JP2006278171A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery excellent in a cycle property, with a large residual capacity even if preserved in a charged state in an environment of high temperature, with little generation of gas. <P>SOLUTION: The nonaqueous electrolyte secondary battery is composed of an anode having an anode activator reversibly storing/releasing lithium, a cathode having a cathode activator reversibly storing/releasing lithium, and a nonaqueous electrolyte. The nonaqueous electrolyte contains vinylene carbonate (VC), and polyvinylformal (PVF) within a range of not less than 0.1 mass% and not more than 5.0 mass% to a total amount of the electrolyte. It is preferable that a content of hydroxyl in a PVF skeleton is 25 mol% or less, a content of an acetyl group in the PVC skeleton is 5 mol% or less, and a weight average molecular weight of the PVF is 100,000 to 150,000. Further, it is preferable that an amount of VC added to the electrolyte is not less than 0.2 mass% and not more than 3.0 mass% to the total amount of the electrolyte. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to a non-aqueous electrolyte secondary battery having improved charge storage characteristics while maintaining cycle characteristics.

  With the rapid spread of portable electronic devices, the required specifications for secondary batteries used in them are becoming stricter year by year, especially those that are small and thin, have high capacity, excellent cycle characteristics, and stable performance. Has been. In the field of secondary batteries, lithium non-aqueous electrolyte secondary batteries, which have a higher energy density than other batteries, are attracting attention, and the proportion of lithium non-aqueous electrolyte secondary batteries shows a significant increase in the secondary battery market. ing.

  This lithium non-aqueous electrolyte secondary battery is composed of a negative electrode in which a negative electrode active material mixture is applied in a film form on both sides of a negative electrode core (current collector) made of an elongated sheet-like copper foil, etc., and an elongated sheet-like aluminum A separator made of a microporous polyolefin film or the like is placed between the positive electrode core material made of foil or the like and coated with a positive electrode active material mixture in the form of a film, and the negative electrode and the positive electrode are insulated from each other by the separator. In the case of a rectangular battery, the wound electrode body is further crushed and formed into a flat shape, and the negative electrode is collected on each of the predetermined portions of the negative electrode and the positive electrode. The electric tab and the positive electrode current collecting tab are connected and accommodated in an exterior of a predetermined shape.

Among the lithium non-aqueous electrolyte secondary batteries, in particular, as a 4V class non-aqueous electrolyte secondary battery having a high energy density, a lithium composite oxide such as LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiFeO ₂ is used as the positive electrode active material. A negative electrode active material made of a carbonaceous material, particularly a negative electrode active material made of graphite, has a discharge potential comparable to that of lithium metal or a lithium alloy, but dendrite grows. Therefore, it is widely used because it has excellent properties such as high safety, excellent initial efficiency, good potential flatness, and high density.

  Nonaqueous solvents (organic solvents) used in such nonaqueous electrolyte secondary batteries must have a high dielectric constant in order to ionize the electrolyte, and must have high ionic conductivity over a wide temperature range. Therefore, carbonates such as propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC), γ-butyrolactone In addition, organic solvents such as lactones, ethers, ketones, esters, etc. are used, and in particular, mixed solvents such as EC and low-viscosity acyclic carbonates such as DMC, DEC, EMC are widely used. Yes.

  However, in nonaqueous electrolyte secondary batteries using these organic solvents, if a carbonaceous material such as graphite or amorphous carbon is used as the negative electrode active material, the organic solvent is reduced and decomposed on the electrode surface during the charge and discharge process. It is known that the negative electrode impedance increases due to the generation of gas, the accumulation of side reaction products, and the like, resulting in a decrease in charge / discharge efficiency, deterioration of charge / discharge cycle characteristics, and the like.

Therefore, conventionally, in order to suppress the reductive decomposition of the organic solvent, various compounds are added to the non-aqueous electrolyte so that the negative electrode active material does not directly react with the organic solvent and is also referred to as a passivation layer. A technique for controlling a negative electrode surface coating (SEI: Solid Electrolyte Interface. Hereinafter referred to as “SEI surface coating”) is important. For example, in Patent Documents 1 and 2 below, at least one selected from vinylene carbonate (VC) and derivatives thereof is added to a non-aqueous electrolyte solution of a non-aqueous electrolyte secondary battery, Before the insertion of lithium into the negative electrode by charging, an SEI surface coating is formed on the negative electrode active material layer to function as a barrier that prevents the insertion of solvent molecules around lithium ions. Yes.
JP 08-045545 A (claims, paragraphs [0009] to [0012], [0023] to [0036]) JP 2001-006729 A (claims, paragraphs [0006] to [0014])

  However, with VC alone, charge / discharge cycle characteristics at room temperature and the like give good results, but on the other hand, it is easily oxidized and decomposed by a positive electrode in a highly oxidized state. It has been found that bubbles are generated on the surface, the battery swells, the effective electrode plate area decreases, and the capacity decreases. In this case, if the amount of VC added is reduced, the charge storage characteristics are improved, but conversely, the charge / discharge cycle characteristics are deteriorated, and therefore cannot be employed.

  The inventor of the present application has made various studies to improve the problems of the non-aqueous electrolyte secondary battery using the non-aqueous electrolyte containing at least VC as described above. As a result, polyvinyl formal (PVF) was added at the same time. Then, due to the synergistic effect of both, the cycle characteristics are maintained well, the capacity remaining rate is greatly improved even when stored in a charged state in a high temperature atmosphere, and a non-aqueous electrolyte secondary battery with a small amount of gas generation is obtained. The inventors have found that the present invention can be obtained and have completed the present invention.

  The reason why such a result is obtained is not yet clear at present, and it is necessary to wait for further research. Perhaps the PVF in the electrolyte on the positive electrode surface is preferentially given priority over VC during charging. It is considered that a polymerized film of PVF is formed on the surface of the positive electrode by oxidation, and this polymerized film is stably present, so that oxidative decomposition of VC is suppressed during charge storage in a high temperature atmosphere.

  That is, an object of the present invention is to provide a non-aqueous electrolyte secondary battery that has excellent cycle characteristics, has a large remaining capacity even when stored in a charged state in a high temperature atmosphere, and has a small amount of gas generation.

  The above object of the present invention can be achieved by the following configurations. That is, the invention of the non-aqueous electrolyte secondary battery according to claim 1 of the present application includes a negative electrode using a negative electrode active material that reversibly absorbs and releases lithium, and a positive electrode active that reversibly absorbs and releases lithium. In a non-aqueous electrolyte secondary battery having a positive electrode using a material and a non-aqueous electrolyte, the non-aqueous electrolyte contains VC and at least 0.1% by mass of PVF with respect to the total amount of the electrolyte It is characterized by including in the range of 5.0 mass% or less.

  In the present invention, the coexistence of VC and PVF is essential in the non-aqueous electrolyte. Among them, when the PVF content is less than 0.1% by mass with respect to the total amount of the electrolytic solution, the cycle characteristics are good, but the amount of gas generation increases, and the capacity remaining ratio after charge storage in a high temperature atmosphere Is much worse. On the other hand, when the PVF content exceeds 5.0% by mass with respect to the total amount of the electrolytic solution, the amount of gas generated is small, and the capacity remaining rate after charge storage in a high-temperature atmosphere gives good results. The ionic conductivity of the liquid is hindered and the cycle characteristics deteriorate rapidly.

  Examples of organic solvents that can be used in the nonaqueous electrolyte secondary battery of the present invention include carbonates, lactones, ethers, and esters. Two or more of these solvents can be used in combination. Specific examples include carbonates such as propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), γ-butyrolactone, γ-valerolactone, γ-dimethoxyethane, tetrahydrofuran, 1,4-dioxane, diethyl carbonate, and the like. From the point of increasing charge and discharge efficiency, a mixed solvent of chain carbonates such as EC and DMC, DEC, EMC, etc. Are preferably used.

Similarly, electrolytes include lithium perchlorate (LiClO ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium borofluoride (LiBF ₄ ), lithium hexafluoroarsenate (LiAsF ₆ ), lithium trifluoromethylsulfonate ( Examples thereof include lithium salts such as LiCF ₃ SO ₃ ) and bistrifluoromethylsulfonylimide lithium [LiN (CF ₃ SO ₂ ) ₂ ]. Of these, LiPF ₆ and LiBF ₄ are preferably used, and the amount dissolved in the non-aqueous solvent is preferably 0.5 to 2.0 mol / l.

Further, as the positive electrode active material, a lithium transition metal composite oxide represented by LixMO ₂ (wherein M is at least one of Co, Ni, and Mn) capable of reversibly occluding and releasing lithium. , _{i.e., LiCoO 2, LiNiO 2, LiNiyCo} 1-y O 2 (y = 0.01~0.99), Li 0.5 MnO 2, LiMnO 2, LiCo x Mn y Ni z O 2 (x + y + z = 1) May be used singly or in combination.

  Similarly, as the negative electrode active material, a mixture with at least one selected from the group consisting of a carbonaceous material, a siliconaceous material, and a metal oxide capable of reversibly occluding and releasing lithium is used.

  The method for synthesizing PVF that can be used in the nonaqueous electrolyte secondary battery of the present invention is not particularly limited, and it is convenient and preferable to produce it by reacting polyvinyl alcohol with formaldehyde.

  The invention according to claim 2 of the present application is the nonaqueous electrolyte secondary battery according to claim 1, wherein the PVF content is 0.2% by mass or more and 3.0% by mass with respect to the total amount of the electrolytic solution. It is characterized by the following. A particularly preferable result is obtained when the content of PVF is in the range of 0.2% by mass to 3.0% by mass with respect to the total amount of the electrolytic solution.

  The invention according to claim 3 of the present application is characterized in that, in the nonaqueous electrolyte secondary battery according to claim 1, the content of hydroxyl groups in the PVF molecular skeleton is 25 mol% or less. When the content of the hydroxyl group in the PVF molecular skeleton is 25 mol% or less, reductive decomposition on the negative electrode surface of PVF is reduced, so that cycle characteristics and high-temperature charge storage characteristics are improved. The effect is recognized even if the hydroxyl group content in the PVF skeleton is small, but the effect of the present invention is sufficiently obtained if the content is 5 mol% or more.

  The invention according to claim 4 of the present application is characterized in that, in the nonaqueous electrolyte secondary battery according to claim 1, the content of acetyl groups in the PVF molecular skeleton is 5 mol% or less. When the content of the acetyl group in the PVF molecular skeleton is 5 mol% or less, reductive decomposition of the PVF on the negative electrode surface is reduced, so that cycle characteristics and high-temperature charge storage characteristics are improved. The effect is recognized even if the content of hydroxyl groups in the PVF skeleton is small, but the effect of the present invention is sufficiently obtained if it is 2 mol% or more.

  The invention according to claim 5 of the present application is characterized in that, in the nonaqueous electrolyte secondary battery according to claim 1, the PVF has a weight average molecular weight of 100,000 to 1,500,000. When the weight average molecular weight of PVF is less than 100,000, the polymer film obtained by the polymerization reaction on the positive electrode surface is difficult to have a high molecular weight, so that the polymer film partially dissolves in the electrolyte during high-temperature charge storage, It becomes difficult to suppress gas generation due to oxidative decomposition of VC. On the other hand, when the weight average molecular weight of PVF exceeds 1,500,000, the concentration gradient is caused in the positive electrode plate, and it is difficult to obtain a uniform polymerized film.

  The invention according to claim 6 of the present application is the nonaqueous electrolyte secondary battery according to claim 1, wherein the content of VC is 0.2% by mass or more and 3.0% by mass with respect to the total amount of the electrolytic solution. It is characterized by the following. When the addition amount of VC is less than 0.2% by mass with respect to the total amount of the electrolyte, the high-temperature charge storage characteristics can maintain good results, but the charge / discharge cycle characteristics deteriorate. Moreover, when the addition amount of VC exceeds 3.0 mass% with respect to electrolyte solution total amount, generation | occurrence | production of gas cannot be suppressed at the time of high temperature charge storage, and high temperature charge storage characteristics will deteriorate.

  The invention according to claim 7 of the present application is characterized in that, in the nonaqueous electrolyte secondary battery according to any one of claims 1 to 6, a laminate exterior body is used for the battery exterior. According to the non-aqueous electrolyte secondary battery having such a configuration, since the mass of the exterior can be reduced and the thickness can be reduced, a small and lightweight non-aqueous solvent secondary battery can be obtained. In addition, when the laminate outer package is used, the effect of the present invention is greatly exhibited because the influence of swelling appears remarkably.

  The present invention relates to a negative electrode using a negative electrode active material that reversibly absorbs and releases lithium, a positive electrode that uses a positive electrode active material that reversibly stores and releases lithium, and a non-aqueous electrolyte. In the water electrolyte secondary battery, the non-aqueous electrolyte contains vinylene carbonate (VC), and polyvinyl formal (PVF) ranges from 0.1% by mass to 5.0% by mass with respect to the total amount of the electrolyte. As described in detail below based on each example and comparative example, it has excellent charge / discharge cycle characteristics, a large capacity remaining rate even when stored in a charged state in a high temperature atmosphere, and gas generation A non-aqueous electrolyte secondary battery exhibiting the excellent effect that the amount is small can be obtained.

  BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the best mode for carrying out the present invention will be described in detail using examples and comparative examples. First, a specific method for manufacturing a non-aqueous electrolyte secondary battery common to the examples and comparative examples will be described. explain.

[Production of positive electrode plate]
A positive electrode active material made of LiCoO _{2 is} a carbon-based conductive agent (for example, 5% by mass) such as acetylene black and graphite, and a binder (for example, 3% by mass) made of polyvinylidene fluoride (PVdF) is N-methyl. What was melt | dissolved in the organic solvent etc. which consist of pyrrolidone was mixed, and it was set as the active material slurry or the active material paste. These active material slurries or active material pastes, in the case of slurry using a die coater, a doctor blade, etc., in the case of paste, a positive electrode core (for example, an aluminum foil or aluminum mesh having a thickness of 20 μm) by a roller coating method or the like The positive electrode plate which apply | coated uniformly on both surfaces and apply | coated the active material layer was formed. Thereafter, the positive electrode plate coated with the active material layer was passed through a drier to remove the organic solvent that was necessary for the preparation of the slurry or paste and dried. After drying, this dried positive electrode plate was rolled by a roll press to obtain a positive electrode plate having a thickness of 0.17 mm.

[Production of negative electrode plate]
A mixture of a negative electrode active material made of graphite and a binder (for example, 3% by mass) made of polyvinylidene fluoride (PVdF) dissolved in an organic solvent made of N-methylpyrrolidone, etc. is mixed to obtain a slurry or paste. . These slurries or pastes are uniformly applied over the entire surface of the negative electrode core (for example, a copper foil having a thickness of 20 μm) by using a die coater, a doctor blade or the like in the case of slurry, or by a roller coating method in the case of paste. The negative electrode plate which apply | coated and applied the active material layer was formed. Thereafter, the negative electrode plate coated with the active material layer was passed through a drier to remove the organic solvent that was necessary when the slurry or paste was prepared, and dried. After drying, the dried negative electrode plate was rolled by a roll press to obtain a negative electrode plate having a thickness of 0.14 mm.

[Production of electrode body]
The positive electrode plate and the negative electrode plate produced as described above are sandwiched with a microporous film (for example, thickness 0.025 mm) made of an inexpensive polyolefin-based resin having low reactivity with an organic solvent, and The electrode plates were overlapped with the center line in the width direction matched. Then, it wound with the winder and taped the outermost periphery, and it was set as the spiral electrode body of an Example and a comparative example. A sheet-like aluminum laminate material having a five-layer structure of resin layer (nylon) / adhesive layer / aluminum alloy layer / adhesive layer / resin layer (polypropylene) was prepared from the electrode body thus fabricated. Inserted into the storage space of aluminum laminate material. After that, the resin layer (polypropylene) on the inner side of the aluminum laminate material on the top portion and the one side portion where the positive electrode current collecting tab and the negative electrode current collecting tab protrude are thermally sealed to form a sealed portion.

[Preparation of electrolyte]
An electrolyte solution was prepared by dissolving in a solvent mixed at a volume ratio of ethylene carbonate (EC) / diethylene carbonate (DEC) = 30/70 at a ratio of 1.0 M-LiPF _6. 1-VC and PVF were added so that it might become a composition as shown in Table 5. 4.0 g of this electrolytic solution was injected from the opening of the outer package produced as described above and sealed, and lithium ion secondary batteries of Examples and Comparative Examples were produced.

[Examples 1-6 and Comparative Examples 1-3]
In Examples 1 to 6 and Comparative Examples 1 to 3, the influence of the addition amount of PVF on the battery characteristics was examined. First, the amount of VC added to the total amount of the electrolytic solution was kept constant at 1.5% by mass, and the amount of PVF added was changed from 0.0% by mass to 6.0% by mass as shown in Table 1. Nine types of batteries 1 to 6 and Comparative Examples 1 to 3 were produced. However, the PVF used has a weight average molecular weight Mw of 1,000,000, a hydroxyl group content of 8 mol%, and an acetyl group content of 3 mol%. In addition, the weight average molecular weight of PVF is a value measured by gel permeation chromatography (GPC) and converted by a calibration curve using standard polystyrene (hereinafter the same).

[Measurement of discharge capacity during one cycle]
First, at 25 ° C., each battery was charged with a constant current of 1 It (1 C), and after the battery voltage reached 4.2 V, it was charged with a constant voltage of 4.2 V for 3 hours. Thereafter, each battery was discharged at a constant current of 1 It at 25 ° C. until the battery voltage reached 2.75 V, and the discharge capacity at this time was determined as the one-cycle discharge capacity.

[Measurement of charge storage characteristics (remaining capacity)]
About each battery which measured the discharge capacity at 1 cycle, it charged with the constant current of 1 It at 25 degreeC, and after the battery voltage reached 4.2V, it charged with the constant voltage of 4.2V for 3 hours. These batteries in each charged state were left in an atmosphere of 80 ° C. for 120 hours, then left in an atmosphere of 25 ° C., and after the battery temperature reached equilibrium, the battery voltage was increased to 2.75 V at a constant current of 1 It. It discharged until it reached | attained, the discharge capacity after standing at high temperature was calculated | required, and the capacity | capacitance residual rate (%) was calculated | required based on the following formulas. The results are shown in Table 1.
Capacity remaining rate (%) = (discharge capacity after standing at high temperature / discharge capacity at one cycle) × 100

[Measurement of charge storage characteristics (gas generation amount)]
The amount of gas generated was measured by quantifying the volume of gas collected by cutting a part of the battery outer case and replacing the liquid in paraffin at 25 ° C for each battery whose capacity remaining rate (%) was measured. went. The results are summarized in Table 1.

[Measurement of cycle characteristics]
In addition, for each battery whose discharge capacity was measured at one cycle, it was charged with a constant current of 1 It at 25 ° C., and after the battery voltage reached 4.2 V, it was charged with a constant voltage of 4.2 V for 3 hours. The battery was discharged at a constant current of 1 It at 25 ° C. until the battery voltage reached 2.75 V. Using this as one cycle, the discharge capacity at 300 cycles was determined, and the cycle characteristic (%) was determined by the following formula. The results are summarized in Table 1.
Cycle characteristics (%) = (discharge capacity at 300 cycles / discharge capacity at one cycle) × 100

  From the results shown in Table 1, the following can be understood. That is, when the VC content with respect to the total amount of the electrolytic solution is 1.5% by mass, the PVF is not present (Comparative Example 1) and the cycle characteristic is 73%, but the gas generation amount after high-temperature charge storage is 12. It is as much as 6 ml, and the capacity remaining rate is as small as 39%. Addition of 0.05% by mass of PVF with respect to the total amount of the electrolyte (Comparative Example 2) slightly improved the gas generation amount and the capacity remaining rate after high-temperature charge storage, but PVF was based on the total amount of the electrolyte. When 0.1 mass% or more was added (Examples 1 to 6), the gas generation amount was greatly improved to 6.1 ml or less, and the capacity remaining rate was 71% or more, and good results were obtained. However, when the amount of PVF added exceeds 5% by mass with respect to the total amount of the electrolytic solution (Comparative Example 3), the cycle characteristics deteriorate. Therefore, the amount of PVF added is 0.1% by mass or more to the total amount of the electrolytic solution 5 The content is preferably not more than mass%, more preferably not less than 0.2 mass% and not more than 3.0 mass%.

[Examples 7 to 12 and Comparative Examples 4 to 5]
In Examples 7 to 12 and Comparative Examples 4 to 5, the effect of the addition amount of VC on the battery characteristics was examined. First, in order to contrast with the battery of Comparative Example 1, a battery of Comparative Example 4 was produced using an electrolytic solution in which neither PVF nor VC was added to the total amount of the electrolytic solution. Next, the PVF addition amount with respect to the total amount of the electrolyte was kept constant at 1.0% by mass, and the VC addition amount was changed from 0.0% by mass to 6.0% by mass as shown in Table 2. Seven types of batteries of Examples 7 to 12 and Comparative Example 5 were produced. However, the PVF used has a weight average molecular weight Mw of 1,000,000, a hydroxyl group content of 8 mol%, and an acetyl group content of 3 mol%. For these eight types of batteries, the amount of gas generation, capacity remaining rate, and cycle characteristics were measured in the same manner as in Examples 1 to 6 to Comparative Examples 1 to 3. The results are shown in Table 2 together with the results of Example 4 and Comparative Example 1.

  From the results shown in Table 2, the following can be understood. That is, when PVF is not added, the amount of VC added is 0.0% by mass (Comparative Example 4) and 1.5% by mass (Comparative Example 1) with respect to the total amount of the electrolyte. When the amount of VC added is increased, the cycle characteristics are improved, but the amount of gas generated after high-temperature charge storage is greatly increased, and the capacity remaining rate is greatly deteriorated. On the other hand, when the amount of PVF added to the total amount of the electrolyte is constant 1.0% by mass, when the amount of VC added is 0.0% by mass, the amount of gas generated after high-temperature charge storage is small and the capacity remaining rate is also good. Although a good result is obtained, the cycle characteristics are significantly deteriorated to 29% or less. However, when the addition amount of VC is 0.2% by mass or more (Examples 4 and 7 to 12), the cycle characteristics are 62% or more and good results can be obtained. However, the addition amount of VC is 3.0% by mass. When the ratio exceeds 20% (Example 12), very good results are obtained in the cycle characteristics, but the amount of gas generated after high-temperature charge storage is slightly increased and the capacity remaining rate is also slightly deteriorated. Therefore, according to the present invention, the coexistence of VC and PVF is essential, and it is understood that the addition amount of VC is preferably 0.2% by mass or more and 3.0% by mass or less.

[Examples 13 to 16]
In Examples 13 to 16, the influence of the hydroxyl content of PVF on the battery characteristics was examined. First, using 4 types of PVF having a weight average molecular weight of 1,000,000 and an acetyl group content of 3 mol% constant and a hydroxyl group content of 5 to 30 mol%, the amount of PVF added to the total amount of the electrolyte: 1.0 mass% Further, the amount of gas generated, the capacity remaining rate, and the cycle characteristics were measured in the same manner as in Examples 1 to 6 to Comparative Examples 1 to 3 using an electrolytic solution with 1.5% by mass of VC added. The results are shown in Table 3 together with the results of Example 4.

  From the results shown in Table 3, the following can be understood. That is, when the hydroxyl content of PVF exceeds 5 mol%, good results are obtained in both the amount of gas generated after high-temperature charge storage, the capacity remaining rate and the cycle characteristics, and the hydroxyl content of PVF is 30 mol%. Although the cycle characteristics tend to be deteriorated, the cycle characteristics are still as good as 67%. Therefore, the hydroxyl content of PVF is preferably 5 mol% or more and 25 mol% or less.

[Examples 17 to 19]
In Examples 17 to 19, the influence of the acetyl group content of PVF on the battery characteristics was examined. First, three types of PVF having a weight average molecular weight of 1,000,000 and a hydroxyl group content of 8 mol% are constant, and an acetyl group content of 2 to 7 mol% are used. Further, the amount of gas generated, the capacity remaining rate, and the cycle characteristics were measured in the same manner as in Examples 1 to 6 to Comparative Examples 1 to 3 using an electrolytic solution with 1.5% by mass of VC added. The results are shown in Table 4 together with the results of Example 4.

  From the results shown in Table 4, the following can be understood. That is, when the acetyl group content of PVF exceeds 2 mol%, good results are obtained in terms of gas generation amount, capacity remaining rate and cycle characteristics after high-temperature charge storage, and the acetyl group content of PVF is 7 mol%. Then, the gas generation amount and the capacity remaining rate tend to be deteriorated, but the gas generation amount is still 5.1 ml and the capacity remaining rate is as good as 81%. Therefore, the acetyl group content of PVF is preferably 2 mol% or more and 5 mol% or less.

[Examples 20 to 25]
In Examples 20 to 25, the influence of PVF weight average molecular weight on battery characteristics was examined. First, 6 types of PVF having a hydroxyl group content of 8 mol% and an acetyl group content of 3 mol% are constant, and a weight average molecular weight of 80,000 to 2.5 million is used. The amount of PVF added to the total amount of the electrolyte: 1.0 mass % And VC addition amount: The amount of gas generation, capacity remaining rate, and cycle characteristics were measured in the same manner as in Examples 1 to 6 to Comparative Examples 1 to 3 using an electrolytic solution with 1.5 mass%. The results are shown in Table 5 together with the results of Example 4.

From the results shown in Table 5, the following can be understood. That is, when the PVF has a weight average molecular weight of 100,000 or more, good results are obtained in terms of gas generation amount, capacity remaining rate, and cycle characteristics after storage at high temperature, and gas generation after storage at high temperature as the weight average molecular weight increases. The amount is reduced and the capacity remaining rate is also good. However, when the weight average molecular weight of PVF is 1,500,000 or more, there is a tendency that the gas generation amount and the capacity remaining rate after storage at high temperature are gradually deteriorated. Therefore, the gas generation amount and the capacity remaining rate after storage at high temperature are deteriorated. However, even if the weight average molecular weight of PVF is 2.5 million, the amount of gas generated after high-temperature charge storage is 6.3 ml, and the residual capacity rate is as good as 78%. Accordingly, the weight average molecular weight of PVF is preferably 100,000 or more and 1,500,000 or less from the results of gas generation after high-temperature storage and capacity remaining.

Claims (7)

  A non-aqueous electrolyte secondary having a negative electrode using a negative electrode active material that reversibly stores and releases lithium, a positive electrode using a positive electrode active material that reversibly stores and releases lithium, and a non-aqueous electrolyte In the battery, the non-aqueous electrolyte contains vinylene carbonate (VC) and contains polyvinyl formal (PVF) in a range of 0.1% by mass to 5.0% by mass with respect to the total amount of the electrolyte. Non-aqueous electrolyte secondary battery characterized.   2. The nonaqueous electrolyte secondary battery according to claim 1, wherein a content of the PVF is 0.2% by mass or more and 3.0% by mass or less with respect to a total amount of the electrolytic solution.   2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the content of hydroxyl groups in the PVF molecular skeleton is 25 mol% or less.   The nonaqueous electrolyte secondary battery according to claim 1, wherein the content of acetyl groups in the PVF molecular skeleton is 5 mol% or less.   2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the PVF has a weight average molecular weight of 100,000 to 1,500,000.   2. The nonaqueous electrolyte secondary battery according to claim 1, wherein a content of the VC is 0.2% by mass or more and 3.0% by mass or less with respect to a total amount of the electrolytic solution.   The nonaqueous electrolyte secondary battery according to any one of claims 1 to 6, wherein a laminate outer package is used for the battery outer package.