<p>This study examines the influence of plasma gas composition on the radiofrequency (RF) sputtering of Li–Mn–O thin films using a LiMn₂O₄ ceramic target. Five gas mixtures with varying Ar/O₂ ratios were employed to investigate their effects on stoichiometric formation, microstructure, and electrochemical behavior. Films deposited in a pure O₂ atmosphere yielded amorphous LiMn₂O₄ with poor electrochemical performance, exhibiting a lithium-ion diffusion coefficient of 8.85 × 10⁻¹³ cm²·s⁻¹. In contrast, deposition under pure Ar produced partial crystalline LiMn₂O₄ cathode, which displayed improved lithium-ion diffusion (1.05 × 10⁻⁹ cm²·s⁻¹) and a moderate specific capacity of 44 mAh·g⁻¹. Notably, a balanced Ar/O₂ plasma atmosphere facilitated the formation of a well-crystalline LiMn₂O₄ cathode, achieving superior performance with a specific capacity of 62 mAh·g⁻¹, fast diffusion kinetics (5.45 × 10⁻⁸ cm²·s⁻¹), and stable cycling behavior. This research work highlights the critical role of plasma composition in tailoring the functional properties of sputtered Li–Mn–O cathode materials.</p>