Molecular Wiring of LiFePO₄ (Olivine)

The carbon-free LiFePO4 (olivine) was modified by a monolayer coverage with [12-(2,5-di-tert-butyl-4-methoxy-phenoxy)-dodecyl]-phosphonic acid, coded DW. Thin film electrodes were bonded with 5% PVDF and deposited on F-doped SnO2 (FTO) support. The surface-modified LiFePO4 can be electrochemically charged via a mechanism called molecular wiring. The DW molecules contacting FTO are reversibly oxidized, which allows that the holes are transported from the FTO via the adsorbed DW monolayer across the whole olivine surface, where a subsequent chemical delithiation occurs toward FePO4. The sole cross-surface hole percolation via adsorbed DW was demonstrated on two reference systems, namely, TiO2 and LiMnPO4 (olivine), which are electrochemically inert in the potential region, where the DW oxidation occurs. The diffusion coefficient of hole transport across the LiMnPO4 surface equals 3 x 10(-9) cm(2)/s, which is ca. three times larger than the corresponding value for mesoporous TiO2. The undoped and carbon-free LiFePO4 can be charged by currents equivalent to ca. C/2 to C/10 exclusively via the monolayer of adsorbed molecules, but the process slows down when the charging progresses. The wiring current is roughly inversely proportional to the level of olivine charging by oxidative delithiation.