Observation of exceptional lithium storage in a colloidal silica-assisted Co(OH)2 anode: Hydride formation

Abstract: Developing electrode materials with high-energy densities based on clear understanding of the electrochemical reaction mechanism for known electrode materials may be regarded as a practical strategy for the improvement of energy storage systems. Here, we introduce a new Co(OH)2 material, synthesized by using colloidal silica, which exhibits an initial charge capacity of 1,112 mAh g-1, about twice its theoretical value based on conversion reaction, and retains its first cycle capacity after 30 cycles. Unlike previous reports, we find that the mechanism for the high and reversible lithium storage capacity of the colloidal silica-assisted Co(OH)2 electrode is not based on a conventional conversion reaction only. The combined results of synchrotron X-ray diffraction and absorption spectroscopy indicate that nanosized Co metal particles and LiOH are generated by conversion reaction at high voltages and subsequent formation of CoxHy, Li2O and LiH by hydride reaction between Co metal, LiOH and other lithium species at low voltages, resulting in a anomalously high capacity beyond theoretical capacity of Co(OH)2. This is further supported by ab initio molecular dynamics simulation, localized scanning transmission electron microscopy and X-ray photoelectron spectroscopy. These findings may be considered in designing advanced nano-structured anode materials for high-energy lithium ion batteries.