Mechanistic\nUnderstanding of Long Duration Fast Charge\nAqueous Zinc Batteries Using Physically Adsorbed Oligomers as Interphases

Polymers\nhave been used as additives in the liquid electrolytes\ntypically used for secondary batteries that utilize metals as anode.\nSuch additives are conventionally argued to improve long-term anode\nperformance by suppressing morphological and hydrodynamic instabilities\nthought to be responsible for out-of-plane and dendritic metal deposition\nduring battery charging. More recent studies have reported that the\npolymer additives provide even more fundamental mechanisms for stabilizing\nmetal electrodeposition through their ability to regulate metal electrodeposit\ncrystallography and, thereby, morphology. Few studies explore how\npolymers carried in a liquid electrolyte achieve these functions,\nand fewer still provide rules for choosing among the various polymer\ntypes, the additive polymer molecular weight (<i>M</i><sub>w</sub>), and concentration in the electrolyte. Here, we investigate\nhow these generally <i>easy-to-control</i> variables influence\nelectrochemical interphase formation inside battery cells and their\nimpact on the morphology and reversibility of Zn electrodes in aqueous\nelectrolytes. We focus on aqueous Zn-iodine electrochemical cells\ncontaining linear polyethylene glycol (PEG) chains as additives and\nfind that in electrolytes where the polymer concentration is maintained\nin the dilute solution regime there is an optimum polymer molecular\nweight (<i>M</i><sub>w</sub> ≈ 1000 Da), above which\nbeneficial effects of polymers on Zn electrode reversibility and Zn–I<sub>2</sub> battery lifetime are progressively lost. By means of optical\nellipsometry and theoretical calculations, we show that the optimal <i>M</i><sub>w</sub> is associated with saturation of the thickness\nof a physiosorbed PEG coating on the Zn metal electrode. Electron\nmicroscopy and X-ray photoelectron spectroscopy analysis of Zn electrodeposits\nformed in such electrolytes reveal that the physiosorbed polymer coating\nhas two primary effectsit regulates the deposit morphology\nand suppresses parasitic reactions between the electrode and electrolyte\ncomponents. The parasitic reactions produce species like ZnO, which\nare known to passivate the Zn electrode and promote nonuniform deposition.\nGalvanostatic cycling measurements in aqueous Zn–I<sub>2</sub> cells containing the PEG additives at the optimal <i>M</i><sub>w</sub> show that the cells maintain very high Coulombic efficiencies\n(≥99%) at current densities as high as 50 mA/cm<sup>2</sup>close to the maximum values permissible across the Celgard\nseparator membranes used in our studies.