Concentration-Tuned Reduction Pathway for Aqueous Lithium–Carbon Dioxide Batteries

Nonaqueous lithium-carbon dioxide (Li-CO₂) batteries exhibit great potential for energy storage but are limited by inherently low rate capability due to sluggish CO₂ reduction kinetics arising from insufficient three-phase interfaces and product passivation layers at the electrode-electrolyte boundary. Aqueous Li-CO₂ systems, leveraging gas-liquid-solid three-phase interfaces for enhanced mass transfer, offer a promising solution to mitigate kinetic bottlenecks and improve rate performance. However, the impact of the electrolyte concentration on reaction selectivity and rate-dependent behavior in such systems remains unexplored. Herein, we systematically investigate the effect of lithium bis((trifluoromethyl)sulfonyl)azanide (LiTFSI) concentration (1-21 M) on the solvation structure and electrochemical performance during the discharge process. The solvation environment is revealed to transition from a free water-dominated salt-in-water structure at low concentrations to a compact ionic cluster water-in-salt structure at high concentrations. At low salt concentrations, disordered three-dimensional deposition of CO₂ reduction products and significant side reactions such as hydrogen evolution result in low electron utilization (∼15%). In contrast, the 21 M electrolyte induces the formation of a dense 2D product layer, enhancing the CO₂ reduction efficiency while effectively suppressing parasitic reactions. This concentration-dependent modulation of the solvation microenvironment reduces interfacial impedance, limits free water activity, and improves Li⁺ transport kinetics, thereby shifting the reaction pathway from side-reaction-dominated to highly selective CO₂ conversion. This work highlights electrolyte concentration as a critical knob to optimize three-phase interface dynamics, offering mechanistic insights to overcome rate-limiting barriers in both aqueous and nonaqueous Li-CO₂ batteries.