Fabrication of Two-Dimensional Heterostructure Electrodes for Intercalation Batteries Via Cation-Driven Assembly
Authors: Pomerantseva Ekaterina 
Published: December 2024

The cation-driven assembly technique was developed and employed to fabricate two-dimensional (2D) heterostructures of lithium preintercalated bilayered vanadium oxide (δ-LixV2O5·nH2O or LVO) and graphene oxide (GO) nanoflakes. These heterostructures were formed using concentrated lithium chloride solution and annealed under vacuum at 200ºC. The presence of Li+ ions from LiCl enhanced the formation of oxide/carbon heterointerfaces, improving structural and electrochemical stability. Annealing at 200ºC allowed to remove interlayer water excess and partially reduce GO, thus improving electron transport through reduced GO (rGO) nanoflakes. By varying the initial GO concentration, the carbon content in the heterostructures could be adjusted, affecting the electrochemical performance. Scanning transmission electron microscopy (STEM) imaging combined with electron energy-loss spectroscopy (EELS) clearly revealed the formation of LVO/rGO heterointerface with rGO nanoflakes with thicknesses down to the nanometer scale embedded between the nanometer-thick layers of LVO.

Electrochemical cycling of the Li+ cation-assembled LVO/rGO heterostructures in Li-ion cells demonstrated enhanced cycling stability and rate performance with increasing rGO content, despite a slight decrease in charge storage capacity. The improved electrochemical stability was attributed to what we have termed the encapsulation effect, which involves the encasing of LVO nanoflakes between the rGO nanoflakes, leading to the suppressed dissolution of LVO in the electrolyte and mitigating the structural distortion of LVO known to be induced by repeated intercalation/deintercalation reactions during electrochemical cycling. Compared to the electrodes prepared by physical mixing of LVO and rGO nanoflakes, the cation assembled LVO/rGO heterostructures showed superior electrochemical stability, highlighting the stabilizing effect of the 2D heterointerface.

The versatility of the cation-driven assembly approach was demonstrated by preparing LVO/rGO heterostructures using different assembling cations: Li+, Na+ and K+. The assembling cations influenced the interlayer spacings and structural water content of the resting LVO/rGO heterostructures. STEM/EELS analysis confirmed the formation of a 2D heterointerface between LVO and rGO for all types of the assembling cations used, with the assembling cations trapped in the interlayer regions. Additionally, STEM/EELS identified a V2O3 phase that formed at the LVO/rGO heterointerface and could stabilize the LVO/rGO heterostructures during cycling. Charge storage mechanism analysis indicated that increased interlayer spacings of the BVO phase and the use of assembling cations to define intercalation sites improved ion diffusion and increased capacities during cycling. Li+ and Na+ cation assembled heterostructures exhibited enhanced ion diffusion and charge storage capacities in their respective charge storage systems (Li-ion and Na-ion cells, respectively). Analyses of our results indicate that the choice of cation for heterostructure assembly can modify the final material structure and tailor ion diffusion and charge storage capacity for specific electrochemical systems, offering a versatile approach for utilizing 2D materials in energy storage applications.