STABILIZING ALKALI METAL ELECTRODEPOSITION VIA NANOSTRUCTURED HYBRID ELECTROLYTE AND INTERPHASE DESIGN FOR RECHARGEABLE METAL BASED BATTERIES

Abstract

Significant advances in the amount of electrical energy that can be stored in electrochemical cells, such as rechargeable batteries require the adoption of high energy metallic anodes including Li, Na, Al, Zn, etc. Such anodes introduce as significant technical challenges because they are known to form rough electrodeposits, loosely termed dendrites, during the device operation. This produces irreversible active material (electrode and electrolyte) losses during normal cell operation and poses safety concerns because the dendrites can proliferate in the inter-electrode space, shorting the cell internally. Though similar phenomenon has been investigated in the more conventional context of metal electroplating, more complex effects can dominate in a battery configuration especially at current densities below the limiting current and in cells where the metal anodes undergo chemical reaction with electrolyte components. In this thesis, a comprehensive materials strategy involving structural and interfacial engineering is pursued to stabilize lithium metal electrodeposition. The strategy is based on guidelines defined by a theoretical linear stability analysis of metal electrodeposition in structured electrolytes. The origin of deposition instability is revealed to involve fundamental features of electrolytes and interfaces near metal anodes, which lead to electro- convective, morphological and chemical instability. I show that the first two instabilities can be addressed by using a nanostructured polymer/ceramic hybrid electrolyte, which exhibits high conductivity, high modulus and the ability to rectify ion transport through confinement. The well-defined nanoporous structure of the electrolytes also confine the length scale of the electrodeposit, which allows surface tension and other weaker forces at the interface to flatten rough electrodeposits, promoting dendrite-free operation. The chemical instability poses a more serious challenge because it is intrinsic to the chemistry of the electrode and electrolyte components; any exposure of one to the other can in principle drive a reaction cascade that ends in unconstrained growth in the cell impedance and premature failure. I show that this challenge can be overcome by the careful design of solid electrolyte interphases (SEIs) that regulate mass transport of reactive electrolyte ingredients and at the same time are able to flex to accommodate volume expansion of the anode. A significant finding is that these features can be realized using electrolyte additives designed to selectively break-down in-situ to form SEI with explicit composition set by the chemistry of the additive. A particularly important example are additives that break down to form halogen salts, which exhibit low surface diffusion barrier and fast interfacial transport. Such materials are shown to be highly effective in improve battery cycle lifetime. A second category of SEI explored in the study are so- called artificial SEI formed by pretreating the metallic electrode with polymer, metals, and metal oxide precursors prior to cell assembly.