Secondary battery and electric device

Abstract

The present application relates to the technical field of electrochemistry. Specifically disclosed are a secondary battery and an electric device. The secondary battery of the present application comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein a positive active material in the positive electrode sheet comprises a lithium-containing material having an olivine structure. The secondary battery is subjected to a charge-discharge test at 25°C and 0.33 C within a voltage range of 2.5-4.25 V to obtain a discharge curve. The discharge curve comprises three discharge plateaus at different potentials, which are sequentially designated as a first discharge plateau, a second discharge plateau and a third discharge plateau in descending order of potential, wherein the potential difference between the second discharge plateau and the third discharge plateau is 23-192 mV; and the discharge capacity of the third discharge plateau accounts for 3.5-88% of the total discharge capacity of the secondary battery. The secondary battery of the present application has a relatively high discharge power at low temperatures and a low SOC and has good energy density.

Description

Secondary batteries and electrical appliances

[0001] Cross-reference to related applications

[0002] This application claims priority to Chinese Patent Application No. 202411641275.3, filed on November 18, 2024, entitled "Secondary Battery and Power-Consuming Device", the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application relates to the field of electrochemical technology, specifically to secondary batteries and electrical devices. Background Technology

[0004] With advancements in modern technology and increasing consumer demand, the application areas of lithium-ion batteries are constantly expanding, from portable electronic devices to electric vehicles and large-scale energy storage systems. These applications not only require batteries to have high energy density, but also place higher demands on their power output capabilities.

[0005] Lithium manganese iron phosphate (LMFP) is gradually becoming a research hotspot in the field of lithium-ion batteries due to its excellent energy density and relative safety. However, despite the good energy density of LMFP materials, its low ion diffusion rate leads to a decrease in battery discharge power, especially in low-temperature environments and when the battery is in a low state of charge (SOC). The internal chemical reaction rate of the battery slows down significantly, which seriously affects the battery discharge power and makes it unable to meet the usage requirements in certain harsh environments.

[0006] Lithium iron phosphate (LFP) materials enable batteries with relatively high power output due to their good conductivity and high electron mobility. However, the blending of LFP and LMFP inevitably leads to a decrease in battery energy density, limiting the application of batteries in scenarios requiring high energy density.

[0007] Therefore, how to maintain or even improve the energy density of batteries to the maximum extent while ensuring their low-temperature power performance has become a technical challenge that urgently needs to be solved. Summary of the Invention

[0008] The purpose of this application is to overcome the shortcomings of the existing technology and provide a secondary battery and an electrical device. In the secondary battery, the positive electrode sheet adopts a lithium-containing material with an olivine structure as the positive electrode active material. By comprehensively controlling the redox reaction characteristics of the positive electrode active material, the secondary battery has high discharge power at low temperature and low SOC and good energy density.

[0009] To achieve the above objectives, in a first aspect of this application, this application provides a secondary battery, including a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive active material layer contains a positive active material, and the positive active material includes a lithium-containing material with an olivine structure.

[0010] The secondary battery was charged and discharged at 0.33C at 25°C to obtain the discharge curve; the discharge curve includes three discharge plateaus with different potentials, which are denoted as the first discharge plateau, the second discharge plateau and the third discharge plateau in order of decreasing potential.

[0011] The potential difference between the second and third discharge platforms is denoted as ΔU, and the range of ΔU is 23 to 192 mV.

[0012] The proportion of the discharge capacity of the third discharge platform and below to the total discharge capacity of the secondary battery is denoted as Q, and the range of Q is 3.5% to 88%.

[0013] In a second aspect, this application provides a secondary battery comprising the positive electrode sheet described above.

[0014] In a third aspect, this application provides an electrical device comprising the aforementioned secondary battery.

[0015] The beneficial effects of this application are as follows:

[0016] This application provides a secondary battery and an electrical device. The positive electrode of this application uses a lithium-containing material with an olivine structure as the positive electrode active material. By comprehensively controlling the redox reaction characteristics of the positive electrode active material, the secondary battery containing this positive electrode has high discharge power at low temperature and low SOC, as well as good energy density. Attached Figure Description

[0017] Figure 1 is a discharge curve of the lithium-ion battery in Example 7.

[0018] Figure 2 shows the dQ / dV-V curve of the lithium-ion battery in Example 7.

[0019] Figure 3 shows the dV / dQ-Q curve of the lithium-ion battery in Example 7. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0021] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0022] In this application, numerical ranges are involved. Unless otherwise specified, the numerical ranges mentioned above are considered continuous and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Optionally, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Furthermore, when multiple ranges are provided to describe features or characteristics, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are included.

[0023] In this application, there are no particular restrictions on the specific dispersion and mixing methods.

[0024] Unless otherwise specified, all reagents or instruments used in this application are commercially available products.

[0025] Secondary batteries

[0026] One embodiment of this application provides a secondary battery, including a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. The positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive active material layer contains a positive active material, which includes a lithium-containing material with an olivine structure.

[0027] The secondary battery was charged and discharged at 25°C at 0.33C in a voltage range of 2.5 to 4.25V to obtain a discharge curve. The discharge curve includes three discharge plateaus with different potentials, which are denoted as the first discharge plateau, the second discharge plateau, and the third discharge plateau in order of decreasing potential.

[0028] The potential difference between the second and third discharge platforms is denoted as ΔU, and the range of ΔU is 23 to 192 mV.

[0029] The proportion of the discharge capacity of the third discharge platform to the total discharge capacity of the secondary battery is denoted as Q, and Q ranges from 3.5% to 88%.

[0030] This application comprehensively controls the redox reaction characteristics of the positive electrode active material, introduces a third discharge platform, and controls the potential difference between the second and third discharge platforms as well as the discharge capacity ratio at and below the third discharge platform, so that the secondary battery containing this positive electrode sheet has high discharge power at low temperature and low SOC and good energy density.

[0031] The relationship between the battery's output power (P) and voltage (U) and current (I) is: P = U × I = (U initial -U 差值 )×I, where U initial U is the initial voltage. 差值 This represents the voltage drop. LMFPs have very high material impedance at low temperatures, and during pulsed discharge, the positive electrode potential drops rapidly, U... 差值 The voltage drop is too large, causing the battery potential to decrease rapidly and resulting in poor power output. Therefore, to improve output power, one approach is to increase the initial voltage and reduce the voltage drop.

[0032] The value of ΔU reflects the difference between the potential of the third discharge plateau and the potential of the second discharge plateau (potential of the second discharge plateau - potential of the third discharge plateau = U value). This study found that the magnitude of ΔU affects both the initial voltage and the voltage drop. A suitable ΔU value helps the battery achieve both a high initial voltage and a low voltage drop. When ΔU is too large, it may introduce an excessively low potential for the third discharge plateau, resulting in a low initial discharge voltage for the secondary battery at low SOC discharge; or it may cause an excessively high voltage for the second plateau, leading to an excessively large voltage drop during battery discharge, thus deteriorating the overall kinetics of the secondary battery. Both of these situations will affect the output power, leading to a reduction in power. When ΔU is too small, i.e., the second and third plateaus are infinitely close, the average operating voltage of the battery decreases, resulting in a decrease in the battery's specific energy density.

[0033] The Q value reflects the capacity proportion of the third discharge platform. If the Q value is too small, the capacity proportion of the introduced third discharge platform is too low, which cannot improve the initial voltage and thus improve the battery's discharge power at low temperature and low SOC. If the Q value is too large, the capacity proportion of the third discharge platform is too large, which will lead to a decrease in the battery's average discharge voltage and a reduction in the battery's overall energy density.

[0034] For lithium-containing materials with an olivine structure, their elemental composition and content, as well as the specific material selection, can all affect the ΔU and Q values. This study found that by controlling ΔU to a range of 23–192 mV and Q to a range of 3.5–88%, a secondary battery containing this positive electrode can achieve excellent low-temperature, low-SOC discharge power, while also possessing high energy density.

[0035] In this application, the values ​​of ΔU and Q can be obtained from the dQ / dV-V and dV / dQ-Q diagrams of the secondary battery containing the positive electrode. Specifically, ΔU and Q can be measured using the following method:

[0036] (1) Capacity setting: The secondary battery was left to stand at 25°C for 120 min, and then discharged at a constant current of 0.33C to 2.5V; after standing for 20 min, it was charged at a constant current and constant voltage of 0.33C to the cutoff voltage of 4.25V and the cutoff current of 0.05C; after standing for 20 min, it was discharged at a constant current of 0.33C to 2.5V; after repeating the above steps once, the discharge QV data and discharge VQ data of the second cycle were obtained;

[0037] (2) Plotting the dQ / dV-V curve: Based on the above discharge QV data, 100 points are sampled at equal intervals as the initial data for Dv and Dq. The voltage and charge data of the nth data point are subtracted from the voltage and charge data of the n+1th data point to obtain dV and dQ data. All data are processed in sequence to obtain a series of dV and dQ data. Then, dQ is divided by dV to obtain dQ / dV. The dQ / dV-V curve is plotted with dQ / dV as the vertical axis and voltage as the horizontal axis.

[0038] (3) In the curve △U:dQ / dV-V, within the range of 3.0 to 4.1V, according to the order of the horizontal axis voltage from low to high, the horizontal axis voltage U3 corresponding to the peak position of the first characteristic peak is the potential of the third discharge platform, the horizontal axis voltage U2 corresponding to the peak position of the second characteristic peak is the potential of the second discharge platform, and the horizontal axis voltage U1 corresponding to the peak position of the third characteristic peak is the potential of the first discharge platform.

[0039] U2-U3 = Potential difference (ΔU) between the second and third discharge platforms;

[0040] (4) Plotting the dV / dQ-Q curve: Based on the above discharge VQ data, 100 points are sampled at equal intervals as the initial data for Dv and Dq. The voltage and charge data of the nth data point are subtracted from the voltage and charge data of the n+1th data point to obtain dV and dQ data. All data are processed in sequence to obtain a series of dV and dQ data. Then, dV is divided by dQ to obtain dV / dQ. The dV / dQ-Q curve is plotted with dV / dQ as the vertical axis and Q as the horizontal axis.

[0041] (5) In the Q: dV / dQ-Q curve, the maximum horizontal axis is the total maximum discharge capacity Q0. The peak on the side closer to Q0 (right side) is defined as peak C. The horizontal axis capacity corresponding to the peak value of the total discharge capacity minus peak C is the discharge capacity of the third discharge platform (denoted as Q3). The proportion of the discharge capacity of the third discharge platform to the total discharge capacity (Q) = Q3 / Q0 × 100%.

[0042] For example, the range of △U can be 23mV, 25mV, 30mV, 40mV, 60mV, 80mV, 100mV, 130mV, 150mV, 160mV, 170mV, 180mV, 185mV, 190mV, or 192mV, or it can be any interval range formed by any two of the above values.

[0043] For example, the range of Q can be 3.5%, 5.0%, 8.0%, 10.0%, 15.0%, 30.0%, 50.0%, 60.0%, 70.0%, 75.0%, 80.0%, 85.0%, 87.0%, or 88.0%, or it can be an interval range formed by any two of the above values.

[0044] As an optional implementation of this application, the range of △U is 50 to 175 mV.

[0045] As an optional implementation of this application, the range of Q is 8.0% to 82%.

[0046] When △U or Q is within the above selectable range, the energy density and low-temperature power of the secondary battery containing this positive electrode are more balanced, and the overall performance is better.

[0047] As an optional embodiment of this application, the potential of the second discharge platform is 3.25 to 3.4V.

[0048] As an optional embodiment of this application, the potential of the third discharge platform is 3.1 to 3.25V.

[0049] As an optional embodiment of this application, the secondary battery is subjected to EIS testing, and the ratio of the positive electrode charge transfer impedance to the total battery impedance is denoted as R, where R ranges from 0.5 to 8%.

[0050] Further research in this application revealed that the positive electrode charge transfer resistance ratio (R value) of the positive electrode sheet has a significant impact on the battery's energy density and discharge power at low temperatures and low SOC. Under low-temperature conditions, the R value is particularly critical to the battery's discharge power. A smaller R value indicates better charge transfer capability of the positive electrode material, helping the battery maintain a higher discharge power at low temperatures. An excessively high R value restricts the charge transfer rate between electrodes, leading to a significant reduction in discharge power at low temperatures and exhibiting poor low-temperature performance. However, the R value should not be too low either. To obtain a low R value, specific modifications to the positive electrode active material or special designs of the electrode sheet are often required, resulting in deterioration of lithium-ion transport and kinetic performance, leading to a lower discharge voltage and consequently, a lower battery energy density. When R is between 0.5% and 8%, a value of 0.9% to 7.5% can be selected, allowing secondary batteries containing this positive electrode sheet to achieve both good discharge power and energy density at low temperatures and low SOC.

[0051] This application does not limit the detection method for R. Those skilled in the art can detect the proportion of positive electrode charge transfer impedance using conventional methods, such as electrochemical impedance spectroscopy (EIS). For example, R can be measured using the following method:

[0052] At room temperature, the secondary battery was discharged at 0.05C to 2.5V and then cut off. It was then charged to 80% SOC (charged at a constant current of 0.33C to 4.25V, then charged at a constant voltage until the current dropped to 0.05C; the discharge capacity of the previous cycle was used as the nominal capacity, and it was discharged at 0.33C to 80% SOC). Subsequently, electrochemical impedance spectroscopy (EIS) was performed on the secondary battery, with a test frequency range of 0.01Hz to 1MHz. After the test, the EIS curve was converted to a DRT curve using the DRTools data conversion tool, and the ratio of the positive electrode charge transfer impedance to the total battery impedance, R, was calculated.

[0053] Among them, the value of positive electrode charge transfer impedance is the peak area of ​​the DRT curve in the range of 0.1s to 1s (the specific integration range is adjusted according to the actual measurement results), and the value of total battery impedance is the sum of the peak areas of all peaks in the DRT curve.

[0054] The R-value is related to various factors, such as the elemental composition of the positive electrode active material, the proportion of element content, and the surface coating. The R-value can be controlled by adjusting the specific type of positive electrode active material, the doping elements and their content, and the carbon coating layer and its thickness.

[0055] As an optional embodiment of this application, the secondary battery satisfies the following relationship: 0.5 ≤ (ΔU / Q) 0.25 ×R≤20; where U is in mV, Q is in %, and R is in %.

[0056] This study found that the potential difference between the second and third discharge platforms, as well as the capacity ratio of the third platform, interact with R. Changes in R indicate changes in battery polarization, which in turn affect the platform voltage difference at the battery terminals; changes in Q lead to kinetic changes in the overall electrode material, which in turn affect R.

[0057] Considering the mutual influence of ΔU, Q, and R, this application further discovers that by comprehensively controlling ΔU, Q, and R, the battery's discharge power and energy density are relatively optimized. Specifically, applying a power of 0.25 to ΔU / Q smooths out data variations, enabling more reasonable data analysis and application. When (ΔU / Q) 0.25 When ×R is too high, it may cause a decrease in the battery's discharge power at low temperatures and low SOC. (△U / Q) 0.25 If the ×R is too low, the energy density of the battery may be relatively low.

[0058] As an optional embodiment of this application, the secondary battery satisfies the following relationship: 1.0 ≤ (ΔU / Q) 0.25 ×R≤13; where U is in mV, Q is in %, and R is in %.

[0059] When ΔU, Q, and R optionally satisfy 1.0 ≤ (ΔU / Q) 0.25 When ×R≤13, the secondary battery containing this positive electrode has relatively better discharge power and energy density.

[0060] As an optional embodiment of this application, the positive electrode active material includes a first active material and a second active material, wherein the first active material includes lithium manganese iron phosphate material and the second active material includes lithium iron phosphate material.

[0061] As an optional implementation of this application, the positive electrode active material includes a first active material and a second active material, both of which include lithium manganese iron phosphate material, and the manganese content in the first active material is greater than the manganese content in the second active material.

[0062] By blending different types of positive electrode active materials, the specific third discharge platform described in this application can be introduced. The positive electrode active material includes a first active material and a second active material, wherein the first active material can be lithium manganese iron phosphate, and the second active material can be lithium iron phosphate or lithium manganese iron phosphate with a lower manganese content than the first active material.

[0063] It should be noted that the manganese content in this application refers to the molar content of manganese relative to the total content of all metal elements other than lithium in the material. For example, the manganese content of the first active material refers to the molar content of manganese relative to the total content of all metal elements other than lithium in the first active material.

[0064] This application does not limit the method for detecting manganese content. Those skilled in the art can detect the manganese content in positive electrode active materials using conventional technical means.

[0065] For example, the manganese content can be detected using the following method:

[0066] The battery is disassembled to obtain the positive electrode sheet, which is then processed to obtain the positive electrode active material powder. The molar content of manganese in the positive electrode active material is obtained by electron microscopy (SEM) and energy dispersive spectroscopy (EDS).

[0067] As an optional implementation of this application, when the first active material includes lithium manganese iron phosphate material, the second active material includes lithium iron phosphate material, and the mass ratio of the first active material to the second active material is ≥1, the range of ΔU is 43~184mV.

[0068] As an optional implementation of this application, when the first active material includes lithium manganese iron phosphate material and the manganese content of the first active material is 50-90 mol%, the second active material includes lithium iron phosphate material, and the mass ratio of the first active material to the second active material is ≥1, the range of ΔU is 123-165 mV.

[0069] When the first active material includes lithium manganese iron phosphate and the second active material includes lithium iron phosphate, if the mass ratio of the first active material to the second active material is ≥1, meaning the proportion of lithium manganese iron phosphate is relatively larger, then lithium manganese iron phosphate will dominate the positive electrode active material. This is especially true when the lithium manganese iron phosphate has a high manganese content, resulting in a relatively high manganese content in the positive electrode active material. In this case, when ΔU meets the above-mentioned selectable range, an excessively high second discharge plateau can be avoided, preventing poor low-temperature power performance caused by deterioration in battery kinetic performance.

[0070] As an optional implementation of this application, when the first active material includes lithium manganese iron phosphate material, the second active material includes lithium iron phosphate material, and the mass ratio of the first active material to the second active material is less than 1 and greater than 0, the range of ΔU is 122 to 192 mV.

[0071] When the first active material includes lithium manganese iron phosphate and the second active material includes lithium iron phosphate, and the proportion of lithium iron phosphate is relatively larger, lithium iron phosphate is the dominant material in the positive electrode active material. In order to ensure that the specific energy density of the battery is in a suitable range, the range of Δ can be selected as 122 to 192 mV.

[0072] As an optional implementation of this application, when the first active material includes lithium manganese iron phosphate material and the manganese content of the first active material is 50-90 mol%, the second active material includes lithium iron phosphate material, and the mass ratio of the first active material to the second active material is less than 1 and greater than 0, the range of ΔU is 137-172 mV.

[0073] As an optional implementation of this application, when both the first active material and the second active material include lithium manganese iron phosphate material, and the manganese content in the first active material is greater than the manganese content in the second active material, and the mass ratio of the first active material to the second active material is ≥1, the range of ΔU is 23 to 144 mV.

[0074] As an optional implementation of this application, when both the first active material and the second active material include lithium manganese iron phosphate material, and the manganese content in the first active material is greater than the manganese content in the second active material, and the mass ratio of the first active material to the second active material is less than 1 and greater than 0, the range of ΔU is 44 to 178 mV.

[0075] When both the first and second active materials include lithium manganese iron phosphate, considering the overall manganese content of the positive electrode active material, the battery's low-temperature discharge power and specific energy density are better when ΔU is controlled within the above-mentioned selectable range.

[0076] As an optional embodiment of this application, the positive electrode active material includes a core and a carbon layer disposed on the outer surface of the core, wherein the core includes lithium manganese iron phosphate or lithium iron phosphate; the average thickness of the carbon layer is H, in nm; the particle diameter of the core is D, in nm, and the aspect ratio of the positive electrode active material is denoted as k = H / (D). 0.7 -0.05, and k ranges from 0.033 to 0.136.

[0077] The chemical structural formula of the lithium manganese iron phosphate is LiMn. x Fe y M z n PO4, where 0 < x < 1, 0 < y < 1, 0 ≤ z < 1, and 2(x + y) + n × z = 2; M is the dopant element, and n is the valence state of element M.

[0078] As an optional embodiment of this application, the doping element may include at least one of vanadium (V), titanium (Ti), tungsten (W), cobalt (Co), and magnesium (Mg).

[0079] The chemical structural formula of the lithium manganese iron phosphate is LiFePO4.

[0080] In this application, the kinetic performance balance of the positive and negative electrodes of the battery is adjusted by regulating the relationship between H and D. H / (D) 0.7 In k = 0.05, H represents the average thickness of the carbon layer, and D represents the particle diameter of the core in the cathode active material. This study found that by controlling the value of k, the adsorption and wetting ability of the carbon-coated cathode active material for the electrolyte, as well as the lithium-ion solid-phase transport rate, can be reflected. A lower k value results in relatively weaker kinetic performance of the cathode, meaning a relatively lower lithium-ion transport rate. Conversely, a higher k value leads to a mismatch between the carbon layer thickness and the core particle size, limiting lithium-ion transport and causing a decrease in power performance. Furthermore, an excessively high k value also results in a small proportion of the cathode active material contributing to overall capacity, reducing the battery's energy density.

[0081] As an optional implementation of this application, the range of k is 0.076 to 0.129.

[0082] As an optional implementation of this application, for positive electrode active materials with lithium manganese iron phosphate core, k ranges from 0.056 to 0.136, and can be optionally from 0.078 to 0.129.

[0083] As an optional implementation of this application, for positive electrode active materials with lithium iron phosphate core, k ranges from 0.033 to 0.125, and can be optionally from 0.076 to 0.116.

[0084] As an optional embodiment of this application, the average thickness (H) of the carbon layer is 4 to 8 nm.

[0085] As an optional embodiment of this application, the average particle diameter (D) of the core is 80 to 200 nm.

[0086] In this application, no limitation is made on the detection methods for H and D. Those skilled in the art can detect the thickness of the carbon layer and the particle diameter of the core using conventional technical means (such as TEM combined with EDS).

[0087] For example, H and D can be detected using the following methods:

[0088] Take an empty positive electrode sheet and soak it in dimethyl carbonate (DMC) solution at room temperature for 4 hours. After soaking, take out the positive electrode sheet and dry it in a vacuum environment. Use a ceramic knife to scrape off the positive electrode active material powder on the surface of the positive electrode sheet.

[0089] The scraped positive electrode active material powder was uniformly dispersed in ethanol and subjected to TEM (transmission electron microscopy) testing. In the TEM observation field, elemental analysis by EDS (energy dispersive X-ray spectroscopy) confirmed whether it was lithium iron phosphate or lithium manganese iron phosphate.

[0090] Draw lines along the long and short axes of the lithium iron phosphate or lithium manganese iron phosphate core in the positive electrode active material particles, measure the diameter, and calculate the average value to obtain the particle diameter of the core of the positive electrode active material.

[0091] Three different locations were selected on a single positive electrode active material to measure the thickness of the carbon layer covering the outer surface of the core, and the average value was calculated to obtain the carbon layer thickness of the positive electrode active material particle.

[0092] Repeat the above measurement steps to ensure that the particle diameter and carbon layer thickness of at least 100 positive electrode active materials are measured, and then calculate the average value to obtain the average particle diameter (D) of the core and the average thickness (H) of the carbon layer.

[0093] As an optional implementation of this application, when both the first active material and the second active material include lithium manganese iron phosphate, the difference between the manganese content in the first active material and the manganese content in the second active material is 20% to 80%.

[0094] This application does not limit the preparation method of lithium manganese iron phosphate material. Those skilled in the art can prepare lithium manganese iron phosphate material using conventional technical means.

[0095] For example, a method for preparing lithium manganese iron phosphate materials may include the following steps:

[0096] Manganese, iron, phosphorus, lithium and carbon sources (if any) are mixed in a certain molar ratio and then ball-milled.

[0097] The ball-milled product was sintered in an atmosphere with an oxygen concentration of less than 150 ppm.

[0098] The sintered material is crushed and then screened to obtain lithium manganese iron phosphate material.

[0099] For example, the preparation method of lithium manganese iron phosphate material may further include the following steps:

[0100] lithium manganese iron phosphate precursor (Mn) a Fe b The PO4 precursor (a+b=1) is mixed with a lithium source and ball-milled. A phosphorus source is then added to adjust the Li / P ratio of the reactants. If necessary, a carbon source is also added. After mixing, the mixture is ball-milled together.

[0101] The ball-milled product was sintered in an atmosphere with an oxygen concentration of less than 150 ppm.

[0102] The sintered material is crushed and then screened to obtain lithium manganese iron phosphate material.

[0103] The preparation method of lithium iron phosphate materials is similar to that of lithium manganese iron phosphate materials, except that no manganese source is added, or FePO4 precursor is used.

[0104] The lithium source may include at least one of lithium hydroxide, lithium carbonate, lithium nitrate, lithium oxalate, lithium dihydrogen phosphate, lithium citrate, or lithium acetate.

[0105] The phosphorus source may include at least one of diammonium hydrogen phosphate, lithium dihydrogen phosphate, ammonium phosphate, or lithium phosphate.

[0106] The iron source may include at least one of ferrous oxalate, ferric hydroxide, ferrous hydroxide, ferric phosphate, ferrous phosphate, ferric acetate, ferrous acetate, ferric carbonate, ferrous carbonate, ferric oxide, ferric oxide, or ferric oxalate.

[0107] The manganese source may include at least one of manganese carbonate, manganese sulfate, manganese nitrate, manganese chloride, manganese oxalate, or manganese acetate.

[0108] The carbon source may include glucose and / or sucrose.

[0109] Ferric manganese phosphate can be used as a source of manganese, iron and phosphorus simultaneously; iron phosphate can be used as a source of manganese, iron and phosphorus simultaneously; the lithium source can include at least one of lithium hydroxide, lithium carbonate, lithium nitrate, lithium oxalate, lithium dihydrogen phosphate, lithium citrate or lithium acetate.

[0110] When preparing lithium manganese iron phosphate materials and / or lithium iron phosphate materials, a certain amount of doping element sources can be mixed with manganese source (if any), iron source, phosphorus source, and lithium source as needed. Doping element sources include vanadium source (vanadium pentoxide), tungsten source (ammonium metatungstate), titanium source (titanium oxide), magnesium source (magnesium carbonate), etc., to obtain lithium manganese iron phosphate materials and / or lithium iron phosphate materials containing a certain amount of doping elements.

[0111] Optionally, the solvent may be at least one of water and ethanol.

[0112] Alternatively, the grinding can be ball milling.

[0113] By adjusting the amount of manganese source, iron source, and phosphoric acid added, the proportion of manganese content in lithium manganese iron phosphate materials can be controlled.

[0114] The thickness of the carbon coating layer can be controlled by adjusting the amount of carbon source added and the ball milling conditions.

[0115] The conductive agent only needs to have suitable electronic conductivity and not cause adverse chemical changes in the battery; this application does not impose any particular limitation on the type of conductive agent. Specifically, the conductive agent can be at least one of carbon nanotubes, carbon black, or graphene.

[0116] The binder is used to improve the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. In this application, the binder can be a conventional choice in the battery field. Specifically, the conductive agent can be at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polyimide (PI), polyacrylic acid (PAA), polyacrylate, polyolefin, sodium carboxymethyl cellulose (CMC), or sodium alginate.

[0117] This application does not impose any particular restrictions on the positive electrode current collector, as long as it is conductive and will not cause adverse chemical changes in the battery, and can be made of, for example: stainless steel, aluminum, nickel, titanium, sintered carbon; or aluminum or stainless steel that has been surface treated with one of carbon, nickel, titanium, silver, etc.

[0118] In this application, the positive electrode sheet can be prepared according to conventional methods in the art. For example, the positive active material, conductive agent and binder are dispersed in a solvent to form a uniform positive electrode slurry, the positive electrode slurry is coated on the positive current collector, and after drying, rolling and other processes, the positive electrode sheet is obtained.

[0119] In addition to the positive electrode, the secondary battery also includes a negative electrode, a separator, and an electrolyte.

[0120] The negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector, wherein the negative active material layer includes a negative active material. Regarding the negative active material, this application embodiment does not specifically limit the type of negative active material, and it can be selected according to actual needs. As an example, the negative active material can be natural graphite, artificial graphite, mesophase microcarbon spheres (MCMB), hard carbon, soft carbon, silicon, or silicon-carbon composites.

[0121] The separator is located between the positive and negative electrode plates, serving to separate them and prevent short circuits caused by contact. The separator can be any material suitable for separators in electrochemical energy storage devices. Specifically, the separator includes at least one of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fibers.

[0122] The electrolyte in this application can be any electrolyte suitable for electrochemical energy storage devices in the art. The electrolyte includes an electrolyte and a solvent, and the electrolyte typically includes a lithium salt.

[0123] Specifically, the lithium salt includes at least one selected from lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalate borate (LiDFOB), lithium dioxalate borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorodioxalate phosphate (LiDFOP), and lithium tetrafluorooxalate phosphate (LiTFOP). The concentration of the electrolyte in the electrolyte solution can be 0.5–5 mol / L.

[0124] Specifically, the solvent includes at least one of ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butyl carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

[0125] Electrical appliances

[0126] One embodiment of this application provides an electrical device comprising the secondary battery described above.

[0127] The electrical device serves as the power source for the electrical device.

[0128] The term "electrical device" refers to any device that can utilize electrical energy and convert it into mechanical energy, thermal energy, light energy, or one or more other energy forms, such as electric motors, electric heaters, and electric light sources. Specifically, it can include, but is not limited to, mobile devices, electric vehicles, electric trains, ships and satellites, and energy storage systems. Mobile devices can include mobile phones, laptops, drones, robot vacuum cleaners, and e-cigarettes; electric vehicles can include pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, and electric trucks.

[0129] The present application is further illustrated below with specific embodiments.

[0130] Example 1

[0131] Example 1 provides a lithium-ion battery, the preparation method of which is as follows:

[0132] (1) Preparation of positive electrode sheet

[0133] (1.1) Preparation of positive electrode active material

[0134] Preparation of the first positive electrode active material:

[0135] According to LiMn x Fe 1-x The molar ratios of Li, Mn, Fe, and P in the chemical formula PO4 are respectively weighed out and mixed with lithium carbonate, manganese carbonate, ferrous oxalate, and diammonium hydrogen phosphate, where the value of x is 0.68, to obtain the mixture.

[0136] Based on the total weight of manganese source (manganese carbonate) and iron source (ferrous oxalate), weigh the doping element source (vanadium, vanadium pentoxide) at a mass ratio of 1000 ppm.

[0137] The mixture and a portion of the dopant source (50 wt.% dopant source) were ball-milled together at a speed of 500 rpm. After 1 hour of ball milling, the remaining dopant source (the remaining 50 wt.% dopant source) was added, and ball milling continued for a total duration of 8 hours.

[0138] The ball-milled product was placed in a tube furnace and sintered under a nitrogen atmosphere at a constant temperature of 700℃ for 10 hours. After cooling to room temperature, lithium manganese iron phosphate material was obtained, which is the first positive electrode active material.

[0139] Preparation of the second positive electrode active material:

[0140] The precursor (FePO4 precursor prepared by solvent method) was mixed with LiOH at a molar ratio of 1:1. The average diameter of the precursor is shown in Table 1. The mixture was ball-milled once at 350 rpm for 3 h. Then, NH4H2PO4 was added to adjust the Li / P ratio of the reactants, as well as a carbon source (glucose). The amount of carbon source added (the mass ratio of carbon source to the total amount of precursor, LiOH and NH4H2PO4) was 5.3 wt.%. The mixture was then ball-milled a second time at 500 rpm for 2 h.

[0141] The ball-milled product was placed in a tube furnace and sintered under a nitrogen atmosphere. The sintering conditions were as follows: the temperature was increased from 25°C to 700°C at a heating rate of 5°C / min. When the sintering temperature reached 300°C and 600°C, it was kept constant for 1 hour.

[0142] After cooling to room temperature, a lithium iron phosphate material with a core of lithium iron phosphate (LiFePO4) and a carbon layer on the outer surface of the core is obtained, which is the second active material.

[0143] (1.2) The first positive electrode active material and the second positive electrode active material are mixed evenly at a mass ratio of 25:75 to obtain the positive electrode active material;

[0144] The positive electrode active material, binder (PVDF), conductive agent SP, and conductive agent CNT are mixed evenly in NMP at a mass ratio of 97:1.5:1:0.5. Then, the mixed positive electrode slurry is evenly coated on aluminum foil and dried in a vacuum furnace at 100°C to obtain a positive electrode sheet. The sheet is then slit and rolled to obtain a positive electrode plate.

[0145] (2) Preparation of negative electrode sheet

[0146] The negative electrode active material (artificial graphite), conductive agent (SP), and binder (carboxymethyl cellulose, CMC) are mixed at a mass ratio of 96.4:1:2.6 and dispersed in deionized water. The negative electrode slurry is prepared by a wet process using a vacuum mixer. The negative electrode slurry is uniformly coated onto the negative electrode current collector (copper foil). The negative electrode current collector coated with the negative electrode slurry is transferred to an oven and dried in a vacuum environment at 100°C for 12 hours. Then, it is rolled and slit to obtain the negative electrode sheet.

[0147] (3) Preparation of electrolyte

[0148] Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a weight ratio of 3:7 to obtain an organic solvent. Then, fully dried lithium salt LiPF6 was dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1.15 mol / L.

[0149] (4) Preparation of the diaphragm

[0150] A polyethylene (PE) diaphragm is used.

[0151] (5) Battery manufacturing

[0152] The prepared positive electrode, separator, and negative electrode are wound to obtain a bare cell without electrolyte injection; the bare cell is placed in an outer packaging foil, and the prepared electrolyte is injected into the dried bare cell. After vacuum sealing, standing, formation, shaping, and sorting, a lithium-ion battery is obtained.

[0153] Examples 2-25 and Comparative Examples 1-8

[0154] Examples 2-25 and Comparative Examples 1-8 each provide a lithium-ion battery, the preparation method of which is basically the same as that of Example 1, the difference being:

[0155] The manganese content (x value) in the first active material is shown in Table 1;

[0156] The types and amounts of doped elements in the first active material are shown in Table 1. The first active materials of some embodiments and comparative examples have no doped elements, which are marked as " / " in Table 1.

[0157] For the examples and comparative examples where the first active material contains doped elements, the ball milling speed and ball milling duration remain unchanged, and other ball milling conditions are shown in Table 1;

[0158] The types of the second active material (LMFP for lithium manganese iron phosphate and LFP for lithium iron phosphate) are shown in Table 1; when the type of the second active material is LMFP, the precursor is Mn. y Fe 1-y PO4 precursor, the values ​​of y are shown in Table 1;

[0159] The average diameter of the precursor used in the preparation of the second active material is shown in Table 1; the amount of carbon source added is shown in Table 1.

[0160] The heating rate and sintering temperature of the second active material are shown in Table 1.

[0161] The mass ratio of the first active material to the second active material is shown in Table 1.

[0162] Table 1

[0163] When the second active material is lithium iron phosphate, the D and H in the lithium iron phosphate materials prepared in each embodiment and comparative example are detected by the following methods:

[0164] Take the positive electrode sheet of the lithium-ion battery in an empty state, soak it in dimethyl carbonate (DMC) solution at room temperature for 4 hours. After soaking, take out the positive electrode sheet and dry it in a vacuum environment. Use a ceramic knife to scrape off the positive active material powder on the surface of the positive electrode sheet.

[0165] The scraped positive electrode active material powder was uniformly dispersed in ethanol and subjected to TEM (transmission electron microscopy) testing. In the TEM observation field, elemental analysis by EDS (energy dispersive X-ray spectroscopy) confirmed whether it was lithium iron phosphate or lithium manganese iron phosphate.

[0166] Draw lines along the long and short axes of the lithium iron phosphate or lithium manganese iron phosphate core in the positive electrode active material particles, measure the diameter, and calculate the average value to obtain the particle diameter of the core of the positive electrode active material.

[0167] Three different locations were selected on a single positive electrode active material to measure the thickness of the carbon layer covering the outer surface of the core, and the average value was calculated to obtain the carbon layer thickness of the positive electrode active material particle.

[0168] Repeat the above measurement steps to ensure that the particle diameter and carbon layer thickness of at least 100 positive electrode active materials are measured, and then calculate the average value to obtain the average particle diameter (D) of the core and the average thickness (H) of the carbon layer.

[0169] k is calculated from D and H, where k = H / (D) 0.7 -0.05.

[0170] The ΔU and Q of lithium-ion batteries are tested using the following methods:

[0171] (1) Capacity setting: The lithium-ion battery was left to stand at 25°C for 120 min, and then discharged at a constant current of 0.33C to 2.5V; after standing for 20 min, it was charged at a constant current and constant voltage of 0.33C to the cutoff voltage of 4.25V and the cutoff current of 0.05C; after standing for 20 min, it was discharged at a constant current of 0.33C to 2.5V; after repeating the above steps once, the discharge QV data and discharge VQ data of the second cycle were obtained.

[0172] (2) Plotting the dQ / dV-V curve: Based on the above discharge QV data, 100 points are sampled at equal intervals as the initial data for Dv and Dq. The voltage and charge data of the nth data point are subtracted from the voltage and charge data of the n+1th data point to obtain dV and dQ data. All data are processed in sequence to obtain a series of dV and dQ data. Then, dQ is divided by dV to obtain dQ / dV. The dQ / dV-V curve is plotted with dQ / dV as the vertical axis and voltage as the horizontal axis.

[0173] (3) In the curve △U:dQ / dV-V, within the range of 3.0 to 4.1V, according to the order of the horizontal axis voltage from low to high, the horizontal axis voltage U3 corresponding to the peak position of the first characteristic peak is the potential of the third discharge platform, the horizontal axis voltage U2 corresponding to the peak position of the second characteristic peak is the potential of the second discharge platform, and the horizontal axis voltage U1 corresponding to the peak position of the third characteristic peak is the potential of the first discharge platform.

[0174] U2-U3 = Potential difference (ΔU) between the second and third discharge platforms;

[0175] (4) Plotting the dV / dQ-Q curve: Based on the above discharge VQ data, 100 points are sampled at equal intervals as the initial data for Dv and Dq. The voltage and charge data of the nth data point are subtracted from the voltage and charge data of the n+1th data point to obtain dV and dQ data. All data are processed in sequence to obtain a series of dV and dQ data. Then, dV is divided by dQ to obtain dV / dQ. The dV / dQ-Q curve is plotted with dV / dQ as the vertical axis and Q as the horizontal axis.

[0176] (5) In the Q: dV / dQ-Q curve, the maximum horizontal axis is the total discharge capacity Q0. The peak on the side closer to Q0 (right side) is defined as peak C. The horizontal axis capacity corresponding to the peak value of the total discharge capacity minus peak C is the discharge capacity of the third discharge platform (denoted as Q3). The proportion of the discharge capacity of the third discharge platform to the total discharge capacity (Q) = Q3 / Q0 × 100%.

[0177] The R value of the positive electrode is tested using the following method:

[0178] At room temperature, the lithium-ion battery was discharged at 0.05C to 2.5V and then cut off, and its charge was adjusted to 80% SOC. Subsequently, the lithium-ion battery was subjected to electrochemical impedance spectroscopy (EIS) testing, with a test frequency range of 0.01Hz to 1MHz. After the test, the EIS curve was converted to a DRT curve using the DRTools data conversion tool, and the ratio of the positive electrode charge transfer impedance to the total battery impedance was calculated, which is R.

[0179] Among them, the value of positive electrode charge transfer impedance is the peak area of ​​the DRT curve in the range of 0.1s to 1s, and the value of total battery impedance is the sum of the peak areas of all peaks in the DRT curve of positive electrode to negative electrode.

[0180] The test results are shown in Table 2.

[0181] Table 2

[0182] The performance of lithium-ion batteries at low temperature and low SOC discharge power and energy density was tested using the following methods:

[0183] (1) Discharge power performance at low temperature and low SOC:

[0184] At room temperature, the lithium-ion battery is charged to 4.25V with a constant current of 0.33C, and then charged with a constant voltage until the current drops to 0.05C. After resting for 5 minutes, the battery is discharged to 2.5V with a constant current of 0.33C. This is one cycle. After two cycles, the battery is charged to 5% SOC based on the discharge capacity of the second cycle as the standard capacity.

[0185] The lithium-ion battery was placed at -20°C and allowed to stand for 2 hours to reach temperature equilibrium. Then, it was pulsed discharged at 0.7C for 180 seconds with a signal acquisition frequency of 100ms. The value of the last voltage acquisition point was measured and is taken as the discharge termination voltage. This application uses the discharge termination voltage of the battery at -20°C and 5% SOC, pulsed at 0.7C for 180 seconds, to demonstrate the discharge power performance at low temperature and low SOC. The discharge termination voltage is determined by the initial voltage and voltage drop. With a constant current, the magnitude of the discharge termination voltage directly determines the discharge power. Therefore, in the embodiments and comparative examples of this application, a higher discharge termination voltage indicates better low-temperature and low-SOC discharge power performance of the battery.

[0186] (2) Energy density performance:

[0187] The lithium-ion battery was charged at 25°C with a constant current of 0.33C to 4.25V, then charged with a constant voltage until the current dropped to 0.05C. After resting for 5 minutes, the battery was discharged at a constant current of 0.33C to 2.5V, which constitutes one cycle. After two cycles, the discharge capacity of the second cycle was recorded and divided by the mass of the positive electrode active material (in electrode preparation, the mass of the electrode is weighed, the mass of the current collector is subtracted, and multiplied by the mass ratio of the positive electrode active material in the positive electrode slurry) to calculate the specific capacity (in MAh / g). Simultaneously, the average discharge voltage of the second cycle (in V) was recorded, and the specific capacity was multiplied by the average voltage to calculate the specific energy density at 25°C (in Wh / kg). This application uses the specific energy density at 25°C to represent the battery's energy density performance.

[0188] Figure 1 is a discharge curve of the lithium-ion battery in Example 7. It can be seen that the discharge curve includes three discharge plateaus with different potentials, which are referred to as the first discharge plateau, the second discharge plateau and the third discharge plateau in order of potential from high to low.

[0189] Figure 2 shows the dQ / dV-V curves of the lithium-ion battery in Example 7. According to the order of voltage on the horizontal axis from low to high, the horizontal axis voltage U3 corresponding to the peak position of the first characteristic peak is the potential of the third discharge plateau, the horizontal axis voltage U2 corresponding to the peak position of the second characteristic peak is the potential of the second discharge plateau, and the horizontal axis voltage U1 corresponding to the peak position of the third characteristic peak is the potential of the first discharge plateau; ΔU=U2-U3=165mV.

[0190] Figure 3 shows the dV / dQ-Q curve of the lithium-ion battery in Example 7. The discharge capacity of the third discharge plateau accounts for 8.3% of the total discharge capacity.

[0191] The test results are shown in Table 3.

[0192] Table 3

[0193] According to the test results in Table 3, it can be seen that the lithium-ion batteries prepared in each embodiment of this application all have good low-temperature, low-SOC discharge power performance and high energy density. The discharge termination voltage at -20℃ and 5% SOC with a 0.7C pulse discharge for 180s is ≥1.212V, and the specific energy density at 25℃ is ≥437Wh / kg. In Comparative Examples 1 to 7, when ΔU or Q exceeds the range of the technical solution of this application, the lithium-ion batteries struggle to simultaneously achieve qualified discharge power performance and energy density.

[0194] As can be seen from Examples 13 to 16, when the value of R is in the range of 0.5% to 8%, especially in the optional range of 0.9% to 7.5%, the overall effect of battery discharge performance and energy density is better.

[0195] As can be seen from Examples 17-19, when a lithium-ion battery optionally satisfies 1.0 ≤ (ΔU / Q) 0.25 When ×R≤13, the battery's low-temperature, low-SOC discharge performance and energy density are relatively more balanced.

[0196] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. A secondary battery comprising a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte, the positive electrode sheet comprising a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector, the positive electrode active material layer containing a positive electrode active material, characterized in that, The positive electrode active material includes a lithium-containing material with an olivine structure; The secondary battery was charged and discharged at 25°C at 0.33C in a voltage range of 2.5 to 4.25V to obtain a discharge curve. The discharge curve includes three discharge plateaus with different potentials, which are denoted as the first discharge plateau, the second discharge plateau, and the third discharge plateau in order of decreasing potential. The potential difference between the second and third discharge platforms is denoted as ΔU, and the range of ΔU is 23 to 192 mV. The proportion of the discharge capacity of the third discharge platform to the total discharge capacity of the secondary battery is denoted as Q, and Q ranges from 3.5% to 88%.

2. The secondary battery according to claim 1, characterized by The range of △U is 50–175 mV.

3. The secondary battery according to claim 1, wherein The range of Q is 8.0% to 82%.

4. The secondary battery according to claim 1, wherein The ratio of positive electrode charge transfer impedance to total battery impedance, denoted as R, is obtained by performing EIS testing on the secondary battery. The range of R is 0.5 to 8%.

5. The secondary battery according to claim 4, wherein The secondary battery satisfies the following relational expression: 0.5 ≤ (ΔU / Q) × R ≤ 20; where the unit of ΔU is mV, the unit of Q is %, and the unit of R is %. 0.25 The secondary battery satisfies the following relational expression: 0.5 ≤ (ΔU / Q) × R ≤ 20; where the unit of ΔU is mV, the unit of Q is %, and the unit of R is %.

6. The secondary battery according to claim 5, wherein The secondary battery satisfies the following relationship: 1.0 ≤ (ΔU / Q) 0.25 × R ≤ 13.

7. The secondary battery according to claim 1, wherein The positive electrode active material includes a first active material and a second active material, wherein the first active material includes lithium manganese iron phosphate and the second active material includes lithium iron phosphate.

8. The secondary battery according to claim 1, wherein The positive electrode active material includes a first active material and a second active material. Both the first and second active materials include lithium manganese iron phosphate material, and the molar percentage of manganese in the first active material relative to the metal elements other than lithium is greater than the molar percentage of manganese in the second active material relative to the metal elements other than lithium.

9. The secondary battery according to claim 7, wherein The positive electrode active material includes a core and a carbon layer disposed on the outer surface of the core, wherein the core includes lithium manganese iron phosphate or lithium iron phosphate; The average thickness of the carbon layer is H, the unit of H is nm; the particle diameter of the inner core is D, the unit of D is nm; let the thickness-diameter ratio of the positive electrode active material k = H / (D 0.7 -0.05, k ranges from 0.033 to 0.

136.

10. The secondary battery according to claim 9, wherein The range of k is 0.076 to 0.

129.

11. The secondary battery according to claim 9, wherein The average thickness of the carbon layer is 4–8 nm, and / or the particle diameter of the core is 80–200 nm.

12. The secondary battery according to claim 8, wherein The difference between the molar percentage of manganese in the first active material and the molar percentage of manganese in the second active material is 20–80 mol%.

13. The secondary battery according to claim 1 or 2, wherein The potential of the second discharge platform is 3.25 to 3.4V, and / or the potential of the third discharge platform is 3.1 to 3.25V.

14. An electrical device, comprising: It includes the secondary battery as described in any one of claims 1 to 13.

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