As the “blood” of lithium-air batteries, the electrolyte plays a crucial role in the success or failure of the battery. In this post, the organic electrolyte systems and solid electrolyte systems commonly used in lithium-air batteries are introduced.
(1) Organic electrolyte system. Usually, the organic electrolyte mainly contains two parts, an organic solvent and a supporting electrolyte. However, the current instability of organic solvents is a major bottleneck in the development of lithium-air batteries. The ideal organic electrolyte should have the following characteristics: stable to superoxide ions, not reacting with any oxygen-reducing substances; high chemical and electrochemical stability, wide chemical window, and able to withstand higher charging voltages; it has good compatibility with lithium metal; low volatility, low viscosity, and high oxygen solubility; low price, etc.
In the early stage of the development history of lithium-air batteries, carbonate solvents, such as propylene carbonate, ethylene carbonate and dimethyl carbonate, are widely used as electrolyte solvents because of their low boiling point, good solubility for lithium salts and high conductivity. However, recent studies have found that ester electrolytes can react with oxygen reduction intermediates in Li-air batteries, accompanied by the production of many by-products, which seriously affects the reversibility of the batteries. And the catalytic activity exhibited by the catalysts in the early studies in the lithium-air battery may be an artifact caused by the catalyst promoting the decomposition of the electrolyte.
Sulfone electrolytes, such as dimethyl sulfoxide (DMSO), etc.; amine electrolytes, such as N,N-dimethylacetamide (DMA), dimethylformamide (DMF), etc.; nitrile electrolytes, such as acetonitrile (CH3CN), trimethylacetonitrile (TMA), etc., the above types of electrolytes have the characteristics of low boiling point, low viscosity, and good solubility for oxygen and lithium salts. As the electrolyte of lithium-air battery, the high DN value (29.8) and strong solvation ability can promote lithium-air battery solution phase discharge and effectively reduce charge-discharge overpotential. However, the above-mentioned electrolytes all react with lithium metal, and the metal anode cannot be directly used to assemble the electrolyte. And a new type of electrolyte system, hexamethylphosphoric triamide (HMPA), has the highest DN value (38.8) and strong solvation ability, and assembled with LiPON-protected lithium negative electrode to form a battery with extremely low charge-discharge overpotential, which is a very potential electrolyte system for lithium-air batteries.
Ether electrolytes, such as tetraethylene glycol dimethyl ether (TEGDME), ethylene glycol dimethyl ether (DME), etc., have good compatibility with lithium anodes and do not react. The oxidation potential window is high (>4.5V), relatively stable to oxygen reduction intermediates, and the main product of the battery reaction is lithium peroxide. However, with the in-depth research, it is found that the decomposition of ether electrolyte will become more and more serious with the battery cycle.
In addition to the solvent, lithium salts also play a pivotal role as an indispensable component of the electrolyte. A good lithium salt has a certain solubility to support the rapid transportation of lithium ions. At the same time, it has good compatibility with solvents, discharge products and intermediate products and other parts of the battery. Among the commonly used lithium salts, LiBF4, LiPF6 and lithium trifluoromethanesulfonic acid imide (LiFSI) will decompose after the battery is charged and discharged, while LiClO4, LiCFSF3, and LiTFSI are relatively stable and have excellent performance.
(2) Solid electrolyte system. Solid-State Electrolyte (SSE) has attracted extensive attention in the battery industry due to its extremely high safety. Compared with organic electrolyte systems, solid electrolytes have obvious advantages: high mechanical strength, can effectively inhibit the growth of lithium dendrites; effectively inhibit the self-discharge behavior caused by the shuttle effect of chemically active substances; reduce the polarization phenomenon caused by the change of ion solubility gradient; high temperature resistance; no leakage, etc.
The ceramic solid electrolytes (Ceramic Solid Electrolyte, CSE) that have been tried in the lithium-air battery system include Li-Al-Ge-PO4 (LAGP), Li-Al-Ti-PO4 (LATP). Most Li solid electrolytes and metallic Li interfaces are thermodynamically unstable. For this reason, Li-ion conductors such as Li3N, LiPON, and PEO are used as protective layers to isolate CSEs and Li metal anodes. Most of the polymer electrolytes use PVDF, PEO, PMMA and other mixed ceramic fillers such as Al2O3, TiO2, SiO2 and ZrO2. The use of these solid electrolytes improves the cycle performance of the battery to a certain extent. However, all-solid-state Li-air batteries still need to solve the contact problem at the solid-solid interface, which is still a great challenge.
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