Pulsed-Laser-Deposited LiMn₂O₄ Thin-Film Solid-State Microbatteries with Extended Voltage Window Cycling

Thin-film microbatteries provide on-chip and surface-mount energy storage for Si-based microsystems, where device area is the primary constraint. Commercial implementations, available for more than 20 years, have largely relied on LiCoO₂ cathodes because they are straightforward to process and package. LiMn₂O₄ offers a cobalt-free alternative; however, in conventional liquid-electrolyte Li-ion cells, its use is constrained by Mn dissolution and capacity fade, especially when the voltage window is widened to access its theoretical capacity of ∼119 μAh·cm^-2·μm^-1 (∼296 mAh·g^-1). Thin-film solid-state architectures can mitigate these limitations and are naturally aligned with footprint-limited applications, where areal capacity and areal energy are the relevant figures of merit. The focus of this study is to examine the device behavior of LiMn₂O₄ thin-film microbatteries operated in a wider voltage window, using a LiPON solid electrolyte and a Li metal anode. Polycrystalline LiMn₂O₄ cathodes (∼850 nm) were grown by pulsed laser deposition with sequential Li₂O enrichment during growth. X-ray diffraction, Raman features, and depth-profiling glow discharge optical emission spectroscopy are consistent with the presence of a Li-rich spinel component formed during deposition. The resulting LiMn₂O₄/LiPON/Li cells, cycled between 2.0 and 4.5 V, deliver up to ∼50 μAh·cm^-2 at low rates; at higher rates, the wider window enables capacities up to ∼4 times those obtained on the same devices in the conventional 3.5-4.5 V window. Impedance measurements are used to track evolution during conditioning and operation. Finally, we provide an overview of relevant LiMn₂O₄ solid-state thin-film microbatteries and outline a tentative route to stabilize the LiMn₂O₄/LiPON interface under wider-window operation.