Secondary battery and electric device

Abstract

The present application relates to the technical field of electrochemistry, and specifically discloses 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. A positive electrode active material in the positive electrode sheet comprises lithium manganese iron phosphate and lithium iron phosphate. In a charging curve obtained by performing charging test on the secondary battery at a rate of 1C at 10°C, a charging plateau having the lowest potential is a first step plateau. The secondary battery satisfies the following relational expression: 2.9≤0.3×Dspan×Q / U≤12.9, wherein Dspan is a particle size distribution coefficient of lithium iron phosphate, U is a potential value of the first step plateau, and Q is the proportion of a charging capacity corresponding to the first step plateau in the total discharge capacity. In the present application, a particle size distribution coefficient of lithium iron phosphate and the voltage plateau characteristics of a positive electrode active material are comprehensively controlled, so that the secondary battery has an excellent low-temperature fast charging capability and a good cycle life.

Description

Secondary batteries and electrical appliances

[0001] Cross-references to related applications

[0002] This application claims priority to Chinese Patent Application No. 202411641271.5, filed on November 18, 2024, entitled "Secondary Battery and Electrical 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 the widespread application of lithium-ion batteries in electric vehicles, portable electronic devices, and energy storage systems, the demand for high energy density and fast charging capabilities is increasing. Lithium manganese iron phosphate (LMFP), as a novel cathode material, has attracted widespread attention due to its excellent energy density.

[0005] However, due to their inherent material properties, traditional LMFP cathode materials have limited lithium-ion insertion and extraction rates during high-rate charging, resulting in low charging efficiency. Especially at low temperatures, the ability of lithium-ions to insert and extract in LMFPs is restricted, leading to poor lithium intercalation capability in LMFP batteries. Consequently, under low-temperature fast-charging conditions, the battery's charging capacity is low, and its fast-charging capability is extremely poor.

[0006] To address this issue, existing technologies generally optimize fast charging capabilities by coating conductive additives, optimizing electrolyte formulations, or altering the microstructure of materials. However, these methods are insufficient to effectively improve the fast charging performance of batteries at low temperatures.

[0007] Therefore, there is a need to develop a rechargeable battery with higher fast charging capability. 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. The secondary battery includes a positive electrode sheet, and the positive electrode active material in the positive electrode sheet includes lithium manganese iron phosphate and lithium iron phosphate. By mixing a portion of lithium iron phosphate into lithium manganese iron phosphate and comprehensively controlling the particle size distribution coefficient of lithium iron phosphate and the voltage plateau characteristics of the positive electrode active material, the secondary battery has excellent low-temperature fast charging capability and good cycle life.

[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, which includes lithium manganese iron phosphate and lithium iron phosphate.

[0010] The secondary battery was charged at 10°C at a 1C rate within a voltage range of 2.5–4.25V to obtain a charging curve; the charging plateau with the lowest potential in the charging curve is the first step plateau.

[0011] The secondary battery satisfies the following relationship: 2.9 ≤ 0.3 × D span ×Q / U≤12.9;

[0012] Wherein, the D span The particle size distribution coefficient of lithium iron phosphate;

[0013] U is the potential value of the first step platform, in V;

[0014] Q represents the proportion of the charging capacity corresponding to the first-step platform to the total discharging capacity, expressed in percentage (%).

[0015] As an optional embodiment of this application, the secondary battery satisfies the following relationship: 5.5 ≤ 0.3 × D span ×Q / U≤8.8.

[0016] As an optional implementation of this application, the range of U is 3.1 to 3.4V.

[0017] As an optional implementation of this application, the range of U is 3.2 to 3.25V.

[0018] As an optional implementation of this application, the range of Q is 10% to 40%.

[0019] As an optional implementation of this application, the range of Q is 20% to 30%.

[0020] As an optional implementation of this application, the D span The range is 2.77 to 3.88.

[0021] As an optional implementation of this application, the D span The range is 2.82 to 3.28.

[0022] As an optional embodiment of this application, the mass ratio of lithium manganese iron phosphate to lithium iron phosphate is (2.3-9):1.

[0023] As an optional embodiment of this application, 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. The negative active material layer contains a negative active material, which includes at least one of natural graphite, artificial graphite, mesophase carbon microspheres, hard carbon, soft carbon, silicon, SiOx, silicon-carbon, and lithium titanate.

[0024] In a second aspect, this application provides an electrical device comprising the aforementioned secondary battery.

[0025] The beneficial effects of this application are as follows:

[0026] This application provides a secondary battery and an electrical device. The secondary battery of this application includes a positive electrode sheet, in which the positive electrode active material includes lithium manganese iron phosphate and lithium iron phosphate. By doping a portion of lithium iron phosphate into lithium manganese iron phosphate and comprehensively controlling the particle size distribution coefficient of lithium iron phosphate and the voltage plateau characteristics of the positive electrode active material, the secondary battery exhibits excellent low-temperature fast charging capability and good high-temperature cycle life. Attached Figure Description

[0027] Figure 1 is a QV curve of the lithium-ion battery in Example 11.

[0028] Figure 2 is a dQ / dV-V curve of the lithium-ion battery in Example 11. Detailed Implementation

[0029] 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.

[0030] 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.

[0031] 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.

[0032] In this application, there are no particular restrictions on the specific dispersion and mixing methods.

[0033] Unless otherwise specified, all reagents or instruments used in this application are commercially available products.

[0034] One embodiment of 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, including lithium manganese iron phosphate (LMFP) and lithium iron phosphate (LFP); wherein the structural formula of LMFP is LiMn. x Fe y M z n PO4; x is greater than 0 and less than 1; y is greater than 0 and less than 1; z is greater than or equal to 0 and less than 1, and x, y, and z satisfy: 2(x+y)+n×z=2; where M is the dopant element and n is the valence state of element M; the structural formula of LFP is LiFePO4.

[0035] The secondary battery was charged at 10°C at a 1C rate within a voltage range of 2.5–4.25V to obtain a charging curve; the charging plateau with the lowest potential in the charging curve is the first step plateau.

[0036] The secondary battery satisfies the following relationship: 2.9 ≤ 0.3 × D span ×Q / U≤12.9;

[0037] Wherein, the D span The particle size distribution coefficient of lithium iron phosphate;

[0038] U is the potential value of the first step platform, in V;

[0039] Q represents the proportion of the charging capacity corresponding to the first-tier platform to the total discharging capacity, expressed in percentage (%).

[0040] This application incorporates a portion of lithium iron phosphate as the positive electrode active material into lithium manganese iron phosphate, and comprehensively controls the particle size distribution coefficient of lithium iron phosphate and the voltage plateau characteristics of the positive electrode active material, thereby enabling the secondary battery to have excellent low-temperature and high-temperature fast charging capabilities. Under low-temperature conditions, fast charging and lithium intercalation are greatly improved, and the battery has good high-temperature cycle life.

[0041] In this application, controlling the voltage plateau characteristics of the positive electrode active material mainly involves controlling the potential value (i.e., U value) of the first step plateau when the battery is charged at 10°C and 1C rate, and Q, which is the ratio of the charging capacity corresponding to the first step plateau to the total discharge capacity (i.e., Q value).

[0042] Those skilled in the art will know that, due to the different charging and discharging voltages of manganese and iron, lithium manganese iron phosphate has two voltage plateaus (manganese plateau and iron plateau) during charging and discharging. Based on the dual voltage plateaus of lithium manganese iron phosphate, this application selectively adjusts the lower potential voltage plateau by mixing it with lithium iron phosphate, so that the U and Q of the lowest potential voltage plateau (i.e., the first step plateau) satisfy the aforementioned relationship.

[0043] This study found that the U-value and Q-value are the main influencing factors on the actual current density and internal resistance of the materials during charging of secondary batteries. According to Joule's law, W = I... 2 Rt (W is heat, t is time, I is current, R is resistance), U value, and Q value affect the heat generated by the battery during charging. By properly controlling the U value and Q value, W can be kept within a suitable range, allowing the secondary battery to continue charging at a high rate in the initial stage of charging, thereby increasing the internal temperature of the battery, improving the subsequent lithium insertion / extraction capability of the positive and negative electrodes, and thus improving the fast charging capability.

[0044] However, this application further discovers that introducing a low-potential first-step plateau through the mixing of LMFP and LFP may also have a certain negative impact on the high-temperature cycle life of the battery. During high-temperature cycling, the active lithium in LMFP is consumed, causing the positive electrode to rapidly lose capacity on the low-potential plateau, thus reducing battery life. Furthermore, when the low-potential plateau accounts for a large portion of the total capacity (i.e., a large Q value), the charging pressure on the high-potential range of LMFP is extremely high. During high-temperature cycling, the positive electrode side is in a high-potential charging state for a long time, which accelerates the degradation of the electrolyte and further affects the battery life.

[0045] D span D is the particle size distribution coefficient of lithium iron phosphate. span = (Dn90-Dn10) / Dn50, where Dn10, Dn50, and Dn90 are the particle sizes corresponding to the cumulative percentage distribution of lithium iron phosphate particles reaching 10%, 50%, and 90%, respectively. span The size of the LFP particle reflects the uniformity of its particle size distribution. The degree of uniformity in LFP particle size distribution affects the particle contact ability between LFPs in the cathode active material, which in turn affects the charging capability of the secondary battery. Furthermore, the cathode active material may undergo volume expansion or contraction during cycling, leading to mechanical stress; especially when the battery is charged at high temperatures, this can exacerbate potential localized overheating, further causing uneven stress in the cathode active material. The uniformity of the lithium iron phosphate particle size distribution affects the stress distribution of the cathode active material during charge-discharge cycles, thus impacting the high-temperature cycling stability of the battery.

[0046] Since both the U-value and Q-value are affected by the D-value of LFP spanTo balance low-temperature fast charging performance and high-temperature cycling performance, D needs to be strictly adjusted. span The relationship between U and Q is controlled to keep it within an appropriate range. This application comprehensively controls D. span The relationship between U and Q satisfies 2.9 ≤ 0.3 × D span The ratio of ×Q / U ≤ 12.9 enables the secondary battery to maintain good high-temperature cycle life and significantly improves charging capability under low-temperature conditions.

[0047] When 0.3×D span When the value of ×Q / U exceeds the upper limit of 12.9, the ability of lithium insertion / extraction at the positive and negative electrodes cannot be significantly improved, and the low-temperature fast charging effect of the secondary battery is not significantly improved; when 0.3×D span When the value of ×Q / U exceeds the lower limit of 2.9, the secondary battery's ability to charge at high rates at low temperatures is insufficient, limiting the improvement of fast charging performance and potentially causing a decrease in the cycle life of the secondary battery.

[0048] For example, in this application, 0.3×D span The value of ×Q / U can be 2.9, 3.0, 4.0, 5.0, 7.0, 9.0, 10.0, 11.0, 12.0, 12.5, or 12.9, or it can be any range formed by any two of the above values.

[0049] In one embodiment, the secondary battery satisfies the following relationship: 5.5 ≤ 0.3 × D span ×Q / U≤8.8.

[0050] Within the range of the above-mentioned relationships, the fast charging capability of secondary batteries is relatively better.

[0051] In one embodiment, the range of U is 3.1 to 3.4V, for example, U can be 3.10V, 3.15V, 3.20V, 3.25V, 3.30V, 3.35V, or 3.40V.

[0052] In one alternative embodiment, the range of U is 3.20 to 3.25 V.

[0053] The potential (U) of the first-stage plateau primarily affects the actual current (I), resistance (R), and charging time (t) of the positive electrode active material during secondary battery charging. Specifically, if the U value is too low, the voltage at which the battery begins high-throughput lithium-ion extraction is extremely low. At this point, the oxidation reaction proceeds almost independently of the high-potential plateau reaction, resulting in a high actual reaction current density, significant battery polarization, and increased internal resistance (R). This is detrimental to the sustained reaction of the first-stage plateau, leading to a very small t value. This makes it impossible to generate heat during initial charging, hindering the improvement of battery operating temperature and reducing the effectiveness of fast charging at low temperatures. While a higher U value can improve battery heating to some extent, an excessively high U value may result in a higher charging voltage, delaying the start of heat generation and reducing the effectiveness of improving internal temperature rise. Furthermore, the first-stage plateau reaction may overlap with the high-potential plateau reaction, leading to uneven lithium extraction between plateaus, greater battery polarization, increased material internal resistance, and insufficient current. This again hinders heat generation and further impedes the improvement of fast charging performance. In addition, excessive polarization may lead to decreased battery stability and reduced cycle life.

[0054] In one embodiment, the range of Q is 10% to 40%, for example, Q can be 10%, 15%, 20%, 25%, 30%, 35%, or 40%.

[0055] In one alternative implementation, the range of Q is 20% to 30%.

[0056] Q refers to the proportion of charging capacity below the first-stage plateau to the total discharging capacity, also known as the first-stage plateau percentage. The value of Q mainly affects the current I and time t during battery charging and heating. A suitable plateau percentage can ensure a larger current I, thereby increasing the heat generated by the battery. When the Q value is too small, the first-stage plateau, as the lowest voltage plateau, has an excessively low plateau percentage. The low SOC plateau needs to withstand a large current density, resulting in insufficient initial high-rate charging capability and hindering the battery's self-heating. When the Q value is too large, the plateau percentage is too high. At low SOC, the battery's internal resistance is low. Although the internal resistance R is small, the low polarization leads to an insufficient temperature rise rate, making it difficult to raise the battery temperature.

[0057] In this application, the values ​​of U and Q can be obtained from the charging curve of the secondary battery.

[0058] Specifically, U and Q can be measured using the following methods:

[0059] (1) Capacity determination: The secondary battery is 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 is charged at a constant current and constant voltage of 0.33C to 4.25V, charged at a constant voltage to 0.05C, and discharged at a constant current of 0.33C to 2.5V; after repeating the above steps once, the capacity Q0 of the second cycle is obtained;

[0060] (2) Plot the charging curve: Charge the above secondary battery to 8% SOC by constant current charging at 0.33C, let it stand at 10℃ for 120min, and then charge it to the upper limit voltage of 4.25V by constant current charging at 1C to obtain the QV curve.

[0061] Based on the QV curve above, the voltage and energy data of the nth data point are subtracted from the voltage and energy data of the (n+1)th data point to obtain dV and dQ data. All data are processed sequentially 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.

[0062] (3) The peak position of the characteristic peak with the minimum voltage in the dQ / dV-V curve (i.e., the potential corresponding to the oxidation peak at the first low potential from low to high potential) is the potential value (U) of the first step plateau, in V.

[0063] The capacity corresponding to the characteristic peak with the minimum voltage in the dQ / dV-V curve is Q1. The proportion of the charging capacity corresponding to the first step plateau to the total charging capacity (Q) = Q1 / Q0 × 100%, where Q is in units of %.

[0064] The values ​​of U and Q are mainly influenced by the composition and structure of the positive electrode, especially by the positive electrode active material. For example, the mixing ratio of LMFP and LFP in the positive electrode active material, the particle coating of LFP, the mixing ratio of raw materials used in LFP preparation, the manganese content in LMFP, the particle morphology and size distribution of the positive electrode active material, and the crystal structure of the positive electrode active material all affect the values ​​of U and Q. The values ​​of U and Q can be controlled by adjusting these factors.

[0065] In one embodiment, the D span The range is 2.77 to 3.88, for example, the D... span The values ​​can be 2.77, 2.80, 2.85, 3.0, 3.2, 3.5, 3.7, 3.75, 3.80, 3.85, or 3.88.

[0066] In one alternative implementation, the D span The range is 2.82 to 3.28.

[0067] This application research found that, D span When the value is within the above selectable range, lithium iron phosphate exhibits suitable particle size distribution uniformity and can synergize well with LMFP. When D span A larger value indicates that lithium iron phosphate has a relatively wider particle size distribution and poorer particle size uniformity, which may lead to poor low-temperature fast charging performance of the battery; when D... span When the value is small, the particle size distribution of lithium iron phosphate is too concentrated, resulting in insufficient effective contact with LFMP particles. This has limited improvement on the battery's fast charging performance. Furthermore, due to the overly concentrated particle size distribution, the stress generated during high-temperature cyclic charging and discharging is difficult to release effectively, leading to a rapid decrease in battery capacity and poor high-temperature cycling performance.

[0068] In one embodiment, the lithium iron phosphate has a Dn90 particle size of 2.63–4.6 μm, a Dn50 particle size of 0.6–1.5 μm, and a Dn10 particle size of 0.3–0.45 μm.

[0069] When the particle size of lithium iron phosphate is within an appropriate range, it contributes to better overall high-temperature cycling and power performance of the battery. When the particle size is too small, it may cause an increase in side reactions, affecting the high-temperature cycling performance of the battery. When the particle size is too large, it may lead to poor kinetic performance of the battery and affect fast charging capability.

[0070] In this application, no limitation is made on the test method for the particle size of lithium iron phosphate. Those skilled in the art can detect the particle size of lithium iron phosphate using conventional technical means.

[0071] For example, the particle size of lithium iron phosphate can be detected using the following method:

[0072] 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. Perform SEM (scanning electron microscope) + EDS (energy dispersive X-ray spectroscopy) test. In the SEM observation field, identify the morphology of LFP particles by EDS and collect elements to confirm the lithium iron phosphate particles.

[0073] The size of lithium iron phosphate particles was measured using SEM images obtained through MEARSURE NANO software. The particle size was collected using the diagonal scribing method. After collecting more than 100 samples, the particle size distribution was statistically analyzed, and the particle size-related parameters of lithium iron phosphate, Dn10, Dn50, and Dn90, were calculated. span = (Dn90-Dn10) / Dn50.

[0074] In one embodiment, the mass ratio of lithium manganese iron phosphate to lithium iron phosphate is (2.3-9):1.

[0075] When the mass ratio of LMFP to LFP in the positive electrode active material is appropriate, it helps the secondary battery to have suitable voltage plateau characteristics and good particle contact ability. It also helps to improve the structural stability of the secondary battery, giving it a better cycle life during cycling.

[0076] In one embodiment, the manganese content of the lithium manganese iron phosphate is 60-90% of the transition metal elements.

[0077] This application does not limit the method for detecting the molar content of manganese as a transition metal element in lithium manganese iron phosphate. Those skilled in the art can detect the manganese content in lithium manganese iron phosphate using conventional technical means.

[0078] For example, the molar content of manganese as a transition metal element in lithium manganese iron phosphate can be detected by the following method:

[0079] The battery was disassembled to obtain the positive electrode sheet, which was then processed to obtain the positive electrode active material powder. The lithium manganese iron phosphate particles were confirmed by SEM (scanning electron microscope) and EDS (energy dispersive X-ray spectroscopy), and the molar content of manganese as a transition metal element in lithium manganese iron phosphate was obtained.

[0080] This application does not limit the preparation method of LMFP. Those skilled in the art can prepare LMFP using conventional techniques, such as hydrothermal or solid-state methods.

[0081] For example, a solid-state preparation method for LMFP may include the following steps:

[0082] Manganese, iron, phosphorus, lithium and carbon sources (if any) are mixed in a certain molar ratio and then ball-milled.

[0083] The ball-milled product was sintered in an atmosphere with an oxygen concentration of less than 150 ppm to obtain LMFP.

[0084] The preparation method of LFP is similar to that of LMFP, except that no manganese source is added.

[0085] For example, the hydrothermal preparation method of LMFP may include the following steps:

[0086] A lithium source, a manganese source, an iron source, a phosphorus source, and a carbon source (if any) are mixed, and the mixture is placed in a reactor for reaction. The product after reaction is sintered under an inert atmosphere to obtain LMFP.

[0087] The preparation method of LFP is similar to that of LMFP, except that no manganese source is added.

[0088] For the hydrothermal preparation method, the reaction conditions in the reactor can be: temperature: 200-300℃; time: 2-3h; pressure: 1.5-2.5MPa; the sintering conditions can be: temperature: 500-800℃; time: 5-15h.

[0089] 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.

[0090] The phosphorus source may include at least one of diammonium hydrogen phosphate, lithium dihydrogen phosphate, ammonium phosphate, or lithium phosphate.

[0091] 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 heptahydrate, ferric oxide, ferric oxide, or ferric oxalate.

[0092] The manganese source may include at least one of manganese carbonate, manganese sulfate, manganese sulfate monohydrate, manganese nitrate, manganese chloride, manganese oxalate, or manganese acetate.

[0093] The carbon source may include glucose and / or sucrose.

[0094] 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.

[0095] In the preparation of LMFP and / or LFP, a certain amount of dopant source can be mixed with manganese source (if any), iron source, phosphorus source, lithium source, etc., as needed. Dopant source sources include vanadium source (vanadium pentoxide), tungsten source (ammonium metatungstate), titanium source (titanium oxide), magnesium source (magnesium carbonate), etc., to obtain LMFP and / or LFP containing a certain amount of dopant element.

[0096] Optionally, the solvent may be at least one of water and ethanol.

[0097] The proportion of manganese content in LMFP can be controlled by adjusting the amount of manganese source, iron source, and phosphoric acid added.

[0098] In the preparation of LFP, the molar ratio of iron source, lithium source and phosphorus source can be 1:(1.001~1.05):(1.001~1.06); the amount of carbon source added can be 5~10 wt.% of the total weight of iron source, lithium source and phosphorus source.

[0099] In addition to the aforementioned positive electrode active material, the positive electrode active material layer may also contain conductive agents and binders.

[0100] 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.

[0101] 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.

[0102] 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.

[0103] The positive electrode sheet of this application 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.

[0104] In one embodiment, 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. The negative active material layer contains a negative active material, which includes at least one of natural graphite, artificial graphite, mesophase carbon microspheres, hard carbon, soft carbon, silicon, SiOx, silicon-carbon, and lithium titanate.

[0105] 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.

[0106] 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.

[0107] 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.

[0108] 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).

[0109] One embodiment of this application provides an electrical device comprising the aforementioned secondary battery. The secondary battery serves as the power supply for the electrical device.

[0110] 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.

[0111] The present application is further illustrated below with specific embodiments.

[0112] Example 1

[0113] Example 1 provides a lithium-ion battery, the preparation method of which is as follows:

[0114] (1) Preparation of positive electrode sheet

[0115] (1.1) Preparation of positive electrode active material

[0116] Preparation of LMFP: according to LiMn 0.75 Fe 0.25 Lithium hydroxide, manganese sulfate monohydrate, ferrous sulfate heptahydrate, and phosphoric acid were weighed out according to the molar ratio of Li, Mn, Fe, and P in the chemical formula of PO4 and mixed. The mixed material was placed in a reaction vessel and reacted at 250℃ and 2MPa pressure for 2.5h. After being taken out, it was sintered at 600℃ for 12h to obtain LMFP (denoted as LMFP-75).

[0117] Preparation of LFP: Ferrous oxalate was dispersed in deionized water, then H2O2 was added and stirred slowly, followed by the addition of Li2CO3 and H3PO4, and a carbon source (sucrose) was added. The mixed material was placed in a reactor and reacted at 220℃ and 2.2MPa pressure for 3 hours to obtain the LFP precursor.

[0118] The molar ratio of ferrous oxalate, lithium carbonate (Li₂CO₃), and phosphoric acid was 1:1.03:1.04; the amount of carbon source added accounted for 7.9 wt.% of the total mass of ferrous oxalate, lithium carbonate, and phosphoric acid.

[0119] The LFP precursor was sintered in an Ar atmosphere under the following conditions: temperature 650℃, time 10h. After sintering, it was crushed and sieved to obtain LFP with particle size distribution coefficients that meet the requirements of Table 2.

[0120] (1.2) Mix the above LMFP and LFP evenly at a mass ratio of 8:2 to obtain the positive electrode active material;

[0121] 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 sheet.

[0122] (2) Preparation of negative electrode sheet

[0123] 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.

[0124] (3) Preparation of electrolyte

[0125] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 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 mol / L.

[0126] (4) Preparation of the diaphragm

[0127] A polyethylene (PE) diaphragm is used.

[0128] (5) Battery manufacturing

[0129] The prepared positive electrode, separator, and negative electrode are wound to obtain a bare cell without electrolyte filling; the bare cell and the cover plate assembly are electrically connected by the adapter piece, the bare cell is inserted into the shell, the cover plate and the shell are welded together to assemble a battery; the prepared electrolyte is injected into the dried bare cell, and after vacuum sealing, standing, formation, shaping and sorting, a lithium-ion battery is obtained.

[0130] Examples 2-14 and Comparative Examples 1-4

[0131] Examples 2-14 and Comparative Examples 1-4 each provide a lithium-ion battery, the preparation method of which is basically the same as that of Example 1, the difference being:

[0132] (1.1) In the preparation of LFP: the molar ratio of ferrous oxalate, lithium carbonate and phosphoric acid is shown in Table 1, the types and amounts of carbon sources are shown in Table 1, and the sintering conditions are shown in Table 1.

[0133] (1.2) LMFP and LFP are mixed evenly according to the mass ratio shown in Table 1.

[0134] also:

[0135] The differences between the preparation methods of lithium-ion batteries in Examples 5, 8, and 12 and Example 1 also include:

[0136] In Examples 5 and 8, LMFP was prepared according to the LiMn... 0.80 Fe 0.20 Lithium hydroxide, manganese sulfate monohydrate, ferrous sulfate heptahydrate, and phosphoric acid were weighed out according to the molar ratio of Li, Mn, Fe, and P in the chemical formula of PO4 and mixed. The mixed material was placed in a reaction vessel and reacted at 200℃ and 3MPa pressure for 3 hours. After being taken out, it was sintered at 580℃ for 10 hours to obtain LMFP (denoted as LMFP-80).

[0137] In Example 12, the preparation of LMFP was carried out according to the LiMn... 0.85 Fe 0.15Lithium hydroxide, manganese sulfate monohydrate, ferrous sulfate heptahydrate, and phosphoric acid were weighed out according to the molar ratio of Li, Mn, Fe, and P in the chemical formula of PO4 and mixed. The mixed material was placed in a reaction vessel and reacted at 300℃ and 2MPa pressure for 2 hours. After being removed, it was sintered at 550℃ for 12 hours to obtain LMFP (denoted as LMFP-85).

[0138] The differences between the lithium-ion battery preparation methods in Example 7 and Example 1 also include:

[0139] In the preparation of the negative electrode sheet, the negative electrode active material is a mixture of silicon-carbon material and artificial graphite, wherein the silicon-carbon material accounts for 15 wt.% of the negative electrode active material, and the silicon element in the silicon-carbon material accounts for 45 wt.%.

[0140] The differences between the lithium-ion battery preparation methods in Example 12 and Example 1 also include:

[0141] In the preparation of the negative electrode sheet, the negative electrode active material is lithium titanate (Li4Ti5O). 12 ).

[0142] Comparative Example 5

[0143] Comparative Example 5 provides a lithium-ion battery, which is prepared in a manner that is basically the same as that in Example 1, except that LFP is not prepared in the preparation of the positive electrode active material, that is, the positive electrode active material is composed only of LMFP-75.

[0144] Table 1

[0145] For the lithium-ion batteries prepared in each embodiment and comparative example, the potential value (U) of the first step plateau, the proportion of the charging capacity below the first step plateau to the total discharge capacity (Q), and the D of LFP in the positive electrode active material are given. span The test was conducted, and the results are shown in Table 2.

[0146] The QV curve and dQ / dV-V curve of the lithium-ion battery in Example 11 are shown in Figure 1 and Figure 2, respectively.

[0147] Table 2

[0148] *: In Comparative Example 5, since LFP is not present, there is also no D for LFP. span Therefore, 0.3 × D is not calculated. span The value of ×Q / U; its positive electrode active material consists of only a single LFMP-75, and the potential value (U value) of the first step plateau is higher than that of other embodiments and comparative examples.

[0149] The performance of lithium-ion batteries at low temperatures and their cycle life were tested using the following methods:

[0150] (1) Low-temperature fast charging performance:

[0151] At 25°C, the lithium-ion battery is discharged to 2.5V at a constant current of 0.33C, then charged to 4.25V at a constant current of 0.33C, charged to 0.05C at a constant voltage, and discharged to 2.5V at a constant current of 0.33C. After repeating the above steps once, the capacity Q0 of the second cycle is obtained.

[0152] The lithium-ion battery was charged to 8% SOC by constant current charging at 0.33C, and then left to stand at 10℃ for 120 minutes. After that, it was charged with constant current at 1C, and the cutoff upper limit voltage was 4.25V to obtain the capacity Q2 of constant current charging at 1C.

[0153] 1C Lithium Intercalation SOC = (Q2 / Q0 × 100% + 8%)SOC;

[0154] The 1C lithium intercalation SOC value reflects the lithium intercalation capability of a lithium-ion battery under low-temperature fast charging conditions. The higher the 1C lithium intercalation SOC value, the stronger the lithium intercalation capability and the better the battery's low-temperature fast charging performance.

[0155] (2) High-temperature cycle life

[0156] The lithium-ion battery was left to stand at 45°C for 120 minutes, then fully charged at 1C constant current and constant voltage with a cutoff voltage of 4.25V and a cutoff current of 0.33C. After standing for 10 minutes, it was discharged at 1C constant current to 2.5V. This constitutes one cycle. 200 cycles (200 rounds) of charge and discharge were performed. The high-temperature cycle capacity retention rate was obtained by dividing the discharge capacity of the 200th round by the full 1C discharge capacity of the 2nd round.

[0157] The cover plate temperature rise of the lithium-ion batteries prepared in Examples 1, 2, and Comparative Examples 1 and 2 was tested using the following method:

[0158] At 25°C, the lithium-ion battery is discharged to 2.5V at a constant current of 0.33C, then charged to 4.25V at a constant current of 0.33C, charged to 0.05C at a constant voltage, and discharged to 2.5V at a constant current of 0.33C. After repeating the above steps once, the capacity Q0 of the second cycle is obtained.

[0159] Connect a temperature sensor to the cover plate of the above-mentioned lithium-ion battery cell; charge the lithium-ion battery at a constant current of 0.33C to adjust the charge to 8% SOC, and after standing at 10℃ for 120 minutes, record the temperature of the cell cover plate as T1℃; charge the lithium-ion battery at a constant current of 1C, with a cutoff upper limit voltage of 4.25V, and record the highest temperature of the cell cover plate during the charging process as T2℃; cover plate temperature rise = T2 - T1.

[0160] Tests showed that the cover temperature rise of the lithium-ion batteries in Examples 1 and 2 was 6.6°C and 5.5°C, respectively; while the cover temperature rise of the lithium-ion batteries in Comparative Examples 1 and 2 was 2.0°C and 1.5°C, respectively. It can be seen that the cover temperature rise of the lithium-ion batteries prepared in these examples was improved to some extent during charging.

[0161] The test results for 1C lithium intercalation SOC and cycle life are shown in Table 3.

[0162] Table 3

[0163] According to the test results in Table 3, it can be seen that the lithium-ion batteries prepared in each embodiment of this application have good low-temperature fast charging performance and high-temperature cycling stability, with a 1C lithium intercalation SOC ≥ 61% and a capacity retention rate ≥ 84% after 200 cycles at 45°C.

[0164] According to Examples 1-9, when 0.3×D span When the value of ×Q / U is between 5.5 and 8.8, the 1C lithium-ion battery exhibits relatively higher lithiation state of charge (SOC) and high-temperature cycle capacity retention. According to Examples 1-7, when D... span When the voltage can optionally meet 2.82–3.28, Q can optionally meet 20–30%, and U can optionally meet 3.2–3.25V, it helps lithium-ion batteries to have both better low-temperature fast charging performance and high-temperature cycle stability.

[0165] According to comparative examples 1-4, when the lithium-ion battery is 0.3×D span When the value of ×Q / U exceeds the scope of the technical solution in this application, its low-temperature fast charging performance deteriorates severely, and the 1C lithium intercalation SOC does not exceed 50%; furthermore, the lithium-ion batteries in Comparative Examples 2 to 4 have poor high-temperature cycle performance, with a capacity retention rate of no more than 76.2% after 200 cycles at 45°C. In Comparative Example 5, the positive electrode active material does not contain LFP, and the battery's fast charging capability is very poor, with a 1C lithium intercalation SOC of only 41.2%.

[0166] 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 characterized by comprising: The device includes 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, which includes lithium manganese iron phosphate and lithium iron phosphate. The secondary battery was charged at 10°C at a 1C rate within a voltage range of 2.5–4.25V to obtain a charging curve; the charging plateau with the lowest potential in the charging curve is the first step plateau. The secondary battery satisfies the following relational expression: 2.9 ≤ 0.3 x D span x Q / U ≤ 12.9; wherein the D span is a particle size distribution coefficient of the lithium iron phosphate; U is the potential value of the first step platform, in V; Q represents the proportion of the charging capacity corresponding to the first-step platform to the total discharging capacity, expressed in percentage (%).

2. The secondary battery according to claim 1, characterized by The secondary battery satisfies the following relational expression: 5.5 ≤ 0.3 x D span x Q / U ≤ 8.

8.

3. The secondary battery according to claim 1 or 2, characterized by The range of U is 3.1 to 3.4 V.

4. The secondary battery according to claim 3, characterized in that, The range of U is 3.2 to 3.25 V.

5. The secondary battery according to claim 1 or 2, characterized in that, The range of Q is 10% to 40%.

6. The secondary battery according to claim 5, characterized in that, The range of Q is 20% to 30%.

7. The secondary battery according to claim 1 or 2, characterized in that, The D span ranges from 2.77 to 3.

88.

8. The secondary battery according to claim 7, characterized in that, The D span ranges from 2.82 to 3.

28.

9. The secondary battery according to claim 1, characterized in that, The mass ratio of lithium manganese iron phosphate to lithium iron phosphate is (2.3~9):

1.

10. The secondary battery according to claim 1, characterized in that, 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. The negative active material layer contains a negative active material, which includes at least one of natural graphite, artificial graphite, mesophase carbon microspheres, hard carbon, soft carbon, silicon, SiOx, silicon-carbon, and lithium titanate.

11. An electrical appliance, characterized in that, It includes the secondary battery as described in any one of claims 1 to 10.

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