Battery and energy storage device

Abstract

A battery and an energy storage device. The battery comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte solution, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer, the negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer, and the electrolyte solution comprises an electrolyte, a solvent and an additive; the ratio of the weight of the negative electrode active material layer per unit area to the weight of the positive electrode active material layer per unit area is z, and the value range of z is 0.46-0.55; and the additive comprises an additive A, and the additive A is a substance containing an oxalic acid structure.

Description

Batteries and energy storage devices

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application is based on the Chinese patent application with application number 202311243633.0 and application date September 25, 2023, and claims the priority of the Chinese patent application. The entire content of the Chinese patent application is hereby introduced into this application as a reference. Technical Field

[0003] The present application relates to the field of battery technology, and in particular to a battery and an energy storage device. Background Art

[0004] With the development of the energy storage industry, battery (such as lithium-ion battery) technology continues to advance, and application scenarios such as wind and solar power generation have put forward requirements for longer battery service life.

[0005] Taking lithium-ion batteries as an example, in the battery's chemical system, as the cycle progresses, the battery life loss is mainly due to the reaction of lithium ions on the surface of the negative electrode, causing the lithium in the battery system to exist in the form of organic compounds or inorganic layers on the surface of the negative electrode, causing the lithium in the active material to be consumed, resulting in the battery capacity decay.

[0006] The reason for lithium consumption is that, on the one hand, during the charging process, the potential of the negative electrode is low, and the electrolyte is easily reduced on the surface of the negative electrode at a low potential, producing lithium-containing organic and inorganic substances, causing the electrolyte to form a negative electrode interface SEI (negative electrode solid electrolyte interface) layer at the negative electrode interface, so that the lithium in the active material is consumed; on the other hand, during the cyclic charge and discharge process, the graphite negative electrode undergoes lithium insertion and removal, resulting in the layered graphite structure always being in the process of expanding and shrinking the interlayer spacing. In this process, the negative electrode interface SEI protective layer formed by the early electrolyte and the negative electrode interface is constantly pulled, constantly exposing new graphite interfaces and direct contact with the electrolyte. The electrolyte continuously reacts on the graphite interface, consuming active lithium and causing the loss of active lithium.

[0007] Summary of the Invention

[0008] The present application aims to solve at least one of the technical problems existing in the prior art. To this end, one purpose of the present application is to propose a battery that, by optimizing the ratio of positive and negative electrodes and adjusting the electrolyte, can reduce the risk of SEI layer rupture and the loss of active materials (such as active lithium) during the battery's cyclic use, thereby reducing capacity decay during the cycle, making the battery have better cycle performance and thus a longer service life.

[0009] The present application also proposes an energy storage device having the above-mentioned battery.

[0010] According to the first embodiment of the present application, the battery includes: a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, the separator is located between the positive electrode sheet and the negative electrode sheet, the positive electrode sheet includes a positive electrode collector and a positive electrode active material layer covering the positive electrode collector, the negative electrode sheet includes a negative electrode collector and a negative electrode active material layer covering the negative electrode collector, and the electrolyte includes an electrolyte, a solvent and an additive; wherein the ratio of the weight of the negative electrode active material layer per unit area to the weight of the positive electrode active material layer per unit area is z, and the value range of z is 0.46 to 0.55; the additive includes additive A, and the additive A is a substance containing an oxalic acid structure.

[0011] According to the battery of the embodiment of the present application, by increasing the ratio of the negative electrode active material layer to the positive electrode active material layer, the expansion amplitude of the negative electrode is smaller during the cycle of the battery, and the SEI layer on the negative electrode interface is pulled less, thereby making it difficult for the negative electrode active material layer to be exposed to the electrolyte, preventing direct contact between the negative electrode active material layer and the electrolyte, and reducing the reduction reaction of the electrolyte, thereby achieving less active lithium loss on the negative electrode surface; and, at the same time, adding an additive A containing an oxalic acid structure to the electrolyte reduces the risk of rupture caused by oxidation of the SEI layer during full charge and discharge of the battery, thereby reducing the loss of active lithium caused by direct contact and reaction between the negative electrode active material layer and the electrolyte, thereby reducing the loss of active lithium on the negative electrode surface; in this way, by optimizing the ratio of the positive and negative electrodes and adjusting the electrolyte at the same time, the battery can reduce the risk of rupture of the SEI layer during cyclic use, reduce the loss of active material (such as active lithium), thereby reducing capacity decay during the cycle, and making the battery have better cycle performance, thereby making the battery have a longer service life.

[0012] According to some embodiments of the present application, the mass percentage of the additive A in the electrolyte is a, and the value range of a is 0.1% to 1%.

[0013] According to some embodiments of the present application, the additive A includes at least one of lithium dioxalatoborate and lithium difluorooxalatoborate.

[0014] According to some embodiments of the present application, the solvent includes solvent B, and the solvent B is a carboxylate solvent.

[0015] According to some optional embodiments of the present application, the mass percentage of the solvent B in the electrolyte is b, and the value range of b is 1% to 30%.

[0016] Optionally, the value range of b is 5% to 15%.

[0017] According to some optional embodiments of the present application, the solvent B includes at least one of methyl acetate, methyl propionate, ethyl acetate, ethyl propionate, propyl formate, propyl acetate and propyl propionate.

[0018] According to some optional embodiments of the present application, the additive includes additive C, and the additive C is a nitrogen-containing heterocyclic substance.

[0019] According to some optional embodiments of the present application, the mass percentage of the additive C in the electrolyte is c, and the value range of c is 0.1% to 0.5%.

[0020] According to some optional embodiments of the present application, the additive C includes at least one of 2-fluoropyrazine, 2,5-difluoropyrazine, 2-fluoropyridazine, 3-fluoropyridazine, 2-fluoropyridine, 3-fluoropyridine, 4-fluoropyridine, 2,3-fluoropyridine and 3,4-fluoropyridine.

[0021] According to some optional embodiments of the present application, the mass percentage of the additive A in the electrolyte is a, the mass percentage of the solvent B in the electrolyte is b, and the mass percentage of the additive C in the electrolyte is c; wherein z, a, b, and c satisfy: 0.25≤(a+c)*(1-b)*100 / z≤2.15.

[0022] An energy storage device according to an embodiment of the second aspect of the present application includes: a battery according to the embodiment of the first aspect of the present application.

[0023] According to the energy storage device of the embodiment of the present application, by providing the above-mentioned battery, the risk of rupture of the SEI layer can be reduced during the cycle of the battery, and the loss of active materials (such as active lithium) can be reduced, thereby reducing the capacity decay during the cycle process, so that the battery has better cycle performance and thus has a longer service life.

[0024] Additional aspects and advantages of the present application will be given in part in the description below, and in part will become obvious from the description below, or will be learned through practice of the present application. DETAILED DESCRIPTION

[0025] The following describes in detail embodiments of the present application. Examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present application and are not to be construed as limiting the present application.

[0026] The following describes a battery according to an embodiment of the present application. In the embodiment of the present application, the battery may be a lithium-ion battery, a sodium-ion battery, or the like.

[0027] According to the first embodiment of the present application, the battery includes: a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, and the separator is located between the positive electrode sheet and the negative electrode sheet.

[0028] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer covering the positive electrode current collector. The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer covering the negative electrode current collector. For example, the positive electrode current collector may be aluminum foil, and the negative electrode current collector may be copper foil. For example, in the case of a lithium-ion battery, the positive electrode active material layer may include lithium iron phosphate, and the negative electrode active material layer may include graphite. The electrolyte includes an electrolyte, a solvent, and additives. The solvent may include an ester solvent. For example, in the case of a lithium-ion battery, the electrolyte may include a lithium salt.

[0029] The ratio of the weight of the negative electrode active material layer per unit area to the weight of the positive electrode active material layer per unit area is z, and the value of z ranges from 0.46 to 0.55. For example, the value of z can be 0.46, 0.47, 0.48, 0.49, 0.50, 0.51, 0.52, 0.53, 0.54, 0.55, etc. By setting the ratio of the weight of the negative electrode active material layer per unit area to the weight of the positive electrode active material layer per unit area to 0.46 to 0.55, this method relatively increases the ratio of the weight of the negative electrode active material layer per unit area to the weight of the positive electrode active material layer per unit area. This reduces the expansion of the negative electrode during battery cycling, and reduces the extent to which the SEI layer on the negative electrode interface is stretched. This makes it difficult for the negative electrode active material layer to be exposed to the electrolyte, preventing direct contact between the negative electrode active material layer and the electrolyte, reducing the reduction reaction of the electrolyte, and thus achieving less loss of active lithium on the negative electrode surface.

[0030] The battery of the embodiment of the present application is described below by taking a lithium-ion battery as an example.

[0031] When the ratio z of the weight of the negative electrode active material layer per unit area to the weight of the positive electrode active material layer per unit area is low, there are too few locations in the negative electrode graphite where lithium can be embedded, and lithium precipitation may occur during battery charging. The precipitated lithium metal then reacts with the electrolyte, resulting in gas production and further aggravated consumption of active lithium. In addition, due to the high ratio of lithium embedded in graphite to graphite, during the cyclic graphite deintercalation and lithium extraction process, the layered graphite structure is in the process of expanding and shrinking the interlayer spacing, the graphite expands to a large extent, and the SEI layer on the negative electrode interface is pulled to a large extent, making the SEI layer on the negative electrode interface easily pulled and damaged, constantly exposing new graphite interfaces to direct contact with the electrolyte. The electrolyte constantly reacts on the graphite interface, consuming active lithium and causing loss of active lithium.

[0032] Since the battery of the embodiment of the present application increases the ratio of the weight of the negative electrode active material layer per unit area to the weight of the positive electrode active material layer per unit area, there are relatively more locations in the graphite in the negative electrode active material layer that can be used for lithium insertion, thereby reducing or avoiding the lithium plating phenomenon that may occur during the battery charging process; and, when the amount of active lithium is constant, the ratio of lithium embedded in the graphite to graphite is reduced, so that during the cyclic graphite deintercalation and lithium insertion process, the layered graphite structure is in the process of expanding and shrinking the interlayer spacing, the expansion amplitude of the graphite is smaller, and the SEI layer on the negative electrode interface is pulled to a smaller extent, so that the negative electrode graphite is difficult to be exposed to the electrolyte, preventing the negative electrode graphite from direct contact with the electrolyte, reducing the reduction reaction of the electrolyte, and achieving less active lithium loss on the negative electrode surface.

[0033] Increasing the ratio z of the weight of the negative electrode active material layer per unit area to the weight of the positive electrode active material layer per unit area can reduce the expansion of the graphite and the extent to which the SEI layer on the negative electrode interface is pulled less, making it difficult for the negative electrode graphite to be exposed to the electrolyte, preventing direct contact between the negative electrode graphite and the electrolyte, and reducing the reduction reaction of the electrolyte, thereby achieving less active lithium loss on the negative electrode surface. However, when the ratio z of the weight of the negative electrode active material layer per unit area to the weight of the positive electrode active material layer per unit area is large, on the one hand, the utilization rate of the negative electrode graphite is reduced, and on the other hand, when the battery is fully discharged, the potential of the negative electrode is relatively high, which may reach the oxidation potential of the SEI layer, causing the SEI layer to rupture during full discharge and then come into direct contact with components such as the electrolyte solvent, reacting and producing gas. Therefore, the ratio of the weight of the negative electrode active material layer per unit area to the weight of the positive electrode active material layer per unit area is set to 0.46-0.55 to reduce the risk of SEI layer rupture overall.

[0034] To better reduce the risk of SEI layer rupture during full charge and discharge of the battery, additive A is added to the electrolyte. That is, the additive in the embodiment of the present application includes additive A, and additive A is a substance containing an oxalic acid structure. Additive A has a high reduction potential at the negative electrode. When the battery is fully discharged, the battery potential is difficult to reach the oxidation potential of the SEI layer it generates, thereby reducing the risk of rupture caused by oxidation of the SEI layer and improving the gas production problem caused by oxidation of the SEI layer when the battery is stored in a fully discharged state.

[0035] It is understood that a battery is fully discharged when the battery is discharged to the lower cut-off voltage, and a battery is fully charged when the battery is charged to the upper cut-off voltage.

[0036] Optionally, additive A includes at least one of lithium bis(oxalatoborate) (LiBOB) and lithium difluorooxalatoborate (LiDFOB). For example, additive A includes lithium bis(oxalatoborate), or additive A includes lithium difluorooxalatoborate, or additive A includes lithium bis(oxalatoborate) and lithium difluorooxalatoborate.

[0037] According to the battery of the embodiment of the present application, by increasing the ratio of the negative electrode active material layer to the positive electrode active material layer, the expansion amplitude of the negative electrode is smaller during the cycle of the battery, and the SEI layer on the negative electrode interface is pulled less, thereby making it difficult for the negative electrode active material layer to be exposed to the electrolyte, preventing direct contact between the negative electrode active material layer and the electrolyte, and reducing the reduction reaction of the electrolyte, thereby achieving less active lithium loss on the negative electrode surface; and, at the same time, adding an additive A containing an oxalic acid structure to the electrolyte reduces the risk of rupture caused by oxidation of the SEI layer during full charge and discharge of the battery, thereby reducing the loss of active lithium caused by direct contact and reaction between the negative electrode active material layer and the electrolyte, thereby reducing the loss of active lithium on the negative electrode surface; in this way, by optimizing the ratio of the positive and negative electrodes and adjusting the electrolyte at the same time, the battery can reduce the risk of rupture of the SEI layer during cyclic use, reduce the loss of active material (such as active lithium), and thus reduce the capacity decay during the cycle, so that the battery has better cycle performance, thereby making the battery have a lower life decay amplitude and a longer service life.

[0038] According to some embodiments of the present application, the mass percentage of additive A in the electrolyte is a, and the value of a ranges from 0.1% to 1%. For example, the value of a can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, etc. If the amount of additive A is too small, it cannot effectively reduce the risk of rupture caused by SEI layer oxidation; if the amount of additive A is too large, since additive A also participates in SEI film formation, additive A's participation in SEI film formation will increase the density of the SEI layer, resulting in higher battery impedance and lower energy efficiency. Therefore, the amount of additive A should not be too large. By setting the mass percentage of additive A in the electrolyte to 0.1% to 1%, the risk of rupture caused by SEI layer oxidation can be effectively reduced, and the overall battery impedance can be lowered and the energy efficiency can be higher.

[0039] According to some embodiments of the present application, the solvent includes solvent B, and solvent B is a carboxylate solvent. As described above, adding additive A to the electrolyte can better reduce the risk of rupture caused by oxidation of the SEI layer, but the participation of additive A in SEI film formation causes the battery impedance to be larger. On the basis of adding additive A to the electrolyte, carboxylate is further added to the electrolyte, that is, the solvent of this embodiment includes solvent B, and solvent B is a carboxylate solvent. Carboxylate has the advantages of large dielectric constant and low viscosity, which allows lithium ions in the battery to be transmitted faster in the pores of the material, which can greatly reduce the impedance of the battery, so that the overall impedance of the battery can be lower and the energy efficiency can be higher. On the basis of adding additive A to the electrolyte, the solvent includes a carboxylate solvent, which can reduce the risk of rupture of the SEI layer while also making the overall impedance of the battery lower and the energy efficiency higher.

[0040] Optionally, solvent B includes at least one of methyl acetate, methyl propionate, ethyl acetate, ethyl propionate, propyl formate, propyl acetate, and propyl propionate. For example, solvent B includes one of methyl acetate, methyl propionate, ethyl acetate, ethyl propionate, propyl formate, propyl acetate, and propyl propionate; or, solvent B includes multiple of methyl acetate, methyl propionate, ethyl acetate, ethyl propionate, propyl formate, propyl acetate, and propyl propionate.

[0041] It should be explained that the “multiple” mentioned in this application refers to two or more.

[0042] According to some optional embodiments of the present application, the mass percentage of solvent B in the electrolyte is b, and the value of b ranges from 1% to 30%. For example, the value of b can be 1%, 3%, 5%, 7%, 10%, 12%, 15%, 18%, 20%, 23%, 25%, 28%, 30%, etc. If the amount of solvent B is too small, the overall impedance of the battery will still be high; if the amount of solvent B is too large, the carboxylate will easily oxidize at the high potential of the positive electrode, resulting in unnecessary side reactions and gas production, which will destroy the stability of the positive electrode and damage the battery interface. Therefore, the amount of solvent B should not be too much. By setting the mass percentage of solvent B in the electrolyte to 1% to 30%, the battery can have a lower impedance and reduce the damage to the positive electrode stability caused by unnecessary side reactions and gas production caused by the oxidation of the carboxylate.

[0043] Optionally, the value of b ranges from 5% to 15%, for example, the value of b can be 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, etc. By setting the mass percentage of solvent B in the electrolyte to 5% to 15%, the content of solvent B can be further optimized, thereby improving the overall performance of the battery.

[0044] According to some optional embodiments of the present application, the additive includes an additive C, and the additive C is a substance containing nitrogen heterocycles. As described above, the battery impedance is reduced by setting the solvent to include a carboxylate solvent, but the carboxylate is easily oxidized at a high potential of the positive electrode, resulting in unnecessary side reactions and gas production, which destroys the stability of the positive electrode and damages the battery interface. On the basis of introducing a carboxylate solvent into the electrolyte, an additive B is further added to the electrolyte, and the additive B is a nitrogen heterocycle. The nitrogen heterocycle can undergo oxidative polymerization on the positive electrode, and a dense CEI (positive electrode solid electrolyte interface) layer is polymerized and grown on the positive electrode interface, which can effectively inhibit the direct contact between the carboxylate and the positive electrode active material layer, and inhibit the oxidation of the carboxylate at the positive electrode. In this way, while reducing the risk of rupture of the SEI layer, the impedance of the battery as a whole can also be lower, the energy efficiency can be higher, and the impact on the stability of the positive electrode can be reduced, thereby improving the overall performance of the battery.

[0045] Optionally, the additive C includes at least one of 2-fluoropyrazine, 2,5-difluoropyrazine, 2-fluoropyridazine, 3-fluoropyridazine, 2-fluoropyridine, 3-fluoropyridine, 4-fluoropyridine, 2,3-fluoropyridine, and 3,4-fluoropyridine. For example, the additive C includes one of 2-fluoropyrazine, 2,5-difluoropyrazine, 2-fluoropyridazine, 3-fluoropyridazine, 2-fluoropyridine, 3-fluoropyridine, 4-fluoropyridine, 2,3-fluoropyridine, and 3,4-fluoropyridine; or, the additive C includes multiple of 2-fluoropyrazine, 2,5-difluoropyrazine, 2-fluoropyridazine, 3-fluoropyridazine, 2-fluoropyridine, 3-fluoropyridine, 4-fluoropyridine, 2,3-fluoropyridine, and 3,4-fluoropyridine.

[0046] According to some optional embodiments of the present application, the mass percentage of additive C in the electrolyte is c, and the value range of c is 0.1% to 0.5%. For example, the value of c can be 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, etc. If the amount of additive C is too small, a dense CEI layer cannot be polymerized and grown on the positive electrode interface, which will not effectively inhibit the oxidation of the carboxylate at the positive electrode caused by the direct contact between the carboxylate and the positive electrode active material layer; if the amount of additive C is too much, the dense CEI layer polymerized and grown on the positive electrode interface is thick, which will increase the impedance to a certain extent. Based on this, setting the mass percentage of additive C in the electrolyte to 0.1% to 0.5% can polymerize and grow a dense CEI layer on the positive electrode interface, effectively inhibit the direct contact between the carboxylate and the positive electrode active material layer, inhibit the oxidation of the carboxylate at the positive electrode, and make the battery impedance lower.

[0047] According to some optional embodiments of the present application, the mass percentage of additive A in the electrolyte is a, the mass percentage of solvent B in the electrolyte is b, and the mass percentage of additive C in the electrolyte is c; wherein z, a, b, and c satisfy the following relationship: 0.25≤(a+c)*(1-b)*100 / z≤2.15. For example, the value of (a+c)*(1-b)*100 / z can be 0.25, 0.40, 0.55, 0.70, 0.85, 1.0, 1.15, 1.30, 1.45, 1.60, 1.75, 1.90, 2.05, 2.15, etc. When (a+c)*(1-b)*100 / z is less than 0.25, the SEI film may oxidize and rupture when the battery is stored at full charge. Furthermore, after the SEI film ruptures, the solvent in the electrolyte easily reacts with the exposed negative electrode active material, resulting in gas production. When (a+c)*(1-b)*100 / z is greater than 2.15, the battery impedance will be relatively high, and lithium deposition is likely to occur, which will exacerbate cycle attenuation and increase impedance.

[0048] By making the ratio z of the weight of the negative electrode active material layer per unit area to the weight of the positive electrode active material layer per unit area, the mass percentage a of the additive A in the electrolyte, the mass percentage b of the solvent B in the electrolyte, and the mass percentage c of the additive C in the electrolyte satisfy the above relationship: 0.25≤(a+c)*(1-b)*100 / z≤2.15, the ratio of the positive and negative electrodes and the adjustment of the electrolyte can be further optimized, the SEI layer on the negative electrode interface can be pulled to a smaller extent, the risk of rupture caused by oxidation of the SEI layer can be reduced, and the gas production problem caused by oxidation of the SEI layer when the battery is stored in a fully discharged state can be improved, thereby reducing the risk of SEI layer rupture as a whole and achieving less active lithium loss on the negative electrode surface; at the same time, the battery impedance is reduced and the energy efficiency is higher.

[0049] An energy storage device according to an embodiment of the second aspect of the present application includes: a battery according to the embodiment of the first aspect of the present application.

[0050] According to the energy storage device of the embodiment of the present application, by providing the above-mentioned battery, the risk of rupture of the SEI layer can be reduced during the cycle of the battery, and the loss of active materials (such as active lithium) can be reduced, thereby reducing the capacity decay during the cycle process, so that the battery has better cycle performance and thus has a longer service life.

[0051] The batteries of the embodiments of the present application are further described below in conjunction with Comparative Examples 1 to 3 and Examples 1 to 19. In the following comparative examples and examples, the batteries are lithium-ion batteries.

[0052] 1. Preparation of lithium-ion batteries:

[0053] The preparation of lithium-ion batteries includes the preparation of positive electrode sheets, the preparation of negative electrode sheets, the preparation of electrolytes and the preparation of diaphragms;

[0054] Preparation of the positive electrode sheet: The positive electrode active material lithium iron phosphate, conductive carbon black SP, and binder PVDF are dispersed in the solvent NMP at a mass ratio of 97:0.7:2.3 and mixed evenly to obtain a positive electrode slurry; the positive electrode slurry is coated on a positive electrode current collector (such as aluminum foil), and after drying, cold pressing, slitting, and cutting, a positive electrode sheet is obtained (wherein the positive electrode active material layer is formed by solidifying the positive electrode slurry);

[0055] Preparation of negative electrode sheet: artificial graphite (negative electrode active material), 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 obtain a negative electrode slurry. The negative electrode slurry is coated on a negative electrode current collector (such as copper foil). After drying, cold pressing, slitting and cutting, a negative electrode sheet is obtained (wherein the negative electrode active material layer is formed by solidifying the negative electrode slurry);

[0056] Electrolyte preparation process: In an argon atmosphere glove box with a moisture content of ≤1ppm, ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate are mixed in a mass ratio of 1:1:1. Then, the dry electrolyte lithium hexafluorophosphate is dissolved in the solvent and stirred until completely dissolved. Fluorinated ethylene carbonate is added and mixed evenly to obtain the electrolyte.

[0057] Preparation of diaphragm: 16um polyethylene film was used as diaphragm;

[0058] The positive electrode sheet, separator, and negative electrode sheet prepared above are stacked in order so that the separator is in the middle of the positive and negative electrodes to separate the positive and negative electrode sheets. After winding, a bare battery cell is obtained. After welding the tabs, the battery cell is assembled into an outer package. After injecting the prepared electrolyte, the battery cell is packaged, allowed to stand, formed, shaped, and capacity tested, and finally a lithium-ion battery is prepared.

[0059] 2. Performance Testing

[0060] The lithium-ion battery prepared above was tested as follows:

[0061] 1) Cyclic performance test

[0062] The battery was tested for charge and discharge cycles on a charge and discharge instrument at a test temperature of 25°C, a cycle rate of 1C (i.e., both the charge rate and discharge rate were 1C), and a charge and discharge voltage range of 2.5V to 3.65V. The capacity retention after each cycle was calculated. The capacity retention calculation formula for cycling at 25°C is: Capacity retention after the nth cycle = (Discharge capacity after the nth cycle / Discharge capacity at the first cycle) * 100%.

[0063] 2) 50% SOC DC resistance (DCR) test

[0064] Place the battery in a constant temperature box at 25℃, discharge it at a constant current of 0.5C to a voltage of 2.5V, let it stand for 5 minutes, then charge it at a constant current of 0.5C to a voltage of 3.65V, and charge it at a constant voltage to a current of 0.025C; let it stand for 5 minutes, discharge it at a constant current of 0.1C to 2.5V, and record the discharge capacity at this time as C1; charge it at a constant current of 0.5C to 3.65V, and charge it at a constant voltage to a current of 0.025C, let it stand for 5 minutes, and discharge it at a constant current of 0.1C1 for 5 hours, and record the voltage V1 at this time; then, discharge it at a constant current of 1C for 30s, and record the voltage V2 at this time; the DC resistance DCR of the battery at 50% SOC is: (V1 V2) / C1.

[0065] 3) Battery expansion rate after 30 days of full storage at 60°C

[0066] The battery was tested on a charge and discharge instrument at a test temperature of 25°C. It was cycled three times at a rate of 1C (i.e., both the charge rate and the discharge rate were 1C), and the third discharge capacity was recorded as C1. The battery was then discharged to 2.5V at a constant current of 0.5C1. The thickness of the lithium-ion battery was tested and recorded as d1. The battery was then stored at 60°C for 30 days, and the monitored thickness was recorded as d2. The battery thickness storage expansion rate after storage at 60°C for 30 days = (d2-d1) / d1×100%.

[0067] The parameters and test data parameters of Comparative Examples 1 to 3 and Examples 1 to 19 are shown in Table 1 below.

[0068] Table 1

[0069] It can be seen from Table 1 above that the electrolytes of Comparative Examples 1 to 3 do not contain additive A, solvent B, and additive C, and their capacity retention rates after 1000 cycles at 25°C are all low, and their expansion rates of the 2.5V storage batteries at 60°C are all high. In Examples 1 to 19, additive A is added to the electrolyte, or additive A and solvent B are added to the electrolyte, or additive A, solvent B, and additive C are added to the electrolyte, and their capacity retention rates after 1000 cycles at 25°C are all high, and their expansion rates of the 2.5V storage batteries at 60°C are all low; and the values ​​of z are all between 0.46 and 0.55, and when the value of z is lower than 0.46, the capacity retention rate after 1000 cycles at 25°C is low and the expansion rate of the 2.5V storage batteries at 60°C is high, and when the value of z is higher than 0.55, the expansion rate of the 2.5V storage batteries at 60°C is high. It can be seen that the battery of the embodiment of the present application has good cycle performance and the impedance of the battery is also at a low level.

[0070] As shown in Table 1, compared to the electrolytes of Comparative Examples 1 to 3, which did not include Additive A, Solvent B, and Additive C, Examples 1 to 8, all of which included Additive A, significantly improved the capacity retention after 1000 cycles at 25°C, and significantly reduced the expansion rate of the battery after 2.5V storage at 60°C. This indicates that when the value of z is between 0.46 and 0.55, the addition of Additive A to the electrolyte can effectively improve the battery's cycling performance and reduce the expansion rate.

[0071] It can be seen from Table 1 above that compared with Examples 1 to 8 in which additive A is added to the electrolyte, Examples 9 and 10 further add solvent B to the electrolyte on the basis of additive A. While maintaining a high capacity retention rate after 1000 cycles at 25°C and a low expansion rate of the 2.5V storage battery at 60°C, the impedance of the battery is reduced.

[0072] It can be seen from Table 1 above that in Examples 11 to 19, compared with the value of (a+c)*(1-b)*100 / z being less than 0.25 or greater than 2.15, when the value of (a+c)*(1-b)*100 / z is between 0.25 and 2.15, the capacity retention rate after 1000 cycles at 25°C is higher and the expansion rate of the 2.5V storage battery at 60°C is lower. Moreover, it can be seen from the data of Examples 11 to 19 that when the value of (a+c)*(1-b)*100 / z is less than 0.25 (for example, Example 17 in Table 1), relative to the value of (a+c)*(1-b)*100 / z between 0.25 and 2.15, the expansion rate of the 60°C 2.5V storage battery is significantly higher, and the capacity retention rate after 1000 cycles at 25°C is acceptable, but slightly lower; when the value of (a+c)*(1-b)*100 / z is greater than 2.15 (for example, Example 19 in Table 1), relative to the value of (a+c)*(1-b)*100 / z between 0.25 and 2.15, the battery impedance is relatively high. If the impedance is too large, lithium plating will occur, thereby affecting the cycle performance.

[0073] Throughout this specification, reference to terms such as "one embodiment," "some embodiments," "illustrative embodiments," "examples," "specific examples," or "some examples" means that the specific features, structures, materials, or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0074] Although the embodiments of the present application have been shown and described, those skilled in the art will appreciate that various changes, modifications, substitutions, and variations may be made to the embodiments without departing from the principles and intent of the present application, and that the scope of the present application is defined by the claims and their equivalents.

Claims

1. A battery, characterized in that: include: A positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the separator is located between the positive electrode sheet and the negative electrode sheet, the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer covering the positive electrode current collector, the negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer covering the negative electrode current collector, and the electrolyte comprises an electrolyte, a solvent and an additive; Among them, the ratio of the weight of the negative electrode active material layer per unit area to the weight of the positive electrode active material layer per unit area is z, and the value range of z is 0.46-0.55; the additive includes additive A, and the additive A is a substance containing oxalic acid structure.

2. The battery according to claim 1, characterized in that The mass percentage content of the additive A in the electrolyte is a, and the value range of a is 0.1% to 1%.

3. The battery according to claim 1 or 2, characterized in that: The additive A includes at least one of lithium dioxalatoborate and lithium difluorooxalatoborate.

4. The battery according to any one of claims 1 to 3, characterized in that The solvent includes solvent B, and the solvent B is a carboxylate solvent.

5. The battery according to claim 4, characterized in that The mass percentage of the solvent B in the electrolyte is b, and the value range of b is 1% to 30%.

6. The battery according to claim 5, characterized in that The value range of b is 5% to 15%.

7. The battery according to any one of claims 4 to 6, characterized in that: The solvent B includes at least one of methyl acetate, methyl propionate, ethyl acetate, ethyl propionate, propyl formate, propyl acetate and propyl propionate.

8. The battery according to any one of claims 4 to 6, characterized in that: The additives include an additive C, and the additive C is a substance containing a nitrogen heterocycle.

9. The battery according to claim 8, characterized in that The mass percentage content of the additive C in the electrolyte is c, and the value range of c is 0.1% to 0.5%.

10. The battery according to claim 8 or 9, characterized in that: The additive C includes at least one of 2-fluoropyrazine, 2,5-difluoropyrazine, 2-fluoropyridazine, 3-fluoropyridazine, 2-fluoropyridine, 3-fluoropyridine, 4-fluoropyridine, 2,3-fluoropyridine and 3,4-fluoropyridine.

11. The battery according to any one of claims 8 to 10, characterized in that: The mass percentage of the additive A in the electrolyte is a, the mass percentage of the solvent B in the electrolyte is b, and the mass percentage of the additive C in the electrolyte is c; Among them, z, a, b, and c satisfy: 0.25≤(a+c)*(1-b)*100 / z≤2.

15.

12. An energy storage device, characterized in that: include: A battery according to any one of claims 1 to 11.

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