Reference electrode and preparation method and application thereof

Abstract

The invention relates to the technical field of secondary batteries, in particular to a reference electrode and a preparation method and application thereof.The reference electrode comprises a current collector and active layers, the active layers are arranged on the two sides of the current collector, and the active layers are made of a sodium-containing positive active material. The reference electrode provided by the invention has good potential stability, does not influence intrinsic charge-discharge electrochemical reaction of the tested battery, and can realize long-time cycle performance monitoring of the sodium-ion battery.

Description

Technical Field

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

[0002] Due to the shortage of lithium resources required for the manufacture of lithium-ion batteries and the similar physical and chemical properties of sodium to lithium, sodium-ion batteries are gradually developing and beginning to replace the lithium-ion battery market. However, sodium-ion batteries have also exposed problems such as gas production and sodium precipitation during their development, and higher requirements have been placed on the performance of sodium-ion batteries. Therefore, in-depth and thorough research is still needed on their reactions and failure mechanisms. The traditional two-electrode battery structure still requires destructive testing such as disassembly of the battery when analyzing failure problems such as sodium precipitation in sodium-ion batteries. It is impossible to perform in-situ detection of the positive and negative electrode states of the battery alone, which greatly affects the efficiency of failure analysis. By introducing a reference electrode to make a three-electrode battery, not only can non-destructive detection of battery failure problems be achieved, but also the potential, impedance, polarization and other properties of the positive or negative electrode of the battery can be tested separately, and the film formation potential, cycle, rate, high and low temperature charge and discharge performance of the battery can be analyzed in situ, as well as the quantification of the impedance of the positive and negative electrodes of the battery, and the formulation of a battery fast charging strategy, etc. Therefore, it is of great significance to develop a stable reference electrode and use it to assemble a reliable sodium-ion three-electrode battery.

[0003] However, there are few reports on three-electrode systems related to sodium-ion batteries. Currently, there are two main methods for assembling sodium-ion three-electrode batteries:

[0004] Solution (1): Place a metal wire with a smaller diameter inside or outside the battery. Commonly used metals include aluminum, copper or silver. The metal wire is placed between the positive and negative electrodes or outside the battery cell. The positive and negative electrodes are separated from the reference electrode by a diaphragm, and the metal wire (reference electrode) is led out to the outside of the battery. Then, the positive electrode and the metal wire are connected to the battery test system at the same time, and a small current is passed for charging to achieve the purpose of sodium plating the metal wire. This is a method of making three electrodes using electrochemical deposition. This reference electrode has poor stability when it is soaked in electrolyte, and this method will change the content of sodium in the active material on the positive electrode side of the battery being tested, affecting the intrinsic electrochemical reaction state of the battery. If the current density is small or the power-on time is short, there may be a problem that the metal wire is not completely covered by metallic sodium, resulting in too little sodium on the metal wire and rapid consumption. As the metallic sodium is consumed, the accuracy of the three-electrode battery decreases. During long-term testing, this method has low accuracy in monitoring voltage changes during the cycle and poor test stability, which affects the test analysis results. If the sodium plating time is extended, problems such as sodium dendrites will occur.

[0005] Solution (2): Based on solution (1), a layer of sodium ion battery positive electrode material is coated on the surface of the metal wire placed inside or outside the battery. The positive electrode material has a stable voltage platform. This is a method of obtaining a stable potential as a reference electrode after sodiumization of the sodium ion battery. However, the positive electrode material coated on the metal filament is often uneven in thickness and poorly dispersed, resulting in sodium precipitation after a period of circulation, making it impossible to effectively monitor the long-term cycle performance of the battery.

[0006] Therefore, it is of great significance to study a reference electrode with good electrode potential stability, which will not affect the intrinsic charge and discharge electrochemical reactions of the battery under test and can realize long-term cycle performance monitoring of sodium-ion batteries. Summary of the invention

[0007] In view of the above-mentioned deficiencies in the prior art, the object of the present invention is to provide a reference electrode and a preparation method and application thereof. The reference electrode provided by the present invention has good potential stability, will not affect the intrinsic charge and discharge electrochemical reaction of the battery under test, and can realize long-term cycle performance monitoring of sodium ion batteries.

[0008] In order to achieve the above object, the present invention adopts the following technical solutions:

[0009] In a first aspect, the present invention provides a reference electrode, comprising a current collector and an active layer, wherein the active layer is disposed on both sides of the current collector, and the material of the active layer comprises a positive electrode active material containing sodium element.

[0010] The reference electrode provided by the present invention has good potential stability, will not affect the intrinsic charge and discharge electrochemical reaction of the tested battery, and can realize long-term cycle performance monitoring of the sodium ion battery.

[0011] Furthermore, the current collector includes at least one of copper foil, aluminum foil and silver foil;

[0012] And / or, the positive electrode active material containing sodium element includes sodium vanadium phosphate;

[0013] And / or, the material of the active layer further includes a conductive agent and a binder;

[0014] And / or, the mass ratio of the positive electrode active material containing sodium element, the conductive agent and the binder in the active layer is (90-98): (0.5-5): (1-5);

[0015] And / or, the thickness of the reference electrode is 50 μm to 200 μm;

[0016] And / or, the thickness of the active layer on both sides is 40 μm to 190 μm;

[0017] And / or, one end of the reference electrode further includes a reference electrode tab; the reference electrode tab is made of an aluminum tab.

[0018] In a second aspect, the present invention provides a method for preparing a reference electrode as described in the first aspect, the preparation method comprising the following steps:

[0019] S1, mixing the positive electrode active material containing sodium element, the conductive agent, the binder and the solvent to obtain a slurry;

[0020] S2. Apply the slurry on both sides of the current collector and obtain the reference electrode by rolling.

[0021] In a third aspect, the present invention provides a three-electrode battery cell, which includes the reference electrode described in the first aspect or the reference electrode prepared by the preparation method described in the second aspect.

[0022] Further, the three-electrode battery cell includes a laminated battery cell or a wound battery cell, wherein the laminated battery cell includes a negative electrode sheet, a first separator, a reference electrode, a second separator and a positive electrode sheet stacked in sequence; the wound battery cell includes a negative electrode sheet, a first separator, a reference electrode, a second separator and a positive electrode sheet stacked and wound in sequence;

[0023] And / or, the number of the reference electrode 1 is 1;

[0024] And / or, the laminated battery cell further comprises a negative electrode sheet, a first separator and a positive electrode sheet stacked in sequence; the wound battery cell further comprises a negative electrode sheet, a first separator and a positive electrode sheet stacked and wound in sequence.

[0025] Furthermore, one end of the positive electrode plate also includes a positive electrode tab, and the positive electrode tab is an aluminum tab;

[0026] And / or, one end of the negative electrode plate further includes a negative electrode tab, and the negative electrode tab is an aluminum tab;

[0027] And / or, the positive electrode tab and the negative electrode tab are located on the same side of the three-electrode battery cell;

[0028] And / or, the reference electrode tab is located on the same side as the positive electrode tab;

[0029] And / or, the negative electrode sheet completely covers the positive electrode sheet, the area of ​​the negative electrode sheet is greater than the area of ​​the positive electrode sheet, and the size of the reference electrode is the same as that of the positive electrode sheet;

[0030] And / or, the positive electrode plate comprises a positive electrode current collector and a positive electrode active material layer disposed on the surface of the positive electrode current collector, wherein the positive electrode active material of the positive electrode active material layer comprises one or more of layered oxides, polymerized anions, and Prussian blue;

[0031] And / or, the negative electrode plate comprises a negative electrode current collector and a negative electrode active material layer disposed on the surface of the negative electrode current collector, wherein the negative electrode active material of the negative electrode active material layer comprises one or more of hard carbon, soft carbon and graphite;

[0032] And / or, the first diaphragm and the second diaphragm each independently include one or two of a polypropylene diaphragm, a polyethylene diaphragm and a glass fiber diaphragm;

[0033] And / or, the thickness of the positive electrode plate is 80 μm-280 μm, and the thickness of the negative electrode plate is 50 μm-300 μm.

[0034] In a fourth aspect, the present invention provides a sodium ion battery, wherein the sodium ion battery comprises the three-electrode battery cell described in the third aspect.

[0035] Furthermore, the sodium ion battery further comprises a shell, the three-electrode battery cell is encapsulated in the shell; the shell is filled with electrolyte; the shell is an aluminum-plastic film shell;

[0036] And / or, the sodium ion battery further comprises a positive electrode tab, a negative electrode tab and a reference electrode tab;

[0037] And / or, the sodium ion battery includes a soft pack battery or a square shell battery.

[0038] In a fifth aspect, the present invention provides a method for preparing a sodium ion battery as described in the fourth aspect, the preparation method comprising the following steps:

[0039] S1. Stacking the reference electrode described in the first aspect or the reference electrode prepared by the preparation method described in the second aspect into a bare cell, separating the reference electrode from the negative electrode sheet using a first diaphragm, and separating the reference electrode from the positive electrode sheet using a second diaphragm, to obtain the three-electrode cell;

[0040] S2, after fixing the three-electrode battery cell, welding the positive electrode tab, the negative electrode tab and the reference electrode tab, and then packaging the three-electrode battery cell with an aluminum-plastic film and injecting an electrolyte to obtain the sodium ion battery.

[0041] Furthermore, the bare cell is a cell composed of the positive electrode sheet, the negative electrode sheet and the first separator;

[0042] And / or, both step S1 and step S2 are performed in a dry environment, wherein the dry environment refers to an environment with a dew point below -35°C;

[0043] And / or, after step S2, the preparation method further comprises the following steps:

[0044] S3, forming and dividing the sodium ion battery;

[0045] S4. Activate the reference electrode.

[0046] Compared with the prior art, the beneficial effects of the present invention include at least one of the following:

[0047] (1) The reference electrode provided by the present invention has good potential stability, will not affect the intrinsic charge and discharge electrochemical reaction of the battery being tested, and can realize long-term cycle performance monitoring of sodium ion batteries.

[0048] (2) The structure of the reference electrode system provided by the present invention does not affect the flatness of the electrode sheet.

[0049] (3) The present invention provides a soft-pack battery reference electrode system and its preparation method to address a series of problems existing in existing sodium ion battery and other three-electrode systems, such as immature technology, poor stability of metal sodium, barrier interference, internal unevenness, and high difficulty in manufacturing. Based on the three-electrode system, the potential change curves of the positive and negative electrodes of the battery at different cycle times can be non-destructively monitored to analyze the capacity attenuation mechanism of full batteries such as sodium ion batteries, so as to better understand the electrochemical reaction information inside the battery.

[0050] (4) The reference electrode provided by the present invention is easy to manufacture and can be coated on a large scale and cut into a specific shape; the NVP reference electrode sheet is implanted into the assembled sodium ion battery, and then the NVP reference electrode sheet is used as the sodium source. The positive electrode sheet is not used as the sodium source during the preparation process, which will not affect the intrinsic charge and discharge electrochemical reactions and related electrochemical properties of the battery under test; and the electrode potential can be kept stable, and the test accuracy is high; by using sodium vanadium phosphate as the reference electrode of the sodium ion battery, when applied to the monitoring of soft-pack batteries, there is no need to activate the reference electrode in advance, and a three-electrode system can be used to activate the reference electrode in situ, and the voltage platform after activation is extremely stable, which greatly reduces the detection process and complexity, is easy to test and promote, and has a wide range of applications, and can also be used in other secondary battery fields, such as lithium ion batteries, potassium ion batteries, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0052] Figure 1 It is a schematic diagram of the structure of the reference electrode provided by the present invention;

[0053] Figure 2 An exploded view of the three-electrode battery cell structure provided by the present invention;

[0054] Figure 3 A schematic diagram of the structure of a sodium ion battery provided by the present invention;

[0055] Figure 4 A full-cell potential change curve diagram of a three-electrode sodium battery provided in Example 1 of the present invention;

[0056] Figure 5 A full-cell potential change curve of a three-electrode sodium battery provided in Comparative Example 1 of the present invention;

[0057] Figure 6 A comparison diagram of the negative electrode-reference electrode potential change curves of the three-electrode sodium battery provided in Example 1 of the present invention and Comparative Example 1;

[0058] Figure 7 A negative electrode-reference electrode potential change curve diagram of a three-electrode sodium battery provided in Comparative Example 2 of the present invention;

[0059] Figure 8 This is a relaxation time distribution analysis result diagram of the three-electrode sodium battery provided in Example 1, Comparative Example 3 and Comparative Example 4 of the present invention.

[0060] Reference numerals:

[0061] 11-current collector; 12-active layer; 1-reference electrode; 2-negative electrode plate; 3-first diaphragm; 4-second diaphragm; 5-positive electrode plate; 13-reference electrode tab; 21-negative electrode tab; 51-positive electrode tab; 6-housing. DETAILED DESCRIPTION

[0062] In order to make the purpose, technical scheme and advantages of the present invention clearer, the technical scheme in the embodiment of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. It should be understood by those skilled in the art that the embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present invention. The process parameters of the following embodiments that do not specify specific conditions are usually based on conventional conditions.

[0063] The endpoints and any values ​​of the ranges disclosed in the present invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be regarded as specifically disclosed in the present invention.

[0064] In a first aspect, the present invention provides a reference electrode 1, such as Figure 1 As shown, the reference electrode 1 includes a current collector 11 and an active layer 12. The active layer 12 is arranged on both sides of the current collector 11. The material of the active layer 12 includes a positive electrode active material containing sodium element.

[0065] The reference electrode provided by the present invention has good potential stability, will not affect the intrinsic charge and discharge electrochemical reaction of the measured battery, and can realize long-term cycle performance monitoring of the sodium ion battery.

[0066] In the above-mentioned reference electrode, as an optional implementation, the current collector 11 includes at least one of copper foil, aluminum foil, and silver foil; preferably aluminum foil.

[0067] In the above-mentioned reference electrode, as an optional embodiment, the positive electrode active material containing sodium element includes sodium vanadium phosphate (Na 3 V 2 (PO 4 ) 3 ).

[0068] In the above-mentioned reference electrode, as an optional implementation, the material of the active layer 12 also includes a conductive agent and a binder.

[0069] In the above-mentioned reference electrode, as an optional implementation, the conductive agent includes at least one of carbon black, acetylene black, conductive graphite, carbon fiber, carbon nanotubes, and graphene.

[0070] In the above-mentioned reference electrode, as an optional implementation, the binder includes at least one of polyvinylidene fluoride (PVDF), polyvinyl alcohol, polytetrafluoroethylene, sodium carboxymethyl cellulose, and styrene-butadiene rubber.

[0071] In the above-mentioned reference electrode, as an optional embodiment, the mass ratio of the sodium-containing positive electrode active material, the conductive agent and the binder in the active layer 12 is (90-98): (0.5-5): (1-5), for example, it can be 90:5:5, 92:4:4, 96:1:3 or 98:1.5:0.5.

[0072] In the above-mentioned reference electrode, as an optional embodiment, the thickness of the reference electrode 1 is 50μm to 200μm. If the thickness of the reference electrode is too large, the diffusion of sodium ions will be limited, the concentration polarization will be aggravated, resulting in potential drift, and the reference electrode will not be able to react to the real potential of the system in real time; the ohmic polarization will increase, the electron conduction path will be extended, and the interface contact resistance will increase; in thick electrodes, only the surface material participates in the reaction, and a "dead zone" is formed inside, resulting in obstructed ion transmission. The unwetted area may cause sodium precipitation or electrolyte decomposition, causing the reference electrode to polarize and destroy its potential stability. If the thickness of the reference electrode is too small, the active material loading is insufficient, resulting in large signal noise, weak test current or voltage signals; weak charge transfer impedance signals, large fitting errors; voltage platform jitters, and equilibrium state is difficult to determine; process control is difficult, and the electrode is easily damaged.

[0073] In the above-mentioned reference electrode, as an optional implementation manner, the thickness of the double-sided active layer 12 is 40 μm to 190 μm.

[0074] In the above-mentioned reference electrode, as an optional embodiment, Figure 3 One end of the reference electrode 1 further includes a reference electrode tab 13 ; the reference electrode tab 13 is made of an aluminum tab.

[0075] In a second aspect, the present invention provides a method for preparing a reference electrode as described in the first aspect, the preparation method comprising the following steps:

[0076] S1, mixing the positive electrode active material containing sodium element, the conductive agent, the binder and the solvent to obtain a slurry;

[0077] S2, coating the slurry on both sides of the current collector 11, and obtaining the reference electrode 1 by rolling.

[0078] In the above-mentioned method for preparing the reference electrode, as an optional implementation, after step S2, the preparation method further includes:

[0079] S3. Welding a pole ear to one end of the reference electrode by welding.

[0080] In the above method for preparing the reference electrode, as an optional implementation, the solvent includes N-methylpyrrolidone (NMP).

[0081] In the above method for preparing the reference electrode, as an optional implementation, in step S1, the solid content of the slurry is 40% to 60%, for example, 40%, 45%, 50%, 55% or 60%.

[0082] In a third aspect, the present invention provides a three-electrode battery cell, such as Figure 2 As shown, the three-electrode cell includes the reference electrode 1 described in the first aspect or the reference electrode 1 prepared by the preparation method described in the second aspect.

[0083] In the above-mentioned three-electrode battery cell, as an optional embodiment, the three-electrode battery cell includes a stacked battery cell or a wound battery cell, wherein the stacked battery cell includes a negative electrode sheet 2, a first separator 3, a reference electrode 1, a second separator 4 and a positive electrode sheet 5 stacked in sequence; the wound battery cell includes a negative electrode sheet 2, a first separator 3, a reference electrode 1, a second separator 4 and a positive electrode sheet 5 stacked and wound in sequence.

[0084] In the above three-electrode battery cell, as an optional implementation, the number of the reference electrode 1 is 1.

[0085] In the above three-electrode battery cell, as an optional implementation, Figure 3 One end of the positive electrode plate 5 further includes a positive electrode tab 51 , and the positive electrode tab 51 is an aluminum tab.

[0086] In the above three-electrode battery cell, as an optional implementation, one end of the negative electrode plate 2 further includes a negative electrode tab 21, and the negative electrode tab 21 is an aluminum tab.

[0087] In the above three-electrode battery cell, as an optional implementation, the positive electrode tab 51 and the negative electrode tab 21 are located on the same side of the three-electrode battery cell.

[0088] In the above three-electrode battery cell, as an optional implementation, the reference electrode tab 13 is located on the same side as the positive electrode tab 51 .

[0089] In the above three-electrode battery cell, as an optional implementation, the negative electrode plate 2 completely covers the positive electrode plate 5 , the area of ​​the negative electrode plate 2 is greater than the area of ​​the positive electrode plate 5 , and the size of the reference electrode 1 is the same as that of the positive electrode plate 5 .

[0090] In the above three-electrode battery cell, as an optional embodiment, the positive electrode plate 5 includes a positive electrode current collector and a positive electrode active material layer arranged on the surface of the positive electrode current collector, wherein the positive electrode active material of the positive electrode active material layer includes one or more of layered oxides, polymerized anions, and Prussian blue.

[0091] In the above three-electrode battery cell, as an optional embodiment, the negative electrode plate 2 includes a negative electrode current collector and a negative electrode active material layer arranged on the surface of the negative electrode current collector, wherein the negative electrode active material of the negative electrode active material layer includes one or more of hard carbon, soft carbon and graphite.

[0092] In the above three-electrode battery cell, as an optional embodiment, the first separator 3 and the second separator 4 each independently include one or two of a polypropylene separator, a polyethylene separator and a glass fiber separator.

[0093] In the above-mentioned three-electrode battery cell, as an optional embodiment, the stacked battery cell also includes a negative electrode plate 2, a first separator 3 and a positive electrode plate 5 stacked in sequence; the wound battery cell also includes a negative electrode plate 2, a first separator 3 and a positive electrode plate 5 stacked and wound in sequence.

[0094] In the above-mentioned three-electrode battery cell, as an optional embodiment, the thickness of the positive electrode plate 5 is 80μm-280μm (for example, it can be 80μm, 100μm, 120μm, 140μm, 160μm, 180μm, 200μm, 220μm, 240μm, 260μm or 280μm), and the thickness of the negative electrode plate 2 is 50μm-300μm (for example, it can be 50μm, 80μm, 100μm, 120μm, 140μm, 160μm or 200μm).

[0095] In a fourth aspect, the present invention provides a sodium ion battery, wherein the sodium ion battery comprises the three-electrode battery cell described in the third aspect.

[0096] In the above sodium ion battery, as an optional embodiment, Figure 3 The sodium ion battery shown further includes a shell 6, in which the three-electrode battery cell is encapsulated; the shell 6 is filled with electrolyte; and the shell 6 is an aluminum-plastic film shell.

[0097] In the above sodium ion battery, as an optional implementation, the sodium ion battery further includes a positive electrode tab 51 , a negative electrode tab 21 and a reference electrode tab 13 .

[0098] In the above-mentioned sodium ion battery, as an optional implementation, the sodium ion battery includes a soft-pack battery or a square-shell battery.

[0099] In a fifth aspect, the present invention provides a method for preparing a sodium ion battery as described in the fourth aspect, the preparation method comprising the following steps:

[0100] S1. Stacking the reference electrode 1 described in the first aspect or the reference electrode 1 obtained by the preparation method described in the second aspect into a bare cell, separating the reference electrode 1 from the negative electrode sheet 2 using a first diaphragm 3, and separating the reference electrode 1 from the positive electrode sheet 5 using a second diaphragm 4, to obtain the three-electrode cell;

[0101] S2, after fixing the three-electrode battery cell, welding the positive electrode tab 51, the negative electrode tab 21 and the reference electrode tab 13, and then packaging the three-electrode battery cell with an aluminum-plastic film and injecting an electrolyte to obtain the sodium ion battery.

[0102] In the above-mentioned method for preparing the sodium ion battery, as an optional implementation, the bare cell is a cell composed of the positive electrode plate 5 , the negative electrode plate 2 and the first separator 3 .

[0103] In the above-mentioned method for preparing a sodium ion battery, as an optional implementation mode, both step S1 and step S2 are performed in a dry environment, and the dry environment refers to an environment with a dew point below -35°C.

[0104] The method for preparing the sodium ion battery, as an optional implementation, after step S2, the method further comprises the following steps:

[0105] S3, forming and dividing the sodium ion battery;

[0106] S4. Activate the reference electrode 1.

[0107] In the present invention, the formation and separation are carried out according to conventional methods.

[0108] The method for preparing the sodium ion battery, as an optional implementation, the activating the reference electrode 1 comprises:

[0109] The positive electrode and negative electrode of the sodium ion battery are composed of a double-electrode battery and charged to 40% to 60% SOC, and then the reference electrode and the negative electrode are composed of a double-electrode battery and charged to the steady voltage (e.g., 3.4V) of the positive active material containing the sodium element with a current of 0.01mA, 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 activation process of the reference electrode is monitored in real time in situ to ensure that the potential of the reference electrode after activation 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.

[0110] The present invention will be further described in detail below with reference to specific embodiments and comparative examples.

[0111] Example 1

[0112] The present embodiment provides a method for preparing a reference electrode, the preparation method comprising the following steps: the current collector adopts an aluminum foil with a thickness of 13 μm, and a sodium vanadium phosphate (NVP) slurry is coated on both sides of the current collector by a coating machine to form a sodium vanadium phosphate (NVP) active layer on both sides of the aluminum foil, and then a roller press is used to roll and die-cut the NVP reference electrode. The total thickness of the NVP reference electrode sheet is 83 μm. The preparation process of the NVP slurry comprises: first, the required slurry components are mixed according to the mass ratio of NVP: conductive agent (acetylene black): polyvinyl alcohol; Vinylidene fluoride (PVDF) is prepared in a ratio of 96.1:2.1:1.8, and then the pretreated (mainly including dehydration and drying) PVDF and solvent N-methylpyrrolidone (NMP) are fully mixed, stirred and dispersed to obtain a glue solution; the conductive agent and the glue solution are put into a stirring device together for mixing and dispersion, and finally the NVP active material is put into the stirring device for mixing and dispersion, and then the slurry viscosity is adjusted by adding the solvent multiple times, and finally the required NVP slurry is prepared, and the solid content of the slurry is 43%.

[0113] This embodiment also provides a method for preparing a three-electrode sodium battery, the preparation method comprising the following steps:

[0114] (1) Electrode preparation

[0115] Positive electrode sheet: The current collector uses an aluminum foil with a thickness of 13μm, and a coating machine is used to coat the sodium nickel iron manganate slurry on both sides of the current collector, and then a roller press is used to roll and die-cut. The total thickness of the positive electrode sheet is 127μm. Among them, the preparation process of the sodium nickel iron manganate slurry includes: first, the mass ratio of the required slurry components is prepared according to the ratio of sodium nickel iron manganate: conductive agent (acetylene black): polyvinylidene fluoride (PVDF) of 96.1:2.1:1.8, and then the pre-treated (mainly including dehydration, drying, etc.) PVDF and solvent N-methylpyrrolidone (NMP) are fully mixed, stirred and dispersed to obtain a glue; the conductive agent and the glue are put into a stirring device together for mixing and dispersion, and finally the sodium nickel iron manganate active material is put into a stirring device for mixing and dispersion, and then the slurry viscosity is adjusted by adding solvents multiple times, and finally the required sodium nickel iron manganate slurry is prepared.

[0116] Negative electrode sheet: The negative electrode current collector uses aluminum foil with a thickness of 13μm, and hard carbon negative electrode slurry is coated on both sides of the negative electrode current collector, followed by roller die cutting. The total thickness of the negative electrode sheet is 209μm. The preparation process of hard carbon negative electrode slurry includes: dissolving sodium carboxymethyl cellulose (CMC) in solvent deionized water to form a glue solution, adding a certain amount of conductive agent carbon black (SP) to the glue solution to form a conductive glue during high-speed dispersion, and then adding negative electrode active material hard carbon to the conductive glue. After high-speed dispersion, deionized water is added to adjust the viscosity, and finally styrene-butadiene rubber (SBR) is added and dispersed evenly to form a negative electrode slurry. According to the mass ratio, hard carbon: CMC: SBR: SP is 94.7:1.5:3.1:0.7.

[0117] The first diaphragm and the second diaphragm: 9+3μm single-sided ceramic coated diaphragm, of which 9μm is a polyethylene-based film and 3μm is a ceramic coating.

[0118] The dimensions of the positive electrode sheet and the reference electrode sheet are both 9.0*5.9 cm, and the dimension of the negative electrode sheet is 9.3 cm*6.2 cm.

[0119] (2) Assembly

[0120] In an environment with a dew point below -35°C, 12 positive electrode sheets and 13 negative electrode sheets are assembled by alternately stacking the negative electrode sheets, the first diaphragm, and the positive electrode sheets to obtain a bare battery cell. The first diaphragm is placed between the positive electrode sheets and the negative electrode sheets. Then, a reference electrode prepared in this embodiment is implanted into the bare battery cell. The first diaphragm separates the reference electrode from the negative electrode sheet. The second diaphragm is used to separate the reference electrode from the positive electrode sheet to assemble into a three-electrode battery cell. The above-mentioned battery cells are then subjected to an insulation test, and the positive electrode tab, negative electrode tab and reference electrode tab of the battery cells that pass the insulation test are fixed and welded, and an aluminum-plastic film shell is used for packaging and electrolyte is injected to obtain a three-electrode sodium ion soft-pack battery with an NVP reference electrode, with a capacity of 1.5Ah, wherein the positive electrode tab is an aluminum tab, the negative electrode tab is an aluminum tab, and the reference electrode tab is an aluminum tab, and the positive electrode tab, the negative electrode tab and the reference electrode tab are located on the same side of the three-electrode battery cell.

[0121] After the three-electrode sodium battery is assembled, it is transferred to a 45°C constant temperature box and placed for 48 hours. It is charged to 3.2V at a rate of 0.02C and 0.05C for formation. Then it is transferred to a 25°C constant temperature box and placed for 48 hours. Finally, the capacity is divided at a charge and discharge rate of 0.2C and 0.33C.

[0122] (3) Activation of the reference electrode

[0123] The positive electrode and the negative electrode of the three-electrode sodium battery are composed of a two-electrode battery and charged to 50% SOC, and then the reference electrode and the negative electrode are composed of a two-electrode battery and charged to a sodium vanadium phosphate steady voltage of 3.4V with a current of 0.01mA, 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 activation process of the reference electrode is monitored in real time in situ to ensure that the potential of the reference electrode after activation is on a stable voltage platform. The present invention can use an auxiliary channel to measure the real-time potential of the positive electrode to the reference electrode.

[0124] Example 2

[0125] The preparation method of the reference electrode provided in this embodiment is basically the same as that of Embodiment 1, except that the total thickness of the NVP reference electrode pole piece is 120 μm.

[0126] A three-electrode sodium battery was prepared by referring to the preparation method of the three-electrode sodium battery provided in Example 1.

[0127] Example 3

[0128] The preparation method of the reference electrode provided in this embodiment is basically the same as that of Embodiment 1, except that the total thickness of the NVP reference electrode plate is 170 μm.

[0129] A three-electrode sodium battery was prepared by referring to the preparation method of the three-electrode sodium battery provided in Example 1.

[0130] Comparative Example 1

[0131] The preparation method of the three-electrode sodium battery provided in this comparative example is basically the same as that in Example 1, except that the reference electrode is different from that in Example 1, and the preparation method of the reference electrode provided in this comparative example includes: adopting the existing metal wire sodium plating method, that is, using the metal wire as a copper wire, inserting the copper wire into the inside of the battery cell, and electrochemically plating sodium, specifically including: fixing two enameled copper wires with a diameter of 0.1 cm on crocodile clips, the crocodile clips are fixed on the iron frame, and the crocodile clips of the two copper wires are respectively connected to the positive and negative electrodes of the electrochemical workstation. The clamped copper wire is placed in a beaker, and 0.5M dilute sulfuric acid is loaded into the beaker, and the length of the copper wire in sulfuric acid is 5cm. Cyclic voltammetry scanning (CV) is performed on the two copper wires, and the scanning potential is ±2V, scanning from the open circuit voltage to 2V, and then scanning from 2V to -2V, and the scanning rate is: first 10V / s, 20 cycles, then 0.5mV / s, 5 cycles. The effect is judged based on the fact that the copper wire surface is translucent and has no dullness, which indicates that it has been cleaned; insert the cleaned copper wire into the battery cell, and lead out the reference electrode ear. After the battery is filled with liquid and packaged in the normal steps, the positive pole of the power supply is connected to the positive pole of the battery, and the negative pole of the power supply is connected to the metal wire reference electrode. A small current is used to charge the reference electrode from the positive pole of the battery. At this time, sodium metal will be deposited on the surface of the metal wire, thereby obtaining a reference electrode with metallic sodium.

[0132] Comparative Example 2

[0133] The preparation method of the three-electrode sodium battery provided in this comparative example is basically the same as that in Example 1, except that the reference electrode is different from that in Example 1. The preparation method of the reference electrode provided in this comparative example includes: dipping the NVP slurry described in Example 1 with aluminum wire, vacuum drying, repeating the dipping three times to ensure that the aluminum wire is coated with the slurry, and drying under the conditions of vacuum drying at 65° C. and a vacuum degree of 0.1 Pa for 2 hours to obtain the reference electrode.

[0134] Comparative Example 3

[0135] The preparation method of the reference electrode provided in this comparative example is basically the same as that of Example 1, except that the total thickness of the NVP reference electrode pole piece is 30 μm.

[0136] A three-electrode sodium battery was prepared by referring to the preparation method of the three-electrode sodium battery provided in Example 1.

[0137] Comparative Example 4

[0138] The preparation method of the reference electrode provided in this comparative example is basically the same as that of Example 1, except that the total thickness of the NVP reference electrode pole piece is 250 μm.

[0139] A three-electrode sodium battery was prepared by referring to the preparation method of the three-electrode sodium battery provided in Example 1.

[0140] Performance Testing

[0141] (1) The three-electrode sodium battery provided in the embodiment and comparative examples 1-2 is subjected to a constant current charge and discharge test to test the three-electrode potential-time curve of the soft-pack sodium ion battery system, including: placing the manufactured battery (soft-pack sodium battery three-electrode system) in a constant temperature box at 25°C, using a test cabinet to connect the charge and discharge circuit between the positive and negative electrodes, while connecting multi-channel 1 to the positive electrode ear and the reference electrode ear to obtain the potential difference between the positive electrode and the reference electrode; multi-channel 2 connects the negative electrode ear and the reference electrode ear to obtain the potential difference between the negative electrode and the reference electrode. Multi-channel 3 connects the positive electrode ear and the negative electrode ear to obtain the potential difference between the positive electrode and the negative electrode. At a 1C rate (1C = 121mAh / g), the data of Example 1 obtained through channel 1 are as follows: Figure 4 The change curve of the potential difference of "positive electrode-NVP potential" is shown in the figure. The data of Example 1 obtained by channel 2 is presented in Figure 4 In the figure, the curve of the potential difference of the negative electrode-NVP reference electrode is shown. The data of Example 1 obtained by channel 3 is as follows: Figure 4 The potential change curve of the whole battery is shown in FIG. Figure 5The potential-time curve of 1C charge and discharge of the three-electrode system of the soft-pack sodium battery with a copper wire reference structure (Comparative Example 1). Under the same test step, the voltage of the negative electrode-reference electrode of Comparative Example 1 dropped sharply, while the potential of Example 1 remained stable, verifying the stability of the NVP reference electrode sheet. Figure 6 A comparison of the potential-time curves of the negative electrode and the reference electrode of the three-electrode sodium battery provided in Example 1 and Comparative Example 1 under 1C charge and discharge, Figure 7 The potential-time curve of the negative electrode-reference electrode of the three-electrode sodium battery provided in Comparative Example 2 under 1C charge and discharge, from Figure 6-7 The stability of the potential of the reference electrode sheet of Example 1 can be more intuitively reflected in the figure. After 10 cycles, the voltage platform is still stable, indicating that the reference electrode provided by the present invention has good potential stability and can realize long-term cycle performance monitoring of sodium ion batteries.

[0142] The full-cell potential change curves of the soft-pack three-electrode sodium batteries prepared in Examples 2 and 3 are similar to that in Example 1.

[0143] (2) Electrochemical impedance spectroscopy and relaxation time distribution analysis

[0144] The soft-pack batteries made in Example 1 and Comparative Examples 3-4 were subjected to a charge-discharge cycle at 50% SOC, 1C / 10, and then charged to 50% SOC at 1C and then kept at 25°C for 2 hours. The working electrode (WE) of the electrochemical workstation was connected to the reference electrode, and the counter electrode (CE) was connected to the negative electrode of the battery. The test was carried out in a constant temperature box (25±1°C) to prevent temperature fluctuations from affecting the impedance data. The data obtained from the impedance test was then analyzed for relaxation time distribution using DRTtools. Figure 8 As shown, the Rct charge transfer impedance fitting of Comparative Example 3 is inaccurate, which is obviously different from Example 1 and Comparative Example 4, affecting the test results; the Rs ohmic impedance and Rsei SEI film impedance of Comparative Example 4 are significantly greater than those of Example 1, indicating that the increase in polarization of the electrode will destroy the stability of its potential.

[0145] The relaxation time distribution analysis results of the soft-pack three-electrode sodium batteries prepared in Examples 2 and 3 are similar to those in Example 1.

[0146] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A reference electrode, characterized in that The reference electrode (1) comprises a current collector (11) and an active layer (12); the active layer (12) is arranged on both sides of the current collector (11); and the material of the active layer (12) comprises a positive electrode active material containing a sodium element.

2. The reference electrode according to claim 1, characterized in that The current collector (11) comprises at least one of copper foil, aluminum foil and silver foil; And / or, the positive electrode active material containing sodium element includes sodium vanadium phosphate; And / or, the material of the active layer (12) further includes a conductive agent and a binder; And / or, the mass ratio of the positive electrode active material containing sodium element, the conductive agent and the binder in the active layer (12) is (90-98): (0.5-5): (1-5); And / or, the thickness of the reference electrode (1) is 50 μm to 200 μm; And / or, the thickness of the active layer (12) on both sides is 40 μm to 190 μm; And / or, one end of the reference electrode (1) further comprises a reference electrode tab (13); the reference electrode tab (13) is made of an aluminum tab.

3. A method for preparing a reference electrode as claimed in claim 1 or 2, characterized in that: The preparation method comprises the following steps: S1, mixing the positive electrode active material containing sodium element, the conductive agent, the binder and the solvent to obtain a slurry; S2. Coating the slurry on both sides of the current collector (11), and rolling to obtain the reference electrode (1).

4. A three-electrode battery cell, characterized in that: The three-electrode cell comprises the reference electrode (1) according to claim 1 or 2 or the reference electrode (1) prepared by the preparation method according to claim 3.

5. The three-electrode battery cell according to claim 4, characterized in that: The three-electrode battery cell comprises a laminated battery cell or a wound battery cell, wherein the laminated battery cell comprises a negative electrode sheet (2), a first separator (3), a reference electrode (1), a second separator (4) and a positive electrode sheet (5) which are sequentially stacked; the wound battery cell comprises a negative electrode sheet (2), a first separator (3), a reference electrode (1), a second separator (4) and a positive electrode sheet (5) which are sequentially stacked and wound; And / or, the number of the reference electrode 1 is 1; And / or, the laminated battery cell further comprises a negative electrode sheet (2), a first separator (3) and a positive electrode sheet (5) which are stacked in sequence; and the wound battery cell further comprises a negative electrode sheet (2), a first separator (3) and a positive electrode sheet (5) which are stacked and wound in sequence.

6. The three-electrode battery cell according to claim 5, characterized in that: One end of the positive electrode plate (5) further includes a positive electrode tab (51), and the positive electrode tab (51) is an aluminum tab; And / or, one end of the negative electrode plate (2) further includes a negative electrode tab (21), and the negative electrode tab (21) is an aluminum tab; And / or, the positive electrode tab (51) and the negative electrode tab (21) are located on the same side of the three-electrode battery cell; And / or, the reference electrode tab (13) is located on the same side as the positive electrode tab (51); and / or, the negative electrode sheet (2) completely covers the positive electrode sheet (5), the area of ​​the negative electrode sheet (2) is greater than the area of ​​the positive electrode sheet (5), and the size of the reference electrode (1) is the same as that of the positive electrode sheet (5); And / or, the positive electrode plate (5) comprises a positive electrode current collector and a positive electrode active material layer disposed on the surface of the positive electrode current collector, wherein the positive electrode active material of the positive electrode active material layer comprises one or more of layered oxides, polymerized anions, and Prussian blue; And / or, the negative electrode plate (2) comprises a negative electrode current collector and a negative electrode active material layer disposed on the surface of the negative electrode current collector, wherein the negative electrode active material of the negative electrode active material layer comprises one or more of hard carbon, soft carbon and graphite; And / or, the first diaphragm (3) and the second diaphragm (4) each independently include one or two of a polypropylene diaphragm, a polyethylene diaphragm and a glass fiber diaphragm; And / or, the thickness of the positive electrode plate (5) is 80 μm-280 μm, and the thickness of the negative electrode plate (2) is 50 μm-300 μm.

7. A sodium ion battery, characterized in that: The sodium ion battery comprises the three-electrode battery cell according to any one of claims 4 to 6.

8. The sodium ion battery according to claim 7, characterized in that: The sodium ion battery further comprises a shell (6), wherein the three-electrode battery cell is encapsulated in the shell (6); an electrolyte is injected into the shell (6); and the shell (6) is an aluminum-plastic film shell; And / or, the sodium ion battery further comprises a positive electrode tab (51), a negative electrode tab (21) and a reference electrode tab (13); And / or, the sodium ion battery includes a soft pack battery or a square shell battery.

9. A method for preparing a sodium ion battery according to claim 7 or 8, characterized in that: The preparation method comprises the following steps: S1. Stacking the reference electrode (1) according to claim 1 or 2 or the reference electrode (1) prepared by the preparation method according to claim 3 into a bare cell, using a first diaphragm (3) to separate the reference electrode (1) from the negative electrode sheet (2), and using a second diaphragm (4) to separate the reference electrode (1) from the positive electrode sheet (5), to obtain the three-electrode cell; S2, after fixing the three-electrode battery cell, welding the positive electrode tab (51), the negative electrode tab (21) and the reference electrode tab (13), then packaging the three-electrode battery cell with an aluminum-plastic film, injecting an electrolyte, and obtaining the sodium ion battery.

10. The method for preparing a sodium ion battery according to claim 9, characterized in that: The bare battery cell is a battery cell composed of the positive electrode sheet (5), the negative electrode sheet (2) and the first separator (3); And / or, both step S1 and step S2 are performed in a dry environment, wherein the dry environment refers to an environment with a dew point below -35°C; And / or, after step S2, the preparation method further comprises the following steps: S3, forming and dividing the sodium ion battery; S4. Activate the reference electrode (1).

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