(Invited) Improving Electrochemical Stability and Rate Capability of Layered Oxides in Li-Ion Batteries Via Controllable Formation of Two-Dimensional Heterointerface

Abstract

The expanding variety of applications relying on portable autonomous power calls for innovative energy storage solutions with high performance. The performance of lithium-ion batteries (LIBs), the most widely used energy storage system today, is limited by the available cathode materials. Layered transition metal oxides show high redox activity in intercalation reactions and relatively high working potentials, making them especially attractive for use as cathodes in batteries. However, the low electronic conductivity of most oxides limits their performance. Expanding interlayer region combined with incorporation of the transition metal ions in high oxidation states, such as V5+, Nb5+, Mo6+, or W6+ in the structural layers, can be an efficient strategy to increase the specific capacity of the cathodes in intercalation batteries, a long sought for capability needed to realize batteries with higher energy densities compared to the performance characteristics achievable today. Utilizing such materials with high operation voltage/specific capacity and fast ion/electron transport as intercalation battery cathodes is attractive for creating next generation batteries with high energy and power density. An oxide material that meets both requirement of forming a layered structure with expanded interlayer spacing and containing a transition metal ion in the high oxidation state within the layers is bilayered (or δ-) vanadium oxide. Unusually for oxides, the two-dimensional bilayered V2O5 slabs are separated by a large interlayer spacing of 11.5 Å stabilized by structural water. It was shown that δ-V2O5·nH2O delivers high specific capacities in both lithium-ion and beyond lithium ion energy storage systems. However, bilayered vanadium oxide electrodes demonstrate poor capacity retention and moderate rate performance. In this presentation, I will demonstrate a new layered material with the regions in its structure built by the alternating layers of vanadium oxide and carbon. The vanadium oxide layers correspond to the bilayers present in δ-V2O5 structure. The carbon layers are created via thermal treatment of organic molecules containing nitrogen atoms, and therefore believed to resemble layers of the nitrogen-doped graphene (NDG). The presence of the NDG layers is supported by a combined scanning transmission electron microscopy (STEM) imaging, Raman spectroscopy, X-ray diffraction (XRD) and thermogravimetric analysis (TGA). The interlayer distance between the vanadium oxide layer is determined to be 10.8 Å by both XRD and STEM, and the NDG layer forms in-between. δ-V2O5·nH2O electrodes, obtained via a sol-gel process, were used as a reference to evaluate electrochemical performance of layered δ-V2O5/NDG heterostructures in Li-ion batteries. δ-V2O5/NDG electrodes showed high energy storage performance, with capacities above 200 mAh g-1 and 36% improvement in capacity retention compared to δ-V2O5·nH2O electrodes in Li-ion cells. Moreover, this heterostructured phase displays significantly improved rate performance, with a 66% increase in capacity retention when current was increased from 20 mA g-1 to 300 mA g-1 compared to the δ-V2O5 reference in Li-ion batteries. The improved electrochemical performance is attributed to the enhanced electron transport through NDG layers and stabilized two-deimensional heterostructured interface between dissimilar layers. This work opens an opportunity to create next generation cathode materials with high specific capacity and advanced electrochemical stability in applications requiring stable Li-ion batteries that can tolerate high current rates.