Abstract
Secondary batteries have become important for smart grid and electric vehicle applications, and massive effort has been dedicated to optimizing the current generation and improving their energy density. Multi‐electron chemistry has paved a new path for the breaking of the barriers that exist in traditional battery research and applications, and provided new ideas for developing new battery systems that meet energy density requirements. An in‐depth understanding of multi‐electron chemistries in terms of the charge transfer mechanisms occuring during their electrochemical processes is necessary and urgent for the modification of secondary battery materials and development of secondary battery systems. In this Review, multi‐electron chemistry for high energy density electrode materials and the corresponding secondary battery systems are discussed. Specifically, four battery systems based on multi‐electron reactions are classified in this review: lithium‐ and sodium‐ion batteries based on monovalent cations; rechargeable batteries based on the insertion of polyvalent cations beyond those of alkali metals; metal–air batteries, and Li–S batteries. It is noted that challenges still exist in the development of multi‐electron chemistries that must be overcome to meet the energy density requirements of different battery systems, and much effort has more effort to be devoted to this.
Highlights
Secondary batteries have become important for smart grid and electric from wind, solar, or nuclear energy, which vehicle applications, and massive effort has been dedicated to optimizing the current generation and improving their energy density
Silicon electrodes with a secondary structure inspired by pomegranate fruit[103] have been successfully fabricated to control SEI formation because it contributes to low Coulombic efficiency. These results indicate that the combination of composite structure and geometry to overcome the challenges confronting silicon anode electrodes still needs further investigation to meet high energy density requirements
Research into lithium-ion batteries has striven for multi-electron reactions, which offer the promise of improved energy density.[6]
Summary
For a given chemical reaction (1), electrochemical energy storage occurs when charge transfers.[7]. According to Equations (1–5), energy density can be improved by i) using electrode materials with high specific capacity, ii) using cathode materials with high redox potential, iii) using anode materials with low redox potential, iv) using active materials that transfer more electrons per molecule.[8a,13a] any corresponding increase in battery voltage may lead to irreversible side reactions in terms of electrolyte decomposition and unfavorable safety problems. Developing multi-electron electrode materials with smaller mole weight may be an effective approach to further increase the energy density, and the concept was first put forward by Xue-Ping Gao and Han-Xi Yang in 2010.[6] The possibility of multi-electron reaction is determined by the characteristics of active materials with a variety of chemical valences in the accessible potential window.
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