Calcium metal batteries are receiving growing research attention due to significant breakthroughs in recent years that have indicated reversible Ca plating/stripping with attractive Coulombic efficiencies (90-95%), once thought to be out of reach. While the Ca anode is often described as being surface filmcontrolled, the ability to access reversible Ca electrochemistry is highly electrolyte-dependent in general, which affects both interfacial chemistry on plated Ca along with more fundamental Ca 2+ /Ca redox properties. This mini-review describes recent progress towards a reversible Ca anode from the point of view of the most successful electrolyte chemistries identified to date. This includes, centrally, what is currently known about the Ca 2+ solvation environment in these systems. Experimental (physico-chemical and spectroscopy) and computational results are summarized for the two major solvent classescarbonates and ethers-that have yielded promising results so far. Current knowledge gaps and opportunities to improve fundamental understanding of Ca 2+ /Ca redox are also identified.

Discovery of new electrochemical redox motifs is essential to expand the design landscape for energy-dense batteries. We report a family of fluorinated reactants based on pentafluorosulfanyl arenes ( R-Ph-SF 5 ) that allow for high electron-transfer numbers (up to 8-e − /reactant) by exploiting multiple coupled redox processes, including extensive S–F bond breaking, yielding capacities of 861 mAh·g reactant −1 and voltages up to ∼2.9 V when used as catholytes in primary Li cells. At a cell level, gravimetric energies of 1,085 Wh·kg −1 are attained at 5 W·kg −1 and moderate temperatures of 50 °C, with 853 Wh·kg −1 delivered at >100 W·kg −1 , exceeding all leading primary batteries based on electrode + electrolyte (substack) mass. Voltage compatibility of R-Ph-SF 5 reactants and carbon monofluoride (CF x ) conversion cathodes further enabled investigation of a hybrid battery containing both fluorinated catholyte and cathode. The hybrid cells reach extraordinarily high cell active mass loading (∼80%) and energy (1,195 Wh·kg −1 ), allowing for significant boosting of substack gravimetric energy of Li−CF x cells by at least 20% while exhibiting good shelf life and safety characteristics.

Fundamental research and practical assembly of rechargeable calcium (Ca) batteries will benefit from an ability to use Ca foil anodes. Given that Ca electrochemistry is considered a surface-film-controlled process, understanding the interface’s role is paramount. This study examines electrochemical signatures of several Ca interfaces in a benchmark electrolyte, Ca(BH4)2/tetrahydrofuran (THF). Preparation methodologies of Ca foils are presented, along with Ca plating/stripping through either pre-existing, native calcium hydride (CaH2), or pre-formed calcium fluoride (CaF2) interfaces. In contrast to earlier work examining Ca foil in other electrolytes, Ca foils are accessible for reversible electrochemistry in Ca(BH4)2/THF. However, the first cyclic voltammetry (CV) cycle reflects persistent, history-dependent behavior from prior handling, which manifests as characteristic interface-derived features. This behavior diminishes as Ca is cycled, though formation of a native interface can return the CV to interface-dominated behavior. CaF2 modification enhances such interface-dominance; however, continued cycling suppresses such features, collectively indicating the dynamic nature of certain Ca interfaces. Cell configuration is also found to significantly influence electrochemistry. With appropriate preparation of Ca foils, the signature of interface-dominated behavior is still present during the first cycle in coin cells, but higher current density compared to three-electrode cells along with moderate cycle life are readily achievable.

Learning how to tailor Ca 2+ speciation and electroactivity is of central importance to engineer next-generation battery electrolytes. Using an exemplar dual-salt electrolyte, Ca(BH 4 ) 2 + Ca(TFSI) 2 in THF, this work examines how to modulate a critical parameter proposed to govern electroactivity, the BH 4 − / Ca 2+ ratio. The introduction of a more-dissociating source of Ca 2+ via Ca(TFSI) 2 drives respeciation of strongly ion-paired Ca(BH 4 ) 2 , confirmed by ionic conductivity, Raman spectroscopy, and reaction microcalorimetry measurements, generating larger populations of charged species and enhancing plating currents. Ca plating is possible when [Ca(TFSI) 2 ] < [Ca(BH 4 ) 2 ], and thus, BH 4 − /Ca 2+ > 1, but a dramatic shutdown of plating activity occurs when [Ca(TFSI) 2 ] > [Ca(BH 4 ) 2 ] (BH 4 − /Ca 2+ < 1), directly evidencing the significance of coordination-shell chemistry on plating activity. Ca(BH 4 ) 2 + TBABH 4 in THF, which enables enrichment of BH 4 − concentrations compared to Ca 2+ , is also examined; ionic conductivity and plating currents also increase compared to Ca(BH 4 ) 2 /THF, with the latter related in part to a decrease in solution resistance. These findings delineate future directions to modulate Ca 2+ coordination toward achieving both high plating activity and reversibility.

Lithium (Li) metal is a compelling replacement for graphite anodes in Li-ion batteries to increase gravimetric energy if the cyclability can be improved. Motivated by high Li Coulombic efficiency (CE) achieved with electrolytes featuring the bis(fluorosulfonyl)imide (FSI − ) anion, this work examined chemically related sulfonyl/sulfamoyl fluoride additives to correlate FSI − -relevant structural features with CE. Across three exemplary carbonate-and glyme-based electrolytes, extended solid, liquid, and gas phase characterizations reveal that Li + coordination is necessary yet insufficient for FSI − derivatives to affect cycling. Beyond coordination, the reactivity of the baseline solvent and the key structural features of the additives are shown to strongly regulate CE, with possession of an N center�common to sulfamoylfluorides and FSI − �consistently leading to higher CE. Some derivatives outperform FSI − in short-term cycling; however, they have difficulty competing with the longevity of FSI − . These results provide insights for developing improved additives in the future through careful consideration of reactant structure and solvent codesign.