Reference electrode and preparation method and application thereof

Abstract

The invention belongs to the technical field of sodium ion batteries, and particularly relates to a reference electrode and a preparation method and application thereof, the reference electrode comprises a metal substrate and a coating layer coating the metal substrate; the coating layer comprises sodium vanadium phosphate, a conductive agent, a binder and a solvent. The reference electrode disclosed by the invention has the beneficial effects that the reference electrode adopts sodium vanadium phosphate with stable potential as a raw material for preparing the reference electrode, so that the problem of poor chemical stability of metal sodium is fundamentally solved, and pre-activation is not needed; the assembled flexibly-packaged sodium ion battery reference electrode system can be directly adopted for in-situ activation, so that the method is easy to test, popularize and apply, and has a wide application field; when the three-electrode battery cell is manufactured, the reference electrode is directly implanted into the battery cell, the manufacturing process is simple and safe, the test stability is improved, and the problems of sodium dendrites, complex sodium modification process and high sodium modification cost are avoided.

Description

Technical Field

[0001] The invention belongs to the technical field of sodium ion batteries, and in particular relates to a reference electrode and a preparation method and application thereof. Background Art

[0002] Sodium-ion batteries have attracted much attention due to their high safety and low price, and can be widely used in energy storage, power, etc. In the research technology of sodium-ion batteries, electrode potential is a very important parameter. By measuring the potential change of a single electrode, detailed information about the internal reaction of the battery can be obtained. In order to obtain detailed information about the electrochemical process of each electrode, one or more reference electrodes can be implanted inside the battery and provide a standard potential, and the potential and electrochemical impedance spectrum of the positive and negative electrodes can be tested in situ.

[0003] The industry generally uses copper wire plated with sodium as a reference electrode for determining sodium analysis in battery cells. The surface of the copper wire is usually protected by a layer of insulating enameled metal wire, and its surface is not conductive, so it is impossible to remove the insulating layer on the surface through an electrochemical reaction for subsequent sodium plating. The prior art generally uses concentrated sulfuric acid to remove the insulating layer on the surface of the copper wire. The specific solution is to soak the copper wire in concentrated sulfuric acid to remove the by-products on the surface of the copper wire, then treat it with clean water, and dry it to obtain the treated copper wire; then put the treated copper wire between the positive and negative electrodes to assemble it into a battery. In the process of sodium analysis, the anode or cathode is used as the positive electrode, and the copper wire electrode is used as the negative electrode. A small current is used for charging and discharging, so that the sodium ions in the electrolyte deposit a layer of sodium on the surface of the copper wire, and then this copper wire sodium-plated electrode is used as a negative reference potential for monitoring.

[0004] However, the existing copper wire sodium electrode plating process has the following disadvantages: the existing constant current sodium plating process causes uneven distribution of sodium on the copper wire surface. The sodium on the copper wire surface is easy to fall off during use, resulting in large fluctuations in the reference electrode voltage and large voltage deviation, which affects the sodium precipitation judgment result; at the same time, the life is short. After several cycles, as the sodium is consumed, inaccurate voltage monitoring will occur, and the practical significance of the reference electrode will be lost. If the sodium plating time is extended, problems such as sodium dendrites will occur; if the sodium metal without surface treatment is directly installed in the battery cell as a reference electrode, the sodium metal surface will react with the electrolyte, resulting in unstable test potential.

[0005] At present, the reference electrode manufacturing technology usually adopts the method of non-in-situ sodiumization, that is, the electrode is placed between two foils to form a battery, and the reference electrode is obtained after two sodiumizations, and then implanted into the battery to be tested. Therefore, the existing reference electrode sodiumization process is complicated and the steps of implanting the battery are cumbersome.

[0006] Therefore, it is of great significance to develop a reference electrode with simple preparation technology, reliable test data, and the ability to timely analyze failed sodium-ion batteries. Summary of the invention

[0007] The present application provides a reference electrode and a preparation method and application thereof, aiming to solve the problems of poor stability, complex sodiumization process and high cost of the sodium reference electrode.

[0008] In a first aspect of the present application, there is provided a reference electrode, comprising a metal substrate and a coating layer coating the metal substrate;

[0009] The coating layer comprises sodium vanadium phosphate, a conductive agent and a binder.

[0010] The advantages and technical effects of the reference electrode of the present application are as follows: the present application uses sodium vanadium phosphate with a stable potential as the raw material for preparing the reference electrode, which overcomes the temperature problems of the metal sodium reference electrode, such as easy drift, short life, and poor stability. The sodium ions of the reference electrode described in the present application can migrate and embed into the positive and negative sides of the NVP reference electrode, thereby achieving one-step in-situ sodiumization, reducing the difficulty of preparing the reference electrode.

[0011] According to some embodiments of the reference electrode described in the present application, the coating layer is arranged along the length direction of the metal substrate, and the length of the coating layer occupies 1 / 3-2 / 3 of the length of the metal substrate.

[0012] According to some embodiments of the reference electrode described in the present application, the metal substrate includes a metal wire and / or an enameled metal wire.

[0013] According to some embodiments of the reference electrode described in the present application, the metal wire includes one or more of a copper wire, an aluminum wire, a platinum wire, a nickel foam and a silver wire.

[0014] According to some embodiments of the reference electrode described in the present application, the enameled metal wire includes one or more of an enameled copper wire, an enameled aluminum wire, an enameled platinum wire and an enameled silver wire.

[0015] According to some embodiments of the reference electrode described in the present application, the mass ratio of the sodium vanadium phosphate, the conductive agent and the binder in the coating layer is (60-95): (2.5-25): (2.5-25).

[0016] According to some embodiments of the reference electrode described in the present application, the conductive agent includes one or more of carbon black, acetylene black, conductive graphite, carbon fiber, carbon nanotube and graphene.

[0017] According to some embodiments of the reference electrode described in the present application, the binder includes one or more of polyvinyl alcohol, polytetrafluoroethylene, sodium carboxymethyl cellulose, polyvinylidene fluoride and styrene-butadiene rubber.

[0018] The second aspect of the present application provides a method for preparing the reference electrode described in the first aspect of the present application, comprising the following steps:

[0019] (1) mixing the sodium vanadium phosphate, the conductive agent, the binder and the solvent to obtain a slurry;

[0020] (2) Using the slurry described in step (1) to coat a metal substrate, followed by drying, to obtain the reference electrode.

[0021] According to some embodiments of the preparation method described in the present application, the preparation method further comprises subjecting the metal substrate to a de-impurity treatment.

[0022] The specific operation of the impurity removal treatment includes: connecting the paired metal substrates to the electrochemical device respectively, and immersing them in a dilute acid electrolyte for electrochemical cleaning to remove impurities on the surface of the metal substrate. The electrochemical cleaning adopted in the present application includes scanning the metal substrate immersed in the dilute acid electrolyte by cyclic voltammetry scanning, removing impurities on the surface of the metal substrate connected to the positive electrode of the electrochemical device through oxygen evolution reaction, and removing impurities on the surface of the metal substrate connected to the negative electrode of the electrochemical device through hydrogen evolution reaction.

[0023] The cyclic voltammetry scan used in the present application includes first using a first scan rate to cycle a first preset number of times, and then using a second scan rate to cycle a second preset number of times, wherein the first scan rate is greater than the second scan rate, and the first preset number of times is greater than the second preset number of times; the connection method of the paired metal substrates and the electrochemical device remains unchanged until the surface of the metal substrate is smooth and has no dullness. The first scan rate used in the present application is 5-10V / s, and the first preset number of times is greater than or equal to 10V / s.

[0024] According to some embodiments of the reference electrode described in the present application, the solvent includes one or more of N-methylpyrrolidone, dimethyl sulfoxide and ethylene glycol dimethyl ether.

[0025] According to some embodiments of the reference electrode described in the present application, the mass ratio of the total mass of the sodium vanadium phosphate, the conductive agent and the binder to the mass of the solvent is (1-1.5):1.

[0026] According to some embodiments of the preparation method described in the present application, the drying is vacuum drying.

[0027] According to some embodiments of the preparation method described in the present application, the drying temperature is 60-70° C., and the drying time is 1-3 hours.

[0028] According to some embodiments of the preparation method described in the present application, the vacuum degree of the vacuum drying is 0.01-10Pa.

[0029] In a third aspect of the present application, a three-electrode battery cell is provided, comprising the reference electrode described in the first aspect of the present application or the reference electrode obtained by the preparation method described in the second aspect of the present application.

[0030] The advantages and technical effects brought by the three-electrode battery cell of the present application are: by using the reference electrode of the present application as the reference electrode, the reference electrode can be directly implanted into the battery cell, the production process is simple and safe, the stability of the test is improved, and the problems of sodium dendrites, complex sodiumization process and high sodiumization cost are avoided.

[0031] According to some embodiments of the three-electrode battery cell described in the present application, the three-electrode battery cell includes a positive electrode sheet, a first separator, a reference electrode, a second separator and a negative electrode sheet stacked in sequence, and the end of the reference electrode provided with a coating layer is arranged between the first separator and the second separator.

[0032] According to some embodiments of the three-electrode battery cell described in the present application, according to the normal operating steps of the sodium ion battery: the positive electrode sheet, the first diaphragm, and the negative electrode sheet are assembled into the battery cell in order, the end of the reference electrode provided with the coating layer is placed in the battery cell body, and the second diaphragm is provided on the outside, and the first diaphragm and the second diaphragm are respectively located on both sides of the reference electrode to keep the reference electrode and the battery cell body in a non-contact state; then the above-mentioned battery cell is subjected to an insulation test, and the battery cell that passes the insulation test is encapsulated in an aluminum shell, injected with liquid, and formed to obtain a three-electrode sodium ion battery with a reference electrode.

[0033] The positive electrode sheet includes one or more of layered oxides, Prussian compounds and polyanionic compounds; the first separator and the second separator each independently include one or both of polypropylene separators and glass fiber separators.

[0034] In a fourth aspect of the present application, a sodium ion battery is provided, comprising the three-electrode battery cell described in the third aspect of the present application.

[0035] The fifth aspect of the present application provides a method for sodiumization of a sodium ion battery as described in the fourth aspect of the present application, comprising the following steps: charging the positive electrode and the negative electrode of the sodium ion battery as a double-electrode battery, charging the reference electrode and the negative electrode as a double-electrode battery to a steady voltage of sodium vanadium phosphate, measuring the real-time potential of the positive electrode to the reference electrode, the real-time potential of the negative electrode to the reference electrode, and monitoring the sodiumization process of the reference electrode in real time in situ to ensure that the potential after sodiumization is on a stable voltage platform. The present application can use an auxiliary channel to measure the real-time potential of the positive electrode to the reference electrode.

[0036] The sodium-ion battery sodiumization method described in the present application benefits from the good ion permeability of the current collector. Sodium ions can migrate and be activated on both sides of the reference electrode, thereby achieving one-step in-situ sodiumization and reducing the difficulty of preparing the reference electrode.

[0037] According to some embodiments of the method for sodiumization of a sodium ion battery described in the present application, a bi-electrode battery composed of a positive electrode and a negative electrode of the sodium ion battery is charged to 50% SOC.

[0038] According to some embodiments of the method for sodiumization of a sodium ion battery described in the present application, the positive electrode and the negative electrode of the sodium ion battery constitute a double-electrode battery and are charged to 3.2V. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is a SEM image of the reference electrode prepared in Example 1 of the present application, wherein Figure 1 a is the SEM image of the reference electrode at 2500 times. Figure 1 b is the SEM image of the reference electrode at 5000 times;

[0040] Figure 2 This is a curve diagram of the charge and discharge of the sodium vanadium phosphate sheet described in Example 1 of the present application;

[0041] Figure 3 This is a schematic diagram of the structure of the battery cell prepared in Example 1 of the present application;

[0042] Figure 4 This is a schematic diagram of the structure of the sodium ion battery prepared in Example 1 of the present application;

[0043] Figure 5 The voltage-time curve of the activation process of the reference electrode prepared in Example 1 of the present application;

[0044] Figure 6 This is the voltage change curve of the sodium ion battery prepared in Example 1 of the present application;

[0045] Figure 7 The potential-time curve of the sodium ion battery 1C charge-discharge standby prepared in Example 1 and Comparative Example 1 of the present application;

[0046] In the figure: 1. aluminum-plastic film shell; 2. positive electrode tab; 3. negative electrode tab; 4. air bag; 5. battery cell; 6. reference electrode tab; 7. reference electrode; 8. positive electrode sheet; 9. negative electrode sheet; 10. first diaphragm; 11. second diaphragm. DETAILED DESCRIPTION

[0047] Embodiments of the present invention are described in detail below, and examples of the embodiments are shown in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and intended to be used to explain the present invention, but should not be understood as limiting the present invention.

[0048] The technical solution of the present application is described in detail below in conjunction with specific embodiments and drawings.

[0049] In the present invention, the terms "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" etc. mean that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the described specific features, structures, materials or characteristics may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

[0050] The embodiment of the present application provides a reference electrode, comprising a metal substrate and a coating layer coating the metal substrate;

[0051] The coating layer comprises sodium vanadium phosphate, a conductive agent and a binder.

[0052] In some embodiments, the coating layer is disposed along the length direction of the metal substrate, and the length of the coating layer occupies 1 / 3-2 / 3 of the length of the metal substrate, such as 1 / 3, 1 / 2, 2 / 3, etc.

[0053] In some embodiments, the metal substrate comprises metal wire and / or enameled metal wire.

[0054] In some embodiments, the metal wire includes one or more of a copper wire, an aluminum wire, a platinum wire, a nickel foam, and a silver wire.

[0055] In some embodiments, the enameled metal wire includes one or more of an enameled copper wire, an enameled aluminum wire, an enameled platinum wire, and an enameled silver wire.

[0056] In some embodiments, the mass ratio of the sodium vanadium phosphate, the conductive agent and the binder in the coating layer is (60-95): (2.5-25): (2.5-25), such as 60:20:20, 60:25:15, 70:15:15, 70:20:10, 80:10:10, 80:12.5:7.5, 90:5:5, 90:6:4, 95:2.5:2.5, 95:3:2.5. The coating layer selects the raw materials with the above mass ratio to ensure the conductive stability of the reference electrode.

[0057] In some embodiments, the conductive agent includes one or more of carbon black, acetylene black, conductive graphite, carbon fiber, carbon nanotube, and graphene.

[0058] In some embodiments, the binder includes one or more of polyvinyl alcohol, polytetrafluoroethylene, sodium carboxymethyl cellulose, polyvinylidene fluoride, and styrene-butadiene rubber.

[0059] The reference electrode of the embodiment of the present application uses sodium vanadium phosphate with a stable potential as the reference electrode, which fundamentally solves the problem of poor chemical stability of metallic sodium, and does not require pre-activation. The assembled soft-pack sodium ion battery reference electrode system can be directly used for in-situ activation, which is easy to test and promote and apply, and has a wide range of applications. It can also be used in the field of other secondary batteries, such as lithium ion batteries, potassium ion batteries, etc.

[0060] The present invention also provides a method for preparing a reference electrode, comprising the following steps:

[0061] (1) mixing the sodium vanadium phosphate, the conductive agent, the binder and the solvent to obtain a slurry;

[0062] (2) Using the slurry described in step (1) to coat a metal substrate, followed by drying, to obtain the reference electrode.

[0063] In some embodiments, the preparation method further comprises subjecting the metal substrate to a de-impurity treatment.

[0064] The specific operation of the impurity removal treatment includes: connecting the paired metal substrates to the electrochemical device respectively, and immersing them in a dilute acid electrolyte for electrochemical cleaning to remove impurities on the surface of the metal substrate. The electrochemical cleaning adopted in the present application includes scanning the metal substrate immersed in the dilute acid electrolyte by cyclic voltammetry scanning, removing impurities on the surface of the metal substrate connected to the positive electrode of the electrochemical device through oxygen evolution reaction, and removing impurities on the surface of the metal substrate connected to the negative electrode of the electrochemical device through hydrogen evolution reaction.

[0065] The cyclic voltammetry scan used in the present application includes first using a first scan rate to cycle a first preset number of times, and then using a second scan rate to cycle a second preset number of times, wherein the first scan rate is greater than the second scan rate, and the first preset number of times is greater than the second preset number of times; the connection method of the paired metal substrates and the electrochemical device remains unchanged until the surface of the metal substrate is smooth and has no dullness. The first scan rate used in the present application is 5-10V / s, and the first preset number of times is greater than or equal to 10V / s.

[0066] In some embodiments, the solvent includes one or more of N-methylpyrrolidone, dimethyl sulfoxide, and ethylene glycol dimethyl ether.

[0067] In some embodiments, the mass ratio of the total mass of the sodium vanadium phosphate, the conductive agent and the binder to the solvent is (1-1.5):1, for example, 1:1, 1.1:1, 1.2:1, 1.3:1, 1.5:1.

[0068] In some embodiments, the drying is vacuum drying.

[0069] In some embodiments, the drying temperature is 60-70° C., such as 60° C., 63° C., 65° C., 68° C., 70° C., and the drying time is 1-3 hours, such as 1 hour, 2 hours, 3 hours. The drying temperature can be selected to ensure sufficient drying.

[0070] In some embodiments, the vacuum degree of the vacuum drying is 0.01-10Pa, for example, 0.01Pa, 0.05Pa, 1.0Pa, 2.5Pa, 3Pa, 5Pa, 6Pa, 8Pa, 9Pa, 10Pa.

[0071] The present application also provides a three-electrode battery cell, including the reference electrode of the present application or the reference electrode obtained by the preparation method of the present application.

[0072] In some embodiments, the three-electrode battery cell includes a positive electrode sheet, a first separator, a reference electrode, a second separator and a negative electrode sheet stacked in sequence, and the end of the reference electrode provided with a coating layer is arranged between the first separator and the second separator.

[0073] In some embodiments, according to the normal operating steps of the sodium ion battery: the positive electrode sheet, the first diaphragm, and the negative electrode sheet are assembled into a battery cell in sequence, the end of the reference electrode provided with a coating layer is placed in the battery cell body, and a second diaphragm is provided on the outside, and the first diaphragm and the second diaphragm are respectively located on both sides of the reference electrode to keep the reference electrode and the battery cell body in a non-contact state; then the above-mentioned battery cell is subjected to an insulation test, and the battery cell that passes the insulation test is encapsulated in an aluminum shell, injected with liquid, and formed to obtain a three-electrode sodium ion battery with a reference electrode.

[0074] The positive electrode sheet includes one or more of layered oxides, Prussian compounds and polyanionic compounds; the first separator and the second separator each independently include one or both of polypropylene separators and glass fiber separators.

[0075] The embodiment of the present application also provides a sodium ion battery, including the three-electrode battery cell of the embodiment of the present application.

[0076] The embodiment of the present application also provides a method for sodiumization of a sodium ion battery, comprising the following steps: the positive electrode and the negative electrode of the sodium ion battery are composed of a two-electrode battery for charging, and then the reference electrode and the negative electrode are composed of a two-electrode battery and charged to the steady voltage of sodium vanadium phosphate, and the real-time potential of the positive electrode to the reference electrode and the real-time potential of the negative electrode to the reference electrode are measured, and the sodiumization process of the reference electrode is monitored in real time in situ to ensure that the potential after sodiumization is on a stable voltage platform. The embodiment of the present application can use an auxiliary channel to measure the real-time potential of the positive electrode to the reference electrode.

[0077] In some embodiments, the positive electrode and the negative electrode of the sodium ion battery forming a bi-electrode battery are charged to 50% SOC.

[0078] In some embodiments, the positive electrode and the negative electrode of the sodium ion battery forming a double-electrode battery are charged to 3.2V.

[0079] The sodium-ion battery sodiumization method of the embodiment of the present application benefits from the good ion permeability of the current collector. Sodium ions can migrate and be activated on both sides of the reference electrode, thereby achieving one-step in-situ sodiumization and reducing the difficulty of preparing the reference electrode.

[0080] The present invention is described in detail below with reference to the embodiments and the accompanying drawings.

[0081] The polyvinyl alcohol and polytetrafluoroethylene used in the examples of this application were purchased from Shanghai Haoyuan Biotechnology Co., Ltd.; the polyvinylidene fluoride used in the examples of this application was purchased from Shanghai Ruichu Biotechnology Co., Ltd.

[0082] Example 1

[0083] A method for preparing a reference electrode:

[0084] (1) Fix two copper wires with a diameter of 20 μm on crocodile clips, which are fixed on an iron frame. The crocodile clips of the two copper wires are connected to the positive and negative electrodes of the electrochemical workstation respectively. Put the clamped copper wires into a beaker filled with 0.5M dilute sulfuric acid. The length of the copper wires in the sulfuric acid is 5 cm. Perform cyclic voltammetry (CV) on the two copper wires. The scanning potential is ±2V, scanning from the open circuit voltage to 2V, and then from 2V to -2V. The scanning speed is: first 10V / s, cycle 20 times, then 0.5mV / s, cycle 5 times. The effect is judged based on the fact that the surface of the copper wire is bright and there is no dullness, which means it has been cleaned.

[0085] Sodium vanadium phosphate (NVP) active material, carbon black (SP), and polyvinylidene fluoride (PVDF) are ground in a mortar at a mass ratio of 90:5:5, and then put into a centrifuge to mix the materials evenly, and then add an appropriate amount of N-methylpyrrolidone (NMP) to mix and stir to obtain a slurry with a solid content of 60%. The pretreated copper wire is dipped in the slurry, vacuum dried, and the dipping is repeated three times to ensure that the copper wire is coated with the slurry. The drying conditions are vacuum dried at 65°C and a vacuum degree of 0.1Pa for 2 hours to obtain the reference electrode. The length of the coating formed by the slurry coating the copper wire accounts for 1 / 3 of the length of the metal substrate.

[0086] The SEM image of the reference electrode is shown in Figure 1 shown. Figure 1 (a) is a 2500-fold SEM image of the reference electrode. Figure 1 (b) is a 5000-fold SEM image of the reference electrode.

[0087] from Figure 1 It can be seen that the copper wire surface has a uniform coating.

[0088] Sodium vanadium phosphate sheets were used to assemble button cells. The assembled sodium vanadium phosphate-sodium button cells were connected to a charge and discharge tester. The charge and discharge current (0.1C) and test voltage (1.5-3.95V) were set to conduct the first week charge and discharge test. After 3 weeks of cycling at 0.1C, the charge and discharge current was set to 0.33C for a cycle performance test. The battery test temperature was 25°C. The NVP-sodium charge and discharge curve was obtained as shown in the figure below: Figure 2 shown.

[0089] from Figure 2 It can be seen that the voltage platform of sodium vanadium phosphate (NVP) is stable at 3.4V.

[0090] The above-mentioned reference electrode is used to prepare a sodium nickel iron manganese oxide-hard carbon three-electrode sodium ion battery:

[0091] According to the normal operation steps of the sodium ion battery: the sodium nickel iron manganese oxide positive electrode sheet and the hard carbon negative electrode sheet are dried in a vacuum drying oven, and the battery cell is assembled in the order of the positive electrode sheet, the polyethylene diaphragm, the reference electrode prepared in Example 1, the polyethylene diaphragm, and the hard carbon negative electrode sheet. The structure is as follows Figure 3 Then the above-mentioned battery cell is subjected to an insulation test, and the battery cell that passes the insulation test is packaged in an aluminum-plastic film shell to form a three-electrode sodium ion battery with a positive electrode tab, a negative electrode tab, and a reference electrode tab, and an air bag inside the battery. The soft-pack three-electrode system is as follows Figure 4 shown.

[0092] After the three-electrode battery was completed, it was transferred to a 45℃ constant temperature box and placed for 48 hours. It was charged to 3.2V at a rate of 0.02C and 0.05C for formation. Then it was transferred to a 25℃ constant temperature box and placed for 48 hours. Finally, the capacity was divided at a charge and discharge rate of 0.2C and 0.33C.

[0093] The method for sodiumization of the above sodium ion battery:

[0094] The positive electrode and negative electrode of the sodium ion battery are combined into a double-electrode battery and charged to a preset voltage of 3.2V. Then, the reference electrode and the negative electrode are combined into a double-electrode battery and charged to the steady voltage of sodium vanadium phosphate (NVP) and then the charging is terminated. The voltage between the two changes with the test time as shown in Figure 5 As shown, after a period of time, the voltage between the two stabilizes at 0.075V, indicating that the reference electrode has entered the platform voltage area. At this time, it reaches 50% SOC, and charging is stopped. The voltage stability value is 3.2V after standing for 10 minutes.

[0095] Example 2

[0096] The difference between Example 2 and Example 1 is that the mass ratio of sodium vanadium phosphate, carbon black and polyvinylidene fluoride is adjusted to 80:10:10, and the other operations remain unchanged. The specific operations are:

[0097] Sodium vanadium phosphate (NVP) active material, carbon black (SP), and polyvinylidene fluoride (PVDF) are ground in a mortar at a mass ratio of 80:10:10, then put into a centrifuge to mix the materials evenly, and then add an appropriate amount of N-methylpyrrolidone (NMP) to mix and stir to obtain a slurry with a solid content of 60%. The pretreated copper wire is dipped in the slurry, put in an oven to dry, and the dipping is repeated three times to ensure that the copper wire is coated with the slurry. The reference electrode is obtained by vacuum drying at 65°C and a vacuum degree of 0.1Pa for 2 hours. The length of the coating formed by the slurry coating the copper wire accounts for 1 / 3 of the length of the metal substrate.

[0098] Example 3

[0099] The only difference between Example 3 and Example 1 is that the length of the coating layer formed by the slurry coating the copper wire accounts for 1 / 2 of the length of the metal substrate.

[0100] The specific operation is as follows: the sodium vanadium phosphate (NVP) active material, carbon black (SP), and polyvinylidene fluoride (PVDF) are ground in a mortar at a mass ratio of 90:5:5, and then put into a centrifuge to mix the materials evenly, and then add an appropriate amount of N-methylpyrrolidone (NMP) to mix and stir to obtain a slurry with a solid content of 60%, and the pretreated copper wire is dipped in the slurry, put in an oven to dry, and the dipping is repeated three times to ensure that the copper wire is coated with the slurry, and vacuum dried at 65°C and a vacuum degree of 0.1Pa for 2h to obtain the reference electrode. The length of the coating formed by the slurry coating the copper wire accounts for 1 / 2 of the length of the metal substrate.

[0101] Comparative Example 1

[0102] The difference between Comparative Example 1 and Example 1 is that the reference electrode is prepared by using the existing metal wire sodium plating method.

[0103] The specific operation is: use copper wire to insert into the battery cell for electrochemical sodium plating, specifically including: use enameled copper wire to insert into the battery cell (the copper wire must also be treated by electrochemical method), and lead out the reference electrode ear. After the battery is filled and packaged in the normal steps, the negative electrode and the metal wire reference electrode are connected, and the negative electrode is charged from the reference electrode with a small current. At this time, sodium metal will be deposited on the surface of the copper wire, thereby obtaining a reference electrode with metallic sodium.

[0104] The reference electrode prepared in Comparative Example 1 was used to prepare a sodium nickel iron manganese oxide-hard carbon three-electrode sodium ion battery, and its preparation method was the same as that in Example 1.

[0105] The sodium ion batteries prepared in Example 1 and Comparative Example 1 were subjected to constant current charge and discharge tests to test the three-electrode potential-time curve of the sodium ion battery system.

[0106] Specific testing methods include:

[0107] The sodium ion battery prepared in Example 1 and Comparative Example 1 was placed in a constant temperature box at 25°C. A test cabinet was used to connect the charge and discharge circuits between the positive and negative electrodes. At the same time, the multi-channel channel 1 of the multi-channel tester was connected to the positive electrode tab and the reference electrode to obtain the potential difference between the positive electrode and the reference electrode. The multi-channel channel 2 of the multi-channel tester was connected to the negative electrode tab and the reference electrode to obtain the potential difference between the negative electrode and the reference electrode. The multi-channel channel 3 of the multi-channel tester was connected to the positive electrode tab and the negative electrode tab to obtain the potential difference between the positive electrode and the negative electrode. At a rate of 1C (1C = 121mAh / g), the data obtained through channel 1 is as follows: Figure 6 The data obtained through channel 2 are shown in Figure 6 The data obtained by channel 3 are as follows: Figure 6 Shown is the voltage curve of the full battery. Figure 6 The data curve further verifies the feasibility of the reference electrode.

[0108] Figure 7 Potential-time curve of 1C charge-discharge standby for sodium battery three-electrode system: Figure 7 It can be seen that under the same test step, after 25000S, the voltage of the negative electrode-sodium prepared in comparative example 1 drops sharply, while the potential of the sodium ion battery prepared in example 1 remains stable, verifying that the reference electrode of the present application has better stability.

[0109] Although the above embodiments have been shown and described, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations on the present invention. Changes, modifications, substitutions and variations of the above embodiments by those of ordinary skill in the art are all within the scope of protection of the present invention.

Claims

1. A reference electrode, characterized in that It comprises a metal substrate and a coating layer coating the metal substrate; The coating layer comprises sodium vanadium phosphate, a conductive agent and a binder.

2. The reference electrode according to claim 1, characterized in that The coating layer is arranged along the length direction of the metal substrate, and the length of the coating layer accounts for 1 / 3-2 / 3 of the length of the metal substrate; And / or, the metal substrate comprises metal wire and / or enameled metal wire; preferably, the metal wire comprises one or more of copper wire, aluminum wire, platinum wire, foamed nickel and silver wire, preferably, the enameled metal wire comprises one or more of enameled copper wire, enameled aluminum wire, enameled platinum wire and enameled silver wire; And / or, the mass ratio of the sodium vanadium phosphate, the conductive agent and the binder in the coating layer is (60-95): (2.5-25): (2.5-25).

3. The reference electrode according to any one of claims 1 to 2, characterized in that The conductive agent includes one or more of carbon black, acetylene black, conductive graphite, carbon fiber, carbon nanotube and graphene; And / or, the binder includes one or more of polyvinyl alcohol, polytetrafluoroethylene, sodium carboxymethyl cellulose, polyvinylidene fluoride and styrene-butadiene rubber.

4. The method for preparing the reference electrode according to any one of claims 1 to 3, characterized in that: The following steps are involved: (1) mixing the sodium vanadium phosphate, the conductive agent, the binder and a solvent to obtain a slurry; (2) Using the slurry described in step (1) to coat the metal substrate, and drying the substrate to obtain the reference electrode.

5. The preparation method according to claim 4, characterized in that: The solvent includes one or more of N-methylpyrrolidone, dimethyl sulfoxide and ethylene glycol dimethyl ether; preferably, the mass ratio of the total mass of the sodium vanadium phosphate, the conductive agent and the binder to the mass ratio of the solvent is (1-1.5):1; And / or, the drying is vacuum drying; preferably, the drying temperature is 60-70° C., and the drying time is 1-3 h; preferably, the vacuum degree of the vacuum drying is 0.01-10 Pa.

6. A three-electrode battery cell, characterized in that: The invention comprises the reference electrode according to any one of claims 1 to 3 or the reference electrode obtained by the preparation method according to any one of claims 4 to 5.

7. The three-electrode battery cell according to claim 6, characterized in that: The three-electrode battery cell comprises a positive electrode sheet, a first separator, a reference electrode, a second separator and a negative electrode sheet which are stacked in sequence, and an end of the reference electrode provided with a coating layer is arranged between the first separator and the second separator.

8. A sodium ion battery, characterized in that: A three-electrode battery cell comprising any one of claims 6 to 7.

9. The method for sodiumization of a sodium ion battery according to claim 8, characterized in that: The method comprises the following steps: the positive electrode and the negative electrode of the sodium ion battery are combined into a double-electrode battery for charging, the reference electrode and the negative electrode are combined into a double-electrode battery and charged to a steady voltage of sodium vanadium phosphate, the real-time potential of the positive electrode to the reference electrode and the real-time potential of the negative electrode to the reference electrode are measured, and the sodiumization process of the reference electrode is monitored in real time in situ to ensure that the potential after sodiumization is on a stable voltage platform.

10. The method for sodiumization of a sodium ion battery according to claim 9, characterized in that: The positive electrode and the negative electrode of the sodium ion battery constitute a double-electrode battery and are charged to 50% SOC.

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