Modification and regulation of electrode/electrolyte interface for high specific energy and long life lithium ion batteries

In recent years, with the rapid developments of high-technology industries such as information technology and electric vehicles, there are urgent need to develop new generations of lithium-ion batteries with higher energy density, longer cycle life and improved safety. In addition to the development of high specific energy cathode and high specific capacity anode materials, regulating the stability of the electrode/electrolyte interface is critical to achieve and balance various performances of the batteries and finally realize their commercial application widely. However, the traditional carbonate electrolytes not only suffer from severe oxidation decomposition when the charging voltage is higher than 4.3 V (vs. Li/Li⁺), but also show poor compatibility with high-capacity silicon or silicon-carbon (Si-C) composite anodes and lithium metal anodes. Moreover, the traditional carbonate electrolytes are flammable, which is still a big safety concern to address for lithium ion battery. Therefore, rational design of electrolytes that match the high voltage cathode, high specific capacity Si-C abode or lithium metal anode, and has better safety property and especially under harsh conditions, has become a decisive factor for the rapid developments of high specific energy lithium-ion batteries. The present work reviews different kinds of liquid electrolytes with functional additives and utilization of solid state electrolytes which are explored by our group in the past 15 years. The design strategies and systematic investigations of the electrolytes recipes include: (1) Developing anti-oxidation solvent systems, for example, applications of new high-voltage nitriles or fluorinated solvents for high-voltage cathodes, such as suberonitrile (SUN), fluoroethylene carbonate (FEC) and ethyl-(2,2,2-trifluoroethyl) carbonate (ETFEC); (2) exploiting some novel multifunctional solvents or additives which contain flame retardant groups or can be used as film-forming agents for high-voltage cathodes. For example, some P-containing compounds such as N,N-diallyl-diethoxy phosphoramide (DADEPA), phenoxy cyclophosphazene (PFPN), and ionic liquid such as N-methyl-N-butylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Py14TFSI), etc. In addition, we have also investigated a series of solid electrolytes such as Garnet-type electrolytes, gel polymer or composite polymer electrolyte with inorganic additives, which could greatly reduce the safety risk of batteries. These works include measurements of the activation energy and ion transport mechanism of oxide solid electrolyte with solid NMR techniques, interface modification of sulfide-type solid state battery with ionic liquids, and interface passivation mechanism of PEO-based polymer electrolyte, etc. Finally, some discussions and future perspectives in developing high-voltage and highly-safe liquid electrolyte and flexible solid-state electrolytes for all solid state batteries are presented. Although the commonly used high-voltage electrolytes are mainly composed of lithium salts, anti-oxidation solvents and functional additives such as nitriles, sulfones, fluoro-ethers or fluoro-carbonates, However, the acting mechanisms of those high-voltage electrolytes, especially solvents, salts and additives, are still not clear, especially the composition, structure and evolution of the electrochemical interfaces are lack of quantitative understanding. Therefore, combining theoretical calculation and advanced in-/ex-situ interface characterization techniques, fully understanding the working mechanism of the additives in high-voltage systems and the effective components of interface film, and designing novel functional electrolyte additives are crucial for the development of a new generation of high-specific energy lithium batteries. In addition, applications of flame-retardant solvents or additives to develop flame-retardant liquid-type electrolytes and developing new solid state electrolytes are both important strategies to reduce the potential safety hazards of lithium ion batteries. The future research should focus on how to improve the compatibility between flame-retardant solvents/additives and the cathode/anode interfaces in liquid electrolytes, minimize the negative impact on battery performance (cycle performance, rate performance, etc.), enhance the ionic conductivity and mechanical flexibilities of the solid electrolytes, optimize the electrolyte phase structure/mechanical properties, and regulate the stability of interfaces to ensure the long-term cycling stability of lithium metal anodes and metal oxide cathodes.