Secondary battery and electric apparatus

Abstract

The present application relates to the technical field of electrochemistry, and specifically discloses a secondary battery and an electric apparatus. The secondary battery in the present application comprises a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte, wherein positive active materials of the positive electrode sheet contain a phosphate material; performing a charging test on the secondary battery at 25°C and at a rate of 2 C, so as to obtain a charging curve, wherein the potential difference between two low potentials is denoted as U; when the potential of the secondary battery is at the lowest potential plateau, a lithium-ion diffusion coefficient corresponding to the negative electrode sheet is r; and the secondary battery satisfies the following relational expression: 0.06≤U×r≤0.189. In the present application, by means of comprehensively regulating the potential difference between two low-potential charging plateaus of a secondary battery under 2 C charging conditions and the lithium-ion diffusion coefficient of a negative electrode sheet under specific conditions, the fast charging performance of the secondary battery is improved, and the charging time of the secondary battery is shortened, thereby prolonging the fast charging cycle life.

Description

Secondary batteries and electrical appliances

[0001] Cross-references to related applications

[0002] This application claims priority to Chinese Patent Application No. 202411641272.X, 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] Lithium-ion batteries are widely used in many fields such as 3C electronic products, electric vehicles and energy storage power stations due to their high energy density, low self-discharge, no memory effect and long cycle life. They are currently a research hotspot in new energy storage and conversion systems.

[0005] Cathode materials have a significant impact on battery performance and cost. Among them, lithium manganese iron phosphate (LMFP), with its olivine structure, inherits the olivine structure of phosphate materials and possesses safety performance comparable to lithium iron phosphate (LFP). Furthermore, the addition of manganese significantly increases the reaction potential of LFP. Therefore, in recent years, LFP has gained an increasingly larger market share in the power battery market.

[0006] However, during battery use, LMFP cathode materials exhibit some limitations during fast charging. Initially, the positive electrode potential is high, but during high-rate charging, the rapid rise in positive electrode potential further drives a rapid drop in negative electrode potential. This rapid potential change leads to a significant shortening of the high-rate charging phase, which is detrimental to battery charging efficiency and prolongs the overall charging time.

[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 power device. By comprehensively controlling the potential difference between the two charging platforms of the secondary battery at low SOC under 2C charging conditions and the lithium ion diffusion coefficient of the corresponding negative electrode when the battery potential is at the first platform, the charging time of the battery is shortened and the fast charging cycle life is improved. The secondary battery has excellent fast charging capability.

[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 a phosphate material.

[0010] A charging curve was obtained by charging the secondary battery at 25°C at a 2C rate within a voltage range of 2.5–4.25V. The charging curve includes three charging plateaus with different potentials, which are sequentially labeled as the first plateau, the second plateau, and the third plateau in ascending order of potential. The potential difference between the first and second plateaus is denoted as U, with the unit of measurement being V.

[0011] When the potential of the secondary battery is at the first plateau, the lithium-ion diffusion coefficient of the corresponding negative electrode is r, measured in units of ×10⁻¹⁰. -8 cm 2 / s;

[0012] The secondary battery satisfies the following relationship: 0.06≤U×r≤0.189.

[0013] As an optional implementation of this application, the secondary battery satisfies the following relationship: 0.124≤U×r≤0.152.

[0014] As an optional implementation of this application, the range of U is 0.07 to 0.19V.

[0015] As an optional implementation of this application, the range of U is 0.13 to 0.16 V.

[0016] As an optional implementation of this application, the range of r is 0.7 × 10⁻⁶. -8 ~1.28×10 -8 cm 2 / s.

[0017] As an optional implementation of this application, the range of r is 0.9 × 10⁻⁶. -8 ~1.05×10 -8 cm 2 / s.

[0018] As an optional implementation of this application, the phosphate material includes lithium manganese iron phosphate and lithium iron phosphate.

[0019] As an optional embodiment of this application, the Dn50 particle size of the lithium iron phosphate is 0.6 to 1.5 μm.

[0020] As an optional embodiment of this application, the Dn50 particle size of the lithium manganese iron phosphate is 60-200 nm.

[0021] As an optional embodiment of this application, the mass ratio of lithium manganese iron phosphate to lithium iron phosphate is (1.5~19):1.

[0022] As an optional embodiment of this application, the mass ratio of lithium manganese iron phosphate to lithium iron phosphate is (2-6):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. By comprehensively controlling the potential difference between the two charging platforms with low potentials under 2C charging conditions and the lithium-ion diffusion coefficient of the negative electrode under specific conditions, this application improves the fast-charging performance of the secondary battery, shortens the charging time, and increases the fast-charging cycle life. Attached Figure Description

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

[0028] Figure 2 is a dQ / dV-V curve of the lithium-ion battery in Example 1. 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 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 phosphate material.

[0035] A charging curve was obtained by charging the secondary battery at 25°C at a 2C rate within a voltage range of 2.5–4.25V. The charging curve includes three charging plateaus with different potentials, which are sequentially labeled as the first plateau, second plateau, and third plateau in ascending order of potential. The potential difference between the first and second plateaus is denoted as U, with the unit of measurement being V.

[0036] When the potential of the secondary battery is at the first plateau, the lithium-ion diffusion coefficient of the corresponding negative electrode is r, measured in units of ×10⁻¹⁰. -8 cm 2 / s;

[0037] The secondary battery satisfies the following relationship: 0.06≤U×r≤0.189.

[0038] In fast-charging technology for secondary batteries, the charging performance of the battery is closely related to the material properties of the electrode materials and the working state of the electrode plates. This study found that, under 2C charging conditions, the key to improving the fast-charging performance of secondary batteries lies in the comprehensive regulation of the potential difference (U) between the two lower charging plateaus of the secondary battery and the lithium-ion diffusion coefficient (r) of the corresponding negative electrode plate when the battery potential is at the first plateau.

[0039] The fast charging mechanism of a battery employs a high charging rate initially, gradually decreasing it until fully charged. During the initial charging phase, the negative electrode exhibits optimal lithium intercalation capability, allowing for the insertion of a large number of lithium ions. Therefore, a high charging rate can be used in the early stages. When the potential difference U is appropriate, the polarization of the positive electrode can be improved during the high-rate charging phase, effectively suppressing the excessively rapid drop in the negative electrode voltage. This ensures sufficient high-rate charging time, reduces the overall charging time, and improves fast charging performance.

[0040] The lithium-ion expansion coefficient r of the negative electrode reflects its lithium intercalation capability. When U and r satisfy the relationship 0.06≤U×r≤0.189, the battery's fast-charging capability is significantly improved, resulting in not only a relatively higher fast-charging rate but also better cycle life under fast-charging conditions.

[0041] When U×r exceeds 0.189, the potential difference at the low potential plateau of the battery is too large, and the lithium-ion diffusion coefficient of the negative electrode is too high. In the initial stage of charging, when the battery has low remaining charge (i.e., low SOC) and a high charging rate, the increased current density causes a large number of lithium ions to accumulate on the surface of the negative electrode, hindering lithium intercalation in the bulk phase and resulting in poor fast charging speed. This lithium-ion accumulation not only affects normal lithium intercalation in the negative electrode but may also cause structural damage to the negative electrode material, thus affecting the battery's stability and cycle life. Furthermore, at high SOC, the positive electrode active material withstands a higher current density. This high load exacerbates the polarization of the positive electrode, making it difficult for lithium ions to escape, thereby prolonging the fast charging time. This not only reduces the battery's charging efficiency but may also increase the thermal risk during high-rate charging and decrease the battery's fast charging cycle life.

[0042] When U×r is below 0.06, the potential difference at the low-potential plateau of the battery is too small, which cannot effectively improve the battery's fast-charging capability. A low lithium-ion diffusion coefficient at the battery's negative electrode may mean that the charge transfer impedance (Rct) of the negative electrode sheet is too large at the first plateau potential. This increased impedance restricts the diffusion ability of lithium ions at the negative electrode interface, making it difficult for lithium ions to be effectively intercalated at the negative electrode. Consequently, at the initial low SOC stage of charging, the high-rate charging time is shortened, the overall charging rate of the battery deteriorates, and the charging time is prolonged.

[0043] For example, in this application, the value of U×r can be 0.060, 0.070, 0.080, 0.090, 0.095, 0.100, 0.110, 0.150, 0.180, or 0.189.

[0044] In one embodiment, the secondary battery satisfies the following relationship: 0.124≤U×r≤0.152.

[0045] When U and r are within the above range, the secondary battery has better fast charging capability, shorter fast charging time, and relatively longer fast charging cycle life.

[0046] For the secondary battery of this application, the potential of the first platform is generally 3.4 to 3.6V, and the potential of the second platform is generally 3.6 to 3.8V.

[0047] In one embodiment, the range of U is 0.07 to 0.19 V.

[0048] In one alternative implementation, the range of U is 0.13 to 0.16 V.

[0049] This study found that when U is within a suitable range, the battery exhibits superior overall fast-charging capability and cycle performance. When U is too large, it indicates an excessively large potential difference between the first and second plateaus. Fast charging at a high SOC level leads to increased electrode polarization, resulting in prolonged charging time. This is especially true when the second plateau potential is too high, causing a rapid drop in the negative electrode potential, leading to poor fast-charging performance. Furthermore, a large U value may indicate an excessively low first plateau potential. For LMFP batteries, the loss of active lithium at the negative electrode is a major factor in cycle capacity decay, resulting in capacity decay at the low-potential positive electrode plateau at the end of discharge. The lower the first plateau potential, the greater the risk of decay, and the lower the battery's cycle capacity retention. When U is too small, the potential difference between the first and second plateaus is too small. Charging at a low SOC level causes the corresponding negative electrode potential to drop rapidly as the positive electrode potential rises, reaching the cutoff potential. This results in a very short period of high-rate charging, with little improvement in the battery's fast-charging rate.

[0050] The value of U is related to factors such as the type, composition, morphological characteristics, and electrochemical properties of the phosphate materials in the positive electrode active material. The value of U can be adjusted by controlling the chemical element composition and ratio of the phosphate materials in the positive electrode active material, as well as the particle morphology, crystal structure, and processing technology.

[0051] In this application, the value of U can be obtained from the charging curve of the secondary battery.

[0052] Specifically, U can be measured using the following methods:

[0053] (1) Capacity setting: 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 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 charging capacity Q0 of the second cycle is obtained;

[0054] (2) Plot the charging curve: After standing at 25℃ for 120 min, charge at a constant current of 2C to the upper limit voltage of 4.25V to obtain the charging capacity Q and voltage V data;

[0055] (3) Perform derivative calculation on the above QV data, and then divide dQ by dV to obtain dQ / dV; plot the dQ / dV-V curve with dQ / dV as the vertical axis and voltage V as the horizontal axis.

[0056] (4) In the dQ / dV-V curve, the peak position of the characteristic peak of the horizontal axis voltage in the range of 3.4V-3.6V is the potential U1 of the first plateau, and the peak position of the characteristic peak of the horizontal axis voltage in the range of 3.6V-3.8V is the potential U2 of the second plateau; the potential difference between the first plateau and the second plateau is U = U2-U1, and the unit is V.

[0057] In one embodiment, the range of r is 0.7 × 10⁻⁶. -8 ~1.28×10 -8 cm 2 / s.

[0058] In one embodiment, the range of r is 0.9 × 10⁻⁶. -8 ~1.05×10 -8 cm 2 / s.

[0059] During the charging process of a lithium-ion battery, the lithium-ion diffusion coefficient (r) of the negative electrode at the first plateau potential directly affects the battery's charging efficiency and stability. The value of r reflects the transport and insertion speed of lithium ions in the negative electrode. When r is within a suitable range, the negative electrode exhibits good lithium-ion transport and insertion / extraction characteristics, and the solid electrolyte interphase (SEI) film on the surface of the negative electrode has good stability, resulting in superior fast charging rate and fast charging cycle performance. When the value of r is too large, exceeding the above-mentioned selectable range, the transport of lithium ions in the bulk phase of the negative electrode is very rapid. At this time, the layered structure of the negative electrode is rapidly "expanded," exposing a large number of active sites, and easily causing SEI film rupture, inducing severe interfacial side reactions, and deteriorating high-temperature stability. When the value of r is large, although the transport rate of lithium ions in the bulk phase is improved, the limitation on lithium insertion in the negative electrode is not only the bulk phase but also the interface. Therefore, at this time, the battery's fast charging performance will not be significantly improved, and when the bulk phase transport is fast, the exposure of active sites caused by structural expansion may induce interfacial side reactions, which may actually reduce interfacial transport and reduce fast charging performance. When the r value is too small, below the above selectable range, the diffusion rate of lithium ions inside the negative electrode is slow, resulting in increased polarization of the negative electrode. The active lithium at the interface does not have enough time to be inserted into the bulk phase, which leads to the risk of lithium plating, deterioration of fast charging performance, and extension of charging time.

[0060] The value of r is affected by various factors, including the negative electrode active material, the design of the negative electrode, and the electrolyte. The value of r can be controlled to meet the scope of this application by adjusting the type, raw materials, processing technology, morphology of the negative electrode active material, and the content of each component in the negative electrode active material layer. This application does not limit the detection method for r; those skilled in the art can use conventional techniques, such as electrochemical impedance spectroscopy (EIS), to detect the ion diffusion coefficient of the negative electrode at a specific first plateau potential.

[0061] For example, r can be detected using the following method:

[0062] (1) Disassemble the positive and negative electrode plates in the secondary battery and reassemble them into a single battery. The separator used is a PE separator. There are no special restrictions on the electrolyte composition. The amount of electrolyte injected should be enough to ensure that there is free electrolyte when the battery is charged to the upper limit voltage. Fix a copper wire on the negative electrode side as the third electrode.

[0063] (2) After the single battery cell is assembled, it is left to stand at 25°C for 120 minutes. It is then charged at a constant current of 0.03C to 4.25V, discharged at a constant current of 0.3C to 2.5V, charged at a constant current of 0.33C to 4.25V, charged at a constant voltage to 0.05C, left to stand at 25°C for 10 minutes, and discharged at a constant current of 0.33C to 2.5V. This process is repeated twice. Then, lithium is plated onto the third electrode using the positive electrode at 0.01C for 6 hours. After lithium plating, the charge is adjusted to the voltage of the first plateau at 0.33C. After standing at 25°C for 30 minutes, EIS testing is continued. The negative electrode and the third electrode are monitored. The testing frequency is 0.01~10. 6 Hz;

[0064] (3) After the test, the EIS data is obtained, the low-frequency slant region is selected, and ω is plotted. -0.5 -The real part of the impedance Z' curve is taken as the slope σ of the straight line in the lowest frequency region; substituting this slope into the formula: DLi + =(R 2 T 2 ) / (2A 2 n 4 F 4 C 2 σ 2 ) to obtain DLi + This is the lithium-ion diffusion coefficient (r); where R is the gas constant (J / mol / K), T is the absolute reaction temperature (K), and A is the area (cm²) of the negative electrode material immersed in the electrolyte. 2 n is the number of electrons in the reaction, F is the Faraday constant (C / mol), and C is the lithium ion molar concentration (mol / cm³). 3 .

[0065] In one embodiment, the phosphate material includes lithium manganese iron phosphate and lithium iron phosphate.

[0066] The phosphate materials used in this application include a blend of lithium iron phosphate (LFP) and lithium manganese iron phosphate (LMFP). LFP and LMFP have different material properties, especially different charge and discharge characteristics. By rationally blending LFP and LMFP, the secondary battery of this application can achieve better fast charging capability.

[0067] During the initial stages of fast charging, the decrease in negative electrode potential is not only affected by lithium-ion intercalation and the interfacial and bulk impedance, but also by the oxidation reaction of the positive electrode at high potentials, which further exacerbates the decrease. When phosphate materials, including LFP and LMFP, are blended, the incorporated LFP preferentially undergoes delithiation over LMFP, forming a lower potential oxidation plateau, thereby mitigating the decrease in negative electrode potential and increasing the lithium-ion intercalation space at the negative electrode. Furthermore, the dissolution of positive electrode metal and its deposition at the negative electrode catalyzes the formation of the SEI film, affecting the interfacial impedance of the negative electrode. This study found that the appropriate amount of manganese dissolution from LMFP can reduce the impedance of the SEI film at the negative electrode. Therefore, the blending of LFP and LMFP contributes to better fast-charging capability of rechargeable batteries.

[0068] In one embodiment, the mass ratio of lithium manganese iron phosphate to lithium iron phosphate is (1.5-19):1.

[0069] In one alternative embodiment, the mass ratio of lithium manganese iron phosphate to lithium iron phosphate is (2-6):1.

[0070] Within the above selection range, the combination of LFP and LMFP is more effective, significantly improving the fast charging capability of secondary batteries, and achieving a more balanced improvement in fast charging speed and fast charging cycle life.

[0071] In one embodiment, the Dn50 particle size of the lithium iron phosphate is 90–200 nm.

[0072] In one alternative embodiment, the Dn50 particle size of the lithium iron phosphate is 150–180 nm.

[0073] In one embodiment, the Dn50 particle size of the lithium manganese iron phosphate is 50-180 nm.

[0074] In one embodiment, the Dn50 particle size of the lithium manganese iron phosphate is 90-120 nm.

[0075] The particle size of LFP and LMFP serves as a morphological characteristic of the cathode active material, influencing not only the U-value but also the overall kinetic performance and stability of the battery. A suitable blend of LFP and LMFP particles helps shorten the lithium-ion transport path and ensures an appropriate contact area between the cathode active material and the electrolyte, improving battery kinetic performance while minimizing interfacial side reactions. Furthermore, during repeated charge-discharge cycles, the cathode active material particles undergo some degree of volume expansion and contraction, especially under fast-charging conditions, where the internal stress is relatively greater. Blending materials with specific particle sizes can improve the mechanical stability of the cathode active material and enhance its fast-charging cycle life.

[0076] The particle size of lithium iron phosphate and lithium manganese iron phosphate can be controlled by adjusting the selection of raw materials and the preparation process.

[0077] In this application, the method for testing the particle size of lithium iron phosphate and lithium manganese iron phosphate is not limited. Those skilled in the art can detect the particle size of lithium iron phosphate and lithium manganese iron phosphate using conventional technical means.

[0078] For example, the particle size of lithium iron phosphate and lithium manganese iron phosphate can be detected using the following methods:

[0079] 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.

[0080] The scraped positive electrode active material powder was uniformly dispersed in ethanol and tested by SEM (scanning electron microscope). Elemental collection was performed in the SEM field of view by EDS (energy dispersive X-ray spectroscopy) spot scanning to confirm lithium iron phosphate particles and lithium manganese iron phosphate particles.

[0081] The size of lithium iron phosphate and lithium manganese iron phosphate particles was measured using MEARSURE NANO software on SEM images. The particle size was collected using the diagonal line method. After collecting more than 100 samples, the particle size distribution was statistically analyzed, and the Dn50 particle size of lithium iron phosphate and lithium manganese iron phosphate was calculated. Dn50 is the particle size corresponding to the cumulative number of particles reaching 50% in the baseline distribution.

[0082] This application does not limit the preparation method of LMFP. Those skilled in the art can prepare LMFP using conventional techniques.

[0083] For example, a method for preparing LMFP may include the following steps:

[0084] Manganese, iron, phosphorus, lithium and carbon sources (if any) are mixed in a certain molar ratio and dispersed in a solvent to carry out the reaction and obtain a mixed solution.

[0085] The above mixture was spray-dried to obtain a powder.

[0086] The powder after spray drying was sintered in an atmosphere with an oxygen concentration of less than 150 ppm.

[0087] The sintered material is crushed and then screened to obtain LMFP.

[0088] Specifically, the preparation method of LMFP may include the following steps:

[0089] Manganese, iron, phosphorus, and lithium sources are mixed in a certain molar ratio and dispersed in deionized water, with the reaction ion concentration controlled at 1–8 M; then a carbon source is added, and the reaction is carried out to obtain a mixed solution.

[0090] The above mixture was spray-dried to obtain a powder.

[0091] The powder after spray drying was sintered in an atmosphere with an oxygen concentration of less than 150 ppm.

[0092] The sintered material is crushed and then screened to obtain LMFP.

[0093] For example, a method for preparing LFP may include the following steps:

[0094] Iron source, phosphorus source, lithium source and carbon source (if any) are mixed in a certain molar ratio and then subjected to mixing and sand milling.

[0095] The product after the above mixture was sand-milled was sintered in an atmosphere with an oxygen concentration of less than 150 ppm.

[0096] The sintered material is crushed and then screened to obtain LFP.

[0097] Optionally, in the LFP preparation method, after the sintering step, a finished product sand milling step can be added as needed.

[0098] Specifically, the preparation method of LFP may include the following steps:

[0099] Iron source, phosphorus source, lithium source and carbon source (if any) are mixed in a certain molar ratio and then subjected to mixing and sand milling; the linear velocity of the mixing and sand milling is 3 to 10 m / s and the time is 2 to 7 h.

[0100] The product after the above sand milling was sintered in an atmosphere with an oxygen concentration of less than 150 ppm;

[0101] The product after the above mixture was sand-milled was sintered in an atmosphere with an oxygen concentration of less than 150 ppm.

[0102] The sintered material is then subjected to a finished product sand mill with a linear velocity of 8–15 m / s and a time of 2–6 h.

[0103] The product after the finished sand mill is crushed and then screened to obtain LFP.

[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] 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.

[0111] The proportion of manganese content in LMFP can be controlled by adjusting the amount of manganese source, iron source, and phosphoric acid added.

[0112] The particle size of LMFP or LFP can be controlled by adjusting the amount and type of carbon source, ball milling conditions, sintering conditions, or by changing crushing conditions and controlling screening conditions.

[0113] In addition to the aforementioned positive electrode active material, the positive electrode active material layer may also contain conductive agents and binders.

[0114] 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.

[0115] 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.

[0116] 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.

[0117] 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.

[0118] 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.

[0119] 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.

[0120] 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.

[0121] 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.

[0122] 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).

[0123] One embodiment of this application provides an electrical device comprising the secondary battery described above.

[0124] The electrical device serves as the power source for the electrical device.

[0125] 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.

[0126] The present application is further illustrated below with specific embodiments.

[0127] Example 1

[0128] Example 1 provides a lithium-ion battery, the preparation method of which is as follows:

[0129] (1) Preparation of positive electrode sheet

[0130] (1.1) Preparation of positive electrode active material

[0131] Preparation of LMFP: according to LiMn a Fe b In the chemical formula PO4, the molar ratios of Li, Mn, Fe, and P (a / b = 72 / 25) were used to weigh out lithium hydroxide, manganese sulfate, ferric chloride, and potassium dihydrogen phosphate, which were then added to deionized water and stirred until completely dissolved. The amount of deionized water was adjusted to control the reaction ion concentration at 1.5 M. The temperature was then raised to 65 °C and maintained for 3 hours at pH 7 to obtain the reaction precursor. The reaction products, LiMn, were then processed... a Fe b 20 wt.% of glucose was weighed from the total PO4 and added to the above solution. The mixture was then stirred and dispersed at 500 rpm for 2 hours. The solution was spray-dried to obtain a dry powder. This powder was then sintered in a tube furnace under a protective atmosphere, starting at room temperature and uniformly increasing to 400°C at a rate of 5°C / min. After holding at this temperature for 5 hours, the temperature was further increased to 600°C and held for 3 hours, before being allowed to cool to room temperature. After grading and screening, LMFP with a particle size Dn50 of 122 nm was obtained, which is the final product LMFP.

[0132] Preparation of LFP: Ferrous oxalate, diamine hydrogen phosphate, and lithium carbonate were weighed and mixed using a Li:Fe:P molar ratio of 1:1:1. Simultaneously, sucrose (10 wt.% of the LFP product weight) was added to the mixture and milled again. The milling process was carried out at a linear speed of 7 m / s for 5 hours. The milled material was then sintered under a protective atmosphere in a tube furnace, starting at room temperature and uniformly heated to 250°C at a rate of 5°C / min. After holding at this temperature for 2 hours, the temperature was further increased to a sintering plateau of 700°C and held at this plateau for 6 hours before being allowed to cool to room temperature. The resulting material was then milled to a finished product size at a linear speed of 10 m / s for 4 hours. After grading and screening, LFP with a particle size Dn50 of 320 nm was obtained, which is the final product, LFP.

[0133] (1.2) LMFP and LFP are mixed at a mass ratio of 2.6:1 to obtain the positive electrode active material;

[0134] The positive electrode active material, binder (PVDF), conductive agent (SP), and conductive agent (CNT) are mixed evenly in NMP at a mass ratio of 96:2:1.5:0.5. Then, the mixed positive electrode slurry is evenly coated on aluminum foil, dried in a vacuum furnace at 100°C, and then rolled to obtain the positive electrode sheet.

[0135] (2) Preparation of negative electrode sheet

[0136] The negative electrode active material (artificial graphite) is mixed with binder (carboxymethyl cellulose, CMC) and conductive agent (SP) at a mass ratio of 96:2.5:1.5 and dispersed in deionized water to form a negative electrode slurry. The negative electrode slurry is then uniformly coated onto the negative electrode current collector (copper foil). The negative electrode current collector coated with the negative electrode slurry is transferred to a vacuum environment in an oven and dried at 100°C for 12 hours. Finally, it is rolled to obtain the negative electrode sheet.

[0137] (3) Preparation of electrolyte

[0138] 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.

[0139] (4) Preparation of the diaphragm

[0140] A polyethylene (PE) diaphragm is used.

[0141] (5) Battery manufacturing

[0142] Assemble the prepared positive electrode, separator, and negative electrode to obtain an unfilled bare cell; place the bare cell in an outer packaging foil, inject the prepared electrolyte into the dried bare cell, and obtain a lithium-ion battery through vacuum sealing, settling, formation, and sorting processes.

[0143] Examples 2-21 and Comparative Examples 1-4

[0144] Examples 2-21 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:

[0145] In the preparation of the positive electrode sheet:

[0146] In step (1.1),

[0147] For the preparation of LMFP:

[0148] LiMn a Fe b The values ​​of a / b in the chemical formula of PO4 are shown in Tables 1-1 and 1-2; the amount of deionized water is adjusted to control the reaction ion concentration to meet the requirements of Tables 1-1 and 1-2; the particle size Dn50 of LMFP meets the requirements of Tables 1-1 and 1-2.

[0149] For the preparation of LFP:

[0150] The linear velocity of the mixing sand mill and the linear velocity of the finished sand mill are shown in Table 1-1 and Table 1-2; the temperature of the sintering platform is controlled as shown in Table 1-1 and Table 1-2; the particle size Dn50 of LFP meets the requirements of Table 1-1 and Table 1-2.

[0151] In step (1.2), LMFP and LFP are mixed according to the mass ratios shown in Tables 1-1 and 1-2;

[0152] In the preparation of the negative electrode sheet:

[0153] The mass of the negative electrode active material, binder, and conductive agent were adjusted as shown in Tables 1-1 and 1-2.

[0154] Table 1-1

[0155] Table 1-2

[0156] For each embodiment and comparative example, U and r were tested; the detection methods for the above items are as follows:

[0157] U detection method:

[0158] 1) Capacity setting: The secondary battery is left to stand at 25°C for 120 minutes, then discharged at a constant current of 0.33C to 2.5V; after standing for 20 minutes, it is charged at a constant current 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 charging capacity Q0 of the second cycle is obtained;

[0159] (2) Plot the charging curve: After standing at 25℃ for 120 min, charge at a constant current of 2C to the upper limit voltage of 4.25V to obtain the charging capacity Q and voltage V data;

[0160] (3) Perform derivative calculation on the above QV data, and then divide dQ by dV to obtain dQ / dV; plot the dQ / dV-V curve with dQ / dV as the vertical axis and voltage V as the horizontal axis.

[0161] (4) In the dQ / dV-V curve, the peak position of the characteristic peak of the horizontal axis voltage in the range of 3.4V-3.6V is the potential U1 of the first plateau, and the peak position of the characteristic peak of the horizontal axis voltage in the range of 3.6V-3.8V is the potential U2 of the second plateau; the potential difference between the first plateau and the second plateau is U = U2-U1, and the unit is V.

[0162] Test method for r:

[0163] (1) Disassemble the positive and negative electrode plates in the secondary battery and reassemble them into a single battery. The separator used is a PE separator. There are no special restrictions on the electrolyte composition. The amount of electrolyte injected should be enough to ensure that there is free electrolyte when the battery is charged to the upper limit voltage. Fix a copper wire on the negative electrode side as the third electrode.

[0164] (2) After the single battery cell is assembled, it is left to stand at 25°C for 120 minutes. It is then charged at a constant current of 0.03C to 4.25V, discharged at a constant current of 0.3C to 2.5V, charged at a constant current of 0.33C to 4.25V, charged at a constant voltage to 0.05C, left to stand at 25°C for 10 minutes, and discharged at a constant current of 0.33C to 2.5V. This process is repeated twice. Then, lithium is plated onto the third electrode using the positive electrode at 0.01C for 6 hours. After lithium plating, the charge is adjusted to the voltage of the first plateau at 0.33C. After standing at 25°C for 30 minutes, EIS testing is continued. The negative electrode and the third electrode are monitored. The testing frequency is 0.01~10. 6 Hz;

[0165] (3) After the test, the EIS data is obtained, the low-frequency slant region is selected, and ω is plotted. -0.5 -The real part of the impedance Z' curve is taken as the slope σ of the straight line in the lowest frequency region; substituting this slope into the formula: DLi + =(R 2 T 2 ) / (2A 2 n 4 F 4 C 2 σ 2 ) to obtain DLi + This is the lithium-ion diffusion coefficient (r); where R is the gas constant (J / mol / K), T is the absolute reaction temperature (K), and A is the area (cm²) of the negative electrode material immersed in the electrolyte. 2 n is the number of electrons in the reaction, F is the Faraday constant (C / mol), and C is the lithium ion molar concentration (mol / cm³). 3 .

[0166] Figure 1 is a charging curve of the lithium-ion battery of Example 12, where SOC (%) represents the charging capacity Q. Figure 2 is the dQ / dV-V curve of the lithium-ion battery of Example 12.

[0167] The test results of each embodiment and comparative example are shown in Table 2.

[0168] Table 2

[0169] The fast charging time and fast charging cycle life of lithium-ion batteries were tested using the following methods: (1) Fast charging time at room temperature from 8% to 80% SOC:

[0170] The lithium-ion battery was discharged to 2V at a constant current of 0.05C and disassembled in an empty state. The disassembly environment was kept at room temperature and low humidity. After disassembly, the positive and negative electrode sheets were immediately sealed in aluminum-plastic bags to isolate them from external moisture and air. The electrode sheets were transferred to a glove box, and after the electrolyte was evaporated, the sheets were cut, weighed, and assembled into a full battery. Copper wires were fixed to the surface of the negative electrode sheet. Subsequently, electrolyte was injected, and the battery was left to stand at room temperature for 24 hours to obtain a full battery.

[0171] The aforementioned full battery was fully charged to 4.25V at 0.05C, then discharged at a constant current of 0.33C to 2.5V. After resting for 10 minutes, it was charged again at a constant current of 0.33C to 4.25V, and then charged at a constant voltage until 0.05C was reached. After resting for 10 minutes, it was discharged at a constant current of 0.33C to 2.5V. Using the charging capacity of the last week as a standard, it was charged at 0.33C to 8% SOC. Constant current charging was then performed at 2C-1.8C-1.6C-1.4C-1.2C-1C-0.8C-0.6C-0.4C-0.3C-0.2C-0.1C-0.05C, with the upper limit voltage of 4.25V and the negative electrode lithium potential of 0mV as the cutoff conditions, until 100% SOC was reached or the current dropped to 0.05C, at which point charging was stopped. The charging time from 8% to 80% SOC was recorded.

[0172] (2) Capacity retention rate after 100cls fast charging cycle at room temperature:

[0173] Using a battery charge / discharge tester, the above-mentioned lithium-ion battery was subjected to charge / discharge cycle testing at 25°C. The charge / discharge regime was as follows: constant current discharge at 0.33C to 2.5V, rest for 10 minutes, constant current charge at 0.33C to 4.25V, constant voltage charge to 0.05C cutoff, rest for 10 minutes, constant current charge at 0.33C to 4.25V, and this charge / discharge cycle was repeated twice. Then, the battery was charged at 25°C with constant current and constant voltage at 1C to 4.3V, and then discharged with constant current at 1C to 2.5V. This 1C charge and 1C discharge cycle was repeated 100 times. The discharge capacity of the first cycle was recorded as Q1, and the discharge capacity of the 100th cycle was recorded as Q100. The capacity retention rate after 100 cls was (Q2 / Q1) × 100%.

[0174] The test results are shown in Table 3.

[0175] Table 3

[0176] According to the test results in Table 3, the lithium-ion batteries prepared in each embodiment of this application have good fast charging performance, with a fast charging time of ≤26.9 min, and good fast charging cycle stability, with a capacity retention rate of ≥94.3% after 100 cls of fast charging cycles.

[0177] Based on Examples 1-4, 10-13, and 17-21, it can be seen that when the potential difference between the first and second platforms of a lithium-ion battery is such that the lithium-ion diffusion coefficient of the corresponding negative electrode plate satisfies 0.125 ≤ U × r ≤ 0.152 when the battery potential is at the first platform, the lithium-ion battery has a shorter fast charging time and a higher capacity retention rate for 100cls fast charging cycles; when U is in the range of 0.11–0.19V, or r is in the range of 0.7 × 10⁻⁶V, the lithium-ion battery exhibits a higher capacity retention rate for 100cls fast charging cycles. -8 ~1.25×10 -8 cm 2 At / s, the battery's fast charging performance is better.

[0178] According to Examples 6-9 and Examples 14-16, it can be seen that when the Dn50 particle size of LFP is in the range of 90-200nm and the Dn50 particle size of LMFP is in the range of 50-180nm, and the mass ratio of LFP to LMFP is (1.5-19):1, lithium-ion batteries can achieve good overall fast charging performance.

[0179] According to comparative examples 1-4, when the U×r of the lithium-ion battery exceeds the range of the technical solution of this application, even if the value of U is between 0.11 and 0.19V and the value of r satisfies 0.7×10 -8 ~1.25×10 -8 cm 2 / s, the fast charging performance of lithium-ion batteries is still relatively poor overall, with excessively long fast charging times or low retention rates after fast charging.

[0180] 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, characterized by, 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 phosphate material. The charging curve of the secondary battery was obtained by charging test at 25°C at a 2C rate within a voltage range of 2.5 to 4.25V. The charging curve includes three charging plateaus with different potentials, which are denoted as the first plateau, the second plateau and the third plateau in order of increasing potential. The potential difference between the first and second platforms is denoted as U, with the unit of measurement being V. The lithium ion diffusion coefficient of the negative electrode tab corresponding to the first platform of the potential of the secondary battery is r, unit: ×10 -8 cm 2 / s; The secondary battery satisfies the following relationship: 0.06≤U×r≤0.

189.

2. The secondary battery according to claim 1, characterized by The secondary battery satisfies the following relationship: 0.124≤U×r≤0.

152.

3. The secondary battery according to claim 1 or 2, characterized by The range of U is 0.07 to 0.19 V.

4. The secondary battery according to claim 3, wherein The range of U is 0.13 to 0.16 V.

5. The secondary battery according to claim 1 or 2, wherein r is in the range of 0.7 x 10 -8 ~ 1.28 x 10 -8 cm 2 / s.

6. The secondary battery according to claim 5, wherein r is in the range of 0.9 x 10 -8 ~ 1.05 x 10 -8 cm 2 / s.

7. The secondary battery according to claim 1 or 2, wherein The phosphate materials include lithium manganese iron phosphate and lithium iron phosphate.

8. The secondary battery according to claim 7, wherein The Dn50 particle size of the lithium iron phosphate is 90-200 nm.

9. The secondary battery according to claim 7, wherein The Dn50 particle size of the lithium manganese iron phosphate is 50-180 nm.

10. The secondary battery according to claim 7, wherein The mass ratio of lithium manganese iron phosphate to lithium iron phosphate is (1.5-19):

1.

11. The secondary battery according to claim 10, wherein The mass ratio of lithium manganese iron phosphate to lithium iron phosphate is (2-6):

1.

12. The secondary battery according to claim 1, wherein 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.

13. An electrical device, characterized by It includes the secondary battery as described in any one of claims 1 to 12.

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