Sodium-ion battery electrolyte, sodium-ion battery and sodium precipitation judgment method of sodium-ion battery

Abstract

The invention relates to a sodium-ion battery electrolyte, a sodium-ion battery and a sodium precipitation judgment method of the sodium-ion battery. The sodium-ion battery electrolyte comprises sodium salt, a carbonic ester organic solvent and a synergistic additive comprising short-chain alcohol ether, inorganic sodium salt and an aromatic compound.

Description

Technical Field

[0001] The present invention 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 method for determining sodium deposition thereof. Background Art

[0002] Currently, lithium-ion batteries have been successfully applied to portable electronic devices and electric vehicles. However, the continuous soaring of lithium salt prices and the uneven geographical distribution of lithium mines have made the use of lithium-ion batteries in large-scale energy storage a difficult problem. Sodium is one of the ideal substitutes for lithium. Sodium has rich reserves and similar physical and chemical properties to lithium. It is urgent to develop sodium storage electrode materials with high performance.

[0003] Due to their rich reserves, low cost, stable structure, and low sodium insertion voltage platform, carbon-based materials such as hard carbon, soft carbon, and graphite have received extensive attention. Among them, hard carbon has become a promising anode material. However, under some abuse conditions (such as fast charging or overcharging) of sodium-ion batteries, the precipitation of metallic sodium is inevitable. The accumulation of metallic sodium on the surface of the carbon anode will lead to increased polarization of the sodium-ion battery. In severe cases, sodium dendrites will pierce the separator, resulting in short circuit of the sodium-ion battery and even explosion accidents. Therefore, it is urgent to develop a method for determining sodium deposition on the carbon anode of sodium-ion batteries, so as to better perform the safety detection and control of the battery, and promote the further development and industrial large-scale application of sodium-ion batteries.

[0004] Currently, there are few reports on sodium deposition on the carbon anode of sodium-ion batteries, and most of them are about the characterization of metallic sodium after the disassembly of sodium-ion batteries (such as the literature Adv. Energy Mater., 2020, 10(3), 201903176 and J. Mater. Chem. A, 2021, 9(41): 23522 - 36). This type of method is destructive to the battery and is not suitable for the safety evaluation of long-term operating sodium-ion batteries. Chinese Patent CN116136572A discloses a non-destructive determination method for whether sodium deposition occurs in a sodium-ion battery, which determines whether sodium deposition occurs and the degree of sodium deposition in a sodium-ion soft-pack battery by analyzing the charging curve of the sodium-ion battery. This method does not require battery disassembly and has the characteristics of non-destruction and intuitiveness, but it lacks attention to the sodium deposition mechanism of sodium-ion batteries. In the literature DOI: 10.13700 / j.bh.1001 - 5965.2023.0051, the researchers monitored, analyzed the failure and mechanism of sodium deposition on the hard carbon anode of sodium-ion batteries, explored the failure behaviors under different degrees of sodium deposition, and analyzed the sodium deposition mechanism of hard carbon with the increase of sodium deposition degree by combining scanning electron microscopy (SEM), galvanostatic intermittent titration technique (GITT), and electrochemical impedance spectroscopy (EIS), providing a comprehensive understanding of sodium deposition on hard carbon.

[0005] However, there is no report on how to regulate the sodium deposition behavior on the carbon negative electrode, especially under overcharge and fast charge conditions, to achieve uniform, dendrite-free, and highly reversible sodium deposition on the surface of the carbon negative electrode, ensuring that the sodium-ion battery does not experience voltage runaway, and related methods for the safe detection and control of sodium deposition on the carbon negative electrode of sodium-ion batteries. Summary of the Invention

[0006] Aiming at the safety problems caused by the sodium deposition behavior of the carbon negative electrode of the above sodium-ion battery under overcharge and fast charge conditions, the present invention provides a sodium-ion battery electrolyte, a sodium-ion battery, and a method for determining sodium deposition thereof.

[0007] In the first aspect, the present invention provides a sodium-ion battery electrolyte, and the sodium-ion battery electrolyte includes: sodium salt, carbonate organic solvent, and a synergistic additive including short-chain alcohol ether, inorganic sodium salt, and aromatic compound.

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

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

[0010] Preferably, the short-chain alcohol ether includes 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 includes one or more of sodium nitrate, sodium trifluoromethanesulfonate, sodium carbonate, sodium chloride, and sodium acetate; the aromatic compound includes one or more of oxybenzophenone, benzophenone, benzil, anthrone, 4-hydroxyacetophenone, tetrahydronaphthone, biphenyl, furan, and cyclohexylbenzene.

[0011] Preferably, in the sodium-ion battery electrolyte, the concentration of the sodium salt is 0.5-3M, preferably 1M; The volume ratio of the synergistic additive to the mixed solution formed by the sodium salt and the carbonate organic solvent is 1:99 to 10:90; Preferably, the molar ratio of the short-chain alcohol ether, inorganic sodium salt, and aromatic compound in the synergistic additive is 0.1-3:0.1-3:0.1-3.

[0012] In the second aspect, the present invention provides a preparation method of the above sodium-ion battery electrolyte, and the preparation method includes the following steps: adding the sodium salt into the carbonate organic solvent to form a mixed solution; then, adding the synergistic additive to the mixed solution and stirring evenly to obtain the sodium-ion battery electrolyte.

[0013] In a third aspect, the present invention provides a sodium-ion battery, which includes a carbon negative electrode sheet, a positive material electrode, a separator, and the above-mentioned sodium-ion battery electrolyte.

[0014] Preferably, the carbon negative electrode sheet includes the following components in parts by weight: 70 to 99 parts of carbon negative electrode active material, 0.5 to 10 parts of conductive agent, and 0.5 to 10 parts of binder; The carbon negative electrode active material includes a composite of one or more of hard carbon materials, soft carbon materials, and graphite; the conductive agent is one or more of acetylene black, Ketjen black, or Super P; the binder is one or more of polyvinylidene fluoride, carboxymethyl cellulose, or polyacrylic acid.

[0015] Preferably, the positive electrode material is one or more of sodium-based transition metal oxides, sodium-based transition metal phosphates, or sodium-based transition metal cyanide compounds; Preferably, 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 vanadium phosphate, fluorovanadium phosphate, or sodium pyrophosphate iron, and the sodium-based transition metal cyanide compound is sodium ferrocyanide or a Prussian blue analogue.

[0016] In a fourth aspect, the present invention provides a method for determining sodium deposition of the above-mentioned sodium-ion battery, and the determination method includes the following steps: S1. After charging and discharging the sodium-ion battery once under set conditions, discharge the battery power. S2. Charge the sodium-ion battery with discharged power at a constant current to a set cut-off voltage at a set temperature and charging rate, then stop, and record the relaxation voltage change of the battery within a set time after charging ends to obtain a voltage-time curve. S3. Differentiate the voltage-time curve to obtain a differential voltage-time curve; determine whether sodium deposition occurs at the negative electrode of the sodium-ion battery when charging to the set cut-off voltage at the set temperature and set charging rate by judging whether there is an inflection point in the differential voltage-time curve; the inflection point is the demarcation point for changing the upward or downward trend direction of a continuous curve or the demarcation point between the concave arc and convex arc of a continuous curve.

[0017] Beneficial effects The present invention adds a short-chain alcohol ether-inorganic sodium salt-aromatic compound synergistic additive to the traditional carbonate-based electrolyte, and the oligomeric ethers and rich NaN derived from the decomposition of the synergistic additive 3Components such as / NaF can crosslink on the surface of the carbon negative electrode to form an elastic solid electrolyte interface (SEI) with fast ion flux and capable of slowly releasing the sodium precipitation stress, greatly improving the stability of the SEI before and after sodium precipitation under overcharge conditions, promoting the uniform deposition of metallic sodium, and enabling uniform, dendrite-free, and highly reversible sodium deposition on the surface of the carbon negative electrode under overcharge and fast charge conditions. In addition, due to the intermolecular interaction with the oligoether, the appropriate aromatic compounds in the synergistic additive can polymerize faster and more easily under overcharge conditions, protecting the sodium-ion battery from voltage runaway by consuming the overcharge current; further analyzing the curve of the relaxation voltage of the sodium-ion battery over time after charging is completed, and judging whether sodium precipitation occurs and the degree of sodium precipitation in the sodium-ion battery by analyzing whether there is an obvious inflection point in the differential voltage-time curve; The technical solution provided by the present invention can significantly improve the safety performance of the sodium-ion battery based on the carbon negative electrode, extend the cycle life of the sodium-ion battery, and this non-destructive detection method can improve the detection efficiency, without disassembling the battery, and can simply and quickly provide an early warning for the safe operation of the sodium-ion battery, thereby providing the possibility for the practical development of the sodium-ion battery system and effectively promoting the industrialization process of the sodium-ion battery. Description of the Drawings

[0018] Figure 1 It is the charging curve graph of the voltage of the sodium-ion battery in Example 1 changing with time at 25°C and a charging rate of 2.5C; Figure 2 is Figure 1 The curve differential dV / dt changes with time curve; Figure 3 It is the battery disassembly diagram of the sodium-ion battery in Example 1 charged to 4V at 25°C and a charging rate of 2.5C; Figure 4 It is the charging curve graph of the voltage of the sodium-ion battery in Example 2 changing with time at 25°C and a charging rate of 2.5C; Figure 5 is Figure 4 The curve differential dV / dt changes with time curve; Figure 6 It is the battery disassembly diagram of the sodium-ion battery in Example 2 charged to 4V at 25°C and a charging rate of 2.5C; Figure 7 It is the charging curve graph of the voltage of the sodium-ion battery in Comparative Example 1 changing with time at 25°C and a charging rate of 2.5C; Figure 8 is Figure 7 The curve differential dV / dt changes with time curve; Figure 9Disassembly diagram of the sodium-ion battery of Comparative Example 1 charged to 4V at 25°C and a charging rate of 2.5C; Figure 10 Charge curve of the sodium-ion battery of Comparative Example 2 showing the change in voltage over time at 25°C and a charging rate of 2.5C; Figure 11 For Figure 10 Curve differential dV / dt versus time curve; Figure 12 Disassembly diagram of the sodium-ion battery of Comparative Example 2 charged to 4V at 25°C and a charging rate of 2.5C; Figure 13 Charge curve of the sodium-ion battery of Comparative Example 3 showing the change in voltage over time at 25°C and a charging rate of 2.5C; Figure 14 For Figure 13 Curve differential dV / dt versus time curve; Figure 15 Disassembly diagram of the sodium-ion battery of Comparative Example 3 charged to 4V at 25°C and a charging rate of 2.5C; Figure 16 Charge curve of the sodium-ion battery of Comparative Example 4 showing the change in voltage over time at 25°C and a charging rate of 2.5C; Figure 17 For Figure 16 Curve differential dV / dt versus time curve; Figure 18 Disassembly diagram of the sodium-ion battery of Comparative Example 4 charged to 4V at 25°C and a charging rate of 2.5C. Detailed implementation manner

[0019] The present invention is further illustrated by the implementation manners. It should be understood that the following implementation manners are only used to illustrate the present invention and do not limit the present invention.

[0020] The present invention obtains a new sodium-ion battery electrolyte by adding a short-chain alcohol ether-inorganic sodium salt-aromatic compound synergistic additive to a traditional carbonate-based 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 surface of the carbon negative electrode, 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 sodium is deposited on the carbon negative electrode of the sodium-ion battery and the degree of sodium deposition, that is, by analyzing the curve of the relaxation voltage of the sodium-ion battery changing over time after charging to judge whether sodium is deposited on the sodium-ion battery and the degree of sodium deposition.

[0021] First, the present invention provides a sodium-ion battery electrolyte. The sodium-ion battery electrolyte includes: a sodium salt, a carbonate organic solvent, and a synergistic additive including a short-chain alcohol ether, an inorganic sodium salt, and an aromatic compound.

[0022] In some embodiments, the sodium salt may include one or more of sodium hexafluorophosphate, sodium perchlorate, sodium tetrafluoroborate, and bis(oxalato)borate sodium.

[0023] In some embodiments, the carbonate organic solvent may include one or more of propylene carbonate, ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, and fluoroethylene carbonate.

[0024] In some embodiments, the short-chain alcohol ether may 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, dipropylene glycol butyl ether, and ethylene glycol monoethyl ether; the inorganic sodium salt may include one or more of sodium nitrate, sodium trifluoromethanesulfonate, sodium carbonate, sodium chloride, and sodium acetate; the aromatic compound may include one or more of oxybenzone, benzophenone, benzil, anthrone, 4-hydroxyacetophenone, tetrahydronaphthone, biphenyl, furan, and cyclohexylbenzene.

[0025] The oligomeric ethers and abundant NaN 3 / NaF and other components derived from the decomposition of the synergistic additive used in the present invention can crosslink 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 capable of slowly releasing the sodium deposition stress, thereby greatly improving the stability of the SEI before and after sodium deposition under overcharge conditions, promoting the uniform deposition of metallic sodium, and enabling uniform, dendrite-free, and highly reversible sodium deposition on the surface of the carbon negative electrode under overcharge and fast charge conditions.

[0026] At the same time, the aromatic compound in the synergistic additive can undergo an electro-polymerization reaction under extreme conditions such as overcharge and fast charge of the sodium-ion battery to form a polymer bridge between the positive and negative electrodes, thereby protecting the sodium-ion battery from voltage runaway by consuming the overcharge current. In addition, the aromatic compound can also form a weak interaction with the short-chain alcohol ether in the synergistic additive to lower the energy barrier for the formation of oligomeric ethers; and can change the solvation structure of sodium ions in the electrolyte to induce uniform and reversible deposition of metallic sodium on the surface of the carbon negative electrode during charge and discharge.

[0027] In some embodiments, in the sodium-ion battery electrolyte, the concentration of the sodium salt can be 0.5 - 3 M, preferably 1 M; the volume ratio of the mixed solution formed by the synergistic additive, the sodium salt, and the carbonate organic solvent can be 1:99 to 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.

[0028] Hereinafter, the preparation method of the above-mentioned sodium-ion battery electrolyte provided by the present invention will be exemplarily described. The preparation method may include the following steps: adding a sodium salt to a carbonate organic solvent to form a mixed solution; then, adding a synergistic additive to the mixed solution and stirring evenly to obtain the sodium-ion battery electrolyte.

[0029] The present invention also provides a sodium-ion battery. Among them, the sodium-ion battery includes: a carbon negative electrode sheet, a positive material electrode, a separator, and the above-mentioned sodium-ion battery electrolyte.

[0030] In some embodiments, the carbon negative electrode sheet may include the following components in parts 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.

[0031] Among them, the carbon negative electrode active material may include a composite of one or more of hard carbon materials, soft carbon materials, and graphite; the conductive agent may be one or more of acetylene black, Ketjen black, or Super P; the binder may be one or more of polyvinylidene fluoride PVDF, carboxymethyl cellulose CMC, or polyacrylic acid PAA.

[0032] In some embodiments, the positive electrode material may be one or more of sodium-based transition metal oxides, sodium-based transition metal phosphates, or sodium-based transition metal cyanide compounds; preferably, the sodium-based transition metal oxide may be sodium cobaltate, sodium manganate, or sodium ferrate, the sodium-based transition metal phosphate may be sodium iron phosphate, sodium vanadium phosphate, fluorovanadium phosphate, or sodium pyrophosphate iron, and the sodium-based transition metal cyanide compound may be sodium ferrocyanide or a Prussian blue analogue.

[0033] Hereinafter, the sodium deposition determination method of the sodium-ion battery provided by the present invention will be exemplarily described. The determination method may include the following steps: S1. After performing a charge-discharge cycle on the sodium-ion battery under set conditions once, discharge the battery to empty; S2. Charge the sodium-ion battery with the discharged battery at a constant current to a set cut-off voltage at a set temperature and charge rate, and then stop. Then record the change in the relaxation voltage (open-circuit voltage) of the battery within a set time after the charging ends to obtain a voltage-time (V-t) curve; S3. Differentiate the voltage-time curve to obtain a U'-time curve (dV / dt-time curve); determine whether sodium deposition occurs when the negative electrode of the sodium-ion battery is charged to the set cut-off voltage at the set temperature and set charging rate by judging whether there is an inflection point in the U'-time curve.

[0034] The inflection point refers to a sudden change in the curve. An inflection point, also known as a point of inflection, mathematically refers to the demarcation point that changes the upward or downward trend direction of a continuous curve. Intuitively, an inflection point is a point where the tangent line crosses the curve (i.e., the demarcation point between the concave arc and the convex arc of a continuous curve). If the function of the curve graph has a second derivative at the inflection point, then the second derivative has different signs (changing from positive to negative or from negative to positive) or does not exist at the inflection point.

[0035] During the charging process of a sodium-ion battery, sodium ions are inserted from the positive electrode material into the negative electrode material, and at the same time, the voltage of the battery will rise; during the discharging process, sodium ions are deintercalated from the negative electrode material, and at the same time, the voltage of the battery will drop. When sodium ions in the negative electrode material are deposited, the voltage drop rate during the static process after charging will increase, and will be faster than the rate without sodium deposition in a sodium-ion battery under the same conditions. If a peak appears in the voltage-time curve, it indicates the presence of sodium deposition, and at the same time, an obvious inflection point will appear in the voltage-time differential curve.

[0036] The present invention analyzes the curve of the relaxation voltage of a sodium-ion battery changing with time after charging by the voltage relaxation method, and further judges whether sodium deposition occurs in the sodium-ion battery and the degree of sodium deposition by analyzing whether an obvious inflection point appears in the differential voltage-time curve. The method is applicable to the determination of sodium deposition on the negative electrode of a sodium-ion battery using carbon-based materials as the negative electrode active substance.

[0037] The following further lists embodiments to illustrate the present invention in detail. It should also be understood that the following embodiments are only used to further illustrate the present invention and cannot be construed as limiting the protection scope of the present invention. Those skilled in the art's non-essential improvements and adjustments based on the above content of the present invention all belong to the protection scope of the present invention. The specific process parameters and the like in the following examples are only an example within a suitable range, that is, those skilled in the art can make selections within a suitable range according to the description in this article, rather than being limited to the specific values in the following examples.

[0038] Example 1

[0039] The electrolyte of the sodium-ion battery, the preparation process of the sodium-ion battery, and the method for judging sodium deposition of the sodium-ion battery provided in this example are as follows: 1. Preparation of electrolyte: Sodium perchlorate is used as the solute, ethylene carbonate (EC), diethyl carbonate (DEC) and fluoroethylene carbonate are used as solvents, and the volume ratio of EC, DEC and fluoroethylene carbonate is 1:1:0.11; the concentration of sodium perchlorate is 1 M, and the volume ratio of the co-additive to the mixed solution formed by the above sodium perchlorate and carbonate organic solvents is 5:95. The molar ratio of triethylene glycol dimethyl ether, sodium nitrate and biphenyl in the co-additive is 3:2:0.5. 2. Electrode coating: (1) Using fast ion conductor Na 3 V 2 (PO 4 ) 3 as the active material to prepare the positive electrode sheet. A slurry containing 80 wt% Na 3 V 2 (PO 4 ) 3 , 10 wt% super P and 10 wt% polyvinylidene fluoride is coated on the aluminum foil to prepare the Na 3 V 2 (PO 4 ) 3 positive electrode; then after drying at 100 °C for 10 h, the electrode film is punched into a pole piece with a diameter of 13 mm; the mass loading of the active material is controlled to be about 3 mg cm -2 . (2) A slurry containing 80 wt% hard carbon, 10 wt% super P and 10 wt% carboxymethyl cellulose is coated on the aluminum foil to prepare the hard carbon negative electrode; then after drying at 100 °C for 10 h, the electrode film is punched into a pole piece with a diameter of 14 mm; the mass loading of the active material is controlled to be about 2.5 mg cm -2 . 3. Assembly of sodium-ion battery: A sodium-ion battery is assembled with a PE separator with a thickness of 9 μm, the electrolyte prepared in step 1, and the positive and negative electrode sheets prepared in step 2. 4. Battery activation: The battery assembled in step 3 is placed in an incubator at 45 °C, and the battery is activated using a blue electrochemical workstation; the activation voltage range is 1.2 - 4.3 V, and the charge and discharge rate is 0.05 C for 3 cycles. 5. Battery cycle stability test: The activated battery is subjected to a 1C, 500-cycle stability test, and the charge and discharge voltage range is 1.2 - 4.3 V; during the test, the instrument automatically records the charge and discharge data and related curves. 6. Set the temperature to 25 °C and the charge rate to 2.5 C. Under this set condition, the activated battery is charged to different cut-off voltages and then stopped charging and left standing to obtain the voltage-time (V-t) curve. 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 is determined that there is sodium precipitation when the battery is charged to the corresponding cut-off voltage at the set temperature and set charging rate.

[0040] Figure 1 It is the charging curve diagram of the sodium-ion battery in Example 1 showing the change of voltage with time at 25 °C and a charging rate of 2.5C. It can be seen from the figure that the voltage-time curve of the sodium-ion battery under static conditions after charging is relatively smooth, and the voltage slowly decreases with the increase of time.

[0041] Figure 2 For Figure 1 is the curve of the differential dV / dt changing with time. It can be seen from the figure that there is no inflection point in the relaxation voltage curve under the charging conditions of a charging temperature of 25 °C, a charging rate of 2.5C, and a cut-off voltage of 4V, proving that there is no sodium precipitation in the battery when it is charged to 4V at a charging rate of 2.5C at 25 °C.

[0042] Figure 3 It is the battery disassembly diagram of the sodium-ion battery in Example 1 charged to 4V at 25 °C and a charging rate of 2.5C. It can be seen from the figure that there is no sodium precipitation on the surface of the carbon negative electrode, and the surface morphology of the electrode remains good.

[0043] Example 2

[0044] The electrolyte of the sodium-ion battery, the preparation process of the sodium-ion battery, and the method for determining sodium precipitation of the sodium-ion battery provided in this example refer to Example 1. The main difference is that the molar ratio of triethylene glycol dimethyl ether, sodium nitrate, and biphenyl in the co-additive in step 1 is 3:2:0.1.

[0045] Figure 4 It is the charging curve diagram of the sodium-ion battery in Example 2 showing the change of voltage with time at 25 °C and a charging rate of 2.5C. It can be seen from the figure that the voltage-time curve of the sodium-ion battery under static conditions after charging is relatively smooth, and the voltage slowly decreases with the increase of time.

[0046] Figure 5 Is Figure 4 the curve of the differential dV / dt changing with time. It can be seen from the figure that there is no obvious inflection point in the curve under the charging conditions of a charging temperature of 25 °C, a charging rate of 2.5C, and a cut-off voltage of 4V, proving that there is no sodium precipitation in the battery when it is charged to 4V at a charging rate of 2.5C at 25 °C.

[0047] Figure 6 It is the battery disassembly diagram of the sodium-ion battery in Example 2 charged to 4V at 25 °C and a charging rate of 2.5C. It can be seen from the figure that Figure 6There is no sodium precipitation on the surface of the carbon negative electrode, and the surface morphology of the electrode remains good.

[0048] Comparative Example 1

[0049] The sodium-ion battery electrolyte, sodium-ion battery preparation process, and sodium precipitation determination method provided in this comparative example refer to Example 1. The main difference is that the molar ratio of triethylene glycol dimethyl ether, sodium nitrate, and biphenyl in the co-additive in Step 1 is 3:2:0.01.

[0050] Figure 7 is the charge curve of the sodium-ion battery in Comparative Example 1 showing the change of voltage with time at 25 °C and a charging rate of 2.5C. It can be seen from the figure that there is a peak in the voltage-time curve of the sodium-ion battery under static conditions after charging. This peak indicates the presence of sodium precipitation.

[0051] Figure 8 is Figure 7 the curve of the differential dV / dt versus time. It can be seen from the figure that there is an inflection point in the curve under the charging conditions of a charging temperature of 25 °C, a charging rate of 2.5C, and a cut-off voltage of 4V, proving that when the battery is charged to 4V at a charging rate of 2.5C at 25 °C, sodium precipitation occurs in this battery.

[0052] Figure 9 is the disassembly diagram of the sodium-ion battery in Comparative Example 1 charged to 4V at 25 °C and a charging rate of 2.5C. It can be seen from the figure that sodium precipitation exists 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 plate.

[0053] Comparative Example 2

[0054] The sodium-ion battery electrolyte, sodium-ion battery preparation process, and sodium precipitation determination method provided in this comparative example refer to Example 1. The main difference is that the molar ratio of triethylene glycol dimethyl ether, sodium nitrate, and biphenyl in the co-additive in Step 1 is 0.01:2:0.5.

[0055] Figure 10 is the charge curve of the sodium-ion battery in Comparative Example 2 showing the change of voltage with time at 25 °C and a charging rate of 2.5C. It can be seen from the figure that there is a peak in the voltage-time curve of the sodium-ion battery under static conditions after charging. This peak indicates sodium precipitation.

[0056] Figure 11 is Figure 10 the curve of the differential dV / dt versus time. It can be seen from the figure that there is an inflection point in the curve under the charging conditions of a charging temperature of 25 °C, a charging rate of 2.5C, and a cut-off voltage of 4V, proving that when the battery is charged to 4V at a charging rate of 2.5C at 25 °C, sodium precipitation occurs in this battery.

[0057] Figure 12 It is the battery disassembly diagram of the sodium-ion battery of Comparative Example 2 when charged to 4V at 25°C and a charging rate of 2.5C. It can be seen from the figure that sodium deposition exists on the surface of the carbon negative electrode, and the active material is peeled off on the surface of the electrode sheet.

[0058] Comparative Example 3

[0059] The electrolyte of the sodium-ion battery, the preparation process of the sodium-ion battery and the method for determining sodium deposition of the sodium-ion battery provided in this comparative example refer to Example 1. The main difference is that the molar ratio of triethylene glycol dimethyl ether, sodium nitrate and biphenyl in the co-additive in Step 1 is 3:0.01:0.5.

[0060] Figure 13 It is the charging curve of the voltage of the sodium-ion battery of Comparative Example 3 changing with time at 25°C and a charging rate of 2.5C. It can be seen from the figure that there is a peak in the voltage-time curve of the sodium-ion battery under the static condition after charging, and this peak indicates sodium deposition.

[0061] Figure 14 It is Figure 13 the curve of the differential dV / dt of [] changing with time. It can be seen from the figure that there is an inflection point in the curve under the charging conditions of a charging temperature of 25°C, a charging rate of 2.5C, and a cut-off voltage of 4V, which proves that sodium deposition exists in the battery when the battery is charged to 4V at a charging rate of 2.5C at 25°C.

[0062] Figure 15 It is the battery disassembly diagram of the sodium-ion battery of Comparative Example 3 when charged to 4V at 25°C and a charging rate of 2.5C. It can be seen from the figure that sodium deposition exists on the surface of the carbon negative electrode, and the active material is peeled off on the surface of the electrode sheet.

[0063] Comparative Example 4

[0064] The electrolyte of the sodium-ion battery, the preparation process of the sodium-ion battery and the method for determining sodium deposition of the sodium-ion battery provided in this comparative example refer to Example 1. The main difference is that the molar ratio of triethylene glycol dimethyl ether, sodium nitrate and biphenyl in the co-additive in Step 1 is 3:2:6.

[0065] Figure 16 It is the charging curve of the voltage of the sodium-ion battery of Comparative Example 4 changing with time at 25°C and a charging rate of 2.5C. It can be seen from the figure that there is a peak in the voltage-time curve of the sodium-ion battery under the static condition after charging, and this peak indicates sodium deposition.

[0066] Figure 17 It is Figure 16The curve differential dV / dt of the curve changes with time. It can be seen from the figure that there is an inflection point in the curve under the charging conditions of a charging temperature of 25 °C, a charging rate of 2.5C, and a cut-off voltage of 4V, which proves that when the battery is charged to 4V at a charging rate of 2.5C at 25 °C, sodium deposition occurs in the battery.

[0067] Figure 18 It is a disassembled view of the sodium-ion battery of Comparative Example 4 charged to 4V at a charging rate of 2.5C at 25 °C. It can be seen from the figure that sodium deposition exists on the surface of the carbon negative electrode, and the active substances on the surface of the electrode sheet are severely peeled off.

[0068] The following Table 1 shows the comparison of the long-term cycle stability performance of the sodium-ion batteries in Examples 1 and 2 and Comparative Examples 1-4: Sodium-ion battery Capacity retention rate after 500 cycles 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％

[0069] It can be seen from Table 1 that the long-term cycle stability of the sodium-ion battery with the sodium deposition behavior of the carbon negative electrode regulated has been improved to a certain extent. In addition, the performance of the sodium-ion batteries assembled with the electrolytes in Comparative Examples 1-3 is inferior to that of Example 1, indicating that each component in the synergistic additive plays an irreplaceable role. The synergistic effect among the three makes the sodium storage performance of the carbon negative electrode the most stable, and has good sodium-ion transmission characteristics. It is not easy to have sodium deposition under fast charging conditions and has higher safety. In Comparative Example 4, an excessive amount of aromatic compound additive was used, which blocked the sodium-ion transmission during the charging process of the battery, and the formed SEI film was uneven and dense, resulting in obvious attenuation of the electrochemical performance of the carbon negative electrode. The stress deformation during the reaction caused the active substances on the electrode sheet to peel off. Further explanation, the synergistic effect of the aromatic compound additive, short-chain alcohol ether, and inorganic sodium salt enables the microstructure of the electrolyte to not only ensure the formation of a uniform, dense and elastic SEI film on the surface of the carbon negative electrode, but also play a protective role under overcharging conditions to prevent the precipitation of sodium.

[0070] The method for controlling the sodium deposition behavior of the carbon negative electrode proposed by the present invention has remarkable effects and is easy to operate. It can effectively improve the long-term cycle stability of the sodium-ion battery based on the carbon negative electrode. Moreover, the non-destructive detection method for sodium deposition of the sodium-ion battery proposed by the present invention can judge whether sodium deposition occurs in the sodium-ion battery and the degree of sodium deposition by analyzing the curve of the relaxation voltage of the sodium-ion battery changing with time after charging is completed. Furthermore, it can improve the safety performance of the sodium-ion battery based on the carbon negative electrode, and this non-destructive detection method can improve the detection efficiency. There is no need to disassemble the battery, and it can provide a warning for the safe operation of the sodium-ion battery simply and quickly, and has high research prospects and application value.

[0071] Although the content of the present invention has been described in detail through the above preferred embodiments, it should be recognized that the above description should not be construed as a limitation of the present invention. After those skilled in the art have read the above content, various modifications and alternatives to the present invention will be obvious. Therefore, the protection scope of the present invention shall be defined by the appended claims.

Claims

1. A sodium-ion battery electrolyte, Characterized in that, The sodium-ion battery electrolyte includes: sodium salt, carbonate organic solvent, and a synergistic additive including short-chain alcohol ether, inorganic sodium salt, and aromatic compound.

2. The sodium-ion battery electrolyte according to claim 1, Characterized in that, The sodium salt includes one or more of sodium hexafluorophosphate, sodium perchlorate, sodium tetrafluoroborate, and bis(oxalato)borate sodium.

3. The sodium-ion battery electrolyte according to claim 1 or 2, Characterized in that, The carbonate organic solvent includes one or more of propylene carbonate, ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, and fluoroethylene carbonate.

4. The sodium-ion battery electrolyte according to any one of claims 1-3, Characterized in that, The short-chain alcohol ether includes 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 includes one or more of sodium nitrate, sodium trifluoromethanesulfonate, sodium carbonate, sodium chloride, and sodium acetate; The aromatic compound includes one or more of oxybenzophenone, benzophenone, benzil, anthrone, 4-hydroxyacetophenone, tetrahydronaphthone, biphenyl, furan, and cyclohexylbenzene.

5. The sodium-ion battery electrolyte according to any one of claims 1-4, Characterized in that, In the sodium-ion battery electrolyte, the concentration of the sodium salt is 0.5-3M, preferably 1M; The volume ratio of the synergistic additive to the mixed solution formed by the sodium salt and the carbonate organic solvent is 1:99 to 10:90; Preferably, the molar ratio of the short-chain alcohol ether, inorganic sodium salt, and aromatic compound in the synergistic additive is 0.1-3:0.1-3:0.1-3.

6. A preparation method of the sodium-ion battery electrolyte according to any one of claims 1-5, Characterized in that, The preparation method includes the following steps: adding the sodium salt into the carbonate organic solvent to form a mixed solution; then, adding the synergistic additive to the mixed solution and stirring evenly to obtain the sodium-ion battery electrolyte.

7. A sodium-ion battery, Characterized in that, The sodium-ion battery includes: a carbon negative electrode sheet, a positive material electrode, a separator, and the above sodium-ion battery electrolyte.

8. The sodium-ion battery according to claim 7, Characterized in that, The carbon negative electrode sheet includes the following components by weight: 70-99 parts of carbon negative electrode active material, 0.5-10 parts of conductive agent, and 0.5-10 parts of binder; The carbon negative electrode active material includes 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; the binder is one or more of polyvinylidene fluoride, carboxymethyl cellulose, or polyacrylic acid.

9. The sodium-ion battery according to claim 7 or 8, Characterized in that, The positive electrode material is one or more of sodium-based transition metal oxides, sodium-based transition metal phosphates, or sodium-based transition metal cyanide compounds; preferably, 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 vanadium phosphate, sodium fluorovanadate, or sodium pyrophosphate iron, and the sodium-based transition metal cyanide compound is sodium ferrocyanide or a Prussian blue analogue.

10. A method for determining sodium deposition in a sodium-ion battery according to any one of claims 7-9, characterized in that the determination method comprises the following steps: S1. After charging and discharging the sodium-ion battery once under set conditions, discharge the battery completely; S2. Charge the sodium-ion battery with the completely discharged battery at a constant current to a set cut-off voltage at a set temperature and charging rate, then stop, and then record the relaxation voltage change of the battery after charging within a set time to obtain a voltage-time curve; S3. Differentiate the voltage-time curve to obtain a differential voltage-time curve; determine whether sodium deposition occurs at the negative electrode of the sodium-ion battery when charging to the set cut-off voltage at the set temperature and set charging rate by judging whether there is an inflection point in the differential voltage-time curve; the inflection point is the demarcation point for changing the upward or downward trend direction of the continuous curve or the demarcation point between the concave arc and the convex arc of the continuous curve.

Images