Positive electrode active material and preparation method therefor, secondary battery cell, and electric device

Abstract

The present disclosure provides a positive electrode active material and a preparation method therefor, a secondary battery cell, and an electric device. The positive electrode active material comprises a first material and a second material, the first material comprises an alkali metal monosulfide, and the second material comprises an alkali metal polysulfide, and is distributed on at least part of the surface of the first material.

Description

Positive electrode active materials and their preparation methods, secondary battery cells and electrical devices

[0001] Cross-reference to related applications

[0002] This disclosure is based on and claims priority to Chinese Patent Application No. 202510194487.X, filed on February 21, 2025, entitled "Positive Electrode Active Material and Preparation Method Thereof, Secondary Battery Cell and Electrical Device", the entire contents of which are incorporated herein by reference. Technical Field

[0003] This disclosure relates to the field of battery technology, and in particular to a positive electrode active material and its preparation method, a secondary battery cell and an electrical device. Background Technology

[0004] In recent years, with the increasingly wide application of rechargeable batteries, they have been widely used in energy storage power systems such as hydropower, thermal power, wind power, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric cars, aerospace, and many other fields. With the application and promotion of rechargeable batteries, people's requirements for their capacity are becoming increasingly higher.

[0005] Therefore, how to improve the capacity of secondary batteries has become an urgent technical problem to be solved. Summary of the Invention

[0006] This disclosure is made in view of the above-mentioned problems, and its purpose is to provide a positive electrode active material and a method for preparing the same, a secondary battery cell and an electrical device, wherein the secondary battery cell prepared by this disclosure has improved capacity.

[0007] To achieve the above objectives, the first aspect of this disclosure provides a positive electrode active material, comprising a first material and a second material, wherein the first material comprises an alkali metal monosulfide, the second material comprises an alkali metal polysulfide, and the second material is distributed on at least a portion of the surface of the first material.

[0008] In this disclosure, the positive electrode active material includes a first material, an alkali metal monosulfide, and a second material, an alkali metal polysulfide. Since the second material can undergo an electrochemical reaction to transform into a third material that can exist stably in the electrolyte, and since the second material is distributed on at least a portion of the surface of the first material, the third material can be formed on at least a portion of the surface of the first material. In this way, the third material can prevent the intermediate products generated by the first material during the conversion reaction from directly contacting the electrolyte, thereby inhibiting the dissolution and shuttle of the intermediate products in the electrolyte, which is beneficial to reducing the consumption of the negative electrode metal layer and thus beneficial to improving the capacity of the secondary battery cell.

[0009] In some embodiments, the first material comprises a material with the chemical formula M2S, and the second material comprises a material with the chemical formula M2S. x The material is given, wherein M includes one of Li, Na, and K, and 2 ≤ x ≤ 8.

[0010] In some embodiments, the first material includes Na2S, and / or the second material includes one or more of Na2S2 and Na2S5.

[0011] In some embodiments, the positive electrode active material includes Na2S2, and the mass percentage of Na2S2 in the positive electrode active material is less than or equal to 10%, and / or, the positive electrode active material includes Na2S5, and the mass percentage of Na2S5 in the positive electrode active material is less than or equal to 10%. This facilitates the formation of a third material on the surface of the first material that can stably exist in the electrolyte, thereby suppressing the dissolution and shuttle of intermediate products generated by the first material during the conversion reaction, and thus improving the capacity of the secondary battery cell.

[0012] In some embodiments, the mass percentage of the first material in the positive electrode active material is greater than the mass percentage of the second material. Optionally, the mass percentage of the first material in the positive electrode active material is 80% to 95%, and optionally, the mass percentage of the second material in the positive electrode active material is 5% to 20%. Since the capacity of alkali metal monosulfides is greater than that of alkali metal polysulfides, a greater mass percentage of the first material in the positive electrode active material than the second material is beneficial for improving the capacity of the secondary battery cell. A mass percentage of the first material in the positive electrode active material within the above-mentioned range is beneficial for improving the capacity of the secondary battery cell. A mass percentage of the second material in the positive electrode active material within the above-mentioned range is beneficial for suppressing the dissolution and shuttle of intermediate products generated by the first material during the conversion reaction through the third material, thereby further improving the capacity of the secondary battery cell.

[0013] In some implementations, the positive electrode active material satisfies one or more of the following characteristics:

[0014] (1) The powder conductivity of the positive electrode active material is 10. -8 S / cm~10 -7 S / cm. This is beneficial for improving electrode reaction kinetics;

[0015] (2) The volume distribution particle size Dv50 of the positive electrode active material is 500 nm to 700 nm. This is beneficial to improving the uniformity and consistency of the conversion reaction, and further beneficial to improving the capacity of the secondary battery cell;

[0016] (3) The particle size distribution (Dv90-Dv10) / Dv50 of the positive electrode active material is 0.5-0.7. This is beneficial to improving the uniformity and consistency of the conversion reaction, and further beneficial to improving the capacity of the secondary battery cell;

[0017] (4) The single particles of the positive electrode active material have a rhombohedral structure. This is beneficial to improving the dispersion uniformity of the positive electrode active material in the positive electrode film layer, and further beneficial to improving the charge and discharge efficiency of the secondary battery cell.

[0018] In some embodiments, the positive electrode active material further includes a third material distributed on at least a portion of the surface of the first material. This allows the third material to prevent direct contact between the intermediate products generated by the first material during the conversion reaction and the electrolyte, thereby suppressing the dissolution and shuttle of the intermediate products and ultimately improving the capacity of the secondary battery cell.

[0019] In some embodiments, the third material comprises an organic compound containing an alkali metal and / or an inorganic compound containing an alkali metal. Thus, the third material can exist stably in the electrolyte and can suppress the dissolution and shuttle of intermediate products generated by the first material during the conversion reaction, which is beneficial for improving the capacity of the secondary battery cell.

[0020] In some embodiments, the alkali metal-containing organic compound includes one or more of R-OM, R-OCO2M, and R-COOM, wherein R is selected from alkyl groups having 1-3 carbon atoms, and M includes one of Li, Na, and K; and / or, the alkali metal-containing inorganic compound includes one or more of alkali metal fluorides, alkali metal carbonates, and alkali metal oxides. Optionally, the alkali metal-containing inorganic compound includes one or more of MF, M2CO3, and M2O, wherein M includes one of Li, Na, and K. Thus, the third material can exist stably in the electrolyte and can suppress the dissolution and shuttle of intermediate products generated by the first material during the conversion reaction, which is beneficial for improving the capacity of the secondary battery cell.

[0021] In some embodiments, the total mass percentage of the second and third materials in the positive electrode active material is 5% to 20%. This is beneficial for suppressing the dissolution and shuttle of intermediate products generated by the first material during the conversion reaction through the third material, and for improving the capacity of the secondary battery cell.

[0022] The second aspect of this disclosure provides a method for preparing a positive electrode active material, used to prepare the positive electrode active material of the first aspect. The method for preparing the positive electrode active material includes the following steps: dissolving elemental sulfur to obtain a sulfur solution; dispersing an alkali metal monosulfide to obtain an alkali metal monosulfide dispersion; adding the sulfur solution to the alkali metal monosulfide dispersion to obtain a mixed solution; and drying the mixed solution to obtain the positive electrode active material. The positive electrode active material includes a first material and a second material. The first material includes an alkali metal monosulfide, and the second material includes an alkali metal polysulfide. The second material is distributed on at least a portion of the surface of the first material.

[0023] Elemental sulfur can react with alkali metal monosulfides to form alkali metal polysulfides. Therefore, in this disclosure, by adding a sulfur solution to an alkali metal monosulfide dispersion, it is beneficial for the sulfur in the sulfur solution to react with some of the solid alkali metal monosulfides in the alkali metal monosulfide dispersion, thereby generating a positive electrode active material containing alkali metal monosulfides and alkali metal polysulfides.

[0024] In addition, by dispersing alkali metal monosulfides to form a dispersion, the agglomerated alkali metal monosulfide powder can be dispersed in the solvent to form nano-sized alkali metal monosulfides. This results in smaller particle sizes of the alkali metal monosulfides, which helps to shorten the transport paths of ions and electrons, thereby improving the ionic conductivity and electronic conductivity of the positive electrode active material and enhancing the cycle performance of the secondary battery cell.

[0025] In some embodiments, elemental sulfur is dissolved in a first solvent, which includes one or more of toluene and carbon disulfide; and / or, alkali metal monosulfides are dispersed in a second solvent, which also includes one or more of toluene and carbon disulfide. Thus, elemental sulfur can dissolve in the first solvent to form a sulfur solution, while alkali metal monosulfides cannot dissolve in the second solvent to form an alkali metal monosulfide dispersion. This facilitates the reaction between sulfur in the sulfur solution and the solid alkali metal monosulfides, thereby generating alkali metal polysulfides on the surface of the alkali metal monosulfides.

[0026] In some embodiments, the mass ratio of elemental sulfur to alkali metal monosulfides is 1:(20-40). This is advantageous because it ensures that the amount of alkali metal polysulfides generated from the reaction of elemental sulfur with alkali metal monosulfides is within a suitable range.

[0027] In some embodiments, the drying process includes rotary evaporation, which includes one or more of the following characteristics: (1) a rotary evaporation vacuum of 0.1 MPa to 1 MPa; (2) a rotary evaporation temperature of 40°C to 80°C; and (3) a rotary evaporation time of 0.5 h to 2 h. Under these conditions, the evaporation rates of the first and second solvents in the mixed solution can be controlled, thereby uniformly generating alkali metal polysulfides on the surface of alkali metal monosulfides.

[0028] In some embodiments, the preparation method further includes the following steps: assembling a positive electrode sheet using a positive electrode active material, an alkali metal counter electrode, and a first electrolyte into a coin cell; and forming a third material on at least a portion of the surface of the first material after the first discharge cycle of the coin cell. Thus, the third material can prevent the direct contact between the intermediate products generated by the first material during the conversion reaction and the electrolyte, thereby inhibiting the dissolution and shuttle of the intermediate products in the electrolyte, which is beneficial for improving the capacity of the secondary battery cell.

[0029] In some embodiments, the first electrolyte includes a first electrolyte salt and a third solvent, the third solvent including a compound capable of undergoing a nucleophilic reaction with the second material. Thus, during the first discharge cycle of the coin cell, the second material can undergo a nucleophilic reaction with both the third solvent and the first electrolyte salt, resulting in an insoluble precipitate that deposits on the surface of the first material to form the third material. This suppresses the dissolution and shuttle of polysulfides, which is beneficial for improving the capacity of the secondary battery cell.

[0030] In some embodiments, the third solvent has a cyclic structure. Thus, during the first discharge cycle of the coin cell, the cyclic third solvent can undergo a nucleophilic reaction with the second material, causing the third solvent to polymerize after ring opening, forming a three-dimensional network structure of the third material. This third material can suppress the dissolution and shuttle of intermediate products during charging and discharging, which is beneficial for improving the capacity of the secondary battery cell.

[0031] In some embodiments, the third solvent includes ester solvents, including one or more of vinylene carbonate, fluoroethylene carbonate, propylene carbonate, and ethylene carbonate.

[0032] In some embodiments, the first electrolyte salt includes one or more of bis(trifluoromethylsulfonyl)imide salt, bis(trifluoromethylsulfonyl)imide salt, hexafluorophosphate, sodium perchlorate salt, and trifluoromethanesulfonate.

[0033] In some embodiments, the concentration of the first electrolyte is 0.1 mol / L to 2 mol / L. This facilitates the formation of a third material on the surface of the first material that can stably exist in the electrolyte, thereby improving the capacity of the secondary battery cell.

[0034] The third aspect of this disclosure provides a secondary battery cell, including a positive electrode sheet, the positive electrode sheet including a positive current collector and a positive electrode film layer located on at least one side of the positive current collector and including a positive electrode active material, the positive electrode active material including the positive electrode active material provided in the first aspect of this disclosure, or including the positive electrode active material prepared by the preparation method provided in the second aspect of this disclosure.

[0035] In some embodiments, the secondary battery cell further includes a negative electrode sheet, which includes a negative current collector and an alkali metal layer located on the surface of the negative current collector, the alkali metal layer including an alkali metal.

[0036] In some embodiments, the thickness of the alkali metal layer is 0.5 mm to 1 mm. This is beneficial for improving the energy density of the secondary battery cell.

[0037] In some embodiments, the secondary battery cell further includes a second electrolyte comprising a second electrolyte salt and a fourth solvent, the fourth solvent being an ether solvent. Ether solvents do not react with alkali metals; therefore, using an ether solvent as the second electrolyte is beneficial for improving the capacity of the secondary battery cell.

[0038] In some embodiments, the fourth solvent includes one or more of tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethylene, and 1,3-dioxolane. Therefore, the aforementioned fourth solvent does not react with alkali metals, which is beneficial for increasing the capacity of the secondary battery cell.

[0039] The fourth aspect of this disclosure provides an electrical device that includes a secondary battery cell as described in the third aspect. Attached Figure Description

[0040] Figure 1 is a schematic diagram of a secondary battery cell according to an embodiment of the present disclosure.

[0041] Figure 2 is an exploded view of a secondary battery cell according to an embodiment of the present disclosure shown in Figure 1.

[0042] Figure 3 is a schematic diagram of a battery module according to an embodiment of the present disclosure.

[0043] Figure 4 is a schematic diagram of a battery pack according to one embodiment of the present disclosure.

[0044] Figure 5 is an exploded view of a battery pack according to an embodiment of the present disclosure, as shown in Figure 4.

[0045] Figure 6 is a schematic diagram of a power supply device using a battery device according to an embodiment of the present disclosure.

[0046] Figure 7 is the X-ray diffraction pattern of the positive electrode active material in Embodiment 1 of this disclosure.

[0047] Figure 8 is a scanning electron microscope image of the positive electrode active material in Embodiment 1 of this disclosure.

[0048] Explanation of reference numerals in the attached drawings: 1 Battery pack; 2 First housing; 3 Second housing; 4 Battery module; 5 Battery cell; 51 Housing; 52 Electrode assembly; 53 Top cover assembly. Detailed Implementation

[0049] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the positive electrode active material, its preparation method, secondary battery cell, and power application device of this disclosure. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand this disclosure and are not intended to limit the subject matter of the claims.

[0050] The "range" disclosed in this disclosure is defined by a lower limit and an upper limit, whereby a given range is defined by selecting a lower limit and an upper limit, which define the boundaries of the particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also expected. Furthermore, if minimum range values ​​1 and 2 are listed, and if maximum range values ​​3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this disclosure, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0051] Unless otherwise specified, all embodiments and optional embodiments of this disclosure can be combined to form new technical solutions.

[0052] Unless otherwise specified, all technical features and optional technical features of this disclosure can be combined to form new technical solutions.

[0053] Unless otherwise specified, all steps of this disclosure may be performed sequentially or randomly, preferably sequentially. For example, if a method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if it is mentioned that the method may also include step (c), it means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0054] Unless otherwise specified, the terminology used in this disclosure has the common meaning as commonly understood by those skilled in the art.

[0055] Unless otherwise specified, the values ​​of the parameters mentioned in this disclosure can be determined using various test methods commonly used in the art, for example, according to the test methods given in this disclosure.

[0056] Currently, ether-based electrolytes, which do not react with the sodium metal anode, are typically used in sodium-sulfur battery systems. However, during the conversion reaction, sulfur, the active material at the cathode, generates an intermediate phase, sodium polysulfide. Sodium polysulfide has high solubility in ether-based electrolytes, and the dissolved sodium polysulfide loses electrical contact with the cathode current collector, preventing further electrochemical reactions and causing capacity loss in the secondary battery. Furthermore, after diffusing to the anode, sodium polysulfide can react chemically with the sodium metal anode to form solid products. These solid products coat the surface of the sodium metal anode, hindering its subsequent insertion into the cathode. This "shuttle effect" of sodium polysulfide further deteriorates the capacity of the secondary battery. Therefore, the use of ether-based electrolytes in current sodium-sulfur battery systems suffers from low capacity.

[0057] In related technologies, on the one hand, adding a catalyst (such as titanium nitride) to the positive electrode active material can promote the conversion rate of sodium polysulfide, an intermediate product during the conversion reaction, and inhibit its continued dissolution and diffusion in ether electrolytes. However, specific catalysts only have adsorption and catalytic effects on specific intermediate products, and solid-phase catalysts are difficult to uniformly disperse in the positive electrode active material when their content is low, and they may also cover the surface of the positive electrode active material, affecting the occurrence of the conversion reaction. On the other hand, an intermediate layer containing polar substances (such as boron nitride) can be set between the positive and negative electrodes to adsorb sodium polysulfide and hinder its diffusion to the negative electrode, but this method will seriously affect the energy density of the secondary battery. In addition, a carbon layer can be coated on the surface of the positive electrode active material to physically block the dissolution and diffusion of sodium polysulfide, but due to defects in the carbon layer, sodium dissolution of polysulfides is still severe, making this method an ineffective means of solving the problem of sodium polysulfide dissolution. In summary, none of the improved methods in related technologies can effectively solve the problem of sodium polysulfide dissolution.

[0058] Based on this, this disclosure proposes a positive electrode active material and its preparation method, a secondary battery cell, and an electrical device. The secondary battery cell prepared by this disclosure has improved capacity. The following provides a more detailed description of this disclosure and optional embodiments.

[0059] Positive electrode active material

[0060] The first aspect of this disclosure provides a positive electrode active material, comprising a first material and a second material, wherein the first material comprises an alkali metal monosulfide and the second material comprises an alkali metal polysulfide, and the second material is distributed on at least a portion of the surface of the first material.

[0061] In this disclosure, the positive electrode active material includes a first material, an alkali metal monosulfide, and a second material, an alkali metal polysulfide. Since the second material can undergo an electrochemical reaction to transform into a third material that can exist stably in the electrolyte, and since the second material is distributed on at least a portion of the surface of the first material, the third material can be formed on at least a portion of the surface of the first material. In this way, the third material can prevent the intermediate products generated by the first material during the conversion reaction from directly contacting the electrolyte, thereby inhibiting the dissolution and shuttle of the intermediate products in the electrolyte, which is beneficial to reducing the consumption of the negative electrode metal layer and thus beneficial to improving the capacity of the secondary battery cell.

[0062] In some embodiments, the second material covers the surface of the first material; alternatively, the second material may partially or completely cover the first material.

[0063] In some embodiments, the second material completely encapsulates the first material, i.e., the first material forms the core and the second material forms the shell, creating a core-shell structure. Thus, the resulting third material, which is stable in the electrolyte, can completely encapsulate the first material. This is more conducive to suppressing the dissolution and shuttle of intermediate products generated by the first material during the conversion reaction, thereby further improving the capacity of the secondary battery cell.

[0064] In some embodiments, the first material comprises a material with the chemical formula M2S, and the second material comprises a material with the chemical formula M2S. x The material, wherein M includes one of Li, Na, and K, 2≤x≤8, and optionally, M includes Li or Na.

[0065] In some implementations, the first material includes Na2S.

[0066] In some embodiments, the second material includes one or more of Na2S2 and Na2S5.

[0067] In some embodiments, the positive electrode active material includes Na2S2, and the mass percentage of Na2S2 in the positive electrode active material is less than or equal to 10%. Optionally, the mass percentage of Na2S2 in the positive electrode active material is less than or equal to 5%. Exemplarily, the mass percentage of Na2S2 in the positive electrode active material is a value within a range of 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any combination thereof.

[0068] In some embodiments, the positive electrode active material includes Na2S5, and the mass percentage of Na2S5 in the positive electrode active material is less than or equal to 10%. Optionally, the mass percentage of Na2S5 in the positive electrode active material is less than or equal to 5%. Exemplarily, the mass percentage of Na2S5 in the positive electrode active material is a value within a range of 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any combination thereof.

[0069] Therefore, it is beneficial to form a third material that can exist stably in the electrolyte on the surface of the first material, thereby suppressing the dissolution and shuttle of intermediate products generated by the first material during the conversion reaction, which in turn helps to improve the capacity of the secondary battery cell.

[0070] In some embodiments, the mass percentage of the first material in the positive electrode active material is greater than that of the second material. Since the capacity of alkali metal monosulfides is greater than that of alkali metal polysulfides, having a greater mass percentage of the first material in the positive electrode active material than that of the second material is beneficial for increasing the capacity of the secondary battery cell.

[0071] In some embodiments, the first material accounts for 80% to 95% of the mass of the positive electrode active material, optionally 88% to 92%. Since the first material, an alkali metal monosulfide, has a large capacity, a mass percentage of the first material in the positive electrode active material within the above range results in a larger capacity for the positive electrode active material, which is beneficial for increasing the capacity of the secondary battery cell. Exemplarily, the mass percentage of the first material in the positive electrode active material is a value within a range of 80%, 82%, 85%, 86%, 88%, 90%, 92%, 95%, or any combination thereof.

[0072] In some embodiments, the second material accounts for 5% to 20% of the mass of the positive electrode active material, optionally 8% to 10%. The mass percentage of the second material in the positive electrode active material within the above range ensures that the mass percentage of the generated third material is within a suitable range. This is beneficial for suppressing the dissolution and shuttle of intermediate products generated by the first material during the conversion reaction, thereby improving the capacity of the secondary battery cell. For example, the mass percentage of the second material in the positive electrode active material is a value within a range of 5%, 6%, 8%, 10%, 12%, 15%, 18%, 20%, or any combination thereof.

[0073] In this disclosure, the mass percentage of the first material in the positive electrode active material can be determined by titration. Specifically, the secondary battery cell is disassembled, the positive electrode sheet is removed, powder is scraped from the positive electrode film, and the scraped powder is sintered to remove the conductive agent and binder, thus obtaining the positive electrode active material. An excess of iodine standard solution is added to a certain amount of positive electrode active material, causing the M2S in the positive electrode active material to react with iodine. After the reaction is complete, the remaining iodine standard solution is titrated with sodium thiosulfate standard solution. The amount of remaining elemental iodine is calculated according to the reaction formula 2NaS2O3 + I2 = Na2S4O6 + 2NaI, and the mass of the alkali metal monosulfide of the first material is calculated according to the reaction formula M2S + I2 = MI + S.

[0074] In some embodiments, the powder conductivity of the positive electrode active material is 10. -8 S / cm~10 -7 S / cm. This is beneficial for improving electrode reaction kinetics. For example, the powder conductivity of the positive electrode active material is 1×10⁻⁶. -8 S / cm, 2×10 -8 S / cm, 5×10 -8 S / cm, 8×10 -8 S / cm, 1×10 -7 The value between S / cm or any two of them.

[0075] In this disclosure, the powder conductivity of the positive electrode active material has a meaning known in the art and can be measured using instruments and methods known in the art. For example, it can be measured using the four-probe method, referring to GB / T 30835-2014. The testing instrument can be a Crystallography ST2263 dual-electrical-measurement digital four-probe tester.

[0076] In some embodiments, the volumetric particle size distribution (Dv50) of the positive electrode active material is 500 nm to 700 nm. Exemplarily, the volumetric particle size distribution (Dv50) is a value within the range of 500 nm, 550 nm, 600 nm, 650 nm, 700 nm, or any combination thereof. In some embodiments, the particle size distribution (Dv90-Dv10) / Dv50 of the positive electrode active material is 0.5 to 0.7. Exemplarily, the particle size distribution (Dv90-Dv10) / Dv50 is a value within the range of 0.5, 0.55, 0.6, 0.65, 0.7, or any combination thereof. This is beneficial for improving the uniformity and consistency of the conversion reaction, and further beneficial for increasing the capacity of the secondary battery cell.

[0077] In this disclosure, the volume distribution particle sizes Dv10, Dv50, and Dv90 of the positive electrode active material represent the particle sizes corresponding to a cumulative volume distribution percentage of 10%, 50%, and 90%, respectively, and can be determined using instruments and methods known in the art. For example, they can be determined using a laser particle size analyzer, referring to GB / T 19077-2016. The testing instrument can be a Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd., UK.

[0078] In some embodiments, the individual particles of the positive electrode active material have a rhombohedral structure. This is beneficial for improving the dispersion uniformity of the positive electrode active material in the positive electrode film layer, and further beneficial for improving the charge and discharge efficiency of the secondary battery cell.

[0079] In this disclosure, the morphology of the positive electrode active material can be observed using a scanning electron microscope (SEM).

[0080] In some embodiments, the positive electrode active material further includes a third material distributed on at least a portion of the surface of the first material. This third material can prevent direct contact between the intermediate products generated by the first material during the conversion reaction and the electrolyte, thereby suppressing the dissolution and shuttle of the intermediate products, which helps reduce the consumption of the negative electrode metal layer and thus improves the capacity of the secondary battery cell.

[0081] In some embodiments, the third material comprises an organic compound containing an alkali metal and / or an inorganic compound containing an alkali metal. Optionally, the organic compound containing an alkali metal comprises one or more of R-OM, R-OCO2M, and R-COOM, wherein R is selected from alkyl groups having 1-3 carbon atoms, and M comprises one of Li, Na, and K; and / or, the inorganic compound containing an alkali metal comprises one or more of alkali metal fluorides, alkali metal carbonates, and alkali metal sulfates. More preferably, the inorganic compound containing an alkali metal comprises one or more of MF, M2CO3, and M2O, wherein M comprises one of Li, Na, and K. Thus, the third material can exist stably in the electrolyte and can suppress the dissolution and shuttle of intermediate products generated by the first material during the conversion reaction, which is beneficial to improving the capacity of the secondary battery cell.

[0082] As an example, R is selected from methyl, ethyl, n-propyl, or isopropyl.

[0083] In some embodiments, the third material may include one or more of CH3-ONa, CH3CH2-ONa, CH3CH2CH2-ONa, CH3-OCO2Na, CH3CH2-OCO2Na, CH3CH2CH2-OCO2Na, CH3-COONa, CH3CH2-COONa, and CH3CH2CH2-COONa.

[0084] In some embodiments, the third material may include one or more of NaF, Na2CO3, and Na2O.

[0085] In some embodiments, the total mass percentage of the second and third materials in the positive electrode active material is 5% to 20%. This is beneficial because the third material helps to suppress the dissolution and shuttle of intermediate products generated by the first material during the conversion reaction, thereby improving the capacity of the secondary battery cell. Exemplarily, the total mass percentage of the second and third materials in the positive electrode active material is a value within a range of 5%, 6%, 8%, 10%, 12%, 15%, 18%, 20%, or any combination thereof.

[0086] In this disclosure, the total mass percentage of the second and third materials in the positive electrode active material can be determined by the following method: Disassemble the secondary battery cell, remove the positive electrode sheet, scrape powder from the positive electrode film, and sinter the scraped powder to remove the conductive agent and binder, thus obtaining the positive electrode active material. Take a certain mass of the positive electrode active material, and determine the mass of the first material in the positive electrode active material using the above titration method. Subtract the mass of the first material from the mass of the positive electrode active material to obtain the total mass of the second and third materials.

[0087] Preparation method of positive electrode active material

[0088] The second aspect of this disclosure provides a method for preparing a positive electrode active material, used to prepare the positive electrode active material of the first aspect. The method for preparing the positive electrode active material includes the following steps: dissolving elemental sulfur to obtain a sulfur solution; dispersing an alkali metal monosulfide to obtain an alkali metal monosulfide dispersion; adding the sulfur solution to the alkali metal monosulfide dispersion to obtain a mixed solution; and drying the mixed solution to obtain the positive electrode active material. The positive electrode active material includes a first material and a second material, the first material including an alkali metal monosulfide, and the second material including an alkali metal polysulfide, the second material being distributed on at least a portion of the surface of the first material.

[0089] Elemental sulfur can react with alkali metal monosulfides to form alkali metal polysulfides. Therefore, in this disclosure, by adding a sulfur solution to an alkali metal monosulfide dispersion, it is beneficial for the sulfur in the sulfur solution to react with some of the solid alkali metal monosulfides in the alkali metal monosulfide dispersion, thereby generating a positive electrode active material containing alkali metal monosulfides and alkali metal polysulfides.

[0090] In addition, by dispersing alkali metal monosulfides to form a dispersion, the agglomerated alkali metal monosulfide powder can be dispersed in the solvent to form nano-sized alkali metal monosulfides. This results in smaller particle sizes of the alkali metal monosulfides, which helps to shorten the transport paths of ions and electrons, thereby improving the ionic conductivity and electronic conductivity of the positive electrode active material and enhancing the cycle performance of the secondary battery cell.

[0091] In some embodiments, elemental sulfur is dissolved in a first solvent, which includes one or more of toluene and carbon disulfide; and / or, alkali metal monosulfides are dispersed in a second solvent, which includes one or more of toluene and carbon disulfide. Optionally, both the first and second solvents include toluene. Thus, elemental sulfur can dissolve in the first solvent to form a sulfur solution, while the alkali metal monosulfides cannot dissolve in the second solvent to form an alkali metal monosulfide dispersion. Adding the sulfur solution to the alkali metal monosulfide dispersion facilitates the reaction between sulfur in the sulfur solution and the solid alkali metal monosulfides, thereby generating alkali metal polysulfides on the surface of the alkali metal monosulfides.

[0092] In some embodiments, the mass ratio of elemental sulfur to the alkali metal monosulfide is 1:(20-40). A mass ratio of elemental sulfur to the alkali metal monosulfide within this range is advantageous because it ensures that the amount of alkali metal polysulfides formed by the reaction of elemental sulfur with the alkali metal monosulfide is within a suitable range. Exemplarily, the mass ratio of elemental sulfur to the alkali metal monosulfide is a value within a range of 1:20, 1:25, 1:30, 1:35, 1:40, or any combination thereof.

[0093] In some embodiments, the drying process includes one or more of rotary evaporation and thermal evaporation. Through the drying process, the first and second solvents in the mixed solution can be completely evaporated, thereby obtaining the positive electrode active material.

[0094] In some embodiments, the drying process includes rotary evaporation, which includes one or more of the following features: (1) a rotary evaporation vacuum of 0.1 MPa to 1 MPa, exemplarily, a value between 0.1 MPa, 0.2 MPa, 0.4 MPa, 0.6 MPa, 0.8 MPa, 1 MPa, or any combination thereof; (2) a rotary evaporation temperature of 40°C to 80°C, exemplarily, a value between 40°C, 50°C, 60°C, 70°C, 80°C, or any combination thereof; and (3) a rotary evaporation time of 0.5 h to 2 h, exemplarily, a value between 0.5 h, 0.8 h, 1 h, 1.3 h, 1.5 h, 1.8 h, 2 h, or any combination thereof. Under the above conditions, the evaporation rates of the first and second solvents in the mixed solution can be controlled, thereby uniformly generating alkali metal polysulfides on the surface of the alkali metal monosulfides.

[0095] In some embodiments, the preparation method further includes the following steps: assembling the positive electrode sheet of the prepared positive electrode active material with an alkali metal counter electrode and a first electrolyte into a coin cell; after the first discharge cycle of the coin cell, forming a third material on at least a portion of the surface of the first material. Thus, the third material can prevent the direct contact between the intermediate products generated by the first material during the conversion reaction and the electrolyte, thereby inhibiting the dissolution and shuttle of the intermediate products in the electrolyte, which is beneficial for reducing the consumption of the negative electrode metal layer, and thus beneficial for increasing the capacity of the secondary battery cell.

[0096] In some embodiments, the first electrolyte includes a first electrolyte salt and a third solvent, the third solvent including a compound capable of undergoing a nucleophilic reaction with the second material. Thus, during the first discharge cycle of the coin cell, the second material can undergo a nucleophilic reaction with both the third solvent and the first electrolyte salt, resulting in an insoluble precipitate that deposits on the surface of the first material to form the third material. This third material further hinders the direct contact between the intermediate products generated during the conversion reaction of the first material and the electrolyte, thereby suppressing the dissolution and shuttle of polysulfides, reducing the consumption of the negative electrode metal layer, and contributing to an increase in the capacity of the secondary battery cell.

[0097] In some embodiments, the third solvent has a cyclic structure. Thus, during the first discharge cycle of the coin cell, the cyclic third solvent can undergo a nucleophilic reaction with the second material, causing the third solvent to polymerize after ring opening, forming a three-dimensional network structure of the third material. This third material can suppress the dissolution and shuttle of intermediate products during charging and discharging, reducing the consumption of the negative electrode metal layer and thus improving the capacity of the secondary battery cell.

[0098] In some embodiments, the third solvent includes an ester solvent, which includes one or more of vinylene carbonate (VC), fluoroethylene carbonate (FEC), propylene carbonate (PC), and ethylene carbonate (EC). Optionally, the ester solvent includes one or more of vinylene carbonate, fluoroethylene carbonate, and propylene carbonate.

[0099] In some embodiments, the first electrolyte salt includes one or more of bis(trifluoromethanesulfonyl)imide salt, hexafluorophosphate, sodium perchlorate salt, and trifluoromethanesulfonate. Optionally, the first electrolyte salt includes bis(trifluoromethanesulfonyl)imide salt.

[0100] In some embodiments, the concentration of the first electrolyte is 0.1 mol / L to 2 mol / L. A concentration within this range facilitates the formation of a third material on the surface of the first material that can stably exist in the electrolyte. This third material can suppress the dissolution and shuttle of intermediate products generated during the conversion reaction of the first material, thereby improving the capacity of the secondary battery cell. For example, the concentration of the first electrolyte is a value within the range of 0.1 mol / L, 0.3 mol / L, 0.5 mol / L, 0.8 mol / L, 1 mol / L, 1.3 mol / L, 1.5 mol / L, 1.8 mol / L, 2 mol / L, or any combination thereof.

[0101] Secondary battery cell

[0102] A third aspect of this disclosure provides a secondary battery cell, including a positive electrode sheet, the positive electrode sheet including a positive current collector and a positive electrode film layer located on at least one side of the positive current collector and including a positive electrode active material, the positive electrode active material including the positive electrode active material provided in the first aspect of this disclosure, or including the positive electrode active material prepared by the preparation method provided in the second aspect of this disclosure.

[0103] In some embodiments, the secondary battery cell further includes a negative electrode sheet, which includes a negative current collector and an alkali metal layer located on the surface of the negative current collector. The alkali metal layer includes an alkali metal. Exemplarily, the alkali metal includes sodium, lithium, potassium, etc.

[0104] In some embodiments, the thickness of the alkali metal layer is 0.5 mm to 1 mm. A thickness within this range is beneficial for increasing the energy density of the secondary battery cell. For example, the thickness of the alkali metal layer is a value within the range of 0.5 mm, 0.6 mm, 0.7 mm, 0.8 mm, 0.9 mm, 1 mm, or any combination thereof.

[0105] In this disclosure, the thickness of the alkali metal layer has a meaning known in the art and can be measured using instruments and methods known in the art. For example, a micrometer can be used for measurement. Specifically, the thickness of the negative electrode sheet is measured at at least 12 different locations along the thickness direction of the negative electrode sheet, and then the average value is taken as the thickness of the negative electrode sheet. The thickness of the negative current collector is then subtracted to obtain the thickness of the alkali metal layer.

[0106] In some embodiments, the secondary battery cell further includes a second electrolyte comprising a second electrolyte salt and a fourth solvent, the fourth solvent being an ether solvent. Ether solvents do not react with alkali metals; therefore, using an ether solvent as the second electrolyte is beneficial for improving the capacity of the secondary battery cell.

[0107] In some embodiments, the fourth solvent includes one or more of tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethylene, and 1,3-dioxolane; optionally, the fourth solvent includes tetraethylene glycol dimethyl ether. Therefore, the aforementioned fourth solvent does not react with alkali metals, which is beneficial for increasing the capacity of the secondary battery cell.

[0108] In some embodiments, the secondary battery cell of this disclosure can be prepared, for example, by the following methods:

[0109] The preparation of a positive electrode active material includes dissolving elemental sulfur to obtain a sulfur solution; dispersing alkali metal monosulfides to obtain an alkali metal monosulfide dispersion; adding the sulfur solution to the alkali metal monosulfide dispersion to obtain a mixed solution; and drying the mixed solution to obtain a positive electrode active material. The positive electrode active material includes a first material and a second material, wherein the first material includes alkali metal monosulfides and the second material includes alkali metal polysulfides.

[0110] A first positive electrode using the above-mentioned positive active material is assembled with an alkali metal counter electrode and a first electrolyte to form a coin cell. The coin cell is subjected to a first discharge to obtain a second positive electrode. The positive active material in the second positive electrode includes a third material, which is distributed on at least a portion of the surface of the first material.

[0111] Disassemble the coin cell after the first discharge cycle, assemble the obtained second positive electrode, separator, and negative electrode into an electrode assembly, inject the second electrolyte, and obtain a secondary battery cell.

[0112] In some embodiments, elemental sulfur is dissolved in a first solvent, which includes one or more of toluene and carbon disulfide; and / or, alkali metal monosulfides are dispersed in a second solvent, which includes one or more of toluene and carbon disulfide. Optionally, both the first and second solvents include toluene.

[0113] In some embodiments, the mass ratio of elemental sulfur to alkali metal monosulfide is 1:(20-40). A mass ratio of elemental sulfur to alkali metal monosulfide within this range is advantageous because it ensures that the amount of alkali metal polysulfides generated from the reaction of elemental sulfur with alkali metal monosulfides is within a suitable range.

[0114] In some embodiments, the drying process includes rotary evaporation, which includes one or more of the following features:

[0115] (1) The rotary evaporation vacuum degree is 0.1 MPa to 1 MPa; (2) The rotary evaporation temperature is 40℃ to 80℃; (3) The rotary evaporation time is 0.5 h to 2 h. Under the above conditions, the evaporation rate of the first solvent and the second solvent in the mixed solution can be controlled, thereby uniformly generating alkali metal polysulfides on the surface of alkali metal monosulfides.

[0116] In some embodiments, the first electrolyte includes a first electrolyte salt and a third solvent, the third solvent including a compound capable of undergoing a nucleophilic reaction with the second material. Thus, during the first discharge cycle of the coin cell, the second material can undergo a nucleophilic reaction with both the third solvent and the first electrolyte salt, resulting in an insoluble precipitate that deposits on the surface of the first material to form the third material. This third material further hinders the direct contact between the intermediate products generated during the conversion reaction of the first material and the electrolyte, thereby suppressing the dissolution and shuttle of intermediate products, reducing the consumption of the negative electrode metal layer, and ultimately improving the capacity of the secondary battery cell.

[0117] In some embodiments, the third solvent has a cyclic structure. Thus, during the first discharge cycle of the coin cell, the cyclic third solvent can undergo a nucleophilic reaction with the second material, causing the third solvent to polymerize after ring opening, forming a three-dimensional network structure of the third material. This third material can suppress the dissolution and shuttle of intermediate products during charging and discharging, reducing the consumption of the negative electrode metal layer and thus improving the capacity of the secondary battery cell.

[0118] In some embodiments, the third solvent includes an ester solvent, which includes one or more of vinylene carbonate, fluoroethylene carbonate, propylene carbonate, and ethylene carbonate. Optionally, the ester solvent includes one or more of vinylene carbonate, fluoroethylene carbonate, and propylene carbonate.

[0119] In some embodiments, the first electrolyte salt includes one or more of bis(trifluoromethanesulfonyl)imide salt, hexafluorophosphate, sodium perchlorate salt, and trifluoromethanesulfonate. Optionally, the first electrolyte salt includes bis(trifluoromethanesulfonyl)imide salt.

[0120] In some embodiments, the concentration of the first electrolyte is 0.1 mol / L to 2 mol / L. A concentration within this range facilitates the formation of a third material on the surface of the first material that can stably exist in the electrolyte. This third material can suppress the dissolution and shuttle of intermediate products generated during the conversion reaction of the first material, thereby improving the capacity of the secondary battery cell.

[0121] In some embodiments, the second electrolyte comprises a second electrolyte salt and a fourth solvent. The fourth solvent includes an ether solvent. Optionally, the fourth solvent includes one or more of tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethylene, and 1,3-dioxolane. More preferably, the fourth solvent includes tetraethylene glycol dimethyl ether. Ether solvents do not react with alkali metals; therefore, using an ether solvent as the second electrolyte is beneficial for improving the capacity of the secondary battery cell.

[0122] Negative electrode sheet

[0123] When the secondary battery is an alkali metal ion secondary battery, the negative electrode sheet includes a negative current collector and a negative active material layer, the negative active material layer including a negative active material. This disclosure does not specify any particular requirements for the negative active material, which can include negative active materials suitable for alkali metal ion secondary batteries in the art. For example, the negative active material can include at least one of carbon materials and silicon materials. Optionally, the carbon material includes one or more of artificial graphite, natural graphite, hard carbon, and soft carbon. Optionally, the silicon material includes one or more of elemental silicon, silicon oxide compounds, silicon carbide compounds, and silicon alloys.

[0124] When the secondary battery is an alkali metal secondary battery, the negative electrode includes a negative current collector and an alkali metal layer. The alkali metal layer can be prepared using methods known in the art.

[0125] In some implementations, the alkali metal layer is formed by the deposition of alkali metal ions on the negative electrode current collector during the charging and discharging process of the battery, and is also known as a negative electrode-free alkali metal secondary battery. A negative electrode-free alkali metal secondary battery typically refers to a secondary battery in which a negative electrode active material layer is not actively set on the negative electrode side during the battery manufacturing process. For example, during the secondary battery manufacturing process, a metal layer, carbon-containing material layer, or silicon-containing material layer is not set on the negative electrode current collector through processes such as deposition or coating. Instead, during the first charge of the battery, alkali metal ions gain electrons at the negative electrode and deposit on the surface of the negative electrode current collector to form a metal phase. During discharge, the metal phase can transform into metal ions and return to the positive electrode, thereby achieving cyclic charging and discharging.

[0126] In some embodiments, the negative electrode sheet may include a negative current collector and a first metal layer disposed on at least one surface of the negative current collector, wherein the metal element in the first metal layer may include one or more alkali metal elements.

[0127] In some embodiments, the metallic material in the first metal layer may include one or more of elemental lithium, lithium alloys, elemental sodium, sodium alloys, elemental potassium, and potassium alloys. The lithium alloy may be an alloy of metallic lithium with other metallic or non-metallic elements. For example, other metallic elements in the lithium alloy may include one or more of tin, zinc, aluminum, magnesium, silver, gold, gallium, indium, and platinum, and non-metallic elements in the lithium alloy may include one or more of boron, carbon, and silicon. The sodium alloy may be an alloy of metallic sodium with other metallic or non-metallic elements. For example, other metallic elements in the sodium alloy may include one or more of tin, zinc, aluminum, magnesium, silver, gold, gallium, indium, and platinum, and non-metallic elements in the sodium alloy may include one or more of boron, carbon, and silicon. The potassium alloy may be an alloy of metallic magnesium with other metallic or non-metallic elements. For example, other metallic elements in the potassium alloy may include one or more of tin, zinc, aluminum, magnesium, silver, gold, gallium, indium, and platinum, and non-metallic elements in the potassium alloy may include one or more of boron, carbon, and silicon.

[0128] In some embodiments, the negative electrode current collector may include a metal foil, a three-dimensional porous current collector, or a composite current collector. Examples of metal foils include copper foil, copper alloy foil, nickel foil, nickel alloy foil, aluminum foil, and aluminum alloy foil. Examples of three-dimensional porous current collectors include copper mesh, nickel mesh, aluminum mesh, copper foam, nickel foam, and aluminum foam. A composite current collector may include a polymer material substrate and a metal material layer formed on at least one surface of the polymer material substrate. Examples of metal materials include, but are not limited to, one or more of copper, copper alloys, aluminum, aluminum alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. Examples of polymer material substrates include, but are not limited to, one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0129] Positive electrode sheet

[0130] The positive electrode sheet 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 includes the positive electrode active material of this disclosure, or the positive electrode active material prepared according to the preparation method of this disclosure.

[0131] 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.

[0132] 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.).

[0133] 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.

[0134] 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.

[0135] 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 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.

[0136] electrolytes

[0137] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This disclosure does not impose specific limitations on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel-like, or entirely solid.

[0138] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0139] In some embodiments, the electrolyte salt may be selected from at least one of hexafluorophosphate, tetrafluoroborate, perchlorate, hexafluoroarsenate, bis(fluorosulfonyl)imide, bis(trifluoromethanesulfonyl)imide, trifluoromethanesulfonate, difluorophosphate, difluorooxalate borate, dioxalate borate, difluorodioxalate phosphate, and tetrafluorooxalate phosphate. As an example, the electrolyte salt may include one of lithium, sodium, and potassium salts.

[0140] In some embodiments, the electrolyte may optionally include 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.

[0141] Separating membrane

[0142] In some embodiments, the secondary battery cell also includes a separator. This disclosure does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0143] 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.

[0144] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0145] In some embodiments, the secondary battery cell may include an outer packaging. This outer packaging can be used to encapsulate the aforementioned electrode assembly and electrolyte.

[0146] In some embodiments, the outer packaging of the secondary battery cell can be a rigid shell, such as a hard plastic shell, aluminum shell, or steel shell. The outer packaging of the secondary battery cell can also be a soft pack, such as a pouch. The soft pack can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0147] This disclosure does not impose any particular limitation on the shape of the secondary battery cell; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 shows a square-structured secondary battery cell 5 as an example.

[0148] In some embodiments, referring to FIG2, the outer packaging may include a housing 51 and a top cover assembly 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the top cover assembly 53 can cover the opening to close the receiving cavity. The positive electrode sheet, negative electrode sheet, and separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery cell 5 may be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0149] In some implementations, the secondary battery cells can be assembled into a battery module. The number of secondary battery cells contained in the battery module can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0150] Figure 3 shows a battery module 4 as an example. Referring to Figure 3, in the battery module 4, multiple battery cells 5 can be arranged sequentially along the length of the battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple secondary battery cells 5 can be fixed in place using fasteners.

[0151] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple secondary battery cells 5 are received.

[0152] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0153] Figures 4 and 5 show a battery pack 1 as an example. Referring to Figures 4 and 5, the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper box 2 and a lower box 3, with the upper box 2 covering the lower box 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0154] Electrical appliances

[0155] The fourth aspect of this disclosure provides an electrical device that includes a secondary battery cell provided in the third aspect of this disclosure.

[0156] Secondary battery cells and the battery modules they are assembled into can be used as power sources for electrical devices or as energy storage units for electrical devices. Electrical devices can include, but are not limited to, mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0157] As an electrical device, you can choose from individual secondary battery cells, battery modules, or battery packs according to your usage requirements.

[0158] Figure 6 shows an example of an electrical device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of the secondary battery cells in this device, a battery pack or battery module can be used.

[0159] Example

[0160] The following describes embodiments of this disclosure. The embodiments described below are exemplary and are only used to explain this disclosure, and should not be construed as limiting this disclosure. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in the art or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0161] Example 1

[0162] Preparation of secondary battery cells:

[0163] (1) Preparation of positive electrode active material

[0164] a. Weigh 0.12 g of elemental sulfur that has been dried under low temperature and vacuum in a glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), add it to 20 mL of toluene solution that has been purged with argon gas, and shake for 3 min to obtain a sulfur solution;

[0165] b. Weigh 3.6g of anhydrous Na2S in a glove box, add it to 20mL of toluene solution purged with argon gas, and shake for 3min to make Na2S uniformly dispersed in toluene to obtain Na2S dispersion.

[0166] c. Add sulfur solution dropwise to Na2S dispersion, with a mass ratio of elemental sulfur to Na2S of 1:30, shake for 5 min to obtain a mixed solution; evaporate the mixed solution by rotary evaporation at 100℃ and 0.5MPa vacuum for 2 h to volatilize toluene to obtain positive electrode active material, wherein the positive electrode active material includes both the first material Na2S and the second materials Na2S2 and Na2S5, and at least a portion of the second material is distributed on the surface of the first material.

[0167] Characterization of positive electrode active materials:

[0168] (a) X-ray diffraction (XRD) test

[0169] The positive electrode active material was tested using an X-ray powder diffractometer (instrument model: Bruker D8 ADVANCE, target material: Cu Kα; voltage and current: 40KV / 40mA, scanning angle range: 10° to 70°).

[0170] Figure 7 shows the X-ray diffraction pattern of the positive electrode active material in Example 1. From the X-ray diffraction pattern, it can be observed that the positive electrode active material contains the first material Na2S, as well as the second materials Na2S2 and Na2S5.

[0171] (b) Volume distribution particle size test

[0172] The volume distribution particle size of the positive electrode active material was tested using a Malvern 2000 (MasterSizer 2000) laser particle size analyzer, following the standard procedure GB / T19077-2016 / ISO 13320:2009. In Example 1, the volume distribution particle size Dv50 of the positive electrode active material was 640 nm, and the particle size distribution (Dv90-Dv10) / Dv50 was 0.56.

[0173] (c) Powder conductivity test

[0174] The resistivity of the positive electrode active material powder was measured using a Crystallography ST2263 dual-electrical-measurement digital four-probe tester, following the standard procedure GB / T30835-2014. The powder resistivity of the positive electrode active material in Example 1 was 8 × 10⁻⁶. -8 S / cm.

[0175] (d) Scanning electron microscopy (SEM) testing

[0176] The microstructure of the positive electrode active material was obtained by SEM testing.

[0177] Figure 8 is a scanning electron microscope image of the positive electrode active material in Example 1. As shown in Figure 8, the positive electrode active material presents as a single particle with a rhombohedral structure.

[0178] The preparation method of the positive active material may also optionally include the following steps:

[0179] d. The above-prepared positive electrode active material, carbon nanotubes, conductive carbon black, and polyvinylidene fluoride are mixed in a weight ratio of 97:1:1:1 and then added to the solvent N-methylpyrrolidone. The mixture is stirred evenly to form a positive electrode slurry. The positive electrode slurry is coated onto the positive electrode current collector aluminum foil to form a positive electrode film. After drying and cold pressing, the first positive electrode sheet is obtained.

[0180] e. Dissolve sodium difluorosulfonamide (NaFSI) in vinylene carbonate (VC) solvent to obtain a first electrolyte with a NaFSI concentration of 1 mol / L;

[0181] f. Assemble the first positive electrode, the sodium metal counter electrode, and the first electrolyte into a coin cell. Discharge the coin cell to 0.5V at a constant current of 0.1C at 25°C. Then disassemble the coin cell to obtain the second positive electrode. The positive active material in the second positive electrode contains a third material. The third material is distributed on at least a portion of the surface of the first material and includes sodium fluoride.

[0182] Characterization of positive electrode active materials:

[0183] (e) Testing of the mass percentage of the first material and the total mass percentage of the second and third materials

[0184] Add 15 mL of 0.2 mol / L iodine standard solution (iodine dissolved in ethanol) to 0.1 g of the positive electrode active material. After reacting for 5 h, titrate the remaining iodine standard solution with 0.1 mol / L sodium thiosulfate standard solution until colorless. Calculate the amount of remaining elemental iodine according to the reaction formula 2NaS₂O₃ + I₂ = Na₂S₄O₆ + 2NaI. Then calculate the mass of the first material Na₂S according to the reaction formula Na₂S + I₂ = NaI + S. Subtract the mass of the first material from the mass of the positive electrode active material to obtain the total mass of the second and third materials. In Example 1, the mass percentage of the first material Na₂S in the positive electrode active material is 84%, and the total mass percentage of the second materials Na₂S₂ and Na₂S₅ and the third material sodium fluoride in the positive electrode active material is 16%.

[0185] (3) Preparation of button cells

[0186] g. Dissolve sodium difluorosulfonamide (NaFSI) in tetraethylene glycol dimethyl ether solvent to obtain a second electrolyte with a NaFSI concentration of 1 mol / L;

[0187] h. Use a sodium metal sheet as the counter electrode;

[0188] i. A polypropylene base film is used as the separator, and the thickness of the separator is 8μm;

[0189] j. Assemble the second positive electrode, sodium metal counter electrode, separator, and second electrolyte prepared above into a button cell in an argon-protected glove box.

[0190] Button cell battery performance test:

[0191] (1) First charging capacity test

[0192] At 25°C, the coin cell prepared above was charged to 3V with a constant current of 0.1C and left to stand for 2 minutes. The charging capacity at this time was recorded as the first charge capacity D0. The test results are recorded in Table 2 below.

[0193] (2) Discharge capacity test

[0194] At 25°C, the coin cell prepared above was charged to 3V with a constant current of 0.1C, left to stand for 5 minutes, and then discharged to 1V with a constant current of 0.1C to obtain the first discharge capacity C1. The above process was repeated 50 times to obtain the discharge capacity C2 of the 50th cycle. The test results are recorded in Table 2 below.

[0195] (3) Cyclic performance test

[0196] At 25°C, the coin cell prepared above was charged to 3V with a constant current of 0.1C, left to stand for 5 minutes, and then discharged to 1V with a constant current of 0.1C to obtain the first discharge capacity C1. The above process was repeated 50 times to obtain the discharge capacity C2 of the 50th cycle.

[0197] The capacity retention rate (%) of a button cell after 50 cycles is calculated as C2 / C1 × 100%, and the test results are recorded in Table 2 below.

[0198] Examples 2 to 9

[0199] The coin cell was prepared using the same method as in Example 1, except that the mass ratio of elemental sulfur to Na2S, and the types of the third solvent and the first electrolyte salt in the first electrolyte were adjusted according to Table 1 below.

[0200] Comparative Example 1

[0201] The coin cell was prepared using the same method as in Example 1, except that the positive electrode active material included only the first material Na2S, and did not include the second materials Na2S2 and Na2S5.

[0202] Comparative Example 2

[0203] The coin cell was prepared using the same method as in Example 1, except that the positive electrode active material included only the second materials Na2S2 and Na2S5, and did not include the first material Na2S.

[0204] The positive electrode active materials prepared in Examples 2 to 9 and Comparative Examples 1 and 2 were characterized using the same test methods as in Example 1, and the coin cells prepared in Examples 2 to 9 and Comparative Examples 1 and 2 were subjected to performance tests.

[0205] Table 1 below shows the relevant parameters of the positive electrode active materials in Examples 1 to 9 and Comparative Examples 1 and 2. Table 2 below shows the performance test results of the coin cells prepared in Examples 1 to 9 and Comparative Examples 1 and 2.

[0206] Table 1

[0207] Table 2

[0208] As can be seen from Tables 1 and 2, compared to Comparative Example 1 (positive electrode active material includes only the first material) and Comparative Example 2 (positive electrode active material includes only the second material), Examples 1 to 9, by controlling the mass ratio of elemental sulfur to Na2S to be 1:(20-40) and using the aforementioned third solvent and first electrolyte salt in the first electrolyte, resulted in positive electrode active materials comprising monosulfides of the alkali metal of the first material and polysulfides of the alkali metal of the second material, with the second material distributed on at least a portion of the surface of the first material, significantly improving the capacity of the secondary battery cells. Compared to Examples 1 to 4, the performance test results of the coin cells in Examples 5 to 9 were slightly worse. This may be due to the smaller amount of third material generated in Examples 5 to 9, or the poorer stability of the third material.

[0209] It should be noted that this disclosure is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same essential structure and achieving the same effect as the technical concept within the scope of this disclosure are included in the technical scope of this disclosure. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, are also included in the scope of this disclosure without departing from the spirit of this disclosure.

Claims

1. A positive electrode active material, comprising a first material and a second material, The first material comprises an alkali metal monosulfide, and the second material comprises a said alkali metal polysulfide; The second material is distributed on at least a portion of the surface of the first material.

2. The positive electrode active material according to claim 1, wherein, The first material includes a material with the chemical formula M2S, and the second material includes a material with the chemical formula M2S. x The material is given, wherein M includes one of Li, Na, and K, and 2 ≤ x ≤ 8.

3. The positive electrode active material according to claim 1 or 2, wherein, The first material includes Na2S; and / or the second material includes one or more of Na2S2 and Na2S5.

4. The positive electrode active material according to any one of claims 1 to 3, wherein, The positive electrode active material includes Na2S2, and the mass percentage of Na2S2 in the positive electrode active material is less than or equal to 10%; and / or, The positive electrode active material includes Na2S5, and the mass percentage of Na2S5 in the positive electrode active material is less than or equal to 10%.

5. The positive electrode active material according to any one of claims 1 to 4, wherein, In the positive electrode active material, the mass percentage of the first material is greater than the mass percentage of the second material; Optionally, the first material accounts for 80% to 95% of the mass of the positive electrode active material; Optionally, the second material accounts for 5% to 20% of the mass of the positive electrode active material.

6. The positive electrode active material according to any one of claims 1 to 5, wherein, The positive electrode active material satisfies one or more of the following characteristics: (1) The powder conductivity of the positive electrode active material is 10. -8 S / cm~10 -7 S / cm; (2) The volume distribution particle size Dv50 of the positive electrode active material is 500nm~700nm; (3) The particle size distribution (Dv90-Dv10) / Dv50 of the positive electrode active material is 0.5 to 0.7; (4) The single particles of the positive electrode active material have a rhombohedral structure.

7. The positive electrode active material according to any one of claims 1 to 6, wherein, The positive electrode active material further includes a third material, which is distributed on at least a portion of the surface of the first material.

8. The positive electrode active material according to claim 7, wherein, The third material includes organic compounds containing the alkali metal and / or inorganic compounds containing the alkali metal.

9. The positive electrode active material according to claim 8, wherein, Organic compounds containing the alkali metal include one or more of R-OM, R-OCO2M, and R-COOM, wherein R is selected from alkyl groups having 1-3 carbon atoms, and M includes one of Li, Na, and K; and / or, Inorganic compounds containing the alkali metal include one or more of the alkali metal fluorides, alkali metal carbonates, and alkali metal oxides; Optionally, the inorganic compound containing the alkali metal includes one or more of MF, M2CO3, and M2O, wherein M includes one of Li, Na, and K.

10. The positive electrode active material according to any one of claims 7 to 9, wherein, The total mass percentage of the second and third materials in the positive electrode active material is 5% to 20%.

11. A method for preparing a positive electrode active material, comprising the following steps: Sulfur is dissolved to obtain a sulfur solution; Alkali metal monosulfides are dispersed to obtain alkali metal monosulfide dispersions. The sulfur solution is added to the dispersion of the alkali metal monosulfide to obtain a mixed solution. The mixed solution is dried to obtain a positive electrode active material. The positive electrode active material includes a first material and a second material. The first material includes the monosulfide of the alkali metal, and the second material includes the polysulfide of the alkali metal. The second material is distributed on at least a portion of the surface of the first material.

12. The preparation method according to claim 11, wherein, The elemental sulfur is dissolved in a first solvent, which includes one or more of toluene and carbon disulfide; and / or The alkali metal monosulfide is dispersed in a second solvent, which includes one or more of toluene and carbon disulfide.

13. The preparation method according to claim 11 or 12, wherein, The mass ratio of the elemental sulfur to the monosulfide of the alkali metal is 1:(20-40).

14. The preparation method according to any one of claims 11 to 13, wherein, The drying process includes rotary evaporation, which includes one or more of the following features: (1) The vacuum degree of rotary evaporation is 0.1 MPa to 1 MPa; (2) The rotary evaporation temperature is 40℃~80℃; (3) The rotary evaporation time is 0.5h to 2h.

15. The preparation method according to any one of claims 11 to 14, wherein, The preparation method further includes the following steps: A coin cell is assembled from a positive electrode sheet using the aforementioned positive electrode active material, an alkali metal counter electrode, and a first electrolyte. After the first discharge of the coin cell, a third material is formed on at least a portion of the surface of the first material.

16. The preparation method according to claim 15, wherein, The first electrolyte comprises a first electrolyte salt and a third solvent; The third solvent includes compounds capable of undergoing nucleophilic reactions with the second material.

17. The preparation method according to claim 16, wherein, The third solvent has a cyclic structure.

18. The preparation method according to claim 16 or 17, wherein, The third solvent includes ester solvents, which include one or more of vinylene carbonate, fluoroethylene carbonate, propylene carbonate, and ethylene carbonate.

19. The preparation method according to any one of claims 16 to 18, wherein, The first electrolyte salt includes one or more of the following: bis(trifluoromethylsulfonyl)imide salt, bis(trifluoromethylsulfonyl)imide salt, hexafluorophosphate, sodium perchlorate salt, and trifluoromethanesulfonate.

20. The preparation method according to any one of claims 15 to 19, wherein, The concentration of the first electrolyte is 0.1 mol / L to 2 mol / L.

21. A secondary battery cell, comprising a positive electrode sheet, the positive electrode sheet comprising a positive current collector and a positive electrode film layer located on at least one side of the positive current collector and comprising a positive active material; The positive electrode active material includes the positive electrode active material according to any one of claims 1 to 10, or includes the positive electrode active material prepared by the preparation method according to any one of claims 11 to 20.

22. The secondary battery cell according to claim 21, wherein, It also includes a negative electrode sheet, which includes a negative current collector and an alkali metal layer located on the surface of the negative current collector, the alkali metal layer including the alkali metal.

23. The secondary battery cell according to claim 22, wherein, The thickness of the alkali metal layer is 0.5 mm to 1 mm.

24. The secondary battery cell according to any one of claims 21 to 23, wherein, It also includes a second electrolyte, which comprises a second electrolyte salt and a fourth solvent; the fourth solvent comprises an ether solvent.

25. The secondary battery cell according to claim 24, wherein, The fourth solvent includes one or more of tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethylene, and 1,3-dioxolane.

26. An electrical device comprising a secondary battery cell according to any one of claims 21 to 25.

Images