Aqueous zinc batteries are attracting interest because of their potential for cost-effective and safe electricity storage. However, metallic zinc exhibits only moderate reversibility in aqueous electrolytes. To circumvent this issue, we study aqueous Zn batteries able to form nanometric interphases at the Zn metal/liquid electrolyte interface, composed of an ion-oligomer complex. In Zn||Zn symmetric cell studies, we report highly reversible cycling at high current densities and capacities (e.g., 160 mA cm−2; 2.6 mAh cm−2). By means of quartz-crystal microbalance, nuclear magnetic resonance, and voltammetry measurements we show that the interphase film exists in a dynamic equilibrium with oligomers dissolved in the electrolyte. The interphase strategy is applied to aqueous Zn||I2 and Zn||MnO2 cells that are charged/discharged for 12,000 cycles and 1000 cycles, respectively, at a current density of 160 mA cm−2 and capacity of approximately 0.85 mAh cm−2. Finally, we demonstrate that Zn||I2-carbon pouch cells (9 cm2 area) cycle stably and deliver a specific energy of 151 Wh/kg (based on the total mass of active materials in the electrode) at a charge current density of 56 mA cm−2.

Aqueous alkaline zinc batteries are of scientific and technological interest because of the potential they offer for cost-effective and safe storage of electrical energy. Poor electrochemical reversibility and shape change of the Zn anode, propensity of Zn to become passivated by surface oxides and hydroxide films upon prolonged exposure to the electrolyte, and electroreduction of water are well-studied but remain unsolved challenges. Here, we create and study electrochemical and transport properties of precise, spatially tunable zwitterionic polymer interphases grown directly on Zn using an initiated-chemical vapor deposition polymerization methodology. In aqueous alkaline media, spatial gradients in compositionfrom the polymer–electrolyte interface to the solid–polymer interfacepromote highly reversible redox reactions at high current density (20 mA cm–2) and high areal capacity (10 mAh cm–2). Via molecular dynamics and experimental analyses, we conclude that the interphases function by regulating the distribution and activity of interfacial water molecules, which simultaneously enables fast ion transport and suppression of surface passivation and the hydrogen evolution reaction. To illustrate the practical relevance of our findings, we study aqueous Zn||NiOOH and Zn||air batteries and observe that zwitterionic polymer interphases produce extended life at high currents and high areal capacity.

Aqueous zinc flow batteries (AZFBs) with high power density and high areal capacity are attractive, both in terms of cost and safety. A number of fundamental challenges associated with out-of-plane growth and undesirable side reactions on the anode side, as well as sluggish reaction kinetics and active material loss on the cathode side, limit practical deployment of these batteries. We investigated artificial interphases created using a simple electrospray methodology as a strategy for addressing each of these challenges. The effectiveness of the electrospray interphases in full cell zinc-iodine flow batteries was evaluated and reported; it is possible to simultaneously achieve high power density [115 milliwatts per square centimeter (mW/cm 2 )] and high areal capacity [25 milliampere hour per square centimeter (mA·hour/cm 2 )]. Last, we extended it to aqueous zinc-bromine and zinc-vanadium flow batteries of contemporary interest. It is again found that high power density (255 and 260 mW/cm 2 , respectively) and high areal capacity (20 mA·hour/cm 2 ) can be simultaneously achieved in AZFBs.

MnO2 is a versatile, cost-effective transition metal oxide that has attracted interest in multiple domains, including as an active cathode material or catalyst for electrochemical energy storage in batteries. Hydrothermal methods are among the most efficient approaches for MnO2 synthesis. These approaches enable facile, versatile production of MnO2 in any of its crystalline phases (α, β, δ, etc.), with the dominant product being determined by reaction conditions such as precursor concentration and temperature. These benefits unfortunately come with impractically low product yields (∼9%) and long reaction times. Here, we report that low-molecular weight, water-soluble polymers function as effective nucleating agents for the hydrothermal synthesis of MnO2. For fundamental reasons, these polymers are also reported to promote large (10- to 40-fold) increases in reaction rate and yield. We evaluate the physical and crystallographic features of the synthesized MnO2 and find that depending upon the reaction conditions, the polymer-assisted synthesis yields dominantly δ-MnO2 or β-MnO2. The as-prepared δ-MnO2 materials with extra interlayer water are studied as electrodes for aqueous Zn-ion battery applications and are shown to support long-duration storage at both moderate and high rates.

Polymers are known to adsorb spontaneously from liquid solutions in contact with high-energy substrates to form configurationally complex, but robust phases that often exhibit higher durability than might be expected from the individual physical bonds formed with the substrate. Rational control of the physical, chemical, and transport properties of such interphases has emerged as a fundamental opportunity for scientific and technological advances in energy storage technology but requires in-depth understanding of the conformation states and electrochemical effect of the adsorbed polymers. Here, we analyze the interfacial adsorption of oligomeric polyethylene glycol (PEG) chains of moderate sizes dissolved in protic and aprotic liquid electrolytes and find that there is an optimum polymer molecular weight of approximately 400 Da at which the highest columbic efficiency is achieved for both Zn and Li deposition. These findings point to a simple, versatile approach for extending the lifetime of batteries.