Carbon-free Mn-doped LiFePO4 cathode for highly transparent thin-film batteries
The search for transparent battery cathodes primarily focuses on patterned electrodes with feature sizes below the optical absorption limit. This significantly limits the electrode capacity, as a large electrode area remains unused to maintain transparency. Herein, we report transparent olivine LiFe0.77Mn0.23PO4 thin-film electrodes discovered through high-throughput continuous-composition-spread sputtering. After investigating six different Mn doping ratios, we found the optimal Mn-doped olivine composition with an enhanced discharge capacity of 45.7 μA h/cm2∙μm without using excessive nanosized features or carbon coating. The thin-film electrode exhibits a clear redox activity for both Fe3+/2+ and Mn3+/2+, resulting in an enhanced average voltage over LiFePO4 composition. A 250-nm-thick film exhibits an optical transmittance of over 80% in the visible region. The results in this study demonstrates that transparent cathode thin films can be developed based on phospho-olivines via doping strategies with high-throughput continuous-composition-spread sputtering methods.