Secondary battery and electric device

Abstract

The invention relates to a secondary battery and an electric device, the secondary battery comprises a positive pole piece, the positive pole piece comprises a positive current collector and a positive film layer arranged on at least one side of the positive current collector, the positive film layer comprises a positive active material, the Dv90 of the positive active material is D1, and the unit is [mu] m; after the secondary battery is circularly charged and discharged for X1 circles, the Dv90 of the positive active material is D2, and the unit is mu m; the ratio of D2 to D1 meets the condition that D2 / D1 is less than 0.95; wherein X1 is any integer from 10 to 20.

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] A first aspect of the present application provides a secondary battery, comprising a positive electrode plate, the positive electrode plate comprising 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 comprising a positive electrode active material, the Dv90 of the positive electrode active material being D1, in units of um; after the secondary battery has been cycled for X1 charge and discharge cycles, the Dv90 of the positive electrode active material being D2, in units of um; the ratio of D2 to D1 satisfies: D2 / D1<0.95; wherein X1 is any integer between 20 and 30.

[0007] The positive electrode active material has a large initial particle size, a small number of active ion pathways, and a long path, which makes it difficult for the electrolyte to fully infiltrate the positive electrode active material in the first cycle of the secondary battery cycle, and the active ions are difficult to completely escape from the positive electrode active material, and the discharge capacity of the battery cannot be effectively exerted. However, as the secondary battery charges and discharges, the lattice of the positive electrode active material continues to expand and contract, and stress continues to accumulate. Cracks appear inside the positive electrode active material at the beginning of the cycle. Under the action of the battery clamp, the cracks extend, and the positive electrode active material particles are broken, which greatly shortens the active ion transmission path and reduces the resistance to active ion escape. The capacity of the positive electrode active material is continuously released, and the discharge capacity of the secondary battery is improved, that is, the discharge capacity "drifts" phenomenon occurs, which improves the problem of high capacity attenuation in the initial stage of the secondary battery cycle. The pursuit of material performance and stability of battery electrochemical performance is a common understanding in the industry. The embodiments of the present application break the industry's prejudice and achieve the phenomenon of "drifting" of the discharge capacity of the secondary battery during the cycle by changing the particle size of the positive electrode active material at the beginning of the cycle, improving the problem of high capacity attenuation in the initial stage of the secondary battery cycle and achieving improved cycle stability of the secondary battery.

[0008] In any embodiment, the ratio of D2 to D1 satisfies: 0.5 ≤ D2 / D1 ≤ 0.9.

[0009] In any embodiment, after the secondary battery undergoes X2 cycles of charge and discharge, the Dv90 of the positive electrode active material is D3, with the unit of um; the ratio of D3 to D2 satisfies: 0.96 ≤ D3 / D2 ≤ 1, and X2 is any integer in the range of 100 - 200.

[0010] The positive electrode active material that is broken in the initial stage of the battery's charge and discharge cycle hardly changes in particle size during subsequent cycles, enabling the discharge capacity that "drifts upward" in the initial stage of the secondary battery to slowly decline during later cycles. It will not cause excessive side reactions with the electrolyte due to the too small particle size after the positive electrode active material breaks, which would otherwise lead to a decrease in the cycle stability of the secondary battery.

[0011] In any embodiment, D1 satisfies: D1 ≥ 7um, optionally, 8um ≤ D1 ≤ 11um.

[0012] The positive electrode active material with D1 within the above range is prone to breakage in the initial stage of the cycle, which is beneficial for the phenomenon of "drifting upward" of the discharge capacity of the secondary battery in the initial stage of the cycle, and improves the problem of high attenuation amplitude of the secondary battery capacity in the initial stage of the cycle.

[0013] In any embodiment, D2 satisfies: 1um ≤ D2 ≤ 10um, optionally, 3um ≤ D2 ≤ 9um.

[0014] The positive electrode active material with D2 within the above range can maintain a stable particle size during subsequent cycles, achieving a slow decline in the battery's discharge capacity and comprehensively improving the cycle life of the secondary battery.

[0015] In any embodiment, the positive electrode active material includes carbon element, and the mass percentage content M of carbon in the positive electrode active material satisfies: 0% < M ≤ 2.5%, optionally, 0.8% ≤ M ≤ 1.4%; further optionally, the carbon element is enriched on the surface of the positive electrode active material.

[0016] When the positive electrode active material contains 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 beneficial to improve the conductivity of the positive electrode active material and to exert the capacity of the positive electrode active material.

[0017] In any embodiment, the positive electrode active material includes one or more of lithium-containing phosphate and lithium-containing transition metal oxide.

[0018] In any embodiment, the positive electrode active material includes a lithium-containing phosphate, and 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)

[0019] 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.

[0020] 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.

[0021] In any embodiment, the secondary battery further comprises a negative electrode plate, the negative electrode plate comprising a negative electrode current collector and a negative electrode film layer disposed on at least one side of the negative electrode current collector, the negative electrode film layer comprising a negative electrode active material, the specific surface area of ​​the negative electrode active material is S, in units of m 2 / g, S meets: 0.9m 2 / g≤S≤1.7m 2 / g, optionally 0.95m 2 / g≤S≤1.2m 2 / g.

[0022] Using negative electrode active materials with a small specific surface area can reduce the relatively small contact area between the negative electrode active material and the electrolyte, further reducing the number of active ions (such as lithium ions) that enter the negative electrode active material to exert capacity per unit time, further improving the polarization of the battery. As the negative electrode active 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, and the battery polarization decreases. By synchronously changing with the specific surface area of ​​the positive electrode active material, the "drift" effect of discharge capacity is further improved; it is also more conducive to suppressing the significant attenuation of the electrochemical performance of the secondary battery in the early stages of the cycle, thereby improving the cycle life of the battery.

[0023] In any embodiment, the Dv90 of the negative electrode active material 负 Meet: 17um≤Dv90 负 ≤35um, optional 18um≤Dv90 负 ≤26um, Dv90 of the negative electrode active material 负 The unit is um.

[0024] The use of large-particle negative electrode active materials requires a longer transmission path for active ions to embed into the center of the negative electrode active material, making it difficult for the negative electrode active material to fully embed lithium, resulting in difficulty in fully utilizing the battery's capacity. As the negative electrode active material expands and contracts during cycling, cracking occurs in the negative electrode active material and its particle size decreases. The transmission path for active ions to embed into the center of the negative electrode active material is shortened, reducing battery polarization. By synchronizing this with the particle size of the positive electrode active material, the "drift" effect of discharge capacity is further improved. This is also conducive to suppressing the significant attenuation of the electrochemical performance of secondary batteries in the early stages of cycling, thereby extending the battery's cycle life.

[0025] In any embodiment, the surface of the negative electrode film layer includes inorganic oxygen-containing acid radicals.

[0026] 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 species in the solid electrolyte interface (SEI), which in turn reduces the overall battery resistance, improves ionic conductivity, and reduces polarization, further enhancing discharge capacity rebound and battery cycling stability.

[0027] In any embodiment, the negative electrode film layer further includes a negative electrode film-forming additive, and the negative electrode film-forming additive includes an inorganic oxygen acid salt.

[0028] 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.

[0029] 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%.

[0030] In any embodiment, the secondary battery further includes an electrolyte, and the ion conductivity of the electrolyte at room temperature is 8 mS / cm to 12 mS / cm, and optionally 9 mS / cm to 10 mS / cm.

[0031] The electrolyte with ionic conductivity within the above range at room temperature can take into account both the increase in battery discharge capacity in the early stage and the improvement in long-cycle performance in the later stage.

[0032] In any embodiment, the secondary battery 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; wherein the cyclic charge and discharge conditions are: charging and discharging at a constant power of 0.5P at 25°C, and a voltage range of 2.0V to 3.6V.

[0033] The secondary battery provided in the embodiment of the present application realizes the depolarization of the positive electrode plate at the beginning of the cycle by virtue of the breakage of the positive electrode active material at the beginning of the cycle, so that the discharge capacity of the secondary battery during the cycle "drifts up" compared with the discharge capacity of the first cycle, prolongs the time for the performance of the secondary battery 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 particularly suitable for energy storage secondary batteries that pay special attention to cycle life.

[0034] 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.

[0035] In any embodiment, the maximum capacity retention rate Q of the secondary battery during cycling satisfies: 100.5%≤Q≤105%.

[0036] In any embodiment, the secondary battery satisfies the following conditions: 0.005% < (Q-1) / (NM) ≤ 0.020%, 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. mThe 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] A second aspect of the present application provides an electrical device comprising the secondary battery of the first aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] FIG1 is a schematic diagram of a discharge capacity-cycle number curve of a secondary battery according to one embodiment of the present application.

[0044] FIG2 is a schematic diagram of the mechanism of “drifting up” of the discharge capacity of a secondary battery during cyclic charge and discharge according to one embodiment of the present application.

[0045] FIG3 is a scanning electron microscope image of a positive electrode active material in a secondary battery according to an embodiment of the present application.

[0046] FIG4 is a scanning electron microscope image of the positive electrode active material in the secondary battery according to one embodiment of the present application after 20 cycles of charge and discharge.

[0047] FIG5 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.

[0048] FIG6 is a schematic diagram of a secondary battery according to an embodiment of the present application.

[0049] FIG. 7 is an exploded view of the secondary battery according to the embodiment of the present application shown in FIG. 6 .

[0050] FIG8 is a schematic diagram of a battery module according to an embodiment of the present application.

[0051] FIG9 is a schematic diagram of a battery pack according to an embodiment of the present application.

[0052] FIG10 is an exploded view of the battery pack shown in FIG9 according to an embodiment of the present application.

[0053] 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.

[0054] 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

[0055] 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.

[0056] " 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.

[0057] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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).

[0062] 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.

[0063] Based on this, the present application proposes a secondary battery, which 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 Dv90 of the positive electrode active material is D1, with a unit of um; after the secondary battery is cycled for X1 cycles of charge and discharge, the Dv90 of the positive electrode active material is D2, with a unit of um; the ratio of D2 to D1 satisfies: D2 / D1<0.95; wherein X1 is any integer between 20 and 30.

[0064] In some embodiments, the conditions for cyclic charge and discharge are: at 25°C, the 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, and this is regarded as one cycle in the cyclic charge and discharge process, and the capacity measured during the discharge process is the discharge capacity. The first cycle in the cyclic charge and discharge process is recorded as the first cycle. 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 2 cyclic charge and discharge processes 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) under normal temperature and stable test environment. The "first cycle" of a secondary battery can also be a cycle before the two batteries reach the corresponding number of cycles of maximum discharge capacity during the cycle.

[0065] 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.

[0066] In the present application, the volume distribution particle size Dv90 of the positive electrode active material can be tested by methods known in the art. As an example, referring to GB / T 19077-2016, a laser particle size analyzer is used for measurement or a scanning electron microscope combined with elemental analysis is used to characterize the particle size obtained by statistical analysis of the positive electrode active material. The laser particle size analyzer can be a Mastersizer 3000 laser particle size analyzer manufactured by Malvern Instruments Ltd., UK. It can be understood that the inorganic components of the positive electrode film layer are mainly positive electrode active materials, so the performance parameters of the positive electrode active materials can be characterized by scraping the positive electrode film layer. D1 can be used to characterize the Dv90 of the positive electrode active material by scraping the positive electrode film layer before the secondary battery starts cyclic charge and discharge; D2 can be used to characterize the Dv90 of the positive electrode active material by scraping the positive electrode film layer after the secondary battery is cyclically charged and discharged for X1 cycles.

[0067] In some embodiments, D2 / D1 can be selected as 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.91, 0.92, 0.93, 0.94 or any range therebetween.

[0068] In some embodiments, X1 can be any integer selected from 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, and 30.

[0069] In some embodiments, the ratio of D2 to D1 satisfies: 0.5≤D2 / D1≤0.9.

[0070] As shown in Figures 2-4, the positive electrode active material has a large initial particle size, with fewer active ion pathways and longer paths. This makes it difficult for the electrolyte to fully penetrate the positive electrode active material during the first cycle of the secondary battery cycle, and it is difficult for the active ions to completely escape from the positive electrode active material, resulting in the battery's discharge capacity not being effectively utilized. However, as the secondary battery undergoes charge and discharge cycles, the lattice of the positive electrode active material continues to expand and contract, and stress continues to accumulate. Cracks appear within the positive electrode active material at the beginning of the cycle, and under the action of the battery cell fixture, the cracks extend, breaking the positive electrode active material particles. This significantly shortens the active ion transmission path and reduces the resistance to active ion escape. The capacity of the positive electrode active material is continuously released, and the discharge capacity of the secondary battery is improved, that is, the discharge capacity "drifts up" phenomenon occurs, which improves the problem of high capacity decay in the early stages of the secondary battery cycle. It is a common understanding in the industry to pursue stable material performance and battery electrochemical performance. The embodiments of the present application break the industry's prejudice and realize the phenomenon of "drifting" of the discharge capacity of the secondary battery during the cycle by changing the particle size of the positive electrode active material at the beginning of the cycle, thereby improving the problem of high capacity attenuation in the early stage of the secondary battery cycle and improving the cycle stability of the secondary battery.

[0071] In some embodiments, after the secondary battery has been cycled X2 times, the Dv90 of the positive electrode active material is D3, with the unit of um; the ratio of D3 to D2 satisfies: 0.96 ≤ D3 / D2 ≤ 1, and X2 is any integer in the range of 100 - 200.

[0072] The positive electrode active material that is broken in the initial stage of battery charge and discharge cycling hardly changes in particle size during subsequent cycling, enabling the discharge capacity that "drifts upward" in the initial stage of the secondary battery to slowly decline during later cycling. It will not cause excessive side reactions with the electrolyte due to the too small particle size after the positive electrode active material breaks, which would instead lead to a decrease in the cycling stability of the secondary battery.

[0073] In some embodiments, D1 satisfies: D1 ≥ 7um, optionally, 8um ≤ D1 ≤ 11um.

[0074] In some embodiments, D1 can be 7um, 8um, 9um, 10um, 11um, 12um, 13um, 14um, 15um or the numerical range between any two of them.

[0075] The positive electrode active material with D1 within the above range is prone to breakage in the initial stage of cycling, which is beneficial for the phenomenon of "drifting upward" of the discharge capacity of the secondary battery in the initial stage of cycling, and improves the problem of high attenuation amplitude of the secondary battery capacity in the initial stage of cycling.

[0076] In some embodiments, D2 satisfies: 1um ≤ D2 ≤ 10um, optionally, 3um ≤ D2 ≤ 9um.

[0077] In some embodiments, D2 can be 1um, 2um, 3um, 4um, 5um, 6um, 7um, 8um, 9um, 10um or the numerical range between any two of them.

[0078] The positive electrode active material with D2 within the above range can maintain a stable particle size during subsequent cycling, achieving a slow decline in the battery discharge capacity, and comprehensively improving the cycling life of the secondary battery.

[0079] In some embodiments, the positive electrode active material includes carbon, and the mass percentage content M of carbon in the positive electrode active material satisfies: 0% < M ≤ 2.5%, optionally, 0.8% ≤ M ≤ 1.4%; further optionally, the carbon element is enriched on the surface of the positive electrode active material.

[0080] 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:

[0081] In some embodiments, the mass percentage C of carbon in the positive electrode active material can be selected to be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.276%, 1.3%, 1.383%, 1.4%, 1.5%, 1.53%, 1.6%, 1.7%, 1.732%, 1,8%, 1.858%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5% or any numerical range therebetween.

[0082] In some embodiments, the carbon element in the positive electrode active material is concentrated on the surface of the positive electrode active material.

[0083] When the positive electrode active material contains carbon elements within the above range, especially when the carbon elements within the above range are enriched on the surface of the positive electrode active material, it is beneficial to improve the conductivity of the positive electrode active material and to maximize the capacity of the positive electrode active material.

[0084] In some embodiments, the positive electrode active material includes one or more of a lithium-containing phosphate and a lithium-containing transition metal oxide.

[0085] 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 / 3Co 1 / 3 Mn 1 / 3 O2 (also referred to as NCM 333 ), LiNi 0.5 Co 0.2 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.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] 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)

[0090] Where m+n+q=1,0 <s<1。

[0091] 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.

[0092] In some embodiments, the positive electrode active material includes a lithium-containing phosphate, 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)

[0093] 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.

[0094] 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.

[0095] 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.).

[0096] 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.

[0097] 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.

[0098] 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.

[0099] 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.

[0100] In some embodiments, the secondary battery further comprises a negative electrode plate, the negative electrode plate comprising a negative electrode current collector and a negative electrode film layer disposed on at least one side of the negative electrode current collector, the negative electrode film layer comprising a negative electrode active material, and the specific surface area of ​​the negative electrode active material is S, in units of m 2 / g, S meets: 0.9m 2 / g≤S≤1.7m 2 / g, optionally 0.95m 2 / g≤S≤1.2m 2 / g.

[0101] In this application, the specific surface area of ​​the negative electrode active material can be tested using methods known in the art. As an example, referring to GB / T 19587-2017, the specific surface area analysis method of nitrogen adsorption is used for testing and the specific surface area is 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. It can be understood that the inorganic component of the negative electrode film layer is mainly the negative electrode active material, so the performance parameters of the negative electrode active material can be characterized by scraping the negative electrode film layer.

[0102] In some embodiments, the specific surface area S of the negative electrode active material may 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.

[0103] Using negative electrode active materials with a small specific surface area can reduce the relatively small contact area between the negative electrode active material and the electrolyte, further reducing the number of active ions (such as lithium ions) that enter the negative electrode active material to exert capacity per unit time, further improving the polarization of the battery. As the negative electrode active 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, and the battery polarization decreases. By synchronously changing with the specific surface area of ​​the positive electrode active material, the "drift" effect of discharge capacity is further improved; it is also more conducive to suppressing the significant attenuation of the electrochemical performance of the secondary battery in the early stages of the cycle, thereby improving the cycle life of the battery.

[0104] In some embodiments, the Dv90 of the negative electrode active material is 负 Meet: 17um≤Dv90 负 ≤35um, optional 18um≤Dv90 负 ≤26um, Dv90 of the negative electrode active material 负 The unit is um.

[0105] 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.

[0106] 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.

[0107] In some embodiments, Dv90 负 The thickness may be 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm, 25 μm, 26 μm, 27 μm, 28 μm, 29 μm, 30 μm, 31 μm, 32 μm, 33 μm, 34 μm, 35 μm or any range therebetween.

[0108] The use of large-particle negative electrode active materials requires a longer transmission path for active ions to embed into the center of the negative electrode active material, making it difficult for the negative electrode active material to fully embed lithium, resulting in difficulty in fully utilizing the battery's capacity. As the negative electrode active material expands and contracts during cycling, cracking occurs in the negative electrode active material and its particle size decreases. The transmission path for active ions to embed into the center of the negative electrode active material is shortened, reducing battery polarization. By synchronizing this with the particle size of the positive electrode active material, the "drift" effect of discharge capacity is further improved. This is also conducive to suppressing the significant attenuation of the electrochemical performance of secondary batteries in the early stages of cycling, thereby extending the battery's cycle life.

[0109] In some embodiments, the surface of the negative electrode film layer includes inorganic oxygen-containing acid radicals.

[0110] 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.

[0111] 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 species in the solid electrolyte interface (SEI), which in turn reduces the overall battery resistance, improves ionic conductivity, and reduces polarization, further enhancing discharge capacity rebound and battery cycling stability.

[0112] In some embodiments, the inorganic oxygen-containing acid radicals include one or more of nitrate and nitrite.

[0113] 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.

[0114] In some embodiments, the negative electrode film layer further includes a negative electrode film-forming additive, and the negative electrode film-forming additive includes an inorganic oxygen acid salt.

[0115] 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 additive can be analyzed by an X-ray diffractometer, and the elements of the negative electrode film 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.

[0116] 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.

[0117] In some embodiments, the negative electrode film-forming additive includes at least one of lithium nitrate and potassium nitrate.

[0118] 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.

[0119] 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%.

[0120] 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.

[0121] 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.

[0122] 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.

[0123] 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.

[0124] 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).

[0125] 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.

[0126] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0127] 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.).

[0128] 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.

[0129] 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.

[0130] In some embodiments, the secondary battery further includes an electrolyte, and the lithium ion conductivity of the electrolyte at room temperature is 8 mS / cm to 12 mS / cm, and optionally 9 mS / cm to 10 mS / cm.

[0131] The ionic conductivity of the electrolyte at room temperature can be tested using methods known in the art. For example, the ionic conductivity of the electrolyte can be tested using a conductivity meter in accordance with the chemical industry standard HG / T 4067.

[0132] In some embodiments, the lithium ion conductivity of the electrolyte at room temperature can be 8 mS / cm, 9 mS / cm, 10 mS / cm, 11 mS / cm, 12 mS / cm, or any range therebetween.

[0133] In some embodiments, room temperature is 25±5°C.

[0134] The electrolyte with ionic conductivity within the above range at room temperature can take into account both the increase in battery discharge capacity in the early stage and the improvement in long-cycle performance in the later stage.

[0135] In some embodiments, the electrolyte solution includes an electrolyte salt and a solvent.

[0136] 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.

[0137] 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.

[0138] In some embodiments, the secondary battery 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; wherein the cyclic charge and discharge conditions are: charging and discharging at a constant power of 0.5P at 25°C, and a voltage range of 2.0V to 3.6V.

[0139] FIG1 also shows a schematic diagram of a cycle curve of a secondary battery according to one embodiment of the present application. Referring to FIG1 , the fact that at least one discharge process during the 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 the current cyclic charge-discharge process has a discharge capacity greater than the discharge capacity C1 of the first cycle of the current cyclic charge-discharge process. In some embodiments, at least three consecutive discharge processes during the cyclic charge-discharge process have a discharge capacity greater than the discharge capacity of the first cycle, where x3 can be any one of 3, 4, 5, 6, 7, 8, 9, or 10.

[0140] It should be noted that the cyclic charge and discharge conditions herein 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.

[0141] 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.

[0142] The secondary battery provided in the embodiment of the present application realizes the depolarization of the positive electrode plate at the beginning of the cycle by virtue of the breakage of the positive electrode active material at the beginning of the cycle, so that the discharge capacity of the secondary battery during the cycle "drifts up" compared with the discharge capacity of the first cycle, prolongs the time for the performance of the secondary battery 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 particularly suitable for energy storage secondary batteries that pay special attention to cycle life.

[0143] 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.

[0144] In some embodiments, the maximum capacity retention rate Q of the secondary battery during cycling satisfies: 100.5%≤Q≤105%.

[0145] Please refer to Figure 1 and Figure 5. 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.

[0146] 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.

[0147] In some embodiments, the secondary battery satisfies the following: 0.005% < (Q-1) / (NM) ≤ 0.020%, 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.

[0148] As shown in FIG5 , 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%.

[0149] Continuing with Figure 5, (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.

[0150] In some embodiments, (Q-1) / (NM) can be selected as 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.

[0151] 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.

[0152] 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.

[0153] 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.

[0154] 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.

[0155] 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.

[0156] 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.

[0157] 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.

[0158] 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.

[0159] 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.

[0160] In some embodiments, the gram capacity of the secondary battery is greater than or equal to 140 mAh / g.

[0161] 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.

[0162] 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.

[0163] In some embodiments, the secondary battery is optionally one or more of a battery cell, a battery module, and a battery pack.

[0164] 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.

[0165] 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.

[0166] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0167] 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.

[0168] 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.

[0169] 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.

[0170] 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.

[0171] FIG8 shows an example battery module 4. Referring to FIG7 , 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.

[0172] Optionally, the battery module 4 may further include a housing having an accommodation space, and the plurality of secondary batteries 5 are accommodated in the accommodation space.

[0173] 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.

[0174] 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.

[0175] A second aspect of the present application provides an electrical device comprising the secondary battery of any embodiment.

[0176] 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.

[0177] As the electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0178] 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.

[0179] 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.

[0180] Example

[0181] 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.

[0182] 1. Preparation method

[0183] Example 1

[0184] 1. Preparation of positive electrode sheet

[0185] The positive electrode active material carbon-coated lithium iron phosphate, conductive carbon black, and binder polyvinylidene fluoride (PVDF) were mixed uniformly in a mass ratio of 97:1:2, and the solvent was N-methylpyrrolidone (NMP) to obtain a positive electrode slurry with a solid content of 63%. The above positive electrode slurry was evenly coated on the positive electrode current collector aluminum foil, and then dried, cold pressed, and cut to obtain the positive electrode sheet. Among them, the Dv90 of the positive electrode active material is 8.1um and the specific surface area is 10.9m 2 / g, and the mass percentage of carbon element in the positive electrode active material is 1.1%.

[0186] Ferrous oxalate is used as the iron source, lithium carbonate is used as the lithium source, ammonium dihydrogen phosphate is used as the phosphorus source, and glucose is used as the carbon source. The iron source, lithium source and phosphorus source are weighed according to the stoichiometric ratio of lithium iron phosphate. The amount of glucose is 5% of the theoretical production mass of lithium iron phosphate. The four are added to the dispersant water at the same time, and ball milled for 8 hours to mix them evenly. The ball-milled dry powder is heated to 730 degrees Celsius under nitrogen protection, kept warm for 10 hours, and cooled to room temperature to obtain a carbon-coated lithium iron phosphate positive electrode active material.

[0187] 2. Preparation of negative electrode sheet

[0188] 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 by mass of negative electrode film-forming additive lithium nitrate are added 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.

[0189] 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.

[0190] 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. The single-side density of the negative electrode is 11mg / cm 2 , compacted density is 1.55g / cm 2 .

[0191] 3. Preparation of electrolyte

[0192] In an argon atmosphere glove box (H2O content <0.1ppm, O2 content <0.1ppm), lithium salt lithium hexafluorophosphate LiPF6 was dissolved in a mixed system 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 and an electrolyte conductivity of 9.94mS / cm.

[0193] 4. Isolation film

[0194] The separator is a 7μm polyethylene (PE) film with an alumina ceramic coating on both sides, with a thickness of 2μm on one side.

[0195] 5. Preparation of batteries

[0196] 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 to obtain the bare battery cell, 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 injected with electrolyte at an injection coefficient of 3.5g / Ah. After vacuum packaging, standing, forming, shaping and other processes, the secondary battery product of Example 1 is obtained, wherein the formation treatment is to charge to 30% SOC (State of Charge) at 0.04C.

[0197] The preparation methods of the batteries of Examples 2-5 are similar to those of Example 1, but the Dv90 of the positive electrode active material is adjusted by changing the feeding sequence, ball milling time, and dispersant type during the preparation of the positive electrode active material. The specific parameters are shown in Table 1.

[0198] The preparation methods of the batteries of Examples 6-8 are similar to those of the battery of Example 1, but the components of the electrolyte are adjusted by adjusting the lithium salt concentration, solvent composition ratio, type and content of additives in the electrolyte, thereby adjusting the conductivity of the electrolyte.

[0199] The preparation method of the battery of Example 9 is similar to that of the battery of Example 1, but no negative electrode film additive is added to the negative electrode film layer.

[0200] The preparation method of the battery of Example 10 is similar to that of the battery of Example 1, but the grinding time in the preparation method of the negative electrode active material is adjusted to 5 h, the Dv90 of the negative electrode active material is 16.5 μm, and the negative electrode active specific surface area is 1.78 m 2 / g.

[0201] The preparation method of the battery of Comparative Example 1 is similar to that of the battery of Example 10, but the positive electrode active material in Comparative Example 1 is lithium iron phosphate, Dv90 is 4.10 μm, and the specific surface area is 12.7 m 2 / g, the surface carbon content in the positive electrode active material is 1.1%; the negative electrode does not contain negative electrode film-forming additives.

[0202] 2. Test Method

[0203] 1. Cycle test

[0204] 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%.

[0205] 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.

[0206] 3. Analysis of test results of various embodiments and comparative examples

[0207] 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 Tables 1 to 3 below.

[0208] Table 1

[0209] Table 2

[0210] Table 3

[0211] Comparison of the embodiment and the comparative example shows that the secondary battery in the embodiment of the present application includes a positive electrode plate, which includes a positive electrode current collector and a positive electrode film layer disposed on at least one side of the positive electrode current collector. The positive electrode film layer includes a positive electrode active material. The Dv90 of the positive electrode active material in its initial state is D1, in units of μm. After 20 cycles of charge and discharge, the Dv90 of the positive electrode active material is D2, in units of μm. The ratio of D2 to D1 satisfies: D2 / D1 < 0.95. This results in a significant capacity increase during the initial cycle of the secondary battery in the present application, reducing the rapid decay of the initial cycle capacity of the secondary battery and effectively improving the cycle stability of the secondary battery. Test results show that after 150 cycles of charge and discharge, the Dv90 of the positive electrode active material in the embodiment is D3, in units of μm. The ratio of D3 to D2 satisfies: 0.96≤D3 / D2≤1.

[0212] From the comparison between Example 1 and Example 9, it can be seen that the inclusion of lithium nitrate as a negative electrode additive in the negative electrode film layer can further enhance the effect of cyclic drift in the discharge capacity of the secondary battery, thereby further improving the cycle life of the secondary battery. XPS testing was performed on the negative electrode film layers of Example 1 and Example 9. The nitrogen element analysis results showed that after the first cycle, the surface of the negative electrode film layer of the secondary battery of Example 1 of the present application included an XPS peak of nitrate or nitrite (binding energy at 403eV-408eV). After the secondary battery cycled a certain number of times (e.g., 200 cycles), the XPS peak of nitrate or nitrite on the surface of the negative electrode film layer decreased or even disappeared, and the spectrum mainly contained the characteristic peaks of lithium nitride.

[0213] From the comparison between Example 1 and Example 10, it can be seen that the specific surface area S of the negative electrode active material in the negative electrode film layer meets 0.95m 2 / g≤S≤1.2m 2 / g, Dv90 of negative electrode active material 负 Meet 18um≤Dv90 负 ≤26um, which enables the negative electrode to cooperate with the positive electrode to produce a more significant capacity drift effect and improve the cycle stability of the secondary battery.

[0214] 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 positive electrode plate, the positive electrode plate 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 Dv90 of the positive electrode active material is D1, with the unit of um; After the secondary battery is cycled for charge and discharge X1 times, the Dv90 of the positive electrode active material is D2, with the unit of um; the ratio of D2 to D1 satisfies: D2 / D1 < 0.95; Wherein, X1 is any integer in 20 - 30.

2. The secondary battery according to claim 1, wherein The ratio of D2 to D1 satisfies: 0.5 ≤ D2 / D1 ≤ 0.

9.

3. The secondary battery according to claim 1, wherein After the secondary battery is cycled for charge and discharge X2 times, the Dv9 of the positive electrode active material is D3, with the unit of um; the ratio of D3 to D2 satisfies: 0.96 ≤ D3 / D2 ≤ 1, and X2 is any integer in 100 - 200.

4. The secondary battery according to any one of claims 1 to 3, characterized in that D1 satisfies: D1 ≥ 7um, optionally, 8um ≤ D1 ≤ 11um.

5. The secondary battery according to any one of claims 1 to 4, characterized in that D2 satisfies: 1um ≤ D2 ≤ 10um, optionally, 3um ≤ D2 ≤ 9um.

6. The secondary battery according to any one of claims 1 to 5, characterized in that The positive electrode active material includes carbon element, and the mass percentage content M of carbon in the positive electrode active material satisfies: 0% < M ≤ 2.5%, optionally, 0.8% ≤ M ≤ 1.4%; further optionally, the carbon element is enriched on the surface of the positive electrode active material.

7. The secondary battery according to any one of claims 1 to 6, characterized in that The positive electrode active material includes one or more of lithium-containing phosphates and lithium-containing transition metal oxides.

8. The secondary battery according to any one of claims 1 to 7, characterized in that The positive electrode active material includes a lithium-containing phosphate, and 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 several of Na, K, Mg; Me includes one or several of Mn, Fe, Co, Ni; M includes one or several 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, Ce; X includes one or several of S, Si, Cl, B, C, N; Y includes one or several of O, F, and any two of A, Me, M, X, and Y do not contain the same element at the same time.

9. The secondary battery according to any one of claims 1 to 8, characterized in that The secondary battery further includes a negative electrode plate, 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, and the negative electrode film layer satisfies at least one of the following conditions: (1) The negative electrode film layer includes a negative electrode active material, and the specific surface area of ​​the negative electrode active material is S, in m 2 / g, S meets: 0.9m 2 / g≤S≤1.7m 2 / g, optionally 0.95m 2 / g≤S≤1.2m 2 / g; (2) Dv90 of the negative electrode active material 负 Meet: 17um≤Dv90 负 ≤35um, optional 18um≤Dv90 负 ≤26um, Dv90 of the negative electrode active material 负 The unit is um.

10. The secondary battery according to claim 9, wherein The surface of the negative electrode film layer includes inorganic oxygen-containing acid radicals.

11. The secondary battery according to any one of claims 9 to 10, characterized in that: The negative electrode film layer further includes a negative electrode film-forming additive, and the negative electrode film-forming additive includes inorganic oxygen-containing acid salts.

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

13. The secondary battery according to any one of claims 9 to 12, characterized in that: Based on the total mass of the negative electrode film layer, the mass ratio p of the negative electrode film-forming additive satisfies: 0.05% ≤ p ≤ 10%, optionally, 0.4% ≤ p ≤ 1%.

14. The secondary battery according to any one of claims 9 to 13, characterized in that: The secondary battery further includes an electrolyte, and the ion conductivity of the electrolyte at room temperature is 8 mS / cm to 12 mS / cm, and can be optionally 9 mS / cm to 10 mS / cm.

15. The secondary battery according to any one of claims 1 to 14, characterized in that The secondary battery has at least one discharge process during the cyclic charge and 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 a voltage range of 2.0V to 3.6V.

16. 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.

17. The secondary battery according to claim 16, wherein: The maximum capacity retention rate Q of the secondary battery during the cycle process satisfies: 100.5%≤Q≤105%.

18. The secondary battery according to any one of claims 1 to 17, characterized in that The secondary battery satisfies: 0.005%<(Q-1) / (NM)≤0.020%, the unit is 1 / lock up; 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 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, in cycles.

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

20. The secondary battery according to any one of claims 1 to 19, 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.

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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