Crystallographic/Morphological Changes and Performance of Lithium Iron Phosphate Deposited By Vacuum Cold Spray

Abstract

Cold spray offers several advantages for material deposition, including the ability to deposit a wide range of materials without the need for high temperatures, minimizing heat-related distortion and maintaining material properties. Additionally, cold spray is a highly efficient and environmentally friendly process, as it reduces waste and energy consumption compared to traditional thermal spray methods. Lithium iron phosphate (LFP) has been gaining popularity as a cathode material for lithium-ion batteries due to its stable cycling performance, lack of oxygen generation, thermal stability, and use of environmentally friendly iron over nickel or cobalt. In this study, LFP was deposited by vacuum cold spray onto aluminum foil substrate to investigate the viability of cold spray as a possible method of manufacturing all-solid-state batteries. Results were obtained through in-situ X-ray diffraction (XRD), scanning electron microscopy (SEM), and in-situ transmission electron microscopy (TEM). Electrochemical results were obtained by assembling cold spray deposited LFP into half-cells with Li-metal anode and liquid electrolyte before being tested on electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and C-Rate. Due to the high impact velocities during cold spray, crystalline LFP was observed to undergo changes to an amorphous phase. Using in-situ X-ray diffraction (XRD), scanning electron microscopy (SEM), and in-situ transmission electron microscopy (TEM) the crystallographic and morphological changes to cold spray deposited LFP were investigated across various temperatures. In-situ XRD initially showed a decrease in crystallinity with crystallinity being restored with elevated temperatures, with in-situ TEM further supporting these findings. Investigation with SEM showed LFP particles undergo deformation upon impact with a mix of intact particles interspersed throughout to create a dense layer. Electrochemical impedance spectroscopy (EIS) of assembled half-cells show low resistance of 90Ω with further reduction to 60Ω upon cycling of the cell. In terms of cycling performance, initial discharge capacities showed 80 mAh/g and 20mAh/g at 0.1C and 1C respectively. These capacities dropped further with repeated cycling. Future work will be focused on improving capacity, annealing post deposition to restore crystallinity, and incorporation of solid electrolyte.