Reversible adsorption with oriented arrangement of a zwitterionic additive stabilizes electrodes for ultralong-life Zn-ion batteries

Abstract

The unstable electrode/electrolyte interface with inhomogeneous Zn deposition and side reactions plagues the practical application of aqueous Zn-ion batteries. Herein, L-carnitine (L-CN) is proposed to stabilize both electrodes and extend the lifespan as an efficient additive. Simultaneous quaternary ammonium cations, COO⁻ anions and hydroxyl groups in a trace amount of added L-CN have a huge synergy in manipulating the behaviors of Zn²⁺ electrochemical deposition/insertion and water molecule activity. Specifically, to accommodate the negative charge on electrodes driven by the electric field, the cationic portion in L-CN automatically orients toward the corresponding electrode surfaces (anode for charging and cathode for discharging), occupying active water sites and realizing reversible adsorption interfaces. The carboxyl anion provides a prominent interaction with Zn²⁺, regulating the Zn²⁺ flux to ensure uniform and fast charge transfer kinetics. Hydroxyl groups effectively capture free-water and coordinated-water through hydrogen bond fixation, thus suppressing side reactions and the cathode dissolution. Noticeably, a trace amount of L-CN additive actualizes ultralong life in the symmetric cell with an 87-fold improvement in cycle life (over 6000 h, 1 mA cm⁻²/1 mA h cm⁻²) for dendrite-free Zn plating/stripping and enables the Zn//V₂O₅ full cell to achieve 3500 cycles with a high capacity retention.