Lithium ion secondary battery and manufacturing method

Abstract

Provided is a lithium ion secondary battery having a high capacity retention rate at a high discharge rate. This lithium ion secondary battery comprises a positive electrode, a negative electrode, and an electrolyte between the positive electrode and the negative electrode. The positive electrode includes lithium vanadium phosphate having an average primary particle diameter of less than 500 nm as a positive electrode active material, and has an electrode density of 1.5 g / cc or less. The negative electrode is pre-doped with lithium ions and has a utilization rate of less than 80%.

Description

Lithium-ion secondary battery and manufacturing method

[0001] This invention relates to lithium-ion secondary batteries and methods for manufacturing them.

[0002] A lithium-ion secondary battery consists of a positive electrode and a negative electrode facing each other in an electrolyte with a separator in between, and charges and discharges according to the direction of absorption and release of lithium ions at the positive and negative electrodes. In a lithium-ion secondary battery, during charging, lithium ions are released from the positive electrode and absorbed into the negative electrode, and during discharge, lithium ions are released from the negative electrode and absorbed into the positive electrode.

[0003] This lithium-ion secondary battery comprises a positive electrode and a negative electrode containing an active material that reversibly absorbs and releases lithium ions, and an electrolyte solution containing a dissolved lithium salt. The positive electrode and the negative electrode are each formed by integrating layers of the active material with a current collector. The positive electrode active material and the current collector, and the negative electrode active material and the current collector, are joined together via a binder using crimping or a doctor blade method, etc.

[0004] For example, lithium-ion secondary batteries typically use lithium nickelate or lithium cobaltate as the positive electrode active material, graphite as the negative electrode active material, and an electrolyte solution in which lithium hexafluoride phosphate is dissolved in a non-aqueous solvent (see, for example, Patent Document 1).

[0005] As a positive electrode active material with higher thermal stability and superior safety, lithium iron phosphate, which has a PO bond as a partial structure in its molecule, is known (see, for example, Patent Document 2). However, the operating voltage of lithium iron phosphate is Li / Li ＋ The voltage is between 3.3V and 3.4V compared to the standard, and batteries using lithium iron phosphate have insufficient energy density and power density.

[0006] In recent years, the development of lithium-ion secondary batteries has become more active due to the rapid spread of digital cameras, smartphones, and portable PCs, rising fuel prices, increased awareness of environmental impact, and expectations for applications in automotive powertrains or smart grid energy storage. In particular, the development of electric vehicles (EVs) and hybrid electric vehicles (HEVs) that use electric motors to assist in part of the drivetrain is accelerating among automobile manufacturers, and these automotive applications require lithium-ion secondary batteries that are not only highly thermally stable but also have high output as their power source.

[0007] Therefore, lithium vanadium phosphate (Li), which has a PO bond like lithium iron phosphate, has high thermal stability. ３ V ２ (PO ４ ) ３ ) is attracting attention (see, for example, Patent Document 3). The operating potential of this lithium vanadium phosphate is Li / Li ＋ The voltage is 3.8V to 4.8V relative to the standard. Furthermore, lithium vanadium phosphate has a Na Super Ionic Conductor (NAS) structure. In addition, lithium vanadium phosphate has a three-dimensional diffusible crystal structure with (V-O-P-O)n bonds. Therefore, when lithium vanadium phosphate is used as the positive electrode active material, high-speed charging and discharging is possible. For this reason, lithium vanadium phosphate is expected to have a higher energy density and higher output than lithium iron phosphate.

[0008] Japanese Patent Publication No. 2011-204571, International Publication No. 2010 / 103821, Japanese Patent Publication No. 2012-36050

[0009] A. Pan et al., “Nano-structured Li3V2(PO4)3 / carbon composite for high-rate lithium-ion batteries”, Electrochemistry Communications, 12, 1674-1677 (2010)

[0010] Lithium vanadium phosphate exhibits good discharge capacity at a discharge rate of less than 80C (for example, Non-Patent Document 1), but when the discharge rate reaches a high discharge rate such as 200C for further higher output, the discharge capacity drops sharply. That is, the capacity retention rate at a high discharge rate such as 200C is low.

[0011] The present invention was proposed to solve the above problems, and its object is to provide a lithium ion secondary battery and a manufacturing method having a high capacity retention rate at a high discharge rate.

[0012] In order to achieve the above object, the lithium ion secondary battery of the embodiment includes lithium vanadium phosphate having an average primary particle diameter of less than 500 nm as a positive electrode active material, a positive electrode having an electrode density of 1.5 g / cc or less, a negative electrode in which lithium ions are pre-doped and the utilization rate is less than 80%, and an electrolyte between the positive electrode and the negative electrode.

[0013] Thereby, the lithium ion secondary battery has a high capacity retention rate at a high discharge rate. That is, even when outputting at a high rate, the dischargeable capacity can be maintained high.

[0014] The electrode density of the positive electrode may be 1.2 g / cc or more and 1.4 g / cc or less.

[0015] The utilization rate may be 20% or more and 50% or less.

[0016] The average primary particle diameter of the lithium vanadium phosphate may be 200 nm or less.

[0017] The average primary particle diameter of the lithium vanadium phosphate may be 200 nm or less, the utilization rate may be 20% or more and 40% or less, and the electrode density of the positive electrode may be 1.2 g / cc or more and 1.3 g / cc or less.

[0018] The lithium vanadium phosphate is Li ３ V ２－ｘ M ｘ (PO ４ ) ３The expression is represented as follows, where M is Mg, Al, Ca, Ti, Cr, Mn, Fe, Co, Ni, Zn, Y, Zr, Nb, Sn, or a plurality of these metallic elements, and x, which is the atomic ratio of M, may be set to 0 ≤ x ≤ 0.2.

[0019] The active material of the negative electrode may be graphite, silicon, or silicon oxide.

[0020] To achieve the above objectives, the manufacturing method of the lithium-ion secondary battery of the embodiment is a manufacturing method of a lithium-ion secondary battery, comprising: a rolling step of rolling lithium vanadium phosphate having an average primary particle diameter of less than 500 nm as a positive electrode active material onto a current collector to an electrode density of 1.5 g / cc or less; a negative electrode forming step of forming a negative electrode active material layer with a utilization rate of less than 80%; and a pre-doping step of pre-doping lithium ions into the negative electrode active material.

[0021] According to the present invention, the capacity retention rate of lithium-ion secondary batteries at 200C is increased.

[0022] The following describes a lithium-ion secondary battery and a manufacturing method according to an embodiment of the present invention. However, the present invention is not limited to the embodiments described below.

[0023] (Lithium-ion secondary battery) This lithium-ion secondary battery comprises a positive electrode and a negative electrode arranged opposite each other, and an electrolyte interposed between the positive and negative electrodes. A separator is provided between the positive and negative electrodes. The separator isolates the positive and negative electrodes to prevent short circuits. The separator may be one that is permeable to the electrolyte. The positive and negative electrodes are Faraday reaction electrodes in which lithium ions are reversibly inserted and removed. The positive and negative electrodes are each formed by integrating layers of active material with a current collector. The positive electrode active material and current collector, and the negative electrode active material and current collector are each joined via a binder using crimping or a doctor blade method.

[0024] (Positive electrode) The positive electrode active material is a granule of lithium vanadium phosphate. Lithium vanadium phosphate has the general formula Li ３ V ２－ｘ M ｘ (PO４ ) ３ It has a NASICON structure represented by . Some of the vanadium may be substituted with a metallic element. In the composition formula, M is Mg, Al, Ca, Ti, Cr, Mn, Fe, Co, Ni, Zn, Y, Zr, Nb, Sn, or several of these metallic elements, and x, which is the atomic ratio of the metallic element M, is 0 ≤ x ≤ 0.2. This lithium vanadium phosphate is typically Li ３ V ２ (PO ４ ) ３ That is the case.

[0025] Preferably, lithium vanadium phosphate has the monoclinic structure, which is the most stable structure. ６ Octahedron and PO ４ The tetrahedron shares the vertices of the oxygen atoms. This VO ６ The octahedron is slightly distorted from the rhombohedral structure, which is a nassicon structure.

[0026] Lithium vanadium phosphate in a monoclinic structure forms a three-dimensional network (V-O-P-O)n within the crystal. Three types of lithium exist within the crystal, each with different sites and different strains. ４ It forms a tetrahedron. LiO ４ The tetrahedrons extend in a chain-like manner along the a-axis, sharing oxygen atoms at their vertices. Open diffusion paths exist along the b-axis and c-axis. Therefore, lithium vanadium phosphate in the monoclinic structure exhibits high ion diffusion properties, similar to the rhombohedral structure.

[0027] The granulated material consists of secondary particles formed by the aggregation of primary particles of lithium vanadium phosphate to create a three-dimensional network structure. The average particle diameter of the primary particles is less than 500 nm. That is, this lithium-ion secondary battery has a positive electrode containing lithium vanadium phosphate as the positive electrode active material, with an average primary particle diameter of less than 500 nm. Preferably, the average particle diameter of the primary particles is 200 nm or less.

[0028] This lithium vanadium phosphate may be mixed with a carbon material that acts as a conductive additive to improve the electrical conductivity of the positive electrode active material. Suitable carbon materials include carbon nanotubes and conductive carbon black having a hollow shell structure (e.g., Ketjenblack®), but carbon nanofibers, carbon black such as acetylene black, amorphous carbon, carbon fibers, natural graphite, artificial graphite, activated carbon, mesoporous carbon, nanoporous carbon, graphene, fullerene, or mixtures thereof are also applicable.

[0029] The average particle size of the primary particles is not limited to this, but can be adjusted by the following method for producing lithium vanadium phosphate. This method is based on the polymerization complex method or the Pecini method. Citric acid and ethylene glycol or polyalcohol contribute to the production of a precursor of lithium vanadium phosphate through the esterification reaction of a chelate compound of vanadium ions and citric acid with ethylene glycol or polyalcohol, and also serve as a material source for the carbon material that is compounded as a conductive additive.

[0030] However, precursors of lithium vanadium phosphate may also be formed by other reactions in the aqueous phase, such as hydrolysis, dehydration condensation, oxidation, and polymerization, and the vanadium and phosphate sources should be selected according to the desired reaction. Typically, the precursor of lithium vanadium phosphate is vanadium phosphate. On the other hand, in the case of the solid-phase method, the average particle size of the primary particles tends to exceed 500 nm, so reactions in the aqueous phase are preferably used.

[0031] Specifically, lithium vanadium phosphate is manufactured by following these steps in this order: an addition step of adding a lithium vanadium phosphate material source; a mixing step of mixing the treatment solution to which the material source has been added; a drying step of concentrating the treatment solution or removing the solvent; a heating step of heating the treated material in an oxidizing atmosphere such as an atmospheric atmosphere; and a firing step of firing the treated material in an inert atmosphere.

[0032] In the addition step, a treatment solution is prepared in which the material source of lithium vanadium phosphate is uniformly dispersed. The lithium source and phosphate source are added to the vanadium source while stirring. If a carbon material is to be combined, the carbon material source is also added. If part of the vanadium is to be replaced with a metal element, the metal source of the metal element is also added. The solvent of the treatment solution is water, and each material source is mixed in the aqueous phase. The aqueous phase also contains a mixture of water and alcohol. Methanol, ethanol, and isopropyl alcohol can be suitably used as the alcohol.

[0033] When employing hydrolysis and complex formation reactions, vanadium acetate, sulfate, nitrate, halogen compounds, or chelating agents can be used as the vanadium source in the addition step. Specifically, as the vanadium source, NH ４ VO ３ , V ２ O ５ , V ２ O ３ , metallic vanadium, V ２ O ４ Examples include vanadium(III) acetylacetonate and vanadium(IV) oxyacetylacetonate. As a source of phosphoric acid, H ３ PO ４ NH ４ H ２ PO ４ , (NH ４ ) ２ HPO ４ , P ２ O ５ and Li ３ PO ４ etc PO ４ Examples of contained compounds include:

[0034] As a lithium source, CH ３ COOLi, LiNO ３ Li ２ CO ３ , LiOH, LiOH・H ２ O, LiCl, Li ２ SO ４ and LiC ３ H ５ O ３The vanadium source, lithium source, and phosphate source should be added in proportions according to the stoichiometric ratio of lithium vanadium phosphate. For example, Li ３ V ２ (PO ４ ) ３ If so, Li, V, and PO ４ The molar ratio should be 3:2:3.

[0035] The carbon material source is a material source that carbonizes into a carbon material during the calcination process. Examples of carbon material sources include organic substances such as polycarboxylic acids, polyhydric alcohols, polymers such as polyvinylpyrrolidone, sugars (such as glucose), and amino acids (such as glutamic acid). Polycarboxylic acids are compounds having two or more carboxyl groups in their molecular structure, such as citric acid, malonic acid, malic acid, and azelaic acid. Examples of polyhydric alcohols include ethylene glycol, diethylene glycol, trimethylene glycol, glycerin, polyvinyl alcohol, and polyethylene glycol. The metal source is Mg, Al, Ca, Ti, Cr, Mn, Fe, Co, Ni, Zn, Y, Zr, Nb, Sn, or multiple types of metal salts, metal oxides, or metal phosphate compounds.

[0036] The following phenomena are promoted during the mixing process: the formation of the lithium vanadium phosphate precursor is promoted; the mixing of the lithium vanadium phosphate precursor with the metal salt or metal oxide is promoted; the adhesion of the carbon material source to the precursor is promoted; and the lithium vanadium phosphate precursor is miniaturized to the nanoparticle level.

[0037] In this mixing process, homogenizers, lithographs, ball mills, bead mills, rod mills, roller mills, agitator mills, planetary mills, hybridizers, and jet mills can be used. Alternatively, mechanochemical treatment may be performed instead of mixing with a homogenizer or the like.

[0038] After the mixing step, the process moves to the drying step. In the drying step, the solvent is evaporated to concentrate the treatment solution, and the carbon coating source attached to the lithium vanadium phosphate precursor is precipitated. The drying method is not particularly limited. Examples of drying methods include high-temperature drying, where the treatment solution is left overnight in an atmosphere of 80 degrees Celsius; reduced-pressure drying using a rotary evaporator; or spray drying, where the solvent is volatilized by spraying the slurry into hot air. A fluidized bed dryer may be used to perform the mixing and drying steps in parallel. A fluidized bed dryer can dry the mixture while stirring and mixing each material source in the aqueous phase.

[0039] After the drying process, the material undergoes a heating process before moving on to the firing process. During the heating process, the material is exposed to air at 100-400°C for 3-5 hours. This heating process removes impurities such as carbon other than the carbon material source. Furthermore, this heating process promotes the dehydration condensation reaction of the carbon material source and the formation of precursors for lithium vanadium phosphate. Therefore, the primary particle size of lithium vanadium phosphate can be adjusted by controlling the heating temperature during the heating process. Note that the heating process is not limited to air; it can be performed within the range of 100-400°C.

[0040] During the firing process, lithium is incorporated into the precursor of lithium vanadium phosphate, producing lithium vanadium phosphate, and the crystallization of lithium vanadium phosphate proceeds. In addition, carbon material sources attached to the precursor are carbonized, and carbon material is generated on the surface of the primary particles of lithium vanadium phosphate. The carbon material covers part or all of the surface of the primary particles, suppressing the bonding between lithium vanadium phosphate particles during the firing process and inhibiting the crystal growth of lithium vanadium phosphate, thus maintaining the fine particle size of the primary particles.

[0041] In this firing process, firing is carried out at 600 to 950°C for 5 to 120 minutes under a non-oxidizing atmosphere such as a nitrogen atmosphere. Non-oxidizing atmospheres include, for example, a low-oxygen atmosphere with an oxygen concentration of about 1000 ppm, as well as inert gas atmospheres and reducing gas atmospheres. Inert gases include noble gases such as Ar and N ２ Examples include H ２ These are some examples.

[0042] By performing the firing process in a non-oxidizing atmosphere and at a temperature range of 600 to 950°C, the burning of carbon material can be suppressed. In an oxidizing atmosphere, carbon material burns away, and lithium vanadium phosphate tends to aggregate. A heat treatment temperature below 600°C is undesirable because the formation of lithium vanadium phosphate is insufficient, and a heat treatment temperature exceeding 950°C is also undesirable because lithium vanadium phosphate aggregates and undergoes structural changes.

[0043] When using carbon material sources involving dehydration condensation reactions such as citric acid and ethylene glycol, the addition of a metal salt or metal oxide during the additive process can form a dense wall that separates the reactions between lithium vanadium phosphate precursors. Through this manufacturing method, the primary particles of lithium vanadium phosphate are reduced in diameter because their growth through melting is inhibited by the carbon material.

[0044] In the manufacturing process of lithium vanadium phosphate granules, from the addition step to the calcination step, it is preferable to add a carbon material source that carbonizes into carbon material during the manufacturing process, but it is preferable not to add the carbon material itself. The carbon material refers to a fibrous carbon material or a particulate carbon material such as a spherical or flaky carbon material, and is not a material source that carbonizes in the manufacturing process of lithium vanadium phosphate. When a carbon material is added, it becomes difficult for the carbon material source to precipitate on the precursor of lithium vanadium phosphate, and the primary particles of lithium vanadium phosphate tend to become coarse.

[0045] Current collectors, which use lithium vanadium phosphate as the positive electrode active material, are typically made of conductive materials such as aluminum, copper, iron, nickel, titanium, steel, and carbon. Aluminum is particularly preferred due to its high thermal and electronic conductivity. The shape of the current collector can be any shape, such as film, foil, plate, mesh, expanded metal, or cylindrical. Current collectors with through holes may also be used.

[0046] A slurry of lithium vanadium phosphate and a binder is applied to this current collector using a doctor blade method or the like, and then dried to form a positive electrode in which layers of lithium vanadium phosphate and the current collector are laminated. Examples of binders include rubbers such as fluororubber, diene rubber, and styrene rubber; fluorine-containing polymers such as polytetrafluoroethylene and polyvinylidene fluoride; celluloses such as carboxymethylcellulose and nitrocellulose; and other materials such as polyolefin resins, polyimide resins, acrylic resins, nitrile resins, polyester resins, phenolic resins, polyvinyl acetate resins, polyvinyl alcohol resins, and epoxy resins. These binders may be used individually or mixed in groups of two or more.

[0047] The electrode density of the positive electrode, i.e., the density of the positive electrode active material layer laminated on the current collector, shall be 1.5 g / cc or less. Preferably, the electrode density of the positive electrode is 1.2 g / cc or more and 1.4 g / cc or less. Particularly preferably, the electrode density of the positive electrode is 1.3 g / cc or less. The electrode density can be controlled in a rolling process in which, after coating the current collector with slurry and drying it, the current collector and the positive electrode active material layer are sandwiched and compressed by press working. Specifically, for example, a roll press machine can be used for press working. The gap between the rolls of the roll press machine can be adjusted to apply pressure so that the electrode density of the positive electrode is 1.5 g / cc or less.

[0048] (Negative electrode) As negative electrode active material, FeO, Fe ２ O ３ Fe ３ O ４ MnO, MnO ２ Mn ２ O ３ Mn ３ O ４ CoO, Co ３ O ４ NiO, Ni ２ O ３ ,TiO,TiO ２ , TiO ２ (B), CuO, NiO, SnO, SnO ２ SiO, SiO ２ RuO ２ WO, WO２ , oxides such as WO3, MoO ３ , metals such as Sn, Si, Al, Zn, LiVO ２ , Li ３ VO ４ , Li ４ Ti ５ O １２ , Sc ２ TiO ５ , Fe ２ TiO ５ and other complex oxides, Li ２．６ Co ０．４ N, Ge ３ N ４ , Zn ３ N ２ , Cu ３ N and other nitrides, Y ２ Ti ２ O ５ S ２ , MoS ２ and the like. Among them, lithium titanate (Li ４ Ti ５ O １２ ) is preferred. [[ID= 54]]

[0049] Further, examples of the negative electrode active material include carbon materials. For example, non-graphitizable carbon, artificial graphite, natural graphite, pyrolytic carbons, pitch cokes, needle cokes, petroleum cokes and other cokes, graphites, glassy carbons, organic polymer compound fired bodies obtained by firing phenolic resins, furan resins, etc. at an appropriate temperature and carbonizing them can be used. In addition, negative electrode active materials such as Si and SiO may be combined with the carbon material and used.

[0050] The negative electrode active material may be laminated on the current collector. The current collector for laminating the negative electrode active material is typically a conductive material such as aluminum, copper, iron, nickel, titanium, steel, carbon, etc. In particular, copper having high thermal conductivity and electron conductivity is preferred. The shape of the current collector can adopt any shape such as film shape, foil shape, plate shape, net shape, expanded metal shape, cylindrical shape, etc. Further, a current collector having a through hole may be used.

[0051] A slurry of a negative electrode active material and a binder is applied to this current collector using a doctor blade method or the like, and then dried to form a negative electrode in which a layer of negative electrode active material and a current collector are laminated. Examples of binders include rubbers such as fluororubber, diene rubber, and styrene rubber; fluorine-containing polymers such as polytetrafluoroethylene and polyvinylidene fluoride; celluloses such as carboxymethylcellulose and nitrocellulose; and other materials such as polyolefin resins, polyimide resins, acrylic resins, nitrile resins, polyester resins, phenolic resins, polyvinyl acetate resins, polyvinyl alcohol resins, and epoxy resins. These binders may be used individually or mixed in groups of two or more.

[0052] The negative electrode active material is pre-doped with lithium ions. For example, an electrical short-circuit method is used in the pre-doping process. In the electrical short-circuit method, a battery element is assembled with the electrodes of the negative electrode active material layer and the lithium metal layer facing each other with a separator in between. This battery element is immersed in an electrolyte solution, and the negative electrode active material layer and the lithium metal layer are externally short-circuited. This external short-circuiting charges the negative electrode active material layer, thereby doping the negative electrode active material with lithium ions. The lithium ions are doped to a charge level of approximately 50% relative to the negative electrode active material. Note that if graphite is used, this pre-doping is Li / Li ＋ It is sufficient for the negative electrode active material to be doped with enough lithium ions to operate in a plateau region of 0.3V or less. Then, by disassembling the battery element, a negative electrode active material pre-doped with lithium ions can be obtained. Alternatively, a method can be used in which lithium is brought into contact with the negative electrode to short-circuit and pre-dope the negative electrode active material with lithium ions.

[0053] Lithium ions are 4.3V (vs. Li / Li) in lithium vanadium phosphate. ＋ ) to 3.0V (vs. Li / Li ＋ ) Capacitance in the high potential range and 3.0V (vs. Li / Li ＋ ) less than 1.2V (vs. Li / Li ＋The lithium vanadium phosphate may be pre-doped such that when the total capacity, including the capacity in the low potential range, is discharged, it has a potential of 1.2 V or less, which is the lower limit of the chargeable and dischargeable potential of lithium vanadium phosphate.

[0054] The utilization rate of the negative electrode active material is set to less than 80%. Numerically, the utilization rate of the negative electrode active material is expressed as a percentage of the reciprocal of the discharge capacity multiplier. For example, the capacity when the charged negative electrode is fully discharged may be taken as the maximum capacity, and the utilization rate of the negative electrode may be set by changing the charging capacity based on the maximum capacity. In other words, the amount of negative electrode active material in the negative electrode and the positive electrode active material in the positive electrode are adjusted so that the 100% discharge capacity of the negative electrode active material is more than 1.25 times the 100% discharge capacity of the positive electrode active material. Preferably, the utilization rate of the negative electrode active material is set to 20% or more and 50% or less. Particularly preferably, the utilization rate of the negative electrode active material is 40% or less. That is, the amount of negative electrode active material in the negative electrode and the positive electrode active material in the positive electrode are adjusted so that the 100% discharge capacity of the negative electrode active material is more than 2 times and 5.0 times or less the 100% discharge capacity of the positive electrode active material.

[0055] As a method for adjusting the amounts of positive electrode active material and negative electrode active material, during the negative electrode active material layer formation process using the doctor blade method or the like, the 100% discharge capacity per unit mass of the negative electrode active material layer, which consists of metal compound particles having a three-dimensional network structure, and the 100% discharge capacity per unit mass of the positive electrode active material layer should be measured, and the masses of the layer of metal compound particles having a three-dimensional network structure and the layer of positive electrode active material should be adjusted so that the 100% discharge capacity of the negative electrode active material layer, which consists of metal compound particles having a three-dimensional network structure, is between 1.25 and 5.0 times the 100% discharge capacity of the positive electrode active material layer. If the density of the negative electrode active material and positive electrode active material supported by the current collector is constant, the adjustment can also be made by adjusting the thickness of the negative electrode active material layer and the positive electrode active material layer.

[0056] The 100% discharge capacity of the negative electrode active material and the 100% discharge capacity of the positive electrode active material are values ​​obtained by the following method. That is, a half-cell is formed by combining a working electrode with a negative electrode active material layer and a lithium counter electrode, and at a rate of 1C, Li / Li ＋The device is charged and discharged in the range of 3V to 1V, and the discharge capacity is measured. This is then converted to the discharge capacity per gram of the negative electrode active material layer. If conductive additives or binders are added to the lithium vanadium phosphate, the measured value is divided by the total weight of the negative electrode active material layer composed of this lithium vanadium phosphate and additives.

[0057] Furthermore, a half-cell is formed by combining a working electrode containing a layer of positive electrode active material with a lithium counter electrode, and a Li / Li ratio at a rate of 1C. ＋ Charge and discharge are performed in the range of 4.3V to 3V, and the discharge capacity is measured. This is then converted to the discharge capacity per gram of positive electrode active material layer. If conductive additives or binders are added to the positive electrode active material, the measured value is divided by the total weight of the positive electrode active material layer composed of these additives.

[0058] (Separator) As a separator to be sandwiched between the positive and negative electrodes, resins such as cellulose and mixed papers such as kraft, Manila hemp, esparto, hemp, and rayon, polyester resins such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and their derivatives, polytetrafluoroethylene resins, polyvinylidene fluoride resins, vinylon resins, aliphatic polyamides, semi-aromatic polyamides, fully aromatic polyamides, polyimide resins, polyethylene resins, polypropylene resins, trimethylpentene resins, polyphenylene sulfide resins, and acrylic resins can be used individually or in mixtures.

[0059] (Electrolyte) The electrolyte is, for example, the electrolyte solution impregnated into the separator. Examples of electrolytes include non-aqueous electrolytes containing lithium salts that serve as lithium ion sources. LiPF ６ LiBF ４ LiClO ４ , LiN (SO ２ CF ３ ) ２ , LiN (SO ２ C ２ F ５ ) ２ CF ３ SO ３ Li, C２ F ５ SO ３ Li, LiN(SO ２ F) ２ , LiC (SO ２ CF ３ ) ３ , LiC (SO ２ C ２ F ５ ) ３ LiPF ３ (CF ３ ) ３ and LiPF ３ (C ２ F ５ ) ３ , or a mixture thereof.

[0060] The following solvents can be used as the electrolyte. These solvents may be used individually or in mixtures of two or more. Examples include cyclic carbonate esters, linear carbonate esters, phosphate esters, cyclic ethers, linear ethers, lactone compounds, linear esters, nitrile compounds, amide compounds, sulfone compounds, etc. Examples of cyclic carbonate esters include ethylene carbonate, propylene carbonate, butylene carbonate, 4-fluoro-1,3-dioxolan-2-one, and 4-(trifluoromethyl)-1,3-dioxolan-2-one, with ethylene carbonate and propylene carbonate being preferred.

[0061] The present invention will be described in more detail based on the following examples. However, the present invention is not limited to the following examples.

[0062] Lithium-ion secondary batteries were fabricated for each example and comparative example. Each example and comparative example differed in the average particle size of the primary particles of lithium vanadium phosphate, which is the positive electrode active material, the electrode density of the positive electrode, and the utilization rate of the negative electrode.

[0063] Lithium vanadium phosphate with average primary particle diameters of 100 nm, 200 nm, 400 nm, 500 nm, and 1500 nm was prepared and used in each example and comparative example. Lithium vanadium phosphate with average primary particle diameters of 100 nm, 200 nm, 400 nm, and 500 nm was prepared by the aqueous phase method, while lithium vanadium phosphate with an average primary particle diameter of 1500 nm was prepared by the solid phase method. The average primary particle diameter was calculated by observing multiple SEM images of the prepared lithium vanadium phosphate using a scanning electron microscope (SEM) and averaging 100 primary particles.

[0064] The material source for lithium vanadium phosphate is ammonium metavanadate (NH₃). ４ VO ３ ), lithium acetate (CH ３ COOLi) and phosphoric acid (H ３ PO ４ ) is ammonium metavanadate (NH ４ VO ３ ) is a vanadium source, and lithium acetate (CH2) ３ COOLi is a lithium source, and phosphoric acid (H ３ PO ４ ) is a source of phosphate.

[0065] In the aqueous phase method, a mixed solvent of distilled water and ethanol was stirred with a magnetic stirrer, and ammonium metavanadate, lithium acetate, and phosphoric acid were added to the distilled water to prepare a treatment solution in which the additives were uniformly dispersed. Each material source was prepared under normal temperature and pressure conditions. ３ V ２ (PO ４ ) ３ The lithium vanadium phosphate was added according to its stoichiometric ratio. Specifically, to a mixed solvent of 10 g of distilled water and 10 g of ethanol, 4.94 g of ammonium metavanadate, 5.01 g of lithium acetate, and 7.40 g of an 85% aqueous phosphoric acid solution were added.

[0066] In the solid-phase method, each material source is prepared under normal temperature and pressure conditions, Li ３ V ２ (PO ４ ) ３The lithium vanadium phosphate was added according to its stoichiometric ratio. Specifically, 5.01 g of lithium acetate and 7.40 g of 85% phosphoric acid aqueous solution were added to 4.94 g of ammonium metavanadate and kneaded in a mortar.

[0067] Furthermore, in the aqueous phase method, citric acid and ethylene glycol were added as carbon sources to act as conductive additives. For lithium vanadium phosphate with an average primary particle size of 100 nm, 19.21 g of citric acid and 21.0 g of ethylene glycol were added. For lithium vanadium phosphate with an average primary particle size of 200 nm, 9.31 g of citric acid and 11.0 g of ethylene glycol were added. For lithium vanadium phosphate with an average primary particle size of 400 nm, 5.50 g of citric acid and 6.32 g of ethylene glycol were added. For lithium vanadium phosphate with an average primary particle size of 500 nm, 4.80 g of citric acid and 5.28 g of ethylene glycol were added. On the other hand, in the solid phase method, Li was added as a carbon source to act as a conductive additive. ３ V ２ (PO ４ ) ３ Carbon black was mixed in to a concentration of 30 wt%.

[0068] In the aqueous phase method, the treatment solution was mixed with a homogenizer at 13,000 rpm for 1 hour. Furthermore, the treatment solution was concentrated and dried using an evaporator, and then vacuum-dried at 80°C.

[0069] After the drying process, the heating process was carried out. For lithium vanadium phosphate with an average primary particle size of 100 nm, the treated material obtained from the drying process was exposed to an air atmosphere at 400°C for 3 hours. For lithium vanadium phosphate with an average primary particle size of 200 nm, the treated material obtained from the drying process was exposed to an air atmosphere at 300°C for 3 hours. For lithium vanadium phosphate with an average primary particle size of 400 nm, the treated material obtained from the drying process was exposed to an air atmosphere at 300°C for 3 hours. For lithium vanadium phosphate with an average primary particle size of 500 nm, the treated material obtained from the drying process was exposed to an air atmosphere at 300°C for 3 hours.

[0070] In the aqueous phase method, after the heating step, the treated material was calcined in a nitrogen atmosphere at 800°C for 2 hours. On the other hand, in the solid phase method, the drying and heating steps were omitted, and the process proceeded directly to the calcination step, in which the treated material was calcined in a nitrogen atmosphere at 800°C for 2 hours.

[0071] Lithium vanadium phosphate was mixed with a binder. Polyvinylidene fluoride was used as the binder. A slurry was obtained by diluting the mixture with N-methylpyrrolidone. The slurry was applied to a current collector and dried, and then rolled to obtain the positive electrode. Aluminum foil was used as the current collector for the positive electrode. In the rolling process, the positive electrode was compressed using a roll press. By adjusting the gap between the rolls of the roll press, positive electrodes with a layer density of positive electrode active material, i.e., an electrode density of 1.2 g / cc, 1.3 g / cc, 1.4 g / cc, 1.5 g / cc, 1.6 g / cc, or 1.9 g / cc were produced.

[0072] Graphite was used as the negative electrode active material. The graphite was laminated onto a copper foil current collector. Carboxymethylcellulose (CMC) and styrene-butadiene rubber (SBR) were used as binders. A slurry was obtained by diluting the mixture with water. The slurry was applied to the current collector, dried, and then rolled to obtain the negative electrode.

[0073] Negative electrodes with utilization rates of 20%, 40%, 50%, 70%, and 80% of the negative electrode active material were prepared. First, the 100% discharge capacity of the positive electrode was measured. Next, the basis weight of the graphite was adjusted so that the 100% discharge capacity matched the measured value of the positive electrode. Using this basis weight that matched the 100% discharge capacity of the positive electrode as a reference, the basis weight corresponding to each utilization rate was calculated by conversion, and negative electrodes for each utilization rate were created according to the converted values. For example, a negative electrode with twice the basis weight of the negative electrode active material of the 100% discharge capacity electrode stacked on top of each other will have a utilization rate of 50%.

[0074] A graphite electrode for pre-doping lithium ions was assembled into the battery element with a lithium metal electrode facing it, separated by a separator. This battery element was immersed in an electrolyte solution, and the graphite electrode and the lithium metal electrode were short-circuited externally. Lithium hexafluoride phosphate (LiPF) was added to 1 L of a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) (EC:DMC:EMC = 3:3:4 (vol ratio)). ６ An electrolyte solution was used, to which 1 mole of the electrolyte solution was added. By short-circuiting the graphite electrode, lithium ions were doped into the graphite electrode. Then, by disassembling the battery element, a pre-doped lithium ion graphite negative electrode was obtained.

[0075] A laminate was formed by sandwiching a separator between the positive and negative electrodes, and this laminate was impregnated with an electrolyte. Rayon was used as the separator. In addition, lithium hexafluoride phosphate (LiPF) was added to 1 L of a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) (EC:DMC:EMC = 3:3:4 (vol ratio)). ６ An electrolyte solution was used, to which 1 mole of the electrolyte solution of ) was added.

[0076] Then, the laminate impregnated with electrolyte was sealed in a laminate film, and lead wires were extended from the positive and negative electrodes to complete the lithium-ion secondary batteries for each example and comparative example.

[0077] (High-Power Capacity Measurement Test) The high-rate discharge capacity of lithium-ion secondary batteries of Examples 1 to 9 and Comparative Examples 1 to 10 was evaluated. These lithium-ion secondary batteries were charged with a fixed current value equivalent to 1C, and then discharged with current values ​​equivalent to 1C and 200C. The discharge capacity at a discharge rate of 1C and the discharge capacity at 200C were measured. Then, with the discharge capacity at a discharge rate of 1C set as 100%, the percentage of the discharge capacity at a discharge rate of 200C (discharge capacity at 200C / discharge capacity at 1C × 100) was calculated as the capacity retention rate at 200C.

[0078] The volume retention rates for Example 1 and Comparative Example 1 are shown in Table 1 below. (Table 1)

[0079] As shown in Table 1 above, in Example 1 and Comparative Example 1, the electrode density was the same at 1.2 g / cc (less than 1.5 g / cc), the negative electrode utilization rate was the same at 40% (less than 80%), and both were identical in that lithium ions were pre-doped into the negative electrode. However, the average particle diameter of the primary particles of lithium vanadium phosphate used in Example 1 was 200 nm (less than 500 nm), whereas in Comparative Example 1 it was 1500 nm, which is outside the range of less than 500 nm.

[0080] Comparing the capacity retention rates of Example 1 and Comparative Example 1, Comparative Example 1 had a capacity retention rate of slightly less than 36% when discharged at 200C, while Example 1's capacity retention rate reached slightly less than 80%, more than double that of Comparative Example 1. From this, it was confirmed that even if the electrode density is 1.5 g / cc or less, the negative electrode utilization rate is less than 80%, and lithium ions are pre-doped into the negative electrode, if the average particle size of the primary particles of lithium vanadium phosphate deviates from the range of less than 500 nm, the discharge capacity at high power will be greatly reduced.

[0081] The volume retention rates for Examples 2 to 4 and Comparative Examples 2 to 5 are shown in Table 2 below. (Table 2)

[0082] As shown in Table 2 above, in Examples 2 to 4, the average particle diameter of the primary particles of lithium vanadium phosphate is 200 nm, which is less than 500 nm, the electrode density is 1.2 to 1.4 g / cc, which is less than 1.5 g / cc, and the negative electrode utilization rate is 40%, which is less than 80%, and lithium ions are pre-doped into the negative electrode. On the other hand, in Comparative Examples 2 to 5, the average particle diameter of the primary particles of lithium vanadium phosphate is 400 nm, 200 nm, or 100 nm, which is less than 500 nm, and the negative electrode utilization rate is 40%, which is less than 80%, and lithium ions are pre-doped into the negative electrode, but the electrode density exceeds 1.5 g / cc, being 1.6 g / cc or 1.9 g / cc.

[0083] Comparing the capacity retention rates of Examples 2 to 4 and Comparative Examples 2 to 5, Comparative Examples 2 to 5 had a capacity retention rate of around 50% when discharged at 200C, while Examples 2 to 4 had a capacity retention rate of at least 70%. From this, it was confirmed that even if the average particle size of the primary particles of lithium vanadium phosphate is less than 500 nm, the negative electrode utilization rate is less than 80%, and lithium ions are pre-doped into the negative electrode, if the electrode density deviates from the range of 1.5 g / cc or less, the discharge capacity at high power will be greatly reduced.

[0084] The volume retention rates for Comparative Examples 6 and 7 are shown in Table 3 below. (Table 3)

[0085] As shown in Table 3 above, Comparative Example 7 has an average particle diameter of 500 nm for the primary particles of lithium vanadium phosphate, which is slightly outside the range of less than 500 nm, and an electrode density of 1.6 g / cc, which is slightly outside the range of 1.5 g / cc or less. Even in this Comparative Example 7, the capacity retention rate when discharged at 200C was significantly below 50%, compared to over 70% for Examples 1 to 4. In Comparative Example 6, where the average particle diameter of the primary particles of lithium vanadium phosphate was 1500 nm, which is significantly outside the range of less than 500 nm, and the electrode density was 1.9 g / cc, which is significantly outside the range of 1.5 g / cc or less, the capacity retention rate was zero, meaning that it could not be discharged at a high rate.

[0086] The volume retention rates for Examples 1, 2, 5, and 6, and Comparative Example 8 are shown in Table 4 below. (Table 4)

[0087] As shown in Table 4 above, in Example 4, the average particle size of the primary particles of lithium vanadium phosphate is 200 nm, which is less than 500 nm, the electrode density is 1.4 g / cc, which is less than 1.5 g / cc, and the negative electrode utilization rate is 40%, which is less than 80%, and lithium ions are pre-doped into the negative electrode. On the other hand, in Comparative Example 8, unlike Example 4, the negative electrode utilization rate is 80%, which is outside the range of less than 80%.

[0088] As a result, Example 4 showed a capacity retention rate of over 70% when discharged at 200C, while Comparative Example 8 showed a capacity retention rate of significantly less than 50% when discharged at 200C. From this, it was confirmed that even if the average particle size of the primary particles of lithium vanadium phosphate is less than 500 nm, the electrode density is 1.5 g / cc or less, and lithium ions are pre-doped into the negative electrode, if the negative electrode utilization rate is not less than 80%, the discharge capacity at high power will be greatly reduced.

[0089] Furthermore, in Example 6, although only a small amount of conductive additive (6 wt%) was added, the average particle size of the primary particles of lithium vanadium phosphate was less than 500 nm, the electrode density was 1.5 g / cc or less, the negative electrode utilization rate was less than 80%, and lithium ions were pre-doped into the negative electrode, resulting in a capacity retention rate approaching 80% when discharged at 200C.

[0090] Furthermore, as shown in Table 4 above, Examples 1, 5, and 6 differ in their negative electrode utilization rates, ranging from 20% to 50%. In Examples 1, 5, and 6, the average particle size of the primary particles of lithium vanadium phosphate is less than 500 nm, the electrode density is 1.5 g / cc or less, the negative electrode utilization rate is less than 80%, and lithium ions are pre-doped into the negative electrode, resulting in a capacity retention rate significantly exceeding 70% when discharged at 200C.

[0091] The volume retention rates for Example 2 and Comparative Examples 9 and 10 are shown in Table 5 below. (Table 5)

[0092] As shown in Table 5 above, in Example 2, the average particle size of the primary particles of lithium vanadium phosphate is 200 nm, which is less than 500 nm; the electrode density is 1.2 g / cc, which is less than 1.5 g / cc; and the negative electrode utilization rate is 40%, which is less than 80%; and lithium ions are pre-doped into the negative electrode. On the other hand, in Comparative Example 9, unlike Example 2, lithium ions are not pre-doped into the negative electrode.

[0093] As a result, while Example 2 achieved a capacity retention rate of nearly 80% when discharged at 200C, Comparative Example 9 achieved a capacity retention rate of less than 30% when discharged at 200C. In Comparative Example 10, the electrode density deviated from the range of 1.5 g / cc or less to 1.6 g / cc, and the negative electrode utilization rate also deviated from the range of less than 80% to 80%, and the lithium ions were not pre-doped. Comparative Example 9, by failing to meet just one condition, has seen its capacity retention rate drop to the same level as Comparative Example 10, which does not meet multiple conditions.

[0094] From this, it was confirmed that even if the average particle size of the primary particles of lithium vanadium phosphate is less than 500 nm, the electrode density is 1.5 g / cc or less, and the negative electrode utilization rate is less than 80%, if lithium ions are not pre-doped into the negative electrode, the discharge capacity at high power will be greatly reduced.

[0095] In other words, as can be seen from Tables 1 to 5, it was confirmed that high capacity can be achieved even when discharged at 200C by satisfying all four conditions: firstly, the average particle size of the primary particles of lithium vanadium phosphate is less than 500 nm; secondly, the electrode density of the positive electrode is 1.5 g / cc or less; thirdly, the utilization rate of the negative electrode is less than 80%; and fourthly, lithium ions are pre-doped into the negative electrode.

[0096] The volume retention rates for each of Examples 7 to 9 are shown in Table 6 below. (Table 6)

[0097] As shown in Table 6 above, Examples 7 to 9, in which the negative electrode utilization rate is less than 80%, exhibit good capacity retention. In particular, Example 7 has a negative electrode utilization rate of 40% or less, and therefore exhibits the best capacity retention rate among Examples 7 to 9. Furthermore, as can be seen by comparing Examples 7 in Table 7 with Tables 1 to 4 above, when the negative electrode utilization rate is 40% or less, the average particle diameter of the primary particles of lithium vanadium phosphate is 200 nm or less, and the electrode density is 1.3 g / cc or less, the capacity retention rate reaches nearly 80%.

[0098] From this, it was confirmed that, in terms of capacity retention, the negative electrode utilization rate is preferably 40% or less, and particularly preferably, the average particle diameter of the primary particles of lithium vanadium phosphate is 200 nm or less, and the electrode density is 1.3 g / cc or less.

Claims

1. A lithium-ion secondary battery comprising: a positive electrode containing lithium vanadium phosphate having an average primary particle diameter of less than 500 nm as the positive electrode active material and having an electrode density of 1.5 g / cc or less; a negative electrode in which lithium ions are pre-doped and the utilization rate is less than 80%; and an electrolyte between the positive electrode and the negative electrode.

2. The lithium-ion secondary battery according to claim 1, characterized in that the electrode density of the positive electrode is 1.2 g / cc or more and 1.4 g / cc or less.

3. The lithium-ion secondary battery according to claim 1, characterized in that the utilization rate is 20% or more and 50% or less.

4. The lithium-ion secondary battery according to claim 1, characterized in that the average primary particle diameter of the lithium vanadium phosphate is 200 nm or less.

5. The lithium-ion secondary battery according to claim 1, characterized in that the average primary particle diameter of the lithium vanadium phosphate is 200 nm or less, the utilization rate is 20% or more and 40% or less, and the electrode density of the positive electrode is 1.2 g / cc or more and 1.3 g / cc or less.

6. The aforementioned lithium vanadium phosphate is Li ３ V ２－ｘ M ｘ (PO ４ ) ３ A lithium-ion secondary battery according to any one of claims 1 to 5, characterized in that M is represented as Mg, Al, Ca, Ti, Cr, Mn, Fe, Co, Ni, Zn, Y, Zr, Nb, Sn, or a plurality of these metallic elements, and x, which is the atomic ratio of M, is 0 ≤ x ≤ 0.

2.

7. The lithium-ion secondary battery according to any one of claims 1 to 5, characterized in that the active material of the negative electrode is graphite, silicon, or silicon oxide.

8. A method for manufacturing a lithium-ion secondary battery, comprising: a rolling step of rolling lithium vanadium phosphate having an average primary particle diameter of less than 500 nm as a positive electrode active material to a current collector with an electrode density of 1.5 g / cc or less; a negative electrode forming step of forming a negative electrode active material layer with a utilization rate of less than 80%; and a pre-doping step of pre-doping lithium ions into the negative electrode active material.

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