Lithium ion battery and electric appliance using the same

Abstract

The application provides a lithium ion battery, which comprises a positive electrode sheet and an electrolyte, and the positive electrode material of the positive electrode sheet comprises a core and a surface layer on the surface of the core; the core comprises a main material with a chemical general formula of Li a MO b , M is at least one of transition metal ions, the valence state of M ions is +X; the surface layer contains A ions, the core contains B ions, A and B are at least one of IA-IVA main group or sub-group elements; the electrolyte comprises BF3 and Li q P r O s F t . Compared with the prior art, the positive electrode material adopted by the application can effectively improve the interface stability of the positive electrode material, and the impedance of the negative electrode SEI film is reduced through the improvement of the electrolyte, so that the positive and negative electrodes further cooperatively improve the low-temperature discharge capacity of the battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of secondary batteries, in particular to a lithium ion battery and an electric appliance using the same. BACKGROUND

[0002] Lithium ion batteries play a crucial role in numerous application fields, especially in mobile communication, electric vehicles, and large-scale energy storage systems. However, with the advancement of technology and the growing market demand, the performance requirements of lithium ion batteries are increasingly high, especially in terms of high-rate charge and discharge performance. When charging at a high rate at room temperature, lithium ion batteries face a series of technical challenges.

[0003] Firstly, the speed of lithium ion migration between electrodes becomes a limiting factor. When the battery is charged at a high rate, the rapid migration of lithium ions leads to a significant increase in the lithium ion concentration gradient between electrodes. Especially on the negative electrode, the limitation of lithium ion intercalation speed leads to the formation of lithium dendrites. Lithium dendrites are a microstructure of metallic lithium, whose growth not only reduces the cycle stability and service life of the battery, but also significantly increases the risk of short circuit and thermal runaway, seriously threatening the safety of the battery.

[0004] Secondly, the volume effect and phase transition problem of electrode materials during lithium deintercalation are also key factors. The positive and negative electrode materials will swell and shrink during charging and discharging, and long-term cycling will cause the material structure to break, affecting the smooth migration of lithium ions. In addition, some positive electrode materials undergo phase transition changes during charging and discharging, which further increases the instability of the material structure. These effects not only affect the energy density and cycle life of the battery, but also can cause the destruction of the solid electrolyte interface (SEI) film. The destruction of the SEI film will cause continuous reaction between the electrode and the electrolyte, which in turn consumes the limited lithium ion resources in the battery, causing electrolyte decomposition. This not only leads to a decrease in battery capacity, but also increases the internal resistance of the battery, further affecting the charge and discharge performance and efficiency of the battery.

[0005] Therefore, it is necessary to provide a technical solution to solve the above problems. SUMMARY

[0006] One of the purposes of the present application is to provide a lithium ion battery to solve the problems of poor positive electrode interface stability, poor SEI film stability, and large negative electrode impedance of the current lithium ion battery in a low temperature environment, in view of the deficiencies of the prior art.

[0007] In order to achieve the above purpose, the present application adopts the following technical solutions:

[0008] A lithium ion battery, comprising a positive electrode sheet and an electrolyte, wherein the positive electrode material of the positive electrode sheet comprises a core and a surface layer located on the surface of the core; wherein the core comprises a chemical formula Lia MO b , a is in the range of 1-3, b is in the range of 1-3, M is at least one of transition metal ions, the valence state of M ion is +X; the surface layer contains A ions, the valence state of the A ions is +Y; the core contains B ions, the valence state of the B ions is +Z; wherein, the A and the B are at least one of IA-IVA main group or sub-group elements; the X, Y, Z satisfy the relationship: 1

[0009] The electrolyte comprises BF3 and Li q P r O s F t , wherein, q+3r-2s-t=0 or q+5r-2s-t=0; the Li q P r O s F t The ratio of the BF3 in the total weight of the electrolyte is E%, and 0.1

[0010] The present application uses Li a MO b as the main material, by controlling the radial distribution of AB ions with different valence states in the positive electrode material, forming a NP structure similar to a semiconductor diode. The surface layer of the positive electrode material contains high-valence A ions, and the inner layer contains low-valence B ions. By taking advantage of the larger repulsive force of A ions and the smaller repulsive force of B ions, the positive electrode material particles form a structure with large lithium ion transmission resistance in the surface layer and small lithium ion transmission resistance in the inner layer. In this way, during charging, lithium ions are released from the inside of the positive electrode material particles to the outside, and need to move from the inner layer with smaller repulsive force to the surface layer with larger repulsive force, which will be subject to corresponding resistance, which can reduce the upper limit of the transmission speed of lithium ions to the outside, thereby reducing the risk of lithium precipitation in the negative electrode. During low-temperature discharge, lithium ions are embedded from the surface of the positive electrode material particles to the inside, and need to move from the surface layer with larger repulsive force to the inner layer with smaller repulsive force, and the transmission speed can be improved, thereby improving the low-temperature discharge capacity of the battery. At the same time, the improved electrolyte contains Li q P r O s F t and BF3, and the compound Li q P r O s F t forms a low-impedance SEI on the surface of the negative electrode, but its dissociation ability is poor, and the amount of low-impedance SEI formed is small. The addition of BF3 can form a complex with it, improve its dissociation ability, further reduce the impedance of the negative electrode, and further improve the low-temperature discharge capacity through the positive and negative electrodes.

[0011] Preferably, the mass content of the M ion is ≥10% based on the mass of the main material.

[0012] Preferably, the weight percentage of the A ion in the surface layer is ≥200 ppm; and / or, an interval extending 1 μm from the surface of the core body to the center thereof is defined as an inner layer region, and the weight percentage of the B ion in the inner layer region is ≥200 ppm. If the content of the A ion in the surface layer is too low, the effect of reducing the lithium ion transmission speed cannot be achieved. Similarly, if the content of the B ion in the core body is too low, the effect of improving the lithium ion transmission speed cannot be achieved. By using the content parameters defined in the present application, the technical effects of simultaneously improving the lithium precipitation of the pole piece and the low-temperature discharge capacity can be achieved.

[0013] On the surface of the positive electrode material, the valence state of the A ion needs to be ≥ the valence state of the M ion to achieve the effect of reducing the lithium ion transmission speed, and the weight percentage of the A ion in the surface layer is ≥200 ppm, which is more obvious, and the content is lower. In the positive electrode, the valence of the B ion needs to be ≤ the valence of the M ion to achieve the effect of promoting the lithium ion transmission speed, and the weight percentage of the B ion in the inner layer region is ≥200 ppm, which is more obvious. When lithium ions are transmitted to the surface, they are inhibited by A ions, and when they are transmitted to the interior, they are promoted by B ions. The existence of the two kinds of ions at the same time can achieve the effect of controlling the speed of lithium ion extraction and embedding, thereby improving the normal temperature large rate charge performance and low temperature discharge capacity of the lithium battery.

[0014] In some embodiments, the surface layer can also appropriately contain B ions, but in order to ensure the effect of the surface layer on inhibiting the lithium ion transmission speed, the content of B ions needs to be controlled. Based on the weight percentage of the A ion in the surface layer being ≥200 ppm, the weight percentage of the B ion in the surface layer needs to be controlled in the range of 0-100 ppm. If it exceeds this range, it will seriously affect the lithium ion transmission inhibition effect of the surface layer. Similarly, the core body can also appropriately contain A ions, but in order to ensure the effect of the core body on improving the lithium ion transmission speed, the content of A ions needs to be controlled. Based on the weight percentage of the B ion in the inner layer region being ≥200 ppm, the weight percentage of the A ion in the inner layer region needs to be controlled in the range of 0-100 ppm. If it exceeds this range, it will seriously affect the lithium ion transmission promotion effect of the core body.

[0015] The preparation method of the positive electrode material comprises the following steps:

[0016] S1, mixing raw materials including a lithium source and an M element precursor, then dispersing in an organic solvent, adding a B element precursor for premixing, and vacuum drying the mixture;

[0017] S2, the mixture in step S1 is divided into two groups a and b, and then each group is sintered to obtain a product and b product;

[0018] S3, the a product and b product prepared in S2 are crushed to obtain a product large particles and b product small particles;

[0019] S4, the large particles and small particles obtained in S3 are mixed according to the mass ratio of (7-9):(1-3), and A element precursor is added and stirred;

[0020] S5, the mixture obtained in S4 is sintered, crushed and cooled to obtain the positive electrode material.

[0021] Preferably, in step S1, the rotation speed of the pre-mixed is 200-400 r / min, and the pre-mixing time is 4-8 h; the temperature of the vacuum drying is 60-100℃, and the drying time is 10-15 h.

[0022] Preferably, in step S2, the temperature of the first sintering is 800-1200℃, the first sintering of the a group mixture adopts two-stage sintering method and the sintering time is 12-14 h, and the first sintering time of the b group mixture is 5.5-7.5 h.

[0023] Preferably, the two-stage sintering method comprises: first, continuously heating at a heating rate of V1℃ / min to D1℃, then sintering at D1℃±50℃ for t1 time, then continuously heating at a heating rate of V2℃ / min to D2℃, then sintering at D2℃±50℃ for t2 time, and finally annealing at a cooling rate of V3℃ / min, wherein V1>V2, V2=V3, D1

[0024] Preferably, in step S4, the stirring speed is 1000-1400 rpm, and the stirring time is 5-8 h.

[0025] Preferably, in step S3, the crushed a product is 5-50 μm large particles, and the crushed b product is 0.5-10 μm small particles.

[0026] Preferably, in step S5, the temperature of the second sintering is 600-800℃, and the sintering time is 8-12 h.

[0027] The second object of the present application is to provide an electric appliance comprising the lithium ion battery.

[0028] Compared with the prior art, the lithium ion battery provided by the application has the advantages that the lithium ion battery provided by the application has remarkable advantages in improving safety performance and charging efficiency by adjusting the composition of the electrolyte and cooperating with the positive plate. The positive material in the positive plate adopts AB ions with different valence states in radial distribution, forms a NP structure similar to a semiconductor diode, and has better structural stability, thermal stability and electrochemical performance. The surface layer of the positive material can effectively reduce the thermal expansion of the positive material caused by temperature change during the process of deintercalating lithium, effectively improve the interface stability of the positive material, and also effectively alleviate the decomposition of the electrolyte and the corrosion of the positive material. The electrolyte used in cooperation contains Li q P r O s F and BF3, the compound Li q P r O s F t forms a complex with BF3, improves the dissociation ability, further reduces the impedance of the negative electrode, and further improves the low-temperature discharge capacity through the cooperation of the positive and negative electrodes. DETAILED DESCRIPTION

[0029] In order to make the technical solutions and advantages of the application clearer, the application and its advantages will be further described in detail below in combination with specific embodiments, but the embodiments of the application are not limited thereto.

[0030] The terms "first", "second" are only used for description purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first", "second" can explicitly or implicitly include one or more of the features. In the description of the application, the meaning of "a plurality of" is two or more, unless otherwise specifically limited.

[0031] The application aims to provide a lithium ion battery, a lithium ion battery comprising a positive plate, a negative plate, a separator and an electrolyte, the positive material of the positive plate comprising a core and a surface layer located on the surface of the core; wherein the core comprises a main material with a chemical formula of Li a MO b , the value range of a is 1-3, the value range of b is 1-3, M is at least one of transition metal ions, the valence state of M ion is +X; the surface layer contains A ions, the valence state of the A ion is +Y; the core contains B ions, the valence state of the B ion is +Z; wherein the A and the B are at least one of IA-IVA main group or sub-group elements; the X, Y, Z satisfy the relationship: 1

[0032] The electrolyte comprises BF3 and Li q P r O s F t Wherein, q+3r-2s-t=0 or q+5r-2s-t=0; the Li q P r O s F t The ratio of the BF3 in the total weight of the electrolyte is E%, and 0.1≤E≤1, and the E and F satisfy the relationship: F≤E≤4F.

[0033] Wherein, the specific value of a can be 1, 1.5, 2, 2.5, 3, and can include but not limited to the above listed values; the specific value of b can be 1, 1.5, 2, 2.5, 3, and can include but not limited to the above listed values.

[0034] The inventor found that the ions of IA~IVA main group or side group elements with different positive valence states have electrostatic repulsion effect on lithium ions, and the higher the valence state is, the greater the electrostatic repulsion is. The outer layer containing high valence state A ions has greater repulsion, while the inner layer containing low valence state B ions has smaller repulsion, and the structure of the positive electrode material particle is formed with large lithium ion transmission resistance in the surface layer and small lithium ion transmission resistance in the inner layer. In this way, during charging, lithium ions are discharged from the inside of the positive electrode material particle to the outside, and need to move from the inner layer with smaller repulsion to the surface layer with greater repulsion, and will be subjected to corresponding resistance, which can reduce the upper limit of the transmission speed of lithium ions to the outside, thereby reducing the risk of lithium precipitation in the negative electrode. During low-temperature discharging, lithium ions are embedded from the surface of the positive electrode material particle to the inside, and need to move from the surface layer with greater repulsion to the inner layer with smaller repulsion, and the transmission speed thereof can be improved, thereby improving the low-temperature discharging capacity of the battery. The compound Li q P r O s F t A low-impedance SEI is formed on the surface of the negative electrode, but its dissociation ability is poor, and the amount of the low-impedance SEI formed is small, and the addition of BF3 can form a complex with it, improve its dissociation ability, further reduce the impedance of the negative electrode, and further improve the low-temperature discharging capacity through the positive and negative electrodes.

[0035] In some embodiments, the preparation method of the positive electrode active material comprises the following steps:

[0036] S1, mixing raw materials including a lithium source and a M element precursor, then dispersing in an organic solvent, adding a B element precursor for premixing, and vacuum drying the mixture;

[0037] S2, the mixture in step S1 is divided into two groups a and b, and then one sintering is carried out respectively to obtain a product and b product;

[0038] S3, the a product and b product prepared in S2 are crushed respectively to obtain a product large particles and b product small particles;

[0039] S4, the large particles and small particles obtained in S3 are mixed according to the mass ratio of (7-9):(1-3), and the A element precursor is added and stirred;

[0040] S5, the mixture obtained in S4 is subjected to secondary sintering, crushing and cooling to obtain the positive electrode material.

[0041] The positive electrode material prepared by the preparation method has better structural stability, thermal stability and electrochemical performance. Among them, the coating material can effectively reduce the thermal expansion caused by temperature rise in the process of deintercalating lithium of the positive electrode material, and effectively reduce the mechanical attenuation of the coating layer, improve the interface stability of the positive electrode material, and also play a good mitigation effect on the decomposition of electrolyte and the corrosion of the positive electrode material, so that the electrochemical performance of the positive electrode material at high voltage is improved.

[0042] In some embodiments, in step S1, the rotation speed of the pre-mixed is 200-400 r / min, which can be 200 r / min, 210 r / min, 220 r / min, 230 r / min, 240 r / min, 250 r / min, 260 r / min, 270 r / min, 280 r / min, 290 r / min, 300 r / min, 310 r / min, 320 r / min, 330 r / min, 340 r / min, 350 r / min, 360 r / min, 370 r / min, 380 r / min, 400 r / min, which can include but not limited to the above listed values; the pre-mixing time is 4-8 h, which can be 4 h, 5 h, 6 h, 7 h, 8 h, which can include but not limited to the above listed values; the temperature of the vacuum drying is 60-100℃, which can be 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, 90℃, 95℃, 100℃, which can include but not limited to the above listed values; the drying time is 10-15 h, which can be 10 h, 11 h, 12 h, 13 h, 14 h, 15 h, which can include but not limited to the above listed values.

[0043] In some embodiments, in step S2, the temperature of the primary sintering is 800-1200℃, specifically 800℃, 850℃, 900℃, 950℃, 1000℃, 1050℃, 1100℃, 1150℃, 1200℃, which can include but is not limited to the above listed values, the primary sintering of the mixture of group a is performed by two-stage sintering method and the sintering time is 12-14h, and the primary sintering time of the mixture of group b is 5.5-7.5h.

[0044] The two-stage sintering method includes: first, continuously heating at a heating rate of V1℃ / min to D1℃, then sintering at D1℃±50℃ for t1 time, then continuously heating at a heating rate of V2℃ / min to D2℃, then sintering at D2℃±50℃ for t2 time, and finally annealing at a cooling rate of V3℃ / min, wherein V1>V2, V2=V3, D1

[0045] In some embodiments, in step S4, the stirring speed of the mixed material is 1000-1400rpm, specifically 1000rpm, 1050rpm, 1100rpm, 1150rpm, 1200rpm, 1250rpm, 1300rpm, 1350rpm, 1400rpm, 1450rpm, 1500rpm, which can include but is not limited to the above listed values; and the stirring time is 5-8h.

[0046] In some embodiments, in step S5, the temperature of the secondary sintering is 600-800℃, specifically 600℃, 650℃, 700℃, 750℃, 800℃, which can include but is not limited to the above listed values; and the sintering time is 8-12h, specifically 8h, 9h, 10h, 11h, 12h, which can include but is not limited to the above listed values.

[0047] In order to make the technical solutions and advantages of the present application clearer, the present application and its beneficial effects will be further described in detail below with specific embodiments, but the embodiments of the present application are not limited thereto.

[0048] Embodiment 1

[0049] The preparation method of the positive electrode material is as follows:

[0050] 1) Primary mixing: Li2CO3, Co3O4 are mixed according to the stoichiometric ratio of Li:Co elements of 1.05:0.993, in order to make up for the loss of lithium under high-temperature sintering, the lithium source is quantified with an excess of 5%, isopropanol is used as a dispersant, and the B element precursor is added. The solid and liquid mixture is premixed in an inclined mixer, the mechanical rotation speed is 300 r / min, and the mixing time is 6 h. After premixing, the above-mentioned materials are vacuum dried at 80°C overnight for 12 h to volatilize the dispersant and moisture.

[0051] 2) Primary sintering: The premixed mixture is divided into a and b groups, and then transferred to a muffle furnace for sintering at 800-1200°C. Specifically, the a group of mixtures adopts a two-stage sintering method: the temperature is first raised at a rate of 3°C / min for the first 3 hours, then maintained at a temperature platform of 500-600°C for 2 h, then raised at a rate of 1.5°C / min for the 5th to 11th hour, then maintained at a temperature platform of 1050±30°C for 2 h, and finally annealed at a rate of 1.5°C / min to obtain the a product after primary sintering. The b group of mixtures is sintered at 1000°C for 6 h to obtain the b product after primary sintering.

[0052] 3) Primary crushing: The a product is crushed with a crusher, the crushing degree is selected as moderate, and the time is 2 h to obtain large particles; the b product is crushed with a crusher, the crushing degree is selected as moderate, and the time is 4 h to obtain small particles.

[0053] 4) Secondary mixing: The large particles and small particles are mixed according to a mass ratio of 8:2, the A element precursor, 500 ppm Al2O3 and 800 ppm LiF (LiF acts as a coating fluxing agent) are added, and the mixture is transferred to a high-speed stirrer, the stirring speed is 1000-1400 r / min, and the stirring time is 6 h.

[0054] 5) Secondary sintering: The above mixture is again transferred to a muffle furnace, the temperature is raised at a rate of 5°C / min until the temperature rises to 750±30°C, sintered for 10 h, and then cooled at a rate of 3°C / min to room temperature to obtain a secondary sintered product.

[0055] 6) Secondary crushing: The secondary sintered product is crushed with a crusher, the crushing degree is selected as moderate, and the time is 4 h to obtain a positive electrode material.

[0056] In this embodiment, the B element precursor is selected as MgF2, and the A element precursor is selected as TiO2.

[0057] Preparation of electrolyte: In an argon-filled glove box, water content <5 ppm, oxygen content <5 ppm, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DMC), ethyl propionate (EP), propyl propionate (PP) were mixed according to the mass ratio EC:PC:DMC:EP:PP=1:1:2:2:2 to obtain an organic solvent; and 13.7wt% of lithium hexafluorophosphate, 1wt% of 1,3-propane sultone and 5wt% of fluoroethylene carbonate were added, and then 0.25wt% of BF3 and 0.25wt% of LiPO2F2 were added and uniformly mixed to obtain the electrolyte.

[0058] Examples 2-6

[0059] Examples 2-6 differ from Example 1 only in the different amounts of LiPO2F2 and BF3 added in the electrolyte, as shown in Table 3.

[0060] The other aspects are the same as those of Example 1 and will not be repeated.

[0061] Comparative Examples 1-6

[0062] Comparative Examples 1-6 differ from Example 1 only in that no BF3 is added, and the amount of LiPO2F2 added is shown in Table 3.

[0063] The other aspects are the same as those of Example 1 and will not be repeated.

[0064] Comparative Examples 7-11

[0065] Comparative Examples 7-11 differ from Example 1 only in that no LiPO2F2 is added, and the amount of BF3 added is shown in Table 3.

[0066] The other aspects are the same as those of Example 1 and will not be repeated.

[0067] Comparative Examples 12-17

[0068] Comparative Examples 12-17 differ from Example 1 in that the amounts of LiPO2F2 and BF3 added in the electrolyte are different, as shown in Table 3.

[0069] The other aspects are the same as those of Example 1 and will not be repeated.

[0070] Comparative Examples 18-23

[0071] Comparative Examples 18-23 differ from Example 1 in that the amounts of LiPO2F2 and BF3 added in the electrolyte are different, and the element parameters of the electrode sheet are different, the amounts of electrolyte added are shown in Table 3, and the parameters of the electrode sheet are shown in Table 1.

[0072] The other aspects are the same as those of Example 1 and will not be repeated.

[0073] The positive electrode sheet and electrolyte formulation of the above Examples 1-6 and Comparative Examples 1-23 were collated as shown in Tables 1-3.

[0074] Table 1

[0075]

[0076] In Table 1, " / " represents that the relevant element is not added.

[0077] Sheet A: No A, B element precursor is added during the preparation of the positive electrode material.

[0078] Sheet B: No B element precursor is added in the primary mixing material during the preparation of the positive electrode material.

[0079] Sheet C: No A element precursor is added in the secondary mixing material during the preparation of the positive electrode material.

[0080] Sheet D: B element precursor MgF2 is added in the primary mixing material and A element precursor TiO2 is added in the secondary mixing material during the preparation of the positive electrode material.

[0081] Sheet E: TiO2 is added in the primary mixing material and MgF2 is added in the secondary mixing material during the preparation of the positive electrode material.

[0082] Sheet F: A, B element precursors are added in the primary mixing material and no A, B element precursors are added in the secondary mixing material during the preparation of the positive electrode material.

[0083] Sheet G: No A, B element precursors are added in the primary mixing material and A, B element precursors are added in the secondary mixing material during the preparation of the positive electrode material.

[0084] After the prepared lithium ion battery was disassembled and cleaned, the positive electrode was cut using CP, and the cross section was tested using SEM-EDS to test the A, B element content on the surface and inside, respectively. Among them, the B element ratio is the weight ratio of B ions in the inner layer area, and the inner layer area is defined as before. The A element ratio is the weight ratio of A ions in the surface layer.

[0085] The results are shown in Table 2:

[0086] Table 2

[0087]

[0088] The collocation of the positive electrode and electrolyte of the related examples is shown in Table 3, wherein the content of BF3 and LiPO2F2 is calculated based on the total mass of the electrolyte.

[0089] Table 3

[0090]

[0091]

[0092]

[0093] The electrical performance of Examples 1-6 and Comparative Examples 1-23 were tested, and the test results are shown in Table 4 below.

[0094] 1. Low-Temperature Discharge Capacity Test of Lithium-ion Batteries: Lithium-ion batteries were placed in a 25°C constant temperature chamber and left to stand for 30 minutes to reach a constant temperature. The batteries were then charged at a constant current of 0.5C to a voltage of 4.48V, followed by a constant voltage charge of 4.48V to a current of 0.05C. After standing for 10 minutes, they were discharged at a constant current of 0.2C to a voltage of 3.0V, and the initial discharge capacity was recorded. The batteries were then charged again at a constant current of 0.5C to a voltage of 4.48V, followed by a constant voltage charge of 4.48V to a current of 0.05C. The chamber temperature was then set to -20°C. After reaching this temperature, the batteries were left to stand for 120 minutes, and then discharged at a constant current of 0.2C to a voltage of 3.0V, and the low-temperature discharge capacity was recorded. The low-temperature discharge capacity ratio was calculated as: Low-temperature discharge capacity / Initial discharge capacity.

[0095] 2. Lithium plating test: The lithium-ion battery was charged at a constant current of XXC to 4.48V, then charged at a constant voltage to a current of 0.05C until fully charged. Next, it was discharged at a constant current of 0.7C to 3.0V, completing one charge-discharge cycle. This charge-discharge cycle was repeated 20 times. Then, the battery was charged at a constant current of XXC to 4.48V, then charged at a constant voltage to a current of 0.05C until fully charged. The cell was then transferred to a glove box for disassembly, and the negative electrode was separated. The presence of lithium dendrites on the surface was observed.

[0096] Table 4

[0097]

[0098]

[0099] Results Analysis: Examples 1-6 and Comparative Examples 1-23 show that adding LiPO2F2 and BF3 simultaneously in a specific ratio to the electrolyte, along with the positive electrode D, can effectively improve the discharge capacity ratio at a low temperature of -10℃. The electrolyte contains Li... q P r O s F and BF3, compound Li q P r O s F t A low-resistance SEI is formed on the negative electrode surface. Combined with BF3, it can form a complex with the SEI, improving its dissociation ability and further reducing the negative electrode impedance. Simultaneously, B ions (Mg²⁺) in the positive electrode... 2+ ) and A ions (Ti4+ ) respectively distributed in the inner layer and the surface layer of the positive electrode material, the battery has higher low-temperature discharge capacity and normal-temperature high-rate charge performance.

[0100] According to the disclosure and teaching of the above description, those skilled in the art of the present application can also make changes and modifications to the above embodiments. Therefore, the present application is not limited to the above specific embodiments, and any obvious improvements, replacements or modifications made by those skilled in the art on the basis of the present application shall fall within the protection scope of the present application. In addition, although some specific terms are used in the present specification, these terms are only for convenience of description and do not constitute any limitation on the present application.

Claims

1. A lithium-ion battery, comprising a positive electrode and an electrolyte, characterized in that, The positive electrode material of the positive electrode sheet includes a core and a surface layer located on the surface of the core; wherein, the core comprises a material with the general chemical formula Li. a MO b The main material has a value of 'a' ranging from 1 to 3, a value of 'b' ranging from 1 to 3, M being Co, and the M ion having a valence state of +X; the surface layer contains A ions with a valence state of +Y; the core contains B ions with a valence state of +Z; wherein A and B are at least one element from group IA to IVA (main or transition elements); and X, Y, and Z satisfy the relationship: 1 < Z < X ≤ Y. The electrolyte comprises BF3 and LiPO2F2, wherein the ratio of LiPO2F2 to the total weight of the electrolyte is F%, 0.1≤F≤1, the ratio of BF3 to the total weight of the electrolyte is E%, 0.1≤E≤1, and the relationship between E and F is: F≤E≤4F.

2. The lithium-ion battery according to claim 1, characterized in that, Based on the mass of the main material, the mass content of the M ions is ≥10%.

3. The lithium-ion battery according to claim 1, characterized in that, The weight percentage of A ions in the surface layer is ≥200ppm; and / or, the region extending 1μm from the surface of the core towards its center is defined as the inner layer region, and the weight percentage of B ions in the inner layer region is ≥200ppm.

4. The lithium-ion battery according to claim 3, characterized in that, The weight percentage of B ions in the surface layer is 0–100 ppm; and / or, the weight percentage of A ions in the inner layer region is 0–100 ppm.

5. The lithium-ion battery according to any one of claims 1 to 4, characterized in that, The preparation method of the positive electrode active material includes the following steps: S1. Mix the main material preparation raw materials, including lithium source and M element precursor, disperse them in an organic solvent, add B element precursor for premixing, and vacuum dry the mixture. S2. Divide the mixture in step S1 into two groups, a and b, and then sinter them once to obtain product a and product b. S3. Crush product a and product b prepared in S2 separately to obtain large particles of product a and small particles of product b. S4. Mix the large and small particles obtained in S3 at a mass ratio of (7-9):(1-3), add the A element precursor, and stir to mix. S5. The mixture obtained in S4 is subjected to secondary sintering, crushing, and cooling to obtain the positive electrode material.

6. The lithium-ion battery according to claim 5, characterized in that, In step S1, the premixing speed is 200-400 r / min and the premixing time is 4-8 h; the vacuum drying temperature is 60-100℃ and the drying time is 10-15 h.

7. The lithium-ion battery according to claim 5, characterized in that, In step S2, the temperature of the first sintering is 800-1200℃, the first sintering of the mixture in group a adopts a two-stage sintering method and the sintering time is 12-14h, and the first sintering time of the mixture in group b is 5.5-7.5h.

8. The lithium-ion battery according to claim 7, characterized in that, The two-stage sintering method includes: first, continuously heating to D1℃ at a heating rate of V1℃ / min, then sintering at D1℃±50℃ for t1 time, then continuously heating to D2℃ at a heating rate of V2℃ / min, then sintering at D2℃±50℃ for t2 time, and finally annealing at a cooling rate of V3℃ / min, wherein V1>V2, V2=V3, D1<D2, and t1=t2.

9. The lithium-ion battery according to claim 5, characterized in that, In step S3, the crushed product a consists of large particles of 5–50 μm, and the crushed product b consists of small particles of 0.5–10 μm.

10. The lithium-ion battery according to claim 5, characterized in that, In step S5, the temperature of the secondary sintering is 600-800℃, and the sintering time is 8-12h.

11. An electrical appliance, characterized in that, Including the lithium-ion battery as described in any one of claims 1 to 4.

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