Tuning the Interlayer Region Chemistry for Enhanced Energy Storage Behavior of Bilayered Vanadium Oxide Electrodes
Authors:   , Gallano Cyra , Shih Wei-Heng 
Published: August 2023

Bilayered vanadium oxide ([delta]-V₂O₅·nH₂O) is an attractive cathode material for Li-ion batteries due its wide interlayer spacing and high theoretical capacity. However, it shows poor cycling stability due to increasing disorder upon cycling. One approach towards improving performance is via chemical pre-intercalation of ions into the interlayer region to stabilize the bilayers. The chemical pre-intercalation of electrochemically cycled ions (e.g. Li⁺ ions for Li-ion batteries) may improve diffusion and result in higher capacities. Stabilizing ions, or ions that do not participate in electrochemical cycling (e.g. Mg²⁺ ions in Li-ion batteries) can act as pillars between the vanadium oxide bilayers and improve capacity retention. This work presents a variety of synthesis techniques to tune the interlayer chemistry and structure of bilayered vanadium oxide and modify its electrochemical performance. In this work, we first demonstrate the synthesis of [delta]-M[x]V₂O₅·nH₂O (M = Li, Na, K, Mg, or Ca) using V₂CT[x] MXene precursor. In this synthesis, a sol-gel technique is used for H₂O₂-assisted transformation of V₂CT[x]. Utilizing this novel precursor, this work presents the first report of two dimensional morphology for Na-, K-, Mg-, and Ca-chemically pre-intercalated [delta]-V₂O₅·nH₂O. MXene-derived [delta]-Li[x]V₂O₅·nH₂O and [delta]-Mg[x]V₂O₅·nH₂O had high initial capacities when cycled in non-aqueous Li-ion half cells, 200 and 192 mAh g⁻¹, respectively. Both materials also exhibited high capacity retention, 89 and 94% capacity retention after 50 cycles. Since this technique is limited in its ability to tune the interlayer region chemistry, the [alpha]-V₂O₅ sol-gel based chemical pre-intercalation synthesis was also developed. This work highlights three major synthesis pitfall for this technique that leads to impurity formation - the "coffee ring effect", overdrying during aging, and excess pressure during hydrothermal treatment. The synthesis procedure for high quality, single phase, chemical pre-intercalation of alkali and alkaline earth metal ions (Li, Na, K, Mg, and Ca) is reported. Building upon these synthesis technique, the interlayer Mg²⁺ ion content in [delta]-Mg[x]V₂O₅·nH₂O is varied by changing the MgCl₂ concentration during hydrothermal treatment. The phase, interlayer spacing, and chemical composition of these materials were characterized using XRD, EDS, and TGA. CV profiles of [delta]-Mg[x]V₂O₅·nH₂O hydrothermally treated in varying MgCl₂ solution concentration show similar electrochemical processes when tested in Li-ion half cells. Mg²⁺ rich and water deficient [delta]-Mg[x]V₂O₅·nH₂O demonstrated the best electrochemical performance. The last portion of this work focuses on the chemical pre-intercalation of multiple ions into the interlayer region via [alpha]-V₂O₅ derived synthesis. There are two approaches for this objective. The first consists of chemical pre-intercalation of one ion during the sol-gel step of synthesis and another during hydrothermal treatment. The second approach uses both desired ions during both steps of synthesis. This work demonstrates the formation of a single phase using both of these approaches for the combinations of Li⁺, K⁺, and Mg²⁺ ions via XRD. EDS was used to quantify Mg²⁺ ion content in [delta]-Li[x]Mg[y]V₂O₅·nH₂O phases. CV profiles were obtained for each [delta]-Li[x]Mg[y]V₂O₅·nH₂O material. Further electrochemical cycling is needed to understand the effects of two ions chemically pre-intercalated into the interlayer region.