Lithium ion secondary battery

Abstract

The present application relates to the technical field of battery, in particular to a lithium ion secondary battery. The lithium ion secondary battery comprises an electrolyte and a negative electrode sheet. The electrolyte comprises methyl trifluoropropionate and a first additive. The content of methyl trifluoropropionate is A, and the content of the first additive is B, based on the total weight of the electrolyte, 10%≤A+B≤35%. The first additive comprises fluoroethylene carbonate and 1,3-propane sultone. The content ratio of fluoroethylene carbonate to 1,3-propane sultone is 1-20. The negative electrode sheet comprises a negative electrode active material. The negative electrode active material comprises a silicon-based material. The average particle size of the silicon-based material is D, and D is 6-20 μm. The lithium ion secondary battery can relieve volume expansion, and has good high-temperature floating and low-temperature discharging performance.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of battery, in particular to a lithium ion secondary battery. BACKGROUND

[0002] With the rapid development of new energy automobile, portable electronic equipment and other markets, the requirement of battery performance is increasing, and silicon-based material is the ideal substitute of traditional carbon-based material (such as graphite) due to its ultra-high theoretical specific capacity (4200mAh / g), and is considered as the first choice of next-generation lithium ion battery negative electrode material; however, silicon-based negative electrode faces the problems of significant volume expansion (volume expansion rate can reach 300%) and low conductivity during charging and discharging, which leads to poor high and low temperature performance, therefore, how to improve the conductivity of silicon-based negative electrode lithium ion secondary battery and relieve volume expansion is crucial to improve the high and low temperature performance of lithium ion secondary battery. SUMMARY

[0003] The present application aims to solve the battery performance deterioration caused by cycle expansion and low conductivity of lithium ion secondary battery (hereinafter referred to as battery) including silicon-based negative electrode, especially to improve the high and low temperature performance of the battery, and proposes a lithium ion secondary battery, by designing the composition of electrolyte, adjusting the sum of the content of methyl trifluoropropionate and the first additive in the electrolyte, and the content ratio of fluoroethylene carbonate and 1,3-propane sulfone lactone in the first additive, enhancing the compactness and stability of SEI film, and constructing stable SEI film; on the other hand, by adjusting the average particle size of silicon-based material, it has higher dispersibility and specific surface area, which helps to increase the contact area of silicon-based material and electrolyte, maintains the structural stability during volume expansion, and improves the high and low temperature performance of the battery.

[0004] In order to solve the above problems, the present application proposes the following technical scheme:

[0005] The lithium ion secondary battery includes electrolyte and negative electrode sheet, the electrolyte includes methyl trifluoropropionate and first additive, the content of methyl trifluoropropionate is A and the content of first additive is B based on the total weight of electrolyte, 10%≤A+B≤35%; the first additive includes fluoroethylene carbonate and 1,3-propane sulfone lactone; the content ratio of fluoroethylene carbonate and 1,3-propane sulfone lactone is 1-20 based on the total weight of electrolyte; the negative electrode sheet includes negative electrode active material, the negative electrode active material includes silicon-based material, and the average particle size of silicon-based material is D, D is 6-20μm.

[0006] Compared with the prior art, the present application has at least the following advantages:

[0007] (1) In the present application, by adjusting the average particle size of the silicon-based material within a suitable range, on the one hand, the cycle expansion of the silicon-based negative electrode can be alleviated, and on the other hand, the infiltration and contact of the electrolyte can be improved, and the ion transmission efficiency can be improved.

[0008] (2) In the present application, the addition of methyl trifluoropropionate and the first additive in the electrolyte can improve the compactness and generation efficiency of the SEI film, improve the oxidation resistance of the electrolyte, and reduce the occurrence of side reactions; and the content relationship between fluoroethylene carbonate and 1,3-propane sulfone lactone in the first additive is regulated, the influence of the addition of fluoroethylene carbonate on high temperature performance is alleviated, and the battery has good high and low temperature performance at the same time.

[0009] (3) The lithium ion secondary battery provided by the present application has good high-temperature floating performance and low-temperature discharge performance.

[0010] The endpoints of the ranges and any values disclosed herein are not limited to the precise values recited as the exact dimensions are not critical to the present application. Any numerical value, however, can be expressed as a range by either adding or subtracting a small percentage (e.g., 1-10%) from the stated value to account for variations, measurement inaccuracies, and the like. For numerical ranges expressed in the format "from X to Y," it is intended that embodiments "X," "Y," and any numerical values therein can be combined with one another to achieve a new numerical range within the scope of this application. In other words, any numerical value implicit in the ranges stated herein is considered part of the disclosure. DETAILED DESCRIPTION

[0011] The specific embodiments of the present application are described in detail below. It should be understood that the specific embodiments described herein are merely illustrative and are not intended to limit the present application.

[0012] A lithium ion secondary battery includes an electrolyte and a negative electrode sheet, the electrolyte includes methyl trifluoropropionate and a first additive, the content of the methyl trifluoropropionate is A, the content of the first additive is B, based on the total weight of the electrolyte, 10%≤A+B≤35%, for example, 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28%, 30%, 32%, 33%, 34% or 35%.

[0013] In an embodiment, 14%≤A+B≤24%.

[0014] In the present application, the first additive includes fluoroethylene carbonate (FEC) and 1,3-propane sulfone lactone (PS).

[0015] In the present application, the electrolyte comprises methyl trifluoropropionate and a first additive. The first additive comprises FEC and PS. FEC can effectively build a SEI film rich in LiF and structurally stable, can effectively alleviate the volume expansion of the silicon-based material, and maintain the stable cycle of the negative electrode sheet. However, the structural stability of FEC is poor at high temperature, and when FEC is added in excess, the fluorine element in FEC dissociates rapidly to generate HF, which reacts with the carbonate in the SEI film and the CEI film to generate CO2, damaging the interface film and exacerbating the decomposition of the electrolyte at the interface; in addition, HF also exacerbates the decomposition of LiPF6 and the solvent, resulting in gas production and high-temperature performance degradation. PS has good chemical stability at high temperature and can remain relatively stable at high temperature and high voltage. The use of FEC and PS together can enhance the stability and density of the SEI film, effectively alleviate the problems of battery gas production and high-temperature performance degradation caused by the poor structural stability of FEC at high temperature, and effectively slow down the dissociation rate of FEC, reduce side reactions, and improve gas production when the temperature rises. Therefore, the use of the first additive is beneficial to improving the volume expansion of the silicon-based material and relatively stabilizing the high-temperature performance of the battery.

[0016] However, adding excessive FEC and PS in the electrolyte will affect the viscosity and ion migration speed of the electrolyte at low temperature. Methyl trifluoropropionate has a low freezing point and can improve the flowability of the electrolyte at low temperature, which is beneficial to the improvement of lithium ion migration speed, thereby improving the performance of the battery at low temperature. In addition, methyl trifluoropropionate can also form a SEI film at the negative electrode end. Therefore, the first additive can complement methyl trifluoropropionate to improve the density and stability of the SEI film and significantly improve the high-temperature floating performance and low-temperature discharge performance of the battery. However, it is not enough to only add methyl trifluoropropionate and the first additive to the electrolyte, and the contents of the two also need to be limited. When the sum of the contents of the two is A+B<10%, the content of methyl trifluoropropionate and / or the first additive in the electrolyte is too small, the SEI film formed at the negative electrode end is insufficient and incomplete, which is not conducive to alleviating the side reactions of the battery during charging and discharging, and is insufficient to improve the high and low temperature performance of the battery; when A+B>35%, the addition of methyl trifluoropropionate and / or the first additive in the electrolyte is excessive, which leads to increased battery gas, increased interface film thickness, and increased impedance, thereby losing active lithium and deteriorating the cycle performance.

[0017] The above technical solution can improve the high and low temperature performance of the battery and the volume expansion during charging and discharging cycle to some extent, but the improvement effect is limited. The present inventors have further found that by further adjusting the content ratio of FEC and PS and the average particle size of the silicon-based material, the volume expansion of the battery can be effectively alleviated, and the battery can have excellent high and low temperature performance. The reason is that:

[0018] First, as described above, the structural stability of FEC is poor at high temperature, while PS has good chemical stability at high temperature. If the content ratio of the two is not limited, the improvement of the high-temperature performance of the battery is limited. When the content ratio of FEC to PS is less than 1, the content of FEC is too low relative to PS. At this time, the SEI film formed by the negative electrode has poor structural stability, which will cause the repeated rupture-generation of the SEI film, consume a large amount of lithium ions, and exacerbate the side reaction between the negative material and the electrolyte, which is not conducive to the improvement of the cycle stability of the battery. When the content ratio of FEC to PS is greater than 20, the content of PS is too low relative to FEC, and PS is not enough to alleviate the gas production of FEC at high temperature, so that the high-temperature performance of the battery is poor.

[0019] Second, by adjusting the average particle size of the silicon-based material, the electrolyte can be combined to alleviate the volume expansion of the silicon-based material during lithiation and improve the lithium ion transmission rate. If the average particle size of the silicon-based material is too large, the transmission rate of lithium ions will be reduced, which is not conducive to the low-temperature performance of the battery. If the average particle size of the silicon-based material is too small, the contact area between the silicon-based material and the electrolyte will increase, which is not conducive to the formation and stability of the SEI (solid electrolyte interface) film, and thus affects the high and low temperature performance of the battery.

[0020] In summary, by synergistically controlling the average particle size of the silicon-based material and the composition of the electrolyte, the battery can have low volume expansion and good high and low temperature performance.

[0021] In the present application, the content ratio of fluoroethylene carbonate to 1,3-propane sultone is 1-20, for example, 1, 2, 3, 4, 5, 6, 8, 10, 12, 14, 16, 17, 18, 19 or 20, based on the total weight of the electrolyte.

[0022] In the present application, the negative electrode sheet comprises a negative electrode active material, and the negative electrode active material comprises a silicon-based material, wherein the average particle size of the silicon-based material is D, and D is 6 μm-20 μm, for example, 6 μm, 6.2 μm, 6.4 μm, 6.6 μm, 6.8 μm, 7 μm, 7.5 μm, 8 μm, 8.5 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm or 20 μm.

[0023] In an embodiment, D is 8 μm-15 μm.

[0024] In the present application, the average particle size of the silicon-based material can be obtained by a conventional test method in the art, for example, by a scanning electron microscope combined with image analysis software, specifically as follows: first, a high-resolution image is obtained by a scanning electron microscope, and then the particle size measurement and distribution statistics are performed by an electron microscope image analysis software such as ImageJ, to obtain the average particle size, and the model of the scanning electron microscope is Sigma300 / 500.

[0025] In the present application, 5%≤B≤30%, for example, 5%, 5.2%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, 10.5%, 11%, 11.5%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 29.2%, 29.4%, 29.6%, 29.8%, or 30%.

[0026] In the present application, 0%<A≤10%, for example, 0.1%, 0.2%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.2%, 9.4%, 9.6%, 9.8%, or 10%.

[0027] In an embodiment, 1≤A≤5%.

[0028] In the present application, 5%≤B≤30%, for example, 5%, 5.2%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, 10.5%, 11%, 11.5%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 29.2%, 29.4%, 29.6%, 29.8%, or 30%.

[0029] In an embodiment, 12%≤B≤23%.

[0030] In the present application, the content of the FEC is 5%-20% based on the total weight of the electrolyte, for example, 5%, 5.2%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, 10.5%, 11%, 11.5%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, or 20%.

[0031] In the present application, the content of PS is 1%-5%, for example, 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5% or 5%, based on the total weight of the electrolyte.

[0032] In the present application, the content of methyl trifluoropropionate, FEC and PS in the electrolyte can be obtained by conventional test methods in the art, for example, by gas chromatography or gas chromatography-mass spectrometry test.

[0033] In the present application, 0.85≤(A+B) / D≤3.75, for example, 0.85, 0.9, 0.95, 1, 1.2, 1.4, 1.6, 1.8, 2, 2.2, 2.4, 2.6, 2.8, 3, 3.2, 3.4, 3.6 or 3.75.

[0034] In an embodiment, 1.3≤(A+B) / D≤2.9.

[0035] In the present application, when the electrolyte is combined with a silicon-based negative electrode, there is a problem of cycle swelling of the silicon-based negative electrode, and further regulating the relationship between the sum of the content of methyl trifluoropropionate and the first additive in the electrolyte and the average particle size of the silicon-based material in the negative electrode sheet can match the composition of the electrolyte with the silicon-based negative electrode. By limiting (A+B) / D to satisfy the above range, a high-efficiency synergy between the fluorinated ethylene carbonate, the first additive and the silicon-based material in the electrolyte can be generated, on the one hand, a uniform and complete interface film is generated on the surface of the silicon-based negative electrode, protecting the silicon-based negative electrode from exacerbating side reactions, thereby relieving the negative electrode swelling; on the other hand, the matching of the electrolyte and the silicon-based material can be improved, and by optimizing the composition of the electrolyte, the adverse effects caused by the particle size effect of the silicon-based material can be alleviated to a certain extent, so that the negative electrode cycle swelling is relieved, and the high-temperature floating and low-temperature discharge performance of the battery is improved.

[0036] In some embodiments, the first additive further comprises ethylene sulfite (DTD). The content of DTD is 0%-2%, for example, 0%, 0.1%, 0.2%, 0.3%, 0.4%, 0.6%, 0.8%, 1%, 1.2%, 1.4%, 1.6%, 1.8% or 2%, based on the total weight of the electrolyte.

[0037] In the present application, DTD can optimize the interface and reduce impedance, so that the electrolyte has good oxidation resistance, reduces the side reactions of the electrolyte, and improves the high and low temperature performance.

[0038] In the present application, the content of DTD in the electrolyte can be obtained by conventional test methods in the art, for example, by gas chromatography or gas chromatography-mass spectrometry test.

[0039] In the present application, the electrolyte can further comprise a second additive, which comprises vinylene carbonate (VC) and / or vinyl ethylene carbonate (VEC).

[0040] In the present application, the content of VC is 0%-2%, for example 0%, 0.1%, 0.2%, 0.3%, 0.4%, 0.6%, 0.8%, 1%, 1.2%, 1.4%, 1.6%, 1.8% or 2%, based on the total weight of the electrolyte.

[0041] In the present application, the content of VEC is 0%-2%, for example 0%, 0.1%, 0.2%, 0.3%, 0.4%, 0.6%, 0.8%, 1%, 1.2%, 1.4%, 1.6%, 1.8% or 2%, based on the total weight of the electrolyte.

[0042] In the present application, the content of VC and VEC in the electrolyte can be obtained by conventional test methods in the art, for example by gas chromatography or gas chromatography-mass spectrometry.

[0043] In the present application, the electrolyte comprises a carbonate solvent, which comprises vinyl carbonate and / or at least one of the following fluorine-substituted or unsubstituted solvents: propylene carbonate, dimethyl carbonate, diethyl carbonate and methyl ethyl carbonate.

[0044] In the present application, the content of the carbonate solvent is C, 30%≤C≤60%, for example 30%, 31%, 32%, 33%, 35%, 37%, 39%, 42%, 45%, 48%, 50%, 52%, 55%, 56%, 57%, 58%, 59% or 60%, based on the total weight of the electrolyte.

[0045] In the present application, the carbonate solvent has a high oxidation potential, which can effectively improve the oxidation resistance of the electrolyte, thereby reducing the side reactions of the electrolyte. When the carbonate solvent with high oxidation resistance is used in combination with methyl trifluoropropionate in the electrolyte, the generation of side reactions in the high-temperature float charging and overcharge tests of the electrolyte can be effectively reduced, the oxidative decomposition of transition metal ions to the electrolyte is reduced, and the high-temperature performance and overcharge safety performance of the battery are further improved.

[0046] In the present application, the content of the carbonate solvent in the electrolyte can be obtained by conventional test methods in the art, for example by gas chromatography or gas chromatography-mass spectrometry.

[0047] In the present application, the median particle size Dv50 of the silicon-based material is 6-20 μm, for example 6 μm, 6.2 μm, 6.4 μm, 6.6 μm, 6.8 μm, 7 μm, 7.5 μm, 8 μm, 8.5 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm or 20 μm.

[0048] In the present application, the silicon-based material has a uniform particle size distribution, which helps to form a stable and thin SEI film and maintain the stability of the battery during the cycle process; if the particle size distribution is uneven, it may cause the storage capacity of some areas to be too high and other areas to be too low, thereby affecting the overall performance of the battery.

[0049] In the present application, the negative electrode active material further comprises a carbon-based material, the particle size Dv'10 of the carbon-based material is 4-11 μm, for example 4 μm, 4.2 μm, 4.4 μm, 4.6 μm, 4.8 μm, 5 μm, 5.2 μm, 5.4 μm, 5.6 μm, 5.8 μm, 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, 8.5 μm, 9 μm, 9.5 μm, 10 μm, 10.5 μm or 11 μm; the median particle size Dv'50 is 8-25 μm, for example 8 μm, 8.2 μm, 8.4 μm, 8.6 μm, 8.8 μm, 9 μm, 9.2 μm, 9.4 μm, 9.6 μm, 9.8 μm, 10 μm, 10.5 μm, 11 μm, 11.5 μm, 12 μm, 12.5 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm or 25 μm.

[0050] In the present application, the particle size and particle size distribution of the carbon-based material help to improve the stability and reactivity of the negative electrode active material, as well as the cycle and rate performance of the battery. When the particle size of the carbon-based material is too large, the adhesion between the carbon-based material and the negative electrode current collector is not strong enough, and loose structures and pores are easily produced, resulting in a decrease in the strength of the carbon-based material and insufficient compactness; in addition, the conductive path between the large particle size carbon-based materials is not smooth, leading to a decrease in conductivity, which in turn causes a decrease in the performance of the negative electrode active material and affects the performance of the battery, such as the rate performance; when the particle size of the carbon-based material is too small, it is more prone to side reactions with the electrolyte and other media, affecting the stability and service life of the negative electrode active material; at the same time, the small particle size carbon-based material is more prone to produce micro-cracks and pulverization during the charge and discharge process, leading to the shedding of the negative electrode active material and capacity decay.

[0051] In the present application, the median particle size Dv50 of the silicon-based material, the particle size Dv'10 and the median particle size Dv'50 of the carbon-based material can be obtained by using a conventional test method in the art, for example, by laser diffraction measurement using a laser particle size analyzer, equipment model Mastersize 3000.

[0052] In the present application, the lithium ion secondary battery comprises a positive electrode sheet, and the positive electrode sheet comprises a positive electrode active material, and the positive electrode active material comprises at least one of Ni element, Co element and Mn element.

[0053] In the present application, the positive electrode active material comprises a substance with a chemical formula of Li a Ni b Co x Mn y A z O2, 0.9≤a<1.1 (for example, 0.9, 0.92, 0.94, 0.96, 0.98, 0.1, 0.15, 0.2, 0.25, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.02, 1.05, 1.07 or 1.09), 0≤b<1 (for example, 0, 0.02, 0.04, 0.06, 0.08, 0.1, 0.15, 0.2, 0.3, 0.4, 0.52, 0.6, 0.7, 0.8, 0.9, 0.92, 0.94, 0.96, 0.98 or 0.99), 0

[0054] In the present application, the content of the Ni element is 50%-100% based on the total weight of the Ni element, Co element, Mn element and A, for example, 50%, 52%, 54%, 56%, 58%, 60%, 62%, 64%, 66%, 68%, 70%, 75%, 80%, 85%, 90%, 92%, 94%, 96%, 98% or 100%.

[0055] In the present application, the positive active material is matched with the electrolyte containing the first additive, which can form a CEI film at the positive electrode end and effectively reduce the dissolution of transition metal ions at the positive electrode end. Since the electrolyte also has high oxidation resistance, the occurrence of electrolyte side reactions is alleviated, the gassing is inhibited, and the high-temperature cycle and high-temperature storage performance of the battery is improved.

[0056] In the present application, the chemical composition of the positive active material can be measured by conventional methods in the art, for example, by the following method: 0.1 g of sample is added to 10 mL of hydrochloric acid for 350℃ electric heating plate digestion for 10 min, cooled and diluted to 100 ml, then diluted 10 times, and then part of the solution is analyzed by ICP optical spectrum analyzer. The corresponding test parameters obtained are used to calculate the chemical composition of the positive active material.

[0057] In the present application, the silicon-based material includes a silicon-carbon material.

[0058] In the present application, the specific surface area of the silicon-carbon material is 0.5m 2 / g-10m 2 / g, for example, 0.5m 2 / g, 0.6m 2 / g, 0.8m 2 / g, 1m 2 / g, 1.2m 2 / g, 1.4m 2 / g, 1.6m 2 / g, 1.8m 2 / g, 2m 2 / g, 2.4m 2 / g, 2.8m 2 / g, 3.2m 2 / g, 3.6m 2 / g, 4m 2 / g, 5m 2 / g, 6m 2 / g, 7m 2 / g, 8m 2 / g, 9m 2 / g, or 10m 2 / g.

[0059] In the present application, the specific surface area of the silicon-carbon material can adjust the contact between the negative active material and the electrolyte and the reaction activity of the negative active material. A larger specific surface area is beneficial to increase the contact area between the negative active material and the electrolyte, form more active sites to participate in the reaction, and thus improve the electrochemical activity of the battery. However, too large specific surface area will lead to the formation of more SEI film, thereby consuming more electrolyte and active lithium, and reducing the first coulombic efficiency.

[0060] In the present application, the specific surface area of the silicon-carbon material can be determined by a conventional test method in the art, for example, by using the Brunauer-Emmett-Teller (BET) test method, for example, using a Tri Star II specific surface analyzer, with N2 as the adsorption gas.

[0061] In the present application, the average sphericity of the silicon-carbon material is 0.5-1, for example, 0.5, 0.52, 0.54, 0.56, 0.58, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 0.96, 0.98 or 1.

[0062] In the present application, regulating the average sphericity of the silicon-carbon material is beneficial to improving the cycle stability and rate performance of the battery; the silicon-carbon material with high average sphericity can more fully utilize the negative active material during the charging and discharging process, thereby improving the cycle stability and rate performance of the battery; at the same time, it has better wettability in the electrolyte, can more effectively contact and react with the electrolyte; in addition, the silicon-carbon material with high average sphericity can also uniformly disperse the mechanical stress during the particle volume expansion process, so that the particles are not easy to break, thereby improving the cycle retention rate of the battery.

[0063] In the present application, the average sphericity can be obtained by analyzing the images of each particle in the SEM photos of the negative active material at a certain magnification (such as 2500 times) using image processing software (such as Image Pro Plus), obtaining the perimeter and area of each particle, respectively calculating the perimeter equivalent radius r1 and area equivalent radius r2 of each particle, then the average sphericity S of each particle = r2 / r1, and then the average sphericity of each particle is quantitatively averaged, that is, the average sphericity of the silicon-carbon material is obtained.

[0064] In the present application, the oil absorption value of the silicon-carbon material is 10 mL / 100 g-100 mL / 100 g, for example, 10 mL / 100 g, 12 mL / 100 g, 30 mL / 100 g, 14 mL / 100 g, 16 mL / 100 g, 18 mL / 100 g, 20 mL / 100 g, 40 mL / 100 g, 50 mL / 100 g, 60 mL / 100 g, 70 mL / 100 g, 80 mL / 100 g, 90 mL / 100 g or 100 mL / 100 g.

[0065] In the present application, the oil absorption value of the silicon-carbon material is related to the liquid retention of the battery. The increase of the oil absorption value improves the liquid retention of the battery. With the consumption of the electrolyte and the occurrence of the side reaction during the cycle of the battery, when the electrolyte is reduced, the lithium ion loses the channel for migration between the positive and negative active materials, which causes the problems such as lithium precipitation, volume expansion, and gas production of the battery, thereby affecting the high and low temperature performance of the battery. The regulation of the oil absorption value within a certain range can make the SEI film more complete and uniform, and improve the structural stability of the silicon-carbon particles in the negative active material.

[0066] In the present application, the oil absorption value of the silicon-carbon material can be measured by the conventional test method in the art, for example, by the following method: the negative active material layer of the negative electrode sheet is scraped off, the adhesive is washed off, and the negative active material powder is obtained after drying (vacuum drying box 100℃, 12h). The mass of the clean beaker and glass rod m1 is measured, 5g of the negative active material powder is added, and the total mass m2 is recorded. The dioctyl phthalate (DOP) is added dropwise with a titration bottle, and is fully stirred. When the mass forms a lump, the addition of DOP is stopped. The total weight of the beaker at this time m3 is measured, and the oil absorption value = (m3-m2) / (m2-m1) x 100. The oil absorption value of the silicon-carbon particles is obtained by weighted average.

[0067] In the present application, the carbon-based material includes at least one of artificial graphite, natural graphite, mesocarbon microbeads, hard carbon, and soft carbon.

[0068] In the present application, the silicon-based material can further include at least one of nanosilicon (Si) and silicon-oxygen material (SiO x (0 < x < 2).

[0069] In the present application, the electrolyte further includes a carboxylic acid ester solvent, and the carboxylic acid ester solvent includes at least one of fluorine-substituted or unsubstituted propyl acetate, n-butyl acetate, isobutyl acetate, n-pentyl acetate, isoamyl acetate, ethyl propionate, n-propyl propionate, methyl butyrate, ethyl butyrate, and n-ethyl butyrate.

[0070] In the present application, the electrolyte further includes a lithium salt, and the lithium salt includes at least one of lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethylsulfonyl)imide (FSI), lithium hexafluoroantimonate, lithium hexafluoroarsenate, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(pentafluoroethylsulfonyl)imide, lithium tris(trifluoromethylsulfonyl)methide, and lithium bis(trifluoromethylsulfonyl)imide.

[0071] In the present application, the electrolyte includes a third additive, and the third additive includes at least one of vinyl sulfate, lithium difluorophosphate (LiPO2F2), lithium difluoro oxalate borate (LiDFOB), lithium difluoro bis-oxalate phosphate, lithium tetrafluoroborate, lithium bis-oxalate borate, butanedinitrile (SN), pentanedinitrile, hexanedinitrile (AND), heptanedinitrile, octanedinitrile, sunflower dinitrile, 1,3,6-hexanetricarboxylic acid, glycerol tricarboxylic acid, 1,2-bis(2-cyanoethoxy)ethane, and propenyl-1,3-sultone.

[0072] In the present application, the positive electrode sheet further includes a positive electrode current collector, a positive electrode conductive agent, and a positive electrode binder, and are all routine selections of those skilled in the art, for example, the positive electrode current collector includes an aluminum foil, the positive electrode binder includes at least one of polyvinylidene fluoride (PVDF), polyacrylate, polyimide, and styrene butadiene rubber, and the positive electrode conductive agent includes at least one of acetylene black, conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes, and graphene.

[0073] In the present application, the negative electrode sheet further includes a negative electrode current collector, a negative electrode conductive agent, and a negative electrode binder, and are all routine selections of those skilled in the art, for example, the negative electrode current collector includes a copper foil, the negative electrode binder includes at least one of polyacrylic acid (PAA), sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), sodium carboxymethyl cellulose, polyimide, styrene butadiene rubber (SBR), and polyvinylidene fluoride, and the negative electrode conductive agent includes at least one of acetylene black, conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes, and graphene.

[0074] In the present application, the lithium ion secondary battery further includes a separator, and the separator is a routine selection in the art, for example, a polyethylene separator.

[0075] In the present application, the lithium ion secondary battery further includes a positive electrode sheet, and the positive electrode sheet includes a positive electrode current collector, a positive electrode conductive agent, and a positive electrode binder, and are all routine selections of those skilled in the art, for example, the positive electrode current collector includes an aluminum foil, the positive electrode binder includes at least one of polyvinylidene fluoride (PVDF), polyacrylate, polyimide, and styrene butadiene rubber, and the positive electrode conductive agent includes at least one of acetylene black, conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes, and graphene.

[0076] In the present application, the negative electrode sheet further comprises a negative electrode current collector, a negative electrode conductive agent and a negative electrode binder, and are all conventional choices for those skilled in the art, for example, the negative electrode current collector comprises a copper foil, the negative electrode binder comprises at least one of polyacrylic acid (PAA), sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), sodium carboxymethyl cellulose, polyimide, styrene butadiene rubber (SBR) and polyvinylidene fluoride, and the negative electrode conductive agent comprises at least one of acetylene black, conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes and graphene.

[0077] In the present application, the upper limit voltage of the lithium ion secondary battery is 4.2 V and above, and the term upper limit voltage has the conventional meaning in the art, generally referring to the maximum voltage value that the battery can safely reach during charging. Continuing to charge beyond the upper limit voltage can cause the battery to overcharge, thereby shortening the battery life and even possibly causing a safety risk.

[0078] It should be noted that the "first", "second" and the like numerical representation in the present application are only used to distinguish different substances or usage, and do not represent the difference in order.

[0079] The present application will be described in detail below by way of examples. The examples described in the present application are only a part of the examples of the present application, but not all the examples. Based on the examples in the present application, all other examples obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0080] In the following examples, the materials used are all commercially available analytical pure without special instructions.

[0081] The following examples are used to illustrate the lithium ion secondary battery of the present application.

[0082] Example 1:

[0083] Preparation of electrolyte: in a glove box (H2O < 0.01 ppm, O2 < 0.01 ppm, Ar atmosphere), carbonate solvent (recorded content X, X = 40%) and propyl propionate (PP) were mixed to form an organic solvent, wherein the carbonate solvent was mixed by ethylene carbonate (EC), propylene carbonate (PC) and diethyl carbonate (DEC) in a ratio of 3:3:14; based on the total weight of the electrolyte, 3% of methyl trifluoropropionate (recorded content A, A = 3%) and 19% of the first additive (recorded content B, B = 19%) were added to the organic solvent, stirred uniformly to form a mixed solvent, wherein the first additive was composed of FEC (content recorded as c1, c1 = 15%), PS (content recorded as c2, c2 = 3%), and DTD (content recorded as c3, c3 = 1%); based on the total weight of the electrolyte, 3% of the third additive and 22% of the lithium salt were added to the obtained electrolyte, wherein the third additive was composed of lithium difluorophosphate: lithium difluoro oxalate borate = 1:2, and the lithium salt was composed of 15% of lithium hexafluorophosphate (LiPF6) and 7% of lithium bis-trifluoromethanesulfonimide (FSI), and after passing the water and free acid detection, the electrolyte was obtained, wherein the weight content of the organic solvent, methyl trifluoropropionate, the first additive, the lithium salt and the third additive added up to 100%.

[0084] Wherein, A + B = 22%, the ratio of the contents of FEC and PS c1 / c2 = 5.

[0085] Preparation of positive electrode sheet:

[0086] The positive electrode active material (811 NCM), conductive carbon black and polyvinylidene fluoride were mixed in a mass ratio of 97.6:1.35:1.05, uniformly placed in N-methyl pyrrolidone (NMP), and stirred uniformly to prepare a positive electrode slurry; the positive electrode slurry was uniformly coated on both sides of the aluminum foil; and after drying and rolling, a positive electrode sheet was obtained.

[0087] Preparation of negative electrode sheet:

[0088] The artificial graphite (median particle size Dv'50 of 11.3 μm), silicon-carbon material (average particle size D of 10.5 μm, median particle size Dv50 of 10.5 μm), conductive carbon black, butadiene rubber and sodium carboxymethyl cellulose were mixed in a mass ratio of 78.5:20.5:0.5:0.25:0.25, deionized water was added, and the mixture was stirred to form a uniform negative electrode slurry; the negative electrode slurry was coated on a copper foil, dried, rolled, cut and cleaned to obtain a negative electrode sheet.

[0089] At this time, (A + B) / D is 2.095.

[0090] Assembly of lithium ion secondary battery:

[0091] The prepared positive electrode sheet, negative electrode sheet and separator (polyethylene film) are placed in sequence with the separator between the positive electrode sheet and the negative electrode sheet, then the tab is welded and the core is wound to obtain a core, then the core is placed in an aluminum plastic film packaging bag, then the injection, formation, two-sealing and sorting processes are carried out to prepare a lithium ion secondary battery, and finally the electrical performance of the battery is tested.

[0092] Example 2:

[0093] Preparation of electrolyte: In a glove box (H2O <0.01 ppm, O2 <0.01 ppm, Ar atmosphere), carbonate solvents (recorded content as X, X = 42%) and propyl propionate (PP) were mixed to form an organic solvent, wherein the carbonate solvents were mixed by ethylene carbonate (EC), propylene carbonate (PC) and diethyl carbonate (DEC) in a ratio of 3:3:14; based on the total weight of the electrolyte, 2% of methyl trifluoropropionate (recorded as A) and 12% of the first additive (recorded as B) were added to the organic solvent and stirred uniformly to form a mixed solvent, wherein the first additive was composed of FEC (content recorded as c1, c1 = 10%), PS (content recorded as c2, c2 = 1%) and DTD (content recorded as c3, c3 = 1%); based on the total weight of the electrolyte, 3% of the third additive and 22% of the lithium salt were added to the obtained electrolyte, wherein the third additive was composed of lithium difluorophosphate: lithium difluoro oxalate borate = 1:2, and the lithium salt was composed of 15% of lithium hexafluorophosphate (LiPF6) and 7% of lithium bis-trifluoromethanesulfonimide (FSI), and stirred uniformly. After passing the moisture and free acid detection, the electrolyte was obtained, wherein the weight content of the organic solvent, methyl trifluoropropionate, the first additive, the lithium salt and the third additive added up to 100%.

[0094] Wherein, A+B = 14%, the ratio of the contents of FEC and PS c1 / c2 = 10.

[0095] Preparation of positive electrode sheet:

[0096] The positive electrode active material (811 NCM), conductive carbon black and polyvinylidene fluoride were mixed in a mass ratio of 97.6:1.35:1.05, uniformly placed in N-methyl pyrrolidone (NMP), and stirred uniformly to prepare a positive electrode slurry; the positive electrode slurry was uniformly coated on both sides of the aluminum foil; and then dried and rolled to obtain a positive electrode sheet.

[0097] Preparation of negative electrode sheet:

[0098] The artificial graphite (median particle size Dv'50 of 12.6 μm), silicon-carbon material (average particle size D of 8 μm, median particle size Dv50 of 8 μm), conductive carbon black, butadiene-styrene rubber and sodium carboxymethyl cellulose are mixed uniformly in a mass ratio of 78.5:20.5:0.5:0.25:0.25, deionized water is added, and the mixture is stirred thoroughly to form a uniform negative electrode slurry; the negative electrode slurry is coated on a copper foil, dried, rolled, cut, and cleaned to obtain a negative electrode sheet.

[0099] At this time, (A+B) / D is 1.75.

[0100] Assembly of the lithium ion secondary battery:

[0101] The prepared positive electrode sheet, negative electrode sheet and separator (polyethylene film) are placed in order, with the separator between the positive electrode sheet and the negative electrode sheet, then the tab is welded and the roll core is obtained by winding, then the roll core is placed in an aluminum plastic film packaging bag, then the liquid injection, formation, two-sealing and sorting processes are carried out, and the lithium ion secondary battery is prepared, finally the electrical performance of the battery is tested.

[0102] Example 3:

[0103] Preparation of the electrolyte: In a glove box (H2O < 0.01 ppm, O2 < 0.01 ppm, Ar atmosphere), carbonate solvents (content X, X = 50%) and propyl propionate (PP) are mixed to form an organic solvent, wherein the carbonate solvents are mixed in a ratio of 3:3:14 by volume of ethylene carbonate (EC), propylene carbonate (PC) and diethyl carbonate (DEC); based on the total weight of the electrolyte, 1% of methyl trifluoropropionate (content A, A = 1%) and 23% of the first additive (content B, B = 23%) are added to the organic solvent, and the mixture is stirred uniformly to form a mixed solvent, wherein the first additive is composed of FEC (content c1, c1 = 18%), PS (content c2, c2 = 4%) and DTD (content c3, c3 = 1%); based on the total weight of the electrolyte, 3% of the third additive and 15% of the lithium salt are added to the obtained electrolyte, wherein the third additive is composed of lithium difluorophosphate: lithium difluoro oxalate borate = 1:2, and the lithium salt is lithium hexafluorophosphate (LiPF6), and the mixture is stirred uniformly. After passing the water content and free acid detection, the electrolyte is obtained, wherein the weight content of the organic solvent, methyl trifluoropropionate, the first additive, the lithium salt and the third additive adds up to 100%.

[0104] Wherein, A+B = 24%, the ratio of the contents of FEC and PS c1 / c2 = 4.5.

[0105] Preparation of the positive electrode sheet:

[0106] The positive electrode active material (811NCM), conductive carbon black and polyvinylidene fluoride were mixed uniformly in a mass ratio of 97.6:1.35:1.05, placed in N-methyl pyrrolidone (NMP), stirred uniformly, and a positive electrode slurry was prepared; the positive electrode slurry was uniformly coated on both sides of an aluminum foil; and the positive electrode sheet was obtained by drying and rolling in sequence.

[0107] Preparation of the negative electrode sheet:

[0108] The artificial graphite (median particle size Dv'50 of 9.2 μm), silicon-carbon material (average particle size D of 8.3 μm, median particle size Dv50 of 8.3 μm), conductive carbon black, butadiene-styrene rubber and sodium carboxymethyl cellulose were mixed uniformly in a mass ratio of 78.5:20.5:0.5:0.25:0.25, deionized water was added, and the mixture was stirred thoroughly to form a uniform negative electrode slurry; the negative electrode slurry was coated on a copper foil, dried, rolled, cut, and cleaned to obtain a negative electrode sheet.

[0109] At this time, (A+B) / D is 2.892.

[0110] Assembly of the lithium ion secondary battery:

[0111] The prepared positive electrode sheet, negative electrode sheet and separator (polyethylene film) were placed in sequence with the separator between the positive electrode sheet and the negative electrode sheet, then the tab was welded and the roll core was obtained by winding, then the roll core was placed in an aluminum plastic film packaging bag, then the processes of liquid injection, formation, second sealing, and sorting were carried out, and the lithium ion secondary battery was prepared, finally the electrical performance of the battery was tested.

[0112] Example 4:

[0113] Preparation of electrolyte: in a glove box (H2O < 0.01 ppm, O2 < 0.01 ppm, Ar atmosphere), carbonate solvent (recorded content X, X = 53%) and propyl propionate (PP) were mixed to form an organic solvent, wherein the carbonate solvent was mixed by ethylene carbonate (EC), propylene carbonate (PC) and diethyl carbonate (DEC) in a ratio of 3:3:14; based on the total weight of the electrolyte, 5% of methyl trifluoropropionate (recorded content A, A = 5%) and 14.5% of the first additive (recorded content B, B = 14.5%) were added to the organic solvent, and stirred uniformly to form a mixed solvent, wherein the first additive was composed of FEC (content recorded as c1, c1 = 12%), PS (content recorded as c2, c2 = 2%), and DTD (content recorded as c3, c3 = 0.5%); based on the total weight of the electrolyte, 3% of the third additive and 15% of lithium salt were added to the obtained electrolyte, wherein the third additive was composed of lithium difluorophosphate: lithium difluoro oxalate borate = 1:1, and the lithium salt was lithium hexafluorophosphate (LiPF6), and stirred uniformly. After passing the water content and free acid detection, the electrolyte was obtained, wherein the weight content of the organic solvent, methyl trifluoropropionate, the first additive, lithium salt and the third additive added up to 100%.

[0114] Wherein, A+B = 19.5%, the ratio of the contents of FEC and PS c1 / c2 = 6.

[0115] Preparation of positive electrode sheet:

[0116] The positive electrode active material (811 NCM), conductive carbon black and polyvinylidene fluoride were mixed in a mass ratio of 97.6:1.35:1.05, uniformly placed in N-methyl pyrrolidone (NMP), and stirred uniformly to prepare a positive electrode slurry; the positive electrode slurry was uniformly coated on the front and back surfaces of an aluminum foil; and the aluminum foil was sequentially subjected to drying and rolling treatment to obtain a positive electrode sheet.

[0117] Preparation of negative electrode sheet:

[0118] The artificial graphite (median particle size Dv'50 of 10.5 μm), silicon-carbon material (average particle size D of 15 μm, median particle size Dv50 of 15 μm), conductive carbon black, butadiene rubber and sodium carboxymethyl cellulose were mixed in a mass ratio of 78.5:20.5:0.5:0.25:0.25, deionized water was added, and the mixture was stirred to form a uniform negative electrode slurry; the negative electrode slurry was coated on a copper foil, and the copper foil was subjected to drying, rolling, cutting and cleaning to obtain a negative electrode sheet.

[0119] At this time, (A+B) / D is 1.3.

[0120] Assembly of lithium ion secondary battery:

[0121] The prepared positive electrode sheet, negative electrode sheet and separator (polyethylene film) are placed in order, with the separator in the middle of the positive electrode sheet and negative electrode sheet, then the tab is welded and the core is wound, then the core is placed in an aluminum plastic film packaging bag, then the injection, formation, two-sealing and sorting processes are carried out, and a lithium ion secondary battery is prepared. Finally, the electrical performance of the battery is tested.

[0122] Example 5 group:

[0123] This group of examples is used to verify the influence brought by the change of "D", which is achieved by adjusting the average particle size of the silicon-carbon material, as follows:

[0124] Example 5a, based on example 1, the difference is that D = 6 μm, the median particle size Dv50 of this silicon-based material is 6 μm, and (A+B) / D is 3.667.

[0125] Example 5b, based on example 1, the difference is that D = 20 μm, the median particle size Dv50 of this silicon-based material is 20 μm, and (A+B) / D is 1.1.

[0126] Example 6 group:

[0127] This group of examples is used to verify the influence brought by the change of "A+B", which is achieved by changing the content of methyl trifluoropropionate and the first additive in the electrolyte, as follows:

[0128] Example 6a, based on example 3, the difference is that A = 5%, B = 23%, at this time, A+B = 28%, and (A+B) / D is 3.373.

[0129] Example 6b, based on example 2, the difference is that A = 1%, B = 12%, at this time, A+B = 13%, and (A+B) / D is 1.625.

[0130] Example 7:

[0131] This example is used to verify the influence brought by the change of "c1 / c2", based on example 1, the difference is that c1 = 18%, c2 = 1%, at this time, c1 / c2 = 18, B = 20%, A+B = 23%, and (A+B) / D is 2.19.

[0132] Example 8 group:

[0133] This group of examples is used to verify the influence brought by the change of "(A+B) / D", as follows:

[0134] Example 8a, based on example 3, the difference is that D = 8 μm, A = 5%, at this time, A+B = 28%, and (A+B) / D is 3.5.

[0135] Example 8b, based on example 2, differs in that D = 15 pm, A = 1 %, in which case A+B = 13 %, (A+B) / D is 0.867.

[0136] Example 9 group:

[0137] This group of examples serves to verify the influence of the variation of "content X of polycarbonate solvent", as follows:

[0138] Example 9a, based on example 1, differs in that the content X of polycarbonate solvent is 30 % based on the total weight of the electrolyte.

[0139] Example 9b, based on example 1, differs in that the content X of polycarbonate solvent is 60 % based on the total weight of the electrolyte, the lithium salt being 15 % of lithium hexafluorophosphate (LiPF6).

[0140] Example 10 group:

[0141] This group of examples serves to verify the influence of the variation of "median particle size Dv'50 of carbon-based material", as follows:

[0142] Example 10a, based on example 1, differs in that the median particle size Dv'50 of carbon-based material is 8 pm.

[0143] Example 10b, based on example 1, differs in that the median particle size Dv'50 of carbon-based material is 25 pm.

[0144] Example 11:

[0145] This example serves to verify the influence of the variation of "A", based on example 1, differing in that A = 10 %, in which case A+B = 29 %, (A+B) / D is 2.762.

[0146] Example 12 group:

[0147] This group of examples serves to verify the influence of the variation of "second additive", as follows:

[0148] Example 12a, based on example 1, differs in that the second additive consists of FEC (content noted cl, cl = 14.5 %), PS (content noted c2, c2 = 3 %), DTD (content noted c3, c3 = 1 %), the second additive consisting of VC (vinylene carbonate, content noted c4, c4 = 0.5 %).

[0149] Example 12b, based on Example 1, the difference is that the first additive is composed of FEC (content is denoted as c1, c1 = 14.5%), PS (content is denoted as c2, c2 = 3%), DTD (content is denoted as c3, c3 = 1%), and the second additive is composed of VEC (ethylene carbonate, content is denoted as c5, c5 = 0.5%).

[0150] In the above examples, the particle size Dv'10 of the carbon-based material is in the range of 4 μm-11 μm, the specific surface area of the silicon-carbon material is in the range of 0.5 m 2 / g-10 m 2 / g, the average sphericity of the silicon-carbon material is in the range of 0.5-1, and the oil absorption value of the silicon-carbon material is in the range of 10 mL / 100 g-100 mL / 100 g.

[0151] Comparative Example 1 group:

[0152] This group of comparative examples is used to verify the influence brought by the change of "D", which is as follows:

[0153] Comparative Example 1a, based on Example 1, the difference is that D = 24.5 μm, the median particle size Dv50 of the silicon-based material is 24.5 μm, and (A+B) / D is 0.898.

[0154] Comparative Example 1b, based on Example 1, the difference is that D = 2.8 μm, the median particle size Dv50 of the silicon-based material is 2.8 μm, and (A+B) / D is 7.857.

[0155] Comparative Example 2 group:

[0156] This group of comparative examples is used to verify the influence brought by the change of "A+B", which is as follows:

[0157] Comparative Example 2a, based on Example 1, the difference is that A = 1% and B = 8% based on the total weight of the electrolyte, wherein c1 = 6.5%, c2 = 1%, and c3 = 0.5%, at this time A+B = 9%, and (A+B) / D is 0.45.

[0158] Comparative Example 2b, based on Example 1, the difference is that A = 10% and B = 27% based on the total weight of the electrolyte, and the lithium salt is 15% lithium hexafluorophosphate (LiPF6), wherein c1 = 20%, c2 = 5%, and c3 = 2%, at this time A+B = 37%, and (A+B) / D is 6.167.

[0159] Comparative Example 3 group:

[0160] This group of comparative examples is used to verify the influence brought by the change of "c1 / c2", which is as follows:

[0161] Comparative Example 3a, based on Example 1, differs in that c1 = 25%, c2 = 1%, c1 / c2 = 25 at this time, B = 27%, A + B = 30%, (A + B) / D is 2.857.

[0162] Comparative Example 3b, based on Example 1, differs in that c1 = 5%, c2 = 6.5%, c1 / c2 = 0.77 at this time, B = 12.5%, A + B = 15.5%, (A + B) / D is 1.476.

[0163] Comparative Example 4:

[0164] Based on Example 1, the difference is that methyl trifluoropropionate is not added in the electrolyte, that is, A = 0%, A + B = 19% at this time, (A + B) / D is 1.81.

[0165] Test Example:

[0166] (1) 45℃ floating test:

[0167] The battery obtained by the inventive example and the comparative example is placed in a 25℃ constant temperature environment, and the electric quantity of the battery is adjusted to 50% SOC. The initial thickness T1 of the battery is tested when the battery is received. The battery is placed in a 45℃ constant temperature environment, and the battery is charged at 0.5C constant current and constant voltage to the upper limit voltage 4.3V constant voltage for 7 days as one cycle. The change in the thermal state thickness expansion of the battery is monitored, and the battery is obviously inflated as the limiting condition. The final thermal state thickness T2 of the battery in the 45℃ floating test is tested. The thickness expansion rate of the lithium battery in the 45℃ floating test = (T2-T1) / T1x100%.

[0168] (2) -30℃ low temperature discharge test:

[0169] The battery obtained by the inventive example and the comparative example is placed in a 25℃ constant temperature environment, and the battery is discharged at 0.2C to 3.0V. The battery is charged at 0.7C constant current and constant voltage to the upper limit voltage 4.3V, and the cutoff current is 0.05C. After the battery is fully charged, it is placed for 5min, and then discharged at 0.5C constant current to the cutoff voltage 3.0V. The 0.5C discharge capacity of the battery at 25℃ is recorded as the initial capacity Q1. In a 25℃ constant temperature environment, the battery is charged at 0.7C constant current and constant voltage to 4.45V, and the cutoff current is 0.05C. The battery is fully charged, and then the fully charged battery is placed in a -30℃ environment for 4H. When the surface temperature of the battery reaches the ambient temperature, it is discharged at 0.5C to 3.0V. The 0.5C discharge capacity of the battery at -30℃ is recorded as Q2, and the -30℃ low temperature discharge capacity retention rate of the battery is calculated as Q2 / Q1x100%.

[0170] (3) 0.25C-10V overcharge test:

[0171] The discharged battery is charged at a constant current of 0.25C to 10V at ambient temperature (25±5)℃, and then charged at a constant voltage, and the charging is stopped when the charging time is limited to 6H.

[0172] Criteria: the battery does not catch fire or explode, which means that the test is passed.

[0173] The performance of the batteries obtained in the examples and the comparative examples is recorded in Table 1.

[0174]

[0175] As shown in Table 1, the lithium ion secondary battery prepared in the present application has good high-temperature floating, low-temperature discharging and overcharging safety compared with the comparative examples.

[0176] The preferred embodiments of the present application are described in detail above, but the present application is not limited thereto. Within the technical concept of the present application, various simple modifications can be made to the technical solutions of the present application, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be considered as disclosed in the present application and fall within the protection scope of the present application.

Claims

1. A lithium-ion secondary battery, characterized by comprising: The lithium ion secondary battery comprises an electrolyte and a negative electrode sheet, the electrolyte comprises methyl trifluoropropionate and a first additive, the content of the methyl trifluoropropionate is A, and the content of the first additive is B, based on the total weight of the electrolyte, 10%≤A+B≤35%; 0%<A≤10%, and, 5%≤B≤30%; The first additive comprises fluoroethylene carbonate and 1,3-propane sultone; the content ratio of the fluoroethylene carbonate to the 1,3-propane sultone is 1-20; The negative electrode sheet comprises a negative electrode active material, the negative electrode active material comprises a silicon-based material, and the average particle size of the silicon-based material is D, D is 6 μm-20 μm; 0.85≤(A+B) / D≤3.

75.

2. The lithium-ion secondary battery according to claim 1, wherein The first additive further comprises ethylene sulfate, and the content of the ethylene sulfate is 0%-2%; And / or, 14%≤A+B≤24%; And / or, the content ratio of the fluoroethylene carbonate to the 1,3-propane sultone is 4.5-10; And / or, D is 8 μm-15 μm.

3. The lithium-ion secondary battery according to claim 1, wherein 1%≤A≤5%； And / or, 12%≤B≤23%; And / or, 1.3≤(A+B) / D≤2.

9.

4. The lithium-ion secondary battery according to claim 1, wherein The content of the fluoroethylene carbonate is 5%-20%, based on the total weight of the electrolyte; And / or, the content of the 1,3-propane sultone is 1%-5%.

5. The lithium-ion secondary battery according to claim 1, wherein The electrolyte comprises a second additive, and the second additive comprises vinylene carbonate and / or vinyl ethylene carbonate; And / or, the electrolyte comprises a carbonate solvent, and the carbonate solvent comprises vinyl carbonate and / or at least one of the following solvents which are substituted or unsubstituted by fluorine: propylene carbonate, dimethyl carbonate, diethyl carbonate and methyl ethyl carbonate.

6. The lithium-ion secondary battery according to claim 5, wherein The content of the carbonate solvent is C, 30%≤C≤60%, based on the total weight of the electrolyte.

7. The lithium-ion secondary battery according to claim 1, wherein The negative electrode active material further comprises a carbon-based material, the particle size Dv'10 of the carbon-based material is 4 μm-11 μm, and the median particle size Dv'50 is 8 μm-25 μm.

8. The lithium-ion secondary battery according to claim 1, wherein The lithium ion secondary battery comprises a positive electrode sheet, and the positive electrode sheet comprises a positive electrode active material, the positive electrode active material comprises at least one of Ni element, Co element and Mn element.

9. The lithium-ion secondary battery according to claim 8, wherein The positive active material includes a compound of the general formula Li a Ni b Co x Mn y A z O2, 0.9≤a<1.1, 0≤b<1, 0 10. The lithium-ion secondary battery according to claim 9, wherein The content of the Ni element is 50%-100%, based on the total weight of the Ni element, the Co element, the Mn element and the A.

11. The lithium-ion secondary battery according to claim 1, wherein The silicon-based material comprises a silicon-carbon material.

12. The lithium-ion secondary battery according to claim 11, wherein The specific surface area of the silicon-carbon material is 0.5 m 2 / g-10 m 2 / g; And / or, the average sphericity of the silicon-carbon material is 0.5-1; And / or, the oil absorption value of the silicon-carbon material is 10 mL / 100 g-100 mL / 100 g.

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