Reference electrode, method for preparing the same, and use thereof

Abstract

The application 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 an active layer, the active layer is arranged on both sides of the current collector, and the material of the active layer comprises a positive electrode active material containing a sodium element. The reference electrode provided by the application has good potential stability, does not affect the intrinsic charge-discharge electrochemical reaction of a measured battery, and can realize long-time cycle performance monitoring of a sodium ion battery.

Description

Technical Field

[0001] This invention relates to the field of secondary battery technology, and in particular to a reference electrode, its preparation method, and its application. Background Technology

[0002] Due to the scarcity of lithium resources needed to manufacture lithium-ion batteries and the similarity of sodium's physicochemical properties to lithium, sodium-ion batteries are gradually developing and beginning to replace lithium-ion batteries in the market. However, while developing, sodium-ion batteries have also exposed problems such as gas production and sodium deposition, placing higher demands on their performance. Therefore, in-depth and thorough research on their reactions and failure mechanisms is still needed. Traditional two-electrode battery structures still require destructive testing such as disassembly when analyzing sodium deposition and other failure problems in sodium-ion batteries, making it impossible to perform in-situ testing of the positive and negative electrode states separately, which greatly affects the efficiency of failure analysis. However, three-electrode batteries, made by introducing a reference electrode, can not only achieve non-destructive testing of battery failure problems, but also test the potential, impedance, polarization, and other properties of the positive or negative electrode separately. This allows for in-situ analysis of the battery's film formation potential, cycle life, rate capability, and high and low temperature charge and discharge performance, as well as quantification of the positive and negative electrode impedance and formulation of fast-charging strategies. Therefore, developing a stable reference electrode and using it to assemble reliable sodium-ion three-electrode batteries is of great significance.

[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] Option (1): A small-diameter metal wire is placed 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, and a separator is used to separate the positive and negative electrodes from the reference electrode. The metal wire (reference electrode) is then led out to the outside of the battery. Then, the positive electrode and the metal wire are connected to the battery testing system simultaneously, and a small current is applied for charging to achieve the purpose of sodium plating on the metal wire. This is a method for fabricating three electrodes using electrochemical deposition. When this reference electrode is wetted by the electrolyte, its stability is poor. Moreover, this method will change the sodium content of the active material on the positive electrode side of the tested battery, affecting the intrinsic electrochemical reaction state of the battery. If the current density or the current-carrying time is small, there may be a problem that the metal wire is not completely covered by the metal sodium, resulting in too little sodium on the metal wire and rapid consumption. As the metal sodium is consumed, the accuracy of the three-electrode battery decreases. During long-term testing, the accuracy of monitoring voltage changes during cycling is low and the test stability is very poor, thus affecting the test analysis results. If the sodium plating time is extended, problems such as sodium dendrites will appear.

[0005] Scheme (2): Based on Scheme (1), a layer of sodium-ion battery positive electrode material is coated onto the surface of a metal wire placed inside or outside the battery. This positive electrode material has a stable voltage platform. This is a method to obtain a stable potential as a reference electrode after sodiumification of the sodium-ion battery. However, the positive electrode material coated on the metal wire is often uneven in thickness and poorly dispersed, causing sodium to precipitate after a period of cycling, 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 that has good electrode potential stability, does not affect the intrinsic charge-discharge electrochemical reaction 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 shortcomings in the prior art, the purpose of this invention is to provide a reference electrode, its preparation method and application. The reference electrode provided by this invention has good potential stability, does 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.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

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

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

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

[0012] And / or, the sodium-containing positive electrode active material 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 sodium-containing positive electrode active material, 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 aluminum.

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

[0019] S1. The sodium-containing positive electrode active material, the conductive agent, the binder, and the solvent are mixed to obtain a slurry;

[0020] S2. The slurry is coated on both sides of the current collector and then rolled to obtain the reference electrode.

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

[0022] Furthermore, the three-electrode cell includes a stacked cell or a wound cell, wherein the stacked cell includes a negative electrode, a first separator, a reference electrode, a second separator, and a positive electrode arranged in sequence; the wound cell includes a negative electrode, a first separator, a reference electrode, a second separator, and a positive electrode arranged in sequence and wound.

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

[0024] And / or, the stacked cell further includes a negative electrode, a first separator, and a positive electrode arranged in sequence; the wound cell further includes a negative electrode, a first separator, and a positive electrode arranged in sequence and wound.

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

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

[0027] And / or, the positive electrode tab and the negative electrode tab are located on the same side of the three-electrode 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 completely covers the positive electrode, the area of ​​the negative electrode is greater than the area of ​​the positive electrode, and the size of the reference electrode is the same as that of the positive electrode;

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

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

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

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

[0034] Fourthly, the present invention provides a sodium-ion battery, the sodium-ion battery comprising the three-electrode cell described in the third aspect.

[0035] Furthermore, the sodium-ion battery also includes a casing, in which the three-electrode cell is encapsulated; the casing is filled with electrolyte; the casing is an aluminum-plastic film casing.

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

[0037] And / or, the sodium-ion battery includes a pouch cell or a prismatic cell.

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

[0039] S1. The reference electrode described in the first aspect or the reference electrode prepared by the preparation method described in the second aspect is stacked into the bare cell, the reference electrode is separated from the negative electrode sheet by the first separator, and the reference electrode is separated from the positive electrode sheet by the second separator to obtain the three-electrode cell.

[0040] S2. After fixing the three-electrode cell, weld the positive electrode tab, negative electrode tab, and reference electrode tab. Then, encapsulate the three-electrode cell with an aluminum-plastic film and inject electrolyte to obtain the sodium-ion battery.

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

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

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

[0044] S3. The sodium-ion battery is formed and tested for capacity.

[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, does 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.

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

[0049] (3) This invention addresses a series of problems existing in current three-electrode systems such as sodium-ion batteries, including immature technology, poor stability of metallic sodium, interference, internal unevenness, and high manufacturing difficulty. It provides a reference electrode system for a pouch cell and its preparation method. Based on this three-electrode system, the potential change curves of the positive and negative electrodes of the battery can be monitored non-destructively at different cycle numbers to analyze the capacity decay mechanism of full cells such as sodium-ion batteries, facilitating a better understanding of 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-discharge electrochemical reaction and related electrochemical characteristics of the battery under test. The electrode potential can be kept stable and the test accuracy is high. By using sodium vanadium phosphate as the reference electrode of sodium-ion battery, it is not necessary to perform separate activation in advance when applied to the monitoring of soft pack battery. The reference electrode can be activated in situ using a three-electrode system. The voltage platform after activation is extremely stable, which greatly reduces the detection process and complexity. It is easy to test and promote application, and has a wide range of applications. It can also be used in other secondary battery fields, such as lithium-ion batteries, potassium-ion batteries, etc. Attached Figure Description

[0051] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

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

[0053] Figure 2 This is an exploded view of the three-electrode cell structure provided by the present invention;

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

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

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

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

[0058] Figure 7 This is a graph showing the change in negative electrode-reference electrode potential of the three-electrode sodium battery provided in Comparative Example 2 of the present invention.

[0059] Figure 8 The diagram shows the relaxation time distribution analysis results of the three-electrode sodium batteries provided in Embodiment 1, Comparative Example 3, and Comparative Example 4 of the present invention.

[0060] Figure label:

[0061] 11-Current collector; 12-Active layer; 1-Reference electrode; 2-Negative electrode; 3-First diaphragm; 4-Second diaphragm; 5-Positive electrode; 13-Reference electrode tab; 21-Negative electrode tab; 51-Positive electrode tab; 6-Outer shell. Detailed Implementation

[0062] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Those skilled in the art should understand that the embodiments described are merely illustrative of the invention and should not be considered as specific limitations thereof. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention. Process parameters not specifically specified in the following embodiments are generally performed under conventional conditions.

[0063] The endpoints and any values ​​of the ranges disclosed in this 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 the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this 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 disposed on both sides of the current collector 11, and the material of the active layer 12 includes a positive electrode active material containing sodium.

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

[0066] In the above-mentioned reference electrode, as an optional embodiment, 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 (Na3V2(PO4)3).

[0068] In the above-mentioned reference electrode, as an optional embodiment, the active layer 12 is further made of a conductive agent and a binder.

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

[0070] In one optional embodiment of the above reference electrode, 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 aforementioned reference electrode, as an optional embodiment, the thickness of the reference electrode 1 is 50 μm to 200 μm. If the thickness of the reference electrode is too large, sodium ion diffusion is restricted, concentration polarization will intensify, leading to potential drift, and the reference electrode cannot reflect the true potential of the system in real time; ohmic polarization increases, the electron conduction path is lengthened, and the interfacial contact resistance increases; only the surface material of the thick electrode participates in the reaction, forming a "dead zone" inside, which hinders ion transport, and the unwetted area may cause sodium precipitation or electrolyte decomposition, causing reference electrode polarization and destroying its potential stability. If the thickness of the reference electrode is too small, the active material loading is insufficient, resulting in high signal noise, weak test current or voltage signals; weak charge transfer impedance signals, large fitting errors; voltage plateau fluctuations, making it difficult to determine the equilibrium state; high process control difficulty, and the electrode is easily damaged.

[0073] In the above-mentioned reference electrode, as an optional embodiment, 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 implementation, such as... Figure 3 The reference electrode 1 further includes a reference electrode tab 13 at one end; the reference electrode tab 13 is made of aluminum.

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

[0076] S1. The sodium-containing positive electrode active material, the conductive agent, the binder, and the solvent are mixed to obtain a slurry;

[0077] S2. The slurry is coated on both sides of the current collector 11 and then rolled to obtain the reference electrode 1.

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

[0079] S3. A tab is welded to one end of the reference electrode by welding.

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

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

[0082] Thirdly, 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. The stacked battery cell includes a negative electrode 2, a first separator 3, a reference electrode 1, a second separator 4, and a positive electrode 5 stacked in sequence. The wound battery cell includes a negative electrode 2, a first separator 3, a reference electrode 1, a second separator 4, and a positive electrode 5 stacked and wound in sequence.

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

[0085] In the above-mentioned three-electrode battery cell, as an optional implementation method, such as... Figure 3 The positive electrode plate 5 shown also includes a positive electrode tab 51 at one end, which 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, which is an aluminum electrode tab.

[0087] In the above-mentioned three-electrode battery cell, as an optional embodiment, 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 cell, as an optional implementation, the negative electrode 2 completely covers the positive electrode 5, the area of ​​the negative electrode 2 is greater than the area of ​​the positive electrode 5, and the size of the reference electrode 1 is the same as that of the positive electrode 5.

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

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

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

[0093] In the above-mentioned three-electrode battery cell, as an optional embodiment, the stacked battery cell further includes a negative electrode 2, a first separator 3, and a positive electrode 5 stacked in sequence; the wound battery cell further includes a negative electrode 2, a first separator 3, and a positive electrode 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 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 2 is 50μm-300μm (for example, it can be 50μm, 80μm, 100μm, 120μm, 140μm, 160μm or 200μm).

[0095] Fourthly, the present invention provides a sodium-ion battery, the sodium-ion battery comprising the three-electrode cell described in the third aspect.

[0096] In the aforementioned sodium-ion battery, as an optional implementation method, such as... Figure 3 The sodium-ion battery shown also includes a casing 6, in which the three-electrode cell is encapsulated; the casing 6 is filled with electrolyte; the casing 6 is an aluminum-plastic film casing.

[0097] In the above-mentioned sodium-ion battery, as an optional embodiment, 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 embodiment, the sodium-ion battery includes a pouch battery or a prismatic battery.

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

[0100] S1. The reference electrode 1 described in the first aspect or the reference electrode 1 obtained by the preparation method described in the second aspect is stacked into the bare cell. The reference electrode 1 is separated from the negative electrode plate 2 by the first separator 3, and the reference electrode 1 is separated from the positive electrode plate 5 by the second separator 4, to obtain the three-electrode cell.

[0101] S2. After fixing the three-electrode cell, weld the positive electrode tab 51, the negative electrode tab 21, and the reference electrode tab 13. Then, encapsulate the three-electrode cell with an aluminum-plastic film and inject electrolyte to obtain the sodium-ion battery.

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

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

[0104] In an optional embodiment of the above-described method for preparing a sodium-ion battery, after step S2, the method further includes the following steps:

[0105] S3. The sodium-ion battery is formed and tested for capacity.

[0106] S4. Activate the reference electrode 1.

[0107] In this invention, the formation and capacity separation are performed according to conventional methods.

[0108] In one optional embodiment of the above-described method for preparing a sodium-ion battery, the activation of the reference electrode 1 includes:

[0109] The sodium-ion battery is configured into a dual-electrode battery by combining the positive and negative electrodes and charged to 40%–60% SOC. Then, a reference electrode and a negative electrode are combined to form another dual-electrode battery and charged with a 0.01 mA current to the stable voltage (e.g., 3.4 V) of the sodium-containing positive electrode active material. The real-time potentials of the positive and negative electrodes relative to the reference electrode are measured. The activation process of the reference electrode is monitored in real-time to ensure that the potential of the activated reference electrode is at a stable voltage platform. In this application, the real-time potential of the positive electrode relative to the reference electrode can be measured using an auxiliary channel.

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

[0111] Example 1

[0112] This embodiment provides a method for preparing a reference electrode, which includes the following steps: A 13μm thick aluminum foil is used as the current collector. A sodium vanadium phosphate (NVP) slurry is coated on both sides of the current collector using a coating machine to form an NVP active layer on both sides of the aluminum foil. Subsequently, the foil is rolled and die-cut to obtain the NVP reference electrode sheet, which has a total thickness of 83μm. The preparation process of the NVP slurry includes: First, the required slurry components are mixed according to the mass ratio of NVP: conductive agent (acetylene black): polyurethane... Vinylidene fluoride (PVDF) was prepared in a ratio of 96.1:2.1:1.8. The pretreated PVDF (mainly including dehydration and drying) and the solvent N-methylpyrrolidone (NMP) were then thoroughly mixed and dispersed to obtain a slurry. The conductive agent and the slurry were then placed in a mixing device for further mixing and dispersion. Finally, the NVP active material was added to the mixing device for further mixing and dispersion. The viscosity of the slurry was adjusted by adding solvent multiple times to finally prepare the desired NVP slurry, which had a solid content of 43%.

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

[0114] (1) Electrode preparation

[0115] Positive electrode sheet: The current collector uses a 13μm thick aluminum foil. A sodium nickel iron manganese slurry is coated on both sides of the current collector using a coating machine, followed by rolling and die-cutting to obtain the positive electrode sheet with a total thickness of 127μm. The preparation process of the sodium nickel iron manganese slurry includes: First, preparing the required slurry components in a mass ratio of sodium nickel iron manganese slurry: conductive agent (acetylene black): polyvinylidene fluoride (PVDF) of 96.1:2.1:1.8. Then, pretreated PVDF (mainly including dehydration and drying) and solvent N-methylpyrrolidone (NMP) are thoroughly mixed and dispersed to obtain a slurry. The conductive agent and slurry are then placed in a mixing device for further mixing and dispersion. Finally, the sodium nickel iron manganese slurry active material is added to the mixing device for further mixing and dispersion. The viscosity of the slurry is adjusted by adding solvent multiple times to finally prepare the required sodium nickel iron manganese slurry.

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

[0117] First diaphragm and second diaphragm: 9+3μm single-sided ceramic-coated diaphragm, wherein 9μm is a polyethylene-based membrane and 3μm is a ceramic coating.

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

[0119] (2) Assembly

[0120] In an environment with a dew point below -35°C, 12 positive electrode plates and 13 negative electrode plates are assembled by alternately stacking negative electrode plates, a first separator, and positive electrode plates to obtain a bare cell. A first separator is placed between the positive and negative electrode plates. Then, a reference electrode prepared in this embodiment is implanted into the bare cell. The first separator separates the reference electrode from the negative electrode plate, and a second separator separates the reference electrode from the positive electrode plate to assemble a three-electrode cell. The aforementioned battery cells are then subjected to insulation testing. After the cells that pass the insulation test are fixed, positive electrode tabs, negative electrode tabs, and reference electrode tabs are welded on. The cells are then encapsulated in an aluminum-plastic film shell and electrolyte is injected to obtain a three-electrode sodium-ion soft-pack battery with an NVP reference electrode and a capacity of 1.5Ah. The positive electrode tab, negative electrode tab, and reference electrode tab are all made of aluminum. The positive electrode tab, negative electrode tab, and reference electrode tab are located on the same side of the three-electrode battery cell.

[0121] After the three-electrode sodium battery was assembled, it was transferred to a 45°C constant temperature chamber and left to stand for 48 hours. It was then charged to 3.2V at 0.02C and 0.05C rates for formation, and then transferred to a 25°C constant temperature chamber and left to stand for 48 hours. Finally, it was charged and discharged at 0.2C and 0.33C rates to complete the capacity testing.

[0122] (3) Activation of the reference electrode

[0123] The positive and negative electrodes of the three-electrode sodium battery are combined to form a two-electrode battery and charged to 50% SOC. Then, a reference electrode and the negative electrode are combined to form another two-electrode battery and charged with a 0.01mA current until the stable sodium vanadium phosphate voltage of 3.4V is reached. The real-time potentials of the positive and negative electrodes relative to the reference electrode are measured. The activation process of the reference electrode is monitored in real-time to ensure that the potential of the activated reference electrode is at a stable voltage platform. In this invention, the real-time potential of the positive electrode relative to the reference electrode can be measured using an auxiliary channel.

[0124] Example 2

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

[0126] A three-electrode sodium battery was prepared according 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 in Example 1, except that the total thickness of the NVP reference electrode sheet is 170 μm.

[0129] A three-electrode sodium battery was prepared according 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. The preparation method of the reference electrode provided in this comparative example includes: using the existing metal wire sodium plating method, that is, using copper wire, inserting the copper wire into the cell for electrochemical sodium plating. Specifically, it includes: fixing two enameled copper wires with a diameter of 0.1 cm onto alligator clips, which are fixed to an iron stand. The alligator clips of the two copper wires are connected to the positive and negative electrodes of the electrochemical workstation, respectively. The clamped copper wires are placed in a beaker containing 0.5 M dilute sulfuric acid, and the length of the copper wire immersed in the sulfuric acid is 5 cm. Cyclic voltammetry (CV) is performed on the two copper wires, with a scanning potential of ±2 V, scanning from the open circuit voltage to 2 V, and then scanning from 2 V to -2 V. The scan rate is: first 10 V / s, 20 cycles, then 0.5 mV / s, 5 cycles. The cleaning effect is judged by the fact that the copper wire surface is clear and free of dark spots, indicating that it has been cleaned. The cleaned copper wire is inserted into the battery cell and the reference electrode tab is led out. After the battery is filled and sealed according to the normal procedure, the positive terminal of the power supply is connected to the positive terminal of the battery, and the negative terminal of the power supply is connected to the reference electrode of the metal wire. A small current is used to charge the reference electrode from the positive terminal of the battery. At this time, sodium metal will be deposited on the surface of the metal wire, thus 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. The difference is 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 an aluminum wire into the NVP slurry described in Example 1, vacuum drying, repeating the dipping three times to ensure that the aluminum wire is coated with the slurry, and drying under vacuum conditions of 65°C and 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 in Example 1, except that the total thickness of the NVP reference electrode sheet is 30 μm.

[0136] A three-electrode sodium battery was prepared according 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 in Example 1, except that the total thickness of the NVP reference electrode sheet is 250 μm.

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

[0140] Performance testing

[0141] (1) Constant current charge-discharge tests were performed on the three-electrode sodium batteries provided in Examples 1 and 2 to test the three-electrode potential-time curves of the pouch sodium battery system. This included: placing the fabricated battery (pouch sodium battery three-electrode system) in a constant temperature chamber at 25°C; connecting the positive and negative electrodes to the charge-discharge circuits using a test cabinet; connecting multiplexer channel 1 to the positive electrode tab and the reference electrode tab to obtain the potential difference between the positive and reference electrodes; connecting multiplexer channel 2 to the negative electrode tab and the reference electrode tab to obtain the potential difference between the negative and reference electrodes; and connecting multiplexer channel 3 to the positive and negative electrode tabs to obtain the potential difference between the positive and negative electrodes. At a 1C rate (1C = 121 mAh / g), the data obtained through channel 1 for Example 1 are as follows: Figure 4 The curve showing the change in the potential difference between the positive electrode and the NVP is illustrated. Data from Example 1, obtained through channel 2, is presented below. Figure 4 The graph shows the change in potential difference between the negative electrode and the NVP reference electrode. Data from Example 1 obtained via channel 3 is shown below. Figure 4 The figure shows the potential change curve of the full cell. Figure 5The potential-time curves of the 1C charge-discharge of the three-electrode system of the soft-pack sodium battery with copper wire reference structure (Comparative Example 1) are shown. Under the same test step, the voltage of the negative electrode-reference electrode of Comparative Example 1 drops sharply, while the potential of Example 1 remains stable, verifying the stability of the NVP reference electrode sheet. Figure 6 This is a comparison of the potential-time curves of the negative electrode and the reference electrode for the three-electrode sodium batteries provided in Example 1 and Comparative Example 1 under 1C charge-discharge. Figure 7 The potential-time curves of the negative electrode-reference electrode of the three-electrode sodium battery provided for Comparative Example 2 under 1C charge-discharge are shown below. Figures 6-7 The voltage plate is still stable after 10 cycles, which more intuitively reflects the stability of the reference electrode potential in Example 1. This indicates 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 those in Example 1.

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

[0144] The pouch cells prepared in Examples 3 and 4 were subjected to one charge-discharge cycle at 50% SOC and 1C / 10, followed by charging at 1C to 50% SOC and then being held 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 tests were conducted in a constant temperature chamber (25±1°C) to avoid the influence of temperature fluctuations on the impedance data. The impedance test data were then analyzed for relaxation time distribution using DRTtools. Figure 8 As shown, the Rct charge transfer impedance of Comparative Example 3 is not accurately fitted and is significantly different from that of 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 increased electrode polarization will disrupt 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, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions 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) includes a current collector (11) and an active layer (12). The active layer (12) is disposed on both sides of the current collector (11). The material of the active layer (12) includes a positive electrode active material containing sodium. The thickness of the reference electrode (1) is 50 μm to 200 μm. The thickness of the active layer (12) on both sides is 40μm~190μm; The sodium-containing positive electrode active material is sodium vanadium phosphate; The active layer (12) also includes a conductive agent and a binder. 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).

2. The reference electrode according to claim 1, characterized in that, The current collector (11) includes at least one of copper foil, aluminum foil, and silver foil; And / or, one end of the reference electrode (1) further includes a reference electrode tab (13); the reference electrode tab (13) is made of aluminum tab.

3. A method for preparing a reference electrode as described in claim 1 or 2, characterized in that, The preparation method includes the following steps: S1. The sodium-containing positive electrode active material, the conductive agent, the binder, and the solvent are mixed to obtain a slurry; S2. The slurry is coated on both sides of the current collector (11) and then rolled to obtain the reference electrode (1).

4. A three-electrode battery cell, characterized in that, The three-electrode cell includes the reference electrode (1) as described in claim 1 or 2 or the reference electrode (1) prepared by the preparation method described in claim 3; the three-electrode cell includes a stacked cell or a wound cell, wherein the stacked cell includes a negative electrode (2), a first separator (3), a reference electrode (1), a second separator (4) and a positive electrode (5) stacked in sequence; the wound cell includes a negative electrode (2), a first separator (3), a reference electrode (1), a second separator (4) and a positive electrode (5) stacked and wound in sequence.

5. The three-electrode cell according to claim 4, characterized in that, The number of reference electrodes (1) is 1; And / or, the stacked cell further includes a negative electrode (2), a first separator (3) and a positive electrode (5) stacked in sequence; the wound cell further includes a negative electrode (2), a first separator (3) and a positive electrode (5) stacked and wound in sequence.

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

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

8. The sodium-ion battery according to claim 7, characterized in that, The sodium-ion battery also includes a casing (6), and the three-electrode cell is encapsulated in the casing (6); the casing (6) is filled with electrolyte; the casing (6) is an aluminum-plastic film casing; And / or, the sodium-ion battery further includes a positive electrode tab (51), a negative electrode tab (21), and a reference electrode tab (13). And / or, the sodium-ion battery includes a pouch cell or a prismatic cell.

9. A method for preparing a sodium-ion battery as described in claim 7 or 8, characterized in that, The preparation method includes the following steps: S1. Stack the reference electrode (1) as described in claim 1 or 2 or the reference electrode (1) prepared by the preparation method described in claim 3 into the bare cell, use the first separator (3) to separate the reference electrode (1) from the negative electrode plate (2), and use the second separator (4) to separate the reference electrode (1) from the positive electrode plate (5) to obtain the three-electrode cell; S2. After fixing the three-electrode cell, weld the positive electrode tab (51), negative electrode tab (21) and reference electrode tab (13). Then, encapsulate the three-electrode cell with an aluminum-plastic film and inject electrolyte to obtain 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 plate (5), the negative electrode plate (2) and the first separator (3); And / or, both steps S1 and S2 are performed in a dry environment, which refers to an environment with a dew point below -35°C; And / or, after step S2, the preparation method further includes the following steps: S3. The sodium-ion battery is formed and tested for capacity. S4. Activate the reference electrode (1).

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