Lithium metal anodes enable substantially higher energy density than current technologies for Li batteries. However, rechargeable Li metal anodes suffer from low Coulombic efficiency (loss of electrochemically active Li), leading to poor cycle life and safety. Engineering the electrolyte formulation to form a stable, well-functioning solid electrolyte interphase (SEI) is a promising approach to improving these performance figures of merit. While design rules have been established for selecting electrolyte solvents and salt anions to establish a more robust SEI, the impact of altering cation identity is not well understood. In this work, we demonstrate that alkali metal additives (here, K+) alter SEI composition and thickness. Through post-mortem elemental analyses, we show that K+ ions do not directly participate in metal electrodeposition, but rather modify the chemical and electrochemical reactivity of the electrode-electrolyte interface. Through a combination of quantitative nuclear magnetic resonance (NMR) spectroscopic characterization and density functional theory (DFT) simulations, we show that decomposition of electrolyte solvent molecules, ethylene carbonate (EC) and dimethyl carbonate (DMC), at the lithium metal surface is suppressed in the presence of a K+ additive. We attribute this to K+ being a softer cation compared to Li+, leading to preferred pair formation between K+ and the soft base carbonates, and thus increased salt-solvent coordination. Electrolyte cation engineering is an underexplored strategy to control the SEI, and we believe that the mechanistic understanding and insight developed in this work will spur further investigation of this promising approach.