Atomic Insights into the Fundamental Interactions in Lithium Battery Electrolytes

Building high-energy-density batteries is urgently demanded in contemporary society because of the continuous increase in global energy consumption and the quick upgrade of electronic devices, which promotes the use of high-capacity lithium metal anodes and high-voltage cathodes. Achieving a stable interface between electrolytes and highly reactive electrodes is a prerequisite to constructing a safe and powerful battery, in which electrolyte regulation plays a decisive role and largely determines the long-term and rate performances. The bulk and interfacial properties of electrolytes are directly determined by the fundamental interactions and the as-derived microstructures in electrolytes. Different from experimental trial-and-error approaches, the rational bottom-up design of electrolytes based on a comprehensive and deep understanding of the fundamental interactions between electrolyte compositions and the structure-function relationship is highly expected to accelerate breaking through the bottleneck in current technology and realizing next-generation Li batteries. In this Account, we afford an overview of our recent attempts toward rational electrolyte design for safe Li batteries based on a comprehensive understanding of the cation-solvent, cation-anion, and anion-solvent interactions in electrolytes. The formation of cation-solvent complexes decreases the reductive stability but increases the oxidative stability of solvent molecules according to frontier molecular orbital theory, whereas the introduction of anions into the Li+ solvation shell has the opposite function in regulating solvent stability compared with cations. The competitive coordination of anions and solvent molecules with cations directly determines the salt solubility in electrolytes and the formation of ion pairs and aggregates, which widely exist in high-concentration electrolytes and stabilize Li metal anodes. An easy and effective route to dissolve lithium nitrate in ester electrolytes is accordingly proposed. Although anions are hardly solvated in routine solvents, solvents with a high acceptor number or an exposed positive charge site are highly expected to enhance the anion-solvent interaction. The solvation of anions will have a strong influence on electrolytes, including regulating the electrolyte solvation structure and stability, increasing the cation transference number, and promoting salt dissociation. The emerging Li bond theory and big-data approaches, combined with first-principles calculations and experimental characterizations, are also expected to promote rational electrolyte design with much reduced time and expense. Collectively, with a comprehensive and deep understanding of the fundamental interactions in electrolytes and the structure-function relationship, bottom-up engineering of Li battery electrolytes is expected to be achieved, accelerating the applications of safe high-energy-density Li batteries. The general principles demonstrated in Li batteries are also supposed to be applicable to other battery systems and even universal electrochemistry in solutions, including fuel cells and various electrocatalyses.