Li4Ti5O12 and LiMn2O4 as promising channels for electrochemical ionic transistors: Electronic and ionic conductivity evolution relative to the state-of-charge

A comprehensive understanding of electronic and ionic conductivity changes in crystalline lithium metal oxide channels during operation is essential for optimizing switching (write-read), speed and minimizing energy consumption in electrochemical ionic transistors for next-generation computing applications. The electrochemical behavior of Li4Ti5O12 (LTO) and LiMn2O4 (LMO) are well studied in battery research, however, their charge-dependent transport properties remain elusive in both bulk and thin-film forms. This study systematically investigates the state-of-charge (SOC) dependent transport properties of bulk pellets and extends the analysis to crystalline thin films. Full lithiation increases transport properties by nine orders of magnitude in LTO pellets and by two orders of magnitude in 100 nm LTO thin films compared to their fully delithiated states. In the 4 V vs. Li+/Li region, delithiation reduced transport properties by two orders of magnitude in LMO pellets and by 1.5 orders of magnitude in 100 nm LMO thin films relative to their lithiated states. Formation of Li2Mn2O4 during cycling in the 3 V vs. Li+/Li region results in an additional four orders of magnitude reduction in transport properties relative to LiMn2O4. DFT+U+V calculations show that LTO and LMO remain electronically insulating across all SOC. The electronic band gap of LTO narrows upon lithiation, whereas LMO exhibits a decreasing gap during delithiation, consistent with experimentally observed electronic conductivity trends. Nevertheless, the band gap alone cannot fully account for electronic conductivity variations at intermediate lithium concentrations, highlighting the need for more detailed computations to account for polaron formation and transport.