A high energy density lithium ion battery

Abstract

The application discloses a high-energy-density lithium ion battery, which comprises a positive electrode sheet and a negative electrode sheet, the positive electrode sheet comprises a positive electrode active material, the positive electrode active material comprises a lithium iron phosphate material and a lithium-rich material, the mass ratio of the lithium-rich material in the positive electrode active material is 0.2%-8%, and the chemical formula of the lithium iron phosphate material is Li 1+x Fe 1‑ y M y (PO4) 1+z , 0<=x<0.1, 0<=y<0.01, 0<=z<0.04, 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 b O 4+c , 0<=a<0.5, 0<=b<0.01, 0<=c<0.03, Z comprises at least one of Ti, V, Zr, Nb, Mg, Mo, W and Al, and the chemical formula of the 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, Y comprises at least one of Ti, V, Al, Mg, Mn and Fe, and the negative electrode active material comprises a silicon negative electrode material, the mass ratio of the silicon negative electrode material in the negative electrode active material is 1%-50%. The application has the advantages of improving the energy density, rate performance and cycle performance of the lithium battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a high-energy-density lithium ion battery. BACKGROUND

[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 future society. Among all new energy systems, solar energy, wind energy, water energy and nuclear energy do not have convenient mobility; while lithium ion batteries, as a portable energy storage form, have their specific irreplaceability in practical application and are widely used.

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

[0004] However, the current energy density of LFP-graphite system has reached the upper limit of the conventional battery design (180Wh / kg), therefore, it is urgent to optimize the system to improve the upper limit of the energy density design of lithium iron phosphate battery. SUMMARY

[0005] In order to improve the energy density of lithium battery while considering its rate performance and cycle performance, the present application provides a high-energy-density lithium ion battery.

[0006] The high-energy-density lithium ion battery provided by the present application adopts the following technical scheme:

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

[0008] The positive electrode 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 electrode active material is 0.2%-8%;

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

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

[0011] The LNO has a chemical formula of Li 2+d Ni 1-e Y e O 2+f , -0.5≤d≤0.5, e≥0, -0.5≤f≤0.5; and 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 ratio of the silicon negative electrode material in the negative electrode active material is 1%-50%.

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

[0014] Firstly, the high initial charge specific capacity of the lithium-rich material can compensate for the consumption of active lithium at the negative electrode due to the formation of SEI film during the initial charging process, thereby increasing the amount of active lithium deintercalation at the negative electrode during discharging, so as to achieve the effect of improving the reversible specific capacity of the lithium battery, and the energy density of the lithium battery reaches 200-280 Wh / kg. Further, the lithium-rich material in the positive electrode system can significantly improve the specific capacity of the positive electrode, greatly reducing the coating thickness or area density of the positive electrode active material layer, and the silicon in the (graphite+silicon) negative electrode system can significantly improve the specific capacity of the negative electrode, greatly reducing the thickness or area density of the negative electrode active material layer. By comprehensively utilizing the above advantages, the (LFP+lithium-rich material) positive electrode system in the application is combined with the (graphite+silicon) negative electrode system, which can significantly improve the energy density of the lithium battery.

[0015] At the same time, due to the significant reduction in the thickness and area density of the positive electrode active material layer and the negative electrode active material layer, the transmission path of electrons and lithium ions in the electrode is shortened, which effectively improves the kinetics of electron and lithium ion mass transfer, reduces the direct current resistance (DCR) of the lithium battery, and further helps to improve the energy efficiency, rate performance and low temperature performance of the lithium battery.

[0016] Secondly, the lithium supplement effect of the lithium-rich material can also improve the cycle life of the lithium battery to some extent, and more importantly, the introduction of the lithium-rich material can change the composition of the SEI film, reduce the direct current resistance (DCR) of the lithium battery, and thus improve the cycle life and energy efficiency of the lithium battery. Since the volume of the lithium-rich material will shrink by 20%-70% after the first charging and deintercalation of lithium, and the volume of LFP will not change substantially (within 5% of the volume change) after the first charging and deintercalation of lithium, the volume shrinkage of the lithium-rich material will form a new porous structure in the positive electrode active material layer of the positive electrode sheet, which will be beneficial to the full infiltration and adsorption of the electrolyte in the positive electrode sheet, improve the liquid retention capacity of the positive electrode sheet, effectively improve the mass transfer kinetics of lithium ions in the electrode, and thus significantly improve the cycle life of the lithium battery.

[0017] Thirdly, by doping elements in the lithium iron phosphate material and doping elements in the lithium-rich material, the structural stability of the lithium iron phosphate material and the lithium-rich material can be improved respectively, and the combination of the two can further improve the rate performance of the lithium battery in cooperation with the silicon negative electrode.

[0018] Fourth, since the potential of silicon anode material to Li+ / Li during lithium intercalation is naturally higher than that of graphite material, the mixed potential of silicon anode material and graphite material is still higher than that of graphite material. Therefore, the potential of the (graphite + silicon) anode system in this invention will not approach the lithium plating potential and cause lithium plating during high-rate charging. It is safer than a simple graphite anode during fast charging, giving the battery 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] Because lithium-rich materials experience volume shrinkage after the first charge and lithium insertion / extraction, with a maximum shrinkage of up to 60%, by controlling the average primary particle size of lithium-rich materials, 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 wetting and has excellent liquid retention performance. This further improves the mass transfer kinetics of lithium ions in the electrode, reduces the liquid phase ohmic polarization caused by the electrolyte, and thus reduces the DC internal resistance (DCR) of the battery, which helps to improve the rate performance and cycle life of the battery.

[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. A dopant source containing doping elements is added to the mixture to obtain a mixed slurry. The mixed slurry is then spray-dried and sintered in an inert atmosphere to obtain the lithium iron phosphate material.

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

[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. A dopant source containing doping elements is added to the mixture to obtain a mixed slurry. The mixed slurry is then spray-dried and sintered in an inert atmosphere to obtain the lithium-rich material.

[0027] The precursor includes nickel phosphate; the doping source containing a doping element 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, the 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, the Z includes any two of Ti, V, and Nb, and the 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 lithium-rich material in combination with 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 conductive agent and a binder, and the mass ratio of the positive electrode active material, the positive electrode conductive agent, and the binder is 95-100:0.5-1.5:1-3.

[0035] Preferably, the positive electrode conductive agent includes at least one of carbon nanotube 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 charging and lithium extraction, and by controlling the average primary particle size of the silicon negative electrode material and the average primary particle size of the graphite material, a suitable pore structure of the silicon negative electrode material can be formed in the positive electrode active material layer, forming a channel structure that is beneficial to electrolyte infiltration and has excellent liquid retention performance. Cooperating with the positive electrode sheet, it can accelerate the transfer of lithium ions between the positive and negative electrode sheets and will not cause lithium precipitation, 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 spherical particles, polycrystalline silicon spherical particles, amorphous silicon spherical particles, single crystal silicon amorphous particles, polycrystalline silicon amorphous particles, and amorphous silicon amorphous particles.

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

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

[0041] Preferably, the negative electrode conductive agent includes conductive carbon black SP; the thickening agent includes sodium carboxymethyl cellulose CMC; and the adhesive includes butyl 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 in the present application is suitable for a button cell, a winding type soft package battery, a laminated type soft package battery, a winding type square hard shell battery, a laminated type square hard shell battery, a winding type cylindrical battery, etc.

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

[0045] 1. Positive electrode sheet preparation

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

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

[0049] 2. Preparation of negative electrode sheet

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

[0052] The above negative electrode slurry is coated on both surfaces of the copper foil (thickness of 4.5-8 μm) by using a transfer coater or an extrusion coater, and the negative electrode sheet (porosity of the negative electrode sheet is 22%-37%) is obtained after drying, rolling, and slitting.

[0053] 3. Preparation of lithium battery

[0054] (1) winding: the positive electrode sheet, the separator, the negative electrode sheet (the separator separates the positive and negative electrode sheets, the negative electrode sheet clamps the positive electrode sheet, and the leading ends of the positive and negative tabs are distributed on the opposite sides) are sequentially stacked to obtain a winding core by manual winding or automatic / semi-automatic winding;

[0055] (2) tab welding: the aluminum tab is ultrasonically welded to the positive leading end, and the nickel tab is ultrasonically welded to the negative leading end, and a PET protective tape is attached to the welding position;

[0056] (3) baking: the winding core is placed in a vacuum environment and baked at 90-120°C for 8-24h, so that the water content of each part of the winding core is reduced to below 400ppm (preferably below 300ppm);

[0057] (4) into the shell (aluminum plastic film sealing): the aluminum plastic film is cut into a rectangle with a certain length and width, tightly wrapped around the winding core, and the edge is heat-sealed by a sealing machine along the positive and negative tabs, leaving a liquid injection port on the side;

[0058] (5) liquid injection: a suitable amount of electrolyte is injected into the aluminum plastic sealing shell, wherein the electrolyte composition is 1.0MLiPF6+EC / EMC / DEC+VC / FEC / DTD and other additives;

[0059] (6) vacuum standing: the winding core after liquid injection is placed in a vacuum oven to maintain negative pressure for 0.5-2h, so that the positive / negative electrode sheet and the separator can be fully soaked in the electrolyte;

[0060] (7) edge sealing: the last opening of the aluminum plastic sealing bag is sealed and welded by a sealing machine, and the sealing position is left a distance from the winding core, which is convenient for storing a small amount of gas generated by the subsequent formation of the battery (i.e. the so-called "gas bag");

[0061] (12) standing: the sealed battery is placed in a 45±5°C oven and allowed to stand for 24-48h, allowing the electrolyte to fully soak into the pores of the positive and negative electrodes and the separator again;

[0062] (13) fixture formation: the lithium battery is placed on the fixture, and a certain current is applied to the battery at a temperature of 45±5°C by the formation cabinet to charge the battery to 20% SOC-70% SOC, so that the negative electrode and the electrolyte generate an SEI film, and a part of the gas byproduct is generated;

[0063] (14) air extraction: the gas bag is punctured and vacuumed by a vacuum air extraction device, and the byproduct gas generated during the formation stage is extracted;

[0064] (15) sealing: heat-seal near the winding core using a sealing machine, and cut off the gas bag to make a complete battery core;

[0065] (16) high temperature aging: the sealed battery is placed at 45±5℃ for 12-72h, so that the SEI film formed on the surface of the negative electrode is more stable;

[0066] (17) capacity matching: through 0.5C rate constant current charging to 3.65V, and maintaining constant voltage at 3.65V until the current is reduced to 0.05C or less, charging cutoff; after standing for 5-30min, discharging to 2.5V at 0.2C rate constant current; then the first coulombic efficiency = capacity of capacity matching / (formation charging capacity + capacity matching charging capacity); charging the battery to a specific state of charge (20% SOC-50% SOC) at 0.5C constant current, and storing at 25℃;

[0067] (18) OCV test: before storing at 25℃, test the open circuit voltage of the battery, recorded as OCV1; after storing at 25℃ 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, select qualified lithium battery through K value, and the battery with too high K value is downgraded or scrapped.

[0068] The preparation process of the positive and negative electrode sheets of the application is very simple, and the homogenization, coating, drying, rolling, slitting / sheeting process commonly used in the conventional industrial production of lithium ion batteries can be used, which can effectively ensure the scalability and cost control of the positive and negative electrode sheets, and has excellent economic value.

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

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

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

[0072] The lithium salt comprises at least one of lithium hexafluorophosphate LiPF6, lithium bisfluorosulfonylimide LiFSI, lithium bis-trifluoromethylsulfonylimide LiTFSI, lithium bis(oxalato)borate LiBOB and lithium difluoro(oxalato)borate LiODFB;

[0073] The additive includes, but is 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, phenyl cycloethane 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, an aluminum alloy shell. BRIEF DESCRIPTION OF DRAWINGS

[0075] Figure 1 The DCR value test diagram of the lithium battery in Example 1, Comparative Example 1 and Comparative Example 3 of the present application.

[0076] Figure 2 The cycle capacity performance test diagram of the lithium battery in Example 1, Comparative Example 1 and Comparative Example 3 of the present application.

[0077] Figure 3 The cross-sectional view of the positive electrode sheet in the lithium battery of Example 1 of the present application.

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

[0079] In order to better understand and implement, the technical solutions of the present application will be described clearly and completely in combination with examples below. Obviously, the described examples are only a part of the embodiments of the present application, not all the embodiments.

[0080] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description herein is for describing the specific embodiments only and is not intended to be limiting of the present application.

[0081] Unless otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as approximations as opposed to being precise. Accordingly, unless indicated otherwise, the numerical parameters set forth in the specification and claims are approximations that can vary depending on the requirements of the desired properties.

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

[0083] As used herein "comprises" and "comprising" are inclusive or open-ended and specify the presence of stated features, integers, steps, or components but do not preclude the presence or addition of one or more other features, integers, steps, components, or groups thereof.

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

[0085] As used in the present specification, "a", "an", "one", and "the" are intended to include "at least one" or "one or more" unless specified otherwise. For example, "a component" means one or more components, and thus, in the implementation of the described embodiments, a plurality of components can be considered and possibly used in implementing the described embodiments.

[0086] Example 1

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

[0088] 1. Preparation of positive electrode sheet

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

[0090] Firstly, the positive electrode conductive agent is mixed with the first part of N-methyl pyrrolidone (NMP) and stirred at a low speed of 3 m / s for 2 h to obtain a positive electrode conductive slurry; secondly, the positive electrode active material (obtained by mixing LFP and LFO) is added into the above positive electrode conductive slurry in steps (at least twice) to be stirred at a low speed of 3 m / s for 2 h; thirdly, the stirring speed is increased to 15 m / s for 2 h to obtain a positive electrode mixed slurry; fourthly, polyvinylidene fluoride (PVDF) is mixed with the second part of N-methyl pyrrolidone (NMP) to obtain a glue solution, which is stirred at a high speed of 16 m / s for 4 h to obtain a premixed slurry; and finally, N-methyl pyrrolidone (NMP) is added into the above premixed slurry to adjust the viscosity to 15000 mPa.s, thereby obtaining the positive electrode slurry.

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

[0092] 2. Preparation of negative electrode sheet

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

[0094] In the first step, the negative electrode active material (a mixture of the graphite material and the silicon negative electrode material) is added into deionized water in steps (at least twice) to obtain a mixed slurry. In the second step, the negative electrode conductive agent (SP) is added into the mixed slurry, and kneading is performed at a low stirring speed of 4 m / s for 1 h to obtain a conductive slurry. In the third step, the thickening agent (CMC) is added into the conductive slurry, and stirring is performed at a low stirring speed of 10 m / s for 2 h to obtain a composite slurry. In the fourth step, the adhesive agent (SBR) is added into the composite slurry, and stirring is performed at a stirring speed of 12 m / s for 4 h to obtain a premixed slurry. In the fifth step, deionized water is added into the premixed slurry, and the viscosity is adjusted to 6000 mPa.s to obtain the negative electrode slurry.

[0095] The negative electrode slurry is coated on both surfaces of a copper foil (with a thickness of 5 μm) by using a transfer coating machine, and a negative electrode sheet (with a porosity of 30%) is obtained after drying, rolling, and slitting.

[0096] 3. Preparation of lithium battery

[0097] (1) Winding: 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 clamps the positive electrode sheet, and the leading ends of the positive and negative tabs are distributed on different sides) are sequentially stacked to obtain a winding core by manual winding or automatic / semi-automatic winding. The surface of the base film is provided with a coating layer with a thickness of 2 μm. The coating layer comprises polyvinylidene fluoride PVDF (with a particle size of 0.5-4 μm) and boehmite powder (with a particle size of 0.5-1 μm).

[0098] (2) Tab welding: the aluminum tab is ultrasonically welded to the positive leading end, and the nickel tab is ultrasonically welded to the negative leading end. The welding positions are pasted with PET protective tape.

[0099] (3) Baking: the winding core is placed in a vacuum environment and baked at 100°C for 16 h to reduce the water content in the winding core to below 400 ppm.

[0100] (4) Shell (aluminum plastic film) packaging: the aluminum plastic film is cut into a rectangle with a certain length and width to tightly wrap the winding core. An edge sealing machine is used to heat and press the edge along the positive and negative tabs, and a liquid injection port is left on the side.

[0101] (5) Liquid injection: an appropriate amount of electrolyte is injected into the aluminum plastic packaging shell. The electrolyte composition is 1.0M LiPF6+EC / EMC / DEC+VC / FEC / DTD and other additives.

[0102] (6) Vacuum standing: after the injection of the core, put it into the vacuum oven to keep the negative pressure for 1h, so that the positive / negative plate and the separator can be fully soaked in the electrolyte;

[0103] (7) Edge sealing: the last opening of the aluminum plastic packaging bag is sealed by a sealing machine, and a distance is left from the core to facilitate the storage of a small amount of gas generated by the battery after the subsequent formation (i.e. the so-called "gas bag");

[0104] (12) Standing: the sealed battery is placed in a 45±5℃ oven for 36h, so that the electrolyte can fully soak into the pores of the positive and negative electrodes and the separator again;

[0105] (13) Fixture formation: the lithium battery is clamped on the fixture, and a certain current is applied to the battery at 45±5℃ by the formation cabinet to charge the battery to 50% SOC, so that the negative electrode reacts with the electrolyte to generate an SEI film, and a part of the gas by-product is generated;

[0106] (14) Gas extraction: the gas bag is punctured and vacuumed by a vacuum gas extraction device to extract the by-product gas generated in the formation stage;

[0107] (15) Sealing: the gas bag is cut off and a complete core is formed by hot pressing and sealing the part close to the core using a sealing machine;

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

[0109] (17) Capacity matching: the battery is charged to 3.65V by 0.5C constant current, and then kept at 3.65V until the current decreases to less than 0.05C; after 20min, the battery is discharged to 2.5V by 0.2C constant current; the first coulombic efficiency = matched discharge capacity / (formation charge capacity + matched continuous charge capacity); the battery is charged to a specific state of charge (40% SOC) by 0.5C constant current, and stored at 25℃;

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

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

[0112] Example 2

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

[0114] Example 3

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

[0116] Example 4

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

[0118] Example 5

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

[0120] Example 6

[0121] The difference between this embodiment and Example 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 , wherein M is Ti and Nb, the doping amount of Ti is 1000 ppm, and the doping amount of Nb is 200 ppm), 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 2000 ppm, and the doping amount of V is 1000 ppm); other steps and parameter settings remain the same as Example 1.

[0122] Example 7

[0123] The difference between this embodiment and Example 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 , wherein M is V and Nb, the doping amount of V is 1000 ppm, and the doping amount of Nb is 500 ppm), a 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 500 ppm, and the doping amount of Nb is 200 ppm); other steps and parameter settings are consistent with those of Example 1.

[0124] Example 8

[0125] The difference between this example and Example 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 500 ppm, and the doping amount of Mg is 200 ppm); other steps and parameter settings are consistent with those of Example 1.

[0126] Example 9

[0127] The difference between this example and Example 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 of Example 1.

[0128] Example 10

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

[0130] Example 11

[0131] The difference between this example and Example 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 of Example 1.

[0132] Example 12

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

[0134] Comparative Example 1

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

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

[0139] Comparative example 2

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

[0141] Comparative example 3

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

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

[0144] Comparative example 4

[0145] The difference between the present comparative example and embodiment 1 is that neither the lithium iron phosphate material nor the lithium-rich material contains a doping element; the other steps and parameter settings are consistent with those of embodiment 1.

[0146] Test method

[0147] I. Energy density test

[0148] The lithium battery in the above examples and comparative examples was subjected to energy density test. The specific test method was as follows: the weight of the lithium battery was weighed and recorded as m; the battery was placed in a clamp, a force of 3000 N was applied, the single battery was charged at a current of 0.33 C to 3.65 V, and the constant voltage charging at 3.65 V was continued until the current decreased to 0.05 C cutoff; it was left for 30 min, discharged at a current of 0.33 C to 2.5 V, left for 30 min, and continuously cycled for 3 times; the discharge capacity (in Ah) and energy E (average value of three cycles) were calculated, and the discharge energy density = E / m (in Wh / kg).

[0149] II. Capacity test of electrode sheet

[0150] The electrode sheet in the above examples and comparative examples was subjected to first charge-discharge capacity test. The specific test method was as follows: the first charge capacity test of the positive electrode was performed. The positive electrode sheet was wiped with a wet wipe on one side until the aluminum foil was exposed, the sheet was dried, and then the sheet was punched and weighed. The mass was recorded as m1. The aluminum foil was punched and weighed by a sheet punching machine, and the mass was recorded as m2. The coating mass m3 = m1-m2. The punched sheet was taken together with lithium sheet, electrolyte and separator to make a button cell. The button cell was placed on a blue cell test cabinet for testing. The test steps were as follows: charged at a current of 0.1 C to 3.75 V, and then charged at a constant voltage of 3.75 V until the current decreased to 0.05 C cutoff. The charge capacity q1 was recorded. The first charge capacity of the positive electrode = q1 / (m1 x positive electrode coating active material ratio); left for 10 min, and then discharged at a current of 0.1 C to 2.0 V cutoff. The discharge capacity q2 was recorded. The first discharge capacity of the positive electrode = q2 / (m1 x positive electrode coating active material ratio).

[0151] The first charge capacity test process of the negative electrode and the button cell preparation process were basically the same as those of the positive electrode. The punched mass of the negative electrode m4, the mass of the copper foil m5, and the mass of the negative electrode coating m6 were recorded. The test steps were as follows: charged at a current of 0.1 C to 0.005 V cutoff. The first charge capacity q3 was recorded. The first charge capacity of the negative electrode = q3 / (m1 x negative electrode coating active material ratio); discharged at a current of 0.1 C to 0.5 V cutoff. The first discharge capacity q4 was recorded. The first discharge capacity of the negative electrode = q4 / (m1 x negative electrode coating active material ratio).

[0152] III. Rate performance test

[0153] The lithium battery in the above examples and comparative examples was subjected to rate performance test, and the specific test method was as follows: the lithium battery was pretreated at 25±2℃, and the steps were as follows: 0.5C current constant current constant voltage charging, cut-off voltage 3.65V, cut-off current 0.05C; standing for 5min; standing for 5min; 0.5C current constant current discharging, cut-off voltage 2.5V; after 13 cycles, 0.1C charging and discharging was carried out once again. The discharge capacity of the last 0.1C was taken as the initial capacity, and was recorded as Q0.

[0154] The capacity retention rate at 1C rate was tested: the battery was fully charged at 0.5C constant current and constant voltage, the cut-off voltage was 3.65V, and then discharged at 1C rate, the discharge capacity Q1 of the battery was recorded, and the 1C capacity retention rate was calculated as Q1 / Q0x100%.

[0155] Four, cycle performance test

[0156] The lithium battery in the above examples and comparative examples was subjected to cycle performance test, and the specific test method was as follows: at 25℃, the voltage range was set to 2.5-3.65V, and the prepared battery was charged and discharged at 1C rate; in the charging process, 1C constant current charging was carried out to 3.65V, and then constant voltage charging was carried out at 3.65V until the current dropped to 0.05C cut-off; in the discharging process, 1C constant current discharging was carried out to 2.5V cut-off; after 1000 cycles, the cycle capacity retention rate was recorded.

[0157] Five, direct current resistance test

[0158] The lithium battery in the above examples and comparative examples was subjected to direct current resistance test, and the specific test method was as follows: at 25℃, the battery of the examples and the comparative examples was adjusted to 95% SOC, and discharged at 1C rate for 10s, the battery voltage U1 before discharging, the current I and the battery voltage U2 at 10s were recorded, and the direct current 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 material and lithium-rich material as positive active material, graphite material and silicon negative electrode material as negative active material, and doping suitable elements in the lithium iron phosphate material and lithium-rich material, the energy density, rate performance and low temperature performance of the lithium battery can be significantly improved, and the collocation of the lithium iron phosphate material and the lithium-rich material can form a pore structure beneficial to the transmission of lithium ions and electrons in the positive active material layer, which is beneficial to improving the cycle performance of the electrolyte on the lithium battery; finally, the collocation of the positive active material and the negative active material in the application can significantly improve the charge and discharge safety performance of the lithium battery.

[0162] In combination with Embodiment 1, Embodiments 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 greater 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, the positive electrode sheet obtains a suitable area density and a pore structure that is beneficial to electrolyte infiltration and has excellent liquid retention performance; and the combination of the two types of positive electrode active material particles helps to improve the compaction density of the positive electrode sheet; the above comprehensive effects can improve the mass transfer kinetics in the electrode, reduce the liquid-phase ohmic polarization caused by the electrolyte, and in turn reduce the direct current resistance DCR of the battery, which helps to improve the rate capability, cycle life and energy density of the battery.

[0163] In combination with Embodiment 1, Embodiments 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 is good for cooperation with the positive electrode sheet, but also a negative electrode active material layer with a suitable pore structure can be formed, which avoids the occurrence of lithium precipitation at the negative electrode during the charging and discharging process of the lithium battery, and in cooperation 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 application and not to limit the protection scope of the present application. Although the present application has been described in detail with reference to the above embodiments, it should be understood by those skilled in the art that the technical solutions of the present application can be modified or replaced by equivalents, and these modifications or replacements are within the protection scope of the present application.

Claims

1. A high energy density lithium-ion battery, characterized by: The battery includes a positive electrode sheet and a negative electrode sheet, the positive electrode sheet includes a positive electrode active material layer, the positive electrode active material layer includes a positive electrode active material, the negative electrode sheet includes a negative electrode active material layer, and the negative electrode active material layer includes a negative electrode active material; The positive electrode active material includes a lithium iron phosphate material and a lithium-rich material, the mass ratio of the lithium-rich material in the positive electrode active material is 0.2%-8%, and the average primary particle size of the lithium-rich material is larger than that of the lithium iron phosphate material; 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 any two of Ti, V, Nb; when the lithium iron phosphate material has Ti doping, the doping amount of Ti is 1000-2000 ppm; when the lithium iron phosphate material has V doping, the doping amount of V is 500-1000 ppm; when the lithium iron phosphate material has Nb doping, the doping amount of Nb is 200-500 ppm; 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; the Z includes any two of Ti, V, Nb; The chemical formula of the 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 includes any two of Ti, V, Mg; When the lithium-rich material is doped with Ti, the doping amount of Ti is 1000-2000ppm; when the lithium-rich material is doped with V, the doping amount of V is 500-1000ppm; when the lithium-rich material is doped with Nb, the doping amount of Nb is 200-500ppm; and when the lithium-rich material is doped with Mg, the doping amount of Mg is 200-500ppm. The negative electrode active material includes a graphite material and a silicon negative electrode material, and the mass ratio of the silicon negative electrode material in the negative electrode active material is 1%-50%.

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

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

4. The high energy density lithium-ion battery of claim 1, wherein: The average primary particle size of the silicon negative electrode material is 0.05-0.5μm, and the average primary particle size of the graphite material is 5-15μm.

5. The high energy density lithium-ion battery of claim 1, wherein: The mass ratio of the silicon negative electrode material in the negative electrode active material is 5%-30%.

6. The high energy density lithium-ion battery of any one of claims 1-5, wherein: The positive electrode sheet has a compact density of 2.2 to 2.81 g / cm 3 , and / or the negative electrode sheet has a compact density of 1.45 to 1.8 g / cm 3 .

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