Low-temperature-resistant lithium iron phosphate battery

Abstract

The invention relates to the technical field of lithium ion batteries. Specifically, the invention provides a low-temperature-resistant lithium iron phosphate battery. By changing the formula and the structure of the positive electrode material, the conductivity of the material and the lithium ion intercalation / deintercalation rate are improved; and by optimizing the formula of the electrolyte and cooperatively using the lithium salt with high conductivity and excellent low-temperature performance, the ionic conductivity of the electrolyte is improved, and the interface impedance of the battery is reduced, so that the low-temperature performance of the lithium iron phosphate battery is improved.

Description

Technical Field

[0001] The present invention relates to the field of lithium ion batteries, and in particular to lithium iron phosphate batteries, especially low temperature resistant lithium iron phosphate batteries. Background Art

[0002] With the development of science and technology, lithium batteries are widely used in various electronic devices, electric vehicles and other fields due to their high energy density, long cycle life and environmental protection. Among them, lithium iron phosphate batteries have become a hot research topic due to their high safety, low cost and environmental friendliness. However, lithium iron phosphate batteries perform poorly at low temperatures, which limits their application in cold regions.

[0003] At low temperatures, the diffusion capacity of ions in the electrode is low, and the ionic conductivity of the electrolyte is low at low temperatures, which makes the low-temperature performance of lithium iron phosphate batteries poor. There are currently several main technical paths to improve the low-temperature performance of lithium iron phosphate:

[0004] 1. Modification of electrolyte: Improve the low-temperature performance of the battery by adjusting the electrolyte formula. The main approach at present is to add low-viscosity solvents, lithium salts with higher ion conductivity or better film-forming additives, which are conducive to improving the battery charging performance and rate performance.

[0005] 2. Modification of lithium iron phosphate materials, such as using carbon nanotubes with higher conductivity as the conductive agent of lithium iron phosphate electrodes. Carbon nanotubes can easily form a good conductive network in the electrode, reducing the polarization of the electrode, thereby improving the low-temperature discharge performance of lithium iron phosphate batteries.

[0006] 3. Improvements in the structural dimensions of the positive and negative electrodes of lithium iron phosphate, such as increasing the axial dimension of the lithium iron phosphate ab plane, reducing the particle size, increasing the spacing between graphite layers, reducing the spacing between graphite layers, and reducing the graphite particle size.

[0007] However, the improvement of low-temperature performance of lithium iron phosphate in the prior art still cannot meet actual needs. Summary of the invention

[0008] Purpose of the Invention

[0009] In view of the problems described in the above background technology section, an object of the present invention is to provide a low-temperature resistant lithium iron phosphate battery.

[0010] Technical Solution

[0011] In order to achieve the above object, the present invention adopts the following technical solutions:

[0012] Solution 1: A lithium iron phosphate battery, the lithium iron phosphate battery comprising a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator, wherein:

[0013] The positive electrode plate comprises a lithium iron phosphate positive electrode material, wherein the lithium iron phosphate positive electrode material comprises a small-particle lithium iron phosphate material and a large-particle lithium iron phosphate material in a mass ratio of (7-9): (3-1), wherein

[0014] The particle size distribution of the small particle lithium iron phosphate material is: D10=about 0.15 to about 0.6 μm, D50=about 0.9 to about 2.5 μm, and D90=about 2.0 to about 7.5 μm;

[0015] and

[0016] The particle size distribution of the large-particle lithium iron phosphate material is: D10=about 2.0 to about 3.5 μm, D50=about 8.0 to about 12.0 μm, and D90=about 16.0 to about 24.1 μm;

[0017] The electrolyte comprises a lithium salt organic solution having a concentration in the range of about 1.2 to about 1.4 mol / L, wherein the lithium salt comprises lithium hexafluorophosphate and another lithium salt, wherein the other lithium salt comprises one or both of lithium tetrafluoroborate and lithium bis(oxalatoborate).

[0018] Option 2: A lithium iron phosphate battery according to Option 1 above, wherein the lithium salt comprises lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4) and lithium dioxalatoborate (LiBOB) in a molar ratio of about 1.0:(0.1-0.3):(0.05-0.15).

[0019] Option 3: A lithium iron phosphate battery according to Option 1 above, wherein the lithium salt comprises LiPF6 and LiBF4 in a molar ratio of about 1.0:(0.1-0.3).

[0020] Option 4: A lithium iron phosphate battery according to Option 1 above, wherein the lithium salt comprises LiPF6 and LiBOB in a molar ratio of about 1.0:(0.05-0.15).

[0021] Option 5: A lithium iron phosphate battery according to any one of Options 1 to 4 above, wherein the electrolyte further contains one or more of vinylene carbonate (VC), fluoroethylene carbonate (FEC), diethylene sulfate (DTD) and lithium difluorophosphate (LiPO2F2) as additives.

[0022] Option 6: The lithium iron phosphate battery according to Option 5 above, wherein the electrolyte contains about 0.9 to about 1.1 wt % VC as an additive based on the total weight of the electrolyte.

[0023] Option 7: The lithium iron phosphate battery according to Option 5 or 6 above, wherein the electrolyte contains about 2.9 to about 3.1 wt % of FEC as an additive based on the total weight of the electrolyte.

[0024] Aspect 8: The lithium iron phosphate battery according to any one of Aspects 5 to 7 above, wherein the electrolyte contains, as an additive, about 1.9 to about 2.1 wt % of DTD based on the total weight of the electrolyte.

[0025] Aspect 9: The lithium iron phosphate battery according to any one of Aspects 5 to 8 above, wherein the electrolyte contains 0 to about 1.0 wt % of LiPO 2 F 2 as an additive based on the total weight of the electrolyte.

[0026] Option 10: A lithium iron phosphate battery according to any one of the above options 1 to 9, wherein the lithium salt organic solution contains one or more of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC) and ethyl acetate (EA) as a solvent.

[0027] Option 11: The lithium iron phosphate battery according to Option 10 above, wherein the solvent of the lithium salt organic solution contains about 26 to about 30 volume % of EC based on the total volume of the solvent.

[0028] Option 12: The lithium iron phosphate battery according to Option 10 or 11 above, wherein the solvent of the lithium salt organic solution contains about 10 to about 14 volume % of PC based on the total volume of the solvent.

[0029] Option 13: A lithium iron phosphate battery according to any one of options 10 to 12 above, wherein the solvent of the lithium salt organic solution contains 22 to 26 volume % of EMC based on the total volume of the solvent.

[0030] Option 14: A lithium iron phosphate battery according to any one of options 10 to 13 above, wherein the solvent of the lithium salt organic solution contains about 18 to about 22 volume % of DMC based on the total volume of the solvent.

[0031] Option 15: A lithium iron phosphate battery according to any one of options 10 to 14 above, wherein the solvent of the lithium salt organic solution contains about 14 to about 18 volume % of EA based on the total volume of the solvent.

[0032] Option 16: A lithium iron phosphate battery according to any one of the above options 1 to 15, wherein the separator is a high-porosity battery separator.

[0033] Option 17: A lithium iron phosphate battery according to Option 16 above, wherein the high-porosity battery separator is a 12μm polypropylene high-porosity battery separator.

[0034] Scheme 18: A lithium iron phosphate battery according to any one of Schemes 1 to 17 above, wherein the negative electrode sheet comprises an aqueous binder, a conductive agent and a negative electrode active material.

[0035] Option 19: According to the lithium iron phosphate battery described in Option 18 above, the negative electrode plate contains carboxymethyl cellulose as an aqueous binder, conductive carbon black as a conductive agent, and graphite as a negative electrode active material.

[0036] Technical Effects

[0037] In the low-temperature resistant lithium iron phosphate battery of the present invention, the conductivity of the material and the lithium ion insertion / extraction rate are improved by changing the formula and structure of the positive electrode material, mainly by adjusting the ratio of the positive electrode material with a specific particle size; further, by optimizing the formula of the electrolyte and using lithium salts with high conductivity and excellent low-temperature performance, the ionic conductivity of the electrolyte is improved, the interfacial impedance of the battery is reduced, and the low-temperature performance of the lithium iron phosphate battery is improved.

[0038] In the low-temperature resistant lithium iron phosphate battery of the present invention, on the one hand, the small-particle lithium iron phosphate material contained in the positive electrode material has a small particle size and high compaction, but its rate performance and low-temperature performance are poor; on the other hand, the large-particle lithium iron phosphate material contained in the positive electrode material has a large particle size, good rate performance and low-temperature performance, but low compaction. Therefore, the large-particle and small-particle lithium iron phosphate positive electrode materials mixed in a specific ratio in the present invention reduce the contact resistance between particles, improve the conductivity and ion insertion / extraction rate, and ensure the energy density, thereby improving the low-temperature performance and rate performance of the battery.

[0039] In the low-temperature resistant lithium iron phosphate battery of the present invention, on the one hand, the electrolyte used may contain LiBOB as a lithium salt, which is more likely to form a passivation film on the surface of the positive electrode material particles, inhibiting electrolyte oxidation, and having a high conductivity, but it also has the disadvantage of low solubility; on the other hand, the electrolyte used may also contain LiBF4 as a lithium salt, which can enhance the film-forming ability of the electrolyte on the electrode, and has good low-temperature cycle performance, but it also has the disadvantage of low conductivity. Therefore, when LiBF4 and LiBoB are used as lithium salts at the same time, the problems of low solubility and low conductivity can be further solved.

[0040] In the low temperature resistant lithium iron phosphate battery of the present invention, the electrolyte used may also contain LiPO2F2 as an additive, wherein PO2F2 - With Li + With lower binding energy, the addition of LiPO2F2 can reduce the viscosity of the electrolyte, increase the conductivity of the electrolyte, and reduce the interfacial impedance of the battery.

[0041] The electrolyte in the low-temperature resistant lithium iron phosphate battery of the present invention still has a relatively high ionic conductivity at low temperatures, for example, the conductivity at a temperature of -20°C can reach even above about 4.0 mS / cm, thereby improving the low-temperature performance of the lithium iron phosphate battery.

[0042] In the low-temperature resistant lithium iron phosphate battery of the present invention, the use of an appropriate amount of large-particle lithium iron phosphate material in the positive electrode material and the electrolyte can further enhance the wetting of the electrolyte with the positive electrode material, while providing more ion transmission channels, thereby improving the low-temperature cycle performance and discharge performance of the battery.

[0043] In summary, the low-temperature-resistant lithium iron phosphate battery of the present invention can still work normally even at an ambient temperature below -40°C, and its discharge capacity retention rate at -40°C reaches more than about 65%, even more than about 70%, or even more than about 75%; the capacity retention rate after 100 cycles at -20°C reaches more than about 72%, even more than about 76%, or even more than about 80%. DETAILED DESCRIPTION

[0044] In order to make the purpose, technical scheme and advantages of the present invention clearer, the technical scheme of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. It should be understood by those skilled in the art that the embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without paying creative work belong to the scope of protection of the present invention. The process parameters of the following embodiments that do not specify specific conditions are usually based on normal conditions.

[0045] The endpoints and any values ​​of the scope disclosed in the present invention are not limited to the precise scope or value, and these scopes or values ​​should be understood to include values ​​close to these scopes or values. The term "about" used in the present invention means that the modified numeral can fluctuate within the range of ±20%, ±15%, ±10%, ±5% or ±2% of the numeral. For numerical ranges, the endpoint values ​​of each scope, the endpoint values ​​of each scope and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in the present invention.

[0046] According to the present invention, a low-temperature resistant lithium iron phosphate battery is provided. The low-temperature resistant lithium iron phosphate battery according to the present invention comprises a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator. The following will describe in detail the various components of the lithium iron phosphate battery.

[0047] Positive electrode:

[0048] The positive electrode plate in the low-temperature resistant lithium iron phosphate battery of the present invention comprises a lithium iron phosphate positive electrode material, which comprises a small-particle lithium iron phosphate material and a large-particle lithium iron phosphate material in a mass ratio of about (7-9):(3-1), for example, about 7:3, about 8:2 or about 9:1.

[0049] Small particle lithium iron phosphate material:

[0050] Here, on the one hand, the small particle lithium iron phosphate material has the following particle size distribution:

[0051] D10 is in the range of about 0.15 to about 0.6 μm, such as about 0.2 μm, about 0.3 μm, about 0.4 μm, or about 0.5 μm;

[0052] D50 is in the range of about 0.9 to about 2.5 μm, such as about 1.2 μm, about 1.3 μm, about 1.8 μm, about 2.1 μm, or about 2.3 μm;

[0053] D90 is in the range of about 2.0 to about 7.5 μm, such as about 3.0 μm, about 4.0 μm, about 5.0 μm, about 6.0 μm, or about 7.0 μm.

[0054] Here, the small-particle lithium iron phosphate material preferably has an irregular morphology. It can be understood by those skilled in the art that "irregular morphology" refers to a morphology that does not conform to traditional geometric shapes and has unique and irregular characteristics. It does not have an obviously regular shape, appearance or structure, but has an irregular outline, boundary or feature. In this case, the particle size distribution of the small-particle lithium iron phosphate material refers to an equivalent particle size distribution.

[0055] Large particle lithium iron phosphate material:

[0056] On the other hand, the large particle lithium iron phosphate material has the following particle size distribution:

[0057] D10 is in the range of about 2.0 to about 3.5 μm, such as about 2.3 μm, about 2.6 μm, about 2.9 μm, or about 3.2 μm;

[0058] D50 is in the range of about 8.0 to about 12.0 μm, such as about 9.0 μm, about 10.0 μm, or about 11.0 μm;

[0059] D90 is in the range of about 16.0 to about 24.1 μm, such as about 18.0 μm, 20.0 μm, or about 22.0 μm.

[0060] Here, the large-particle lithium iron phosphate material preferably has a roughly regular spherical shape. It should be understood here that the roughly regular spherical shape does not only include a perfect spherical shape, but also includes an ellipsoid or an oblate spheroid, or also includes a combined shape in which a part of another protruding sphere, ellipsoid or oblate spheroid is attached to the sphere, ellipsoid or oblate spheroid. In this case, the particle size distribution of the large-particle lithium iron phosphate material refers to an equivalent particle size distribution.

[0061] In addition, the positive electrode may also include any other materials commonly used in the art, such as current collectors, binders and / or conductive agents. For example, the positive electrode may include aluminum foil as a current collector; polyvinylidene fluoride (such as PVDF with a molecular weight in the range of about 300,000 to about 600,000 Daltons) as a binder; and carbon black and / or carbon nanotubes (such as conductive carbon black SP and carbon nanotubes in a mass ratio of about 1:1) as a conductive agent.

[0062] Lithium salt in electrolyte:

[0063] In another aspect, the electrolyte comprises a lithium salt organic solution, the lithium salt comprises lithium hexafluorophosphate and another lithium salt, and the other lithium salt comprises one or both of lithium tetrafluoroborate and lithium bis(oxalatoborate).

[0064] In one aspect herein, when the lithium salt comprises lithium tetrafluoroborate, the molar amount of lithium tetrafluoroborate is about 10% to about 30%, such as about 15%, about 20% or about 25%, of the molar amount of lithium hexafluorophosphate.

[0065] On the other hand, when the lithium salt comprises the lithium bis(oxalatoborate), the molar amount of the lithium bis(oxalatoborate) is about 5% to about 15%, for example about 7.5%, about 10% or about 12.5%, of the molar amount of the lithium hexafluorophosphate.

[0066] For example, in some preferred embodiments, the lithium salt may comprise lithium hexafluorophosphate and lithium tetrafluoroborate in a molar ratio of about 1.0:(0.1-0.3), such as about 1.0:0.2; or lithium hexafluorophosphate and lithium bis(oxalatoborate) in a molar ratio of about 1.0:(0.05-0.15), such as about 1.0:0.1; particularly preferably, lithium hexafluorophosphate, lithium tetrafluoroborate and lithium bis(oxalatoborate) in a molar ratio of about 1.0:(0.1-0.3):(0.05-0.15), such as about 1.0:0.2:0.1.

[0067] In the electrolyte, the total concentration of the lithium salt organic solution is in the range of about 1.2 to about 1.4 mol / L, for example, about 1.25 mol / L, about 1.3 mol / L, or about 1.35 mol / L.

[0068] Additives in the electrolyte:

[0069] The electrolyte may also contain additives commonly used in the art. In some preferred embodiments of the present invention, the additive may be selected from one or more of vinylene carbonate (VC), fluoroethylene carbonate (FEC), vinyl sulfate (DTD) and lithium difluorophosphate (LiPO2F2) commonly used in the art.

[0070] For example, in some preferred embodiments, the electrolyte may contain about 0.9 to about 1.1 wt % VC; and / or about 2.9 to about 3.1 wt % FEC; and / or about 1.9 to about 2.1 wt % DTD; and / or 0 to about 1.0 wt % LiPO2F2, based on the total weight of the electrolyte.

[0071] Solvents in the electrolyte:

[0072] The solvent contained in the electrolyte is not particularly limited, and a commonly used solvent in the art can be used. For example, in some preferred embodiments of the present invention, the solvent contained in the lithium salt organic solution can be selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC) and ethyl acetate (EA).

[0073] For example, in some further preferred embodiments, the electrolyte may contain about 26 to about 30 volume % EC; and / or about 10 to about 14 volume % PC; and / or about 22 to about 26 volume % EMC; and / or about 18 to about 22 volume % DMC; and / or about 14 to about 18 volume % EA, based on the total volume of the solvent used in the electrolyte.

[0074] Diaphragm:

[0075] In the low temperature resistant lithium iron phosphate battery of the present invention, the separator used is not particularly limited, and the separator commonly used in the art can be used. However, in some preferred embodiments, the separator can be a high porosity battery separator, in particular a 12 μm polypropylene high porosity battery separator.

[0076] Negative electrode:

[0077] In the low temperature resistant lithium iron phosphate battery of the present invention, the negative electrode plate used comprises a composition consisting of an aqueous binder, a conductive agent and a negative electrode active material located on a negative electrode current collector. In a further preferred embodiment, the negative electrode current collector can be selected from copper foil; the aqueous binder can be selected from carboxymethyl cellulose; the conductive agent can be selected from conductive carbon black; and the negative electrode active material can be selected from graphite.

[0078] The present invention is further described in detail below with reference to specific embodiments and comparative examples.

[0079] The chemical materials used in the examples were all commercially available analytically pure products.

[0080] Example 1

[0081] Preparation of positive electrode:

[0082] Polyvinylidene fluoride (PVDF) having a molecular weight range of about 30 to about 600,000 Daltons is mixed with N-methylpyrrolidone (NMP) as a solvent to prepare a positive electrode glue having a solid content of about 6% by weight. A composite conductive agent containing conductive carbon black (SP) and carbon nanotubes (CNT) (mass ratio of about 1:1) is added to the obtained positive electrode glue and dispersed at high speed to obtain a positive electrode conductive glue. About 97% by weight of lithium iron phosphate material based on the weight of the positive electrode conductive glue is added to the obtained positive electrode conductive glue, and dispersed at high speed to obtain a positive electrode slurry having a viscosity in the range of about 6000 to about 8000 mPa·s. The lithium iron phosphate material comprises a small-particle lithium iron phosphate material and a large-particle lithium iron phosphate material, wherein the small-particle lithium iron phosphate material has an irregular shape, and the large-particle lithium iron phosphate material has a roughly spherical shape, and the weight ratio of the two is about 8:2. The particle size distribution of the small-particle lithium iron phosphate material is: D10 = 0.4-0.6 μm, D50 = 1.2-1.4 μm, D90 = 5.4-5.6 μm, and the particle size distribution of the large-particle lithium iron phosphate material is: D10 = 2.5-2.7 μm, D50 = 9.4-9.6 μm, D90 = 23.9-24.1 μm. The obtained positive electrode slurry is evenly coated on both surfaces of a carbon-coated aluminum foil with a thickness of about 13 μm, and after rolling, die-cutting and drying, a double-sided surface density of about 358 g / m 2 The positive electrode.

[0083] Preparation of negative electrode sheet:

[0084] Carboxymethyl cellulose (CMC) as an aqueous binder is added to deionized water to prepare a negative electrode glue with a solid content of about 1.8% by weight. About 1.0% by weight of a conductive agent containing conductive carbon black (SP) based on the weight of the negative electrode glue is added to the obtained negative electrode glue and dispersed at high speed to obtain a negative electrode conductive glue. About 96.2% by weight of graphite as a negative electrode active material based on the weight of the negative electrode conductive glue is added to the obtained negative electrode conductive glue, and dispersed at high speed to obtain a negative electrode slurry with a viscosity ranging from about 4000 to about 6000 mPa·s. The obtained negative electrode slurry is evenly coated on both surfaces of a copper foil with a thickness of about 6 μm, and a double-sided surface density of about 168 g / m is obtained after rolling, die-cutting, and drying. 2The negative electrode.

[0085] Preparation of lithium-ion batteries

[0086] The positive and negative electrode sheets and the separator are stacked, assembled, baked, injected, formed, and divided into different volumes in the order of separator-negative electrode-separator-positive electrode-separator to form a 2.9Ah low-temperature resistant lithium iron phosphate soft pack battery, wherein the separator is a 12μm polypropylene high-porosity battery separator. The injection coefficient adopted is about 3.6g / Ah. The lithium salt concentration in the electrolyte used is about 1.3mol / L, and the lithium salt contains LiPF6, LiBF4 and LiBOB in a weight ratio of about 1.0:0.2:0.1. The solvent of the electrolyte includes ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC) and ethyl acetate (EA), wherein the volume fractions of EC, PC, EMC, DMC and EA in the solvent are about 28% by volume, about 12% by volume, about 24% by volume, about 20% by volume and about 16% by volume, respectively. The electrolyte also contains additives, wherein the additives used include vinylene carbonate (VC), fluoroethylene carbonate (FEC), vinyl sulfate (DTD) and lithium difluorophosphate (LiPO2F2). The additives are added in the following proportions: VC accounts for about 1% by weight of the electrolyte, FEC accounts for about 3.0% by weight of the electrolyte, DTD accounts for about 2.0% by weight of the electrolyte, and LiPO2F2 accounts for about 1.0% by weight of the electrolyte.

[0087] Example 2

[0088] A lithium ion battery was prepared in a similar process to Example 1, except that the ratio of small particles to large particles in the lithium iron phosphate material was set to about 9:1.

[0089] Example 3

[0090] A lithium ion battery was prepared in a similar process to Example 1, except that the ratio of small particles to large particles in the lithium iron phosphate material was set to about 7:3.

[0091] Example 4

[0092] A lithium ion battery was prepared in a similar process to Example 1, except that the electrolyte contained only LiPF6 and LiBOB as the lithium salt in a weight ratio of about 1.0:0.1.

[0093] Example 5

[0094] A lithium ion battery was prepared in a similar process to Example 1, except that the electrolyte contained only LiPF6 and LiBF4 as the lithium salt in a weight ratio of about 1.0:0.2.

[0095] Example 6

[0096] A lithium ion battery was prepared in a similar process to Example 1, except that LiPO 2 F 2 was not added as an additive to the electrolyte.

[0097] Comparative Example 1

[0098] A lithium-ion battery was prepared in a similar process to that of Example 1, except that the lithium iron phosphate contained only the small-particle lithium iron phosphate material but not the large-particle lithium iron phosphate material.

[0099] Comparative Example 2

[0100] A lithium ion battery was prepared in a similar process to Example 1, except that only LiPF6 was included in the electrolyte as the lithium salt.

[0101] The materials used in the above Examples 1 to 6 and Comparative Examples 1 to 2 are summarized in Table 1 below.

[0102] Table 1:

[0103]

[0104] The electrical properties of the lithium-ion batteries prepared by the above Examples 1 to 6 and Comparative Examples 1 to 2 were tested according to the provisions of the Chinese national standard GB / T31484-2015 "Requirements and Test Methods for Cycle Life of Power Batteries for Electric Vehicles". The specific test conditions include: charging at a constant current of about 1C at a temperature of about 25°C±2°C, switching to constant voltage charging after the battery voltage reaches about 3.65V, stopping charging after the current drops to about 0.05C, and standing for about 1 hour after charging.

[0105] The test results are shown in Table 2 below:

[0106] Table 2

[0107]

[0108] From the performance data in Table 2, we can see that:

[0109] Among Examples 1 to 6, the -20°C conductivity of the electrolyte of the lithium ion battery of Example 1 is the largest (about 4.14 mS / cm), and the battery's -20°C cycle 100 cycle capacity retention rate (about 83.6%) and 3C discharge retention rate are both the largest (about 88.9%).

[0110] The viscosity of the electrolyte of the lithium ion batteries of Examples 1 to 6 and Comparative Example 1 is between about 1.28 and about 1.49 mPa·s, which is much lower than the viscosity of the electrolyte of the lithium ion battery of Comparative Example 2 (about 1.65 mPa·s). This to a certain extent indicates that the electrolyte containing specific components (LiPF6 and LiBF4 and / or LiBOB in a specific weight ratio) in the lithium ion battery of the present invention can have better ionic conductivity than the electrolyte of Comparative Example 2 (containing only LiPF6).

[0111] From the comparison between Example 4 and Comparative Example 2, it can be seen that when a certain amount of LiBOB is contained in the electrolyte (Example 4), the -20°C conductivity of the electrolyte is improved (from about 2.15 mS / cm to about 3.64 mS / cm), the low temperature performance (-40°C discharge capacity retention rate) of the battery is improved (from about 45.4% to about 65.3%), and the cycle performance (-20°C cycle 100 cycles capacity retention rate) is improved (from about 61.2% to about 72.1%).

[0112] From the comparison between Example 5 and Comparative Example 2, it can be seen that although the -20°C conductivity of the lithium ion battery of Example 5 is the lowest among all the examples and comparative examples (only about 1.69 mS / cm), the -20°C cycle 100-cycle capacity retention rate of the battery of Example 5 is improved compared with that of Comparative Example 2 (from about 61.2% to about 75.3%), indicating that adding LiBF4 to the electrolyte can help improve the low-temperature performance of the battery.

[0113] From the comparison between Example 1 and Example 6, it can be seen that by adding LiPO2F2 to the electrolyte, the -20°C conductivity of the electrolyte will increase (from about 3.85mS / cm to about 4.14mS / cm), and the low temperature performance of the battery (-40°C discharge capacity retention rate) will also increase (from about 70.2% to about 79.2%), indicating that the addition of LiPO2F2 can also appropriately improve the low temperature performance of the battery.

[0114] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions claimed to be protected by the present invention.

Claims

1. A lithium iron phosphate battery, comprising a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator, characterized in that: The positive electrode plate comprises a lithium iron phosphate positive electrode material, wherein the lithium iron phosphate positive electrode material comprises a small-particle lithium iron phosphate material and a large-particle lithium iron phosphate material in a mass ratio of (7-9): (3-1), wherein The particle size distribution of the small-particle lithium iron phosphate material is: D10=0.15 to 0.6 μm, D50 = 0.9 to 2.5 μm, D90 = 2.0 to 7.5 μm; and The particle size distribution of the large-particle lithium iron phosphate material is: D10=2.0 to 3.5 μm, D50=8.0 to 12.0 μm, D90=16.0 to 24.1 μm; The electrolyte comprises a lithium salt organic solution with a concentration in the range of 1.2 to 1.4 mol / L, the lithium salt comprises lithium hexafluorophosphate and another lithium salt, and the other lithium salt comprises one or both of lithium tetrafluoroborate and lithium bis(oxalatoborate).

2. The lithium iron phosphate battery according to claim 1, characterized in that: The lithium salt comprises: Lithium hexafluorophosphate, lithium tetrafluoroborate and lithium bis(oxalato)borate in a molar ratio of 1.0:(0.1-0.3):(0.05-0.15); or Lithium hexafluorophosphate and lithium tetrafluoroborate in a molar ratio of 1.0:(0.1-0.3); or The molar ratio of lithium hexafluorophosphate and lithium bis(oxalatoborate) is 1.0:(0.05-0.15).

3. The lithium iron phosphate battery according to claim 1, characterized in that: The electrolyte further comprises one or more of vinylene carbonate, fluoroethylene carbonate, vinyl sulfate and lithium difluorophosphate as additives.

4. The lithium iron phosphate battery according to claim 3, characterized in that: The electrolyte contains as an additive, based on the total weight of the electrolyte: 0.9 to 1.1 wt % vinylene carbonate; and / or 2.9 to 3.1 wt. % fluoroethylene carbonate; and / or 1.9 to 2.1 wt% vinyl sulfate; and / or 0 to 1.0 wt% lithium difluorophosphate.

5. The lithium iron phosphate battery according to claim 1, characterized in that: The lithium salt organic solution contains one or more of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, dimethyl carbonate and ethyl acetate as a solvent.

6. The lithium iron phosphate battery according to claim 5, characterized in that: The solvent of the lithium salt organic solution comprises, based on the total volume of the solvent: 26 to 30% by volume of ethylene carbonate; and / or 10 to 14% by volume of propylene carbonate; and / or 22 to 26% by volume of ethyl methyl carbonate; and / or 18 to 22% by volume of dimethyl carbonate; and / or 14 to 18% by volume of ethyl acetate.

7. The lithium iron phosphate battery according to any one of claims 1 to 6, characterized in that: The separator is a high-porosity battery separator.

8. The lithium iron phosphate battery according to claim 7, characterized in that: The high porosity battery separator is a 12 μm polypropylene high porosity battery separator.

9. The lithium iron phosphate battery according to any one of claims 1 to 6, characterized in that: The negative electrode plate comprises an aqueous binder, a conductive agent and a negative electrode active material.

10. The lithium iron phosphate battery according to claim 9, characterized in that: The negative electrode sheet includes carboxymethyl cellulose as an aqueous binder, conductive carbon black as a conductive agent, and graphite as a negative electrode active material.

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