A lithium-ion battery

Abstract

To overcome the problem that existing lithium-ion batteries cannot simultaneously achieve wide-temperature-range application performance at both high and low temperatures, this invention provides a lithium-ion battery comprising a positive electrode, a negative electrode, and an electrolyte. The positive electrode comprises a positive electrode material layer, which includes a positive electrode active material selected from lithium iron phosphate with a carbon coating. The negative electrode comprises a negative electrode material layer, which includes a negative electrode active material selected from graphite. The electrolyte comprises a solvent, a lithium salt, and additives. The additives include a first additive and a second additive, wherein the first additive is an unsaturated cyclic carbonate, and the second additive is a sulfur-containing cyclic ester.

Description

Technical Field

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

[0002] High-temperature environments are one of the important reasons for the performance degradation of lithium iron phosphate batteries. Under high-temperature conditions, the interfacial side reactions inside the battery are accelerated. For example, on the positive electrode side, the side reactions between lithium iron phosphate material and electrolyte are accelerated, which leads to damage to the surface of the positive electrode material, the dissolution of iron in the structure, and the deposition and destruction of the SEI film on the negative electrode surface. This further leads to accelerated consumption of electrolyte and increased battery impedance, resulting in performance degradation. On the negative electrode side, high-temperature conditions cause the SEI film itself to degrade more rapidly, which also exacerbates the side reactions between graphite negative electrode material and electrolyte, further accelerating electrolyte consumption and increasing battery impedance.

[0003] To improve the high-temperature performance of lithium iron phosphate batteries, the current mainstream technology in the industry is electrolyte functional additive modification. This involves introducing film-forming functional additives into the electrolyte to pre-construct a thermally stable interfacial protective film at the electrode / electrolyte interface, thereby suppressing interfacial side reactions under high-temperature conditions. Existing technologies commonly use vinylene carbonate (VC), 1,3-propenesulfonyl lactone (PES), and 1,3-propanesulfonyl lactone (PS) as high-temperature film-forming additives. While these additives can suppress electrode interfacial side reactions to some extent and improve the cycle life and capacity retention of the battery under high-temperature conditions, they still have insurmountable technical drawbacks: the interfacial film formed by these additives is too dense and has high lithium-ion conductivity, significantly increasing the battery's bulk impedance. This not only severely limits the battery's charge / discharge rate performance and power output capability but also leads to significant degradation of the battery's electrochemical performance in low-temperature and room-temperature environments, failing to achieve the battery's wide-temperature-range adaptability.

[0004] Therefore, improving the high-temperature cycle performance of lithium iron phosphate batteries while taking into account battery impedance and wide temperature range adaptability is a battery technology problem that urgently needs to be solved. Summary of the Invention

[0005] To address the problem that existing lithium-ion batteries struggle to achieve performance across a wide temperature range, from high to low temperatures, this invention provides a lithium-ion battery.

[0006] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows: This invention provides a lithium-ion battery, comprising a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes a positive electrode material layer, which includes a positive electrode active material. The positive electrode active material is selected from lithium iron phosphate with a carbon coating layer on its surface. The specific surface area of ​​the positive electrode active material is S', in m³ / s. 2 / g, the median particle size of the positive electrode active material in volume distribution is D 50 The unit is μm, the carbon mass percentage of the positive electrode active material is ε, the negative electrode includes a negative electrode material layer, the negative electrode material layer includes a negative electrode active material, the negative electrode active material is selected from graphite, and the specific surface area of ​​the negative electrode active material is S'', the unit is m. 2 / g, the median particle size of the negative electrode active material in volume distribution is D 50 '', the unit is μm, and the degree of graphitization of the negative electrode active material is G; The electrolyte includes a solvent, a lithium salt, and an additive. The additive includes a first additive and a second additive. The first additive is an unsaturated cyclic carbonate, and the second additive is a sulfur-containing cyclic ester. The mass percentage of the first additive is W1 and the mass percentage of the second additive is W2, based on the total mass of the electrolyte. The lithium-ion battery meets the following conditions: 10≤ ≤25, and 9.5≤S'≤15.8, 0.85≤D 50 '≤1.25, 0.95%≤ε≤1.60%, 0.88≤S'' ≤1.64, 8.0≤D 50 ''≤14, 92.5%≤G≤95%, 0.5%≤W1≤5.0%, 0.2%≤W2≤3.0%.

[0007] Optionally, the lithium-ion battery meets the following conditions: 11≤ ≤20.

[0008] Optionally, the lithium-ion battery satisfies at least one of the following conditions: (1) 10.0 ≤ S' ≤ 14.5; (2) 0.9≤D 50 ≤1.2; (3) 1.0%≤ε≤1.55%; (4) 0.95≤S'' ≤1.57; (5) 8.5≤D 50 ≤13.5; (6) 93%≤G≤94.5%; (7) 0.5%≤W1≤4.5%; (8) 0.2%≤W2≤2.5%.

[0009] Optionally, the volume distribution particle size D of the positive electrode active material 10 '0.32~0.46μm, D 90 'Within 1.8~4.0μm, D99 ' is 4.05~7.8μm, and (D 90 '-D 10 ') / D 50 The value is 1.4 to 3.5.

[0010] Optionally, the positive electrode active material is obtained by agglomeration of multiple primary particles, wherein the average particle size D of the primary particles is... 50 1 The range is 0.1~0.49μm.

[0011] Optionally, the volume distribution particle size D of the negative electrode active material 10 '' is 4.1~6.8μm, D 90 '' is 14.5~24.0μm, D 99 '' is 22.0~33.8μm, D 01 ''≥2.0μm, and (D 90 ''-D 10 '') / D 50 The value is 1.05~1.60.

[0012] Optionally, the first additive includes at least one of vinylene carbonate, ethylene ethylene carbonate, and methyl vinylene carbonate.

[0013] Optionally, the second additive includes at least one of 1,3-propenesulfonate lactone, 1,3-propanesulfonate lactone, 1,3-propanediol cyclosulfonate, and 1,4-butanesulfonate lactone.

[0014] Optionally, the additive further includes a third additive, which includes at least one of 1,3,2-dioxothiacyclopentane-2,2-dioxide, 4-methyl-1,3,2-dioxothiacyclopentane-2,2-dioxide, 4-propyl-[1,3,2]dioxothiacyclopentane-2,2-dioxide, methane disulfonate, tris(trimethylsilane) phosphate, triphenyl phosphite, tris(trimethylsilane) phosphate, tris(trimethylsilane) borate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, fluoroethylene carbonate, and fluoroethylene carbonate.

[0015] Optionally, based on the total mass of the electrolyte, the mass percentage of the third additive is 0.5% ≤ W3 ≤ 4.0%; The lithium-ion battery meets the following conditions: 3.5%≤W1+W2+W3≤8.0%.

[0016] Optionally, the solvent includes ethylene carbonate, propylene carbonate, and diethyl carbonate, wherein, based on the total mass of the electrolyte, the weight percentage of ethylene carbonate is 15-30%, the weight percentage of propylene carbonate is 1-15%, and the weight percentage of diethyl carbonate is 30-55%.

[0017] According to the lithium-ion battery provided by the present invention, the inventors have discovered that by adjusting the specific surface area S' of the positive electrode active material and the median particle size D of the volume distribution of the positive electrode active material... 50 ', Carbon mass percentage ε of the positive electrode active material, Specific surface area S'' of the negative electrode active material, Median particle size D of the volume distribution of the negative electrode active material 50 The graphitization degree G of the negative electrode active material, the mass percentage W1 of the first additive, and the mass percentage W2 of the second additive satisfy the condition 10 ≤ ≤25, and 9.5≤S'≤15.8, 0.85≤D 50 '≤1.25, 0.95%≤ε≤1.60%, 0.88≤S'' ≤1.64, 8.0≤D 50 When the content of additives is ≤14%, 92.5%≤G≤95%, 0.5%≤W1≤5.0%, and 0.2%≤W2≤3.0%, the resulting lithium-ion battery can achieve both good high-temperature cycle performance and low-temperature discharge performance, while effectively controlling the battery's bulk impedance. It is speculated that this is because the first and second additives decompose and form films on the surfaces of the positive and negative electrodes during the battery formation stage. Factors such as the specific surface area, particle size, and carbon coating amount of the positive electrode active material, and the particle size and graphitization degree of the negative electrode active material, have a significant impact on the quality of the solid electrolyte interface film formed by the first and second additives on the positive and negative electrode surfaces during the battery formation stage. In particular, the carbon coating amount of the positive electrode and the graphitization degree of the negative electrode both determine the active sites and defect extent on the positive and negative electrode surfaces. The defects in both the carbon coating layer and the graphite material of the negative electrode will lead to the decomposition and film formation effect of the first and second additives on the positive and negative electrode surfaces during the formation stage. This will regulate the proportion of decomposition products from the first and second additives in the solid electrolyte interface film on both sides of the positive and negative electrodes, as well as the uniformity and density of the film formation. Therefore, through extensive experimental fitting, the inventors found that when the above parameters meet the above conditions, it is beneficial to improve the uniformity and density of the solid electrolyte interface film on the positive and negative electrode surfaces, thereby simultaneously improving the ion conduction performance and high-temperature stability of the solid electrolyte interface film on the positive and negative electrode surfaces. Under the premise of improving the high-temperature performance of lithium-ion batteries, the ion transport efficiency at low and room temperatures is guaranteed, and the low-temperature discharge capability of lithium-ion batteries is improved simultaneously. Detailed Implementation

[0018] To make the technical problems solved, the technical solutions, and the beneficial effects of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0019] This invention provides a lithium-ion battery, comprising a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes a positive electrode material layer, which includes a positive electrode active material. The positive electrode active material is selected from lithium iron phosphate with a carbon coating layer on its surface. The specific surface area of ​​the positive electrode active material is S', in m³ / s. 2 / g, the median particle size of the positive electrode active material in volume distribution is D 50 The unit is μm, the carbon mass percentage of the positive electrode active material is ε, the negative electrode includes a negative electrode material layer, the negative electrode material layer includes a negative electrode active material, the negative electrode active material is selected from graphite, and the specific surface area of ​​the negative electrode active material is S'', the unit is m. 2 / g, the median particle size of the negative electrode active material in volume distribution is D 50 '', the unit is μm, and the degree of graphitization of the negative electrode active material is G; The electrolyte includes a solvent, a lithium salt, and an additive. The additive includes a first additive and a second additive. The first additive is an unsaturated cyclic carbonate, and the second additive is a sulfur-containing cyclic ester. The mass percentage of the first additive is W1 and the mass percentage of the second additive is W2, based on the total mass of the electrolyte. The lithium-ion battery meets the following conditions: 10≤ ≤25, and 9.5≤S'≤15.8, 0.85≤D 50 '≤1.25, 0.95%≤ε≤1.60%, 0.88≤S''≤1.64, 8.0≤D 50 ''≤14, 92.5%≤G≤95%, 0.5%≤W1≤5.0%, 0.2%≤W2≤3.0%.

[0020] When the specific surface area S' and the median particle size D of the positive electrode active material are adjusted... 50 ', Carbon mass percentage ε of the positive electrode active material, Specific surface area S'' of the negative electrode active material, Median particle size D of the volume distribution of the negative electrode active material 50 The graphitization degree G of the negative electrode active material, the mass percentage W1 of the first additive, and the mass percentage W2 of the second additive satisfy the condition 10 ≤ ≤25, and 9.5≤S'≤15.8, 0.85≤D 50'≤1.25, 0.95%≤ε≤1.60%, 0.88≤S'' ≤1.64, 8.0≤D 50 When the content of additives is ≤14%, 92.5%≤G≤95%, 0.5%≤W1≤5.0%, and 0.2%≤W2≤3.0%, the resulting lithium-ion battery can achieve both good high-temperature cycle performance and low-temperature discharge performance, while effectively controlling the battery's bulk impedance. It is speculated that this is because the first and second additives decompose and form films on the surfaces of the positive and negative electrodes during the battery formation stage. Factors such as the specific surface area, particle size, and carbon coating amount of the positive electrode active material, as well as the particle size and graphitization degree of the negative electrode active material, have a significant impact on the quality of the solid electrolyte interface film formed by the first and second additives on the positive and negative electrode surfaces during the battery formation stage. In particular, the carbon coating amount of the positive electrode and the graphitization degree of the negative electrode both determine the degree of defect exposure of the carbon material on the positive and negative electrode surfaces. The defects in both the carbon coating layer and the graphite material of the negative electrode will lead to the decomposition and film formation effect of the first and second additives on the positive and negative electrode surfaces during the formation stage. This will regulate the proportion of decomposition products from the first and second additives in the solid electrolyte interface film on both sides of the positive and negative electrodes, as well as the uniformity and density of the film formation. Therefore, through extensive experimental fitting, the inventors found that when the above parameters meet the above conditions, it is beneficial to improve the uniformity and density of the solid electrolyte interface film on the positive and negative electrode surfaces, thereby simultaneously improving the ion conduction performance and high-temperature stability of the solid electrolyte interface film on the positive and negative electrode surfaces. Under the premise of improving the high-temperature performance of lithium-ion batteries, the ion transport efficiency at low and room temperatures is guaranteed, and the low-temperature discharge capability of lithium-ion batteries is improved simultaneously.

[0021] In some embodiments, the lithium-ion battery satisfies the following conditions: 11≤ ≤20.

[0022] When the above parameters further meet this condition, the matching between positive and negative electrode materials and electrolyte additives can be further optimized to maximize the synergistic effect, more effectively balance the stability of the interface film and the ion transport capability, and further improve the wide temperature range performance of the battery.

[0023] In some embodiments, the specific surface area S' of the positive electrode active material can be 9.5 m². 2 / g, 10.0m 2 / g, 11.2m 2 / g, 12.0m 2 / g, 12.8m 2 / g, 13.5m 2 / g, 14.5m 2 / g, 15.0m 2 / g, 15.8m 2 / g or any two of the above ranges.

[0024] In a preferred embodiment, the specific surface area S' of the positive electrode active material satisfies 10.0 ≤ S' ≤ 14.5.

[0025] By controlling the specific surface area of ​​the positive electrode active material within the aforementioned range, the reactivity of the positive electrode and the degree of side reactions can be effectively balanced. When S' is too high, there are too many surface active sites on the positive electrode active material, which will aggravate the side reactions between the positive electrode and the electrolyte at high temperatures, promote the dissolution of Fe elements, and thus lead to the deposition of transition metals on the surface of the negative electrode, destroying the stability of the solid electrolyte interfacial film and accelerating capacity decay. When S' is too low, the reactivity of the positive electrode is insufficient, the lithium ion insertion and extraction kinetics are poor, which will lead to a decrease in the rate performance of the battery. At the same time, polarization at low temperatures will be aggravated, deteriorating the low-temperature discharge performance.

[0026] In some embodiments, the specific surface area S' of the positive electrode active material can be obtained by testing the following methods: Referring to GB / T 19587-2017, 2.0±0.5g of sample was placed in a sample tube and degassed at 200℃ for 120min. Then, the specific surface area data was obtained by multi-point BET method with P / P0 values ​​of 0.05, 0.1, 0.15, 0.20, 0.25, and 0.3. The instrument model was Bestech 3H-2000PS4.

[0027] In some embodiments, the median particle size D of the positive electrode active material in the volume distribution 50 It can be 0.85μm, 0.90μm, 0.95μm, 1.00μm, 1.05μm, 1.10μm, 1.15μm, 1.20μm, 1.25μm or any two of the above.

[0028] In a preferred embodiment, the median particle size D of the positive electrode active material in its volume distribution 50 'Satisfies 0.9≤D 50 ≤1.2.

[0029] The appropriate particle size of the positive electrode active material can effectively optimize the lithium-ion transport path and surface reaction characteristics, while also affecting the formation of the solid electrolyte interfacial film. When D50' is too small, the particle size of the positive electrode particles is too small, which leads to a sharp increase in specific surface area, thereby exacerbating the aforementioned side reactions. At the same time, the agglomeration effect of small particles increases the difficulty of electrolyte wetting. Conversely, when D50' is too large, the lithium-ion transport path inside the positive electrode particles is too long, which increases concentration polarization, leading to a decrease in the rate performance and low-temperature performance of the battery. In addition, the volume change stress of large particles is more likely to cause particle cracks, accelerating the destruction and reorganization of the solid electrolyte interfacial film during cycling, resulting in electrolyte consumption and increased impedance.

[0030] In some embodiments, the carbon mass percentage ε of the positive electrode active material can be 0.95%, 1.00%, 1.10%, 1.20%, 1.30%, 1.40%, 1.50%, 1.60%, or any range between the above two.

[0031] In a preferred embodiment, the carbon mass percentage ε of the positive electrode active material satisfies: 1.0% ≤ ε ≤ 1.55%.

[0032] The carbon coating content is crucial to the performance of the cathode. When ε is too low, the carbon coating cannot completely cover the surface of the lithium iron phosphate particles, causing some lithium iron phosphate to come into direct contact with the electrolyte, exacerbating the dissolution of Fe and side reactions, affecting the uniformity of the solid electrolyte interfacial film, and resulting in insufficient electronic conductivity, increasing the impedance of the cathode. Conversely, when ε is too high, excessive carbon will occupy the volume of the cathode active material, reducing the energy density of the battery. At the same time, excessive carbon will increase the number of active sites on the surface, exacerbating the decomposition side reactions of the electrolyte, which is detrimental to high-temperature stability.

[0033] In some embodiments, the volume distribution particle size D of the positive electrode active material 10 '0.32~0.46μm, D 90 'Within 1.8~4.0μm, D 99 ' is 4.05~7.8μm, and (D 90 '-D 10 ') / D 50 The value is 1.4 to 3.5.

[0034] By controlling the particle size distribution of the positive electrode active material, the uniformity of the particles can be ensured. A narrow particle size distribution can avoid the aggravation of side reactions caused by too many small particles, and also avoid the insufficient transport kinetics caused by too many large particles, thereby improving the reaction uniformity of the positive electrode particles, reducing local polarization, and thus optimizing the uniformity of the interface film.

[0035] In some embodiments, the volume distribution particle size D of the positive electrode active material 10 '、D 50 '、D 90 'and D 99 This can be obtained through testing using the following methods: Referring to GB / T 19077.1-2016, 0.05-0.50 g of sample was ultrasonically dispersed in anhydrous ethanol for 5 min. The dispersed sample was then added to a Malvern 3000 laser particle size analyzer for testing. The particle size data were obtained under the following conditions: solvent: deionized water; refractive index: 1.52; absorptivity: 1.0; stirring speed: 2000-3000 rpm; and occlusion: 8%-12%.

[0036] In some embodiments, the positive electrode active material is obtained by agglomeration of multiple primary particles, wherein the average particle size D of the primary particles is... 50 1 The range is 0.1~0.49μm.

[0037] The particle size of the primary particles ensures that the agglomerated secondary particles have a suitable pore structure, which is beneficial for electrolyte wetting, shortens the lithium ion transport distance, and improves the reaction kinetics of the positive electrode active material.

[0038] In some embodiments, the lithium iron phosphate can be modified by doping with doping elements selected from at least one of Mg, Al, Ti, Zr, Nb, Mn, Zn, and V. The molar percentage of the doping element does not exceed 5% of the total molar amount of lithium iron phosphate. Element doping can further improve the electronic conductivity and crystal structure stability of the lithium iron phosphate.

[0039] In some embodiments, the carbon source of the carbon coating layer may be an organic carbon source, including but not limited to at least one of glucose, sucrose, citric acid, polyvinyl alcohol, polyethylene glycol, phenolic resin, asphalt, graphene, and carbon nanotubes; the carbon coating layer is an amorphous carbon layer, and under the premise of satisfying the carbon content ε of the present invention, the density and conductivity of the carbon layer can be controlled by the type of carbon source and the coating process.

[0040] In some embodiments, the lithium iron phosphate has an olivine-type crystal structure with a crystal phase purity of ≥98%, which can ensure the reversibility of lithium-ion insertion / extraction and structural stability of the material.

[0041] In some embodiments, the positive electrode includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. The positive electrode material layer, in addition to the positive electrode active material, also includes a conductive agent and a binder, specifically comprising: Conductive agent: can be selected from at least one of conductive carbon black, acetylene black, Ketjen black, superconducting carbon black, carbon nanotubes, carbon fibers, graphene, and flake graphite; the mass percentage of the conductive agent is 0.5% to 5.0% based on the total mass of the positive electrode material layer.

[0042] The binder may be selected from at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-hexafluoropropylene copolymer, polyacrylate, polyimide, and polyacrylonitrile; the binder has a mass percentage of 1.0% to 4.0% based on the total mass of the cathode material layer.

[0043] Positive current collector: Aluminum foil can be used, including but not limited to smooth aluminum foil, etched aluminum foil, and porous aluminum foil. The thickness of the aluminum foil can be controlled within the range of 8μm to 20μm. A conductive undercoating layer can be provided on the surface of the current collector. The thickness of the undercoating layer is 0.5μm to 2μm to further reduce the contact resistance between the positive electrode and the current collector.

[0044] In some embodiments, the specific surface area S'' of the negative electrode active material can be 0.88 m². 2 / g, 0.95m 2 / g, 1.05m 2 / g, 1.20m 2 / g, 1.35m 2 / g, 1.52m 2 / g, 1.64m 2 / g, or any range between the two mentioned above.

[0045] In a preferred embodiment, the specific surface area S'' of the negative electrode active material satisfies 0.95≤S''≤1.57.

[0046] The specific surface area of ​​the negative electrode directly affects its side reactions and film formation process. When S'' is too high, there are too many active sites on the surface of the negative electrode, which leads to excessive consumption of additives during the formation process, forming an excessively thick solid electrolyte interfacial film, increasing the battery's bulk impedance, and exacerbating electrolyte consumption, resulting in capacity decay during cycling. Conversely, when S'' is too low, the reactivity of the negative electrode is insufficient, and the lithium-ion insertion / extraction kinetics are poor, leading to a decrease in the battery's rate performance. At the same time, insufficient active sites on the surface cannot guarantee uniform film formation of additives, resulting in insufficient stability of the solid electrolyte interfacial film.

[0047] In some embodiments, the specific surface area S'' of the negative electrode active material can be obtained by a method similar to that used for the specific surface area S' of the positive electrode active material.

[0048] In some embodiments, the median particle size D of the negative electrode active material in the volume distribution 50 The value can be 8.0μm, 9.0μm, 10.0μm, 11.0μm, 12.0μm, 13.0μm, 14.0μm, or any two of the above.

[0049] In a preferred embodiment, the median particle size D50'' of the volume distribution of the negative electrode active material satisfies: 8.5 ≤ D 50 ≤13.5.

[0050] The particle size of the negative electrode graphite also needs to be matched to the overall system. When D50'' is too small, the specific surface area of ​​the negative electrode particles is too large, which will exacerbate the aforementioned side reactions; while when D50'' is too large, the transport path of lithium ions inside the negative electrode particles is too long, increasing polarization and degrading rate and low-temperature performance.

[0051] In some embodiments, the degree of graphitization G of the negative electrode active material can be 92.5%, 93.0%, 93.5%, 94.0%, 94.5%, 95.0%, or any range between the above two.

[0052] In a preferred embodiment, the degree of graphitization G of the negative electrode active material satisfies: 93% ≤ G ≤ 94.5%.

[0053] The degree of graphitization determines the electronic conductivity and surface reactivity of graphite. When G is too high, the degree of order in graphite is too high, and there are insufficient active sites on the surface, which is not conducive to the adsorption of additives and film formation, resulting in insufficient stability of the solid electrolyte interfacial film. When G is too low, the degree of disorder in graphite is too high, and there are too many surface defects, which will aggravate side reactions, reduce electronic conductivity, increase the impedance of the negative electrode, and degrade the performance of the battery.

[0054] In some embodiments, the degree of graphitization G of the negative electrode active material can be obtained by testing using the following methods: The negative electrode active material was ground uniformly, and then analyzed using a Cu target K on an X-ray diffractometer. α X-rays, wavelength λ = 1.54 Å, test range 10~80°, step size 0.1°, scan rate 10° / min, to obtain X-ray diffraction data, then according to Bragg's equation: 2dsin i = l Calculate the interplanar spacing d (002) Using the formula G=(3.440-2 d (002) The degree of graphitization G is calculated as (3.440-3.3354)×100%.

[0055] In some embodiments, the volume distribution particle size D of the negative electrode active material 10 '' is 4.1~6.8μm, D 90 '' is 14.5~24.0μm, D 99 '' is 22.0~33.8μm, D 01 ''≥2.0μm, and (D 90 ''-D 10 '') / D 50 The value is 1.05~1.60.

[0056] A suitable negative electrode particle size distribution can ensure the uniformity of negative electrode particles, avoid excessive side reactions of small particles, and avoid the transport resistance of large particles, thereby improving the uniformity of negative electrode reaction and optimizing the formation quality of solid electrolyte interfacial film.

[0057] In some embodiments, the volume distribution particle size D of the negative electrode active material 10 ''、D 50 ''、D 90 '' and D 01 It can be obtained by a method similar to that used for measuring the volume distribution and particle size of positive electrode active materials.

[0058] In some embodiments, the main component of the negative electrode active material is graphite, and various graphite materials suitable for lithium iron phosphate batteries can be selected. Auxiliary modification methods can also be used, specifically including: Graphite type options: The graphite can be selected from at least one of artificial graphite, natural graphite, and modified natural graphite; wherein artificial graphite can be obtained by graphitization treatment using petroleum coke, needle coke, and pitch coke as raw materials, and natural graphite can be modified natural graphite with spheroidization and surface coating treatment.

[0059] Surface modification features: The graphite surface can be provided with an amorphous carbon coating layer, an oxidation treatment layer, and a doping modification layer, and the thickness of the coating layer can be controlled within the range of 1nm~10nm.

[0060] Composite active materials: In the negative electrode active materials, other negative electrode active materials, including but not limited to silicon-based materials, silicon-oxygen materials, and lithium titanate, may be combined without affecting the core parameters of the present invention. The mass ratio of the composite components shall not exceed 10% of the total mass of the negative electrode active materials, so as to help improve the capacity or structural stability of the negative electrode.

[0061] In some embodiments, the negative electrode includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. The negative electrode material layer, in addition to the negative electrode active material, also includes a conductive agent, a binder, and a thickener, specifically including: Conductive agent: can be selected from at least one of conductive carbon black, acetylene black, Ketjen black, carbon nanotubes, carbon fibers, and graphene; the mass percentage of the conductive agent is 0.3% to 3.0% based on the total mass of the negative electrode material layer.

[0062] Adhesive: can be selected from at least one of styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), polyacrylic acid, sodium polyacrylate, lithium polyacrylate, polyimide, and polyurethane; the adhesive has a mass percentage content of 0.5% to 3.0% based on the total mass of the negative electrode material layer.

[0063] Thickener: can be selected from at least one of sodium carboxymethyl cellulose (CMC), hydroxypropyl methyl cellulose, carboxyethyl cellulose, and polyvinyl alcohol; the mass percentage of the thickener is 0.5% to 2.0% based on the total mass of the negative electrode material layer.

[0064] Negative electrode current collector: Copper foil can be used, including but not limited to smooth copper foil, electrolytic copper foil, rolled copper foil, and porous copper foil; a conductive undercoating can be provided on the surface of the current collector to further reduce the contact resistance between the negative electrode and the current collector.

[0065] In this application, the various parameters of the positive electrode active material can be achieved through process control during preparation: For carbon-coated lithium iron phosphate, the particle size can be controlled by adjusting the holding time and sintering temperature during the sintering process, as well as subsequent crushing and shaping processes. For example, by adjusting the speed and processing time of a turbine pulverizer or an airflow vortex micronizer, particle shaping can be achieved, and the desired particle size (D) can be obtained. 50 1 The particle size and specific surface area can be adjusted by controlling the porosity of the precursor and the atmosphere during the sintering process; the carbon coating content can be controlled by the amount of carbon source added and the reaction time of the coating process, thereby obtaining a positive electrode active material that meets the parameter range of this application.

[0066] For graphite active materials of negative electrode, the degree of graphitization can be controlled by the sintering temperature and holding time of the graphitization sintering process. For example, by adjusting the temperature of high-temperature graphitization treatment between 2800℃ and 3200℃, the required degree of graphitization G can be obtained. Its particle size and specific surface area can be controlled by the crushing and grading process of graphite raw materials, so as to obtain negative electrode active materials that meet the parameter range of this application.

[0067] In some embodiments, the mass percentage content W1 of the first additive can be any two of 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, 5.0%, or more.

[0068] In a preferred embodiment, the mass percentage content W1 of the first additive satisfies: 0.5% ≤ W1 ≤ 4.5%.

[0069] As an unsaturated cyclic carbonate, the first additive preferentially decomposes with the solvent on the negative electrode surface to form the main component of the solid electrolyte interfacial film, thus improving the stability of the interfacial film. When W1 is too low, the amount of the first additive is insufficient and cannot completely cover the active sites on the negative electrode surface, resulting in incomplete film formation and inability to effectively suppress side reactions. Conversely, when W1 is too high, excessive first additive will decompose excessively, forming an overly thick solid electrolyte interfacial film, increasing interfacial impedance, hindering lithium-ion transport, and degrading low-temperature and rate performance.

[0070] In some embodiments, the first additive includes at least one of vinylene carbonate, ethylene ethylene carbonate, and methyl vinylene carbonate.

[0071] In some embodiments, the mass percentage content W2 of the second additive can be any two of 0.2%, 0.5%, 0.8%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, or more.

[0072] In a preferred embodiment, the mass percentage content W2 of the second additive satisfies: 0.2% ≤ W2 ≤ 2.5%.

[0073] The second additive, a sulfur-containing cyclic ester, works synergistically with the first additive to improve the thermal stability of the solid electrolyte interfacial film on the electrode surface. The sulfur-containing groups enhance the strength of the interfacial film while suppressing side reactions and Fe dissolution on the positive electrode side. When W2 is too low, the amount of the second additive is insufficient to effectively improve the high-temperature stability of the interfacial film and suppress side reactions at high temperatures. Conversely, when W2 is too high, excessive sulfur-containing additives lead to an overabundance of sulfur components in the interfacial film, increasing interfacial impedance. Furthermore, excessive sulfur-containing substances can trigger side reactions, resulting in problems such as gas production.

[0074] In some embodiments, the second additive includes at least one of 1,3-propenesulfonate lactone, 1,3-propanesulfonate lactone, 1,3-propanediol cyclosulfonate, and 1,4-butanesulfonate lactone.

[0075] In some embodiments, the additive further includes a third additive, which includes at least one of 1,3,2-dioxothiacyclopentane-2,2-dioxide, 4-methyl-1,3,2-dioxothiacyclopentane-2,2-dioxide, 4-propyl-[1,3,2]dioxothiacyclopentane-2,2-dioxide, methylene disulfonate, tris(trimethylsilane) phosphate, triphenyl phosphite, tris(trimethylsilane) phosphate, tris(trimethylsilane) borate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, fluoroethylene carbonate, and fluoroethylene carbonate.

[0076] The third additive can further synergize with the first and second additives to optimize the composition of the interfacial film, further improve the stability and ion conductivity of the interfacial film, and at the same time inhibit the decomposition of lithium salts and improve the stability of the electrolyte.

[0077] In some embodiments, the mass percentage of the third additive is 0.5% ≤ W3 ≤ 4.0% based on the total mass of the electrolyte.

[0078] In a specific embodiment, the mass percentage content W3 of the third additive can be any two of the following: 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, or more.

[0079] In some embodiments, the lithium-ion battery satisfies the following conditions: 3.5%≤W1+W2+W3≤8.0%.

[0080] By adjusting the total content of the three additives, the synergistic effect of the additives can be ensured. This ensures sufficient film formation and suppresses side reactions, while preventing excessive additives from causing an overly thick interfacial film and increasing impedance. When the total content is too low, the amount of additives is insufficient, resulting in incomplete film formation; when the total content is too high, excessive additives will lead to excessive interfacial impedance and degrade battery performance.

[0081] In some embodiments, the solvent includes at least one of ethylene carbonate EC, dimethyl carbonate DMC, diethyl carbonate DEC, methyl ethyl carbonate EMC, methyl trifluoroethyl carbonate FEMC, propylene carbonate PC, methyl acetate MA, ethyl propionate EP, ethyl acetate EA, tetrahydrofuran THF, 1,3-dioxolane DOL, ethylene glycol dimethyl ether DME, diethylene glycol dimethyl ether DEGDME, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether TTE, dimethyl sulfoxide DMSO, sulfolane DMSO2, adiponitrile ADN, succinate SN, and fluorobenzene FB.

[0082] In a preferred embodiment, the solvent includes ethylene carbonate, propylene carbonate, and diethyl carbonate, wherein the weight percentage of ethylene carbonate is 15-30%, the weight percentage of propylene carbonate is 1-15%, and the weight percentage of diethyl carbonate is 30-55% based on the total mass of the electrolyte.

[0083] This solvent system effectively balances the dielectric constant and viscosity of the electrolyte: ethylene carbonate has a high dielectric constant, which promotes lithium salt dissociation and improves ionic conductivity; propylene carbonate has good low-temperature performance, which improves the low-temperature fluidity of the electrolyte; diethyl carbonate has low viscosity, which improves lithium-ion transport efficiency, and also has a high boiling point, which can reduce performance degradation caused by insufficient electrolyte due to solvent vaporization inside the battery at high temperatures. The combination of these three components effectively improves the wide-temperature-range performance of the electrolyte, making it suitable for the battery system of this invention.

[0084] In some embodiments, the lithium salt includes at least one of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium difluorooxalate borate (LiODFB), lithium di(oxalate borate) (LiBOB), lithium tetrafluorooxalate phosphate (LiOTFP), lithium tetrafluoroborate (LiBF4), and lithium perchlorate (LiClO4).

[0085] In some embodiments, the lithium salt accounts for 9.8 to 20.4% of the weight of the electrolyte.

[0086] In some embodiments, the electrolyte has an ionic conductivity ≥7.0 mS / cm at 25°C.

[0087] In some embodiments, the lithium-ion battery further includes a separator located between the positive electrode and the negative electrode.

[0088] In some embodiments, the diaphragm is selected from at least one of polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), and polyimide (PI).

[0089] In some embodiments, at least one surface of the diaphragm may be provided with a modified coating, including but not limited to a ceramic coating, a polymer bonding coating, and a composite conductive coating; wherein the inorganic particles of the ceramic coating may be selected from at least one of alumina, boehmite, magnesium oxide, silicon oxide, and titanium oxide; and the material of the polymer bonding coating may be selected from at least one of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyacrylate, and polyimide.

[0090] The present invention will be further illustrated by the following examples.

[0091] Table 1 Continued from Table 1 Example 1 This embodiment illustrates the lithium-ion battery and its preparation method disclosed in this invention, and includes the following steps: 1. Preparation of positive electrode sheet Carbon-coated lithium iron phosphate positive electrode active material, conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) were added to N-methylpyrrolidone (NMP) solvent at a mass ratio of 96.5:1.5:2.0. The mixture was thoroughly stirred in a vacuum mixer until homogeneous to obtain a positive electrode slurry. The positive electrode slurry was uniformly coated on both sides of the positive electrode current collector aluminum foil, dried under vacuum at 120℃, cold-pressed, and then cut and welded with aluminum electrode tabs to obtain the positive electrode sheet. The specific surface area S' and median particle size D of the positive electrode active material were measured. 50 '、D10 '、D 90 '、D 99 The percentage of carbon by mass ε is shown in Table 1.

[0092] 2. Preparation of negative electrode sheet Graphite anode active material, conductive agent carbon black, binder styrene-butadiene rubber (SBR), and thickener sodium carboxymethyl cellulose (CMC) were added to a deionized water solvent in a mass ratio of 95.5:1.5:1.5:1.5. The mixture was thoroughly stirred in a vacuum mixer until homogeneous to obtain anode slurry. The anode slurry was uniformly coated on both sides of the anode current collector copper foil, dried under vacuum at 110℃, cold-pressed, and then cut and welded with copper electrode tabs to obtain the anode sheet. The specific surface area S'', median particle size D50'', and D of the anode active material were measured. 10 ''、D 90 ''、D 99 ''、D 01 The degree of graphitization, G, is shown in Table 1.

[0093] 3. Electrolyte preparation In a dry argon atmosphere glove box with a water content of <10ppm and an oxygen content of <1ppm, ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) were mixed evenly at a mass ratio of 25:10:45. Lithium hexafluorophosphate (LiPF6) was added to the mixed solvent and stirred until completely dissolved, with the mass percentage of LiPF6 in the electrolyte being 12.5%. Then, additives with the mass percentages shown in Table 1 were added and stirred until completely homogeneous to obtain the electrolyte.

[0094] 4. Diaphragm A porous polyethylene (PE) composite membrane is used.

[0095] 5. Lithium-ion battery assembly The prepared positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. After winding, a bare cell is obtained. The bare cell is placed in an aluminum-plastic film outer packaging and vacuum-baked at 85°C for 24 hours to remove moisture. The electrolyte prepared above is injected, and after vacuum sealing and standing at 25°C for 24 hours, a formation process is performed: it is charged to 3.0V with a constant current of 0.05C, then charged to 3.65V with a constant current of 0.2C, and then stood at room temperature for 12 hours before being vacuum-sealed a second time to obtain a soft-pack lithium-ion battery.

[0096] Examples 2-28 Examples 2-28 illustrate the lithium-ion battery and its preparation method disclosed in this invention, including most of the operational steps in Example 1, with the following differences: The specific surface area S' and volume distribution particle size D of the positive electrode active material50 The following parameters are listed in Table 1: D50'', D10'', D90'', D99'', carbon mass percentage ε, specific surface area S'' of negative electrode active material, median particle size of volume distribution D50'', D10'', D90'', D99'', D01'', degree of graphitization G, and the selection and mass content of additives in the electrolyte.

[0097] Comparative Examples 1-21 Comparative Examples 1-21 are used to illustrate the lithium-ion battery and its preparation method disclosed in this invention, including most of the operation steps in Example 1, the difference being: The specific surface area S' and volume distribution particle size D of the positive electrode active material 50 The following parameters are listed in Table 1: D50'', D10'', D90'', D99'', carbon mass percentage ε, specific surface area S'' of negative electrode active material, median particle size of volume distribution D50'', D10'', D90'', D99'', D01'', degree of graphitization G, and the selection and mass content of additives in the electrolyte.

[0098] Performance testing The lithium-ion batteries prepared above were subjected to the following performance tests: 1. High-temperature cycling performance test The lithium-ion battery was placed in a constant temperature environment of 55°C and charged at a constant current of 1C to 3.65V, then charged at a constant voltage until the current dropped to 0.05C, and allowed to stand for 30 minutes. It was then discharged at a constant current of 1C to 2.5V and allowed to stand for 30 minutes. This constitutes one complete charge-discharge cycle. The above cycle was repeated 1000 times. The discharge capacity of the first cycle was recorded as the initial discharge capacity, and the discharge capacity of the 1000th cycle was recorded as the discharge capacity after the cycle. The high-temperature cycle capacity retention rate was calculated using the following formula: High-temperature cycling capacity retention (%) = (Discharge capacity at 1000th cycle / Initial discharge capacity) × 100% 2. Low-temperature discharge performance test The lithium-ion battery was charged at a constant current of 0.5C to 3.65V at 25℃, then charged at a constant voltage until the current dropped to 0.05C, and left to stand for 30 minutes. The battery was then transferred to a constant temperature environment of -20℃ and left to stand for 4 hours, followed by constant current discharge at 0.5C to 2.5V. The discharge capacity was recorded as the low-temperature discharge capacity. Simultaneously, the rated capacity of the battery at 0.5C discharge at 25℃ was recorded. The low-temperature discharge capacity retention rate was calculated using the following formula: Low-temperature discharge capacity retention rate (%) = (-20℃ low-temperature discharge capacity / 25℃ rated capacity) × 100% 3. Battery body impedance test The lithium-ion battery was charged to 27% SOC at a constant current of 0.5C in an environment of 25℃ and left to stand for 2 hours. It was then discharged using a 2C current for 10 seconds. The voltages V1 and V2 before and after discharge were recorded. The DCIR, expressed in mΩ, is the ratio of the voltage difference between V1 and V2 to the 2C current. The following formula can be used to calculate the DCIR: DCIR = (V1 - V2) / 2C.

[0099] The test results are entered into Table 2.

[0100] Table 2 The test results of Examples 1-21 and Comparative Examples 1-21 show that when the specific surface area, median particle size of the volume distribution, and carbon mass percentage of the positive electrode active material, the specific surface area, median particle size of the volume distribution, and degree of graphitization of the negative electrode active material, as well as the mass percentage of the first and second additives in the electrolyte, simultaneously meet the condition defined in this invention: 10 ≤ ≤25, and 9.5≤S'≤15.8, 0.85≤D 50 '≤1.25, 0.95%≤ε≤1.60%, 0.88≤S'' ≤1.64, 8.0≤D 50 When ''≤14, 92.5%≤G≤95%, 0.5%≤W1≤5.0%, and 0.2%≤W2≤3.0%, lithium-ion batteries can simultaneously achieve excellent high-temperature cycle performance, low-temperature discharge performance, and low bulk impedance, effectively solving the problem that lithium iron phosphate batteries in the prior art cannot simultaneously achieve wide-temperature-range application performance; however, when any of the above parameters exceeds the range defined by this invention or does not satisfy the relationship 10≤ When the temperature is ≤25, the battery's high-temperature cycle performance, low-temperature discharge performance, or body impedance will all deteriorate to varying degrees, making it impossible to achieve a balance among the three.

[0101] As can be seen from the test results of Examples 1-11 and Examples 22-25, under the premise of meeting the basic parameter range of the present invention, further controlling the volume distribution particle size D10', D90', D99' and the particle size distribution width (D90'-D10') / D50' of the positive electrode active material within the preferred range of the present invention can further improve the high-temperature cycle performance and low-temperature discharge performance of lithium-ion batteries, further reduce the battery's bulk impedance, and optimize the battery's overall electrochemical performance.

[0102] As can be seen from the test results of Examples 1-11 and Examples 26-28, under the premise of meeting the basic parameter range of the present invention, further controlling the volume distribution particle size D10'', D90'', D99'', D01'' and the particle size distribution width (D90''-D10'') / D50'' of the negative electrode active material within the preferred range of the present invention can further improve the high-temperature cycle performance and low-temperature discharge performance of the lithium-ion battery, and further reduce the battery's body impedance, thereby further improving the battery's comprehensive performance over a wide temperature range.

[0103] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A lithium-ion battery, characterized in that, The electrode comprises a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes a positive electrode material layer, which includes a positive electrode active material. The positive electrode active material is selected from lithium iron phosphate with a carbon coating. The specific surface area of ​​the positive electrode active material is S', in m³. 2 / g, the median particle size of the positive electrode active material in volume distribution is D 50 The unit is μm, the carbon mass percentage of the positive electrode active material is ε, the negative electrode includes a negative electrode material layer, the negative electrode material layer includes a negative electrode active material, the negative electrode active material is selected from graphite, and the specific surface area of ​​the negative electrode active material is S'', the unit is m. 2 / g, the median particle size of the negative electrode active material in volume distribution is D 50 '', the unit is μm, and the degree of graphitization of the negative electrode active material is G; The electrolyte includes a solvent, a lithium salt, and an additive. The additive includes a first additive and a second additive. The first additive is an unsaturated cyclic carbonate, and the second additive is a sulfur-containing cyclic ester. The mass percentage of the first additive is W1 and the mass percentage of the second additive is W2, based on the total mass of the electrolyte. The lithium-ion battery meets the following conditions: 10≤ ≤25, and 9.5≤S'≤15.8, 0.85≤D 50 '≤1.25, 0.95%≤ε≤1.60%, 0.88≤S'' ≤1.64, 8.0≤D 50 ''≤14, 92.5%≤G≤95%, 0.5%≤W1≤5.0%, 0.2%≤W2≤3.0%.

2. The lithium-ion battery according to claim 1, characterized in that, The lithium-ion battery meets the following conditions: 11≤ ≤20。 3. The lithium-ion battery according to claim 1, characterized in that, The lithium-ion battery satisfies at least one of the following conditions: （1）10.0≤S'≤14.5； （2）0.9≤D 50 '≤1.2； (3) 1.0%≤ε≤1.55%; （4）0.95≤S'' ≤1.57； （5）8.5≤D 50 ''≤13.5； （6）93%≤G≤94.5%； （7）0.5%≤W1≤4.5%； （8）0.2%≤W2≤2.5%。 4. The lithium-ion battery according to claim 1, characterized in that, The volume distribution particle size D of the positive electrode active material 10 '0.32~0.46μm, D 90 'Within 1.8~4.0μm, D 99 ' is 4.05~7.8μm, and (D 90 '-D 10 ') / D 50 The value is 1.4 to 3.

5.

5. The lithium-ion battery according to claim 1, characterized in that, The positive electrode active material is obtained by agglomeration of multiple primary particles, the average particle size D of which is... 50 1 The range is 0.1~0.49μm.

6. The lithium-ion battery according to claim 1, characterized in that, The volume distribution particle size D of the negative electrode active material 10 '' is 4.1~6.8μm, D 90 '' is 14.5~24.0μm, D 99 '' is 22.0~33.8μm, D 01 ''≥2.0μm, and (D 90 ''-D 10 '') / D 50 The value is 1.05~1.

60.

7. The lithium-ion battery according to claim 1, characterized in that, The first additive includes at least one of vinylene carbonate, ethylene ethylene carbonate, and methyl vinylene carbonate.

8. The lithium-ion battery according to claim 1, characterized in that, The second additive includes at least one of 1,3-propenesulfonate lactone, 1,3-propanesulfonate lactone, 1,3-propanediol cyclosulfonate, and 1,4-butanesulfonate lactone.

9. The lithium-ion battery according to claim 1, characterized in that, The additive further includes a third additive, which comprises at least one of 1,3,2-dioxothiacyclopentane-2,2-dioxide, 4-methyl-1,3,2-dioxothiacyclopentane-2,2-dioxide, 4-propyl-[1,3,2]dioxothiacyclopentane-2,2-dioxide, methane disulfonate, tri(trimethylsilane) phosphate, tri(trimethylsilane) phosphate, triphenyl phosphite, tri(trimethylsilane) borate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, fluoroethylene carbonate, and fluoroethylene carbonate.

10. The lithium-ion battery according to claim 9, characterized in that, Based on the total mass of the electrolyte, the mass percentage of the third additive is 0.5% ≤ W3 ≤ 4.0%; The lithium-ion battery meets the following conditions: 3.5%≤W1+W2+W3≤8.0%.

11. The lithium-ion battery according to claim 1, characterized in that, The solvent includes ethylene carbonate, propylene carbonate, and diethyl carbonate. Based on the total mass of the electrolyte, the weight percentage of ethylene carbonate is 15-30%, the weight percentage of propylene carbonate is 1-15%, and the weight percentage of diethyl carbonate is 30-55%.

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