Anode materials for lithium-air batteries
At present, most lithium-air battery researches focus on the oxygen electrode reaction of the positive electrode, and the side reactions caused by the extremely strong reducibility of the metal lithium negative electrode make the chemical and electrochemical reactions in the lithium-air battery more complicated. Because both the electrolyte and the oxygen diffused from the positive electrode will react with the metallic lithium. At the same time, the side reaction products generated on the lithium negative electrode will also diffuse to the positive electrode side, interfering with the oxygen reaction of the positive electrode. In addition, lithium dendrites may be formed on the lithium negative electrode, which reduces the safety performance of the battery, thereby hindering the practical progress of lithium-air batteries. With the increase of the number of cycles of the battery, the SEI film is continuously generated and decomposed, and the coulombic efficiency of the battery is continuously decreased due to the continuous consumption of metal lithium and electrolyte. Therefore, the improvement of the stability of the metal lithium anode directly determines whether the high specific energy density of the lithium-air battery can be achieved. Simply using lithium metal as the negative electrode material obviously cannot meet the demand for long cycle and high stability of lithium-air batteries. The measures to solve the safety and stability of lithium anode mainly include the following aspects:
(1) Alternative counter/reference electrodes. Due to the above problems of lithium metal negative electrodes, the most direct way is to ban them. For example, lithium iron phosphate (LiFePO4) is used to replace metal lithium as the negative electrode of lithium-air batteries, so as to avoid side reactions between metal lithium and electrolyte, and purify the reaction system of lithium-air batteries. Similar to it is Li4Ti5O12.
Although this method of replacing the metal negative electrode with lithium ion intercalation chemicals reduces the occurrence of side reactions between the negative electrode and the electrolyte to a certain extent, it also greatly reduces the working voltage of the battery, which is not conducive to the application and development of the battery. Another option is to use lithium alloy composite electrodes, such as LixSi, LixSn and LixAl electrodes. Although the working voltage of the battery has been improved, the cycle performance degradation and O2 stability caused by the volume expansion effect of metal alloy materials need to be further improved.
(2) Solid electrolyte interface (SEI) modification. During the working process, the lithium metal negative electrode will spontaneously react with the electrolyte to form a solid electrolyte interface layer (Solid Electrolyte Interphase, SEl) mixed with inorganic and organic compounds on the surface of the electrode. The SEI has the characteristics of electronic insulation and ionic conductivity, and the SEI with stable electrochemical and mechanical properties can effectively prevent the occurrence of side reactions between the metal lithium anode and the electrolyte, inhibit the growth of lithium dendrites, and improve the Coulombic efficiency of the anode. For this reason, constructing an in-situ SEI film on the surface of the lithium anode has become an important measure to protect the lithium anode. At present, new supporting electrolytes (mostly fluorine-containing electrolyte salts), additives (such as LiNO3, LiOH, AlI3, CsI, fluoroethylene carbonate, vinylene carbonate, N,N-dimethyltrifluoroethyl), high-concentration lithium salt electrolyte systems, etc. are mainly used to improve the stability of solid electrolyte membranes. In addition, in the lithium-air battery, the metal lithium negative electrode is also required to be inert to oxygen. To this end, the constructed SEI film should also have the function of resisting the chemical corrosion of O2 and the superoxide oxides of the discharge reaction product, which puts forward higher requirements for improving the stability of the metal anode.
(3) Composite protective layers (CPLs). In addition to the in-situ growth of SEI films, artificial composite protective layers (GPLs) can also be constructed on the lithium anode. Such protective layers have certain mechanical strength and flexibility, can store liquid electrolytes and provide transport channels for lithium ions, and can also slow down the shuttle effect caused by O2 in the electrolyte or the redox mediator, so as to protect the negative electrode. Their composition is mainly composed of polymers and ceramic fillers, such as PVDF-ZrO2, PVDF-Al2O3, etc. In addition, the use of an oxygen-inert solid electrolyte to completely coat lithium metal is expected to solve the problem of limited interfacial ion conduction and promote the application and development of lithium anodes.