Dissolved lithium activity confines the layered-to-rock-salt transition while preserving residual lithium removal in high-nickel positive electrode washing.

Aqueous washing is necessary for high-nickel layered positive electrodes, yet the medium that removes residual LiOH and Li2CO3 also draws lithium from the lattice and converts the near-surface layered phase into an inactive rock-salt structure. Whether this conversion protects or harms the positive electrode is settled within a few nanometers, since a thin layer shields the surface while an overgrown one blocks lithium transport. We propose a chemical-equilibrium description of washing in which the activity of dissolved lithium sets the position of the interfacial Li+/H+ exchange, and the binding of water into Li+ hydration shells lowers the activity of free water available for proton transfer, together governing the thickness of the reconstructed layer. In LiNi0.90Co0.05Mn0.05O2, a lithium-rich medium displaces this equilibrium toward the reactant side and suppresses lattice lithium extraction without compromising residual lithium removal, an outcome inaccessible to a concentration-gradient description. This control holds the reconstructed layer within the protective range, whereas water washing drives it beyond and accelerates capacity fade. The lithium-rich medium yields a positive electrode that cycles above the pristine reference in graphite full cells, rendering the reconstructed surface phase a tunable variable rather than an inevitable degradation product.