Molten Hydroxide Electrodeposition of Transition Metal Oxide Cathodes for Electrochemical Energy Storage

Abstract

State-of-the-art synthesis for layered oxide cathodes for Li and Na-ion battery involves prolonged high temperature (>700°C) multistep processing for long reaction times (~24 hours) under high oxygen pressure, followed by slurry casting after mixing with binders and additives. Here, we demonstrate an intermediate temperature (250-350°C) molten hydroxide-based electrodeposition process to grow alkali ion (Li+, Na+) intercalated transition metal oxides across multiple transition metal chemistries of the Li system (from layered LiCoO2 to cobalt-less orthorhombic LiMnO2, layered Li2MnO3, spinel LiNixMn1-xO4, LiMn2O4,) and Na system (O3 and P2 NaxCoO2, O’3 NaxMnO2) in thick film form factors. The highly textured ([110]||ND), dense (>95%) electroplated LiCoO2 cathodes can perform at ultrahigh thickness of ~200 µm (areal capacity ~13.6 mAh/cm2) in comparison to 40-60 µm for state-of-the-art slurry cast cathodes (areal capacity ~3-4 mAh/cm2 with a porosity of ~20%), a fivefold increase in areal capacity and volumetric energy density. We discuss the interplay of bulk crystallographic orientation of the electro-active material (LiCoO2), surface terminating planes interacting with the electrolyte, and electrochemically grown conformal surface protective layers on the electrochemical-chemo-mechanical degradation mechanisms during intercalation and deintercalation for high voltage operation (>4.2 V vs. Li). We also demonstrate electrochemically controllable isovalent transition metal doping and substitution in ceramic oxides (LiCoxMn1-xO2) by electrodeposition. Our findings highlight the influence of the unique molten hydroxide-based solution chemistry platform and electrochemical processing parameters, on regulating the phase assemblage, designing the microstructure, and controlling crystallographic orientation during electrocrystallization.