Sodium ion battery

Abstract

In order to overcome the technical problems of high impedance and low rate capability of the existing sodium ion battery added with an inorganic sodium supplement, the invention provides a sodium ion battery, the sodium ion battery comprises a non-aqueous electrolyte, a positive electrode and a negative electrode, the non-aqueous electrolyte comprises a boron-containing additive, the positive electrode comprises a positive electrode active material layer, the positive electrode active material layer comprises a sodium supplement additive, and the negative electrode comprises a boron-containing additive. The sodium-supplementing additive is a sodium-rich transition metal oxide, and the sodium-ion battery meets the following relational expressions: 10 < = B * C / A < = 60, 1 < = A < = 5, 5 < = B < = 10, 9 < = C < = 12; wherein A is the mass content of the boron-containing additive in the non-aqueous electrolyte, and the unit is%; b is the mass content of the sodium supplement additive in the positive active material layer, and the unit is%; and C is alkalinity of the positive active material layer. The sodium ion battery provided by the invention has the effects of improving the first efficiency of the battery, reducing the impedance and improving the rate capability.

Description

Technical Field

[0001] The invention belongs to the technical field of batteries, and in particular relates to a sodium ion battery. Background Art

[0002] Sodium-ion batteries focus on the electrochemical energy storage application market, and are positioned as the main force of large-scale energy storage, a supplement to lithium-ion batteries, and a substitute for lead-acid batteries. Sodium-ion batteries have the characteristics of low voltage platform and low energy density. In sodium-ion batteries, the negative electrode material is mainly hard carbon material. Since the negative electrode material has a large specific surface area and more defects, the active sodium ion consumption in the positive electrode sheet during the first charging process is high, the battery's first efficiency is reduced, and the utilization rate of the battery's active substances is reduced, resulting in low battery energy density.

[0003] In order to solve the problem that the active sodium ion consumption of sodium ion in the first charge process of sodium ion battery is high, resulting in the reduction of the first efficiency of the battery, the current practice is to add positive electrode sodium supplement to the positive electrode sheet to supplement the sodium ion battery with sodium, so as to make up for the irreversible loss when the SEI film is formed, reduce the irreversible capacity, and improve the first efficiency and energy density of the battery. The main sodium supplement methods reported so far are: (1) cold pressed sodium sheet method; (2) electrodeposition method; (3) additive method. The additive method does not require changing the existing process and is simple to operate. At present, the common sodium supplement additives are mainly inorganic salts, such as sodium oxalate, sodium nickelate, sodium phosphide, sodium carbonate, etc. However, this type of sodium supplement additive generally has the following problems: after the reaction, the inorganic sodium supplement in the positive electrode sheet will cause the positive electrode alkalinity to increase, and the residual alkali on the surface will cause the sodium ion conduction impedance to be higher, which will increase the internal resistance of the battery and reduce the battery rate performance. Summary of the invention

[0004] In order to solve the technical problems of high impedance and low rate performance of existing sodium ion batteries with inorganic sodium supplement, the present application provides a sodium ion battery.

[0005] The technical solution adopted by the present invention to solve the above technical problems is as follows:

[0006] On the one hand, the present application provides a sodium ion battery, comprising a non-aqueous electrolyte, a positive electrode and a negative electrode, wherein the non-aqueous electrolyte comprises a boron-containing additive, the positive electrode comprises a positive electrode active material layer, the positive electrode active material layer comprises a sodium supplement additive, and the sodium supplement additive is a sodium-rich transition metal oxide.

[0007] The sodium ion battery satisfies the following relationship:

[0008] 10≤B×C / A≤60, and 1≤A≤5, 5≤B≤10, 9≤C≤12;

[0009] Wherein, A is the mass content of the boron-containing additive in the non-aqueous electrolyte, in %;

[0010] B is the mass content of the sodium supplement additive in the positive electrode active material layer, in %;

[0011] C is the basicity of the positive electrode active material layer.

[0012] Preferably, the sodium ion battery satisfies the following relationship: 15≤B×C / A≤50.

[0013] Preferably, based on 100% of the mass of the non-aqueous electrolyte, the mass content A of the boron-containing additive is 1% to 3%.

[0014] Preferably, the mass content B of the sodium supplement additive in the positive electrode active material layer is 6% to 9%.

[0015] Preferably, the alkalinity C of the positive electrode active material layer is 9.5 to 11.5. Preferably, the boron-containing additive includes one or more of trimethyl borate, triethyl borate, tri(trimethylsilyl)borate, sodium dioxalatoborate, and sodium difluorooxalatoborate.

[0016] Preferably, the sodium-rich transition metal oxide comprises a compound represented by Formula 1,

[0017] Na x M y O z Formula 1,

[0018] Among them, 1≤x≤5, 0≤y≤1, 1≤z≤5, and M is selected from one of Ni, Mn, and Fe.

[0019] Preferably, the sodium-rich transition metal oxide includes one or more of NaNiO2, Na2NiO2, Na2MnO3, and Na5FeO4.

[0020] Preferably, the non-aqueous electrolyte further comprises an electrolyte salt, an auxiliary additive and a non-aqueous organic solvent.

[0021] The electrolyte salt includes at least one of sodium hexafluorophosphate, sodium trifluoromethylsulfonyl imide, sodium trifluoromethylsulfonate, sodium tetrafluoroborate, sodium perchlorate, sodium hexafluoroarsenate, sodium trifluoroacetate, sodium tetraphenylborate and sodium bis(trifluoromethylsulfonyl)imide;

[0022] The mass content of the electrolyte salt is 7% to 13% based on the mass of the electrolyte solution being 100%;

[0023] The auxiliary additive includes at least one of 1,3-propane sultone, 1,4-butane sultone, fluoroethylene carbonate and difluoroethylene carbonate;

[0024] Taking the mass of the electrolyte as 100%, the mass content of the additive is 0.5% to 4%;

[0025] The non-aqueous organic solvent includes at least one of carbonates having 3 to 5 carbon atoms, carboxylates having 2 to 6 carbon atoms, and ethers having 4 to 10 carbon atoms.

[0026] Preferably, the positive electrode active material layer includes a positive electrode active material, and the positive electrode active material includes at least one of a layered transition metal oxide, a Prussian compound, a phosphate compound, and a sulfate compound; the negative electrode active material layer includes a negative electrode active material, and the negative electrode active material includes one or more of a carbon-based material, a silicon-based material, and a tin-based material;

[0027] Preferably, the chemical formula of the layered transition metal oxide compound is Na x L y O z , 0<x≤1, 0<y≤1, 1<z≤2, L can be selected from at least one of Cr, Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, and V;

[0028] The chemical formula of the Prussian compound is Na x＇ M[M′(CN)6] y＇ ·z'H2O, M is a transition metal, M' is a transition metal, 0 <x'≤2,0<y'≤1,0<z'≤20:

[0029] The chemical formula of the phosphate compound is Na3(RO 1-x＇＇ PO4)2F 1+2x＇＇ or Na2R′PO4F, wherein 0≤x′′≤1, R is selected from at least one of Al, V, Ge, Fe, and Ga, and R′ is selected from at least one of Fe and Mn;

[0030] The chemical formula of the sulfate compound is Na2A(SO4)2·2H2O, where A is selected from at least one of Cr, Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, and V.

[0031] Beneficial effects:

[0032] The sodium ion battery provided by the present application comprises a sodium-supplementing additive of a sodium-rich transition metal oxide in the positive electrode active material layer. During the battery charging and discharging process, the sodium-supplementing agent decomposes in the positive electrode to release active sodium, thereby increasing the reversible capacity of the battery and improving the initial efficiency of the battery. At the same time, a boron-containing additive is added to the electrolyte. The decomposition process of the positive electrode sodium-supplementing additive promotes the boron-containing additive in the electrolyte to participate in the positive electrode interface reaction, thereby forming a boride-rich conductive film on the electrode surface, thereby improving interface ion transport, reducing impedance and improving the battery rate performance. There is a certain relationship between the mass content A of the boron-containing additive in the electrolyte, the mass content B of the sodium-supplementing additive in the positive electrode active material layer and the alkalinity C of the positive electrode active material layer, that is, the sodium-supplementing additive, the boron-containing additive and the alkalinity of the positive electrode active material layer satisfy the relationship 10≤B×C / A≤60, which can play a good synergistic role and improve the first efficiency, impedance and rate performance of the battery. The specific mechanism is as follows: A, B, and C satisfy the relationship 10≤B×C / A≤60, and the sodium-supplementing additive is a sodium-rich transition metal oxide, which releases active sodium during the electrochemical reaction and produces a high-valent oxidation state transition metal oxide. The oxide of transition metal element may promote the boron atom of the boron-containing additive to participate in the reaction. On the one hand, the high-valent oxidation state transition metal oxide is fixed on the interface to reduce the reduction of free transition metal oxide at the negative electrode. On the other hand, a boron-containing positive electrode interface film is generated to improve the density of the interface film and reduce the battery impedance. The positive electrode active material layer containing the sodium supplement additive has alkalinity. The boron atom center of the boron-containing additive acts as a Lewis acid to neutralize the alkalinity. The positive electrode with higher alkalinity can promote the reaction of the boron-containing additive and further improve the interface, thereby improving the first efficiency, impedance and rate performance of the battery. DETAILED DESCRIPTION

[0033] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0034] On one hand, the present application provides a sodium ion battery, comprising a non-aqueous electrolyte, a positive electrode and a negative electrode, wherein the non-aqueous electrolyte comprises a boron-containing additive, the positive electrode comprises a positive electrode active material layer, the positive electrode active material layer comprises a sodium supplement additive, and the sodium supplement additive is a sodium-rich transition metal oxide.

[0035] The sodium ion battery satisfies the following relationship:

[0036] 10≤B×C / A≤60, and 1≤A≤5, 5≤B≤10, 9≤C≤12;

[0037] Wherein, A is the mass content of the boron-containing additive in the non-aqueous electrolyte, in %;

[0038] B is the mass content of the sodium supplement additive in the positive electrode active material layer, in %;

[0039] C is the basicity of the positive electrode active material layer.

[0040] According to the sodium ion battery provided by the present application, the positive electrode active material layer contains a sodium-supplementing additive of a sodium-rich transition metal oxide. During the charge and discharge process of the battery, the sodium-supplementing agent decomposes in the positive electrode to release active sodium, thereby increasing the reversible capacity of the battery and improving the initial efficiency of the battery. At the same time, a boron-containing additive is added to the electrolyte. The decomposition process of the positive electrode sodium-supplementing additive promotes the boron-containing additive in the electrolyte to participate in the positive electrode interface reaction, thereby forming a boride-rich conductive film on the electrode surface, thereby improving interface ion transport, reducing impedance and improving the battery rate performance. After extensive research, the inventors found that there is a certain relationship between the mass content A of the boron-containing additive in the electrolyte, the mass content B of the sodium-supplementing additive in the positive electrode active material layer and the alkalinity C of the positive electrode active material layer. The higher the content of the sodium-supplementing additive, the higher the first efficiency of the battery. However, the greater the alkalinity of the positive electrode active material layer, the higher the sodium ion conduction impedance and the worse the battery rate performance. Therefore, a higher content of boron-containing additive is needed to compensate for this. The inventors verified through extensive research that the use of boron-containing additives in the electrolyte can synergistically play an interface regulation role during the decomposition of the sodium-supplementing agent, thereby improving the interfacial ion transport caused by the excessive alkalinity of the positive electrode active material layer. When the relationship 10≤B×C / A≤60 is satisfied, the battery has a higher first efficiency, and the impedance performance and rate performance are improved.

[0041] The sodium-supplementing additive, the boron-containing additive, and the alkalinity of the positive electrode active material layer in the present application satisfy the relationship 10≤B×C / A≤60, which can play a good synergistic role and improve the battery's first efficiency, impedance and rate performance. The specific mechanism is as follows: A, B, and C satisfy the relationship 10≤B×C / A≤60, the sodium-supplementing additive is a sodium-rich transition metal oxide, which releases active sodium during the electrochemical reaction and simultaneously produces an oxide containing a high-valent oxidized transition metal element. The oxide may promote the boron atoms of the boron-containing additive to participate in the reaction. On the one hand, the high-valent oxidized transition metal oxide is fixed on the interface to reduce the reduction of the free transition metal oxide at the negative electrode. On the other hand, a boron-containing positive electrode interface film is generated to improve the density of the interface film and reduce the battery impedance. The positive electrode active material layer containing the sodium-supplementing additive has alkalinity, and the boron atom center of the boron-containing additive acts as a Lewis acid to neutralize the alkalinity. The positive electrode with higher alkalinity can promote the reaction of the boron-containing additive and further improve the interface, thereby improving the battery's first efficiency, impedance and rate performance.

[0042] Specifically, when the relationship B×C / A is less than 10, the alkalinity of the positive electrode active material layer or the content of sodium supplement additives is low, and the first efficiency of the battery is deteriorated; or the amount of boron-containing additives in the electrolyte is high, and the interface film continues to thicken during the cycle of the sodium ion battery, the cycle DCIR deteriorates, and the battery impedance increases. When the relationship B×C / A is greater than 60, the alkalinity of the positive electrode active material layer or the content of sodium supplement additives is high, or the amount of boron-containing additives is insufficient, the effect of improving the ion transmission caused by the residual alkalinity of the positive electrode is poor, and the battery rate performance is not improved.

[0043] In some preferred embodiments, the sodium ion battery satisfies the following relationship: 15≤B×C / A≤50. The battery satisfies this preferred relationship, while ensuring that the battery has a high initial efficiency, the interface membrane formed at the positive electrode interface has high ion conductivity, a high sodium ion transmission rate, low impedance, and good rate performance.

[0044] In some embodiments, based on 100% of the mass of the non-aqueous electrolyte, the mass content A of the boron-containing additive is 1% to 5%.

[0045] The mass content A of the boron-containing additive added to the electrolyte is between 1% and 5%. The decomposition process of the positive electrode sodium supplement additive promotes the boron-containing additive in the electrolyte to participate in the positive electrode interface reaction, forming a boride-rich conductive film on the electrode surface, improving the interface ion transmission, reducing the impedance, and improving the battery's rate performance. If A < 1%, the alkalinity of the positive electrode active material layer cannot be completely neutralized during the battery reaction, and the high impedance improvement effect caused by the positive electrode sodium supplement additive is not good. If A > 5%, the excessive content of the boron-containing additive is too high, the thickness of the interface film formed with the positive electrode interface is thick, the sodium ion transmission distance is increased, and the DCIR is continuously increased during the cycle, and the battery impedance increases. Specifically, the mass content A of the boron-containing additive added to the electrolyte can be 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, 5.0%, etc., as long as A is between 1% and 5%.

[0046] In some preferred embodiments, the mass content A of the boron-containing additive is 10% to 3% based on the mass of the non-aqueous electrolyte being 100%. The boron-containing additive in the electrolyte within this preferred range can not only neutralize the positive electrode alkalinity, but also better cooperate with the positive electrode sodium supplement additive to form a denser and more structurally stable interface film at the positive electrode interface, and the battery has lower impedance and better rate performance.

[0047] In some embodiments, the mass content B of the sodium supplement additive in the positive electrode active material layer is 5% to 10%, and the content of active sodium ions in the positive electrode active material layer is relatively high, which improves the first efficiency of the battery. If B < 5%, the content of the positive electrode sodium supplement additive is too low, there are fewer active sodium ions in the positive electrode active material layer, and the first efficiency of the battery is low; if B > 10%, the content of the positive electrode sodium supplement additive is relatively high, and the high content of the sodium supplement additive increases the alkalinity of the positive electrode active material layer, resulting in a larger battery impedance and deteriorated rate performance. Specifically, the mass content B of the sodium supplement additive in the positive electrode active material layer can be 5%, 5.5%, 6.0%, 6.5%, 7.0%, 7.5%, 8.0%, 8.5%, 9.0%, 9.5%, 10.0%, etc., as long as B is between 5% and 10%.

[0048] In some preferred embodiments, the mass content B of the sodium supplement additive in the positive electrode active material layer is 6% to 9%. In this preferred embodiment, it can not only improve the first efficiency of the battery, but also reduce the battery impedance and improve the battery rate performance.

[0049] In some embodiments, the alkalinity C of the positive electrode active material layer is 9 to 12. When the alkalinity of the positive electrode active material layer is in the range of 9 to 12, the corresponding content of the positive electrode sodium supplement additive can improve the first efficiency of the battery while avoiding excessive residual alkali on the positive electrode surface causing an increase in battery impedance, which has the effect of improving battery impedance and rate performance. If C < 9, the content of the positive electrode sodium supplement additive is low, the positive electrode capacity is low, and the first efficiency is low; if C > 12, excessive residual alkali on the positive electrode surface causes an increase in battery impedance, and the battery impedance increases and the rate performance decreases. Specifically, the alkalinity C of the positive electrode active material layer can be 9, 9.5, 10, 10.5, 11.0, 11.5, 12.0, etc., as long as C is in the range of 9 to 12.

[0050] In some preferred embodiments, the basicity C of the positive electrode active material layer is 9.5-11.5.

[0051] In some embodiments, the boron-containing additive includes one or more of trimethyl borate, triethyl borate, tris(trimethylsilyl)borate, sodium dioxalatoborate, and sodium difluorooxalatoborate.

[0052] In some embodiments, the sodium-rich transition metal oxide comprises a compound represented by Formula 1,

[0053] Na x M y O z Formula 1,

[0054] Among them, 1≤x≤5, 0≤y≤1, 1≤z≤5, and M is selected from one of Ni, Mn, and Fe.

[0055] In some preferred embodiments, the sodium-rich transition metal oxide includes one or more of NaNiO2, Na2NiO2, Na2MnO3, and Na5FeO4.

[0056] In some embodiments, the non-aqueous electrolyte further comprises an electrolyte salt, wherein the electrolyte salt comprises at least one of sodium hexafluorophosphate, sodium trifluoromethylsulfonyl imide, sodium trifluoromethylsulfonate, sodium tetrafluoroborate, sodium perchlorate, sodium hexafluoroarsenate, sodium trifluoroacetate, sodium tetraphenylborate and sodium bis(trifluoromethylsulfonyl)imide;

[0057] Based on the mass of the electrolyte being 100%, the mass content of the electrolyte salt is 7% to 13%.

[0058] Electrolyte salts are added to the electrolyte, which have high solubility in non-aqueous organic solvents and are easy to dissociate, to ensure that the electrolyte has high ionic conductivity and promote the redox reaction of the battery.

[0059] In some embodiments, the non-aqueous electrolyte further includes an auxiliary additive, the auxiliary additive including at least one of 1,3-propane sultone, 1,4-butane sultone, fluoroethylene carbonate and difluoroethylene carbonate, and the mass content of the additive is 0.5% to 4% based on the mass of the electrolyte as 100%.

[0060] In some embodiments, the non-aqueous electrolyte further comprises a non-aqueous organic solvent.

[0061] The non-aqueous organic solvent includes at least one of carbonates having 3 to 5 carbon atoms, carboxylates having 2 to 6 carbon atoms, and ethers having 4 to 10 carbon atoms.

[0062] Preferably, the carbonate solvent having 3 to 5 carbon atoms includes cyclic carbonates or chain carbonates having 3 to 5 carbon atoms, wherein the cyclic carbonates include but are not limited to at least one of ethylene carbonate (EC), vinylene carbonate (VC), vinylethylene carbonate (VEC), propylene carbonate (PC), γ-butyrolactone (GBL), and butylene carbonate (BC); the chain carbonates may specifically be but are not limited to dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and dipropyl carbonate (DPC).

[0063] Preferably, the carboxylic acid ester having 2 to 6 carbon atoms includes, but is not limited to, at least one of methyl acetate (MA), ethyl acetate (EA), propyl acetate (EP), butyl acetate, and propyl propionate (PP).

[0064] Preferably, the ether solvent having 4 to 10 carbon atoms includes cyclic ethers or chain ethers having 4 to 10 carbon atoms, and the cyclic ethers include but are not limited to at least one of 1,3-dioxolane (DOL), 1,4-dioxolane (DX), tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-CH3-THF), and 2-trifluoromethyltetrahydrofuran (2-CF3-THF); the chain ethers include but are not limited to at least one of dimethoxymethane (DMM), 1,2-dimethoxyethane (DME), and diethylene glycol dimethyl ether (TEGDME).

[0065] In some embodiments, the positive electrode active material layer includes a positive electrode active material, and the positive electrode active material is selected from at least one of a layered transition metal oxide, a Prussian compound, a phosphate compound, and a sulfate compound;

[0066] The negative electrode includes a negative electrode active material, and the negative electrode active material includes one or more of a carbon-based material, a silicon-based material, and a tin-based material. The carbon-based material includes at least one of hard carbon, soft carbon, carbon nanotubes, graphite, graphene, expanded graphite, or mesophase carbon microspheres.

[0067] In some embodiments, the chemical formula of the layered transition metal oxide compound is Na x L y O z , 0<x≤1, 0<y≤1, 1<z≤2, L can be selected from at least one of Cr, Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, and V.

[0068] In a preferred embodiment, the chemical formula of the layered transition metal oxide is Na p Ni m Mn n M (1-m-n) O q , where 0<p≤1, 0≤m<1, 0<m+n<1, 1<q≤2.

[0069] In a more preferred embodiment, the positive electrode active material is Na p＇ Ni m＇ Mn n＇ Fe (1-m＇-n＇) O2, wherein 0<p'≤1, 0≤m'<1, 0<m'+n'<1, 1<q'≤2. Further, preferably NaNi 0.3 Mn 0.3 Fe 0.4 O2、NaNi 0.4 Mn 0.4 Fe 0.2 O2、NaNi 0.5 Mn 0.35 Fe0.15 O2.

[0070] In a more preferred embodiment, the chemical formula of the layered transition metal oxide is NaNi m Co n Mn p At least one of O2(m+n+p=1, 0≤m≤1, 0≤n≤1, 0≤p≤1).

[0071] In some embodiments, the molecular formula of the Prussian compound is Na x＇ M[M′(CN)6] y＇ ·z'H2O, M is a transition metal, M' is a transition metal, 0 <x'≤2,0<y'≤1,0<z'≤20。

[0072] In a more preferred embodiment, the Prussian compound is Na x＇ Mn[Fe(CN)6] y＇ ·z'H2O or Na x＇ Fe[Fe(CN)6] y＇ ·z'H2O, 0<x'≤2, 0<y'≤1, 0<z'≤20.

[0073] In some embodiments, the chemical formula of the phosphate compound is Na3(RO 1-x＇＇ PO4)2F 1+2x＇＇ Or Na2R′PO4F, wherein 0≤x′′≤1, R is selected from at least one of Al, V, Ge, Fe, and Ga, and R′ is selected from at least one of Fe and Mn.

[0074] In a more preferred embodiment, the chemical formula of the phosphate compound is Na3(VPO4)2F3, Na3(VOPO4)2F, Na2FePO4F, or Na2MnPO4F.

[0075] In some embodiments, the chemical formula of the sulfate compound is Na2A(SO4)2·2H2O, where A is selected from at least one of Cr, Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, and V.

[0076] The present invention is further described below by way of examples.

[0077] Example 1

[0078] This embodiment is used to illustrate the sodium ion battery disclosed in this application.

[0079] (1) Preparation of positive electrode: The positive electrode active material NaNi was prepared in a mass ratio of 89:6:2:3. 0.3 Fe 0.3Mn 0.4 O2, sodium supplement additive Na2NiO2, conductive carbon black Super-P and binder polyvinylidene fluoride (PVDF) are mixed, and then dispersed in an appropriate amount of N-methyl-2-pyrrolidone (NMP) to obtain positive electrode slurry; the obtained slurry is evenly coated on both sides of aluminum foil, and after drying, calendering and vacuum drying, an aluminum lead wire is welded with an ultrasonic welder to obtain a positive electrode sheet.

[0080] (2) Preparation of negative electrode: hard carbon, conductive carbon black Super-P, binder styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) are mixed in a mass ratio of 94:1:2.5:2.5, and then dispersed in an appropriate amount of deionized water to obtain negative electrode slurry; the slurry is coated on both sides of a copper foil, dried, rolled and vacuum dried, and a nickel lead wire is welded on with an ultrasonic welder to obtain a negative electrode sheet.

[0081] (3) Preparation of electrolyte: Prepare a solution of solvent ethylene carbonate, solvent ethyl methyl carbonate, sodium salt sodium hexafluorophosphate, boron-containing additive sodium difluorooxalate borate, and auxiliary additive vinyl sulfate in a mass ratio of 30:55:12:2:1.

[0082] (4) Preparation of sodium ion battery: A separator is placed between the positive electrode sheet and the negative electrode sheet prepared above, and then a sandwich structure consisting of the positive electrode sheet, the negative electrode sheet and the separator is wound, and then the wound body is flattened and placed in an aluminum foil packaging bag to assemble a battery.

[0083] The alkalinity test method of the positive electrode active material layer is as follows: the positive electrode active material layer is separated from the positive electrode, and then the positive electrode active material layer is dispersed in deionized water by ultrasonication for 30 minutes, and its pH value is tested by a pH meter, and the pH value is the alkalinity C value.

[0084] It should be noted that when testing the alkalinity of the positive electrode active material layer, it is acceptable to test its pH value before or after charging. To test the alkalinity of the positive electrode active material layer of a charged battery, the battery needs to be discharged to 0% SOC and then disassembled, cleaned with dimethyl carbonate and vacuum dried, and then tested using the above-mentioned alkalinity test method for the positive electrode active material layer.

[0085] Examples 2-18 and Comparative Examples 1-11

[0086] The difference between Examples 2-18 and Comparative Examples 1-11 and Example 1 is that in the positive electrode preparation step, the type and content of the sodium supplement additive are different, the type and content of the boron-containing additive in the electrolyte are different, and the alkalinity C of the positive electrode active material layer is different, as shown in Table 1. The rest is the same as the preparation method of Example 1.

[0087] Table 1 Parameters of Examples and Comparative Examples

[0088]

[0089]

[0090] Performance Testing

[0091] The sodium ion batteries prepared in the above examples and comparative examples were subjected to the following performance tests.

[0092] First effect test:

[0093] The battery was placed at 45°C for 2 h, charged at a constant current of 0.2C to 3.95V, then charged at a constant voltage to a current of 0.03C, and the charging capacity C1 was recorded; the battery was discharged at a constant current of 0.2C to 1.5V, and the discharge capacity C2 was recorded.

[0094] First effect (%) = C2 / C1×100%.

[0095] 2C rate test:

[0096] Place the battery at 45℃ for 2h, charge it to 3.95V at a rate of 0.2C, charge it to 0.03C at a constant voltage, discharge it to 1.5V at a current of 0.2C, and record the discharge capacity C3. Charge it to 3.95V at a rate of 0.2C, charge it to 0.03C at a constant voltage, and discharge it to 1.5V at a current of 2C, and record the discharge capacity C4.

[0097] 2C rate retention rate (%) = C4 / C3×100%.

[0098] Cyclic DCIR growth rate test:

[0099] (1) First, place the formed battery at 45℃ for 2h, discharge it to 1.5V at 0.5C, then charge it at 0.5C for 1h, and test the DCIR value R1 at 0.1C and 0.5C rates.

[0100] (2) Then charge at a constant current rate of 0.5C to 3.95V, then charge at a constant voltage to a current of 0.03C, and then discharge at a constant current of 1C to 1.5V, and cycle the charge and discharge for 200 times.

[0101] (3) Finally, discharge at 0.5C to 1.5V, then charge at 0.5C for 1h, and test the DCIR value R2 at 0.1C and 0.5C rates.

[0102] Cyclic DCIR growth rate (%) = (R2-R1) / R1×100%.

[0103] The test results are shown in Table 2-4.

[0104] Table 2 Performance data of examples and comparative examples

[0105]

[0106] It can be seen from Tables 1 and 2 that, compared with Example 10, the mass content A of the boron-containing additive in the electrolyte of Comparative Examples 1-2 is lower than 1% or higher than 5%, and the cycle DCIR growth rate of the battery is too high. It is speculated that A is too low, and the high impedance improvement effect caused by the positive electrode sodium supplement additive is not good. A is too high, and the thickness of the interface film formed with the positive electrode interface is thicker, and the sodium ion transmission distance increases. Comparative Examples 3-4 and Examples 8-9 are compared. The mass content B of the sodium supplement additive in the positive electrode active material layer is lower than 5%, the sodium supplement additive is less, and the battery first efficiency is low. B is higher than 10%, and the battery 2C rate retention rate is low and the DCIR is high, indicating that the content of the positive electrode sodium supplement additive is high, and the alkalinity of the positive electrode active material layer increases, resulting in a large battery impedance and poor rate performance. Comparative Examples 5-6 and Examples 1 and 9 are compared. The alkalinity C of the positive electrode active material layer is greater than 12, and the DCIR of the battery is large, indicating that too much residual alkali on the positive electrode surface causes the battery impedance to increase and the rate performance to decrease; if C is less than 9, the battery first efficiency is low.

[0107] Comparison between Examples 1-11 and Comparative Examples 7-8 shows that A, B, and C not only need to satisfy 1≤A≤5, 5≤B≤10, and 9≤C≤12, but also need to satisfy the relationship 10≤B×C / A≤60. The battery has good first efficiency and rate performance, and a low cycle DCIR growth rate. It is speculated that the sodium-supplementing additive releases active sodium during the electrochemical reaction, and simultaneously produces an oxide containing a high-valent oxidized transition metal element. The oxide may promote the boron atoms of the boron-containing additive to participate in the reaction. On the one hand, the high-valent oxidized transition metal oxide is fixed on the interface to reduce the reduction of the free transition metal oxide at the negative electrode. On the other hand, a boron-containing positive electrode interface film is generated to improve the density of the interface film and reduce the battery impedance. The positive electrode active material layer containing the sodium-supplementing additive has alkalinity, and the boron atom center of the boron-containing additive acts as a Lewis acid to neutralize the alkalinity. The positive electrode with higher alkalinity can promote the reaction of the boron-containing additive, further improve the interface, and thus improve the first efficiency, impedance and rate performance of the battery. A comparison between Examples 1-11 and Comparative Examples 9-11 shows that as long as one of A, B, and C does not satisfy 1≤A≤5, 5≤B≤10, and 9≤C≤12, even if A, B, and C satisfy the relationship 10≤B×C / A≤60, the prepared battery will have low first efficiency, poor rate performance, and a high DCIR growth rate during the battery cycle.

[0108] By comparison with Examples 1-11, when the mass content A of the boron-containing additive is between 1% and 3%, the mass content B of the sodium-supplementing additive in the positive electrode active material layer is between 6% and 9%, the alkalinity C of the positive electrode active material layer is within the range of 9.5 to 11.5, and the following relationship 15≤B×C / A≤50 is satisfied, the battery has a high rate retention rate and a low DCIR after 200 cycles.

[0109] Table 3 Data table of Example 1 and Examples 12-14

[0110]

[0111]

[0112] It can be seen from Tables 1 and 3 that by changing the type of sodium supplement additive, as long as the compound shown in 1 is met, Na x M y O z Formula 1, wherein 1≤x≤5, 0≤y≤1, 1≤z≤5, and M is selected from one of Ni, Mn, and Fe; all have the same effect of improving the first efficiency and rate performance of the battery and reducing the battery impedance.

[0113] Table 4 Data table of Example 1 and Examples 15-18

[0114]

[0115] It can be seen from Tables 1 and 4 that changing the type of boron-containing additive, the boron-containing additive is selected from one or more of trimethyl borate, triethyl borate, tri(trimethylsilyl)borate, sodium dioxalate borate, and sodium difluorooxalate borate, all of which have the same effect, and work together with the sodium supplement additive to improve the battery's first efficiency, rate performance, and reduce battery impedance.

[0116] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A sodium ion battery, characterized in that: The invention comprises a non-aqueous electrolyte, a positive electrode and a negative electrode, wherein the non-aqueous electrolyte comprises a boron-containing additive, the positive electrode comprises a positive electrode active material layer, the positive electrode active material layer comprises a sodium-supplementing additive, and the sodium-supplementing additive is a sodium-rich transition metal oxide; The sodium ion battery satisfies the following relationship: 10≤B×C / A≤60, and 1≤A≤5, 5≤B≤10, 9≤C≤12; Wherein, A is the mass content of the boron-containing additive in the non-aqueous electrolyte, in %; B is the mass content of the sodium supplement additive in the positive electrode active material layer, in %; C is the basicity of the positive electrode active material layer.

2. The sodium ion battery according to claim 1, characterized in that The sodium ion battery satisfies the following relationship: 15≤B×C / A≤50.

3. The sodium ion battery according to claim 1, characterized in that: Based on 100% of the mass of the non-aqueous electrolyte, the mass content A of the boron-containing additive is 1% to 3%.

4. The sodium ion battery according to claim 1, characterized in that The mass content B of the sodium supplement additive in the positive electrode active material layer is 6-9%.

5. The sodium ion battery according to claim 1, characterized in that: The basicity C of the positive electrode active material layer is 9.5 to 11.

5.

6. The sodium ion battery according to claim 1, characterized in that: The boron-containing additive includes one or more of trimethyl borate, triethyl borate, tri(trimethylsilyl)borate, sodium dioxalate borate, and sodium difluorooxalate borate.

7. The sodium ion battery according to claim 1, characterized in that: The sodium-rich transition metal oxide includes a compound shown in Formula 1, Na x M y O z Formula 1 Among them, 1≤x≤5, 0≤y≤1, 1≤z≤5, and M is selected from one of Ni, Mn, and Fe.

8. The sodium ion battery according to claim 7, characterized in that: The sodium-rich transition metal oxide includes one or more of NaNiO2, Na2NiO2, Na2MnO3, and Na5FeO4.

9. The sodium ion battery according to claim 1, characterized in that: The non-aqueous electrolyte further comprises an electrolyte salt, an auxiliary additive and a non-aqueous organic solvent. The electrolyte salt includes at least one of sodium hexafluorophosphate, sodium trifluoromethylsulfonyl imide, sodium trifluoromethylsulfonate, sodium tetrafluoroborate, sodium perchlorate, sodium hexafluoroarsenate, sodium trifluoroacetate, sodium tetraphenylborate and sodium bis(trifluoromethylsulfonyl)imide; The mass content of the electrolyte salt is 7% to 13% based on the mass of the electrolyte solution being 100%; The auxiliary additive includes at least one of 1,3-propane sultone, 1,4-butane sultone, fluoroethylene carbonate and difluoroethylene carbonate; Taking the mass of the electrolyte as 100%, the mass content of the additive is 0.5% to 0.4%; The non-aqueous organic solvent includes at least one of carbonates having 3 to 5 carbon atoms, carboxylates having 2 to 6 carbon atoms, and ethers having 4 to 10 carbon atoms.

10. The sodium ion battery according to claim 1, characterized in that: The positive electrode active material layer includes a positive electrode active material, and the positive electrode active material includes at least one of a layered transition metal oxide, a Prussian compound, a phosphate compound, and a sulfate compound; the negative electrode active material layer includes a negative electrode active material, and the negative electrode active material includes one or more of a carbon-based material, a silicon-based material, and a tin-based material; Preferably, the chemical formula of the layered transition metal oxide compound is Na x L y O z , 0<x≤1, 0<y≤1, 1<z≤2, L can be selected from at least one of Cr, Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, and V; The chemical formula of the Prussian compound is Na x＇ M[M′(CN)6] y＇ ·z'H2O, M is a transition metal, M' is a transition metal, 0 <x'≤2,0<y'≤1,0<z'≤20: The chemical formula of the phosphate compound is Na3(RO 1-x＇＇ PO4)2F 1+2x＇＇ or Na2R′PO4F, wherein 0≤x′′≤1, R is selected from at least one of Al, V, Ge, Fe, and Ga, and R′ is selected from at least one of Fe and Mn; The chemical formula of the sulfate compound is Na2A(SO4)2·2H2O, where A is selected from at least one of Cr, Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, and V.

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