A lithium-ion battery

Abstract

The application relates to the technical field of batteries, in particular to a lithium ion battery. The lithium ion battery comprises a positive electrode, a negative electrode and a non-aqueous electrolyte; the positive electrode comprises a positive electrode material layer containing a positive electrode active material, the positive electrode active material comprises a lithium iron phosphate composite material composed of lithium iron phosphate and a carbon coating layer wrapped on the surface of the lithium iron phosphate; the positive electrode material layer is doped with Ni elements with a mass content of c ppm, and 50<=c<=2000; the mass percentage content d% of carbon elements in the carbon coating layer satisfies 0.5<=d<=3; the non-aqueous electrolyte comprises ethoxy (pentafluoro) cyclotriphosphazene with a mass percentage content of a% and a carbonate additive with a mass percentage content of b%; the carbonate additive comprises at least one of vinylene carbonate and vinyl ethylene carbonate; the lithium ion battery satisfies 1<=d*x<=500; 0.5<=a<=15, and 0.05<=b<=3. The lithium ion battery can have low initial impedance, low impedance growth rate and excellent fast-charging long-cycle performance.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically to a lithium-ion battery. Background Technology

[0002] With the global new energy vehicle industry's large-scale upgrading and improvement, power batteries have moved beyond simply pursuing basic performance indicators such as high safety, long cycle life, and low cost. They are now entering a new stage of high-quality development that balances ultra-fast charging, high energy density, wide temperature range adaptability, long lifespan, and high safety redundancy. Compared to ternary lithium battery systems, lithium iron phosphate materials possess higher structural stability and thermal safety performance, with significant advantages in total lifespan cost. This aligns perfectly with the core development needs of the current new energy industry, prioritizing cost reduction, efficiency improvement, safety, and large-scale promotion. It also represents the core mainstream route for the subsequent low-carbon, localized, and mass-produced development of power battery and energy storage battery technologies.

[0003] In the current technological development of the industry, fast charging performance has become a core requirement for the upgrading and iteration of lithium iron phosphate (LFP) power batteries and a key competitive arena for industrialization. The core pain points of charging anxiety and insufficient charging convenience for downstream new energy vehicle end-users have driven the evolution of LFP batteries from the traditional 1C-2C conventional rate to ultra-high rate fast charging of 4C and above. However, long-term ultra-high rate fast charging cycles result in severe degradation of electrode interface stability. The surface structure of the positive electrode material is prone to microcracks and collapse, causing iron ion dissolution. Furthermore, during high-current fast charging, the internal polarization of the battery increases sharply, and the local temperature rise of the cell is too rapid. Under the catalysis of metal ions and high temperature and high polarization, the electrolyte will continue to decompose and side reactions will intensify, leading to a continuous increase in the core interface impedance. This directly causes rapid capacity decay and a significant reduction in cycle life, making it difficult to meet the requirements of the entire life cycle of power batteries and energy storage devices. Therefore, developing lithium-ion secondary battery electrolytes that can effectively suppress iron ion dissolution and interface side reactions during fast charging cycles is of great significance for balancing battery impedance growth and long-cycle performance. Summary of the Invention

[0004] Based on this, the purpose of the present invention is to provide a lithium-ion battery to solve the problems of rapid impedance growth and poor high-rate long-cycle performance of lithium-ion secondary batteries.

[0005] The present invention adopts the following technical solution: A lithium-ion battery includes a positive electrode, a negative electrode, and a non-aqueous electrolyte; the positive electrode includes a positive electrode material layer containing a positive electrode active material, the positive electrode active material including a lithium iron phosphate composite material composed of lithium iron phosphate and a carbon coating layer wrapped around the surface of the lithium iron phosphate; the positive electrode active material is also doped with Ni element; With the positive electrode material layer as 100% of the mass, the mass content of Ni element c ppm satisfies 50≤c≤2000, and the mass percentage content of carbon element d% in the carbon coating layer satisfies 0.5≤d≤3. The non-aqueous electrolyte comprises ethoxy(pentafluoro)cyclotriphosphazene and carbonate additives; the carbonate additives are at least one of vinylene carbonate and ethylene ethylene carbonate. Based on the mass of the non-aqueous electrolyte, the mass percentage of the ethoxy(pentafluoro)cyclotriphosphazene is a%, and the mass percentage of the carbonate additive is b%. The lithium-ion battery meets the following conditions: 1≤d× ≤500; and 0.5≤a≤15, 0.05≤b≤3.

[0006] Even though phosphate structures are highly stable, lithium iron phosphate batteries experience structural collapse and Fe ion dissolution at the positive electrode during high-rate (≥4C) cycling due to the high lithium ion concentration gradient and migration. After Fe ions dissolve, they not only catalyze solvent decomposition, causing byproducts to cover the positive and negative electrode interface film, but also migrate to the negative electrode under the action of the battery, accelerating the decomposition of the SEI film. Ultimately, this leads to increased battery impedance and intensified heat generation, resulting in significant degradation of battery performance. To suppress the damage to the positive electrode structure and the dissolution of Fe ions during high-rate cycling, the lithium-ion battery provided in this invention uses ethoxy(pentafluoro)cyclotriphosphazene (PFPN) and carbonate additives as additives in the non-aqueous electrolyte, and limits the content of Ni and carbon elements in ethoxy(pentafluoro)cyclotriphosphazene, carbonate additives, and the positive electrode material layer. Through extensive experiments, the inventors discovered that when the mass percentage of ethoxy(pentafluoro)cyclotriphosphazene (a), the mass percentage of carbonate additives (b), the mass percentage of Ni element in the positive electrode material layer (c), and the mass percentage of carbon element in the carbon coating layer (d) satisfy 1 ≤ d × When the impedance is ≤500, and 0.5≤a≤15, 0.05≤b≤3, 50≤c≤2000, and 0.5≤d≤3, the battery can achieve a balance between low initial impedance, low impedance growth, and excellent fast-charging and long-cycle performance. The presumed reason is that, on the one hand, doping the positive electrode material layer with Ni can reduce the unit cell size and shorten the Li... + Transmission path and make Li + The channels are more regular, the migration barrier is lowered, and Fe is induced to form. 2+ with Fe 3+Mixed oxidation states create internal electronic conduction channels, reducing the band gap and charge transfer impedance. More importantly, the Ni-O bond is stronger than the Fe-O bond, thus suppressing lattice distortion and enhancing structural rigidity, thereby improving the overall structural stability of the cathode and reducing structural collapse and Fe ion dissolution during cycling. On the other hand, PFPN can form a highly stable CEI film on the cathode and can complex and capture dissolved Fe ions, and reduce side reactions by eliminating free radicals H·, suppressing impedance growth during high-rate cycling. However, the introduction of Ni leads to an increase in residual alkali in the cathode and exacerbates side reactions. PFPN cathode film formation impedance is relatively high, so the content of both elements cannot be excessive. Otherwise, its beneficial effects cannot be fully realized. Therefore, further appropriate carbon coating on the surface of the positive electrode active material not only helps reduce battery impedance but also inhibits direct contact between residual alkali on the positive electrode surface and the electrolyte. Furthermore, the film morphology of PFPN on the carbon-coated positive electrode surface and within the pores of the carbon material is more dense and uniform, which can reduce the film-forming impedance of PFPN. In addition to improvements on the positive electrode side, protection on the negative electrode side is equally crucial. Ethylene carbonate or ethylene ethylene carbonate can form a highly stable interfacial film on the negative electrode and play an excellent role in film repair during cycling, inhibiting the catalytic decomposition of Ni and Fe ions deposited after the SEI film. Through the synergistic effect of the above parameters, the negative effects caused by metal ion dissolution can be significantly suppressed.

[0007] In summary, to meet the performance requirements of high-rate lithium iron phosphate (LFP) lithium-ion batteries in the power sector, a series of methods are needed to suppress the dissolution of transition metal ions from the source. On the one hand, the structural stability of LFP materials can be improved and the lithium-ion migration barrier can be reduced by doping elements at the process end, and a carbon coating layer can be prepared on the surface of the positive electrode active material to improve conductivity and suppress metal ion dissolution. On the other hand, a combination of PFPN and carbonate additives can be used at the electrolyte end to suppress Fe ion dissolution and reduce its destructive effect on the negative electrode interface film. Ultimately, this achieves a balance between excellent battery impedance growth and ultra-long high-rate cycle performance.

[0008] When the relation d× When the value is less than 1, it indicates that the PFPN addition is too low, or the content of vinylene carbonate (VC) / ethylene ethylene carbonate (VEC) is too low, or the Ni doping is too low, or the carbon coating layer fails to completely cover the positive electrode active material. These conditions will prevent the material or electrolyte end from providing sufficient improvement. Under fast charging conditions, the possibility of metal ion dissolution increases significantly at the material end, and side reactions and interfacial film impedance increase significantly at the electrolyte end. This vicious cycle leads to rapid increase in battery impedance and rapid capacity decay during cycling. When the relationship d× A value >500 indicates excessive PFPN addition, excessive VC / VEC content, excessive Ni doping, or excessively thick carbon coating. Excessive PFPN and VC / VEC not only lead to an excessively thick battery interface film and high initial impedance, but also significantly increase electrolyte costs, hindering commercial applications. Excessively high impedance significantly increases the safety hazard of lithium plating and causes a series of negative effects such as heat generation, polarization, and increased side reactions. Excessive Ni doping causes mixing, uneven distribution, accelerated Ni dissolution, and intensified side reactions. An excessively thick carbon coating not only fails to improve film quality but also prevents PFPN from fully participating in CEI film formation, introducing more defect potentials and exacerbating film damage and side reactions during fast charging. Ultimately, all these factors degrade battery impedance, impedance growth, and electrochemical performance.

[0009] Preferably, the lithium-ion battery satisfies 20 ≤ d × ≤200.

[0010] The positive electrode film-forming additive PFPN has a cyclic structure. During the battery formation stage, it can preferentially oxidize and decompose to form a highly stable CEI film rich in F and N, and complex metal ions in the electrolyte to reduce the dissolution and migration of metal ions. If the mass percentage 'a' of PFPN in the non-aqueous electrolyte is too high, it will degrade the electrolyte viscosity and lead to an excessively thick positive electrode CEI film during film formation. Moreover, the excess PFPN cannot play an additional improvement role, resulting in excessively high initial battery impedance, which increases sharply during fast charging due to heat, leading to faster battery impedance growth and worse cycle performance. If the mass percentage 'a' of PFPN in the non-aqueous electrolyte is too low, it is difficult for it to play its role in film formation and metal ion complexation, and it cannot improve battery performance.

[0011] Specifically, the mass percentage a% of PFPN in the non-aqueous electrolyte can be 0.5%, 1%, 3%, 5%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 15%, or any combination of these values; preferably, the mass percentage a% of PFPN is 5% to 10%. Within this range, PFPN can preferentially oxidize and decompose during the battery formation stage to form a dense CEI film rich in F and N, which can effectively suppress the decomposition of the electrolyte at the positive electrode interface and the corrosion of the positive electrode active material by HF under fast charging, high polarization, and heat generation conditions, thereby reducing the increase in positive electrode impedance and the dissolution of Fe ions.

[0012] Carbonate additives VC and VEC are excellent anode film-forming additives and have been widely used in lithium-ion batteries based on lithium iron phosphate. Both possess cyclic structures and, during the formation stage, form polyvinyl carbonate or cross-linked polycarbonate network structures, as well as inorganic / organic composite SEI film components such as alkyl lithium carbonate (ROCO2Li) and Li2CO3 through ring-opening. This significantly improves the stability and toughness of the anode interfacial film, suppresses anode-side side reactions, and has a film repair function. The disadvantage is their relatively high film-forming impedance; higher concentrations significantly degrade the initial impedance of the battery. When used in combination with PFPN, VC and VEC have higher reduction potentials, preferentially causing PFPN to decompose and form a film on the anode side. However, PFPN still participates in anode film formation and forms P / F / N-containing components, enhancing SEI film stability. The P-containing components are more beneficial for lithium-ion migration, thus reducing interfacial film impedance.

[0013] Specifically, the mass percentage b% of carbonate additives (VC or VEC) in the non-aqueous electrolyte can be 0.05%, 0.08%, 0.1%, 0.3%, 0.5%, 0.8%, 1%, 1.3%, 1.5%, 1.8%, 2%, 2.3%, 2.5%, 2.8%, 3%, or any combination of these values. If the b value is too high, the negative electrode film-forming resistance will be too high, leading to easy lithium plating during fast charging, excessive heat generation, and exacerbated side reactions, causing safety hazards and battery performance degradation. If the b value is too low, the stability of the negative electrode SEI film will be insufficient, and there will be no good film repair during cycling. Side reactions will cause a rapid increase in resistance during cycling and consume active lithium. If metal ions migrate and deposit in large quantities at the negative electrode under the action of the battery, the performance degradation will be further aggravated. Preferably, the mass percentage b% of carbonate additives (VC or VEC) in the non-aqueous electrolyte is 0.5% to 1.5%. Within this range, carbonate additives can form a superior interfacial film at the negative electrode and better suppress side reactions at the negative electrode.

[0014] Ni doping in positive electrode active material LiFePO4 2+ Mainly replaces Fe 2+ Sites that can induce the formation of Fe 2+ with Fe 3+ Mixed valence forms internal electron conduction channels, reducing the band gap, lowering charge transfer impedance, and improving the intrinsic conductivity of the material. It can also reduce the unit cell size, thereby increasing structural rigidity and shortening the Li-C-C-Tc ratio. + The transmission path is improved and Fe ion dissolution is reduced, thus significantly improving the fast charging performance of the battery. In addition, doping with Ni will slightly raise the voltage platform and increase the battery energy density. Adding PFPN to the electrolyte also has the effect of inhibiting dissolution and complexing of Ni ions. The disadvantage is that the reactivity of Ni ions after dissolution is higher than that of Fe ions, and the more Ni doping, the more residual alkali on the positive electrode surface, which in turn catalyzes solvent decomposition and increases side reactions.

[0015] Specifically, the Ni element mass content c ppm can be 50 ppm, 100 ppm, 200 ppm, 300 ppm, 400 ppm, 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm, 1000 ppm, 1300 ppm, 1500 ppm, 1800, 2000 ppm, or any combination of these values. If the Ni element mass content c value is too high, Ni ion dissolution will be accelerated, which will degrade battery performance and increase battery cost, and may also lead to process problems such as uneven doping concentration; if the Ni element mass content c value is too low, its advantages cannot be effectively utilized, and the improvement on material conductivity, structural stability, battery rate, and cycle performance will not be as expected. Preferably, the Ni element mass content c ppm is 200 ppm to 1000 ppm. Within this range, the positive effects of increased electronic conductivity, lattice contraction, and voltage rise brought about by nickel doping can be fully released. At the same time, it limits the amount of residual alkali generated in the cathode, weakens the catalytic decomposition effect of dissolved Ni ions on the electrolyte, and, together with the complexing and passivation effect of PFPN, balances the material's fast charging capability and long-cycle stability, avoiding the continuous deterioration of side reactions caused by high doping ratio.

[0016] Carbon coating is an effective method to improve the conductivity of the positive electrode active material LiFePO4. Simultaneously, carbon coating during the formation stage can influence the film-forming reaction pathway and morphology of multi-element additives, improving the quality and uniformity of the interfacial film. Tight coating on the positive electrode surface also effectively isolates metal ions from dissolution and residual alkali on the positive electrode surface, preventing direct contact and catalytic electrolyte decomposition. Therefore, appropriate carbon coating combined with excellent multi-element film-forming additive PFPN and Ni doping of the positive electrode material can achieve better suppression of side reactions and metal ion dissolution. However, the carbon coating layer itself has a porous structure and high specific surface area. An excessively thick carbon layer introduces more defects and reaction sites, exacerbating side reactions. If the d-value is too high, the carbon coating layer becomes too thick, and byproducts from side reactions will degrade battery impedance and ultimately worsen battery cycle performance. If the d-value is too low, it is difficult to completely coat the entire surface of the positive electrode active material, leading to uneven CEI film formation and interfacial ion migration rates, ultimately degrading various electrochemical performance characteristics.

[0017] Specifically, the mass percentage of carbon element d% can be 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, 2.3%, 2.5%, 2.8%, 3%, or any combination of these values; preferably, the mass percentage of carbon element d% is 1% to 2%. Within this range, the thickness of the carbon coating layer is moderate, which can completely and continuously coat all lithium iron phosphate particles, ensuring overall electronic conductivity, uniformly controlling the PFPN additive film formation process to generate a dense and uniform CEI film, and synergistically suppressing lattice distortion and iron dissolution with Ni doping; at the same time, the specific surface area of ​​the carbon layer is controllable, which will not introduce too many defect sites, effectively reducing the accumulation of electrolyte decomposition byproducts, suppressing the increase in battery impedance during cycling, and balancing battery rate performance, interface stability, and long cycle life.

[0018] In some embodiments, the non-aqueous electrolyte includes an organic solvent, which is one or more of cyclic carbonates, linear carbonates, carboxylic acid esters, and ethers.

[0019] In a preferred embodiment, the cyclic carbonate includes one or more of vinylene carbonate, propylene carbonate, ethylene carbonate, and butene carbonate.

[0020] In a preferred embodiment, the linear carbonate includes one or more of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, and methyl propyl carbonate.

[0021] In a preferred embodiment, the carboxylic acid ester includes one or more of methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, methyl butyrate, methyl isobutyrate, methyl trimethylacetate, and ethyl trimethylacetate.

[0022] In a preferred embodiment, the ether includes one or more of ethylene glycol dimethyl ether, 1,3-dioxolane, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0023] In a more preferred embodiment, the organic solvent is a mixture of ethylene carbonate, diethyl carbonate, and methyl ethyl carbonate.

[0024] In some embodiments, the non-aqueous electrolyte further includes auxiliary additives selected from at least one of cyclic carbonate compounds, cyclic sulfate compounds, sulfonyl lactone compounds, phosphate compounds, borate ester compounds, and nitrile compounds, excluding vinylene carbonate and ethylene ethylene carbonate.

[0025] In some embodiments, the content of the auxiliary additives is 0.01% to 30% based on the total mass of the non-aqueous electrolyte as 100%.

[0026] In some embodiments, the cyclic carbonate compound other than vinylene carbonate or ethylene carbonate is selected from at least one of methylene ethylene carbonate, fluoroethylene carbonate, trifluoromethyl ethylene carbonate, difluoroethylene carbonate, or a compound represented by structural formula 1 below: Structural Formula 1, In structural formula 1, R 21 R 22 R 23 R 24 R 25 R 26 Each is independently selected from one of the following: hydrogen atom, halogen atom, or C1-C5 group.

[0027] In some preferred embodiments, the compound represented by structural formula 1 includes at least one of the compounds represented by compounds 1-1 to 1-6 below: .

[0028] In some embodiments, the cyclic sulfate compound is selected from vinyl sulfate, 4-methylvinyl sulfate, propylene sulfate, etc. , , At least one of them.

[0029] In some preferred embodiments, the sulfonyl lactone compound is selected from 1,3-propanesulfonyl lactone, 1,4-butanesulfonyl lactone, propenyl-1,3-sulfonyl lactone, etc. At least one of them.

[0030] In some embodiments, the phosphate ester compounds include saturated phosphate ester compounds and unsaturated phosphate ester compounds, wherein the saturated phosphate ester compounds include tris(trimethylsilane) phosphate esters, and the unsaturated phosphate ester compounds include compounds represented by structural formula 2 below: Structural Formula 2, In structural formula 2, R 31 R 32 R 33 Each is independently selected from C1-C5 saturated hydrocarbon groups, unsaturated hydrocarbon groups, halohydrocarbon groups, and -Si(C m H 2m+1 )3, m is a natural number from 1 to 3, and R 31 R 32 R 33 At least one of them is an unsaturated hydrocarbon group.

[0031] In some preferred embodiments, the compound represented by structural formula 2 includes at least one of the following: triargyl phosphate, diallyl methyl phosphate, diallyl ethyl phosphate, diallyl propyl phosphate, diallyl trifluoromethyl phosphate, diallyl-2,2,2-trifluoroethyl phosphate, diallyl-3,3,3-trifluoropropyl phosphate, diallyl hexafluoroisopropyl phosphate, triallyl phosphate, diallyl methyl phosphate, diallyl ethyl phosphate, diallyl propyl phosphate, diallyl trifluoromethyl phosphate, diallyl-2,2,2-trifluoroethyl phosphate, diallyl-3,3,3-trifluoropropyl phosphate, and diallyl hexafluoroisopropyl phosphate.

[0032] In some embodiments, the borate ester compounds include tris(trimethylsilane)borate and tris(triethylsilane)borate.

[0033] In some embodiments, the nitrile compound includes at least one selected from succinic acid, glutaronitrile, ethylene glycol bis(propionitrile) ether, hexanetrionitrile, adiponitrile, heptanonitrile, octanilide, nonadionitrile, and sebaconitrile.

[0034] In some preferred embodiments, the content of any one of the optional substances in the auxiliary additive in the non-aqueous electrolyte is less than 10%, preferably 0.1% to 5%, and more preferably 0.1% to 3%. Specifically, the content of any one of the optional substances in the auxiliary additive can be 0.05%, 0.08%, 0.1%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 7.8%, 8%, 8.5%, 9%, 9.5%, or 10%.

[0035] In some embodiments, when the auxiliary additive is selected from fluoroethylene carbonate, the content of fluoroethylene carbonate is 0.05% to 30% based on the total mass of the non-aqueous electrolyte as 100%.

[0036] In some embodiments, the non-aqueous electrolyte further includes lithium salts.

[0037] In some preferred embodiments, the lithium salt is selected from LiPF6, LiBOB, LiDFOB, LiPO2F2, LiBF4, LiSbF6, LiAsF6, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiC(SO2CF3)3, LiN(SO2F)2, LiClO4, LiAlCl4, LiCF3SO3, and Li2B. 10 Cl 10At least one of the following: lithium salts of lower aliphatic carboxylic acids.

[0038] In some embodiments, the concentration of the lithium salt is 0.1~8 mol / L. Specifically, the concentration of the lithium salt can be within the range of 0.1 mol / L, 0.3 mol / L, 0.5 mol / L, 1.0 mol / L, 1.5 mol / L, 2.0 mol / L, 2.5 mol / L, 3.0 mol / L, 3.5 mol / L, 4.0 mol / L, 4.5 mol / L, 5.0 mol / L, 5.5 mol / L, 6.0 mol / L, 6.5 mol / L, 7.0 mol / L, 7.5 mol / L, 8.0 mol / L, or any combination thereof. In a preferred embodiment, the concentration of the lithium salt is 0.5~2.5 mol / L.

[0039] In some embodiments, the positive electrode further includes a positive electrode current collector, and the positive electrode material layer is disposed on the surface of the positive electrode current collector.

[0040] The positive current collector is selected from a metallic material that can conduct electrons; preferably, the positive current collector includes at least one of Al, Ni, tin, copper, and stainless steel; in a more preferred embodiment, the positive current collector is selected from aluminum foil.

[0041] In some embodiments, the positive electrode material layer further includes a positive electrode binder and a positive electrode conductive agent, and the positive electrode active material, the positive electrode binder and the positive electrode conductive agent are blended to obtain the positive electrode material layer.

[0042] In some embodiments, based on the total mass of the positive electrode material layer as 100%, the mass percentage of the positive electrode binder is 0.5% to 3%, and the mass percentage of the positive electrode conductive agent is 0.5% to 3%.

[0043] In some embodiments, the positive electrode binder includes at least one of polyvinylidene fluoride, copolymers of polyvinylidene fluoride, polytetrafluoroethylene, copolymers of polyvinylidene fluoride-hexafluoropropylene, copolymers of tetrafluoroethylene-hexafluoropropylene, copolymers of tetrafluoroethylene-perfluoroalkyl vinyl ethers, copolymers of ethylene-tetrafluoroethylene, copolymers of polyvinylidene fluoride-tetrafluoroethylene, copolymers of polyvinylidene fluoride-trifluoroethylene, copolymers of polyvinylidene fluoride-trichloroethylene, copolymers of polyvinylidene fluoride-fluorinated vinylidene ether, copolymers of polyvinylidene fluoride-hexafluoropropylene-tetrafluoroethylene, thermoplastic polyimide, thermoplastic resins such as polyethylene and polypropylene, acrylic resins, and styrene-butadiene rubber.

[0044] In some embodiments, the positive electrode conductive agent is one or more selected from carbon nanotubes, SP, and graphite powder. Preferably, the positive electrode conductive agent is carbon nanotubes.

[0045] In some embodiments, the carbon nanotube content is 0.1% to 2% of the total weight of the cathode material layer. Preferably, the carbon nanotube content is 0.5% to 2% of the total weight of the cathode material layer.

[0046] In some embodiments, the compaction density of the positive electrode is 2.0 g / cm³. 3 ~4.4 g / cm 3 Preferably, the compaction density of the positive electrode is 2.3 g / cm³. 3 ~4.2 g / cm 3 .

[0047] In some embodiments, the bifacial density of the positive electrode is 20 mg / cm³. 2 ~70 mg / cm 2 Preferably, the bifacial density of the positive electrode is 30 mg / cm³. 2 ~50 mg / cm 2 .

[0048] In some embodiments, the compaction density of the negative electrode is 1.0 g / cm³. 3 ~2.0 g / cm 3 Preferably, the compaction density of the negative electrode is 1.4 g / cm³. 3 ~1.8 g / cm 3 .

[0049] In some embodiments, the bifacial density of the negative electrode is 10 mg / cm³. 2 ~35 mg / cm 2 Preferably, the bifacial density of the negative electrode is 15 mg / cm³. 2 ~30 mg / cm 2 .

[0050] In some embodiments, the negative electrode includes a negative electrode material layer comprising a negative electrode active material, wherein the negative electrode active material includes one or more of silicon-based negative electrodes and carbon-based negative electrodes.

[0051] In a preferred embodiment, the carbon-based anode may include graphite, hard carbon, soft carbon, graphene, mesophase carbon microspheres, etc. The graphite includes, but is not limited to, one or more of natural graphite, artificial graphite, amorphous carbon, carbon-coated graphite, graphite-coated graphite, and resin-coated graphite.

[0052] In some embodiments, the natural graphite may be flake graphite, flaky graphite, soil graphite, and / or graphite particles obtained by using these graphites as raw materials and subjecting them to spheroidization, densification, or other treatments.

[0053] In some embodiments, the artificial graphite can be obtained by graphitizing organic materials such as coal tar pitch, heavy crude oil from coal, atmospheric residue, heavy crude oil from petroleum, aromatic hydrocarbons, nitrogen-containing cyclic compounds, sulfur-containing cyclic compounds, polystyrene, polyvinyl chloride, polyvinyl alcohol, polyacrylonitrile, polyvinyl butyral, natural polymers, polyphenylene sulfide, polyphenylene ether, furfuryl alcohol resin, phenolic resin, and imide resin at high temperatures. The amorphous carbon can be amorphous carbon particles obtained by heat treatment at a temperature range (400~2200℃) using easily graphitizable carbon precursors such as tar and pitch as raw materials, or amorphous carbon particles obtained by heat treatment using difficult-to-graphitize carbon precursors such as resin as raw materials.

[0054] In some embodiments, the carbon-coated graphite can be obtained by mixing natural graphite and / or artificial graphite with carbon precursors such as tar, asphalt, and resin, and performing heat treatment once or more in the range of 400°C to 2300°C. The resulting natural graphite and / or artificial graphite are used as the core graphite, and amorphous carbon is used to coat it to obtain a carbon-graphite composite. The carbon-graphite composite can be in the form where the entire or part of the surface of the core graphite is coated with amorphous carbon, or it can be in the form where multiple primary particles are composited using carbon derived from the aforementioned carbon precursors as a binder. Alternatively, carbon-graphite composites can be obtained by reacting hydrocarbon gases such as benzene, toluene, methane, propane, and aromatic volatile components with natural graphite and / or artificial graphite at high temperatures, causing carbon to deposit on the graphite surface. The graphite-coated graphite can also be obtained by mixing natural graphite and / or artificial graphite with carbon precursors such as tar, asphalt, and resin, which are easily graphitized organic compounds, and performing heat treatment once or more in the range of approximately 2400°C to 3200°C. Graphite-coated graphite can be obtained by using the obtained natural graphite and / or artificial graphite as the core graphite and coating the entire or part of the surface of the core graphite with graphitides. The resin-coated graphite can be obtained by mixing natural graphite and / or artificial graphite with resin, drying at a temperature below 400°C, using the resulting natural graphite and / or artificial graphite as the core graphite, and coating the core graphite with resin. The aforementioned organic compounds such as tar and pitch resin can be selected from carbonizable organic compounds found in coal-based heavy crude oil, direct-flow heavy crude oil, decomposed petroleum heavy crude oil, aromatic hydrocarbons, N-ring compounds, S-ring compounds, polystyrene, organic synthetic polymers, natural polymers, thermoplastic resins, and thermosetting resins.

[0055] In some embodiments, the negative electrode material layer further includes a negative electrode binder and a negative electrode conductive agent, which may be the same as the positive electrode binder and positive electrode conductive agent, respectively, and will not be described in detail here.

[0056] In some embodiments, the separator is located between the positive electrode and the negative electrode. The separator can be a conventional separator, such as a ceramic separator, a polymer separator, a non-woven fabric separator, or an inorganic-organic composite separator, including but not limited to single-layer PP (polypropylene), single-layer PE (polyethylene), double-layer PP / PE, double-layer PP / PP, and triple-layer PP / PE / PP separators.

[0057] Compared with the prior art, the present invention has the following advantages: The lithium iron phosphate (LFP) system suffers from low intrinsic conductivity, high initial impedance, and cathode metal ion dissolution, leading to significant heat generation and side reactions during fast-charging cycles, resulting in rapid impedance increase and capacity decay. The lithium-ion battery provided in this invention incorporates an appropriate amount of Ni ions into the LFP active material and forms a carbon coating layer on the LFP surface. Ethoxy(pentafluoro)cyclotriphosphazene (PFPN) and carbonate additives are used as additives in the non-aqueous electrolyte. Through combined control of the material and electrolyte aspects, the negative effects of metal ions are greatly reduced, and the fast-charging performance of the battery is improved. Furthermore, the mass percentages of ethoxy(pentafluoro)cyclotriphosphazene (a), carbonate additives (b), Ni element (c), and carbon element in the carbon coating layer (d) are limited to satisfy 1 ≤ d × When the impedance is ≤500 and 0.5≤a≤15, 0.05≤b≤3, 50≤c≤2000, and 0.5≤d≤3, the battery can achieve a balance between low initial impedance, low impedance growth rate, and excellent fast-charging long-cycle performance. Detailed Implementation

[0058] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention.

[0059] Example 1 This embodiment uses the preparation of a lithium-ion battery as an example to illustrate the present invention, including the following operation steps: 1. Preparation of positive electrode sheet Step 1: Weigh the following materials in a mass ratio of 94:3:3: lithium iron phosphate (LFP), a positive electrode active material doped with Ni and having a carbon coating, composite conductive agent (Super P+ carbon nanotubes CNT), and binder polyvinylidene fluoride (PVDF).

[0060] PVDF, used as a binder, is added to NMP solvent and stirred thoroughly to obtain PVDF adhesive.

[0061] Step 2: Using N-methyl-2-pyrrolidone (NMP) as a solvent, add PVDF binder to the solvent and stir continuously until completely dissolved and the system is homogeneous and transparent to obtain PVDF adhesive solution.

[0062] Step 3: Add lithium iron phosphate cathode active material and composite conductive agent (Super P+CNT) to the prepared PVDF adhesive in batches. After high-speed stirring and dispersion and degassing treatment, a lithium iron phosphate cathode slurry with stable viscosity and no particle agglomeration is obtained.

[0063] Step 4: The lithium iron phosphate cathode slurry is evenly coated on the surface of the cathode current collector aluminum foil, and then dried in an oven at high temperature to remove NMP solvent, rolled and compacted to increase the compaction density of the electrode sheet, and then die-cut / slit into the specified size to obtain the finished lithium iron phosphate cathode sheet.

[0064] The Ni content in the cathode material layer is 500 ppm; the carbon content is 1%.

[0065] 2. Preparation of negative electrode sheet Step 1: Weigh out each material according to the negative electrode sheet ratio of graphite (Shanghai Shanshan, FSN-1): conductive carbon (super P): sodium carboxymethyl cellulose (CMC): styrene-butadiene rubber (SBR) = 96.3:1.0:1.2:1.5 (mass ratio).

[0066] Step 2: First, add CMC to pure water at a solid content of 1.5% and stir thoroughly (e.g., stirring time 120 minutes) to prepare a transparent CMC solution.

[0067] Step 3: Add conductive carbon (super P) to the CMC adhesive solution and stir thoroughly (e.g., stirring time 90 min) to prepare the conductive adhesive.

[0068] Step 4: Continue adding graphite and stir thoroughly to obtain the desired negative electrode slurry.

[0069] Step 5: The prepared negative electrode slurry is evenly coated on copper foil, and then dried, rolled, die-cut or slit to obtain the negative electrode sheet.

[0070] 3. Preparation of non-aqueous electrolytes Ethylene carbonate (EC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC) were mixed in a mass ratio of EC:DEC:EMC = 1:1:1. Based on the mass of the non-aqueous electrolyte, 7% ethoxy(pentafluoro)cyclotriphosphazene (PFPN) and 1% vinylene carbonate were added. Then, lithium hexafluorophosphate (LiPF6) was added to a molar concentration of 1 mol / L.

[0071] 4. Lithium-ion battery cell manufacturing The prepared positive electrode sheet and the prepared negative electrode sheet are assembled into a stacked soft-pack battery cell.

[0072] 5. Electrolyte injection and formation of battery cells In a glove box with the dew point controlled below -40°C, the electrolyte prepared above was injected into the cell, vacuum sealed, and left to stand for 72 hours. Then, the first charge was performed according to the following steps: 0.05C constant current charging for 180 minutes, 0.1C constant current charging for 120 minutes, 0.2C constant current charging for 120 minutes, followed by a second vacuum sealing, and then a full charge at 0.2C (100% SOC). After resting at room temperature for 72 hours, a full discharge at 0.2C (0% SOC) was performed.

[0073] Examples 2-17 and Comparative Examples 1-14 Examples 2-17 and Comparative Examples 1-14 are used to illustrate the non-aqueous electrolyte and battery disclosed in this invention, including most of the operating steps in Example 1, with the differences being: the content of ethoxy(pentafluoro)cyclotriphosphazene, the type and content of carbonate additives, the content of Ni element in the positive electrode material layer, the content of carbon element, the type and content of auxiliary additives, and the relationship d× The values ​​are shown in Tables 1 and 2.

[0074] Performance testing The lithium-ion batteries prepared in each embodiment and comparative example were subjected to performance testing according to the following methods: 1. Room temperature fast charging cycle performance test: At 25°C, the lithium-ion batteries prepared in the examples and comparative examples were charged at a constant current and constant voltage of 1C (cutoff current 0.05C) and discharged at a 1C rate for three cycles. The capacity of the last cycle was taken as the initial 100% SOC capacity. After being charged at a constant current of 4C to 80% SOC, the batteries were fully charged at a constant current and constant voltage of 1C (cutoff current 0.05C) and discharged at a 1C rate. Full charge and discharge cycle tests were performed within the commonly used charge and discharge cutoff voltage range for the corresponding system (e.g., 2.5-3.65V for LFP / artificial graphite system) until the capacity of the lithium-ion battery decayed to 80% of the initial capacity, and the number of cycles was recorded.

[0075] 2. Internal resistance test at 25℃: Charge the battery at a constant current to 50% of its capacity at room temperature (25℃), then set the temperature to 0℃ and maintain this temperature for 6 hours.

[0076] Charge at 0.1C constant current for 10 seconds and then let stand for 40 seconds; discharge at 0.1C constant current for 10 seconds and then let stand for 40 seconds, and record the termination voltage V1. Charge at 0.2C constant current for 10s and then let stand for 40s; discharge at 0.2C constant current for 10s and then let stand for 40s, and record the termination voltage V2. Charge at 0.5C constant current for 10 seconds and then let stand for 40 seconds; discharge at 0.5C constant current for 10 seconds and then let stand for 40 seconds, and record the termination voltage V3. Plot a straight line with current as the x-axis and voltage as the y-axis. The slope of the line is the impedance at 0°C.

[0077] Test Results Table 1 shows the parameters of the lithium-ion batteries prepared in Examples 1-14 and Comparative Examples 1-14. The differences between Examples 2-14 and Comparative Examples 1-14 and Example 1 are the relevant parameters in Table 1. The rest are the same as those recorded in Example 1. The specific differences are: the Ni content in the cathode material, the carbon content in the carbon coating layer, the ethoxy(pentafluoro)cyclotriphosphazene (PFPN) content in the non-aqueous electrolyte, and the types and contents of carbonate additives and the relationship d× The values ​​are shown in Table 1.

[0078] Table 1 Note: In the table, "VC 0.5+VEC 0.5" means that the content of VC in the non-aqueous electrolyte is 0.5% and the content of VEC is 0.5%; in the table, "VC1+VEC2" means that the content of VC in the non-aqueous electrolyte is 1% and the content of VEC is 2%; "-" in the table means that the item is not present; "VC" represents vinylene carbonate; "VEC" represents ethylene ethylene carbonate.

[0079] As shown in the test results in Table 1, the lithium-ion battery provided by this invention uses ethoxy(pentafluoro)cyclotriphosphazene (PFPN) and carbonate additives as additives in the non-aqueous electrolyte, and limits the following relationships between the mass percentages of ethoxy(pentafluoro)cyclotriphosphazene (a%), carbonate additives (b%), Ni element (c ppm), and carbon element in the carbon coating layer (d%): 1 ≤ d × When the impedance is ≤500 and 0.5≤a≤15, 0.05≤b≤3, 50≤c≤2000, and 0.5≤d≤3, lithium-ion batteries can balance low initial impedance, low impedance growth rate, and excellent fast-charging and long-cycle performance.

[0080] The test results of Example 1 and Comparative Examples 1-14 show that when the mass percentage of ethoxy(pentafluoro)cyclotriphosphazene in the lithium-ion battery is a%, the mass percentage of carbonate additives is b%, the mass percentage of Ni element is c ppm, and the mass percentage of carbon element in the carbon coating layer is d%, or the relationship is d× When any one or more of the specified ranges are not met, the overall structural stability of the positive electrode is poor, and structural collapse and Fe ion dissolution may occur during cycling. It is also impossible to form a highly stable interface film on the negative electrode, which ultimately degrades the battery impedance, impedance growth and electrochemical performance.

[0081] When the mass percentages of ethoxy(pentafluoro)cyclotriphosphazene in the lithium-ion battery (a%), carbonate additives (b%), Ni (c ppm), and carbon in the carbon coating (d%) further satisfy: 20 ≤ d × When the values ​​are ≤200, 5≤a≤10, 0.5≤b≤1.5, 200≤c≤1000, and 1≤d≤2, the doped Ni element can effectively improve the structural stability of lithium iron phosphate materials and reduce the lithium ion migration barrier. Carbon coating can improve conductivity and inhibit metal ion dissolution. The appropriate amount of PFPN and carbonate additives can form an interface film of suitable thickness, enabling lithium-ion batteries to better balance low initial impedance, low impedance growth rate, and excellent fast charging and long cycle performance.

[0082] Table 2 shows the parameters of the lithium-ion batteries prepared in Examples 1 and 15-17. The difference between Examples 15-17 and Example 1 lies in the relevant parameters in Table 2; all other parameters are the same as those in Example 1. The specific difference is the type of auxiliary additives. The test results are shown in Table 2.

[0083] Table 2 Note: "-" in the table indicates that the item is not present; "HTCN" in the table represents 1,3,6-hexanetrionitrile; "SN" represents vinyl sulfate; "TMSB" represents tris(trimethylsilyl)borate; "PS" represents 1,3-propanesulfonyl lactone.

[0084] Table 2 shows the test results, which indicate that when the mass percentage of ethoxy(pentafluoro)cyclotriphosphazene in the lithium-ion battery is a%, the mass percentage of carbonate additives is b%, the mass percentage of Ni element is c ppm, and the mass percentage of carbon element in the carbon coating layer is d%, the relationship between them is d× When the corresponding conditions are met, adding different types of auxiliary additives can still enable lithium-ion batteries to maintain low initial impedance, low impedance growth rate, and excellent fast-charge long-cycle performance. This demonstrates that the battery system provided by this invention is universally applicable to different types of auxiliary additives.

[0085] The present invention has been further described above with reference to specific embodiments. However, it should be understood that the specific description herein should not be construed as limiting the nature and scope of the present invention. Various modifications made to the above embodiments by those skilled in the art after reading this specification are all within the scope of protection of the present invention.

Claims

1. A lithium-ion battery, characterized in that, It includes a positive electrode, a negative electrode, and a non-aqueous electrolyte; the positive electrode includes a positive electrode material layer containing a positive electrode active material, the positive electrode active material including a lithium iron phosphate composite material composed of lithium iron phosphate and a carbon coating layer wrapped on the surface of lithium iron phosphate; the positive electrode active material is also doped with Ni element; With the positive electrode material layer as 100% of the mass, the mass content of Ni element c ppm satisfies 50≤c≤2000, and the mass percentage content of carbon element d% in the carbon coating layer satisfies 0.5≤d≤3. The non-aqueous electrolyte comprises ethoxy(pentafluoro)cyclotriphosphazene and carbonate additives; the carbonate additives comprise at least one of vinylene carbonate and ethylene ethylene carbonate. Based on the mass of the non-aqueous electrolyte, the mass percentage of the ethoxy(pentafluoro)cyclotriphosphazene is a%, and the mass percentage of the carbonate additive is b%. The lithium-ion battery meets the following conditions: 1≤d× ≤500; and 0.5≤a≤15, 0.05≤b≤3.

2. The lithium-ion battery as described in claim 1, characterized in that, The lithium-ion battery satisfies: 20 ≤ d × ≤200.

3. The lithium-ion battery as described in claim 1, characterized in that, The mass percentage a% of the ethoxy(pentafluoro)cyclotriphosphazene satisfies 5 ≤ ​​a ≤ 10.

4. The lithium-ion battery as described in claim 1, characterized in that, The mass percentage b% of the carbonate additive satisfies 0.5 ≤ b ≤ 1.

5.

5. The lithium-ion battery as described in claim 1, characterized in that, The mass content of Ni element, c ppm, satisfies 200 ≤ c ≤ 1000.

6. The lithium-ion battery as described in claim 1, characterized in that, The mass percentage of carbon element d% satisfies 1≤d≤2.

7. The lithium-ion battery as described in claim 1, characterized in that, The non-aqueous electrolyte includes an organic solvent, which is one or more of cyclic carbonates, linear carbonates, carboxylic acid esters, and ethers.

8. The lithium-ion battery as described in claim 1, characterized in that, The non-aqueous electrolyte also includes auxiliary additives, which are selected from at least one of the following: cyclic carbonate compounds, cyclic sulfate compounds, sulfonyl lactone compounds, phosphate compounds, borate compounds, and nitrile compounds, excluding vinylene carbonate and ethylene ethylene carbonate.

9. The lithium-ion battery as described in claim 8, characterized in that, Based on the total mass of the non-aqueous electrolyte as 100%, the content of the auxiliary additives is 0.01% to 30%.

10. The lithium-ion battery as described in claim 8, characterized in that, The cyclic carbonate compounds other than vinylene carbonate or ethylene carbonate are selected from at least one of methylene ethylene carbonate, fluoroethylene carbonate, trifluoromethyl ethylene carbonate, difluoroethylene carbonate, or compounds represented by structural formula 1 below: Structural Formula 1, In structural formula 1, R 21 R 22 R 23 R 24 R 25 R 26 Each is independently selected from one of the following: hydrogen atom, halogen atom, C1-C5 group; and / or, The cyclic sulfate compounds are selected from vinyl sulfate, 4-methylvinyl sulfate, propylene sulfate, etc. , , At least one of them; and / or, The sulfonyl lactone compounds are selected from 1,3-propanesulfonyl lactone, 1,4-butanesulfonyl lactone, propenyl-1,3-sulfonyl lactone, and others. At least one of them; and / or, The phosphate ester compounds include saturated phosphate ester compounds and unsaturated phosphate ester compounds, wherein the saturated phosphate ester compounds include tris(trimethylsilane) phosphate esters, and the unsaturated phosphate ester compounds include compounds represented by structural formula 2 below: Structural Formula 2, In structural formula 2, R 31 R 32 R 33 Each is independently selected from C1-C5 saturated hydrocarbon groups, unsaturated hydrocarbon groups, halohydrocarbon groups, and -Si(C m H 2m+1 )3, m is a natural number from 1 to 3, and R 31 R 32 R 33 At least one of them is an unsaturated hydrocarbon group; and / or, The borate ester compounds include tris(trimethylsilane)borate and tris(triethylsilane)borate; and / or, The nitrile compounds include at least one of butadionitrile, glutaronitrile, ethylene glycol bis(propionitrile) ether, hexanetrionitrile, adiponitrile, heptacyanide, octadionitrile, nonadionitrile, and sebaconitol.

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