Sodium ion battery composite reference electrode and preparation method and application thereof

Abstract

The invention belongs to the technical field of battery materials, and particularly relates to a sodium ion battery composite reference electrode and a preparation method and application thereof. The sodium ion battery composite reference electrode provided by the invention comprises a conductive substrate, a first active coating and a second non-conductive coating, the first active coating coats the surface of the conductive base material; the second non-conductive coating coats the surface of the first active coating; the second non-conductive coating comprises a polybenzimidazole polymer; the polybenzimidazole polymer comprises at least one of poly [2, 2 '-(m-phenylene)-5, 5'-bis-benzimidazole], poly [2, 2 '-(p-phenylene)-5, 5'-bis-benzimidazole] and poly (2, 5-benzimidazole). The sodium-ion battery composite reference electrode provided by the invention is applied to a sodium-ion battery without adding an additional diaphragm, and meanwhile, the tested cathode potential is smaller than the real potential deviation, so that the stability of the reference electrode can be improved, and the stable service life can be prolonged.

Description

Technical Field

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

[0002] Lithium-ion battery three-electrode technology has become a mature method for cell system development and failure analysis, such as lithium plating window detection, full electrical impedance decomposition, and positive and negative electrode charge and discharge behavior. In recent years, the rapid industrialization of sodium-ion batteries has replaced lithium-ion batteries, and the demand for comprehensive electrochemical performance testing of sodium-ion batteries has become increasingly prominent.

[0003] However, the existing three-electrode technology requires the introduction of a multi-layer diaphragm, which greatly increases the concentration polarization between the reference electrode and the counter electrode, causing distortion between the test potential results and the actual value; and the existing technical solution has the disadvantage of poor sodium plating stability, making it difficult to monitor long-term cyclic potential changes.

[0004] In the prior art, the preparation methods of sodium-ion battery reference electrodes mainly include the metal wire electrochemical sodium plating method, the built-in metal sodium sheet method, and the copper foam-molten sodium method. Among them, the metal wire electrochemical sodium plating method is the simplest to operate and is currently the most widely used method for preparing three-electrode reference electrodes for sodium-ion batteries. This method directly applies the sodium-ion battery metal wire sodium plating reference electrode technology to sodium-ion batteries. The specific technical details include: the metal wire is embedded in the battery cell after being etched with concentrated acid, and a dense layer of sodium metal is deposited on the surface of the metal wire by electrochemical deposition, thereby obtaining a sodium-ion battery reference electrode with a constant potential. However, unlike long lithium crystals that can form a dense lithium metal layer on the surface of the metal wire, the petal-shaped sodium metal is accumulated on the surface of the metal wire in the form of fluffy powder, with a large contact area with the electrolyte, which can easily lead to the detachment or inactivation of the sodium metal. The sodium-ion battery reference electrode has poor stability and a short stable service life. Summary of the Invention

[0005] Therefore, the technical problem to be solved by the present invention is to overcome the defects of poor stability and short stable service life of the existing sodium ion battery reference electrode, thereby providing a sodium ion battery composite reference electrode and its preparation method and application.

[0006] The present invention provides a sodium ion battery composite reference electrode, which comprises a conductive substrate, a first active coating and a second non-conductive coating;

[0007] The first active coating is coated on the surface of the conductive substrate;

[0008] The second non-conductive coating is coated on the surface of the first active coating;

[0009] The second non-conductive coating comprises a polybenzimidazole polymer;

[0010] The polybenzimidazole polymer includes at least one of poly[2,2'-(m-phenylene)-5,5'-bisbenzimidazole] (mPBI), poly[2,2'-(p-phenylene)-5,5'-bisbenzimidazole] (pPBI), and poly(2,5-benzimidazole) (ABPBI).

[0011] Preferably, the second non-conductive coating further comprises a porous silicate mineral that has been acid activated;

[0012] The porous silicate mineral is selected from at least one of sepiolite and halloysite;

[0013] and / or, the porosity of the acid-activated porous silicate mineral is 30%-80%;

[0014] And / or, the specific surface area of ​​the porous silicate mineral treated by acid activation is 300-800m 2 / g.

[0015] Preferably, the method for preparing the acid-activated porous silicate mineral comprises: placing the porous silicate mineral in a 20%-60% dilute hydrochloric acid solution, heating and stirring at 70-100° C. for 5-12 hours, washing, and drying to obtain the acid-activated porous silicate mineral;

[0016] Preferably, the second non-conductive coating comprises, by mass percentage, 20%-80% of a polybenzimidazole polymer and 20%-80% of an acid-activated porous silicate mineral;

[0017] And / or, the thickness of the second non-conductive coating layer is 5-20 μm.

[0018] Preferably, the first active coating layer comprises a positive electrode active material containing sodium, a conductive agent and a binder;

[0019] Preferably, the first active coating layer comprises, by mass percentage, 70%-98% of a positive electrode active material containing sodium, 1%-10% of a conductive agent, and 1%-20% of a binder;

[0020] The positive electrode active material containing sodium element includes at least one of sodium vanadium phosphate, sodium manganese vanadium phosphate, sodium ferrous vanadium phosphate, sodium iron phosphate, and sodium fluorophosphate;

[0021] Preferably, the positive electrode active material containing sodium element is sodium sulfate;

[0022] And / or, the binder includes at least one of polyvinylidene fluoride (PVDF), styrene-butadiene rubber, carboxymethyl cellulose, sodium carboxymethyl cellulose, polyacrylonitrile, and polyacrylic acid;

[0023] Preferably, the binder is polyvinylidene fluoride;

[0024] and / or, the conductive agent comprises at least one of conductive carbon black (SP), conductive graphite, carbon nanotubes, acetylene black or graphene;

[0025] And / or, the first active coating layer has a thickness of 3-20 μm.

[0026] Preferably, the conductive substrate comprises at least one of metal wire, enameled metal wire, and carbon fiber;

[0027] The metal wire includes at least one of copper wire, aluminum wire, platinum wire and silver wire;

[0028] The enameled metal wire includes at least one of enameled copper wire, enameled aluminum wire, enameled platinum wire, and enameled silver wire;

[0029] Preferably, the conductive substrate has a diameter of 10-200 μm.

[0030] The present invention provides a method for preparing the above-mentioned sodium ion battery composite reference electrode, comprising the following steps:

[0031] 1) immersing the exposed end of the conductive substrate into the first active slurry and drying after the immersion is completed to obtain a reference electrode having one end coated with the first active coating;

[0032] 2) immersing the end of the reference electrode coated with the first active coating obtained in step 1) into a second non-conductive coating slurry comprising a polybenzimidazole polymer, and drying after the immersion to obtain a sodium ion battery composite reference electrode.

[0033] Preferably, the solvent in the second non-conductive coating slurry comprising the polybenzimidazole polymer in step 2) is at least one of methanesulfonic acid, N,N-dimethylformamide, N,N-dimethylacetamide, and N-methyl-2-pyrrolidone (NMP);

[0034] The solid content of the second non-conductive coating slurry comprising the polybenzimidazole polymer is 5%-30%.

[0035] The solid content of the second non-conductive coating slurry of the present invention is the total mass content of the other components in the second non-conductive coating slurry except the solvent. For example, when the second non-conductive coating includes only a polybenzimidazole polymer, the solid content of the second non-conductive coating slurry is the total mass content of the polybenzimidazole polymer in the second non-conductive coating slurry; when the second non-conductive coating includes only a polybenzimidazole polymer and an acid-activated porous silicate mineral, the solid content of the second non-conductive coating slurry is the total mass content of the polybenzimidazole polymer and the acid-activated porous silicate mineral in the second non-conductive coating slurry;

[0036] Preferably, the solvent in the first active slurry in step 1) is at least one of methanesulfonic acid, N,N-dimethylformamide, N,N-dimethylacetamide, and N-methyl-2-pyrrolidone;

[0037] The solid content of the first active slurry is 5%-30%.

[0038] The solid content of the first active slurry of the present invention is the total mass content of the other components in the first active slurry except the solvent. For example, when the first active slurry only includes the positive electrode active material containing the sodium element, the conductive agent and the binder, the solid content of the first active slurry is the total mass content of the positive electrode active material containing the sodium element, the conductive agent and the binder in the first active slurry.

[0039] Preferably, the exposed end of the conductive substrate in step 1) is an end where the conductive material is exposed;

[0040] Optionally, both ends of the carbon fiber are exposed, and both ends of the metal wire are exposed;

[0041] Optionally, the enameled metal wire is pre-treated to form a bare end;

[0042] Optionally, the pretreatment includes placing both ends of the enameled metal wire in a sulfuric acid solution for 10-120 minutes to remove the surface enameling layer, 0.5-3 cm at one end and 3-7 cm at the other end, to obtain the exposed ends.

[0043] Optionally, the mass concentration of sulfuric acid in the sulfuric acid solution is 50%-98%;

[0044] Optionally, the exposed end of the conductive substrate is further cleaned with deionized water or ethanol before use;

[0045] Optionally, before immersing the exposed end of the conductive substrate into the first active slurry in step 1), the step further includes wetting the exposed end of the conductive substrate with a solvent of the first active slurry.

[0046] Wetting can reduce metal surface tension during the coating process and prevent leakage (i.e., preventing partial areas from not being coated with the first active slurry). For example, when N-methyl-2-pyrrolidone is used as the solvent for the first active slurry, the exposed end of the conductive substrate can be wetted with N-methyl-2-pyrrolidone before being immersed in the first active slurry.

[0047] The present invention provides a sodium ion battery, which includes the sodium ion battery composite reference electrode described above or the sodium ion battery composite reference electrode prepared by the above preparation method.

[0048] One end of the sodium ion battery composite reference electrode coated with the first active coating and the second non-conductive coating is placed in the middle of the soft-pack battery core without adding an additional diaphragm. After preparing a three-electrode soft-pack battery with the positive electrode and the negative electrode, the reference electrode is activated. The specific steps of reference electrode activation are: the external positive power line is connected to the reference electrode ear, the external negative power line is connected to the negative electrode ear, and charging is performed at 1-10μA for 1-5h until the potential remains constant.

[0049] Optionally, the middle portion of the pole core includes the middle portion in the thickness direction, the middle portion in the length direction, and the middle portion in the width direction of the pole core, and the exposed end (3 to 7 cm) of the metal wire / enameled metal wire is led out of the shell and connected to the external pole lug;

[0050] Optionally, after the three-electrode soft pack containing the reference electrode undergoes liquid injection, impregnation, chemical formation, and volume separation similar to a normal two-electrode soft pack, a reference electrode activation step can be performed. The specific steps for reference electrode activation are: connecting an external positive power cable to the reference electrode tab and an external negative power cable to the negative electrode tab, and charging at 1-10 μA for 1-5 hours until the potential remains constant, thereby obtaining a partially desodiumized composite reference electrode.

[0051] Optionally, the positive electrode of the soft pack battery is a sodium layered oxide Na(Ni 1 / 3 Fe 1 / 3 Mn 1 / 3 )O2, the negative electrode is hard carbon. The exposed end of the reference electrode is led out from the shell, and the external tab forms the reference electrode tab, thereby preparing a three-electrode soft-pack battery;

[0052] The technical solution of the present invention has the following advantages:

[0053] 1. A sodium ion battery composite reference electrode provided by the present invention, a sodium ion battery composite reference electrode, comprising a conductive substrate, a first active coating and a second non-conductive coating; the first active coating is coated on the surface of the conductive substrate; the second non-conductive coating is coated on the surface of the first active coating; the second non-conductive coating comprises a polybenzimidazole polymer; the polybenzimidazole polymer comprises at least one of poly[2,2'-(m-phenylene)-5,5'-bisbenzimidazole] (mPBI), poly[2,2'-(p-phenylene)-5,5'-bisbenzimidazole] (pPBI), and poly(2,5-benzimidazole) (ABPBI). Polybenzimidazole polymers are used as components of the second non-conductive coating. Polybenzimidazole polymers are excellent engineering plastics with outstanding insulation properties. Their volume resistivity is more than 10 times that of ceramics, and they can block the passage of electrons. The prepared composite reference electrode can be placed directly in the middle of the battery cell without the need for an additional separator. Polybenzimidazole polymers also have high mechanical strength, heat resistance, and acid and alkali corrosion resistance. On the one hand, they can tightly bind the inner first active coating layer and prevent the active material of the first active coating from detaching from the reference and causing an internal short circuit. On the other hand, they are resistant to electrolyte corrosion (for example, HF and other corrosive components generated by side reactions in the electrolyte), preventing the second non-conductive coating from being corroded by HF in the electrolyte and detaching from the reference (if the second non-conductive coating only uses silicon dioxide, it will be corroded by HF in the electrolyte and detach from the reference), causing the first active coating to directly contact the negative electrode and short-circuit and fail. At the same time, polybenzimidazole polymers have large free volume and nanochannels, which can absorb up to 2 times their own weight of liquid, which can increase the liquid retention and prevent the electrolyte around the reference electrode from drying up and causing distortion of the detection potential. The weakly alkaline imidazole ring in the polybenzimidazole polymer can absorb the Na + The formation of anion-cation pairs enhances ion conduction, thereby achieving the insulation performance of the battery composite reference electrode while also enabling stable ion passage. The sodium-ion battery composite reference electrode provided by the present invention can be used in sodium-ion batteries without the need for an additional diaphragm. Furthermore, the measured negative electrode potential deviates less from the actual potential, improving the stability of the reference electrode and extending its stable service life.

[0054] 2. In the sodium ion battery composite reference electrode provided by the present invention, the second non-conductive coating further comprises a porous silicate mineral treated with acid activation; the porous silicate mineral is selected from at least one of sepiolite and halloysite; and / or the porosity of the porous silicate mineral treated with acid activation is 30%-80%; and / or the specific surface area of ​​the porous silicate mineral treated with acid activation is 300-800 m 2 / g. Unlike the tortuous pore structures of alumina and silica, sepiolite and halloysite have a chain-layered structure with a one-dimensional nanopore structure running through the entire structure. After acid activation, Mg impurities in the skeleton can be removed, the channel pore size and porosity can be expanded, and the specific surface area can be increased from <200m 2 / g increased to >300m 2 / g; Its large specific surface area and high porosity can not only increase the liquid retention capacity, but also construct Na + Fast transport channel, shortening Na + At the same time, polybenzimidazole polymers and acid-activated porous silicate minerals are mixed and wrapped with each other to form the optimal Na + transmission pathway, enhancing the Na + The rapid transmission of the composite reference electrode also makes it have a strong liquid retention ability. On the one hand, it can prevent the electrolyte around the reference electrode from drying up and causing the detection potential distortion. At the same time, the composite reference electrode can improve the Na + The potential shift caused by transmission obstruction has the disadvantage of the real electrode potential. The sodium ion battery composite reference electrode of the present application has good isolation from the positive and negative electrodes. The porous silicate mineral with high porosity and acid activation treatment and the polybenzimidazole organic polymer synergistically form a second non-conductive coating, which can further increase the electrolyte absorption and synergistically construct the Na + Fast diffusion channel, synergistically for Na + Diffusion boosting reduces the concentration polarization of the reference electrode, resulting in smaller potential offsets and further improving the accuracy and service life of the test potential.

[0055] 3. The composite reference electrode used in the present invention has a non-conductive surface and can replace the diaphragm. When preparing sodium-ion batteries, no additional diaphragm is required between the reference electrode and the positive or negative electrode, which greatly reduces the difficulty of three-electrode packaging, reduces concentration polarization, and increases test accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0057] Figure 1 Schematic diagram of the cross-section structure of the composite reference electrode for sodium ion batteries prepared in Example 1 of the present invention at the coating layer;

[0058] Reference numerals: 1-substrate, 2-first active coating, 3-second non-conductive coating;

[0059] Figure 2 Cycling potential diagram of a soft-pack battery prepared with the sodium-ion battery composite reference electrode of Example 1 at 25°C and 1C / 2C;

[0060] Figure 3 Comparison of the cycling potentials of the soft-pack batteries prepared with the sodium-ion battery composite reference electrodes of Example 1 and Comparative Example 1 at 25°C and 1C / 2C. DETAILED DESCRIPTION

[0061] The following examples are provided for a better understanding of the present invention and are not intended to limit the best mode of implementation. They do not limit the content and scope of protection of the present invention. Any product identical or similar to the present invention obtained by anyone under the guidance of the present invention or by combining the features of the present invention with other prior arts shall fall within the scope of protection of the present invention.

[0062] If no specific experimental steps or conditions are specified in the examples, the conventional experimental steps or conditions described in the literature in this field can be used. If the manufacturer of the reagents or instruments is not specified, they are all commercially available conventional reagents.

[0063] The poly[2,2'-(m-phenylene)-5,5'-bisbenzimidazole] (mPBI), poly[2,2'-(p-phenylene)-5,5'-bisbenzimidazole] (pPBI), and poly(2,5-benzimidazole) (ABPBI) used in the examples and comparisons of the present invention can be prepared by existing technologies in the art or can be purchased. Among them, poly[2,2'-(m-phenylene)-5,5'-bisbenzimidazole] (mPBI) can be prepared according to the preparation method described in Ayakody, JRP; Chung, SH; Durantino, L.; Zhang, H.; Xiao, L.; Benicewicz, BC; Greenbaum, SGNMR Studies of Mass Transport in High-Acid-Content Fuel Cell Membranes Based on Phosphoric Acid and Polybenzimidazole. J. Electrochem. Soc. 2007, 154(2), B242. Poly[2,2'-(p-phenylene)-5,5'-bisbenzimidazole] (pPBI) can be prepared according to the preparation method described in Kim, TH; Lim, TW; Lee, JCHigh-Temperature Fuel Cell Membranes Based on Mechanically Stable Para-Ordered PolybenzimidazolePrepared by Direct Casting. J. Power The preparation method described in Sources 2007, 172(1), 172-179. Poly(2,5-benzimidazole) (ABPBI) can be prepared according to the preparation method described in Chaudhari HD, Illathvalappil R, Kurungot S, et al. Preparation and investigations of ABPBI membrane for HT-PEMFC byimmersion precipitation method[J]. J. Membr. Sci., 2018, 564: 211-217.

[0064] Example 1

[0065] This embodiment provides a method for preparing a composite reference electrode for a sodium ion battery, comprising the following steps:

[0066] 1) A 15 cm, 50 μm diameter enameled copper wire was cut as a substrate, and one end (3 cm) and the other end (5 cm) were immersed in 98% concentrated sulfuric acid for 60 minutes to remove the enameled copper wire, followed by rinsing with deionized water for 10 minutes. The wire was then cleaned and air-dried to obtain an enameled copper wire with bare ends. Sodium vanadium phosphate, conductive agent (SP), and binder (PVDF) were weighed and mixed in a mass ratio of 80%:10%:10% sodium vanadium phosphate, SP, and PVDF, and then dissolved in NMP. The mixture was thoroughly stirred to obtain a first active slurry having a solid content of 20%. The 3 cm exposed end of the pretreated enameled copper wire was moistened with NMP and then immersed in the first active slurry for 15 seconds until the exposed end of the enameled copper wire was completely coated with the first active slurry. After natural drying, a reference electrode having one end coated with a first active coating having a thickness of 7 μm was obtained.

[0067] 2) Weigh 5 g of sepiolite powder, heat and stir at 90°C in 40% by mass dilute hydrochloric acid for 8 h, wash with deionized water and dry to obtain acid-treated sepiolite with a specific surface area of ​​520 m 2 / g, porosity 65%; poly[2,2'-(m-phenylene)-5,5'-bisbenzimidazole]: acid-treated sepiolite = 40%: 60% by mass ratio, weigh poly[2,2'-(m-phenylene)-5,5'-bisbenzimidazole] and acid-treated sepiolite, mix them, and then dissolve them in NMP, and stir them thoroughly at 50°C to obtain a second non-conductive slurry, the solid content of the second non-conductive slurry is 30%; the reference electrode coated with the first active coating at one end obtained in step 1) is immersed in the second non-conductive coating slurry for 10S, so that the non-conductive coating slurry completely covers the first active coating, and the sodium ion battery composite reference electrode is obtained after natural drying. The thickness of the second non-conductive coating is 10um. The structural schematic of the cross section of the sodium ion battery composite reference electrode at the coating is as follows Figure 1 As shown, it includes a substrate 1, a first active coating 2, and a second non-conductive coating 3.

[0068] Example 2

[0069] This embodiment provides a method for preparing a composite reference electrode for a sodium ion battery, which differs from embodiment 1 only in that sodium vanadium phosphate is replaced with an equal mass of sodium manganese vanadium phosphate in step 1).

[0070] Example 3

[0071] This embodiment provides a method for preparing a composite reference electrode for a sodium ion battery. The only difference from Example 1 is that in step 2), poly(2,5-benzimidazole) is used in equal mass to replace poly[2,2'-(m-phenylene)-5,5'-bisbenzimidazole].

[0072] Example 4

[0073] This embodiment provides a method for preparing a composite reference electrode for a sodium ion battery. The only difference between the embodiment 1 and the embodiment 1 is that in step 2), an acid-treated halloysite of equal mass is used to replace the acid-treated sepiolite. The acid-treated halloysite is prepared by weighing 5 g of halloysite powder, heating and stirring at 85° C. in 45% mass fraction dilute hydrochloric acid for 8.5 h, washing with deionized water, and drying to obtain the acid-treated halloysite having a specific surface area of ​​400 m 2 / g, and the porosity is 54%.

[0074] Example 5

[0075] This embodiment provides a method for preparing a composite reference electrode for a sodium ion battery, which differs from Example 1 only in that in step 2), "according to the mass ratio of poly[2,2'-(m-phenylene)-5,5'-bisbenzimidazole]: acid-treated sepiolite = 40%:60%, weighing poly[2,2'-(m-phenylene)-5,5'-bisbenzimidazole] and acid-treated sepiolite and mixing" is replaced by "according to the mass ratio of poly[2,2'-(m-phenylene)-5,5'-bisbenzimidazole]: acid-treated sepiolite = 20%:80%, weighing poly[2,2'-(m-phenylene)-5,5'-bisbenzimidazole] and acid-treated sepiolite and mixing".

[0076] Example 6

[0077] This embodiment provides a method for preparing a composite reference electrode for a sodium ion battery, which differs from Example 1 only in that in step 2), "according to the mass ratio of poly[2,2'-(m-phenylene)-5,5'-bisbenzimidazole]: acid-treated sepiolite = 40%:60%, weighing poly[2,2'-(m-phenylene)-5,5'-bisbenzimidazole] and acid-treated sepiolite and mixing" is replaced by "according to the mass ratio of poly[2,2'-(m-phenylene)-5,5'-bisbenzimidazole]: acid-treated sepiolite = 60%:40%, weighing poly[2,2'-(m-phenylene)-5,5'-bisbenzimidazole] and acid-treated sepiolite and mixing".

[0078] Example 7

[0079] This embodiment provides a method for preparing a composite reference electrode for a sodium ion battery, which differs from Example 1 only in that in step 2), "according to the mass ratio of poly[2,2'-(m-phenylene)-5,5'-bisbenzimidazole]: acid-treated sepiolite = 40%:60%, weighing poly[2,2'-(m-phenylene)-5,5'-bisbenzimidazole] and acid-treated sepiolite and mixing" is replaced by "according to the mass ratio of poly[2,2'-(m-phenylene)-5,5'-bisbenzimidazole]: acid-treated sepiolite = 80%:20%, weighing poly[2,2'-(m-phenylene)-5,5'-bisbenzimidazole] and acid-treated sepiolite and mixing".

[0080] Example 8

[0081] This embodiment provides a method for preparing a composite reference electrode for a sodium ion battery. The only difference from Example 1 is that in step 2), poly[2,2'-(m-phenylene)-5,5'-bisbenzimidazole] is dissolved in NMP and thoroughly stirred at 50°C to obtain a second non-conductive slurry. The solid content of the second non-conductive inorganic oxide slurry is 30%. The end of the reference electrode coated with the first active coating obtained in step 1) is immersed in the second non-conductive coating slurry for 10 seconds to completely cover the first active coating. After natural drying, the composite reference electrode for a sodium ion battery is obtained. The thickness of the second non-conductive coating is 12 μm.

[0082] Example 9

[0083] This embodiment provides a method for preparing a composite reference electrode for a sodium ion battery. The method differs from Example 1 only in that, in step 2), poly[2,2'-(m-phenylene)-5,5'-bisbenzimidazole] and sepiolite are weighed and mixed in a mass ratio of 40%:60% poly[2,2'-(m-phenylene)-5,5'-bisbenzimidazole] and sepiolite, then dissolved in NMP and thoroughly stirred at 50°C to obtain a second non-conductive slurry having a solid content of 30%. The end of the reference electrode coated with the first active coating obtained in step 1) is immersed in the second non-conductive coating slurry for 10 seconds, allowing the non-conductive coating slurry to completely cover the first active coating. The electrode is then air-dried to obtain the composite reference electrode for a sodium ion battery. The thickness of the second non-conductive coating is 10 μm.

[0084] Comparative Example 1

[0085] This comparative example provides a method for preparing a composite reference electrode for a sodium ion battery. The difference between this method and Example 1 is that step 2) is based on the mass ratio of silicon dioxide (specific surface area 120m 2 / g, porosity 30%: binder (PVDF) = 90%: 10%, weighing silica and PVDF, mixing, and then dissolving in NMP, stirring at 50 ° C to obtain a second non-conductive slurry, the mass content of silica in the second non-conductive slurry is 30%; the first active coating end of the reference electrode obtained in step 1) coated with the first active coating is immersed in the second non-conductive coating slurry for 10 seconds, so that the non-conductive coating slurry completely covers the first active coating, and the sodium ion battery composite reference electrode is obtained after natural drying.

[0086] Comparative Example 2

[0087] This comparative example provides a method for preparing a composite reference electrode for a sodium ion battery. The difference between the comparative example and the embodiment 1 is that step 2) 5 g of sepiolite powder is weighed, heated and stirred at 90° C. for 8 h in 40% by mass fraction dilute hydrochloric acid, and then washed and dried with deionized water to obtain acid-treated sepiolite with a specific surface area of ​​520 m 2 / g, porosity 65%; acid-treated sepiolite: binder (PVDF) = 90%:10% by mass, weighing the acid-treated sepiolite and PVDF, mixing them, then dissolving them in NMP, and fully stirring at 50°C to obtain a second non-conductive slurry, wherein the mass content of the acid-treated sepiolite in the second non-conductive slurry is 30%; immersing the first active coating end of the reference electrode obtained in step 1) having one end coated with the first active coating into the second non-conductive coating slurry for 10 seconds, so that the non-conductive coating slurry completely covers the first active coating, and naturally drying to obtain the sodium ion battery composite reference electrode.

[0088] Comparative Example 3

[0089] This comparative example provides a method for preparing a composite reference electrode for a sodium ion battery. The difference between the comparative example and the example 1 is that in step 2), 5 g of halloysite powder is weighed, heated and stirred at 85° C. for 8.5 h in 45% by mass dilute hydrochloric acid, and then washed and dried with deionized water to obtain acid-treated halloysite with a specific surface area of ​​400 m 2 / g, with a porosity of 54%; acid-treated halloysite: binder (PVDF) = 90%: 10% by mass, weighing the acid-treated halloysite and PVDF, mixing them, and then dissolving them in NMP. After fully stirring at 50° C., a second non-conductive slurry is obtained, and the mass content of the acid-treated halloysite in the second non-conductive slurry is 30%; the reference electrode having one end coated with the first active coating obtained in step 1) is immersed in the second non-conductive coating slurry for 10 seconds, so that the non-conductive coating slurry completely covers the first active coating, and the sodium ion battery composite reference electrode is obtained after natural drying.

[0090] Comparative Example 4

[0091] This comparative example provides a method for preparing a sodium ion battery reference electrode, comprising the following steps:

[0092] 1) A 15 cm, 50 μm diameter enameled copper wire was cut as the substrate. One end (3 cm) and the other end (5 cm) were immersed in 98% concentrated sulfuric acid for 60 min to remove the enameled copper wire. The wire was then rinsed with deionized water for 10 min, rinsed, and air-dried to obtain the enameled copper wire with both ends exposed.

[0093] 2) The exposed end of the enameled copper wire (3 cm) is placed in the middle of the length direction of the soft-pack battery core as a reference electrode. The reference electrode is located between the positive electrode and the negative electrode. A diaphragm is set between the reference electrode and the positive electrode, and between the reference electrode and the negative electrode. The positive electrode of the soft-pack battery is a sodium layered oxide Na(Ni 1 / 3 Fe 1 / 3 Mn 1 / 3 )O2 is, the negative electrode is hard carbon, and the diaphragm material is polypropylene (PP). The exposed end of the reference electrode (5cm exposed end) is led out from the shell, and the external pole ear forms the reference electrode pole ear; after the three-electrode soft-pack battery is prepared, the reference electrode is plated with sodium; the sodium plating steps are: the external positive power line is connected to the positive ear of the three-electrode battery, the external negative power line is connected to the reference electrode pole ear, and sodium is plated at 20μA for 4h; then the external positive power line is connected to the negative ear of the three-electrode battery, and the external negative power line is still connected to the reference electrode pole ear, and sodium is plated at 20μA for 4h to obtain a sodium ion battery reference electrode. This comparative example also obtains a three-electrode battery containing a sodium ion battery reference electrode.

[0094] Test Case

[0095] The reference electrodes prepared in Examples 1-9 and Comparative Examples 1-3 of the present invention, coated with the first active coating and the second non-conductive coating, were placed in the middle of the length direction of the soft-pack battery core without adding an additional diaphragm. The positive electrode of the soft-pack battery was a sodium layered oxide Na(Ni 1 / 3 Fe 1 / 3 Mn 1 / 3 )O2, and the negative electrode is hard carbon. The exposed end of the reference electrode (5cm exposed end) is led out of the shell and connected to an external tab to form the reference electrode tab. After preparing the three-electrode soft-pack battery, the reference electrode is activated. The specific steps of reference electrode activation are as follows: connect the external positive power line to the reference electrode tab and the external negative power line to the negative electrode tab, and charge at 4μA for 2 hours until the potential remains constant.

[0096] The three-electrode battery containing the sodium ion battery reference electrode prepared in Comparative Example 4 was subjected to reference electrode activation. The specific steps of reference electrode activation were: the external positive power line was connected to the reference electrode tab, the external negative power line was connected to the negative electrode tab, and charging was performed at 4 μA for 2 h until the potential remained constant.

[0097] The three-electrode soft-pack battery prepared above was tested in an environment of 25°C with a voltage range of 1.5V-4.0V. On a blue-electric electrochemical workstation, 1C rate charging, stopping charging, keeping the battery in an open circuit state and standing for 10min, and then 2C rate discharge, stopping discharge, keeping the battery in an open circuit state and standing for 10min were used as one cycle, which took 1 hour and 50 minutes, for a total of 2000 cycles, and the electrode potential changes were monitored. If the reference electrode potential, the negative electrode potential, and the positive electrode potential can be measured in the first cycle, the three-electrode potential can be successfully detected; the stability (potential fluctuation amplitude) of the reference electrodes prepared in Examples 1-9 and Comparative Examples 1-3 of the present invention was evaluated, and the stable test time was recorded. The stability evaluation standard is: the difference between the lowest potential of the negative electrode relative to the reference electrode in any two cycles within every 100h is ≤10mV, that is, the reference electrode is stable; the difference between the lowest potential of the negative electrode relative to the reference electrode in any two cycles within every 100h is >10mV, that is, the reference electrode is poorly stable. The drift difference between the lowest potential at 1000 hours and the lowest potential at 10 hours, and the drift difference between the lowest potential at 2000 hours and the lowest potential at 10 hours were recorded. The test results are shown in Table 1.

[0098] In the cycle test process of Example 1, the cycle potential diagram within the first 10 hours at 25°C and 1C / 2C is as follows: Figure 2 As shown, the figure includes the potential of the negative electrode relative to the reference electrode (negative electrode-reference electrode potential), the potential of the positive electrode relative to the reference electrode (positive electrode-reference electrode potential) and the total potential; the potential of the negative electrode relative to the reference electrode (negative electrode-reference electrode potential) of Example 1 and Comparative Example 1 in the first 8 hours at 25°C and 1C / 2C is compared as shown in Figure 3 shown.

[0099] Table 1

[0100]

[0101]

[0102] Note: + indicates that the minimum potential drift difference between 10 hours and 10 hours is greater, or the minimum potential drift difference between 2000 hours and 10 hours is greater. / indicates that the reference electrode can be stably tested for less than 100 hours, and subsequent testing is meaningless, that is, no test is performed.

[0103] The lowest potential is the weak point of the entire battery system. Generally, the lowest potential of the negative electrode relative to the reference electrode needs to be >0mV; if it is <0mV, metallic sodium deposition will occur on the surface of the negative electrode, resulting in a sharp decrease in capacity and even causing safety problems. The lowest potential of the negative electrode-reference electrode is the most direct and important evaluation parameter.

[0104] Small concentration polarization means that the lowest potential will be higher, closer to the actual negative electrode potential. When evaluating the charging capacity of a battery cell, a battery with large concentration polarization will be misled by the test data (the negative electrode potential reaches 0mV, while the lowest negative electrode potential of a battery with small concentration polarization, which is closer to the actual negative electrode potential, is >0mV at this time), causing the electrochemical performance to be underestimated. Figure 3 It can be seen that the reference electrode in Example 1 of the present application has small concentration polarization.

[0105] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will readily appreciate that other variations or modifications based on the above descriptions are possible. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A composite reference electrode for a sodium ion battery, characterized in that: The sodium ion battery composite reference electrode comprises a conductive substrate, a first active coating and a second non-conductive coating; The first active coating is coated on the surface of the conductive substrate; The second non-conductive coating is coated on the surface of the first active coating; The second non-conductive coating comprises a polybenzimidazole polymer; The polybenzimidazole polymer includes at least one of poly[2,2'-(m-phenylene)-5,5'-bisbenzimidazole], poly[2,2'-(p-phenylene)-5,5'-bisbenzimidazole], and poly(2,5-benzimidazole).

2. The sodium ion battery composite reference electrode according to claim 1, characterized in that The second non-conductive coating further comprises a porous silicate mineral that has been activated with acid; The porous silicate mineral is selected from at least one of sepiolite and halloysite; and / or, the porosity of the acid-activated porous silicate mineral is 30%-80%; And / or, the specific surface area of ​​the porous silicate mineral treated by acid activation is 300-800m 2 / g.

3. The sodium ion battery composite reference electrode according to claim 2, characterized in that The method for preparing the acid-activated porous silicate mineral comprises: placing the porous silicate mineral in a 20%-60% dilute hydrochloric acid solution, heating and stirring at 70-100° C. for 5-12 hours, washing, and drying to obtain the acid-activated porous silicate mineral; Preferably, the second non-conductive coating comprises, by mass percentage, 20%-80% of a polybenzimidazole polymer and 20%-80% of an acid-activated porous silicate mineral; And / or, the thickness of the second non-conductive coating layer is 5-20 μm.

4. The sodium ion battery composite reference electrode according to any one of claims 1 to 3, characterized in that The first active coating layer comprises a positive electrode active material containing sodium, a conductive agent and a binder; Preferably, the first active coating layer comprises, by mass percentage, 70%-98% of a positive electrode active material containing sodium, 1%-10% of a conductive agent, and 1%-20% of a binder; The positive electrode active material containing sodium element includes at least one of sodium vanadium phosphate, sodium manganese vanadium phosphate, sodium ferrous vanadium phosphate, sodium iron phosphate, and sodium fluorophosphate; And / or, the binder includes at least one of polyvinylidene fluoride, styrene-butadiene rubber, carboxymethyl cellulose, sodium carboxymethyl cellulose, polyacrylonitrile, and polyacrylic acid; and / or, the conductive agent comprises at least one of conductive carbon black, conductive graphite, carbon nanotubes, acetylene black or graphene; And / or, the first active coating layer has a thickness of 3-20 μm.

5. The sodium ion battery composite reference electrode according to any one of claims 1 to 4, characterized in that The conductive substrate includes at least one of metal wire, enameled metal wire, and carbon fiber; The metal wire includes at least one of copper wire, aluminum wire, platinum wire and silver wire; The enameled metal wire includes at least one of enameled copper wire, enameled aluminum wire, enameled platinum wire, and enameled silver wire; Preferably, the conductive substrate has a diameter of 10-200 μm.

6. A method for preparing a composite reference electrode for a sodium ion battery according to any one of claims 1 to 5, characterized in that: The following steps are involved: 1) immersing the exposed end of the conductive substrate into the first active slurry and drying after the immersion is completed to obtain a reference electrode having one end coated with the first active coating; 2) immersing the end of the reference electrode coated with the first active coating obtained in step 1) into a second non-conductive coating slurry comprising a polybenzimidazole polymer, and drying after the immersion to obtain a sodium ion battery composite reference electrode.

7. The method for preparing a composite reference electrode for a sodium ion battery according to claim 6, wherein: The solvent in the second non-conductive coating slurry comprising the polybenzimidazole polymer in step 2) is at least one of methanesulfonic acid, N,N-dimethylformamide, N,N-dimethylacetamide, and N-methyl-2-pyrrolidone; The solid content of the second non-conductive coating slurry comprising the polybenzimidazole polymer is 5%-30%.

8. The method for preparing a composite reference electrode for a sodium ion battery according to claim 6 or 7, wherein: In step 1), the solvent in the first active slurry is at least one of methanesulfonic acid, N,N-dimethylformamide, N,N-dimethylacetamide, and N-methyl-2-pyrrolidone; The solid content of the first active slurry is 5%-30%.

9. The method for preparing a composite reference electrode for a sodium ion battery according to any one of claims 6 to 8, characterized in that: The exposed end of the conductive substrate in step 1) is the end where the conductive material is exposed; Optionally, both ends of the carbon fiber are exposed, and both ends of the metal wire are exposed; Optionally, the enameled metal wire is pre-treated to form a bare end; Optionally, the pretreatment includes placing both ends of the enameled metal wire in a sulfuric acid solution for 10-120 minutes to remove the surface enameling layer, 0.5-3 cm at one end and 3-7 cm at the other end, to obtain the exposed ends; Optionally, the exposed end of the conductive substrate is further cleaned with deionized water or ethanol before use; Optionally, before immersing the exposed end of the conductive substrate into the first active slurry in step 1), the step further includes wetting the exposed end of the conductive substrate with a solvent of the first active slurry.

10. A sodium ion battery, characterized in that: The sodium ion battery comprises the sodium ion battery composite reference electrode according to any one of claims 1 to 5 or the sodium ion battery composite reference electrode prepared by the preparation method according to any one of claims 6 to 9.

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