Copper Vanadium Sulfide as an Anode Material in Sodium-Ion Batteries

Abstract

As the modern world is moving from non-renewable energy sources to renewable sources, such as wind and solar, energy storage devices are necessary in more than just portable electronics. Large-scale, stationary energy storage is required to combat the intermittent supply of renewable energy. Li-ion batteries, which are the current state of art in energy storage devices, will not be viable in the long term for such an application as it requires a large quantity of scarce raw materials (e.g. Li, Co). To reduce the cost and increase the long-term sustainability of batteries, more abundant energy storage materials are required. Sodium is the 6th most abundant element in the earth, making it significantly more abundant than Li. Na shares a similar chemistry to Li, helping advance research into sodium-ion batteries (SIBs). However, there are many challenges associated with SIBs, such as slower diffusion kinetics and reduced cycling stability due to the larger iconic radius of Na ions (1.02 Å (Na+) vs 0.76Å (Li+)) (1). This issue leads to incompatibility of most current anode materials used in LIBs, necessitating the identification of alternative materials (2). Transition metal sulfides (TMS) have great potential as anode materials for SIBs due to their high theoretical capacities as a result of conversion-based reactions occuring during cycling (3). However, conversion-based materials suffer from volume expansion during the sodiation and desodation process (4). CuS (covellite) has attracted a lot of attention and has a theoretical capacity of 560mA.h/g and electrical conductivity of ∼10−3Scm−1 (2,5). Unfortunately, CuS requires significant modifications to enhance its structural stability such as the formation of rGO composites or intricate morphological alteration to mitigate volume expansion during cycling. Herein, we address the addition of a second transition metal, vanadium, to form a ternary material (Cu3VS4) that enables cycling stability. Cu3VS4 is produced via a colloidal hot injection method to allow for high precision control over the morphology and phase purity. Factors such as ligand exchange, voltage profile, electrolyte selection, phase transformation (via in-situ and ex-situ studies), and SEI layer formation are also investigated to understand their contributions to the long-term stability of the cell. This study provides a mechanistic insight into the factors that affect the long-term stability of the material, taking a step closer to the production of sustainable materials for energy storage devices.