Secondary battery and electric device

Abstract

Provided in the present application are a secondary battery and an electric device. During the cyclic charge-discharge process, the growth rate of the discharge direct-current internal resistance of the secondary battery at 10% SOC during the first-cycle discharge process relative to the discharge direct-current internal resistance of the secondary battery at 90% SOC in the same cycle is B1, the growth rate of the discharge direct-current internal resistance of the secondary battery at 10% SOC during the Xth-cycle discharge process relative to the discharge direct-current internal resistance of the secondary battery at 90% SOC in the same cycle is B2, and the difference between B1 and B2 satisfies: 0.1<B1-B2≤0.35, X being any integer between 10 and 20.

Description

Secondary battery and electrical device Technical field

[0001] This application relates to the technical field of secondary batteries, and particularly to a secondary battery and an electrical device. Background technique

[0002] In recent years, as the application range of lithium-ion batteries has become increasingly wide, lithium-ion batteries are widely used in energy storage power systems such as hydroelectric, thermal, wind, and solar power stations, as well as in multiple fields such as power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, and aerospace.

[0003] With the continuous expansion of the application scenarios of secondary batteries, higher requirements are put forward for the cycle life of secondary batteries.

[0004] Summary of the invention

[0005] This application is made in view of the above problems, and its purpose is to provide a secondary battery with a high cycle life.

[0006] To achieve the above object, this application provides a secondary battery. During the cyclic charge and discharge process, the growth rate of the discharge DC internal resistance of the secondary battery at 10% SOC during the first-cycle discharge process relative to the discharge DC internal resistance at 90% SOC in this cycle is B1, and the growth rate of the discharge DC internal resistance of the secondary battery at 10% SOC during the X-th cycle discharge process relative to the discharge DC internal resistance at 90% SOC in this cycle is B2. The difference between B1 and B2 satisfies: 0.1 < B1 - B2 ≤ 0.35, where X is any integer from 10 to 20.

[0007] The growth rate (B2) of the discharge DC internal resistance at 10% SOC relative to the discharge DC internal resistance at 90% SOC during the Xth cycle of the secondary battery in the embodiment of the present application is much smaller than the growth rate (B1) of the discharge DC internal resistance at 10% SOC relative to the discharge DC internal resistance at 90% SOC during the first cycle, indicating that the transport resistance of active ions in the solid phase actually decreases during the cycling process. Unlike secondary batteries in the prior art, the secondary battery provided by the embodiment of the present application faces higher solid phase transport resistance during the first discharge cycle, which prevents the active ions from fully embedding into the positive electrode active material and prevents the secondary battery from fully utilizing its discharge capacity. However, during the Xth cycle, the transport resistance of active ions in the solid phase actually decreases compared to the first cycle, reducing the resistance to the active ions embedding back into the positive electrode active material, allowing the secondary battery to fully utilize its discharge capacity. During the cycling process, the secondary battery experiences a phenomenon of "drifting" in discharge capacity as the number of cycles increases, extending the time it takes for the secondary battery's performance to decay, which is beneficial for improving the cycle life of the secondary battery and is particularly suitable for energy storage secondary batteries where cycle life is a priority. In any embodiment, B1 satisfies: 0.35≤B1≤0.80.

[0008] The DC internal resistance of the secondary battery in the embodiment of the present application when discharged to 10% SOC during the first discharge process is significantly higher than the DC internal resistance when discharged to 90% SOC. This means that the secondary battery faces a higher solid-phase transmission resistance during the first cycle discharge process, resulting in a low level of re-embedding of active ions in the positive electrode active material, and the capacity cannot be fully utilized, which provides a possibility for improving the level of re-embedding of active ions in the positive electrode active material during subsequent cycles, that is, improving the discharge capacity.

[0009] In any embodiment, the secondary battery includes a positive electrode plate, the positive electrode plate includes a positive electrode current collector and a positive electrode film layer arranged on at least one side of the positive electrode current collector, the positive electrode film layer includes a positive electrode active material, and the positive electrode active material includes a carbon element. Based on the total mass of the positive electrode active material, the mass content C of the carbon element in the positive electrode active material satisfies: 1.2%≤C≤2.3%, optionally, 1.6%≤C≤2.0%, and further optionally, the carbon element is enriched on the surface of the positive electrode active material.

[0010] 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 difficult for the positive electrode active material to be effectively infiltrated into the electrolyte during the first cycle of the secondary battery, and the number of initial active ion pathways in the positive electrode film layer is small. As the degree of discharge increases, the transmission resistance of lithium ions in the solid phase increases, and the resistance to lithium ion reinsertion increases sharply. Therefore, during the first cycle of discharge, the growth rate B1 of the discharge DC internal resistance of the secondary battery at 10% SOC is higher than that of the discharge DC internal resistance of the secondary battery at 90% SOC, and the high DC internal resistance at 10% SOC makes it difficult for the active ions embedded in the negative electrode to be completely embedded back into the electrode, and the discharge capacity of the secondary battery is difficult to fully exert. However, as the secondary battery undergoes charge and discharge cycles, the positive electrode active material continues to expand and contract, causing the positive electrode active material to be gradually wetted by the electrolyte, the number of ion channels to increase, and the transmission resistance of lithium ions in the solid phase to decrease. As a result, during the Xth cycle, the growth rate B2 of the discharge DC internal resistance of the secondary battery at 10% SOC relative to the discharge DC internal resistance at 90% SOC is actually lower than B1 during the first cycle, and the discharge capacity of the secondary battery is improved compared to the discharge capacity after the first cycle.

[0011] In any embodiment, the positive electrode active material comprises a carbon coating layer, and a ratio D of the thickness of the carbon coating layer to the particle major diameter of the positive electrode active material is 0.0019≤D≤0.0150, optionally, 0.0025≤D≤0.009.

[0012] The positive electrode active material having a ratio D of the thickness of the carbon coating layer to the particle length of the positive electrode active material within the above range can achieve an "increase" in discharge capacity while making the discharge capacity of the secondary battery have a lower attenuation rate during a long cycle, thereby comprehensively improving the cycle life of the secondary battery.

[0013] In any embodiment, the thickness H of the carbon coating layer satisfies: 2.5 nm ≤ H ≤ 10 nm, optionally, 3 nm ≤ H ≤ 8 nm.

[0014] In any embodiment, the volume distribution particle size Dv50 of the positive electrode active material satisfies: 0.5 μm ≤ Dv50 正 ≤2.5um, optionally, 1um≤Dv50 正 ≤1.5um.

[0015] Volume distribution particle size Dv50 正The positive electrode active material within the above range can form an effective combination with the carbon coating layer, making 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, so the battery's discharge capacity cannot be effectively exerted. However, as the secondary battery's charge and discharge cycles, the lattice of the positive electrode active material continues to expand and contract, and the ion channels in the carbon coating layer continue to increase, which reduces the resistance to active ion embedding and improves the discharge capacity of the secondary battery. That is, during the cycle, the discharge capacity appears to be higher than the discharge capacity of the first cycle, and the discharge capacity "drifts up". This improves the problem of high capacity attenuation in the initial stage of the secondary battery cycle and comprehensively improves the cycle life of the secondary battery.

[0016] In any embodiment, the positive electrode active material includes one or more of lithium-containing phosphates and lithium-containing transition metal oxides.

[0017] In any embodiment, the lithium-containing phosphate includes a component Li shown in formula I x A y Me a M b P 1-c X c Y z (Formula I)

[0018] 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; Y includes one or more of O and F; and any two of A, Me, M, X, and Y do not include the same element at the same time.

[0019] 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.

[0020] 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 specific surface area of ​​the negative electrode active material is S, in units of m 2 / g, S meets: 0.9m 2 / g≤S≤1.8m 2 / g, optionally 0.9m 2 / g≤S≤1.2m 2 / g.

[0021] 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, and reducing the increase in the DC internal resistance of the secondary battery in the early stages of the cycle as the number of cycles increases. 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 for active ions to embed in the negative electrode material increases, and the negative electrode polarization decreases. By combining this with 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, and improving the cycle life of the battery.

[0022] In any embodiment, the Dv90 of the negative electrode active material is recorded as Dv90 负 , unit is um, meeting: 16μm≤Dv90 负 ≤35μm, optionally, 18μm≤Dv90 负 ≤26μm.

[0023] 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. This results in a higher DC internal resistance when the secondary battery is discharged to 10% SOC during the first cycle, which in turn prevents the battery from fully utilizing its capacity in the early stages of the charge and discharge cycle. As the negative electrode active material expands and contracts during the cycle, cracking occurs in the negative electrode active material, shortening the transmission path for active ions to embed into the center of the negative electrode active material, reducing battery polarization, and further enhancing the "drift" effect of discharge capacity through interaction with the electrolyte infiltration level of the positive electrode active material. This is more conducive to suppressing the significant attenuation of the secondary battery's electrochemical performance in the early stages of the cycle, thereby improving the cycle life of the secondary battery.

[0024] In any embodiment, the secondary battery includes an electrolyte, the electrolyte contains an electrolyte additive, the electrolyte additive includes a fluorine-containing additive, and the fluorine-containing additive includes one or more of fluorinated carbonates, fluorinated phosphates, fluorinated carbamates, fluorinated sulfates, fluorinated carboxylates, fluorinated ethers, fluorinated nitriles, fluorinated silanes, and fluorinated sulfones.

[0025] In any embodiment, during the cyclic charge and discharge process, the discharge capacity of at least one discharge process is greater than the discharge capacity C1 of the first cycle.

[0026] The discharge capacity of the secondary battery during the cycle "drifts up" compared to the discharge capacity in the first cycle, which prolongs the time for the secondary battery performance to decay and slows down the performance decay rate of the secondary battery during the initial discharge process, which is beneficial to improving the cycle life of the secondary battery. It is especially suitable for energy storage secondary batteries that pay special attention to cycle life.

[0027] In any embodiment, the discharge capacity of the discharge process at the Xth cycle during the cyclic charge and discharge process is greater than the discharge capacity C1 of the first cycle, wherein the test conditions of the cyclic charge and discharge are: charging and discharging at a constant power of 0.5P at 25°C, and a voltage range of 2.0V to 3.6V.

[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 performance decay rate of the secondary battery during the initial discharge process 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: 0.001% < (Q-1) / (NM) ≤ 0.12%, the unit is 1 / cycle; represents the maximum capacity retention rate of the secondary battery, that is, 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.

[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] 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.

[0035] 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.

[0036] 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.

[0037] A second aspect of the present application provides an electrical device comprising the secondary battery of the first aspect of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] FIG1 is a transmission electron microscope image of a positive electrode active material according to an embodiment of the present application.

[0039] FIG2 is a scanning electron microscope image of a positive electrode active material according to an embodiment of the present application.

[0040] FIG3a is a comparison diagram of the DC internal resistance of an embodiment of the present application after the first cycle and 10 cycles; FIG3b is a comparison diagram of the DC internal resistance of a comparative example of the present application after the first cycle and 10 cycles.

[0041] FIG. 4 is a schematic diagram showing the relationship between discharge capacity and cycle number of a secondary battery according to an embodiment of the present application.

[0042] FIG5 is a schematic diagram showing the cycle capacity retention rate versus the number of cycles of a secondary battery according to an embodiment of the present application.

[0043] FIG6 is a schematic diagram of a secondary battery according to an embodiment of the present application.

[0044] FIG. 7 is an exploded view of the secondary battery according to the embodiment of the present application shown in FIG. 6 .

[0045] FIG8 is a schematic diagram of a battery module according to an embodiment of the present application.

[0046] FIG9 is a schematic diagram of a battery pack according to an embodiment of the present application.

[0047] FIG10 is an exploded view of the battery pack shown in FIG9 according to an embodiment of the present application.

[0048] FIG11 is a schematic diagram of an electrical device using a secondary battery as a power source according to an embodiment of the present application.

[0049] 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

[0050] 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.

[0051] " 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.

[0052] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] Unless otherwise specified, in this application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following conditions satisfies the condition "A or B": 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).

[0057] As shown in FIG. 4, in the prior art, secondary batteries are prone to a significant decrease in the discharge capacity during the initial use and storage, resulting in difficulty in further improving the cycle life of the secondary batteries. This phenomenon is particularly significant in energy storage secondary batteries that pay special attention to the cycle life.

[0058] Based on this, the present application proposes a secondary battery. During the cyclic charge and discharge process, the growth rate of the discharge DC internal resistance of the secondary battery at 10% SOC during the first-cycle discharge process relative to the discharge DC internal resistance at 90% SOC in this cycle is B1, and the growth rate of the discharge DC internal resistance of the secondary battery at 10% SOC during the X-th cycle discharge process relative to the discharge DC internal resistance at 90% SOC in this cycle is B2. The difference between B1 and B2 satisfies: 0.1 < B1 - B2 ≤ 0.35, where X is any integer from 10 to 20.

[0059] In some embodiments, the conditions for cyclic charge and discharge are as follows: at 25°C, the battery is charged at a constant power of 0.5P to 3.6V and discharged at a constant power of 0.5P to 2.0V, which is taken as one cycle in the cyclic charge and discharge process. The first cycle in the cyclic charge and discharge process is recorded as the first-cycle charge and discharge. It should be noted that the secondary battery is not necessarily a battery that has not been cycled after formation, but also includes batteries that have been cycled, as long as the battery can perform at least X times of cyclic charge and discharge processes under the charge and discharge conditions known in the art. Therefore, the "first-cycle charge and discharge" of the 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), or it can also be a certain cycle before the cycle corresponding to the maximum discharge capacity of the secondary battery during the cycle process. It can be understood that the conditions for cyclic charge and discharge defined here are only the test conditions for cyclic charge and discharge, and do not specifically refer to the rated parameters of the secondary battery.

[0060] In some embodiments, X can be optionally any integer among 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20.

[0061] In this article, SOC (State of charge) is used to reflect the remaining capacity of the battery. Its numerical value is defined as the ratio of the remaining capacity to the total capacity of the battery. It is usually expressed as a percentage and its value range is 0% to 100%. When SOC = 0, it means that the battery is fully discharged and the battery is discharged to 2.0V; when SOC = 1, it means that the battery is fully charged and the battery is charged to 3.6V.

[0062] The discharge DC resistance (DCR) of a secondary battery can be tested by methods known in the art. As an example, a Kester electrochemical workstation is used to test the DC internal resistance (DCR) of a secondary battery. The secondary battery is discharged to different SOCs using the above charge and discharge process, and the stable voltage is recorded as V t0 , discharge at a rate of 0.5C for 60s and record the instantaneous voltage at the end of 60s discharge as V t1 , the pulse current used for 60s discharge is I dmax , calculate the DC internal resistance DCR of the secondary battery by the following formula:

[0063] In some embodiments, B1-B2 can be selected as 0.15, 0.2, 0.25, 0.3, 0.35 or any range of values ​​therebetween.

[0064] The DC internal resistance (DCR) of a secondary battery can be used to characterize the resistance to the extraction and insertion of active ions. During the discharge process, the discharge depth increases at 10% SOC compared to 90% SOC, the resistance to the extraction and insertion of active ions increases, and the discharge DC internal resistance increases. The growth rate of the discharge DC internal resistance of a secondary battery at 10% SOC relative to the discharge DC internal resistance at 90% SOC mainly depends on the transport resistance of the active ions in the solid phase. The greater the solid-phase transport resistance of the active ions in the active material, the greater the resistance to the extraction / insertion of the active ions during deep discharge, and the more significant the increase in the DC internal resistance of the secondary battery, that is, the higher the growth rate of the discharge DC internal resistance of the secondary battery at 10% SOC relative to the discharge DC internal resistance at 90% SOC.

[0065] As shown in Figure 3b, in the secondary battery of the prior art, as the number of cycles increases, the growth rate of the discharge DC internal resistance of the secondary battery at 10% SOC relative to the discharge DC internal resistance at 90% SOC remains essentially unchanged, that is, the discharge DC internal resistance at 10% SOC and the discharge DC internal resistance at 90% SOC increase synchronously with the cycle of the secondary battery, and the transport resistance of active ions in the solid phase does not change significantly during the cycle. As shown in Figure 3a, the growth rate B2 of the discharge DC internal resistance at 10% SOC relative to the discharge DC internal resistance at 90% SOC in the secondary battery of the embodiment of the present application during the Xth cycle is much smaller than the growth rate B1 of the discharge DC internal resistance at 10% SOC relative to the discharge DC internal resistance at 90% SOC during the first cycle, indicating that the transport resistance of active ions in the solid phase actually decreases during the cycle. Unlike the secondary batteries of the prior art, the secondary battery provided by the embodiment of the present application faces a higher solid phase transport resistance during the first discharge cycle, which prevents the active ions from being fully embedded in the positive electrode active material, and the discharge capacity of the secondary battery cannot be fully utilized. However, during the Xth cycle, the transmission resistance of active ions in the solid phase decreases compared to the first cycle, which reduces the resistance of active ions to embed back into the positive electrode active material, and the discharge capacity of the secondary battery is exerted to a greater extent. During the cycle process, the discharge capacity of the secondary battery "drifts" as the number of cycles increases, which prolongs the time before the performance of the secondary battery decays, which is beneficial to improving the cycle life of the secondary battery. It is especially suitable for energy storage secondary batteries that pay special attention to cycle life.

[0066] In some embodiments, B1 satisfies: 0.35≤B1≤0.8.

[0067] In some embodiments, B1 can be selected as 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.80 or any range therebetween.

[0068] As shown in Figure 3b, in the secondary battery of the prior art, during the first discharge cycle, the DC internal resistance of the secondary battery when discharged to 10% SOC is slightly higher than the DC internal resistance when discharged to 90% SOC. The DC internal resistance of the secondary battery in the embodiment of the present application when discharged to 10% SOC during the first discharge cycle is significantly higher than the DC internal resistance at 90% SOC. This means that the secondary battery faces a high solid-phase transport resistance during the first cycle discharge, resulting in a low level of active ion re-intercalation in the positive electrode active material, and the capacity cannot be fully utilized. This provides the possibility of increasing the level of active ion re-intercalation in the positive electrode active material during subsequent cycles, that is, increasing the discharge capacity.

[0069] In some embodiments, the secondary battery includes a positive electrode plate, the positive electrode plate includes a positive electrode current collector and a positive electrode film layer arranged on at least one side of the positive electrode current collector, the positive electrode film layer includes a positive electrode active material, and the positive electrode active material includes a carbon element. Based on the total mass of the positive electrode active material, the mass content C of the carbon element in the positive electrode active material satisfies: 1.2%≤C≤2.3%, optionally, 1.6%≤C≤2.0%, and further optionally, the carbon element is enriched on the surface of the positive electrode active material.

[0070] 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:

[0071] In some embodiments, the mass percentage C of carbon in the positive electrode active material can be selected as 1.2%, 1.276%, 1.3%, 1.383%, 1.5%, 1.53%, 1.6%, 1.732%, 1.858%, 2.0%, 2.1%, 2.2%, 2.3% or any numerical range therebetween.

[0072] 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 difficult for the positive electrode active material to be effectively infiltrated into the electrolyte during the first cycle of the secondary battery, and the number of initial active ion pathways in the positive electrode film is small. As the degree of discharge increases, the transmission resistance of lithium ions in the solid phase increases, and the resistance to lithium ion reinsertion increases sharply. Therefore, during the first cycle of discharge, the growth rate B1 of the discharge DC internal resistance of the secondary battery at 10% SOC is higher than that of the discharge DC internal resistance of the secondary battery at 90% SOC. The high DC internal resistance at 10% SOC makes it difficult for the active ions embedded in the negative electrode to be completely reinserted into the electrode, making it difficult for the secondary battery to fully utilize its discharge capacity. However, as the secondary battery undergoes charge and discharge cycles, the positive electrode active material continues to expand and contract, causing the positive electrode active material to be gradually wetted by the electrolyte, the number of ion channels to increase, and the transmission resistance of lithium ions in the solid phase to decrease. As a result, during the Xth cycle, the growth rate B2 of the discharge DC internal resistance of the secondary battery at 10% SOC relative to the discharge DC internal resistance at 90% SOC is actually lower than B1 during the first cycle, and the discharge capacity of the secondary battery is improved compared to the discharge capacity after the first cycle.

[0073] In some embodiments, the mass content C of the carbon element in the positive electrode active material satisfies: 1.6%≤C≤2.0%.

[0074] As shown in Figure 5, while a higher carbon coating content can increase the difference (B1-B2) between the growth rate of the discharge DC internal resistance at 10% SOC relative to that at 90% SOC in the first cycle and the growth rate of the discharge DC internal resistance at 10% SOC relative to that at 90% SOC in the Xth cycle, leading to a further improvement in the maximum discharge capacity retention rate Q during cycling (i.e., from Q2 to Q1), it also results in a significant loss in the absolute capacity and energy density of the secondary battery. Furthermore, the coating layer cannot break down during the battery's cyclic charge and discharge, which has a persistent negative impact on the ion transport of the positive electrode film, resulting in a higher rate of discharge capacity decay after reaching the maximum discharge capacity. The absolute value of the slope of line segment E, (Q-1) / (NM), is used to characterize the rate of discharge capacity decay after reaching the maximum discharge capacity, i.e., the rate of decay changes from E2 to E1.

[0075] The positive electrode active material within the above range can achieve sufficient "upward drift" of the discharge capacity while making the discharge capacity of the secondary battery have a lower attenuation rate during a long cycle, thereby comprehensively improving the cycle life of the secondary battery.

[0076] In some embodiments, the positive electrode active material comprises a carbon coating layer. In some embodiments, a ratio D of a thickness H of the carbon coating layer to a particle major diameter of the positive electrode active material is 0.0019≤D≤0.0150, optionally, 0.0025≤D≤0.009.

[0077] In some embodiments, the ratio D of the thickness of the carbon coating layer to the particle major diameter of the positive electrode active material may be 0.0019, 0.002, 0.0025, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, 0.010, 0.0150, or any range therebetween.

[0078] The thickness of the carbon coating layer and the particle length of the positive electrode active material can be characterized by transmission electron microscopy (TEM) testing. A small amount of positive electrode active material sample is dispersed in a mixed solution containing ethanol and water. After ultrasonication for 30 minutes, the suspension is dropped on the carbon film, and the sample on the carbon film is observed using a transmission electron microscope. The particle length of the positive electrode active material is measured at a low magnification. The carbon coating layer on the surface of the positive electrode active material is observed at a high magnification. As shown in Figure 1, the carbon coating layer mainly includes amorphous carbon. There is no regular arrangement between the carbon atoms in the amorphous carbon structure, while the positive electrode active material is a crystalline material. Under a transmission electron microscope, the positive electrode active material presents regular lattice fringes. Therefore, the thickness of the carbon coating layer can be tested using methods known in the art. As an example, it can be obtained by testing the sample using a transmission electron microscope (TEM). The original image obtained from the above TEM test is opened in Digital Micrograph software, and the carbon coating layer is identified by the lattice fringes to measure its thickness.

[0079] The positive electrode active material having a ratio D of the thickness of the carbon coating layer to the particle length of the positive electrode active material within the above range can achieve an "increase" in discharge capacity while making the discharge capacity of the secondary battery have a lower attenuation rate during a long cycle, thereby comprehensively improving the cycle life of the secondary battery.

[0080] In some embodiments, the thickness H of the carbon coating layer satisfies: 2.5 nm ≤ H ≤ 10 nm, optionally, 3 nm ≤ H ≤ 8 nm.

[0081] In some embodiments, the thickness H of the carbon coating layer of the positive electrode active material may be 2.5 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, or any range therebetween.

[0082] In some embodiments, the volume distribution particle size Dv50 of the positive electrode active material satisfies: 0.5um≤Dv50 正≤2.5um, optionally, 1um≤Dv50 正 ≤1.5um.

[0083] As used herein, the term "Dv50" refers to the particle size corresponding to the cumulative volume distribution number of particles reaching 50% in a particle size distribution curve.

[0084] In this application, the volume distribution particle size Dv50 of the positive electrode active material can be measured using methods known in the art. As an example, referring to GB / T 19077-2016, it can be measured using a laser particle size analyzer. The testing instrument can be a Mastersizer 3000 laser particle size analyzer manufactured by Malvern Instruments Ltd., UK.

[0085] In some embodiments, the volume distribution particle size Dv50 of the positive electrode active material is 正 The options can be 0.5μm, 0.6μm, 0.69μm, 0.7μm, 0.8μm, 0.9μm, 1μm, 1.1μm, 1.2μm, 1.3μm, 1.4μm, 1.5μm, 1.6μm, 1.7μm, 1.8μm, 1.9μm, 2.0μm, 2.1μm, 2.2μm, 2.0μm, 2.1μm, 2.2μm, 2.3μm, 2.4μm, 2.5μm or any numerical range between two of them.

[0086] Volume distribution particle size Dv50 正 The positive electrode active material within the above range can form an effective combination with the carbon coating layer, making 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, so the battery's discharge capacity cannot be effectively exerted. However, as the secondary battery's charge and discharge cycles, the lattice of the positive electrode active material continues to expand and contract, and the ion channels in the carbon coating layer continue to increase, which reduces the resistance to active ion embedding and improves the discharge capacity of the secondary battery. That is, during the cycle, the discharge capacity appears to be higher than the discharge capacity of the first cycle, and the discharge capacity "drifts up". This improves the problem of high capacity attenuation in the initial stage of the secondary battery cycle and comprehensively improves the cycle life of the secondary battery.

[0087] In some embodiments, the positive electrode active material includes one or more of a lithium-containing phosphate and a lithium-containing transition metal oxide.

[0088] In some embodiments, the positive electrode active material may adopt the positive electrode active material for batteries that is well known in the art. As an example, the positive electrode active material may include at least one of the following materials: lithium-containing phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also referred to as NCM 333 ), LiNi 0.5 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.

[0089] 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.

[0090] 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.

[0091] 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.

[0092] In this article, "lithium-rich manganese-based material" refers to a material whose main component includes 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), where m+n+q=1,0 <s<1。

[0093] 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.

[0094] 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)

[0095] 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; Y includes one or more of O and F; and any two of A, Me, M, X, and Y do not include the same element at the same time.

[0096] 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.

[0097] 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.).

[0098] 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.

[0099] 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.

[0100] 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.

[0101] 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.

[0102] 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≤1.8m 2 / g, optionally 0.9m 2 / g≤S≤1.2m 2 / g.

[0103] 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.

[0104] 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, 1.8m 2 / g or any range of values ​​between them.

[0105] 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, and reducing the increase in the DC internal resistance of the secondary battery in the early stages of the cycle as the number of cycles increases. 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 for active ions to embed in the negative electrode material increases, and the negative electrode polarization decreases. By combining this with 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, and improving the cycle life of the battery.

[0106] In some embodiments, the Dv90 of the negative electrode active material is recorded as Dv90 负 , unit is um, satisfying: 16um≤Dv90 负 ≤35um, optional 18um≤Dv90 负 ≤26um.

[0107] 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.

[0108] 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.

[0109] In some embodiments, Dv90 负 The options include 16μm, 17μm, 18μm, 19μm, 20μm, 21μm, 22μm, 23μm, 24μm, 25μm, 26μm, 27μm, 28μm, 29μm, 30μm, 31μm, 32μm, 33μm, 34μm, 35μm or any numerical range therebetween.

[0110] 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. This results in a higher DC internal resistance when the secondary battery is discharged to 10% SOC during the first cycle, which in turn prevents the battery from fully utilizing its capacity in the early stages of the charge and discharge cycle. As the negative electrode active material expands and contracts during the cycle, cracking occurs in the negative electrode active material, shortening the transmission path for active ions to embed into the center of the negative electrode active material, reducing battery polarization, and further enhancing the "drift" effect of discharge capacity through interaction with the electrolyte infiltration level of the positive electrode active material. This is more conducive to suppressing the significant attenuation of the secondary battery's electrochemical performance in the early stages of the cycle, thereby improving the cycle life of the secondary battery.

[0111] 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.

[0112] 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).

[0113] 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.

[0114] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0115] 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.).

[0116] As an example, the negative electrode current collector has two surfaces facing each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two facing surfaces of the negative electrode current collector.

[0117] 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.

[0118] In some embodiments, the secondary battery includes an electrolyte, the electrolyte contains an electrolyte additive, the electrolyte additive includes a fluorine-containing additive, and the fluorine-containing additive includes one or more of fluorinated carbonates, fluorinated phosphates, fluorinated carbamates, fluorinated sulfates, fluorinated carboxylates, fluorinated ethers, fluorinated nitriles, fluorinated silanes, and fluorinated sulfones.

[0119] In some embodiments, the electrolyte further includes an electrolyte salt and a solvent.

[0120] 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.

[0121] 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.

[0122] In some embodiments, during the cyclic charge and discharge process, the discharge capacity of at least one discharge process is greater than the discharge capacity C1 of the first cycle, wherein the test conditions of the cyclic charge and discharge are: charging and discharging at a constant power of 0.5P at 25°C, and a voltage range of 2.0V to 3.6V.

[0123] 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.

[0124] The discharge capacity of the secondary battery during the cycle "drifts up" compared to the discharge capacity in the first cycle, which prolongs the time for the secondary battery performance to decay and slows down the performance decay rate of the secondary battery during the initial discharge process, which is beneficial to improving the cycle life of the secondary battery. It is especially suitable for energy storage secondary batteries that pay special attention to cycle life.

[0125] In some embodiments, the discharge capacity of the discharge process at the Xth cycle in the cyclic charge and discharge process is greater than the discharge capacity C1 of the first cycle.

[0126] 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.

[0127] Please refer to Figure 4 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.

[0128] 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.

[0129] In some embodiments, the maximum capacity retention rate Q of the secondary battery during cycling satisfies: 100.2%≤Q≤105%.

[0130] The secondary battery has a high maximum capacity retention rate. The discharge capacity of the secondary battery increases to a certain extent during the cycle, which further slows down the performance decay rate of the secondary battery during the initial discharge process and can further improve the cycle life of the secondary battery.

[0131] In some embodiments, the secondary battery satisfies the following conditions: 0.001% < (Q-1) / (NM) ≤ 0.12%, 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.

[0132] 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%.

[0133] 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.

[0134] In some embodiments, (Q-1) / (NM) can be selected as 0.001%, 0.002%, 0.003%, 0.004%, 0.005%, 0.006%, 0.007%, 0.008%, 0.009%, 0.01%, 0.011%, 0.012%, 0.013%, 0.014%, 0.015%, 0.016%, 0.017%, 0.018%, 0.019%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, 0.11%, 0.12%, or any range therebetween.

[0135] 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.

[0136] In some embodiments, (Q-1) / (NM) ≤ 0.015%.

[0137] 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.

[0138] 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.

[0139] 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.

[0140] 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.

[0141] 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.

[0142] 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.

[0143] 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.

[0144] 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.

[0145] In some embodiments, the secondary battery has a gram capacity greater than or equal to 140 mAh / g.

[0146] 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.

[0147] 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.

[0148] In some embodiments, the secondary battery is optionally one or more of a battery cell, a battery module, and a battery pack.

[0149] 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.

[0150] 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.

[0151] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0152] 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.

[0153] 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.

[0154] 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.

[0155] 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.

[0156] Figure 8 shows an example battery module 4. Referring to Figure 8 , within the battery module 4, multiple secondary batteries 5 may be arranged sequentially along the length of the battery module 4. Of course, any other arrangement is also possible. Furthermore, the multiple secondary batteries 5 may be secured together using fasteners.

[0157] 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.

[0158] 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.

[0159] 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.

[0160] A second aspect of the present application provides an electrical device comprising the secondary battery of any embodiment.

[0161] 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.

[0162] As the electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0163] 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.

[0164] 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.

[0165] Example

[0166] 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.

[0167] 1. Preparation method

[0168] Example 1

[0169] 1. Preparation of positive electrode sheet:

[0170] The prepared lithium iron phosphate, conductive carbon black, and binder polyvinylidene fluoride (PVDF) are thoroughly stirred and mixed in an N-methylpyrrolidone solvent system in a weight ratio of 97:1:2 to obtain a positive electrode slurry; the above positive electrode slurry is evenly coated on the positive electrode current collector aluminum foil, and then dried, cold pressed, and cut to obtain a positive electrode sheet.

[0171] Preparation of carbon-coated lithium iron phosphate P1 as the positive electrode active material:

[0172] A liquid phase method is used to dissolve iron sources such as ferric nitrate or ferric sulfate, lithium sources such as lithium nitrate or lithium hydroxide, and phosphoric acid or ammonium dihydrogen phosphate as raw materials in water, and then the mixture is kept at a high temperature and high pressure of 180°C in a sealed kettle for 4 hours, and then cooled at a predetermined rate, filtered, washed, and dried to obtain a precursor.

[0173] The prepared precursor was dry-mixed and ball-milled with 10% glucose carbon source at a mixing and grinding speed of 400 rpm and a grinding time of 3 h. The ball-milled powder was then placed in a high-temperature furnace protected by N2 and sintered at a temperature of 600 ° C for 2 h to obtain carbon-coated LFP material, which was then crushed and graded.

[0174] The Dv50 of the positive electrode active material is 1.2 μm. The positive electrode active material is lithium iron phosphate with a carbon coating layer. The thickness H of the carbon coating layer is 3.2 nm. The mass content X of the carbon element is 1.7% based on the total mass of the positive electrode active material. The ratio D of the thickness of the carbon coating layer to the particle length of the positive electrode active material is 0.00267. The specific surface area of ​​the positive electrode active material is 10.6 m 2 / g.

[0175] 2. Preparation of negative electrode sheet

[0176] The negative electrode active material graphite, conductive carbon black, binder styrene-butadiene rubber (SBR), and thickener sodium carboxymethyl cellulose (CMC-Na) are fully stirred and mixed in a deionized water solvent system in a mass ratio of 96:2:1:1 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.

[0177] The specific surface area of ​​the negative electrode active material is 1.78m 2 / g, Dv90 is 16.5um.

[0178] 3. Preparation of electrolyte

[0179] In an argon atmosphere glove box (H2O content <0.1ppm, O2 content <0.1ppm), 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.

[0180] 4. Isolation film

[0181] A 9 μm polyethylene (PE) film was used as the separator.

[0182] 5. Preparation of batteries

[0183] 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, and shaping, 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.

[0184] The preparation methods of the batteries of Examples 2-5 are similar to those of the battery of Example 1, but the carbon content of the positive electrode active material is adjusted by adjusting the amount of carbon source added and the sintering temperature. Specific parameters are shown in Table 1.

[0185] The batteries of Examples 6-7 were prepared similarly to the battery of Example 1, but the Dv50 of the positive electrode active material was adjusted by adjusting the hydrothermal reaction time of the precursor and the time of mixing and grinding the precursor and the glucose carbon source;

[0186] In Example 6, the hydrothermal reaction time and ball milling time were controlled to 3h and 4h, the Dv50 of the positive electrode active material was 0.9μm, the addition amount of the carbon source and the sintering temperature were controlled to 15% and 560°C, and the carbon percentage was 2.3%.

[0187] In Example 7, the hydrothermal reaction time and ball milling time were controlled to 5 h and 2.3 h, the Dv50 of the positive electrode active material was 1.8 μm, the amount of carbon source added and the sintering temperature were controlled to 15% and 580° C., and the carbon content was 2.3%.

[0188] The battery of Comparative Example 1 was prepared similarly to the battery of Example 1. The Dv50 of the positive electrode active material particles was 1.1 μm. The lithium iron phosphate had a carbon coating layer on its surface. The thickness of the carbon coating layer was 2 nm, and the mass content of the carbon element was 1.16% based on the total mass of the lithium iron phosphate. The ratio of the thickness of the carbon coating layer to the major diameter of the positive electrode active material particles was 0.0018.

[0189] Table 1

[0190] 2. Test Method

[0191] 1. Cycle test

[0192] At 25°C, the battery is charged at a constant power of 0.5P to 3.6V and discharged at a constant power of 0.5P to 2.0V. The obtained discharge capacity is recorded as the discharge capacity C1 of the first cycle. Repeat the above steps for the same battery, and at the same time record the discharge capacity Cn of the battery after the nth cycle. Then, the battery capacity retention rate Pn = Cn / C1 × 100%.

[0193] The maximum discharge capacity C during the cyclic charge and discharge process M The ratio of the maximum discharge capacity C to the discharge capacity C1 of the first cycle is used as the maximum capacity retention rate Q. The number of cycles corresponding to the maximum capacity retention rate Q is M, and the number of cycles corresponding to the capacity retention rate when it decays from the maximum capacity retention rate Q to basically 100% is N. When the battery capacity retention rate reaches 80%, stop the test and record the number of cycles.

[0194] III. Analysis of test results of each embodiment and comparative example

[0195] Prepare the batteries of each embodiment and comparative example according to the above method, and measure various performance parameters. The results are shown in Table 2 below.

[0196] Table 2

[0197] From the comparison between the embodiments and the comparative examples, it can be seen that during the cyclic charge and discharge process of the secondary battery in the embodiments, the growth rate of the discharge DC internal resistance of the secondary battery at 10% SOC during the first-cycle discharge process relative to the discharge DC internal resistance at 90% SOC in this cycle is B1, and the growth rate of the discharge DC internal resistance of the secondary battery at 10% SOC during the 10th-cycle discharge process relative to the discharge DC internal resistance at 90% SOC in this cycle is B2. The difference between B1 and B2 satisfies: 0.1 < B1 - B2 ≤ 0.35. The secondary battery in the embodiments realizes the "upward drift" of the discharge capacity during the cycle compared with the comparative example, and improves the cycle life of the battery.

[0198] When the carbon content C of the positive active material in the secondary battery satisfies: 1.6% ≤ C ≤ 2.0%, the discharge capacity of the secondary battery shows a slow decay, that is, a low (Q - 1) / (N - M), which is beneficial to further improving the cycle life of the secondary battery.

[0199] It should be noted that this application is not limited to the above embodiments. The above embodiments are only examples, and embodiments with the same structure and the same function and effect as the technical idea within the technical solution scope of this application are all included in the technical scope of this application. In addition, within the scope of not departing from the main idea of this application, various modifications that can be thought of by those skilled in the art to the embodiments and other ways constructed by combining some constituent elements of the embodiments are also included in the scope of this application.

Claims

1. A secondary battery, characterized in that: During the cyclic charge and discharge process, the growth rate of the discharge DC internal resistance of the secondary battery at 10% SOC during the first-cycle discharge process relative to the discharge DC internal resistance at 90% SOC in this cycle is B1, and the growth rate of the discharge DC internal resistance of the secondary battery at 10% SOC during the Xth-cycle discharge process relative to the discharge DC internal resistance at 90% SOC in this cycle is B2. The difference between B1 and B2 satisfies: 0.1 < B1 - B2 ≤ 0.35, where X is any integer from 10 to 20.

2. The secondary battery according to claim 1, wherein B1 satisfies: 0.35 ≤ B1 ≤ 0.

80.

3. The secondary battery according to claim 1 or 2, 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 disposed on at least one side of the positive electrode current collector, the positive electrode film layer includes a positive electrode active material, the positive electrode active material includes carbon element, based on the total mass of the positive electrode active material, the mass content C of carbon element in the positive electrode active material satisfies: 1.2% ≤ C ≤ 2.3%, optionally, 1.6% ≤ C ≤ 2.0%, and further optionally, the carbon element is enriched on the surface of the positive electrode active material.

4. The secondary battery according to claim 3, wherein The positive electrode active material includes a carbon coating layer, and the ratio D of the thickness of the carbon coating layer to the particle major axis of the positive electrode active material is 0.0019 ≤ D ≤ 0.0150, optionally, 5. The secondary battery according to claim 4, wherein ​ 6. The secondary battery according to any one of claims 3 to 5, characterized in that The volume distribution particle size Dv50 of the positive electrode active material 正 Meet: 0.5um≤Dv50 正 ≤2.5um, optionally, 1um≤Dv50 正 ≤1.5um.

7. The secondary battery according to any one of claims 3 to 6, characterized in that ​ 8. The secondary battery according to claim 7, 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) ​ 9. The secondary battery according to any one of claims 1 to 8, characterized in that The negative electrode sheet 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 specific surface area of ​​the negative electrode active material is S, in units of m 2 / g, S meets: 0.9m 2 / g≤S≤1.8m 2 / g, optionally 0.9m 2 / g≤S≤1.2m 2 / g.

10. The secondary battery according to claim 9, wherein The Dv90 of the negative electrode active material is recorded as Dv90 负 , unit is um, meeting: 16μm≤Dv90 负 ≤35μm, optionally, 18μm≤Dv90 负 ≤26μm.

11. The secondary battery according to any one of claims 1 to 10, characterized in that: ​ 12. The secondary battery according to any one of claims 1 to 11, characterized in that: ​ 13. The secondary battery according to any one of claims 1 to 12, characterized in that: During the charge and discharge cycle, the discharge capacity of the discharge process at the Xth cycle is greater than the discharge capacity C1 of the first cycle.

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) ≤ 0.12%, with the unit being 1 / cycle; indicating 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.

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 comprising the secondary battery according to any one of claims 1 to 18.

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