JP6618386B2 - Lithium ion secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte battery, particularly a lithium ion secondary battery.

  Nonaqueous electrolyte batteries have been put to practical use as automobile batteries including hybrid cars and electric cars. Lithium ion secondary batteries are used as such on-vehicle power supply batteries. Lithium ion secondary batteries are required to have various characteristics such as output characteristics, energy density, capacity, lifetime, and high temperature stability. In particular, various improvements have been made to the battery structure, the electrodes, and the electrolytic solution in order to improve safety during overcharging of the battery.

For example, Patent Document 1 proposes to use Li ₂ B ₁₂ F _x Z _12-x as an electrolyte in order to improve voltage rise and safety during battery overcharge. In Patent Document 1, Li ₂ B ₁₂ F _x Z _12-x is said to be capable of improving the safety by developing a redox shuttle mechanism when the battery is overcharged. In addition to Patent Document 1, many attempts have been made to use a specific electrolyte or a highly viscous electrolytic solution in order to improve safety during overcharging of the battery.

WO2013-54676

  If a specific electrolyte is used in order to enhance the safety of the battery or an electrolytic solution having a high viscosity is used, there is a problem that the output of the battery is lowered. Particularly in applications where high output charge / discharge characteristics are required, such as a vehicle battery, a decrease in output can be a major problem. In addition, it is very difficult to improve both the battery output under normal temperature and low temperature environments. At room temperature where the viscosity of the electrolyte is low, the more lithium salt is present in the power generation element, the lower the resistance value of the battery, but at low temperatures where the viscosity of the electrolyte is high, This is because the resistance value of the battery increases.

  Accordingly, an object of the present invention is to provide a lithium ion secondary battery that is excellent in output characteristics particularly at low temperatures.

  A lithium ion secondary battery according to an embodiment of the present invention includes a positive electrode in which a positive electrode active material layer is disposed on a positive electrode current collector, a negative electrode in which a negative electrode active material layer is disposed on a negative electrode current collector, a separator, and an electrolyte solution Is a lithium ion secondary battery including a power generation element including the inside of the exterior body. Here, the amount of the lithium salt present in the negative electrode active material layer is in the range of 0.36 to 0.52 mol / L, and the electrolyte contains propylene carbonate, and the output and capacity of the lithium ion secondary battery The value of the ratio (W / Wh) is 35-80.

  The lithium ion secondary battery of the present invention is a high capacity battery that can be charged and discharged at a high current density (that is, a high rate) at low temperatures.

FIG. 1 is a schematic cross-sectional view showing a lithium ion secondary battery according to an embodiment of the present invention.

  Embodiments of the present invention will be described below. In the embodiment, the positive electrode is a thin plate in which a positive electrode active material layer is formed by applying or rolling and drying a mixture of a positive electrode active material, a binder, and, if necessary, a conductive additive on a positive electrode current collector such as a metal foil. Or it is a sheet-like battery member. The negative electrode is a thin plate-like or sheet-like battery member in which a negative electrode active material layer is formed by applying a mixture of a negative electrode active material, a binder, and, if necessary, a conductive additive to a negative electrode current collector. The separator is a film-like battery member for separating the positive electrode and the negative electrode and ensuring the conductivity of lithium ions between the negative electrode and the positive electrode. The electrolytic solution is an electrically conductive solution in which an ionic substance is dissolved in a solvent. In this embodiment, a nonaqueous electrolytic solution can be used in particular. The power generation element including the positive electrode, the negative electrode, the separator, and the electrolytic solution is a unit of the main constituent member of the battery. Usually, the positive electrode and the negative electrode are laminated via the separator, and this laminate is immersed in the electrolytic solution. Has been.

  The lithium ion secondary battery of the embodiment is configured such that the power generation element is included in the exterior body, and preferably the power generation element is sealed inside the exterior body. The term “sealed” means that the power generation element is encased in an exterior body material so as not to touch the outside air. That is, the exterior body has a bag shape capable of sealing the power generation element therein.

The positive electrode that can be used in all embodiments includes a positive electrode in which a positive electrode active material layer including a positive electrode active material is disposed on a positive electrode current collector. Preferably, the positive electrode has a positive electrode active material layer obtained by applying or rolling a mixture of a positive electrode active material, a binder, and optionally a conductive additive to a positive electrode current collector made of a metal foil such as an aluminum foil, and drying. ing. In each embodiment, the positive electrode active material layer preferably contains a lithium / nickel composite oxide as the positive electrode active material. The lithium-nickel based composite oxide is a general formula Li _x Ni _y Me _(1-y) O ₂ (where Me is Al, Mn, Na, Fe, Co, Cr, Cu, Zn, Ca, K, It is a transition metal composite oxide containing lithium and nickel, represented by at least one metal selected from the group consisting of Mg and Pb.

The positive electrode active material layer can further contain a lithium / manganese composite oxide as a positive electrode active material. Examples of the lithium / manganese composite oxide include a zigzag layered structure lithium manganate (LiMnO ₂ ) and spinel type lithium manganate (LiMn ₂ O ₄ ). By using a lithium-manganese composite oxide in combination, the positive electrode can be produced at a lower cost. In particular, it is preferable to use spinel type lithium manganate (LiMn ₂ O ₄ ) which is excellent in terms of stability of the crystal structure in an overcharged state.

In particular, the positive electrode active material layer may include a lithium nickel manganese cobalt composite oxide having a layered crystal structure represented by a general formula Li _x Ni _y Co _z Mn _(1-yz) O ₂ as a positive electrode active material. preferable. Here, x in the general formula is 1 ≦ x ≦ 1.2, y and z are positive numbers that satisfy y + z <1, and the value of y is 0.5 or less. Note that when the proportion of manganese increases, it becomes difficult to synthesize a single-phase composite oxide, so 1-yz ≦ 0.4 is desirable. Further, since the cost increases and the capacity decreases when the proportion of cobalt increases, it is desirable to satisfy z <y and z <1-yz. In order to obtain a high capacity battery, it is particularly preferable to satisfy y> 1-yz and y> z.

  Carbon fibers such as carbon nanofibers, carbon blacks such as acetylene black and ketjen black, activated carbon, graphite, mesoporous carbon, fullerenes, carbon nanotubes and other carbon materials as conductive aids optionally used in the positive electrode active material layer Is mentioned. In addition, electrode additives generally used for electrode formation, such as thickeners, dispersants, and stabilizers, can be appropriately used for the positive electrode active material layer.

  As binders used for the positive electrode active material layer, conductive resins such as fluororesins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and polyvinyl fluoride (PVF), polyanilines, polythiophenes, polyacetylenes, polypyrroles, etc. Polymer, synthetic rubber such as styrene butadiene rubber (SBR), butadiene rubber (BR), chloroprene rubber (CR), isoprene rubber (IR), acrylonitrile bradiene rubber (NBR), or carboxymethylcellulose (CMC), xanthan gum, guar gum Polysaccharides such as pectin can be used.

  The negative electrode that can be used in all embodiments includes a negative electrode in which a negative electrode active material layer containing a negative electrode active material is disposed on a negative electrode current collector. Preferably, the negative electrode has a negative electrode active material layer obtained by applying or rolling a mixture of a negative electrode active material, a binder, and optionally a conductive additive to a negative electrode current collector made of a metal foil such as copper foil, and drying. ing. In each embodiment, the negative electrode active material includes graphite. In particular, when graphite is contained in the negative electrode active material layer, there is an advantage that the output of the battery can be improved even when the remaining capacity (SOC) of the battery is low. Graphite is a carbon material of hexagonal hexagonal plate crystal, and is sometimes referred to as graphite or graphite. The graphite is preferably in the form of particles. Further, as the negative electrode active material, amorphous carbon may be contained, and in some cases, a mixture of graphite and amorphous carbon may be used. Amorphous carbon is a carbon material that has a structure in which microcrystals are randomly networked and may be partially similar to graphite, and is entirely amorphous. . Examples of the amorphous carbon include carbon black, coke, activated carbon, carbon fiber, hard carbon, soft carbon, and mesoporous carbon. The amorphous carbon is preferably in the form of particles. When a mixed carbon material containing both the above graphite particles and amorphous carbon particles is used, the battery regeneration performance is improved.

  Examples of conductive aids that are optionally used in the negative electrode active material layer include carbon fibers such as carbon nanofibers, carbon blacks such as acetylene black and ketjen black, carbon materials such as activated carbon, mesoporous carbon, fullerenes, and carbon nanotubes. It is done. In addition, electrode additives generally used for electrode formation, such as a thickener, a dispersant, and a stabilizer, can be appropriately used for the negative electrode active material layer.

  As a binder used for the negative electrode active material layer, fluororesins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and polyvinyl fluoride (PVF), conductive materials such as polyanilines, polythiophenes, polyacetylenes, and polypyrroles. Polymers, synthetic rubbers such as styrene butadiene rubber (SBR), butadiene rubber (BR), chloroprene rubber (CR), isoprene rubber (IR), acrylonitrile bradiene rubber (NBR), or carboxymethyl cellulose (CMC), xanthan gum, guar gum Polysaccharides such as pectin can be used.

The electrolyte used in all embodiments of the present specification is a non-aqueous electrolyte, which is dimethyl carbonate (hereinafter referred to as “DMC”), diethyl carbonate (hereinafter referred to as “DEC”), ethyl methyl carbonate (hereinafter referred to as “DMC”). Hereinafter referred to as "EMC"), chain carbonate such as di-n-propyl carbonate, di-t-propyl carbonate, di-n-butyl carbonate, di-isobutyl carbonate, or di-t-butyl carbonate, and propylene A mixture containing a carbonate (hereinafter referred to as “PC”) and a cyclic carbonate such as ethylene carbonate (hereinafter referred to as “EC”) is preferable. The electrolytic solution is obtained by dissolving a lithium salt such as lithium hexafluorophosphate (LiPF ₆ ), lithium borofluoride (LiBF ₄ ), or lithium perchlorate (LiClO ₄ ) in such a carbonate mixture.

  The electrolytic solution is preferably a mixed solvent containing a cyclic carbonate and a chain carbonate. In particular, PC is a solvent having a low freezing point, and is suitably used for improving the output of the battery at a low temperature. In this embodiment as well, it is very preferable that the electrolyte contains PC. However, it is known that PC has a slightly low compatibility with graphite used as a negative electrode active material. EC is a solvent having a high polarity and a high dielectric constant, and is widely used as a constituent component of an electrolytic solution for a lithium ion secondary battery. However, since EC has a high melting point (freezing point) and is a solid at room temperature, handling is somewhat difficult. DMC is a solvent having a large diffusion coefficient and low viscosity. However, since DMC has a high melting point (freezing point), the electrolytic solution may solidify at a low temperature. Similarly to DMC, EMC is a solvent having a large diffusion coefficient and low viscosity. As described above, the constituent components of the electrolytic solution have different characteristics, and in order to improve the output at low temperature of the battery, it is important to use them as an appropriate mixed solvent in consideration of these balances. .

  In addition to this, the electrolytic solution may contain a cyclic carbonate compound as an additive. Vinylene carbonate (VC) is mentioned as a cyclic carbonate used as an additive. Moreover, you may use the cyclic carbonate compound which has a halogen as an additive. These cyclic carbonates are also compounds that form a protective film for the positive electrode and the negative electrode during the charge / discharge process of the battery. In particular, it is a compound that can prevent an attack on a positive electrode active material containing a lithium / nickel composite oxide by a sulfur-containing compound such as the above-mentioned disulfonic acid compound or disulfonic acid ester compound. Examples of the cyclic carbonate compound having halogen include fluoroethylene carbonate (FEC), difluoroethylene carbonate, trifluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, and trichloroethylene carbonate. Fluoroethylene carbonate, which is a cyclic carbonate compound having a halogen and an unsaturated bond, is particularly preferably used.

  The electrolytic solution may further contain a disulfonic acid compound as an additive. The disulfonic acid compound is a compound having two sulfo groups in one molecule, and includes a disulfonate compound in which the sulfo group forms a salt with a metal ion, or a disulfonate compound in which the sulfo group forms an ester. . One or two of the sulfo groups of the disulfonic acid compound may form a salt with the metal ion or may be in an anionic state. Examples of disulfonic acid compounds include methanedisulfonic acid, 1,2-ethanedisulfonic acid, 1,3-propanedisulfonic acid, 1,4-butanedisulfonic acid, benzenedisulfonic acid, naphthalenedisulfonic acid, biphenyldisulfonic acid, and these And salts (lithium methanedisulfonate, lithium 1,3-ethanedisulfonate, etc.) and anions thereof (methanedisulfonate anion, 1,3-ethanedisulfonate anion, etc.). Examples of the disulfonic acid compound include disulfonic acid ester compounds, such as methanedisulfonic acid, 1,2-ethanedisulfonic acid, 1,3-propanedisulfonic acid, 1,4-butanedisulfonic acid, benzenedisulfonic acid, naphthalenedisulfonic acid, Alternatively, chain disulfonic acid esters such as alkyl diesters or aryl diesters of biphenyl disulfonic acid; and cyclic disulfonic acid esters such as methylenemethane disulfonic acid ester, ethylenemethane disulfonic acid ester, and propylene methane disulfonic acid ester are preferably used. Methylenemethane disulfonate (MMDS) is particularly preferably used.

  The separator used in all the embodiments is composed of an olefin resin layer. The olefin resin layer is a layer composed of polyolefin obtained by polymerizing or copolymerizing α-olefin such as ethylene, propylene, butene, pentene, hexene and the like. In the embodiment, a structure having pores that are closed when the battery temperature rises, that is, a layer composed of a porous or microporous polyolefin is preferable. When the olefin resin layer has such a structure, even if the battery temperature rises, the separator is closed (shut down), and the ion flow can be cut off. In order to exert a shutdown effect, it is very preferable to use a porous polyethylene film. The separator may optionally have a heat-resistant fine particle layer. At this time, the heat-resistant fine particle layer provided for preventing abnormal heat generation of the battery is composed of inorganic fine particles having a heat-resistant temperature of 150 ° C. or more and stable in electrochemical reaction. As such inorganic fine particles, inorganic oxides such as silica, alumina (α-alumina, β-alumina, θ-alumina), iron oxide, titanium oxide, barium titanate, zirconium oxide; boehmite, zeolite, apatite, kaolin, Mention may be made of minerals such as spinel, mica and mullite. Thus, a ceramic separator having a heat-resistant layer can also be used.

  Here, a lithium salt derived from the electrolytic solution is present in the negative electrode active material layer of the lithium ion battery of the embodiment. The abundance of the lithium salt is preferably 0.36 to 0.52 mol / L. Here, “the amount of the lithium salt present in the negative electrode active material layer” is substantially the product of the ratio of the vacancies present in the negative electrode active material and the lithium salt concentration of the electrolyte. That is, since the electrolytic solution exists in the void (void) portion in the negative electrode active material layer, only the void volume portion of the volume of the negative electrode active material layer needs to be considered. In order to increase the lithium salt present in the negative electrode active material layer, several methods are conceivable. First, increasing the pore volume of the negative electrode active material layer can be mentioned. This is because if the number of voids (voids) present in the negative electrode active material layer is large, more electrolytic solution can enter the negative electrode active material layer. On the other hand, the lithium salt present in the negative electrode active material layer can be increased by dissolving a high concentration lithium salt in the electrolytic solution. When the amount of the lithium salt present in the negative electrode active material layer is in the range of 0.36 to 0.52 mol / L, the direct current resistance value of the battery is reduced, and the battery can be charged / discharged with high input / output.

In particular, the amount of lithium salt per square root (root T) of the thickness (T) of the negative electrode active material layer is preferably 0.064 to 0.092 mol / (L · μm ^1/2 ). Here, “the amount of the lithium salt per square root (root T) of the thickness (T) of the negative electrode active material layer” substantially means the porosity present in the negative electrode active material and the lithium salt concentration of the electrolytic solution. And the square root of the thickness of the negative electrode active material layer. As described above, since the electrolyte is present in the void (void) portion in the negative electrode active material layer, the amount of lithium salt present in the void volume portion of the volume of the negative electrode active material layer may be taken into consideration. . In addition, in the negative electrode active material layer at the time of charging / discharging at a high current density, the pseudo thickness of the negative electrode active material layer that can participate in the electrode reaction can be said to be substantially a diffusible distance of lithium ions. . The concentration of the electrolyte in the battery power generation element is macroscopically uniform. However, during the charging / discharging of the battery, lithium ions move between the positive and negative electrodes, and it is considered that a concentration distribution of the electrolytic solution occurs microscopically on the electrode surface and inside the electrode. Since the amount of lithium ions required during steady state and the amount of lithium ions required during discharge at maximum power are different, when considering diffusion of electrolyte, the system is in an unsteady state (ie, the concentration in diffusion varies with time) ) Must be considered. That is, the diffusion state of the electrolyte solution inside the electrode is expressed by Fick's second law diffusion equation. Fick's second law diffusion equation is that the change in concentration with time is proportional to the second derivative of position and concentration. According to this, the thickness of the negative active material layer is the thickness of the square root negative active material layer. It can be inferred that the square root of affects the thickness at which the electrolyte can diffuse. In this embodiment, when the amount of the lithium salt per square root (root T) of the thickness (T) of the negative electrode active material layer is 0.064 to 0.092 mol / (L · μm ^1/2 ), It has been found that the direct current resistance of the battery is reduced and the battery can be charged and discharged with high input / output.

  Here, the structural example of the lithium ion secondary battery concerning embodiment is demonstrated using drawing. FIG. 1 shows an example of a cross-sectional view of a lithium ion secondary battery. The lithium ion secondary battery 10 includes a negative electrode current collector 11, a negative electrode active material layer 13, a separator 17, a positive electrode current collector 12, and a positive electrode active material layer 15 as main components. In FIG. 1, the negative electrode active material layer 13 is provided on both surfaces of the negative electrode current collector 11 and the positive electrode active material layer 15 is provided on both surfaces of the positive electrode current collector 12, but only on one side of each current collector. An active material layer can also be formed. The negative electrode current collector 11, the positive electrode current collector 12, the negative electrode active material layer 13, the positive electrode active material layer 15, and the separator 17 are constituent units of one battery, that is, a power generation element (unit cell 19 in the figure). A plurality of such unit cells 19 are stacked via the separator 17. The extending portion extending from each negative electrode current collector 11 is collectively bonded onto the negative electrode lead 25, and the extending portion extending from each positive electrode current collector 12 is collectively bonded to the positive electrode lead 27. An aluminum plate is preferably used as the positive electrode lead, and a copper plate is preferably used as the negative electrode lead, and in some cases, it may have a partial coating with another metal (for example, nickel, tin, solder) or a polymer material. The positive electrode lead and the negative electrode lead are welded to the positive electrode and the negative electrode, respectively. A battery formed by laminating a plurality of single cells in this manner is packaged by an outer package 29 so that the welded negative electrode lead 25 and positive electrode lead 27 are drawn out to the outside. An electrolytic solution 31 is injected into the exterior body 29. The exterior body 29 has a shape in which the peripheral edge portion is heat-sealed.

  The ratio (W / Wh) between the output and capacity of the lithium ion secondary battery according to the embodiment is preferably 35-80. A lithium ion battery having an output capacity ratio in such a range is conveniently used particularly as a vehicle-mounted battery or a stationary battery.

<Production of negative electrode>
As the negative electrode active material, natural graphite powder having a BET specific surface area of 3 m ² / g was used as the negative electrode active material. This graphite powder, carbon black powder having a BET specific surface area of 62 m ² / g (hereinafter referred to as “CB”) (made by TIMCAL, SC65) as a conductive auxiliary agent, and vinylidene fluoride resin (hereinafter referred to as “PVDF” as a binder resin). (Made by Kureha, # 7200) in a solid content mass ratio of graphite: conductive aid: binder = 93: 2: 5 and mixed with N-methylpyrrolidone (hereinafter referred to as “NMP”) as a solvent. Added). These materials were uniformly mixed and dispersed to prepare a slurry. The obtained slurry was applied onto a 10 μm thick copper foil serving as a negative electrode current collector so that the weight after drying was 4.1 mg / cm ² per side. Next, the electrode was heated at 125 ° C. for 10 minutes to evaporate NMP, thereby forming a negative electrode active material layer. Furthermore, the electrode was pressed so that the porosity of the negative electrode active material layer was 40%, and a negative electrode in which the negative electrode active material layer was applied on one side of the negative electrode current collector was produced.

<Preparation of positive electrode>
Lithium / nickel composite oxide (nickel / cobalt / lithium manganate (“NCM523”, ie, nickel: cobalt: manganese = 5: 2: 3)) having an average particle diameter of 8 μm and a BET specific surface area of 62 m ² as a conductive aid / G CB (manufactured by TIMCAL, SC65) and PVDF (manufactured by Kureha, # 7200) as a binder resin are mixed so that NCM523: CB: PVDF is in a ratio of 90: 5: 5 in terms of solid content mass ratio. To the NMP solvent. Further, oxalic anhydride (molecular weight 90) as an organic moisture scavenger was added to this mixture in an amount of 0.03 parts by mass with respect to 100 parts by mass of the solid content obtained by removing NMP from the above mixture, followed by planetary dispersion mixing. By carrying out for 30 minutes, these materials were disperse | distributed uniformly and the slurry was produced. The obtained slurry was applied on a 12 μm thick aluminum foil serving as a positive electrode current collector so that the weight after drying was 6 mg / cm ² per side. Next, the electrode was heated at 125 ° C. for 10 minutes to evaporate NMP, thereby forming a positive electrode active material layer. Furthermore, the electrode was pressed so that the porosity of the positive electrode active material layer was 30%, and a positive electrode in which the positive electrode active material layer was applied on one side of the positive electrode current collector was produced.

<Separator>
A 25 μm-thick ceramic separator made of polypropylene and a heat-resistant fine particle layer using alumina as the heat-resistant fine particles was used.

<Electrolyte>
Lithium hexafluorophosphate (LiPF ₆ ) as an electrolyte salt in a mixed non-aqueous solvent in which ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) are mixed at 25: 5: 70 (volume ratio) Was dissolved so as to have each concentration shown in Table 1, and then MMDS as an additive was dissolved so as to be 1% by weight. These non-aqueous mixed solvents were used as electrolyte solutions.

<Production of lithium ion secondary battery>
The positive electrode plate produced as described above was cut into a rectangle having a size of 200 mm × 140 mm, and the opposing negative electrode plate was cut into a rectangle having a size of 204 mm × 144 mm. Ten layers of the above-described negative electrode plate and positive electrode plate arranged on both surfaces of a polypropylene porous separator so that both active material layers overlap with the separator interposed therebetween were obtained to obtain an electrode laminate. And the aluminum-made positive electrode lead terminal was ultrasonically welded to the foil part of the electrode laminated body. Similarly, a nickel negative electrode lead terminal was ultrasonically welded to the negative electrode plate on the negative electrode foil portion. This electrode laminate was wrapped with two aluminum laminate films, and the three sides were bonded together by heat fusion except for one of the long sides. Each electrolytic solution shown in Table 1 was poured into the electrode laminate and separator so as to have a liquid amount of 145%, impregnated with vacuum, and then the opening was sealed by heat sealing under reduced pressure. By stopping, a laminated lithium ion battery was created. After the initial charge of this multilayer lithium ion battery, aging was performed at 45 ° C. for several days, and multilayer lithium ion batteries of Examples 1 to 7 and Comparative Examples 1 and 2 shown in Table 1 were obtained.

<First charge / discharge efficiency and battery capacity>
The first charging / discharging performed the constant current constant voltage (CC-CV) charge by the atmospheric temperature of 25 degreeC, 0.1C electric current, and the upper limit voltage 4.2V. Thereafter, aging was performed at 45 ° C. for several days. Thereafter, constant current discharge was performed at a current of 0.2 C up to 3.0 V. The initial charge / discharge efficiency is the ratio between the amount of electricity discharged and the initial charge capacity (0.2 C) when CC-CV charge is performed again to a battery voltage of 4.2 V and the battery voltage is discharged at 3.0 C to 3.0 V. (Discharge capacity / initial charge capacity). The capacity (Ah) of the battery is the discharge capacity (the product of discharge current value and time when 0.2 C is discharged from 4.2 V to 3.0 V), and the Wh capacity is calculated from the product of discharge output and time. Asked.

<Remaining capacity (SOC)>
The remaining capacity (SOC, State of charge) is a value representing the amount of charge with respect to the capacity of the battery in the operating voltage range of the battery as a percentage. In this example, the operating voltage range of the battery was 3.0 V (SOC 0%) to 4.2 V (SOC 100%) as the SOC range.

<SOC adjustment>
CC charging was performed from the state of the battery voltage of 3 V with a current of 0.2 C so as to obtain a desired charge amount (SOC) with respect to the battery capacity obtained above. The battery voltage after standing for 1 hour in this state was defined as the voltage value at the desired SOC.

<Maximum battery output>
An SOC 50% battery was discharged at 25 ° C. for 10 seconds at various rates. The results of each discharge experiment were plotted on a graph with the current value on the horizontal axis and the battery voltage after 10 seconds on the vertical axis. The plotted points were connected, and the current value corresponding to the desired lower limit voltage value (battery voltage 3 V in the present example) was set to I _Max by the extrapolation method. The maximum output W of the battery was obtained by multiplying this I _{Max by} the average voltage of the voltage at the start SOC and the voltage at the final SOC.

<Output capacity ratio (W / Wh)>
The output capacity ratio was calculated by the ratio between the maximum output and the capacity measured above (maximum output / capacity).

<DC resistance of battery>
The battery resistance is determined by preparing a battery with a remaining battery capacity (SOC) of 50%, performing a constant current discharge at 10 A at 25 ° C. for 10 seconds, and measuring the voltage at the end of the discharge to determine the direct current resistance value (DCR of the battery). ) Similarly, the DC resistance value (DCR) of the battery was determined by performing a constant current discharge at 10 A at 0 ° C. for 10 seconds and measuring the voltage at the end of the discharge. The volume of the battery was measured in accordance with JIS Z 8807 “Method for measuring density and specific gravity of solids—Method for measuring density and specific gravity by submerged weighing method”.

  Table 1 shows the results of the above evaluations on the stacked lithium ion secondary batteries of Examples 1 to 7 and Comparative Examples 1 and 2. The DC resistance value is a relative value with respect to the DC resistance value measured in Comparative Example 1.

  The battery in which the concentration of the lithium salt in the negative electrode active material layer is within the range defined by the present invention has a lower DC resistance value than the battery of the reference example that does not satisfy this range. In particular, reduction of the battery resistance value at a low temperature (0 ° C.) is a great merit when used as a vehicle battery. By reducing the resistance value of the battery, it is possible to charge and discharge the battery with high input / output.

  As mentioned above, although the Example of this invention was described, the said Example was only an example of Embodiment of this invention, and in the meaning which limits the technical scope of this invention to specific embodiment or a specific structure. Absent.

DESCRIPTION OF SYMBOLS 10 Lithium ion secondary battery 11 Negative electrode collector 12 Positive electrode collector 13 Negative electrode active material layer 15 Positive electrode active material layer 17 Separator 25 Negative electrode lead 27 Positive electrode lead 29 Exterior body 31 Electrolyte

Claims (4)

A positive electrode having a positive electrode active material layer disposed on a positive electrode current collector;
A negative electrode having a negative electrode active material layer disposed on a negative electrode current collector;
A separator;
An electrolyte,
A lithium ion secondary battery including a power generation element including
The positive electrode active material contains only lithium nickel cobalt manganese composite oxide having a layered crystal structure represented by the general formula Li _x Ni _y Co _z Mn _(1-yz) O ₂
The negative electrode active material is graphite;
The amount of lithium salt present in the negative electrode active material layer is in the range of 0.36 to 0.52 mol / L,
The electrolyte contains propylene carbonate;
The value (W / Wh) of the output and capacity of the lithium ion secondary battery is 35-80.
The lithium ion secondary battery.   The lithium ion secondary battery according to claim 1, wherein the electrolytic solution contains ethyl methyl carbonate and diethyl carbonate.   The lithium ion secondary battery according to claim 1 or 2, wherein a ratio of the propylene carbonate with respect to a weight of the electrolytic solution is 5 to 10%. The amount of the lithium salt per square root (root T) of the thickness (T) of the negative electrode active material layer is 0.064 to 0.092 mol / L · μm ^1/2. A lithium ion secondary battery according to claim 1.

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