Ssnmr Studies of Li3V2(PO4)3/C Cathode Reactions: Structural Evolution, Dynamics and LVP-Carbon Interface

Abstract

Monoclinic lithium vanadium phosphate Li3V2 (PO4)3(LVP) is a highly promising cathode material for lithium-ion batteries. In the past decades, extensive efforts have been made to improve the electrochemical performance of LVP by carbon coating and cation doping[1]. In-depth understanding of the local structures and charge-discharge mechanism of LVP can provide valuable information for the design of cation doping or surface modification. SSNMR has been demonstrated a powerful tool of local structure analysis and dynamics. However, due to sever line broadening and signal coalescence caused by the heating effect of carbon in the cathode composite, SSNMR study of LVP material was only limited to samples prepared by chemical delithiation[2], which cannot fully represent the real electrochemical process. We hereby present the first SSNMR study of LVP/C cathode reaction. The electrodes have been prepared in a binder free and carbon additive free fashion, in order to reduce the heating effect to a minimum during NMR spectra acquisition. A variety of SSNMR techniques have been combined to obtain structural information from different aspects. The structural changes of the polyhedron framework upon delithiation-lithiation were traced by 7Li and 31P NMR. The dynamic processes involving Li exchange and migration were monitored by variable temperature 7Li NMR and EIS technique. 13C-7Li REDOR NMR and HRTEM were performed to investigate Li-O-C connectivity and the short range structure at the LVP-carbon interface. The NMR results have elucidated the driving force of the asymmetric phase formation during charge-discharge within the voltage window of 4.3-4.8 volts and provided experimental evidence of direct chemical bonding between LVP and carbon coating. Our study helps the battery community to understand the (de)lithiation mechanism of LVP cathode, and more important, the role of carbon coating. [1]. Rui X, Yan Q, Skyllas-Kazacos M, et al. Li3V2(PO4)3 cathode materials for lithium-ion batteries: A review[J]. J. Power Sources , 2014, 258(21):19-38. [2]. Yin S C, Grondey H, Strobel P, et al. Charge ordering in lithium vanadium phosphates: electrode materials for lithium-ion batteries.[J]. J. Am. Chem. Soc. , 2003, 125(2):326-7.