Ion transport modulations by alkyl chain length control of quaternary ammonium additives enabling stable cycling of aqueous Zn batteries

The key challenges of Zn metal anode in aqueous batteries include dendritic growth and hydrogen evolution reaction (HER). Herein, quaternary ammonium electrolyte additives are rationally designed to synergistically modulate the transport behaviors of different ions and solve these issues. With appropriate hydrophobic alkyl chain length, butyltrimethylammonium (C 4 ) functions as an effective shielding agent to isolate water into smaller clusters in the electrolyte. It disrupts the continuous hydrogen bonding network and blocks the facile proton conduction via the Grotthuss mechanism, thereby inhibiting HER. Meanwhile, the positively charged nitrogen head attracts anions and slows down their transport. It results in almost doubled Zn 2+ transference number and ensures sufficient cation supply to the interface. These effects enable dense and uniform morphology during both plating and stripping processes with the coulombic efficiency reaching 99.88 %. More importantly, the C 4 additive also largely extends the cycling stabilities of full cells with a commercial MnO 2 cathode, i.e. 86.9 % capacity retention after 700 cycles with N/P = 3 and 92.5 % capacity retention after 380 cycles with anode-free configuration. They are superior to sudden and rapid capacity drop after only 360 and 50 cycles, respectively, in the baseline electrolyte.