Phase structure changes of MnP anode material during electrochemical lithiation and delithiation process

Manganese phosphide anode material is successfully prepared by a high-temperature solid-phase synthesis process, and its phase structure changes during cycling are revealed in this work. The results derived from X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) combined with selection area electron diffraction (SAED) show that the prepared MnP powder has a single crystalline structure covered with an amorphous oxide layer. Galvanostatic charge/discharge test results indicate that in the voltage range between 0.01 V and 2.00 V at the current density of 50 mA g(-1), the MnP anode delivers an initial lithiation and delithiation capacity of 1104 mAh g(-1) and 870 mAh g(-1), respectively. After 50 cycles, the retained capacity reaches 287 mAh g(-1). XRD results indicate that the single crystalline MnP phase is transformed to an amorphous LixMnyPz phase during the initial lithiation process. During the following cycles, the content of the MnP phase is gradually reduced, and the content of the amorphous LixMnyPz phase is continuously accumulated. The amorphous LixMnyPz and Mn2P phases residued in the anode act as the buffer matrix for the MnP active material to suppress the decrease of the lithiation and delithiation capacity during cycling. This amorphous structure is believed to be responsible for the reversible lithiation and delithiation after decade cycles. (c) 2013 Elsevier Ltd. All rights reserved.