Secondary battery and electric device

Abstract

Provided in the present application are a secondary battery and an electric device. During the cyclic charge-discharge process of the secondary battery, the discharge capacity of at least one discharge process is greater than the discharge capacity C1 of the first cycle, wherein the conditions for cyclic charge and discharge are: performing charge and discharge at a constant power of 0.5P at 25°C, and the cyclic test voltage ranging from 2.0V to 3.6V. Compared with the first-cycle discharge capacity of the secondary battery, the discharge capacity thereof during the cycle process exhibits an "upward drift", thereby delaying the onset of performance degradation of the secondary battery, reducing the performance degradation speed of the secondary battery during an initial discharge process, and helping to improve the cycle life of the secondary battery. Thus, the present application is particularly suitable for energy-storage secondary batteries where cycle life is of particular concern.

Description

Secondary batteries and electrical devices Technical Field

[0001] The present application relates to the technical field of secondary batteries, and in particular to a secondary battery and an electrical device. Background Art

[0002] In recent years, as the application scope of lithium-ion batteries has become increasingly wider, lithium-ion batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power stations, as well as power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace and other fields.

[0003] With the continuous expansion of secondary battery application scenarios, higher requirements are placed on the cycle life of secondary batteries.

[0004] Summary of the Invention

[0005] The present application has been made in view of the above-mentioned problems, and an object of the present application is to provide a secondary battery having a long cycle life.

[0006] In order to achieve the above-mentioned purpose, the present application provides a secondary battery, in which the discharge capacity of at least one discharge process during the cyclic charge and discharge process is greater than the discharge capacity C1 of the first cycle; wherein the conditions of the cyclic charge and discharge are: charging and discharging at a constant power of 0.5P at 25°C, and the cycle test voltage range is 2.0V~3.6V.

[0007] Therefore, the discharge capacity of the secondary battery of the present application "drifts up" during the cycle compared to the discharge capacity of the first cycle, which prolongs the time for the secondary battery performance to decay and slows down the performance decay rate of the secondary battery during the initial discharge process, which is beneficial to improving the cycle life of the secondary battery and is especially suitable for energy storage secondary batteries that pay special attention to cycle life.

[0008] In any embodiment, during the cyclic charge and discharge process, the maximum capacity retention rate Q of the secondary battery satisfies: 100.1%≤Q<110%; wherein the maximum capacity retention rate Q represents the maximum discharge capacity C during the cyclic charge and discharge process. m The ratio of the discharge capacity C1 to the first cycle.

[0009] The secondary battery has a high maximum capacity retention rate. The discharge capacity of the secondary battery increases to a certain extent during the cycle, which further slows down the performance decay rate of the secondary battery during the initial discharge process and can further improve the cycle life of the secondary battery.

[0010] In any embodiment, the maximum capacity retention rate Q of the secondary battery during cycling satisfies: 100.2%≤Q≤105%.

[0011] In any embodiment, the secondary battery satisfies the following conditions: 0.001% < (Q-1) / (NM) ≤ 1.20%, with the unit being 1 / cycle; wherein Q represents the maximum capacity retention rate of the secondary battery, i.e., the maximum discharge capacity C during the cyclic charge and discharge process. m The ratio of the discharge capacity C1 of the first cycle to the discharge capacity C1 of the first cycle; M represents the number of cycles corresponding to the maximum capacity retention rate Q of the secondary battery, in cycles; N represents the number of cycles N corresponding to the capacity retention rate of the secondary battery decaying from the maximum capacity retention rate Q to basically 100% during the cyclic charge and discharge process, in cycles.

[0012] For secondary batteries with (Q-1) / (NM) within the above range, the discharge capacity has a slower decay rate during the drift-up stage, which can effectively improve the overall cycle stability of the secondary battery and comprehensively improve the cycle life of the secondary battery.

[0013] In any embodiment, during the cyclic charge and discharge process, the number of cycles corresponding to the maximum capacity retention rate of the secondary battery is M, in units of cycles, and M satisfies: 10≤M≤1000.

[0014] The secondary battery can achieve its maximum capacity retention rate only after a certain number of cycles, further extending the time before the secondary battery performance decays, improving the phenomenon of performance decay during the initial discharge process of the secondary battery, and being beneficial to improving the cycle life of the secondary battery. It is especially suitable for energy storage secondary batteries that pay special attention to cycle life.

[0015] In any embodiment, during the cyclic charge and discharge process, the number of cycles corresponding to the capacity retention rate of the secondary battery decaying from the maximum capacity retention rate to substantially 100% is N, in units of cycles, and N satisfies: 20≤N≤2000.

[0016] After a certain number of cycles, the discharge capacity of the secondary battery is equal to the discharge capacity of the first cycle, which further prolongs the time for the secondary battery performance to decay, improves the phenomenon of performance decay of the secondary battery during the initial discharge process, and is beneficial to further improve the cycle life of the secondary battery. It is especially suitable for energy storage secondary batteries that pay special attention to cycle life.

[0017] In any embodiment, the ratio of the polarization voltage of the secondary battery after the first discharge cycle to the polarization voltage of the secondary battery after M cycles is 1.08-1.45.

[0018] Polarization voltage can be used to characterize the degree of polarization of a battery. A higher polarization voltage often means a more severe polarization phenomenon in the battery. Polarization refers to the phenomenon that the electrode potential changes with the applied current density. Battery polarization can be caused by a variety of reasons. For example, the charge exchange rate (electrochemical reaction rate) on the electrode surface is less than the electron transfer rate in the electrode, which will cause electrochemical polarization; the diffusion rate of active ions in the electrolyte is less than their consumption or generation rate on the electrode surface, which will cause concentration polarization; due to the formation of oxide films, passivation films or other high-resistance insoluble corrosion products on the electrode surface, the system resistance increases, which will cause ohmic polarization. The more severe the battery polarization phenomenon, the worse the battery's ability to release capacity and output electrical work. Therefore, in the prior art, people try to pursue depolarization in battery manufacturing and use. The embodiments of the present application break the prejudice of the prior art. Through the design of materials and electrode pieces, the battery has a relatively high polarization level in the initial state, and gradually depolarizes during the cyclic charge and discharge process, so that the secondary battery can have a capacity drift during the cycle, thereby restraining the rapid decay of the secondary battery discharge capacity in the early stage of the cycle and improving the cycle life of the secondary battery.

[0019] In any embodiment, the secondary battery includes a negative electrode plate, the negative electrode plate includes a negative electrode current collector and a negative electrode film layer disposed on at least one side of the negative electrode current collector, and the surface of the negative electrode film layer includes inorganic oxygen-containing acid radicals.

[0020] The presence of inorganic oxygen-containing radicals on the surface of the negative electrode film gives the battery a high resistance at the beginning of the cycle, resulting in a large ohmic polarization. As the secondary battery cycles, the inorganic oxygen-containing radicals on the surface of the negative electrode film continuously evolve into lithium-containing inorganic substances in the solid electrolyte interface (SEI), which in turn reduces the overall battery resistance, improves ionic conductivity, and reduces polarization, achieving discharge capacity rebound and improving battery cycling stability.

[0021] In any embodiment, the inorganic oxygen-containing acid radicals include one or more of nitrate and nitrite.

[0022] The nitrate and phosphite on the surface of the negative electrode film gradually participate in the formation of the solid electrolyte membrane (SEI membrane) during the battery cycle, and evolve into inorganic components such as lithium oxide and lithium nitride, which can effectively improve the ionic conductivity of the SEI membrane, reduce the overall polarization of the battery, achieve an increase in the battery capacity level during the cycle, and improve the battery's cycle stability.

[0023] In any embodiment, the negative electrode film layer includes a negative electrode film-forming additive. Optionally, the negative electrode film-forming additive includes an inorganic oxygen acid salt.

[0024] The above-mentioned negative electrode film-forming additives can gradually participate in the formation of the solid electrolyte membrane (SEI membrane) during the battery cycle, evolving into inorganic components such as lithium oxide and lithium nitride, effectively improving the ionic conductivity of the SEI membrane, reducing the overall polarization of the battery, and realizing the improvement of the battery capacity level during the cycle process, thereby improving the cycle stability of the battery.

[0025] In any embodiment, the inorganic oxygen-containing acid salt includes one or more of lithium nitrate, sodium nitrate, potassium nitrate, rubidium nitrate, cesium nitrate, lithium phosphate, lithium nitrite, sodium nitrite, and potassium nitrite.

[0026] In any embodiment, the inorganic oxygen-containing acid salt includes at least one of lithium nitrate and potassium nitrate.

[0027] In any embodiment, the negative electrode film layer includes a negative electrode material, and the specific surface area of ​​the negative electrode material is S, in m 2 / g, S meets: 0.9m 2 / g≤S≤1.7m 2 / g, optionally 0.9m 2 / g≤S≤1.2m 2 / g.

[0028] Compared to the negative electrode materials commonly used in the prior art, the negative electrode material in this application has a relatively small specific surface area, resulting in a relatively small contact area between the negative electrode material and the electrolyte, which in turn reduces the number of active ions (such as lithium ions) that enter the negative electrode material per unit time to exert its capacity, resulting in large polarization and low kinetic performance of the battery. As the negative electrode material expands and contracts during the cycle, the negative electrode sheet cracks and the electrolyte continues to infiltrate, the number of ion channels through which active ions are embedded in the negative electrode material increases, the battery polarization decreases, and the discharge capacity increases. This suppresses the significant attenuation of the electrochemical performance of the secondary battery in the early stages of the cycle, which is beneficial to improving the battery cycle life.

[0029] In any embodiment, the volume distribution particle size Dv90 of the negative electrode material is recorded as Dv90 负 , unit is μm, Dv90 负 Meet: 17μm≤Dv90 负 ≤35μm, optionally, 18μm≤Dv90 负 ≤26μm.

[0030] Compared to commonly used anode materials in the prior art, the anode material in this application has a relatively large particle size, which requires a longer transmission path for active ions to embed into the center of the anode material. This makes it difficult for the anode material to fully embed lithium, which in turn hinders the full utilization of the battery's capacity. As the anode material expands and contracts during cycling, it cracks and its particle size decreases. This shortens the transmission path for active ions to embed into the center of the anode material, reduces battery polarization, and improves discharge capacity. This prevents significant attenuation of the secondary battery's electrochemical performance during the initial stages of cycling.

[0031] In any embodiment, based on the total mass of the negative electrode film layer, the mass proportion p of the negative electrode film-forming additive satisfies: 0.05%≤p≤10%, optionally, 0.4%≤p≤1%.

[0032] The negative electrode film-forming additives within the above-mentioned content range will neither affect the energy density of the battery due to excessive addition; nor will they be precipitated from the negative electrode film layer due to excessive addition, thereby affecting the conductivity of the electrolyte; and can fully play the role of forming the SEI film, gradually evolving into inorganic components such as lithium oxide and lithium nitride during the battery cycle, effectively improving the ionic conductivity of the SEI film, reducing the overall polarization of the battery, achieving an increase in the battery capacity level during the cycle, and improving the cycle stability of the battery.

[0033] In any embodiment, the secondary battery includes a positive electrode plate, the positive electrode plate includes a positive electrode current collector and a positive electrode film layer arranged on at least one side of the positive electrode current collector, the positive electrode film layer includes a positive electrode active material, the positive electrode active material includes a carbon element, and the mass percentage C of carbon in the positive electrode active material satisfies: 1.2%≤C≤2.3%, optionally, 1.3%≤C≤2.0%, and further optionally, the carbon element is enriched on the surface of the positive electrode active material.

[0034] In any embodiment, the positive electrode active material includes a carbon coating layer, and a thickness H of the carbon coating layer satisfies: 2.5 nm ≤ H ≤ 10 nm, optionally, 3 nm ≤ H ≤ 8 nm.

[0035] When the positive electrode active material contains a carbon coating layer within the above range or a carbon element within the above range, especially when the carbon element within the above range is enriched on the surface of the positive electrode active material, it is difficult for the positive electrode active material to be effectively infiltrated in the electrolyte during the first cycle of the secondary battery. The initial number of active ion pathways in the positive electrode film layer is small, the ion transmission rate of the positive electrode active material is limited, and the secondary battery has a large polarization. Therefore, in the first cycle of the secondary battery cycle, during the discharge process, it is difficult for the active ions embedded in the negative electrode to be completely embedded back into the electrode, and the discharge capacity of the secondary battery is difficult to fully exert. However, as the secondary battery is charged and discharged, the positive electrode active material continues to expand and contract, causing the positive electrode active material to be gradually infiltrated by the electrolyte, the number of ion channels increases, and the proportion of active ions embedded in the negative electrode embedded back into the positive electrode increases, thereby improving the discharge capacity of the battery. Although a larger coating thickness can further improve the maximum discharge capacity retention rate Q during cycling, it will result in excessive loss of the secondary battery's absolute capacity and energy density. Furthermore, the coating cannot break down during the secondary battery's charge and discharge cycles, which will have a continuous negative impact on the ion transport in the positive electrode film, causing the secondary battery's discharge capacity to decay at a high rate. Positive electrode active materials within the above range can achieve a "drift" in discharge capacity while taking into account the secondary battery's absolute capacity and internal resistance, allowing the secondary battery to maintain a certain capacity level over long cycles, thereby comprehensively improving the secondary battery's cycle life.

[0036] In any embodiment, the volume distribution particle size Dv50 of the positive electrode active material is recorded as Dv50 正 , unit is μm, Dv50 正 Meet: 0.5μm≤Dv50 正 ≤2.5μm.

[0037] Volume distribution particle size Dv50 正 The positive electrode active material within the above range has a small number of initial active ion pathways and a long path, making it difficult for the electrolyte to fully penetrate the positive electrode active material during the first cycle of the secondary battery cycle, and for the active ions to be completely released from the positive electrode active material, resulting in the battery's discharge capacity not being effectively utilized. However, as the secondary battery's charge and discharge cycles progress, the positive electrode active material is continuously soaked by the electrolyte, the ion transport resistance of the positive electrode active material decreases, and the depth of ion release in the positive electrode active material increases, further promoting the continuous release of the capacity of the positive electrode active material and improving the discharge capacity of the secondary battery. This phenomenon, known as the "drift" of discharge capacity, improves the problem of high capacity decay in the initial stage of the secondary battery cycle.

[0038] In any embodiment, the positive electrode active material includes one or more of lithium-containing phosphates, lithium cobaltate, lithium manganate, and lithium-rich manganese-based materials.

[0039] In any embodiment, the lithium-containing phosphate includes a component Li shown in formula I x A y Me a M b P 1-c X c Y z (Formula I)

[0040] Wherein, 0≤x≤1.3, 0≤y≤1.3, and 0.9≤x+y≤1.3; 0.9≤a≤1.5, 0≤b≤0.5, and 0.9≤a+b≤1.5; 0≤c≤0.5; 3≤z≤5; A includes one or more of Na, K, and Mg; Me includes one or more of Mn, Fe, Co, and Ni; M includes one or more of B, Mg, Al, Si, S, Ca, Sc, Ti, V, Cr, Cu, Zn, Sr, Y, Zr, Nb, Mo, Cd, Sn, Sb, Te, Ba, Ta, W, Yb, La, and Ce; X includes one or more of S, Si, Cl, B, C, and N; and Y includes one or more of O and F, wherein any two of A, Me, M, X, and Y do not include the same element at the same time.

[0041] Although the above-mentioned positive electrode active materials have good cycling stability, the discharge capacity of the secondary battery tends to decay significantly in the early stages of cycling, making it difficult to further improve the lifespan of the secondary battery. The embodiments of the present application can effectively improve the rapid decay of battery capacity in the early stages of cycling of secondary batteries containing the above-mentioned positive electrode active materials, thereby further improving the lifespan of the secondary battery.

[0042] In any embodiment, the compacted density of the negative electrode sheet is 1.4 g / cm 3 -1.67g / cm 3 The single-side density of the negative electrode sheet is 8 mg / cm 2 -15mg / cm 2 The compaction density of the positive electrode is 2.5g / cm 3 -2.7g / cm 3 The single-side density of the positive electrode sheet is 18 mg / cm 2 -30mg / cm 2 .

[0043] In any embodiment, the secondary battery includes an electrolyte, the electrolyte contains an electrolyte additive, the electrolyte additive includes a fluorine-containing additive, and the fluorine-containing additive includes one or more of fluorinated carbonates, fluorinated phosphates, fluorinated carbamates, fluorinated sulfates, fluorinated carboxylates, fluorinated ethers, fluorinated nitriles, fluorinated silanes, and fluorinated sulfones.

[0044] The electrolyte additives can further improve the oxidation resistance and stability of the electrolyte, which is beneficial to further improve the cycle life of the secondary battery.

[0045] In any embodiment, based on the total mass of the electrolyte, the mass content A of the electrolyte additive is 0.1%≤A≤5%, optionally 0.5%≤A≤2%.

[0046] The second aspect of the present application further provides an electric device comprising the secondary battery according to the third aspect of the present application.

[0047] The electric device has a long battery life. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] FIG1 is a schematic diagram of a discharge capacity-cycle number curve of a secondary battery according to one embodiment of the present application.

[0049] FIG2 is a schematic diagram of a discharge capacity retention rate-cycle number cycle curve of a secondary battery according to an embodiment of the present application.

[0050] FIG3 is a schematic diagram of a discharge capacity retention rate-cycle number cycle curve of a secondary battery according to an embodiment of the present application and a comparative example.

[0051] FIG4 is a polarization voltage curve diagram of a secondary battery according to an embodiment of the present application and a comparative example.

[0052] FIG5 is a transmission electron microscope image of a positive electrode active material of a secondary battery according to an embodiment of the present application.

[0053] FIG6 is a schematic diagram of a secondary battery according to an embodiment of the present application.

[0054] FIG. 7 is an exploded view of the secondary battery according to the embodiment of the present application shown in FIG. 6 .

[0055] FIG8 is a schematic diagram of a battery module according to an embodiment of the present application.

[0056] FIG9 is a schematic diagram of a battery pack according to an embodiment of the present application.

[0057] FIG10 is an exploded view of the battery pack shown in FIG9 according to an embodiment of the present application.

[0058] FIG. 11 is a schematic diagram of an electric device using a secondary battery as a power source according to an embodiment of the present application.

[0059] Explanation of reference numerals: 1 battery pack; 2 upper case; 3 lower case; 4 battery module; 5 secondary battery; 51 housing; 52 electrode assembly; 53 top cover assembly. DETAILED DESCRIPTION

[0060] Below, the embodiments of the secondary battery and the electrical device of the present application are described in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0061] " range " disclosed in the present application is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0062] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0063] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0064] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0065] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0066] Unless otherwise specified, the term "or" is used in this application to be inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0067] As shown in FIG1 , secondary batteries in the prior art are prone to a significant drop in discharge capacity during initial use and storage, making it difficult to further increase the cycle life of the secondary batteries. This phenomenon is particularly pronounced in energy storage secondary batteries, where cycle life is a key concern.

[0068] Based on this, the present application proposes a secondary battery, in which the discharge capacity of at least one discharge process during the cyclic charge and discharge process is greater than the discharge capacity of the first cycle; wherein the conditions for the cyclic charge and discharge are: charging and discharging at a constant power of 0.5P at 25°C, and the cycle test voltage range is 2.0V~3.6V.

[0069] In some embodiments, the cyclic charge-discharge conditions are as follows: at 25°C, the battery is charged at a constant power of 0.5P to 3.6V, and then discharged at a constant power of 0.5P to 2.0V. This constitutes one cycle of the cyclic charge-discharge process, and the capacity measured during the discharge process is the discharge capacity. During the cyclic charge-discharge process, the discharge capacity of the first cycle under stable test temperature, environment, and other test conditions is recorded as the discharge capacity of the first cycle. Alternatively, the first cycle in which the discharge capacity of the secondary battery changes steadily can be regarded as the first cycle.

[0070] It should be noted that the secondary battery is not necessarily a battery that has not been cycled after formation, but also includes a battery that has been cycled, as long as the battery can undergo at least two cycles of charge and discharge under charge and discharge conditions known in the art. The "first cycle" of a secondary battery can be the first cycle test performed by the relevant experimental operator on the secondary battery (which can be a secondary battery that has been cycled). The "first cycle" of a secondary battery can also be a cycle before the second battery reaches the maximum discharge capacity corresponding to the number of cycles during the cycle. The cyclic charge and discharge conditions here refer to the test conditions of the secondary battery during the cyclic charge and discharge process, and do not specify the rated or preferred cyclic charge and discharge mode of the secondary battery.

[0071] FIG1 also shows a schematic diagram of a cycle curve for a secondary battery according to one embodiment of the present application. Referring to FIG1 , the fact that at least one discharge process during a cyclic charge-discharge process has a discharge capacity greater than the discharge capacity of the first cycle means that at least one discharge process during this cyclic charge-discharge process has a discharge capacity greater than the discharge capacity C1 of the first cycle of this cyclic charge-discharge process. In some embodiments, at least x consecutive discharge processes during the cyclic charge-discharge process have a discharge capacity greater than the discharge capacity of the first cycle, where x can be any integer between 3 and 10.

[0072] It can be understood that in the embodiment of the present application, the fact that the discharge capacity of the secondary battery is greater than the discharge capacity of the first cycle in at least one discharge process during the cyclic charge and discharge process does not mean that the discharge capacity of the second cycle of the secondary battery during the cyclic charge and discharge process must be greater than the discharge capacity of the first cycle.

[0073] The discharge capacity of the secondary battery during the cycle "drifts up" compared to the discharge capacity in the first cycle, which prolongs the time for the secondary battery performance to decay and slows down the performance decay rate of the secondary battery during the initial discharge process, which is beneficial to improving the cycle life of the secondary battery. It is especially suitable for energy storage secondary batteries that pay special attention to cycle life.

[0074] In some embodiments, during the cyclic charge and discharge process, the maximum capacity retention rate Q of the secondary battery satisfies: 100.1%≤Q<110%; wherein the maximum capacity retention rate Q represents the ratio of the maximum discharge capacity during the cyclic charge and discharge process to the discharge capacity of the first cycle.

[0075] Please refer to Figure 1 and Figure 2. The maximum capacity retention rate Q of the secondary battery represents the maximum discharge capacity C during the cyclic charge and discharge process. m The ratio of the discharge capacity C1 to the first cycle.

[0076] In some embodiments, during the cyclic charge and discharge process, the maximum capacity retention rate Q of the secondary battery can be selected to be 100.1%, 100.2%, 100.3%, 100.4%, 100.5%, 100.6%, 100.7%, 100.8%, 100.9%, 101%, 102%, 103%, 104%, 105%, 106%, 107%, 108%, 109%, 109.5% or any numerical range therebetween.

[0077] In some embodiments, the maximum capacity retention rate Q of the secondary battery during cycling satisfies: 100.2%≤Q≤105%.

[0078] The secondary battery has a high maximum capacity retention rate. The discharge capacity of the secondary battery increases to a certain extent during the cycle, which further slows down the performance decay rate of the secondary battery during the initial discharge process and can further improve the cycle life of the secondary battery.

[0079] In some embodiments, please continue to refer to Figure 2, the secondary battery satisfies: 0.001% < (Q-1) / (NM) ≤ 0.120%, with the unit being 1 / cycle; wherein Q represents the maximum capacity retention rate of the secondary battery, that is, the ratio of the maximum discharge capacity Cm during the cyclic charge and discharge process to the discharge capacity C1 of the first cycle; M represents the number of cycles corresponding to the maximum capacity retention rate Q of the secondary battery, with the unit being cycles; N represents the number of cycles N corresponding to the time when the capacity retention rate of the secondary battery decays from the maximum capacity retention rate Q to basically 100% during the cyclic charge and discharge process, with the unit being cycles.

[0080] As shown in FIG2 , the number of cycles N corresponding to when the capacity retention rate of the secondary battery decays from the maximum capacity retention rate Q to substantially 100% means that during the cyclic charge and discharge process, the maximum capacity retention rate of the secondary battery in the Nth cycle is slightly greater than or equal to 100%; during the cyclic charge and discharge process after the Nth cycle, the maximum capacity retention rate of the secondary battery is less than 100%.

[0081] Continuing with Figure 2, (Q-1) / (NM) represents the absolute value of the slope of line segment E, which indicates the rate at which the secondary battery's capacity retention rate decays during the battery's discharge capacity ramp-up phase. The smaller (Q-1) / (NM), the slower the rate at which the secondary battery's capacity retention rate decays.

[0082] In some embodiments, (Q-1) / (NM) may be 0.002%, 0.003%, 0.004%, 0.005%, 0.006%, 0.007%, 0.008%, 0.009%, 0.010%, 0.011%, 0.012%, 0.0125%, 0.013%, 0.014%, 0.015%, 0.016%, 0.017%, 0.018%, 0.019%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, 0.11%, 0.12%, or any range therebetween.

[0083] For secondary batteries with (Q-1) / (NM) within the above range, the discharge capacity has a slower decay rate during the drift-up stage, which can further amplify the cycle improvement effect through the slow decay of capacity, thereby comprehensively improving the cycle life of the secondary battery.

[0084] In some embodiments, during the cyclic charge and discharge process, the number of cycles corresponding to the maximum capacity retention rate of the secondary battery is M, in units of cycles, and M satisfies: 10≤M≤1000.

[0085] In some embodiments, M can be selected as 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000 or any range therebetween.

[0086] The secondary battery can achieve its maximum capacity retention rate only after a certain number of cycles, further extending the time before the secondary battery performance decays, improving the phenomenon of performance decay during the initial discharge process of the secondary battery, and being beneficial to improving the cycle life of the secondary battery. It is especially suitable for energy storage secondary batteries that pay special attention to cycle life.

[0087] In some embodiments, during the cyclic charge and discharge process, the number of cycles corresponding to the capacity retention rate of the secondary battery decaying from the maximum capacity retention rate to substantially 100% is N, in units of cycles, and N satisfies: 20≤N≤2000.

[0088] In some embodiments, N can be selected as 20, 30, 40, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500, 1600, 1700, 1800, 1900, 2000 or any range therebetween.

[0089] After a certain number of cycles, the discharge capacity of the secondary battery is equal to the discharge capacity of the first cycle, indicating that the discharge capacity of the secondary battery can not only be effectively improved but also slowly decayed, which is beneficial to further improve the cycle life of the secondary battery. It is especially suitable for energy storage secondary batteries that pay special attention to cycle life.

[0090] In some embodiments, during the cyclic charge and discharge process, the ratio of the polarization voltage of the secondary battery after the first discharge cycle to the polarization voltage of the secondary battery after M cycles is 1.08-1.45.

[0091] Polarization voltage refers to the potential at which the electrode potential shifts during charging or discharging. Polarization voltage can be measured using any test method known in the art. For example, after each discharge cycle, the battery is set to a cutoff voltage of V1 = 2.0V. After standing for 30 minutes, the voltage rebounds to above 2.0V. This voltage is recorded as V2, and V2-V1 is the polarization voltage.

[0092] In some embodiments, during the cyclic charge and discharge process, the ratio of the polarization voltage of the secondary battery after the first discharge cycle to the polarization voltage of the secondary battery after M cycles can be selected as 1.08, 1.10, 1.15, 1.20, 1.25, 1.30, 1.35, 1.40, 1.45 or any numerical range therebetween.

[0093] Polarization voltage can be used to characterize the degree of polarization of the battery. A higher polarization voltage often means a more severe polarization phenomenon in the battery. Polarization refers to the phenomenon that the electrode potential changes with the change of the applied current density. Battery polarization can be caused by a variety of reasons. For example, the charge exchange rate (electrochemical reaction rate) on the electrode surface is less than the electron transfer rate in the electrode, which will cause electrochemical polarization; the diffusion rate of active ions in the electrolyte is less than their consumption or generation rate on the electrode surface, which will cause concentration polarization; due to the formation of oxide film, passivation film or other high-resistance insoluble corrosion products on the electrode surface, the system resistance increases, which will cause ohmic polarization. The more severe the battery polarization phenomenon, the worse the battery's ability to release capacity and output electrical work. Therefore, in the existing technology, people try to pursue a low polarization level of secondary batteries in the initial state during battery manufacturing and use. The embodiments of the present application break the prejudice of the prior art. The battery has a high polarization level in the initial state and achieves significant "depolarization" during the cyclic charge and discharge process (that is, the ratio of the polarization voltage of the secondary battery after the first discharge cycle of the present application to the polarization voltage after the secondary battery has been cycled M times is greater than 1.08), so that the secondary battery can have a capacity drift during the cycle, thereby restraining the rapid decay of the discharge capacity of the secondary battery in the early stage of the cycle and improving the cycle life of the secondary battery.

[0094] In some embodiments, the secondary battery includes a negative electrode plate, the negative electrode plate includes a negative electrode current collector and a negative electrode film layer disposed on at least one side of the negative electrode current collector, and the surface of the negative electrode film layer includes inorganic oxygen-containing acid radicals.

[0095] Inorganic oxygen-containing acid radicals on the surface of the negative electrode film can be detected by X-ray photoelectron spectroscopy (XPS). As an example, an ESCALab220i-XL X-ray photoelectron spectroscopy instrument was used for detection. The negative electrode was used as the sample, an Al target was used as the X-ray source, and a power of 300W was selected. X-rays were used to bombard the surface of the negative electrode film to excite the inner electrons or valence electrons of atoms or molecules. C1s (284.8eV) was used as the XPS reference peak to analyze the surface elements and groups.

[0096] The presence of inorganic oxygen-containing radicals on the surface of the negative electrode film contributes to a high resistance in the initial stages of battery cycling, leading to a large ohmic polarization. As the secondary battery cycles, these inorganic oxygen-containing radicals on the surface of the negative electrode film continuously evolve into lithium-containing inorganic species within the solid electrolyte interphase (SEI), reducing the overall battery resistance, increasing ionic conductivity, and reducing polarization, ultimately achieving discharge capacity rebound and improving battery cycling stability.

[0097] In some embodiments, the inorganic oxygen-containing acid radicals include one or more of nitrate and nitrite.

[0098] The nitrate and nitrite on the surface of the negative electrode film gradually participate in the formation of the solid electrolyte membrane (SEI membrane) during the battery cycle, and evolve into inorganic components such as lithium oxide and lithium nitride. They can effectively improve the ionic conductivity of the SEI membrane, reduce the overall polarization of the battery, achieve an increase in the battery capacity level during the cycle, and improve the battery's cycle stability.

[0099] In some embodiments, the negative electrode film layer includes a negative electrode film-forming additive. Optionally, the negative electrode film-forming additive includes an inorganic oxygen acid salt.

[0100] The type of negative electrode film-forming additive in the negative electrode film layer can be tested by any method in the art. As an example, the crystal form of the negative electrode film layer additive can be analyzed by an X-ray diffractometer, and the elements of the negative electrode film-forming additive can be assisted by energy spectrometer, infrared, and X-ray photoelectron spectroscopy, while the molecular structure can be finely detected by combining a mass spectrometer. For example, a certain amount of powder sample is scraped from the surface layer of the negative electrode film layer, the powder is soaked in anhydrous ethanol for 30 minutes, and dried in an oven at 60°C for 10 minutes to facilitate mass spectrometry analysis. The pretreated sample is introduced into a mass spectrometer to determine the type of additive in the negative electrode film layer. In some embodiments, the inorganic oxygen-containing acid salt includes one or more of lithium nitrate, sodium nitrate, potassium nitrate, rubidium nitrate, cesium nitrate, lithium phosphate, lithium nitrite, sodium nitrite, and potassium nitrite.

[0101] In some embodiments, the inorganic oxygen-containing acid salt includes at least one of lithium nitrate and potassium nitrate.

[0102] The above-mentioned negative electrode film-forming additives can gradually participate in the formation of the solid electrolyte membrane (SEI membrane) during the battery cycle, evolving into inorganic components such as lithium oxide and lithium nitride, effectively improving the ionic conductivity of the SEI membrane, reducing the overall polarization of the battery, and realizing the improvement of the battery capacity level during the cycle process, thereby improving the cycle stability of the battery.

[0103] In some embodiments, the negative electrode film layer includes a negative electrode material, and the specific surface area of ​​the negative electrode material is S, in m 2 / g, S meets: 0.9m 2 / g≤S≤1.7m 2 / g, optionally 0.9m 2 / g≤S≤1.2m 2 / g.

[0104] In some embodiments, the term "negative electrode material" refers to a mixed material containing a negative electrode active material in the negative electrode film layer. In some embodiments, the term "negative electrode material" refers to the negative electrode active material in the negative electrode film layer. It is understood that due to the high proportion of negative electrode active material in the negative electrode film layer, the specific surface area and volume distribution particle size of the mixed material in the negative electrode film layer are substantially the same as those of the negative electrode active material.

[0105] In this application, the specific surface area of ​​the negative electrode material can be measured using methods known in the art. As an example, the specific surface area of ​​the negative electrode material can be measured using the nitrogen adsorption specific surface area analysis method according to GB / T 19587-2017 and calculated using the BET (Brunauer Emmett Teller) method. The testing instrument can be a Tri-Star 3020 specific surface area pore size analyzer from Micromeritics, USA.

[0106] In some embodiments, the specific surface area S of the negative electrode material can be selected to be 0.9 m 2 / g, 0.95m 2 / g、1m 2 / g, 1.1m 2 / g, 1.2m 2 / g, 1.3m 2 / g, 1.4m 2 / g, 1.5m 2 / g, 1.6m 2 / g, 1.7m 2 / g or any range of values ​​between them.

[0107] Compared to the negative electrode materials commonly used in the prior art, the negative electrode material in this application has a relatively small specific surface area, resulting in a relatively small contact area between the negative electrode material and the electrolyte, which in turn reduces the number of active ions (such as lithium ions) that enter the negative electrode material per unit time to exert its capacity, resulting in large polarization and low kinetic performance of the battery. As the negative electrode material expands and contracts during the cycle, the negative electrode sheet cracks and the electrolyte continues to infiltrate, the number of ion channels through which active ions are embedded in the negative electrode material increases, the battery polarization decreases, and the discharge capacity increases. This suppresses the significant attenuation of the electrochemical performance of the secondary battery in the early stages of the cycle, which is beneficial to improving the battery cycle life.

[0108] In some embodiments, the volume distribution particle size Dv90 of the negative electrode material is recorded as Dv90负 , unit is μm, Dv90 负 Meet: 17μm≤Dv90 负 ≤35μm, optionally, 18μm≤Dv90 负 ≤26μm.

[0109] As used herein, the term "Dv90" refers to the particle size corresponding to the 90% cumulative volume distribution number of particles in a particle size distribution curve.

[0110] In this application, the volume distribution particle size Dv90 of the negative electrode material can be measured using methods known in the art. As an example, referring to GB / T 19077-2016, it can be measured using a laser particle size analyzer. The testing instrument can be a Mastersizer 3000 laser particle size analyzer manufactured by Malvern Instruments Ltd., UK.

[0111] In some embodiments, Dv90 负 The options include 17μm, 18μm, 19μm, 20μm, 21μm, 22μm, 23μm, 24μm, 25μm, 26μm, 27μm, 28μm, 29μm, 30μm, 31μm, 32μm, 33μm, 34μm, 35μm or any numerical range therebetween.

[0112] Compared to commonly used anode materials in the prior art, the anode material in this application has a relatively large particle size, which requires a longer transmission path for active ions to embed into the center of the anode material. This makes it difficult for the anode material to fully embed lithium, which in turn hinders the full utilization of the battery's capacity. As the anode material expands and contracts during cycling, it cracks and its particle size decreases. This shortens the transmission path for active ions to embed into the center of the anode material, reduces battery polarization, and improves discharge capacity. This prevents significant attenuation of the secondary battery's electrochemical performance during the initial stages of cycling.

[0113] In some embodiments, based on the total mass of the negative electrode film layer, the mass proportion p of the negative electrode film-forming additive satisfies: 0.05%≤p≤10%, optionally 0.4%≤p≤1%.

[0114] The mass proportion of the negative electrode film-forming additive in the negative electrode film layer can be tested by any method in the art. As an example, a certain amount of sample is obtained from the negative electrode film layer, and the sample is pretreated by filtration, concentration or derivatization to facilitate mass spectrometry analysis. The pretreated sample is introduced into a mass spectrometer to determine the type of film-forming additive in the negative electrode film layer. The sample is ionized by ionization, and the ionization instrument includes but is not limited to electrospray ionization (ESI) or matrix-assisted laser desorption / ionization (MALDI). The ionized sample enters the mass analyzer, and the mass of the film-forming additive is determined by measuring the mass of the ions. In order to obtain sufficiently accurate mass data, a high-resolution mass spectrometer may need to be used in the quantitative process. The mass of the additive is calculated based on the type and mass of the additive obtained from the analysis and the total mass of the sample.

[0115] Based on the total mass of the negative electrode film layer, the mass proportion p of the negative electrode film-forming additive can be selected as 0.05%, 0.1%, 0.2%, 0.4%, 0.5%, 0.7%, 1%, 1.2%, 1.5%, 1.7%, 2%, 2.2%, 2.5%, 2.7%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or any numerical range between two of them.

[0116] The negative electrode film-forming additives within the above-mentioned content range will neither affect the energy density of the battery due to excessive addition; nor will they be precipitated from the negative electrode film layer due to excessive addition, thereby affecting the conductivity of the electrolyte; and can fully play the role of forming the SEI film, gradually evolving into inorganic components such as lithium oxide and lithium nitride during the battery cycle, effectively improving the ionic conductivity of the SEI film, reducing the overall polarization of the battery, achieving an increase in the battery capacity level during the cycle, and improving the cycle stability of the battery.

[0117] In some embodiments, the negative electrode material includes a negative electrode active material, and the negative electrode active material can be a negative electrode active material for a battery that is well known in the art. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, lithium titanate, etc. The silicon-based material can be selected from at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material can be selected from at least one of elemental tin, tin oxides, and tin alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries can also be used. These negative electrode active materials can be used alone or in combination of two or more.

[0118] In some embodiments, the negative electrode film layer may further include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0119] In some embodiments, the negative electrode film layer may further include a conductive agent, which may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0120] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0121] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one surface of the polymer base material. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0122] As an example, the negative electrode current collector has two surfaces opposite to each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0123] In some embodiments, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0124] In some embodiments, a secondary battery includes a positive electrode plate, the positive electrode plate includes a positive electrode current collector and a positive electrode film layer arranged on at least one side of the positive electrode current collector, the positive electrode film layer includes a positive electrode active material, the positive electrode active material includes a carbon element, and the mass percentage C of carbon in the positive electrode active material satisfies: 1.2%≤C≤2.3%, optionally, 1.3%≤C≤2.0%.

[0125] The mass percentage C of carbon in the positive electrode active material is the mass of the carbon element in the positive electrode active material divided by the mass of the positive electrode active material. The mass percentage C of carbon in the positive electrode active material can be obtained by testing in any known manner. As an example, the carbon content in the positive electrode active material can be tested by thermogravimetric method. Take a sample larger than 100g and weigh it, the mass of the sample is m2; place it in a high-temperature furnace at 1200℃ and burn it in pure oxygen or air to convert the carbon element in the sample into carbon dioxide. The carbon dioxide produced by the combustion is collected by a cooling device, and the mass m1 of the collected carbon dioxide is measured using a weighing device; the mass content C of the carbon element in the positive electrode active material is calculated by the following formula:

[0126] In some embodiments, the mass percentage C of carbon in the positive electrode active material can be selected as 1.2%, 1.276%, 1.3%, 1.383%, 1.5%, 1.53%, 1.732%, 1.858%, 2.0%, 2.1%, 2.2%, 2.3% or any range therebetween.

[0127] In some embodiments, the carbon element in the positive electrode active material is concentrated on the surface of the positive electrode active material.

[0128] In some embodiments, the positive electrode active material includes a carbon coating layer, and a thickness H of the carbon coating layer satisfies: 2.5 nm ≤ H ≤ 10 nm, optionally, 3 nm ≤ H ≤ 8 nm.

[0129] The carbon coating can be characterized using transmission electron microscopy (TEM). A small amount of the positive electrode active material sample is dispersed in a mixture of ethanol and water. After 30 minutes of sonication, the suspension is dropped onto a carbon film and the sample is observed using TEM. At high magnification, the surface region, including the lattice fringes, exhibits long-range disorder and short-range order in the coating.

[0130] The carbon coating layer mainly includes amorphous carbon. There is no regular arrangement between the carbon atoms in the amorphous carbon structure, while the positive electrode active material is a crystalline material. Under a transmission electron microscope, the positive electrode active material presents regular lattice stripes. Therefore, the thickness of the carbon coating layer can be tested by methods known in the art. As an example, it can be obtained by transmission electron microscopy (TEM) testing. A thin slice with a thickness of about 20 to 50 nm is cut from the middle of the graphite negative electrode active material particle body by a focused ion beam (FIB), and then a TEM test is performed on the thin slice to obtain the original TEM test image. The original image obtained by the above TEM test is opened in the Digital Micrograph software, and the carbon coating layer is identified by the lattice stripes to measure its thickness.

[0131] In some embodiments, the thickness H of the carbon coating layer of the positive electrode active material may be 2.5 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, or any range therebetween.

[0132] When the positive electrode active material contains a carbon coating layer within the above range or a carbon element within the above range, especially when the carbon element within the above range is enriched on the surface of the positive electrode active material, it is difficult for the positive electrode active material to be effectively infiltrated in the electrolyte during the first cycle of the secondary battery. The initial number of active ion pathways in the positive electrode film layer is small, the ion transmission rate of the positive electrode active material is limited, and the secondary battery has a large polarization. Therefore, in the first cycle of the secondary battery cycle, during the discharge process, it is difficult for the active ions embedded in the negative electrode to be completely embedded back in the positive electrode, and the discharge capacity of the secondary battery is difficult to fully exert. However, as the secondary battery is charged and discharged, the positive electrode active material continues to expand and contract, causing the positive electrode active material to be gradually infiltrated by the electrolyte, the number of ion channels increases, and the proportion of active ions embedded in the negative electrode embedded back in the positive electrode increases, thereby improving the discharge capacity of the battery. Although a larger coating thickness can further improve the maximum discharge capacity retention rate Q during cycling, it will result in excessive loss of the secondary battery's absolute capacity and energy density. Furthermore, the coating cannot break down during the secondary battery's charge and discharge cycles, which will have a continuous negative impact on the ion transport in the positive electrode film, causing the secondary battery's discharge capacity to decay at a high rate. Positive electrode active materials within the above range can achieve a "drift" in discharge capacity while taking into account the secondary battery's absolute capacity and internal resistance, allowing the secondary battery to maintain a certain capacity level over long cycles, thereby comprehensively improving the secondary battery's cycle life.

[0133] In some embodiments, the carbon coating layer of the positive electrode active material is prepared by chemical vapor deposition (CVD).

[0134] This method can effectively control the thickness of the coating layer, improve the uniformity of the coating layer, and achieve uniform regulation of the polarity of the electrode.

[0135] In some embodiments, the volume distribution particle size Dv50 of the positive electrode active material is recorded as Dv50 正 , unit is μm, Dv50 正 Meet: 0.5μm≤Dv50 正 ≤2.5μm.

[0136] As used herein, the term "Dv50" refers to the particle size corresponding to the cumulative volume distribution number of particles reaching 50% in a particle size distribution curve.

[0137] In this application, the volume distribution particle size Dv50 of the positive electrode active material can be measured using methods known in the art. As an example, referring to GB / T 19077-2016, it can be measured using a laser particle size analyzer. The testing instrument can be a Mastersizer 3000 laser particle size analyzer manufactured by Malvern Instruments Ltd., UK.

[0138] In some embodiments, the volume distribution particle size Dv50 of the positive electrode active material is 正 The options can be 0.5μm, 0.6μm, 0.69μm, 0.7μm, 0.8μm, 0.9μm, 1μm, 1.1μm, 1.2μm, 1.3μm, 1.4μm, 1.5μm, 1.6μm, 1.7μm, 1.8μm, 1.9μm, 2μm, 2.1μm, 2.2μm, 2.3μm, 2.4μm, 2.5μm or any numerical range therebetween.

[0139] Volume distribution particle size Dv50 正 The positive electrode active material within the above range has a small number of initial active ion pathways and a long path, making it difficult for the electrolyte to fully penetrate the positive electrode active material and the active ions to be completely released from the positive electrode active material during the first cycle of the secondary battery cycle, and the battery's discharge capacity cannot be effectively utilized. However, as the secondary battery's charge and discharge cycles progress, the carbon coating of the positive electrode active material is continuously soaked by the electrolyte, the ion transport resistance of the positive electrode active material decreases, and the embedding depth in the positive electrode active material increases, further promoting the continuous release of the capacity of the positive electrode active material, and improving the discharge capacity of the secondary battery. This phenomenon, that is, the discharge capacity "drifts up," improves the problem of high capacity decay in the initial stage of the secondary battery cycle.

[0140] In some embodiments, the positive electrode active material may adopt the positive electrode active material for batteries that is well known in the art. As an example, the positive electrode active material may include at least one of the following materials: lithium-containing phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also referred to as NCM 333 ), LiNi 0.5 Co0.2 Mn 0.3 O2 (also referred to as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also referred to as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2 (also referred to as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2 (also referred to as NCM 811 ), lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.15 Al 0.05 O2) and its modified compounds. Examples of olivine-structured lithium-containing phosphates may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon.

[0141] In some embodiments, the positive electrode active material includes one or more of lithium-containing phosphates, lithium cobaltate, lithium manganate, and lithium-rich manganese-based materials.

[0142] In this article, "lithium cobalt oxide" refers to materials that mainly include the structural characteristics of lithium cobalt oxide, including but not limited to lithium cobalt oxide (LiCoO2) and its doped modified materials and coated modified materials.

[0143] In this article, "lithium manganate" refers to a material mainly including a lithium manganate structure, including but not limited to LiMn2O4 and its doped modified materials and coated modified materials.

[0144] In this document, "lithium-rich manganese-based material" refers to a material whose main component is manganese oxide. In some embodiments, the lithium-rich manganese-based material includes a composition shown in Formula II: sLi2MnO3·(1-s)LiNi m Co n Mn q O2 (Formula II)

[0145] Where m+n+q=1,0 <s<1。

[0146] In this article, "lithium-containing phosphate" refers to a material whose main component includes lithium phosphate salt, including but not limited to lithium iron phosphate, lithium manganese iron phosphate, lithium nickel phosphate, lithium manganese phosphate, lithium cobalt phosphate and their doped modified materials and coated modified materials.

[0147] In some embodiments, the lithium-containing phosphate includes a component Li shown in Formula I x A y Me a M b P 1-c X c Y z (Formula I)

[0148] Wherein, 0≤x≤1.3, 0≤y≤1.3, and 0.9≤x+y≤1.3; 0.9≤a≤1.5, 0≤b≤0.5, and 0.9≤a+b≤1.5; 0≤c≤0.5; 3≤z≤5; A includes one or more of Na, K, and Mg; Me includes one or more of Mn, Fe, Co, and Ni; M includes one or more of B, Mg, Al, Si, S, Ca, Sc, Ti, V, Cr, Cu, Zn, Sr, Y, Zr, Nb, Mo, Cd, Sn, Sb, Te, Ba, Ta, W, Yb, La, and Ce; X includes one or more of S, Si, Cl, B, C, and N; and Y includes one or more of O and F, wherein any two of A, Me, M, X, and Y do not include the same element at the same time.

[0149] Although the above-mentioned positive electrode active materials have good cycling stability, the discharge capacity of the secondary battery tends to decay significantly in the early stages of cycling, making it difficult to further improve the lifespan of the secondary battery. The embodiments of the present application can effectively improve the rapid decay of battery capacity in the early stages of cycling of secondary batteries containing the above-mentioned positive electrode active materials, thereby further improving the lifespan of the secondary battery.

[0150] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0151] As an example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive electrode current collector.

[0152] In some embodiments, the positive electrode film layer may further optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin.

[0153] In some embodiments, the positive electrode film layer may further include a conductive agent. For example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0154] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0155] In some embodiments, the compacted density of the negative electrode sheet is 1.4 g / cm 3 -1.67g / cm 3 The single-side density of the negative electrode sheet is 8 mg / cm 2 -15mg / cm 2 The compaction density of the positive electrode is 2.5g / cm 3 -2.7g / cm 3 The single-side density of the positive electrode sheet is 18 mg / cm 2 -30mg / cm 2 .

[0156] In some embodiments, the compaction density of the negative electrode sheet can be selected to be 1.4 g / cm 3 , 1.5g / cm 3 , 1.6g / cm 3 , 1.67g / cm 3 or any range of values ​​between them.

[0157] In some embodiments, the single-side density of the negative electrode sheet can be selected to be 8 mg / cm 2 , 9mg / cm 2 、10mg / cm 2 , 11mg / cm 2 , 12mg / cm 2 、13mg / cm 2 、14mg / cm 2 、15mg / cm 2 or any range of values ​​between them.

[0158] In some embodiments, the compaction density of the positive electrode sheet can be selected to be 2.5 g / cm 3 , 2.6g / cm 3 , 2.7g / cm 3 or any range of values ​​between them.

[0159] In some embodiments, the single-side density of the positive electrode sheet can be selected to be 18 mg / cm 2 、19mg / cm 2 , 20mg / cm 2 , 21mg / cm 2 , 22mg / cm 2 , 23mg / cm 2 , 24mg / cm 2 , 25mg / cm 2 , 26mg / cm 2 , 27mg / cm 2 , 28mg / cm 2 , 29mg / cm 2 、30mg / cm 2 or any range of values ​​between them.

[0160] In some embodiments, the secondary battery includes an electrolyte, the electrolyte contains an electrolyte additive, the electrolyte additive includes a fluorine-containing additive, and the fluorine-containing additive includes one or more of fluorinated carbonates, fluorinated phosphates, fluorinated carbamates, fluorinated sulfates, fluorinated carboxylates, fluorinated ethers, fluorinated nitriles, fluorinated silanes, and fluorinated sulfones.

[0161] The electrolyte additives can further improve the oxidation resistance and stability of the electrolyte, which is beneficial to further improve the cycle life of the secondary battery.

[0162] In some embodiments, based on the total mass of the electrolyte, the mass content A of the electrolyte additive is 0.1%≤A≤5%, optionally 0.5%≤A≤2%.

[0163] In some embodiments, based on the total mass of the electrolyte, the mass content A of the electrolyte additive may be 0.1%, 0.3%, 0.5%, 1%, 2%, 3%, 4%, 5% or any range therebetween.

[0164] In some embodiments, the electrolyte solution includes an electrolyte salt and a solvent.

[0165] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0166] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

[0167] In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0168] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0169] In some embodiments, the gram capacity of the secondary battery is greater than or equal to 140 mAh / g.

[0170] In some embodiments, the gram capacity of the secondary battery can be selected as 140 mAh / g, 141 mAh / g, 142 mAh / g, 143 mAh / g, 144 mAh / g, 145 mAh / g, 146 mAh / g, 147 mAh / g, 148 mAh / g, 149 mAh / g, 150 mAh / g, or any range therebetween.

[0171] The gram capacity of a secondary battery is calculated by dividing its maximum discharge capacity by the mass of its positive electrode active material. The secondary battery provided in the embodiments of this application not only has a long cycle life, but also has a gram capacity that is substantially consistent with that of energy storage batteries in the prior art, making the secondary battery provided in the embodiments of this application highly valuable.

[0172] In some embodiments, the secondary battery is optionally one or more of a battery cell, a battery module, and a battery pack.

[0173] In addition, the secondary battery, battery module, battery pack, and electric device of the present application will be described below with reference to the drawings as appropriate.

[0174] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0175] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0176] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0177] The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square, or any other shape. For example, FIG6 shows a secondary battery 5 with a square structure as an example.

[0178] In some embodiments, referring to Figure 7, the outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can be formed into an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0179] In some embodiments, secondary batteries can be assembled into a battery module. The number of secondary batteries contained in the battery module can be one or more. The specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0180] Figure 8 shows an example battery module 4. Referring to Figure 7 , in the battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of the battery module 4. Of course, they can also be arranged in any other manner. Furthermore, the multiple secondary batteries 5 can be secured using fasteners.

[0181] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0182] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery pack.

[0183] Figures 9 and 10 illustrate an example battery pack 1. Referring to Figures 9 and 10 , the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box comprises an upper case 2 and a lower case 3. The upper case 2 can be placed over the lower case 3 to form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0184] A second aspect of the present application provides an electrical device comprising the secondary battery of any embodiment.

[0185] The secondary battery can be used as a power source or an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships, satellites, and energy storage systems.

[0186] As the electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0187] Figure 11 shows an example of an electric device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery, a battery pack or battery module can be used.

[0188] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a secondary battery as a power source.

[0189] Example

[0190] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0191] 1. Preparation method

[0192] Example 1

[0193] 1. Preparation of negative electrode active material graphite S1

[0194] The preparation method for the negative electrode active material, graphite S1, is as follows: The graphite preparation process consists of four steps: crushing and pretreatment of the raw petroleum coke, granulation, graphitization, and screening and demagnetization. The pretreatment stage includes raw material mixing and pulverization, while granulation involves pyrolysis granulation and ball milling and screening. The graphitization process uses a box furnace.

[0195] The pulverization pretreatment time in Example 1 was 2.4 h, and the specific surface area of ​​the negative electrode active material graphite was 1.06 m 2 / g, Dv90 is 21.6μm.

[0196] 2. Preparation of negative electrode sheet

[0197] The negative electrode active material graphite S1, conductive carbon black, binder styrene-butadiene rubber (SBR), and thickener sodium carboxymethyl cellulose (CMC-Na) are thoroughly stirred in a deionized water solvent system at a mass ratio of 96:2:1:1. Lithium nitrate, a negative electrode film-forming additive, is added at a mass ratio of 0.005 to the negative electrode active material graphite and stirred to mix evenly to obtain a negative electrode slurry. The negative electrode slurry is evenly coated on the negative electrode current collector copper foil, and then dried, cold pressed, and cut to obtain a negative electrode sheet. The single-side density of the negative electrode sheet is 11 mg / cm 2 , compacted density is 1.55g / cm 2 .

[0198] 3. Preparation of carbon-coated lithium iron phosphate P1, a cathode active material

[0199] The preparation method of the positive electrode active material lithium iron phosphate P1 is as follows: ferrous sulfate, phosphoric acid and hydrogen peroxide are reacted by carbon thermal reduction method to prepare the precursor iron phosphate, the reaction time is 3.0 hours, and the reaction temperature is 500℃;

[0200] The prepared precursors iron phosphate and lithium carbonate were mixed and ground at a stirring speed of 500 rpm for 3 h, and then spray-dried and sintered.

[0201] A carbon coating layer is obtained by depositing it on the above-mentioned sintered product by CVD method for 3.3 hours. The specific operation is: lithium iron phosphate powder is placed in a reaction chamber, carbon-containing precursor gas methane and protective gas argon are introduced, and the reaction is carried out under high temperature conditions to make carbon atoms deposit on the surface of the lithium iron phosphate particles to form a coating layer. The positive electrode active material forming the coating layer is sintered again, shaped, crushed and iron-removed to obtain the final material.

[0202] The Dv50 of the positive electrode active material is 1.20 μm, the surface of the lithium iron phosphate has a carbon coating layer, the thickness of the carbon coating layer is 3.2 nm, and the carbon content is 1.732% based on the total mass of the positive electrode active material.

[0203] 4. Preparation of positive electrode sheet

[0204] The positive electrode active material lithium iron phosphate P1, conductive carbon black, and binder polyvinylidene fluoride (PVDF) were thoroughly stirred and mixed in an N-methylpyrrolidone solvent system at a weight ratio of 97:0.7:2.3 to obtain a positive electrode slurry. The positive electrode slurry was evenly coated on the positive electrode current collector aluminum foil, and then dried, cold pressed, and cut to obtain a positive electrode sheet. The single-side density of the positive electrode sheet is 21mg / cm 2 , compacted density is 2.55g / cm 2 .

[0205] 5. Preparation of electrolyte

[0206] In an argon atmosphere glove box (H2O content <10ppm, O2 content <1ppm), the lithium salt lithium hexafluorophosphate (LiPF6) was dissolved in a mixture of organic solvents ethylene carbonate (EC) and diethyl carbonate (DEC) (EC:DEC volume ratio of 3:7) and stirred evenly to obtain an electrolyte with a lithium salt concentration of 1 mol / L. Electrolyte additives vinylene carbonate (VC) and fluoroethylene carbonate (FEC) were then added, with the mass content of each electrolyte additive in the electrolyte being 1% of the total mass of the electrolyte.

[0207] 6. Isolation film

[0208] The separator is a 7μm polyethylene (PE) film with an alumina ceramic coating on both sides, with a thickness of 2μm on one side.

[0209] 7. Preparation of batteries

[0210] The positive electrode sheet, the separator, and the negative electrode sheet are stacked in order, with the separator placed between the positive and negative electrode sheets to isolate the positive and negative electrode sheets. The bare battery cell is wound and the tabs are welded. The bare battery cell is placed in an outer packaging aluminum shell, dried at 85°C for 6 hours, and then the electrolyte is injected at an injection coefficient of 3.5g / Ah. After vacuum packaging, standing, forming, shaping and other processes, a secondary battery product is obtained, wherein the formation treatment is to charge to 30% state of charge (SOC) at 0.04C.

[0211] The preparation method of the battery of Example 2-3 is similar to that of the battery of Example 1, but the type of negative electrode film-forming additive is adjusted. The specific parameters are shown in Table 1.

[0212] The preparation methods of the batteries of Examples 4-6 are similar to those of Example 1, but the pulverization time during the pretreatment of the negative electrode active material graphite is adjusted, thereby regulating the specific surface area and Dv90 of the prepared negative electrode active material graphite.

[0213] In Example 4, the grinding time is 2 h, the negative electrode active material graphite is S2, and its specific surface area is 0.94 m 2 / g, Dv90 is 20.1μm.

[0214] In Example 5, the pulverization time is 2.8 h, the negative electrode active material graphite is S3, and its specific surface area is 1.2 m 2 / g, Dv90 is 25.5μm.

[0215] In Example 6, the pulverization time is 3.5 h, the negative electrode active material graphite is S4, and its specific surface area is 1.65 m 2 / g, Dv90 is 18.5μm.

[0216] The preparation methods of the batteries of Examples 7-9 are similar to those of the battery of Example 1, but the mass ratio of the negative electrode film-forming additive to the negative electrode active material is adjusted. The specific parameters are shown in Table 1.

[0217] The preparation methods of the batteries of Examples 10-14 are similar to those of Example 1, except that the reaction time for preparing the precursor iron phosphate, the stirring speed for mixed grinding of the precursor and iron phosphate, and the deposition time parameters during the preparation of the positive electrode active material are adjusted, and the carbon content and Dv50 of the positive electrode active material are adjusted.

[0218] In Example 10, the reaction time and stirring speed were controlled to be 3 h and 500 rpm, the positive electrode active material was P2, its Dv50 was 1.3 μm, the deposition time was 2.8 h, and the carbon content was 1.28%.

[0219] In Example 11, the reaction time and stirring speed were controlled to be 1 h and 700 rpm, the positive electrode active material was P3, its Dv50 was 0.69 μm, the deposition time was 3.5 h, and the carbon content was 1.86%.

[0220] In Example 12, the reaction time and stirring speed were controlled to be 3.3 h and 450 rpm, the positive electrode active material was P4, its Dv50 was 1.5 μm, the deposition time was 3.2 h, and the carbon content was 1.53%.

[0221] In Example 13, the reaction time and stirring speed were controlled to 4.5 h and 450 rpm, the positive electrode active material was P5, its Dv50 was 2.1 μm, the deposition time was 2.8 h, and the carbon content was 1.28%.

[0222] The battery preparation method of Comparative Example 1 is basically the same as that of Example 1, with the following differences:

[0223] The preparation method of the negative electrode active material is to adjust the pulverization time to 5h, and the specific surface area of ​​the negative electrode active material is 1.78m 2 / g, Dv90 is 16.5μm.

[0224] In the preparation method of the positive electrode active material, the reaction time and stirring speed are controlled to 3h and 450rpm, the deposition time is 1.0h, the Dv50 of the positive electrode active material is 1.1μm, and the lithium iron phosphate surface of the positive electrode active material has a carbon coating layer with a thickness of 2nm. Based on the total mass of the positive electrode active material, the mass content of carbon is 1.16%.

[0225] The negative electrode sheet does not include negative electrode additives.

[0226] Table 1: Preparation parameters of batteries of Examples 1-13 and Comparative Example 1

[0227] 2. Test Method

[0228] 1. Cycle test

[0229] At 25°C, charge the battery at a constant power of 0.5P to 3.6V, then discharge it at a constant power of 0.5P to 2.0V. The resulting discharge capacity is recorded as the discharge capacity C1 of the first cycle. Repeat these steps for the same battery and record the discharge capacity Cn after the nth cycle. The battery capacity retention rate after each cycle is Pn = Cn / C1 × 100%.

[0230] The maximum discharge capacity C during the cycle charge and discharge process m The ratio of the discharge capacity C1 to the first cycle is taken as the maximum capacity retention rate Q. The number of cycles corresponding to the maximum capacity retention rate Q is M. The number of cycles corresponding to the capacity retention rate decaying from the maximum capacity retention rate Q to basically 100% is N. When the battery capacity retention rate reaches 80%, the test is stopped and the number of cycles is recorded.

[0231] 2. Polarization voltage test

[0232] During the test, after each cycle of discharge, the potential of the battery is at the cutoff voltage V1 = 2.0V. After standing for 30 minutes, the voltage rebounds to above 2.0V, and the voltage V2 at this time is recorded. Vpolarization = V1-V2 is the polarization voltage.

[0233] 3. Gram capacity of secondary battery

[0234] The maximum discharge capacity C of the secondary battery during the cycle charge and discharge process m Divide by the mass of the positive electrode active material to obtain the gram capacity of the secondary battery.

[0235] 3. Analysis of test results of various embodiments and comparative examples

[0236] Batteries of various examples and comparative examples were prepared according to the above methods, and various performance parameters were measured. The results are shown in Table 2 below.

[0237] Table 2: Performance of batteries of Examples 1-13 and Comparative Example 1

[0238] Figure 3 is a cycle curve diagram of Example 1 and the comparative example. As can be seen from the figure, the secondary battery in the embodiment of the present application has a discharge capacity greater than the discharge capacity C1 of the first cycle in at least one discharge process during the cyclic charge and discharge process, which significantly improves the cycle stability of the secondary battery. Figure 4 is a polarization voltage comparison diagram of Example 1 and the comparative example. As can be seen from the figure, the secondary battery in the embodiment has a higher polarization in the initial cycle stage. As the cycle proceeds, the secondary battery undergoes a significant depolarization process, and the discharge capacity is improved. Figure 5 is a transmission electron microscope image of the positive electrode active material of Example 1. As can be seen from the figure, the positive electrode active material used in the embodiment of the present application has a relatively high volume distribution particle size Dv50 and a high coating carbon content, which is conducive to the capacity drift of the secondary battery during the cyclic charge and discharge process. XPS nitrogen spectrum testing shows that under initial conditions, the surface of the negative electrode film layer of the secondary battery of the present application includes XPS peaks of nitrate or nitrite (binding energy ranges from 403eV to 408eV). After a certain number of cycles (e.g., 200), the XPS peaks of nitrate or nitrite on the surface of the negative electrode film layer decrease or even disappear, and the spectrum primarily contains characteristic peaks of lithium nitride. Furthermore, the test results show that the gram capacity of the secondary battery in Example 1 is 144.4 mAh / g, while the gram capacity of the secondary battery in Comparative Example 1 is 144 mAh / g, with the gram capacity levels of the two being comparable.

[0239] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A secondary battery, characterized in that: During the cyclic charge and discharge process, there is at least one discharge process in which the discharge capacity is greater than the discharge capacity C1 of the first cycle; The cyclic charge and discharge conditions are as follows: charge and discharge at a constant power of 0.5P at 25°C, and the cycle test voltage range is 2.0V to 3.6V.

2. The secondary battery according to claim 1, wherein During the cyclic charge and discharge process, the maximum capacity retention rate Q of the secondary battery satisfies: 100.1%≤Q<110%; The maximum capacity retention rate Q represents the maximum discharge capacity C during the cyclic charge and discharge process. m The ratio of the discharge capacity C1 to the first cycle.

3. The secondary battery according to claim 2, wherein: The maximum capacity retention rate Q of the secondary battery during the cycle process satisfies: 100.2%≤Q≤105%.

4. The secondary battery according to any one of claims 1 to 3, characterized in that The secondary battery satisfies: 0.001%<(Q-1) / (NM)≤0.120%, with the unit being 1 / cycle; Where Q represents the maximum capacity retention rate of the secondary battery, that is, the maximum discharge capacity C during the cyclic charge and discharge process. m The ratio of the discharge capacity C1 of the first cycle to the discharge capacity C1 of the first cycle; M represents the number of cycles corresponding to the maximum capacity retention rate Q of the secondary battery, in cycles; N represents the number of cycles N corresponding to the capacity retention rate of the secondary battery decaying from the maximum capacity retention rate Q to basically 100% during the cyclic charge and discharge process, in cycles.

5. The secondary battery according to any one of claims 1 to 4, characterized in that During the cyclic charge and discharge process, the number of cycles corresponding to the maximum capacity retention rate of the secondary battery is M, in cycles, and M satisfies: 10≤M≤1000.

6. The secondary battery according to any one of claims 1 to 5, characterized in that During the cyclic charge and discharge process, the number of cycles corresponding to when the capacity retention rate of the secondary battery decays from the maximum capacity retention rate to substantially 100% is N, in units of cycles, and N satisfies: 20≤N≤2000.

7. The secondary battery according to any one of claims 1 to 6, characterized in that During the cyclic charge and discharge process, the ratio of the polarization voltage of the secondary battery after the first discharge cycle to the polarization voltage of the secondary battery after M cycles is 1.08-1.

45.

8. The secondary battery according to any one of claims 1 to 7, characterized in that The secondary battery includes a negative electrode plate, which includes a negative electrode current collector and a negative electrode film layer disposed on at least one side of the negative electrode current collector. The surface of the negative electrode film layer includes inorganic oxygen-containing acid radicals.

9. The secondary battery according to claim 8, characterized in that The inorganic oxygen-containing acid radicals include one or more of nitrate and nitrite.

10. The secondary battery according to any one of claims 1 to 9, characterized in that The negative electrode film layer includes a negative electrode film-forming additive. Optionally, the negative electrode film-forming additive includes an inorganic oxygen acid salt.

11. The secondary battery according to claim 10, wherein The inorganic oxygen-containing acid salt includes one or more of lithium nitrate, sodium nitrate, potassium nitrate, rubidium nitrate, cesium nitrate, lithium phosphate, lithium nitrite, sodium nitrite, and potassium nitrite.

12. The secondary battery according to claim 10 or 11, characterized in that: The inorganic oxygen-containing acid salt includes at least one of lithium nitrate and potassium nitrate.

13. The secondary battery according to any one of claims 8 to 12, characterized in that: The negative electrode film layer includes a negative electrode material, and the negative electrode material satisfies at least one of the following conditions: (1) The specific surface area of ​​the negative electrode material is S, in m 2 / g, S meets: 0.9m 2 / g≤S≤1.7m 2 / g, optionally 0.9m 2 / g≤S≤1.2m 2 / g; (2) The volume distribution particle size Dv90 of the negative electrode material is recorded as Dv90 负 , unit is μm, Dv90 负 Meet: 17μm≤Dv90 负 ≤35μm, optionally, 18μm≤Dv90 负 ≤26μm.

14. The secondary battery according to claim 13, wherein: Based on the total mass of the negative electrode film layer, the mass proportion p of the negative electrode film-forming additive satisfies: 0.05%≤p≤10%, optionally, 0.4%≤p≤1%.

15. The secondary battery according to any one of claims 1 to 14, characterized in that The secondary battery includes a positive electrode sheet, the positive electrode sheet includes a positive electrode current collector and a positive electrode film layer provided on at least one side of the positive electrode current collector, the positive electrode film layer includes a positive electrode active material, and the positive electrode active material satisfies at least one of the following conditions: (1) The positive electrode active material includes carbon, and the mass percentage C of carbon in the positive electrode active material satisfies: 1.2%≤C≤2.3%, optionally, 1.3%≤C≤2.0%, and further optionally, the carbon is enriched on the surface of the positive electrode active material; (2) The positive electrode active material includes a carbon coating layer, and the thickness H of the carbon coating layer satisfies: 2.5 nm ≤ H ≤ 10 nm, optionally, 3 nm ≤ H ≤ 8 nm; (3) The volume distribution particle size Dv50 of the positive electrode active material is recorded as Dv50 正 , unit is μm, Dv50 正 Meet: 0.5μm≤Dv50 正 ≤2.5μm.

16. The secondary battery according to claim 15, characterized in that The positive electrode active material includes one or more of lithium-containing phosphate, lithium cobaltate, lithium manganate, and lithium-rich manganese-based materials.

17. The secondary battery according to claim 15, characterized in that The lithium-containing phosphate includes a component Li shown in formula I x A y Me a M b P 1-c X c Y z (Formula I) Among them, 0≤x≤1.3, 0≤y≤1.3, and 0.9≤x+y≤1.3; 0.9≤a≤1.5, 0≤b≤0.5, and 0.9≤a+b≤1.5; 0≤c≤0.5; 3≤z≤5; A includes one or more of Na, K, and Mg. A, Me, M, X and Y include one or more of the following: A, Me, M, X and Y include one or more of the following: Mn, Fe, Co and Ni; M includes one or more of the following: B, Mg, Al, Si, S, Ca, Sc, Ti, V, Cr, Cu, Zn, Sr, Y, Zr, Nb, Mo, Cd, Sn, Sb, Te, Ba, Ta, W, Yb, La and Ce; X includes one or more of the following: S, Si, Cl, B, C and N; Y includes one or more of the following: O and F, wherein any two of A, Me, M, X and Y do not include the same element at the same time.

18. The secondary battery according to any one of claims 15 to 17, characterized in that The compaction density of the negative electrode sheet is 1.4 g / cm 3 -1.67g / cm 3 The single-side density of the negative electrode sheet is 8 mg / cm 2 -15mg / cm 2 The compaction density of the positive electrode is 2.5g / cm 3 -2.7g / cm 3 The single-side density of the positive electrode sheet is 18 mg / cm 2 -30mg / cm 2 .

19. The secondary battery according to any one of claims 1 to 18, characterized in that The secondary battery includes an electrolyte, the electrolyte contains an electrolyte additive, the electrolyte additive includes a fluorine-containing additive, and the fluorine-containing additive includes one or more of fluorinated carbonates, fluorinated phosphates, fluorinated carbamates, fluorinated sulfates, fluorinated carboxylates, fluorinated ethers, fluorinated nitriles, fluorinated silanes, and fluorinated sulfones.

20. The secondary battery according to claim 19, wherein Based on the total mass of the electrolyte, the mass content A of the electrolyte additive is 0.1%≤A≤5%, optionally 0.5%≤A≤2%.

21. An electrical device, characterized in that: A secondary battery comprising the secondary battery according to any one of claims 1 to 20.

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