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

Nonaqueous lithium–carbon dioxide (Li-CO 2 ) batteries exhibit great potential for energy storage but are limited by inherently low rate capability due to sluggish CO 2 reduction kinetics arising from insufficient three-phase interfaces and product passivation layers at the electrode–electrolyte boundary. Aqueous Li–CO 2 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 CO 2 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 CO 2 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 CO 2 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–CO 2 batteries.