battery.
For batteries, there is a growing demand for improving high rate characteristics and pulse characteristics as well as high energy density. Charging and discharging at a high rate increases the load on the active material, and as a result, it is difficult to maintain the cycle life with the conventional technology.

In addition, depending on the equipment used, even in such a high rate discharge, the flatness of the battery voltage in the charge / discharge curve is required. When lithium cobaltate (LiCoO ₂ ) and lithium nickelate (LiNiO ₂ ), which are positive electrode active materials having a layer structure, are used, an essentially S-shaped charge / discharge curve is obtained, It is difficult to maintain a flat charge / discharge voltage at a high rate. Further, since the positive electrode active material repeatedly expands and contracts in the layer direction along with charge and discharge, the cycle life particularly in high rate charge and discharge is shortened due to stress caused by this.

On the other hand, from the viewpoint of remaining capacity detection, the charge / discharge curve obtained using the positive electrode active material is recognized to have a relatively flat shape, and it is necessary to accurately measure a narrow potential range. The substance was unsuitable for detecting the remaining capacity. In particular, when lithium is inserted into the negative electrode by charging, the potential of the negative electrode suddenly drops to around 0.1 V, and thereafter, lithium is occluded at a substantially constant potential. Regarding the positive electrode active material, since LiMn ₂ O ₄ having a spinel structure draws a flatter charge / discharge curve, it remains in comparison with lithium cobaltate (LiCoO ₂ ) and lithium nickelate (LiNiO ₂ ) having a layer structure. Not suitable for capacity detection.

In general, in order to recognize the remaining capacity of a nonaqueous electrolyte secondary battery, for example, as represented by the method described in Patent Document 1, current and time are detected in addition to voltage, and based on these information. Thus, the remaining capacity of the battery is recognized by performing an operation in the integrated circuit.
As a charge termination monitor, Patent Document 2 proposes using LiMn ₂ O ₄ as a positive electrode active material and using Li ₄ Ti ₅ O ₁₂ and natural graphite as a negative electrode active material. This technology makes it possible to monitor the end of charging by providing a potential difference stage at the negative electrode potential. The potential changes at 1.5 V and Li ₄ Ti ₅ O ₁₂ where the potential changes at 1.5 _V. A negative electrode combined with natural graphite is disclosed.

Here, a battery system using a positive electrode including a conventional positive electrode active material having a spinel structure and a negative electrode including a lithium-containing titanium oxide having a spinel skeleton as the negative electrode will be described below.
For example, in Patent Document 3, the composition _{formula: Li 1 + x Mn 2-} xy M y O z ( wherein, 0 ≦ x ≦ 0.2,0.2 ≦ y ≦ 0.6,3.94 ≦ z ≦ 4.06, M is a positive electrode active material expressed by nickel or nickel, which is an essential component, and at least one selected from aluminum and a transition element), and does not contain NiO as an impurity A method for synthesizing active materials has been proposed. Specifically, it is described that after a mixture of a manganese compound and a metal M compound is fired at 900 ° C. to 1100 ° C., the mixture is fired again together with a lithium compound.
However, in this method, since the reaction between manganese and the metal M is a reaction between the solid and the solid, it is difficult to uniformly dissolve both of them. Moreover, since baking is performed at a high temperature of 900 ° C. or higher, the reactivity with lithium is lowered thereafter, and it is difficult to obtain a target positive electrode active material.

Patent Document 4 is represented by a general formula: Li _{x + y} M _z Mn _{2 -yz} O ₄ , has a spinel crystal structure, M represents a transition metal, and 0 ≦ x <1, 0 ≦ y <0.33. , 0 <z <1, a positive electrode active material that is charged and discharged at a potential of 4.5 V or higher with respect to Li / Li + is disclosed.
Patent Document 5 discloses a battery including a positive electrode containing Li ₂ MnO ₃ or LiMnO ₂ as an active material, and a negative electrode containing Li _4/3 Ti _5/3 O ₄ or LiTi ₂ O ₄ containing lithium as an active material. Patent Document 6 discloses a battery including a positive electrode including lithium cobaltate (LiCoO ₂ ) and a negative electrode including lithium titanate (Li _4/3 Ti _5/3 O ₄ ). Yes. Patent Document 7 discloses a negative electrode containing lithium or a lithium alloy or a spinel-type lithium-titanium oxide as an active material, and a spinel-type lithium-manganese oxide Li _4/3 Mn _{5 /} as an active material. A battery including a positive electrode containing ₃ O ₄ and an electrolytic solution containing a mixed solvent of two or more components containing LiN (CF ₃ SO ₂ ) ₂ and ethylene carbonate as solutes is disclosed.

In Patent Document 8, lithium-containing transition metal oxide (LiA _x B _1-x O ₂ ) is used as a positive electrode active material, and lithium titanium oxide (Li _4/3 Ti _5/3 O ₄ ) is used as a negative electrode active material. ) And the ratio of the actual capacity of the negative electrode active material to the actual capacity of the positive electrode active material is 0.5 or less. In Patent Document 9, lithium manganese oxide (Li _4/3 Mn _5/3 O ₄ ) is used as a positive electrode active material, and lithium titanium oxide (Li _4/3 Ti _5/3 O ₄ ) and lithium are used as a negative electrode. It is disclosed that the molar ratio of lithium titanium oxide to lithium manganese oxide is 1.0 or less and the molar ratio of lithium to lithium titanium oxide is 1.5 or less.

Furthermore, Patent Document 10 discloses that the positive electrode has a composition formula Li _1-x A _x Ni _{1- y My} O ₂ (A is one or more selected from alkali metals and alkaline earth metals excluding Li; 1 or more selected from Co, Mn, Al, Cr, Fe, V, Ti and Ga, 0 ≦ x ≦ 0.2, 0.05 ≦ y ≦ 0.5), and the average particle size is 0.00. It contains a positive electrode active material composed of secondary particles obtained by agglomerating primary particles of 5 μm or more, and the negative electrode is represented by a composition formula: LiaTibO 4 (0.5 ≦ a ≦ 3, 1 ≦ b ≦ 2.5). Including a lithium-titanium composite oxide as a negative electrode active material.

Furthermore, for example, Patent Document 11, the composition _{formula: Li 1 + x M y Mn} 2-xy O 4-z (M is Ti, V, Cr, Fe, Co, Ni, Zn, Cu, W, Mg, And one or more of Al and 0 ≦ x ≦ 0.2, 0 ≦ y <0.5, 0 ≦ z <0.2), and by a powder X-ray diffraction method using CuKα rays (400) A positive electrode containing a lithium manganese composite oxide having a half-width of a diffraction peak of 0.02θ or more and 0.1θ or less (θ is a diffraction angle) and having an octahedral shape with primary particles as a positive electrode active material and a composition formula LiaTibO 4 ( A battery including a negative electrode including a lithium titanium composite oxide represented by 0.5 ≦ a ≦ 3 and 1 ≦ b ≦ 2.5) as a negative electrode active material is disclosed.
JP 11-072544 A JP 2000-348725 A JP 11-321951 A JP-A-9-147867 JP-A-7-320784 JP 7-335261 A JP-A-10-27609 JP 10-27626 A Japanese Patent Laid-Open No. 10-27627 Japanese Patent Laid-Open No. 2001-243952 JP 2001-210324 A

  However, the conventional techniques as described above cannot sufficiently solve the above-described problems of improving the high rate characteristics and pulse characteristics. For example, when charging / discharging at a high rate, the load on the active material is increased, causing structural breakdown and the like, and it becomes difficult to maintain the cycle life. In addition, lithium cobaltate and graphite materials having a layer structure cause stress to the active material and bleeding of the electrolyte from between the electrodes by repeating large expansion and contraction in the layer direction with charge and discharge, In particular, the cycle life in high rate charge / discharge is shortened. Therefore, in order to extend the battery life as described above, it is a big problem to suppress the expansion / contraction of the active material.

  In addition, it is desirable that the battery used as the power source of the electronic device has a flat discharge curve, and such voltage flatness is required even in such a high rate discharge. However, the battery discharge curve that has been put to practical use up to now has either an S-shape and the voltage gradually decreases, or is flat but the battery voltage rapidly decreases at the end of discharge. It is. In the former case, it is not difficult to monitor the remaining capacity, but there is a problem that the voltage is preferably flat. On the other hand, in the latter case, the voltage change is very small until the end of discharge, and as a result, it is very difficult to monitor the remaining capacity of the battery. Therefore, obtaining a battery capable of appropriately monitoring the remaining capacity has been one of the problems.

  Further, for example, there is an increasing demand for large batteries such as a battery pack (pack battery) obtained by connecting a plurality of conventional batteries as described above in series. Is generated and a temperature distribution is formed, and there is a difference between the temperature of the outer battery and the temperature of the inner battery. Specifically, there is a problem that the temperature of the outer battery is lowered, and thereby the polarization (resistance) of the outer battery becomes larger than that of the assembled battery, and in particular, the overcharge cycle life is inferior. . As a result, the overcharge cycle life as an assembled battery is also inferior.

Therefore, in order to solve the above problems, the present invention provides a composition of positive electrode active material, crystal structure and synthesis method, selection of battery system, electrolyte, current collector material of positive and negative electrodes, separator, and positive and negative electrode active materials. It is an object of the present invention to provide a non-aqueous electrolyte secondary battery with improved rate characteristics, cycle life, safety and storage stability by designing a battery by optimizing the capacity ratio and the like. The present invention also provides a non-aqueous electrolyte secondary battery using a positive electrode active material having a flat charge / discharge curve and having a step in the voltage intentionally at the end of discharge to facilitate monitoring of the remaining capacity. To do.
Furthermore, the present invention is such that even when a plurality of nonaqueous electrolyte secondary batteries are connected in series to form an assembled battery, a temperature distribution is not easily formed in the assembled battery, and in particular, it has excellent overcharge cycle characteristics. Another object is to provide a non-aqueous electrolyte secondary battery.

In order to solve the above problems, the present invention provides:
A single cell comprising a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte;
The positive electrode active material has the formula: Li ₂ ± α [Me] ₄ O _8-x (where 0 ≦ α <0.4, 0 ≦ x <2, Me is Mn, Ni, Cr, Fe, Co and A transition metal element including at least one selected from the group consisting of Cu) and having a spinel-related crystal,
The negative electrode active material includes a titanium oxide having a spinel structure ,
The weight ratio of the negative electrode active material to the positive electrode active material is 0.5 or more and less than 1.2,
Provided is a nonaqueous electrolyte secondary battery characterized in that the nonaqueous electrolyte contains acetonitrile.
The non-aqueous electrolyte secondary battery may include a plurality of the single cells connected in series or in parallel, and can also be used as an assembled battery or a module battery.

The positive electrode active material preferably has a 2 × 2 superlattice in a transition metal element phase.
In the positive electrode active material, it is preferable that the ratio of Mn to other transition metal elements is substantially 3: 1.
The positive electrode active material preferably has Li and / or Me at the 16 (c) site in the F d3m space group.
It is preferable that the positive electrode active material has a charge / discharge curve having a potential step of 0.2 to 0.8V.
In the positive electrode active material, it is preferable that a lattice constant assigned to a cubic crystal is 8.3 mm or less.
The positive electrode active material preferably includes a mixture of crystal particles having a particle diameter of 0.1 to 8 μm and secondary particles of the crystal particles having a particle diameter of 2 to 30 μm.

It is preferred before Symbol titanium oxide is Li ₄ Ti ₅ O _12.
It is preferable that the positive electrode and the negative electrode have a current collector made of aluminum or an aluminum alloy.
The non-aqueous electrolyte contains at least one selected from the group consisting of propylene carbonate, γ-butyl lactone, γ-valerolactone, methyl diglyme, sulfolane, trimethyl phosphate, triethyl phosphate, and methoxymethyl ethyl carbonate. preferable.

The separator is preferably made of a nonwoven fabric.
It is preferable that the nonwoven fabric is composed of at least one selected from the group consisting of polyethylene, polypropylene, and polybutylene terephthalate .
The single cell preferably has a practical charge / discharge region of 2.5 V to 3.5 V and a practical average voltage of 3 V class.
Moreover, it is preferable that the said single cell has an operation discharge curve which has the level | step difference of 0.2V-0.8V.

According to the battery system of the present invention, the flatness of the discharge voltage, high rate discharge characteristics, pulse characteristics, cycle life during high rate charge / discharge, and the like can be dramatically improved in a balanced manner. In the above embodiments, the secondary battery of the present invention as a battery for portable devices has been described. However, the present invention relates to a power source for an electric tool and a hybrid vehicle for which high charge / discharge and cycle life are strongly demanded. The present invention can also be applied to a large battery of a drive system used for a power source, a power source for an electric vehicle, and the like, and can exhibit its characteristics.
According to the present invention, an inexpensive nickel-manganese composite oxide having a flat and high voltage can be effectively used as a positive electrode active material, and by using a titanium oxide for the negative electrode, it is excellent in 3V class high rate characteristics and cycle life. A good nonaqueous electrolyte secondary battery can be provided.
Furthermore, according to the present invention, even when a plurality of nonaqueous electrolyte secondary batteries are connected in series to form an assembled battery, a temperature distribution is not formed in the assembled battery, and particularly overcharge cycle characteristics. It is possible to realize a nonaqueous electrolyte secondary battery excellent in the above.

The present invention optimizes design items such as a novel composition and synthesis method of a positive electrode active material, other battery constituent materials, and a capacity ratio between the positive electrode active material and the negative electrode active material, thereby achieving a flat charge / discharge voltage and an excellent In addition, a non-aqueous electrolyte secondary battery having high rate characteristics and cycle life can be provided.
In addition, if an appropriate battery system is designed using the positive electrode active material of the present invention, a potential step can be formed freely in the vicinity of the end of discharge. Thus, an alarm function can be added to the nonaqueous electrolyte secondary battery according to the present invention so that the remaining capacity can be accurately grasped and the loss of the power supply capacity can be accurately notified.
Further, since the positive electrode active material according to the present invention draws a flat discharge curve, it draws a flat discharge curve, for example, by using Li ₄ Ti ₅ O ₁₂ as a negative electrode, a flat shape preferable for an electronic device is obtained. A battery that draws a discharge curve can be obtained.
In addition, since the battery according to the present invention exhibits a voltage of 3V class, for example, a camera, a digital camera, a game machine, a portable MD player, and the like, instead of a combination of a conventional lithium primary battery and two dry batteries, The remarkable effect that it can be used for electronic devices, such as a headphone stereo, is acquired.
Furthermore, the nonaqueous electrolyte secondary battery of the present invention can realize an assembled battery and a module battery that are particularly excellent in overcharge cycle characteristics.

1. Synthesis of Positive Electrode Active Material in the Present Invention The present invention has a composition formula: Li _{2 ± α} [Me] ₄ O _8-x (where 0 ≦ α <0.4, 0 ≦ x <2, Me represents Mn and Ni , A transition metal element including at least one selected from the group consisting of Cr, Fe, Co, and Cu), and a reaction in a charge / discharge region is a two-phase reaction, and relates to a positive electrode active material. The composition formula preferably satisfies 0 ≦ x <1.3.
In the following, the positive electrode active material according to the present invention will be described using Li [Ni _1/2 Mn _3/2 ] O ₄ as a representative. In addition, these explanations can be said also about the positive electrode active material which has another composition within the range of the said composition formula.

Li [Ni _1/2 Mn _3/2 ] O ₄ is a mixture of oxides, hydroxides and / or carbonates containing constituent elements so as to have a predetermined composition, and the resulting mixture is fired ( It is possible to synthesize by performing the first firing. However, in this case, it is necessary to make the particle diameters of the respective raw materials the same, and sufficient mixing to make the reaction uniform, and the synthesis requires advanced powder technology.
On the other hand, Li [Ni _1/2 Mn _3/2 ] O ₄ can also be synthesized by coprecipitating nickel and manganese as hydroxides or carbonates in an aqueous solution. In this case, since nickel and manganese which are difficult to disperse can be uniformly dispersed in advance in the particles, the synthesis can be performed relatively easily.

Therefore, in the following synthesis examples, the eutectic compound obtained as a hydroxide was used, and lithium hydroxide was used as the lithium compound. After mixing these sufficiently, the obtained mixture is fired (first firing). The reaction can be more reliably performed by molding a mixture of hydroxide and lithium hydroxide by eutectic into pellets.
Here, FIG. 1 shows a mixture of a eutectic compound and a lithium compound at 1000 ° C. (a), 900 ° C. (b), 800 ° C. (c), 700 ° C. (d) and 600 ° C. (e) in air. It is a graph which shows the electrochemical property of the positive electrode active material obtained by baking for 12 hours (1st baking). Specifically, [Ni _1/4 Mn _3/4 ] (OH) ₂ obtained by eutectic and LiOH · H ₂ O are sufficiently mixed to obtain a mixture, and then the mixture is formed into a pellet. and, firing the resulting molded body was prepared _{Li [Ni 1/2 Mn 3/2] O} 4.

The electrochemical characteristics were measured by preparing test cells as follows.
First, 80 parts by weight of Li [Ni _1/2 Mn _3/2 ] O ₄ , 10 parts by weight of acetylene black as a conductive agent, and 10 parts by weight of polyvinylidene fluoride (PVdF) as a binder, The resulting mixture is diluted with N-methyl-2-pyrrolidone (NMP) to obtain a paste. This paste was applied on a current collector made of aluminum foil. The current collector after application was dried in a vacuum at 60 ° C. for 30 minutes, and then cut into a size of 15 mm × 20 mm. Thereafter, the current collector after cutting was further dried in vacuum at 150 ° C. for 14 hours to obtain a test electrode.

As the counter electrode, an electrode obtained by pressure bonding a lithium metal sheet on stainless steel was used. As the separator, a polyethylene porous film was used, and the electrolyte was ethylene carbonate (EC) and dimethyl carbonate (DMC) mixed at 3: 7 (volume ratio), and 1.0 M LiPF ₆ was added to the obtained mixed solvent. Obtained by dissolution.
These were superposed in the order of a test electrode, a separator and metallic lithium, and after pouring, a test cell was produced by tightening from the outside with an appropriate strength with a stainless steel holder. Charging / discharging of the obtained test cell was repeated between 3.0 V and 5.0 V at a current density of 0.17 mA / cm ² .

As can be seen from FIG. 1, even when using a positive electrode active material obtained by firing at any temperature, the discharge voltage is as high as 4.6 V to 4.8 V with respect to lithium metal, and charge / discharge of about 125 mAh / g. Indicates capacity. It can also be seen that the higher the firing temperature, the better the polarization characteristics.
It can also be seen that the voltage step near 4V increases regularly as the temperature increases. In the present invention, by utilizing such a phenomenon, it is possible to provide a battery capable of detecting the remaining capacity in conformity with an electronic device. In other words, the timing for detecting the remaining capacity required for the battery can be controlled by variously changing the firing temperature. This step appears in the vicinity of 4V, and its width is 0.2V to 0.8V and is not a large change of several V. Therefore, even if a battery having such a step is used in an electronic device, the power source of the electronic device is Troubles that run out do not occur.

From the above, the positive electrode active material according to the present invention is produced by mixing the raw material mixture or eutectic compound and the lithium compound, performing the first firing, and then gradually lowering the ambient temperature (slow cooling). can do. The conditions for the first firing and slow cooling are as follows.
First firing lower limit: 600 ° C., preferably 900 ° C.
Upper limit: 1000 ° C
Time: 2 to 72 hours slow cooling rate Lower limit: 4.5 ° C / min
Upper limit: 10 ° C / min

2. As described above, the higher the firing temperature, the smaller the polarization, but in this case, the width of the 4V step region becomes large. Of course, it is desirable that the polarization can be kept small and the width of the 4V region can be freely controlled. For this purpose, the present inventors have intensively studied synthetic methods.
FIG. 2 shows a TG curve (thermogravimetric analysis) of the positive electrode active material after the first firing. The positive electrode active material used here is Li [Ni _1/2 Mn _3/2 ] O ₄ obtained by firing at a low temperature of 500 ° C. The positive electrode active material was heated by increasing the temperature between 700 ° C. and 850 ° C. by 50 ° C., the positive electrode active material was held at each temperature, and the temperature was increased stepwise. Similar temperature control was performed when the temperature was lowered. The heating rate was 10 ° C./min and the atmosphere was in the air.

  In FIG. 2, a is the temperature, b is the weight changed when the temperature is raised, and c is the weight changed when the temperature is lowered. From FIG. 2, up to 400 ° C., an irregular weight loss that is considered to be related to moisture is observed. As the temperature increases from 400 ° C. to 1000 ° C., the weight decreases monotonically in the range of 700 ° C. to 1000 ° C. On the other hand, looking at the change in weight during the temperature lowering process, the weight increased (recovered) from 1000 ° C. to 800 ° C., which was the same as the weight decreased during the temperature increase, following the speed of this experiment. It can also be seen that up to 700 ° C., although the speed is delayed, the weight is almost completely recovered. This increase in weight is considered to be that oxygen once released at a high temperature is returned to the positive electrode active material by re-baking (second baking), that is, reoxidation of the positive electrode active material. Therefore, when performing the process for returning the weight, it is suggested that the temperature of the positive electrode active material obtained after the first firing is preferably lowered at a rate of 10 ° C./min or less.

Next, FIG. 3 shows a charge / discharge curve of the positive electrode active material obtained by performing the first firing at 1000 ° C. for 12 hours and then performing the second firing at 700 ° C. for 48 hours. As a result, this positive electrode active material has a charge / discharge capacity of about 135 mAh / g, has a stage of about 15 mAh / g in the vicinity of 4 V, and is excellent in polarization characteristics.
From the above, even if it is a positive electrode active material obtained by firing at a high temperature of 1000 ° C. (first firing) once, by re-firing (second firing) at a temperature lower than that, for example, 700 ° C. The step near 4 V can be controlled to be equivalent to the step of the positive electrode active material at 700 ° C. as shown in FIG.
Moreover, since the positive electrode active material which passed through the 1st baking and the 2nd baking is once baked at 1000 degreeC, it has the developed crystal particle which does not have a pore, Therefore, it has a high packing density. Moreover, this positive electrode active material is excellent in polarization characteristics.

As described above, in the positive electrode active material according to the present invention, from the viewpoint of voltage step and polarization characteristics, the raw material mixture or eutectic compound and lithium compound are mixed, and the first firing is performed, and then the second firing is performed. It is preferable to manufacture by. Preferred conditions for the first firing and the second firing are as follows.
First firing lower limit: 600 ° C., preferably 900 ° C.
Upper limit: 1000 ° C
Time: 2 to 72 hours Second firing Lower limit: 350 ° C., preferably 650 ° C.
Upper limit: 950 ° C, preferably 850 ° C
Time: 2 to 72 hours

In view of the results shown in FIG. 2 and the evaluation of electrochemical characteristics, the first firing is performed at 600 to 1000 ° C. or more, preferably 900 to 1000 ° C., then rapidly cooled to 350 to 950 ° C., and then the second firing is performed at 350 to It turns out that it is preferable to carry out at 950 degreeC, Preferably it is 650-850 degreeC.
By using such conditions, it is possible to improve the polarization characteristics of the obtained positive electrode active material, and at the same time appropriately control the step appearing in the vicinity of 4 V of the charge / discharge curve. In the above experiment, the temperature increase rate during firing was 7.5 ° C./min, and the temperature decrease rate was 4.5 ° C./min.

(3) Control of particle form of active material When the positive electrode active material is applied to a battery, the particle form of the positive electrode active material is an important factor. It is no exaggeration to say that this is done by controlling the form. Paying attention to this point, the present inventors have advanced earnestly research on preferred particle morphology and control of the positive electrode active material according to the present invention. As described above, in the method for producing a positive electrode active material according to the present invention, it is preferable to perform the first firing once at a high temperature (900 ° C. or higher) and then perform the second firing for the purpose of reoxidation.

  Then, the cross-sectional shape of the particles of the positive electrode active material according to the present invention obtained by performing the first baking under conditions of 1000 ° C. and 12 hours and the second baking under conditions of 700 ° C. and 48 hours was photographed by SEM. . The obtained SEM photograph is shown in FIG. 4 (a) (magnification: 30,000 times). 4B is an SEM photograph of the same material as the positive electrode active material of FIG. 4A except that the second baking is not performed.

From FIG. 4, it can be seen that the crystal particles of the positive electrode active material are developed because it is once fired at 1000 ° C. Moreover, although this particle | grain is a primary particle | grain of 2-3 micrometers, it turns out that it is the particle | grain filled without the pore at all inside the said particle | grain.
Furthermore, the particle form (particularly the shape of the outer shape) of the positive electrode active material greatly affects the coating density and the packing density when the electrode plate is produced using the positive electrode active material. In Japanese Patent Laid-Open No. 2001-210324, a proposal relating to particle morphology is made, and specifically, it is described that the shape of primary particles is octahedral. The reason for the positive electrode active material according to the present invention will be described below. However, it is preferable that the positive electrode active material has a shape completely different from that of the octahedron.

First, a method for controlling the particle morphology will be described with reference to the production of Li [Ni _1/2 Mn _3/2 ] O ₄ . In addition, even if it was the positive electrode active material which has another composition, the same tendency was shown within the scope of the present invention.
(I) Case 1 (FIG. 5)
The first firing was performed by raising the temperature from room temperature to 1000 ° C. in about 3 hours and holding at 1000 ° C. for 12 hours.
After the first firing, the temperature was lowered from 1000 ° C. to room temperature in 2 hours (slow cooling rate 8 ° C./min).

(Ii) Case 2 (Fig. 6)
The first firing was performed by raising the temperature from room temperature to 1000 ° C. in about 3 hours and holding at 1000 ° C. for 12 hours.
The second baking was performed by lowering the temperature from 1000 ° C. to 700 ° C. over 30 minutes and holding at 700 ° C. for 48 hours.
After the second baking, the temperature was lowered from 700 ° C. to room temperature in 1.5 hours (slow cooling rate 7.5 ° C./min).

(Iii) Case 3 (Fig. 7)
The first firing was performed by raising the temperature from room temperature to 1000 ° C. in about 3 hours and holding at 1000 ° C. for 12 hours.
After the first firing, it was rapidly cooled from 1000 ° C. to room temperature.
The second firing was performed by raising the temperature to 700 ° C. in about 1 hour and holding at 700 ° C. for 48 hours.
After the second baking, the temperature was lowered from 700 ° C. to room temperature in 1.5 hours.

(Iv) Case 4 (Fig. 8)
The first firing was performed by raising the temperature from room temperature to 1000 ° C. in about 3 hours and holding at 1000 ° C. for 12 hours.
After the first firing, it was rapidly cooled from 1000 ° C. to room temperature.

Broadly speaking, a rapid cooling process was used in cases 3 and 4, and a reoxidation (second firing) process at 700 ° C. was used in cases 2 and 3.
In FIGS. 5-8, the SEM photograph of the positive electrode active material particle manufactured on the conditions of cases 1-4 was shown. As can be seen from these SEM images, it is clear that the particle form of the positive electrode active material according to the present invention is not an octahedron. Although it is difficult to express such a form, it can be said that the positive electrode active material includes rhomboid dodecahedron or rhombobic dodecahedron. More specifically, it can be said that the positive electrode active material is a positive electrode active material including a tetradecahedron having eight hexagons and six tetragonal faces. 7 and 8, (a) used a magnification of 10,000 times, and (b) used a magnification of 30000 times.

It can be seen that the quenching process greatly affects the control of the particle morphology. The edges of the positive electrode active material particles obtained by cases 1 and 2 are sharp, but the positive edges of the positive electrode active material particles obtained by cases 3 and 4 are round. That is, it turns out that a ridge side can be rounded by taking a rapid cooling process by this.
When the positive electrode active material obtained in Cases 1 to 4 is applied to the electrode plate for a battery, the use of the positive electrode active material with a rounded edge improves the fluidity of the powder or coating paste, so that the density is high. The filling with was possible.

  From the above, the positive electrode active material according to the present invention is completely different from the octahedral particles proposed by the prior art, and is composed of rhomboid dodecahedron, rhombobic dodecahedron, It includes a tetradecahedron having a hexagon and six tetragonal faces, and it is considered that improved battery characteristics are exhibited by this particle form. Moreover, the positive electrode active material according to the present invention is preferably composed of crystal particles having a particle size of about 0.1 to 8 μm and secondary particles of crystal particles having a particle size of 2 to 30 μm as the whole powder. .

As described above, the positive electrode active material according to the present invention is manufactured by mixing the raw material mixture or the eutectic compound and the lithium compound, performing the first firing, and quenching from the viewpoint of controlling the particle form. Is preferred. Moreover, you may perform 2nd baking after rapid cooling. The conditions for the first firing, the rapid cooling, and the second firing are as follows.
First firing lower limit: 600 ° C., preferably 900 ° C.
Upper limit: 1000 ° C
Time: 2 to 72 hours Rapid cooling rate 10 ° C./min or more, preferably 20 ° C./min or more,
More preferably 50 ° C./min or more Second firing Lower limit: 350 ° C., preferably 650 ° C.
Upper limit: 950 ° C, preferably 850 ° C
Time: 2 to 72 hours

(4) Crystal structure, X-ray diffraction pattern and FT-IR signal of the positive electrode active material Regarding the crystal structure, the positive electrode active material according to the present invention has a spinel-framework-structure. FIG. 9 shows X-ray diffraction patterns of the positive electrode active material according to the present invention manufactured at various first firing temperatures. (A)-(e) are the X-ray-diffraction patterns of the positive electrode active material manufactured by performing 1st baking at 600 degreeC, 700 degreeC, 800 degreeC, 900 degreeC, and 1000 degreeC, respectively. The composition of the positive electrode active material is Li [Ni _1/2 Mn _3/2 ] O ₄ .
If the Miller index is assigned assuming a cubic crystal in the obtained X-ray diffraction pattern, it can be assigned to all peaks as shown in FIG. FIG. 9 shows that the peak in the first firing at a high temperature is sharper and the crystallinity is improved.

  Next, the FT-IR measurement results of the positive electrode active materials shown in FIGS. 9A to 9E are shown in FIGS. 10A to 10E, respectively. In the case of the positive electrode active material (b) at 700 ° C., 8 sharp peaks are observed, and it can be seen that the peaks become broad even at temperatures above 700 ° C. or below 700 ° C. This indicates that the first baking at 700 ° C. is preferable for the crystal arrangement.

A substantially similar X-ray diffraction pattern can be obtained by the second baking regardless of whether or not quenching is performed. In order to consider the crystal structure in more detail, the X-ray diffraction patterns of the positive electrode active materials obtained in the above cases 3 and 4 were compared.
FIG. 11 shows X-ray diffraction patterns of the positive electrode active material (a) obtained in the case 3 and the positive electrode active material (b) obtained in the case 4, and FIG. 12 shows the measurement results of these FT-IRs. showed that. The difference between these samples is whether or not they are undergoing a reoxidation process. With this in mind, as a result of structural analysis of the X-ray diffraction pattern of the quenched sample, the following becomes clear in the present invention, which is considered to be a factor that exerts the effects of the present invention.

The positive electrode active material according to the present invention has a composition formula: Li _{2 ± α} [Me] ₄ O _8-x (where 0 ≦ α <0.4, 0 ≦ x <2, preferably 0 ≦ x <1. 3, Me: transition metal containing Mn and at least one selected from the group consisting of Ni, Cr, Fe, Co and Cu). In the following, for ease of understanding, explanation will be made based on a specific example with α = 0.
In the atomic arrangement of the spinel structure belonging to the space group Fd3m of LiMn ₂ O ₄ , the lithium element occupies the 8a site, the transition metal element Me (Mn) occupies the 16d site, and oxygen occupies the 32e site. However, the 16c site is usually empty. The positive electrode active material according to the present invention is characterized in that an element is arranged at the 16c site.
That is, in the positive electrode active material of the present invention, the voltage step in the discharge curve is controlled by controlling the amount of elements entering the 16c site.

  When analyzing the X-ray diffraction pattern of the sample of the positive electrode active material produced by the first firing and the rapid cooling (second firing, that is, without reoxidation), Me is about 1/5 Me at the 8a site, 16c. It was found that the X-ray diffraction pattern could be well fitted assuming that there were 2/5 on the site and 7/4 on the 16d site. From this, when the temperature rises to 1000 ° C., oxygen in the spinel structure is desorbed, the transition metal is reduced, and a considerable amount of lithium element and transition metal element in the vicinity of the surface of the positive electrode active material are 8a site and 16c, respectively. It is thought to move to the site. By this phenomenon, a rock salt structure is formed in a part of the spinel structure of the positive electrode active material according to the present invention.

Since the sample obtained through the first firing and the rapid cooling has not undergone the reoxidation step in which oxygen is reintroduced, it is represented by Li _1.2 Me _2.4 O ₄ as judged from the result of the TG curve described above. Me includes Ni and Mn in a ratio of 1: 3.

Further, from the X-ray diffraction pattern shown in FIG. 11, the positive electrode active material (a) obtained through re-oxidation (second baking) at 700 ° C. after the first baking and rapid cooling, and the first baking (1000 ° C.) In the cathode active material (c) obtained through subsequent re-oxidation (second baking) at 700 ° C. after lowering the temperature, these rock salt structures reversibly return to the spinel structure by re-oxidation (second baking). You can see that Such flexibility of the crystal structure of the positive electrode active material contributes to stabilization of the crystal structure when the positive electrode active material is stressed by a high-rate charge / discharge cycle or the like, and as a result, a long life is expected to be achieved. .
Furthermore, when the FT-IR measurement results shown in FIG. 12 are observed, eight peaks are clearly observed for the positive electrode active materials (a) and (c) obtained through the reoxidation (second firing) step. Is done.

  In contrast, the X-ray diffraction pattern and the FT-IR measurement results of the positive electrode active material obtained by simply performing the first firing and reoxidizing when the temperature is lowered are shown in FIGS. 11 and 12 (b), respectively. And shown in (d). From the X-ray diffraction patterns of these positive electrode active materials, it appears that the positive electrode active materials obtained by performing reoxidation (second baking) have a spinel-like crystal structure. However, the results of FT-IR measurement are clearly different, and eight peaks are not clearly observed. These eight peaks are theoretically unpredictable from the local symmetry of Fd3m having a spinel structure. Therefore, it is also possible to identify the positive electrode active material according to the present invention from the measurement result of FT-IR, and this method is used when identifying the positive electrode active material in which the voltage step in the charge / discharge curve is substantially eliminated. It is valid.

Next, composition formula: Li _{2 ± α} [Me] ₄ O _8-x (where 0 ≦ α <0.4, 0 ≦ x <2, preferably 0 ≦ x <1.3, Me is Mn And transition metal elements including at least one selected from the group consisting of Ni, Cr, Fe, Co, and Cu), the values of α and x will be described.
The α value is a factor that is varied to control grain growth. When the α value is smaller than 2 in the stoichiometric composition, particle growth during synthesis can be suppressed, and the surface area tends to increase. Conversely, increasing the α value can promote particle growth. Therefore, the particles are designed according to the characteristics required for the battery. In this case, the particle growth can be controlled by changing the composition ratio of lithium. The range of the α value is substantially about ± 0.4, and if the range (variation width) becomes larger than this, the function of the original positive electrode active material may be impaired.

On the other hand, with respect to the value of X, as described above, the positive electrode active material obtained through the first baking and rapid cooling at 1000 ° C. is represented by Li _1.2 Me _2.4 O ₄ , and therefore the x value is 1. It is converted to 33. Further, since the amount of oxygen returns to the stoichiometric composition by reoxidation (second baking), x is considered to be 2, but the upper limit of x is substantially 1.3. Therefore, the present inventors set the range of x as 0 ≦ x <1.3 in consideration of the return of oxygen by reoxidation.

  Here, the state of each atomic site in the crystal structure of the positive electrode active material according to the present invention is shown in FIG. FIG. 13 is a diagram schematically showing the state of elements occupying each site with respect to the x value. As shown in FIG. 13, by introducing various elements into each site and effectively using originally empty sites, it becomes possible to freely control voltage steps appearing in the discharge curve.

When considered together with the measurement results such as XAFS, in the step, the 4V region is caused by an electrochemical reaction corresponding to Mn ³⁺ → Mn ⁴⁺ , and the 5V (4.7V) region is Ni ²⁺ → This is considered to be caused by the electrochemical reaction of Ni ⁴⁺ . It was found that the two positive electrode active material samples obtained through the rapid cooling described above can also freely control the step by performing reoxidation after the rapid cooling several times.

When the positive electrode active material according to the present invention is identified by an X-ray diffraction pattern or a unit cell, the following points are mentioned from the above description. Moreover, in order to obtain a positive electrode active material with few steps (substantially hardly seen), it is preferable to consider the following points.
FIG. 14 shows the change in the unit cell constant of the positive electrode active material produced through rapid cooling. As a result, the lattice constant is 8.33 A or less, preferably 8.25 A or less, and more preferably 8.2 A or less.

  As for the ratio of Mn to other transition metal elements, the ratio of 3: 1 was the best when judged from the capacity and the shape of the discharge curve. The details are unknown, but in the case of 3: 1, the transition metal layer in the framework of the spinel structure can form a [2 × 2] superlattice, and this effect has an effect. It is thought that. In addition, since a superlattice spot in this direction is observed by electron diffraction measurement, formation of a [2 × 2] superlattice can be confirmed.

  Japanese Patent Application Laid-Open No. 9-147867 describes a high-voltage positive electrode active material, but only discloses a composition and a simple structure, and does not disclose any preferable manufacturing method and temperature range. . Specifically, it only describes that the raw materials are simply mixed and fired, and only a wide firing temperature is described. On the other hand, the positive electrode active material according to the present invention is a novel material that has not been obtained in the past because it exhibits superior effects obtained on the basis of such a positive electrode active material according to the prior art. JP-A-9-147867 does not describe that the particle morphology is freely controlled according to the conditions in the production method as in the present invention.

Focusing particularly on the crystal structure, JP-A-9-147867 describes that Mn in LiMn ₂ O ₄ having an ideal spinel structure is replaced with a transition metal or Li. This is a description focusing only on the 16d site, and it is clearly stated in the text that it is clearly different from LiNiVO ₄ and the like. That is, Japanese Patent Laid-Open No. 9-147867 is equivalent to the description that no atoms enter the 8a site and the originally empty 16d site.

  On the other hand, in the present invention, by appropriately controlling the conditions of the production method, a rock salt structure is formed in a part of the positive electrode active material using these sites, and the reoxidation (second firing) is performed. The structure is deliberately controlled. That is, the rock salt structure and the spinel-related structure exist in the same crystal, and the ratio is freely controlled. In the case of a positive electrode active material that has only a spinel-related structure and has virtually no voltage step in the discharge curve, whether or not eight peaks are clearly observed in the FT-IR measurement. Becomes an identification signal.

(5) Two-phase reaction A battery showing a flat discharge curve is more advantageous for the device used. Generally, when the charge / discharge reaction of the positive electrode active material is a one-phase reaction, the shape of the discharge curve is S-shaped according to the Nernst equation. In a layer structure material such as lithium cobaltate or lithium nickelate, a two-phase reaction partially proceeds, but a one-phase reaction proceeds in most cases. Therefore, it essentially shows an S-shaped discharge curve. For this reason, especially at the end of high-rate discharge, a large voltage drop occurs together with polarization, and it is difficult to obtain a flat discharge curve.

  On the other hand, when charging / discharging proceeds by a two-phase reaction, the discharge curve is essentially flat. In this sense, a positive electrode active material that performs a two-phase reaction in the entire charge / discharge region is preferable. In FIG. 15, the X-ray-diffraction pattern accompanying charging / discharging of the positive electrode active material which concerns on this invention was shown. (A) to (m) in FIG. 15 are 15 mAh / g, 30 mAh / g, 50 mAh / g, 60 mAh / g, 70 mAh / g, 75 mAh / g, 80 mAh / g, 90 mAh / g, 100 mAh / g, 105 mAh, respectively. The discharge capacities are shown as / g, 110 mAh / g, 120 mAh / g, and 136.3 mAh / g. In FIG. 15, when the change of the peaks at (111), (311), and (400) is observed, it can be seen that splitting is observed and the two-phase reaction proceeds.

For easier understanding, FIG. 16 shows the change in lattice constant obtained from FIG. 15 assuming a cubic crystal. In the portion where two lattice constants exist, the lattice constant is calculated on the assumption that two phases coexist.
FIG. 16 shows that the discharge of the positive electrode active material according to the present invention is divided into the first half and the second half, and the two-phase reaction proceeds in each. In LiMn ₂ O ₄ having a conventional spinel structure, a two-phase reaction proceeds in a half region, but a one-phase reaction proceeds in the remaining region, and a two-phase reaction does not proceed in the entire region. That is, the positive electrode active material according to the present invention in which the two-phase reaction proceeds in the entire region shows a discharge curve with extremely good flatness, unlike the conventional one.

(6) 3V class non-aqueous electrolyte secondary battery using oxide negative electrode and residual capacity detection Advantages of using the positive electrode active material according to the present invention for the positive electrode and titanium oxide having a spinel structure for the negative electrode will be described. . The positive electrode active material according to the present invention has a large reversible capacity and excellent polarization characteristics as compared with a conventional 4.5 V class spinel positive electrode active material.
When Li ₄ Ti ₅ O ₁₂ (Li [Li _1/3 Ti _5/3 ] O ₄ ) is used for the negative electrode, a 3V class battery can be configured.

Japanese Laid-Open Patent Publication No. 2001-210324 proposes using a titanium-based oxide for the negative electrode. However, only the positive electrode active material showing the positive electrode capacity in the potential range of 4.3 V to 3.5 V is described in the text of the publication. This is merely a positive electrode active material obtained by adding a small amount of an additive element to the conventional LiMn ₂ O ₄ and improving the cycle life, and the positive electrode active material having a charge / discharge region of 4.7 V according to the present invention. It is clearly different from the substance. Therefore, the battery system described in Japanese Patent Laid-Open No. 2001-210324 is a 2.5 V class battery system.
On the other hand, the battery system according to the present invention has an actual charge / discharge range of 2.5V to 3.5V, which is the same region as a 3V-class lithium primary battery currently commercially available. In addition, the battery system according to the present invention is versatile and advantageous from the viewpoint that only one battery needs to be used in a device that uses two dry batteries.

  In other words, the difference in battery voltage of 0.5 V due to the battery system appears as a significant practical advantage or handicap in the market, whereas the 2.5 V class battery system described in Japanese Patent Laid-Open No. 2001-210324 is substantially different. The big value is not recognized. JP-A-9-147867 proposes a positive electrode active material having a charge / discharge potential of 4.5 V or more. In addition, a battery system using carbon as a negative electrode is disclosed, and an object of the technology is to realize a lithium ion battery having a high voltage of 4.5V, which is an object of the battery system according to the present invention. Is different.

  In the positive electrode active material according to the present invention, the voltage step at the end of discharge in the discharge curve can be freely controlled. Accordingly, if the battery system is appropriately selected, the remaining capacity can be detected. As described above, it is more advantageous for electronic devices to have a flat discharge curve (discharge voltage), but this is disadvantageous from the viewpoint of detecting the remaining capacity. On the other hand, according to the present invention, it is possible to design a positive electrode active material that has a flat discharge curve and can freely control the voltage step at the end of discharge in the discharge curve.

Therefore, Li ₄ Ti ₅ O ₁₂ (Li [Li _1/3 Ti _5/3 ] O ₄ ) is used as the negative electrode active material from the viewpoint that the negative electrode is desired to have a flat discharge curve. It is advantageous.
This Li ₄ Ti ₅ O 12 (Li [Li _1/3 Ti _5/3 ] O ₄ ) and the positive electrode active material according to the present invention have substantially the same capacity density. Therefore, if these are used, it is possible to make the thicknesses of the positive electrode plate and the negative electrode plate coincide when manufacturing the battery. This is also advantageous in terms of battery characteristics. In a commercially available battery system using LiCoO ₂ / graphite or LiMn ₂ O ₄ / graphite, the capacity density of the negative electrode is high, so that there is a large difference between the thickness of the positive electrode plate and the thickness of the negative electrode plate. Due to this difference, a difference also occurs in the degree of diffusion of the electrolyte into the electrode. As a result, there is a problem in that the rate balance between the positive electrode and the negative electrode is deviated, and a load is applied to one of the electrode plates, resulting in faster deterioration. Also from this viewpoint, it can be seen that a battery system combining the positive electrode active material according to the present invention and Li ₄ Ti ₅ O ₁₂ (Li [Li _1/3 Ti _5/3 ] O ₄ ) is preferable.

The negative electrode active material exhibits a flat charge / discharge curve of 1.55 V based on metallic lithium. FIG. 17 shows the charge / discharge behavior of a battery system using Li [Ni _1/2 Mn _3/2 ] O ₄ as the positive electrode and Li [Li _1/3 Ti _5/3 ] O ₄ as the negative electrode. FIG. 18 shows the cycle life of the battery system up to 200 cycles. In FIG. 17, the horizontal axis indicates the discharge capacity per unit weight of the positive electrode active material. The charge / discharge conditions were a current density of 0.17 mA / cm ² and a constant current charge / discharge between 0V and 3.5V.

  As is clear from FIG. 17, the battery system according to the present invention exhibits a flat charge / discharge voltage with an average voltage of about 3.2 V, and at the same time, a voltage step appears at the end of discharge. By using this step, it is possible to exhibit an accurate display function of remaining capacity or a power-off alarm function. The actual charging / discharging range of this battery system is 2.5 V to 3.5 V, which is the same region as a 3 V class lithium primary battery currently used commercially.

FIG. 19 shows the load characteristics of this battery system. In Figure 19 (a) ~ (f), respectively ^{0.1mA / cm 2, 0.17mA / cm} 2, 0.33mA / cm 2, 0.67mA / cm 2, 1.0mA / cm 2 and 1. The discharge behavior at a current density of 67 mA / cm ² is shown. FIG. 19 also confirms that a step in the discharge voltage appears clearly even when the load changes greatly.
On the other hand, it is possible to prevent this step from appearing. An example is shown in FIG. It can be seen that no obvious steps appear even when the load is increased. As the positive electrode active material at this time, a material obtained through first firing at 1000 ° C. and second firing (reoxidation) at 700 ° C. was used. Incidentally, (a) ~ (e), respectively ^{0.17mA / cm 2, 0.33mA / cm} 2, 1.0mA / cm 2, 1.67mA / cm 2 and 3.33mA / cm ² current density in FIG. 20 It shows the discharge behavior at.

  Further, graphite expands and contracts greatly with charge and discharge, whereas the negative electrode active material is a strain-free material that does not expand and contract with charge and discharge. The positive electrode active material according to the present invention does not significantly expand / contract due to charging / discharging, and by using these combinations, a battery system having substantially no expansion / contraction can be designed. That is, the deterioration of the cycle life, rate characteristics and temperature characteristics caused by factors such as deterioration of the active material due to expansion / contraction and leakage of the electrolytic solution out of the battery system are dramatically improved.

Here, in FIG. 21, the result of having measured the expansion and contraction accompanying charging / discharging with the dilate meter was shown. The thicknesses of the positive electrode plate and the negative electrode plate were 60 μm and 110 μm, respectively, and the change in thickness per combination (one stack) of one positive electrode plate and one negative electrode plate was measured.
From FIG. 21, it can be seen that expansion / contraction appears in a form corresponding to charge / discharge, and the measurement can be performed with high accuracy. The amount of change is about 1 μm, which is only 0.6% per battery. The negative electrode Li [Li _1/3 Ti _5/3 ] O ₄ is known as an unstrained material having no expansion / contraction, so even if this amount is subtracted, the thickness of the positive electrode plate only changes by 2%. Absent. When a conventional LiCoO ₂ / graphite battery is charged, the positive electrode expands by about 5% and the negative electrode by about 20%, whereas the battery according to the present invention has an extremely low degree of expansion / contraction. Recognize. Such extremely small expansion / contraction associated with charging / discharging is a cause of a long cycle life. According to the present invention, in particular, the cycle life when charging / discharging at a high rate is dramatically improved as compared with the conventional battery system.

(7) Battery capacity design When designing the capacity load of a battery, the capacity of either the positive electrode or the negative electrode is regulated. This is intentionally designed from the application of the equipment used and the characteristics of the material. In the 3V class battery system according to the present invention, it is more preferable to regulate the capacity of the negative electrode. Specifically, the ratio (weight) of the negative electrode active material to the positive electrode active material is 0.5 or more and less than 1.2. In the case of 1.2, it seems that the positive electrode active material is regulated formally, but the theoretical charge / discharge capacity per gram of the negative electrode active material is equal to the theoretical charge / discharge capacity per gram of the positive electrode active material. Therefore, the negative electrode active material is substantially regulated.
The reason why the battery system regulated by the negative electrode is more preferable is as follows. That is, the potential of the positive electrode is about 4.7 V, but some positive electrodes have poor oxidation resistance depending on the electrolyte used. Therefore, it is disadvantageous from the stability of the electrolyte to terminate the charging by increasing the potential of the positive electrode. In addition, if the lithium element is completely extracted from the positive electrode active material, oxygen is gradually released, leading to deterioration of the active material and oxidation of the electrolyte due to the released oxygen, which is expected to cause cycle life and deterioration of battery characteristics. Is done.

(8) Current collector of positive electrode plate and negative electrode plate In lithium ion secondary batteries currently on the market, aluminum is usually used as the current collector of the positive electrode, and copper is used as the current collector of the negative electrode. This is because a current collector having excellent corrosion resistance is used in consideration of the potential of each electrode. Japanese Patent Application Laid-Open Nos. 9-147867 and 2001-210324 also specify that aluminum and copper are used as current collectors for the positive electrode and the negative electrode, respectively.
In the nonaqueous electrolyte secondary battery according to the present invention, it is preferable to use aluminum or an aluminum alloy for both the positive electrode and the negative electrode. The reason for this is as follows.

  First, by using aluminum with respect to copper, the weight of the battery can be reduced, and at the same time, the cost can be reduced. In a commercially available battery system using graphite as a negative electrode, it was impossible to use aluminum as a current collector because the potential of graphite was as low as 0.2 V or less with respect to lithium metal. This is because aluminum starts to react with lithium ions at a potential higher than the potential at which the graphite of the negative electrode is charged and discharged. However, in the battery system according to the present invention, since the charge / discharge potential of the negative electrode is as high as 1.5 V, it is possible to use aluminum that does not react unless the potential falls below that potential. Further, when the potential of the negative electrode is increased due to deep discharge or the like using copper, copper ions are eluted in the electrolytic solution. This copper ion precipitates on the negative electrode prior to the lithium insertion reaction by recharging, and inhibits the lithium insertion reaction. As a result, lithium is deposited as a metal in the form of needle crystals on the negative electrode surface. This causes a decrease in battery safety and a decrease in cycle life. On the other hand, elution and reprecipitation of metal ions do not occur when aluminum is used.

  Further, when a charger or the like fails in the negative electrode rate limiting battery system, the battery is overcharged and excess lithium is supplied to the negative electrode. At this time, if the current collector of the negative electrode is copper, excess lithium is deposited on the negative electrode. Such needle-like crystal lithium metal reduces the overcharge safety of the battery. However, since aluminum has a sufficient capacity to store lithium, when aluminum is used for the current collector of the negative electrode, it must be stored in the current collector without depositing lithium metal on the negative electrode during overcharge. Is possible. As a result, the overcharge safety of the battery is not reduced.

(9) Nonaqueous Electrolyte A preferable electrolytic solution in the 3V class nonaqueous electrolyte secondary battery according to the present invention will be described. The organic solvent used for the electrolytic solution has a potential window. The potential window is a measure of oxidation resistance and reducibility, and the wider the potential window, the more stable the organic solvent. In general LiCoO ₂ / graphite-based non-aqueous electrolyte secondary batteries, oxidation resistance is required to be close to 4.5 V, which is the charge / discharge potential of cobalt, and reduction resistance is required to be close to 0 V, which is the charge / discharge potential of graphite. (The potential is based on lithium metal. The same shall apply hereinafter.) Therefore, selecting and using an organic solvent that does not have a potential window that satisfies these conditions has been avoided.

  In particular, when improving the reduction resistance by using graphite for the negative electrode, it has been difficult to use a lactone organic solvent. Propylene carbonate is also difficult to use because it is simultaneously decomposed during charging and discharging of graphite. Since these solvents are inexpensive and have a large dielectric constant, they have the ability to sufficiently dissolve solutes (salts), and even though they are useful solvents with excellent oxidation resistance, It is difficult to use. In addition, although trimethyl phosphate and triethyl phosphate have a fire extinguishing action and are excellent in safety, it is difficult to use them for the same reason.

In the battery system according to the present invention, all the solvents having the useful characteristics as described above can be used. In the non-aqueous electrolyte secondary battery according to the present invention, Li ₄ Ti ₅ O ₁₂ (Li [Li _1/3 Ti _5/3 ] O ₄ ) is used for the negative electrode instead of graphite, so the potential on the negative electrode side is 1.5 V. Therefore, the reduction resistance required for the solvent is drastically reduced. Moreover, it becomes possible to use a solvent such as propylene carbonate which is decomposed on the negative electrode surface due to charge / discharge characteristic of graphite as an extremely effective solvent.

  On the other hand, the potential of the positive electrode rises to 4.7 V or higher, but the oxidation resistance of these solvents is 5 V or higher, so that they can be used without any problem. In addition, sulfolane having excellent oxidation resistance, methyldiglyme, and the like are considered to be suitable solvents for the battery system. Conventionally used solvents such as DEC (dimethyl carbonate), MEC (methyl ethyl carbonate), and DMC (dimethyl carbonate) can also be used as diluents for highly viscous solvents.

  Especially, in this invention, it is preferable to use the solvent containing acetonitrile (AN). This is because acetonitrile has a high dielectric constant, can dissolve a large amount of solute (salt), and has a low viscosity, so that it can be used as an electrolyte having very high conductivity. Therefore, when an assembled battery consisting of a plurality of single cells is produced, heat is generated along with charge / discharge to form a temperature distribution, or the temperature of the outer single cell and the temperature of the inner single cell Even if there is a difference between the two cells, since the electrolyte using acetonitrile has high conductivity, the influence of the temperature between the single cells is reduced. That is, even if the temperature changes somewhat, it does not appear as a large difference in charge / discharge behavior. As a result, the overcharge cycle life can be particularly moderately maintained.

The solute that can be used in the present invention is not particularly limited, and conventionally used LiPF ₆ , LiBF ₄ , lithium salts of organic anions, and the like can also be used. In a conventional general LiCoO ₂ / graphite-based non-aqueous electrolyte secondary battery, in order to use graphite or dissolve a solute, EC (ethylene carbonate) having a high dielectric constant and extremely high viscosity is used as a low-viscosity solvent. A mixed solvent diluted with has been widely used. In the battery system according to the present invention, it is possible to select an optimal electrolytic solution according to the desired characteristics of the device used without limiting the electrolytic solution to be used for the reasons described above.

(10) Separator Conventionally, in a general LiCoO ₂ / graphite battery, a porous film made of polyethylene or propylene is used as a separator. This separator is quite expensive because it is obtained by molding a polymer material by melt extrusion and stretching the resulting molded product in a biaxial direction to produce a thin porous film. The main reason for requiring this film is considered as follows.

  The potential of the graphite used for the negative electrode drops to a point close to the potential at which lithium metal is deposited. This causes various inconveniences. In some cases, lithium is deposited minutely on a part of the surface of graphite due to rapid charging or charging at low temperature. In addition, cobalt or impurity metals may be eluted due to excessive floating charge and deposited on the negative electrode.

In such a case, in the porous film having fine pores, it is possible to suppress acicular metal deposition with a physical force, whereas in a non-woven fabric or the like having a large pore diameter, the porous film can be suppressed in a short time. Causes a short circuit. In addition, in order to ensure overcharge safety assuming a failure of the charger, the separator has a shutdown function in order to suppress an increase in battery temperature during overcharge. This is a function to stop the current flowing between the electrodes by shrinking and crushing the fine pores of the separator at a constant temperature (about 135 ° C.). For these reasons, an expensive porous film has been used in conventional LiCoO ₂ / graphite batteries.

  On the other hand, in the battery system according to the present invention, since the potential of the negative electrode is 1.5 V and the potential at which lithium is deposited, there is almost no problem as described above. When aluminum is used as the current collector for the negative electrode, lithium is occluded, so that there is no such problem of metal deposition. In addition, since the positive electrode of the present invention does not contain excessive lithium element as in the cobalt system, this battery system is extremely excellent in overcharge safety. That is, a highly accurate shutdown function that a porous film has is not required. For these reasons, in this battery system, it is possible to use a non-woven fabric preferably by using aluminum or an aluminum alloy for the current collector of the negative electrode.

  Since the nonwoven fabric has a large amount of liquid retention, the rate characteristics, particularly the pulse characteristics, can be dramatically improved. Further, since a sophisticated and complicated process such as a porous film is not required, the choice of separator material is widened and inexpensive. Considering application to the present battery system, as a material constituting the separator, for example, polyethylene, polypropylene, polybutylene terephthalate, and a mixed material of these materials are preferably used. Polyethylene or polypropylene is stable in the electrolyte, and polybutylene terephthalate is preferred when strength at high temperatures is required. The fiber diameter is preferably about 1 to 3 [mu] m, and those in which some fibers are fused together by a heated calendar roll treatment are effective in reducing the thickness and increasing the strength.

(11) Nonaqueous Electrolyte Secondary Battery Hereinafter, other constituent materials that can be used when producing a nonaqueous electrolyte (lithium) secondary battery using the positive electrode active material of the present invention will be described.
The conductive agent in the positive electrode mixture used for producing the positive electrode in the present invention is not particularly limited as long as it is an electron conductive material that does not cause a chemical change in the constituted battery. For example, graphite such as natural graphite (such as flake graphite) and artificial graphite, carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black and thermal black, and conductive such as carbon fiber and metal fiber Organic fibers such as conductive fibers, metal powders such as carbon fluoride, copper, nickel, aluminum, and silver, conductive whiskers such as zinc oxide and potassium titanate, conductive metal oxides such as titanium oxide, and polyphenylene derivatives Examples thereof include a conductive material. These may be used alone or in any mixture as long as the effects of the present invention are not impaired.

Among these, artificial graphite, acetylene black, and nickel powder are particularly preferable. Although the addition amount of a electrically conductive agent is not specifically limited, 1 to 50 weight% is preferable and especially 1 to 30 weight% is preferable. In the case of carbon or graphite, 2 to 15% by weight is particularly preferable.
A preferable binder in the positive electrode mixture in the present invention is a polymer having a decomposition temperature of 300 ° C. or higher. For example, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), tetrafluoroethylene-hexafluoroethylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene- Perfluoroalkyl vinyl ether copolymer (PFA), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene copolymer, ethylene-tetrafluoroethylene copolymer (ETFE resin), polychlorotri Fluoroethylene (PCTFE), vinylidene fluoride-pentafluoropropylene copolymer, propylene-tetrafluoroethylene copolymer, ethylene-chlorotrifluoroethylene copolymer ( CTFE), vinylidene fluoride - hexafluoropropylene - tetrafluoroethylene copolymer and vinylidene fluoride - perfluoromethylvinylether - and the like tetrafluoroethylene copolymer. These may be used alone or in any mixture as long as the effects of the present invention are not impaired.
Particularly preferred among these are polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE).

The current collector of the positive electrode is not particularly limited as long as it is an electronic conductor that does not cause a chemical change in the constructed battery. Examples of the material constituting the current collector include stainless steel, nickel, aluminum, titanium, various alloys, and carbon, as well as composites obtained by treating the surface of aluminum or stainless steel with carbon, nickel, titanium, or silver. Can also be used.
In particular, aluminum or an aluminum alloy is preferable. The surface of these materials can also be oxidized. Moreover, you may give an unevenness | corrugation to the collector surface by surface treatment. The shape may be one used in the field of batteries, and examples include foils, films, sheets, nets, punched ones, lath bodies, porous bodies, foams, fiber groups, and nonwoven fabrics. . The thickness is not particularly limited, but a thickness of 1 to 500 μm is preferably used.

As the negative electrode active material (negative electrode material) that can be used in the present invention, it is particularly preferable to use a titanium oxide such as Li ₄ Ti ₅ O ₁₂ (Li [Li _1/3 Ti _5/3 ] O ₄ ). preferable. If this negative electrode is used, a 3V class battery can be obtained, the conventional problems are solved, and the battery performance is dramatically improved as described above. On the other hand, it is naturally possible to use only the positive electrode active material according to the present invention. In that case, the following negative electrode can be used.

  The negative electrode material may be any compound that can occlude and release lithium ions, such as lithium, lithium alloy, alloy, intermetallic compound, carbonaceous material, organic compound, inorganic compound, metal complex, and organic polymer compound. These can be used alone or in any combination within a range not impairing the effects of the present invention.

Examples of the lithium alloy include Li-Al alloys, Li-Al-Mn alloys, Li-Al-Mg alloys, Li-Al-Sn alloys, Li-Al-In alloys, Li-Al-Cd. Alloy, Li—Al—Te alloy, Li—Ga alloy, Li—Cd alloy, Li—In alloy, Li—Pb alloy, Li—Bi alloy, Li—Mg alloy, etc. . In this case, the lithium content is preferably 10% by weight or more.
Examples of the alloy and the intermetallic compound include a compound of transition metal and silicon and a compound of transition metal and tin, and a compound of nickel and silicon is particularly preferable.

  Examples of carbonaceous materials include coke, pyrolytic carbons, natural graphite, artificial graphite, mesocarbon microbeads, graphitized mesophase microspheres, vapor-grown carbon, glassy carbons, carbon fibers (polyacrylonitrile-based, pitch-based) , Cellulose-based, vapor-grown carbon-based), amorphous carbon, and organic baked carbon. These may be used alone or in any combination as long as the effects of the present invention are not impaired. Among these, graphite materials such as graphitized mesophase spherules, natural graphite, and artificial graphite are preferable.

In addition to carbon, the carbonaceous material may include different types of compounds such as O, B, P, N, S, SiC, and B ₄ C. The content is preferably 0 to 10% by weight.
Examples of inorganic compounds include tin compounds and silicon compounds. Examples of inorganic oxides include tungsten oxide, molybdenum oxide, niobium oxide, vanadium oxide, and iron oxide in addition to the titanium oxide described above. Etc.

Further, as the inorganic chalcogenide, inorganic chalcogenides such as iron sulfide, molybdenum sulfide, and titanium sulfide can be used.
Examples of the organic polymer compound include polymer compounds such as polythiophene and polyacetylene, and examples of the nitride include cobalt nitride, copper nitride, nickel nitride, iron nitride, and manganese nitride.
These negative electrode materials may be used in combination, for example, a combination of carbon and an alloy, or a combination of carbon and an inorganic compound.

The average particle size of the carbonaceous material used in the present invention is preferably 0.1 to 60 μm. More preferably, it is 0.5-30 micrometers. The specific surface area is preferably ¹ to 10 m ² / g. From the viewpoint of the crystal structure, graphite having a carbon hexagonal plane interval (d002) of 3.35 to 3.40 mm and a crystallite size (LC) in the c-axis direction of 100 mm or more is preferable.
In the present invention, since the positive electrode active material contains Li, a negative electrode material (such as carbon) that does not contain Li can be used. Moreover, when a small amount of Li (about 0.01 to 10 parts by weight with respect to 100 parts by weight of the negative electrode material) is contained in such a negative electrode material that does not contain Li, a part of Li reacts with an electrolyte or the like. Even if it becomes inactive, it can be supplemented with Li contained in the negative electrode material, which is preferable.

  As described above, in order to contain Li in the negative electrode material, for example, a heated and melted lithium metal is applied onto a current collector obtained by crimping the negative electrode material, and the negative electrode material is impregnated with Li, or an electrode group in advance. Lithium metal may be stuck inside by pressure bonding or the like, and the anode material may be electrochemically doped with Li in the electrolyte.

  The conductive agent in the negative electrode mixture is not particularly limited as long as it is an electron conductive material that does not cause a chemical change in the constituted battery, similarly to the conductive agent in the positive electrode mixture. Further, when a carbonaceous material is used for the negative electrode material, the carbonaceous material itself has electronic conductivity, and therefore it may or may not contain a conductive agent.

  The binder in the negative electrode mixture may be either a thermoplastic resin or a thermosetting resin, but a preferred binder is a polymer having a decomposition temperature of 300 ° C. or higher. For example, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), styrene butadiene rubber, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene copolymer, ethylene-tetrafluoroethylene copolymer (ETFE resin), polychlorotrifluoroethylene (PCTFE), fluorine Vinylidene fluoride-pentafluoropropylene copolymer, propylene-tetrafluoroethylene copolymer, ethylene-chlorotrifluoroethylene copolymer (ECTFE), vinylidene fluoride- Hexafluoropropylene - tetrafluoroethylene copolymer and vinylidene fluoride - perfluoromethylvinylether - may be mentioned, such as tetrafluoroethylene copolymer. Styrene butadiene rubber, polyvinylidene fluoride, styrene butadiene rubber, and the like can also be used.

When a titanium oxide such as Li ₄ Ti ₅ O ₁₂ (Li [Li _1/3 Ti _5/3 ] O ₄ ) is used as the negative electrode active material as the negative electrode current collector, aluminum or an aluminum alloy should be used. Is particularly preferred for the reasons described above.

  Moreover, when using other negative electrode active materials, the following can be used. If it is an electronic conductor which does not raise | generate a chemical change in the comprised battery, there will be no restriction | limiting in particular. Examples of the material constituting the current collector include stainless steel, nickel, copper, titanium, and carbon, as well as the surface of copper or stainless steel treated with carbon, nickel, titanium, or silver, and an Al—Cd alloy. Used. In particular, copper or a copper alloy is preferable. The surface of these materials may be oxidized. Moreover, you may provide an unevenness | corrugation in the collector surface by surface treatment.

As the shape of the negative electrode current collector, for example, a foil, a film, a sheet, a net, a punched one, a lath body, a porous body, a foamed body, a molded body of a fiber group, and the like are used. The thickness is not particularly limited, but a thickness of 1 to 500 μm is preferably used.
In addition to the conductive agent and the binder, a filler, a dispersant, an ionic conductive agent, a pressure enhancer, and other various additives can be used for the electrode mixture. Any filler can be used as long as it is a fibrous material that does not cause a chemical change in the constructed battery. Usually, olefin polymers such as polypropylene and polyethylene, fibers such as glass and carbon are used. Although the addition amount of a filler is not specifically limited, 0 to 30 weight% is preferable.

  The positive electrode and the negative electrode in the present invention are introduced for the purpose of improving the adhesion between the current collector and the mixture layer, conductivity, cycle characteristics and charge / discharge efficiency, in addition to the mixture layer containing the cathode active material or the anode material. It may have a protective layer to be introduced for the purpose of mechanical protection or chemical protection of the undercoat layer or the mixture layer. The undercoat layer and the protective layer can contain binders, conductive agent particles, non-conductive particles, and the like.

As the separator, for example, when a titanium oxide such as Li ₄ Ti ₅ O ₁₂ (Li [Li _1/3 Ti _5/3 ] O ₄ ) is used as the negative electrode active material, a nonwoven fabric is particularly preferable as described above. When other negative electrode active materials are used, the following can be used. An insulating microporous thin film having a large ion permeability and a predetermined mechanical strength is used. Moreover, it is preferable to have a function of closing the hole at 80 ° C. or higher and increasing the resistance. Sheets and nonwoven fabrics made from olefin polymers such as polypropylene and polyethylene, glass fibers, or the like, which are resistant to organic solvents and hydrophobic, are used.

  The pore diameter of the separator is desirably in a range in which the active material, the binder, the conductive agent and the like desorbed from the electrode sheet do not permeate, and is preferably 0.1 to 1 μm, for example. In general, the thickness of the separator is preferably 10 to 300 μm. The porosity is determined according to the permeability of the electrons and ions, the material, and the film pressure, but is generally preferably 30 to 80%. In addition, the use of a flame-retardant material such as glass or a metal oxide film or a non-combustible material further improves the safety of the battery.

As the non-aqueous electrolyte that can be used in the present invention, when a titanium oxide such as Li ₄ Ti ₅ O ₁₂ (Li [Li _1/3 Ti _5/3 ] O ₄ ) is used as the negative electrode active material, for example. It is particularly preferable to use the aforementioned electrolytic solution. When other negative electrodes are used, the following electrolytic solution can be used.

  The electrolytic solution is composed of a solvent and a lithium salt dissolved in the solvent. Preferred solvents are esters alone or mixed esters. Of these, cyclic carbonates, cyclic carboxylic acid esters, acyclic carbonates and aliphatic carboxylic acid esters are preferred. Furthermore, a mixed solvent containing a cyclic carbonate and an acyclic carbonate, a mixed solvent containing a cyclic carboxylic acid ester, and a mixed solvent containing a cyclic carboxylic acid ester and a cyclic carbonate are preferable.

Specific examples of the solvent and other solvents used in the present invention are exemplified below.
Examples of the ester used for the non-aqueous solvent include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and vinylene carbonate (VC), dimethyl carbonate (DMC), and diethyl carbonate (DEC). Acyclic carbonates such as ethyl methyl carbonate (EMC) and dipropyl carbonate (DPC), aliphatic carbons such as methyl formate (MF), methyl acetate (MA), methyl propionate (MP) and ethyl propionate (MA) And cyclic carboxylic acid esters such as acid esters and γ-butyrolactone (GBL).
EC, PC and VC are particularly preferred as the cyclic carbonate, GBL and the like are particularly preferred as the cyclic carboxylic acid ester, and DMC, DEC and EMC are preferred as the acyclic carbonate. Moreover, it is preferable that aliphatic carboxylic acid ester is included as needed. The aliphatic carboxylic acid ester is preferably contained in a range of 30% or less, more preferably 20% or less of the total solvent weight.

Further, the solvent of the electrolytic solution of the present invention may contain a known aprotic organic solvent in addition to containing 80% or more of the ester.
Examples of the lithium salt dissolved in the above-described solvent include LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAlCl ₄ , LiSbF ₆ , LiSCN, LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , Li (CF ₃ SO ₂ ) ₂ , LiAsF _6. , Lithium chloroborane such as LiN (CF ₃ SO ₂ ) ₂ , LiB ₁₀ Cl _10, lithium lower aliphatic carboxylate, lithium tetraphenylborate, LiN (CF ₃ SO ₂ ) (C ₂ F ₅ SO ₂ ) and LiN ( Examples include imides such as CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ and LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ). These can be used alone or in any combination as long as they do not impair the effects of the present invention. Among them, it is particularly preferable to add LiPF _6.

A nonaqueous electrolyte particularly preferable in the present invention is an electrolyte solution containing at least ethylene carbonate and ethyl methyl carbonate and containing LiPF ₆ as a lithium salt, or an electrolyte solution containing at least acetonitrile and containing LiBF ₄ or LiPF ₆ as a lithium salt. is there. In addition, an electrolytic solution containing GBL as a main solvent is also preferable. In this case, an additive such as VC is added in several percent, and LiBF ₄ other than LiPF ₆ and LiN (C ₂ F ₅ SO ₂ ) ₂ are used as lithium salts. It is preferable to use a mixed salt.
The amount of the electrolyte added to the battery is not particularly limited, but a necessary amount may be used depending on the amount of the positive electrode active material and the negative electrode material and the size of the battery. The amount of lithium salt dissolved in the non-aqueous solvent is not particularly limited, but is preferably 0.2 to 2 mol / liter. In particular, it is more preferably 0.5 to 1.5 mol / liter.

  This electrolytic solution is usually used by impregnating or filling a separator such as a porous polymer, a glass filter, or a nonwoven fabric. In order to make the electrolyte nonflammable, a halogen-containing solvent such as carbon tetrachloride or ethylene trifluoride chloride can be contained in the electrolyte. In addition, carbon dioxide gas can be included in the electrolyte in order to make it suitable for high-temperature storage.

Moreover, the following solid electrolyte can also be used. The solid electrolyte is classified into an inorganic solid electrolyte and an organic solid electrolyte.
As the inorganic solid electrolyte, for example, a Li nitride, a halide, an oxyacid salt, and the like are well known.
Among _{these, Li 4 SiO 4, Li 4} SiO 4 -LiI-LiOH, xLi 3 PO 4- (1-x) Li 4 SiO 4, Li 2 SiS 3, Li 3 PO 4 -Li 2 S-SiS 2 , And phosphorus sulfide compounds are effective.
As the organic solid electrolyte, polymer materials such as polyethylene oxide, polypropylene oxide, polyphosphazene, polyaziridine, polyethylene sulfide, polyvinyl alcohol, polyvinylidene fluoride and polyhexafluoropropylene, and derivatives, mixtures and composites thereof are effective. is there.

  Moreover, the gel electrolyte which made the organic solid electrolyte contain the said non-aqueous electrolyte can also be used. Examples of the organic solid electrolyte include polymer matrices such as polyethylene oxide, polypropylene oxide, polyphosphazene, polyaziridine, polyethylene sulfide, polyvinyl alcohol, polyvinylidene fluoride and polyhexafluoropropylene, and derivatives, mixtures and composites thereof. The material is effective. In particular, a copolymer of vinylidene fluoride and hexafluoropropylene or a mixture of polyvinylidene fluoride and polyethylene oxide is preferable.

  As the shape of the battery, any of a coin type, a button type, a sheet type, a cylindrical type, a flat type and a square type may be adopted. When the shape of the battery is a coin type or a button type, the positive electrode active material or the negative electrode material mixture is mainly compressed into a pellet shape. The thickness and diameter of the pellet may be determined depending on the size of the battery.

  In the case where the shape of the battery is a sheet type, a cylindrical type, or a square type, the mixture containing the positive electrode active material or the negative electrode material is mainly applied (coated), dried and compressed on the current collector. As a coating method, a general method can be used. Examples thereof include a reverse roll method, a direct roll method, a blade method, a knife method, an extrusion method, a curtain method, a gravure method, a bar method, a casting method, a dip method, and a squeeze method. Of these, blade method, knife method and extrusion method are preferred.

  The application is preferably performed at a speed of 0.1 to 100 m / min. Under the present circumstances, the surface state of a favorable coating layer can be obtained by selecting the said coating method according to the physical property and dryness of the solution of a mixture. Application of the mixture to the current collector may be performed on each side of the current collector, or may be performed simultaneously on both sides. Moreover, it is preferable to provide an application layer on both sides of the current collector, and the application layer on one surface may be composed of a plurality of layers including a mixture layer. The mixture layer includes a binder, a conductive material, and the like in addition to a material related to insertion and release of lithium ions such as a positive electrode active material or a negative electrode material. In addition to the mixture layer, a protective layer containing no active material, an undercoat layer provided on the current collector, an intermediate layer provided between the mixture layers, and the like may be provided. The layer not containing these active materials preferably contains conductive particles, insulating particles, a binder, and the like.

  The coating method may be continuous, intermittent, or striped. The thickness, length and width of the coating layer can be determined depending on the size of the battery and the like, but the thickness of the coating layer on one side is particularly preferably 1 to 2000 μm in a compressed state after drying.

  As a method for drying or dehydrating a mixture pellet or sheet, a generally employed method can be used. In particular, it is preferable to use hot air, vacuum, infrared rays, far infrared rays, electron beams and low-humidity air alone or in combination.

The temperature is preferably in the range of 80 to 350 ° C, particularly preferably in the range of 100 to 250 ° C. The water content of the entire battery is preferably 2000 ppm or less, and the water content of each of the positive electrode mixture, the negative electrode mixture and the electrolyte is preferably 500 ppm or less from the viewpoint of cycleability.
As a sheet pressing method, a generally adopted method can be used, and a mold pressing method or a calendar pressing method is particularly preferable. The pressing pressure is not particularly limited, but is preferably 0.2 to 3 t / cm ² . The press speed of the calendar press method is preferably 0.1 to 50 m / min.

The pressing temperature is preferably room temperature to 200 ° C. The ratio of the width of the positive electrode sheet to the negative electrode sheet is preferably 0.9 to 1.1. In particular, 0.95-1.0 is preferable.
When the negative electrode comprising the positive electrode and titanium oxide of the present invention is used, the content ratio of the positive electrode active material and the negative electrode material is particularly preferably a capacity-limited design ratio on the negative electrode side as described above. However, when only the positive electrode active material according to the present invention is used, it cannot be limited because it differs depending on the type of compound and the formulation of the mixture. Can be set.

In addition, the wound body of the electrode in the present invention does not necessarily have a true cylindrical shape, and may have a prismatic shape such as a long cylindrical shape or a rectangular shape whose cross section is an ellipse.
Hereinafter, the present invention will be described by way of representative examples, but the present invention is not limited thereto.

Experimental example 1

Three types of positive electrode active material samples were prepared under the synthesis conditions shown in (3) above. By forming a mixture obtained by sufficiently mixing [Ni _1/4 Mn _3/4 ] (OH) ₂ and LiOH · H ₂ O obtained by eutectic into pellets and firing the pellets A positive electrode active material was obtained. Therefore, the composition of the obtained positive electrode active material is Li [Ni _1/2 Mn _3/2 ] O ₄ . Note that the amount of oxygen varied depending on the synthesis conditions. The electrochemical measurement of the obtained positive electrode active material was performed by the method shown in the above (1).

(I) Production Example 1
The ambient temperature was raised from room temperature to 1000 ° C. in about 3 hours, held at 1000 ° C. for 12 hours, and lowered from 1000 ° C. to room temperature in 2 hours.
(Ii) Production Example 2
The ambient temperature is raised from room temperature to 1000 ° C. in about 3 hours, held at 1000 ° C. for 12 hours, lowered from 1000 ° C. to 700 ° C. in 30 minutes, held at 700 ° C. for 48 hours, and from 700 ° C. to room temperature is 1 Lowered in 5 hours.
(iii) Production Example 3
The ambient temperature was raised from room temperature to 1000 ° C. in about 3 hours, held at 1000 ° C. for 12 hours, and rapidly cooled from 1000 ° C. to room temperature. Further, the ambient temperature was raised to 700 ° C. in about 1 hour, maintained at 700 ° C. for 48 hours, and lowered from 700 ° C. to room temperature in 1.5 hours.

  The electrochemical behaviors of the positive electrode active materials obtained in Production Examples 1 to 3 are shown in FIGS. FIG. 22 shows that any positive electrode active material shows a charge / discharge curve with little polarization and very good flatness. Further, the positive electrode active material (a) of Production Example 1 shows a step at the end of discharge, and the remaining capacity can be detected by utilizing this step. This level difference is not a large level of several V, and it is possible to detect the effective remaining capacity without causing power down due to power shortage in the device used. In the case of the positive electrode active material (b) obtained by reoxidation at 700 ° C., this step can be almost eliminated. Therefore, by controlling the temperature and time of the reoxidation step, the step at the end of discharge can be freely controlled within this range. Similarly, almost no step is observed in the positive electrode active material (c) obtained by reoxidation through the rapid cooling step. It can be seen that the material is more highly polarized and flat by controlling the particles in the rapid cooling process as described above. Furthermore, it can be filled with high density.

  Although the case where the combination of Ni and Mn was used was shown above, the discharge capacity was measured when the transition metal shown in Table 1 was used. Firing was performed under the conditions of Production Example 3 above. Similarly, the ratio of Mn to other transition metals was 3: 1. Table 1 shows the discharge capacity obtained when each positive electrode active material was used. From Table 1, it can be seen that positive electrode active materials having similar characteristics were obtained although there were capacity differences.

  The ratio of Mn to other transition metal species was the best when the ratio was 3: 1. When the ratio of the transition metal species was increased or decreased from this, the capacity of the high potential decreased.

Experimental example 2

A positive electrode active material according to the present invention is used for the positive electrode, and Li ₄ Ti ₅ O ₁₂ (Li [Li _1/3 Ti _5/3 ] O ₄ ) is used for the negative electrode as the negative electrode active material to produce a 3V class battery. did. The negative electrode plate and the positive electrode plate were produced in the same manner with the same mixing ratio. A 25 μm nonwoven fabric made of polybutylene terephthalate was used as the separator. Moreover, the area of the electrode was 3 cm ^2, and an organic electrolytic solution in which 1 mol of LiPF ₆ was dissolved in a mixed solvent of EC / DEC (3/7) was used as the electrolytic solution. As the positive electrode active material, the one obtained in Case 3 was used.

FIG. 23 shows the discharge behavior of this battery system, and FIG. 24 shows the high rate characteristics. 23 and 24, this battery system is a 3V battery, which is a battery system with extremely good polarization characteristics. Furthermore, the potential shape also shows extremely high flatness that has not been achieved in the past.
FIG. 25 shows the pulse discharge characteristics. FIG. 25 shows a pulse characteristic having the same width from the beginning of discharge to almost the end, which is clearly different from the conventional one in which the pulse polarization gradually increases from the end of discharge. Such potential flatness and excellent polarization characteristics are considered to have a great effect by optimizing the synthesis method of the positive electrode active material and realizing the all-region two-phase reaction.

Experimental example 3

  FIG. 26 shows a front view with a cross section of a part of the cylindrical battery produced in this experimental example. An electrode plate group 4 obtained by winding a laminate of a positive electrode plate, a separator and a negative electrode plate in a spiral shape is housed in a battery case 1. A positive electrode lead 5 is drawn from the positive electrode plate and connected to the sealing plate 2, and a negative electrode lead 6 is drawn from the negative electrode plate and connected to the bottom of the battery case 1. For the battery case and the lead plate, a metal or alloy having an organic electrolyte resistance and an electron conductivity can be used. For example, a metal such as iron, nickel, titanium, chromium, molybdenum, copper, aluminum, or an alloy thereof is used. In particular, the battery case is preferably obtained by processing a stainless steel plate or an Al—Mn alloy plate, and the positive electrode lead is preferably made of aluminum. The negative electrode lead is preferably composed of nickel or aluminum. In addition, various engineering plastics and a combination of these and a metal can be used for the battery case in order to reduce the weight.

  Insulating rings 7 are respectively provided on the upper and lower portions of the electrode plate group 4. And electrolyte solution is inject | poured and the battery case 1 is sealed using the sealing body 3. FIG. At this time, a safety valve can be provided on the sealing plate 2 or the sealing body 3. In addition to the safety valve, various conventionally known safety elements may be provided. For example, a fuse, bimetal, PTC element, or the like can be used as the overcurrent prevention element. In addition to the safety valve, as a countermeasure against an increase in the internal pressure of the battery case, a method of cutting the battery case, a method of cracking the gasket, a method of cracking the sealing plate, or a method of cutting the lead plate can be used. Further, the charger may be provided with a protection circuit incorporating measures against overcharge and overdischarge, or the protection circuit may be connected independently.

  As a method for welding the cap, the battery case, the sheet, and the lead plate, a conventionally known method such as DC or AC electric welding, laser welding, or ultrasonic welding can be used. Moreover, as a sealing agent for sealing, a conventionally known compound or mixture such as asphalt can be used.

The positive electrode plate was produced as follows. To 85 parts by weight of the positive electrode active material powder of the present invention, 10 parts by weight of carbon powder as a conductive agent and 5 parts by weight of polyvinylidene fluoride resin as a binder were mixed. The obtained mixture was dispersed in dehydrated N-methylpyrrolidinone to obtain a slurry. This slurry was applied onto a positive electrode current collector made of an aluminum foil, dried and rolled, and then cut into a predetermined size. A negative electrode plate was produced in the same manner as the positive electrode plate, except that Li ₄ Ti ₅ O ₁₂ (Li [Li _1/3 Ti _5/3 ] O ₄ ) was used instead of the positive electrode active material.

Also, a styrene butadiene rubber binder may be used. In the present invention, titanium oxide is mainly used as the negative electrode active material. However, when a carbonaceous material is mainly used, the carbonaceous material and the styrene-butadiene rubber binder are mixed at a weight ratio of 100: 5. The resulting mixture was applied to both sides of a copper foil, dried, rolled, and then cut into a predetermined size to obtain a negative electrode plate.
As the separator, a nonwoven fabric or a microporous film made of polyethylene was used.

The organic electrolyte used was a solution of 1.0 mol / liter of LiPF ₆ in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate in a volume ratio of 3: 7. The cylindrical battery had a diameter of 14.1 mm and a height of 50.0 mm.
By using the positive electrode active material according to the present invention, the remaining capacity alarm is facilitated. From this viewpoint, the size of the step at the end of discharge was adjusted by the reoxidation temperature.
In case 3 above, only the reoxidation (second firing) temperature was changed. The above-described cylindrical battery was fabricated using Li ₄ Ti ₅ O ₁₂ (Li [Li _1/3 Ti _5/3 ] O ₄ ) for the negative electrode. These batteries were discharged at a 1C rate, and the remaining capacity was detected when the battery voltage reached 2.7V. Thereafter, the remaining capacity when discharged to 2 V was measured. Table 2 shows the results. The numerical value is shown by the ratio of the remaining capacity to the total battery capacity.

From the results in Table 2, the remaining capacity alarm can be more easily realized by the battery system of the present invention without the need for complicated electronic circuits or calculations. Also, the remaining capacity alarm timing can be set freely.

Experimental Example 4

  The capacity design of the positive and negative electrodes was studied. The ratio of the amount of positive electrode active material to the amount of negative electrode active material per unit area was varied, the cycle life of the cylindrical battery was measured, and the results are shown in Table 3. Regarding charging / discharging cycle conditions, charging was performed at a constant current and a constant voltage of 3.5 V, and the maximum current was 1 C. The end condition was a time cut of 2 hours from the start of charging. The discharge was a constant current discharge of 2 C and a 2.0 V cut. Table 3 shows the number of cycles in which the retention capacity decreases to 95% with respect to the initial capacity. From Table 3, it can be seen that the cycle life is reduced when the capacity ratio is 1.2 or more. Therefore, it is preferable that the balance between the positive electrode and the negative electrode is substantially negative electrode rate-determined (negative electrode capacity regulation). If the positive electrode material is increased more than necessary, the battery capacity decreases, so it is preferable that the capacity ratio is substantially 0.5 to 1.2.

Experimental Example 5

Here, positive and negative electrode current collectors were examined. When graphite is used for the negative electrode, it is common to use copper as a current collector (core material) for reasons such as electric potential.
As described above, when Li ₄ Ti ₅ O ₁₂ (Li [Li _1/3 Ti _5/3 ] O ₄ ) is used for the negative electrode, an aluminum core can be used. According to the present invention, it has been found that, in addition to weight reduction and cost reduction, there are merits of improving safety. The reason for this is as follows. That is, when a battery is overcharged due to a failure of a charger or the like, lithium metal is deposited on the surface of the negative electrode, which causes a reduction in safety. When Li ₄ Ti ₅ O ₁₂ (Li [Li _1/3 Ti _5/3 ] O ₄ ) is used for the negative electrode, the charge / discharge potential is as high as 1.5 V, which is considerably higher than 0 V of the lithium deposition potential. When copper is used as the material, lithium metal may be deposited on the surface. On the other hand, when aluminum is used, the core material occludes lithium and does not deposit as lithium metal. Table 4 shows an overcharge test of a cylindrical battery manufactured using each current collector, and the maximum temperature of the battery surface at that time was measured. In the overcharge test, constant current overcharge was performed at a current value of 1.5C.

  From Table 4, it can be seen that the use of an aluminum core can suppress battery heat generation during overcharge. As described above, by using the aluminum core material in the battery system of the present invention, a 3V class battery can be produced, and the battery can be reduced in weight, cost, and safety.

Here, preferred electrolytes for the battery system according to the present invention were examined.
In the battery using graphite as the negative electrode, there were many restrictions on the electrolyte. In particular, it has been difficult to use a lactone organic solvent from the viewpoint of reduction resistance. Propylene carbonate is also difficult to use because it is simultaneously decomposed during charging and discharging of graphite. Since these solvents are inexpensive and have a high dielectric constant, they are also useful solvents that have sufficient ability to dissolve solutes and are excellent in oxidation resistance. For the same reason, it is difficult to use trimethyl phosphate and triethyl phosphate. These solvents have a fire extinguishing action and are excellent in safety. On the other hand, these useful solvents can be used in the present invention.

  In addition, for the necessity of forming a protective film on the graphite surface and for the purpose of dissolving the solute, the electrolytic solution is based on ethylene carbonate (EC) having a very high viscosity. The present invention does not require this EC. In other words, by using this battery system, it is a battery having a high voltage of 3V, but by not using graphite, the selection range of the electrolyte can be dramatically expanded. Table 5 shows preferable electrolytes in the battery system. Table 5 shows the capacities obtained by variously changing the electrolytic solution as an index when the capacity obtained in the conventional electrolytic solution system is 100. For comparison, the results are shown in which a cylindrical battery was fabricated and evaluated in the same manner using lithium cobalt oxide for the positive electrode and a graphite material for the negative electrode.

In Table 5, the EC / DEC (3/7) display of the mixed solvent indicates that the composition ratio of the mixed solvent of EC and DEC is 3: 7. The capacity obtained with this electrolyte was set to 100 for each battery system.
From Table 5, almost no high capacity is exhibited in the conventional battery system using graphite, and an electrolyte system that could not be used in the present invention can be used without any problem in the present invention, and an inexpensive and safer battery can be obtained. I understand that. Also, a mixed solvent of these solvents or a combination of conventionally used solvent types can be used.

Experimental Example 7

Here, preferred separators in the present invention were examined.
The battery system of the present invention does not require a highly functional separator such as a porous film. If a non-woven fabric is used, the overcharge safety may be lowered because the shutdown function is lowered. On the other hand, in the case of a non-woven fabric, an improvement in pulse performance can be expected especially because the electrolyte solution holding ability is higher than that of a porous film.

  Using a nonwoven fabric made of various polymer materials shown in Table 6 as a separator, a cylindrical battery according to the present invention was produced in the same manner as described above. Table 6 shows the pulse discharge characteristics and the maximum value of the battery surface temperature during overcharge. Pulse discharge is a simple pulse of 1A 5 seconds on and 5 seconds off. When the pulse discharge time obtained using a conventional PE porous film is 100, the pulse of the battery using other separators The discharge time is indicated by an index. The overcharge was performed by a constant current overcharge of 1.5C.

  From Table 6, it can be seen that in this battery system, the safety of overcharge is almost the same as that of the prior art and the pulse discharge time can be greatly improved by using a non-woven fabric. Moreover, it turns out that the fall of the voltage by a pulse current can also be improved by using a nonwoven fabric.

Experimental Example 8

In this experimental example, except that the nonaqueous electrolyte shown in Table 7 was used, three cylindrical batteries (single cells) were produced in the same manner as in Experimental Example 3, and the three single cells were connected in series. An assembled battery was produced.
Since the charging condition per unit cell is a constant voltage and a constant current of 3.5 V, the charging condition of the assembled battery is a constant voltage and a constant current of 10.5 V. In addition, the maximum current was set to 2C, and the assembled battery was placed in a thermostat at 0 ° C. for charging. The constant voltage holding time was 12 hours. The discharge was performed at a constant current of 2 C until the end voltage reached 2V.
The charging and discharging were repeated for 100 cycles with a rest of 5 minutes, and the capacity retention rate after 100 cycles was determined.

As can be seen from Table 7, when a solvent containing acetonitrile is used for the non-aqueous electrolyte, formation of a temperature distribution in the assembled battery can be suppressed, battery deterioration due to the temperature distribution can be reduced, and cycle life can be reduced. (Capacity maintenance rate) can be more reliably suppressed.
In particular, in the nonaqueous electrolyte secondary battery in this experimental example, since the battery capacity is regulated by the capacity of the negative electrode, the potential of the negative electrode is polarized from 1.5 V to the negative side with charging. When the charge is overcharged (overcharged), the potential decreases rapidly. However, when the polarization is large, the potential decrease tends to become more prominent. On the other hand, when the potential is 1.0 V or less, a side reaction with respect to Li ₄ Ti ₅ O ₁₂ used in the negative electrode starts, which may reduce the cycle life.
On the other hand, since acetonitrile contained in the non-aqueous electrolyte used in this experimental example is excellent in conductivity, polarization (resistance) does not increase even in a single cell located outside, and even if an overcharge cycle is performed, Li Deterioration of ₄ Ti ₅ O ₁₂ can be suppressed more reliably.

As described above, according to the battery system of the present invention, the flatness of the discharge voltage, the high rate discharge characteristics, the pulse characteristics, the cycle life during high rate charge / discharge, and the like can be dramatically improved in a balanced manner. In the above embodiments, the secondary battery of the present invention as a battery for portable devices has been described. However, the present invention relates to a power source for an electric tool and a hybrid vehicle for which high charge / discharge and cycle life are strongly demanded. The present invention can also be applied to a large battery of a drive system used for a power source, a power source for an electric vehicle, and the like, and can exhibit its characteristics.
According to the present invention, an inexpensive nickel-manganese composite oxide having a flat and high voltage can be effectively used as a positive electrode active material, and by using a titanium oxide for the negative electrode, it is excellent in 3V class high rate characteristics and cycle life. A good nonaqueous electrolyte secondary battery can be provided.
Furthermore, according to the present invention, even when a plurality of nonaqueous electrolyte secondary batteries are connected in series to form an assembled battery, a temperature distribution is not formed in the assembled battery, and particularly overcharge cycle characteristics. It is possible to realize a nonaqueous electrolyte secondary battery excellent in the above.

The mixture of the eutectic compound and the lithium compound is calcined in air at 1000 ° C. (a), 900 ° C. (b), 800 ° C. (c), 700 ° C. (d), and 600 ° C. (e) for 12 hours. It is a graph which shows the electrochemical characteristic of the positive electrode active material which concerns on this invention obtained by 1 baking). It is a figure which shows the TG curve (thermogravimetric analysis) of the positive electrode active material after 1st baking. It is a figure which shows the charging / discharging curve of the positive electrode active material which concerns on this invention obtained by performing 1st baking of 1000 degreeC and 12 hours, and performing 2nd baking of 700 degreeC and 48 hours. It is a SEM photograph which shows the cross section of the positive electrode active material (a) based on this invention, and the conventional positive electrode active material (b). 4 is a SEM photograph of positive electrode active material particles produced under the conditions of Case 1. 4 is a SEM photograph of positive electrode active material particles manufactured under the conditions of Case 2. 4 is a SEM photograph of positive electrode active material particles produced under the conditions of Case 3. 4 is a SEM photograph of positive electrode active material particles produced under the conditions of Case 4. It is a figure which shows the X-ray-diffraction pattern of the positive electrode active material which concerns on this invention manufactured at various 1st baking temperatures. It is a figure which shows the measurement result of FT-IR of the positive electrode active material which concerns on this invention manufactured at various 1st baking temperatures. It is a figure which shows the X-ray-diffraction pattern of the positive electrode active material manufactured on various conditions. It is a figure which shows the FT-IR measurement result of the positive electrode active material synthesize | combined on various conditions. It is a figure which shows the mode of each atomic site in the crystal structure of the positive electrode active material which concerns on this invention. It is a figure which shows the change of the unit cell constant of the positive electrode active material manufactured through rapid cooling. It is a figure which shows the X-ray-diffraction pattern change accompanying charging / discharging of the positive electrode active material which concerns on this invention. It is a figure which shows the change of the lattice constant accompanying charging / discharging of the positive electrode active material which concerns on this invention. It is a figure which shows the charging / discharging behavior of the battery system which concerns on this invention. It is a figure which shows the cycle life to 200 cycles of the battery system which concerns on this invention. It is a figure which shows the load characteristic of the battery system which concerns on this invention. It is a figure which shows the high-rate discharge characteristic (no level | step difference) of the battery system which concerns on this invention. It is a figure which shows the result of the dilate meter which measured the expansion and contraction accompanying charging / discharging. It is a figure which shows the electrochemical behavior of the positive electrode active material based on this invention produced through rapid cooling. It is a figure which shows the discharge behavior of the battery system which concerns on this invention. It is a figure which shows the high rate characteristic of the battery system which concerns on this invention. It is a figure which shows the pulse discharge characteristic of the battery system which concerns on this invention. It is the front view which made a part of cylindrical battery produced in the example of the present invention a section.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Battery case 2 Sealing plate 3 Sealing body 4 Electrode plate group 5 Positive electrode lead 6 Negative electrode lead 7 Insulation ring

Claims (15)

A single cell comprising a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte;
The positive electrode active material has the formula: Li ₂ ± α [Me] ₄ O _8-x (where 0 ≦ α <0.4, 0 ≦ x <2, Me is Mn, Ni, Cr, Fe, Co and A transition metal element including at least one selected from the group consisting of Cu) and having a spinel-related crystal,
The negative electrode active material includes a titanium oxide having a spinel structure ,
The weight ratio of the negative electrode active material to the positive electrode active material is 0.5 or more and less than 1.2,
A non-aqueous electrolyte secondary battery, wherein the non-aqueous electrolyte contains acetonitrile.   The nonaqueous electrolyte secondary battery according to claim 1, comprising a plurality of the single cells connected in series or in parallel.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the positive electrode active material has a 2 × 2 superlattice in a transition metal element phase.   The nonaqueous electrolyte secondary battery according to claim 1, wherein in the positive electrode active material, a ratio of Mn to other transition metal elements is substantially 3: 1. The non-aqueous electrolyte secondary battery according to claim 1 , wherein the positive electrode active material has Li and / or Me at 16 (c) sites in a space group of Fd3m.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the positive electrode active material has a charge / discharge curve having a step of a potential of 0.2 to 0.8V.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the positive electrode active material has a lattice constant assigned to cubic crystals of 8.3 μm or less.   The positive electrode active material includes a mixture of crystal particles having a particle diameter of 0.1 to 8 μm and secondary particles of the crystal particles having a particle diameter of 2 to 30 μm. Non-aqueous electrolyte secondary battery. The non-aqueous electrolyte secondary battery according to claim 1, wherein the titanium oxide is Li ₄ Ti ₅ O ₁₂ .   The nonaqueous electrolyte secondary battery according to claim 1, wherein the positive electrode and the negative electrode have a current collector made of aluminum or an aluminum alloy.   The nonaqueous electrolyte contains at least one selected from the group consisting of propylene carbonate, γ-butyl lactone, γ-valerolactone, methyl diglyme, sulfolane, trimethyl phosphate, triethyl phosphate, and methoxymethyl ethyl carbonate; The nonaqueous electrolyte secondary battery according to claim 1.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the separator is made of a nonwoven fabric. The nonwoven fabric of polyethylene, polypropylene and be composed of at least one member selected from the group consisting of polybutylene terephthalate, the non-aqueous electrolyte secondary battery according to claim 1 2, wherein.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the single cell has a practical charge / discharge region of 2.5 V to 3.5 V and a practical average voltage of 3 V class.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the single cell has an operating discharge curve having a step of a potential of 0.2 V to 0.8 V.