Energy storage devices

Abstract

To suppress a decrease in energy density and capacity density while suppressing a change in volume of the entire cell.SOLUTION: A power storage device includes a negative electrode having a carbon material as a negative electrode active material, a positive electrode having a lithium composite oxide as a positive electrode active material represented by the general formula LiaPxMyO2-zAz (M is one or more of Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, and Ge, and A is one or more of F, Cl, Br, I, and S, and satisfies 0<a<2, 0<x≤0.09, 0<y<1, 0≤z<2), and having an electrode density of 1 g / cm3 or more and 2.4 g / cm3 or less, and an ion conductive medium interposed between the negative electrode and the positive electrode and conducting lithium ions.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an electricity storage device. [Background technology]

[0002] Currently, the common high-capacity lithium-ion batteries (LIBs) for automotive use use layered oxides (e.g., LiNi x Co y Mn 1-x-y LIBs using lithium ion battery (LiO2) and carbon materials such as graphite as the negative electrode active material are widely used. In LIBs with this configuration, the entire cell can expand excessively during repeated charging and discharging, which can cause deterioration such as particle cracking and loss of conductive paths. In addition, measures such as externally restraining the cell must be taken to suppress cell expansion, which can result in a decrease in capacity density and energy density. To address this problem, for example, LIBs using lithium titanate, which has small volumetric changes during charging and discharging, as the negative electrode active material (Non-Patent Document 1), and Lithium ion battery (Li-ion battery), which has small volumetric changes during charging and discharging, have been developed. 1+x Ti y V z Proposals have been made to use O2 as the positive electrode active material (Patent Document 1), to use a binder that follows the volume change of the electrode structure during charge and discharge (Patent Document 2), and to place a Li buffer layer between the negative electrode and the electrolyte and a porous layer between the negative electrode and the current collector (Patent Document 3). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2023-68548 [Patent Document 2] Japanese Patent Publication No. 2020-145193 [Patent Document 3] Japanese Patent Publication No. 2022-136024 [Non-patent literature]

[0004] [Non-Patent Document 1] Toshiba SCiB General Catalog 2301 (https: / / www.global.toshiba / content / dam / toshiba / jp / products-solutions / battery / scib / pdf / SCiB_toshiba.pdf) Summary of the Invention [Problem to be solved by the invention]

[0005] However, in Non-Patent Document 1, the oxidation-reduction potential of lithium titanate is about 1.5 V versus lithium, which is higher than that of a carbon anode, which is about 0.1 V, resulting in a corresponding decrease in energy density. Furthermore, while Patent Document 1 can suppress volumetric changes in the positive electrode, the volumetric changes can still be large when viewed as a whole cell. In Patent Document 2, the binder can follow the volumetric changes in the electrode structure that accompany charge and discharge, but it cannot suppress the volumetric changes of the whole cell. Patent Document 3 has the problem of an increased number of parts, resulting in a decrease in capacity density.

[0006] The present invention has been made to solve these problems, and its main object is to suppress the change in the volume of the entire cell while suppressing the decrease in energy density and capacity density. [Means for solving the problem]

[0007] In order to achieve the above-mentioned object, the present inventors have conducted extensive research and have found that by using a carbon anode and a cathode containing a specific lithium composite oxide as a cathode active material in an electricity storage device and adjusting the electrode density of the cathode, it is possible to suppress a change in the volume of the entire cell while suppressing a decrease in the energy density and capacity density of the cell, and have thus completed the invention disclosed in this specification.

[0008] That is, the electricity storage device disclosed in this specification is a negative electrode having a carbon material as a negative electrode active material; General formula Li a P x My O 2-z A z (However, M is one or more of Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, and Ge, A is one or more of F, Cl, Br, I, and S, and 0 < a < 2, 0 < x ≤ 0.09, 0 < y < 1, 0 ≤ z < 2 are satisfied.) Using the lithium composite oxide represented by this as the positive electrode active material, with an electrode density of 1 g / cm 3 or more and 2.4 g / cm 3 or less, a positive electrode, An ion conduction medium that is interposed between the negative electrode and the positive electrode and conducts lithium ions, is provided. <000026°> Alternatively, the power storage device disclosed in this specification has a negative electrode using a carbon material as the negative electrode active material, Using a lithium composite oxide having a cubic crystal structure as the positive electrode active material, the positive electrode active material has a unit cell volume of V of the crystal structure during charging C PEAU , and a unit cell volume of V of the crystal structure during discharging D PEAU When this is defined as, ΔV PEAU =(1 - V C PEAU / V D PEAU ) × 100, the volume change rate ΔV PEAU is 6% or more and 10% or less, and with an electrode density of 1 g / cm 3 or more and 2.4 g / cm<00°025>or less, a positive electrode, [[ID=4°]] An ion conduction medium that is interposed between the negative electrode and the positive electrode and conducts lithium ions, <°000265>is provided. [Effect of the Invention]

[0010] <00002°0>In the present disclosure, it is possible to suppress the volume change of the entire cell while suppressing the decrease in capacity density and energy density. The reason for this effect is presumed to be as follows. In the present disclosure, a carbon material with a low redox potential is used as the negative electrode active material, thereby suppressing the decrease in energy density compared to when lithium titanate or the like is used. Carbon negative electrodes undergo large volume changes during charge and discharge. However, when a positive electrode containing a predetermined lithium composite oxide and formed with a predetermined electrode density is used as the counter electrode, the volume change of the positive electrode during charge and discharge cancels out the volume change of the carbon negative electrode, thereby suppressing the volume change of the entire cell. This makes it possible to omit measures such as externally restraining the cell, and it is presumed that this also suppresses the decrease in capacity density and energy density. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 2 is a schematic diagram showing an example of the configuration of an electricity storage device 20. [Figure 2] FIG. 1 is an explanatory diagram of a cell used in a simulation. [Figure 3] FIG. 1 is an explanatory diagram showing the results of operando XRD measurement. [Figure 4] Graph showing simulation results. [Figure 5] Graph showing simulation results. [Figure 6] Graph showing experimental results. [Figure 7] 1A and 1B are explanatory diagrams showing the state of a lithium ion battery during discharging and charging. DETAILED DESCRIPTION OF THE INVENTION

[0012] The power storage device of the present disclosure includes a negative electrode having a carbon material as a negative electrode active material, a positive electrode having a predetermined lithium composite oxide as a positive electrode active material, and an ion conductive medium interposed between the negative electrode and the positive electrode and conducting lithium ions. This power storage device may be, for example, a lithium ion battery.

[0013] The negative electrode may be formed, for example, by mixing a negative electrode active material, a binder, and optionally a conductive material, adding an appropriate solvent to form a paste-like negative electrode material, applying it to the surface of a current collector, drying it, and compressing it to increase electrode density as needed. The negative electrode active material is a carbon material. The carbon material is preferably one that can absorb and release lithium, and examples of such carbon materials include cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, and carbon fibers. Among these, graphites (graphites) such as artificial graphite and natural graphite are preferred because they have an operating potential close to that of metallic lithium and can be charged and discharged at a high operating voltage. The binder contained in the negative electrode serves to bind the active material particles and conductive material particles together. Examples of binders include fluorine-containing resins such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and fluorine-containing rubber; thermoplastic resins such as polypropylene and polyethylene; ethylene propylene diene monomer (EPDM) rubber, sulfonated EPDM rubber, and natural butyl rubber (NBR), either alone or in a mixture of two or more. Water-based binders such as cellulose-based binders and aqueous dispersions of styrene butadiene rubber (SBR) can also be used. The conductive material contained in the negative electrode is not particularly limited as long as it is an electronically conductive material that does not adversely affect the battery performance of the negative electrode. For example, graphite such as natural graphite (scale graphite, flake graphite) and artificial graphite, acetylene black, carbon black, ketjen black, carbon whiskers, needle coke, carbon fiber, and metals (copper, nickel, aluminum, silver, gold, etc.) can be used alone or in a mixture of two or more. Among these, carbon black and acetylene black are preferred as the conductive material from the viewpoints of electronic conductivity and coatability. Examples of solvents that can be used to disperse the negative electrode active material, binder, conductive material, etc. include organic solvents such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethylenetriamine, N,N-dimethylaminopropylamine, ethylene oxide, and tetrahydrofuran. Alternatively, a dispersant, a thickener, etc. may be added to water, and the active material may be slurried with a latex such as SBR.As the thickener, polysaccharides such as carboxymethyl cellulose and methyl cellulose can be used alone or in a mixture of two or more types. Application methods include, for example, roller coating such as an applicator roll, screen coating, doctor blade coating, spin coating, and bar coating. Any of these can be used to obtain any desired thickness and shape. Materials such as copper, nickel, stainless steel, titanium, aluminum, calcined carbon, conductive polymers, conductive glass, and Al-Cd alloys can be used as the negative electrode current collector. Materials such as copper whose surfaces have been treated with carbon, nickel, titanium, or silver for the purpose of improving adhesion, conductivity, and reduction resistance can also be used. These surfaces can also be subjected to oxidation treatment. Current collector shapes include foil, film, sheet, net, punched or expanded, lath, porous, foamed, and fibrous structures. The thickness of the current collector is, for example, 1 to 500 μm. The electrode density of the negative electrode is 1 g / cm. 3 More than 2g / cm 3 It may be less than or equal to 1.3 g / cm 3 More than 1.7g / cm 3 It may be less than or equal to 1.4 g / cm 3 More than 1.6g / cm 3 The electrode density is the electrode density excluding the current collector. The active material ratio in the negative electrode (excluding the current collector) may be 90% by mass or more and 99.5% by mass or less, or 95% by mass or more and 99% by mass or less.

[0014] The positive electrode may be formed by mixing a positive electrode active material, a conductive material, and a binder, adding an appropriate solvent to form a paste-like positive electrode mixture, applying it to the surface of a current collector, drying it, and compressing it to increase electrode density as needed. The positive electrode active material is a lithium composite oxide. The lithium composite oxide may contain, for example, one or more of Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, and Ge. The lithium composite oxide may also contain P. The lithium composite oxide may also contain one or more of F, Cl, Br, I, and S. The lithium composite oxide may be, for example, the lithium composite oxide disclosed in JP 2023-77491 A. Such lithium composite oxides are preferred because of their high capacity density.

[0015] The positive electrode active material is of the general formula Li a P x M y O 2-z A zIt may also be a lithium composite oxide represented by the following formula. However, M is one or more of Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, and Ge; A is one or more of F, Cl, Br, I, and S; and 0 < a < 2, 0 < x ≤ 0.09, 0 < y < 1, and 0 ≤ z < 2 are satisfied. In this lithium composite oxide, the content x of P preferably satisfies 0.02 ≤ x ≤ 0.08, more preferably 0.03 ≤ x ≤ 0.08. Further, the content y of the metal element M preferably satisfies 0.65 ≤ y < 1, more preferably 0.7 ≤ y ≤ 0.95. Also, the content z of the element A preferably satisfies 0 ≤ z ≤ 1, more preferably 0 ≤ z ≤ 0.5 or less. The content a of Li may satisfy 1 ≤ a ≤ 1.25 in the state where charge and discharge have not been performed. This lithium composite oxide may contain at least one of the above-mentioned transition metal elements (Ti, V, Cr, Mn, Fe, Co, Ni, Cu) as the metal element M, and preferably contains at least Mn. This lithium composite oxide may contain the element A (that is, the content z of the element A satisfies 0 < z). In the case of containing the element A (especially F), when charge and discharge are repeated, there is an advantage that it is easy to estimate the state of charge (SOC) from the potential because the changes in the discharge curve and dQ / dV curve are small. When F is included as the element A, the content z of F preferably satisfies 0 < z < 0.5, and may satisfy 0.3 ≤ z < 0.5. In this lithium composite oxide, it is preferable that the metal element M and P are uniformly distributed in the element distribution measured by STEM / EDX. Further, when this lithium composite oxide contains the element A, it is preferable that the metal element M, P, and element A are uniformly distributed in the element distribution measured by STEM / EDX. When the metal element M, P, and element A are uniformly distributed, it is preferable because the carrier ions (for example, lithium ions) of the power storage device are occluded and released into the lithium composite oxide without bias.

[0016] The positive electrode active material may be a lithium composite oxide having a cubic crystal structure. Of these, the positive electrode active material is preferably a lithium composite oxide having a disordered rock salt structure (also referred to as a random rock salt structure). The disordered rock salt structure exhibits a peak pattern in an XRD spectrum with four diffraction peaks in the 2θ ranges of 35° to 39°, 42° to 48°, 62° to 68°, and 80° to 84°. Furthermore, due to the disordered structure, the diffraction peaks are relatively broad, with the main peak at 2θ = 42° to 48° having a half-width of 1.5° or more and the peak at 2θ = 62° to 68° having a half-width of 2° or more.

[0017] The volume of the unit cell of the crystalline structure of the positive electrode active material during charging is V C PEAU , the volume of the unit cell of the crystal structure during discharge is V D PEAU When this is done, ΔV PEAU =(1-V C PEAU / V D PEAU ) × 100 PEAU It is preferable that the volume change rate ΔV is 6% or more and 10% or less. PEAU is preferably 7% or more, more preferably 8% or more. PEAU may be set to 9% or less.

[0018] The conductive material, binder, solvent, etc. used in the positive electrode can be the same as those exemplified for the positive electrode. Examples of current collectors include aluminum, titanium, stainless steel, nickel, iron, calcined carbon, conductive polymers, conductive glass, and the like. For the purpose of improving adhesion, conductivity, and oxidation resistance, materials such as aluminum and copper whose surfaces have been treated with carbon, nickel, titanium, or silver can also be used. These surfaces can also be subjected to oxidation treatment. Examples of the shape of the current collector include foil, film, sheet, net, punched or expanded, lath, porous, foamed, and fibrous formations. The thickness of the current collector is, for example, 1 to 500 μm. The shape of the current collector can be the same as that of the negative electrode.

[0019] The electrode density of the positive electrode is 1 g / cm 3 More than 2.4g / cm 3 The electrode density of the positive electrode is preferably high from the viewpoint of increasing the capacity density and suppressing the volume shrinkage of the cell due to charge and discharge. For example, it is preferably 1.1 g / cm 3 More than 1.2 g / cm is preferable. 3 The electrode density of the positive electrode is preferably low, for example, 2.0 g / cm, from the viewpoint of suppressing volume expansion of the cell during charge and discharge. 3 Preferably less than 1.8 g / cm 3 The following is more preferable. The electrode density is the electrode density excluding the current collector. The active material ratio in the positive electrode (excluding the current collector) may be 80% by mass or more and 97% by mass or less, or 85% by mass or more and 95% by mass or less. The capacity ratio of the negative electrode to the positive electrode may be more than 1.0 and 1.2 or less, or 1.05 or more and 1.15 or less.

[0020] The ion-conducting medium may be a non-aqueous electrolyte solution containing a supporting salt containing lithium and a non-aqueous solvent. Examples of the solvent for the non-aqueous electrolyte solution include carbonates, esters, ethers, nitriles, furans, sulfolanes, and dioxolanes, and these can be used alone or in combination. Specific examples of carbonates include cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, butylene carbonate, and chloroethylene carbonate; chain carbonates such as dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl-n-butyl carbonate, methyl-t-butyl carbonate, di-i-propyl carbonate, and t-butyl-i-propyl carbonate; cyclic esters such as γ-butyl lactone and γ-valerolactone; chain esters such as methyl formate, methyl acetate, ethyl acetate, and methyl butyrate; ethers such as dimethoxyethane, ethoxymethoxyethane, and diethoxyethane; nitriles such as acetonitrile and benzonitrile; furans such as tetrahydrofuran and methyltetrahydrofuran; sulfolanes such as sulfolane and tetramethylsulfolane; and dioxolanes such as 1,3-dioxolane and methyldioxolane. Of these, a combination of a cyclic carbonate and a chain carbonate is preferred.

[0021] Examples of supporting salts include LiPF, LiBF, LiAsF, LiCF, SO, LiN(CF, SO), LiC(CF, SO), LiSbF, LiSiF, LiAlF, LiSCN, LiClO, LiCl, LiF, LiBr, LiI, and LiAlCl. From the viewpoint of electrical properties, it is preferable to use a combination of one or more salts selected from the group consisting of inorganic salts such as LiPF, LiBF, LiAsF, and LiClO, and organic salts such as LiCF, SO, LiN(CF, SO), and LiC(CF, SO). The concentration of this supporting salt in the nonaqueous electrolyte is preferably 0.1 mol / L to 5 mol / L, and more preferably 0.5 mol / L to 2 mol / L. When the concentration of the dissolving supporting electrolyte is 0.1 mol / L or more, a sufficient current density can be obtained, and when it is 5 mol / L or less, the electrolyte can be made more stable.

[0022] The power storage device of the present disclosure may include a separator between the negative electrode and the positive electrode. The separator is not particularly limited as long as it has a composition that can withstand the range of use of the power storage device, but examples thereof include polymer nonwoven fabrics such as polypropylene nonwoven fabrics and polyphenylene sulfide nonwoven fabrics, and thin microporous films of olefin resins such as polyethylene and polypropylene. These may be used alone or in combination.

[0023] In the electricity storage device of the present disclosure, the rate of change in the total volume of the negative electrode and positive electrode before and after charging and discharging is preferably within ±2%, more preferably within ±1.5%, and even more preferably within ±1%.

[0024] The power storage device of the present disclosure preferably has a discharge capacity per cell volume of 200 Ah / L or more, more preferably 220 Ah / L or more. The power storage device of the present disclosure may have a discharge capacity per cell volume of 300 Ah / L or less.

[0025] The energy storage device of the present disclosure is not particularly limited, and examples thereof include coin type, button type, sheet type, laminated type, cylindrical type, flat type, rectangular type, etc. Further, it may be applied to large-sized ones in which a plurality of such energy storage devices are connected in series and used for electric vehicles or the like. FIG. 1 is a schematic diagram showing an example of the energy storage device 20 of the present embodiment. This energy storage device 20 includes a sheet-like negative electrode 26 in which a negative electrode composite material 25 is formed on the surface of a current collector 24, a sheet-like positive electrode 23 in which a positive electrode composite material 22 is formed on a current collector 21, a separator 28 provided between the negative electrode 26 and the positive electrode 23, and a non-aqueous electrolyte 29 filling the space between the negative electrode 26 and the positive electrode 23. In this energy storage device 20, a separator 28 is sandwiched between the negative electrode 26 and the positive electrode 23, and these are wound and inserted into a cylindrical case 32, and a negative electrode terminal 36 connected to the negative electrode 26 and a positive electrode terminal 34 connected to the positive electrode 23 are disposed. The negative electrode composite material 25 contains a carbon material such as graphite as a negative electrode active material. The positive electrode composite material 22 contains, for example, a lithium composite oxide represented by the general formula Li a P x M y O 2-z A z (However, M is one or more of Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, and Ge, A is one or more of F, Cl, Br, I, and S, and 0 < a < 2, 0 < x ≤ <0.09, 0 < y < 1, 0 ≤ z < 2 are satisfied.) as a positive electrode active material. The electrode density of the positive electrode (density of the positive electrode composite material 22) is 1 g / cm 3 or more and 2.4 g / cm 3 or less.

[0026] In the power storage device of the present embodiment described in detail above, it is possible to suppress a decrease in the volume change of the entire cell while suppressing a decrease in the capacity density and the energy density. The reason for obtaining such an effect is presumed as follows. In the power storage device of the present embodiment, since a carbon material having a low redox potential is used as the negative electrode active material, it is possible to suppress a decrease in the energy density more than when using lithium titanate or the like. Although the carbon negative electrode has a large volume change during charge and discharge, when a positive electrode containing a predetermined lithium composite oxide and formed at a predetermined density is used as the counter electrode, the volume change of the carbon negative electrode is canceled out by the volume change of the positive electrode during charge and discharge. Therefore, it is possible to suppress the volume change of the entire cell. As a result, it is presumed that it is possible to omit measures such as restraining the cell from the outside, and thus it is also possible to suppress a decrease in the capacity density and the energy density.

[0027] Note that the present disclosure is not limited to the above-described embodiments at all, and it is needless to say that the present disclosure can be implemented in various modes as long as it belongs to the technical scope of the present disclosure.

[0028] For example, the present disclosure may be as shown in any of the following [1] to [7]. [1] A negative electrode using a carbon material as a negative electrode active material, a lithium composite oxide represented by the general formula Li a P x M y O 2-z A z (However, M is one or more of Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, and Ge, A is one or more of F, Cl, Br, I, and S, and 0 < a < 2, 0 < x ≤ 0.09, 0 < y < 1, 0 ≤ z < ( satisfying 2.)) as a positive electrode active material, a positive electrode having an electrode density of 1 g / cm 3 or more and 2.4 g / cm 3 or less, and an ion conduction medium interposed between the negative electrode and the positive electrode and conducting lithium ions. [2] The positive electrode active material has a cubic crystal structure, and the volume of the unit cell of the crystal structure during charging is V C PEAU , and the volume of the unit cell of the crystal structure during discharging is V D PEAUWhen this is done, ΔV PEAU =(1-V C PEAU / V D PEAU ) × 100 PEAU The electricity storage device according to [1], wherein the ratio of the total capacitance to the total capacitance is 6% or more and 10% or less. [3] A negative electrode using a carbon material as the negative electrode active material and a lithium composite oxide having a cubic crystal structure as the positive electrode active material, wherein the volume of the unit cell of the crystal structure during charging is V C PEAU , the volume of the unit cell of the crystal structure during discharge is V D PEAU When this is done, ΔV PEAU =(1-V C PEAU / V D PEAU ) × 100 PEAU is 6% or more and 10% or less, and the electrode density is 1g / cm 3 More than 2.4g / cm 3 and an ion-conductive medium interposed between the negative electrode and the positive electrode and conducting lithium ions. [4] The electricity storage device according to any one of [1] to [3], wherein the negative electrode active material is graphite. [5] The density of the positive electrode is 1.2 g / cm 3 More than 1.8g / cm 3 The electricity storage device according to any one of [1] to [4] below. [6] The electricity storage device according to any one of [1] to [5], wherein the rate of change in the total volume of the negative electrode and the positive electrode before and after charge and discharge is within ±2%. [7] The electricity storage device according to any one of [1] to [6], which has a discharge capacity per cell volume of 200 Ah / L or more. [Example]

[0029] Below, examples in which a lithium ion battery, which is an example of an electricity storage device of the present disclosure, is specifically examined will be described as examples. Of Experimental Example 1, Experimental Examples 1-1 to 1-8 and 1-15 correspond to working examples, and Experimental Examples 1-9 to 1-14 correspond to comparative examples. Furthermore, Experimental Examples 2 to 4 all correspond to comparative examples.

[0030] [Experimental Example 1 (Experimental Examples 1-1 to 1-14)] 1. Simulation of cell volume change rate As shown in Figure 2, we investigated a cell consisting of a positive electrode and a negative electrode facing each other in the vertical direction. The volume of the positive electrode active material is V PEA [cm 3 ] and the volume of the negative electrode active material is V NEA [cm 3 ]. The volume of the positive electrode composite is V PE [cm 3 ] and the volume of the negative electrode composite is V NE [cm 3 ]. Also, a certain unit area S [cm 2 Consider the cell cut out by ] and set the positive electrode thickness to l PE [μm] and the negative electrode thickness is l NE The cell volume V of the part whose volume changes during charging and discharging is all [cm 3 ] is defined by the following formula (1), and V before and after charge and discharge is all The rate of change of ΔV all I researched the following.

[0031]

number

[0032] The positive and negative electrode active materials are + The volume of the crystal lattice changes before and after the insertion and removal of the ions. The volume of the unit cell of the crystal structure of the positive electrode active material is V PEAU [Å 3 ] and the volume of the unit cell of the crystal structure of the negative electrode active material is V NEAU [Å 3 ] and V PEAU and V NEAUwas calculated from the peak position of X-ray diffraction (XRD). PEAU V C PEAU , V during discharge PEAU V D PEAU Then, V before and after charging and discharging PEAU The rate of change of ΔV PEAU is expressed by the following formula (2).

[0033]

number

[0034] The positive electrode active material used in the simulation was Li 0.90 Mn 0.84 P 0.04 O2(LMPO). From the pre-operando XRD experiment described later, ΔV of LMPO PEAU The capacity per LMPO, Q, was calculated to be 8.73%. PEA The capacity was 346mAh / g. 0.90 Mn 0.84 P 0.04 O2(LMPO) was confirmed by XRD to have a cubic crystal structure (specifically, a disordered rock salt structure).

[0035] The negative electrode active material used in the simulation was standard natural graphite. The true density of natural graphite is 2.2 g / cm3 during discharge and charge. 3 to 2 g / cm 3 That is, V before and after charging and discharging NEAU The rate of change of ΔV NEAU The capacity per unit of natural graphite, Q NEA is set to 350 mAh / g based on S. Schweidler, L. de Biasi, A. Schiele, P. Hartmann, T. Brzesinski, J. Janek, The Journal of Physical Chemistry C 2018, 122, 8829-8835.

[0036] In the cell used in the simulation, the active material ratio of the positive electrode w PEA The electrode density of the positive electrode, c PEA is 1.0g / cm 3 ~3.6g / cm 3 The ratio of the active material in the negative electrode, w NEA is 98 mass %, and the mass of the active material of the negative electrode ρ NEA is 7.5 mg / cm 2 , negative electrode density c NEA is 1.5g / cm 3 The capacity ratio of the negative electrode to the positive electrode was 1.1. The active material weight of the positive electrode was ρ PEA and placed it there.

[0037] Based on the above assumptions, the active material weight ρ of the positive electrode is calculated from the following formula (3): PEA [mg / cm 2 ] and calculate the positive electrode thickness l from the following equation (4) PE [μm] is calculated, and from the result, the positive electrode volume V PE asked for.

[0038]

number

[0039] In addition, the negative electrode thickness l is calculated from the following equation (6): NE [μm] is calculated, and from the result, the negative electrode volume V NE asked for.

[0040]

number

[0041] Applying the above results to the above formula (1), the cell volume V of the part whose volume changes during charging and discharging is all was calculated.

[0042] Furthermore, when the volume of the active material changes during charging, it is most likely that the positive and negative electrodes change in the same manner as the change rate. Based on the above calculations, the change rate of the cell volume before and after charging and discharging, ΔV all ΔV all is ΔV all =(V all (Before charging) -V all (After charging))×100 / V all (Before charging) was calculated from the formula. In addition, the capacity per cell volume Q [Ah / L] was calculated from the following formula (8).

[0043]

number

[0044] 2. Preparation of the Positive Electrode As the positive electrode active material, Li 0.90 Mn 0.84 P 0.04 LiMnO2 (LMPO) was synthesized. The raw material, orthorhombic LiMnO2, was placed in a zirconia pot together with zirconia balls and milled in a planetary ball mill at 560 rpm for 28 hours to obtain LiMnO2 powder with a disordered rock salt phase. The raw materials, LiMnO2 obtained by the above method and Li3PO4, were mixed in an agate bowl so that the weight fraction of Li3PO4 was 5%, and then the mixture was placed in a zirconia pot together with zirconia balls and milled in a planetary ball mill at 600 rpm for 36 hours to obtain LiMnO2 powder with a disordered rock salt phase. 0.90 P 0.04 Mn 0.84 O2 positive electrode active material was synthesized. 85% by mass of the synthesized LMPO positive electrode active material, 10% by mass of carbon black (CB, TB5500, manufactured by Tokai Carbon), and 5% by mass of a binder (polyvinylpyrrolidone, manufactured by Kishida) were mixed, and the mixture was coated on an Al foil and dried to prepare a positive electrode.

[0045] 3. Operando XRD of the Positive Electrode Operando XRD was performed on the positive electrode fabricated as described above to investigate the change in the crystal lattice constant during charge and discharge. A half-cell was fabricated by combining the LMPO positive electrode with a metallic Li counter electrode, a separator (E20MMS), and an electrolyte. XRD patterns were obtained at BL33XU of SPring-8 while conducting constant current and constant voltage charge and discharge measurements. The electrolyte was a mixed solvent containing 30 vol% ethylene carbonate (EC), 40 vol% dimethyl carbonate (DMC), and 30 vol% ethyl methyl carbonate (EMC), to which LiPF6 was added to a concentration of 1M. Constant current and low voltage charge and discharge measurements were performed at 4.8 to 1.5 V vs. Li + The test was carried out by adjusting the current value to 20 mA / g per mass of the positive electrode active material in the potential range of 0.1 V / Li.

[0046] Figure 3 shows the results of operando XRD measurements. Figure 3 also shows the change in lattice constant calculated from the operando XRD pattern. From this, it was found that LMPO has a current of 4.075 A during charging and 4.17 A during discharging, and the ΔV of LMPO PEAU was calculated to be 8.73%.

[0047] 4. Preparation of the Negative Electrode A negative electrode was prepared by mixing 98% by mass of graphite, 1% by mass of carboxymethyl cellulose, and 1% by mass of PVdF, applying the mixture to a Cu foil, and drying the mixture. NEA is 7.5 mg / cm 2 and the negative electrode density c NEA is 1.5g / cm 3 It was decided.

[0048] 5.Measuring the thickness of lithium-ion batteries A full-cell lithium-ion battery was fabricated using the LMPO positive electrode and graphite negative electrode prepared as described above. The full cell was constructed by placing the positive and negative electrodes facing each other in a 60 × 100 mm Al laminate, with a polyethylene separator between them. The electrolyte was the same as that used in the half-cell. The charge / discharge cutoff voltage was 4.7 to 1.4 V, and the current value was 20 mA / g of positive electrode active material mass. The cell thickness was measured at four points during the initial charge and discharge using a thickness gauge. The cell volume was calculated by subtracting the thickness of the current collector, Al laminate, and separator from the measured thickness. The results were calculated using the average of the four measurements.

[0049] [Experimental Example 2] The positive electrode active material is LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 The same procedure as in Experimental Example 1 was carried out except that O2 (NCM111) was used. Based on the data in A.O. Kondrakov, et. al., The Journal of Physical Chemistry C 2017, 121, 3286-3294, the ΔV PEAU is 1.16%, and Q PEA was set to 149mAh / g.

[0050] [Experimental Example 3] The positive electrode active material is LiNi 0.8 Co 0.1 Mn 0.1 The same procedure as in Experimental Example 1 was carried out except that O2 (NCM811) was used. Based on the data in A.O. Kondrakov, et. al., The Journal of Physical Chemistry C 2017, 121, 3286-3294, the ΔV PEAU is 1.45%, and Q PEA was set to 189mAh / g.

[0051] [Experimental Example 4] The same procedure as in Experimental Example 1 was carried out except that the positive electrode active material was LiMn2O4. Based on the data in T. Ohzuku, et al., J. Electrochem. Soc. 1990, 137, 769-775, ΔV PEAU is 7.45%, and Q PEA was set to 110mAh / g.

[0052] [Results and Discussion] Figure 4 shows the positive electrode density c PE V before and after charging and discharging when all The results of a simulation of the change rate (%) of the positive electrode active material and the capacity per cell volume (Ah / L) are shown in Figure 4A. 0.90 Mn 0.84 P 0.04 When O2(LMPO) was used (Experimental Example 1), Figure 4B shows the LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 When O2(NCM111) was used (Experimental Example 2), Figure 4C shows the results of LiNi 0.8 Co 0.1 Mn 0.1 FIG. 4D shows the results when O2 (NCM811) was used (Experimental Example 3), and FIG. 4E shows the results when LiMn2O4 was used (Experimental Example 4).

[0053] These results indicate that, regardless of the positive electrode density, when NCM111, NCM811, or LiMn2O4 is used as the positive electrode active material and graphite is used for the negative electrode, there are no conditions under which the cell volume change rate before and after charge and discharge falls within ±1%. On the other hand, when LMPO is used as the positive electrode active material, the positive electrode density is 1.4 to 1.6 g / cm 3 Nearby is V all It was found that the rate of change was within ±1%.

[0054] 4A to 4D, V before and after charging and discharging all The rate of change of ΔV all The relationship between the discharge capacity per cell volume and the capacity per cell volume is shown in Figure 5. Table 1 summarizes the simulation results for the cases where each positive electrode active material was used under conditions 1 to 7 shown in the figure.

[0055] [Table 1]

[0056] Condition 1 (LMPO positive electrode with density 1.4 g / cm 3 In V all The change in volume was -0.1%, indicating that almost no volume change occurred. The capacity Q per cell volume was 226 Ah / L. Condition 2 (LMPO positive electrode with a density of 3.0 g / cm 3 In the case of the battery used in the above example, the capacity per cell volume Q is expected to be the highest at 312 Ah / L, but V all The rate of change of ΔV all The battery life is 3.1%, which means that deterioration is expected over the long term, and external restraints are necessary. Conditions 3, 4, 5, and 6 are combinations of positive and negative electrodes that are currently commonly used in automotive lithium-ion batteries. all The rate of change of ΔV all In addition, in condition 7 (LiMn2O4 positive electrode with a density of 3.0 g / cm3), the 3 Even in the case of all The rate of change of ΔV all The change in volume was -1.5%, outside the ±1% range, indicating a large volume change. Furthermore, the capacity per cell volume was 190 Ah / L, lower than when NCM-based or LMPO-based cathodes were used. Table 2 summarizes the change in cell volume and capacity per volume relative to the cathode density when using an LMPO cathode. It was found that a cathode density of 1.2 to 1.8% is preferable to keep the cell volume change rate within the range of -1 to 1%.

[0057] [Table 2]

[0058] Full cells were fabricated using LMPO and NCM811 positive electrodes and graphite negative electrodes, and the changes in cell volume during charge and discharge were estimated using the thickness measurements described above. The results are shown in Figure 6 and Table 3. The cell using the LMPO positive electrode expanded by 1.7% during charge, but showed almost no change in cell volume during discharge. On the other hand, the cell using the NCM811 positive electrode expanded by 16% during charge and contracted by 16% during discharge. While this value was larger than the cell expansion / shrinkage predicted by the simulation, it is believed that this was largely due to measurement error. The cell expansion / shrinkage generally matched the simulation. From the above, it was found that in conventional high-capacity lithium-ion batteries, as shown in Figure 7A, the expansion / shrinkage of the carbon negative electrode cannot be fully absorbed during charge / discharge, resulting in large expansion / shrinkage of the entire cell. In contrast, in the lithium-ion battery disclosed herein, the expansion / shrinkage of the carbon negative electrode can be absorbed by the expansion / shrinkage of the positive electrode during charge / discharge, as shown in Figure 7B, thereby suppressing overall cell expansion / shrinkage.

[0059] [Table 3] [Industrial Applicability]

[0060] The present disclosure is applicable to technical fields such as the battery industry. [Explanation of symbols]

[0061] 20 Energy storage device, 21 Current collector, 22 Positive electrode composite, 23 Positive electrode, 24 Current collector, 25 Negative electrode composite, 26 Negative electrode, 28 Separator, 29 Non-aqueous electrolyte, 32 Cylindrical case, 34 Positive electrode terminal, 36 Negative electrode terminal.

Claims

1. a negative electrode having a carbon material as a negative electrode active material; General formula Li a P x M y O 2-z A z (wherein M is one or more of Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, and Ge, and A is one or more of F, Cl, Br, I, and S, and 0<a<2, 0<x≦0.09, 0<y<1, 0≦z<2 are satisfied), and the positive electrode active material is a lithium composite oxide represented by the following formula: 3 2.4g / cm or more 3 a positive electrode, an ion conductive medium interposed between the negative electrode and the positive electrode and conducting lithium ions; An electricity storage device comprising:

2. The positive electrode active material has a cubic crystal structure, and the volume of a unit cell of the crystal structure during charging is V C PEAU , the volume of the unit cell of the crystal structure during discharge is V D PEAU When this is done, ΔV PEAU = (1 - V C PEAU / V D PEAU ) × 100 PEAU is 6% or more and 10% or less, The electricity storage device according to claim 1 .

3. a negative electrode having a carbon material as a negative electrode active material; A lithium composite oxide having a cubic crystal structure is used as a positive electrode active material, and the volume of a unit cell of the crystal structure during charging is V C PEAU , the volume of the unit cell of the crystal structure during discharge is V D PEAU When this is done, ΔV PEAU = (1 - V C PEAU / V D PEAU ) × 100 PEAU is 6% or more and 10% or less, and the electrode density is 1 g / cm 3 2.4g / cm or more 3 a positive electrode, an ion conductive medium interposed between the negative electrode and the positive electrode and conducting lithium ions; An electricity storage device comprising:

4. The negative electrode active material is graphite. The electricity storage device according to any one of claims 1 to 3.

5. The electrode density of the positive electrode is 1.2 g / cm 3 1.8g / cm or more 3 Below is the The electricity storage device according to any one of claims 1 to 3.

6. The rate of change in the total volume of the negative electrode and the positive electrode before and after charge and discharge is within ±2%. The electricity storage device according to any one of claims 1 to 3.

7. The discharge capacity per cell volume is 200 Ah / L or more. The electricity storage device according to any one of claims 1 to 3.

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