Nonaqueous electrolyte energy storage element and energy storage device

Abstract

A nonaqueous electrolyte power storage element according to one aspect of the present invention is provided with a positive electrode having a lithium-transition metal composite oxide comprising manganese, and a negative electrode comprising graphite, wherein the ratio accounted for by manganese in the metal elements other than lithium in the lithium-transition metal composite oxide is 20 mol% or less, and the state of charge of the graphite in a charged condition is less than 0.54.

Description

[Technical Field]

[0001] The present invention relates to a non-aqueous electrolyte electricity storage element and an electricity storage device. [Background technology]

[0002] Due to their high energy density, non-aqueous electrolyte secondary batteries, such as lithium ion secondary batteries, are widely used in electronic devices such as personal computers and communication terminals, as well as automobiles. Non-aqueous electrolyte secondary batteries generally have a pair of electrodes electrically isolated by a separator and a non-aqueous electrolyte interposed between the electrodes, and are configured to charge and discharge by transferring ions between the electrodes. In addition to non-aqueous electrolyte secondary batteries, capacitors such as lithium ion capacitors and electric double layer capacitors are also widely used as non-aqueous electrolyte energy storage elements.

[0003] Conventionally, lithium transition metal composite oxides with an α-NaFeO2 type crystal structure have been investigated as positive electrode active materials for non-aqueous electrolyte storage elements, and non-aqueous electrolyte secondary batteries using LiCoO2 have been widely put to practical use. However, LiCoO2 is expensive to manufacture and difficult to supply stably. Therefore, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 Positive electrode active materials containing manganese and the like together with cobalt, such as O2, have been developed and put to practical use (see Patent Document 1). On the other hand, carbon materials such as graphite are widely used as negative electrode active materials (see Patent Document 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-18678 [Patent Document 2] Japanese Patent Application Laid-Open No. 2005-222933 Summary of the Invention [Problem to be solved by the invention]

[0005] Non-aqueous electrolyte energy storage elements are required to be resistant to a decrease in discharge capacity even after repeated charge and discharge, i.e., to have a high capacity retention rate after charge and discharge cycles. An improvement in the capacity retention rate after charge and discharge cycles is also expected in non-aqueous electrolyte energy storage elements having a positive electrode containing a lithium transition metal composite oxide containing manganese, which can be supplied stably at low cost.

[0006] An object of the present invention is to provide a nonaqueous electrolyte electricity storage element and electricity storage device that have a high capacity retention rate after charge / discharge cycles. [Means for solving the problem]

[0007] A nonaqueous electrolyte storage element according to one aspect of the present invention includes a positive electrode having a lithium transition metal composite oxide containing manganese, and a negative electrode having graphite, wherein the ratio of manganese to metal elements other than lithium in the lithium transition metal composite oxide is 20 mol % or less, and the depth of charge of the graphite in a charged state is less than 0.54. [Effects of the Invention]

[0008] According to one aspect of the present invention, it is possible to provide a nonaqueous electrolyte electricity storage element and electricity storage device having a high capacity retention rate after charge / discharge cycles. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a perspective view showing one embodiment of a nonaqueous electrolyte electricity storage element. [Figure 2] FIG. 2 is a schematic diagram showing one embodiment of an electricity storage device configured by assembling a plurality of nonaqueous electrolyte electricity storage elements. DETAILED DESCRIPTION OF THE INVENTION

[0010] First, an outline of the nonaqueous electrolyte electricity storage element and electricity storage device disclosed in this specification will be described.

[0011] A nonaqueous electrolyte storage element according to one aspect of the present invention includes a positive electrode having a lithium transition metal composite oxide containing manganese, and a negative electrode having graphite, wherein the ratio of manganese to metal elements other than lithium in the lithium transition metal composite oxide is 20 mol % or less, and the depth of charge of the graphite in a charged state is less than 0.54.

[0012] The nonaqueous electrolyte storage element has a high capacity retention rate after charge-discharge cycling. While the reason for this is unclear, the following reasons are presumed. Generally, the decrease in the capacity retention rate of a nonaqueous electrolyte storage element with charge-discharge cycling is due to a decrease in electronic conductivity caused by cracking of the positive electrode active material. Meanwhile, in the manganese-containing lithium transition metal composite oxide used as the positive electrode active material, the higher the manganese content, the lower the electronic conductivity. Therefore, by using a manganese-containing lithium transition metal composite oxide with a low manganese content, sufficient electronic conductivity can be ensured even when cracking of the lithium transition metal composite oxide occurs with charge-discharge cycling. Furthermore, one cause of cracking of the positive electrode active material is repeated pressure application due to expansion and contraction of the negative electrode active material caused by repeated charge-discharge cycles. Therefore, by reducing the depth of charge of the negative electrode active material, graphite expansion in the charged state is suppressed, making the lithium transition metal composite oxide used as the positive electrode active material less likely to crack. It is presumed that this is the reason why the nonaqueous electrolyte storage element has a high capacity retention rate after charge-discharge cycles.

[0013] "Graphite" refers to graphite that has an average lattice spacing (d 002 ) is a carbon material having a particle size of 0.33 nm or more and less than 0.34 nm. The "discharged state" of a carbon material refers to a state in which the carbon material, which is the negative electrode active material, is discharged so that lithium ions that can be absorbed and released during charging and discharging are sufficiently released. For example, in a single-electrode battery using a negative electrode containing a carbon material as the negative electrode active material as the working electrode and metallic Li as the counter electrode, this state is one in which the open circuit voltage is 0.7 V or more.

[0014] The "depth of charge of graphite in a charged state" refers to the ratio of the amount of charge per mass of graphite in a charged state to the theoretical capacity per mass of graphite. The "theoretical capacity" refers to the maximum amount of electricity that a unit mass of active material can store in a predicted electrochemical reaction. In this specification, the theoretical capacity of graphite is 372 mAh / g. The "charged state" refers to a state in which a nonaqueous electrolyte storage element is charged to a rated upper limit voltage to ensure a predetermined rated capacity. If the rated capacity is unclear, the "charged state" refers to a state in which the nonaqueous electrolyte storage element is charged to a cut-off voltage at which the charging operation is stopped when charging is performed using a charge control device employed by the nonaqueous electrolyte storage element. For example, a typical example of the "charged state" here is a state in which the nonaqueous electrolyte storage element is charged at a constant current of 1 / 3 C to a rated upper limit voltage or a cut-off voltage, and then charged at a constant voltage of 0.01 C at the rated upper limit voltage or the cut-off voltage. Specifically, the "charged electricity quantity per mass of graphite in a charged state" is measured by the following procedure. (1) The target non-aqueous electrolyte storage element is discharged to the lower limit voltage (SOC 0%) when the rated capacity is obtained, and then disassembled in a glove box controlled to an argon atmosphere with an oxygen concentration of 5 ppm or less. (2) In the glove box, the positive and negative electrodes are removed and a small pouch cell (I) is assembled. (3) After charging the small pouch cell (I) to the above-mentioned charged state, it is discharged at a constant current of 0.01 C to the lower limit voltage when the rated capacity is obtained in the nonaqueous electrolyte storage element. (4) In a glove box controlled to an argon atmosphere with an oxygen concentration of 5 ppm or less, the small pouch cell (I) is disassembled, the negative electrode is removed, and a small pouch cell (II) is assembled with lithium metal placed as the counter electrode. (5) The small pouch cell (II) is further discharged at a current of 0.01 C until the voltage reaches 2.0 V, and the negative electrode is adjusted to a fully discharged state. (6) The sum of the discharged electricity quantities in (3) and (5) above is divided by the mass of graphite in the negative electrode active material layer of the positive and negative electrode opposing portion in the small pouch cell (I) to obtain the charged electricity quantity per mass of graphite in the charged state. Here, the reduction reaction in which lithium ions etc. are absorbed into the negative electrode active material is called "charging," and the oxidation reaction in which lithium ions etc. are released from the negative electrode active material is called "discharging."

[0015] It is preferable that the lithium transition metal composite oxide further contains nickel and cobalt, which further increases the capacity retention rate after charge-discharge cycling.

[0016] A nonaqueous electrolyte electricity storage element, an electricity storage device, a method for manufacturing a nonaqueous electrolyte electricity storage element according to one embodiment of the present invention, and other embodiments will be described in detail below. Note that the names of the components (elementary components) used in each embodiment may differ from the names of the components (elementary components) used in the background art.

[0017] <Non-aqueous electrolyte energy storage element> A nonaqueous electrolyte storage element (hereinafter also simply referred to as "storage element") according to one embodiment of the present invention comprises an electrode assembly having a positive electrode, a negative electrode, and a separator, a nonaqueous electrolyte, and a container that accommodates the electrode assembly and the nonaqueous electrolyte. The electrode assembly is typically a stacked type in which a plurality of positive electrodes and a plurality of negative electrodes are stacked with separators interposed therebetween, or a wound type in which a positive electrode and a negative electrode are stacked with separators interposed therebetween and wound. The nonaqueous electrolyte is present in a state contained in the positive electrode, the negative electrode, and the separator. As an example of a nonaqueous electrolyte storage element, a nonaqueous electrolyte secondary battery (hereinafter also simply referred to as "secondary battery") will be described.

[0018] (positive electrode) The positive electrode has a positive electrode substrate and a positive electrode active material layer disposed on the positive electrode substrate directly or via an intermediate layer.

[0019] The positive electrode substrate is electrically conductive. Whether or not it has "electrical conductivity" is determined by whether the volume resistivity measured in accordance with JIS-H-0505 (1975) is 10 -2The threshold value is Ω·cm. The material of the positive electrode substrate is a metal such as aluminum, titanium, tantalum, or stainless steel, or an alloy of these. Among these, aluminum or an aluminum alloy is preferred from the viewpoints of potential resistance, high conductivity, and cost. Positive electrode substrates include foils, vapor-deposited films, meshes, and porous materials, with foil being preferred from the viewpoint of cost. Therefore, aluminum foil or aluminum alloy foil is preferred as the positive electrode substrate. Examples of aluminum or aluminum alloys include A1085, A3003, and A1N30, as specified in JIS-H-4000 (2014) or JIS-H4160 (2006).

[0020] The average thickness of the positive electrode substrate is preferably 3 μm to 50 μm, more preferably 5 μm to 40 μm, even more preferably 8 μm to 30 μm, and particularly preferably 10 μm to 25 μm. By setting the average thickness of the positive electrode substrate within the above range, the strength of the positive electrode substrate can be increased while increasing the energy density per volume of the secondary battery.

[0021] The intermediate layer is a layer disposed between the positive electrode substrate and the positive electrode active material layer. The intermediate layer contains a conductive agent such as carbon particles to reduce the contact resistance between the positive electrode substrate and the positive electrode active material layer. The configuration of the intermediate layer is not particularly limited, and may contain, for example, a binder and a conductive agent.

[0022] The positive electrode active material layer contains a positive electrode active material and, if necessary, optional components such as a conductive agent, a binder, a thickener, and a filler.

[0023] The positive electrode active material contains a lithium transition metal composite oxide containing manganese. This lithium transition metal composite oxide preferably further contains at least one of nickel and cobalt, and more preferably both nickel and cobalt. The use of a lithium transition metal composite oxide containing these transition metal elements can increase the discharge capacity, etc.

[0024] The upper limit of the manganese ratio relative to the metal elements excluding lithium in the lithium transition metal composite oxide is 20 mol%, or may be 19 mol%, preferably 18 mol%, more preferably 17 mol%, even more preferably 16 mol%, and even more preferably 15 mol%. By having the manganese ratio at or below the upper limit, the electronic conductivity of the lithium transition metal composite oxide can be improved, and the capacity retention rate of the nonaqueous electrolyte energy storage element after charge / discharge cycling can be increased. On the other hand, the lower limit of the manganese ratio is preferably 1 mol%, or may be 2 mol%, 3 mol%, or 4 mol%, more preferably 5 mol%, or may be more preferably 6 mol%, 7 mol%, 8 mol%, or 9 mol%, even more preferably 10 mol%, and even more preferably 11 mol%, 12 mol%, 13 mol%, 14 mol%, or 15 mol%. By having the manganese ratio at or above the lower limit, costs can be reduced. The manganese ratio can be within a range that combines any of the upper and lower limits described above.

[0025] The ratio of nickel to the metal elements excluding lithium in the lithium transition metal composite oxide is preferably 20 mol% to 80 mol%, or 25 mol% to 75 mol%, more preferably 30 mol% to 70 mol%, and sometimes even more preferably 35 mol% to 68 mol%, more preferably 40 mol% to 65 mol%, even more preferably 45 mol% to 60 mol%, and even more preferably 47 mol% to 55 mol%. By keeping the nickel ratio within this range, it is possible to further increase the capacity retention rate after charge / discharge cycling.

[0026] The ratio of cobalt to the metal elements excluding lithium in the lithium transition metal composite oxide may be 1 mol% to 60 mol%, preferably 5 mol% to 60 mol%, more preferably 10 mol% to 50 mol%, sometimes more preferably 15 mol% to 48 mol%, even more preferably 20 mol% to 45 mol%, sometimes even more preferably 25 mol% to 43 mol%, and even more preferably 30 mol% to 40 mol%. By keeping the cobalt ratio within the above range, it is possible to further increase the capacity retention rate after charge / discharge cycling. Furthermore, by keeping the cobalt ratio below the above upper limit, it is possible to achieve cost reduction.

[0027] The lithium transition metal composite oxide may further contain a metal element other than lithium, manganese, nickel, and cobalt, such as aluminum, etc. However, the ratio of the total amount of manganese, nickel, and cobalt to the metal elements other than lithium in the lithium transition metal composite oxide is preferably 90 mol % or more, more preferably 99 mol % or more, and even more preferably substantially 100 mol %.

[0028] The lithium transition metal composite oxide is preferably a lithium transition metal composite oxide having a layered α-NaFeO2 type crystal structure. Also, the lithium transition metal composite oxide is preferably a compound represented by the following formula 1:

[0029] Li 1+α Me 1-α O2···1 In formula 1, Me represents two or more metal elements containing Mn at a ratio of 20 mol % or less, and α satisfies 0≦α<1.

[0030] In formula 1, α may be 0 or more and 0.5 or less, 0 or more and 0.3 or less, or 0 or more and 0.1 or less. In addition to Mn, Me preferably further contains at least one of Ni and Co, and more preferably further contains both Ni and Co. As suitable ratios of each and the total amount of Mn, Ni, and Co in Me, the respective ratios described above as suitable ratios of each and the total amount of manganese, nickel, and cobalt in metal elements excluding lithium in the lithium transition metal composite oxide can be adopted. Me may further contain a metal element other than Mn, Ni, and Co.

[0031] The surface of the lithium transition metal composite oxide may be coated with another material. Examples of other materials for coating the surface include compounds containing aluminum, tungsten, boron, etc., and boron-containing compounds are preferred. Examples of compounds containing aluminum, tungsten, boron, etc. include oxides of these. When the surface is coated with such a material, the capacity retention rate of the nonaqueous electrolyte energy storage element after charge-discharge cycles can be further increased.

[0032] The content of the lithium transition metal composite oxide in the positive electrode active material layer, which contains manganese and in which the ratio of manganese to the metal elements excluding lithium is 20 mol % or less, is preferably 50 mass % to 99 mass %, more preferably 70 mass % to 98 mass %, and even more preferably 80 mass % to 95 mass %. By setting the content of the lithium transition metal composite oxide within the above range, it is possible to achieve both high energy density and manufacturability of the positive electrode active material layer, while improving the capacity retention rate after charge / discharge cycling.

[0033] The positive electrode active material may further contain a positive electrode active material other than the lithium transition metal composite oxide. As the other positive electrode active material, various conventionally known positive electrode active materials can be used. However, the content of the lithium transition metal composite oxide in the total positive electrode active material contained in the positive electrode active material layer is preferably 90% by mass or more, more preferably 99% by mass or more. By using a lithium transition metal composite oxide containing manganese and having a manganese ratio of 20 mol % or less to the metal elements excluding lithium as the positive electrode active material, the electron conductivity can be sufficiently increased, thereby further improving the capacity retention rate after charge / discharge cycling.

[0034] The content of all the positive electrode active materials in the positive electrode active material layer is preferably 50% by mass to 99% by mass, more preferably 70% by mass to 98% by mass, and even more preferably 80% by mass to 95% by mass. By setting the content of all the positive electrode active materials within the above range, both high energy density and manufacturability of the positive electrode active material layer can be achieved.

[0035] The positive electrode active material is usually in the form of particles (powder). The average particle size of the positive electrode active material is preferably, for example, 0.1 μm or more and 20 μm or less. By setting the average particle size of the positive electrode active material to the above lower limit or more, the positive electrode active material becomes easier to manufacture and handle. By setting the average particle size of the positive electrode active material to the above upper limit or less, the electronic conductivity of the positive electrode active material layer is improved. Note that when a composite of the positive electrode active material and another material is used, the average particle size of the composite is taken as the average particle size of the positive electrode active material. The "average particle size" refers to the value at which the volume-based cumulative distribution calculated in accordance with JIS-Z-8819-2 (2001) is 50% based on the particle size distribution measured by laser diffraction / scattering in a diluted solution obtained by diluting particles with a solvent in accordance with JIS-Z-8825 (2013).

[0036] To obtain powders with a predetermined particle size, grinders, classifiers, etc. are used. Grinding methods include, for example, methods using a mortar, ball mill, sand mill, vibration ball mill, planetary ball mill, jet mill, counter jet mill, swirling airflow jet mill, or sieves. Wet grinding in the presence of water or an organic solvent such as hexane can also be used during grinding. As classification methods, sieves, air classifiers, etc. are used as needed for both dry and wet methods.

[0037] The conductive agent is not particularly limited as long as it is a material having electrical conductivity. Examples of such conductive agents include carbonaceous materials, metals, and conductive ceramics. Examples of carbonaceous materials include graphite, non-graphitic carbon, and graphene-based carbon. Examples of non-graphitic carbon include carbon nanofibers, pitch-based carbon fibers, and carbon black. Examples of carbon black include furnace black, acetylene black, and ketjen black. Examples of graphene-based carbon include graphene, carbon nanotubes (CNTs), and fullerenes. The conductive agent may be in the form of powder or fiber. As the conductive agent, one of these materials may be used alone, or two or more may be mixed. These materials may also be used in combination. For example, a composite of carbon black and CNTs may be used. Among these, carbon black is preferred from the viewpoints of electronic conductivity and coatability, and acetylene black is particularly preferred.

[0038] The content of the conductive agent in the positive electrode active material layer is preferably 1% by mass to 10% by mass, more preferably 3% by mass to 9% by mass. By setting the content of the conductive agent within this range, the energy density of the secondary battery can be increased.

[0039] Examples of binders include thermoplastic resins such as fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), polyethylene, polypropylene, polyacrylic, and polyimide; elastomers such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), and fluororubber; and polysaccharide polymers.

[0040] The binder content in the positive electrode active material layer is preferably 1% by mass to 10% by mass, more preferably 2% by mass to 9% by mass. By setting the binder content within this range, the active material can be stably maintained.

[0041] Examples of thickeners include polysaccharide polymers such as carboxymethyl cellulose (CMC), methyl cellulose, etc. When the thickener has a functional group that reacts with lithium or the like, this functional group may be deactivated in advance by methylation or the like.

[0042] The filler is not particularly limited, and examples of the filler include polyolefins such as polypropylene and polyethylene, inorganic oxides such as silicon dioxide, alumina, titanium dioxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicates, hydroxides such as magnesium hydroxide, calcium hydroxide, and aluminum hydroxide, carbonates such as calcium carbonate, sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium sulfate, nitrides such as aluminum nitride and silicon nitride, substances derived from mineral resources such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, and artificial products thereof.

[0043] The positive electrode active material layer may contain typical non-metallic elements such as B, N, P, F, Cl, Br, and I; typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, and Ba; and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Nb, and W as components other than the positive electrode active material, conductive agent, binder, thickener, and filler.

[0044] (Negative electrode) The negative electrode has a negative electrode substrate and a negative electrode active material layer disposed on the negative electrode substrate directly or via an intermediate layer. The configuration of the intermediate layer is not particularly limited and can be selected from the configurations exemplified for the positive electrode above, for example.

[0045] The negative electrode substrate is conductive. Metals such as copper, nickel, stainless steel, and nickel-plated steel, alloys thereof, and carbonaceous materials are used as the material of the negative electrode substrate. Among these, copper or copper alloys are preferred. Examples of the negative electrode substrate include foil, vapor-deposited film, mesh, and porous material, with foil being preferred from the viewpoint of cost. Therefore, copper foil or copper alloy foil is preferred as the negative electrode substrate. Examples of copper foil include rolled copper foil and electrolytic copper foil.

[0046] The average thickness of the negative electrode substrate is preferably 2 μm to 35 μm, more preferably 3 μm to 30 μm, even more preferably 4 μm to 25 μm, and particularly preferably 5 μm to 20 μm. By setting the average thickness of the negative electrode substrate within the above range, the strength of the negative electrode substrate can be increased while increasing the energy density per volume of the secondary battery.

[0047] The negative electrode active material layer contains a negative electrode active material. The negative electrode active material layer contains optional components such as a conductive agent, a binder, a thickener, and a filler, as needed. The optional components such as the conductive agent, the binder, the thickener, and the filler can be selected from the materials exemplified for the positive electrode above.

[0048] The negative electrode active material layer may contain typical non-metallic elements such as B, N, P, F, Cl, Br, and I; typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, and Ba; and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Ta, Hf, Nb, and W as components other than the negative electrode active material, conductive agent, binder, thickener, and filler.

[0049] The negative electrode active material contains graphite. By containing graphite in the negative electrode active material, it is possible to increase the capacity retention rate after charge / discharge cycles. Furthermore, by containing graphite in the negative electrode active material, it is possible to design the depth of charge with high precision. Examples of graphite include natural graphite and artificial graphite. Artificial graphite is preferred from the viewpoint of being able to obtain a material with stable physical properties.

[0050] The depth of charge of graphite, which is the negative electrode active material in a charged state, is less than 0.54, preferably 0.53 or less, and more preferably 0.52 or less. Having a depth of charge of the graphite of less than 0.54 can improve the capacity retention rate after charge / discharge cycling. On the other hand, the depth of charge of the graphite is preferably 0.40 or more, and may be 0.41 or more, more preferably 0.42 or more, and even more preferably 0.43 or more or 0.44 or more. It is further preferably 0.45 or more, 0.46 or more, 0.47 or more, even more preferably 0.48 or more, 0.49 or more, and particularly preferably 0.50 or more. Having a depth of charge of the graphite equal to or greater than the lower limit can improve the energy density of the nonaqueous electrolyte storage element. The depth of charge of the graphite can be within a range that combines any of the upper and lower limits described above.

[0051] The depth of charge of graphite, which is the negative electrode active material in a charged state, can be adjusted, for example, by changing the ratio of the mass of graphite per unit area in the negative electrode active material layer to the mass of the positive electrode active material per unit area in the positive electrode active material layer.

[0052] The graphite content in the negative electrode active material layer is preferably 60% by mass to 99% by mass, more preferably 90% by mass to 98% by mass, and even more preferably 95% by mass or more. By setting the graphite content within this range, both high energy density and manufacturability of the negative electrode active material layer can be achieved, and the capacity retention rate after charge / discharge cycling can be further improved.

[0053] The negative electrode active material may contain a negative electrode active material other than graphite. As the other negative electrode active material, various conventionally known negative electrode active materials can be used. However, the content of graphite in all the negative electrode active materials contained in the negative electrode active material layer is preferably 90% by mass or more, more preferably 99% by mass or more, and may be substantially 100% by mass. By using graphite as the main negative electrode active material in this way, the capacity retention rate after charge / discharge cycles can be further improved. Furthermore, by setting the content of graphite in the negative electrode active material to be equal to or greater than the above-mentioned lower limit, the depth of charge of the graphite in the charged state can be precisely designed.

[0054] The content of all the negative electrode active materials in the negative electrode active material layer is preferably 60% by mass to 99% by mass, more preferably 90% by mass to 98% by mass. By setting the content of all the negative electrode active materials within the above range, both high energy density and manufacturability of the negative electrode active material layer can be achieved.

[0055] Graphite and other negative electrode active materials are usually in the form of particles (powders). The average particle size of the negative electrode active material can be, for example, 1 nm or more and 100 μm or less. This average particle size may be 1 μm or more and 100 μm or less. By setting the average particle size of the negative electrode active material to be equal to or greater than the above lower limit, the negative electrode active material can be easily manufactured or handled. By setting the average particle size of the negative electrode active material to be equal to or less than the above upper limit, the electronic conductivity of the negative electrode active material layer is improved. To obtain powder with a predetermined particle size, a pulverizer, a classifier, or the like is used. The pulverization method and the powder classification method can be selected, for example, from the methods exemplified for the positive electrode above.

[0056] The porosity of the negative electrode active material layer is preferably 30% to 60%, more preferably 40% to 50%. By ensuring that the porosity of the negative electrode active material layer is within this range, charge / discharge performance can be improved. The porosity of the negative electrode active material layer can be adjusted by the strength of the press used to form the negative electrode active material layer, the material constituting the negative electrode active material layer, and other factors. The "porosity" of the negative electrode active material layer is a volume-based value measured using a mercury porosimeter. Specifically, the negative electrode that was fully discharged using the procedure for measuring the "charged electricity quantity per mass of graphite in a charged state" described above was removed and immersed in dimethyl carbonate for 5 minutes twice to thoroughly wash the nonaqueous electrolyte adhering to the negative electrode. The washed negative electrode was then dried under reduced pressure at room temperature overnight before being used for measurement.

[0057] (separator) The separator can be appropriately selected from known separators. Examples of separators that can be used include separators consisting of only a substrate layer and separators in which a heat-resistant layer containing heat-resistant particles and a binder is formed on one or both surfaces of the substrate layer. Examples of the shape of the substrate layer of the separator include woven fabric, nonwoven fabric, and porous resin film. Among these shapes, porous resin films are preferred from the viewpoint of strength, and nonwoven fabrics are preferred from the viewpoint of non-aqueous electrolyte retention. Materials for the substrate layer of the separator are preferably polyolefins such as polyethylene and polypropylene from the viewpoint of shutdown function, and polyimide and aramid from the viewpoint of oxidative decomposition resistance. A composite material of these resins may also be used for the substrate layer of the separator.

[0058] The heat-resistant particles contained in the heat-resistant layer preferably exhibit a mass loss of 5% or less when heated from room temperature to 500°C in an air atmosphere at 1 atmosphere pressure, and more preferably exhibit a mass loss of 5% or less when heated from room temperature to 800°C. Examples of materials exhibiting a mass loss of a predetermined value or less include inorganic compounds. Examples of inorganic compounds include oxides such as iron oxide, silicon oxide, aluminum oxide, titanium oxide, zirconium oxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicate; nitrides such as aluminum nitride and silicon nitride; carbonates such as calcium carbonate; sulfates such as barium sulfate; sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium titanate; covalently bonded crystals such as silicon and diamond; mineral-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, as well as artificial products thereof. As the inorganic compound, these substances may be used alone or in the form of a complex, or two or more of them may be used in combination. Among these inorganic compounds, silicon oxide, aluminum oxide, or aluminosilicate is preferred from the viewpoint of the safety of the energy storage device.

[0059] The porosity of the separator is preferably 80% by volume or less from the viewpoint of strength, and is preferably 20% by volume or more from the viewpoint of discharge performance. Here, "porosity" refers to a volume-based value measured with a mercury porosimeter.

[0060] The separator may be a polymer gel composed of a polymer and a non-aqueous electrolyte. Examples of polymers include polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, polyvinylpyrrolidone, and polyvinylidene fluoride. The use of a polymer gel has the effect of suppressing leakage. The separator may be a combination of the porous resin film or nonwoven fabric described above and a polymer gel.

[0061] (non-aqueous electrolyte) The nonaqueous electrolyte can be appropriately selected from known nonaqueous electrolytes. The nonaqueous electrolyte may be a nonaqueous electrolytic solution. The nonaqueous electrolytic solution contains a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent.

[0062] The non-aqueous solvent can be appropriately selected from known non-aqueous solvents. Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, carboxylic acid esters, phosphate esters, sulfonic acid esters, ethers, amides, and nitriles. Non-aqueous solvents in which some of the hydrogen atoms contained in these compounds have been substituted with halogens may also be used.

[0063] Examples of cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), vinylethylene carbonate (VEC), chloroethylene carbonate, fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), styrene carbonate, 1-phenylvinylene carbonate, 1,2-diphenylvinylene carbonate, etc. Among these, EC is preferred.

[0064] Examples of the chain carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diphenyl carbonate, trifluoroethyl methyl carbonate, bis(trifluoroethyl) carbonate, etc. Among these, EMC is preferred.

[0065] It is preferable to use a cyclic carbonate or a chain carbonate as the non-aqueous solvent, and it is more preferable to use a combination of a cyclic carbonate and a chain carbonate. The use of a cyclic carbonate can promote dissociation of the electrolyte salt and improve the ionic conductivity of the non-aqueous electrolyte. The use of a chain carbonate can reduce the viscosity of the non-aqueous electrolyte. When a cyclic carbonate and a chain carbonate are used in combination, the volume ratio of the cyclic carbonate to the chain carbonate (cyclic carbonate:chain carbonate) is preferably in the range of, for example, 5:95 to 50:50.

[0066] Lithium salts are used as the electrolyte salt. Examples of lithium salts include inorganic lithium salts such as LiPF6, LiPO2F2, LiBF4, LiClO4, and LiN(SO2F)2; lithium oxalate salts such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalateborate (LiFOB), and lithium bis(oxalate)difluorophosphate (LiFOP); and lithium salts having a halogenated hydrocarbon group such as LiSO3CF3, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(SO2CF3)(SO2C4F9), LiC(SO2CF3)3, and LiC(SO2C2F5)3. Among these, inorganic lithium salts are preferred, and LiPF6 is more preferred.

[0067] The content of electrolyte salt in the non-aqueous electrolyte is 0.1 mol / dm at 20°C and 1 atmosphere. 3 More than 2.5mol / dm 3 It is preferable that the value is 0.3 mol / dm or less. 3 More than 2.0mol / dm 3 It is more preferable that it is 0.5 mol / dm or less. 3 More than 1.7mol / dm 3 It is more preferable that it is 0.7 mol / dm or less. 3 More than 1.5mol / dm 3 By setting the content of the electrolyte salt within the above range, the ionic conductivity of the non-aqueous electrolyte can be increased.

[0068] The non-aqueous electrolyte may contain additives in addition to the non-aqueous solvent and the electrolyte salt. Examples of the additives include aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; partial halides of the aromatic compounds such as 2-fluorobiphenyl, o-cyclohexylfluorobenzene, and p-cyclohexylfluorobenzene; halogenated anisole compounds such as 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole, and 3,5-difluoroanisole; vinylene carbonate, methylvinylene carbonate, ethylvinylene carbonate, succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, and cyclohexanedicarboxylic anhydride; and ethylene sulfite. Examples of the additives include propylene sulfite, dimethyl sulfite, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, ethylene sulfate, sulfolane, dimethyl sulfone, diethyl sulfone, dimethyl sulfoxide, diethyl sulfoxide, tetramethylene sulfoxide, diphenyl sulfide, 4,4'-bis(2,2-dioxo-1,3,2-dioxathiolane), 4-methylsulfonyloxymethyl-2,2-dioxo-1,3,2-dioxathiolane, thioanisole, diphenyl disulfide, dipyridinium disulfide, 1,3-propene sultone, 1,3-propane sultone, 1,4-butane sultone, 1,4-butene sultone, perfluorooctane, tristrimethylsilyl borate, tristrimethylsilyl phosphate, and tetrakistrimethylsilyl titanate. These additives may be used alone or in combination of two or more.

[0069] The content of the additive contained in the non-aqueous electrolyte is preferably 0.01% by mass to 10% by mass, more preferably 0.1% by mass to 7% by mass, even more preferably 0.2% by mass to 5% by mass, and particularly preferably 0.3% by mass to 3% by mass. By setting the content of the additive within the above range, it is possible to improve the capacity retention performance or cycle performance after high-temperature storage, and further improve safety.

[0070] The non-aqueous electrolyte may be a solid electrolyte, or a non-aqueous electrolytic solution and a solid electrolyte may be used in combination.

[0071] The solid electrolyte can be selected from any material that has lithium ion conductivity and is solid at room temperature (e.g., 15°C to 25°C). Examples of the solid electrolyte include sulfide solid electrolytes, oxide solid electrolytes, oxynitride solid electrolytes, and polymer solid electrolytes. Examples of the sulfide solid electrolyte include Li2S-P2S5, LiI-Li2S-P2S5, and Li 10 Ge-P2S 12 etc.

[0072] The shape of the nonaqueous electrolyte storage element of this embodiment is not particularly limited, and examples thereof include cylindrical batteries, prismatic batteries, flat batteries, coin batteries, and button batteries.

[0073] FIG. 1 shows a nonaqueous electrolyte storage element 1 as an example of a prismatic battery. The figure is a see-through view of the inside of the container. An electrode assembly 2 having a positive electrode and a negative electrode wound with a separator sandwiched between them is housed in a prismatic container 3. The positive electrode is electrically connected to a positive electrode terminal 4 via a positive electrode lead 41. The negative electrode is electrically connected to a negative electrode terminal 5 via a negative electrode lead 51.

[0074] <Electricity storage device> The nonaqueous electrolyte energy storage element of this embodiment can be mounted as an energy storage unit (battery module) comprising a plurality of nonaqueous electrolyte energy storage elements 1 in an automobile power source such as an electric vehicle (EV), a hybrid electric vehicle (HEV), or a plug-in hybrid electric vehicle (PHEV), a power source for electronic devices such as a personal computer or a communication terminal, or a power storage power source, etc. In this case, the technology of the present invention may be applied to at least one nonaqueous electrolyte energy storage element included in the energy storage unit.

[0075] An electricity storage device according to one embodiment of the present invention includes one or more nonaqueous electrolyte electricity storage elements according to one embodiment of the present invention. Fig. 2 shows an example of an electricity storage device 30 in which electricity storage units 20, each of which is an assembly of two or more electrically connected nonaqueous electrolyte electricity storage elements 1, are further assembled. The electricity storage device 30 may include a bus bar (not shown) that electrically connects two or more nonaqueous electrolyte electricity storage elements 1, a bus bar (not shown) that electrically connects two or more electricity storage units 20, and the like. The electricity storage unit 20 or the electricity storage device 30 may include a status monitoring device (not shown) that monitors the status of one or more nonaqueous electrolyte electricity storage elements.

[0076] <Method of manufacturing nonaqueous electrolyte energy storage element> The method for manufacturing the nonaqueous electrolyte storage element of this embodiment can be appropriately selected from known methods. The manufacturing method includes, for example, preparing an electrode assembly, preparing a nonaqueous electrolyte, and housing the electrode assembly and the nonaqueous electrolyte in a container. Preparing the electrode assembly includes preparing a positive electrode and a negative electrode, and forming the electrode assembly by stacking or winding the positive electrode and the negative electrode with a separator interposed therebetween.

[0077] The method for placing the non-aqueous electrolyte in the container can be appropriately selected from known methods. For example, when a non-aqueous electrolyte solution is used as the non-aqueous electrolyte, the non-aqueous electrolyte solution may be poured into the container through an inlet formed in the container, and then the inlet may be sealed.

[0078] <Other embodiments> The nonaqueous electrolyte storage element of the present invention is not limited to the above-described embodiments, and various modifications may be made without departing from the spirit of the present invention. For example, the configuration of one embodiment may be added to the configuration of another embodiment, or part of the configuration of one embodiment may be replaced with the configuration of another embodiment or well-known technology. Furthermore, part of the configuration of one embodiment may be deleted. Also, well-known technology may be added to the configuration of one embodiment.

[0079] In the above embodiment, the nonaqueous electrolyte storage element is used as a chargeable and dischargeable nonaqueous electrolyte secondary battery (lithium ion secondary battery), but the nonaqueous electrolyte storage element may be of any type, shape, size, capacity, etc. The present invention can also be applied to various secondary batteries, electric double layer capacitors, lithium ion capacitors, and other capacitors.

[0080] In the above embodiment, the electrode assembly in which the positive electrode and the negative electrode are stacked with a separator interposed therebetween has been described, but the electrode assembly may not include a separator. For example, the positive electrode and the negative electrode may be in direct contact with each other in a state in which a non-conductive layer is formed on the active material layer of the positive electrode or the negative electrode. [Example]

[0081] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0082] [Example 1] (Preparation of positive electrode) Positive electrode active material LiNi 0.45 Co 0.35 Mn 0.20 A positive electrode mixture paste was prepared using O2, acetylene black (AB) as a conductive agent, polyvinylidene fluoride (PVDF) as a binder, and N-methylpyrrolidone (NMP) as a dispersion medium. The mass ratio of the positive electrode active material, AB, and PVDF was 93:3.5:3.5 (solid content equivalent). The positive electrode mixture paste was applied to both sides of aluminum foil as a positive electrode substrate and dried. Then, a roll press was performed to obtain a positive electrode.

[0083] (Preparation of negative electrode) A negative electrode mixture paste was prepared using graphite as the negative electrode active material, styrene-butadiene rubber (SBR) as the binder, carboxymethyl cellulose (CMC) as the thickener, and water as the dispersion medium. The mass ratio of graphite, SBR, and CMC was 97.5:1.5:1 (solid content equivalent). The negative electrode mixture paste was applied to both sides of copper foil as the negative electrode substrate and dried. Then, a roll press was performed to obtain a negative electrode. The ratio of the mass of graphite per unit area in the negative electrode active material layer to the mass of positive electrode active material per unit area in the positive electrode active material layer was adjusted so that the depth of charge of the graphite in the charged state was 0.52.

[0084] (Non-aqueous electrolyte) A 1.2 mol / dm 3 LiPF6 was dissolved in the solution at a concentration of 1000 to obtain a non-aqueous electrolyte solution.

[0085] (separator) The separator was a microporous polyolefin film.

[0086] (Assembly of non-aqueous electrolyte energy storage element) The positive electrode, negative electrode, and separator were used to obtain a wound electrode assembly, which was then housed in a container, filled with a non-aqueous electrolyte solution, and sealed to obtain a non-aqueous electrolyte storage element (secondary battery) of Example 1.

[0087] [Examples 2 to 3 and Comparative Examples 1 to 6] The nonaqueous electrolyte storage elements of Examples 2 and 3 and Comparative Examples 1 to 6 were obtained in the same manner as in Example 1, except that the positive electrode active material shown in Table 1 was used and the ratio of the mass of graphite per unit area in the negative electrode active material layer to the mass of the positive electrode active material per unit area in the positive electrode active material layer was adjusted so that the depth of charge of the graphite in the charged state would be the value shown in Table 1.

[0088] [evaluation] (1) Initial discharge capacity confirmation test An initial discharge capacity confirmation test was performed on each of the obtained nonaqueous electrolyte storage elements under the following conditions. In a thermostatic chamber at 25°C, constant current charging was performed with a charging current of 1.0 C and a charge cut-off voltage of 4.10 V, followed by constant voltage charging at 4.10 V. The charge was terminated after a total charge time of 3 hours. A 10-minute rest period was then provided. Constant current discharge was performed with a discharge current of 1.0 C and a discharge cut-off voltage of 2.50 V. The initial discharge capacity was measured. (2) Capacity retention rate after 2000 hours of charge / discharge cycles Next, the following charge-discharge cycle test was conducted. In a thermostatic chamber at 60°C, the nonaqueous electrolyte storage element was charged at a constant current of 4.0C to a quantity of electricity that was 85% of the initial discharge capacity, adjusting the SOC to 85%. Thereafter, without a rest period, the nonaqueous electrolyte storage element was discharged at a constant current of 4.0C to a quantity of electricity that was 70% of the initial discharge capacity, adjusting the SOC to 15%. Subsequently, the nonaqueous electrolyte storage element was charged at a constant current of 4.0C to a quantity of electricity that was 70% of the initial discharge capacity, adjusting the SOC to 85%. Thereafter, without a rest period, the nonaqueous electrolyte storage element was discharged at a constant current of 4.0C to a quantity of electricity that was 70% of the initial discharge capacity, adjusting the SOC to 15%. This charge-discharge cycle was conducted for 2000 hours. A discharge capacity confirmation test was then conducted using the same method as in (1) above. The discharge capacity after 2000 hours of charge-discharge cycles was measured. The percentage of the discharge capacity after 2000 hours of charge-discharge cycles to the initial discharge capacity was calculated to be the capacity retention rate (%) after 2000 hours of charge-discharge cycles. The results are shown in Table 1.

[0089] [Table 1]

[0090] As shown in Table 1, in each of the nonaqueous electrolyte storage elements of Examples 1 to 3, in which the positive electrode active material was a lithium transition metal composite oxide in which the ratio of manganese to the metal elements excluding lithium was 20 mol % or less and the negative electrode active material was graphite, the depth of charge in the charged state was less than 0.54, the capacity retention rate after 2000 hours of charge-discharge cycling exceeded 80%. As described above, the nonaqueous electrolyte storage element was shown to have a high capacity retention rate after charge-discharge cycling when it had a positive electrode containing a lithium transition metal composite oxide containing manganese and a negative electrode containing graphite, and when the ratio of manganese to the metal elements excluding lithium in the lithium transition metal composite oxide was 20 mol % or less and the depth of charge of the graphite in the charged state was less than 0.54. [Industrial Applicability]

[0091] The present invention can be applied to nonaqueous electrolyte electricity storage elements and electricity storage devices used as power sources for electronic devices such as personal computers and communication terminals, automobiles, and the like. [Explanation of symbols]

[0092] 1. Non-aqueous electrolyte energy storage element 2 Electrode body 3 containers 4 Positive terminal 41 Positive lead 5 Negative terminal 51 Negative lead 20 Energy storage unit 30 Energy storage device

Claims

1. a positive electrode having a lithium transition metal composite oxide containing manganese; a negative electrode having graphite; Equipped with the ratio of manganese to metal elements other than lithium in the lithium transition metal composite oxide is 20 mol % or less; The nonaqueous electrolyte electricity storage element has a depth of charge of the graphite in a charged state of less than 0.

54.

2. 2. The nonaqueous electrolyte electricity storage element according to claim 1, wherein the lithium transition metal composite oxide further contains nickel and cobalt.

3. 3. The nonaqueous electrolyte storage element according to claim 1, wherein the ratio of manganese to the metal elements other than lithium in said lithium transition metal composite oxide is 1 mol % or more.

4. 4. The nonaqueous electrolyte storage element according to claim 1, wherein the graphite has a depth of charge of 0.40 or more in a charged state.

5. An electricity storage device comprising one or more nonaqueous electrolyte electricity storage elements according to claim 1 .

Images