Sodium ion secondary battery

Abstract

In order to overcome the problem of poor cycling stability of a sodium ion battery in the prior art, the invention provides a sodium ion secondary battery, which comprises an electrolyte and a negative electrode, the electrolyte comprises additives, the additives comprise a first additive and a second additive, the first additive is a phosphorus-containing compound, and the second additive comprises ethylene sulfate, 1, 2-propylene glycol and a catalyst. The solvent is at least one of 1, 3-propane sultone; the negative electrode comprises a negative electrode active material, and the phosphorus-containing compound is selected from at least one of compounds as shown in a structural formula 1 or a structural formula 2; in the imgabs0 #, R1, R2, R3, R4, R5, R6 and R7 are respectively and independently selected from a cyano group, an ester group, an unsaturated alkyl group with 1 to 4 carbon atoms, an alkyl group with 1 to 4 carbon atoms and a halogenated alkyl group with 1 to 4 carbon atoms; the sodium ion secondary battery satisfies the following relational expressions: 0.8 < = (a + b) * c / 2 < = 3.5, 0.5% < = a% < = 3%, 1% < = b% < = 4%, and 0.85 g / cm < 3 > < = c < = 1.15 g / cm < 3 >; the sodium ion secondary battery provided by the invention has the effect of improving the cycle performance and the rate capability of the battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of sodium-ion batteries, and particularly relates to a sodium-ion secondary battery. Background Art

[0002] In recent years, lithium batteries have been applied in many fields such as mobile phones, computers, wearable devices, electric vehicles, two-wheel bicycles, power tools, street lamps, etc. In recent years, the consumption of lithium resources has shown a large usage amount and a fast consumption speed, and the growth of lithium production cannot meet the growth of consumption: This is because, on the one hand, lithium resources are limited and mainly exist in the form of spodumene ore and salt lake lithium, and on the other hand, lithium cannot be extracted from salt lake lithium in winter. Sodium is widely sourced, rich in reserves, and the reserves of sodium are 420 times that of lithium, and the price is much lower than that of lithium. In recent years, with the crazy rise in lithium prices, sodium-ion batteries are expected to receive wide attention with a cost 30 - 50% lower than that of lithium-ion batteries, especially in the fields of energy storage, hybrid power, and replacing lead-acid batteries, sodium-ion batteries have attractive application prospects.

[0003] Due to the characteristics of sodium ions themselves, hard carbon materials are considered to be ideal anode materials for sodium-ion batteries. It has been found that when organic solvents in the electrolytes of traditional lithium batteries, such as ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), propylene carbonate (PC), etc., are added to sodium-ion batteries, a rough, uneven, and relatively thick incomplete solid electrolyte film is formed on the surface of the anode of the sodium-ion battery. During the battery cycling process, the solid electrolyte film continuously breaks and grows, and the internal resistance of the battery continuously increases with cycling, resulting in poor cycling stability of the battery. Summary of the Invention

[0004] The problem to be solved by the present invention is the poor cycling stability of existing sodium-ion batteries, and the present application provides a sodium-ion secondary battery.

[0005] To solve the above technical problems, the present application provides a sodium-ion secondary battery, including an electrolyte and an anode. The electrolyte includes additives, and the additives include a first additive and a second additive. The first additive is a phosphorus-containing compound, and the second additive includes at least one of vinylene sulfate and 1,3-propane sultone; the anode includes an anode active material.

[0006] The phosphorus-containing compound is selected from at least one of the compounds shown in Structural Formula 1 or Structural Formula 2;

[0007]

[0008] Wherein, R1, R2, R3, R4, R5, R6, and R7 are each independently selected from a cyano group, an ester group, an unsaturated hydrocarbon group with 1 - 4 carbon atoms, an alkyl group with 1 - 4 carbon atoms, and a haloalkyl group with 1 - 4 carbon atoms;

[0009] The sodium-ion secondary battery satisfies the following relationship:

[0010] 0.8 ≤ (a + b) × c / 2 ≤ 3.5, 0.5% ≤ a% ≤ 3%, 1% ≤ b% ≤ 4%, 0.85 g / cm 3 ≤ c ≤ 1.15 g / cm 3 ;

[0011] wherein, a% is the mass percentage of the first additive in the electrolyte;

[0012] b% is the mass percentage of the second additive in the electrolyte;

[0013] c is the tap density of the negative electrode active material, with the unit of g / cm 3 .

[0014] Preferably, the sodium-ion secondary battery satisfies the following relationship: 1.35 ≤ (a + b) × c / 2 ≤ 2.5.

[0015] Preferably, the mass percentage a% of the first additive in the electrolyte ranges from 1% to 2%.

[0016] Preferably, the mass percentage b% of the second additive in the electrolyte ranges from 2% to 3%.

[0017] Preferably, the range of the tap density c of the negative electrode active material is 0.9 g / cm 3 ≤ c ≤ 1 g / cm 3 .

[0018] Preferably, the phosphorus-containing compound is selected from at least one of the following compounds:

[0019]

[0020]

[0021] Preferably, the electrolyte further includes a sodium salt, and the sodium salt includes one or more of NaClO4, NaBF4, NaPF6, NaBOB, NaODFB, NaAsF6, CF3COONa, NaB(C6H5)4, Na[(FSO2)2N], Na[(CF3SO2)2N];

[0022] Based on the total mass of the electrolyte being 100%, the mass content of the sodium salt is 10 - 15%.

[0023] Preferably, the non-aqueous organic solvent includes one or more of carbonate compounds, carboxylate compounds, and ether compounds.

[0024] Preferably, the carbonate compound includes a cyclic or linear carbonate having 3 to 5 carbon atoms. The cyclic carbonate includes one or more of ethylene carbonate, vinylene carbonate, ethylene ethylene carbonate, propylene carbonate, γ-butyrolactone, and butylene carbonate; the linear carbonate includes one or more of dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, and dipropyl carbonate;

[0025] The carboxylic acid ester compound includes a carboxylic acid ester having 2 to 6 carbon atoms. The carboxylic acid ester includes one or more of methyl acetate, ethyl acetate, propyl acetate, butyl acetate, and propyl propionate;

[0026] The ether compound includes a cyclic ether or a linear ether having 4 to 10 carbon atoms. The cyclic ether includes one or more of 1,3-dioxolane, 1,4-dioxane, tetrahydrofuran, 2-methyltetrahydrofuran, and 2-trifluoromethyltetrahydrofuran; the linear ether includes one or more of dimethoxymethane, 1,2-dimethoxyethane, and diglyme;

[0027] Based on the mass of the electrolyte being 100%, the mass percentage of the non-aqueous organic solvent is 75% to 85%.

[0028] Preferably, the additive further includes an auxiliary additive. The auxiliary additive includes a fluorinated carbonate. Preferably, the fluorinated carbonate includes fluorinated ethylene carbonate;

[0029] Based on the mass of the electrolyte being 100%, the mass percentage of the fluorinated carbonate is 1% to 5%.

[0030] Preferably, the negative electrode active material is selected from one or more of hard carbon and soft carbon;

[0031] The sodium secondary battery further includes a positive electrode. The positive electrode includes a positive electrode active material. The positive electrode active material is selected from a sodium-containing layered oxide, a sodium-containing polyanion compound, and a sodium-containing Prussian blue compound;

[0032] The sodium-containing layered oxide is selected from NaiMO2, where 0 < i ≤ 1, and M is selected from one or more of V, Cr, Mn, Fe, Co, Ni, and Cu;

[0033] The sodium-containing polyanion compound is selected from Na3V2(PO4)2F3;

[0034] The sodium-containing Prussian blue compound is selected from Na i＇ Mn[Fe(CN)6] 1-m·□m· nH2O, where 0 ≤ i' ≤ 2, 0 ≤ m ≤ 1, and 0 ≤ n ≤ 20.

[0035] Beneficial effects:

[0036] Compared with the prior art, the sodium-ion secondary battery provided by the present application satisfies the relationship 0.8 ≤ (a + b) × c / 2 ≤ 3.5, where 0.5% ≤ a% ≤ 3%, 1% ≤ b% ≤ 4%, and 0.85 g / cm 3 ≤ c ≤ 1.15 g / cm 3 , and has the following effects: The sodium-ion battery satisfies the relationship 0.8 ≤ (a + b) × c / 2 ≤ 3.5, and at the same time, the compaction density c of the negative electrode active material is limited to be within the range of 0.085 to 1.15 g / cm 3 . This can enable the particles of the negative electrode active material to be in full contact with each other without blocking the ion movement channels, ensure good electrical conductivity of electrons and fast ion movement during high-current discharge, and reduce discharge polarization; at the same time, the negative electrode active material, in cooperation with the first additive and the second additive, can form a SEI film with a stable structure, good toughness, appropriate thickness, high sodium-ion transmission rate, and low impedance at the negative electrode interface. The formed SEI film also has the effect of inhibiting the decomposition caused by the contact between the electrolyte and the positive electrode active material and the negative electrode active material, reducing the irreversible loss of active sodium, improving the stability of the electrolyte, and thus improving the cycle stability and rate performance of the battery. Detailed implementation manners

[0037] In order to make the technical problems, technical solutions, and beneficial effects solved by the present invention clearer, the present invention will be further described in detail below with reference to 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.

[0038] A sodium-ion secondary battery provided by the present application includes an electrolyte and a negative electrode. The electrolyte includes additives, and the additives include a first additive and a second additive. The first additive is a phosphorus-containing compound, and the second additive includes at least one of vinylene sulfate and 1,3-propane sultone; the negative electrode includes a negative electrode active material,

[0039] The phosphorus-containing compound is selected from at least one of the compounds shown in Structural Formula 1 or Structural Formula 2;

[0040]

[0041] wherein, R1, R2, R3, R4, R5, R6, and R7 are each independently selected from a cyano group, an ester group, an unsaturated hydrocarbon group with 1 to 4 carbon atoms, an alkyl group with 1 to 4 carbon atoms, and a haloalkyl group with 1 to 4 carbon atoms;

[0042] The sodium-ion secondary battery satisfies the following relationship:

[0043] 0.8 ≤ (a + b) × c / 2 ≤ 3.5, 0.5% ≤ a% ≤ 3%, 1% ≤ b% ≤ 4%, 0.85 g / cm 3 ≤ c ≤ 1.15 g / cm 3 ;

[0044] Wherein, a% is the mass percentage content of the first additive in the electrolyte;

[0045] b% is the mass percentage content of the second additive in the electrolyte;

[0046] c is the tap density of the negative electrode active material, unit g / cm 3 .

[0047] Specifically, the C1-C4 unsaturated hydrocarbon groups can be straight-chain or branched unsaturated hydrocarbon groups with carbon-carbon double bonds such as vinyl, propenyl, butenyl, etc., and can also be unsaturated hydrocarbon groups with carbon-carbon triple bonds such as ethynyl, butynyl, etc.

[0048] Similarly, the C1-C4 alkyl groups can be straight-chain saturated hydrocarbon groups such as methyl, ethyl, butyl, etc., and can also be branched saturated hydrocarbon groups such as isobutyl. The haloalkyl group means that at least one hydrogen atom in the alkyl group is replaced by a halogen, and the halogen includes fluorine, chlorine, bromine, iodine, etc.

[0049] Aiming at the problem of poor cycle stability of existing sodium ion batteries, the inventors have found through a large number of studies that a first additive with a mass percentage content of a% and a second additive with a mass content of b% are added to the electrolyte of the sodium ion battery, and the sodium ion secondary battery satisfies the relationship 0.8 ≤ (a + b) × c / 2 ≤ 3.5, where 0.5% ≤ a% ≤ 3%, 1% ≤ b% ≤ 4%, 0.85 g / cm 3 ≤ c ≤ 1.15 g / cm 3 , c is the tap density of the negative electrode active material, and has the following effects: the sodium ion battery satisfies the relationship 0.8 ≤ (a + b) × c / 2 ≤ 3.5, and at the same time limits the tap density c of the negative electrode active material to be in the range of 0.085 - 1.15 g / cm 3 so that the particles of the negative electrode active material can be in full contact without blocking the ion movement channels, which can ensure good conductivity of electrons and fast ion movement during high-current discharge, and reduce discharge polarization; at the same time, the negative electrode active material cooperates with the first additive and the second additive to form a SEI film with a stable structure, good toughness, moderate thickness, high sodium ion transmission rate and low impedance at the negative electrode interface. The formed SEI film also has the effect of inhibiting the decomposition caused by the contact between the electrolyte and the positive electrode active material and the negative electrode active material, reducing the irreversible loss of active sodium, improving the stability of the electrolyte, and thus improving the cycle stability and rate performance of the battery.

[0050] If the relation (a + b) × c / 2 of the sodium-ion secondary battery is less than 0.8, the contents of the first additive and the second additive are low and cannot effectively participate in the formation of the SEI film at the negative electrode interface, the conductivity of the electrolyte is too low, and the cycle performance stability of the battery is poor; if the relation (a + b) × c / 2 of the sodium-ion secondary battery is greater than 3.5, the contents of the first additive and the second additive are high, the viscosity of the electrolyte increases, the electrolyte is excessively consumed to participate in film formation, the thickness of the formed SEI film is too large, sodium ions are excessively consumed, the initial efficiency of the battery decreases, the impedance increases, and the rate performance decreases.

[0051] Specifically, the value of the relation (a + b) × c / 2 of the sodium-ion secondary battery can be, for example, 0.8, 1.0, 1.2, 1.5, 1.7, 2.0, 2.2, 2.4, 2.5, 2.7, 2.9, 3.0, 3.2, 3.4, 3.5, etc., as long as the sodium-ion secondary battery satisfies the relation 0.8 ≤ (a + b) × c / 2.

[0052] In some preferred embodiments, the sodium-ion secondary battery satisfies the following relation: 1.35 ≤ (a + b) × c / 2 ≤ 2.5.

[0053] Specifically, when the sodium-ion secondary battery satisfies the relation 1.35 ≤ (a + b) × c / 2 ≤ 2.5, the first additive and the second additive cooperate with the negative electrode active material to form an SEI film with a more stable structure, higher toughness, lower impedance, higher sodium-ion transmission rate, and appropriate thickness at the negative electrode interface, which is more conducive to improving the cycle performance and rate performance of the battery.

[0054] In some embodiments, the range of the mass percentage content a% of the first additive in the electrolyte is 0.5% to 3%.

[0055] Specifically, the phosphorus-containing compound is used as the first additive. During the charge and discharge process of the battery, it can form a film on the surface of the negative electrode of the battery, and because the formed SEI film is more stable, it can further inhibit the side reaction of the decomposition of the electrolyte caused by the contact of the non-aqueous electrolyte with the positive electrode active material and the negative electrode active material, significantly improve the stability of the electrode and the electrolyte, reduce the irreversible loss of active sodium, enhance the reversibility of the cycle, and promote the cycle stability.

[0056] If the mass content of the first additive in the electrolyte is less than 0.5%, the phosphorus-containing compound cannot effectively participate in film formation, the film formation quality is poor, the stability of the positive and negative electrode interfaces is poor, the side reaction is aggravated, the battery capacity decays rapidly, and the cycle performance is poor; if the mass content of the first additive is greater than 3%, the content of the phosphorus-containing compound is too high, the phosphorus-containing compound participates in film formation excessively, the thickness of the SEI film formed at the negative electrode interface is too thick, the impedance of the battery increases significantly, and the cycle performance deteriorates severely.

[0057] The mass percentage content a% of the first additive in the electrolyte can take values such as 0.5%, 0.8%, 1.0%, 1.2%, 1.5%, 1.8%, 2.0%, 2.3%, 2.5%, 2.8%, 3.0%, etc., as long as the value of a% is within the range of 0.5% to 3%.

[0058] In some preferred embodiments, the range of the mass percentage content a% of the first additive in the electrolyte is 1% to 2%.

[0059] When the mass content of the first additive is within the range of 1% to 2%, it is beneficial to form a more stable and ductile SEI film at the negative electrode interface, effectively inhibit the side reaction of electrolyte decomposition, and improve the cycle stability of the battery.

[0060] In some embodiments, the range of the mass percentage content b% of the second additive in the electrolyte is 1% to 4%.

[0061] Specifically, the second additive is selected from at least one of vinylene sulfate (DTD) and 1,3 - propane sultone (RPS). Adding the second additive to the electrolyte can participate in the formation of the negative electrode SEI film earlier than other components in the electrolyte, improve the quality of the negative electrode film formation, increase the transmission rate of sodium ions on the interface film, reduce the impedance and growth rate of the interface film, reduce electrolyte decomposition, and effectively improve the cycle stability of the battery while improving the rate performance.

[0062] If the mass percentage content b% of the second additive is less than 1%, the mass content of the second additive is too low, and there are fewer additives participating in the film formation, resulting in a decrease in the conductivity of the electrolyte, an overly thin and low - stability SIE film formed at the negative electrode interface, and the performance of the battery cannot be significantly improved; if the mass percentage content b% of the second additive is greater than 4%, the content of the second additive is too high, too many additives participate in the film formation, the thickness of the formed SEI film increases, and at the same time, too many second additives increase the reaction with the electrolyte, causing a sharp increase in the viscosity of the electrolyte and deteriorating the rate and cycle performance of the battery.

[0063] The value of the mass percentage content b% of the second additive in the electrolyte can be 1%, 1.2%, 1.5%, 1.8%, 2.0%, 2.3%, 2.5%, 2.8%, 3.0%, 3.4%, 3.5%, 3.6%, 3.8%, 4.0%, etc., as long as b% is within the range of 1% to 4%.

[0064] In some preferred embodiments, the range of the mass percentage content b% of the second additive in the electrolyte is 2% to 3%.

[0065] The mass content of the second additive in the electrolyte is in the range of 2% to 3%, which can form a SEI film with a moderate thickness, high stability, and high ion transport rate at the negative electrode interface, improve the transport rate of sodium ions on the interface film, reduce the impedance and growth rate of the interface film, reduce the decomposition of the electrolyte, and effectively improve the cycle stability of the battery while improving the rate performance.

[0066] In some embodiments, the tap density c of the negative electrode active material ranges from 0.85 to 1.15 g / cm 3 .

[0067] Specifically, through multiple experiments, it is found that when the tap density c of the negative electrode active material is in the range of 0.85 to 1.15 g / cm 3 , the particles can be in full contact without blocking the ion movement channels, which is beneficial to the formation of the SEI film by the phosphorus-containing compound on the surface of the negative electrode material, ensuring good electrical conductivity and fast ion movement during high-current discharge, reducing discharge polarization, increasing the capacity density, and improving the rate and cycle performance of the battery.

[0068] When the tap density c of the negative electrode active material is less than 0.85 cm 3 , the particle spacing of the negative electrode active material is too large, the contact probability and contact area between particles decrease, the conductivity decreases, affecting high-current discharge, increasing discharge polarization, and deteriorating the rate and cycle performance of the battery; when the tap density c of the negative electrode active material is greater than 1.15 cm 3 , the distance between particles decreases, the ion movement channels are reduced or blocked, which is not conducive to the rapid movement of a large number of ions, deteriorating the rate performance of the battery and reducing the discharge capacity.

[0069] The tap density c of the negative electrode active material can be, for example, 0.85 g / cm 3 , 0.9 g / cm 3 , 0.95 g / cm 3 , 1.0 g / cm 3 , 1.05 g / cm 3 , 1.1 g / cm 3 , 1.15 g / cm 3 , etc., as long as C is in the range of 0.85 to 1.15 g / cm 3 it is acceptable.

[0070] In some preferred embodiments, the tap density c of the negative electrode active material ranges from 0.9 to 1.0 g / cm 3 .

[0071] The tap density c of the negative electrode active material is in the range of 0.9 to 1.0 g / cm 3Within this range, the particles of the negative electrode active material are in sufficient contact with each other without blocking the ion movement channels. The negative electrode has higher conductivity, which is beneficial for the phosphorus-containing compound to participate in the formation of the SEI film on the surface of the negative electrode material, improving the ion transport rate at the negative electrode interface, reducing the discharge polarization, increasing the capacity density, and improving the rate performance and cycle performance of the battery.

[0072] In some embodiments, the phosphorus-containing compound is selected from at least one of the following compounds:

[0073]

[0074] It can be understood that the above Compounds 1 - 10 are only some of the listed compounds. As long as the compounds satisfy the structural formulas 1 and 2, they belong to the second additive of the present application and have the same effect.

[0075] In some embodiments, the electrolyte further includes a sodium salt, and the sodium salt includes one or more of sodium perchlorate (NaClO4), sodium tetrafluoroborate (NaBF4), sodium hexafluorophosphate (NaPF6), NaBOB, NaODFB, NaAsF6, sodium trifluoroacetate (CF3COONa), sodium tetraphenylborate (NaB(C6H5)4), sodium bis(fluorosulfonyl)imide (Na[(FSO2)2N]), sodium bis(trifluoromethylsulfonyl)imide (Na[(CF3SO2)2N]);

[0076] Based on the total mass of the electrolyte being 100%, the mass content of the sodium salt is 10 - 15%.

[0077] In some embodiments, the non-aqueous organic solvent includes one or more of carbonate compounds, carboxylate compounds, and ether compounds.

[0078] In some preferred embodiments, the carbonate compounds include cyclic or chain carbonates with 3 - 5 carbon atoms. The cyclic carbonates include one or more of ethylene carbonate, vinylene carbonate, ethylene vinylene carbonate, propylene carbonate, γ-butyrolactone, and butylene carbonate; the chain carbonates include one or more of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and dipropyl carbonate;

[0079] The carboxylate compounds include carboxylates with 2 - 6 carbon atoms, and the carboxylates include one or more of methyl acetate, ethyl acetate, propyl acetate, butyl acetate, and propyl propionate;

[0080] The ether compounds include cyclic ethers or chain ethers having 4 to 10 carbon atoms. The cyclic ethers include one or more of 1,3-dioxolane, 1,4-dioxane, tetrahydrofuran, 2-methyltetrahydrofuran, and 2-trifluoromethyltetrahydrofuran; the chain ethers include one or more of dimethoxymethane, 1,2-dimethoxyethane, and diethylene glycol dimethyl ether.

[0081] Based on the mass of the electrolyte being 100%, the mass percentage of the non-aqueous organic solvent is 75% to 85%.

[0082] The non-aqueous organic solvent dissolves sodium salts, additives, and auxiliary additives. If the content of the non-aqueous organic solvent is less than 75%, the solubility of the sodium salt decreases, the viscosity of the electrolyte increases, the battery impedance increases, and the battery rate performance decreases; if the content of the non-aqueous organic solvent is higher than 85%, the proportion of the additives and auxiliary additives decreases, affecting the film-forming reaction of the additives, the surface film of the electrode material is thinner, and the battery cycle performance decreases.

[0083] In some embodiments, the additive further includes an auxiliary additive, the auxiliary additive includes fluorinated carbonate, and the fluorinated carbonate includes fluoroethylene carbonate (FEC).

[0084] Based on the mass of the electrolyte being 100%, the mass percentage of the fluorinated carbonate is 1% to 5%.

[0085] Specifically, the mass percentage of the fluorinated carbonate added to the electrolyte can be 1%, 1.5%, 1.9%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, 5.0%, as long as the mass percentage of the added fluorinated carbonate is between 1% and 5%.

[0086] In some embodiments, the negative electrode active material is selected from one or more of hard carbon and soft carbon.

[0087] In some embodiments, the sodium ion secondary battery further includes a positive electrode, the positive electrode includes a positive electrode active material, and the positive electrode active material is selected from sodium-containing layered oxides, sodium-containing polyanion compounds, and sodium-containing Prussian blue compounds.

[0088] The sodium-containing layered oxide is selected from NaiMO2, where 0 < i ≤ 1, and M is selected from one or more of V, Cr, Mn, Fe, Co, Ni, and Cu.

[0089] The sodium-containing polyanion compound is selected from Na3V2(PO4)2F3.

[0090] The sodium-containing Prussian blue compound is selected from Na i＇ Mn[Fe(CN)6] 1-m· □m· nH2O, where 0 ≤ i' ≤ 2, 0 ≤ m ≤ 1, 0 ≤ n ≤ 20.

[0091] For Na i＇ Mn[Fe(CN)6] 1-m·□m· The "□" in nH2O represents a hole.

[0092] On the other hand, the present application provides a method for preparing a sodium-ion secondary battery, including the following steps: Preparation of the positive electrode: uniformly mixing a positive electrode active material, a positive electrode binder, a positive electrode conductive agent, and a first solvent, coating the mixture on a substrate, and removing the first solvent to obtain the positive electrode;

[0093] Preparation of the negative electrode: uniformly mixing a negative electrode active material, a negative electrode binder, a negative electrode conductive agent, and a second solvent, coating the mixture on a substrate, and removing the second solvent to obtain the negative electrode;

[0094] Preparation of the electrolyte: uniformly mixing a sodium salt, an additive, an auxiliary additive, and a non-aqueous organic solvent to obtain the electrolyte, where the additive includes a first additive and a second additive;

[0095] The first additive is a phosphorus-containing compound, and the second additive includes at least one of ethylene sulfate and 1,3-propane sultone;

[0096] The phosphorus-containing compound is selected from at least one of the compounds shown in Structural Formula 1 or Structural Formula 2;

[0097]

[0098] Wherein, R1, R2, R3, R4, R5, R6, and R7 are each independently selected from a cyano group, an ester group, an unsaturated hydrocarbon group with 1 to 4 carbon atoms, an alkyl group with 1 to 4 carbon atoms, and a haloalkyl group with 1 to 4 carbon atoms;

[0099] Assembling the positive electrode, the negative electrode, and the electrolyte to obtain a sodium-ion secondary battery.

[0100] Wherein, the prepared sodium-ion secondary battery satisfies the following relationship: 0.8 ≤ (a + b) × c / 2 ≤ 3.5, 0.5% ≤ a% ≤ 3%, 1% ≤ b% ≤ 4%, 0.85 g / cm 3 ≤ c ≤ 1.15 g / cm 3 ;

[0101] Wherein, a% is the mass percentage content of the first additive in the electrolyte;

[0102] b% is the mass percentage content of the second additive in the electrolyte;

[0103] c is the tap density of the negative electrode active material, with the unit g / cm 3。

[0104] In some embodiments, the positive electrode further includes a positive current collector and a positive electrode material layer, and the positive electrode material layer is disposed on the surface of the positive current collector.

[0105] The positive current collector is selected from metal materials capable of conducting electrons. Preferably, the positive current collector includes one or more of Al, Ni, tin, copper, and stainless steel. In a more preferred embodiment, the positive current collector is selected from aluminum foil.

[0106] In some embodiments, the positive electrode material layer includes a positive electrode binder and a positive electrode conductive agent. The positive electrode binder includes one or more of polyvinylidene fluoride, copolymers of vinylidene fluoride, polytetrafluoroethylene, copolymers of vinylidene fluoride - hexafluoropropylene, copolymers of tetrafluoroethylene - hexafluoropropylene, copolymers of tetrafluoroethylene - perfluoroalkyl vinyl ether, copolymers of ethylene - tetrafluoroethylene, copolymers of vinylidene fluoride - tetrafluoroethylene, copolymers of vinylidene fluoride - trifluoroethylene, copolymers of vinylidene fluoride - trichloroethylene, copolymers of vinylidene fluoride - fluoroethylene, copolymers of vinylidene fluoride - hexafluoropropylene - tetrafluoroethylene, thermoplastic polyimides, thermoplastic resins such as polyethylene and polypropylene; acrylic resins; and styrene - butadiene rubbers.

[0107] The positive electrode conductive agent includes one or more of conductive carbon black, conductive carbon spheres, conductive graphite, conductive carbon fibers, carbon nanotubes, graphene, or reduced graphene oxide.

[0108] In some embodiments, the negative electrode further includes a negative current collector and a negative electrode material layer, and the negative electrode material layer is disposed on the surface of the negative current collector. The material of the negative current collector may be the same as that of the positive current collector, which will not be elaborated here. Preferably, the negative current collector is selected from copper foil.

[0109] In some embodiments, the negative electrode material layer includes a negative electrode binder and a negative electrode conductive agent. The negative electrode binder and the negative electrode conductive agent may be the same as the positive electrode binder and the positive electrode conductive agent respectively, which will not be elaborated here.

[0110] In some embodiments, the first solvent includes N - methylpyrrolidone, deionized water, acetone, propanol, ethanol, etc. Preferably, the first solvent is N - methylpyrrolidone. The second solvent includes N - methylpyrrolidone, deionized water, acetone, propanol, ethanol, etc. Preferably, the second solvent is deionized water.

[0111] In some embodiments, the secondary battery further includes a separator, and the separator is located between the positive electrode and the negative electrode.

[0112] The diaphragm can be an existing conventional diaphragm, such as a ceramic diaphragm, a polymer diaphragm, a non-woven fabric, an inorganic-organic composite diaphragm, etc., including but not limited to single-layer PP (polypropylene), single-layer PE (polyethylene), double-layer PP / PE, double-layer PP / PP, and triple-layer PP / PE / PP diaphragms.

[0113] The following will further explain the specific embodiments of the present invention through examples, but it does not indicate that the protection scope of the present invention is limited to the scope described in the examples.

[0114] Example 1

[0115] This example is used to illustrate the sodium-ion secondary battery disclosed in this application.

[0116] The components in the electrolyte are as follows: based on 100% of the electrolyte mass, the mass content of the non-aqueous organic solvent is 82.5%, where the non-aqueous organic solvent is ethylene carbonate, propylene carbonate, and ethyl methyl carbonate with a mass ratio of 20:10:70. The mass percentage content of the first additive is a%, and the mass percentage content of the second additive is b%. An auxiliary additive is added, and the values of a and b and the addition amount of the auxiliary additive are shown in Table 1. The mass content of the sodium salt sodium hexafluorophosphate is 10% - 15%.

[0117] Prepare the electrolyte: Add the above non-aqueous organic solvent, first additive, second additive, and sodium salt into a stirring container and mix them evenly to obtain the electrolyte.

[0118] Prepare a sodium-ion secondary battery, including the following steps:

[0119] (1) Prepare the positive electrode: Take the positive electrode active material (NaNi 0.7 Co 0.15 Mn 0.15 O2), conductive carbon black Super-P, and binder polyvinylidene fluoride (PVDF) in a mass ratio of 93:4:3 for mixing, and then disperse them in an appropriate amount of N-methyl-2-pyrrolidone (NMP) to obtain the positive electrode slurry; coat the obtained slurry evenly on both sides of the aluminum foil, dry, calender, and vacuum dry it, and then weld the aluminum lead wire with an ultrasonic welder to obtain the positive electrode plate, and the thickness of the electrode plate is between 120 - 150 μm.

[0120] (2) Prepare the negative electrode: According to the mass ratio of 94:1:2.5:2.5, take the negative electrode active material with a compaction density c of 0.9 g / m 3The hard carbon, conductive carbon black Super-P, binder styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC) are mixed, and then they are dispersed in an appropriate amount of deionized water to obtain the negative electrode slurry; the slurry is coated on both sides of the copper foil, dried, calendered, and vacuum dried, and then a nickel lead-out wire is welded with an ultrasonic welder to obtain the negative electrode plate, and the thickness of the electrode plate is between 120-150 μm.

[0121] (3) Preparation of the sodium-ion secondary battery: A three-layer separator with a thickness of 20 μm is placed between the positive electrode and the negative electrode prepared above, and then the sandwich structure composed of the positive electrode, the negative electrode, and the separator is wound, and then the wound body is flattened and placed in an aluminum foil packaging bag. After that, the battery is assembled according to the existing preparation method.

[0122] Examples 2-24 and Comparative Examples 1-10

[0123] The differences between Examples 2-24, Comparative Examples 1-10 and Example 1 are that the types of the first additive, the mass percentage content a% of the first additive, the types and mass percentage content b% of the second additive, the compaction density c of the negative electrode active material, and the types and mass contents of the auxiliary additives are different, as shown in Table 1 specifically.

[0124] Table 1 Electrolyte and battery parameter table of Examples 1-15 and Comparative Examples 1-10

[0125]

[0126]

[0127] Battery performance test:

[0128] (1) 25°C normal temperature cycle test:

[0129] The batteries prepared in Examples 1-24 and Comparative Examples 1-10 are placed under the condition of 25°C normal temperature, charged at a constant current of 0.7C to 3.95V, then charged at a constant voltage of 3.95V, with the cut-off current of 0.03C, and then discharged at a constant current of 1C to 1.5V, and cycled 200 weeks in this way;

[0130] Calculate the capacity retention rate at 200 weeks = the discharge capacity at the 200th week / the average value of the cyclic discharge capacities in the 1st - 3rd weeks × 100%.

[0131] (2) 45°C high temperature cycle test:

[0132] The batteries prepared in Examples 1-24 and Comparative Examples 1-10 were placed under the condition of 45 °C, charged at a constant current of 0.7C to 3.95V, then charged at a constant voltage until the charging current dropped to 0.03C, and then discharged at a constant current of 1C to 1.5V. This cycle was repeated 200 times; the discharge capacity in the first week and the discharge capacity in the 200th week were recorded.

[0133] Calculate the capacity retention rate after 200 weeks = discharge capacity in the 200th week / discharge capacity in the first week × 100%.

[0134] (3) Discharge capacity ratio at 3C rate

[0135] The batteries prepared in Examples 1-24 and Comparative Examples 1-10 were placed under the condition of 25 °C, charged at a constant current of 0.2C to 3.95V, then charged at a constant voltage until the charging current dropped to 0.03C, and then discharged at a constant current of 3C to 1.5V. The discharge capacity C1 of the battery was recorded. Similarly, it was charged at a constant current of 0.2C to 3.95V, then charged at a constant voltage until the charging current dropped to 0.03C, and then discharged at a constant current of 0.2C to 1.5V. The discharge capacity C2 of the battery was recorded.

[0136] Discharge capacity ratio at 3C rate = C1 / C2 × 100%.

[0137] The specific test data of the electrical performance of the above batteries are shown in Tables 2-5.

[0138] Table 2 Test data of battery performance of Examples 1-15 and Comparative Examples 1-10

[0139]

[0140]

[0141] As can be seen from Tables 1 and 2, when comparing Example 1 with Comparative Examples 1-2, only the first additive or the second additive was added to the electrolyte, and the battery had poor cycling performance and rate performance, and the electrolyte conductivity was low; when comparing Example 1 with Comparative Examples 3-10, the mass content a% of the first additive added to the electrolyte was not within the range of 0.5% to 3%, and the battery had poor cycling performance and rate performance, and the electrolyte conductivity was low; when the mass content b% of the second additive in the electrolyte was not within the range of 1% to 4%, the battery had poor cycling performance and rate performance, and the electrolyte conductivity was low; when the negative electrode compaction density c was not within the range of 0.85 to 1.15 g / cm 3 range, the cycling performance and rate performance of the battery were even worse; it shows that when the first additive and the second additive are added to the electrolyte, the addition amount a% of the first additive is within the range of 0.5% to 3%, the mass content b of the second additive is within the range of 1% to 4%, and the negative electrode compaction density c is within the range of 0.85 to 1.15 g / cm 3Within a certain range, and the sodium-ion secondary battery satisfies the relationship 0.8 ≤ (a + b) × c / 2 ≤ 3.5. The battery has good cycling performance and rate performance. It is speculated that the sodium-ion secondary battery satisfies the relationship 0.8 ≤ (a + b) × c / 2 ≤ 3.5, 0.5% ≤ a% ≤ 3%, 1% ≤ b% ≤ 4%, 0.85 g / cm 3 ≤ c ≤ 1.15 g / cm 3 This can enable the particles of the negative electrode active material to be in full contact without blocking the ion movement channels, ensuring good electrical conductivity of electrons and fast ion movement during high-current discharge, and reducing discharge polarization. At the same time, the negative electrode active material, in cooperation with the first additive and the second additive, can form a SEI film with a stable structure, good toughness, appropriate thickness, high sodium-ion transmission rate, and low impedance at the negative electrode interface. The formed SEI film also has the effect of inhibiting the decomposition caused by the contact between the electrolyte and the positive and negative active materials, reducing the irreversible loss of active sodium, improving the stability of the electrolyte, and enhancing the cycling stability and rate performance of the battery.

[0142] Comparing Examples 1 - 4, when the mass content a% of the first additive in the electrolyte is in the range of 1% - 2%, the battery has a higher cycling capacity retention rate, a higher rate discharge specific capacity, and a higher electrolyte conductivity. Comparing Examples 1, 5 - 7, when the mass content b% of the second additive is in the range of 2% - 3%, the battery has better cycling performance and rate performance. Comparing Examples 1, 8 - 10, when the negative electrode compaction density c is in the range of 0.9 - 1.0 g / cm 3 the battery has better cycling performance and rate performance. Comparing Examples 1 - 15, 0.5% ≤ a% ≤ 3%, 1% ≤ b% ≤ 4%, 0.85 g / cm 3 ≤ c ≤ 1.15 g / cm 3 and the sodium-ion secondary battery satisfies the relationship 1.35 ≤ (a + b) × c / 2 ≤ 2.5. The first additive, the second additive, and the negative electrode active material cooperate to form a SEI film with a more stable structure, higher toughness, lower impedance, higher sodium-ion transmission rate, and appropriate thickness at the negative electrode interface, which is more conducive to improving the cycling performance and rate performance of the battery.

[0143] Table 3 Data table of battery performance test for Examples 1, 16 - 17

[0144]

[0145] By comparing Tables 1 and 3, when the second additive added to the electrolyte is vinylene sulfate or 1,3 - propanesultone, it can also improve the cycling capacity retention rate and high-rate discharge specific capacity of the battery.

[0146] Table 4 Data table of battery performance test for Examples 1, 18 - 23

[0147]

[0148] By comparing Tables 1 and 4, it can be seen that by changing the compound type of the first additive, as long as the compounds shown in Structural Formula 1 or Structural Formula 2 are satisfied, they all have the same effect, can form a film on the surface of the battery negative electrode, and because the formed SEI film is more stable, it can further inhibit the side reaction of electrolyte decomposition caused by the contact of the non-aqueous electrolyte with the positive electrode active material and the negative electrode active material, significantly improve the stability of the electrode and the electrolyte, reduce the irreversible loss of active sodium, enhance the reversibility of the cycle, and promote the cycle stability.

[0149] Table 5 Data Sheet of Battery Performance Test for Examples 1 and 24

[0150]

[0151] By comparing Tables 1 and 5, it can be known that by adding an auxiliary additive to the electrolyte, the auxiliary additive can cooperate with the first additive and the second additive to improve the room temperature cycle capacity retention rate, high temperature cycle capacity retention rate, high rate discharge specific capacity of the battery and the conductivity of the electrolyte.

[0152] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A sodium-ion secondary battery, characterized in that, It includes an electrolyte and a negative electrode. The electrolyte includes additives, and the additives include a first additive and a second additive. The first additive is a phosphorus-containing compound, and the second additive includes at least one of vinylene sulfate and 1,3-propane sultone; the negative electrode includes a negative electrode active material, The phosphorus-containing compound is selected from at least one of the compounds shown in Structural Formula 1 or Structural Formula 2; wherein, R1, R2, R3, R4, R5, R6, and R7 are each independently selected from a cyano group, an ester group, an unsaturated hydrocarbon group with 1 to 4 carbon atoms, an alkyl group with 1 to 4 carbon atoms, and a halogenated alkyl group with 1 to 4 carbon atoms; The sodium-ion secondary battery satisfies the following relationship: 0.8 ≤ (a + b) × c / 2 ≤ 3.5, 0.5% ≤ a% ≤ 3%, 1% ≤ b% ≤ 4%, 0.85 g / cm 3 ≤ c ≤ 1.15 g / cm 3 ; wherein, a% is the mass percentage content of the first additive in the electrolyte; b% is the mass percentage content of the second additive in the electrolyte; c is the tap density of the negative electrode active material, unit: g / cm 3 .

2. The sodium-ion secondary battery according to claim 1, characterized in that, The sodium-ion secondary battery satisfies the following relationship: 1.35 ≤ (a + b) × c / 2 ≤ 2.

5.

3. The sodium ion secondary battery according to claim 1, characterized in that, The mass percentage content a% of the first additive in the electrolyte ranges from 1% to 2%.

4. The sodium-ion secondary battery according to claim 1, wherein, The mass percentage content b% of the second additive in the electrolyte ranges from 2% to 3%.

5. The sodium-ion secondary battery according to claim 1, characterized in that, The tap density c of the negative electrode active material ranges from 0.9 g / cm 3 ≤ c ≤ 1 g / cm 3 .

6. The sodium ion secondary battery according to claim 1, wherein The phosphorus-containing compound is selected from at least one of the following compounds:

7. The sodium ion secondary battery according to claim 1, characterized in that The electrolyte further includes a sodium salt, and the sodium salt includes one or more of NaClO4, NaBF4, NaPF6, NaBOB, NaODFB, NaAsF6, CF3COONa, NaB(C6H5)4, Na[(FSO2)2N], and Na[(CF3SO2)2N]; Based on the total mass of the electrolyte being 100%, the mass content of the sodium salt is 10 to 15%.

8. The sodium ion secondary battery according to claim 1, characterized in that, The non-aqueous organic solvent includes one or more of carbonate compounds, carboxylate compounds, and ether compounds; Preferably, the carbonate compounds include cyclic or chain carbonates with 3 to 5 carbon atoms. The cyclic carbonates include one or more of ethylene carbonate, vinylene carbonate, ethylene vinyl carbonate, propylene carbonate, γ-butyrolactone, and butylene carbonate; the chain carbonates include one or more of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and dipropyl carbonate; The carboxylate compounds include carboxylates with 2 to 6 carbon atoms, and the carboxylates include one or more of methyl acetate, ethyl acetate, propyl acetate, butyl acetate, and propyl propionate; The ether compounds include cyclic ethers or chain ethers with 4 to 10 carbon atoms. The cyclic ethers include one or more of 1,3-dioxolane, 1,4-dioxane, tetrahydrofuran, 2-methyltetrahydrofuran, and 2-trifluoromethyltetrahydrofuran; the chain ethers include one or more of dimethoxymethane, 1,2-dimethoxyethane, and diethylene glycol dimethyl ether; Based on the mass of the electrolyte being 100%, the mass percentage content of the non-aqueous organic solvent is 75% to 85%.

9. The sodium ion secondary battery according to claim 1, wherein The additives further include auxiliary additives, and the auxiliary additives include fluorinated carbonates. Preferably, the fluorinated carbonates include fluorinated ethylene carbonate; Based on the mass of the electrolyte being 100%, the mass percentage of the fluorinated carbonate is 1% to 5%.

10. The sodium-ion secondary battery according to claim 1, wherein The negative electrode active material is selected from one or more of hard carbon and soft carbon; The sodium ion secondary battery further includes a positive electrode, the positive electrode includes a positive electrode active material, and the positive electrode active material is selected from sodium-containing layered oxides, sodium-containing polyanion compounds, and sodium-containing Prussian blue compounds; The sodium-containing layered oxide is selected from NaiMO2, where 0 < i ≤ 1, and M is selected from one or more of V, Cr, Mn, Fe, Co, Ni, and Cu; The sodium-containing polyanion compound is selected from Na3V2(PO4)2F3; The sodium-containing Prussian blue compound is selected from where 0 ≤ i' ≤ 2, 0 ≤ m ≤ 1, and 0 ≤ n ≤ 20.

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