High-energy-density lithium ion battery

Abstract

The high-energy-density lithium ion battery comprises a positive plate and a negative plate, the positive plate comprises a positive active material, the positive active material comprises a lithium iron phosphate material and a lithium-rich material, and the mass ratio of the lithium-rich material in the positive active material is 0.2%-8%; the chemical formula of the lithium iron phosphate material is Li1 + xFe1-yMy (PO4) 1 + z, x is greater than or equal to 0 and less than 0.1, y is greater than or equal to 0 and less than 0.01, z is greater than or equal to 0 and less than 0.04, and M comprises at least one of Ti, V, Zr, Nb, Mg, Mo, W and Al; the lithium-rich material comprises at least one of LFO and LNO; the chemical formula of the LFO is Li < 5 + a > Fe < 1-b > Z O < 4 + c, 0 < = a < 0.5, 0 < = b < 0.01, and 0 < = c < 0.03; z comprises at least one of Ti, V, Zr, Nb, Mg, Mo, W and Al; the chemical formula of the LNO is Li < 2 + > dNi < 1-e > YeO < 2 + > f,-0.5 < = d < = 0.5, e > = 0,-0.5 < = f < = 0.5; y comprises at least one of Ti, V, Al, Mg, Mn and Fe; the negative electrode active material comprises a silicon negative electrode material, and the silicon negative electrode material accounts for 1%-50% of the mass of the negative electrode active material. The method has the advantages of improving the energy density, the rate capability and the cycle performance of the lithium battery.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a high energy density lithium ion battery. Background Art

[0002] In recent years, with the gradual consumption of traditional fossil energy and the increasingly serious global warming problem, people are increasingly aware of the importance of new energy in the future society. Among all the new energy systems, solar energy, wind energy, hydropower, nuclear energy, etc. do not have convenient mobility; while lithium-ion batteries, as a lightweight energy storage form, have their specific irreplaceable nature in practical applications and are therefore widely used.

[0003] Lithium-ion batteries have the advantages of high energy density, low self-discharge, long cycle life, safety and environmental protection, and are therefore widely used in consumer electronics, electric vehicles, power and communication energy storage, etc. According to the different positive electrode materials, lithium-ion batteries can be divided into lithium cobalt oxide batteries, lithium manganese oxide batteries, ternary lithium batteries, lithium iron phosphate batteries, etc., corresponding to the positive electrode materials lithium cobalt oxide (LiCoO2, LCO), lithium manganese oxide (LiMn2O4, LMO), ternary nickel cobalt manganese oxide (NMC) and ternary nickel cobalt aluminum oxide (NCA), lithium iron phosphate (LiFePO4, LFP). Among them, lithium cobalt oxide LCO has the highest charge and discharge voltage, but the price of the main element Co is relatively high, and it is mainly used in consumer electronics; spinel lithium manganese oxide LMO is relatively cheap, but the energy density and cycle life are slightly lower, and it is mainly used in electric two-wheeled vehicles; ternary nickel cobalt manganese oxide NMC and ternary nickel cobalt aluminum oxide NCA have higher energy density and price, and are mainly used as high-end passenger car power batteries; lithium iron phosphate LFP positive electrode material is widely used in passenger car power batteries, commercial vehicle power batteries, and power & communication energy storage batteries due to its stable structure, abundant reserves of the main elements Fe&P, low price, ultra-long cycle life, and high safety.

[0004] However, the current energy density of the LFP-graphite system is close to the design upper limit of conventional batteries (180Wh / kg). Therefore, it is urgent to optimize the system to increase the energy density design upper limit of lithium iron phosphate batteries. Summary of the invention

[0005] In order to improve the energy density of a lithium battery while taking into account its rate performance and cycle performance, the present invention provides a high energy density lithium ion battery.

[0006] The present invention provides a high energy density lithium ion battery, which adopts the following technical solution:

[0007] A high energy density lithium ion battery, comprising a positive electrode sheet and a negative electrode sheet, wherein the positive electrode sheet comprises a positive electrode active material layer, wherein the positive electrode active material layer comprises a positive electrode active material, and the negative electrode sheet comprises a negative electrode active material layer, wherein the negative electrode active material layer comprises a negative electrode active material;

[0008] Wherein, the positive electrode active material comprises lithium iron phosphate material and lithium-rich material, and the mass proportion of the lithium-rich material in the positive electrode active material is 0.2%-8%;

[0009] The chemical formula of the lithium iron phosphate material LFP is Li 1+x Fe 1-y M y (PO4) 1+z , 0≤x<0.1, 0≤y<0.01, 0≤z<0.04; the M includes at least one of Ti, V, Zr, Nb, Mg, Mo, W, and Al;

[0010] The lithium-rich material includes at least one of LFO and LNO; wherein the chemical formula of the LFO is Li 5+a Fe 1- b Z b O 4+c , 0≤a<0.5, 0≤b<0.01, 0≤c<0.03; Z includes at least one of Ti, V, Zr, Nb, Mg, Mo, W, and Al;

[0011] The chemical formula of LNO is Li 2+d Ni 1-e Y e O 2+f , -0.5≤d≤0.5, e≥0, -0.5≤f≤0.5; the Y comprises at least one of Ti, V, Al, Mg, Mn, and Fe;

[0012] The negative electrode active material comprises a graphite material and a silicon negative electrode material, and the mass proportion of the silicon negative electrode material in the negative electrode active material is 1%-50%.

[0013] The first coulombic efficiency of lithium iron phosphate material LFP is relatively high (generally above 96%), while the first coulombic efficiency of the negative electrode active material obtained by combining graphite material and silicon negative electrode material is relatively low (generally below 90%). Therefore, the reversible gram capacity of the lithium battery is restricted by the first coulombic efficiency of the negative electrode.

[0014] First, the present invention introduces lithium-rich materials into the positive electrode active material. The high first charge gram capacity of the lithium-rich materials can compensate for the active lithium consumption caused by the formation of SEI film at the negative electrode during the first charge process, thereby increasing the amount of active lithium deintercalation at the negative electrode during the discharge process, thereby achieving the effect of increasing the reversible gram capacity of the lithium battery, and the energy density of the lithium battery reaches 200-280Wh / kg; further, since the lithium-rich material in the positive electrode system can significantly increase the gram capacity of the positive electrode, greatly reducing the coating thickness or surface density of the positive electrode active material layer, and the silicon in the (graphite + silicon) negative electrode system can significantly increase the gram capacity of the negative electrode, greatly reducing the thickness or surface density of the negative electrode active material layer; combining the above advantages, the use of the (LFP + lithium-rich material) positive electrode system in the present invention with the (graphite + silicon) negative electrode system can significantly improve the energy density of the lithium battery.

[0015] At the same time, since the thickness and surface density of the positive electrode active material layer and the negative electrode active material layer are greatly reduced, the transmission path of electrons and lithium ions in the electrode is shortened, which will effectively improve the kinetic performance of electron and lithium ion mass transfer and reduce the DC internal resistance (DCR) of the lithium battery, thereby helping to improve the energy efficiency, rate performance and low temperature performance of the lithium battery.

[0016] Second, the lithium-replenishing effect of lithium-rich materials can also improve the cycle life of lithium batteries to a certain extent. More importantly, the introduction of lithium-rich materials can change the SEI film components and reduce the direct current resistance (DCR) of lithium batteries, thereby improving the cycle life and energy efficiency of lithium batteries; since the volume of lithium-rich materials will shrink by 20%-70% after the first charging and deintercalation of lithium, while the volume of LFP will basically not change after the first charging and deintercalation of lithium (the volume change is within 5%), therefore, due to the volume shrinkage of lithium-rich materials, a new porous structure will be formed in the positive electrode active material layer of the positive electrode sheet of the present invention, which will be conducive to the electrolyte to fully infiltrate and adsorb in the positive electrode sheet, improve the liquid retention capacity of the positive electrode sheet, and effectively improve the mass transfer kinetics of lithium ions in the electrode, thereby significantly improving the cycle life of the lithium battery.

[0017] Third, by doping elements in lithium iron phosphate materials and lithium-rich materials, not only can the structural stability of lithium iron phosphate materials and lithium-rich materials be improved separately, but the combination of the two with the silicon negative electrode can further improve the rate performance of lithium batteries.

[0018] Fourth, since the potential of silicon negative electrode material to Li+ / Li when embedded with lithium is naturally higher than that of graphite material, the mixed potential of silicon negative electrode material and graphite material is still higher than that of graphite material. Therefore, the potential of the (graphite + silicon) negative electrode system in the present invention is still not close to the lithium deposition potential and cause lithium deposition when charged at a high rate. The safety during fast charging is better than that of a pure graphite negative electrode, which enables the battery to have higher fast charging performance.

[0019] Preferably, the average primary particle size of the lithium iron phosphate material is 0.1-2 μm, and the average primary particle size of the lithium-rich material is 0.8-25 μm.

[0020] Since lithium-rich materials will shrink in volume after the first charge and deintercalation of lithium, with the maximum shrinkage reaching 60%, by controlling the average primary particle size of the lithium-rich material, the size of the new pore structure formed in the positive electrode active material layer can be adjusted to form a pore structure that is conducive to electrolyte infiltration and has excellent liquid retention performance, thereby further improving the mass transfer kinetics of lithium ions in the electrode, reducing the liquid phase ohmic polarization caused by the electrolyte, and thereby reducing the DC internal resistance DCR of the battery, which helps to improve the battery's rate performance and cycle life.

[0021] Preferably, the doping amount of M in the lithium iron phosphate material is 500-8000 ppm, and / or the doping amount of Z in the lithium-rich material is 200-5000 ppm, and / or the doping amount of Y in the lithium-rich material is 200-5000 ppm.

[0022] Preferably, the lithium iron phosphate material is prepared by a method comprising the following steps:

[0023] The precursor is mixed with lithium carbonate and deionized water, and then ground, and a doping source containing a doping element is added thereto to obtain a mixed slurry; the mixed slurry is spray-dried and then sintered in an inert atmosphere to obtain the lithium iron phosphate material;

[0024] Wherein, the precursor includes iron phosphate; the doping source containing doping elements includes at least one of nano titanium oxide, soluble organic titanium, vanadium pentoxide, ammonium metavanadate, nano zirconium oxide, niobium pentoxide, magnesium oxide, molybdenum oxide, tungsten oxide, and nano aluminum oxide.

[0025] Preferably, the lithium-rich material is prepared by a method comprising the following steps:

[0026] The precursor is mixed with lithium carbonate and deionized water, and then ground, and a doping source containing a doping element is added thereto to obtain a mixed slurry; the mixed slurry is spray-dried and then sintered in an inert atmosphere to obtain the lithium-rich material;

[0027] Among them, the precursor includes nickel phosphate; the doping source containing doping elements includes at least one of nano titanium oxide, soluble organic titanium, vanadium pentoxide, ammonium metavanadate, nano zirconium oxide, niobium pentoxide, magnesium oxide, molybdenum oxide, tungsten oxide, nano aluminum oxide, manganese oxide, and iron oxide.

[0028] Preferably, in the lithium iron phosphate material, M includes any two of Ti, V and Nb.

[0029] Preferably, in the lithium iron phosphate material, the doping amount of Ti is 1000-2000 ppm, the doping amount of V is 500-1000 ppm, and the doping amount of Nb is 200-500 ppm.

[0030] By further optimizing the type of M and controlling the doping amount of the doping element, not only the structural stability of the lithium iron phosphate material can be improved, but also the rate performance of the lithium iron phosphate material can be improved.

[0031] Preferably, in the lithium-rich material, Z includes any two of Ti, V, and Nb, and Y includes any two of Ti, V, and Mg.

[0032] Preferably, in the lithium-rich material, the doping amount of Ti is 1000-2000 ppm, the doping amount of V is 500-1000 ppm, the doping amount of Nb is 200-500 ppm, and the doping amount of Mg is 200-500 ppm.

[0033] By further optimizing the type of Z and controlling the doping amount of the doping element, not only the structural stability of the lithium-rich material can be improved, but also the stability of the combination of the lithium-rich material and the lithium iron phosphate material can be improved, which helps to improve the rate performance of the lithium battery.

[0034] Preferably, the positive electrode active material layer further includes a positive electrode conductor and a binder, and the mass ratio of the positive electrode active material, the positive electrode conductor and the binder is 95-100:0.5-1.5:1-3.

[0035] Preferably, the positive electrode conductor includes at least one of carbon nanotubes CNT and conductive carbon black SP; and the binder includes polyvinylidene fluoride PVDF.

[0036] Preferably, the average primary particle size of the silicon negative electrode material is 0.05-0.5 μm, and the average primary particle size of the graphite material is 5-15 μm.

[0037] The silicon negative electrode material will shrink in volume after the first charge and deintercalation of lithium. By controlling the average primary particle size of the silicon negative electrode material and the average primary particle size of the graphite material, the silicon negative electrode material can form a suitable pore structure in the positive electrode active layer, forming a pore structure that is conducive to electrolyte infiltration and has excellent liquid retention performance. In conjunction with the positive electrode sheet, it accelerates the transfer of lithium ions between the positive and negative electrode sheets without the occurrence of lithium plating, significantly improving the kinetic performance and cycle performance of the lithium battery.

[0038] Preferably, the silicon negative electrode material includes at least one of single crystal silicon spherical particles, polycrystalline silicon spherical particles, amorphous silicon spherical particles, single crystal silicon-like spherical particles, polycrystalline silicon-like spherical particles, amorphous silicon-like spherical particles, single crystal silicon amorphous particles, polycrystalline silicon amorphous particles, and amorphous silicon amorphous particles.

[0039] Preferably, the mass proportion of the silicon negative electrode material in the negative electrode active material is 5%-30%.

[0040] Preferably, the negative electrode active material layer further comprises a negative electrode conductor, a thickener and an adhesive, and the mass ratio of the negative electrode active material, the negative electrode conductor, the thickener and the adhesive is 95-98:0.1-1.5:1-2:1-2.

[0041] Preferably, the negative electrode conductor includes conductive carbon black SP; the thickener includes sodium carboxymethyl cellulose CMC; and the adhesive includes styrene-butadiene rubber SBR.

[0042] Preferably, the compaction density of the positive electrode sheet is 2.2-2.81 g / cm 3 , and / or, the compaction density of the negative electrode sheet is 1.45-1.8 g / cm 3 .

[0043] Preferably, the lithium-ion battery of the present invention is suitable for button batteries, wound soft-pack batteries, laminated soft-pack batteries, wound square hard-shell batteries, laminated square hard-shell batteries, wound cylindrical batteries, etc.

[0044] Preferably, the lithium-ion battery is prepared by a method comprising the following steps:

[0045] 1. Preparation of positive electrode

[0046] In the positive electrode slurry, the mass ratio of lithium iron phosphate material LFP, lithium-rich material, carbon nanotube CNT, conductive graphite SP, and polyvinylidene fluoride PVDF is 96.0:1.5:0.5:0.5:2.0;

[0047] The first step is to mix the positive electrode conductive agent with the first part of N-methylpyrrolidone (NMP), and stir at a low speed of 2.0-5.0 m / s for 0.5-3 h to obtain a positive electrode conductive slurry; the second step is to add the positive electrode active material (LFP and lithium-rich material mixed) to the above positive electrode conductive slurry in steps (at least twice) and stir at a low speed of 2.0-5.0 m / s for 0.5-3 h; the third step is to continue stirring at a speed of 10 m / s-25 The mixture is stirred at a high speed of 0.5-3 h at a linear speed of m / s to obtain a positive electrode mixed slurry; the third step is to mix polyvinylidene fluoride (PVDF) with the second part of N-methylpyrrolidone (NMP) to obtain a glue solution, and stir it at a linear speed of 10m / s-25m / s for 1-8h to obtain a premixed slurry; the fifth step is to add N-methylpyrrolidone (NMP) to the above premixed slurry and adjust the viscosity to 5000-25000mPa.s to obtain a positive electrode slurry.

[0048] The positive electrode slurry is coated on both surfaces of a carbon-coated aluminum foil (thickness 10-15 μm) using a transfer coater or an extrusion coater, and the positive electrode sheet (the porosity of the positive electrode sheet is 23%-40%) is obtained after drying, rolling and slitting.

[0049] 2. Negative electrode preparation

[0050] In the negative electrode slurry, the mass ratio of graphite material, silicon negative electrode material, conductive graphite SP, carboxymethyl cellulose CMC, and styrene butadiene rubber SBR is 96.0:1.5:0.5:0.5:2.0;

[0051] The first step is to add the negative electrode active material (graphite material and silicon negative electrode material mixed) to deionized water in steps (at least twice) to obtain a mixed slurry; the second step is to add the negative electrode conductive agent (SP) to the above mixed slurry, stir it at a low speed of 2.0-5.0m / s for 0.5-3h to knead it to obtain a conductive slurry; the third step is to add the thickener (CMC) to the above conductive slurry, stir it at a low speed of 5-15m / s for 0.5-3h to obtain a composite slurry; the fourth step is to add the adhesive (SBR) to the above composite slurry, stir it at a linear speed of 5-20m / s for 1-8h to obtain a premixed slurry; the fifth step is to add deionized water to the above premixed slurry and adjust the viscosity to 3000-8000mPa.s to obtain the negative electrode slurry.

[0052] The negative electrode slurry is coated on both surfaces of a copper foil (thickness 4.5-8 μm) using a transfer coater or an extrusion coater, and then dried, rolled and cut to obtain a negative electrode sheet (the porosity of the negative electrode sheet is 22%-37%).

[0053] 3. Lithium battery preparation

[0054] (1) Winding: stack the positive electrode sheet, separator, and negative electrode sheet (the separator separates the positive and negative electrode sheets, the negative electrode sheet sandwiches the positive electrode sheet, and the lead ends of the positive and negative electrode ears are distributed on opposite sides) in sequence, and wind manually or automatically / semi-automatically to obtain a winding core;

[0055] (2) Tab welding: Ultrasonic weld the aluminum tab to the positive terminal and the nickel tab to the negative terminal, and apply PET protective tape to the welded joints.

[0056] (3) Baking: placing the core in a vacuum environment and baking at 90-120° C. for 8-24 hours to reduce the moisture content of each part inside the core to below 400 ppm (preferably below 300 ppm);

[0057] (4) Shelling (sealing with aluminum-plastic film): Cut the aluminum-plastic film into rectangles of a certain length and width, tightly wrap the above-mentioned core, and use a sealing machine to heat-press and seal the edges along the positive and negative ears, leaving a liquid injection port on the side;

[0058] (5) Liquid injection: inject an appropriate amount of electrolyte into the aluminum-plastic packaging shell, wherein the electrolyte composition is 1.0MLiPF6+EC / EMC / DEC+VC / FEC / DTD and other additives;

[0059] (6) Vacuum standing: Place the injected core into a vacuum incubator and evacuate to maintain negative pressure for 0.5-2 hours to allow the positive / negative electrode sheets and separator to be completely soaked in the electrolyte;

[0060] (7) Sealing: The last opening of the aluminum-plastic packaging bag is sealed with a sealing machine, and a certain distance is left between the sealing point and the roll core to facilitate the storage of a small amount of gas generated by the battery after subsequent formation (commonly known as the "air bag");

[0061] (12) Standing: Place the sealed battery in a 45±5°C incubator for 24-48 hours to allow the electrolyte to fully infiltrate the pores of the positive and negative electrodes and the separator again;

[0062] (13) Fixture formation: The lithium battery is clamped and charged in a formation cabinet at 45±5°C by applying a certain current to the battery to charge it to 20% SOC-70% SOC, so that the negative electrode and the electrolyte form a SEI film and a portion of gas by-products are generated;

[0063] (14) Vacuuming: Use vacuum pumping equipment to puncture the air bag and then evacuate the by-product gas generated in the formation stage;

[0064] (15) Sealing: Use a sealing machine to perform heat-pressure sealing at the part close to the core, and cut off the air bag to make a complete battery cell;

[0065] (16) High temperature aging: The sealed battery is placed at 45±5°C for 12-72 hours to make the SEI film formed on the negative electrode surface more stable;

[0066] (17) Capacity division: Continue to charge to 3.65V at a constant current of 0.5C, and maintain a constant voltage at 3.65V until the current decreases to below 0.05C and the charging is cut off; after standing for 5-30 minutes, discharge to 2.5V at a constant current of 0.2C; the first coulomb efficiency = capacity division discharge capacity / (formation charge capacity + capacity division continuous charge capacity); charge the battery to a specific state of charge (20% SOC-50% SOC) at a constant current of 0.5C and store at 25°C;

[0067] (18) OCV test: Before storage at 25°C, test the open circuit voltage of the battery, recorded as OCV1; after storage at 25°C for a period of time t (24-72h), test the open circuit voltage of the battery again, recorded as OCV2; then the battery self-discharge K value = (OCV1-OCV2) / t, and qualified lithium batteries are screened by K value. Batteries with too high K value are downgraded or scrapped.

[0068] The preparation process of the positive and negative electrode sheets of the present invention is very simple, and can adopt the slurrying, coating, drying, rolling, slitting / sheeting processes commonly used in the conventional industrial production of lithium-ion batteries, which can effectively ensure the large-scale manufacturability and cost control of the positive and negative electrode sheets and have excellent economic value.

[0069] Preferably, the base film comprises at least one of polyethylene PE and polypropylene PP; the base film has a thickness of 5-20 μm and a porosity of 0.4-0.6.

[0070] Preferably, a coating layer is provided on at least one surface of the base film, and the thickness of the coating layer is 1-4 μm; the coating layer includes granular glue and ceramic powder, the granular glue includes at least one of polyvinylidene fluoride PVDF, polymethyl methacrylate PMMA, and polyacrylic acid PAA, and the particle size of the granular glue is 0.5-4 μm; the ceramic powder includes at least one of boehmite powder and alumina powder, and the particle size of the ceramic powder is 0.5-1 μm.

[0071] Preferably, the electrolyte comprises a lithium salt, a solvent and an additive, and the solvent comprises at least one of ethylene carbonate EC, diethylene carbonate DEC, ethyl methyl carbonate EMC, dimethyl carbonate DMC, propylene carbonate PC, and methyl acetate MA;

[0072] The lithium salt includes at least one of lithium hexafluorophosphate LiPF6, lithium bis(fluorosulfonyl)imide LiFSI, lithium bis(trifluoromethylsulfonyl)imide LiTFSI, lithium bis(oxalatoborate) LiBOB, and lithium difluorooxalatoborate LiODFB;

[0073] The additives include but are not limited to at least one of vinylene carbonate VC, vinyl ethylene carbonate VEC, propylene sulfite PS, fluoroethylene carbonate FEC, vinyl sulfate DTD, dimethyl sulfate DMS, diethyl oxalate DEP, trimethyl borate TMB, fluorobenzene FB, biphenyl BP, and phenylcyclohexane CHB.

[0074] Preferably, the lithium battery further comprises a packaging material; the packaging material comprises any one of an aluminum-plastic film, a steel shell, an aluminum shell, and an aluminum alloy shell. BRIEF DESCRIPTION OF THE DRAWINGS

[0075] Figure 1 This is a DCR value test diagram of the lithium battery in Example 1, Comparative Example 1, and Comparative Example 3 of the present invention.

[0076] Figure 2 It is a test diagram of the cycle capacity performance of the lithium batteries in Example 1, Comparative Example 1 and Comparative Example 3 of the present invention.

[0077] Figure 3 This is a cross-sectional view of the positive electrode sheet in the lithium battery of Example 1 of the present invention.

[0078] Figure 4 This is a cross-sectional view of the positive electrode sheet in the lithium battery of Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0079] For better understanding and implementation, the technical solution of the present invention will be clearly and completely described below in conjunction with embodiments. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments.

[0080] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used in the specification of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0081] Unless otherwise indicated, all numerical values ​​expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as modified by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth herein are approximations that can vary depending upon the desired properties desired to be obtained.

[0082] As used herein, "and / or" means one or all of the mentioned elements.

[0083] As used herein, “including” and “comprising” encompasses the case where only the stated elements are present as well as the case where there are other elements other than the stated elements.

[0084] All percentages in the present invention are by weight, unless otherwise specified.

[0085] Unless otherwise indicated, "a", "an", "an" and "the" used in this specification are intended to include "at least one" or "one or more". For example, "a component" means one or more components, so more than one component may be considered and may be employed or used in the implementation of the embodiment.

[0086] Example 1

[0087] The lithium-ion battery is prepared by a method comprising the following steps:

[0088] 1. Preparation of positive electrode

[0089] In the positive electrode slurry, lithium iron phosphate material LFP (Li 1.05 Fe 0.995 M 0.005 (PO4) 1.02 , where M is Ti and V, the doping amount of Ti is 1500ppm, the doping amount of V is 700ppm), lithium-rich material LFO (Li 5.3 Fe 0.995 Z 0.005 O 4.02 , wherein Z is Ti and Nb, the doping amount of Ti is 1000ppm, the doping amount of Nb is 500ppm), the mass ratio of carbon nanotubes CNT, conductive graphite SP, and polyvinylidene fluoride PVDF is 96.0:1.5:0.5:0.5:2.0;

[0090] The first step is to mix the positive electrode conductive agent with the first part of N-methylpyrrolidone (NMP), and stir at a low speed of 3m / s for 2h to obtain a positive electrode conductive slurry; the second step is to add the positive electrode active material (LFP and LFO mixed) to the above positive electrode conductive slurry in steps (at least twice) and stir at a low speed of 3m / s for 2h; the third step is to continue to stir at a high speed of 15m / s for 2h to obtain a positive electrode mixed slurry; the third step is to mix polyvinylidene fluoride (PVDF) with the second part of N-methylpyrrolidone (NMP) to obtain a glue, and stir at a high speed of 16m / s for 4h to obtain a premixed slurry; the fifth step is to add N-methylpyrrolidone (NMP) to the above premixed slurry and adjust the viscosity to 15000mPa.s to obtain a positive electrode slurry.

[0091] The positive electrode slurry was coated on both surfaces of carbon-coated aluminum foil (the thickness of the aluminum foil was 12 μm) using a transfer coater, and the positive electrode sheet was obtained after drying, rolling and slitting (the porosity of the positive electrode sheet was 32%).

[0092] 2. Negative electrode preparation

[0093] In the negative electrode slurry, the mass ratio of graphite material, silicon negative electrode material, conductive graphite SP, carboxymethyl cellulose CMC, and styrene butadiene rubber SBR is 96.0:1.5:0.5:0.5:2.0;

[0094] The first step is to add the negative electrode active material (graphite material and silicon negative electrode material mixed) to deionized water in steps (at least twice) to obtain a mixed slurry; the second step is to add the negative electrode conductive agent (SP) to the above mixed slurry, stir it at a low speed of 4m / s for 1h to knead it, and obtain a conductive slurry; the third step is to add a thickener (CMC) to the above conductive slurry, stir it at a low speed of 10m / s for 2h to obtain a composite slurry; the fourth step is to add an adhesive (SBR) to the above composite slurry, stir it at a linear speed of 12m / s for 4h to obtain a premixed slurry; the fifth step is to add deionized water to the above premixed slurry and adjust the viscosity to 6000mPa.s to obtain a negative electrode slurry.

[0095] The negative electrode slurry was coated on both surfaces of a copper foil (5 μm thick) using a transfer coater, and the negative electrode sheet (the porosity of the negative electrode sheet was 30%) was obtained after drying, rolling and slitting.

[0096] 3. Lithium battery preparation

[0097] (1) Winding: stacking the positive electrode sheet, the base film, and the negative electrode sheet (the base film separates the positive and negative electrode sheets, the negative electrode sheet sandwiches the positive electrode sheet, and the lead ends of the positive and negative electrode ears are distributed on opposite sides) in sequence, and winding manually or automatically / semi-automatically to obtain a winding core; wherein a coating layer is provided on the surface of the base film, and the thickness of the coating layer is 2 μm; the coating layer comprises polyvinylidene fluoride PVDF (particle size is 0.5-4 μm) and boehmite powder (particle size is 0.5-1 μm);

[0098] (2) Tab welding: Ultrasonic weld the aluminum tab to the positive terminal and the nickel tab to the negative terminal, and apply PET protective tape to the welded joints.

[0099] (3) Baking: The core is placed in a vacuum environment and baked at 100° C. for 16 h to reduce the moisture content of each part inside the core to less than 400 ppm;

[0100] (4) Shelling (sealing with aluminum-plastic film): Cut the aluminum-plastic film into rectangles of a certain length and width, tightly wrap the above-mentioned core, and use a sealing machine to heat-press and seal the edges along the positive and negative ears, leaving a liquid injection port on the side;

[0101] (5) Liquid injection: inject an appropriate amount of electrolyte into the aluminum-plastic packaging shell, wherein the electrolyte composition is 1.0MLiPF6+EC / EMC / DEC+VC / FEC / DTD and other additives;

[0102] (6) Vacuum standing: Place the injected core into a vacuum incubator and evacuate to maintain negative pressure for 1 hour to allow the positive / negative electrode sheets and separator to be completely soaked in the electrolyte;

[0103] (7) Sealing: The last opening of the aluminum-plastic packaging bag is sealed with a sealing machine, and a certain distance is left between the sealing point and the roll core to facilitate the storage of a small amount of gas generated by the battery after subsequent formation (commonly known as the "air bag");

[0104] (12) Standing: Place the sealed battery in a 45±5°C incubator for 36 h to allow the electrolyte to fully infiltrate the pores of the positive and negative electrodes and the separator again;

[0105] (13) Formation on a fixture: The lithium battery is placed on a fixture and charged in a formation cabinet at 45±5°C with a certain current to charge the battery to 50% SOC, so that the negative electrode reacts with the electrolyte to form a SEI film and a portion of gas byproducts are generated;

[0106] (14) Vacuuming: Use vacuum pumping equipment to puncture the air bag and then evacuate the by-product gas generated in the formation stage;

[0107] (15) Sealing: Use a sealing machine to perform heat-pressure sealing at the part close to the core, and cut off the air bag to make a complete battery cell;

[0108] (16) High temperature aging: The sealed battery is placed at 45±5°C for 45 h to make the SEI film formed on the negative electrode surface more stable;

[0109] (17) Capacity division: Continue to charge to 3.65V at a constant current rate of 0.5C, and maintain a constant voltage at 3.65V until the current decreases to below 0.05C and the charging is cut off; after standing for 20 minutes, discharge to 2.5V at a constant current rate of 0.2C; the first coulomb efficiency = capacity division discharge capacity / (formation charge capacity + capacity division continuous charge capacity); charge the battery to a specific state of charge (40% SOC) at a constant current of 0.5C and store at 25°C;

[0110] (18) OCV test: Before storage at 25°C, test the open circuit voltage of the battery, recorded as OCV1; after storage at 25°C for a period of time t (24-72h), test the open circuit voltage of the battery again, recorded as OCV2; then the self-discharge K value of the lithium battery = (OCV1-OCV2) / t, and qualified lithium batteries are screened by the K value. Batteries with too high a K value are downgraded or scrapped.

[0111] The DCR value test graph and the cycle capacity performance test graph of the lithium battery in this embodiment are respectively as follows: Figure 1 , Figure 2 As shown; the cross-sectional view of the positive electrode sheet of the lithium battery in this embodiment is as shown Figure 3 shown.

[0112] Example 2

[0113] The difference between this embodiment and Embodiment 1 is that in the positive electrode slurry, the mass ratio of LFP, LFO, CNT, SP and PVDF is 95.0:2.5:0.5:0.5:2.0; the other steps and parameter settings are consistent with those in Embodiment 1.

[0114] Example 3

[0115] The difference between this embodiment and Embodiment 1 is that in the positive electrode slurry, the mass ratio of LFP, LFO, CNT, SP and PVDF is 91.0:6.5:0.5:0.5:2.0; the other steps and parameter settings are consistent with those in Embodiment 1.

[0116] Example 4

[0117] The difference between this embodiment and Embodiment 1 is that in the negative electrode slurry, the mass ratio of graphite material, silicon negative electrode material, SP, CMC, and SBR is 91.0:6.5:0.5:0.5:2.0; other steps and parameter settings are consistent with Embodiment 1.

[0118] Example 5

[0119] The difference between this embodiment and Embodiment 1 is that in the negative electrode slurry, the mass ratio of graphite material, silicon negative electrode material, SP, CMC, and SBR is 86.0:10.5:0.5:1.5:1.5; other steps and parameter settings are consistent with Embodiment 1.

[0120] Example 6

[0121] The difference between this embodiment and embodiment 1 is that the positive electrode active material used in the positive electrode slurry is lithium iron phosphate material LFP (Li1Fe 0.999 M 0.001 (PO4) 1.04 , where M is Ti and Nb, the doping amount of Ti is 1000ppm, the doping amount of Nb is 200ppm), lithium-rich material LFO (Li5Fe 0.09 Z 0.01 O 4.03 , wherein Z is Ti and V, the doping amount of Ti is 2000ppm, and the doping amount of V is 1000ppm); the other steps and parameter settings are consistent with those in Example 1.

[0122] Example 7

[0123] The difference between this embodiment and embodiment 1 is that the positive electrode active material used in the positive electrode slurry is lithium iron phosphate material LFP (Li1.01 Fe 0.99 M 0.01 (PO4) 1.01 , where M is V and Nb, the doping amount of V is 1000ppm, the doping amount of Nb is 500ppm), lithium-rich material LFO (Li 5.5 Fe 0.99 Z 0.01 O 4.3 , wherein Z is V and Nb, the doping amount of V is 500ppm, and the doping amount of Nb is 200ppm); the other steps and parameter settings are consistent with those in Example 1.

[0124] Example 8

[0125] The difference between this embodiment and embodiment 1 is that the lithium-rich material is LNO (Li 2.2 Ni 0.95 Y 0.05 O 2.1 , wherein Y is V and Mg, the doping amount of V is 500ppm, and the doping amount of Mg is 200ppm); the other steps and parameter settings are consistent with those in Example 1.

[0126] Example 9

[0127] The difference between this embodiment and embodiment 1 is that the average primary particle size of the lithium-rich material LFO is 0.5 μm, and the average primary particle size of the lithium iron phosphate material is 3 μm; other steps and parameter settings are consistent with those in embodiment 1.

[0128] Example 10

[0129] The difference between this embodiment and embodiment 1 is that the compaction density of the positive electrode sheet is 3g / cm 3 ; Other steps and parameter settings are consistent with Example 1.

[0130] Embodiment 11

[0131] The difference between this embodiment and embodiment 1 is that the average primary particle size of the silicon negative electrode material is 1.5 μm, and the average primary particle size of the graphite material is 30 μm; other steps and parameter settings are consistent with those in embodiment 1.

[0132] Example 12

[0133] The difference between this embodiment and embodiment 1 is that the compaction density of the negative electrode sheet is 2g / cm 3 ; Other steps and parameter settings are consistent with Example 1.

[0134] Comparative Example 1

[0135] The difference between this embodiment and embodiment 1 is that the positive electrode active material does not contain lithium-rich material, and the negative electrode active material does not contain silicon negative electrode material;

[0136] Specifically, in the positive electrode slurry, the mass ratio of LFP, CNT, SP, and PVDF is 97.0:0.5:0.5:2.0; in the negative electrode slurry, the mass ratio of graphite, SP, CMC, and SBR is 96.5:0.5:1.5:1.5;

[0137] The other steps and parameter settings are consistent with those in Example 1.

[0138] The DCR value test graph and the cycle capacity performance test graph of the lithium battery in this comparative example are shown as follows: Figure 1 , Figure 2 As shown; the cross-sectional view of the positive electrode sheet of the lithium battery in this comparative example is as shown Figure 4 shown.

[0139] Comparative Example 2

[0140] The difference between this embodiment and embodiment 1 is that the positive electrode active material does not contain lithium-rich material. Specifically, in the positive electrode slurry, the mass ratio of LFP, CNT, SP, and PVDF is 97.0:0.5:0.5:2.0; other steps and parameter settings are consistent with embodiment 1.

[0141] Comparative Example 3

[0142] The difference between this embodiment and embodiment 1 is that the negative electrode active material does not contain silicon negative electrode material. Specifically, in the negative electrode slurry, the mass ratio of graphite, SP, CMC, and SBR is 96.5:0.5:1.5:1.5; other steps and parameter settings are consistent with embodiment 1.

[0143] The DCR value test graph and the cycle capacity performance test graph of the lithium battery in this comparative example are shown as follows: Figure 1 , Figure 2 shown.

[0144] Comparative Example 4

[0145] The difference between this comparative example and Example 1 is that neither the lithium iron phosphate material nor the lithium-rich material contains doping elements; other steps and parameter settings are consistent with Example 1.

[0146] Test Method

[0147] 1. Energy density test

[0148] The energy density test was performed on the lithium batteries in the above embodiments and comparative examples. The specific test method was as follows: weigh the weight of the lithium battery to be tested, recorded as m; place the battery in a fixture, apply a force of 3000N, charge the single cell at a constant current of 0.33C to 3.65V, and continue to charge at a constant voltage of 3.65V until the current drops to 0.05C and is cut off; leave it for 30 minutes, discharge it at a constant current of 0.33C to 2.5V, leave it for 30 minutes, and cycle it for 3 consecutive times; calculate the discharge capacity (in Ah) and energy E (average value of three cycles), discharge energy density = E / m (in Wh / kg).

[0149] 2. Electrode Gram Capacity Test

[0150] The electrodes in the above-mentioned embodiments and comparative examples are subjected to the first charge and discharge capacity-gram test, and the specific test method is as follows: for the first charge-gram capacity test of the positive electrode, the positive electrode electrode is taken and one side is wiped with a wet wipe until the bare foil is exposed, and after the electrode is dried, it is punched out with a punching machine and weighed, and the mass is recorded as m1. In addition, the bare aluminum foil is punched out with a punching machine and weighed, and the mass is recorded as m2, and the coating mass m3=m1-m2; the punched electrode is taken and made into a button battery together with a lithium sheet, an electrolyte, and a diaphragm. The button cell was placed on a blue-electric test cabinet for testing. The test steps were as follows: charge at a constant current of 0.1C to 3.75V, then charge at a constant voltage of 3.75V until the current drops to 0.05C and the charging capacity is recorded as q1. The first charge gram capacity of the positive electrode = q1 / (m1×the proportion of active substances in the positive electrode coating); let it stand for 10 minutes, discharge at a constant current of 0.1C to 2.0V and the discharge capacity is recorded as q2. The first discharge gram capacity of the positive electrode = q2 / (m1×the proportion of active substances in the positive electrode coating);

[0151] The test process of the negative electrode's first charge capacity in grams is basically the same as the button battery preparation process and the positive electrode. The mass of the negative electrode sheet is recorded as m4, the mass of the bare copper foil is recorded as m5, and the mass of the negative electrode coating is recorded as m6. The test steps are as follows: 0.1C constant current charging to 0.005V cutoff, the first charging capacity is recorded as q3, then the negative electrode's first charge capacity in grams = q3 / (m1×the proportion of active substances in the negative electrode coating); 0.1C discharge to 0.5V cutoff, the first discharge capacity is recorded as q4, then the negative electrode's first discharge capacity in grams = q4 / (m1×the proportion of active substances in the negative electrode coating).

[0152] 3. Rate performance test

[0153] The lithium battery in the above embodiment and comparative example was tested for rate performance. The specific test method is as follows: the lithium battery was pre-treated at 25±2°C, and the steps are as follows: charging at 0.5C constant current and constant voltage, with a cut-off voltage of 3.65V and a cut-off current of 0.05C; standing for 5 minutes; standing for 5 minutes; discharging at 0.5C constant current, with a cut-off voltage of 2.5V; after 13 cycles, charging and discharging at 0.1C once again. The last 0.1C discharge capacity is taken as the initial capacity, recorded as Q0;

[0154] Test the capacity retention rate at 1C rate: the battery is fully charged at 0.5C constant current and constant voltage, with a cut-off voltage of 3.65V, and then discharged at 1C rate constant current, record the battery's discharge capacity Q1, and calculate the 1C capacity retention rate = Q1 / Q0×100%.

[0155] 4. Cycle performance test

[0156] The lithium batteries in the above embodiments and comparative examples were subjected to a cycle performance test. The specific test method was as follows: at 25°C and a voltage range of 2.5-3.65V, the prepared batteries were charged and discharged at a rate of 1C; during the charging process, 1C constant current was charged to 3.65V, and then 3.65V constant voltage was charged until the current dropped to 0.05C and then cut off; during the discharging process, 1C constant current was discharged to 2.5V and then cut off; the cycle was 1000 times, and the cycle capacity retention rate was recorded.

[0157] 5. DC internal resistance test

[0158] The lithium batteries in the above embodiments and comparative examples were subjected to a DC internal resistance test. The specific test method was as follows: at 25° C., the batteries in the embodiments and comparative examples were charged to 95% SOC, discharged at a current rate of 1C for 10 seconds, and the battery voltage U1 before discharge, the current I, and the battery voltage U2 after 10 seconds of discharge were recorded. The DC internal resistance DCR was calculated according to the formula R=(U1-U2) / I.

[0159] Table 1

[0160]

[0161] In combination with Examples 1-8, Comparative Examples 1-4 and Table 1, it can be seen that by using lithium iron phosphate materials and lithium-rich materials as positive electrode active materials, and graphite materials and silicon negative electrode materials as negative electrode active materials, and doping suitable elements in the lithium iron phosphate materials and lithium-rich materials, the energy density, rate performance and low temperature performance of the lithium battery can be significantly improved. In addition, the combination of lithium iron phosphate materials and lithium-rich materials can form a pore structure in the positive electrode active material layer that is conducive to lithium ion transmission and electron transmission, which is beneficial to improving the cycle performance of the electrolyte for the lithium battery; finally, the combination of the positive electrode active material and the negative electrode active material in the present invention can significantly improve the charging and discharging safety performance of the lithium battery.

[0162] In combination with Examples 1, 9-10 and Table 1, it can be seen that by controlling the average primary particle size of the lithium-rich material in the positive electrode active material to be larger than the average primary particle size of the lithium iron phosphate material, the size of the pore structure formed in the positive electrode active material layer can be adjusted, and the positive electrode sheet can be adjusted to obtain a suitable surface density and form a pore structure that is conducive to electrolyte infiltration and has excellent liquid retention performance; and the combination of the two types of particle positive electrode active materials helps to improve the compaction density of the positive electrode sheet; the above combined effects can improve the mass transfer kinetics in the electrode, reduce the liquid phase ohmic polarization caused by the electrolyte, and thereby reduce the DC internal resistance DCR of the battery, which helps to improve the battery's rate performance, cycle life and energy density.

[0163] In combination with Example 1, Examples 11-12 and Table 1, it can be seen that by controlling the average primary particle size of the silicon negative electrode material and the average primary particle size of the graphite material in the negative electrode active material, not only can the negative electrode sheet obtain a compaction density range that cooperates well with the positive electrode sheet, but also a negative electrode active material layer with a suitable pore structure can be formed to avoid lithium precipitation at the negative electrode during the charge and discharge process of the lithium battery. In combination with the positive electrode sheet, the kinetic performance and cycle performance of the lithium battery can be significantly improved.

[0164] The above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the protection scope of the present invention. Although the present invention is described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the technical solutions of the present invention can be modified or equivalently replaced, but these modifications or replacements are all within the protection scope of the present invention.

Claims

1. A high energy density lithium ion battery, characterized in that: The invention comprises a positive electrode sheet and a negative electrode sheet, wherein the positive electrode sheet comprises a positive electrode active material layer, wherein the positive electrode active material layer comprises a positive electrode active material, and the negative electrode sheet comprises a negative electrode active material layer, wherein the negative electrode active material layer comprises a negative electrode active material; Wherein, the positive electrode active material comprises lithium iron phosphate material and lithium-rich material, and the mass proportion of the lithium-rich material in the positive electrode active material is 0.2%-8%; The chemical formula of the lithium iron phosphate material LFP is Li 1+x Fe 1-y M y (PO4) 1+z , 0≤x≤0.1, 0≤y≤0.01, 0≤z≤0.04; the M includes at least one of Ti, V, Zr, Nb, Mg, Mo, W, and Al; The lithium-rich material includes at least one of LFO and LNO; wherein the chemical formula of the LFO is Li 5+a Fe 1-b Z b O 4+c , 0≤a≤0.5, 0≤b≤0.01, 0≤c≤0.03; Z includes at least one of Ti, V, Zr, Nb, Mg, Mo, W, and Al; The chemical formula of LNO is Li 2+d Ni 1-e Y e O 2+f , -0.5≤d≤0.5, e≥0, -0.5≤f≤0.5; the Y comprises at least one of Ti, V, Al, Mg, Mn, and Fe; The negative electrode active material comprises a graphite material and a silicon negative electrode material, and the mass proportion of the silicon negative electrode material in the negative electrode active material is 1%-50%.

2. The high energy density lithium ion battery according to claim 1, characterized in that: The average primary particle size of the lithium iron phosphate material is 0.1-2 μm, and the average primary particle size of the lithium-rich material is 0.8-25 μm.

3. The high energy density lithium ion battery according to claim 1, characterized in that: The doping amount of M in the lithium iron phosphate material is 500-8000 ppm, and / or the doping amount of Z in the lithium-rich material is 200-5000 ppm, and / or the doping amount of Y in the lithium-rich material is 200-5000 ppm.

4. The high energy density lithium ion battery according to claim 3, characterized in that: In the lithium iron phosphate material, M includes any two of Ti, V, and Nb.

5. The high energy density lithium ion battery according to claim 4, characterized in that: In the lithium iron phosphate material, the doping amount of Ti is 1000-2000 ppm, the doping amount of V is 500-1000 ppm, and the doping amount of Nb is 200-500 ppm.

6. The high energy density lithium ion battery according to claim 3, characterized in that: In the lithium-rich material, Z includes any two of Ti, V, and Nb, and Y includes any two of Ti, V, and Mg.

7. The high energy density lithium ion battery according to claim 6, characterized in that: In the lithium-rich material, the doping amount of Ti is 1000-2000 ppm, the doping amount of V is 500-1000 ppm, the doping amount of Nb is 200-500 ppm, and the doping amount of Mg is 200-500 ppm.

8. The high energy density lithium ion battery according to claim 1, characterized in that: The average primary particle size of the silicon negative electrode material is 0.05-0.5 μm, and the average primary particle size of the graphite material is 5-15 μm.

9. The high energy density lithium ion battery according to claim 1, characterized in that: The silicon negative electrode material accounts for 5% to 30% by mass of the negative electrode active material.

10. The high energy density lithium ion battery according to any one of claims 1 to 9, characterized in that: The compaction density of the positive electrode sheet is 2.2-2.81 g / cm 3 , and / or, the compaction density of the negative electrode sheet is 1.45-1.8 g / cm 3 .

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