Topological and Physicochemical Origins of the Preferential Lithium Solvation of Organic Additives in Aqueous Electrolytes

Aqueous lithium-ion batteries (LIBs) have gained attention due to their superior power and safety performance compared with conventional organic electrolyte-based LIBs. However, the battery reversibility of pure water electrolytes is limited. Modifying the Li⁺ solvation structure using organic additives holds significance to optimize the overall performance, leading to changes in ion deposition, the formation of a favorable solid-electrolyte interphase, and the electrochemical stability window. The first step toward such a modification is to identify the preferential lithium solvation of additive molecules. In this study, molecular dynamics simulations combining the graph neural networks technique were used to quantify the preferential solvation of small organic molecules to replace water molecules in the Li⁺ solvation shells. The analysis of millions of small-molecule additives reveals that those containing nitrogen-containing functional groups, particularly amino groups, effectively reduce the water coordination number of Li⁺ (WCNLi). Introducing oxygen-containing groups to nitrile/amine-containing molecules enhances this effect. Quantum chemistry calculations show that oxygen-nitrogen synergy increases molecular polarity, further lowering WCNLi. Further comprehensive interpretable analysis reveals two principles for an additive's ability to reduce WCNLi. The first is the atom group such as carbonyl and amino with strong binding affinity with Li⁺ that direct coordinate Li⁺; the second is the atom group such as amine and hydroxyl acting as hydrogen bond donors to engage the surrounding water network, which indirectly weakens the Li⁺-water interaction. This study provides significant insights into additive molecule screening and design for aqueous systems.