With the global energy reduction and environmental degradation, the development of environmentally friendly new energy sources has received widespread attention. Among them, new energy materials play a great role in guiding and supporting. Battery materials mainly involve positive electrode, negative electrode, electrolyte and phase diaphragm, which are important parts of electricity storage and power supply. At present, battery materials are developing to the nanometer level, and nanomaterials have special microscopic shape and structure, high embedded lithium capacity and energy density, and long cycle life. However, the problems of nanoparticle agglomeration and size control, small contact area between electrode material and electrolyte, and corrosion inhibition of electrolyte need to be solved by surfactants.
Surfactants can change the crystalline shape of metal oxides, inhibit hydrogen precipitation, dendrite growth and corrosion, delay electrode passivation, and act as a “microreaction cell” and prevent the agglomeration of nanoparticles in emulsions. The research of surfactants in the preparation of battery materials and the recycling of used batteries is reviewed.
Surfactant molecules are composed of hydrophobic and hydrophilic groups, which can be used as a “microreactor” to prepare nanoparticles by forming ordered combinations of micelles, anti-micelles and vesicles in solution. The surfactants can prevent the agglomeration of nanoparticles, control the size of nanoparticles, and improve the electrical properties of the electrodes made by forming a directional arrangement of carriers. Surfactants play an important role in the production of battery catalysts, electrode materials, as corrosion inhibitors, and in battery recycling.
I. Application in the production of battery catalyst
Surfactants play an important role in the production of ion-exchange membrane fuel cell catalysts, which are mainly made by microemulsion method. The microemulsion method includes orthogel microemulsion system and antigel microemulsion system.
The main disadvantage of the orthoglomerate microemulsion system is that the particles are difficult to be separated and purified from the microemulsion, and then a large amount is consumed in the production process.
The inverse micellar method is used in the production of proton exchange membrane fuel cell electrocatalysts, and compared with other chemical methods, the prepared particles are less prone to agglomeration, have controllable size and good dispersion. This method is a promising method for nanoparticle preparation with simple equipment and process.
The role of surfactants in the preparation of nanoparticles by anti-micellar method mainly includes: (1) forming anti-micellar system to control particle size; (2) reducing particle agglomeration; (3) controlling the shape and crystal shape of particles, etc.
Second, the application in the preparation of electrode materials
The main surfactants used in the preparation of electrode materials are CTAB, sodium 2-ethylhexane sulfosuccinate, nonylphenol polyoxyethylene ether (NPE), ethylene oxide (EO) and propylene oxide (PO) triblock copolymer EO100PO70EO100, P123 (E020P070EO20), PEO600, Tween 80, Span, TritonX-100, sorbitan monooleate anhydrous, SDBS, SDS, cyclohexane and fluoroalkyl quaternary ammonium salts, oleic acid and sodium 4-styrenesulfonate.
Commonly used methods include phase transfer method, precipitation method, self-assembly method, template method, sol-gel method, etc.
1、Make positive electrode material
MnO2 with different structures and electrochemical properties can be produced using SDBS, CTAB, TritonX-100 and Brij56[C16H33 (OCH2CH2)8H]. 100 had good discharge capacity and cycling performance, while the nanoscale MnO2 cathode material made by electro-precipitation method using Brij56 as electrolyte showed good cycling performance and high discharge capacity.
P123, sodium 4-styrenesulfonate, CTAB, oleic acid and kerosene mixed melt solution can be used to prepare LiCoO2 electrode materials for Li battery. The sulfur-polypyrrole (S-PPy) composite was made by using sodium 4-styrenesulfonate as the stencil and FeCl3 as the oxidizer, and was used as the positive electrode of Li/S-PPy battery. When LiFePO4 was used as battery cathode by hydrothermal method using CTAB as template, its discharge capacity was improved, cost was reduced and toxicity was decreased. The LiFePO4/TiO2 composite electrode made of LiFePO4 and nano-scale TiO2/graphite composite material by mixed molten liquid of oleic acid and kerosene has lower energy density, almost no loss in 700 cycles, current efficiency up to 100%, durability, low cost and good safety than the conventional lithium battery.
From the above, it can be seen that surfactants play a great role in the preparation of positive electrode materials, especially the LiFePO4/TiO2 composite electrode prepared by oleic acid and kerosene mixed with molten liquid active agent has good performance.
2、Make negative electrode material
Surfactants can not only control the particle size and order the crystal arrangement, but also control the porosity and improve the electrical properties of the electrodes made.
Ulagappan et al. were the first to successfully synthesize tin-based mesoporous materials with a pore size of 3.2 nm using the cationic surfactant 2-ethylhexane sulfosuccinate as a templating agent, and the surface of CuO modified with the anionic surfactant CTAB showed an ordered needle-like crystal structure, which increased the contact area between CuO and electrolyte. The amount of CTAB can be adjusted to control the porosity of tin-phosphate material and improve the battery cycle performance.
The complex surfactant has a synergistic effect on corrosion inhibition. The Cu-Sn nanoparticles were assembled into lithium-ion battery anodes with high cycle capacity and reversible specific capacity by controlling the size of Cu-Sn particles with the dosage of TritonX-100 and n-ethanol. The non-ionic surfactant containing polyoxyethylene and indium hydroxide additive can significantly slow down the self-discharge of the battery and improve the electrochemical performance of the rechargeable alkaline manganese battery. The nano-tin dioxide/carbon composites and nano-tin oxide/carbon composites were fabricated in ethanolic solution containing P123, and showed better cycling performance compared with general nano-tin-based materials as the anode of lithium-ion batteries.
3、For organic substitute mercury corrosion inhibitor
Self-discharge of zinc electrode in alkaline manganese battery is one of the main reasons that affects the working life of the battery. Nowadays, people generally use mercury or mercury salt to suppress self-discharge. Mercury is not only highly toxic and seriously harmful to the environment, but also accelerates the deformation of the zinc electrode. Therefore, it is necessary to eliminate or replace the toxic mercury in the battery.
At present, there are two main measures for mercury-free alkaline manganese batteries: the addition of corrosion inhibitors to the electrolyte and the addition of corrosion inhibitors to the zinc cathode. The buffers used in the study are mainly inorganic corrosion inhibitors and organic mercury-substituting corrosion inhibitors, of which organic mercury-substituting corrosion inhibitors are mainly anti-corrosive surfactants.
The corrosion inhibition performance of zinc in KOH solution was investigated by six different hydrophilic chain surfactants, among which decyl(octa)oxyethylene ether phosphate potassium salt could reduce the hydrogen precipitation of zinc powder by 70.6% and the corrosion inhibition efficiency could reach 78.7%. However, it is difficult to improve the electrochemical performance of zinc electrodes with a single additive, and most researchers are now focusing on the search for efficient compounding additives with good results.
By comparing the corrosion inhibition effect of organic additives YLZX (alkyl trimethyl ammonium bromide with alkyl number from 12 to 16), YZ (alkyl phenol ethoxylate with alkyl ethoxy sulfate with alkyl number from 10 to 17), lauryl ethoxylate and alkyl ethoxy sulfate, PEO400, SDS and Tween-20 on zinc electrode, it can be seen that YLZX and YZ have obvious effect of inhibiting hydrogen precipitation and dendrite and do not affect the discharge performance of zinc electrode. SDBS and Tween-20 had excellent corrosion inhibition, delayed passivation effect and obvious synergistic effect, and the discharge capacity of zinc electrode was significantly improved, and the corrosion inhibition efficiency reached 83.26% at the optimum compounding addition.
The above is the study of corrosion inhibitor added to electrolyte on zinc electrode corrosion inhibition effect, while the study of adding in electrode material is less. The addition of SDBS and CTAB to zinc powder has been studied, and the addition of both has significantly improved the charging and discharging efficiency and capacity retention of the battery and prolonged its cycle life, but the addition of organic corrosion inhibitors to the negative electrode can reduce the self-discharge of zinc-nickel batteries, but still cannot meet the requirements of commercialization of zinc-nickel batteries.
4、Application in battery recycling
The surfactant formed an emulsion to improve the mobility of some metals. An emulsion film formed by (kerosene), surfactant (Span80), carrier and ammonia in the internal aqueous phase was used to separate and enrich cadmium ions from used nickel-cadmium batteries, and an industrial scale-up experiment was conducted in a 100-L reactor, and the migration rate of cadmium could reach 93.3%, while that of nickel was only 14.6%. This method can better achieve the separation of cadmium and nickel ions.
Surfactant can also reduce the activation energy of the interface reaction between carbon and manganese oxide, which has a great influence on the reduction rate. The waste batteries were sorted, crushed and sieved, and the manganese dioxide was made into pellets with the reducing agent carbon, sintered and reduced in an induction furnace to make manganese steel. The experimental results show that the process can be directly applied to the treatment of waste batteries in the production of cast steel.
5、Conclusion and outlook
Surfactants have important applications in batteries, which can change the crystalline shape of metal oxides, inhibit hydrogen precipitation and dendrite growth, play a role in corrosion inhibition, and act as a “micro-reaction cell” in emulsions to prevent agglomeration. The surfactants used are mainly anionic, cationic, nonionic and special surfactants, which are mainly used in proton exchange membrane fuel cells, lithium-ion batteries, alkaline manganese batteries, zinc-air batteries and nickel-cadmium batteries. The composite electrode prepared by mixed molten liquid active agent of oleic acid and kerosene is more ideal. The nano-lithium-ion battery anode made in ethanol solution containing P123 showed more excellent cycling performance, but there are still some problems.
Although surfactants have improved the cycling performance and charging/discharging efficiency of electrodes, there is still a certain distance from practical use, for example, the charging/discharging efficiency is still not above 99.5% for many electrodes.
In fact, surfactants can be used as trapping agents, inhibitors, flotation agents, etc., and the flotation method can be used to sort out metals in battery recycling, and surfactants will play a greater role in battery recycling in the future. In addition, surfactants are mainly conventional, and their cost is high, and there is too little research on new surfactants, such as new polycarboxylic acid system, naphthalene system and biosurfactants with good dispersion performance, etc. The electrical properties of battery materials have not been studied.
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