Recrystallized lithium oxalate and preparation method thereof, positive plate, secondary battery and electric device

Abstract

The invention provides recrystallized lithium oxalate and a preparation method thereof, a positive plate, a secondary battery and an electric device. The preparation method comprises the following steps: S1, mixing lithium oxalate with water to obtain a lithium oxalate supersaturated solution; step S2, recrystallizing the lithium oxalate supersaturated solution to obtain recrystallized lithium oxalate; wherein the mass concentration of the lithium oxalate supersaturated solution is 1.1-1.3 times of the mass concentration of the lithium oxalate saturated solution. Recrystallized lithium oxalate with smaller granularity is formed through a recrystallization technology, so that the reduction of surface electron and lithium ion transmission paths of recrystallized lithium oxalate is facilitated, the oxygenolysis of recrystallized lithium oxalate is facilitated, the decomposition potential of recrystallized lithium oxalate is effectively reduced, and the lithium supplement effect of recrystallized lithium oxalate is improved; therefore, the capacity and the cycling stability of the secondary battery can be improved.

Description

Technical Field

[0001] This invention relates to the field of secondary battery technology, and more specifically, to a recrystallized lithium oxalate and its preparation method, a positive electrode, a secondary battery, and an electrical device. Background Technology

[0002] During the first charge cycle of a lithium-ion battery, the solid electrolyte interphase (SEI) film formed on the negative electrode surface irreversibly consumes the active lithium in the battery system, leading to a decrease in battery energy density and a shortened cycle life. Pre-lithiation technology is considered an effective solution to the problem of active lithium loss in batteries. From a process perspective, pre-lithiation technology is mainly divided into negative electrode pre-lithiation and positive electrode pre-lithiation, among which positive electrode pre-lithiation has high safety and simple process. Lithium oxalate (Li2C2O4) has become an excellent positive electrode lithium replenishing agent due to its advantages such as low theoretical oxidation decomposition potential (3.0 V), high chemical stability, and high specific capacity. However, when Li2C2O4 is added as a lithium replenishing material in positive electrode pre-lithiation in related technologies, the oxidation decomposition potential of Li2C2O4 is as high as 4.7V, which is difficult to meet the requirements of commonly used positive electrode materials in current lithium-ion batteries. Summary of the Invention

[0003] The main objective of this invention is to provide a recrystallized lithium oxalate and its preparation method, a positive electrode, a secondary battery, and an electrical device, in order to solve the problem that the high oxidation decomposition potential of lithium oxalate in the prior art leads to poor lithium replenishment effect.

[0004] To achieve the above objectives, according to one aspect of the present invention, a method for preparing recrystallized lithium oxalate is provided, the method comprising: step S1, mixing lithium oxalate and water to obtain a lithium oxalate supersaturated solution; step S2, recrystallizing the lithium oxalate supersaturated solution to obtain recrystallized lithium oxalate; wherein the mass concentration of the lithium oxalate supersaturated solution is 1.1 to 1.3 times the mass concentration of the lithium oxalate saturated solution.

[0005] Furthermore, the temperature of the lithium oxalate supersaturated solution is 20~30℃.

[0006] Furthermore, the recrystallization process is repeated 2 to 3 times.

[0007] Further, step S2 includes: sequentially stirring a lithium oxalate supersaturated solution and lithium oxalate seed crystals and allowing them to stand for a first time to obtain recrystallized lithium oxalate; preferably, the stirring speed of the first stirring is 100~200 rpm; and / or, the stirring time of the first stirring is 30~60 min; and / or, the temperature of the first standing is 20~30℃; and / or, the standing time of the first standing is 8~12 h; and / or, the mass ratio of lithium oxalate seed crystals to lithium oxalate is (0.02~0.05):1.

[0008] Further, step S2 includes: sequentially subjecting a supersaturated lithium oxalate solution and a first dispersant to a second stirring and a second settling to obtain recrystallized lithium oxalate; preferably, the mass ratio of the first dispersant to lithium oxalate is (0.05~0.1):1; and / or, the first dispersant is a first polymer dispersant; more preferably, the number average molecular weight of the first polymer dispersant is 1000~200000 g / mol; and / or, the first polymer dispersant is selected from any one or more of polyethylene glycol, polypropylene glycol, polyvinylpyrrolidone, polyvinyl alcohol, and polyethylene oxide; and / or, the second stirring speed is 300~600 rpm; and / or, the second stirring time is 30~60 min; and / or, the second settling temperature is 20~30℃; and / or, the second settling time is 4~8 h.

[0009] Further, step S2 includes: mixing a supersaturated lithium oxalate solution and a second dispersant, followed by sequential cooling and a third settling to obtain recrystallized lithium oxalate; preferably, the mass ratio of the second dispersant to lithium oxalate is (0.05~0.1):1; and / or, the second dispersant is a second polymer dispersant; more preferably, the number average molecular weight of the second polymer dispersant is 1000~200000 g / mol; and / or, the second polymer dispersant is selected from any one or more of polyethylene glycol, polypropylene glycol, polyvinylpyrrolidone, polyvinyl alcohol, and polyethylene oxide; and / or, the cooling rate is 5~10℃ / min; and / or, the cooling time is 200~400s; and / or, the third settling temperature is 20~30℃; and / or, the third settling time is 8~12h.

[0010] According to another aspect of the present invention, a recrystallized lithium oxalate is provided, which is prepared by the aforementioned preparation method; preferably, the decomposition potential of the recrystallized lithium oxalate is 3.65~3.67V; and / or, the particle size of the recrystallized lithium oxalate is 2~5μm; and / or, the specific surface area of ​​the recrystallized lithium oxalate is 5~10m². 2 / g; and / or, the particle size distribution width of recrystallized lithium oxalate is 3 < D90 / D10 < 6.

[0011] According to another aspect of the present invention, a positive electrode sheet is provided, comprising a positive electrode active layer and a current collector, wherein the positive electrode active layer comprises the aforementioned recrystallized lithium oxalate, positive electrode active material, binder and conductive agent; preferably, the total mass ratio of recrystallized lithium oxalate, positive electrode active material, binder and conductive agent to the mass ratio of recrystallized lithium oxalate is 100:(2~6).

[0012] According to another aspect of the present invention, a secondary battery is provided, comprising a positive electrode, an electrolyte, and a negative electrode, wherein the positive electrode is the aforementioned positive electrode.

[0013] According to another aspect of the present invention, an electrical device is provided, which includes the aforementioned secondary battery.

[0014] By applying the technical solution of this invention, lithium oxalate is mixed with water in a certain proportion to form a supersaturated solution. Insufficient supersaturation makes it difficult to drive the formation of crystal nuclei, while excessive supersaturation, although it can rapidly generate a large number of crystal nuclei, is not conducive to the formation of larger, purer crystals. Instead, it easily generates small and irregular grains, reducing the purity and performance of the crystalline product. Controlling the mass concentration of the lithium oxalate supersaturated solution within the above-mentioned range helps to ensure a moderate crystal nucleus formation rate, which ensures sufficient crystal nucleus density without triggering explosive nucleation. This is beneficial for controlling the size distribution of the crystalline particles, concentrating them within the optimal particle size range, thereby improving the lithium replenishment performance of recrystallized lithium oxalate. Forming smaller-sized recrystallized lithium oxalate through recrystallization technology helps to reduce the electron and lithium-ion transport paths on the surface of recrystallized lithium oxalate, which is beneficial for the oxidative decomposition of recrystallized lithium oxalate. This effectively reduces the decomposition potential of recrystallized lithium oxalate and improves the lithium replenishment effect, thereby helping to improve the capacity and cycle stability of the secondary battery. Attached Figure Description

[0015] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0016] Figure 1 SEM images of recrystallized lithium oxalate in Example 1 of this application are shown;

[0017] Figure 2 The XRD comparison diagrams of commercially available lithium oxalate and recrystallized lithium oxalate of this application are shown. Detailed Implementation

[0018] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0019] As analyzed in the background section of this application, the prior art has the problem that the oxidation decomposition potential of lithium oxalate is too high, resulting in poor lithium replenishment effect. In order to solve the above problems, this application provides recrystallized lithium oxalate and its preparation method, positive electrode sheet, secondary battery and power device.

[0020] In a typical embodiment of this application, a method for preparing recrystallized lithium oxalate is provided. The method includes: step S1, mixing lithium oxalate and water to obtain a supersaturated lithium oxalate solution; step S2, recrystallizing the supersaturated lithium oxalate solution to obtain recrystallized lithium oxalate; wherein the mass concentration of the supersaturated lithium oxalate solution is 1.1 to 1.3 times that of the saturated lithium oxalate solution.

[0021] This application involves mixing lithium oxalate with water in a specific ratio to form a supersaturated solution. Insufficient supersaturation hinders the formation of crystal nuclei, while excessive supersaturation, although rapidly generating a large number of crystal nuclei, is detrimental to the formation of larger, purer crystals, instead easily producing small and irregular grains, reducing the purity and performance of the crystalline product. Controlling the mass concentration of the lithium oxalate supersaturated solution within the aforementioned range helps to achieve a moderate crystal nucleus formation rate, ensuring sufficient nucleus density without triggering explosive nucleation. This facilitates control over the size distribution of the crystalline particles, concentrating them within the optimal particle size range, thereby improving the lithium replenishment performance of recrystallized lithium oxalate. Forming smaller-particle recrystallized lithium oxalate through recrystallization technology helps reduce the electron and lithium-ion transport paths on the surface of recrystallized lithium oxalate, promoting its oxidative decomposition, effectively lowering its decomposition potential, and improving its lithium replenishment effect, thus contributing to increased capacity and cycle stability of the secondary battery.

[0022] In some embodiments of this application, the temperature of the above-mentioned lithium oxalate supersaturated solution is 20~30°C.

[0023] Controlling the temperature of the lithium oxalate supersaturated solution within the above range helps promote the stable formation of crystal nuclei and subsequent slow crystal growth.

[0024] In some embodiments of this application, the above-mentioned recrystallization is repeated 2 to 3 times.

[0025] Controlling the number of recrystallization cycles within the aforementioned range helps to further optimize the size distribution of the crystallized particles, making the particle size more concentrated. This helps to reduce the electron and lithium-ion transport paths on the surface of recrystallized lithium oxalate, thereby improving its oxidation decomposition efficiency, reducing the decomposition potential, and facilitating lithium replenishment at lower voltages.

[0026] In some embodiments of this application, step S2 includes: sequentially stirring a lithium oxalate supersaturated solution and lithium oxalate seed crystals and allowing them to stand for a first time to obtain recrystallized lithium oxalate; preferably, the stirring speed of the first stirring is 100~200 rpm; and / or, the stirring time of the first stirring is 30~60 min; and / or, the temperature of the first standing is 20~30℃; and / or, the standing time of the first standing is 8~12 h; and / or, the mass ratio of lithium oxalate seed crystals to lithium oxalate is (0.02~0.05):1.

[0027] Controlling the speed and duration of the first stirring within the aforementioned range helps improve the dispersibility of lithium oxalate seed crystals in a supersaturated lithium oxalate solution. Controlling the temperature and time of the first settling period within the aforementioned range helps promote crystal precipitation, reduce crystal size, and improve crystal uniformity. Controlling the mass ratio of lithium oxalate seed crystals to lithium oxalate within the aforementioned range helps improve crystal uniformity, thereby enhancing the lithium replenishment effect of recrystallized lithium oxalate.

[0028] In some embodiments of this application, the average particle size of the lithium oxalate seeds is 2~5 μm.

[0029] Controlling the average particle size of lithium oxalate seed crystals within the above-mentioned range helps to form appropriately sized grains, thereby improving the lithium replenishment effect of recrystallized lithium oxalate.

[0030] In some embodiments of this application, step S2 includes: sequentially subjecting a supersaturated lithium oxalate solution and a first dispersant to a second stirring and a second settling to obtain recrystallized lithium oxalate; preferably, the mass ratio of the first dispersant to lithium oxalate is (0.05~0.1):1; and / or, the first dispersant is a first polymer dispersant; more preferably, the number average molecular weight of the first polymer dispersant is 1000~200000 g / mol; and / or, the first polymer dispersant is selected from any one or more of polyethylene glycol, polypropylene glycol, polyvinylpyrrolidone, polyvinyl alcohol, and polyethylene oxide; and / or, the second stirring speed is 300~600 rpm; and / or, the second stirring time is 30~60 min; and / or, the second settling temperature is 20~30℃; and / or, the second settling time is 4~8 h.

[0031] Controlling the mass ratio of the first dispersant to lithium oxalate within the aforementioned range helps to control the grain size within a suitable range, thereby improving the lithium supplementation effect of recrystallized lithium oxalate. Controlling the number-average molecular weight and type of the first polymeric dispersant within the aforementioned range helps to form recrystallized lithium oxalate with higher uniformity. Controlling the second stirring speed and time within the aforementioned range helps to promote the dispersibility of the first dispersant in the supersaturated lithium oxalate solution. Controlling the second settling temperature and time within the aforementioned range helps to control the grain size within a suitable range, thereby helping to reduce the decomposition potential of recrystallized lithium oxalate.

[0032] In some embodiments of this application, step S2 includes: mixing a supersaturated lithium oxalate solution and a second dispersant, followed by sequential cooling and a third settling to obtain recrystallized lithium oxalate; preferably, the mass ratio of the second dispersant to lithium oxalate is (0.05~0.1):1; and / or, the second dispersant is a second polymer dispersant; more preferably, the number average molecular weight of the second polymer dispersant is 1000~200000 g / mol; and / or, the second polymer dispersant is selected from any one or more of polyethylene glycol, polypropylene glycol, polyvinylpyrrolidone, polyvinyl alcohol, and polyethylene oxide; and / or, the cooling rate is 5~10℃ / min; and / or, the cooling time is 200~400s; and / or, the third settling temperature is 20~30℃; and / or, the third settling time is 8~12h.

[0033] Controlling the mass ratio of the second dispersant to lithium oxalate within the aforementioned range helps to control the grain size within a suitable range, thereby improving the lithium supplementation effect of recrystallized lithium oxalate. Controlling the number-average molecular weight and type of the second polymeric dispersant within the aforementioned range helps to form recrystallized lithium oxalate with higher uniformity. Controlling the cooling rate and time, and the third settling temperature and time within the aforementioned ranges helps to control the grain size within a suitable range, thereby helping to reduce the decomposition potential of recrystallized lithium oxalate.

[0034] In some embodiments of this application, step S2 further includes drying and crushing the recrystallized crystalline particles sequentially to obtain recrystallized lithium oxalate.

[0035] In another typical embodiment of this application, a recrystallized lithium oxalate is provided, which is prepared by the aforementioned preparation method; preferably, the decomposition potential of the recrystallized lithium oxalate is 3.65~3.67V; and / or, the particle size of the recrystallized lithium oxalate is 2~5μm; and / or, the specific surface area of ​​the recrystallized lithium oxalate is 5~10m². 2 / g; and / or, the particle size distribution width of recrystallized lithium oxalate is 3 < D90 / D10 < 6.

[0036] Since the recrystallized lithium oxalate described above is prepared using the method of this application, it exhibits a smaller grain size and a lower decomposition potential. Recrystallized lithium oxalate with the aforementioned size and decomposition potential demonstrates superior lithium replenishment performance.

[0037] In some embodiments of this application, the shape of the recrystallized lithium oxalate is selected from at least one of needle-like, flake-like, and irregular particles.

[0038] In another typical embodiment of the present application, a positive electrode sheet is provided, which includes a positive electrode active layer and a current collector. The positive electrode active layer includes the aforementioned recrystallized lithium oxalate, a positive electrode active material, a binder, and a conductive agent; preferably, the ratio of the total mass of the recrystallized lithium oxalate, the positive electrode active material, the binder, and the conductive agent to the mass of the recrystallized lithium oxalate is 100:(2 - 6).

[0039] Since the above positive electrode sheet contains the recrystallized lithium oxalate of the present application, the positive electrode sheet has a relatively high capacity and relatively stable cycle performance. Controlling the ratio of the total mass of the recrystallized lithium oxalate, the positive electrode active material, the binder, and the conductive agent within the above range helps to further improve the capacity and cycle performance of the positive electrode sheet.

[0040] In another typical embodiment of the present application, a secondary battery is provided, which includes a positive electrode sheet, an electrolyte, and a negative electrode sheet, and the positive electrode sheet is the aforementioned positive electrode sheet.

[0041] The above secondary battery has a relatively high capacity and excellent cycle stability.

[0042] The positive electrode active material may include, but is not limited to, lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), lithium cobalt phosphate (LiCoPO4), lithium iron pyrophosphate (Li2FeP2O7), lithium cobalt oxide (LiCoO2), spinel-type lithium manganese oxide (LiMn2O4), spinel-type lithium nickel manganese oxide (LiNi 0.5 Mn 1.5 O4), layered lithium manganese oxide (LiMnO2), lithium nickel oxide (LiNiO2), lithium niobate (LiNbO2), lithium ferrite (LiFeO2), lithium manganate (LiMgO2), lithium calcium oxide (LiCaO2), lithium copper oxide (LiCuO2), lithium zinc oxide (LiZnO2), lithium molybdate (LiMoO2), lithium tantalate (LiTaO2), lithium tungstate (LiWO2), lithium nickel cobalt aluminum oxide (LiNi x Co y Al 1-x-y O2, 0 < x < 1, 0 < y < 1, 0 < x + y < 1, such as LiNi 0.8 Co 0.15 Al 0.05 O2), lithium nickel cobalt manganese oxide (LiNi x Co y Mn 1-x-y O2, 0 < x < 1, 0 < y < 1, 0 < x + y < 1, such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiNi 0.5 Co 0.2 Mn 0.3O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.8 Co 0.1 Mn 0.1 At least one of the following: O2, lithium-rich materials (e.g., lithium-rich nickel-cobalt-manganese oxide), manganese oxide (MnO2), vanadium oxide, sulfur oxide, silicate oxide, and their respective modified compounds. These materials may be used alone or in combination of two or more.

[0043] The modified compounds for the above-mentioned positive electrode active materials can be modified by doping, surface coating, or both doping and coating.

[0044] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including the positive electrode material of the first aspect of this application.

[0045] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0046] The positive electrode film includes the positive electrode material.

[0047] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0048] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0049] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0050] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive electrode active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0051] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector, the negative electrode film layer including a negative electrode active material.

[0052] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0053] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene, polyethylene terephthalate, polybutylene terephthalate, polystyrene, polyethylene, etc.).

[0054] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. Silicon-based materials may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0055] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may be selected from at least one of styrene-butadiene rubber, polyacrylic acid, sodium polyacrylate, polyacrylamide, polyvinyl alcohol, sodium alginate, polymethacrylic acid, and carboxymethyl chitosan.

[0056] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0057] In some embodiments, the negative electrode film layer may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose).

[0058] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto a negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0059] The electrolyte acts as a conductor of ions between the positive and negative electrodes. The electrolyte in this application is formed by adding additives to a common electrolyte in the art. There are no specific limitations on the types of common electrolytes in the art; they can be selected according to requirements. The additives promote the decomposition of the lithium-replenishing material in the positive electrode material at a decomposition potential of less than 3.67V.

[0060] In some embodiments, the concentration of the additive in the electrolyte is 0.02~0.1 mol·L⁻¹. -1 The additives may be soluble redox media, including sulfides, sulfates, nitrates, iodides, bromides, nitrites, or chlorides; the additives are at least one of potassium thiosulfate, sodium thiosulfate, lithium thiosulfate, potassium iodide, lithium iodide, sodium iodide, lithium nitrate, sodium nitrate, ferric nitrate, ferrous nitrate, magnesium nitrate, calcium nitrate, aluminum nitrate, zinc nitrate, copper nitrate, silver nitrate, tin nitrate, ammonium nitrate, lithium bromide, potassium bromide, sodium bromide, lithium nitrite, sodium nitrite, ferrous nitrite, ferrous nitrite, magnesium nitrite, calcium nitrite, aluminum nitrite, zinc nitrite, copper nitrite, silver nitrite, tin nitrite, ammonium nitrite, potassium chloride, lithium chloride, and sodium chloride.

[0061] For example, when the additive is NaNO2, during secondary battery charging, NaNO2 loses electrons to generate NO2 on the positive electrode side. NO2 oxidizes the lithium replenishment material Li2C2O4 to generate LiNO2 and CO2. This electrochemical-chemical reaction mechanism promotes the decomposition of Li2C2O4 in the positive electrode active material at low voltage, and synergistically promotes the decomposition of the pre-lithiated lithium replenishment material Li2C2O4, releasing sufficient active lithium to compensate for the irreversible lithium loss at the negative electrode. Meanwhile, on the negative electrode side, Li2C2O4 is reduced and decomposed at 1.25V, forming a nitrogen-containing SEI film on the negative electrode surface to inhibit electrolyte decomposition and reduce active lithium loss during cycling. When charged to a high voltage (>3.44V), NO2... - Lithium oxalate loses electrons and oxidizes to form NO2. The NO2 product then oxidizes lithium oxalate to form lithium nitrite and CO2. This electrochemical-chemical reaction mechanism lowers the decomposition voltage of lithium oxalate, thus improving its utilization rate. At high voltages (>3.4V), NO2... -It is thermodynamically unstable and will lose electrons and decompose to produce NO2.

[0062] In some embodiments, the concentration of the additive in the electrolyte is 0.02 mol·L⁻¹. -1 0.03 mol·L -1 0.05 mol·L -1 0.06 mol·L -1 0.07 mol·L -1 0.09 mol·L -1 0.1 mol·L -1 Additives assist positive electrode materials in replenishing lithium during application, and in secondary batteries, additives play a role in negative electrode film formation and promoting the decomposition of positive electrode lithium replenishment materials.

[0063] In some embodiments, the electrolyte is an electrolyte solution, which includes an electrolyte salt and a solvent.

[0064] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0065] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0066] In some embodiments, the electrolyte may optionally include other additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0067] Therefore, the additive in this application reduces the oxidation decomposition voltage of the lithium-replenishing material to 3.67V when assisting in lithium replenishment at the positive electrode, and can provide more than 380mAh·g. -1The irreversible specific capacity, oxidation decomposition voltage, and good compatibility with existing commercial lithium-ion battery cathodes and electrolytes enable lithium oxalate to be used for cathode replenishment. The suitable operating voltage of the secondary battery in this application, with a charging cutoff voltage of 4.3V, matches well with existing commercial lithium-ion cathodes and electrolytes, allowing sufficient active lithium to be released without charging to a high voltage. Furthermore, the secondary battery in this application has high lithium replenishment efficiency, providing an additional 27.4 mAh·g on the first charge. -1 The charging capacity.

[0068] Meanwhile, most cathode lithium replenishment additives in related technologies leave residues after lithium replenishment, which affects the specific capacity of the cathode and reduces the energy density of the entire battery. Compared with the lithium replenishment materials in related technologies, the secondary battery of this application has low residues after lithium replenishment, and its impact is negligible. Moreover, the lithium replenishment method is simple, has good compatibility with current industrialization, does not require large-scale modification of the existing lithium-ion battery preparation system, and has good application prospects.

[0069] In some embodiments, the secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0070] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0071] In this application, an electrolyte with additives is used to promote the decomposition of lithium replenishment materials at a decomposition potential of less than 3.67V, thereby compensating for the large initial irreversible active lithium loss in the secondary battery.

[0072] In another typical embodiment of this application, an electrical device is provided, which includes the aforementioned secondary battery.

[0073] The secondary battery assembly obtained by applying the cathode material provided in this application can be used in power-consuming devices to improve the performance of the devices. The power-consuming device in this application can compensate for the large initial irreversible loss of active lithium. It can not only efficiently provide enough active lithium to compensate for the irreversible lithium loss of the anode, but also has the advantages of being compatible with the charging cutoff voltage of current lithium-ion batteries and having low residue after lithium replenishment and decomposition. In addition, the application of the cathode material significantly improves the reversible specific capacity of the full cell in the first cycle and significantly improves the subsequent cycle stability.

[0074] The beneficial effects of this application will be further illustrated below with reference to the embodiments.

[0075] Example 1

[0076] Commercially available lithium oxalate was mixed with water at 25°C to obtain a supersaturated lithium oxalate solution. The supersaturation of the lithium oxalate supersaturated solution was in the metastable region, and its mass concentration was 1.3 times that of the saturated lithium oxalate solution. The supersaturated lithium oxalate solution was placed in a beaker, and lithium oxalate seed crystals with an average particle size of 3 μm were added. The mass ratio of lithium oxalate seed crystals to lithium oxalate was 0.03:1. The mixture was stirred at 100 rpm for 30 min at 25°C, and then allowed to stand at 25°C for 9 h for recrystallization. This recrystallization was repeated three times to obtain recrystallized lithium oxalate. The recrystallized lithium oxalate was then dried in a vacuum drying oven at 50°C. After drying, it was ball-milled to a particle size of 2 μm, and the specific surface area of ​​the recrystallized lithium oxalate was 7.2 m². 2 / g; The particle size distribution width of recrystallized lithium oxalate is D90 / D10=5.3.

[0077] Example 2

[0078] The difference from Example 1 is that a supersaturated lithium oxalate solution was placed in a beaker and polyethylene glycol (number average molecular weight 1000 g / mol) was added. The mass ratio of polyethylene glycol to lithium oxalate was 0.05:1. The mixture was stirred at 350 rpm for 30 min at 25°C, and then allowed to stand at 25°C for 6 h for recrystallization. This process was repeated three times to obtain recrystallized lithium oxalate. The recrystallized lithium oxalate was then dried in a vacuum drying oven at 50°C to obtain recrystallized lithium oxalate with a particle size of 2 μm and a specific surface area of ​​6.8 m². 2 / g; The particle size distribution width of recrystallized lithium oxalate is D90 / D10=4.3.

[0079] Example 3

[0080] The difference from Example 1 is that a supersaturated lithium oxalate solution was placed in a beaker and polyethylene glycol (number-average molecular weight of 1000 g / mol) was added. The mass ratio of polyethylene glycol to lithium oxalate was 0.05:1. Nucleation was induced by cooling at a rate of 8°C / min for 330 s. Recrystallization was then performed by standing at 25°C for 9 h. This process was repeated three times to obtain recrystallized lithium oxalate. The recrystallized lithium oxalate was then dried in a vacuum drying oven at 50°C to obtain recrystallized lithium oxalate with a particle size of 2 μm and a specific surface area of ​​7.8 m². 2 / g; The particle size distribution width of recrystallized lithium oxalate is D90 / D10=5.6.

[0081] Example 4

[0082] The difference from Example 1 is that the average particle size of the lithium oxalate seeds is 2 μm, and the mass ratio of lithium oxalate seeds to lithium oxalate is 0.02:1, ultimately yielding recrystallized lithium oxalate with a specific surface area of ​​8.1 m². 2 / g; The particle size distribution width of recrystallized lithium oxalate is D90 / D10=3.7.

[0083] Example 5

[0084] The difference from Example 1 is that the average particle size of the lithium oxalate seeds is 5 μm, and the mass ratio of lithium oxalate seeds to lithium oxalate is 0.05:1, ultimately yielding recrystallized lithium oxalate with a specific surface area of ​​7.0 m². 2 / g; The particle size distribution width of recrystallized lithium oxalate is D90 / D10=4.1.

[0085] Example 6

[0086] The difference from Example 1 is that the average particle size of the lithium oxalate seeds is 6 μm, and the mass ratio of lithium oxalate seeds to lithium oxalate is 0.06:1, ultimately yielding recrystallized lithium oxalate with a specific surface area of ​​4.3 m². 2 / g; The particle size distribution width of recrystallized lithium oxalate is D90 / D10=6.5.

[0087] Example 7

[0088] The difference from Example 1 is that, at 25°C, commercially available lithium oxalate and water were mixed to obtain a supersaturated lithium oxalate solution. The supersaturation of the lithium oxalate supersaturated solution was in the metastable region, and its mass concentration was 1.3 times that of the saturated lithium oxalate solution. The supersaturated lithium oxalate solution was placed in a beaker, and lithium oxalate seed crystals with an average particle size of 3 μm were added. The mass ratio of lithium oxalate seed crystals to lithium oxalate was 0.03:1. After stirring at 100 rpm for 30 min at 25°C, the mixture was allowed to stand at 25°C for 9 h for recrystallization to obtain recrystallized lithium oxalate. The recrystallized lithium oxalate was then dried in a vacuum drying oven at 50°C. After drying, it was ball-milled to a particle size of 2 μm, and the specific surface area of ​​the recrystallized lithium oxalate was 7.1 m². 2 / g; The particle size distribution width of recrystallized lithium oxalate is D90 / D10=5.4.

[0089] Example 8

[0090] The difference from Example 2 is that the number average molecular weight of polyethylene glycol is 100,000 g / mol, and the mass ratio of polyethylene glycol to lithium oxalate is 0.07:1, ultimately yielding recrystallized lithium oxalate.

[0091] Example 9

[0092] The difference from Example 2 is that the number average molecular weight of polyethylene glycol is 200,000 g / mol, and the mass ratio of polyethylene glycol to lithium oxalate is 0.1:1, ultimately yielding recrystallized lithium oxalate.

[0093] Example 10

[0094] The difference from Example 2 is that the number average molecular weight of polyethylene glycol is 300,000 g / mol, and the mass ratio of polyethylene glycol to lithium oxalate is 0.2:1, ultimately yielding recrystallized lithium oxalate.

[0095] Example 11

[0096] The difference from Example 3 is that the number average molecular weight of polyethylene glycol is 100,000 g / mol, and the mass ratio of polyethylene glycol to lithium oxalate is 0.07:1, ultimately yielding recrystallized lithium oxalate.

[0097] Example 12

[0098] The difference from Example 3 is that the number average molecular weight of polyethylene glycol is 200,000 g / mol, and the mass ratio of polyethylene glycol to lithium oxalate is 0.1:1, ultimately yielding recrystallized lithium oxalate.

[0099] Example 13

[0100] The difference from Implementation 1 is that, at 25°C, commercially available lithium oxalate and water are mixed to obtain a lithium oxalate supersaturated solution. The supersaturation of the lithium oxalate supersaturated solution is in the metastable region, and its mass concentration is 1.1 times that of the saturated lithium oxalate solution. The lithium oxalate supersaturated solution is placed in a beaker, and lithium oxalate seed crystals with an average particle size of 3 μm are added. The mass ratio of lithium oxalate seed crystals to lithium oxalate is 0.03:1. After stirring at 200 rpm for 60 min at 25°C, it is allowed to stand at 25°C for 9 h for recrystallization. The recrystallization is repeated twice to obtain recrystallized lithium oxalate. The recrystallized lithium oxalate is placed in a vacuum drying oven and dried at 50°C. After drying, it is ball-milled to a particle size of 2 μm.

[0101] Example 14

[0102] The difference from Implementation 2 is that polyvinylpyrrolidone (number average molecular weight of 1000 g / mol) is used instead of polyethylene glycol to finally obtain recrystallized lithium oxalate.

[0103] Comparative Example 1

[0104] The difference from Example 1 is that the mass concentration of the lithium oxalate supersaturated solution is 1.4 times that of the lithium oxalate saturated solution, and recrystallized lithium oxalate is finally obtained.

[0105] Comparative Example 2

[0106] The difference from Example 1 is that recrystallization is omitted, and commercially available lithium oxalate is directly ball-milled to a particle size of 2μm.

[0107] Battery manufacturing

[0108] The recrystallized lithium oxalate or lithium oxalate prepared in the examples and comparative examples were used as lithium supplementers. Lithium iron phosphate, lithium supplementer, binder (polyvinylidene fluoride), and conductive agent (super conductive carbon black) were mixed in a mass ratio of 93:2:2.5:2.5. Based on the positive electrode material, the lithium supplementer had a mass fraction of 2 wt.%. N-methylpyrrolidone dispersant was added to form a slurry, which was then uniformly coated onto aluminum foil. After drying, the slurry was pressed into sheets, punched, and dried in a drying oven at 80°C for 12 hours before use. A lithium metal sheet was used as the counter electrode. A three-component mixed solvent of 1 mol / L LiPF6 electrolyte was used as the solvent, namely a mixture of ethylene carbonate: dimethyl carbonate: methyl ethyl carbonate in a volume ratio of 1:1:1, with NaNO2 as the additive at a concentration of 0.02 mol·L⁻¹. -1 A CR2032 coin cell was assembled in an argon glove box in Microna, using a Celgard 2400 microporous polypropylene membrane as the separator.

[0109] The batteries prepared in the examples and comparative examples were subjected to charge-discharge tests on the Blue Battery Testing System, specifically including:

[0110] First, the prepared battery was left to stand at 25°C for 10 hours, and then charged and discharged at 0.5C constant current at 25°C with the charging and discharging voltage limited to 2.0~3.8V. Its additional capacity and capacity retention rate after 200 cycles were tested. The test results are shown in Table 1.

[0111] The recrystallized lithium oxalate or lithium oxalate prepared in the examples and comparative examples were subjected to decomposition potential tests. Specifically, a potential scan was applied by linear scanning voltammetry. When the current suddenly increased, the corresponding potential was the decomposition potential. The test results are shown in Table 1.

[0112] Table 1

[0113]

[0114] As shown in Table 1, the battery in this application can compensate for significant initial irreversible lithium loss, efficiently providing sufficient active lithium to compensate for the irreversible lithium loss at the negative electrode. Furthermore, the battery using the lithium replenishment material exhibits a significantly improved reversible specific capacity in the first cycle and a marked improvement in subsequent cycle stability.

[0115] Figure 1 The following is a SEM image of recrystallized lithium oxalate from Example 1 of this application. Figure 1 It can be seen that the recrystallized lithium oxalate has a regular morphology and good size uniformity.

[0116] Figure 2 The XRD comparison diagrams of commercially available lithium oxalate and recrystallized lithium oxalate of this application are shown. The commercially available lithium oxalate is C-Li2C2O4, and the recrystallized lithium oxalate is re-Li2C2O4. It can be seen from the figure that the recrystallized lithium oxalate has a higher degree of crystallinity.

[0117] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects:

[0118] This application involves mixing lithium oxalate with water in a specific ratio to form a supersaturated solution. Insufficient supersaturation hinders the formation of crystal nuclei, while excessive supersaturation, although rapidly generating a large number of crystal nuclei, is detrimental to the formation of larger, purer crystals, instead easily producing small and irregular grains, reducing the purity and performance of the crystalline product. Controlling the mass concentration of the lithium oxalate supersaturated solution within the aforementioned range helps to achieve a moderate crystal nucleus formation rate, ensuring sufficient nucleus density without triggering explosive nucleation. This facilitates control over the size distribution of the crystalline particles, concentrating them within the optimal particle size range, thereby improving the lithium replenishment performance of recrystallized lithium oxalate. Forming smaller-particle recrystallized lithium oxalate through recrystallization technology helps reduce the electron and lithium-ion transport paths on the surface of recrystallized lithium oxalate, promoting its oxidative decomposition, effectively lowering its decomposition potential, and improving its lithium replenishment effect, thus contributing to increased capacity and cycle stability of the secondary battery.

[0119] The above are merely embodiments of the present invention and are not intended to limit the invention. Those skilled in the art will recognize that the present invention can have various modifications and variations. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing recrystallized lithium oxalate, characterized in that, The preparation method includes: Step S1: Mix lithium oxalate and water to obtain a supersaturated lithium oxalate solution; Step S2: Recrystallize the supersaturated lithium oxalate solution to obtain recrystallized lithium oxalate; The mass concentration of the lithium oxalate supersaturated solution is 1.1 to 1.3 times that of the lithium oxalate saturated solution.

2. The preparation method according to claim 1, characterized in that, The temperature of the lithium oxalate supersaturated solution is 20~30℃.

3. The preparation method according to claim 1 or 2, characterized in that, The recrystallization process is repeated 2 to 3 times.

4. The preparation method according to any one of claims 1 to 3, characterized in that, Step S2 includes: sequentially stirring the lithium oxalate supersaturated solution and lithium oxalate seed crystals and allowing them to stand for a first time to obtain the recrystallized lithium oxalate. Preferably, the first stirring speed is 100~200 rpm; and / or, the first stirring time is 30~60 min; And / or, the temperature of the first settling period is 20~30℃; and / or, the time of the first settling period is 8~12h; And / or, the mass ratio of the lithium oxalate seed crystals to the lithium oxalate is (0.02~0.05):

1.

5. The preparation method according to any one of claims 1 to 3, characterized in that, Step S2 includes: sequentially stirring the lithium oxalate supersaturated solution and the first dispersant for a second time and then allowing them to stand to obtain the recrystallized lithium oxalate. Preferably, the mass ratio of the first dispersant to the lithium oxalate is (0.05~0.1):1; and / or, the first dispersant is a first polymer dispersant; more preferably, the number average molecular weight of the first polymer dispersant is 1000~200000 g / mol; and / or, the first polymer dispersant is selected from any one or more of polyethylene glycol, polypropylene glycol, polyvinylpyrrolidone, polyvinyl alcohol and polyethylene oxide; And / or, the second stirring speed is 300~600 rpm; and / or, the second stirring time is 30~60 min; And / or, the temperature of the second settling period is 20~30℃; and / or, the time of the second settling period is 4~8h.

6. The preparation method according to any one of claims 1 to 3, characterized in that, Step S2 includes: mixing the lithium oxalate supersaturated solution and the second dispersant, and then sequentially cooling and allowing it to stand to obtain the recrystallized lithium oxalate. Preferably, the mass ratio of the second dispersant to the lithium oxalate is (0.05~0.1):1; and / or, the second dispersant is a second polymer dispersant; more preferably, the number average molecular weight of the second polymer dispersant is 1000~200000 g / mol; and / or, the second polymer dispersant is selected from any one or more of polyethylene glycol, polypropylene glycol, polyvinylpyrrolidone, polyvinyl alcohol and polyethylene oxide; And / or, the cooling rate is 5~10℃ / min; and / or, the cooling time is 200~400s; And / or, the temperature of the third settling period is 20~30℃; and / or, the time of the third settling period is 8~12h.

7. A recrystallized lithium oxalate, characterized in that, The recrystallized lithium oxalate is prepared by the preparation method according to any one of claims 1 to 6; preferably, the decomposition potential of the recrystallized lithium oxalate is 3.65~3.67V; and / or, the particle size of the recrystallized lithium oxalate is 2~5μm; and / or, the specific surface area of ​​the recrystallized lithium oxalate is 5~10m². 2 / g; and / or, the particle size distribution width of the recrystallized lithium oxalate is 3 < D90 / D10 < 6.

8. A positive electrode sheet, comprising a positive electrode active layer and a current collector, characterized in that, The positive electrode active layer includes the recrystallized lithium oxalate as described in claim 7, the positive electrode active material, the binder, and the conductive agent; preferably, the ratio of the total mass of the recrystallized lithium oxalate, the positive electrode active material, the binder, and the conductive agent to the mass of the recrystallized lithium oxalate is 100:(2~6).

9. A secondary battery, comprising a positive electrode, an electrolyte, and a negative electrode, characterized in that, The positive electrode is the positive electrode as described in claim 8.

10. An electrical appliance, characterized in that, The electrical device includes the secondary battery as described in claim 9.

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