Secondary battery and electric device

Abstract

A secondary battery and an electric device. The secondary battery comprises a negative electrode sheet. During the cyclic charge-discharge process of the secondary battery, the charge transfer resistance of the negative electrode sheet after at least one discharge process is less than the charge transfer resistance of the negative electrode sheet after the first cycle of the secondary battery.

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 wide, 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 any embodiment, the present application provides a secondary battery, which includes a negative electrode plate. After at least one complete discharge process during the cyclic charge and discharge process of the secondary battery, the charge transfer resistance of the negative electrode plate is smaller than the charge transfer resistance of the negative electrode plate after the first cycle of the secondary battery.

[0007] In the secondary battery of the prior art, as the cycle reaction proceeds, side reactions continue to occur at the interface between the negative electrode plate and the electrolyte, causing the charge transfer resistance (Rct) to increase with the increase in the number of cycles. The secondary battery provided by the present application first decreases the charge transfer resistance Rct at the beginning of the cycle, indicating that the initial plate has a high electrode polarization, and the charge transfer rate on the electrode surface increases with the increase in the number of cycles of the secondary battery during the cyclic charge and discharge process. High electrode polarization means that the charge transfer rate on the electrode surface is less than the transfer rate of electrons in the electrode, so that charge transfer becomes the electrochemical reaction rate and becomes the rate-controlling step of the electrochemical reaction, resulting in a decrease in 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 plates, the battery plates have a high polarization level in the initial state, and gradually "depolarize" during the cyclic charge and discharge process, so that the secondary battery can have a capacity drift during the cyclic charge and discharge process, 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.

[0008] In any embodiment, during the cyclic charge-discharge process, after 10 cycles of charge-discharge, the charge transfer resistance of the negative electrode plate of the secondary battery is less than the charge transfer resistance of the negative electrode plate after the first cycle of the secondary battery. This means that the secondary battery provided by the embodiment of the present application undergoes a stable depolarization process in the early stages of the cycle, thereby improving the discharge capacity of the secondary battery in the early stages of the cycle and extending the cycle life.

[0009] In any embodiment, the negative electrode plate includes a negative electrode current collector and a negative electrode film layer provided on at least one side of the negative electrode current collector, the negative electrode film layer includes a negative electrode active material, and the Dv90 of the negative electrode active material is recorded as Dv90 负 , unit is um, satisfying: 20um≤Dv90 负 ≤50um, optional 25um≤Dv90 负 ≤36um.

[0010] Compared to the negative electrode active materials commonly used in the prior art, the negative electrode active materials in this application have a relatively large particle size, which requires a longer transmission path for active ions to be embedded in the center of the negative electrode active material. The relative aggregation of active ions on the surface of the negative electrode active material increases the difficulty of charge transfer from the electrolyte to the electrode, thereby increasing the charge transfer resistance Rct. As the negative electrode active material expands and contracts during the cycle, the negative electrode active material cracks and its particle size decreases. The transmission path for active ions to be embedded in the center of the negative electrode active material is shortened, the polarization of the electrode is reduced, and the negative electrode active material is more easily fully lithium-intercalated, resulting in an increase in the discharge capacity as the number of secondary battery cycles increases. This also suppresses the significant attenuation of the secondary battery's electrochemical performance in the early stages of the cycle.

[0011] In any embodiment, the specific surface area of ​​the negative electrode active material is S, in m 2 / g, S meets: 0.5m 2 / g≤S≤3m 2 / g, optionally 1.0m 2 / g≤S≤2m 2 / g.

[0012] Compared with the negative electrode active materials commonly used in the prior art, the negative electrode active material in this application has a relatively small specific surface area, resulting in a relatively small contact area between the negative electrode active material and the electrolyte, thereby reducing the number of active ions (such as lithium ions) that enter the negative electrode active material to exert capacity per unit time, resulting in large polarization of the negative electrode plate. As the negative electrode active material expands and contracts during the cycle, the negative electrode plate cracks and the electrolyte continues to infiltrate, the number of ion channels through which active ions are embedded in the negative electrode active material increases, the polarization of the negative electrode plate decreases, and the charge transfer resistance Rct decreases with the increase in the number of cycles, thereby increasing the discharge capacity of the battery during the cyclic charge and discharge process; suppressing the significant attenuation of the discharge capacity of the secondary battery in the early stage of the cycle, which is beneficial to improving the cycle life of the battery.

[0013] In any embodiment, the surface of the negative electrode film layer includes inorganic oxygen-containing acid radicals.

[0014] The surface of the negative electrode film layer initially contains inorganic oxygen-containing acid radicals; however, after a certain number of cycles, the content of inorganic oxygen-containing acid radicals on the surface of the negative electrode film layer decreases or even disappears. This means that the negative electrode sheet in the initial state has a high resistance, which is not conducive to the charge transfer between the electrolyte and the negative electrode sheet. As the secondary battery cycles, the inorganic oxygen-containing acid radicals on the surface of the negative electrode film layer continuously evolve into lithium-containing inorganic substances in the solid electrolyte membrane (SEI), thereby reducing the resistance of the negative electrode sheet, increasing the ionic conductivity, reducing the charge transfer resistance Rct, and reducing polarization, which is conducive to achieving the rebound of the discharge capacity and improving the battery cycle stability.

[0015] In any embodiment, the inorganic oxygen-containing acid radicals include one or more of nitrate and nitrite.

[0016] 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.

[0017] In any embodiment, the negative electrode film layer includes a negative electrode film-forming additive, and the negative electrode film-forming additive includes an inorganic oxygen acid salt.

[0018] 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.

[0019] 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, and evolve into inorganic components such as lithium oxide and lithium nitride, effectively improving the ionic conductivity of the SEI membrane, reducing the charge transfer impedance Rct of the negative electrode sheet, reducing the negative electrode polarization, and realizing the improvement of the battery capacity level during the cycle process, thereby improving the cycle stability of the battery.

[0020] In any embodiment, the inorganic oxygen-containing acid salt includes at least one of lithium nitrate and potassium nitrate.

[0021] 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, 0.3%≤p≤1%.

[0022] The negative electrode film-forming additives within the above content range can not only improve the polarization of the electrode at the beginning of the cycle and increase the charge transfer impedance of the electrode, but also gradually evolve into inorganic components such as lithium oxide and lithium nitride during the battery cycle, effectively improving the ionic conductivity of the SEI film and realizing the improvement of the battery capacity level during the cycle; it will not affect the energy density of the battery and the conductivity of the electrolyte due to excessive addition, so that the secondary battery can maintain a certain discharge capacity level for a long time, and comprehensively improve the cycle stability of the battery.

[0023] In any embodiment, the secondary battery includes a positive electrode plate, the positive electrode plate includes a positive electrode collector and a positive electrode film layer arranged on at least one side of the positive electrode collector, the positive electrode film layer includes a positive electrode active material, and the positive electrode active material includes one or more of lithium-containing phosphates and lithium-containing transition metal oxides.

[0024] In any embodiment, the lithium-containing phosphate comprises a component represented by formula I: Li x A y Me a M b P 1-c X c Y z (Formula I)

[0025] 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.

[0026] In any embodiment, 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; wherein the cyclic charge and discharge conditions are: charging and discharging at a constant power of 0.5P at 25°C, and the voltage range of the cyclic charge and discharge is 2.0V~3.6V.

[0027] By reducing the charge transfer resistance of the secondary battery during the cyclic charge and discharge process, the discharge capacity of the secondary battery during the cycle will "drift up" compared to the discharge capacity in the first cycle, extending the time for the secondary battery performance to decay, and slowing 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.

[0028] 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.

[0029] 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 attenuation rate of the discharge capacity of the secondary battery in the early stage of the cycle and can further improve the cycle life of the secondary battery.

[0030] In any embodiment, the maximum capacity retention rate Q of the secondary battery during cycling satisfies: 100.2%≤Q≤105%.

[0031] In any embodiment, the secondary battery satisfies the following conditions: 0.001% < (Q-1) / (NM) ≤ 0.02%, 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] A second aspect of the present application provides an electrical device comprising the secondary battery of the first aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] FIG1 is a schematic diagram of a discharge capacity-cycle number curve of a secondary battery according to an embodiment of the present application.

[0037] 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.

[0038] FIG3 a is a Nyquist diagram of a secondary battery according to a certain embodiment of the present application after the first cycle and after 10 cycles; FIG3 b is a Nyquist diagram of a secondary battery according to a comparative example of the present application after the first cycle and after 10 cycles.

[0039] Figure 4a is the nitrogen element spectrum of the XPS test on the surface of the negative electrode film layer of the negative electrode plate of a certain embodiment of the present application after the first cycle; Figure 4b is the nitrogen element spectrum of the XPS test on the surface of the negative electrode film layer of the negative electrode plate of the same embodiment of the present application after 200 cycles; Figure 4c is the nitrogen element spectrum of the XPS test on the surface of the negative electrode film layer of the negative electrode plate of the comparative example of the present application after the first cycle.

[0040] FIG5 is a schematic diagram of a secondary battery according to an embodiment of the present application.

[0041] FIG. 6 is an exploded view of the secondary battery according to the embodiment of the present application shown in FIG. 5 .

[0042] FIG7 is a schematic diagram of a battery module according to an embodiment of the present application.

[0043] FIG8 is a schematic diagram of a battery pack according to an embodiment of the present application.

[0044] FIG. 9 is an exploded view of the battery pack shown in FIG. 8 according to an embodiment of the present application.

[0045] FIG. 10 is a schematic diagram of an electric device using a secondary battery as a power source according to an embodiment of the present application.

[0046] 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

[0047] 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 may be cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structures 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.

[0048] " 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.

[0049] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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).

[0054] 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.

[0055] Based on this, the present application proposes a secondary battery, which includes a negative electrode plate. After at least one complete discharge process in the cyclic charging and discharging process of the secondary battery, the charge transfer resistance of the negative electrode plate is smaller than the charge transfer resistance of the negative electrode plate after the first cycle of the secondary battery.

[0056] In this application, fully discharged refers to a secondary battery in a fully discharged state. In some embodiments, the cyclic charge and discharge conditions are: charge and discharge at a constant power of 0.5P at 25°C, and the voltage range of the cyclic charge and discharge is 2.0V to 3.6V. In other words, the fully discharged state refers to the state of discharging to 2.0V at a constant power of 0.5P at 25°C.

[0057] 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 the charge and discharge conditions known in the art. Therefore, 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 two batteries reach the maximum discharge capacity corresponding to the number of cycles in the cycle process. The charge transfer resistance of the negative electrode of the secondary battery after the discharge process in the cyclic charge and discharge process can be tested using a method known in the art. As an example, at 25°C, the secondary battery is charged to 3.6V at a constant power of 0.5P and discharged to 2.0V at a constant power of 0.5P, which is used as one cycle in the cyclic charge and discharge process. In the cyclic charge and discharge process, the first cycle of the cycle under stable test temperature, environment and other test conditions is used as the first cycle of the secondary battery.

[0058] The secondary battery discharged to 2.0V after different number of cycles was disassembled, the negative electrode piece was taken out, and assembled into a symmetrical electrochemical system. A small amplitude sine wave was applied to the electrochemical system using a Coster electrochemical workstation, and the frequency of the applied small amplitude sine wave was changed from 0.1Hz to 10000Hz to obtain a series of impedances at different frequencies. The real part of the impedance was the horizontal axis, the imaginary part was the vertical axis, and each point represented a different frequency to obtain a Nyquist diagram, as shown in Figure 3. The data on the left side of the figure was obtained by testing under high frequency conditions, which is called the high frequency zone. The electrode process in the high frequency zone is controlled by the charge transfer process. The arc of the high frequency zone is fitted according to the following equivalent circuit using Zview software (charge transfer resistance Rct is connected in series with Weber impedance Zw and then connected in parallel with the electrode solution interface double layer capacitance Cd, and then connected in series with ohmic resistance RΩ). The diameter of the semicircle obtained by fitting can be used as the charge transfer resistance of the electrochemical system, that is, the charge transfer resistance Rct of the negative electrode piece of the secondary battery provided in this application. As shown in Figure 3a, the secondary battery provided in the embodiment of the present application has a charge transfer resistance of the negative electrode plate after at least one discharge process during the cyclic charge and discharge process that is smaller than the charge transfer resistance of the negative electrode plate after the first cycle of the secondary battery; while the charge transfer resistance (Rct) of the secondary battery in the prior art increases during the cycle process.

[0059] In the secondary batteries of the prior art, as the cycle reaction proceeds, side reactions continue to occur at the interface between the negative electrode plate and the electrolyte, causing the charge transfer resistance (Rct) to increase with the increase in the number of cycles. The charge transfer resistance Rct of the secondary battery provided by the present application first decreases at the beginning of the cycle, indicating that the initial plate has a high electrode polarization, and the charge transfer rate on the electrode surface increases with the increase in the number of cycles of the secondary battery during the cyclic charge and discharge process. High electrode polarization means that the charge transfer rate on the electrode surface is less than the transfer rate of electrons in the electrode, so that charge transfer becomes the rate-controlling step of the electrochemical reaction rate, resulting in a decrease in the battery's ability to release capacity and output electrical work. Therefore, in the prior art, people try their best 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 plates, the battery plates have a high polarization level in the initial state, and gradually "depolarize" during the cyclic charge and discharge process, so that the secondary battery can have a capacity drift during the cyclic charge and discharge process, 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.

[0060] In some embodiments, during the cyclic charge and discharge process, after 10 cycles of charge and discharge, the charge transfer resistance of the negative electrode plate of the secondary battery is less than the charge transfer resistance of the negative electrode plate after the first cycle of the secondary battery.

[0061] This means that the secondary battery provided by the embodiment of the present application undergoes a stable depolarization process at the early stage of the cycle, so that the discharge capacity of the secondary battery at the early stage of the cycle is improved and the cycle life is increased. In some embodiments, the negative electrode plate includes a negative electrode current collector and a negative electrode film layer provided on at least one side of the negative electrode current collector, the negative electrode film layer includes a negative electrode active material, and in some embodiments, the Dv90 of the negative electrode active material is recorded as Dv90 负 , unit is um, satisfying: 20um≤Dv90 负 ≤50um, optional 25um≤Dv90 负 ≤36um.

[0062] 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.

[0063] In this application, the volume distribution particle size Dv90 of the negative 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.

[0064] In some embodiments, Dv90负 The options include 20μm, 21μm, 22μm, 23μm, 24μm, 25μm, 26μm, 27μm, 28μm, 29μm, 30μm, 31μm, 32μm, 33μm, 34μm, 35μm, 36μm, 37μm, 38μm, 39μm, 40μm, 41μm, 42μm, 43μm, 44μm, 45μm, 46μm, 47μm, 48μm, 49μm, 50μm or any numerical range between two of them.

[0065] Compared to the negative electrode active materials commonly used in the prior art, the negative electrode active materials in this application have a relatively large particle size, which requires a longer transmission path for active ions to embed into the center of the negative electrode active material. The relative aggregation of active ions on the surface of the negative electrode active material increases the difficulty of lithium ion transfer from the electrolyte to the negative electrode plate, thereby increasing the charge transfer resistance Rct. As the negative electrode active material expands and contracts during the cycle, the negative electrode active material cracks and its particle size decreases. The transmission path for active ions to embed into the center of the negative electrode active material is shortened, the polarization of the plate is reduced, and the negative electrode active material is more easily fully embedded with lithium, resulting in an increase in the discharge capacity as the number of secondary battery cycles increases. This also suppresses the significant attenuation of the secondary battery's electrochemical performance in the early stages of the cycle.

[0066] In some embodiments, the specific surface area of ​​the negative electrode active material is S, in m 2 / g, S meets: 0.5m 2 / g≤S≤3m 2 / g, optionally 1.0m 2 / g≤S≤2m 2 / g.

[0067] In this application, the specific surface area of ​​the negative electrode active material can be measured using methods known in the art. As an example, the specific surface area of ​​the negative electrode active material can be measured using the nitrogen adsorption specific surface area analysis method in accordance with 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, Inc., USA.

[0068] In some embodiments, the specific surface area S of the negative electrode active material may be 0.5 m 2 / g, 0.6m 2 / g, 0.7m 2 / g, 0.8m 2 / g, 0.9m 2 / g, 0.95m 2 / g、1m 2 / g, 1.1m2 / 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, 1.8m 2 / g, 2m 2 / g、3m 2 / g or any range of values ​​between them.

[0069] Compared with the negative electrode active materials commonly used in the prior art, the negative electrode active material in this application has a relatively small specific surface area, resulting in a relatively small contact area between the negative electrode active material and the electrolyte, thereby reducing the number of active ions (such as lithium ions) that enter the negative electrode active material to exert capacity per unit time, resulting in large polarization of the negative electrode plate. As the negative electrode active material expands and contracts during the cycle, the negative electrode plate cracks and the electrolyte continues to infiltrate, the number of ion channels through which active ions are embedded in the negative electrode active material increases, the polarization of the negative electrode plate decreases, and the charge transfer resistance Rct decreases with the increase in the number of cycles, thereby increasing the discharge capacity of the battery during the cyclic charge and discharge process; suppressing the significant attenuation of the discharge capacity of the secondary battery in the early stage of the cycle, which is beneficial to improving the cycle life of the battery.

[0070] In some embodiments, the surface of the negative electrode film layer includes inorganic oxygen-containing acid radicals.

[0071] 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.

[0072] The XPS test results show that the surface of the negative electrode film layer includes inorganic oxygen-containing acid radicals after the first cycle; and after a certain number of cycles, the content of inorganic oxygen-containing acid radicals on the surface of the negative electrode film layer decreases or even disappears, and the spectrum mainly contains characteristic peaks of lithium nitride. This means that the negative electrode sheet in the initial state has a high resistance, which is not conducive to the charge transfer between the electrolyte and the negative electrode sheet. With the cycle of the secondary battery, the inorganic oxygen-containing acid radicals on the surface of the negative electrode film layer continue to evolve into lithium-containing inorganic substances in the solid electrolyte membrane (SEI), thereby reducing the resistance of the negative electrode sheet, increasing the ionic conductivity, reducing the charge transfer resistance Rct, and reducing the polarization, which is conducive to achieving the rebound of the discharge capacity and improving the battery cycle stability.

[0073] In some embodiments, the inorganic oxygen-containing acid radicals include one or more of nitrate and nitrite.

[0074] As shown in Figures 4a and 4b, the XPS test results of one embodiment of the present application show that after the first cycle, the surface of the negative electrode film layer contains a nitrite peak at 403-408 eV. After a certain number of cycles (200 cycles), the nitrite content on the surface of the negative electrode film layer decreases or even disappears, and the spectrum mainly contains the characteristic peak of lithium nitride at around 398 eV. In contrast, as shown in Figure 4c, the XPS peak of nitrite on the surface of the negative electrode film layer in the prior art is absent after the first cycle.

[0075] 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.

[0076] In some embodiments, the negative electrode film layer includes a negative electrode film-forming additive, and the negative electrode film-forming additive includes an inorganic oxygen acid salt.

[0077] 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 an energy spectrometer, infrared, and X-ray photoelectron spectroscopy, while the molecular structure can be finely detected by 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.

[0078] In some embodiments, the inorganic oxygen-containing acid salt includes at least one of lithium nitrate and potassium nitrate.

[0079] 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, and evolve into inorganic components such as lithium oxide and lithium nitride, effectively improving the ionic conductivity of the SEI membrane, reducing the charge transfer impedance Rct of the negative electrode sheet, reducing the negative electrode polarization, and realizing the improvement of the battery capacity level during the cycle process, thereby improving the cycle stability of the battery.

[0080] 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, 0.3%≤p≤1%.

[0081] 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 proportion of the negative electrode film-forming additive is calculated based on the type and mass of the additive obtained by analysis and the total mass of the sample.

[0082] 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%, 0.3%, 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.

[0083] The negative electrode film-forming additives within the above content range can not only improve the polarization of the electrode at the beginning of the cycle and increase the charge transfer impedance of the electrode, but also gradually evolve into inorganic components such as lithium oxide and lithium nitride during the battery cycle, effectively improving the ionic conductivity of the SEI film and realizing the improvement of the battery capacity level during the cycle; it will not affect the energy density of the battery and the conductivity of the electrolyte due to excessive addition, so that the secondary battery can maintain a certain discharge capacity level for a long time, and comprehensively improve the cycle stability of the battery.

[0084] 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.

[0085] 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).

[0086] 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.

[0087] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0088] 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.).

[0089] 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.

[0090] 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.

[0091] In some embodiments, the secondary battery includes a positive electrode plate, the positive electrode plate includes a positive electrode collector and a positive electrode film layer arranged on at least one side of the positive electrode collector, the positive electrode film layer includes a positive electrode active material, and the positive electrode active material includes one or more of lithium-containing phosphates and lithium-containing transition metal oxides.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] 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)

[0096] Where m+n+q=1,0 <s<1。

[0097] 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.

[0098] 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)

[0099] 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.

[0100] 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.

[0101] 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.).

[0102] 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.

[0103] 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.

[0104] 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.

[0105] 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.

[0106] In some embodiments, 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; wherein the cyclic charge and discharge conditions are: charging and discharging at a constant power of 0.5P at 25°C, and the voltage range of the cyclic charge and discharge is 2.0V to 3.6V.

[0107] FIG1 also shows a schematic diagram of a cycle curve of a secondary battery according to an embodiment of the present application. Referring to FIG1 , the fact that there is at least one discharge process in the cyclic charge and discharge process in which the discharge capacity is greater than the discharge capacity of the first cycle means that there is at least one discharge process in this cyclic charge and discharge process in which the discharge capacity is greater than the discharge capacity C1 of the first cycle of this cyclic charge and discharge process. In some embodiments, there are at least r consecutive discharge processes in the cyclic charge and discharge process in which the discharge capacity is greater than the discharge capacity of the first cycle, and r can be any value selected from 3, 4, 5, 6, 7, 8, 9, and 10. It should be noted that the cyclic charge and discharge conditions herein refer to the test conditions of the secondary battery in the cyclic charge and discharge process, and do not specify the rated or preferred cyclic charge and discharge mode of the secondary battery.

[0108] By reducing the charge transfer resistance of the secondary battery during the cyclic charge and discharge process, the discharge capacity of the secondary battery during the cycle will "drift up" compared to the discharge capacity in the first cycle, extending the time for the secondary battery performance to decay, and slowing 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.

[0109] 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 maximum discharge capacity C during the cyclic charge and discharge process. m The ratio of the discharge capacity C1 to the first cycle.

[0110] 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.

[0111] 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.

[0112] In some embodiments, the maximum capacity retention rate Q of the secondary battery during cycling satisfies: 100.2%≤Q≤105%.

[0113] 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 attenuation rate of the discharge capacity of the secondary battery in the early stage of the cycle and can further improve the cycle life of the secondary battery.

[0114] In some embodiments, the secondary battery satisfies the following conditions: 0.001% < (Q-1) / (NM) ≤ 0.02%, 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.

[0115] 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%.

[0116] 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.

[0117] In some embodiments, (Q-1) / (NM) can be selected as 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%, or any range therebetween.

[0118] 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.

[0119] 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.

[0120] 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.

[0121] 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.

[0122] 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.

[0123] In some embodiments, N can be selected from 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500, 1600, 1700, 1800, 1900, 2000, or any range therebetween.

[0124] 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.

[0125] In some embodiments, the secondary battery has a gram capacity greater than or equal to 140 mAh / g.

[0126] In some embodiments, the gram capacity of the secondary battery can be selected as 140mAh / g, 141mAh / g, 142mAh / g, 143mAh / g, 144mAh / g, 145mAh / g, 146mAh / g, 147mAh / g, 148mAh / g, 149mAh / g, 150mAh / g, or any range therebetween.

[0127] The gram capacity of a secondary battery is calculated by dividing the maximum discharge capacity of the secondary battery cell by the mass of the secondary battery's 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.

[0128] In some embodiments, a secondary battery includes an electrolyte. The electrolyte conducts ions between the positive and negative electrodes. This application does not specifically limit the type of electrolyte, and the electrolyte may be selected based on needs. For example, the electrolyte may be liquid, gel, or solid.

[0129] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0130] 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.

[0131] 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.

[0132] In some embodiments, the electrolyte may further include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0133] 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.

[0134] 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.

[0135] 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.

[0136] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0137] 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.

[0138] 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, FIG5 shows a secondary battery 5 having a square structure as an example.

[0139] In some embodiments, referring to Figure 6, 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.

[0140] In some embodiments, the secondary battery may be a secondary battery cell or may be assembled into a battery module. The number of secondary batteries contained in the battery module may be one or more, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery module.

[0141] Figure 7 shows an example battery module 4. Referring to Figure 6 , 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.

[0142] 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.

[0143] 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.

[0144] Figures 8 and 9 illustrate an example battery pack 1. Referring to Figures 7 and 8 , the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the 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.

[0145] In addition, the present application also provides an electrical device, which includes the secondary battery provided in the present application. The secondary battery can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but is not limited thereto.

[0146] As the electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0147] Figure 10 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.

[0148] 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.

[0149] Example

[0150] 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.

[0151] 1. Preparation method

[0152] Example 1

[0153] 1. Preparation of negative electrode sheet

[0154] 95.5 parts by mass of negative electrode active material graphite S1, 2 parts by mass of conductive carbon black, 1 part by mass of binder styrene-butadiene rubber (SBR), and 1 part by mass of thickener sodium carboxymethyl cellulose (CMC-Na) are fully stirred in a deionized water solvent system, and then 0.5 parts of negative electrode film-forming additive lithium nitrate are added and stirred 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 The preparation method of artificial graphite as the negative electrode active material is as follows:

[0155] 1) Pretreatment: Artificial graphite raw material needle coke and pitch are mixed in a mass ratio of 100:5. After mixing, the material is transferred to a hopper via a vacuum feeder. The hopper is then placed in a grinding mill for ultrafine grinding, grinding the material with a particle size of 5-20 mm to a particle size of 5-10 μm. The crushed product is placed in a pulverizer for mechanical pulverization; the pulverizer speed is 10,000-25,000 rpm, and the pulverization time is 5-10 minutes. After airflow grinding, the material is collected in a cyclone dust collector to obtain the desired particle size.

[0156] 2) Secondary granulation: The collected materials are placed in a kiln for carbonization at a temperature of 1100°C for 24 hours. Stirring is performed during carbonization to achieve bonding and obtain secondary particles. After carbonization, ball milling and screening are performed.

[0157] 3) Graphitization: The graphitization temperature is 2800°C, and the heating and holding time is 800h.

[0158] 4) Ball milling and screening: After graphitization, the material is tested and the appropriate vibration and particle size distribution are obtained. Then, electric demagnetization is used to generate a magnetic field by applying electricity to adsorb and demagnetize the magnetic material. The specific surface area of ​​the product is 1.06m 2 / g, Dv90 is 28.2um.

[0159] 2. Preparation of positive electrode sheet

[0160] The positive electrode active material, lithium iron phosphate, the conductive agent, carbon black, and the binder, polyvinylidene fluoride (PVDF), were mixed uniformly in a weight ratio of 97:1:2. The solvent was N-methylpyrrolidone (NMP) with a solid content of 63% 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 slit to obtain the positive electrode sheets. The positive electrode active material, lithium iron phosphate, was purchased commercially and had a Dv50 of 1.15 μm. The lithium iron phosphate surface was coated with a carbon coating with a thickness of 1.5 nm and a carbon content of 1.10% by weight.

[0161] 3. Preparation of electrolyte

[0162] 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.

[0163] 4. Isolation film

[0164] The separator is a 7μm polyethylene (PE) film with an alumina ceramic coating on both sides, with a thickness of 2μm on one side.

[0165] 5. Preparation of batteries

[0166] 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.

[0167] The preparation methods of the batteries of Examples 2-5 are similar to those of the battery of Example 1, but the type or content of the negative electrode film-forming additive is adjusted. The specific parameters are shown in Table 1.

[0168] The preparation method of the battery of Example 6 is similar to that of the battery of Example 1, but no negative electrode film-forming additive is added. The specific parameters are shown in Table 1.

[0169] The preparation methods of the batteries of Examples 7-12 are similar to those of Example 1, except that the time, rotation speed, and other related parameters of the mechanical crushing, secondary granulation, and ball milling screening of the graphite during the preparation process are adjusted. For graphites with similar particle sizes, the surface morphology of the graphite is adjusted by adjusting the rotation speed to regulate the specific surface area of ​​the graphite; graphites with different active specific surface areas and DV90 are screened out. The specific parameters of the graphite are shown in Table 1.

[0170] The preparation method of the battery of Comparative Example 1 is similar to that of Example 1, but no negative electrode film-forming additive is added to the negative electrode slurry. The preparation method of graphite is basically the same as that of Example 1, but the relevant parameters of the graphite in the particle crushing, secondary granulation and ball milling screening processes are adjusted to obtain an active specific surface area of ​​1.82 m 2 / g, and graphite with a Dv90 of 16.7 um was used as the negative electrode active material of Comparative Example 1.

[0171] Table 1: Related parameters of batteries of Examples 1-12 and Comparative Example 1

[0172] 2. Test Method

[0173] 1. Cycle test

[0174] 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%.

[0175] 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.

[0176] 2. Gram capacity of secondary battery

[0177] 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.

[0178] 3. Analysis of test results of various embodiments and comparative examples

[0179] 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.

[0180] Table 2: Performance of the batteries in Examples 1-12 and Comparative Examples

[0181] A comparison of the examples and comparative examples demonstrates that the secondary batteries provided by the examples of the present application undergo at least one complete discharge process during their cyclic charge and discharge cycles. After the complete discharge process, the charge transfer resistance of the negative electrode plate is lower than that of the negative electrode plate after the initial cycle, which is beneficial for improving the cycle life of the secondary battery. Furthermore, test results indicate that the gram capacity of the secondary battery in Example 1 is 143.4 mAh / g, while that of the secondary battery in Comparative Example 1 is 144 mAh / g, representing comparable gram capacity levels.

[0182] 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: The secondary battery includes a negative electrode plate. The secondary battery has at least one full discharge process during the cyclic charge and discharge process. After the full discharge process, the charge transfer resistance of the negative electrode plate is smaller than the charge transfer resistance of the negative electrode plate after the first cycle of the secondary battery.

2. The secondary battery according to claim 1, wherein During the cyclic charge and discharge process, after 10 cycles of charge and discharge, the charge transfer resistance of the negative electrode plate of the secondary battery is less than the charge transfer resistance of the negative electrode plate after the first cycle of the secondary battery.

3. The secondary battery according to claim 1, wherein The negative electrode plate includes a negative electrode current collector and a negative electrode film layer provided on at least one side of the negative electrode current collector. The negative electrode film layer includes a negative electrode active material. The Dv90 of the negative electrode active material is recorded as Dv90 负 , unit is um, satisfying: 20um≤Dv90 负 ≤50um, optional 25um≤Dv90 负 ≤36um.

4. The secondary battery according to claim 3, wherein The specific surface area of ​​the negative electrode active material is S, in m 2 / g, S meets: 0.5m 2 / g≤S≤3m 2 / g, optionally 1.0m 2 / g≤S≤2m 2 / g.

5. The secondary battery according to any one of claims 1 to 4, characterized in that The surface of the negative electrode film layer includes inorganic oxygen-containing acid radicals.

6. The secondary battery according to claim 5, characterized in that The inorganic oxygen-containing acid radicals include one or more of nitrate and nitrite.

7. The secondary battery according to any one of claims 1 to 6, characterized in that The negative electrode film layer includes a negative electrode film-forming additive, and the negative electrode film-forming additive includes an inorganic oxygen acid salt.

8. The secondary battery according to claim 7, 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.

9. The secondary battery according to claim 7 or 8, characterized in that: The inorganic oxygen-containing acid salt includes at least one of lithium nitrate and potassium nitrate.

10. The secondary battery according to any one of claims 7 to 9, characterized in that 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, 0.3%≤p≤1%.

11. The secondary battery according to any one of claims 1 to 10, characterized in that The secondary battery includes a positive electrode plate, which includes a positive electrode collector and a positive electrode film layer arranged on at least one side of the positive electrode collector. The positive electrode film layer includes a positive electrode active material, and the positive electrode active material includes one or more of lithium-containing phosphates and lithium-containing transition metal oxides.

12. The secondary battery according to claim 11, wherein 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) 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.

13. The secondary battery according to any one of claims 1 to 12, 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 conditions for the cyclic charge and discharge are: charging and discharging at a constant power of 0.5P at 25°C, and the voltage range of the cyclic charge and discharge is 2.0V to 3.6V.

14. The secondary battery according to any one of claims 1 to 13, characterized in that 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.

15. The secondary battery according to any one of claims 1 to 14, characterized in that The maximum capacity retention rate Q of the secondary battery during the cycle process satisfies: 100.2%≤Q≤105%.

16. The secondary battery according to any one of claims 1 to 15, characterized in that The secondary battery satisfies: 0.001%<(Q-1) / (NM)≤0.02%, the unit being 1 / circuit; 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.

17. The secondary battery according to any one of claims 1 to 16, 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.

18. The secondary battery according to any one of claims 1 to 17, 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.

19. An electrical device, characterized in that: A secondary battery selected from any one of claims 1 to 18 is included.

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