Electrolyte and energy storage device

Abstract

An electrolyte and an energy storage device. The electrolyte comprises, in terms of the mass of the electrolyte: a fluorinated solvent having a mass percentage content of a; a cyclic carbonate having a mass percentage content of b; and lithium hexafluorophosphate having a mass percentage content of c, wherein 1%≤a≤30%, 25%≤b≤45%, and 7%≤c≤10%. An energy storage device prepared from the electrolyte can be ensured to have relatively good overcharge safety performance while having relatively large single-cell capacity.

Description

Electrolyte and energy storage device

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on June 30, 2023, with application number 2023107991733 and application name “Electrolyte and Energy Storage Device”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present application relates to the technical field of energy storage devices, and in particular to an electrolyte and an energy storage device. Background Art

[0003] In existing energy storage devices, large-capacity cells offer advantages over smaller cells in terms of cost and energy density. However, due to their poor heat dissipation, large-capacity cells are prone to heat accumulation during overcharge safety testing. This heat accumulation can easily trigger a chain reaction within the cell, leading to safety issues such as cell failure and fire, and thus compromising the overcharge safety performance of the energy storage device.

[0004] Summary of the Invention

[0005] The purpose of the embodiments of the present application is to provide an electrolyte and an energy storage device, wherein the energy storage device made using the electrolyte can ensure good overcharge safety performance when having a large single cell capacity.

[0006] An embodiment of the present application provides an electrolyte, which includes, based on the mass of the electrolyte, a fluorinated solvent with a mass percentage a, a cyclic carbonate with a mass percentage b, and lithium hexafluorophosphate with a mass percentage c, wherein 1%≤a≤30%, 25%≤b≤45%, and 7%≤c≤10%.

[0007] The embodiment of the present application provides an electrolyte, which introduces a fluorinated solvent into the electrolyte to improve the oxidation resistance of the electrolyte and reduce the heat generated by the oxidation reaction of the electrolyte during overcharging. At the same time, when introducing the fluorinated solvent, in order to solve the problem that the fluorinated solvent is prone to reduction reaction at the negative electrode, the embodiment of the present application adjusts and reduces the content of lithium hexafluorophosphate in the electrolyte. While ensuring that there is sufficient lithium ion conductivity in the electrolyte, it can reduce the content of Lewis acid formed by the reaction of lithium hexafluorophosphate in the electrolyte with water in the electrolyte, thereby making the solid electrolyte interface film (SEI film) formed on the surface of the negative electrode plate and the positive electrode surface passivation film (CEI film) formed on the surface of the positive electrode plate less corroded, while reducing the Lewis acid's capture of F on the fluorinated solvent, reducing the defluorination effect of the fluorinated solvent, and making the fluorinated solvent play a better role in the overcharging process. In addition, by increasing the content of cyclic carbonate in the electrolyte and utilizing the property that cyclic carbonate can undergo ring-opening polymerization at the negative electrode to form a dense SEI film, the problem of poor competitive film formation caused by the introduction of fluorinated solvents into the electrolyte is solved. At the same time, high concentrations of cyclic carbonates can stabilize the negative electrode while preventing the consumption and oxidation of fluorinated solvents on the negative electrode side, stabilizing the content of fluorinated solvents in the electrolyte, thereby indirectly improving the oxidation resistance of the electrolyte, and further solving the overcharge safety performance of the energy storage device while improving the charge and discharge cycle performance of the energy storage device.

[0008] An embodiment of the present application also provides an energy storage device, comprising a positive electrode plate and the electrolyte as described above. DETAILED DESCRIPTION

[0009] The following is a clear and complete description of the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, not all of them. Based on the embodiments of this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0010] An embodiment of the present application provides an energy storage device, comprising a housing, a battery cell, and an electrolyte. Specifically, the battery cell is mounted on the inner side of the housing, and the electrolyte is injected into the inner side of the housing and infiltrates the battery cell. Exemplarily, the overall capacity of the energy storage device is not less than 280 Ah (Ampere-Hour). Exemplarily, the energy storage device may be a lithium-ion battery.

[0011] The battery cell includes a positive electrode sheet, a separator, a negative electrode sheet, a positive tab, and a negative tab. The separator is located between the positive and negative electrode sheets, and the positive tab is electrically connected to the positive electrode sheet, while the negative tab is electrically connected to the negative electrode sheet. For example, the positive electrode sheet, separator, and negative electrode sheet can be stacked in sequence, wound to form a bare cell, and then the positive and negative tabs are welded to produce the battery cell.

[0012] Wherein, the positive electrode plate includes a positive electrode current collector and a positive electrode active layer covering the surface of the positive electrode current collector. In the positive electrode plate, the coating density of the positive electrode active layer is tg / 1540.25mm2, which satisfies 0.25≤t≤0.4. Wherein, t represents the coating weight of the positive electrode material layer. Exemplarily, the positive electrode current collector is aluminum foil. The positive electrode material layer includes materials such as positive electrode active materials and additives. Exemplarily, the positive electrode active material is lithium iron phosphate. Wherein, the additives in the positive electrode material layer include adhesives and conductive agents. Exemplarily, the adhesive is polyvinylidene difluoride (PVDF) and the conductive agent is conductive carbon black SP (Super P).

[0013] Specifically, the coating weight t of the positive electrode active layer can be a value such as 0.25, 0.3, 0.4, and any value between the above values. The smaller the coating weight t of the positive electrode active layer, the lower the energy density of the obtained energy storage device will be. At the same time, more battery core materials, such as current collectors and shells, will be consumed, which will increase the cost of energy storage. The larger the coating weight t of the positive electrode active layer, the lower the cost of energy storage will be. However, if t is too large and exceeds the range value, it will cause problems with the wetting of the material, which will lead to the occurrence of lithium precipitation and gas production, thereby affecting the overcharge safety performance and charge and discharge cycle performance of the energy storage device.

[0014] The diaphragm can be an existing battery diaphragm. Exemplarily, the diaphragm is a polyethylene film. The negative electrode plate includes a negative electrode current collector and a negative electrode active layer covering the surface of the negative electrode current collector. Exemplarily, the negative electrode current collector is aluminum foil. The negative electrode active layer includes materials such as a negative electrode active material and additives. Exemplarily, the negative electrode active material is graphite. The additives in the negative electrode active layer include materials such as a thickener, an adhesive, and a conductive agent. Exemplarily, the adhesive is styrene butadiene rubber (SBR), and the conductive agent is conductive carbon black SP (Super P).

[0015] The electrolyte includes a solvent and a lithium salt. The solvent includes a fluorinated solvent and a cyclic carbonate, and the lithium salt includes lithium hexafluorophosphate (LiPF6). Exemplarily, based on the mass of the electrolyte, the mass percentage of the fluorinated solvent is a, 1%≤a≤30%, the mass percentage of the cyclic carbonate is b, 25%≤b≤45%, and the mass percentage of lithium hexafluorophosphate is c, 7%≤c≤10%. In some embodiments, 3%≤a≤15%. It should be understood that conventional electrolyte solvents such as dimethyl carbonate, ethyl methyl carbonate, etc. can also be added to the electrolyte. It is understandable that additives such as vinyl sulfate (1,3,2-dioxathiolane 2,2-dioxide, DTD) can also be added to the electrolyte.

[0016] Specifically, the mass percentage content a of the fluorinated solvent can be 1%, 3%, 5%, 10%, 15%, 30%, and any value in between. If the content a of the fluorinated solvent is too low, the improvement in the overcharge safety performance of the formed energy storage device will be insufficient. If the content a of the fluorinated solvent is too high, it will affect the viscosity and conductivity of the electrolyte, thereby deteriorating the charge-discharge cycle performance. By controlling the content a of the fluorinated solvent to meet 1% ≤ a ≤ 30%, it is beneficial to ensure that the formed energy storage device has both good overcharge safety performance and charge-discharge cycle performance.

[0017] The mass percentage of cyclic carbonate is b, which can be 25%, 27%, 35%, 40%, 45%, and any other values ​​between the above values. The main function of cyclic carbonate is to form a film on both the positive and negative electrodes. When the content b of cyclic carbonate is too little, it cannot protect the positive and negative electrodes, and will have an adverse effect on the overcharge safety performance and the charge and discharge cycle performance. When the content b of cyclic carbonate is too much, the negative electrode film may be too thick, and excessive film formation may cause problems at the negative electrode interface. Once the interface has problems, the overcharge safety performance and the charge and discharge cycle performance will deteriorate. By controlling the content b of cyclic carbonate to meet 25%≤b≤45%, it is helpful to ensure that the formed energy storage device has good overcharge safety performance and charge and discharge cycle performance.

[0018] The mass percentage of lithium hexafluorophosphate, c, can be 7%, 7.5%, 8%, 9%, 10%, and any other value in between. If the lithium hexafluorophosphate content, c, is too low, the main impact is on the conductivity of the electrolyte, causing problems with ion transport and thus worsening the charge-discharge cycle performance of the resulting energy storage device. If the lithium hexafluorophosphate content, c, is too high, the viscosity of the electrolyte increases, which is also detrimental to conductivity. It also produces more acidic substances such as PF5 and POF3, damaging SEI and CEI, thereby affecting the charge-discharge cycle performance of the resulting energy storage device. By controlling the lithium hexafluorophosphate content, c, to meet 7% ≤ c ≤ 10%, it is beneficial to ensure that the resulting energy storage device has both good overcharge safety and charge-discharge cycle performance.

[0019] Wherein, the fluorinated solvent includes at least one of fluorinated carbonate, fluorinated carboxylate and fluorinated ether. Exemplarily, the fluorinated carbonate is selected from at least one of Chemical Formula (I-1) to Chemical Formula (I-7):

[0020] Exemplarily, the fluorocarboxylate is selected from at least one of Chemical Formula (I-8) to Chemical Formula (I-17):

[0021] The fluoroether is selected from at least one of Chemical Formula (I-18) to Chemical Formula (I-21):

[0022] The cyclic carbonate is selected from at least one of ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), and vinylene carbonate (VC).

[0023] Among them, the lithium salt can also include at least one of lithium difluorophosphate (LiDFP), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bistrifluoromethanesulfonimide (LiTFSI), lithium difluorooxalyldifluoroborate (LiODFB), lithium bis(oxalate)borate (LiBOB), and lithium tetrafluoroborate (LiBF4).

[0024] In energy storage devices, increasing the coating weight t of the positive electrode active layer and producing large-capacity battery cells have the benefits of increasing the overall energy density of the battery cell and reducing the battery cell cost, but it also poses challenges to the battery cell's safety performance. On the one hand, the high coating weight of the positive electrode active layer gives the battery cell a greater electrode ohmic impedance. The increased battery cell impedance causes the battery cell to generate more Joule heat during the charging process. On the other hand, when the capacity of the single battery cell is above 280Ah, the large-capacity battery cell further gains advantages in cost and energy density over the small-capacity cell. However, due to the poor heat dissipation of large-capacity batteries, heat easily accumulates in the battery cell during charging. The accumulated heat can easily cause a chain chemical reaction within the battery cell, leading to safety issues such as battery cell failure and fire.

[0025] During the overcharge safety test, the voltage of the energy storage device's battery cells will rise to a maximum voltage of 1.5V during continuous charging. At this time, the battery cell's voltage increases during continuous charging, and the electrolyte undergoes an oxidation reaction at the positive electrode. The reaction heat generated by the reaction accumulates in the battery cell. When the heat reaches a certain level, the reaction between the electrolyte and the positive electrode plate is further intensified under the action of high temperature. At the same time, the SEI film (Solid Electrolyte Interphase) formed on the negative electrode plate begins to decompose due to the high temperature. The decomposition reaction of the SEI film generates reaction heat. At the same time, because the SEI film decomposes at high temperature, the fully charged negative electrode directly contacts the electrolyte, and the electrolyte undergoes a reduction reaction at the negative electrode, further releasing heat and accumulating heat. Under the influence of the high temperature caused by this heat, the separator in the battery cell melts and ruptures, causing the positive and negative electrodes to contact. At this time, the battery cell may catch fire or explode, thereby affecting the safety performance of the battery cell.

[0026] To address the safety issue of heat accumulation in large-capacity battery cells, which can cause chain chemical reactions within the cells and lead to cell failure and fire, the present invention provides an electrolyte solution comprising 1% to 30% by mass of a fluorinated solvent, 25% to 45% by mass of a cyclic carbonate, and 7% to 10% by mass of lithium hexafluorophosphate. The introduction of the fluorinated solvent into the electrolyte improves the electrolyte's oxidation resistance and reduces the heat generated by oxidation reactions during overcharging. At the same time, when introducing the fluorinated solvent, in order to solve the problem that the fluorinated solvent is prone to reduction reaction at the negative electrode, the embodiment of the present application adjusts and reduces the content of lithium hexafluorophosphate in the electrolyte. While ensuring that there is sufficient lithium ion conductivity in the electrolyte, it can reduce the content of Lewis acid formed by the reaction of lithium hexafluorophosphate in the electrolyte with water in the electrolyte, thereby making the solid electrolyte interface film (SEI film) formed on the surface of the negative electrode plate and the positive electrode surface passivation film (CEI film) formed on the surface of the positive electrode plate less corroded, and at the same time reducing the Lewis acid's capture of F on the fluorinated solvent, reducing the defluorination effect of the fluorinated solvent, and allowing the fluorinated solvent to play a better role in the overcharge process. In addition, by increasing the content of cyclic carbonate in the electrolyte and utilizing the property that cyclic carbonate can undergo ring-opening polymerization at the negative electrode to form a dense SEI film, the problem of poor competitive film formation caused by the introduction of fluorinated solvents into the electrolyte is solved. At the same time, high concentrations of cyclic carbonates can stabilize the negative electrode while preventing the consumption and oxidation of fluorinated solvents on the negative electrode side, stabilizing the content of fluorinated solvents in the electrolyte, thereby indirectly improving the oxidation resistance of the electrolyte, and further solving the overcharge safety performance of the energy storage device while improving the charge and discharge cycle performance of the energy storage device.

[0027] In addition, the coating weight t of the positive electrode active layer, the mass percentage a of the fluorinated solvent, the mass percentage b of the cyclic carbonate, and the mass percentage c of the lithium salt satisfy the following relationship: 0.07 ≤ (1-ab) * c / t ≤ 0.288. Wherein, "*" represents a multiplication sign. Specifically, (1-ab) * c / t can be 0.07, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21, 0.27, 0.288, and any value in between. When (1-ab)*c / t is less than 0.07, due to the combined effects of several factors, lithium deposition may occur in the pores or on the negative electrode layer, causing a large amount of reaction between the dendrites deposited from the negative electrode and the electrolyte during overcharge, generating a large amount of reaction heat, resulting in unsatisfactory overcharge test results. At the same time, during the cycling process, due to the occurrence of lithium deposition, on the one hand, dead lithium is produced, causing irreversible loss of capacity, and on the other hand, due to the reaction between the deposited lithium and the electrolyte, by-products, gas generation and other interface problems are caused, resulting in unsatisfactory cycling performance of the energy storage device. When (1-ab)*c / t>0.288, there may be insufficient oxidation resistance of the electrolyte and insufficient passivation protection of the electrolyte for the positive and negative electrodes. At this time, the electrolyte cannot play a role in improving overcharge performance. By controlling the condition 0.07≤(1-ab)*c / t≤0.288, the prepared energy storage device can be guided to have a large-capacity single cell while ensuring that the cell overcharge safety test performance rating meets the standards and has good charge and discharge cycle performance, thereby guiding the preparation of an energy storage device with good overcharge safety performance and good charge and discharge cycle performance.

[0028] The following is an experimental demonstration using specific examples. Examples 1-23 and Comparative Examples 1-12 each provide an energy storage device, which is exemplified by a lithium-ion battery and is prepared in the following steps:

[0029] Preparation of positive electrode sheet: The positive electrode active material lithium iron phosphate, conductive carbon black SP, and binder PVDF are dispersed in the solvent NMP (N-Methylpyrrolidone) in a mass ratio of 97:0.7:2.3 and mixed evenly to obtain positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector aluminum foil, and after drying, cold pressing, slitting, and cutting, the positive electrode sheet is obtained. The positive electrode slurry coating forms the positive electrode active layer, and the coating density of the positive electrode active layer is tg / 1540.25mm 2 , t represents the coating quality of the positive electrode active layer.

[0030] Preparation of the negative electrode sheet: The negative electrode active material, artificial graphite, conductive carbon SP, thickener CMC, and binder SBR, are dispersed in deionized water at a mass ratio of 96.5:0.5:1:2 and mixed evenly to form a negative electrode slurry. The slurry is then applied to the negative electrode current collector aluminum foil. After drying, cold pressing, slitting, and cutting, the negative electrode sheet is obtained. The negative electrode slurry coating forms the negative electrode active layer.

[0031] Preparation of the electrolyte: In an argon atmosphere glove box with a moisture content of ≤1 ppm, solvent A, dimethyl carbonate, and ethyl methyl carbonate were mixed in a mass ratio of 1:1:1. Dry lithium hexafluorophosphate (LiPF6) electrolyte was then dissolved in the solvent and stirred until completely dissolved. Solvent A was then added and mixed thoroughly to obtain the electrolyte. Solvent A comprised a fluorinated solvent and a cyclic carbonate.

[0032] Preparation of diaphragm: 16um polyethylene film was used as the diaphragm.

[0033] Preparation of a lithium-ion battery: The prepared positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator positioned between the positive and negative electrode sheets to separate them. After winding, a bare cell is obtained. The positive and negative tabs are welded to form a cell. The cell is assembled into a casing, and after injecting the prepared electrolyte, the cell is packaged, allowed to stand, formed, shaped, and capacity tested to finally produce a lithium-ion battery.

[0034] In the energy storage devices provided in Examples 1-23 and Comparative Examples 1-12, the capacity of the individual cells, the coating weight t of the positive electrode active layer, the content of solvent A, and the lithium salt LiPF6 are shown in Table 1. The energy storage devices of Examples 1-23 and Comparative Examples 1-12 were subjected to charge-discharge cycle performance tests and safety overcharge tests, and the results are shown in Table 1.

[0035] The charge-discharge cycle performance test involves running the energy storage device through a charge-discharge tester at a temperature of 25°C, a cycle rate of 1C (i.e., both the charge and discharge rates are 1C), and a charge voltage between 2.5V and 3.65V. The capacity retention rate after each cycle is then calculated. The formula for calculating the capacity retention rate at 25°C is: Capacity retention after the nth cycle = (Discharge capacity after the nth cycle / Discharge capacity at the first cycle) * 100%.

[0036] The safety overcharge test process is as follows: the energy storage device is tested on a charge-discharge instrument and allowed to stand for 5 hours at (25±2)°C. It is then discharged at 1C to 2.5V and then charged to 3.65V and allowed to stand for 10 minutes. The device is then transferred to an overcharge tester and charged at a constant current of 1C until the voltage reaches 5.475V or the charging time reaches 1 hour. The device is then observed for 1 hour, and any swelling, leakage, smoke, fire, or explosion is recorded. Rating: No leakage is considered Level 1 compliance; leakage but no smoke is considered Level 2 compliance; leakage and smoke but no fire or explosion are considered Level 3 compliance; fire or explosion are considered Level 4 compliance.

[0037] Table 1 Composition and performance of energy storage devices of Examples 1-23 and Comparative Examples 1-12

[0038] As can be seen from Table 1, by comparing Comparative Examples 1 to 4, no fluorinated solvents were added to the electrolytes of the energy storage devices of Comparative Examples 1 to 4. As the capacity of the single cell increases, the rating of the energy storage device in the safe overcharge test gradually increases, and the overcharge safety performance rating of the energy storage device gradually fails to meet the standard. When the capacity of the single cell is 280Ah, the overcharge safety performance rating of the energy storage device is as low as level four, and the overcharge safety performance does not meet the standard. At the same time, as the capacity of the single cell increases, the capacity retention rate of the energy storage device after 1000 cycles at 25°C gradually decreases. In Examples 1 to 23, fluorinated solvents and cyclic carbonates were added to the electrolytes of the energy storage devices at the same time, and the capacity of the single cell can reach more than 280Ah. At the same time, the overcharge safety performance ratings of the energy storage devices all meet the standards. The experimental results show that when no fluorinated solvents are added to the electrolyte of the energy storage device, as the capacity of the single cell increases, the overcharge safety performance and charge-discharge cycle performance will deteriorate.

[0039] By comparing Comparative Examples 4 to 7, the energy storage devices in Comparative Examples 4 to 7 do not have a fluorinated solvent added to the electrolyte. When the capacity of the single cell is a large capacity of 280Ah, as the coating amount t of the positive electrode active layer increases, the overcharge safety performance of the energy storage device remains poor, and the overcharge safety performance basically does not meet the standard. At the same time, the capacity retention rate gradually decreases after 1000 cycles at 25°C. The experimental results show that when no fluorinated solvent is added to the energy storage device, the overcharge safety performance of the energy storage device basically does not meet the standard when the single cell capacity is large. At the same time, the increase in the coating amount t of the positive electrode active layer will lead to poor charge and discharge cycle performance of the energy storage device.

[0040] By comparing Comparative Example 4 with Examples 1 to 20, when the capacity of the energy storage device's single cells and the coating weight t of the positive electrode active layer are the same, the addition of a fluorinated solvent and a cyclic carbonate to the electrolytes of Examples 1 to 20 of the present application can improve the overcharge safety performance of the energy storage device or improve the capacity retention rate after 1000 cycles at 25°C. The experimental results show that the simultaneous addition of a fluorinated solvent and a cyclic carbonate to the electrolyte is beneficial for improving the overcharge safety performance of the energy storage device or improving the charge-discharge cycle performance of the energy storage device, thereby improving the overall performance of the energy storage device.

[0041] Specifically, by comparing Examples 1 to 6, the energy storage devices of Examples 1 to 6, with the same single cell capacity and the same coating weight t of the positive electrode active layer, all improved the overcharge safety performance of the energy storage devices by adding different fluorinated solvents to the electrolyte. Specifically, the fluorinated solvents added in Examples 2 to 6 were more effective than the fluorinated solvent added in Example 1.

[0042] By comparing Examples 7 to 9, fluorinated solvents are added to the electrolytes of the energy storage devices of Examples 7 to 9, and as the content a of the fluorinated solvent gradually increases, the overcharge safety performance of the energy storage device is significantly improved. The experimental results show that, on the basis of adding cyclic carbonate to the electrolyte, the addition of fluorinated solvents and increasing the content of fluorinated solvents in the electrolyte can improve the overcharge safety performance of the energy storage device. This may be because the fluorinated solvent has antioxidant ability, which can greatly improve the oxidation resistance of the electrolyte. During the high-voltage process of overcharging, the oxidation reaction of the electrolyte on the positive electrode is reduced, that is, the amount of oxidation reaction of the electrolyte with the positive electrode during the overcharging process is greatly reduced, and the reaction heat generated by the oxidation reaction is reduced, so that the battery cell accumulates less heat at the beginning. At this time, the reduction in heat accumulation can reduce the possibility of decomposition of the solid electrolyte interface film (SEI film) on the negative electrode side, thereby prompting the energy storage device to maintain a healthier state during overcharging, thereby improving the performance of the energy storage device in the overcharge safety test. Illustratively, the mass fraction a of the fluorinated solvent in the electrolyte is 1% to 30%. Preferably, the mass fraction a of the fluorinated solvent in the electrolyte is 3% to 15%.

[0043] By comparing Examples 10 to 14, it is found that fluorinated solvents and cyclic carbonates are added to the electrolytes of the energy storage devices of Examples 10 to 14. At the same time, as the content of lithium salt LiPF6 (lithium hexafluorophosphate) in the electrolyte gradually decreases, the overcharge safety performance level of the energy storage devices of Examples 10 to 14 gradually improves to level one. The experimental results show that reducing the content of lithium salt LiPF6 in the electrolyte can further improve the overcharge safety performance of the energy storage device. This may be because LiPF6 in the lithium salt of the electrolyte is easily decomposed to generate Lewis acids such as PF5 (phosphorus pentafluoride) in the presence of moisture in the electrolyte. This Lewis acid may not only attack the solid electrolyte interface film (SEI film) on the negative electrode side and the passivation film (CEI film) on the positive electrode surface, causing the electrolyte and the positive electrode plate or the negative electrode plate to directly contact and react, but also may have the possibility of capturing fluoride ions in the fluorinated solvent. Therefore, when the electrolyte contains a smaller amount of LiPF6, less Lewis acids such as PF5 are produced in the electrolyte, which reduces the attack of the acid on the positive electrode CEI film and the negative electrode SEI film, so that the SEI film formed on the surface of the negative electrode and the CEI film formed on the surface of the positive electrode are less corroded. At the same time, the Lewis acid captures F from the fluorinated solvent, which protects the fluorinated solvent from damage and allows the fluorinated solvent to play a role in the overcharging process of the energy storage device, thereby further improving the overcharging safety performance of the energy storage device.

[0044] Continuing to observe Examples 7 to 9, although the addition of fluorinated solvents within a certain content range improves the overcharge safety performance of the energy storage devices in Examples 7 to 10, as the content of fluorinated solvents increases, the capacity retention rate of the energy storage device after 1000 cycles at 25°C gradually decreases. This may be due to the competition between the fluorinated solvents and the cyclic carbonate for film formation on the negative electrode side. Continuing to observe Examples 10 to 14, although reducing the content of lithium salt LiPF6 in the electrolyte can improve the overcharge safety performance of Examples 10 to 14, the capacity retention rate of the energy storage device after 1000 cycles at 25°C gradually decreases. It can be seen that the overcharge safety performance of the energy storage device may be improved by adding fluorinated solvents within a certain content range and reducing the content of lithium salt LiPF6, but this strategy may have the potential for a vicious cycle.

[0045] Comparing Examples 15 to 18, the electrolytes of the energy storage devices in Examples 15 to 18 all contained a fluorinated solvent and a cyclic carbonate. As the content of the cyclic carbonate solvent in the electrolyte of the energy storage device increased, the capacity retention rate of the energy storage device after 1000 cycles at 25°C gradually increased while maintaining good overcharge safety performance. The experimental results show that by increasing the content of the cyclic carbonate solvent in the electrolyte, the overcharge safety performance issue of the energy storage device can be resolved while ensuring the charge and discharge cycle performance of the energy storage device. This is because the fluorinated solvent is easily reduced on the negative electrode side before a dense SEI film is formed on the negative electrode side. The SEI film formed by the reduction product is not dense enough, which may lead to a decrease in the electrical performance of the energy storage device. By adding a high concentration of cyclic carbonate to the electrolyte, the cyclic carbonate can undergo ring-opening polymerization at the negative electrode to form a dense SEI film, solving the problem of poor film formation caused by the introduction of the fluorinated solvent into the electrolyte. At the same time, high concentrations of cyclic carbonates can not only stabilize the negative electrode, but also prevent the consumption and oxidation of fluorinated solvents on the negative electrode side, stabilize the content of fluorinated solvents in the electrolyte, and thus indirectly improve the oxidation resistance of the electrolyte. While solving the overcharge safety performance of the energy storage device, it is beneficial to improve the charge and discharge cycle performance of the energy storage device.

[0046] By comparing Comparative Examples 8 to 12 with Examples 1 to 23, although a fluorinated solvent, a cyclic carbonate and a lithium salt LiPF6 are added to the electrolyte of the energy storage device of Comparative Example 8, the content a of the fluorinated solvent is less than 1%, the content b of the cyclic carbonate is less than 25%, and the content c of the lithium salt LiPF6 exceeds 10%. The overcharge safety performance of the obtained energy storage device does not meet the standard, and the charge-discharge cycle performance is poor. No fluorinated solvent is added to the electrolyte of the energy storage device of Comparative Example 9, and the content b of the cyclic carbonate is less than 25%. The overcharge safety performance of the obtained energy storage device is poor. The content a of the fluorinated solvent in the electrolyte of the energy storage device of Comparative Example 10 exceeds 30%, and the charge-discharge cycle performance of the obtained energy storage device is very poor. The content c of the lithium salt LiPF6 in the electrolyte of the energy storage device of Comparative Example 11 is less than 7%, and the charge-discharge cycle performance of the obtained energy storage device is very poor. The cyclic carbonate content b in the electrolyte of the energy storage device of Comparative Example 12 exceeded 45%, resulting in poor overcharge safety performance for the resulting energy storage device. In contrast, the electrolytes of the energy storage devices of Examples 1 to 23, by adding a fluorinated solvent, a cyclic carbonate, and a lithium salt, LiPF6, and controlling the fluorinated solvent content a in the electrolyte to satisfy 1% ≤ a ≤ 30%, the cyclic carbonate content b to satisfy 25% ≤ b ≤ 45%, and the lithium salt LiPF6 content c to satisfy 7% ≤ c ≤ 10%, ensure the overcharge safety performance of the energy storage device, or guarantee the capacity retention rate after 1000 cycles at 25°C, thereby guaranteeing the charge-discharge cycle performance of the energy storage device.

[0047] In addition, by comparing Example 7 and Example 22 with other examples (Examples 1 to 6, Examples 8 to 21, and Example 23), the battery cell capacity of the energy storage device in Example 7 is 280Ah, which is a large capacity, (1-ab)*c / t=0.31>0.288, and the energy storage device has good charge and discharge cycle performance, but the overcharge safety performance level is level four, and the overcharge safety performance does not meet the standard. The battery cell capacity of the energy storage device in Example 22 is 280Ah, which is a large capacity, (1-ab)*c / t=0.056<0.288, and the overcharge safety performance level is level three, and the overcharge safety performance just meets the standard. In the energy storage devices of other embodiments, the relationship is satisfied: 0.07≤(1-ab)*c / t≤0.288. The prepared energy storage device has a large single cell capacity while the overcharge safety performance level reaches level two or level one, and the overcharge safety performance is very good. Experimental results show that the embodiment of the present application satisfies the following relationship by controlling the coating weight t of the positive electrode active layer, the mass percentage content a of the fluorinated solvent, the mass percentage content b of the cyclic carbonate, and the mass percentage content c of the lithium salt LiPF6: 0.07≤(1-ab)*c / t≤0.288. The energy storage device obtained has a large single cell capacity and meets the overcharge safety test performance rating, thus having good overcharge safety performance.

[0048] The above disclosure is only a preferred embodiment of the present application, and it is certainly not intended to limit the scope of the rights of the present application. A person skilled in the art can understand that all or part of the processes of the above embodiments and equivalent changes made in accordance with the claims of the present application are still within the scope of the present application.

Claims

1. An electrolyte, characterized in that: Calculated by the mass of the electrolyte, the electrolyte includes a fluorinated solvent with a mass percentage of a, a cyclic carbonate with a mass percentage of b, and lithium hexafluorophosphate with a mass percentage of c, wherein 1%≤a≤30%, 25%≤b≤45%, and 7%≤c≤10%.

2. The electrolyte according to claim 1, characterized in that The cyclic carbonate is selected from at least one of ethylene carbonate, propylene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, and vinylene carbonate.

3. The electrolyte according to claim 1, characterized in that The fluorinated solvent includes at least one of fluorinated carbonate, fluorinated carboxylate and fluorinated ether.

4. The electrolyte according to claim 3, characterized in that The fluorinated carbonate is selected from at least one of the chemical formulas (I-1) to (I-7): The fluorocarboxylate is selected from at least one of the chemical formulas (I-8) to (I-17): The fluoroether is selected from at least one of chemical formula (I-18) to chemical formula (I-21):

5. The electrolyte according to any one of claims 1 to 4, characterized in that: The electrolyte includes a lithium salt, the lithium salt includes the lithium hexafluorophosphate, and the lithium salt further includes at least one of lithium difluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium difluorooxalatoborate, lithium dioxalatoborate, and lithium tetrafluoroborate.

6. An energy storage device, characterized in that: The invention comprises a positive electrode sheet and an electrolyte as claimed in any one of claims 1 to 5.

7. The energy storage device according to claim 6, characterized in that: The positive electrode plate comprises a current collector and a positive electrode active layer covering the surface of the positive electrode current collector. In the positive electrode plate, the coating density of the positive electrode active layer is tg / 1540.25mm 2 , and satisfies the following relationship: 0.07≤(1-ab)*c / t≤0.

288.

8. The energy storage device according to claim 7, characterized in that: 0.25≤t≤0.4。 9. The energy storage device according to claim 7, characterized in that: The material in the positive electrode active layer includes lithium iron phosphate.

10. The energy storage device according to any one of claims 6 to 9, characterized in that: The capacity of the energy storage device is not less than 280Ah.

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