Sputtered Porous Li-Fe-P-O Film Cathodes Prepared by Radio Frequency Sputtering for Li-ion Microbatteries

The increasing demands from micro-power applications call for the development of the electrode materials for Li-ion microbatteries using thin-film technology. Porous Olivine-type LiFePO₄ (LFP) and NASICON-type Li₃Fe₂(PO₄)₃ have been successfully fabricated by radio frequency (RF) sputtering and post-annealing treatments of LFP thin films. The microstructures of the LFP films were characterized by X-ray diffraction and scanning electron microscopy. The electrochemical performances of the LFP films were evaluated by cyclic voltammetry and galvanostatic charge-discharge measurements. The deposited and annealed thin film electrodes were tested as cathodes for Li-ion microbatteries. It was found that the electrochemical performance of the deposited films depends strongly on the annealing temperature. The films annealed at 500 °C showed an operating voltage of the porous LFP film about 3.45 V vs. Li/Li⁺ with an areal capacity of 17.9 µAh cm^-2 µm^-1 at C/5 rate after 100 cycles. Porous NASICON-type Li₃Fe₂(PO₄)₃ obtained after annealing at 700 °C delivers the most stable capacity of 22.1 µAh cm^-2 µm^-1 over 100 cycles at C/5 rate, with an operating voltage of 2.8 V vs. Li/Li⁺. The post-annealing treatment of sputtered LFP at 700 °C showed a drastic increase in the electrochemical reactivity of the thin film cathodes vs. Li⁺, leading to areal capacity ~9 times higher than as-deposited film (~27 vs. ~3 µAh cm^-2 µm^-1) at C/10 rate.