Sodium-ion battery electrolyte, sodium-ion battery, and sodium precipitation determination method thereof

Abstract

The present application relates to a sodium-ion battery electrolyte, a sodium-ion battery and a sodium precipitation determination method thereof. The sodium-ion battery electrolyte comprises a sodium salt, a carbonate organic solvent and a synergistic additive comprising a short-chain alcohol ether, an inorganic sodium salt and an aromatic compound.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of electrochemical energy storage batteries, and particularly relates to a sodium ion battery electrolyte, a sodium ion battery and a sodium precipitation determination method thereof. BACKGROUND

[0002] At present, lithium ion batteries have been successfully applied to portable electronic devices and electric vehicles. However, the continuous surge in the price of lithium salts and the uneven geographical distribution of lithium mines make it difficult for lithium ion batteries to be used in large-scale energy storage. Sodium is one of the ideal substitutes for lithium, and the sodium reserves are abundant and have similar physical and chemical properties to lithium, so it is urgent to develop high-performance sodium storage electrode materials.

[0003] Due to abundant reserves, low cost, stable structure and low sodium insertion voltage platform, carbon-based materials such as hard carbon, soft carbon and graphite have attracted widespread attention, and hard carbon has become a commercially promising negative electrode material. However, under some abuse conditions (such as fast charging or overcharging), the precipitation of metallic sodium in sodium ion batteries is difficult to avoid. The accumulation of metallic sodium on the surface of the carbon negative electrode will lead to the intensification of sodium ion battery polarization. In severe cases, sodium dendrites will pierce the separator, causing a short circuit in the sodium ion battery, and even an explosion accident. Therefore, it is urgent to develop a sodium ion battery carbon negative electrode sodium precipitation determination method, so as to better detect and control the safety of the battery, and to promote the further development and large-scale industrial application of sodium ion batteries.

[0004] At present, there are few reports on sodium precipitation of carbon negative electrodes in sodium ion batteries, and most of them are about the characterization of metallic sodium after disassembly of sodium ion batteries (such as documents Adv. Energy Mater., 2020, 10(3), 201903176 and J. Mater. Chem. A, 2021, 9(41): 23522-36), which is destructive to the battery and is not suitable for safety evaluation of long-term running sodium ion batteries. Chinese patent CN116136572A discloses a non-destructive determination method for sodium precipitation of sodium ion batteries, which analyzes the charging curve of the sodium ion battery to determine whether sodium precipitation occurs in the sodium ion soft package battery and the degree of sodium precipitation. This method does not need to disassemble the battery and has the characteristics of non-destructive and intuitive, but lacks attention to the sodium precipitation mechanism of sodium ion batteries. In the document DOI: 10.13700 / j.bh.1001-5965.2023.0051, the researchers monitored, analyzed the failure and mechanism of sodium precipitation of hard carbon negative electrodes in sodium ion batteries, and explored the failure behavior under different degrees of sodium precipitation. Combined with scanning electron microscopy (SEM), constant current intermittent titration method (GITT) and electrochemical impedance (EIS), the sodium precipitation mechanism of hard carbon was analyzed with the increase of sodium precipitation degree, which provided a comprehensive understanding of sodium precipitation of hard carbon.

[0005] However, how to regulate the sodium deposition behavior of the carbon negative electrode, especially under overcharge and fast charging conditions, to achieve uniform, dendrite-free, and highly reversible sodium deposition on the surface of the carbon negative electrode, and to ensure that the sodium ion battery does not lose control of the voltage, there is no report on the related method for safety detection and control of sodium deposition of the carbon negative electrode of the sodium ion battery. SUMMARY

[0006] In view of the safety problems caused by the sodium deposition behavior of the carbon negative electrode of the sodium ion battery under overcharge and fast charging conditions, the application provides a sodium ion battery electrolyte, a sodium ion battery and a sodium deposition determination method thereof.

[0007] In a first aspect, the application provides a sodium ion battery electrolyte, which comprises: a sodium salt, a carbonate organic solvent and a synergistic additive comprising a short-chain alcohol ether, an inorganic sodium salt and an aromatic compound.

[0008] Preferably, the sodium salt comprises one or more of sodium hexafluorophosphate, sodium perchlorate, sodium tetrafluoroborate and sodium bis(oxalato)borate.

[0009] Preferably, the carbonate organic solvent comprises one or more of propylene carbonate, ethylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate and fluorinated ethylene carbonate.

[0010] Preferably, the short-chain alcohol ether comprises one or more of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, diethylene glycol monomethyl ether, propylene glycol methyl ether, diethylene glycol ethyl ether and dipropylene glycol butyl ether.

[0011] The inorganic sodium salt comprises one or more of sodium nitrate, sodium triflate, sodium carbonate, sodium chloride and sodium acetate.

[0012] The aromatic compound comprises one or more of oxybenzone, benzophenone, benzil, anthrone, p-hydroxyacetophenone, tetralone, biphenyl, furan and cyclohexylbenzene.

[0013] Preferably, in the sodium ion battery electrolyte, the concentration of the sodium salt is 0.5-3M, preferably 1M.

[0014] The volume ratio of the mixed solution formed by the synergistic additive, the sodium salt and the carbonate organic solvent is 1:99-10:90.

[0015] Preferably, the molar ratio of the short-chain alcohol ether, the inorganic sodium salt and the aromatic compound in the synergistic additive is 0.1-3:0.1-3:0.1-3.

[0016] In a second aspect, the present application provides a preparation method of the sodium-ion battery electrolyte, which comprises the following steps: adding a sodium salt into a carbonate organic solvent to form a mixed solution; then, adding a synergistic additive into the mixed solution, and stirring uniformly to obtain the sodium-ion battery electrolyte.

[0017] In a third aspect, the present application provides a sodium-ion battery, which comprises a carbon negative electrode sheet, a positive material electrode, a separator and the sodium-ion battery electrolyte.

[0018] Preferably, the carbon negative electrode sheet comprises the following components by weight: 70-99 parts of a carbon negative electrode active material, 0.5-10 parts of a conductive agent and 0.5-10 parts of a binder.

[0019] The carbon negative electrode active material comprises a composite of one or more of hard carbon material, soft carbon material and graphite; the conductive agent is one or more of acetylene black, ketjen black or Super P; and the binder is one or more of polyvinylidene fluoride, carboxymethyl cellulose or polyacrylic acid.

[0020] Preferably, the positive material is one or more of sodium-based transition metal oxide, sodium-based transition metal phosphate or sodium-based transition metal cyanide compound.

[0021] Preferably, the sodium-based transition metal oxide is sodium cobaltate, sodium manganate or sodium ferrite; the sodium-based transition metal phosphate is sodium iron phosphate, sodium vanadium phosphate, sodium fluorovanadate or sodium pyrophosphate; and the sodium-based transition metal cyanide compound is sodium ferrocyanide or Prussian blue analogue.

[0022] In a fourth aspect, the present application provides a sodium precipitation determination method of the sodium-ion battery, which comprises the following steps:

[0023] S1, charging and discharging the sodium-ion battery once under a set condition, and then emptying the battery power;

[0024] S2, performing constant current charging on the sodium-ion battery with emptied battery power under a set temperature and charging rate to a set cut-off voltage, then stopping, recording the relaxation voltage change of the battery within a set time after the charging is completed, and obtaining a voltage-time curve;

[0025] S3, differentiating the voltage-time curve to obtain a differential voltage-time curve; determining whether there is sodium precipitation when the negative electrode of the sodium-ion battery is charged to the set cut-off voltage under the set temperature and the set charging rate by judging whether there is an inflection point in the differential voltage-time curve; the inflection point is a demarcation point of changing the upward or downward trend direction of a continuous curve or a demarcation point of a concave arc and a convex arc of a continuous curve.

[0026] Advantages

[0027] The present application adds a short-chain alcohol ether-inorganic sodium salt-aromatic compound synergistic additive in the traditional carbonate electrolyte, and the synergistic additive decomposes oligoether and rich components such as NaN3 / NaF, which can cross-link on the carbon negative electrode surface to form an elastic solid electrolyte interface (SEI) with fast ion flux and can release sodium stress, greatly improving the stability of the SEI before and after sodium deposition under overcharge conditions, promoting the uniform deposition of metal sodium, so that under overcharge and fast charge conditions, uniform, dendrite-free, and highly reversible sodium deposition can be achieved on the carbon negative electrode surface. In addition, thanks to the intermolecular interaction with oligoether, the appropriate aromatic compound in the synergistic additive can undergo polymerization more quickly and easily under overcharge conditions, thereby protecting the sodium ion battery from voltage runaway by consuming the overcharge current; further analyzing the relaxation voltage-time curve of the sodium ion battery after charging, whether the sodium ion battery appears sodium deposition and the degree of sodium deposition can be judged by analyzing whether the differential voltage-time curve appears an obvious inflection point;

[0028] The technical scheme provided by the present application can significantly improve the safety performance of sodium ion batteries based on carbon negative electrodes, prolong the cycle life of sodium ion batteries, and the non-destructive testing method can improve the detection efficiency, without disassembling the battery, and can quickly and simply provide early warning for the safe operation of sodium ion batteries, thereby providing the possibility for the practical development of the sodium ion battery system and effectively promoting the industrialization process of sodium ion batteries. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 The voltage-time curve of the sodium ion battery in Example 1 under a 2.5C charging rate at 25°C is shown in the charging curve graph;

[0030] Figure 2 The curve differential dV / dt of the voltage-time curve of the sodium ion battery in Example 1 under a 2.5C charging rate at 25°C is shown in the charging curve graph; Figure 1 The curve differential dV / dt of the voltage-time curve of the sodium ion battery in Example 1 under a 2.5C charging rate at 25°C is shown in the charging curve graph;

[0031] Figure 3 The battery disassembly graph of the sodium ion battery in Example 1 under a 2.5C charging rate at 25°C charged to 4V is shown in the charging curve graph;

[0032] Figure 4 The voltage-time curve of the sodium ion battery in Example 2 under a 2.5C charging rate at 25°C is shown in the charging curve graph;

[0033] Figure 5 The curve differential dV / dt of the voltage-time curve of the sodium ion battery in Example 2 under a 2.5C charging rate at 25°C is shown in the charging curve graph; Figure 4 The curve differential dV / dt of the voltage-time curve of the sodium ion battery in Example 2 under a 2.5C charging rate at 25°C is shown in the charging curve graph;

[0034] Figure 6 The battery disassembly graph of the sodium ion battery in Example 2 under a 2.5C charging rate at 25°C charged to 4V is shown in the charging curve graph;

[0035] Figure 7 Charge profile of the voltage as a function of time for the sodium-ion battery of Comparative Example 1 at 25°C at a 2.5C charge rate;

[0036] Figure 8 differential of the curve dV / dt as a function of time for Figure 7

[0037] Figure 9 Battery disassembly of the sodium-ion battery of Comparative Example 1 charged to 4V at 25°C at a 2.5C charge rate;

[0038] Figure 10 Charge profile of the voltage as a function of time for the sodium-ion battery of Comparative Example 2 at 25°C at a 2.5C charge rate;

[0039] Figure 11 differential of the curve dV / dt as a function of time for Figure 10

[0040] Figure 12 Battery disassembly of the sodium-ion battery of Comparative Example 2 charged to 4V at 25°C at a 2.5C charge rate;

[0041] Figure 13 Charge profile of the voltage as a function of time for the sodium-ion battery of Comparative Example 3 at 25°C at a 2.5C charge rate;

[0042] Figure 14 differential of the curve dV / dt as a function of time for Figure 13

[0043] Figure 15 Battery disassembly of the sodium-ion battery of Comparative Example 3 charged to 4V at 25°C at a 2.5C charge rate;

[0044] Figure 16 Charge profile of the voltage as a function of time for the sodium-ion battery of Comparative Example 4 at 25°C at a 2.5C charge rate;

[0045] Figure 17 differential of the curve dV / dt as a function of time for Figure 16

[0046] Figure 18 Battery disassembly of the sodium-ion battery of Comparative Example 4 charged to 4V at 25°C at a 2.5C charge rate. DETAILED DESCRIPTION

[0047] The present application is further illustrated by the following examples which should not be construed as limiting the present application. ​​​​

[0048] The present application obtains a new sodium ion battery electrolyte by adding a short-chain alcohol ether-inorganic sodium salt-aromatic compound synergistic additive in a traditional carbonate electrolyte. Under overcharge conditions, through the synergistic effect of the synergistic additive, uniform, dendrite-free and highly reversible sodium deposition can be achieved on the carbon negative electrode surface, and at the same time, the overcharge current is consumed to protect the sodium ion battery from voltage runaway. Further, the voltage relaxation method is used to detect whether the sodium ion battery carbon negative electrode deposits sodium and the degree of sodium deposition, that is, by analyzing the curve of the relaxation voltage of the sodium ion battery after charging with time to determine whether the sodium ion battery deposits sodium and the degree of sodium deposition.

[0049] Firstly, the present application provides a sodium ion battery electrolyte. The sodium ion battery electrolyte comprises: a sodium salt, a carbonate organic solvent, and a synergistic additive comprising a short-chain alcohol ether, an inorganic sodium salt, and an aromatic compound.

[0050] In some embodiments, the sodium salt can include one or more of sodium hexafluorophosphate, sodium perchlorate, sodium tetrafluoroborate, and sodium bis(oxalato)borate.

[0051] In some embodiments, the carbonate organic solvent can include one or more of propylene carbonate, ethylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, and fluoroethylene carbonate.

[0052] In some embodiments, the short-chain alcohol ether can include one or more of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, diethylene glycol monomethyl ether, propylene glycol methyl ether, diethylene glycol ethyl ether, and dipropylene glycol butyl ether; the inorganic sodium salt can include one or more of sodium nitrate, sodium triflate, sodium carbonate, sodium chloride, and sodium acetate; and the aromatic compound can include one or more of oxybenzone, benzophenone, benzil, anthrone, p-hydroxyacetophenone, tetralone, biphenyl, furan, and cyclohexylbenzene.

[0053] The synergistic additive used in the present application can decompose the derived oligoether and abundant components such as NaN3 / NaF to cross-link on the surface of the carbon negative electrode of the sodium ion battery to form an elastic solid electrolyte interface (SEI) with fast ion flux and slow release of sodium deposition stress, thereby greatly improving the stability of the SEI before and after sodium deposition under overcharge conditions, promoting uniform deposition of metallic sodium, and enabling the carbon negative electrode surface to achieve uniform, dendrite-free, and highly reversible sodium deposition under overcharge and fast charging conditions.

[0054] Meanwhile, the aromatic compound in the synergistic additive can undergo electropolymerization under extreme conditions such as overcharge, fast charging and the like of the sodium ion battery, form a polymer bridge between the positive and negative electrodes, and thus protect the sodium ion battery from voltage runaway by consuming overcharge current. In addition, the aromatic compound can form a weak interaction with the short-chain alcohol ether in the synergistic additive, reduce the potential barrier of oligomeric ether formation, and change the sodium ion solvent structure in the electrolyte to induce uniform and reversible deposition of metal sodium on the surface of the carbon negative electrode during charging and discharging.

[0055] In some embodiments, the concentration of the sodium salt in the sodium ion battery electrolyte can be 0.5-3M, preferably 1M; the volume ratio of the mixed solution formed by the synergistic additive, the sodium salt and the carbonate organic solvent can be 1:99-10:90; preferably, the molar ratio of the short-chain alcohol ether, the inorganic sodium salt and the aromatic compound in the synergistic additive can be 0.1-3:0.1-3:0.1-3.

[0056] Hereinafter, the preparation method of the above-mentioned sodium ion battery electrolyte provided by the present application is exemplarily described. The preparation method can include the following steps: adding a sodium salt into a carbonate organic solvent to form a mixed solution; then, adding a synergistic additive into the mixed solution and stirring uniformly to obtain the sodium ion battery electrolyte.

[0057] The present application also provides a sodium ion battery. The sodium ion battery comprises a carbon negative electrode sheet, a positive material electrode, a separator and the above-mentioned sodium ion battery electrolyte.

[0058] In some embodiments, the carbon negative electrode sheet can comprise the following components by weight: 70-99 parts of a carbon negative electrode active material, 0.5-10 parts of a conductive agent and 0.5-10 parts of a binder.

[0059] The carbon negative electrode active material can comprise a composite of one or more of hard carbon material, soft carbon material and graphite; the conductive agent can be one or more of acetylene black, ketjen black or Super P; and the binder can be one or more of polyvinylidene fluoride (PVDF), carboxymethyl cellulose (CMC) or polyacrylic acid (PAA).

[0060] In some embodiments, the positive electrode material can be one or more of sodium-based transition metal oxide, sodium-based transition metal phosphate or sodium-based transition metal cyanide compound; preferably, the sodium-based transition metal oxide can be sodium cobaltate, sodium manganate or sodium ferrite, the sodium-based transition metal phosphate can be sodium iron phosphate, sodium vanadium phosphate, sodium fluorovanadate phosphate or sodium ferric pyrophosphate, and the sodium-based transition metal cyanide compound can be sodium ferrocyanide or Prussian blue analogue.

[0061] The sodium ion battery sodium precipitation determination method provided by the present application is exemplarily described below. The determination method can include the following steps:

[0062] S1, charging and discharging the sodium ion battery once under the set conditions, and emptying the battery capacity;

[0063] S2, after the sodium ion battery with empty battery capacity is charged to the set cut-off voltage under the set temperature and charge rate, stopping, then recording the relaxation voltage (open circuit voltage) change of the battery within the set time after charging, obtaining the voltage-time (V-t) curve;

[0064] S3, differentiating the voltage-time curve to obtain the U'-time curve (dV / dt-time curve); by judging whether there is an inflection point in the U'-time curve, it is determined whether there is sodium precipitation in the negative electrode of the sodium ion battery when the negative electrode is charged to the set cut-off voltage under the set temperature and set charge rate.

[0065] The inflection point refers to a sudden change in the curve. The inflection point, also known as the inflection point, refers to the dividing point of the continuous curve changing the upward or downward trend direction in mathematics. Intuitively, the inflection point is the point where the tangent line crosses the curve (i.e. the dividing point of the concave arc and the convex arc of the continuous curve). If the function of the curve graph has a second derivative at the inflection point, the second derivative at the inflection point changes sign (from positive to negative or from negative to positive) or does not exist.

[0066] During the charging process of the sodium ion battery, sodium ions are embedded in the negative electrode material from the positive electrode material, and at the same time the voltage of the battery will rise. During the discharging process, sodium ions are de-embedded from the negative electrode material, and at the same time the voltage of the battery will decrease. When sodium ions in the negative electrode material are precipitated, the voltage drop rate during the standing process after charging will increase, which will be faster than the rate without sodium precipitation in the sodium ion battery under the same conditions. If the voltage-time curve appears a peak, it indicates that there is sodium precipitation, and at the same time the voltage-time differential curve will appear a clear inflection point.

[0067] The present application analyzes the curve of the relaxation voltage of the sodium ion battery changing with time after charging by the voltage relaxation method, and further analyzes whether the differential voltage-time curve appears a clear inflection point to determine whether the sodium ion battery appears sodium precipitation and the degree of sodium precipitation. The method is suitable for determining the sodium precipitation of the negative electrode of the sodium ion battery using carbon-based materials as the negative electrode active material.

[0068] The following further illustrates the embodiments to explain the present application in detail. It should also be understood that the following embodiments are only used to further illustrate the present application and cannot be understood as a limitation on the scope of protection of the present application. Some non-essential improvements and adjustments made by those skilled in the art according to the above content of the present application all belong to the protection scope of the present application. The specific process parameters in the following examples are only one example in the appropriate range, i.e. those skilled in the art can select within the appropriate range according to the description herein, and are not limited to the specific values in the following examples.

[0069] Example 1

[0070] The sodium ion battery electrolyte, sodium ion battery preparation process and sodium ion battery sodium precipitation determination method provided in the present embodiment are as follows:

[0071] 1. Preparation of electrolyte: sodium perchlorate as solute, ethylene carbonate (EC), diethyl carbonate (DEC) and fluoroethylene carbonate as solvent, the volume ratio of EC, DEC and fluoroethylene carbonate is 1:1:0.11; the concentration of sodium perchlorate is 1M, the volume ratio of the mixed solution of the synergistic additive and the above sodium perchlorate and carbonate organic solvent is 5:95, and the molar ratio of triethylene glycol dimethyl ether, sodium nitrate and diphenyl in the synergistic additive is 3:2:0.5.

[0072] 2. Coating of electrode sheet: (1) Na3V2(PO4)3 positive electrode sheet was prepared by using fast ion conductor Na3V2(PO4)3 as active material, and the slurry containing 80wt% Na3V2(PO4)3, 10wt% super P and 10wt% polyvinylidene fluoride was coated on aluminum foil; then after drying at 100℃ for 10h, the electrode film was punched into a sheet with a diameter of 13mm; the mass loading of active material was controlled to be about 3mg cm -2 .(2) The slurry containing 80wt% hard carbon, 10wt% super P and 10wt% carboxymethyl cellulose was coated on aluminum foil to prepare a hard carbon negative electrode; then after drying at 100℃ for 10h, the electrode film was punched into a sheet with a diameter of 14mm; the mass loading of active material was controlled to be about 2.5mg cm -2 .

[0073] 3. Assembly of sodium ion battery: a PE separator with a thickness of 9μm, the electrolyte prepared in step 1 and the positive and negative electrode sheets prepared in step 2 were assembled to obtain a sodium ion battery.

[0074] 4. Activation of battery: the battery assembled in step 3 was sent into a 45℃ constant temperature box, and a blue electric charge-discharge instrument was used to activate the battery; the activation voltage range was 1.2-4.3V, and the charge-discharge rate was 0.05C; the charge-discharge was carried out for 3 weeks.

[0075] 5. Battery cycle stability test: The activated battery is subjected to a 1C, 500-cycle cycle stability test, with a charge and discharge voltage range of 1.2 to 4.3V; the instrument automatically records the charge and discharge data and related curves during the test.

[0076] 6. Set the temperature to 25℃ and the charging rate to 2.5C. Under these conditions, charge the activated battery to different cutoff voltages, stop charging, and let it stand to obtain the voltage-time (Vt) curve.

[0077] 7. Differentiate the voltage-time curve to obtain the dV / dt-time curve. If there is an inflection point in the dV / dt-time curve, it indicates that sodium deposition occurs when the battery is charged to the corresponding cutoff voltage at the set temperature and set charging rate.

[0078] Figure 1 The graph shows the voltage-time curve of the sodium-ion battery in Example 1 at 25°C and a charging rate of 2.5C. As can be seen from the graph, the voltage-time curve of the sodium-ion battery under static conditions after charging is relatively smooth, and the voltage decreases slowly with increasing time.

[0079] Figure 2 for Figure 1 The curve shows the variation of the differential dV / dt over time. The graph shows that the relaxation voltage curve under charging conditions of 25℃, 2.5C charging rate, and 4V cutoff voltage has no inflection point, proving that at 25℃, when the battery is charged to 4V at a charging rate of 2.5C, sodium deposition does not occur.

[0080] Figure 3 This is a disassembled image of the sodium-ion battery from Example 1, charged to 4V at 25°C and a 2.5C charging rate. As can be seen from the image, there is no sodium deposition on the carbon negative electrode surface, and the electrode surface morphology remains intact.

[0081] Example 2

[0082] The sodium-ion battery electrolyte, sodium-ion battery preparation process, and sodium precipitation determination method provided in this embodiment are the same as in Example 1. The main difference is that the molar ratio of triethylene glycol dimethyl ether, sodium nitrate, and biphenyl in the synergistic additives in step 1 is 3:2:0.1.

[0083] Figure 4 The graph shows the voltage-time curve of the sodium-ion battery in Example 2 at 25°C and a charging rate of 2.5C. As can be seen from the graph, the voltage-time curve of the sodium-ion battery under static conditions after charging is relatively smooth, and the voltage decreases slowly with increasing time.

[0084] Figure 5 yes Figure 4The voltage-time curve of the sodium-ion battery of Example 1 under the charging condition of 25℃, 2.5C charging rate and 4V cut-off voltage is shown in FIG. 2. As can be seen from the figure, the voltage-time curve under the charging condition of 25℃, 2.5C charging rate and 4V cut-off voltage does not have an obvious inflection point, proving that there is no sodium precipitation in the battery when the battery is charged to 4V at 25℃ and 2.5C charging rate.

[0085] Figure 6 The battery disassembly diagram of the sodium-ion battery of Example 1 under the charging condition of 25℃, 2.5C charging rate and 4V cut-off voltage is shown in FIG. 3. As can be seen from the figure, Figure 6 The surface of the carbon negative electrode of the sodium-ion battery of Example 1 does not have sodium precipitation phenomenon, and the surface morphology of the electrode sheet is good.

[0086] Comparative Example 1

[0087] The sodium-ion battery electrolyte, sodium-ion battery preparation process and sodium-ion battery sodium precipitation determination method provided in the present comparative example refer to Example 1, the main difference being that the molar ratio of triethylene glycol dimethyl ether, sodium nitrate and diphenyl in the synergistic additive in step 1 is 3:2:0.01.

[0088] Figure 7 The voltage-time curve of the sodium-ion battery of Comparative Example 1 under the charging condition of 25℃, 2.5C charging rate is shown in FIG. 4. As can be seen from the figure, the voltage-time curve of the sodium-ion battery under the resting condition after charging has a peak, which indicates that there is sodium precipitation.

[0089] Figure 8 The voltage-time curve of the sodium-ion battery of Comparative Example 1 under the charging condition of 25℃, 2.5C charging rate is shown in FIG. 4. As can be seen from the figure, the voltage-time curve of the sodium-ion battery under the resting condition after charging has a peak, which indicates that there is sodium precipitation. Figure 7 The differential curve dV / dt of the voltage-time curve of the sodium-ion battery of Comparative Example 1 under the charging condition of 25℃, 2.5C charging rate and 4V cut-off voltage is shown in FIG. 5. As can be seen from the figure, the curve under the charging condition of 25℃, 2.5C charging rate and 4V cut-off voltage has an inflection point, proving that there is sodium precipitation in the battery when the battery is charged to 4V at 25℃ and 2.5C charging rate.

[0090] Figure 9 The battery disassembly diagram of the sodium-ion battery of Comparative Example 1 under the charging condition of 25℃, 2.5C charging rate and 4V cut-off voltage is shown in FIG. 6. As can be seen from the figure, there is sodium precipitation on the surface of the carbon negative electrode, and a small amount of active material is peeled off from the surface of the carbon negative electrode sheet.

[0091] Comparative Example 2

[0092] The sodium-ion battery electrolyte, sodium-ion battery preparation process and sodium-ion battery sodium precipitation determination method provided in the present comparative example refer to Example 1, the main difference being that the molar ratio of triethylene glycol dimethyl ether, sodium nitrate and diphenyl in the synergistic additive in step 1 is 0.01:2:0.5.

[0093] Figure 10The graph shows the voltage-time curve of the sodium-ion battery in Comparative Example 2 at 25°C and a charging rate of 2.5C, as a function of time. As can be seen from the graph, the voltage-time curve of this sodium-ion battery under static conditions after charging exhibits a peak, indicating sodium deposition.

[0094] Figure 11 for Figure 10 The curve shows the variation of the differential dV / dt over time. The graph shows an inflection point under the charging conditions of 25℃, 2.5C, and 4V cutoff voltage, indicating that sodium deposition occurs in the battery when charged to 4V at 25℃ and a 2.5C charging rate.

[0095] Figure 12 This is a disassembly diagram of the sodium-ion battery from Comparative Example 2, charged to 4V at 25℃ and a charging rate of 2.5C. The diagram shows sodium deposition on the carbon negative electrode surface and active material stripping from the electrode surface.

[0096] Comparative Example 3

[0097] The sodium-ion battery electrolyte, sodium-ion battery preparation process, and sodium precipitation determination method provided in this comparative example are the same as in Example 1. The main difference is that the molar ratio of triethylene glycol dimethyl ether, sodium nitrate, and biphenyl in the synergistic additives in step 1 is 3:0.01:0.5.

[0098] Figure 13 The graph shows the voltage-time curve of the sodium-ion battery in Comparative Example 3 at 25℃ and a charging rate of 2.5C. As can be seen from the graph, a peak appears in the voltage-time curve of this sodium-ion battery under static conditions after charging, indicating sodium deposition.

[0099] Figure 14 for Figure 13 The curve shows the variation of the differential dV / dt over time. The graph shows an inflection point under the charging conditions of 25℃, 2.5C, and 4V cutoff voltage, indicating that sodium deposition occurs in the battery when charged to 4V at 25℃ and a 2.5C charging rate.

[0100] Figure 15 This is a disassembly diagram of the sodium-ion battery (Comparative Example 3) charged to 4V at 25℃ and a charging rate of 2.5C. The diagram shows sodium deposition on the carbon negative electrode surface and active material stripping from the electrode surface.

[0101] Comparative Example 4

[0102] The sodium-ion battery electrolyte, sodium-ion battery preparation process and sodium-ion battery sodium precipitation determination method provided by the comparative example are the same as those of example 1, the main difference is that the molar ratio of triethylene glycol dimethyl ether, sodium nitrate and diphenyl in the synergistic additive in step 1 is 3:2:6.

[0103] Figure 16 The voltage-time charge curve of the sodium-ion battery of comparative example 4 at 25°C and 2.5C charge rate is shown in the figure. As can be seen from the figure, the voltage-time curve of the sodium-ion battery after charging under static conditions appears a peak, which indicates that sodium is precipitated.

[0104] Figure 17 The curve of the differential dV / dt of Figure 16 As can be seen from the figure, the curve under the charging condition of charging temperature 25°C, charging rate 2.5C and cut-off voltage 4V has an inflection point, which proves that when the battery is charged to 4V at 25°C and 2.5C, the battery has sodium precipitation.

[0105] Figure 18 The battery disassembly diagram of the sodium-ion battery of comparative example 4 at 25°C and 2.5C charge rate is shown in the figure. As can be seen from the figure, sodium is precipitated on the surface of the carbon negative electrode, and the active material on the surface of the electrode sheet is seriously peeled off.

[0106] Table 1 below is a comparison of the long-term cycle stability performance of the sodium-ion batteries in examples 1, 2 and comparative examples 1-4:

[0107] Sodium-ion battery Cycling 500 cycles capacity retention Example 1 96.2％ Example 2 80.4％ Comparative Example 1 52.6％ Comparative Example 2 47.5％ Comparative Example 3 41.1％ Comparative Example 4 35.3％

[0108] As can be seen from table 1, the long-term cycle stability of the sodium-ion battery after the sodium precipitation behavior of the carbon negative electrode is regulated is improved to a certain extent, and the performance of the sodium-ion battery assembled with the electrolyte in comparative examples 1-3 is poorer than that of example 1, which shows that each component in the synergistic additive plays an irreplaceable role, and the synergistic effect among the three makes the sodium storage performance of the carbon negative electrode most stable, and has better sodium ion transmission characteristics, and is not prone to sodium precipitation under fast charging conditions, and has higher safety. In comparative example 4, an excess of aromatic compound additive is used, which hinders the transmission of sodium ions during charging, and the formed SEI film is not uniform and dense, resulting in a significant decay of the electrochemical performance of the carbon negative electrode, and the stress deformation in the reaction process causes the active material on the electrode sheet to peel off. Further, the synergistic effect of the aromatic compound additive, the short-chain alcohol ether and the inorganic sodium salt makes the microstructure of the electrolyte not only ensure the formation of a uniform and dense SEI film with elasticity on the surface of the carbon negative electrode, but also play a protective role under overcharging conditions, hindering the precipitation of sodium.

[0109] The sodium precipitation behavior control method has remarkable effect, is easy to operate, and can effectively improve the long-term cycle stability of the sodium ion battery based on the carbon negative electrode; the sodium ion battery sodium precipitation nondestructive detection method can judge whether the sodium ion battery has sodium precipitation and the degree of sodium precipitation by analyzing the curve of the relaxation voltage of the sodium ion battery after charging with time, thereby improving the safety performance of the sodium ion battery based on the carbon negative electrode, and the nondestructive detection method can improve the detection efficiency, does not need to disassemble the battery, can simply and quickly provide early warning for the safe operation of the sodium ion battery, and has high research prospect and application value.

[0110] Although the content of the present application has been described in detail by the above preferred embodiments, it should be recognized that the above description should not be considered as limiting the present application. After reading the above content, various modifications and substitutions of the present application will be apparent to those skilled in the art. Therefore, the protection scope of the present application should be defined by the appended claims.

Claims

1. A sodium-ion battery electrolyte, characterized in that, The sodium ion battery electrolyte comprises a sodium salt, a carbonate organic solvent and a synergistic additive comprising a short-chain alcohol ether, an inorganic sodium salt and an aromatic compound. The short-chain alcohol ether comprises one or more of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, diethylene glycol monomethyl ether, propylene glycol methyl ether, diethylene glycol ethyl ether, dipropylene glycol butyl ether and ethylene glycol monoethyl ether. The inorganic sodium salt comprises one or more of sodium nitrate, sodium triflate, sodium carbonate, sodium chloride and sodium acetate. The aromatic compound comprises one or more of oxybenzone, benzophenone, benzil, anthrone, p-hydroxyacetophenone, tetralone and biphenyl. The molar ratio of the short-chain alcohol ether, the inorganic sodium salt and the aromatic compound in the synergistic additive is 0.1-3:0.1-3:0.1-3.

2. The sodium-ion battery electrolyte of claim 1, wherein, The sodium salt comprises one or more of sodium hexafluorophosphate, sodium perchlorate, sodium tetrafluoroborate and sodium bis(oxalato)borate.

3. The sodium-ion battery electrolyte of claim 1, wherein, The carbonate organic solvent comprises one or more of propylene carbonate, ethylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate and fluorinated ethylene carbonate.

4. The sodium-ion battery electrolyte of claim 1, wherein, The concentration of the sodium salt in the sodium ion battery electrolyte is 0.5-3M. The volume ratio of the mixed solution of the synergistic additive, the sodium salt and the carbonate organic solvent is 1:99-10:

90.

5. The sodium-ion battery electrolyte of claim 4, wherein, The concentration of the sodium salt in the sodium ion battery electrolyte is 1M.

6. A method of preparing the sodium-ion battery electrolyte of claim 1, characterized in that, The preparation method comprises the following steps: adding the sodium salt into the carbonate organic solvent to form a mixed solution; then, adding the synergistic additive into the mixed solution and stirring uniformly to obtain the sodium ion battery electrolyte.

7. A sodium-ion battery, characterized in that, The sodium ion battery comprises a carbon negative electrode sheet, a positive electrode material, a separator and the sodium ion battery electrolyte according to claim 1.

8. The sodium-ion battery of claim 7, wherein, The carbon negative electrode sheet comprises the following components by weight: 70-99 parts of a carbon negative electrode active material, 0.5-10 parts of a conductive agent and 0.5-10 parts of a binder. The carbon negative electrode active material comprises a composite of one or more of hard carbon material, soft carbon material and graphite; the conductive agent is one or more of acetylene black, ketjen black and Super P; and the binder is one or more of polyvinylidene fluoride, carboxymethyl cellulose and polyacrylic acid.

9. The sodium-ion battery of claim 7, wherein, The positive electrode material is one or more of a sodium-based transition metal oxide, a sodium-based transition metal phosphate or a sodium-based transition metal cyanide compound.

10. The sodium-ion battery of claim 9, wherein, The sodium-based transition metal oxide is sodium cobaltate, sodium manganate or sodium ferrate; the sodium-based transition metal phosphate is sodium iron phosphate, sodium vanadate phosphate, fluorinated sodium vanadate phosphate or sodium ferric pyrophosphate; and the sodium-based transition metal cyanide compound is sodium ferrocyanide or Prussian blue analogues.

11. A method of determining sodium precipitation of the sodium-ion battery according to claim 7, characterized by, The determination method comprises the following steps: S1. After the sodium ion battery is subjected to one cycle of charging and discharging under a set condition, the battery capacity is emptied; S2. After the sodium ion battery with the emptied battery capacity is subjected to constant current charging to a set cut-off voltage under a set temperature and charging rate, the charging is stopped, then the relaxation voltage change of the battery after the charging is completed within a set time is recorded to obtain a voltage-time curve; S3, differentiating the voltage-time curve to obtain a differential voltage-time curve; determining whether sodium precipitation exists when the negative electrode of the sodium ion battery is charged to the set cut-off voltage at the set temperature and the set charge rate by judging whether an inflection point exists in the differential voltage-time curve; the inflection point is a demarcation point of changing the upward or downward trend direction of a continuous curve or a demarcation point of a concave arc and a convex arc of a continuous curve.

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