阅读说明：本技术 提升高镍正极循环稳定性的陶瓷纤维隔膜及电池应用 (Ceramic fiber diaphragm for improving cycle stability of nickel anode and application of battery ) 是由 蒋建 李顺 刘奕霏 于 2021-06-02 设计创作，主要内容包括：本发明涉及了一种新型功能化无机硅酸铝陶瓷纤维隔膜。该隔膜以耐火粘土为原料,通过熔融、甩丝等过程制得,能有效提升高镍正极(镍酸锂LiNiO-2、高镍三元LiNi-(0.8)Co-(0.1)Mn-(0.1)O-2和LiNi-(0.8)Co-(0.15)Al-(0.05))的循环稳定性。当运用于高镍锂离子电池时,其具备的应用优势如下：①陶瓷纤维具有出色的阻燃性和机械韧性,能防止高温变形和锂枝晶刺穿；②隔膜中的硅酸铝能中和电解液中产生的HF,所生成的铝、硅等成分均匀沉积在高镍正极颗粒表面,可有效阻碍电解液/电极界面副反应的进一步发生。该陶瓷纤维隔膜可大幅延长高镍正极循环寿命,所组装的电池具有更高安全性,具备一定的实用潜力和商业前景。(The invention relates to a novel functionalized inorganic aluminum silicate ceramic fiber diaphragm. The diaphragm is prepared by taking refractory clay as a raw material through processes of melting, throwing and the like, and can effectively improve a nickel anode (lithium nickelate LiNiO) 2 High nickel ternary LiNi 0.8 Co 0.1 Mn 0.1 O 2 And LiNi 0.8 Co 0.15 Al 0.05 ) The cycle stability of (c). When the lithium ion battery is applied to a high nickel lithium ion battery, the application advantages of the lithium ion batteryThe following were used: the ceramic fiber has excellent flame retardance and mechanical toughness, and can prevent high-temperature deformation and lithium dendrite penetration; secondly, the aluminum silicate in the diaphragm can neutralize HF generated in the electrolyte, and the generated components such as aluminum, silicon and the like are uniformly deposited on the surface of the high-nickel anode particles, so that the further occurrence of the side reaction of the electrolyte/electrode interface can be effectively prevented. The ceramic fiber diaphragm can greatly prolong the cycle life of the high-nickel anode, and the assembled battery has higher safety and certain practical potential and commercial prospect.)

1. The ceramic fiber diaphragm for improving the cycling stability of the nickel anode and the application of the battery are characterized in that: a method of making a battery containing a ceramic fiber separator for improving the cycling stability of a nickel anode, comprising the steps of:

(1) preparing a ceramic fiber material: hard refractory clay is used as a raw material and is put into a resistance furnace to be heated and melted; then, throwing the fiber to a stainless steel substrate in a fiber form with the aid of a throwing machine, and obtaining the reticular fiber cloth in a rapid cooling mode;

(2) manufacturing a ceramic fiber diaphragm: cutting the fiber cloth into ceramic fiber diaphragm wafers by a button cell slicer;

(3) preparing a positive plate: mixing a high-nickel anode material, a conductive agent and binder powder according to a certain proportion, adding a proper amount of organic solvent, and stirring at room temperature to obtain black viscous slurry; coating the slurry on a current collector electrode by using a scraper, and drying to obtain a high-nickel positive plate;

(4) assembling and testing the battery: and (3) tightly stacking the positive electrode shell, the high-nickel positive electrode plate, the electrolyte, the ceramic fiber diaphragm, the lithium metal or carbon negative electrode, the gasket, the elastic sheet and the negative electrode shell in sequence and packaging the stacked positive electrode shell, the high-nickel positive electrode plate, the electrolyte, the ceramic fiber diaphragm and the lithium metal or carbon negative electrode with a sealing machine, and carrying out performance detection on the assembled full battery.

2. The ceramic fiber separator for improving the cycle stability of a high nickel positive electrode and the battery application as claimed in claim 1, whereinIn the step (1), the hard refractory clay is Al₂O₃·SiO₂·H₂O; the diameter of the fiber in the step (1) is in the range of 0.4 to 4 μm; the molar ratio of aluminum to silicon in the fiber cloth in the step (1) is 2:1, and the skeleton structure density of the fiber cloth is 100-³。

3. The ceramic fiber separator and battery application for improving the cycle stability of the high-nickel positive electrode according to claim 1, wherein the heating temperature of the resistance furnace in the step (1) is 2000 ℃; the rotating speed of the wire throwing machine in the step (1) is 5000-7000 RPM.

4. The ceramic fiber membrane for improving the cycle stability of the high-nickel positive electrode and the battery application as claimed in claim 1, wherein the diameter of the ceramic fiber membrane disc in the step (2) is 16 mm, and the thickness of the ceramic fiber membrane disc is 0.05 mm.

5. The ceramic fiber membrane for improving the cycle stability of a high-nickel positive electrode and the battery application of the ceramic fiber membrane are claimed in claim 1, wherein in the step (3), the conductive agent is acetylene black, the adhesive is polyvinylidene fluoride (PVDF), the organic solvent is N-methyl pyrrolidone, and the current collector electrode is aluminum foil.

6. The ceramic fiber membrane for improving the cycle stability of the high-nickel positive electrode and the battery as claimed in claim 1, wherein in the step (3), the mass ratio of the positive electrode material, the conductive agent and the binder is 9:0.5: 0.5.

7. The ceramic fiber membrane and battery application for improving the cycle stability of a high nickel positive electrode according to claim 1, wherein the high nickel positive electrode material in the step (3) comprises LiNiO₂High nickel ternary LiNi_0.8Co_0.1Mn_0.1O₂And LiNi_0.8Co_0.15Al_0.05O₂One or more of them.

8. The ceramic fiber membrane for improving the cycle stability of a high-nickel positive electrode and the battery of claim 1, wherein in the step (4), the electrolyte is LiPF₆The solution is dissolved in a mixed solution of ethylene carbonate EC, diethyl carbonate DEC and ethyl methyl carbonate EMC solvent with the volume ratio of 1:1:1, and the molar concentration is 1.0 mol/L.

9. The ceramic fiber separator and battery application for improving cycle stability of a high nickel positive electrode according to claim 1, wherein in the step (4), the carbon negative electrode is graphitized carbon.

Technical Field

The invention belongs to the technical field of lithium ion batteries, and particularly relates to a ceramic fiber diaphragm for improving the cycle stability of a nickel anode and application of the ceramic fiber diaphragm.

Background

The development of lithium ion batteries with high energy density, long endurance and high safety for providing power guarantees for electric automobiles, large unmanned aerial vehicles and the like is always the key point of research and development of the electrochemical energy storage industry. As an important component of lithium ion batteries, the production cost and the comprehensive performance of the batteries are seriously influenced by the anode material. LiCoO as traditional anode material₂The method faces the difficulties of high price (high cobalt content), limited specific energy (180 Wh/kg), and the like, and is difficult to meet the requirement of the market development of power batteries. In contrast, high nickel positive electrode materials such as lithium nickelate LiNiO₂High nickel ternary LiNi_0.8Co_0.1Mn_0.1O₂(NCM 811) and high-nickel ternary LiNi_0.8Co_0.15Al_0.05(NCA), because of the large increase in nickel content, the cobalt content is significantly reduced, resulting in a reduction in the production cost of the battery system and higher operating voltage and energy density (-280 Wh/kg). Therefore, the high nickel positive electrode is regarded as an electrode material with practical prospect in a new generation of power lithium ion battery.

However, the development of the application of the high nickel cathode material faces many challenges such as poor cycle stability and large potential safety hazard, which are mainly due to the following reasons: residual water molecules in the electrolyte can generate side reaction with the electrolyte to generate HF (LiPF)₆→LiF+PF₅，PF₅+H₂O→POF₃+2 HF) and corrode the surface area of the positive electrode particles, causing the dissolution of transition metals such as nickel in the material, and the cycle stability of the positive electrode is affected (refer to Converting tertiary HF in electrolytes in o a high purity fluorinated interphase on catalysts, j. mater. chem. a, 2018). ② due to Ni²⁺(0.69A) and Li⁺Approximate (0.76A) ionic radius, Ni²⁺Easy migration to Lithium vacancy, occurrence of cation mixed discharge phenomenon, and irreversible transformation of partial Layered structure to spinel structure, and continuous attenuation of specific capacity of high-nickel positive electrode (refer to porous Lithium-Ion Diffusion in Layered Cathode Materials by Lithium reduction Li)⁺/Ni²⁺Antisite Defects for High-Rate Li-Ion Batteries. Research, 2019）。When the battery is in a fully charged state, the high-nickel anode is easy to decompose at high temperature to generate oxygen and release the oxygen together with gasified electrolyte, so that the thermal runaway phenomenon of the battery is easy to cause great potential safety hazard.

In order to solve the above problems, a method for improving the stability of a high nickel positive electrode and the safety of a battery by using a ceramic fiber diaphragm is proposed. The specific experimental method is as follows: with hard refractory clay Al₂O₃·SiO₂·H₂O is taken as a raw material, the raw material is placed into a resistance furnace to be heated and gradually melted to form a molten mass, the molten mass is thrown onto a stainless steel substrate in a fiber form under the assistance of a thread throwing machine, and the network-shaped fiber cloth is obtained through a rapid cooling mode; and finally, cutting the fiber membrane into a ceramic fiber membrane wafer by using a battery slicer. In the nickelic material, we use nickelic ternary NCM811 and lithium nickelate LiNiO₂For example, when the ceramic fiber diaphragm is applied to a high-nickel lithium ion battery, on one hand, the ceramic fiber diaphragm has excellent insulativity and flame retardance, and cannot deform even under an extreme high-temperature environment (800 ℃), so that the short circuit phenomenon caused by heating of the battery can be effectively avoided; on the other hand, the reticular structure formed by the ceramic fibers in a staggered manner endows the diaphragm with higher mechanical toughness and strength, can effectively block the growth of lithium dendrites, and avoids the phenomenon of battery short circuit caused by the fact that the dendrites pierce the diaphragm. In addition, the aluminum silicate in the diaphragm can be used as an HF remover to completely neutralize HF generated in the electrolyte, and generated flocculent aluminum, silicon and other components can be uniformly deposited and adhered to the surface of the high-nickel anode particles, so that the further occurrence of side reactions of an electrolyte/anode interface is avoided. This results in a high nickel positive electrode with excellent long cycle stability and higher safety of the assembled full cell. The scheme has the advantages of easily available raw materials, low price, convenience for large-scale batch production, and certain practical potential and commercial prospect.

Disclosure of Invention

In view of this, the object of the invention is: an inorganic aluminum silicate ceramic fiber diaphragm material capable of pertinently improving the cycle stability of a high-nickel positive electrode is provided and developed.

In order to achieve the purpose, the technical scheme provided by the invention is as follows:

1. the development of a battery containing a ceramic fiber separator for improving the cycling stability of a nickel anode comprises the following steps:

(1) preparing a ceramic fiber material: hard refractory clay is taken as a raw material, is put into a resistance furnace to be heated and melted, is thrown onto a stainless steel substrate in a fiber form under the assistance of a wire throwing machine, and is rapidly cooled to obtain network-shaped fiber cloth;

(2) manufacturing a ceramic fiber diaphragm: cutting the fiber cloth into ceramic fiber diaphragm wafers by a button cell slicer;

(3) preparing a positive plate: mixing a high-nickel positive electrode material, a conductive agent and binder powder according to a certain proportion, adding a proper amount of organic solvent, stirring at room temperature to prepare black viscous slurry, coating the slurry on a current collector electrode by using a scraper, and drying to obtain a high-nickel positive electrode sheet;

(4) assembling and testing the battery: and (3) tightly stacking the positive electrode shell, the high-nickel positive electrode plate, the electrolyte, the ceramic fiber diaphragm, the lithium metal or carbon negative electrode, the gasket, the elastic sheet and the negative electrode shell in sequence and packaging the stacked positive electrode shell, the high-nickel positive electrode plate, the electrolyte, the ceramic fiber diaphragm and the lithium metal or carbon negative electrode with a sealing machine, and carrying out performance detection on the assembled full battery.

Further, in the step (1), the hard refractory clay is Al₂O₃·SiO₂·H₂O, the diameter of the fiber is 0.4 to 4 mu m, the molar ratio of aluminum to silicon in the fiber cloth is 2:1, and the skeleton structure density of the fiber cloth is 100-³。

Further, in the step (1), the heating temperature of the resistance furnace is 2000 ℃, and the rotation speed of the wire throwing machine is 5000-7000 RPM.

Further, in the step (2), the diameter of the ceramic fiber diaphragm wafer is 16 mm, and the thickness of the ceramic fiber diaphragm wafer is 0.05 mm.

Further, in the step (3), the conductive agent is acetylene black, the adhesive is polyvinylidene fluoride (PVDF), the organic solvent is N-methylpyrrolidone, and the current collector electrode is aluminum foil.

Further, in the step (3), the mass ratio of the positive electrode material, the conductive agent and the binder is 9:0.5: 0.5.

Further, in the step (3), the high-nickel positive electrode material includes lithium nickelate LiNiO₂High nickel ternary LiNi_0.8Co_0.1Mn_0.1O₂And LiNi_0.8Co_0.15Al_0.05O₂One or more of them.

Further, in the steps (4) and (5), the electrolyte is LiPF₆The solution is dissolved in a mixed solution of ethylene carbonate EC, diethyl carbonate DEC and ethyl methyl carbonate EMC solvent with the volume ratio of 1:1:1, and the molar concentration is 1.0 mol/L.

Further, in the step (5), the carbon negative electrode is graphitized carbon.

2. The invention has the beneficial effects that: discloses a novel ceramic fiber diaphragm material which has low cost and can improve the cycle stability of a high-nickel anode; the diaphragm has excellent insulativity and flame retardance, and cannot deform under a high-temperature environment; the interlaced net structure of the fibers endows the diaphragm with higher mechanical toughness and strength, and can effectively block the growth of lithium dendrites; the aluminum silicate in the diaphragm can be used as an HF remover to completely neutralize HF generated in the electrolyte, and generated flocculent aluminum, silicon and other components can be uniformly deposited and adhered to the surface of the high-nickel anode particles, so that the further occurrence of side reaction of an electrolyte/anode interface is avoided. Among high nickel cathode materials, we use NCM811 and LiNiO₂For example, it is matched to a ceramic fiber membrane. The verification proves that the diaphragm can obviously slow down the specific capacity attenuation of the high-nickel anode, and effectively solves the problems of unstable cycle, poor safety and the like of a full battery based on the high-nickel anode. The fiber diaphragm has the advantages of easily obtained raw materials, low price, simple and efficient preparation process, and practical prospect and commercial value.

Drawings

In order to make the object, technical scheme and beneficial effect of the invention more clear, the invention provides the following drawings for explanation:

FIG. 1 is an optical photograph of a ceramic fiber separator used in example 1;

FIG. 2 is a photograph taken by a scanning electron microscope of (a) a ceramic fiber separator used in example 1 and a comparison of the same with (b) a conventional PE separator in a heat resistance test;

FIG. 3 is a graph of (a) cycle curve and (b) rate performance of a lithium metal battery assembled in accordance with example 2 matching a ceramic fiber separator with an NCM811 pole piece, lithium metal electrode;

FIG. 4 shows a ceramic fiber separator and LiNiO in example 2₂Matching (a) a cycle curve diagram and (b) a rate performance diagram of the assembled lithium metal battery by the positive plate and the lithium metal electrode;

fig. 5 is (a) a cycle life diagram and (b) a charge-discharge graph of a full battery assembled by matching the ceramic fiber separator of example 2 with the NCM811 positive electrode and the carbon negative electrode;

FIG. 6 is a transmission electron micrograph of the NCM 811/lithium metal battery assembled with the ceramic fiber separator of example 3 taken after 100 cycles of cycle (a) of the positive electrode of NCM811, and X-ray energy line scan test chart and line scan analysis result chart (b).

Detailed Description

Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

Example 1 preparation of ceramic fiber separator

(1) Preparing ceramic fibers: the raw material of hard refractory clay Al₂O₃·SiO₂·H₂And (3) putting the O into a resistance furnace, heating the O to 2000 ℃, gradually melting the O to form a molten mass, and then throwing the O to a stainless steel substrate in a fiber form under the conditions of a wire throwing machine of 5000-7000 RPM. Under the environment that the relative humidity is not higher than 40 percent, the reticular fiber cloth is obtained by a rapid cooling mode, and the density of the skeleton structure of the reticular fiber cloth is 100-600 kg/m³。

(2) Preparing a ceramic fiber diaphragm: the thickness of the ceramic fiber cloth is cut to be 0.05 mm, and then the ceramic fiber cloth is cut into a ceramic fiber diaphragm wafer with the diameter of 16 mm by a button cell slicer.

Fig. 1 is a macroscopic morphology optical photograph of a conventional PE diaphragm and a ceramic fiber diaphragm, and it can be seen from the image that the PE diaphragm has a smooth surface, while the ceramic fiber diaphragm has a rough surface and obvious network stripes on the surface.

FIG. 2 (a) The scanning electron microscope photograph of the ceramic fiber membrane shows that the ceramic fiber membrane is formed by interlacing ceramic fibers and has a net structure, and the diameter of the fibers is about 0.4 to 4 mm. FIG. 2 (b) is an optical photograph showing the temperature resistance test of a conventional PE separator and a conventional ceramic fiber separator, in which the two separators are subjected to heat treatment (rate of temperature rise: 30)^oC/min) was carried out to observe their heat shrinkage deformation. Conventional PE separator at 90 ^oCDeformation occurs during the process, and melting occurs at 130 ℃; in contrast, the ceramic fiber separator is at 800 ^oCStill has no deformation and has excellent thermal stability.

Example 2 ceramic fiber membranes were separately mixed with high nickel ternary LiNi_0.8Co_0.1Mn_0.1O₂ (NCM 811) Positive electrode and lithium nickelate LiNiO₂And (4) matching the positive electrodes, assembling into a button cell and testing

(1) Manufacturing a positive pole piece: mixing the positive plate, the conductive agent carbon black and the binder PVDF according to the mass ratio of 9:0.5:0.5, adding a proper amount of N-methyl pyrrolidone, and uniformly grinding to obtain black viscous slurry. Uniformly coating the slurry on an aluminum foil current collector by using a scraper, 110 ^oCAnd (5) drying for 12 hours in vacuum to obtain the positive pole piece. The positive plate is high-nickel ternary NCM811 and lithium nickelate LiNiO₂。

(2) And (3) manufacturing a negative plate: the lithium metal sheet and the carbon negative electrode sheet were purchased commercially and dried for direct use.

(3) Lithium metal battery assembly and testing: high nickel ternary NCM811 or lithium nickelate LiNiO₂A positive electrode sheet as a positive electrode, a lithium metal sheet as a negative electrode, LiPF₆And (3) dissolving a mixed solution (molar concentration: 1.0 mol/L) of ethylene carbonate EC, diethyl carbonate DEC and methyl ethyl carbonate EMC solvent with a volume ratio of 1:1:1 as an electrolyte, using aluminum silicate ceramic fiber as a diaphragm, and assembling the positive plate, the diaphragm, the lithium plate, the gasket, the elastic sheet and the negative plate in a glove box according to the stacking sequence to obtain the lithium metal battery. The lithium metal battery is set aside for 6 h and then tested, the test content comprises a cycle performance diagram, a rate performance diagram and the like, and the performance results are shown in fig. 3 and fig. 4.

(4) Assembling and testing the full battery: the NCM811 pole piece is used as a positive electrode, graphite carbon is used as a negative electrode, and LiPF₆A mixed solution (molar concentration: 1.0 mol/L) of ethylene carbonate EC, diethyl carbonate DEC and ethyl methyl carbonate EMC solvent dissolved in a volume ratio of 1:1:1 was used as an electrolyte, and ceramic fibers were used as separators, and stacked and assembled in order in step (3) in a glove box to obtain a full cell. The battery was left for 6 h and then tested, the test contents include a cycle life chart and a charge-discharge curve chart, and the performance results are shown in fig. 5.

FIG. 3 (a) shows a graph comparing the cycle performance of a high nickel ternary NCM811// lithium metal battery assembled with a conventional PE separator and a ceramic fiber separator. The tested cells were cycled for 600 cycles at a current density of 1C (i.e., 1C =274 mA/g). As can be seen from the graph, the capacity of the battery using the PE separator rapidly decayed as the cycle progressed, and the capacity retention rate was only 35% after 600 cycles. The capacity of the battery adopting the ceramic fiber diaphragm is not greatly changed in the circulation process, and the capacity retention rate can reach 88% after 600 circles. Fig. 3 (b) is a graph comparing rate performance of a battery assembled by a conventional PE separator and a ceramic fiber separator at different current densities. Under the charge-discharge multiplying power of 0.5, 1, 2C and the like, the specific capacity of the two is not greatly different. Under the conditions of large charge-discharge multiplying power of 7.5 and 10C, the specific capacity of the traditional PE diaphragm-based battery is only 76 mAh g^-1And 50 mAh g^-1And the capacity of the battery based on the ceramic fiber diaphragm is 90 mAh g respectively^-1And 66 mAh g^-1This indicates that the NCM811 lithium metal battery using the ceramic fiber separator has more excellent rate performance.

FIG. 4 (a) shows LiNiO, a lithium nickelate membrane assembled by a conventional PE membrane and a ceramic fiber membrane₂Lithium metal battery cycling performance is compared. The lithium metal battery was cycled for 500 cycles at a current density of 1C. As can be seen from the graph, the capacity of the battery using the PE separator rapidly decayed as the cycle progressed, and the capacity retention rate was only 26% after 500 cycles. Compared with the prior art, the battery based on the ceramic fiber diaphragm has little specific capacity change in the circulation process, and the capacity retention rate is still as high as 82% after 500 circles. FIG. 4 (b) shows a conventional PE separator and a ceramic fiber separatorMembrane-assembled lithium nickelate LiNiO₂Graph comparing rate performance of lithium metal battery under different current density. Under different charge and discharge multiplying power, the battery capacity based on the ceramic diaphragm is higher than that of the battery based on the traditional PE diaphragm. Even under higher multiplying power such as 4.5C and the like, the capacity of the lithium metal battery based on the ceramic fiber diaphragm is still as high as 160 mAh g^-1This indicates that lithium nickelate LiNiO using a ceramic fiber separator is used₂The lithium metal battery has more excellent rate performance.

FIG. 5 (a) is a comparison graph of cycle life of a high nickel ternary NCM811// graphitic carbon negative electrode full cell assembled with a conventional PE separator and ceramic fiber separator. As the cycling proceeded, no significant decrease in the specific capacity of the cells assembled with the ceramic fiber separator occurred. After 600 cycles, the capacity retention rate is still 81 percent, which is much higher than the test result of the PE diaphragm battery (the capacity retention rate is only 22 percent after 600 cycles). Fig. 5 (b) is a charge and discharge graph of the full cell assembled with the ceramic fiber separator. Along with the increase of the number of charging and discharging circles of the battery, the charging and discharging platform of the battery is not obviously changed, and the specific capacity is not greatly reduced, which shows that the NCM 811/carbon full battery adopting the ceramic fiber diaphragm has excellent cycling stability.

Example 3 microstructure observation of NCM811// lithium metal battery with ceramic fiber separator after cycling 100 cycles for the positive electrode

(1) Disassembling the button battery: placing the circulated button cell with the negative electrode facing upwards in a groove of a cell disassembling machine, closing a valve, and pressurizing to 100 kg/cm²And then held for 30 seconds. And opening the valve, taking out the disassembled battery, and clamping the positive plate out by using tweezers.

(2) Transmission electron microscope test: scraping the active matter on the positive plate, putting the positive plate into a sample tube, dripping a proper amount of alcohol into the tube, and ultrasonically dispersing the sample. And (3) taking a small amount of mixed liquid by using a liquid-transferring gun, dropwise adding the mixed liquid onto a copper net, and then carrying out transmission electron microscope detection on the sample.

FIG. 6 (a) is a TEM photograph of the positive electrode after 100 cycles of the NCM 811/lithium metal battery equipped with a ceramic fiber separator. At the microscopic scale, a floc coverage of the surface of the NCM811 particles was clearly observed. Fig. 6 (b) shows the X-ray energy line scan test results of the NCM811 particles after 100 cycles (the line scan direction extends from the inside to the outside of the particles and passes through the flocculent layer on the surface of the NCM811 particles). As shown in fig. 6 (c), the results of line scan analysis showed that the particles were mainly composed of Ni, Co, Mn (Ni content was highest), and no signals of other elements (such as Al, Si, etc.) were detected. However, in the flocculent layer region, Al and Si element signals can be clearly observed, and the results fully confirm that the generated substances containing aluminum, silicon and the like are uniformly deposited and adhered on the surface of the high-nickel anode particles, so as to play a role in blocking the side reaction of the electrolyte/electrode interface.