New spotlight for electrolyte design: Anion chemistry

Researchers spotlighted anion chemistry in high-energy-density battery electrolytes, delving into the molecular-level and interfacial engineering perspectives to elucidate the role of anions in electrolyte transport dynamics and interfacial kinetics. The progress and limitations of current characterization techniques for anion chemistry in electrolytes were also discussed. This review underscores the critical importance of anion chemistry in electrolyte design, providing a theoretical foundation and unique perspective for developing electrolytes for next-generation high-energy-density batteries.

This review systematically elucidates the core fundamentals of anion chemistry in electrolytes. The three principal interactions among electrolyte components—Coulombic attraction between anions and cations, ion-dipole interactions or hydrogen bonding between anions and solvents, and dipole-dipole interactions between solvents were analyzed. The changes in redox properties and lithium-ion transport dynamics following anion-regulated solvation structures are also discussed in detail. After anions participate in the solvation structure, their own reduction stability decreases, while that of the solvent increases. Simultaneously, the oxidation potential of anions is lower than that of the solvent, causing them to undergo preferential oxidation decomposition during charging and discharging. This characteristic lays the foundation for the formation of an inorganic-rich electrode/electrolyte interphase.

Based on the core coordination mechanism described above, researchers have summarized strategies for achieving anion-modulated solvation structures. These include salt design (optimization of traditional salts like LiNO₃ and novel salts like LiCTFSI), solvent-anion interaction regulation (hydrogen bonding/ion-dipole interaction), and anion acceptors—emerging methods enabling precise control over anion solvation structures.

Anion chemistry significantly enhances battery interface stability by governing electrode/electrolyte interface reactions. At the anode interface, the lower LUMO of anions prioritizes reduction on anode surfaces like lithium metal and silicon, forming an inorganic-rich SEI layer. This interphase layer exhibits high mechanical strength and excellent lithium-ion conductivity, effectively suppressing lithium dendrite growth and solvent decomposition. At the cathode interface, anions with higher HOMO preferentially oxidize on high-voltage cathode surface, contributing to the formation of an inorganic-rich CEI layer. This layer blocks direct solvent contact with cathode materials, inhibiting solvent decomposition and transition metal ions leaching.

Finally, the discussion addresses the current progress and limitations of characterization techniques available for studying anion chemistry in electrolytes. Overall, existing technologies still face the core challenge of insufficient spatiotemporal resolution, making it difficult to capture picosecond-scale dynamic evolution and nanometer-scale long-range structural information in real time.

Anion chemistry has fundamentally transformed the traditional paradigm of electrolyte design, elevating anions from “passive counterions” to “active regulators”. By precisely controlling the structure and properties of anions, combined with novel electrolyte system design and solvent environment optimization, a comprehensive balance will be achieved between electrolyte ion dynamics, temperature adaptability, and interfacial stability.