Plasma enhanced atomic layer deposition of thin film Li<inf>1+x</inf>Mn<inf>2-x</inf>O<inf>4</inf>for realization of all solid-state 3D lithium-ion microbatteries

The plasma enhanced atomic layer deposition (PEALD) process for MnO2 was demonstrated, exhibiting self-limiting growth as well as stable composition and stable growth rate over a temperature window of 205-265 °C. The PEALD process for MnO2 was combined with the thermal ALD process for LiOH to synthesize Li1+xMn2-xO4 thin film cathodes, where the stoichiometry was effectively controlled to allow for crystallization in the electrochemically active spinel phase. A 3D nanostructure consisting of an Li1+xMn2-xO4 cathode layer, ALD LixAlySizO solid electrolyte, and SiGe nanowire anode was fabricated and characterized via TEM. The Li1+xMn2-xO4 thin films maintained 66% of the areal capacity upon a 100× increase in the rate (4-360 μA cm-2) as well as 97% capacity retention over 100 cycles at ∼5C. The ALD Li1+xMn2-xO4 thin films exhibited a volumetric capacity of 52 μAh cm-2 μm-1 at a C-rate of ∼0.5C and, coupled with the high operating voltage (4.0 V), offer some of best areal energy densities for ALD thin film cathodes, making it a viable material for integration with 3D lithium-ion microbatteries.