Regulating the space charge at interface by negative polar biomolecule enabling ultrastable Zn anode chemistry

• The negative xanthan gum can reduce space charge near the anode-electrolyte interface and form H 2 O-poor electrical double layer to regulate the Zn deposition behaviors. • The Zn//Zn symmetric cells in ZnSO 4 electrolyte with xanthan gum deliver an ultra-long cycle life (>4700 h). • Xanthan gum can break the association between H 2 O and Zn 2+ to reconstruct solvated structures of Zn ions to suppress HER. The irreversibility of Zn metal anode arising from uncontrollable dendrite growth and hydrogen evolution reaction, especially at high current densities, has challenged the commercialization application of aqueous Zn-ion batteries. Here we report biomolecule additive, named xanthan gum, to solve these issues through making use of dissociated electronegative groups to reduce space charge near the anode-electrolyte interface, tuning solvation structure and form H 2 O-poor electrical double layer to regulate the Zn deposition behaviors. Meanwhile, the dissolved xanthan gum with multiple polar groups can break the association between H 2 O and Zn 2+ to reconstruct solvated structures of Zn ions to suppress hydrogen evolution reaction (HER). Therefore, the Zn//Zn symmetric cells in ZnSO 4 electrolyte with xanthan gum deliver an ultra-long cycle life (>4700 h), and also present excellent cycling performances in other electrolytes (Zn(OAc) 2 and Zn(OTf) 2 ). More importantly, a high coulombic efficiency of 99.7 % is achieved using Zn//Cu half cells tested and excellent cycling life for KVO//Zn full cells. This work provides a guidance to design the electrolyte additive for highly reversible Zn metal batteries.