Composite positive electrode sheet, lithium secondary battery, and activation method of lithium secondary battery

Abstract

The application provides a composite positive electrode sheet, a lithium secondary battery and an activation method of the lithium secondary battery. The composite positive electrode sheet comprises a current collector and a coating layer; the oxidation potential of a lithium supplementing substance in the coating layer is greater than 3.55 V, and the positive electrode active substance in the coating layer comprises LiFePO4. The activation method comprises the following steps: after the lithium secondary battery starts to attenuate, once static, charging to an upper limit voltage, twice static, then discharging to a voltage less than or equal to 3.65 V, and the battery normally works; wherein, the activation is repeated when the next attenuation of the battery occurs; the range of the upper limit voltage is 3.65-4.4 V and does not include the two end values. Through the fine matching of the de-lithiation potential of the positive electrode active substance and the lithium supplementing substance and the control of the activation potential, the application realizes the selective activation of the lithium supplementing substance, thereby making up for the consumption of active lithium due to the repeated repair of SEI in the cycle process of the lithium secondary battery, and finally realizing the super-long cycle life.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion batteries, and relates to a composite positive electrode sheet, a lithium secondary battery and an activation method of the lithium secondary battery. BACKGROUND

[0002] The gradual depletion of petrochemical energy and the increasing severity of the greenhouse effect have prompted researchers to continuously explore sustainable and clean energy. Since Sony first commercialized lithium ion batteries in 1991, lithium ion batteries have been widely used in 3C digital products and power batteries. Because lithium ion batteries have more advantages than traditional lead-acid batteries in cycle life, volume energy density and mass energy density, many technical personnel have been working on the commercialization of lithium ion batteries in the past 20 years.

[0003] The actual available capacity and cycle life of the full battery are closely related to the first coulomb efficiency of the lithium ion battery and the formation of the negative electrode SEI film. The first efficiency of widely used graphite is between 90% and 95%, and the first irreversible efficiency of hard carbon and silicon-carbon materials with higher energy density is more than 15%, mainly because a large amount of active lithium is consumed in the process of forming a solid-state electrolyte film (SEI), thereby causing the reversible capacity of the entire cell to greatly decrease relative to the ideal reversible capacity. In addition, during the charging and discharging cycle process of the cell, the SEI is continuously generated due to the repeated expansion and contraction of the negative electrode, which causes the continuous slow consumption of active lithium, which is the most common capacity decay mode of lithium ion batteries.

[0004] In order to solve this problem, many researchers have proposed various methods of supplementing lithium, which is also one of the current research hotspots.

[0005] The existing lithium supplement mainly includes positive electrode lithium supplement, negative electrode lithium supplement and third electrode lithium supplement. The positive electrode lithium supplement mainly composites a certain proportion of high specific capacity single substance or mixture containing lithium during the preparation of the positive electrode sheet, and significantly improves the charging capacity of the positive electrode sheet through the first charging activation, although the first efficiency of the negative electrode does not change, but the comprehensive reversible capacity increases. Among them, mainly include lithium-rich compounds, nano-composites based on conversion reaction and binary lithium compounds, etc. The negative electrode lithium supplement is mainly lithium foil or inert lithium powder, which forms a primary cell with the negative electrode active material after being compounded with the negative electrode and providing active lithium to the negative electrode active material spontaneously after liquid injection. Some also use an internal third electrode lithium metal to supplement active lithium. In general, through additional lithium supplement, the first efficiency of the cell and the total amount of active lithium that can be run can be improved.

[0006] Both lithium metal and lithium powder have a low potential and high chemical reactivity, are easy to burn in the air, have great processing safety problems, and have compatibility problems with existing solvents and binders, for example, stabilized lithium metal powder (SLMP) will react with the commonly used sizing solvent NMP, which poses an explosion hazard.

[0007] CN103401016A discloses a high-energy-density lithium ion battery, which comprises a positive electrode sheet, a negative electrode sheet, a separator, a positive electrode lithium supplement layer between the positive electrode sheet and the separator, an electrolyte, a first polymer conductive layer between the positive electrode sheet and the positive electrode lithium supplement layer, and a second polymer conductive layer between the positive electrode lithium supplement layer and the separator. The positive electrode sheet comprises a positive electrode current collector and a positive electrode film, and the negative electrode sheet comprises a negative electrode current collector and a negative electrode film. The positive electrode film of the positive electrode sheet at least contains a first positive electrode active material with high initial efficiency performance and a second positive electrode active material with resistance to excessive lithium intercalation performance. This method has great processing safety problems, and the lithium metal has high reactivity with the electrolyte. Since the lithium element can react with the electrolyte, it is easy to cause damage to the electrolyte system, and finally affect the performance of the entire battery.

[0008] CN111370657A discloses a positive electrode lithium supplement material, a preparation method and application thereof. The positive electrode lithium supplement material comprises a first lithium-containing compound selected from a combination of one or more compounds of formula I: Li x A y Formula I wherein A is selected from C, N, O, P, S, 0. In the first circle, all the activation is activated, and the reversible capacity of the first circle is improved to a certain extent. Although the initial capacity is improved in this method, the amount of negative electrode must be increased to ensure the excess coefficient of the negative electrode, which can improve the energy density to a certain extent, but is not helpful for improving the cycle life.

[0009] Therefore, how to further improve the cycle performance of the lithium secondary battery is a technical problem to be solved. SUMMARY

[0010] The purpose of the present application is to provide a composite positive electrode sheet, a lithium secondary battery and an activation method of the lithium secondary battery. The present application realizes selective activation of the lithium supplement material by fine matching of the de-lithiation potential of the positive electrode active material and the lithium supplement material and control of the activation potential, thereby making up for the consumption of active lithium due to repeated repair of SEI in the cycle process of the lithium secondary battery, and finally realizing ultra-long cycle life.

[0011] To achieve the purpose of the present application, the following technical solutions are adopted:

[0012] In a first aspect, the present application provides a composite positive electrode sheet, which comprises a current collector and a coating layer. The coating layer comprises a positive electrode active material, a lithium supplement material, a binder and a conductive agent. The oxidation potential of the lithium supplement material is greater than 3.55V, and the positive electrode active material comprises LiFePO4.

[0013] For example, the oxidation potential can be 3.6V, 3.7V, 3.8V, 3.9V, 4V, 4.1V, 4.2V, 4.3V, 4.4V, 4.5V, 4.6V, 4.7V or 4.8V, etc.

[0014] In the present application, the oxidation potential of the lithium supplementing substance is the lithium potential.

[0015] In the present application, when the positive active material is lithium iron phosphate material, the oxidation potential of the lithium supplementing substance is above 3.55V, so that only the positive active material participates in the electrochemical reaction or the positive active material and a very small part of the lithium supplementing additive participates in the electrochemical reaction within the normal charging and discharging voltage range of 2.5-3.65V.

[0016] In the present application, by limiting the oxidation potential of the lithium supplementing substance, only the positive active material participates in the electrochemical reaction or the positive active material and a very small part of the lithium supplementing additive participates in the electrochemical reaction within the normal voltage range of 2.5-3.65V, and most of the lithium supplementing additive participates in the electrochemical reaction under a specific activation process to provide active lithium ions and supplement the consumption of active lithium by the negative electrode, thereby recovering a certain proportion of capacity. By activating the lithium supplementing additive after the capacity decays to a certain extent, the problem of insufficient negative electrode excess coefficient or the need to increase the negative electrode load caused by the activation of all lithium supplementing additives in the formation stage of the conventional lithium supplementing method is solved, and the cycle life is greatly prolonged.

[0017] The current collector of the composite positive electrode plate can be selected from aluminum foil, the conductive agent can be selected from any one or a combination of at least two of conductive carbon black, carbon fiber, acetylene black, ketjen black, graphene or carbon nanotube, and the binder can be selected from any one or a combination of at least two of fluorine resin type binder, polypropylene resin type binder, fiber type binder, rubber type binder or polyimide type binder.

[0018] In the present application, the preparation method of the composite positive electrode plate is not specially limited, and any conventional and achievable technical solution can be used.

[0019] Preferably, the lithium supplementing substance includes Li2NiO2 and / or Li5FeO5.

[0020] In the present application, Li2NiO2 or Li5FeO5 is selected as the lithium supplementing substance. On the one hand, Li2NiO2 and Li5FeO5 can also be used as positive active materials, so that the capacity provided by the composite positive electrode plate will not be significantly reduced when used within the normal voltage range, and at the same time, they can also be used as lithium supplementing additives to further increase the total amount of active lithium that can be provided by the composite positive electrode plate during the whole life cycle, thereby prolonging the cycle life.

[0021] Preferably, the oxidation potential of the lithium supplementing substance is 3.6-4.4V, for example 3.6V, 3.65V, 3.7V, 3.75V, 3.8V, 3.85V, 3.9V, 3.95V, 4V, 4.05V, 4.1V, 4.15V, 4.2V, 4.25V, 4.3V, 4.35V or 4.4V, etc.

[0022] In the present application, the oxidation potential of the lithium supplementing substance is in the range of 3.6-4.4V, the charge specific capacity is greater than 170mAh / g, and the discharge specific capacity at 2.5V is less than 50% of the total charge specific capacity. It can be further matched with the lithium iron phosphate positive electrode active material to realize that the capacity provided by the composite positive electrode sheet in the normal use voltage range will not be significantly reduced, and at the same time, it also serves as a lithium supplementing additive to further improve the total amount of active lithium that can be provided by the composite positive electrode sheet in the whole life cycle, thereby prolonging the cycle life.

[0023] Preferably, the lithium supplementing substance accounts for 0.5-10% of the coating, for example 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%, etc., preferably 0.5-5%.

[0024] In the present application, more than 10% of the lithium supplementing substance will cause the capacity provided by the composite positive electrode sheet in the normal use voltage range to be significantly reduced. When it is in the range of 0.5-5%, it is more conducive to achieving the best balance point of the capacity provided by the composite positive electrode sheet in the normal use voltage range and the capacity provided in the whole life cycle.

[0025] In the second aspect, the present application provides a lithium secondary battery, which comprises the positive electrode sheet as described in the first aspect, a negative electrode sheet, a separator and an electrolyte.

[0026] The selection of the negative electrode sheet, the separator and the electrolyte of the lithium secondary battery provided by the present application can be conventional selection, for example, the active substance in the negative electrode sheet can embed and remove lithium ions, which can be selected from any one or a combination of at least two of carbon material, lithium metal, silicon or tin; the separator can be selected from a sheet or non-woven fabric obtained from polypropylene, glass fiber or polyethylene.

[0027] The lithium secondary battery provided by the present application can be a liquid battery or a solid battery.

[0028] In the third aspect, the present application further provides an activation method of the lithium secondary battery as described in the second aspect, which comprises:

[0029] After the lithium secondary battery begins to degrade, it is first left to stand, then charged to the upper limit voltage, then left to stand again, and then discharged to a voltage ≤3.65V, such as 3.65V, 3.6V, 3.5V, 3.4V, 3.3V, 3.2V, 3.1V, 3V, 2.9V, 2.8V, 2.7V, 2.6V, 2.5V, 2.4V or 2.3V, and the battery will work normally.

[0030] The battery is reactivated when it experiences its next degradation. The upper limit voltage range is 3.65 to 4.4V and does not include the two endpoint values. For example, the upper limit voltage can be 3.65V, 3.7V, 3.75V, 3.8V, 3.85V, 3.9V, 3.95V, 4V, 4.05V, 4.1V, 4.15V, 4.2V, 4.25V, 4.3V, 4.35V, or 4.4V, etc.

[0031] This invention achieves selective activation of the lithium replenishing material by precisely matching the delithiation potential of the positive electrode active material and the lithium replenishing material, as well as controlling the activation potential. This compensates for the consumption of active lithium due to repeated SEI repair during the cycling process of lithium secondary batteries, ultimately achieving an ultra-long cycle life.

[0032] In this invention, the upper limit voltage is greater than 3.65V because the normal charge and discharge voltage range is 2.5-3.65V. If the activation voltage is less than or equal to 3.65V, the lithium supplementation additive cannot be activated. At the same time, it must be less than 4.4V because a voltage higher than 4.4V will cause significant decomposition and gas production of the electrolyte, increasing the internal resistance.

[0033] The activation method for lithium secondary batteries provided by this invention,

[0034] Preferably, the settling time is 1 to 30 minutes, such as 1 minute, 5 minutes, 10 minutes, 15 minutes, 20 minutes, 25 minutes, or 30 minutes.

[0035] Preferably, the second settling time is 1 to 30 minutes, such as 1 minute, 5 minutes, 10 minutes, 15 minutes, 20 minutes, 25 minutes, or 30 minutes.

[0036] Preferably, when activation is repeated, the upper limit voltage value of the later activation should be greater than the upper limit voltage of the previous activation.

[0037] In this invention, during the repeated activation process, the upper limit voltage of each activation is higher than the upper limit voltage of the previous one. This is because the previous activation has already activated all the capacity below the upper limit voltage of this activation, and the lithium additive only has the capacity above the upper limit voltage of this activation that has not been activated.

[0038] Preferably, the voltage at which the battery normally operates is 2.5-3.65V, such as 2.5V, 2.6V, 2.7V, 2.8V, 2.9V, 3V, 3.1V, 3.2V, 3.3V, 3.4V, 3.5V, 3.6V or 3.65V, etc.

[0039] As a preferred technical solution, the activation method comprises:

[0040] After the lithium secondary battery starts to decay, it is first left for 1-30min, then charged to the upper limit voltage, secondly left for 1-30min, and then discharged to a voltage of ≤3.65V, and the battery normally operates at 2.5-3.65V;

[0041] Wherein, the upper limit voltage value of the first activation after the next decay of the battery is greater than the upper limit voltage of the previous activation; the range of the upper limit voltage is 3.65-4.4V and does not include the two end values.

[0042] In the present application, when the positive electrode of the lithium secondary battery is a lithium iron phosphate material and the lithium supplementing substance is Li2NiO2, the above-mentioned activation method is used to selectively activate the lithium supplementing substance, thereby making up for the consumption of active lithium due to repeated repair of SEI during the cycle process of the lithium secondary battery, and finally achieving an ultra-long cycle life.

[0043] Compared with the prior art, the present application has the following beneficial effects:

[0044] The present application selectively activates the lithium supplementing substance by fine matching of the de-lithiation potential of the positive active material and the lithium supplementing substance and control of the activation potential, thereby making up for the consumption of active lithium due to repeated repair of SEI during the cycle process of the lithium secondary battery, enabling the capacity of the battery to be further improved after the first decay through re-activation, rather than further decay on the basis of the original decay, and finally achieving an ultra-long cycle life. BRIEF DESCRIPTION OF DRAWINGS

[0045] Figure 1 The cycle performance comparison chart of the lithium secondary battery provided in Example 1 and Comparative Example 1. DETAILED DESCRIPTION

[0046] The technical solutions of the present application will be further described below through specific embodiments. Those skilled in the art should understand that the embodiments are only to help understand the present application and should not be regarded as specific limitations on the present application.

[0047] Example 1

[0048] The present embodiment provides a lithium secondary battery, which comprises a composite positive electrode sheet, a negative electrode sheet, a separator and an electrolyte.

[0049] The preparation method of the composite positive electrode sheet is as follows:

[0050] LiFePO4, Li2NiO2, conductive carbon black and polyvinylidene fluoride were dispersed in N-methylpyrrolidone to form a positive active material slurry, wherein the weight ratio of LiFePO4: Li2NiO2: conductive carbon black: polyvinylidene fluoride was 94.7: 1.8: 1.6: 1.9, the slurry was coated on both surfaces of a 13 μm thick aluminum foil, and then dried at 90°C for 10 minutes, the coated double-sided density of the dried aluminum foil was 33.2 mg / cm 2 , and then punched into a tab with a tab ear 70.5 mm wide and 81.0 mm high after rolling;

[0051] The negative electrode tab was prepared as follows:

[0052] Graphite, conductive carbon black, hydroxymethyl cellulose and butadiene-styrene rubber were dispersed in deionized water to form a negative active material slurry, wherein the weight ratio of graphite: conductive carbon black: hydroxymethyl cellulose: butadiene-styrene rubber was 96: 0.8: 1.2: 2, the slurry was coated on both surfaces of a copper foil (thickness 8 μm), and then dried at 60°C for 10 minutes to form a negative active material layer, the double-sided coating surface density of the dried copper foil was 16.0 mg / cm 2 , and then punched into a tab with a tab ear 74.5 mm wide and 85.0 mm high after rolling;

[0053] Preparation of a lithium secondary battery:

[0054] The above positive and negative electrodes and a polyethylene separator were stacked into a core package (thickness 5.5 mm, width 75.0 mm, height 89.5 mm) of a lithium ion secondary battery, the core package was packaged in an aluminum-plastic film bag, then LiPF6 was dissolved in a mixed solvent of EC / PC / EMC = 35:5:60 (mass ratio) to form a non-aqueous electrolyte with a concentration of 1 mol / L, the electrolyte was injected into the aluminum-plastic film bag in an amount of 4.5 g / Ah and sealed. After standing for 24 h, formation was carried out at 45°C, then the gas was extracted under negative pressure and re-sealed, then charged at 1.0C to 3.65V, constant voltage to 0.05C at 3.65V, then discharged at 1.0C to 2.5V, to obtain an initial discharge capacity Co.

[0055] The lithium secondary battery was subjected to a cycle performance test:

[0056] After 1500 cycles of charging and discharging at 1C between 2.5 and 3.65V, the battery was attenuated;

[0057] After the attenuation of the battery, the battery was activated, and the activation method was as follows:

[0058] (1) standing for 5 min;

[0059] (2) 1.0C constant current charging to 4.3V, 4.3V constant voltage to 0.05C;

[0060] (3) resting for 5min;

[0061] (4) 1.0C discharging to 2.5V; after the activation, the battery continues to perform the cycle performance test.

[0062] Example 2

[0063] The difference between this example and Example 1 is that in this example, the lithium supplementing substance is Li5FeO5.

[0064] The preparation method of the remaining lithium secondary battery is consistent with that of Example 1.

[0065] The cycle performance test is performed on the lithium secondary battery:

[0066] After 1500 cycles of charging and discharging at 1C under 2.5-3.65V, the battery is attenuated, and is activated;

[0067] After the attenuation of the battery, the battery is activated, and the activation method is as follows:

[0068] (1) resting for 5min;

[0069] (2) 1.0C constant current charging to 4.3V, 4.3V constant voltage to 0.05C;

[0070] (3) resting for 5min;

[0071] (4) 1.0C discharging to 2.5V; after the activation, the battery continues to perform the cycle performance test.

[0072] Example 3

[0073] The difference between this example and Example 1 is that in this example, the mass ratio of the lithium supplementing substance Li2NiO2 in the coating is 5%, and the mass ratio of the positive active material LiFePO4 is adjusted to 91.5%.

[0074] The preparation method of the remaining lithium secondary battery is consistent with that of Example 1.

[0075] The cycle performance test is performed on the lithium secondary battery:

[0076] After 1500 cycles of charging and discharging at 1C under 2.5-3.65V, the battery is attenuated, and is activated;

[0077] After the attenuation of the battery, the battery is activated, and the activation method is as follows:

[0078] (2) 1.0C constant current charging to 4.3V, 4.3V constant voltage to 0.05C;

[0079] (3) resting for 5 min;

[0080] (4) 1.0C discharging to 2.5V; after the activation, the battery continues to perform the cycle performance test.

[0081] Example 4

[0082] The difference between this example and Example 1 is that in this example, the mass ratio of the lithium supplementing substance Li2NiO2 in the coating is 0.5%, and the mass ratio of the positive active material LiFePO4 is adjusted to 96%.

[0083] The preparation method of the remaining lithium secondary battery is consistent with that of Example 1.

[0084] The cycle performance test is performed on the lithium secondary battery:

[0085] The battery is attenuated after 1500 cycles of charging and discharging at 1C under 2.5-3.65V, and is activated;

[0086] After the battery is attenuated, the battery is activated, and the activation method is as follows:

[0087] (1) resting for 5 min;

[0088] (2) 1.0C constant current charging to 4.3V, 4.3V constant voltage to 0.05C;

[0089] (3) resting for 5 min;

[0090] (4) 1.0C discharging to 2.5V; after the activation, the battery continues to perform the cycle performance test.

[0091] Example 5

[0092] The difference between this example and Example 1 is that in this example, the mass ratio of the lithium supplementing substance Li2NiO2 in the coating is 10%, and the mass ratio of the positive active material LiFePO4 is adjusted to 86.5%.

[0093] The preparation method of the remaining lithium secondary battery is consistent with that of Example 1.

[0094] The cycle performance test is performed on the lithium secondary battery:

[0095] The battery is attenuated after 1500 cycles of charging and discharging at 1C under 2.5-3.65V, and is activated;

[0096] After the battery attenuates, the battery is activated, and the activation method is as follows: (1) stand for 5 min;

[0097] (2) 1.0C constant current charging to 4.3V, 4.3V constant voltage to 0.05C;

[0098] (3) stand for 5 min;

[0099] (4) 1.0C discharging to 2.5V; after the activation is completed, the battery continues to perform the cycle performance test.

[0100] Comparative Example 1

[0101] The difference between this comparative example and Example 1 is that no lithium supplement is added in this comparative example, and the mass ratio of the positive active material LiFePO4 is adjusted to 96.5%.

[0102] The preparation method of the remaining lithium secondary battery is consistent with that of Example 1.

[0103] The cycle performance test is performed on the lithium secondary battery:

[0104] The battery is attenuated after 1500 cycles of charging and discharging at 1C in the voltage range of 2.5-3.65V without activation.

[0105] Figure 1 The cycle performance comparison chart of the lithium secondary batteries provided by Example 1 and Comparative Example 1 is shown in FIG. Figure 1 As can be seen from FIG., the capacity retention rate of both is reduced by 96.2% after 1500 cycles, and the capacity retention rate of Example 1 is increased to 99.8% after activation. Subsequently, it will start to attenuate from 99.8% in the normal voltage range, thereby significantly improving the overall cycle life.

[0106] Comparative Example 2

[0107] In this comparative example, the preparation of the lithium secondary battery and the cycle performance test are consistent with those of Example 1.

[0108] In the activation method, the upper limit voltage after charging is 3.55V, and the remaining steps are consistent with those of Example 1.

[0109] Comparative Example 3

[0110] In this comparative example, the preparation of the lithium secondary battery and the cycle performance test are consistent with those of Example 1.

[0111] In the activation method, the upper limit voltage after charging is 4.5V, and the remaining steps are consistent with those of Example 1.

[0112] Comparative Example 4

[0113] The positive active material in the present comparative example is LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2.

[0114] The rest of the preparation and activation method remain consistent with Example 1.

[0115] The initial capacity, capacity after 1500 cycles, initial capacity when the cycle is performed again (C j ), and capacity retention rate after activation (relative to the initial capacity) in Examples 1-5 and Comparative Examples 1-4 are listed in Table 1.

[0116] Table 1

[0117]

[0118] From the data results of Example 1 and Example 2, it can be known that when LiFePO4 and Li2NiO2 or LiFePO4 and Li5FeO5 are selected for activation, the capacity retention rate of the battery after activation is higher, and the initial capacity is also at a high level.

[0119] From the data results of Example 1 and Example 5, it can be known that when the mass ratio of the lithium supplement is too high, the initial capacity may be too low, which leads to the initial capacity of the composite positive plate not meeting the design requirements.

[0120] From the data results of Example 1 and Comparative Example 1, it can be known that when no lithium supplement is added to the positive plate and no activation is performed, the battery will not be able to recover after the capacity decreases, but will continue to decrease further. With the further cycle, the capacity retention rate will quickly reach 80%, and the overall cycle life is short.

[0121] From the data results of Example 1 and Comparative Examples 2 and 3, it can be known that when the upper limit voltage during the activation process is too low or too high, the capacity cannot be restored to a high level after the activation process.

[0122] From the data results of Example 1 and Comparative Example 4, it can be known that when the positive active material is a non-lithium iron phosphate material, the oxidation potential of the lithium supplement does not match, and the voltage selection in the activation method is also not suitable, which leads to the initial capacity of the battery at a very low level under the normal use potential of 2.5-3.65V.

[0123] In summary, the application realizes selective activation of the lithium supplement material by fine matching of the de-lithiation potential of the positive active material and the lithium supplement material and control of the activation potential, thereby making up for the consumption of active lithium due to repeated repair of SEI in the lithium secondary battery cycle process, so that the battery realizes further capacity improvement after initial attenuation through re-activation, instead of further attenuation on the basis of the original attenuation, and finally realizes super-long cycle life.

[0124] The applicant declares that the above is only a specific embodiment of the application, but the protection scope of the application is not limited thereto, and those skilled in the art should understand that any changes or replacements that can be easily thought of by those skilled in the art within the technical scope disclosed by the application fall within the protection scope and disclosure scope of the application.

Claims

1. A lithium secondary battery, characterized by comprising: The lithium secondary battery comprises a composite positive electrode sheet, a negative electrode sheet, a separator and an electrolyte; The composite positive electrode sheet comprises a current collector and a coating layer; the coating layer comprises a positive active material, a lithium supplementing material, a binder and a conductive agent, the oxidation potential of the lithium supplementing material is 3.6-4.4V, and the positive active material comprises LiFePO4; The activation method of the lithium secondary battery comprises: After the lithium secondary battery starts to decay, it is once statically placed, charged to an upper limit voltage, twice statically placed, then discharged to a voltage ≤3.65V, and normally works; Wherein, the activation is repeatedly performed when the next decay occurs, and the upper limit voltage of the last time is greater than that of the previous activation; The range of the upper limit voltage is 3.65-4.4V and does not include the two end values; The voltage for the normal work of the battery is 2.5-3.65V.

2. The lithium secondary battery according to claim 1, characterized by The lithium supplementing material comprises Li2NiO2.

3. The lithium secondary battery according to claim 1, characterized by The lithium supplementing material accounts for 0.5-10% of the coating layer.

4. The lithium secondary battery according to claim 3, characterized by The lithium supplementing material accounts for 0.5-5% of the coating layer.

5. The lithium secondary battery according to claim 1, characterized by The time for the once static placement is 1-30min.

6. The lithium secondary battery according to claim 1, characterized by The time for the twice static placement is 1-30min.

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