Nanosized Sn 4 P 3 with a layered structure was synthesized by a mechanochemical method, and electrochemical and local structural characteristics of tin phosphide during charge/discharge were studied for its use as an anode material for lithium secondary batteries. As the amount of lithium insertion increased, tin phosphide was converted into lithium phosphides followed by lithiumtin alloy formation, which was confirmed by differential capacity plots and X-ray absorption spectroscopic ͑XAS͒ analysis. Based on X-ray diffraction, XAS, and electrochemical data, a three-step reaction mechanism of Sn 4 P 3 with lithium was suggested. Tin phosphide showed a good cyclability and retained a fairly large capacity of 370 mAh/g up to 50 cycles when cycled within a limited voltage window.

The reaction mechanism of lithium insertion into Mg 2 Si was studied using various analytic techniques including electrochemical measurements, X-ray diffraction (XRD), and Auger electron spectroscopy (AES). Electrochemical tests demonstrated that 1 mol Mg 2 Si reacted with 3.9 mol Li from which the initial capacity obtained was approximately 1370 mAh/g. Ex situ XRD and AES data showed that lithium intercalated into the Mg 2 Si lattice first followed by alloying with Si and Mg. The degradation mechanism of Mg 2 Si during cycling was investigated because the Mg 2 Si materials degraded rapidly within ten cycles. The electrode material disintegrated and Li remained within the active material after ten cycles. The XRD and scanning electron microscope data suggested that the degradation mechanism of Mg 2 Si was due to the volume change during the alloying/dealloying reaction, and the volume expansion/contraction made the Mg 2 Si electrode materials electrically isolated.

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