There is widespread interest in determining the structural features of redox-active electrochemical energy storage materials that enable simultaneous high power and high energy density. Here, we present the discovery that confined interlayer water in crystalline tungsten oxide hydrates, WO3·nH2O, enables highly reversible proton intercalation at subsecond time scales. By comparing the structural transformation kinetics and confined water dynamics of the hydrates with anhydrous WO3, we determine that the rapid electrochemical proton intercalation is due to the ability of the confined water layers to isolate structural transformations to two dimensions while stabilizing the structure along the third dimension. As a result, these water layers provide both structural flexibility and stability to accommodate intercalation-driven bonding changes. This provides an alternative explanation for the fast energy storage kinetics of materials that incorporate structural water and provides a new strategy for enabling high power and high energy density with redox-active layered materials containing confined fluids.

Lithium metal anodes can largely enhance the energy density of rechargeable batteries because of the high theoretical capacity and the high negative potential. However, the problem of lithium dendrite formation and low Coulombic efficiency (CE) during electrochemical cycling must be solved before lithium anodes can be widely deployed. Herein, a new atomic layer deposition (ALD) chemistry to realize the low-temperature synthesis of homogeneous and stoichiometric lithium fluoride (LiF) is reported, which then for the first time, as far as we know, is deposited directly onto lithium metal. The LiF preparation is performed at 150 °C yielding 0.8 Å/cycle. The LiF films are found to be crystalline, highly conformal, and stoichiometric with purity levels >99%. Nanoindentation measurements demonstrate the LiF achieving a shear modulus of 58 GPa, 7 times higher than the sufficient value to resist lithium dendrites. When used as the protective coating on lithium, it enables a stable Coulombic efficiency as high as 99.5% for over 170 cycles, about 4 times longer than that of bare lithium anodes. The remarkable battery performance is attributed to the nanosized LiF that serves two critical functions simultaneously: (1) the high dielectric value creates a uniform current distribution for excellent lithium stripping/plating and ultrahigh mechanical strength to suppress lithium dendrites; (2) the great stability and electrolyte isolation by the pure LiF on lithium prevents parasitic reactions for a much improved CE. This new ALD chemistry for conformal LiF not only offers a promising avenue to implement lithium metal anodes for high-capacity batteries but also paves the way for future studies to investigate failure and evolution mechanisms of solid electrolyte interphase (SEI) using our LiF on anodes such as graphite, silicon, and lithium.