High Reversibility of Zinc Metal Anode for High-Energy Aqueous Batteries
Authors: Wang Fei , Wang Chunsheng , Xu Kang 
Published: February 2020

Since the invention of the first battery by Volta in 1796, metallic zinc has been regarded as an ideal anode material for the aqueous batteries systems for its high theoretical capacity (820 mAh/g), low negative potential (-0.762 V vs. SHE), abundance, low toxicity and the intrinsic safety advantages that arise from nonflammable aqueous electrolytes. Recently, rechargeable batteries using zinc metal anode have been investigated extensively.
However, an important barrier of the Zn-based batteries is the poor cycle life, which mainly derives from the drawbacks of the Zn metal anode and the electrolyte. The cyclability of the traditional alkaline Zn-based batteries is mainly restricted by dendrite growth, high solubility of discharge product (i.e. zincate) in the electrolytes, water loss from the liquid electrolyte, electrolyte depletion caused by the narrow electrochemical window. In most previous studies, the zinc-based aqueous batteries suffered from low columbic efficiency (CE) even using the high rate to minimize the side reaction, which means the Zn-based batteries usually require regular topping up with electrolyte. Significant excessive zinc has to been used to keep the cycle stability, results in sub-optimal utilization of the zinc theoretical capacity, as in the case of the lithium metal anode. The goal of achieving high CE in aqueous zinc metal batteries remained allusive.
The poor reversibility and low CE of the Zn metal anode is closely related with the Zn (II) cation solvate structure in the aqueous electrolyte. The hydration effects of the Zn (II) cation in water is so significant that the zinc hydroxide is easily forms. The slight but non-ignorable water decomposition caused by the narrow stability window produces more hydroxyl ion and certainly aggravates the formation of zinc hydroxide. Zinc hydroxide converts into insoluble zinc oxide (ZnO) when the solubility limit of the hydroxide species is reached. Formation of solid ZnO can be a difficult process to reverse during recharge, as it relies on the resolubilization of the Zn species back into the electrolyte prior to reduction.
We report the development of a highly concentrated neutral electrolyte that alters Zn(II) solvation structure resulting in the dendrite-free plating of Zn metal with high CE. The suppression of the zinc hydration was achieved through the formation of [Zn-TFSI] solvation structure instead of the [Zn-6H2O]2+ solvation structure. The solvation structure change is ascribed to the introduction of the TFSI- anion, which has strong coordination to the Zn (II) cation in concentrated electrolytes. The solvation structure change was investigated via a combination of IR spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, density functional theory (DFT) calculations and molecular dynamics (MD) simulations using polarizable force fields. We demonstrate an exceptional performance of zinc metal cells containing Zn-based aqueous electrolyte that delivered an unprecedented high practical energy density of 300Wh/kg (based on both the cathode and anode electrodes). This study opens an avenue for the highly efficient utilization of zinc metal electrodes for advanced energy storage applications while the fundamental knowledge gained can also be applied to other metal anodes.