Secondary battery, preparation method thereof and electronic device

Abstract

The invention provides a secondary battery and a preparation method thereof, and an electronic device, the secondary battery comprises a positive electrode plate, the positive electrode plate comprises a positive electrode current collector and a positive electrode material layer, the positive electrode material layer comprises a positive electrode active material and a lithium supplement agent, the positive electrode active material comprises lithium ferric manganese phosphate, and the lithium supplement agent comprises a lithium-rich manganese-based compound; in a full discharge state, the mass ratio k1 of the lithium element to the manganese element of the positive electrode material layer is 1.12-1.91; in a dQ / dV curve, a first characteristic peak, a second characteristic peak and a third characteristic peak are included in a range of 3.0 V to 4.1 V; the peak position of the first characteristic peak is 3.5 V + / -0.05 V, the peak position of the second characteristic peak is 4.0 V + / -0.05 V, and the peak position of the third characteristic peak is 3.8 V + / -0.05 V. The positive electrode material layer comprises the lithium-rich manganese-based compound, the value of k1 is within the range, a dQ / dV curve comprises the characteristic peak, and the secondary battery has good cycle performance.

Description

Technical Field

[0001] This application relates to the field of electrochemical technology, and in particular to a secondary battery and its preparation method, as well as an electronic device. Background Technology

[0002] Lithium iron manganese phosphate batteries, by doping manganese into lithium iron phosphate cathode active materials, have a higher voltage platform and energy density, which helps to improve the voltage and power output of secondary battery systems. They are one of the important development directions for future small power batteries and portable energy storage for home use.

[0003] However, secondary batteries using lithium iron manganese phosphate as the positive electrode active material have poor cycle stability and currently cannot fully meet the cycle life requirements of applications such as small power batteries and portable energy storage for home use. Therefore, improving the cycle life of lithium iron manganese phosphate batteries is an urgent problem to be solved. Summary of the Invention

[0004] The purpose of this application is to provide a secondary battery, its preparation method, and an electronic device to improve the cycle life of lithium iron manganese phosphate batteries. The specific technical solution is as follows:

[0005] It should be noted that the invention description in this application uses lithium iron manganese phosphate batteries as an example of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium iron manganese phosphate batteries.

[0006] The first aspect of this application provides a secondary battery comprising a positive electrode sheet, the positive electrode sheet including a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector, the positive electrode material layer including a positive electrode active material and a lithium replenishing agent, the positive electrode active material including lithium iron manganese phosphate, and the lithium replenishing agent including a lithium-rich manganese-based compound; in the fully discharged state, the mass ratio k1 of lithium to manganese in the positive electrode material layer is 1.12 to 1.91; in the dQ / dV curve, a first characteristic peak, a second characteristic peak, and a third characteristic peak are included in the range of 3.0V to 4.1V; the peak position of the first characteristic peak is 3.5V ± 0.05V, the peak position of the second characteristic peak is 4.0V ± 0.05V, and the peak position of the third characteristic peak is 3.8V ± 0.05V. The cathode material layer in this application includes a lithium-rich manganese-based compound, and the mass ratio k1 of lithium and manganese is within the above range. The dQ / dV curve includes a first characteristic peak, a second characteristic peak, and a third characteristic peak, resulting in a secondary battery with good cycle performance.

[0007] In some embodiments of this application, the mass ratio k2 of iron to manganese in the cathode material layer is from 0.05 to 0.77. By introducing a lithium-rich manganese-based compound into the cathode material layer, and with the molar ratio k2 of iron to manganese within the above range, it is beneficial to improve the cycle stability of the secondary battery, thereby improving the cycle life and capacity retention of the secondary battery.

[0008] In some embodiments of this application, the average particle size of lithium iron manganese phosphate particles is D1, and the average particle size of lithium-rich manganese-based compound particles is D2, where 310nm≤D1≤600nm and 11.5μm≤D2≤34.5μm. When D1 and D2 are within the above ranges, the resulting secondary battery exhibits improved cycle stability, longer cycle life, and higher capacity retention. Furthermore, it facilitates the formation of a uniformly dispersed positive electrode material layer, thereby improving the charge / discharge rate and cycle stability of the secondary battery.

[0009] The second aspect of this application provides a method for preparing a secondary battery according to any of the foregoing embodiments, comprising the following steps: preparing a positive electrode sheet; the method for preparing the positive electrode sheet comprises the following steps: mixing lithium iron manganese phosphate, a first lithium supplementing agent, a second lithium supplementing agent, a conductive agent, a binder, and a solvent to obtain a positive electrode material layer slurry, and depositing the positive electrode material layer slurry on at least one surface of a positive electrode current collector to obtain a positive electrode sheet; wherein, the first lithium supplementing agent is yLi2O·(1-y)Li 2 / 3 Mn 1 / 3O 5 / 6 0.3≤y≤0.75, the second lithium supplement is zLi2MnO3·(1-z)LiCo 1 / 3 Ni 1 / 3 Mn 1 / 3 O2, 0.3≤z≤0.5; based on the mass of the positive electrode material layer, the mass percentage content W1 of the first lithium replenishing agent is 0.1% to 2.5%, and the mass percentage content W2 of the second lithium replenishing agent is 0.2% to 6.7%. In some embodiments of this application, 0.3%≤W1≤1%. In some embodiments of this application, 0.5%≤W2≤4.7%. Adding the above-mentioned mass percentages of the first and second lithium replenishing agents during the preparation of the positive electrode sheet is beneficial for achieving segmented lithium replenishment, improving the poor cycle performance of lithium iron manganese phosphate batteries due to large overpotentials, thereby improving the cycle stability of the secondary battery, increasing its capacity retention rate, and taking cost into consideration.

[0010] In some embodiments of this application, lithium iron manganese phosphate is LiMn x Fe 1-xPO4, 0.55≤x≤0.95; based on the mass of the cathode material layer, the mass percentage content W3 of lithium iron manganese phosphate is 88.8% to 95.3%. In some embodiments of this application, 90.8%≤W3≤93.3%. By selecting the above-mentioned lithium iron manganese phosphate as the cathode active material and controlling W3 within the above range, it is beneficial to improve the cycle stability of the secondary battery, increase the cycle life and capacity retention of the secondary battery, and balance energy density and cost.

[0011] In some embodiments of this application, the particle size of lithium iron manganese phosphate satisfies: 255nm ≤ Dv501 ≤ 500nm, 446nm ≤ Dv901 ≤ 910nm. In some embodiments of this application, the particle size of the first lithium supplement meets the following criteria: 420nm ≤ Dv502 ≤ 600nm, 850nm ≤ Dv902 ≤ 1000nm. In some embodiments of this application, the particle size of the second lithium supplement meets the following criteria: 10μm ≤ Dv503 ≤ 30μm, 17.5μm ≤ Dv903 ≤ 51μm. By adjusting the Dv50 and Dv90 of lithium iron manganese phosphate, the first lithium supplement, and the second lithium supplement to the above ranges, the particle size distribution among lithium iron manganese phosphate, the first lithium supplement, and the second lithium supplement can be improved, thereby facilitating the construction of a more stable cathode material layer structure, improving ion transport, and thus improving the cycle performance of the secondary battery.

[0012] A third aspect of this application provides an electronic device comprising a secondary battery as described in any embodiment of the first aspect or a secondary battery prepared in any embodiment of the second aspect.

[0013] The beneficial effects of this application are:

[0014] This application provides a secondary battery and its preparation method, as well as an electronic device. The secondary battery includes a positive electrode sheet, which includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. The positive electrode material layer includes a positive active material and a lithium replenishing agent. The positive active material includes lithium iron manganese phosphate. In the fully discharged state, the mass ratio k1 of lithium to manganese in the positive electrode material layer is 1.12 to 1.91. In the dQ / dV curve, a first characteristic peak, a second characteristic peak, and a third characteristic peak are included in the range of 3.0V to 4.1V. The peak position of the first characteristic peak is 3.5V ± 0.05V, the peak position of the second characteristic peak is 4.0V ± 0.05V, and the peak position of the third characteristic peak is 3.8V ± 0.05V. The positive electrode material layer includes a positive active material and a lithium replenishing agent, and the molar ratio of iron to manganese in the positive electrode material layer is within the above-mentioned range, which helps to alleviate the problem of poor cycle stability caused by the loss of active lithium during cycling and extends the cycle life of the secondary battery.

[0015] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other embodiments can be obtained based on these drawings.

[0017] Figure 1 The dQ / dV curves of the lithium-ion battery in Example 1-1 are shown.

[0018] Figure 2 The dQ / dV curves are for the lithium-ion battery in Comparative Example 3. Detailed Implementation

[0019] The technical solutions of this application will be clearly and completely described below with reference to the embodiments and accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0020] It should be noted that, in the specific embodiments of this application, lithium iron manganese phosphate batteries are used as an example of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium iron manganese phosphate batteries.

[0021] Secondary batteries, such as lithium iron manganese phosphate (LMP) batteries, have been widely studied due to their high energy density, good thermal stability, and relatively environmental friendliness. However, secondary batteries using LMP as the positive electrode active material have poor cycle stability and currently cannot fully meet the cycle life requirements of applications such as small power batteries and portable energy storage for home use. One reason is that LMP positive electrode materials contain Mn+ during charging and discharging. 2+ / Mn 3+ Phase and Fe 2+ / Fe 3+ The abrupt change in phase voltage plateau causes a large overpotential in lithium iron manganese phosphate batteries within the voltage range of 3.4V to 4.1V, which is detrimental to the cycling of lithium iron manganese phosphate batteries. On the other hand, research on the causes of capacity loss in lithium iron manganese phosphate batteries has revealed that the loss of active lithium is one of the important reasons for the cycle degradation of lithium iron manganese phosphate batteries.

[0022] Based on the above problems, this application proposes a secondary battery and its preparation method and electronic device, which helps to alleviate the problem of large overpotential of 3.4V to 4.1V in lithium iron manganese phosphate cathode active material during charge and discharge. At the same time, it can also make up for the loss of active lithium during cycling, thereby helping to extend the cycle life of the secondary battery and improve the capacity retention rate.

[0023] The first aspect of this application provides a secondary battery comprising a positive electrode sheet, the positive electrode sheet including a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. The positive electrode material layer includes a positive electrode active material and a lithium replenishing agent. The positive electrode active material includes lithium iron manganese phosphate, and the lithium replenishing agent includes a lithium-rich manganese-based compound. In a fully discharged state, the mass ratio k1 of lithium to manganese in the positive electrode material layer is 1.12 to 1.91. For example, k1 can be 1.12, 1.2, 1.28, 1.36, 1.44, 1.52, 1.59, 1.67, 1.75, 1.83, 1.91, or a range consisting of any two ratios therebetween. In a fully discharged state, lithium ions from the lithium-rich manganese-based compound in the positive electrode material layer can be replenished to the positive electrode along with lithium ions from the positive electrode active material, thereby mitigating the loss of active lithium during cycling. When the value of k1 is too large, such as greater than 1.91, too much lithium is added to the secondary battery. After the first charge, the excess lithium may exist in an unstable form, potentially causing side reactions and affecting cycle life under long-term cycling. When the value of k1 is too small, such as less than 1.12, it indicates that the lithium added to the positive electrode material layer is insufficient to compensate for the loss of active lithium during cycling, resulting in poor cycle performance of the secondary battery. Therefore, introducing a lithium-rich manganese-based compound into the positive electrode material layer, with k1 within the above-mentioned range, is beneficial for better compensating for the loss of active lithium during cycling and improving cycle performance.

[0024] The secondary battery of this application includes a first characteristic peak, a second characteristic peak, and a third characteristic peak in the dQ / dV curve within the range of 3.0V to 4.1V; the peak position of the first characteristic peak is 3.5V ± 0.05V, the peak position of the second characteristic peak is 4.0V ± 0.05V, and the peak position of the third characteristic peak is 3.8V ± 0.05V. For example, the peak position of the first characteristic peak can be 3.45V, 3.46V, 3.47V, 3.48V, 3.49V, 3.5V, 3.51V, 3.52V, 3.53V, 3.54V, 3.55V, or a range consisting of any two values ​​in between. For example, the position of the second characteristic peak can be 3.95V, 3.96V, 3.97V, 3.98V, 3.99V, 4V, 4.01V, 4.02V, 4.03V, 4.04V, 4.05V, or any range of two values ​​in between. Similarly, the position of the third characteristic peak can be 3.75V, 3.76V, 3.77V, 3.78V, 3.79V, 3.8V, 3.81V, 3.82V, 3.83V, 3.84V, 3.85V, or any range of two values ​​in between. In the above dQ / dV curve, the first and second characteristic peaks are characteristic peaks of the positive electrode active material lithium iron manganese phosphate, while the third characteristic peak located between the first and second characteristic peaks is a characteristic peak of lithium-rich manganese-based compounds. The presence of the aforementioned characteristic peaks in the dQ / dV curve of the secondary battery indicates the presence of lithium-rich manganese-based compounds in the cathode material layer. This helps alleviate the problem of large overpotentials (3.4V to 4.1V) in the lithium iron manganese phosphate cathode active material during charge and discharge, and can also compensate for the loss of active lithium during the cycle of the secondary battery, thereby improving the cycle performance of the lithium iron manganese phosphate battery.

[0025] Therefore, the cathode material layer in this application includes a lithium-rich manganese-based compound, and the mass ratio k1 of lithium and manganese is within the above range. The dQ / dV curve includes a first characteristic peak, a second characteristic peak, and a third characteristic peak, resulting in a secondary battery with good cycle performance.

[0026] In some embodiments of this application, the mass ratio k2 of iron to manganese in the cathode material layer is from 0.05 to 0.77. For example, k2 can be 0.05, 0.12, 0.19, 0.27, 0.34, 0.41, 0.48, 0.55, 0.63, 0.7, 0.77, or any combination of two values ​​within this range. By introducing a lithium-rich manganese-based compound into the cathode material layer, and ensuring that the molar ratio k2 of iron to manganese is within the aforementioned range, it is beneficial to improve the cycle stability of the secondary battery, thereby improving its cycle life and capacity retention.

[0027] The second aspect of this application provides a method for preparing a secondary battery according to the aforementioned embodiments, comprising the following steps: preparing a positive electrode sheet. The method for preparing the positive electrode sheet comprises the following steps: mixing lithium iron manganese phosphate, a first lithium supplement agent, a second lithium supplement agent, a conductive agent, a binder, and a solvent to obtain a positive electrode material layer slurry; and depositing the positive electrode material layer slurry on at least one surface of a positive electrode current collector to obtain a positive electrode sheet. Wherein, the first lithium supplement agent is yLi₂O·(1-y)Li 2 / 3 Mn 1 / 3 O 5 / 6 0.3≤y≤0.75, the second lithium supplement is zLi2MnO3·(1-z)LiCo 1 / 3 Ni 1 / 3 Mn 1 / 3 O2, 0.3 ≤ z ≤ 0.5. For example, y can be 0.3, 0.4, 0.5, 0.6, 0.7, 0.75, or any two values ​​within this range; z can be 0.3, 0.4, 0.5, or any two values ​​within this range. Specifically, the first lithium supplement can be, but is not limited to, 0.3Li2O·0.7Li 2 / 3 Mn 1 / 3 O 5 / 6 0.4Li₂O·0.6Li 2 / 3 Mn 1 / 3 O 5 / 6 0.6Li₂O·0.4Li 2 / 3 Mn 1 / 3 O 5 / 6 0.75Li₂O·0.25Li 2 / 3 Mn 1 / 3 O 5 / 6 At least one of the following; the second lithium supplement may include, but is not limited to, 0.3Li₂MnO₃·0.7LiCo. 1 / 3Ni 1 / 3 Mn 1 / 3 O2, 0.38Li2MnO3·0.62LiCo 1 / 3 Ni 1 / 3 Mn 1 / 3O2, 0.4Li2MnO3·0.6LiCo 1 / 3 Ni1 / 3Mn 1 / 3 O2 and 0.5Li2MnO3·0.5LiCo 1 / 3 Ni 1 / 3Mn 1 / 3At least one of O2. Based on the mass of the positive electrode material layer, the mass percentage content W1 of the first lithium supplement is 0.1% to 2.5%, and the mass percentage content W2 of the second lithium supplement is 0.2% to 6.7%. For example, W1 can be 0.1%, 0.3%, 0.6%, 0.8%, 1.1%, 1.3%, 1.5%, 1.8%, 2%, 2.3%, 2.5%, or any two values ​​thereof; W2 can be 0.2%, 0.9%, 1.5%, 2.2%, 2.8%, 3.5%, 4.1%, 4.8%, 5.4%, 6.1%, 6.7%, or any two values ​​thereof. In some embodiments of this application, 0.3% ≤ W1 ≤ 1%. In some embodiments of this application, 0.5% ≤ W2 ≤ 4.7%.

[0028] The secondary battery prepared by the above method exhibits several advantages. Firstly, the first lithium replenishing agent in the positive electrode slurry tends to release a large amount of active lithium in the early stages of battery cycling. This compensates for the rapid loss of active lithium during the initial cycles, reducing capacity loss in the positive electrode active material. Simultaneously, the lithium ions extracted from the first lithium replenishing agent are consumed in the early stages of cycling, such as during formation. Therefore, after charge-discharge cycles, the original first lithium replenishing agent is essentially absent from the positive electrode. Secondly, the second lithium replenishing agent tends to release active lithium slowly and uniformly in subsequent cycles and has a longer cycle life. This helps replenish the active lithium lost in later cycles, mitigating capacity decay caused by lithium consumption and thus improving the cycle stability and capacity retention of the secondary battery. Furthermore, the second lithium replenishing agent exhibits lithium extraction / intercalation activity in the 3.4V to 4.1V voltage range, which helps alleviate the problem of large overpotentials in this range, further improving cycle stability, cycle life, and capacity retention. Since the secondary lithium replenisher tends to release active lithium slowly during cycling and has a long cycle life, the original secondary lithium replenisher is still present in the positive electrode after the secondary battery has undergone charge-discharge cycles.

[0029] In the cathode material layer, the mass percentages of the first and second lithium replenishers affect the cycle performance of the secondary battery. When W1 is too small, the active lithium released by the first lithium replenisher is insufficient to compensate for the active lithium lost in the early stages of the secondary battery cycle, causing the capacity retention rate to decrease rapidly in the early stages of the cycle, thus reducing the cycle life of the secondary battery. Conversely, when W1 is too large, it reduces the overall conductivity of the cathode material layer, which is detrimental to the electron transport rate in the secondary battery and reduces the power density of the secondary battery. Therefore, by adjusting W1 within the above range, the first lithium replenisher is more effective in replenishing the active lithium lost in the early stages of the secondary battery cycle, which is beneficial to improving the cycle life and capacity retention rate of the secondary battery. W2 mainly affects the performance of the secondary battery in the later stages of the cycle. When W2 is too small, the active lithium released by the second lithium replenisher cannot effectively replenish the active lithium lost in the later stages of the secondary battery cycle, resulting in a decrease in the cycle life and capacity retention rate of the secondary battery. However, if W2 is too high, it will reduce the proportion of positive electrode active material in the secondary battery, thus lowering the energy density. Furthermore, since the second lithium supplement is a compound containing relatively expensive nickel and cobalt, an excessively high W2 will increase the raw material cost of the secondary battery. Therefore, controlling W2 within the aforementioned range not only results in a secondary battery with better cycle life and capacity retention, but also improves the battery's electrical performance while controlling the amount of the relatively expensive nickel and cobalt-containing second lithium supplement, achieving a balance between cost and efficiency. Thus, adding the corresponding mass fractions of the first and second lithium supplements during the preparation of the positive electrode sheet can address the characteristics of lithium iron manganese phosphate positive electrodes, facilitating segmented lithium supplementation, mitigating the problem of large overpotentials affecting cycle life in the 3.4V to 4.1V voltage range, and addressing cycle degradation due to the consumption of active lithium. This improves the cycle stability of the secondary battery, enhances its capacity retention, and controls the amount of cobalt and nickel-containing raw materials, balancing cost and efficiency.

[0030] In some embodiments of this application, lithium iron manganese phosphate is LiMn x Fe 1-x PO4, 0.55 ≤ x ≤ 0.95. For example, x can be 0.55, 0.6, 0.7, 0.8, 0.9, 0.95, or a range of any two values ​​in between. For example, lithium iron manganese phosphate can include, but is not limited to, LiMn. 0.95 Fe 0.05 PO4, LiMn 0.8 Fe 0.2 PO4, LiMn 0.7 Fe 0.3 PO4, LiMn 0.6 Fe 0.4 PO4, LiMn 0.55 Fe0.45 PO4. Based on the mass of the cathode material layer, the mass percentage content W3 of lithium iron manganese phosphate is 88.8% to 95.3%. In some embodiments of this application, 90.8% ≤ W3 ≤ 93.3%. For example, W3 can be 88.8%, 89.5%, 90.1%, 90.8%, 91.4%, 92.1%, 92.7%, 93.4%, 94%, 94.7%, 95.3%, or any two values ​​within this range. By selecting the above-mentioned lithium iron manganese phosphate as the cathode active material and controlling W3 within the above range, it is beneficial to cooperate with the first lithium replenisher and the second lithium replenisher to achieve segmented lithium replenishment, which is beneficial to improve the cycle stability of the secondary battery, increase the cycle life and capacity retention of the secondary battery, and, thanks to the high energy density of the lithium iron manganese phosphate cathode active material, the resulting secondary battery has a high energy density while also considering cost.

[0031] This application does not impose any particular limitation on the types of conductive agents and binders, as long as they can achieve the purpose of this application. For example, the conductive agent may include, but is not limited to, at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, graphene, metallic materials, or conductive polymers. The conductive carbon black may include, but is not limited to, at least one of acetylene black or Ketjen black. The aforementioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The aforementioned carbon fibers may include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or carbon nanofibers. The aforementioned metallic materials may include, but are not limited to, metal powders and / or metal fibers. Specifically, the metal may include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The aforementioned conductive polymer may include, but is not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. The binder may include, but is not limited to, at least one of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyimide, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, polyimide, polyamide-imide, styrene-butadiene rubber, or polyvinylidene fluoride.

[0032] This application does not impose any particular limitation on the mass percentage content of the conductive agent and binder in the positive electrode material layer, as long as the purpose of this application can be achieved. For example, based on the mass of the positive electrode material layer, the mass percentage content of the conductive agent is 0.5% to 8%, and the mass percentage content of the binder is 0.5% to 8%.

[0033] In some embodiments of this application, the particle size of lithium iron manganese phosphate satisfies: 255nm ≤ Dv501 ≤ 500nm, 446nm ≤ Dv901 ≤ 910nm. For example, Dv501 can be 255nm, 270nm, 290nm, 300nm, 320nm, 350nm, 380nm, 400nm, 430nm, 450nm, 470nm, 500nm, or any two values ​​thereof; Dv901 can be 446nm, 492nm, 539nm, 585nm, 632nm, 678nm, 724nm, 771nm, 817nm, 864nm, 910nm, or any two values ​​thereof.

[0034] In some embodiments of this application, the particle size of the first lithium supplement meets the following conditions: 420nm ≤ Dv502 ≤ 600nm, 850nm ≤ Dv902 ≤ 1000nm. For example, Dv502 can be 420nm, 440nm, 460nm, 480nm, 500nm, 520nm, 540nm, 560nm, 580nm, 600nm, or any two values ​​thereof; Dv902 can be 850nm, 880nm, 900nm, 920nm, 950nm, 980nm, 1000nm, or any two values ​​thereof.

[0035] In some embodiments of this application, the particle size of the second lithium supplement meets the following conditions: 10 μm ≤ Dv503 ≤ 30 μm, 17.5 μm ≤ Dv903 ≤ 51 μm. For example, Dv503 can be 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, 22 μm, 24 μm, 26 μm, 28 μm, 30 μm, or any two values ​​thereof; Dv903 can be 17.5 μm, 20.9 μm, 24.2 μm, 27.6 μm, 30.9 μm, 34.3 μm, 37.6 μm, 41 μm, 44.3 μm, 47.7 μm, 51 μm, or any two values ​​thereof.

[0036] By adjusting the Dv50 and Dv90 of lithium iron manganese phosphate, the first lithium supplementer, and the second lithium supplementer to the above range, the particle size distribution among lithium iron manganese phosphate, the first lithium supplementer, and the second lithium supplementer can be improved, which is conducive to constructing a more stable cathode material layer structure, improving ion transport, and thus improving the cycle performance of the secondary battery.

[0037] In this application, Dv501 refers to the particle size that reaches 50% of the volumetric size from the smallest particle size side in the volumetric particle size distribution of lithium iron manganese phosphate, and Dv901 refers to the particle size that reaches 90% of the volumetric size from the smallest particle size side in the volumetric particle size distribution of lithium iron manganese phosphate. Similarly, Dv502 and Dv902 refer to the corresponding particle size parameters in the volumetric particle size distribution of the first lithium supplement, and Dv503 and Dv903 refer to the corresponding particle size parameters in the volumetric particle size distribution of the second lithium supplement.

[0038] In some embodiments of this application, the average particle size of the lithium iron manganese phosphate particles is D1, and the average particle size of the lithium-rich manganese-based compound particles is D2, where 310nm ≤ D1 ≤ 600nm and 11.5μm ≤ D2 ≤ 34.5μm. For example, D1 can be 310nm, 339nm, 368nm, 397nm, 426nm, 455nm, 484nm, 513nm, 542nm, 571nm, 600nm, or any two values ​​within this range; D2 can be 11.5μm, 13.8μm, 16.1μm, 18.4μm, 20.7μm, 23μm, 25.3μm, 27.6μm, 29.9μm, 32.2μm, 34.5μm, or any two values ​​within this range. The positive electrode material layers D1 and D2 within the above range can replenish active lithium in a timely manner during the secondary battery cycle, thereby improving the cycle stability of the secondary battery and increasing the capacity retention rate. At the same time, it is also beneficial to obtain a positive electrode material layer with uniformly dispersed conductive agent, and accelerate the electron transfer speed between the positive electrode material layer and the negative electrode material layer, thereby improving the charge and discharge rate and cycle stability of the secondary battery.

[0039] This application does not impose any particular restrictions on the positive electrode current collector, as long as it can achieve the purpose of this application. For example, it may include aluminum foil, aluminum alloy foil, or composite current collectors (such as aluminum-carbon composite current collectors). In this application, positive electrode current collectors of different thicknesses can be purchased and selected by measuring with a micrometer to determine the required thickness.

[0040] This application does not impose any particular limitations on the thickness of the positive electrode current collector and the positive electrode material layer, as long as the purpose of this application can be achieved. For example, the thickness of the positive electrode current collector can be 5 μm to 20 μm, and the thickness of the positive electrode material layer can be 30 μm to 120 μm.

[0041] Optionally, the positive electrode may further include a conductive layer located between the positive current collector and the positive electrode material layer. The composition of the conductive layer is not particularly limited and can be any conductive layer commonly used in the art. The conductive layer includes a conductive agent and a binder. This application does not impose any particular limitation on the conductive agent and binder in the conductive layer; for example, it can be at least one of the aforementioned conductive agents and binders.

[0042] In this application, the secondary battery further includes a negative electrode sheet, which includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. The phrase "the negative electrode material layer is disposed on at least one surface of the negative electrode current collector" means that the negative electrode material layer can be disposed on one surface of the negative electrode current collector along its thickness direction, or on two surfaces of the negative electrode current collector along its thickness direction. It should be noted that the "surface" here can be the entire surface area of ​​the negative electrode current collector, or only a portion thereof; this application does not have any particular limitation, as long as the purpose of this application is achieved.

[0043] This application does not impose any particular restrictions on the negative electrode current collector, as long as it can achieve the purpose of this application. For example, it may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or composite current collector. For example, the composite current collector may be lithium copper composite current collector, carbon copper composite current collector, nickel copper composite current collector, titanium copper composite current collector, etc.

[0044] The negative electrode material layer includes a negative electrode active material. This application does not impose any particular limitation on the negative electrode active material, as long as it can achieve the purpose of this application. For example, the negative electrode active material may include, but is not limited to, natural graphite, artificial graphite, mesophase micro carbon spheres, hard carbon, soft carbon, silicon, silicon-carbon composites, Li-Sn alloys, Li-Sn-O alloys, Sn, SnO, SnO2, and spinel-structured lithiated TiO2-Li4Ti5O. 12 Or at least one of Li-Al alloys.

[0045] In some embodiments of this application, the negative electrode material layer may further include a conductive agent and a binder. This application does not impose any particular restrictions on the types of conductive agents and binders, as long as they achieve the purpose of this application. For example, it may be at least one of the aforementioned conductive agents and binders. This application does not impose any particular restrictions on the mass ratio of the negative electrode active material, conductive agent, and binder in the negative electrode material layer. Those skilled in the art can select according to actual needs, as long as the purpose of this application is achieved.

[0046] This application does not impose any particular limitation on the thickness of the negative electrode material layer, as long as it achieves the purpose of this application. For example, the thickness of the negative electrode material layer can be from 30 μm to 120 μm. This application also does not impose any particular limitation on the thickness of the negative electrode current collector, as long as it achieves the purpose of this application. For example, the thickness of the negative electrode current collector can be from 4 μm to 15 μm.

[0047] Optionally, the negative electrode sheet may further include a conductive layer located between the negative electrode current collector and the negative electrode material layer. This application does not impose any particular limitation on the composition of the conductive layer, which can be a conductive layer commonly used in the art. For example, the conductive layer may include a conductive agent and a binder. This application does not impose any particular limitation on the conductive agent and binder in the conductive layer; for example, it can be at least one of the aforementioned conductive agents and binders.

[0048] In this application, the secondary battery also includes a separator. This application does not impose any particular limitation on the separator, as long as it achieves the purpose of this application. For example, the separator material may include, but is not limited to, at least one of polyethylene (PE), polyolefins (PO) primarily composed of polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid. The separator type may include at least one of woven membrane, nonwoven membrane, microporous membrane, composite membrane, rolled membrane, or spun membrane.

[0049] In some embodiments of this application, the diaphragm may include a substrate layer and a surface treatment layer. The substrate layer may be a nonwoven fabric, membrane, or composite membrane with a porous structure, and the material of the substrate layer may include at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Optionally, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane may be used.

[0050] Optionally, a surface treatment layer is provided on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by mixing polymers and inorganic substances.

[0051] In some embodiments of this application, the inorganic layer comprises inorganic particles and a binder. This application does not particularly limit the inorganic particles; for example, the inorganic particles may include at least one selected from alumina, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. This application does not particularly limit the binder; for example, the binder may be at least one of the binders described above. In some embodiments of this application, the polymer layer comprises a polymer, the polymer material of which includes at least one selected from polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, or polyvinylidene fluoride or poly(vinylidene fluoride-hexafluoropropylene).

[0052] In this application, there is no particular limitation on the thickness of the diaphragm, as long as it can achieve the purpose of this application. For example, the thickness of the diaphragm can be from 3 μm to 30 μm.

[0053] In this application, the secondary battery also includes an electrolyte, which includes lithium salts and non-aqueous solvents.

[0054] This application does not impose any particular limitation on the lithium salt, as long as it achieves the purpose of this application. For example, the lithium salt may include, but is not limited to, at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalato)borate (LiBOB), or lithium difluoroborate. This application also does not impose any particular limitation on the content of the lithium salt in the electrolyte, as long as it achieves the purpose of this application.

[0055] This application does not impose any particular restrictions on non-aqueous solvents, as long as they can achieve the purpose of this application. For example, non-aqueous solvents may include, but are not limited to, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents.

[0056] The aforementioned carbonate compounds may include, but are not limited to, at least one of chain carbonate compounds, cyclic carbonate compounds, or fluorocarbonate compounds. The aforementioned chain carbonate compounds may include, but are not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), or methyl ethyl carbonate (MEC). The aforementioned cyclic carbonates may include, but are not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), or vinyl ethylene carbonate (VEC). Fluorocarbonate compounds may include, but are not limited to, at least one of fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethylethylene carbonate. The aforementioned carboxylic acid ester compounds may include, but are not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valproic acid lactone, or caprolactone. The aforementioned ether compounds may include, but are not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. Other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate. This application does not impose any particular limitation on the content of non-aqueous solvents in the electrolyte, as long as the purpose of this application is achieved.

[0057] The secondary battery also includes a casing for housing the positive electrode, separator, negative electrode, and electrolyte, as well as other components known in the field of secondary batteries. This application does not limit the scope of these other components. This application does not impose any particular limitation on the casing; it can be a casing known in the art, as long as it achieves the purpose of this application. For example, the casing can be a rigid casing or a flexible casing. The material of the rigid casing can be metal; this application does not limit the type of metal and can use known metal rigid casings, as long as they achieve the purpose of this application. The flexible casing can be a metal plastic film, such as aluminum-plastic film, steel-plastic film, etc.

[0058] The fabrication process of the secondary battery described in this application is well known to those skilled in the art, and this application does not impose any particular limitations. For example, the fabrication process of the secondary battery may include, but is not limited to, the following steps: stacking a separator, a positive electrode, a separator, and a negative electrode in sequence, and performing operations such as winding and folding as needed to obtain a wound electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain the secondary battery. Alternatively, stacking a separator, a positive electrode, a separator, and a negative electrode in sequence, and then fixing the four corners of the entire stacked structure with tape to obtain a stacked electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain the secondary battery. In addition, overcurrent protection elements, conductive plates, etc., may be placed in the housing as needed to prevent pressure rise and overcharging / discharging inside the secondary battery.

[0059] A third aspect of this application provides an electronic device comprising a secondary battery as described in any embodiment of the first aspect or a secondary battery prepared in any embodiment of the second aspect.

[0060] This application does not specifically limit the type of electronic device; it can be any electronic device known in the prior art. In some embodiments of this application, the electronic device may include, but is not limited to, laptops, pen-based computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, household large-capacity batteries, and lithium-ion capacitors.

[0061] Example

[0062] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0063] Test methods and equipment:

[0064] Characteristic peak test of dQ / dV curve:

[0065] At 25℃, the lithium-ion battery was charged at a constant current of 0.2C to the charging cutoff voltage of 4.3V, and then charged at a constant voltage of 4.3V until the current was 0.02C, reaching a fully charged state, or 100% SOC. The fully charged lithium-ion battery was then left to stand at 25℃ for 5 minutes, and then discharged at a constant current of 1C to 2.8V, reaching a fully discharged state, or 0% SOC. The dQ / dV curves of the lithium-ion battery from the fully charged state to the fully discharged state were recorded.

[0066] Observe the dQ / dV curve. The peak appearing in the range of 3.45V to 3.55V is the first characteristic peak, the peak appearing in the range of 3.95V to 4.05V is the second characteristic peak, and the peak appearing in the range of 3.75V to 3.85V is the third characteristic peak. If the corresponding characteristic peak exists, it is marked as "yes"; if it does not exist, it is marked as "no".

[0067] Cyclic capacity retention test:

[0068] At 25℃, the lithium-ion battery was charged at a constant current of 0.2C to 4.3V, then charged at a constant voltage of 4.3V to a current of 0.02C, allowed to stand for 5 minutes, and then discharged at a constant current of 1C to 2.8V. This constitutes one charge-discharge cycle. The discharge capacity of the first cycle was recorded as C0. This process was repeated for 1000 charge-discharge cycles, and the discharge capacity after the 1000th cycle was recorded as C. 1000 .

[0069] Capacity retention rate of a lithium-ion battery after 1000 cycles = (C 1000 / C0)×100%.

[0070] k1, k2 tests: At 25℃, the lithium-ion battery was charged to 4.3V at a constant current of 0.2C, then charged to 0.02C at a constant voltage of 4.3V, left to stand for 5 minutes, and then discharged to 2.8V at a constant current of 1C. The lithium-ion battery was disassembled under an argon atmosphere, and the positive electrode was soaked in dimethyl carbonate solvent for 2 hours and dried at 60℃ for 1 hour to obtain the positive electrode sample.

[0071] A sample of the positive electrode material layer was scraped from the positive electrode sheet and placed in a digestion vessel. 10 mL of aqua regia (a mixture of concentrated hydrochloric acid and concentrated nitric acid at a volume ratio of 3:1) was added, and the vessel was shaken for 30 minutes before digestion. The digested sample was then brought to volume using a volumetric flask. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used. Characteristic spectra were emitted through plasma excitation, and the masses of Li, Mn, and Fe in the positive electrode material layer were determined by detecting these spectra. The mass ratio of Li to Mn (k1) and the mass ratio of Fe to Mn (k2) were calculated.

[0072] Dv50 and Dv90 tests:

[0073] Add 0.02g of lithium iron manganese phosphate sample to a 50mL clean beaker, add 20mL of deionized water, and then add 1mL of 1% sodium dodecyl sulfate surfactant to disperse the sample in the water. Sonicate the sample in a 120W ultrasonic cleaner for 5 minutes, and then test the particle size distribution using a Malvern particle size analyzer (Master Sizer 2000). In the volume-based particle size distribution of the lithium iron manganese phosphate sample, starting from the smallest particle size, the particle size reaching 50% of the volumetric accumulation is designated as Dv501, and the particle size reaching 90% of the volumetric accumulation is designated as Dv901.

[0074] Similarly, the first lithium replenishment sample was prepared and tested using the above method to obtain Dv502 and Dv902; then the second lithium replenishment sample was prepared and tested using the above method to obtain Dv503 and Dv903.

[0075] D1 and D2 tests:

[0076] At an ambient temperature of 25℃, a fully loaded lithium-ion battery was disassembled, and the negative electrode was removed and soaked in dimethyl carbonate (DMC) for 20 minutes. The positive electrode was then placed in an oven and dried at 80℃ for 12 hours to obtain a positive electrode sample. 10g of positive electrode material powder was scraped from the positive electrode sample and calcined at 300℃ under vacuum for 2 hours to obtain a positive electrode material layer sample containing positive active material and a second lithium supplementer. 0.02g of the positive electrode material layer sample was added to a 5mL clean sample tube, followed by 20mL of anhydrous ethanol to disperse the sample. The sample was ultrasonically cleaned for 5 minutes using a 120W ultrasonic cleaner, and then dropped onto a silicon wafer. After drying, D1 and D2 were measured using a scanning electron microscope. Using the elemental analysis function of a scanning electron microscope, select 10 particles containing lithium and 10 particles containing iron, and measure their circumscribed circle diameters, which are then recorded as D1a, D2a, D3a up to D10a. The average of these ten dimensions is then calculated as D1. The testing method for D2 is basically the same as that for D1, except that when selecting samples, samples containing lithium but not iron should be chosen for size testing.

[0077] Example 1-1

[0078] <Preparation of the positive electrode>

[0079] Lithium iron manganese phosphate LiMn 0.6 Fe 0.4 PO4, first lithium supplement 0.5Li2O·0.5Li 2 / 3 Mn 1 / 3 O 5 / 6 The second lithium supplement is 0.38Li₂MnO₃·0.62LiCo. 1 / 3 Ni1 / 3 Mn 1 / 3 O2, conductive carbon black (SP) as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder were mixed in a mass ratio of 93:0.6:2.5:2.2:1.7. N-methylpyrrolidone (NMP) was added as a solvent to prepare a slurry with a solid content of 64 wt%. After vacuum stirring, a positive electrode material layer slurry was obtained. This positive electrode material layer slurry was uniformly coated onto one surface of a 12 μm thick aluminum foil used as a positive electrode current collector. The coating was then dried at 60°C to obtain a positive electrode sheet with a single-sided coating of the positive electrode material layer. The single-sided coating weight of the positive electrode material layer was 140 mg / 1540.25 mm. 2 Then, the above steps are repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of positive electrode material. After drying at 120℃, it is cold-pressed, then cut and welded with tabs to obtain a positive electrode sheet with a size of 172mm×6000mm for use. The thickness of the positive electrode material layer on one side is 40μm.

[0080] <Preparation of Negative Electrode Sheets>

[0081] Artificial graphite (negative electrode active material), styrene-butadiene rubber (binder), and acetylene black (conductive agent) were mixed in a mass ratio of 97.4:1.4:1.2. Deionized water was added as a solvent to prepare a slurry with a solid content of 45 wt%. The slurry was then stirred evenly in a vacuum mixer to obtain the negative electrode slurry. The negative electrode slurry was uniformly coated onto one surface of a 6 μm thick copper foil used as a negative electrode current collector and dried at 120 °C to obtain a negative electrode sheet with a single-sided coating of negative electrode material. The single-sided coating weight of the negative electrode material layer was 120 mg / 1540.25 mm. 2 The above steps are then repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coating of negative electrode material. After drying at 120℃, it is cold-pressed, then cut and welded with tabs to obtain a negative electrode sheet with a size of 176mm×6400mm for use. The thickness of the single-sided negative electrode material layer is 61μm.

[0082] <Preparation of Electrolyte>

[0083] In an environment with a water content of less than 10 ppm, dimethyl carbonate, diethyl carbonate, and ethylene carbonate were mixed in a mass ratio of 1:1:1 to obtain an organic solvent. Then, the electrolyte salt LiPF6 was added to the organic solvent and mixed thoroughly to obtain the electrolyte. Based on the mass of the electrolyte, the electrolyte salt comprised 12.5% ​​by mass, with the remainder being the organic solvent.

[0084] <Preparation of the diaphragm>

[0085] A porous polyethylene film with a thickness of 7μm (provided by Celgard) is used.

[0086] <Preparation of Lithium-ion Batteries>

[0087] The prepared separator, positive electrode, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The four corners of the stacked structure are then secured with tape to obtain the stacked electrode assembly. The electrode assembly is placed in an aluminum-plastic film outer packaging foil and dehydrated at 80°C. The prepared electrolyte is then injected, followed by vacuum sealing, settling, formation, shaping, and capacity testing to obtain a soft-pack lithium-ion battery. The formation temperature is 80°C, and the settling time is 2 hours.

[0088] Examples 1-2 to Examples 1-18

[0089] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 1-1.

[0090] Examples 2-1 to 2-4

[0091] Except for adjusting the relevant preparation parameters according to Table 2, the rest is the same as in Examples 1-1.

[0092] Comparative Examples 1 to 3

[0093] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 1-1.

[0094] Table 1

[0095] As can be seen from Examples 1-1 to 1-18 and Comparative Examples 1 to 3, the positive electrode of the lithium-ion battery in these examples includes a first lithium replenishing agent yLi2O·(1-y)Li. 2 / 3 Mn 1 / 3 O 5 / 6 0.3≤y≤0.75 and the second lithium supplement zLi2MnO3·(1-z)LiCo 1 / 3 Ni 1 / 3 Mn 1 / 3The lithium-ion battery contains O2, with the first lithium supplementer W1 containing 0.1% to 2.5% and the second lithium supplementer W2 containing 0.2% to 6.7%. Under fully discharged conditions, the mass ratio k1 of lithium to manganese in the positive electrode material layer is 1.12 to 1.91, and the dQ / dV curve includes the first, second, and third characteristic peaks. The resulting lithium-ion battery exhibits high cycle capacity retention. In contrast, in Comparative Example 1, the value of W1 exceeds the upper limit of the aforementioned W1, resulting in lower cycle capacity retention for the resulting lithium-ion battery. Comparative Example 2, which does not use the second lithium supplementer, also exhibits low cycle capacity retention. Comparative Example 3, which does not use either the first or second lithium supplementer, also exhibits low cycle capacity retention. This demonstrates that the lithium-ion battery conforming to this application possesses good cycle performance. Specifically, Figure 1 The dQ / dV curves of the lithium-ion battery in Example 1-1 are shown below. Figure 1 As can be seen, the lithium-ion battery of Example 1-1 includes a third characteristic peak in addition to the first and second characteristic peaks; Figure 2 For the dQ / dV curve of the lithium-ion battery in Comparative Example 3, from... Figure 2 As can be seen, the lithium-ion battery in Comparative Example 3 only includes the first and second characteristic peaks, but does not include the third characteristic peak.

[0096] The mass ratio k2 of iron and manganese in the cathode material layer affects the cycle performance of the lithium-ion battery. As can be seen from Examples 1-1 to 1-18, when the value of k2 is within the range specified in this application, the resulting lithium-ion batteries exhibit high cycle capacity retention. This demonstrates that when the value of k2 is within the range specified in this application, the resulting lithium-ion battery possesses good cycle performance.

[0097] Lithium iron manganese phosphate (LiMn) x Fe 1-x The value of x in PO4 affects the cycle performance of lithium-ion batteries. As can be seen from Examples 1-1, 1-16 to 1-18, when the value of x is within the range of this application, the cycle capacity retention rate of the obtained lithium-ion batteries is consistently high. This demonstrates that when the value of x is within the range of this application, the obtained lithium-ion batteries exhibit good cycle performance.

[0098] The content of lithium iron manganese phosphate (W3) affects the cycle performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-9, when the value of W3 is within the range specified in this application, the resulting lithium-ion batteries exhibit high cycle capacity retention. This demonstrates that when the value of W3 is within the range specified in this application, the resulting lithium-ion batteries possess good cycle performance.

[0099] Table 2

[0100] The particle sizes Dv501 and Dv901 of lithium iron manganese phosphate affect the cycle performance of lithium-ion batteries. As can be seen from Examples 1-1, 2-1 to 2-2, when the values ​​of Dv501 and Dv901 are within the range specified in this application, the resulting lithium-ion batteries exhibit high cycle capacity retention. This demonstrates that when the values ​​of Dv501 and Dv901 are within the range specified in this application, the resulting lithium-ion batteries possess good cycle performance.

[0101] The average particle size D1 of lithium iron manganese phosphate particles affects the cycle performance of lithium-ion batteries. As can be seen from Examples 1-1, 2-1 to 2-2, when the value of D1 is within the range specified in this application, the resulting lithium-ion batteries exhibit high cycle capacity retention. This demonstrates that when the value of D1 is within the range specified in this application, the resulting lithium-ion batteries possess good cycle performance.

[0102] The particle sizes Dv502 and Dv902 of the first lithium supplement affect the cycle performance of lithium-ion batteries. As can be seen from Examples 1-1, 2-3 to 2-4, when the values ​​of Dv502 and Dv902 are within the range of this application, the resulting lithium-ion batteries exhibit high cycle capacity retention. This demonstrates that when the values ​​of Dv502 and Dv902 are within the range of this application, the resulting lithium-ion batteries possess good cycle performance.

[0103] The particle sizes Dv503 and Dv903 of the second lithium supplement affect the cycle performance of lithium-ion batteries. As can be seen from Examples 1-1, 2-1 to 2-2, when the values ​​of Dv503 and Dv903 are within the range of this application, the resulting lithium-ion batteries exhibit high cycle capacity retention. This demonstrates that when the values ​​of Dv503 and Dv903 are within the range of this application, the resulting lithium-ion batteries possess good cycle performance.

[0104] The average particle size D2 of lithium-rich manganese-based compounds affects the cycle performance of lithium-ion batteries. As can be seen from Examples 1-1, 2-1 to 2-2, when the D2 value is within the range specified in this application, the resulting lithium-ion batteries exhibit high cycle capacity retention. This demonstrates that when the D2 value is within the range specified in this application, the resulting lithium-ion batteries possess good cycle performance.

[0105] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

[0106] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, or article.

[0107] The various embodiments in this specification are described in a related manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

Claims

1. A secondary battery, characterized in that, include: A positive electrode sheet, the positive electrode sheet comprising a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector, the positive electrode material layer comprising a positive electrode active material and a lithium supplementing agent, the positive electrode active material comprising lithium iron manganese phosphate, the lithium supplementing agent comprising a lithium-rich manganese-based compound; In the fully discharged state, the mass ratio k1 of lithium and manganese in the positive electrode material layer of the secondary battery is 1.12 to 1.

91. The dQ / dV curve includes a first characteristic peak, a second characteristic peak, and a third characteristic peak in the range of 3.0V to 4.1V; the peak position of the first characteristic peak is 3.5V ± 0.05V, the peak position of the second characteristic peak is 4.0V ± 0.05V, and the peak position of the third characteristic peak is 3.8V ± 0.05V.

2. The secondary battery according to claim 1, wherein, The mass ratio k2 of iron and manganese in the positive electrode material layer is 0.05 to 0.

77.

3. The secondary battery according to claim 1 or 2, wherein, The average particle size of the lithium iron manganese phosphate particles is D1, and the average particle size of the lithium-rich manganese-based compound particles is D2, where 310nm≤D1≤600nm and 11.5μm≤D2≤34.5μm.

4. A method for preparing a secondary battery as described in any one of claims 1 to 3, comprising the following steps: Preparation of positive electrode sheet; The method for preparing the positive electrode sheet includes the following steps: mixing the lithium iron manganese phosphate, a first lithium supplement agent, a second lithium supplement agent, a conductive agent, a binder and a solvent to obtain a positive electrode material layer slurry; depositing the positive electrode material layer slurry on at least one surface of the positive electrode current collector to obtain the positive electrode sheet on at least one surface of the positive electrode current collector. The first lithium replenishing agent is yLi₂O·(1-y)Li 2 / 3 Mn 1 / 3 O 5 / 6 0.3≤y≤0.75, the second lithium supplement is zLi2MnO3·(1-z)LiCo 1 / 3 Ni 1 / 3 Mn 1 / 3 O2, 0.3≤z≤0.5; Based on the mass of the cathode material layer, the mass percentage content W1 of the first lithium replenishing agent is 0.1% to 2.5%, and the mass percentage content W2 of the second lithium replenishing agent is 0.2% to 6.7%.

5. The method for preparing a secondary battery according to claim 4, wherein it satisfies at least one of the following characteristics: （1）0.3%≤W1≤1%； （2）0.5%≤W2≤4.7%。 6. The method for preparing a secondary battery according to claim 4, wherein, The lithium iron manganese phosphate is LiMn. x Fe 1-x PO4, 0.55 ≤ x ≤ 0.95; Based on the mass of the cathode material layer, the mass percentage W3 of the lithium iron manganese phosphate is 88.8% to 95.3%.

7. The method for preparing a secondary battery according to claim 6, wherein, 90.8%≤W3≤93.3%。 8. The method for preparing a secondary battery according to any one of claims 4 to 7, wherein, The particle size of the lithium iron manganese phosphate satisfies: 255nm≤Dv501≤500nm, 446nm≤Dv901≤910nm.

9. The method for preparing a secondary battery according to any one of claims 4 to 7, wherein it satisfies at least one of the following characteristics: (1) The particle size of the first lithium replenishing agent satisfies: 420nm≤Dv502≤600nm, 850nm≤Dv902≤1000nm; (2) The particle size of the second lithium supplement meets the following requirements: 10μm≤Dv503≤30μm, 17.5μm≤Dv903≤51μm.

10. An electronic device comprising a secondary battery according to any one of claims 1 to 3 or a secondary battery obtained by a method for preparing a secondary battery according to any one of claims 4 to 9.

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