Lithium-ion secondary battery and manufacturing method

Abstract

To increase the capacity of lithium-ion secondary batteries. [Solution] The lithium-ion secondary battery comprises a positive electrode containing a lithium vanadium phosphate positive electrode active material layer, a negative electrode containing a lithium ion-predoped negative electrode active material layer, and an electrolyte between the positive electrode and the negative electrode. This lithium-ion secondary battery is manufactured by a positive electrode manufacturing step of manufacturing a positive electrode containing a lithium vanadium phosphate, a negative electrode manufacturing step of manufacturing a negative electrode containing a negative electrode active material, and a pre-doping step of pre-doping the negative electrode active material.

Description

Technical Field

[0001] The present invention relates to a lithium ion secondary battery and a manufacturing method thereof.

Background Art

[0002] A lithium ion secondary battery is formed by opposing a positive electrode and a negative electrode in an electrolytic solution through a separator, and is charged and discharged according to the direction of occlusion and release of lithium ions in the positive and negative electrodes. In a lithium ion secondary battery, lithium ions are released from the positive electrode and occluded in the negative electrode during charging, and lithium ions are released from the negative electrode and occluded in the positive electrode during discharging.

[0003] This lithium ion secondary battery has a positive electrode and a negative electrode containing an active material that reversibly occludes and releases lithium ions, and an electrolytic solution in which a lithium salt is dissolved. The positive electrode and the negative electrode are each formed by integrating a layer of the active material with a current collector. The positive electrode active material and the current collector, as well as the negative electrode active material and the current collector, are joined via a binder using, for example, pressure bonding or the doctor blade method.

[0004] For example, in a lithium ion secondary battery, it is mainstream to use lithium nickelate or lithium cobaltate for the positive electrode active material, graphite for the negative electrode active material, and an electrolytic solution in which lithium hexafluorophosphate is dissolved in a non-aqueous solvent for the electrolyte (see, for example, Patent Document 1).

[0005] In recent years, due to the rapid spread of digital cameras, smartphones, and portable PCs, the rise in fuel prices and the increasing awareness of environmental impact, and further the expectation of application for power for automobiles or energy storage for smart grids, the development of lithium ion secondary batteries has become active. In particular, the development of electric vehicles (EVs) and hybrid electric vehicles (HEVs) that assist part of the drive with an electric motor is progressing rapidly among each automobile manufacturer, and for these automotive applications, a large-capacity lithium ion secondary battery is required as the power source.

[0006] In lithium-ion secondary batteries, a potential difference is sometimes created between the positive and negative electrodes by pre-doping the negative electrode active material with lithium ions to lower the negative electrode potential to around 0V (vs. Li / Li+). This pre-doping of the negative electrode active material with lithium ions is called pre-doping (see, for example, Patent Document 2). This pre-doping maintains a potential difference between the positive and negative electrodes even when the positive electrode potential decreases due to discharge, allowing the positive electrode capacity to be utilized with high efficiency. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2011-204571 [Patent Document 2] Japanese Patent Publication No. 2019-145386 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] However, when lithium nickelate is used as the positive electrode active material, discharge is possible between 3.0V (vs. Li / Li+) and 4.2V (vs. Li / Li+) relative to Li, but almost no capacity is exhibited below 3.0V (vs. Li / Li+). Similarly, when lithium cobalt oxide is used as the positive electrode active material, discharge is possible between 3.0V (vs. Li / Li+) and 4.3V (vs. Li / Li+) relative to Li, but almost no capacity is exhibited below 3.0V. Therefore, even if the negative electrode potential is maintained near 0V (vs. Li / Li+) by pre-doping when the positive electrode is near 3.0V (vs. Li / Li+), it does not contribute to increasing the capacity of lithium-ion secondary batteries.

[0009] This invention was proposed to solve the above-mentioned problems. Its objective is to increase the capacity of lithium-ion secondary batteries. [Means for solving the problem]

[0010] To achieve the above object, a lithium-ion secondary battery according to an embodiment of the present invention includes a positive electrode containing lithium vanadium phosphate in a positive electrode active material layer, a negative electrode containing a negative electrode active material pre-doped with lithium ions in a negative electrode active material layer, and an electrolyte between the positive electrode and the negative electrode.

[0011] The capacity of lithium vanadium phosphate is about 110 mAh / g in the charge-discharge range from 4.3 V (vs. Li / Li + / V 3+ ) to 3.0 V (vs. Li / Li + ) in the oxidation-reduction of V + . The capacity of lithium nickelate (NCA) is about 170 mAh / g in the charge-discharge range from 3.0 V (vs. Li / Li + ) to 4.3 V (vs. Li / Li + ), and the capacity of lithium cobaltate (LCO) is 150 mAh / g in the discharge range from 3.0 V (vs. Li / Li + ) to 4.3 V (vs. Li / Li + ). Therefore, the capacity of lithium vanadium phosphate in the charge-discharge range from 4.3 V (vs. Li / Li + ) to 3.0 V (vs. Li / Li + ) is less than the capacities of lithium nickelate and lithium cobaltate.

[0012] However, lithium vanadium phosphate has an electrochemical capacity at a high potential in the range from 4.3 V (vs. Li / Li 4+ / V 3+ and V 3+ / V 2+ oxidation-reduction from 4.3 V (vs. Li / Li + ) to 3.0 V (vs. Li / Li + ) and at a low potential from less than 3.0 V (vs. Li / Li + ) to 1.2 V (vs. Li / Li + ). That is, lithium vanadium phosphate can be charged and discharged in the range from 1.8 V (vs. Li / Li + ) to 1.2 V (vs. Li / Li + ) even below 3 V, and its capacity is 108 mAh / g.

[0013] Therefore, lithium vanadium phosphate is used as the positive electrode active material, and the charge / discharge range is 4.3V (vs. Li / Li), which combines the high and low potential ranges. + ) to 1.2V (vs. Li / Li + If expanded to this extent, the effective capacity of lithium vanadium phosphate would be around 218 mAh / g, surpassing lithium nickelate and lithium cobaltate.

[0014] However, if the negative electrode active material is graphite, for example, the potential of the graphite from which lithium ions have been removed is approximately 1.2V (vs. Li / Li + ) Also, if the negative electrode active material is, for example, SiO, the potential of the SiO from which lithium ions have been removed is approximately 1.5V (vs.Li / Li + Therefore, lithium vanadium phosphate is 1.8V (vs. Li / Li + ) to 1.2V (vs. Li / Li + Even if you try to operate it at a low potential of 3.0V (vs. Li / Li), the potential difference between the positive and negative electrodes quickly reaches 0V, and lithium vanadium phosphate is used at 3.0V (vs. Li / Li + ) to 1.2V (vs. Li / Li + It cannot be operated at low potentials below ).

[0015] Therefore, lithium ions are pre-doped into the negative electrode active material. This pre-doping results in a lithium vanadium phosphate voltage of 4.3V (vs.Li / Li + ) to 1.2V (vs. Li / Li + While discharging a capacity equivalent to the capacity in the operating range, the operating potential range of the negative electrode active material should be at least 1.2V (vs. Li / Li + Keep it below this limit. This will result in a lithium-ion secondary battery that can utilize the capacity of lithium vanadium phosphate, which is around 218 mAh / g, with a high utilization rate, thereby increasing the capacity of lithium-ion secondary batteries.

[0016] That is, the negative electrode has a potential of 4.3V (vs. Li / Li) +) to 1.2V (vs. Li / Li + The negative electrode has a negative electrode capacity greater than or equal to the positive electrode capacity that emerges before the voltage drops to ), and the negative electrode active material has a negative electrode capacity equivalent to the positive electrode capacity when discharged, and the potential is at least 1.2V (vs.Li / Li + The lithium ions may be pre-doped to maintain the following levels:

[0017] Furthermore, the negative electrode has a lithium vanadium phosphate of 1.2V (vs.Li / Li + The negative electrode active material has a negative electrode capacity greater than or equal to the positive electrode capacity that emerges before the negative electrode capacity drops to 60%, and the negative electrode active material may be pre-doped with a charge level of 60% or more. By pre-doping the negative electrode active material with a charge level of 60% or more, when a negative electrode capacity equivalent to the positive electrode capacity is discharged, the potential is at least 1.2V (vs.Li / Li + Maintain the following:

[0018] The aforementioned lithium vanadium phosphate is Li3V 2-x M x The formula is represented as (PO4)3, where M is Mg, Al, Ca, Ti, Cr, Mn, Fe, Co, Ni, Zn, Y, Zr, Nb, Sn, or a combination of these metallic elements, and x, which is the atomic ratio of M, may be such that 0 ≤ x ≤ 0.2.

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

[0020] Furthermore, in order to achieve the above objectives, the method for manufacturing a lithium-ion secondary battery according to an embodiment of the present invention comprises a positive electrode manufacturing step for creating a positive electrode containing lithium vanadium phosphate, a negative electrode manufacturing step for creating a negative electrode containing a negative electrode active material, and a pre-doping step for pre-doping lithium ions into the negative electrode active material. [Effects of the Invention]

[0021] According to the present invention, the lithium-ion secondary battery is 4.3V (vs.Li / Li + ) to 1.2V (vs. Li / Li+ It operates within the voltage range of ) and has a high charge / discharge capacity. [Brief explanation of the drawing]

[0022] [Figure 1] This graph shows the relationship between the potential and capacitance of the positive and negative electrodes during discharge when lithium vanadium phosphate is used as the positive electrode active material and a negative electrode active material that is not pre-doped with lithium ions. [Figure 2] This graph shows the relationship between the potential and capacitance of the positive and negative electrodes during discharge when lithium vanadium phosphate is used as the positive electrode active material and lithium ion-predoped negative electrode active material is used. [Figure 3] (a) is a graph showing the discharge results of the lithium-ion secondary battery of Example 1, and (b) is a graph showing the discharge results of the lithium-ion secondary battery of Comparative Example 1. [Modes for carrying out the invention]

[0023] 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.

[0024] A 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.

[0025] The positive electrode active material is lithium vanadium phosphate. Lithium vanadium phosphate has the general formula Li3V 2-x M xIt has a nasicone structure represented as (PO4)3. Some of the vanadium may be substituted with a metallic element. In the empirical formula, M is Mg, Al, Ca, Ti, Cr, Mn, Fe, Co, Ni, Zn, Y, Zr, Nb, Sn, or several of these metallic elements, and the atomic ratio x of the metallic element M is 0 ≤ x ≤ 0.2. This lithium vanadium phosphate is typically Li3V2(PO4)3.

[0026] 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. Alternatively, lithium vanadium phosphate particles and a carbon material that acts as a conductive additive may be compounded, and this composite may be used as the positive electrode active material. Furthermore, a negative electrode active material such as Si or SiO may be used in combination with the carbon material.

[0027] The negative electrode has 4.3V (vs. Li / Li + ) to 3.0V (vs. Li / Li + Capacitance at high potentials in the range of ) and 3.0V (vs. Li / Li + ) to 1.2V (vs. Li / Li + The negative electrode active material is equipped with a capacity greater than or equal to the sum of the capacity at low potentials (vs. Li / Li). The negative electrode active material equipped with the negative electrode is pre-doped with lithium ions. When the lithium ions discharge the sum of the electrochemical capacity at high potential and low potential of lithium vanadium phosphate, they reach the lower limit of the charge / discharge potential of lithium vanadium phosphate, which is 1.2V (vs. Li / Li). + ) are pre-doped to have the following potentials.

[0028] Preferably, the negative electrode active material is pre-doped with lithium ions to a charge level of 50% or more. This ensures that when the combined capacity of the high-potential and low-potential electrochemical capacities of lithium vanadium phosphate is discharged, the negative electrode reaches the lower limit of the charge / discharge potential of lithium vanadium phosphate, which is 1.2V (vs.Li / Li). + ) It has a potential of the following magnitude.

[0029] As for the type of negative electrode active material, within the operating potential range of 0V (vs. Li / Li + ) to 1.2V (vs. Li / Li + The material is not particularly limited as long as the following conditions are met. For example, examples of negative electrode active materials include graphite such as natural graphite or artificial graphite, soft carbon, hard carbon, silicon, or silicon oxides such as SiO.

[0030] The current collector, which laminates the positive electrode active material and the negative electrode active material, is typically made of a conductive material such as aluminum, copper, iron, nickel, titanium, steel, or carbon. Aluminum or copper, which have high thermal and electronic conductivity, are particularly preferred. For example, aluminum may be used for the current collector that laminates the positive electrode active material, and copper may be used for the current collector that laminates the negative electrode active material. The shape of the current collector can be any shape, such as film, foil, plate, mesh, expanded metal, or cylindrical. A current collector with through holes may also be used.

[0031] A slurry of positive electrode active material or negative electrode active material with a binder is coated onto this current collector using a doctor blade method or the like, and then dried to form a positive electrode with a layer of positive electrode active material and a current collector laminated together, and a negative electrode with a layer of negative electrode active material and a current collector laminated together. 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 in mixtures of two or more types.

[0032] As a separator sandwiched between the positive and negative electrodes, resins such as cellulose and mixed papers including 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.

[0033] The electrolyte is, for example, the electrolyte solution impregnated into the separator. Examples of electrolyte solutions include non-aqueous electrolytes containing lithium salts that serve as lithium ion sources. Lithium salts include LiPF6, LiBF4, LiClO4, LiN(SO2CF2)2, LiN(SO2C2F5)2, and CF3SO3L. i C2F5SO3Li, LiC(SO2CF3) 3、 These are LiC(SO2C2F5)3, LiPF3(CF3)3, and LiPF3(C2F5)3, or mixtures thereof.

[0034] 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.

[0035] Figure 1 shows the operation of such lithium-ion secondary batteries. Figure 1 is a graph showing the relationship between the potential and capacity of the positive and negative electrodes during discharge when lithium vanadium phosphate is used as the positive electrode active material and a negative electrode active material that is not pre-doped with lithium ions. Figure 2 is a graph showing the relationship between the potential and capacity of the positive and negative electrodes during discharge when lithium vanadium phosphate is used as the positive electrode active material and a negative electrode active material that is pre-doped with lithium ions.

[0036] As shown in the positive electrode discharge curve Dc in Figure 1, lithium vanadium phosphate has a V on the oxidized side relative to the trivalent state. 4+ / V 3+ Due to the de-insertion of lithium ions in this region, 4.3V (vs.Li / Li + ) to 3.0V (vs. Li / Li + It has a capacity in the range of ). When the vanadium ion is tetravalent, lithium vanadium phosphate is LiV 2-x M x (PO4)3. 4.3V(vs.Li / Li + ) to 3.0V (vs. Li / Li + Within the range of ), during the discharge process, one of the two vanadium atoms changes from a state where both are tetravalent to a trivalent vanadium atom, and then the other vanadium atom also changes to a trivalent state.

[0037] Furthermore, as shown in the positive electrode discharge curve Dc, in lithium vanadium phosphate, the reduction side V is based on the trivalent state. 3+ / V 2+ By de-insertion of lithium ions in this region, 3.0V (vs. Li / Li + ) less than 1.2V (vs. Li / Li + It has a capacity in the range of ). When the vanadium ion is divalent, lithium vanadium phosphate is Li5V 2-x M x (PO4)3. 3.0V(vs.Li / Li + ) less than 1.2V (vs. Li / Li + Within the range of ), the discharge process changes both vanadium atoms from a trivalent state to a divalent state.

[0038] Thus, in lithium vanadium phosphate, the valence of the vanadium ion is 2 to 4, so V 4+ / V 3+ and V 3+ / V 2+ Due to the dual oxidation-reduction reaction, 4.3V (vs.Li / Li + ) to 1.2V (vs. Li / Li + Within this range, it has capacity at both the high potential on the oxidation side and the low potential on the reduction side, relative to the trivalent state. Therefore, if a lithium-ion secondary battery can be operated down to 1.2V, its capacity will be increased.

[0039] However, as shown in the discharge curve Da of the negative electrode in Figure 1, if the capacity of the negative electrode active material is matched to the capacity expressed at the oxidation potential of lithium vanadium phosphate, the potential of the negative electrode will rise sharply before lithium vanadium phosphate can express its capacity at the low potential on the reduction side, and the potential difference between the positive and negative electrodes will become zero.

[0040] Therefore, as shown in Figure 2, the negative electrode has a voltage of 4.3V (vs.Li / Li) in lithium vanadium phosphate. + ) to 3.0V (vs. Li / Li + Capacitance in the high potential range and from less than 3.0V to 1.2V (vs. Li / Li +The negative electrode active material is mounted on the negative electrode so that its capacity is greater than or equal to the sum of its capacity in the low potential range of ) and its capacity in the low potential range. When the sum of the capacity of lithium vanadium phosphate at high potential and low potential is discharged, the lower limit of the dischargeable potential of lithium vanadium phosphate is 1.2V (Li / Li + The lithium ions are pre-doped to have a potential below a certain value. This allows the lithium-ion secondary battery to exhibit capacity in a voltage range of 4.2V to 1.2V when operated. Therefore, this lithium-ion secondary battery has a high capacity.

[0041] Furthermore, the lithium vanadium phosphate used in this lithium-ion secondary battery may be manufactured by any method, and the manufacturing method is not particularly limited. Similarly, the lithium-ion pre-doped negative electrode active material may be manufactured by any method, and the manufacturing method is not particularly limited.

[0042] For example, it is manufactured through an addition step of adding a material source of lithium vanadium phosphate, 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, and a firing step of firing the treated product in an inert atmosphere.

[0043] In the addition step, a processing solution is prepared in which the material sources of electrode material 1 are uniformly dispersed. The lithium source and phosphoric acid source are added to the vanadium source while stirring. The solvent of the processing 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.

[0044] When employing hydrolysis and complex formation reactions, vanadium acetate, sulfate, nitrate, halogen compounds, or chelating agents can be used as vanadium sources in the addition step. Specifically, vanadium sources include NH4VO3, V2O5, V2O3, metallic vanadium, V2O4, vanadium(III) acetylacetonate, and vanadium(IV) oxyacetylacetonate. Phosphate sources include PO4-containing compounds such as H3PO4, NH4H2PO4, (NH4)2HPO4, P2O5, and Li3PO4.

[0045] Lithium sources include CH3COOLi, LiNO3, Li2CO3, LiOH, LiOH·H2O, LiCl, Li2SO4, and LiC3H5O3. Vanadium, lithium, and phosphorus sources should be added in proportions corresponding to the stoichiometric ratio of lithium vanadium phosphate. For example, in the case of Li3V2(PO4)3, the molar ratio of Li, V, and PO4 should be 3:2:3.

[0046] The following phenomenon is promoted in the mixing process: namely, the formation of a precursor of lithium vanadium phosphate is promoted. Homogenizers, lithographs, ball mills, bead mills, rod mills, roller mills, agitated mills, planetary mills, hybridizers, and jet mills can be used in this mixing process. Alternatively, mechanochemical treatment may be performed instead of mixing with a homogenizer or the like.

[0047] After the mixing process, the process moves to the drying process. The drying process can be, for example, high-temperature drying, where the processed liquid is left overnight in an atmosphere of 80 degrees Celsius; reduced-pressure drying using a rotary evaporator; or spray drying, where the solvent is evaporated by spraying the slurry into hot air. A fluidized bed dryer may be used to carry out the mixing and drying processes in parallel. A fluidized bed dryer can dry the mixture while stirring and mixing each material source in the aqueous phase.

[0048] After the drying process, the process moves to the calcination stage. In the calcination stage, lithium is incorporated into the precursor of lithium vanadium phosphate, producing lithium vanadium phosphate, and the crystallization of lithium vanadium phosphate proceeds. In this calcination stage, calcination 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. Examples of inert gases include noble gases such as Ar and N2. Examples of reducing gases include H2.

[0049] The lithium-ion-predoped negative electrode active material is prepared, for example, by slurry mixing or electrical short-circuiting. In the slurry mixing method, the negative electrode active material and metallic lithium slurry are mixed using a ball mill and a planetary mixer.

[0050] 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 process charges the negative electrode active material layer, doping it with lithium ions. The lithium ions dope the negative electrode active material to a charge level of approximately 60%. Then, by disassembling the battery element, the lithium-ion-pre-doped negative electrode active material is 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. [Examples]

[0051] 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.

[0052] (Example 1) A lithium-ion secondary battery of Example 1 was fabricated. First, the positive electrode active material and the negative electrode active material were generated, then the positive electrode and negative electrode were fabricated, and a battery element was formed with a separator interposed between the positive electrode and the negative electrode, and the battery element was impregnated with an electrolyte.

[0053] The positive electrode active material is lithium vanadium phosphate. The material sources for lithium vanadium phosphate are ammonium metavanadate (NH4VO3), lithium acetate (CH3COOLi), and phosphoric acid (H3PO4). Ammonium metavanadate (NH4VO3) is the vanadium source, lithium acetate (CH3COOLi) is the lithium source, and phosphoric acid (H3PO4) is the phosphoric acid source.

[0054] 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 added under normal temperature and pressure conditions according to the stoichiometric ratio of lithium vanadium phosphate in Li3V2(PO4)3. 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.

[0055] The treatment solution was concentrated and dried using an evaporator, and then vacuum-dried at 80°C. After the drying process, the process moved to a heating step, where the treated material obtained in the drying step was exposed to an atmospheric environment at 300°C for 5 hours. Subsequently, the treated material was calcined in a nitrogen atmosphere at 800°C for 2 hours. Through these steps, lithium vanadium phosphate was obtained as the positive electrode active material.

[0056] The negative electrode active material is graphite pre-doped with lithium ions at a charge level of 60% or higher. The graphite has a D50 particle size of 8 μm as measured by laser diffraction and a specific surface area of ​​5 m² as measured by BET. 2 Spherical graphite particles of a weight of / g were used.

[0057] A slurry was prepared by mixing the positive electrode active material and binder with N-methylpyrrolidone (NMP), and a slurry was prepared by mixing the negative electrode active material and binder with water. Polyvinylidene fluoride (PVDF) was used as the binder for the positive electrode, and carboxymethylcellulose (CMC) and styrene-butadiene rubber (SBR) were used for the negative electrode. The theoretical capacity per unit weight of the positive electrode active material was 4.3V(vs.Li / Li). + ) to 3.0V (vs. Li / Li + The capacitance is 132 mAh / g in the potential range of ), and the theoretical capacity per unit weight of the negative electrode active material is 372 mAh / g. The positive electrode active material is 3.0V (vs. Li / Li + ) less than (vs. Li / Li + The capacitance in the potential range up to 3.0V (vs.Li / Li) is 3.0V (vs.Li / Li + ) to 4.3V (vs. Li / Li + Since the capacitance in the potential range is theoretically the same, the positive electrode capacitance (mAh) should be manufactured to be 50% or less of the negative electrode capacitance (mAh), and it was manufactured to be 40%. The positive electrode capacitance (mAh) at this time is 4.3V (vs.Li / Li + ) to 3.0V (vs. Li / Li + This was calculated from the positive electrode capacity within the potential range. Furthermore, since the negative electrode active material is pre-doped with lithium ions at a 60% charge level, even if the voltage reaches 1.2V when operating as a lithium-ion secondary battery, the negative electrode will not reach full discharge. In other words, the negative electrode has room to operate.

[0058] The slurry was applied to etched aluminum foil and copper foil, and the solvent (NMP, water) was removed. This created a positive electrode by laminating a layer of positive electrode active material on the aluminum foil surface, and a negative electrode by laminating a layer of negative electrode active material on the copper foil surface. A laminate was formed by sandwiching a separator between the positive and negative electrodes, and this laminate pair was impregnated with an electrolyte. Rayon was used as the separator. In addition, an electrolyte was used prepared by adding 1 mole of lithium hexafluoride phosphate (LiPF6) 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%)).

[0059] 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 battery of Example 1.

[0060] (Comparative Example 1) A lithium-ion secondary battery of Comparative Example 1 was fabricated in correspondence with Example 1. The lithium-ion secondary battery of Comparative Example 1 is identical to that of Example 1, except for the negative electrode. That is, the positive electrode active material is lithium vanadium phosphate, but the negative electrode active material is graphite that has not been pre-doped with lithium ions. In Comparative Example 1, the negative electrode active material was fabricated using the same manufacturing method as in Example 1, without the pre-doping process.

[0061] (Discharge test) Discharge tests were performed on the lithium-ion secondary batteries of Example 1 and Comparative Example 1. The discharge capacity (mAh / g) of the lithium-ion secondary batteries of Example 1 and Comparative Example 1 was measured when discharged at a discharge current density of 50 mA / g with an operating voltage of 4.2V to 1.2V. The discharge capacity (mAh / g) at this time is the discharge capacity per unit weight of the positive electrode.

[0062] Figure 3(a) is a graph showing the discharge results of the lithium-ion secondary battery of Example 1, and Figure 3(b) is a graph showing the discharge results of the lithium-ion secondary battery of Comparative Example 1. The horizontal axis of the graph represents the discharge capacity, and the vertical axis represents the voltage of the lithium-ion secondary battery.

[0063] As shown in Figure 3(b), the lithium-ion secondary battery of Comparative Example 1 operated until the voltage dropped from 4.2V to 3.0V, and its discharge capacity was 108mAh / g. In contrast, as shown in Figure 3(a), the lithium-ion secondary battery of Example 1 operated until the voltage dropped from 4.2V to 1.2V, and its discharge capacity was 218mAh / g. Thus, it was confirmed that the lithium-ion secondary battery in which the positive electrode active material was lithium vanadium phosphate and the negative electrode active material was pre-doped with lithium ions achieved approximately twice the capacity.

[0064] (Example 2) Next, a lithium-ion secondary battery of Example 2 was fabricated. The negative electrode active material in Example 2 was hard carbon. The hard carbon had a D50 particle size of 3 μm as measured by laser diffraction. The hard carbon was pre-doped with lithium ions at a charge level of 60%. Except for the fact that the negative electrode active material was hard carbon, the lithium-ion secondary battery of Example 2 had the same configuration as that of Example 1 and was fabricated using the same manufacturing method and conditions.

[0065] (Comparative Example 2) A lithium-ion secondary battery of Comparative Example 2 was fabricated in accordance with Example 2. The lithium-ion secondary battery of Comparative Example 2 uses hard carbon as the negative electrode active material, just like Example 2, but this hard carbon is not pre-doped with lithium ions. The lithium-ion secondary battery of Comparative Example 2 was fabricated using the same manufacturing method and conditions as Example 2, but without the pre-doping process.

[0066] (Example 3) Next, a lithium-ion secondary battery of Example 3 was fabricated. The negative electrode active material in Example 3 was Si particles. The Si particles had a D50 particle size of 0.1 μm as measured by laser diffraction. The Si particles were pre-doped with lithium ions at a charge level of 60%. Except for the fact that the negative electrode active material was Si particles, the lithium-ion secondary battery of Example 3 had the same configuration as that of Example 1 and was fabricated using the same manufacturing method and conditions.

[0067] (Comparative Example 3) A lithium-ion secondary battery of Comparative Example 3 was fabricated in accordance with Example 3. The lithium-ion secondary battery of Comparative Example 3 is the same as in Example 2 in that it uses Si particles as the negative electrode active material, but these Si particles are not pre-doped with lithium ions. The lithium-ion secondary battery of Comparative Example 3 was fabricated using the same manufacturing method and conditions as Example 3, but without the pre-doping process.

[0068] (Example 4) Next, a lithium-ion secondary battery of Example 4 was fabricated. The negative electrode active material in Example 4 was SiO particles. The SiO particles had a D50 particle size of 5 μm as measured by laser diffraction. The SiO particles were pre-doped with lithium ions at a charge level of 60%. Except for the negative electrode active material being SiO particles, the lithium-ion secondary battery of Example 4 had the same configuration as that of Example 1 and was fabricated using the same manufacturing method and conditions.

[0069] (Comparative Example 4) A lithium-ion secondary battery of Comparative Example 4 was fabricated in correspondence with Example 4. The lithium-ion secondary battery of Comparative Example 4 is the same as in Example 2 in that it uses SiO particles as the negative electrode active material, but these SiO particles are not pre-doped with lithium ions. The lithium-ion secondary battery of Comparative Example 4 was fabricated using the same manufacturing method and conditions as Example 4, but without the pre-doping process.

[0070] (Example 5) Next, a lithium-ion secondary battery of Example 5 was fabricated. The negative electrode of Example 5 was made of the same negative electrode material as in Example 1, and the electrode thickness was also the same. However, the electrode density of the negative electrode was 1.2 times that of the negative electrode of Example 1. At this time, the negative electrode capacity of Example 5 was 1.2 times that of the negative electrode of Example 1. In addition, the same amount of lithium ions as in Example 1 was pre-doped into the negative electrode active material. At this time, the negative electrode of Example 5 was fabricated using the same manufacturing method and conditions as Example 1, except that lithium ions were pre-doped into the negative electrode at a charge rate of 50% relative to the negative electrode capacity.

[0071] (Discharge test) Discharge tests were performed on the lithium-ion secondary batteries of Examples 2 to 4 and Comparative Examples 2 to 4 using the same methods and conditions as in Example 1 and Comparative Example 1. First, the discharge capacity of Comparative Example 2 was 108 mAh / g, the discharge capacity of Comparative Example 3 was 106 mAh / g, and the discharge capacity of Comparative Example 4 was 106 mAh / g.

[0072] In contrast, the discharge capacity of Example 2 was 220 mAh / g, the discharge capacity of Example 3 was 210 mAh / g, and the discharge capacity of Example 4 was 214 mAh / g. That is, Examples 2 to 4 had more than twice the discharge capacity of the corresponding lithium secondary batteries of Comparative Examples 2 to 4.

[0073] Thus, even when hard carbon, Si particles, or SiO particles are used instead of graphite as the negative electrode active material, a high-capacity lithium-ion secondary battery can be obtained by providing a positive electrode containing lithium vanadium phosphate and a negative electrode containing a negative electrode active material pre-doped with lithium ions.

[0074] The discharge capacity of Example 5 was 216 mAh / g. Even in Example 5, where the negative electrode capacity is 1.2 times that of Example 1, a high-capacity lithium-ion secondary battery can be obtained by providing a negative electrode containing a lithium-ion pre-doped negative electrode active material.

Claims

1. A positive electrode containing lithium vanadium phosphate, A negative electrode containing a lithium-ion pre-doped negative electrode active material, The electrolyte between the positive electrode and the negative electrode, To be equipped, A lithium-ion secondary battery characterized by the following features.

2. The negative electrode has a potential of 4.3V (vs. Li / Li) of lithium vanadium phosphate. ＋ ) to 1.2V (vs. Li / Li ＋ It has a negative electrode capacity greater than the positive electrode capacity that appears before it drops to ) When a negative electrode capacitance equivalent to the positive electrode capacitance is discharged to the negative electrode active material, the negative electrode potential is at least 1.2V (vs. Li / Li ＋ ) The lithium ions are pre-doped to maintain the following levels: A lithium-ion secondary battery according to claim 1, characterized by the above.

3. The negative electrode is the lithium vanadium phosphate with a voltage of 1.2 V (vs. Li / Li ＋ It has a negative electrode capacity greater than the positive electrode capacity that appears before it drops to ) The aforementioned negative electrode active material is pre-doped with a charge level of 50% or more. A lithium-ion secondary battery according to claim 1, characterized by the above.

4. The lithium vanadium phosphate is Li ３ V ２－ｘ M ｘ (PO ４ ) ３ represented by The aforementioned M is Mg, Al, Ca, Ti, Cr, Mn, Fe, Co, Ni, Zn, Zr, Nb, Sn, or a plurality of these metallic elements, and the atomic ratio of M, x, is 0 ≤ x ≤ 0.

2. A lithium-ion secondary battery according to any one of claims 1 to 3, characterized by the above.

5. The negative electrode active material is graphite, silicon, or silicon oxide. A lithium-ion secondary battery according to any one of claims 1 to 3, characterized by the above.

6. A cathode fabrication process for producing a cathode containing lithium vanadium phosphate, The system includes a negative electrode manufacturing step for producing a negative electrode containing a negative electrode active material. The negative electrode active material includes a pre-doping step of pre-doping it with lithium ions. A method for manufacturing lithium-ion secondary batteries characterized by the following.

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