Lithium Intercalation Mechanism and Critical Role of Structural Water in Layered H2V3O8 High-Capacity Cathode Material for Lithium-Ion Batteries

Abstract

H2V3O8 (HVO) is a promising high-capacity cathode material for lithium-ion batteries (LIBs). It allows reversible two-electron transfer during electrochemical lithium cycling processes, yielding a very attractive theoretical capacity of 378 mAh g–1. While an abundant number of research works exclusively proved the outstanding electrochemical lithium storage properties of H2V3O8, structural changes during the intercalation process have not been scrutinized, and the crystallographic positions occupied by the guest species have not been revealed yet. However, an in-depth understanding of structural changes of cathode materials is essential for developing new materials and improving current materials. Aimed at providing insights into the storage behavior of HVO, in this work, we employed a combination of high-resolution synchrotron X-ray and neutron diffraction to accurately describe the crystal structures of both pristine and lithiated H2V3O8. In HVO, hydrogen is located on one single-crystallographic site in a waterlike arrangement, through which bent asymmetric hydrogen bonds across adjacent V3O82– chains are established. The role played by water in network stabilization was further examined by density functional theory (DFT) calculations. Easy hydrogen-bonding switch of structural water upon lithium intercalation not only allows better accommodation of intercalated lithium ions but also enhances Li-ion mobility in the crystal host, as evidenced by magic-angle spinning (MAS) NMR spectroscopy. Facile conduction pathways for Li ions in the structure are deduced from bond valence sum difference mapping. The hydrogen bonds mitigate the volume expansion/contraction of vanadium layers during Li intercalation/deintercalation, resulting in improved long-term structural stability, explaining the excellent performance in rate capability and cycle life reported for this high-energy cathode in LIBs. This study suggests that many hydrated materials can be good candidates for electrode materials in not only implemented Li technology but also emerging rechargeable batteries.