Chemical pre-intercalation is a wet chemistry method that allows for the insertion of positively charged inorganic ions into the crystal structure of electrode materials prior to electrochemical cycling. This synthetic methodology was proven to be efficient for layered electrode materials. In our work, we use bilayered vanadium oxide (δ-V₂O₅) as a high capacity intercalation host. Chemical pre-intercalation synthesis approach is based on a sol-gel process in which the vanadium oxide bilayers grow through a sequence of hydroxylation and condensation reactions. During this process, chemically preintercalated inorganic ions are added into the reaction mixture, directing formation of novel, metastable structures with controllable, large interlayer spacing. Post-synthesis treatment, such as aging, hydrothermal treatment, or annealing, can be used to further tune structure, composition, and electrochemical properties of the synthesized materials.

This method can be utilized to investigate two distinct directions in electrochemical systems. First, the effect of pre-intercalation of electrochemically active, charge-carrying ions (for example Li⁺ for Li-ion batteries) into the open interlayer spacing of a layered host structure can be studied with the aim to understand if ionic mobility and specific capacity can be improved. We have previously demonstrated that chemical pre-intercalation of Na⁺ ions into the structure of bilayered vanadium oxide results in record high initial capacities above 350 mAh g^-1 in Na-ion cells. This performance is attributed to the expanded interlayer spacing and predefined diffusion pathways achieved by the insertion of charge-carrying ions prior to cycling [1].

Second, the effect of pre-intercalation of electrochemically inactive, stabilizing ions (for example Mg²⁺ ions for Li-ion batteries) on electrochemical stability of the layered phase upon cycling in various ion-based systems can be determined. We have recently shown that d-V₂O₅ can be preintercalated with a range of inorganic ions, including alkali (Li⁺, Na⁺, K⁺) and alkali-earth (Mg²⁺and Ca²⁺) ions, via our chemical pre-intercalation approach [2]. By altering the pre-intercalated ion, the interlayer spacing of the synthesized materials can be controlled between 9.62 Å to 13.40 Å. A correlation between hydrated ionic radii and interlayer spacing was for the first time described [2]. We demonstrated that ion-stabilization can indeed be used to improve the electrochemical stability of the bilayered phase in Li-ion and Na-ion batteries and that larger interlayer spacing can lead to improved electrochemical performance. Doubly charged Mg-preintercalated δ-Mg_xV₂O₅, which has the largest interlayer spacing of 13.40 Å, shows the highest capacity retentions of 81.8% and 68.3% in Li-ion and Na-ion batteries, respectively.

In this work, we will first present a systematic study of the effect of chemical pre-intercalation of charge-carrying ions in three alkali-ion intercalation-based energy storage systems (δ-Li_xV₂O₅ in Li-ion cells, d-Na_xV₂O₅ in Na-ion cells, and δ-K_xV₂O₅ in K-ion cells). The initial specific capacities vary depending on ion system, with δ-Li_xV₂O₅ material exhibiting a capacity of 250 mAh g^-1 in Li-ion cells and δ-Na_xV₂O₅ material exhibiting a capacity of 365 mAh g^-1 in Na-ion cells, when cycled in 1.5 – 4.0 V vs. Li/Li⁺ and Na/Na⁺ range, respectively. The mechanism of charge storage differs depending on the charge-carrying ion, with Li-ion and Na-ion cells demonstrating predominantly pseudocapacitive behavior with the sloped shape of charge/discharge curves, and K-ion cells revealing diffusion limited intercalation processes characterized by distinct plateaus on charge/discharge curves. Therefore, K-preintercalated bilayered vanadium oxide was cycled in a voltage range of 2.0 – 4.3 V vs. K/K⁺ and demonstrated a capacity of 238 mAh g^-1. Our results provide an insight on how the mobility of the charge carriers is affected by their size while the charge is maintained the same.

Second, the role of chemical pre-intercalated non-cycling ions on electrochemical stability will be discussed. Indeed the larger fraction of chemically preintercalated stabilizing ions can create stronger bonds with the layers of the host material, thus improving its stability. But at the same time, additional positive charge held by stabilizing ions decreases the charge-storage capabilities of the electrode materials. Therefore, an optimum amount of the stabilizing ions need to be determined to achieve high capacity and high electrochemical stability simultaneously. We will demonstrate the effect of stabilizing ion content on electrochemical performance of δ-Mg_xV₂O₅ in alkali-ion intercalation-based energy storage systems. This work demonstrates the efficacy and versatility of the unique chemical pre-intercalation approach to synthesize a wide-range of high-capacity, advanced layered electrode materials with controllable electrochemical performance.

1. Clites, M., B.W. Byles, Pomerantseva, E., Journal of Materials Chemistry A, 2016. 4(20), 7754.
2. Clites, M.; Pomerantseva, E., in preparation, 2017.