JP7782534B2 - Energy storage devices - Google Patents

Description

The present invention relates to an electricity storage device.

Currently, typical high-capacity lithium-ion batteries (LIBs) for automotive applications use layered oxides (e.g., LiNi _x Co _y Mn _1-xy O ₂ ) as the positive electrode active material and carbon materials such as graphite as the negative electrode active material. LIBs with this configuration can experience excessive expansion of the entire cell during repeated charge and discharge cycles, resulting in degradation such as particle cracking and loss of conductive paths. Furthermore, measures such as externally restraining the cell are required to suppress cell expansion, which can result in reduced capacity density and energy density. To address these problems, for example, a battery using lithium titanate as the negative electrode active material, which _exhibits little volume change during charge and discharge (Non-Patent Document 1), a battery using Li1 _{+ xTiyVzO2} as the positive electrode active material, which exhibits little volume change during charge and discharge (Patent Document 1), _a battery using a binder that follows the volume change of the electrode structure that accompanies charge and discharge (Patent Document 2), and a battery in which a Li buffer layer is disposed between the negative electrode and the electrolyte and a porous layer is disposed between the negative electrode and the current collector (Patent Document 3) have been proposed.

JP 2023-68548 A Japanese Patent Application Laid-Open No. 2020-145193 Japanese Patent Application Laid-Open No. 2022-136024

Toshiba SCiB General Catalog 2301 (https://www.global.toshiba/content/dam/toshiba/jp/products-solutions/battery/scib/pdf/SCiB_toshiba.pdf)

However, in Non-Patent Document 1, the oxidation-reduction potential of lithium titanate is approximately 1.5 V versus lithium, which is higher than the approximately 0.1 V of a carbon anode, resulting in a corresponding decrease in energy density. Furthermore, while Patent Document 1 is able to 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 is able to follow the volumetric changes in the electrode structure that occur during charging and discharging, but is unable to suppress volumetric changes in the entire cell. Patent Document 3 has the problem of an increased number of parts, resulting in a decrease in capacity density.

The present invention was made to solve these problems, and its main purpose is to suppress changes in the volume of the entire cell while preventing a decrease in energy density and capacity density.

To achieve the above-mentioned objectives, the inventors conducted extensive research. They discovered that by using a carbon anode and a cathode with a specific lithium composite oxide as the cathode active material in an electricity storage device and adjusting the electrode density of the cathode, they were able to suppress changes in the overall volume of the cell while also suppressing decreases in the cell's energy density and capacity density, leading to the completion of the invention disclosed in this specification.

That is, the electricity storage device disclosed in this specification is
a negative electrode having a carbon material as a negative electrode active material;
a positive electrode having a positive electrode active material made of a lithium composite oxide represented by the 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), and an electrode density of 1 g/cm ³ or more and 2.4 g/cm ³ or less;
an ion conductive medium interposed between the negative electrode and the positive electrode and conducting lithium ions;
It is equipped with the following.

Alternatively, the electricity storage device disclosed in the present specification is
a negative electrode having a carbon material as a negative electrode active material;
a positive electrode having a volume change rate ΔV PEAU of 6% or more and 10% or less, expressed as ΔV _PEAU = (1 - V ^C _PEAU / V ^D _PEAU ) × 100, where V ^C _PEAU is the volume of a unit cell of the crystal structure during charging and V ^D _PEAU is the volume of a unit cell of the crystal _structure during discharging; and an electrode density of 1 g/cm ³ or more and 2.4 g/cm ³ or less;
an ion conductive medium interposed between the negative electrode and the positive electrode and conducting lithium ions;
It is equipped with the following.

The present disclosure makes it possible to suppress volume changes throughout the cell while suppressing decreases in capacity density and energy density. The reason for this effect is believed 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 decreases in energy density more effectively than 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 specified lithium composite oxide and formed with a specified 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 volume changes throughout the cell. This makes it possible to omit measures such as externally restraining the cell, which is believed to also suppress decreases in capacity density and energy density.

FIG. 2 is a schematic diagram showing an example of the configuration of an electricity storage device 20. FIG. 1 is an explanatory diagram of a cell used in a simulation. FIG. 10 is an explanatory diagram showing the results of operando XRD measurement. Graph showing simulation results. Graph showing simulation results. Graph showing experimental results. 1A and 1B are explanatory diagrams showing the state of a lithium ion battery during discharging and charging.

The electricity storage device disclosed herein comprises a negative electrode using a carbon material as the negative electrode active material, a positive electrode using a specified lithium composite oxide as the positive electrode active material, and an ion-conductive medium interposed between the negative electrode and positive electrode and conducting lithium ions. This electricity storage device may be, for example, a lithium-ion battery.

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 of the negative electrode material, applying it to the surface of a current collector, drying it, and compressing it to increase the 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, such as 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 the conductive material particles together. For example, fluorine-containing resins such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and fluorine-containing rubber, or thermoplastic resins such as polypropylene and polyethylene, ethylene propylene diene monomer (EPDM) rubber, sulfonated EPDM rubber, and natural butyl rubber (NBR) can be used alone or as a mixture of two or more. Also, aqueous binders such as cellulose-based binders and aqueous dispersions of styrene butadiene rubber (SBR) can be used. The conductive material contained in the negative electrode is not particularly limited as long as it is an electron-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 as a mixture of two or more. Among these, carbon black and acetylene black are preferred as conductive materials 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. Examples of thickeners that can be used include polysaccharides such as carboxymethyl cellulose and methyl cellulose, either alone or as a mixture of two or more. Examples of application methods include roller coating such as an applicator roll, screen coating, a doctor blade method, spin coating, and a bar coater. Any of these methods can be used to create a desired thickness and shape. The negative electrode current collector can be made of copper, nickel, stainless steel, titanium, aluminum, calcined carbon, conductive polymers, conductive glass, Al—Cd alloys, etc., or, for example, copper whose surface has been treated with carbon, nickel, titanium, silver, etc. for the purpose of improving adhesion, conductivity, and reduction resistance. These surfaces can also be oxidized. The current collector may be in the form of a foil, film, sheet, net, punched or expanded, lath, porous, foam, or fiber group. The thickness of the current collector is, for example, 1 to 500 μm. The electrode density of the negative electrode may be 1 g/cm ³ or more and 2 g/cm ³ or less, 1.3 g/cm ³ or more and 1.7 g/cm ³ or less, or 1.4 g/cm ³ or more and 1.6 g/cm ³ or less. The electrode density is the electrode density of the portion excluding the current collector. The proportion of the active material 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.

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 as needed to increase electrode density. 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 due to their high capacity density.

The positive electrode active material may be a lithium composite oxide represented by the general formula Li _a P _x M _y O _2-z A _z . Here, 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 the following conditions are satisfied: 0<a<2, 0<x≦0.09, 0<y<1, and 0≦z<2. In this lithium composite oxide, the P content x preferably satisfies 0.02≦x≦0.08, and more preferably 0.03≦x≦0.08. The content y of the metal element M preferably satisfies 0.65≦y<1, and more preferably 0.7≦y≦0.95. The content z of the element A preferably satisfies 0≦z≦1, and more preferably 0≦z≦0.5. The Li content a may satisfy 1≦a≦1.25 in a state where no charge or discharge is performed. This lithium composite oxide may contain at least one of the transition metal elements (Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) described above as the metal element M, and preferably contains at least Mn. This lithium composite oxide may also contain element A (i.e., the content z of element A satisfies 0<z). Those containing element A (particularly F) have the advantage that the change in the discharge curve and dQ/dV curve is small when charge and discharge are repeated, making it easy to estimate the state of charge (SOC) from the potential. When F is included as element A, the content z of F preferably satisfies 0<z<0.5, and may also satisfy 0.3≦z<0.5. In this lithium composite oxide, it is preferable that the metal elements M and P are uniformly distributed in the element distribution measured by STEM/EDX. Furthermore, when this lithium composite oxide contains element A, it is preferable that metal elements M, P, and element A are uniformly distributed in the element distribution measured by STEM/EDX. Uniform distribution of metal elements M, P, and element A is preferable because carrier ions (e.g., lithium ions) of the electricity storage device are absorbed and released into and from the lithium composite oxide without bias.

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 called a random rock salt structure). A 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.

The positive ^electrode active material preferably has a volume change rate ΔV _PEAU expressed as ΔV _PEAU = (1 - V ^C _PEAU / V ^D _PEAU ) × 100 of 6% or more and 10% or less, where V C _PEAU is the volume of a unit cell of the crystalline structure during charging and V ^D PEAU is the volume of a unit cell of the crystalline _{structure during discharging. The volume change rate ΔV PEAU is} preferably 7% or more, more preferably 8% or more. Alternatively, the volume change rate ΔV _PEAU may be 9% or less.

The conductive materials, binders, solvents, etc. used in the positive electrode can be the same as those exemplified for the positive electrode. Current collectors can be made of aluminum, titanium, stainless steel, nickel, iron, baked carbon, conductive polymers, conductive glass, etc., as well as aluminum or copper whose surfaces have been treated with carbon, nickel, titanium, silver, etc. to improve adhesion, conductivity, and oxidation resistance. These surfaces can also be oxidized. 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 shape of the current collector can be the same as that of the negative electrode.

The electrode density of the positive electrode is 1 g/cm ³ or more and 2.4 g/cm ³ or less. The electrode density of the positive electrode is preferably higher from the viewpoint of increasing the capacity density and suppressing volumetric shrinkage of the cell during charge and discharge. For example, 1.1 g/cm ³ or more is preferable, and 1.2 g/cm ³ or more is more preferable. The electrode density of the positive electrode is preferably lower from the viewpoint of suppressing volumetric expansion of the cell during charge and discharge. For example, 2.0 g/cm ³ or less is preferable, and 1.8 g/cm ³ or less is more preferable. The electrode density is the electrode density of the portion excluding the current collector. The active material ratio in the positive electrode (excluding the current collector) may be 80 mass% or more and 97 mass% or less, or 85 mass% or more and 95 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.

The ion-conducting medium may be a non-aqueous electrolyte solution containing a supporting salt containing lithium and a non-aqueous solvent. Examples of solvents for non-aqueous electrolyte solutions include carbonates, esters, ethers, nitriles, furans, sulfolanes, and dioxolanes, which may 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 cyclic carbonates and chain carbonates is preferred.

Examples of supporting salts include _LiPF6 , LiBF4, _LiAsF6 , _LiCF3SO3 , LiN _{( CF3SO2 ) 2} , LiC( _{CF3SO2 ) 3 ,} LiSbF6, _LiSiF6 , LiAlF4, LiSCN, _{LiClO4 ,} LiCl, _LiF , LiBr, LiI, and _LiAlCl4 . Among these, it is _preferable from the viewpoint of electrical properties to _use a combination of one or _more salts selected from the group consisting of inorganic salts such as _LiPF6 , _LiBF4 , _LiAsF6 , and _LiClO4 , and organic salts such as _LiCF3SO3 , LiN( _CF3SO2 ) ₂ , and LiC( _CF3SO2 ) ₃ . The concentration of this supporting salt in the nonaqueous electrolyte is preferably 0.1 mol/L or more and 5 mol/L or less, and more preferably 0.5 mol/L or more and 2 mol/L or less. When the concentration of the dissolving supporting salt 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 more stabilized.

The power storage device of the present disclosure may include a separator between the negative electrode and positive electrode. The separator is not particularly limited as long as its composition can withstand the range of use of the power storage device. Examples of the separator include polymer nonwoven fabrics such as polypropylene nonwoven fabrics and polyphenylene sulfide nonwoven fabrics, and thin microporous membranes made of olefin resins such as polyethylene and polypropylene. These may be used alone or in combination.

In the energy storage device disclosed herein, 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%.

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

The electricity storage device of the present disclosure is not particularly limited, and examples thereof include coin-shaped, button-shaped, sheet-shaped, laminated, cylindrical, flat, and rectangular types. Furthermore, a plurality of such electricity storage devices may be connected in series to form large devices for use in electric vehicles and the like. FIG. 1 is a schematic diagram illustrating an example of an electricity storage device 20 according to this embodiment. The electricity storage device 20 includes a sheet-shaped negative electrode 26 having a negative electrode composite 25 formed on the surface of a current collector 24, a sheet-shaped positive electrode 23 having a positive electrode composite 22 formed on a current collector 21, a separator 28 disposed between the negative electrode 26 and the positive electrode 23, and a nonaqueous electrolyte solution 29 filling the space between the negative electrode 26 and the positive electrode 23. The electricity storage device 20 is formed by sandwiching the separator 28 between the negative electrode 26 and the positive electrode 23, winding them, and inserting them into a cylindrical case 32. 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 therein. The negative electrode composite 25 contains a carbon material such as graphite as a negative electrode active material. The positive electrode composite 22 contains, for example, a lithium composite oxide represented by the general formula Li _a P _x M _y O _2-z A _z (where 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, and 0≦z<2 are satisfied) as a positive electrode active material. The electrode density of the positive electrode (density of the positive electrode composite 22) is 1 g/cm ³ or more and 2.4 g/cm ³ or less.

The electricity storage device of this embodiment, as described above in detail, can suppress volumetric changes throughout the cell while suppressing decreases in capacity density and energy density. The reason for this effect is believed to be as follows: In the electricity storage device of this embodiment, a carbon material with a low redox potential is used as the negative electrode active material, which suppresses decreases in energy density more than when lithium titanate or the like is used. Carbon negative electrodes undergo large volumetric changes during charge and discharge. However, when a positive electrode containing a specified lithium composite oxide and formed to a specified density is used as the counter electrode, the volumetric changes of the positive electrode during charge and discharge cancel out the volumetric changes of the carbon negative electrode, thereby suppressing volumetric changes throughout the cell. This makes it possible to omit measures such as externally restraining the cell, which is believed to suppress decreases in capacity density and energy density.

It goes without saying that the present disclosure is in no way limited to the above-described embodiments, and can be implemented in various forms as long as they fall within the technical scope of the present disclosure.

For example, the present disclosure may be any of the following [1] to [7].
[1] An electricity storage device comprising: a negative electrode using a carbon material as a negative electrode active material; a positive electrode using a _{lithium composite} oxide represented by the general formula _{LiaPxMyO2 -zAz (} 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) as a positive electrode active material, the positive electrode 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.
[2] The electricity storage device according to [1], wherein the positive electrode active material has a cubic crystal structure, and a volume change rate ΔV PEAU expressed as ΔV _PEAU = (1 - V ^C _PEAU / V ^{D PEAU ) × 100 is 6% or more and 10% or less, where V C} _PEAU ^is _the volume of a unit cell of the crystal structure during charging and V ^D _PEAU is the volume of a unit cell of the crystal _structure during discharging.
[3] An electricity storage device comprising: a negative electrode using a carbon material as a negative electrode active material; a positive electrode using a lithium composite oxide having a cubic crystal structure as a positive electrode active material, wherein the positive electrode active material has a volume change rate ΔV _PEAU of 6% or more and 10% _{or less, expressed as ΔV PEAU} ⁼ ₍ 1 - V ^C _PEAU / V ^D _PEAU ) × 100, where V ^C _PEAU is the volume of a unit cell of the crystal structure during charging and ^{V D} PEAU is the volume of a unit cell of the crystal structure during discharging; and an ion conductive medium interposed between the negative electrode and the positive electrode, which conducts 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 electricity storage device according to any one of [1] to [4], wherein the density of the positive electrode is 1.2 g/cm ³ or more and 1.8 g/cm ³ or less.
[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], wherein the discharge capacity per cell volume is 200 Ah/L or more.

The following describes, as an example, a specific example of a lithium-ion battery, which is an example of the energy storage device of the present disclosure. 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.

[Experimental Example 1 (Experimental Examples 1-1 to 1-14)]
1. Simulation of Cell Volume Change Rate As shown in Figure 2, a cell consisting of a positive electrode and a negative electrode facing each other in the vertical direction was studied. The volume of the positive electrode active material was defined as V _PEA [cm ³ ], and the volume of the negative electrode active material was defined as V _NEA [cm ³ ]. The volume of the positive electrode composite was defined as V _PE [cm ³ ], and the volume of the negative electrode composite was defined as V _NE [cm ³ ]. Considering a cell cut out with a certain unit area S [cm ² ], the positive electrode thickness was defined as l _PE [μm], and the negative electrode thickness was defined as l _NE [μm]. The cell volume V _all [cm ³ ] of the portion whose volume changes during charge and discharge was defined by the following equation (1), and the rate of change in V _all , ΔV _{all ,} before and after charge and discharge was investigated.

The volume of the crystal lattice of the positive electrode active material and the negative electrode active material changes before and after charge and discharge, i.e., before and after deintercalation and insertion of Li ⁺ . 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 ]. V _PEAU and V _NEAU were calculated from the peak positions of X-ray diffraction (XRD). If V _PEAU during charge is V ^C _PEAU and V _PEAU during discharge is V ^D _PEAU , the change rate ΔV _PEAU in V _PEAU before and after charge and discharge is expressed by the following formula (2):

The positive electrode active material used in the simulation in Experimental Example 1 was Li _0.90 Mn _0.84 P _0.04 O ₂ (LMPO). From the pre-operando XRD experiment described below, the ΔV _PEAU of LMPO was calculated to be 8.73%. Furthermore, from the charge/discharge measurement results during the pre-operando XRD experiment, the capacity Q _PEA per LMPO was determined to be 346 mAh/g. XRD confirmed that this Li _0.90 Mn _0.84 P _0.04 O ₂ (LMPO) had a cubic crystal structure (specifically, a disordered rock salt structure).

The negative electrode active material used in the simulation was standard natural graphite. The true density of natural graphite changes from 2.2 g/ ^cm3 to 2 g/ ^cm3 during discharge and charge. In other words, the change rate of _VNEAU before and after charge and discharge, _ΔVNEAU , was assumed to be -10%. The capacity _QNEA per unit of natural graphite was assumed to be 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.

In the cell used in the simulation, the active material ratio w _PEA of the positive electrode was set to 90 mass%. The electrode density c _PEA of the positive electrode was varied in the range of 1.0 g/cm ³ to 3.6 g/cm ^3. The active material ratio w _NEA of the negative electrode was set to 98 mass%, the active material weight ρ _NEA of the negative electrode was set to 7.5 mg/cm ² , and the electrode density c _NEA of the negative electrode was set to 1.5 g/cm ^3. The capacity ratio of the negative electrode to the positive electrode was set to 1.1. The active material weight ρ _PEA of the positive electrode was set to

Based on the above assumptions, the active material coverage ρ _PEA [mg/cm ² ] of the positive electrode was calculated using the following formula (3), and the positive electrode thickness l _PE [μm] was calculated using the following formula (4). From these results, the positive electrode volume V _PE was calculated using the following formula (5).

Furthermore, the negative electrode thickness l _NE [μm] was calculated from the following formula (6), and the negative electrode volume V _NE was calculated from the result, as expressed by the following formula (7).

The above results were substituted into the above formula (1) to calculate the cell volume V _all of the portion whose volume is variable during charging and discharging.

Furthermore, it was considered most likely that when the volume of the active material changes during charging, the positive and negative electrodes each change at the same rate, and the rate of change in cell volume ΔV _all was calculated based on the above calculations. ΔV _all was calculated using the formula: ΔV _all = (V _all (before charging) - V _all (after charging)) × 100/V _all (before charging). Furthermore, the capacity per cell volume Q [Ah/L] was calculated using the following formula (8):

2. Preparation of Positive Electrode Li _0.90 Mn _0.84 P _0.04 O ₂ (LMPO) was synthesized as the positive electrode active material. Orthorhombic LiMnO ₂ as the raw material was sealed in a zirconia pot together with zirconia balls and ground in a planetary ball mill at 560 rpm for 28 hours to obtain LiMnO ₂ powder with a disordered rock salt phase. The raw materials LiMnO ₂ and Li ₃ PO ₄ obtained by the above method were mixed in an agate bowl so that the weight fraction of Li ₃ PO ₄ was 5%, and then sealed in a zirconia pot together with zirconia balls and ground in a planetary ball mill at 600 rpm for 36 hours to synthesize Li _0.90 P _0.04 Mn _0.84 O ₂ positive electrode active material with a cubic phase. A mixture of 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) was applied to an Al foil and dried to prepare a positive electrode.

3. Operando XRD of the positive electrode
Operando XRD was performed on the positive electrode prepared as described above to examine the change in the crystal lattice constant during charge and discharge. A half-cell was prepared by combining the LMPO positive electrode and a metallic Li counter electrode with a separator (E20MMS) and electrolyte. XRD patterns were obtained using a Spring-8 BL33XU battery 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 1 M. Constant-current, low-voltage charge and discharge measurements were performed in the potential range of 4.8 to 1.5 V vs. Li ⁺ /Li, adjusting the current value to 20 mA/g per mass of positive electrode active material.

The results of operando XRD measurements are shown in Figure 3. The change in lattice constant calculated from the operando XRD patterns is also shown in Figure 3. From these results, it was found that LMPO had a current of 4.075 A during charging and 4.17 A during discharging, and the ΔV _PEAU of LMPO was calculated to be 8.73%.

4. Preparation of Negative Electrode: A mixture of 98% by weight of graphite, 1% by weight of carboxymethyl cellulose, and 1% by weight of PVdF was coated on Cu foil and dried to prepare a negative electrode. The active material weight per unit area of the negative _electrode, ρ, was 7.5 mg/ ^cm² , and the negative electrode density, c _, was 1.5 g/ ^cm³ .

5. Thickness Measurement of Lithium-Ion Battery 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 fabricated by placing the positive and negative electrodes facing each other in a 60 x 100 mm Al laminate with a polyethylene separator between them. The electrolyte was the same as that used in the half-cell. The end voltage during charge and discharge was 4.7 to 1.4 V, and the current value was 20 mA/g per mass of positive electrode active material. The thickness of the cell was measured at four points using a thickness gauge during the initial charge and discharge. 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.

[Experimental Example 2]
_{The procedure} was the same as in Experimental Example 1, except that the positive electrode active material was _LiNiCoMnO ( _NCM111 ). Based on the data in A.O. Kondrakov et al., The Journal of Physical Chemistry C 2017, 121, 3286-3294, _ΔVPEAU of NMC111 was set to 1.16% and _QPEA was set to 149 mAh/g.

[Experimental Example 3]
The same _procedure as in Experimental Example 1 was repeated except that the positive electrode active material was _{LiNi0.8Co0.1Mn0.1O2} ( _NCM811 ). Based on the data in A.O. Kondrakov et _al ., The Journal of Physical Chemistry C 2017, 121, 3286-3294, _ΔVPEAU of NCM811 was set to 1.45% and _QPEA was set to 189 mAh/g.

[Experimental Example 4]
The same procedure as in Experimental Example 1 was repeated 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 of LiMn2O4} was set to 7.45% and Q _PEAU was set to 110 mAh/g.

[Results and Discussion]
Figure 4 shows the results of a simulation using the above formula to determine the change in V _all (%) before and after charge/discharge and the capacity per cell volume (Ah/L) when the positive electrode density c _PE is changed. Figure 4A shows the results when Li _0.90 Mn _0.84 P _0.04 O ₂ (LMPO) was used as the positive electrode active material (Experimental Example 1), Figure 4B shows the results when LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (NCM111) was used (Experimental Example 2), Figure 4C shows the results when LiNi _0.8 Co _0.1 Mn _0.1 O ₂ (NCM811) was used (Experimental Example 3), and Figure 4D shows the results when LiMn ₂ O ₄ was used (Experimental Example 4).

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, it was found that the change rate in V _all falls within ±1% when the positive electrode density is around 1.4 to 1.6 g/ ^cm3 .

4A to 4D are shown in Fig. 5, which is a representation of the relationship between the rate of change ΔV _all in V _all before and after charge and discharge and the discharge capacity per cell volume. Table 1 also summarizes the simulation results under conditions 1 to 7 shown in the figures for the cases where each positive electrode active material was used.

Under condition 1 (using an LMPO positive electrode with a density of 1.4 g/cm ³ ), the rate of change in V _all was −0.1%, indicating almost no volume change. The capacity Q per cell volume was 226 Ah/L. Under condition 2 (using an LMPO positive electrode with a density of 3.0 g/cm ³ ), the capacity Q per cell volume was 312 Ah/L, which is the highest capacity expected, but the rate of change in V _all , ΔV _all , was 3.1%, indicating that long-term degradation is expected and external constraints, etc., are required. Conditions 3, 4, 5, and 6 are combinations of positive and negative electrodes currently commonly used in automotive lithium-ion batteries, but all of them showed a rate of change in V _all , ΔV _all , of 2% or more, indicating the need for external constraints. Furthermore, under condition 7 (using a LiMn ₂ O ₄ positive electrode with a density of 3.0 g/cm ³ ), the rate of change in V _all , ΔV _all , was −1.5%, outside the ±1% range, indicating a large volume change. The capacity per cell volume was also 190 Ah/L, which was lower than when an NCM-based or LMPO-based cathode was used. Table 2 summarizes the change in cell volume and capacity per volume relative to the cathode density when an LMPO cathode was used. It was found that a cathode density of 1.2 to 1.8% is preferable to keep the cell volume change rate to -1 to 1%.

Full cells were actually fabricated using LMPO and NCM811 positive electrodes and graphite negative electrodes, and the change in cell volume during charge and discharge was 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 there was almost no change in cell volume during discharge. On the other hand, when the NCM811 positive electrode was used, cell expansion of 16% during charge and cell contraction of 16% during discharge were confirmed. While this was a larger value than the cell expansion and contraction rate predicted by the simulation, it is believed that this was largely due to measurement error, and the cell expansion and contraction generally tended to be consistent with the simulation. From the above, it has been found that in conventional high-capacity lithium-ion batteries, as shown in Figure 7A, the expansion and contraction of the carbon anode cannot be fully absorbed during charge and discharge, resulting in large expansion and contraction of the cell as a whole, whereas in the lithium-ion battery of the present disclosure, as shown in Figure 7B, the expansion and contraction of the carbon anode can be absorbed by the expansion and contraction of the positive electrode during charge and discharge, thereby suppressing expansion and contraction of the cell as a whole.

This disclosure can be used in technical fields such as the battery industry.

20: Electricity 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 (7)

a negative electrode having a carbon material as a negative electrode active material;
a positive electrode having a positive electrode active material made of a lithium composite oxide represented by the 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), and an electrode density of 1 g/cm ³ or more and 2.4 g/cm ³ or less;
an ion conductive medium interposed between the negative electrode and the positive electrode and conducting lithium ions;
An electricity storage device comprising: the positive electrode active material has a cubic crystal structure, and a volume change rate ΔV PEAU expressed by ΔV _PEAU = (1 − ^{V C PEAU} _/ V ^D _PEAU ) × 100 is 6% or more and 10% _or less, where V ^C _PEAU is the volume of a unit cell of the crystal structure during charging and V D _PEAU is the volume of a unit cell of the crystal structure during discharging;
The electricity storage device according to claim 1 . a negative electrode having a carbon material as a negative electrode active material;
a positive electrode having a volume change rate ΔV PEAU of 6% or more and 10% or less, expressed as ΔV _PEAU = (1 - V ^C _PEAU / V ^D _PEAU ) × 100, where V ^C _PEAU is the volume of a unit cell of the crystal structure during charging and V ^D _PEAU is the volume of a unit cell of the crystal _structure during discharging; and an electrode density of 1 g/cm ³ or more and 2.4 g/cm ³ or less;
an ion conductive medium interposed between the negative electrode and the positive electrode and conducting lithium ions;
An electricity storage device comprising: The negative electrode active material is graphite.
The electricity storage device according to any one of claims 1 to 3. The electrode density of the positive electrode is 1.2 g/cm ³ or more and 1.8 g/cm ³ or less.
The electricity storage device according to any one of claims 1 to 3. 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. 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.