(352d) Rational Design of Solid-Liquid Interphases for Reactive Metal Batteries

Advancement in energy storage technologies is necessary owing to the growing demand for portable consumer electronics and also for the global consensus of energy policy shift to renewable resources. In this regard, metal based batteries comprising of a reactive metallic anode (like Li, Na, Al) have gained significant attention because of their promise of improving the anode-specific capacity by 10-fold compared to the current state-of-art Li-ion battery that uses a graphitic anode. However, in these metal batteries, the issue of sudden short-circuits by dendrite growth as well as rapid fade in battery capacity due to internal side reactions limit their practical usage. Such failures modes have been characterized to occur due to the uneven deposition of metal ions, however, a striking fact in the electrodeposition literature is that, certain metals like, Magnesium do not form dendrites. In this study, we use theoretical and experimental tools to compare the surface properties of Mg with that of alkali metals to rationally design the solid-liquid interphase of lithium and sodium electrodes. We, specifically, use Density Functional Theory (DFT) calculations to quantify the diffusion energy barrier of ions on Mg, Li, Na surfaces and interestingly it is seen that the diffusion barrier of Mg (0.02eV/atom) is several fold lower than Li (0.14eV/atom) or Na (0.16eV/atom) metals. In fact, the diffusion barrier of Li₂CO₃, Li₂O (the commonly found compounds in lithium interface) is even higher, thus indicating that electrodeposition in such batteries can lead to the formation of dendrites. However, in quest for finding stable interfaces, we observed that most metal halides (LiF, LiBr, NaF etc.) have much lower diffusion barrier. In other words, lithium or sodium anode with halide-rich interface can lead to stable electrodeposition similar to Mg deposition. We further utilize the energy values from DFT calculation in Classical Nucleation Theory (CNT) to determine the microscopic deposit size for different metals and interfaces at various operating conditions (like current density, overpotential and temperature). The predictions from the coupled DFT-CNT model were validated using ex-situ scanning electron microscopy as well as in-situ X-ray imaging. The nucleation pattern, indeed, showed a strike difference between carbonate-rich and halide-rich lithium interfaces, which were utilized to calculate the experimental diffusion barriers. In order to understand the macroscopic implications of the interfacial ion transport, we performed in-situ optical microscopy of electrodeposition to directly visualize the morphology of dendrite growth in lithium and sodium batteries, without and with halide generating additives. Electrodeposition in usual electrolytes led to accelerated dendrite growth capable of causing short-circuits, while with halide-rich interfaces a more compact and smooth deposition was observed. Finally, the concept of halide-protected anodes was utilized in lithium and sodium metal batteries with high energy density cathodes that demonstrated extended capacity retention and longer cycle life.