Ion Intercalation into Vanadium Sulfides for Battery Applications
Authors: Egbu James , Martin Benjamin 
Published: February 2020

Global battery manufacturing capacity will more than double by 2021 to about 280,000 megawatt-hours.1 Rechargeable batteries make up a significant fraction of battery manufacturing. A rechargeable battery cell is made up of two electrodes, the anode and the cathode, separated by an electrolyte. For solid electrodes, an electrolyte-permeable separator separates them. Chemical reactions at the anode and the cathode produces two components; ionic and electronic. The electrolyte conducts the movement of ions inside the cell while forcing electrons to pass through outside of the cell. Current collectors at the anode and the cathode deliver electric current to and from the external circuit. During discharge, electrons and ions flow from the anode to the cathode. On charge, an applied electric field forces the electrons and ion to flow back from the cathode to the anode.2 Lithium-ion technology has dominated the rechargeable battery industry because of their high energy density, and portability.
High cost, safety issues, and charge storage capacity limit lithium-ion technology. These problems have sparked new interest in rechargeable batteries with multivalent ions like magnesium ion (Mg-ion). Magnesium is divalent, cheaper than lithium, easy to handle, and environmentally friendly. It has 1.85 times the theoretical volumetric capacity of lithium.3 Passivation and choice of cathode materials have hindered Mg-ion technology. Passivation produces a surface thin film on the metal anode that prevents reversibility of magnesium ions. Electrolytes from magnesium organo-halo-aluminate complexes in ether solvents have significantly reduced passivation problems.4 The choice of cathode material remains elusive. Theoretical predictions have shown high magnesium ion mobility in six-coordination, spinel close-packed frameworks.5 Mg-ion has been reported to favor octahedral environment in oxides and sulfides.6 This project focuses on investigating vanadium sulfides compounds, AxV5S8, AxV6S8, and V5S8 as candidate cathode materials for Mg-ion battery. A is an alkali metal, or monovalent cation and x = 0 – 0.8.
AxV6S8 has a hexagonal unit cell formed from a distorted VS6 octahedral sharing faces and edges, with large hexagonal channels capable of intercalating ions.7 AxV5S8 has a monoclinic structure. It has a three-dimensional framework made from slightly distorted VS6 octahedral sharing edges and faces, with rectangular tunnels prime for ion insertion.8 Both AxV5S8 and AxV6S8 have short V-V zigzag chains that make both metallically conductive.5,6 V5S8 has a monoclinic structure with every three-quarter of metal atom position vacant in every layer of the NiAs structure.9 Stoichiometry mixtures of vanadium sulfide compounds are synthesized in evacuated sealed quartz tubes. The products are handled and analyzed under argon. Samples of AxV6S8, AxV5S8, and V5S8 are characterized using powder XRD. Electrochemical properties of the vanadium sulfide compounds are studied using cyclic voltammetry and chronopotentiometry. Cell assembly consists of lithium metal anode, 80 % vanadium sulfide compound cathode, 1 M LiClO4 in 1:1 EC:DEC electrolyte, glass frit, Kynar, and graphite.
XRD pattern for as prepared KxV5S8 matched with a known KxV5S8 pattern in the International Center for Diffraction Data database (ICDD). KxV6S8 XRD pattern showed KxV5S8 phase peaks when matched with the ICDD database, indicating phase impurity. Potassium is not a big enough metal to make the AxV6S8 framework. The XRD patterns of RbxV5S8, RbxV6S8, and V5S8 matched with ICDD database patterns for each compound without showing any phase impurities. Cyclic voltammetry and chronopotentiometry data with a lithium anode and the lithium-ion electrolyte indicate that KxV5S8 and RbxV5S8 are not electrochemically active. V5S8 is electrochemically active. It shows significant oxidation and reduction peaks during cyclic voltammetry with a lithium anode. In the future, RbxV6S8 will be investigated for electrochemical activity. Intercalation reactions of magnesium ion will be investigated in the electrochemically active compounds of vanadium sulfide compounds for battery applications.

Figure 1