Lithium iron phosphate (LiFePO4, LFP) is one of the main cathode materials for lithium-ion batteries on the market; however, its implementation in all-solid-state thin-film batteries remains challenged by transport and interfacial limitations, as well as by compatibility and reactivity issues arising from thin film fabrication processes. In this work, carbon-free LiFePO4 thin films with thicknesses between 120 and 300 nm were deposited by pulsed laser deposition (PLD) and investigated as cathodes in LFP/LiPON/Li all-solid-state thin-film batteries. Structural and morphological analyses confirm the growth of phase-pure, crystalline LiFePO4 films without post deposition annealing. Electrochemical measurements reveal reversible lithium insertion and extraction, yet the theoretical capacity of LiFePO4 is not fully accessed under most operating conditions. When cycled at elevated temperature (50 °C), the full cells show a clear enhancement in capacity utilization, with the 200 nm-thick cathode delivering an areal capacity of 7.2 μA.h.cm-2 (100 mA.h.g-1) at 5 μA.h.cm-2. The limited utilization of the active material may partially originate from interfacial phenomena at the LFP/Pt current-collector interface. From an application perspective, the achieved areal capacities and current densities fall within the operational range required for low-power autonomous microsystems, highlighting the potential relevance of LFP/LiPON thin-film batteries for Internet-of-Things applications.
Freitas et al. (Tue,) studied this question.