A lithium-ion battery

Abstract

The application belongs to the technical field of batteries, and particularly relates to a lithium ion battery. Compared with the prior art, the application improves the fast charging and high-temperature storage performance of a lithium iron phosphate battery by comprehensively controlling the positive and negative electrode sheet design and electrolyte. On the one hand, by adjusting the particle size distribution of the lithium iron phosphate material particles, the lithium ion transmission path is more uniform, the consistency of the lithium ion transmission rate is improved, and the overall fast charging performance of the battery is improved. On the other hand, the application uses a carboxylic acid ester solvent with lower viscosity, which significantly reduces the viscosity of the electrolyte, greatly improves the liquid-phase lithium ion transmission capacity, and matches the positive electrode material to improve the overall fast charging performance of the battery. Finally, by selecting a chain-shaped binder and appropriately matching the unsaturated carbonate content, the negative electrode active material is protected, and the electrolyte and negative electrode side reactions are significantly reduced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of batteries, and particularly relates to a lithium ion battery. BACKGROUND

[0002] Lithium ion batteries are widely used in digital cameras, mobile phones, tablet computers, notebook computers and other electronic devices due to their high working voltage, large energy density, no memory effect and other advantages, and are constantly developing towards the field of new energy vehicles. However, compared with the traditional fuel vehicle which can be filled up in a few minutes, the new energy vehicle needs several hours to be fully charged, and under the premise that charging piles have not been popularized, fast charging technology has become one of the important factors affecting the rapid popularization of electric vehicles, and fast charging batteries have become another development direction of power lithium ion batteries.

[0003] Lithium iron phosphate battery is a kind of lithium ion battery with high safety. Compared with other lithium ion batteries, lithium iron phosphate battery has the advantages of high cycle life, high temperature stability, low internal resistance, high energy density, etc., and in addition, lithium iron phosphate battery can maintain a relatively stable voltage during charging and discharging.

[0004] At present, the laboratory test normal temperature cycle life of commercial lithium iron phosphate (LiFePO4) positive electrode lithium ion battery can reach 1500-2400 times, but more and more studies show that the cycle performance of the battery at high temperature decays sequentially. However, the working environment of electric vehicles is complex, and the power system is usually a battery block composed of multiple batteries in series and parallel, which is affected by installation position, ventilation condition and high-rate charging and discharging demand, etc. Combined with the environmental conditions of road driving, the high temperature environment caused by heat accumulation is an inevitable application condition for lithium ion batteries used in electric vehicles. Moreover, during the fast charging process, the battery will also generate relatively high temperature, and high temperature will accelerate the chemical reaction inside the battery, resulting in the consumption of battery capacity. SUMMARY

[0005] Therefore, the technical problem to be solved by the application is to provide a lithium ion battery with high high-temperature cycle stability.

[0006] The application provides a lithium ion battery, which comprises a positive electrode, a negative electrode and an electrolyte.

[0007] The positive electrode comprises a positive electrode active material; the positive electrode active material comprises lithium iron phosphate; the particle size of the positive electrode active material satisfies (D90+D10) / D50=a, and the a is 2.0-6.0.

[0008] The negative electrode comprises a negative electrode active layer; the negative electrode active layer comprises a negative electrode active material and a negative electrode binder; the negative electrode binder comprises a chain binder; and the mass percentage content of the chain binder in the negative electrode active layer is n.

[0009] The electrolyte comprises a solvent and an additive; the solvent comprises a carboxylic acid ester solvent; the additive comprises an unsaturated ester additive; the mass percentage of the carboxylic acid ester solvent in the electrolyte is m; the mass percentage of the unsaturated ester additive in the electrolyte is p.

[0010] The negative electrode and the electrolyte satisfy (p+n) / m=b; the b is 0.05-1.5.

[0011] Preferably, the a is 3.0-4.0;

[0012] And / or, the b is 0.1-1.

[0013] Preferably, the n is 0.01%-5%;

[0014] And / or, the m is 6%-60%;

[0015] And / or, the p is 1%-6%;

[0016] And / or, the range of n / p is 0.0025-4.

[0017] Preferably, the chain-like binder is selected from one or more of polyacrylic acid, styrene butadiene rubber, sodium polyacrylate, polyacrylamide, polyvinyl alcohol and polymethylacrylic acid;

[0018] And / or, the carboxylic acid ester solvent is selected from one or more of methyl acetate, ethyl formate and ethyl acetate (EA); the unsaturated ester additive is selected from one or more of vinylene carbonate (VC), fluoroethylene carbonate (FEC), vinyl ethylene carbonate (VEC), allyl ethylene carbonate (AEC), vinyl acetate (VA) and catechol carbonate (CC).

[0019] Preferably, the additive further comprises a sulfur-containing additive; the sulfur-containing additive is selected from sulfonic acid lactone and / or sulfuric acid ester; the mass percentage of the sulfur-containing additive in the electrolyte is 0.01%-3%.

[0020] Preferably, the sulfur-containing additive is selected from methane disulfonate methylene.

[0021] Preferably, the additive further comprises a siloxane additive; the mass percentage of the siloxane additive in the electrolyte is 0.01%-2.0%.

[0022] Preferably, the siloxane additive is selected from tris(trimethylsilyl) phosphite and / or tris(trimethylsilyl) borate.

[0023] Preferably, the solvent further comprises a cyclic carbonate and / or a chain carbonate; the mass percentage of the cyclic carbonate in the electrolyte is 20% to 40%; the mass percentage of the chain carbonate in the electrolyte is 0% to 80%.

[0024] Preferably, the electrolyte further comprises a lithium salt; the lithium salt is selected from one or more of lithium hexafluorophosphate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonylimide, lithium bis-trifluoromethanesulfonylimide, lithium trifluoromethanesulfonate, lithium difluoro oxalate borate, lithium difluoro oxalate borate, lithium difluoro di-oxalate phosphate and lithium tetrafluoro oxalate phosphate.

[0025] Preferably, the D90 of the positive electrode active material is 2.0 to 6.0 microns; and / or, the D10 of the positive electrode active material is 0.3 to 0.6 microns; and / or, the D50 of the positive electrode active material is 0.4 to 1.2 microns.

[0026] Compared with the prior art, the present application improves the fast charging and high-temperature storage performance of the lithium iron phosphate battery by comprehensively controlling the positive and negative electrode sheet design and the electrolyte. On the one hand, by controlling the D50 of the positive electrode particles and the range of (D10+D90) / D50, the particle size distribution of the lithium iron phosphate material particles is adjusted, so that the lithium ion transmission path is more uniform, the consistency of the lithium ion transmission rate is improved, the overall fast charging performance of the battery is improved, and the problems of increased side reactions and poor high-temperature storage performance caused by too small particle size of the particles are avoided. By controlling the particle size distribution of the positive electrode material, the lithium ion deintercalation rate is improved. On the other hand, as a lithium ion transmission medium, the electrolyte also affects its lithium ion transmission capacity. The present application uses a more low-viscosity carboxylic acid ester solvent, which significantly reduces the viscosity of the electrolyte and greatly improves the liquid-phase lithium ion transmission capacity. At the same time, the positive electrode material is matched to improve the overall fast charging performance of the battery. However, the side reactions of the carboxylic acid ester solvent and the electrolyte increase under high-temperature conditions, so the present application protects the negative electrode active material by selecting a chain binder and appropriately compounding unsaturated carbonate content, which significantly reduces the side reactions of the electrolyte and the negative electrode. Moreover, the unsaturated carbonate additive can form a solid electrolyte interphase film (SEI) at the negative electrode to protect the negative electrode active material and avoid side reactions between the negative electrode active material and the electrolyte. However, too much addition of the unsaturated carbonate additive will cause poor negative electrode kinetics. The present application reduces the addition of the unsaturated carbonate additive by matching the chain binder to achieve the purpose of protecting the negative electrode active material, which can reduce the negative electrode interface side reactions while avoiding excessive impedance that causes lithium precipitation and deteriorates the fast charging performance. In summary, the present application controls the relationship between the carboxylic acid ester content and the chain binder and the carboxylic acid ester solvent to improve the electrolyte viscosity, improve the battery fast charging performance, avoid the increase in side reactions between the electrolyte and the negative electrode material caused by the increase in carboxylic acid ester content, and balance the battery fast charging and high-temperature storage performance. DETAILED DESCRIPTION

[0027] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments of the present application, all the other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0028] The present application provides a kind of lithium ion battery, including positive pole, negative pole and electrolyte;The positive pole includes positive pole active material;The positive pole active material includes lithium iron phosphate;The D50 of the positive pole active material is 0.4~1.2 μm;The particle size of the positive pole active material satisfies (D90+D10) / D50=a, and the a is 2.0~6.0;The negative pole includes negative pole active layer;The negative pole active layer includes negative pole active material and negative pole binder;The negative pole binder includes chain binder;The mass percentage content of the chain binder in negative pole active layer is n;The electrolyte includes solvent and additive;The solvent includes carboxylic acid ester solvent;The additive includes unsaturated ester additive;The mass percentage content of the carboxylic acid ester solvent in electrolyte is m;The mass percentage content of the unsaturated ester additive in electrolyte is p;The negative pole and electrolyte satisfy (p+n) / m=b;The b is 0.05~1.5.

[0029] In the present application, the chain binder refers to the polymer that can be used as binder with long chain structure, which is formed by monomers of polymer connected with each other through polymerization reaction.

[0030] The battery fast charging performance is not only determined by the electrolyte, but also by the positive active material, the present application controls the positive active material particle size D50 and (D90+D10) / D50 range, so that the positive active material can realize fast transmission of lithium ions, and can also avoid the side reaction caused by the electrolyte. Wherein D50 is the particle size corresponding to the cumulative volume percentage of 50% of the active material, unit: mu m; D90 is the particle size corresponding to the cumulative volume percentage of 90% of the active material, unit: mu m; D10 is the particle size corresponding to the cumulative volume percentage of 10% of the active material, unit: mu m. The smaller D10 is, the smaller the particle size is. The larger D90 value is, the larger the particle size is. D50 represents the median particle size. When D50 is in a suitable range, the overall particle size range is moderate. The (D10+D90) / D50 value cannot be too large. Too large indicates that there is a large particle size, which increases the lithium ion transmission path and is not conducive to the rapid release of lithium ions. The ratio cannot be too small. If the ratio is too small, the particle size is too small. Although the lithium ion transmission rate is large, the contact area between the positive electrode and the electrolyte is small, which will increase the side reaction and cause the high-temperature storage performance to deteriorate.

[0031] The present application controls the positive electrode particle D50 and the range of (D10+D90) / D50 to adjust the particle size distribution of lithium iron phosphate material particles, so that the lithium ion transmission path is more uniform, the consistency of lithium ion transmission rate is improved, and the overall fast charging performance of the battery is improved. At the same time, it avoids the increase of side reaction caused by too small particle size, and the deterioration of high-temperature storage performance. By controlling the particle size distribution of the positive electrode material, the lithium ion deintercalation rate is improved. The electrolyte as a lithium ion transmission medium also affects its lithium ion transmission capacity. The use of a more low-viscosity carboxylic acid ester solvent significantly reduces the viscosity of the electrolyte and greatly improves the liquid-phase lithium ion transmission capacity. At the same time, the positive electrode material is matched to improve the overall fast charging performance of the battery. However, the side reaction between the carboxylic acid ester solvent and the electrolyte increases at high temperature. By selecting a chain adhesive and appropriately compounding the content of unsaturated carbonate, the negative active material is protected, and the side reaction between the electrolyte and the negative electrode is significantly reduced. The unsaturated carbonate additive can form a solid electrolyte interface film (SEI) at the negative electrode to protect the negative active material and avoid the side reaction between the negative active material and the electrolyte. However, too much addition will cause the negative electrode kinetics to deteriorate. By matching a chain adhesive to reduce the addition amount of unsaturated carbonate, the purpose of protecting the negative active material is achieved, which can reduce the negative electrode interface side reaction while avoiding excessive impedance leading to lithium precipitation and deteriorating the fast charging performance. By controlling the relationship between the content of the carboxylic acid ester solvent and the chain adhesive and the carboxylic acid ester solvent, the electrolyte viscosity is improved to improve the battery fast charging performance, and the increase of the side reaction between the electrolyte and the negative electrode caused by the increase of the content of the carboxylic acid ester solvent is avoided, and the battery fast charging and high-temperature storage performance are considered.

[0032] In one specific embodiment of the present invention, the positive electrode active material D90 is preferably 2.0~6.0 μm, more preferably 2.0~3.4 μm; in some specific embodiments of the present invention, the D90 of the positive electrode active material is specifically 2.83 μm, 2.04 μm, 2.91 μm, 3.31 μm, 3.24 μm, 2.1 μm, 3.78 μm or 5.95 μm.

[0033] In one specific embodiment of the present invention, the D10 of the positive electrode active material is preferably 0.3~0.6μm, 0.3~0.5μm, and more preferably μm; in some specific embodiments of the present invention, the D10 of the positive electrode active material is specifically 0.33 μm, 0.37 μm, 0.31 μm, 0.34 μm or 0.49 μm.

[0034] In one specific embodiment of the present invention, the D50 of the positive electrode active material is preferably 0.4~1.2 μm, more preferably 0.8~1.2 μm; in some specific embodiments of the present invention, the D50 of the positive electrode active material is specifically 0.91 μm, 0.8 μm, 0.96 μm, 1.19 μm, 0.94 μm, 1.06 μm or 0.95 μm.

[0035] In this invention, unless otherwise specified, particle size is measured using a Malvern 3000 laser particle size analyzer.

[0036] In one specific embodiment of the present invention, the value of 'a' is 2.0~6.0. Optionally, 'a' can be 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, or any two of the above values. By satisfying the above range of 'a', the particle size distribution of lithium iron phosphate particles can be made more uniform, lithium ion transport can be uniform, and the presence of excessively large particles can be avoided, thereby avoiding affecting the lithium ion transport rate and improving the overall fast charging performance of the battery. In one specific embodiment of the present invention, 'a' is 3.0~4.0. By controlling 'a' within this range, side reactions between the positive electrode and the electrolyte can be further avoided, and the battery can maintain better fast charging performance. In some embodiments of the present invention, 'a' is specifically 3.47, 3.01, 3.35, 3.80, 3.93, 2.02, 4.41, or 5.93.

[0037] In one specific embodiment of the present invention, the positive electrode includes a positive current collector and a positive active layer attached to the surface of the positive current collector; the positive active layer includes a positive active material; the mass of the positive active material is preferably 80% to 98% of the mass of the positive active layer; optionally, the mass of the positive active material is 80%, 85%, 90%, 95%, 97%, 98% of the mass of the positive active layer or a range between any two of the above values.

[0038] In another specific embodiment of the present invention, the positive electrode active layer preferably further includes a positive electrode conductive agent and a positive electrode binder; the mass of the positive electrode conductive agent is preferably 0.01% to 3% of the mass of the positive electrode active layer; optionally, the mass of the positive electrode conductive agent is 0.01%, 0.05%, 0.1%, 0.5%, 1%, 1.5%, 2.0%, 2.5%, 3.0% of the mass of the positive electrode active layer, or a range between any two of the above values; the positive electrode conductive agent comprises at least one of conductive carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. Carbon black is preferred in this invention. The mass of the positive electrode binder is preferably 0.01% to 5% of the mass of the positive electrode active layer; optionally, the mass of the positive electrode binder is 0.01%, 0.05%, 0.1%, 0.5%, 1%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, 5.0% of the mass of the positive electrode active layer, or a range between any two of the above values; the positive electrode binder is preferably PVDF.

[0039] In one specific embodiment of the present invention, the molecular weight of the PVDF is preferably 60~100 W; optionally, the molecular weight of the PVDF is 60 W, 65 W, 70 W, 75 W, 80 W, 85 W, 90 W, 95 W, 100 W or any two of the above values.

[0040] In one specific embodiment of the present invention, the negative electrode includes a negative electrode current collector and a negative electrode active layer attached to the surface of the negative electrode current collector; the negative electrode active layer includes a negative electrode active material and a negative electrode binder; the mass of the negative electrode active material is preferably 80% to 98% of the mass of the negative electrode active layer; optionally, the mass of the negative electrode active material is 80%, 85%, 90%, 94%, 95%, 97%, 98% of the mass of the negative electrode active layer or a range between any two of the above values; the negative electrode active material is preferably natural graphite, artificial graphite, mesophase carbon microspheres, hard carbon, soft carbon, silicon, or SiO2. x Silicon-carbon and Li4Ti5O 12 One or more of them.

[0041] In a specific embodiment of the present invention, the negative electrode binder comprises a chain binder; the mass percentage n of the chain binder in the negative electrode active layer is preferably 0.01% to 5.0%; specifically, the mass percentage n of the chain binder without benzene rings in the negative electrode active layer is preferably 0.01% to 5.0%; optionally, n is 0.01%, 0.05%, 0.1%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, 5%, or any two of the above values. The range of n is specified; further, n is more preferably 1.5% to 5.0%; in some embodiments provided by the present invention, n is specifically 3.0%, 1.5%, 4.0%, 2.5%, 5.0%, 0.1% or 2.8%; the chain adhesive is preferably one or more of polyacrylic acid (PAA), styrene-butadiene rubber (SBR), sodium polyacrylate, polyacrylamide, polyvinyl alcohol and polymethacrylic acid. Among these chain adhesives, polyacrylic acid is more preferred because it has an appropriate molecular weight, is simple to synthesize, has lower cost and uses less.

[0042] In one specific embodiment of the present invention, the molecular weight of the polyacrylic acid is preferably 30-100 W; optionally, the molecular weight of the polyacrylic acid is 30 W, 35 W, 40 W, 45 W, 50 W, 55 W, 60 W, 65 W, 70 W, 75 W, 80 W, 85 W, 90 W, 95 W, 100 W or any two of the above values.

[0043] In one specific embodiment of the present invention, the particle size of the styrene-butadiene rubber is preferably 100~200 nm; optionally, the particle size of the styrene-butadiene rubber is 100 nm, 110 nm, 120 nm, 130 nm, 140 nm, 150 nm, 160 nm, 170 nm, 180 nm, 190 nm, 200 nm or any two of the above values.

[0044] In a specific embodiment of the present invention, the negative electrode active layer preferably further includes a negative electrode conductive agent; the mass of the negative electrode conductive agent is preferably 0.01% to 3% of the mass of the negative electrode active layer; optionally, the mass of the negative electrode conductive agent is 0.01%, 0.05%, 0.1%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0% of the mass of the negative electrode active layer or a range between any two of the above values; the negative electrode conductive agent can be any conductive agent well known to those skilled in the art, and there are no special limitations. In the present invention, at least one of conductive carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers is preferred, and conductive agent SP is more preferred.

[0045] In one specific embodiment of the present invention, the electrolyte includes a solvent and additives; the solvent includes carboxylic acid ester solvents; to improve the fast-charging performance of the battery, it is necessary to increase the liquid phase conduction of the battery system. Reducing viscosity is one of the most effective means to improve liquid phase conduction. However, traditional chain carbonates such as ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC), although having low viscosity, have a short temperature range, and their viscosity increases rapidly at low temperatures. Therefore, the present invention uses carboxylic acid ester solvents with a wider temperature range and lower viscosity. The mass percentage m of the carboxylic acid ester solvent in the electrolyte is preferably 6%~60%, more preferably 12%~60%, even more preferably 20%~60%, and most preferably 40%~60%. Excessive carboxylic acid ester content leads to a sharp deterioration in high-temperature performance, particularly severe high-temperature gas generation, which may cause the battery valve to open, worsening safety performance. Conversely, a low carboxylic acid ester content results in minimal reduction in electrolyte viscosity, relatively stable liquid phase conduction, and less benefit from fast charging. In some embodiments provided by this invention, the content m of the carboxylic acid ester solvent in the electrolyte is specifically 6%, 12%, 20%, 22%, 40%, or 60%. The carboxylic acid ester solvent is preferably one or more of methyl acetate, ethyl formate, and ethyl acetate.

[0046] In one specific embodiment of the present invention, the solvent further includes cyclic carbonates and chain carbonates; the mass percentage of the cyclic carbonates in the electrolyte is preferably 20% to 40%; optionally, the mass percentage of the cyclic carbonates in the electrolyte is 20%, 25%, 30%, 35%, 40%, or any two of the above values; the cyclic carbonates can be any cyclic carbonates well known to those skilled in the art, and there are no special limitations. In the present invention, ethylene carbonate (EC), propylene carbonate (PC), or fluoroethylene carbonate is preferred. FEC (Fen ester); the mass percentage of the chain carbonate in the electrolyte is preferably 0% to 80%; optionally, the mass percentage of the chain carbonate in the electrolyte is 0%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% or any two of the above values; the chain carbonate can be any chain carbonate well known to those skilled in the art, and there are no special limitations. In this invention, it is preferably one or more of dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC).

[0047] In one specific embodiment of the present invention, the solvent further includes cyclic carbonates and chain carbonates; the mass ratio of the cyclic carbonates to the chain carbonates is preferably (1~5):(5~9), more preferably (2~4):(6~8), and even more preferably 3:7.

[0048] In one specific embodiment of the present invention, the additive includes unsaturated ester additives; the content p of the unsaturated ester additives in the electrolyte is preferably 1%~6%, more preferably 2%~6%, even more preferably 3%~6%, and most preferably 3.5%~5.0%; in some embodiments of the present invention, the content p of the unsaturated ester additives in the electrolyte is specifically 1%, 3.5%, 4%, 4.5%, 5%, or 6%; the unsaturated ester additives are preferably one or more of vinylene carbonate (VC), fluoroethylene carbonate (FEC), ethylene ethylene carbonate (VEC), allyl ethylene carbonate (AEC), vinyl acetate (VA), and catechol carbonate (CC).

[0049] In this invention, the b-value reflects the relationship between the content of chain binder and unsaturated ester additives (functional groups containing unsaturated bonds, such as unsaturated alkenyl and alkynyl groups) and the content of carboxylic acid ester solvents. While a high content of carboxylic acid ester solvents can reduce electrolyte viscosity and improve ionic conductivity, their addition accelerates the deterioration of the high-temperature performance of lithium iron phosphate batteries, such as high-temperature cycling, high-temperature gas generation, and high-temperature storage. Adding unsaturated additives can form a better SEI film on the negative electrode side, preventing the continued occurrence of side reactions of the carboxylic acid ester solvent on the negative electrode side. However, excessive addition of unsaturated ester additives can increase impedance, dramatically increase initial DCR, and easily lead to interfacial lithium plating and storage degradation. Matching the negative electrode binder with the chain binder can better coat the negative electrode material, reducing the contact between the negative electrode material and the electrolyte at high temperatures. Simultaneously, adding the chain binder can reduce the amount of unsaturated ester additives, thus achieving a significant reduction in DCR with only a slight reduction in the unsaturated ester additive content, achieving high passivation on the negative electrode side. This invention controls the amount of unsaturated ester additives and chain binders. Unsaturated ester additives can form a dense SEI film on the negative electrode, reducing side reactions between the negative electrode and the electrolyte. Chain binders can form a better coating on the negative electrode. By comprehensively adjusting the content of carboxylic acid ester solvents, unsaturated ester additives, and chain binders, the ionic conductivity can be improved, fast charging performance can be enhanced, and high-temperature performance can be taken into account.

[0050] In one specific embodiment of the present invention, b is preferably 0.05~1.5, more preferably 0.1~1. By controlling the preferred range of b to 0.1~1, better initial film formation protection on the negative electrode side can be achieved, thereby improving the high-temperature storage calendar life of the battery. In some embodiments of the present invention, b is specifically 0.05, 0.4, 0.1, 0.21, 0.3, 0.33, 0.92, or 1.47.

[0051] In one specific embodiment of the present invention, the n / p ratio is controlled to be between 0.0025 and 5 to avoid both excessive unsaturated ester additives leading to increased impedance and excessive chain binders resulting in poor lithium-ion transport performance and fast-charging performance. Optionally, n / p can be 0.0025, 0.005, 0.01, 0.02, 0.05, 0.1, 0.3, 0.5, 0.6, 0.7, 0.9, 1, 1.5, 2, 2.5, 3, 3.5, 4, or any two of the above values. In some embodiments of the present invention, n / p is specifically 0.6, 0.33, 0.89, 0.7, 0.83, 0.1, 0.47, or 0.625.

[0052] In one specific embodiment of the present invention, the additive further includes a sulfur-containing additive; the sulfur-containing additive is preferably a sulfonyl lactone and / or a sulfate ester; the mass percentage of the sulfur-containing additive in the electrolyte is preferably 0.01% to 3%; optionally, the mass percentage of the sulfur-containing additive in the electrolyte is 0.01%, 0.05%, 0.1%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, or any two of the above values. Adding a sulfur-containing additive can further reduce side reactions between the electrolyte and the negative electrode; the sulfur-containing additive can form an SEI film with inorganic components at the negative electrode, which is not easily decomposed at high temperatures, resulting in a more stable SEI, thereby reducing the battery's DCR and improving the battery's high-temperature performance. In another specific embodiment of the present invention, the sulfur-containing additive is specifically methylene methane disulfonate (MMDS).

[0053] In one specific embodiment of the present invention, the additive further includes a siloxane additive; the mass percentage of the siloxane additive in the electrolyte is preferably 0.01% to 2.0%; optionally, the mass percentage of the siloxane additive in the electrolyte is 0.01%, 0.05%, 0.1%, 0.3%, 0.5%, 0.8%, 1.0%, 1.5%, 2.0%, or any two of the above values; such additives can modify the initial SEI film formation, resulting in a SEI film with better ion conductivity, which can significantly reduce the initial DCR of the battery. In another specific embodiment of the present invention, the siloxane additive is specifically tris(trimethylsilane) phosphite (TMSP) and / or tris(trimethylsilane) borate (TMSB).

[0054] In one specific embodiment of the present invention, the electrolyte further includes a lithium salt; the concentration of the lithium salt in the electrolyte is preferably 0.5~1.5 mol / L to ensure ion transport in the electrolyte; optionally, the concentration of the lithium salt in the electrolyte is 0.5 mol / L, 0.8 mol / L, 1.0 mol / L, 1.2 mol / L, 1.4 mol / L, 1.5 mol / L or any two of the above values; the lithium salt is preferably one or more of lithium hexafluorophosphate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorooxalateborate, lithium dioxalateborate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0055] In one specific embodiment of the present invention, the lithium-ion battery further includes a separator for separating the positive electrode and the negative electrode; the separator is preferably one or more of polypropylene (PP), polyethylene (PE) and polypropylene-phenolic resin composite materials.

[0056] The present invention also provides a method for preparing the above-mentioned lithium-ion battery, comprising the following steps:

[0057] S1) Preparation of positive electrode active material: A lithium source, an iron source, and a phosphorus source are mixed evenly, ground, and spray-dried to obtain a precursor material; the precursor material is then sintered at high temperature in a protective atmosphere and crushed to obtain the positive electrode active material; specifically, the particle size D50 of the lithium source, iron source, and phosphorus source is controlled between 100 and 150 nm; the grinding method can be any method known to those skilled in the art and is not particularly limited, but in this invention, coarse grinding and fine grinding are preferred sequentially; the grinding is carried out until the particle size of the mixture is 370 to 420 nm; the spray drying temperature is preferably 300°C to 350°C; the protective atmosphere can be any protective atmosphere known to those skilled in the art and is not particularly limited, but in this invention, nitrogen is preferred; the high-temperature sintering temperature is preferably 750°C to 850°C. In this invention, the adjustment method for D10, D90, and D50 of lithium iron phosphate is not limited; for example, it can be adjusted by grinding time, rotation speed, and sintering temperature.

[0058] In a specific embodiment of this invention, the preparation of the positive electrode active material is as follows: A qualified lithium source, iron source, and phosphorus source (with a precursor D50 controlled at 100-150 nm) are mixed into a mixture. This mixture is then subjected to two steps: coarse grinding and fine grinding (to 370-420 nm). After fine grinding, the precursor is spray-dried at 300-350°C for a very short time to form a powdered precursor. The spray-dried powder is then sent to a sealed roller kiln for sintering. According to the sintering process requirements, the sintering temperature in each zone is set between 750°C and 850°C under a nitrogen protective atmosphere. The roller conveyor then moves the loading sagger forward, simultaneously completing the sintering. After being pulverized by a certain airflow, the material is cooled to obtain lithium iron phosphate material, which is the positive electrode active material.

[0059] S2) Preparation of positive electrode sheet: The above-mentioned lithium iron phosphate material is used as the main material, and is mixed evenly with positive electrode binder and positive electrode conductive agent, and dispersed in NMP to obtain positive electrode slurry; the positive electrode slurry is coated on aluminum foil to obtain double-sided coated positive electrode sheet; then it is rolled and cut to obtain positive electrode sheet; the amount and type of lithium iron phosphate, positive electrode binder and positive electrode conductive agent are the same as described above, and will not be repeated here.

[0060] S3) Preparation of negative electrode sheet: The negative electrode active material, negative electrode conductive agent, and negative electrode binder are mixed evenly and dispersed in deionized water to obtain a negative electrode slurry; the negative electrode slurry is coated on copper foil to obtain a double-sided coated electrode sheet; then it is rolled and cut to obtain a negative electrode sheet; the amount and type of negative electrode active material, negative electrode conductive agent and negative electrode binder are the same as described above, and will not be repeated here.

[0061] S4) Preparation of electrolyte: This includes a solvent and an electrolyte salt dissolved in the solvent, various additives, etc. The electrolyte salt includes lithium salt; the types and amounts of the solvent, lithium salt, and additives are the same as described above, and will not be repeated here.

[0062] S5) Assembly and formation: The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. Then, they are wound to obtain a bare cell. The bare cell is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a lithium-ion battery is obtained.

[0063] To further illustrate the present invention, the following describes in detail a lithium-ion battery provided by the present invention with reference to embodiments; the molecular weight of PVDF used in the embodiments and comparative examples is about 70 W; the molecular weight of PAA is about 60 W; the particle size of SBR is 120~180 nm; and the particle size of artificial graphite is 9~12 μm.

[0064] All reagents used in the following examples are commercially available.

[0065] Examples 1-13 and Comparative Examples 1-5

[0066] S1) Preparation of cathode material (specific conditions are shown in Tables 1 and 2): Qualified lithium, iron, and phosphorus sources (precursor D50 controlled at 100-150 nm) are mixed into a mixture. This mixture is then subjected to two steps: coarse grinding and fine grinding (to 370-420 nm). After fine grinding, the powder is spray-dried at 300-350°C to form a powdered precursor. The spray-dried powder is then sent to a sealed roller kiln for sintering. The sintering temperature for each zone is set between 750°C and 850°C according to the sintering process requirements. The roller conveyor then moves the loading sagger forward, simultaneously completing the sintering. After being pulverized by a certain airflow, it is cooled to obtain lithium iron phosphate material.

[0067] S2) Positive electrode preparation: The above-mentioned lithium iron phosphate material is used as the main material and mixed evenly with positive electrode binder PVDF and positive electrode conductive agent SP at a mass ratio of 97:1.5:1.5, and dispersed in NMP to obtain a positive electrode slurry; the positive electrode slurry is coated on aluminum foil to obtain a double-sided coated positive electrode sheet; then rolled and cut to obtain a positive electrode sheet with an areal density of 380 g / m³. 2 The compacted density is 2.6 g / cm³. 3 .

[0068] S3) Negative electrode preparation: Artificial graphite (94% by mass) is used as the negative electrode active material. The negative electrode conductive agent SP (1% by mass) and the negative electrode binder (SBR + PAA total mass percentage of 5%, where the PAA dosage is shown in Table 1) are mixed evenly and dispersed in deionized water to obtain a negative electrode slurry. This negative electrode slurry is then coated onto copper foil to obtain a double-sided coated electrode. After rolling and cutting, the negative electrode sheet is obtained, with an areal density of 170 g / m². 2 The compacted density is 1.6 g / cm³. 3 .

[0069] S4) Electrolyte preparation: This includes organic solvents and electrolyte salts dissolved in the organic solvents, various additives, etc. (details are shown in Table 1). The organic solvents include conventional carbonate solvents and carboxylic acid ester solvents, of which the carboxylic acid ester solvent EA accounts for 6%~60% of the total mass of the electrolyte (details are shown in Table 1), and the remaining solvents are conventional carbonate solvents EC and DMC (the mass ratio of EC to DMC is 3:7) to supplement the total electrolyte volume to 100%; the electrolyte salt includes lithium hexafluorophosphate, and the content of the electrolyte salt is 1.3 mol / L to ensure lithium ion transport in the electrolyte.

[0070] S5) Assembly and Formation: The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. The stacked electrodes are then used to obtain a bare cell, with the positive electrode measuring 75mm × 75mm and the negative electrode measuring 80mm × 80mm. The bare cell is placed in an outer packaging shell, dried to a satisfactory condition, and then injected with electrolyte. After vacuum sealing, standing, and formation (0.05C rate, constant current charging for 120min, standing for 10min, then 0.33C charging for 120min, followed by 0.33C charging to 3.65V cutoff, to obtain the formed lithium-ion battery), a lithium-ion battery is obtained.

[0071] The performance of the obtained lithium-ion batteries was tested, and the results are shown in Table 2.

[0072] Test methods for carboxylic acid ester solvent content and unsaturated carbonate additive mass content:

[0073] The battery was discharged using a battery charging and discharging device under the following conditions: current 0.33C, cutoff voltage 2.5V. After recording the battery number / barcode, the battery was disassembled and the electrolyte collected in a glove box (H2O≤1 ppm, O2≤10ppm). There are three methods for collecting the electrolyte: After removing the battery cover, ① if there is free electrolyte, collect it into a 10mL sample tube using a pipette and seal it with sealant to prevent leakage. ② if there is no free electrolyte, a hydraulic press (Beijing Heng'ao Technology Co., Ltd.'s FY-30 hydraulic press) can be used to continuously pressurize until free electrolyte appears. Collect the electrolyte into a sample tube and seal it. ③ Add an appropriate amount of dichloromethane extractant to the battery and record the dichloromethane content. After adding dichloromethane, put the battery into an aluminum-plastic bag and seal it with a heat sealer. Transfer it to an ultrasonic oscillator and let it stand for 12 hours to allow the electrolyte in the electrode to mix thoroughly with the dichloromethane. Then, use a pipette to draw the mixture of dichloromethane and electrolyte into a 5mL sample tube and seal the sample tube with sealing glue. Inject the collected electrolyte sample into an Agilent Intuvo 9000 gas chromatograph and ion chromatograph using a microsyringe to test the electrolyte composition. By comparing the GC-MS spectrum of the electrolyte to be tested with the standard GC-MS spectrum, the additive components in the electrolyte to be tested are determined. Then, the content of each component is determined based on the peak area of ​​each component in the electrolyte to be tested.

[0074] Test method for adhesive content:

[0075] The adhesive was tested using an infrared instrument and a Netzsch STA449F5 simultaneous thermal analyzer.

[0076] Test methods for D10, D50, and D90:

[0077] Disassemble the empty battery, remove the positive electrode sheet, soak the positive electrode sheet in dimethyl carbonate (DMC) for 2 hours, and then dry it; scrape the positive electrode sheet to remove powder, calcine the scraped powder in an atmosphere furnace for 4 hours, and remove surface adhesives, etc., using a Malvern 3000 laser particle size analyzer.

[0078] Performance testing methods:

[0079] Fast charging performance testing method:

[0080] Using copper wire as a reference electrode, after normalization and calibrating to constant capacity, the battery was charged to 0% SOC. Then, lithium was plated on the three-electrode copper wire at a rate of 0.01C for 10 hours on the positive side. After lithium plating, the following fast charging test was performed.

[0081] The battery is charged at a constant current of 0.33C to the upper limit voltage of 3.65V, and then charged at a constant voltage until the current is less than or equal to 0.05C; then discharged at 0.33C to the lower limit voltage of 2.75V. The above steps are repeated 3 times, and the capacity discharged in the third cycle is taken as the battery discharge capacity.

[0082] After resting for 10 minutes, discharge at 1C to 2.5V, rest for 10 minutes, and charge at 0.33C to 10% SOC. Then charge at 4C to 3.6V or the auxiliary voltage to 0V, and record the time t0. Then charge at 0.4C in descending order, and charge at 3.6C, 3.2C, 2.8C, 2.4C, 2.0C, 1.6C, 1.2C, 0.8C, and 0.4C. The cutoff condition for each charge is to charge to 3.6V or the auxiliary voltage to 0V. Record the time t1. t1-t0 is the fast charging time.

[0083] Capacity recovery rate after 60℃ storage for 90 days:

[0084] The battery is charged at a constant current of 0.33C to the upper limit voltage of 3.65V, and then charged at a constant voltage until the current is less than or equal to 0.05C; then discharged at 0.33C to the lower limit voltage of 2.75V, and the above steps are repeated 3 times. The capacity discharged in the third cycle is taken as the battery discharge capacity; then charged at 0.33C to the upper limit voltage of 3.65V, and after the cut-off current is less than or equal to 0.05C, it is placed in a 60℃ oven.

[0085] After 90 days, the battery is discharged at 0.33C. The capacity of the first discharge cycle is recorded as C1, and the capacity of the third discharge cycle at 0.33C is recorded as C2. C2 / C0×100% is the capacity recovery rate of the battery after 90 days of storage at 60℃.

[0086] Table 1 Specific parameters of lithium-ion batteries

[0087]

[0088] Table 2. Specific parameters and performance test results of lithium-ion batteries

[0089]

[0090] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A lithium-ion battery, characterized in that, Includes positive electrode, negative electrode and electrolyte; The positive electrode includes a positive electrode active material; the positive electrode active material includes lithium iron phosphate; the particle size of the positive electrode active material satisfies (D90+D10) / D50=a, where a is 3.0~4.0; The negative electrode includes a negative electrode active layer; the negative electrode active layer includes a negative electrode active material and a negative electrode binder; the negative electrode binder includes a chain-like binder; the mass percentage of the chain-like binder in the negative electrode active layer is n; The electrolyte comprises a solvent and an additive; the solvent comprises a carboxylic acid ester solvent; the additive comprises an unsaturated ester additive; the mass percentage of the carboxylic acid ester solvent in the electrolyte is m; the mass percentage of the unsaturated ester additive in the electrolyte is p. The negative electrode and the electrolyte satisfy (p+n) / m=b; where b is 0.1~1; The additives also include sulfur-containing additives; the sulfur-containing additives are selected from sulfonyl lactones and / or sulfate esters; the mass percentage of the sulfur-containing additives in the electrolyte is 0.5% to 2.5%; The value of n is 1.5% to 5%; The value of m is 40%~60%; The p value is 2%~4.5%; The additives also include siloxane additives; the mass percentage of the siloxane additives in the electrolyte is 0.01% to 2.0%. The solvent further includes cyclic carbonates and / or chain carbonates; the cyclic carbonates have a mass percentage content of 20% to 40% in the electrolyte; the chain carbonates have a mass percentage content of 0% to 80% in the electrolyte. The D90 of the positive electrode active material is 2.0~6.0 μm; The D10 of the positive electrode active material is 0.3~0.6 μm; The D50 of the positive electrode active material is 0.4~1.2 μm.

2. The lithium-ion battery according to claim 1, characterized in that, The range of n / p is 0.0025 to 4.

3. The lithium-ion battery according to claim 1, characterized in that, The chain adhesive is selected from one or more of polyacrylic acid, styrene-butadiene rubber, sodium polyacrylate, polyacrylamide, polyvinyl alcohol, and polymethacrylic acid; And / or, the carboxylic acid ester solvent is selected from one or more of methyl acetate, ethyl formate, and ethyl acetate; the unsaturated ester additive is selected from one or more of vinylene carbonate, fluoroethylene carbonate, ethylene ethylene carbonate, allyl ethylene carbonate, vinyl acetate, and catechol carbonate.

4. The lithium-ion battery according to claim 1, characterized in that, The sulfur-containing additive is selected from methylene disulfonate.

5. The lithium-ion battery according to claim 1, characterized in that, The siloxane additive is selected from tris(trimethylsilane) phosphite and / or tris(trimethylsilane) borate.

6. The lithium-ion battery according to claim 1, characterized in that, The electrolyte further includes lithium salts; the lithium salts are selected from one or more of lithium hexafluorophosphate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

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