Unique lithiation and delithiation processes of nanostructured metal silicides

We report that TiSi(2) nanonet exhibits considerable activities in the reversible lithiation and delithiation processes, although bulk-sized titanium silicide is known to be inactive when used as an electrode material for lithium ion batteries. The detailed mechanism of this unique process was studied using electrochemical techniques including the electrochemical impedance spectroscopy (EIS) method. By systematic characterizations of the Nyquist plots and comparisons with the microstructure examinations, we identified the main reason for the activities as the layered crystal structure that is found stable only in TiSi(2) nanonets. The layer structure is characterized by the existence of a Si-only layer, which exhibits reactivity when exposed to lithium ions. Control studies where TiSi(2) nanowires and TiSi(2)/Si heteronanostructures were involved, respectively, were performed. Similar to bulk TiSi(2), TiSi(2) nanowires show limited reactivity in lithium ion insertion and deinsertion; the EIS characteristics of TiSi(2)/Si heteronanostructures, on the other hand, are distinctly different from those of TiSi(2) nanonets. The result supports our proposed TiSi(2) nanonet lithiation mechanism. This discovery highlights the uniqueness of nanoscale materials and will likely broaden the spectrum of electrode material choices for electrochemical energy storage.