Positive electrode sheet, secondary battery, and electric device

Abstract

The application discloses a positive electrode sheet, a secondary battery and an electric device. The positive electrode sheet comprises a positive electrode active material, the positive electrode active material comprises manganese lithium oxide and a ternary material; and the positive electrode sheet satisfies 46.7 < Z / X < 189, wherein Z is the specific surface area of the positive electrode sheet, the unit is cm 2 / g, and X is the mass ratio of the ternary material to the mass of the positive electrode active material. The composition and the specific surface area of the positive electrode active material are controlled, and the high-temperature cycle performance of the battery is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electrochemistry, in particular to a positive electrode sheet, a secondary battery and an electric device. BACKGROUND

[0002] In a secondary battery, lithium manganese iron phosphate (LMFP) is a promising positive electrode material because it has a high voltage window, excellent energy density and good cycle stability, but when LMFP is used as a positive electrode material, the intrinsic electronic / ionic conductivity of Mn 3+ / Mn 2+ is low, and because Mn 3+ / Mn 2+ is a two-phase reaction, it has a large lattice mismatch and is more affected by polarization. On the other hand, it may be due to the John-Teller effect of Mn 3+ , which generates Mn 2+ and Mn 4+ . Mn dissolution easily occurs, high temperature accelerates the dissolution of Mn in the LMFP electrode material into the electrolyte, and the decomposed Mn 2+ conducts through the electrolyte and deposits on the surface of the negative electrode graphite, which destroys the SEI film in the subsequent cycle process, and the SEI film is continuously regenerated and repaired, consuming a large amount of active lithium. At the same time, the positive electrode material produces a manganese-deficient phase due to manganese dissolution, and the lithium ion diffusion is hindered in the subsequent charge and discharge process, increasing the polarization of the battery and increasing the capacity loss, resulting in fast high-temperature capacity decay and poor capacity retention during the charge and discharge cycle. SUMMARY

[0003] The purpose of the present application is to provide a positive electrode sheet, a secondary battery and an electric device, and the composition and active specific surface area of the positive electrode active material can improve the high-temperature cycle performance of the battery.

[0004] Therefore, the present application provides a positive electrode sheet, which comprises a positive electrode active material, the positive electrode active material comprises lithium manganese iron oxide and a ternary material; the positive electrode sheet satisfies 46.7 < Z / X < 189, wherein Z is the active specific surface area of the positive electrode sheet, unit: cm 2 / g, and X is the ratio of the mass of the ternary material to the mass of the positive electrode active material.

[0005] Further, 0.3 < X < 0.9.

[0006] Further, 44.8 < Z < 56.7.

[0007] Further, the chemical formula of the ternary material comprises Li a Ni x Co y Mz A e O2, M includes Mn, Al, wherein 0.8≤a<1.2, 0.3≤x<1, 0.1≤y≤0.3, 0.1≤z≤0.4, 0≤e≤0.1, A includes at least one of Zr, Sr, Ti, B, Mg, Sn, W, Y, Ba, Nb, Mo, Ta, Si, La, Er, Nd, Gd, Ce.

[0008] Further, the ternary material has a layered structure, and 0.6≤x≤0.7.

[0009] Further, the chemical formula of the manganese lithium iron oxide includes Li c Mn b Fe 1-b PO4, wherein 0<b<1, 0.8<c<1.2.

[0010] Further, 104<Z*PD<177, wherein PD is the compactness density of the positive electrode sheet, and the unit is g / cm 3 .

[0011] Further, 2.2≤PD≤3.5.

[0012] The application also provides a secondary battery, which includes the positive electrode sheet.

[0013] The application also provides a power device, which includes the secondary battery as a power supply of the power device.

[0014] The application has the following beneficial effects: compared with the prior art, the secondary battery of the application includes a positive electrode active material, and the positive electrode active material includes a manganese lithium iron oxide and a ternary material; the positive electrode sheet satisfies 46.7<Z / X<189, Z is the specific surface area of the positive electrode sheet, the unit is cm 2 / g, and X is the ratio of the mass of the ternary material to the mass of the positive electrode active material. The application reduces the side reaction of the positive electrode active material and the electrolyte, improves the high-temperature cycle performance of the battery, reduces the polarization of the battery, and stabilizes the high-capacity release of the battery. DETAILED DESCRIPTION

[0015] The technical solutions in the embodiments of the present application will be described clearly and completely below. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of the present application. In addition, in the description of the present application, the term "comprising" means "including but not limited to". Various embodiments of the present application can exist in a range form; it should be understood that the description in a range form is only for the convenience and brevity, and should not be understood as a hard limitation on the scope of the present application; therefore, it should be considered that the described range has been specifically disclosed all possible sub-ranges and single values in the range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers in the range, such as 1, 2, 3, 4, 5 and 6, regardless of the range. In addition, whenever a numerical range is indicated in the present application, it refers to any cited number (fraction or integer) in the indicated range.

[0016] In order to solve the problems of fast capacity attenuation and poor rate performance of the secondary battery in the prior art, the present application provides a positive electrode sheet. It should be noted that the description order of the following embodiments is not limited as the preferred order of the embodiments.

[0017] The positive electrode sheet of the present application comprises a positive electrode active material, and the positive electrode active material comprises manganese lithium oxide and ternary material; the positive electrode sheet satisfies: 46.7 < Z / X < 189, wherein Z is the specific surface area of the active material of the positive electrode sheet, unit: cm 2 / g, and X is the ratio of the mass of the ternary material to the mass of the positive electrode active material.

[0018] During the charging process of the battery, the active material of the positive electrode sheet, such as lithium ion, sodium ion, etc., undergoes oxidation reaction to release electrons under the action of electric field, completes charge exchange on the surface of the positive electrode active material, and then enters the electrolyte to migrate to the negative electrode through the separator, and is embedded into the inside of the active material of the negative electrode. In the present embodiment, the manganese lithium oxide and the ternary material are mixed as the positive electrode active material of the positive electrode sheet. Compared with the conventional manganese lithium oxide, the appropriate mixing ratio of the manganese lithium oxide and the ternary material can improve the specific surface area of the active material of the positive electrode sheet, reduce the side reaction of the manganese lithium oxide and the electrolyte, at the same time, the ternary material can also increase the conductivity of the positive electrode sheet, reduce the polarization phenomenon of the battery, stabilize the high capacity performance of the battery and improve the high temperature cycle performance of the battery.

[0019] The active specific surface area of the positive electrode tab of the present application is different from the specific surface area of the positive electrode tab in the conventional technology. The active specific surface area of the positive electrode tab of the present application, i.e. the active specific surface area of the positive electrode tab in the real reaction in the charging and discharging process, can more accurately represent the number of active sites of the positive electrode tab in the charging and discharging process. The size of the active specific surface area of the positive electrode tab is related to the type, particle size and specific surface area of the positive active material, and is also related to the porosity and roughness of the positive electrode tab. The porosity and roughness of the positive electrode tab can be adjusted by changing the proportion of each component (such as positive active material, binder and conductive agent) in the slurry preparation process or the cold pressing pressure. In the positive electrode tab of the present embodiment, when the ratio of Z / X is between 46.7 and 189, the high-temperature cycle performance of the battery can be improved, and the polarization phenomenon of the battery is reduced, and the high-capacity release of the battery is stabilized. Wherein, Z represents the active specific surface area of the positive electrode tab, and X represents the ratio of the mass of the ternary material to the mass of the positive active material. When the ratio of Z to X is greater than 189, the electrochemical activity of the positive electrode tab is too high, the active particles contact too much electrolyte, the side reaction is increased, a large amount of active lithium is consumed, and the diffusion of lithium ions is indirectly hindered because the ternary material is mixed in a small proportion and is completely coated by manganese-iron lithium oxide with poor conductivity. When the ratio of Z to X is less than 46.7, the cost of preparing the battery is high, and the high-temperature cycle performance of the battery is affected.

[0020] In order to further improve the electrochemical performance of the battery, in some embodiments, 49.8 < Z / X < 133, and further, the value of Z / X is any one or any two of 49.8, 65.4, 90.5, 129.6 and 133.

[0021] The greater the active specific surface area of the positive electrode tab, the more active sites in the positive electrode tab, the faster the exchange speed of active substances and electrons, and the better the kinetic performance of the battery. However, too many active sites will increase the side reaction of active materials and electrolyte, and deteriorate the cycle performance of the battery. Too few active sites will lead to a decrease in chemical reactions in the battery system, resulting in poor battery capacity and cycle performance. Based on this, in some embodiments of the present application, 44.8 < Z < 56.7.

[0022] In the present embodiment, when the active specific surface area of the positive electrode tab is greater than 56.7, the electrochemical activity of the positive electrode tab is too high, the active particles contact too much electrolyte, the side reaction is increased, a large amount of active lithium is consumed, and the battery performance is deteriorated. When the active specific surface area of the positive electrode tab is less than 44.8 cm 2 / g, the chemical reactions in the battery system will be decreased, the deintercalation efficiency of lithium ions will be decreased, and the battery capacity and cycle performance will be decreased.

[0023] To keep the chemical reaction rate in the battery in a more reasonable range, so that the cycle performance of the battery is kept in a more optimal state, in some embodiments, 47.8 < Z < 50.8.

[0024] In some embodiments, 0.3 < X < 0.9. Controlling the ratio X of the mass of lithium manganese iron oxide to the mass of the positive electrode active material in the above range can increase the compatibility of the positive electrode material with the electrolyte, reduce the side reaction of lithium manganese iron oxide with the electrolyte, and also improve the conductivity of the positive electrode plate and reduce the polarization of the battery. To further enable the positive electrode plate to have more reasonable conductivity and active sites, and to improve the performance of the battery, in some embodiments, 0.4 < X < 0.8.

[0025] In some embodiments, the chemical formula of the ternary material includes Li a Ni x Co y M z A e O2, M includes Mn, Al, wherein 0.8≤a<1.2, 0.3≤x<1, 0.1≤y≤0.3, 0.1≤z≤0.4, 0≤e≤0.1, A contains at least one of Zr, Sr, Ti, B, Mg, Sn, W, Y, Ba, Nb, Mo, Ta, Si, La, Er, Nd, Gd, Ce.

[0026] In some embodiments, the ternary material has a layered structure. The above-mentioned ternary material has the advantages of high voltage platform, high energy density, high tap density, good conductivity, mature production process, etc., which can effectively improve the electrochemical performance of the battery and reduce the production cost. In addition, the layered structure of the ternary material can make the stability of the ternary material better, improve the cycle performance and safety performance of the battery. In some embodiments, 0.6≤x≤0.7, the nickel content is kept in this range, which can further improve the cycle stability of the battery and improve the comprehensive performance of the battery. If the nickel content is too low, the battery capacity will decrease, and if the nickel content is too high, the lithium-nickel mixing in the ternary material will be serious, which will affect the cycle stability of the battery.

[0027] In some embodiments, the chemical formula of the lithium manganese iron oxide includes Li c Mn b Fe 1-b PO4, wherein 0 < b < 1, 0.8 < c < 1.2. That is, the lithium manganese iron oxide includes lithium manganese iron phosphate. Lithium manganese iron phosphate has the advantages of energy density and low-temperature performance compared to lithium iron phosphate, and has an olivine structure compared to ternary materials, which is more stable during charging and discharging, and has better safety and cycle stability than ternary materials. Blending lithium manganese iron phosphate with ternary materials can balance the energy density and stability of the materials, which is beneficial to improving the performance of the battery.

[0028] In some embodiments, 104 < Z*PD < 177, wherein PD is the compacted density of the positive electrode tab, in g / cm 3 In the embodiments, the value of Z*PD is controlled to be between 104 and 177, which can effectively improve the long-term cycle performance of the battery. In some embodiments, the value of Z*PD is any value in 104, 130, 145, 160, 177 or a range of any two values.

[0029] In some embodiments, 2.2 < PD < 3.5. Controlling the compacted density of the positive electrode tab in the above range can balance the energy density of the positive electrode tab and its wettability to the electrolyte, so that the battery energy density can be effectively improved while the battery polarization can be reduced.

[0030] In some embodiments, the positive electrode tab includes a positive electrode current collector, and the positive electrode active material is arranged on at least one surface of the positive electrode current collector. The positive electrode current collector can be selected according to actual needs, and is preferably an aluminum foil.

[0031] In some embodiments, the positive electrode tab further includes a conductive agent and a binder, and the types and contents of the conductive agent and the binder are not particularly limited and can be selected according to actual needs. In some embodiments, the conductive agent can include conductive carbon black, carbon nanotubes, graphene, etc., and the binder can include polyvinylidene fluoride.

[0032] The application also provides a secondary battery including a positive electrode tab, a negative electrode tab, a separator, and an electrolyte. The positive electrode tab includes a positive electrode active material, and the positive electrode active material includes a manganese lithium oxide and a ternary material; the positive electrode tab satisfies 46.7 < Z / X < 189, wherein Z is the specific surface area of the positive electrode tab, in cm 2 / g, and X is the ratio of the mass of the ternary material to the mass of the positive electrode active material.

[0033] In some embodiments, the negative electrode tab includes a negative electrode current collector and a negative electrode active material, a binder, and a conductive agent covering the negative electrode current collector. The types and contents of the negative electrode active material, the binder, and the conductive agent are not particularly limited and can be selected according to actual needs.

[0034] In some embodiments, the negative electrode material can be selected from one or more of graphite, soft carbon, hard carbon, carbon fiber, silicon-based material, and tin-based material.

[0035] In some embodiments, the negative electrode material is graphite.

[0036] In some embodiments, the main components of the electrolyte include lithium salt, organic solvent and additives. The types and compositions of the lithium salt and the organic solvent are not particularly limited and can be selected according to actual needs. The lithium salt can include lithium hexafluorophosphate, lithium bisfluorosulfonylimide and the like, the solvent can include ethylene carbonate, methyl ethyl carbonate, dimethyl carbonate and propyl propionate and the like, and the additives can include lithium difluorophosphate, lithium bisoxalate borate and succinonitrile and the like.

[0037] In some embodiments, the separator material can be any separator material commonly used, such as polyethylene, polypropylene, polyvinylidene fluoride and a separator composed of a plurality of composite films.

[0038] In some embodiments, the separator material is a polypropylene film.

[0039] In some embodiments, the preparation method of the secondary battery includes the following steps:

[0040] The positive electrode sheet, the separator and the negative electrode sheet are sequentially stacked, the separator is between the positive electrode sheet and the negative electrode sheet to play a role of isolation, then the bare battery cell is wound and is loaded into a shell, then the water is removed by baking at 65-95°C, the electrolyte is injected, the opening is sealed, and after the processes of standing, hot and cold pressing, formation, clamp and capacity distribution, the secondary battery is obtained.

[0041] The application further provides a power consumption device including the secondary battery.

[0042] In some embodiments, the power consumption device of the application is, but is not limited to, a backup power supply, an electric motor, an electric vehicle, an electric motorcycle, a power-assisted bicycle, a bicycle, an electric tool, a household large storage battery and the like.

[0043] In the application, the specific surface area of the positive electrode sheet can be obtained by the following test method, which includes the following steps:

[0044] The positive electrode sheet to be tested, a reference electrode, a negative electrode sheet and an electrolyte are assembled into a coin-type half cell, wherein the electrolyte contains an electrochemical redox probe molecule with a concentration of C, the redox potential of the probe molecule is 2-4 V, and the probe molecule is dissolved in the electrolyte.

[0045] A series of linear sweep voltammetry curves of different coin-type half cells at different scan rates V are tested by using an electrochemical workstation, and the oxidation peak current i p (oxidation) and the reduction peak current i p (reduction) are obtained, then the peak current i p of the obtained series of coin-type half cells is plotted against the square root of the scan rate V, and the slope K is obtained; according to the Randles-Sevick equation ip=2.69*10 5 *n 2 / 3cD 1 / 2 AV 1 / 2 , wherein ip is the peak current, n is the number of electrons transferred, D is the diffusion coefficient, D(oxidation) = 1.41*10 -6 cm 2 / s, D(reduction) = 1.26*10 -6 cm 2 / s, v is the scan rate, and A is the active surface area of the positive electrode tab. The active surface area A of the positive electrode tab divided by the weight m of the positive electrode tab is the active specific surface area of the positive electrode tab.

[0046] When a certain potential is applied, electrons are transferred from the current collector to the active sites on the surface of the active material in the positive electrode tab, and the probe molecules undergo redox reactions.

[0047] In the above test method, the three electrodes are glassy carbon electrodes. The electrolyte is a common electrolyte, including electrolyte salt and organic solvent; including one or several of cyclic carbonates, carboxylic acid esters, etc.

[0048] In the above test method, the redox potential of the probe molecule is 3V-4V. The concentration of the probe molecule is 0.03mol / L-0.08mol / L; further preferably 0.05mol / L.

[0049] In the above test method, the probe molecule is one of hexaammine ruthenium, ferric chloride, and ferrocene, and is further preferably ferrocene, because ferrocene has excellent reversibility in the battery system due to its low redox potential (3V-3.4V), and the peak position is not affected by the positive and negative electrode systems. The method tests the cyclic voltammetry curve of the ferrocene solution at different scan rates on the surface of the active material, obtains the peak current of the oxidation peak and the reduction peak, obtains the slope by fitting the current i and the scan rate V, and calculates the reaction active specific surface area of the tab. This method uses iron ions as a pointer element, and the oxidation / reduction sites of iron ions on the surface of the tab as active sites of the tab. According to the Randles-Sevick equation and the cyclic voltammetry curve, a linear relationship is established to obtain the active reaction area.

[0050] In the above test method, the scan rate is 0.01mv / s-3.50mv / s, the scan voltage range is 2.9-3.5v, D(oxidation) = 1.41*10 -6 cm 2 / s, D(reduction) = 1.26*10 -6 cm 2 / s.

[0051] The present application has undergone multiple tests, and now a part of the test results are taken as a reference to further describe the invention in detail. The following will be described in detail in combination with specific examples. It should be understood that these examples are only used to illustrate the present application and not to limit the scope of the present application.

[0052] Example 1

[0053] Preparation of positive electrode sheet: LMFP (LiMn 0.5 Fe 0.5 PO4), ternary material (LiNi 0.5 Co 0.2 Mn 0.3 O2) were mixed in a mass ratio of 7:3 to obtain a positive active material, and then the obtained positive active material, conductive agent carbon black, and binder PVDF were mixed in a mass ratio of 96.5:2.3:1.2 and added with solvent NMP. Vacuum stirring was performed until the system was uniform, and a positive electrode slurry was obtained. Then, the positive electrode slurry was uniformly coated on the upper and lower surfaces of the positive current collector aluminum foil. After air drying at room temperature, the positive electrode slurry was transferred to an oven and subjected to cold pressing (cold pressing pressure was 1.70 MPa) and slitting processes to obtain a positive electrode sheet. The obtained positive electrode sheet had a tap density of 3.0 g / cm 3 , and an active specific surface area of the positive electrode sheet was 56.7 cm 2 / g.

[0054] Preparation of negative electrode sheet: The negative active material graphite, conductive agent CNT, thickening agent CMC, and binder SBR were mixed in a mass ratio of 96.5:0.8:0.9:1.8 and added with deionized water. Vacuum stirring was performed until the system was uniform, and a negative electrode slurry was obtained. Then, the negative electrode slurry was uniformly coated on the upper and lower surfaces of the negative current collector copper foil. After air drying at room temperature, the negative electrode slurry was transferred to an oven for further drying. After cold pressing and slitting, a negative electrode sheet was obtained, and the tap density was 1.6 g / cm 3 .

[0055] Preparation of electrolyte: lithium hexafluorophosphate with ethylene carbonate as the solvent was prepared as the electrolyte

[0056] Preparation of lithium ion secondary battery: the above-mentioned positive electrode sheet, negative electrode sheet, and polyethylene film separator were stacked in the order of positive electrode sheet / separator / negative electrode sheet, and then subjected to winding, shell entering, packaging, and drying. After adding the electrolyte, the lithium ion secondary battery was obtained after vacuum packaging, standing, formation, and capacity distribution processes.

[0057] Example 2

[0058] The preparation method is the same as that in Example 1, except that in the process of preparing the positive electrode plate, the mass ratio of LMFP and ternary material is 6:4, the mass ratio of positive electrode active material, conductive agent carbon black and binder PVDF in the positive electrode slurry is 96.5:2.35:1.15, and the cold pressing pressure is 1.74 MPa.

[0059] Example 3

[0060] The preparation method is the same as that in Example 1, except that in the process of preparing the positive electrode plate, the mass ratio of LMFP and ternary material is 5:5, the mass ratio of positive electrode active material, conductive agent carbon black and binder PVDF in the positive electrode slurry is 96.5:2.4:1.1, and the cold pressing pressure is 1.78 MPa.

[0061] Example 4

[0062] The preparation method is the same as that in Example 1, except that in the process of preparing the positive electrode plate, the mass ratio of LMFP and ternary material is 3:7, the mass ratio of positive electrode active material, conductive agent carbon black and binder PVDF in the positive electrode slurry is 96.5:2.45:1.05, and the cold pressing pressure is 1.84 MPa.

[0063] Example 5

[0064] The preparation method is the same as that in Example 1, except that in the process of preparing the positive electrode plate, the mass ratio of LMFP and ternary material is 2:8, the mass ratio of positive electrode active material, conductive agent carbon black and binder PVDF in the positive electrode slurry is 96.5:2.5:1.0, and the cold pressing pressure is 1.88 MPa.

[0065] Example 6

[0066] The preparation method is the same as that in Example 1, except that in the process of preparing the positive electrode plate, the mass ratio of LMFP and ternary material is 4:6, the mass ratio of positive electrode active material, conductive agent carbon black and binder PVDF in the positive electrode slurry is 96.5:2.55:0.95, and the cold pressing pressure is 1.91 MPa.

[0067] Example 7

[0068] The preparation method is the same as that in Example 1, except that in the process of preparing the positive electrode plate, the mass ratio of LMFP and ternary material is 1:9, the mass ratio of positive electrode active material, conductive agent carbon black and binder PVDF in the positive electrode slurry is 96.5:2.6:0.9, and the cold pressing pressure is 1.92 MPa.

[0069] Examples 8-14

[0070] The preparation method is the same as that in Example 3, except that the positive electrode active material is coated in different amounts to obtain positive electrode plates with different compaction densities.

[0071] Example 15:

[0072] The preparation method is the same as that in Example 1, except that during the preparation of the positive electrode plate, the ternary material is selected from LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2.

[0073] Example 16:

[0074] The preparation method is the same as that in Example 1, except that during the preparation of the positive electrode plate, the ternary material is selected from LiNi 0.8 Co 0.1 Mn 0.1 O2.

[0075] Example 17:

[0076] The preparation method is the same as that in Example 1, except that during the preparation of the positive electrode plate, the ternary material is selected from LiNi 0.6 Co 0.2 Mn 0.2 O2.

[0077] Example 18:

[0078] The preparation method is the same as that in Example 1, except that during the preparation of the positive electrode plate, the ternary material is selected from LiNi 0.7 Co 0.2 Mn 0.1 O2.

[0079] Example 19: The preparation method is the same as that in Example 1, except that during the preparation of the positive electrode plate, the ternary material is selected from LiNi 0.4 Co 0.2 Mn 0.4 O2.

[0080] Comparative Example 1:

[0081] The preparation method is the same as that in Example 1, except that the positive electrode plate does not contain the ternary material.

[0082] Comparative Example 2:

[0083] The preparation method is the same as that in Example 1, except that the mass ratio of LMFP to the ternary material is 5:5, and the Dv50 is 0.5 μm and 4 μm, respectively.

[0084] Comparative Example 3:

[0085] The preparation method is the same as that in Example 1, except that the mass ratio of LMFP to ternary material is 4:6, and the Dv50 is 6 μm and 10 μm, respectively.

[0086] High-temperature cycle performance test of lithium ion secondary battery:

[0087] The lithium ion secondary battery was left to stand for 10 minutes at 45℃, and then charged at a constant current of 1C rate to 4.25V, and then charged at a constant voltage until the current was less than or equal to 0.05C, and then left to stand for 10 minutes, and then discharged at a constant current of 1C rate to 2.8V. This is one charge-discharge cycle. The discharge capacity at this time is recorded as the discharge capacity of the first cycle of the lithium ion secondary battery. The lithium ion secondary battery was subjected to 500 cycles of charge-discharge test according to the above method, and the discharge capacity at the 500th cycle was recorded. The capacity retention rate (%) of the lithium ion secondary battery after 500 cycles of 45℃, 1C / 1C is (discharge capacity at the 500th cycle) / (discharge capacity at the first cycle) x 100%.

[0088] Test method of internal resistance of lithium ion secondary battery:

[0089] The lithium ion secondary battery was left to stand for 5 minutes at 25℃, and then charged at a constant current of 1C rate to 4.2V, and then charged at a constant voltage until the current was less than or equal to 0.05C, at which time the state of charge (SOC) of the battery was 100%, and then left to stand for 5 minutes, and then discharged at a constant current of 1C rate, and the state of charge (SOC) of the lithium ion secondary battery was adjusted to 50%. The lithium ion secondary battery with 50% SOC was left to stand for another 10 minutes, and then discharged at a constant current of 4C rate for 30 seconds. The voltage U1 at the last second of standing, the voltage U2 at the last second of 4C rate constant current discharge, and the current I of 4C rate constant current discharge were recorded. The internal resistance of the lithium ion secondary battery = (U2-U1) / I.

[0090] Table 1 Parameters and test results of Examples 1-19 and Comparative Examples 1-3 of the application

[0091]

[0092]

[0093] Result analysis: The mixture of lithium manganese iron phosphate and ternary material as the active material of the positive electrode sheet can significantly improve the high-temperature cycle performance of the battery. When Z / X is in the range of 46.7-189, the cycle performance of the battery is better. If Z / X is not in the above range, for example, Comparative Example 2 and Comparative Example 3, the high-temperature cycle performance of the battery is significantly deteriorated.

[0094] In the above examples, the description of each example has its own focus, and the parts not described in detail in a certain example can be referred to the relevant description of other examples.

[0095] The above describes in detail the positive electrode plate, the secondary battery and the electric device provided by the embodiment of the application. The principles and implementation manners of the application are described by applying specific examples. The above description of the embodiments is only used to help understand the method of the application and its core idea. Meanwhile, for those skilled in the art, according to the idea of the application, the specific implementation manners and application ranges will be changed. In summary, the content of the specification should not be understood as a limitation of the application.

Claims

1. A positive electrode sheet, characterized by, The positive electrode plate comprises a positive electrode active material, the positive electrode active material comprises manganese lithium iron oxide and a ternary material; the positive electrode plate satisfies: 44.8 < Z < 56.7, 46.7 < Z / X < 189, wherein Z is the specific surface area of the positive electrode plate, unit: cm 2 / g, X is the ratio of the mass of the ternary material to the mass of the positive electrode active material; The active specific surface area of the positive electrode sheet is obtained by the following test method, which comprises: Assembling the positive electrode sheet to be tested, a reference electrode, a negative electrode sheet and an electrolyte into a button half-cell, wherein the electrolyte contains an electrochemical redox probe molecule with a concentration c, the redox potential of the electrochemical redox probe molecule is 2V-4V, and the electrochemical redox probe molecule comprises one of hexaammine ruthenium, ferric chloride and ferrocene; the concentration c of the electrochemical redox probe molecule is 0.03mol / L-0.08mol / L; The linear scan volt-ampere curve of the button-type half cell at different scanning speeds V is tested by using an electrochemical workstation to obtain a peak current i p , the peak current i p includes an oxidation peak current i p氧化 or a reduction peak current i p还原 , the scanning speed V is 0.01 mv / s to 3.50 mv / s, and the scanning voltage range is 2.9 V to 3.5 V; with the peak current i p plotting against the square root of the sweep rate V gives a slope; According to the Randles-Sevick equation: i p = 2.69*10 5 n 2 / 3 cD 1 / 2 AV 1 / 2 , the active surface area A of the positive electrode sheet was calculated, where n is the number of electron transfer, D is the diffusion coefficient, which includes D 氧化 or D 还原 , D 氧化 = 1.41*10 -6 cm 2 / s, D 还原 = 1.26*10 -6 cm 2 / s; Dividing the active surface area A of the positive electrode sheet by the mass of the positive electrode sheet to obtain the active specific surface area Z of the positive electrode sheet.

2. The cathode electrode of claim 1, wherein, 0.3＜X＜0.9。 3. The cathode electrode of claim 1, wherein The chemical formula of the ternary material includes Li a Ni x Co y M z A e O2, M includes Mn, Al, wherein 0.8≤a<1.2, 0.3≤x<1, 0.1≤y≤0.3, 0.1≤z≤0.4, 0≤e≤0.1, A contains at least one of Zr, Sr, Ti, B, Mg, Sn, W, Y, Ba, Nb, Mo, Ta, Si, La, Er, Nd, Gd, Ce.

4. The cathode electrode of claim 3, wherein, The ternary material has a layered structure, and 0.6≤x≤0.

7.

5. The cathode sheet of claim 1, wherein, The chemical formula of the lithium manganese iron oxide includes Li c Mn b Fe 1-b PO4, wherein 0 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 6. The positive electrode plate according to any one of claims 1 to 5, characterized by, 104< Z*PD < 177, where PD is the compacted density of the positive electrode sheet in g / cm 3 .

7. The cathode electrode of claim 6, wherein, 2.2≤PD≤3.

5.

8. A secondary battery characterized by comprising: The positive electrode sheet according to any one of claims 1-7.

9. An electrical device, characterized by The secondary battery according to claim 8, which is used as a power supply for the power utilization device.

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