Spherical Si-Nb2O5-C Composite As Anode Materials for Li Secondary Batteries

Abstract

In recent years, Li-ion batteries have attracted much attention as the demand for electronics and electric vehicles has increased. Currently, graphite is used as anode materials for Li-ion batteries because of its good cycle performance and reasonable cost. However, graphite has a relatively low specific capacity of 372 mAhg-1 due to its lithium storage mechanism. The low theoretical capacity of graphite does not meet the consumer's need for batteries with high capacity. Therefore, development of new anode materials with high capacity are strongly required. Among diverse Li storage materials that can replace graphite, Li-alloy based materials including group IV elements have been widely researched due to their high theoretical capacity. In particular, silicon has attracted great attention as anode materials for Li-ion batteries due to its high capacity and natural abundance. However, silicon anodes generally suffer from volume expansion during the Li insertion and extraction process, which results in poor cycle performance. To overcome the problem, strategies to suppress the volume expansion of silicon should be suggested. In this work, we synthesized carbon-coated niobium oxide-silicon composites. Niobium oxide (Nb2O5) has various crystalline polymorphs such as pseudo-hexagonal (TT-Nb2O5), orthorhombic (T-Nb2O5), tetragonal (M-Nb2O5) and monoclinic (H-Nb2O5). These various crystalline polymorphs of Nb2O5 can be prepared by varying the heat treatment temperature, of which orthorhombic structure is favorable for fast Li ions storage. T-Nb2O5 exhibits excellent rate performance during cycling due to its Li insertion/desertion mechanism. However, T-Nb2O5 has a relatively low capacity and thus, silicon and T-Nb2O5 were combined to obtain a high capacity and stable cycle performance. In addition, to increase the electrical conductivity of both silicon and T-Nb2O5, carbon incorporation was employed. First, niobium oxide-silicon composite with micrometer-sized spherical structure was prepared by a solvothermal method. In the microsphere, nanosized Si and Nb2O5 were uniformly distributed. Then, the heat treatment was done with naphthalene at 800℃ to perform both phase transformation to T-Nb2O5 for fast Li transport and carbon coating for cycle performance and enhanced electrical conductivity. As a result, a nanometer-thick carbon layer was coated on the surface. Electrochemical test results exhibited that the synthesized electrode materials showed a high reversible capacity, excellent cycle life, and good rate performance as anode materials for Li rechargeable batteries.