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 o􀆯ers a cobalt-free alternative, but 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 behaviour of LiMn2O4 thin film microbatteries operated in a wider voltage window, using a LiPON solid electrolyte and a Li metal anode. Polycrystalline ~850 nm LiMn2O4 cathodes were grown by Pulsed Laser Deposition with sequential Li2O enrichment during growth. X-ray di􀆯raction, 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 thinfilm microbatteries and outline a tentative route to stabilize the LiMn2O4/LiPON interface under widerwindow operation.