The development of fast cycling Li-ion electrode materials calls for a fundamental understanding of how crystal structure, composition, and material architecture influence ionic and electronic transport. Recently, the family of Nb-containing Wadsley-Roth shear oxides has shown exceptional rate capability in Li-ion electrodes, even with bulk morphologies that typically have very slow lithiation kinetics in other battery materials. However, shear oxides that do not contain Nb have received little attention. Here, we characterize the properties and performance of the shear oxide V9Mo6O40 (VMO). Using synthetic routes based on sol-gel chemistry, we have produced bulk and nanoporous VMO powders with high purity. Electrochemical testing shows reversible cycling with good rate capability for the bulk material and excellent high-rate cycling for the nanoporous architecture. Using operando X-ray diffraction, we demonstrate that VMO undergoes a previously unreported order-to-disorder transition during its first lithiation, whereby the initially highly crystalline material becomes amorphous or ordered only at very short length scales. This disordered material appears to be responsible for the cycling performance of VMO, and its structure and properties are of fundamental interest, since most amorphous compounds show poor ionic and electronic conductivity. However, characterizing the structure of battery materials after cycling is complicated by the need for conductive carbon and binder additives in composite electrodes. Using electron microscopy, X-ray photoelectron spectroscopy, and electrochemical impedance spectroscopy, we have investigated the disordered VMO phase. Despite the dramatic transformation of the crystal structure during the first lithiation, particle morphology and composition are largely retained. Moreover, the charge transfer resistance of composite electrodes is reasonably low and doesn’t change significantly as a function of lithium content, which suggests that the disordered VMO is both electronically and ionically conductive. Overall, this work sheds new insight on the previously unknown order-to-disorder transition and offers a proof-of-concept for fast-charging in disordered materials.

Understanding the order-to-disorder transition in the fast-charging Li-ion electrode material V9Mo6O40

ACS Spring 2022

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