Reversible intercalation of the charge carrying ions remains the most reliable mechanism of the operation of rechargeable energy storage devices. While Li-ion batteries currently dominate the market, as the number of applications requiring batteries, including large-scale storage technologies, grows, it becomes obvious that new solutions, utilizing more abundant elements than lithium, are needed. Na-ion batteries (SIBs), an electrical energy storage system that belongs to beyond lithium ion (BLI) battery family, is an attractive candidate as they operate due to reversible intercalation of more abundant Na⁺ ions. However, Na⁺ ion is larger and heavier than Li⁺ ion, which can affect diffusion and cause deteriorated performance compared to that of Li-ion batteries. In addition, Na-ion batteries operate at lower voltages than Li-ion batteries, and thus higher capacity cathode materials are needed to increase the energy density of SIBs. Layered transition metal oxides with relatively high working voltages and large interlayer spacing, enabling a large number of intercalation sites and well-defined two-dimensional pathways for ion diffusion, are considered as attractive candidates for applications as cathodes in BLI batteries. In this presentation, we will demonstrate how electrochemical activity of the bilayered vanadium oxide in Na-ion batteries can be controlled through the modification of its chemical composition and interlayer spacing achieved in a versatile synthesis approach, called chemical pre-intercalation.

Chemical pre-intercalation is a sol-gel-based process developed in our laboratory to insert specific types and amounts of guest species, such as inorganic ions and organic molecules, into the crystal structure of host electrode material leading to unprecedented compositional diversity. Post sol-gel process treatments, such as aging, hydrothermal treatment and thermal treatment, can be used to further tune chemical composition and structure of the synthesized materials [1]. We use vanadium pentoxide as the host material due to the ability of vanadium to reduce from V⁵⁺ to V³⁺, which is accompanied by the transfer of two electrons resulting in high specific capacity needed for cathodes in BLI batteries. More specifically, we focus on a unique allotropic modification of vanadium pentoxide, so-called bilayered (or δ-) V₂O₅ which is characterized by unusually large for oxides interlayer spacing of >9 Å. We will discuss the importance of the synthesis parameters, such as an extended aging step and addition of chemically pre-intercalated species during hydrothermal treatment process, for the formation of bilayered V₂O₅. We have successfully used chemical pre-intercalation to synthesize ion-preintercalated δ-M_xV₂O₅ (M = Li, Na, K, Mg, Ca) with tunable interlayer spacing ranging from 9.6 Å for δ-K_xV₂O₅ to 13.4 Å for δ-Mg_xV₂O₅. Chemical pre-intercalation of organic molecules, such as cetrimonium bromide (CTAB), allowed to expand the interlayer spacing further up to >30 Å. We will report the maximum content of chemically preintercalated species achieved in our work and discuss strategies for tuning the degree of chemical preintercaltion and how it affects electrochemical performance.

Bilayered δ-Na_xV₂O₅ demonstrated a record high initial capacity of 365 mAh/g in Na-ion cells, however this capacity decayed quickly [1]. We will show that low-temperature thermal annealing improves electrochemical stability of chemically pre-intercalated bilayered V₂O₅, which was attributed to the increased crystallinity of the materials. Annealing was carried out at 260⁰C under vacuum to prevent phase transformations and preserve the bilayered crystal structure. We also showed that electrochemical stability of the bilayered vanadium oxide can be improved through chemical pre-intercalation of electrochemically inactive, or stabilizing, ions (Li⁺, K⁺, Mg²⁺, and Ca²⁺). While the highest initial capacity (~350-365 mAh g^-1) in Na-ion cells was exhibited by δ-Li_xV₂O₅ and δ-Na_xV₂O₅, the greatest capacity retention (68% after 50 cycles at C/10) was demonstrated by δ-Mg_xV₂O₅ with the largest interlayer spacing (13.40 Å) achieved in this study. The highest capacity retention at higher current rates (~50% when current rate was changed from C/10 to 1C) were exhibited by Li- and Mg-stabilized phases. We will discuss the role of chemically preintercalated ion size and charge in electrochemical performance of δ-M_xV₂O₅ (M = Li, Na, K, Mg, Ca). We will also report our findings on the effect of chemical pre-intercalation of organic molecules on structure and electrochemical performance of the bilayered vanadium oxide in Na-ion batteries.

1. Clites, Byles, Pomerantseva, J. Mater. Chem. A 4 (2016) 7754.