Lithium ion battery

Abstract

The invention belongs to the technical field of batteries, and particularly relates to a lithium ion battery. Compared with the prior art, the rapid charging and high-temperature storage performance of the lithium iron phosphate battery is improved by comprehensively controlling the design of the positive and negative pole pieces and the electrolyte. On one hand, by adjusting the particle size distribution of the lithium iron phosphate material particles, the lithium ion transmission path is more uniform, the lithium ion transmission rate consistency is improved, and the overall fast charging performance of the battery is improved; and on the other hand, the carboxylic ester solvent with lower viscosity is adopted, the viscosity of the electrolyte is obviously reduced, the liquid-phase lithium ion transmission capability is greatly improved, and meanwhile, a positive electrode material is matched, so that the comprehensive fast charging performance of the battery is improved. And finally, by selecting a chain binder and compounding a proper amount of unsaturated carbonic ester, the protection of the negative electrode active material is realized, and the side reaction of the electrolyte and the negative electrode side is obviously reduced.

Description

Technical Field

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

[0002] Lithium-ion batteries are widely used in various electronic devices such as digital cameras, mobile phones, tablet computers, and laptops due to their advantages such as high operating voltage, high energy density, and no memory effect, and are constantly developing in the field of new energy vehicles. However, compared with traditional fuel vehicles that take a few minutes to fill up, new energy vehicles take several hours to fully charge. Under the premise that charging piles are not yet popular, fast charging technology has become one of the important factors affecting the rapid popularization of electric vehicles, and fast charging batteries have also become another development direction of power lithium-ion batteries.

[0003] Lithium iron phosphate battery is a highly safe lithium-ion battery. 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. In addition, lithium iron phosphate battery can maintain a relatively stable voltage during charging and discharging.

[0004] At present, commercial lithium iron phosphate (LiFePO 4 )The laboratory test of positive lithium-ion batteries shows that the cycle life at room temperature can reach 1500~2400 times, but more and more studies have shown that the cycle performance of batteries at high temperatures decays gradually. However, the working environment of electric vehicles is complex. The power system is usually a battery resistor composed of multiple batteries connected in series and parallel. It is affected by the installation location, ventilation conditions and high-rate charging and discharging requirements. 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 for electric vehicles. In addition, during fast charging, the battery will also generate relatively high temperatures. High temperatures will accelerate the chemical reactions inside the battery, resulting in the consumption of battery capacity. Summary of the invention

[0005] In view of this, the technical problem to be solved by the present invention is to provide a lithium ion battery with higher high temperature cycle stability.

[0006] The present invention provides a lithium ion battery, comprising a positive electrode, a negative electrode and an electrolyte;

[0007] 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, and 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; the mass percentage of the chain binder in the negative electrode active layer is n;

[0009] The electrolyte includes a solvent and an additive; the solvent includes a carboxylic acid ester solvent; the additive includes 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; and b is 0.05-1.5.

[0011] Preferably, a is 3.0-4.0;

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

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

[0014] And / or, said m is 6% to 60%;

[0015] And / or, said p is 1% to 6%;

[0016] And / or, n / p ranges from 0.0025 to 4.

[0017] Preferably, the chain binder is selected from one or more of polyacrylic acid, styrene-butadiene rubber, sodium polyacrylate, polyacrylamide, polyvinyl alcohol and polymethacrylic 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 ethyl 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 sultone and / or sulfate; the mass percentage of the sulfur-containing additive in the electrolyte is 0.01% to 3%.

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

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

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

[0023] Preferably, the solvent further comprises cyclic carbonate and / or 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] And / or, the electrolyte further includes a lithium salt; the lithium salt is selected from one or more of lithium hexafluorophosphate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorobis(oxalatophosphate) and lithium tetrafluorooxalatophosphate.

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

[0026] Compared with the prior art, the present invention improves the fast charging and high-temperature storage performance of lithium iron phosphate batteries by comprehensively controlling the design of positive and negative electrode plates and electrolytes. On the one hand, the present invention adjusts the particle size distribution of lithium iron phosphate material particles by controlling the D50 of the positive electrode particles and the range of (D10+D90) / D50, so that 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. At the same time, it can also avoid the problem of increased side reactions and poor high-temperature storage performance caused by too small particle size, and by controlling the particle size distribution of the positive electrode material, the lithium ion deintercalation rate is improved. On the other hand, the electrolyte, as a lithium ion transmission medium, will also affect its lithium ion transmission capacity. The present invention uses a carboxylic acid ester solvent with a 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 comprehensive fast charging performance of the battery. However, under high temperature conditions, the carboxylic acid ester solvent increases the side reaction with electrolysis. Therefore, the present invention protects the negative electrode active material by selecting a chain binder and compounding an appropriate unsaturated carbonate content, and significantly reduces the electrolyte and negative electrode side reactions. In addition, the unsaturated carbonate additive can form a solid electrolyte interface 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, if it is added in too much amount, the kinetic performance of the negative electrode will deteriorate. The present invention achieves the purpose of protecting the negative electrode active material by matching a chain binder to reduce the amount of unsaturated carbonate added. It can reduce the side reactions at the negative electrode interface while avoiding excessive impedance that leads to lithium precipitation in the battery and deterioration of fast charging performance. In summary, the present invention improves the viscosity of the electrolyte and improves the fast charging performance of the battery by controlling the relationship between the content of carboxylate solvents and the chain binder and the carboxylate solvent, avoids the increase of side reactions between the electrolyte and the negative electrode material due to the increase of the content of carboxylate solvents, and takes into account both fast charging and high temperature storage performance of the battery. DETAILED DESCRIPTION

[0027] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0028] The present invention provides a lithium ion battery, comprising a positive electrode, a negative electrode and an electrolyte; the positive electrode comprises a positive electrode active material; the positive electrode active material comprises lithium iron phosphate; the D50 of the positive electrode active material is 0.4-1.2 μm; the particle size of the positive electrode active material satisfies (D90+D10) / D50=a, and a is 2.0-6.0; 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; the mass percentage of the chain 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; and b is 0.05-1.5.

[0029] In the present invention, the chain binder refers to a polymer having a long chain structure that can be used as a binder. The long chain structure is formed by monomers of the polymer being connected to each other through a polymerization reaction.

[0030] The fast charging performance of the battery depends not only on the electrolyte, but also on the positive active material. The present invention controls the particle size D50 of the positive active material and the range of (D90+D10) / D50 so that the positive active material can achieve fast lithium ion transmission and avoid side reactions caused by the electrolyte. Where D50 is the particle size corresponding to 50% of the cumulative volume distribution percentage in the active material, in μm; D90 is the particle size corresponding to 90% of the cumulative volume distribution percentage in the active material, in μm; D10 is the particle size corresponding to 10% of the cumulative volume distribution percentage in the active material, in μm. The smaller the D10, the smaller the particle size. The larger the D90 value, the larger the particle size. D50 represents the median particle size. D50 is in the appropriate range, indicating that the overall particle size range is moderate. The (D10+D90) / D50 value cannot be too large. Too large indicates the presence of large particles. Large particle size increases the lithium ion transmission path, which 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 high, the particle size is too small, and the contact area between the positive electrode and the electrolyte is small, which will increase the side reactions and deteriorate the high-temperature storage performance.

[0031] The present invention adjusts the particle size distribution of lithium iron phosphate material particles by controlling the D50 of the positive electrode particles and the range of (D10+D90) / D50, so that 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; at the same time, the particle size is avoided to be too small, the side reaction increases, and the high-temperature storage performance deteriorates; 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, will also affect its lithium ion transmission capacity. The use of a lower-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 comprehensive fast charging performance of the battery. However, the carboxylic acid ester solvent increases the side reaction with electrolysis under high temperature conditions. By selecting a chain binder and compounding an appropriate unsaturated carbonate content, the protection of the negative electrode active material is achieved, and the electrolyte and negative electrode side side reactions are significantly reduced. The unsaturated carbonate additive can form a solid electrolyte interface 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, if too much is added, the negative electrode kinetics will deteriorate. By using a chain binder to reduce the amount of unsaturated carbonate added, the purpose of protecting the negative electrode active material can be achieved. While reducing the negative electrode interface side reactions, it can avoid excessive impedance leading to lithium deposition in the battery and deterioration of fast charging performance. By controlling the relationship between the carboxylate solvent content and the chain binder and carboxylate solvent, the electrolyte viscosity is improved to improve the battery fast charging performance, avoiding the increase in side reactions between the electrolyte and the negative electrode material due to the increase in the carboxylate solvent content, and taking into account both the battery fast charging and high temperature storage performance.

[0032] In a specific embodiment provided by the present invention, the D90 of the positive electrode active material is preferably 2.0~6.0 μm, more preferably 2.0~3.4 μm; in some specific embodiments provided by 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 a specific embodiment provided by the present invention, the D10 of the positive electrode active material is preferably 0.3~0.6μm 0.3~0.5μm, more preferably μm; in some specific embodiments provided by 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 a specific embodiment provided by 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 provided by 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 the present invention, unless otherwise specified, the particle size is measured using a Malvern 3000 laser particle size analyzer.

[0036] In a specific embodiment provided by the present invention, the range 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 a range between any two of the above values; by making the particle size meet the above range of a, the particle size distribution of lithium iron phosphate particles can be made more uniform, the lithium ion transmission is uniform, and the presence of particles with excessively large particle sizes is avoided, thereby avoiding affecting the lithium ion transmission rate and improving the overall fast charging performance of the battery. In a specific embodiment provided by the present invention, a is 3.0~4.0. By controlling a within this range, the side reactions of the positive electrode and the electrolyte can be further avoided, and the battery can maintain better fast charging performance; in some embodiments provided by 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 a specific embodiment provided by the present invention, the positive electrode includes a positive electrode collector and a positive electrode active layer attached to the surface of the positive electrode collector; the positive electrode active layer includes a positive electrode active material; the mass of the positive electrode active material is preferably 80% to 98% of the mass of the positive electrode active layer; optionally, the mass of the positive electrode active material is 80%, 85%, 90%, 95%, 97%, 98% of the mass of the positive electrode active layer or a range between any two of the above values.

[0038] In another specific embodiment provided by 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 includes at least one of conductive carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers. In the present invention, carbon black is preferred. 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 a specific embodiment provided by 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 a range between any two of the above values.

[0040] In a specific embodiment provided by the present invention, the negative electrode comprises 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 comprises 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, SiO x , Silicon Carbon and Li 4 Ti 5 O 12 One or more of .

[0041] In a specific embodiment provided by the present invention, the negative electrode binder includes a chain binder; the mass percentage n of the chain binder in the negative electrode active layer is preferably 0.01%~5.0%; specifically, the mass percentage n of the chain binder without benzene ring in the negative electrode active layer is preferably 0.01%~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. ; further, the n is more preferably 1.5% to 5.0%; in some embodiments provided by the present invention, the n is specifically 3.0%, 1.5%, 4.0%, 2.5%, 5.0%, 0.1% or 2.8%; the chain binder is preferably one or more of polyacrylic acid (PAA), styrene-butadiene rubber (SBR), sodium polyacrylate, polyacrylamide, polyvinyl alcohol and polymethacrylic acid. Among these chain binders, polyacrylic acid is more preferably used because it has an appropriate molecular weight, is simple to synthesize, has a lower cost and is used in a smaller amount.

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

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

[0044] In a specific embodiment provided by the present invention, the negative electrode active layer preferably also 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 a conductive agent well known to those skilled in the art, without any special restrictions. In the present invention, it is preferably at least one of conductive carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers, and more preferably a conductive agent SP.

[0045] In a specific embodiment provided by the present invention, the electrolyte includes a solvent and an additive; the solvent includes a carboxylic acid ester solvent; in order to improve the fast charging performance of the battery, the liquid phase conductivity of the battery system needs to be increased. Reducing viscosity is one of the most effective means to improve liquid phase conductivity. However, although traditional chain carbonates such as ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) have lower viscosity, their temperature range is shorter and the viscosity increases rapidly at low temperatures. Therefore, the present invention uses a carboxylic acid ester solvent with a wider temperature range and lower viscosity. The mass percentage m of the carboxylate solvent in the electrolyte is preferably 6%~60%, more preferably 12%~60%, more preferably 20%~60%, and most preferably 40%~60%; when the carboxylate content is too much, the high-temperature performance will deteriorate sharply, especially the high-temperature gas production will be serious, which may cause the battery valve to open and deteriorate the safety performance; when the carboxylate content is low, the viscosity of the electrolyte does not decrease significantly, the liquid phase conduction is equivalent, and the fast charging benefit is small; in some embodiments provided by the present invention, the content m of the carboxylate solvent in the electrolyte is specifically 6%, 12%, 20%, 22%, 40% or 60%; the carboxylate solvent is preferably one or more of methyl acetate, ethyl formate and ethyl acetate.

[0046] In a specific embodiment provided by the present invention, the solvent further comprises a cyclic carbonate and a chain carbonate; the mass percentage of the cyclic carbonate in the electrolyte is preferably 20% to 40%; optionally, the mass percentage of the cyclic carbonate in the electrolyte is 20%, 25%, 30%, 35%, 40% or a range between any two of the above values; the cyclic carbonate is a cyclic carbonate well known to those skilled in the art, and there is no special limitation. In the present invention, it is preferably ethylene carbonate (EC) or propylene carbonate (PC) or fluoroethylene carbonate. The linear carbonate is preferably present in the electrolyte in a mass percentage of 0% to 80%; optionally, the linear carbonate is present in the electrolyte in a mass percentage of 0%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% or in a range between any two of the above values; the linear carbonate is any linear carbonate known to those skilled in the art without any particular limitation, and in the present invention, preferably is one or more of dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC).

[0047] In a specific embodiment provided by the present invention, the solvent further comprises a cyclic carbonate and a chain carbonate; the mass ratio of the cyclic carbonate to the chain carbonate is preferably (1-5):(5-9), more preferably (2-4):(6-8), and even more preferably 3:7.

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

[0049] In the present invention, the b value reflects the relationship between the content of chain binders and unsaturated ester additives (functional groups containing unsaturated bonds, such as unsaturated alkenyl, alkynyl, etc.) and the content of carboxylic acid ester solvents. When the content of carboxylic acid ester solvents is relatively high, although the viscosity of the electrolyte can be reduced and the ionic conductivity can be improved, the addition of carboxylic acid ester solvents will accelerate the deterioration of the high-temperature performance of lithium iron phosphate batteries, such as high-temperature cycling, high-temperature gas production, and high-temperature storage. After adding unsaturated bond additives, a good SEI film can be formed on the negative electrode side to prevent the continuous occurrence of side reactions of carboxylic acid ester solvents on the negative electrode side. However, if the amount of unsaturated ester additives added is too much, the impedance will increase, the initial DCR will increase sharply, and it is easy to cause interface lithium precipitation and storage diving. By matching the negative electrode binder with the chain binder, the negative electrode material can be better coated, reducing the contact between the negative electrode material and the electrolyte at high temperature. At the same time, the addition of a chain binder can reduce the amount of unsaturated ester additives, so that the content of unsaturated ester additives can be slightly reduced to achieve the purpose of significantly reducing DCR, and achieving high passivation on the negative electrode side. The present invention controls the amounts of unsaturated ester additives and chain binders. The unsaturated ester additives can form a dense SEI film on the negative electrode to reduce the side reactions between the negative electrode and the electrolyte. The chain binder can form a better coating on the negative electrode. The contents of the carboxylic acid ester solvents, the unsaturated ester additives and the chain binder are comprehensively adjusted to improve the ionic conductivity, enhance the fast charging performance and take into account the high temperature performance.

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

[0051] In a specific embodiment provided by the present invention, n / p is controlled in the range of 0.0025~5 to avoid too much unsaturated ester additives, which increases impedance, and too much chain binder, which leads to poor lithium ion transmission performance and poor fast charging performance; optionally, n / p is 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 a range between any two of the above values; in some embodiments provided by 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 a specific embodiment provided by the present invention, the additive further includes a sulfur-containing additive; the sulfur-containing additive is preferably sultone and / or sulfate; the mass percentage of the sulfur-containing additive in the electrolyte is preferably 0.01%~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 a range between any two of the above values. The addition of sulfur-containing additives can further reduce the 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 easy to decompose at high temperature, resulting in a more stable SEI, thereby reducing the battery cell DCR and improving the high temperature performance of the battery. In another specific embodiment provided by the present invention, the sulfur-containing additive is specifically methylene disulfonate (MMDS).

[0053] In a specific embodiment provided by 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 a range between any two of the above values; such additives can modify the initial SEI film formation, and the formed SEI film has better ion conductivity, which can significantly reduce the initial DCR of the battery. In another specific embodiment provided by the present invention, the siloxane additive is specifically tris(trimethylsilyl)phosphite (TMSP) and / or tris(trimethylsilyl)borate (TMSB).

[0054] In a specific embodiment provided by 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 of 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 a range between 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 difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate and lithium tetrafluorooxalatophosphate.

[0055] In a specific embodiment provided by 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 material.

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

[0057] S1) Preparation of positive electrode active material: the lithium source, iron source and phosphorus source are mixed uniformly, ground, and spray-dried to obtain a precursor material; the precursor material is sintered at high temperature in a protective atmosphere, and crushed to obtain a positive electrode active material; specifically, the particle size D50 of the lithium source, iron source and phosphorus source is controlled at 100-150 nm; the grinding method can be a method well known to those skilled in the art, and there is no special restriction. In the present invention, it is preferably coarse grinding and fine grinding in sequence; the mixed material is ground to a particle size of 370-420 nm; the spray drying temperature is preferably 300°C-350°C; the protective atmosphere can be a protective atmosphere well known to those skilled in the art, and there is no special restriction. In the present invention, it is preferably nitrogen; the temperature of the high-temperature sintering is preferably 750°C-850°C. In the present invention, there is no limitation on the adjustment method of D10, D90 and D50 of lithium iron phosphate. For example, it can be adjusted by grinding time, rotation speed and sintering temperature.

[0058] In a specific embodiment provided by the present invention, the preparation of the positive electrode active material is specifically as follows: the qualified lithium source, iron source and phosphorus source (precursor D50 is controlled at 100~150 nm) are mixed into a mixed material, and the mixed material is completed by coarse grinding and fine grinding (to 370~420 nm). After fine grinding, the powder precursor is processed into a powder precursor in a very short time at 300℃~350℃ by spray drying, and the powder after spraying is sent to a sealed roller kiln for sintering. According to the sintering process requirements, the sintering temperature of each temperature zone is set between 750℃ and 850℃ under a nitrogen protection atmosphere, and then the roller rolls to drive the loading sagger forward to complete the sintering at the same time. After a certain airflow crushing, it is cooled to obtain the 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, mixed evenly with the positive electrode binder and the positive electrode conductive agent, and dispersed in NMP to obtain a positive electrode slurry; the positive electrode slurry is coated on an aluminum foil to obtain a double-sided coated positive electrode sheet; then, the positive electrode sheet is obtained by rolling and cutting; wherein the amount and type of the lithium iron phosphate, the positive electrode binder and the 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, the negative electrode conductive agent and the negative electrode binder are uniformly mixed and dispersed in deionized water to obtain a negative electrode slurry; the negative electrode slurry is coated on a copper foil to obtain a double-sided coated electrode sheet; then, the negative electrode sheet is obtained by rolling and cutting; the amount and type of the negative electrode active material, the negative electrode conductive agent and the negative electrode binder are the same as described above and will not be repeated here.

[0061] S4) Preparation of electrolyte: including solvent and 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 those described above and will not be repeated here.

[0062] S5) Assembly and formation: stack the positive electrode sheet, separator and negative electrode sheet in order, so that the separator is between the positive and negative electrode sheets to play an isolating role, and then wind them to obtain a bare cell; place the bare cell in an outer packaging shell, inject electrolyte after drying, and obtain a lithium-ion battery after vacuum packaging, standing, formation, shaping and other processes.

[0063] To further illustrate the present invention, a lithium ion battery provided by the present invention is described in detail below in conjunction with an embodiment; the molecular weight of the PVDF used in the embodiment and the comparative example 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] The reagents used in the following examples are all commercially available.

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

[0066] S1) Preparation of positive electrode materials (specific conditions are shown in Table 1 and Table 2): The qualified lithium source, iron source and phosphorus source (precursor D50 is controlled at 100~150 nm) are mixed into a mixed material, and the mixed material is completed by coarse grinding and fine grinding (to 370~420nm). After fine grinding, it is treated into a powder precursor at 300℃~350℃ by spray drying. After spraying, the powder is sent to a sealed roller kiln for sintering. According to the sintering process requirements, the sintering temperature of each temperature zone is set between 750℃ and 850℃, and then the roller rolling drives the loading sagger forward to complete the sintering at the same time. After a certain air flow crushing, it is cooled to obtain lithium iron phosphate material.

[0067] S2) Preparation of positive electrode sheet: The above-mentioned lithium iron phosphate material is used as the main material, mixed evenly with the positive electrode binder PVDF and the positive electrode conductive agent SP in 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, the surface density of which is 380 g / m 2 , compacted density is 2.6 g / cm 3 .

[0068] S3) Preparation of negative electrode sheet: The negative electrode active material artificial graphite (mass percentage 94%), the negative electrode conductive agent SP (mass percentage 1%), and the negative electrode binder (the total mass percentage of SBR+PAA is 5%, wherein the amount of PAA is shown in Table 1) are mixed evenly and dispersed in deionized water to obtain a negative electrode slurry; the negative electrode slurry is coated on a copper foil to obtain a double-sided coated electrode sheet; then, the negative electrode sheet is rolled and cut to obtain a negative electrode sheet, and the surface density of the negative electrode sheet is 170 g / m 2 , compacted density is 1.6 g / cm 3 .

[0069] S4) Preparation of electrolyte: including organic solvent and electrolyte salt dissolved in the organic solvent, various additives, etc. (as shown in Table 1). The organic solvent includes conventional carbonate solvent and carboxylate solvent, wherein the carboxylate solvent EA accounts for 6% to 60% of the total mass of the electrolyte (as shown in Table 1), and the remaining solvent is conventional carbonate solvent EC and DMC (the mass ratio of EC to DMC is 3:7) to replenish the total amount of electrolyte to 100%; wherein, the electrolyte salt includes lithium salt lithium hexafluorophosphate, and the content of the electrolyte salt is 1.3 mol / L to ensure the lithium ion transmission of the electrolyte.

[0070] S5) Assembly and formation: stack the above-mentioned positive electrode sheet, isolation film, and negative electrode sheet in order, so that the isolation film is between the positive and negative electrode sheets to play an isolating role, and then stack the sheets to obtain a bare battery cell, wherein the positive electrode sheet has a size of 75mm×75mm, and the negative electrode sheet has a size of 80mm×80mm; place the bare battery cell in an outer packaging shell, inject electrolyte after drying, and undergo vacuum packaging, standing, and formation (0.05C rate, constant current charging for 120min, standing for 10min and then charging at 0.33C for 120min, and then charging at 0.33C to 3.65V to obtain a formed lithium-ion battery) to obtain a lithium-ion battery.

[0071] The performance of the obtained lithium-ion battery 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] Use battery charging and discharging equipment to discharge the battery. Discharge conditions: current 0.33C, cut-off voltage 2.5V. Record the battery number / barcode and place in a glove box (H 2 O≤1 ppm, O 2 ≤10ppm) to collect the electrolyte. There are three ways to collect the electrolyte: After removing the battery cover, ① if there is free electrolyte, use a pipette to collect the electrolyte into a 10mL sample tube and seal it with sealant to prevent electrolyte leakage. ② If there is no free electrolyte, use a hydraulic press (Beijing Hengaode Technology Co., Ltd. FY-30 hydraulic press) 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 for oscillation + let it stand for 12 hours to allow the electrolyte in the electrode to be fully mixed with 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 Agilent Intuvo 9000 gas chromatograph and ion chromatograph with a microinjector to test the electrolyte composition, and determine the additive components in the electrolyte to be tested by comparing the GC-MS spectrum of the electrolyte to be tested with the standard GC-MS spectrum, and then determine the content of each component according to the peak area of ​​each component in the electrolyte to be tested.

[0074] Test method for binder content:

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

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

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

[0078] Performance testing method:

[0079] Fast charging performance test method:

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

[0081] The battery is charged at a constant current of 0.33C to an upper voltage limit 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 a lower voltage limit of 2.75V, and the above steps are repeated 3 times, and the capacity discharged in the third cycle is taken as the battery discharge capacity;

[0082] Let it stand for 10 minutes, discharge at 1C to 2.5V, let it stand for 10 minutes, charge at 0.33C to 10% SOC; then charge at 4C to 3.6V or auxiliary voltage 0V, record time t0, then charge at 0.4C, charge at 3.6C, 3.2C, 2.8C, 2.4C, 2.0C, 1.6C, 1.2C, 0.8C, 0.4C, the cutoff condition for each charge is charging to 3.6V or auxiliary voltage 0V; record time t1, t1-t0 is the fast charging time.

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

[0084] The battery is charged at a constant current of 0.33C to an 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 a lower limit voltage of 2.75V, and the above steps are repeated 3 times, with the capacity discharged in the third cycle as the battery discharge capacity; then charged at 0.33C to an 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°C oven;

[0085] After 90 days, the battery was discharged at 0.33C, and the capacity of the first discharge cycle was recorded as C1, and the capacity of the third discharge cycle was recorded as C2; C2 / C0×100% is the capacity recovery rate of the battery stored at 60°C for 90 days.

[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 solution of the present invention rather than to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims

1. A lithium ion battery, characterized in that: including a positive electrode, a negative electrode and an 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, and a is 2.0-6.0; 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; the mass percentage of the chain binder in the negative electrode active layer is n; The electrolyte includes a solvent and an additive; the solvent includes a carboxylic acid ester solvent; the additive includes 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; and b is 0.05-1.

5.

2. The lithium-ion battery according to claim 1, characterized in that The a is 3.0-4.0; And / or, b is 0.1~1.

3. The lithium-ion battery according to claim 1, characterized in that The n is 0.01%~5%; And / or, said m is 6% to 60%; And / or, said p is 1% to 6%; And / or, n / p ranges from 0.0025 to 4.

4. The lithium ion battery according to claim 1, characterized in that: The chain binder 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, vinyl ethylene carbonate, allyl ethyl carbonate, vinyl acetate and catechol carbonate.

5. The lithium-ion battery according to claim 1, characterized in that: The additives also include sulfur-containing additives; the sulfur-containing additives are selected from sultone and / or sulfate; the mass percentage of the sulfur-containing additive in the electrolyte is 0.01% to 3%.

6. The lithium-ion battery according to claim 5, characterized in that: The sulfur-containing additive is selected from methylene methanedisulfonate.

7. The lithium-ion battery according to claim 1, characterized in that: The additives also include a siloxane additive; the mass percentage of the siloxane additive in the electrolyte is 0.01% to 2.0%.

8. The lithium-ion battery according to claim 7, characterized in that: The siloxane additive is selected from tris(trimethylsilyl)phosphite and / or tris(trimethylsilyl)borate.

9. The lithium-ion battery according to claim 1, characterized in that: The solvent further comprises cyclic carbonate and / or 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%; And / or, the electrolyte further includes a lithium salt; the lithium salt is selected from one or more of lithium hexafluorophosphate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorobis(oxalatophosphate) and lithium tetrafluorooxalatophosphate.

10. The lithium ion battery according to claim 1, characterized in that: The D90 of the positive electrode active material is 2.0-6.0 μm; and / or, the D10 of the positive electrode active material is 0.3-0.6 μm; and / or, the D50 of the positive electrode active material is 0.4-1.2 μm.

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