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 . 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-MxV2O5 (M = Li, Na, K, Mg, Ca) with tunable interlayer spacing ranging from 9.6 Å for δ-KxV2O5 to 13.4 Å for δ-MgxV2O5. 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-NaxV2O5 demonstrated a record high initial capacity of 365 mAh/g in Na-ion cells, however this capacity decayed quickly . 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-LixV2O5 and δ-NaxV2O5, the greatest capacity retention (68% after 50 cycles at C/10) was demonstrated by δ-MgxV2O5 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-MxV2O5 (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.