Highly Concentrated Salt Electrolyte for a Highly Stable Aqueous Dual-Ion Zinc Battery.

A zinc metal anode for zinc-ion batteries is a promising alternative to solve safety and cost issues in lithium-ion batteries. The Zn metal is characterized by its high theoretical capacity (820 mAh g-1), low redox potential (0.762 V vs SHE), low toxicity, high abundance on Earth, and high stability in water. Taking advantage of the stability of Zn in water, an aqueous Zn ion battery with low cost, high safety, and easy-to-handle features can be developed. To minimize water-related parasitic reactions, this work utilizes a highly concentrated salt electrolyte (HCE) with dual salts─1 m Zn(OTf)2 + 20 m LiTFSI. MD simulations prove that Zn2+ is preferentially coordinated with O in the TFSI- anion from HCE instead of O in H2O. HCE has a broadened electrochemical stability window due to suppressed H2 and O2 evolution. Some advanced ex situ and in situ/in operando analysis techniques have been applied to evaluate the morphological structure and the composition of the in situ formed passivation layer. A dual-ion full Zn||LiMn2O4 cell employing HCE has an excellent capacity retention of 92% after 300 cycles with an average Coulombic efficiency of 99.62%. Meanwhile, the low concentration electrolyte (LCE) cell degrades rapidly and is short-circuited after 66 cycles with an average Coulombic efficiency of 96.91%. The battery's excellent cycling performance with HCE is attributed to the formation of a stable anion-derived solid-electrolyte interphase (SEI) layer. On the contrary, the high free water activity in LCE leads to a water-derived interfacial layer with unavoidable dendrite growth during cycling.

Tuning Dynamically Formed Active Phases and Catalytic Mechanisms of In Situ Electrochemically Activated Layered Double Hydroxide for Oxygen Evolution Reaction.

The active phase and catalytic mechanisms of Ni-based layered double hydroxide (LDH) materials for oxygen evolution reaction (OER) have no common consensus and remain controversial. Moreover, engineering the site activity and the number of active sites of LDHs is an efficient approach to advance the OER activity, as the thickness and stacking structure of the LDHs layer limit the catalytic activity. This work presents an interesting in situ approach of tuning the site activity and number of active sites of NiMn-LDHs, which exhibit the superior OER performance (onset overpotential of 0.17 V and overpotential of 0.24 V at 10 mA cm-2). The fundamental mechanistic insights and active phases during the OER process are characterized by in operando techniques along with the computational density functional theory calculations, revealing that the Ni site constitutes the OER activity and the dynamically generated NiOOH moiety is the active phase. We also prove that Ni sites undergo a reversible oxidation state under the working conditions to create active NiOOH species which catalyze the water to generate oxygen. These findings suggest that the Ni(III) phase in NiMn-LDHs is the OER active site and Mn promotes the electronic properties of Ni sites. Utilizing in situ/in operando techniques and theoretical calculation, we find that the in situ intercalation of guest anions allows the expansion of the LDH layers and keeps the active NiOOH species under the oxidation state of +3 via electron coupling, which ultimately tunes the site populations and site activity toward the superior OER activity, respectively. This work thus targets to provide insight into strategies to design the next generation of highly active catalysts for water electrolysis and fuel cell technologies.