Bilayered vanadium oxide, δ-V2O5·nH2O, is a promising electrode material for Na-ion batteries due to its large interlayer spacing, 11.5 Å, that allows for insertion of many charge-carrying ions. Previously, δ-V2O5·nH2O electrodes have shown high capacities in Na-ion batteries1-3. However, capacity fade is common when synthesized via cost effective, sol-gel routes. Poor cycling stability is attributed to the loss of V-O layers’ stacking order upon cycling1. Therefore, methods to improve the structural stability of the δ-V2O5·nH2O phase are necessary for its utilization as Na-ion cathodes. A synthesis approach known as chemical pre-intercalation allows for the insertion of inorganic cations into the structure of electrode materials prior to electrochemical cycling. Previously, we have demonstrated that chemical pre-intercalation of Na-ions into the bilayered phase results in high initial capacities above 350 mAh g-1 in Na-ion cells3. In this study, we focus on the incorporation of low-temperature annealing to increase structural and electrochemical stability of the bilayered phase in Na-ion batteries. We demonstrate that annealing can lead to increased crystallinity leading to increased cycling stability. This result shows how synthesis approaches affect the structure of the bilayered vanadium oxide phase and can lead to increased electrochemical stability in Na-ion cells.
Chemical pre-intercalation is a low-temperature, scalable synthesis method that utilizes a sol-gel process to form layered oxides with positively-charged species inserted between the layers. We have shown that this approach can be used to successfully intercalate Li+ , Na+ , K+ , Mg2+, and Ca2+ ions into the crystal structure of bilayered vanadium oxide (δV2O5).1 Through this ion-intercalation, the interlayer spacing of the δ-MgxV2O5 (M=Li, Na, K, Mg, and Ca) structure can be controlled between 9.6 Å (δ-KxV2O5) and 13.4 Å (δ-MgxV2O5).1 Moreover, the expanded spacing achieved for the δ-MgxV2O5 phase corresponded to increased electrochemical stability in both Li- and Na-ion cells.[1] While this study identified a correlation between expanded interlayer spacing and improved electrochemical stability over cycling, chemical pre-intercalation of ions does not allow for expansion beyond that exhibited by the δ-MgxV2O5 structure. In this work, we show that further expansion of the interlayer spacing can be achieved via pre-intercalation of positivelycharged linear, organic cations. We report synthesis of hybrid inorganic/organic materials with a 1D nanobelt morphology. The layered structure of the hybrids is confirmed by both XRD and TEM analysis. δ-V2O5 preintercalated with cetyltrimethylammonia ions, CTA+ , demonstrated the interlayer spacings of all samples (31 Å), more than twice larger than the largest interlayer spacing achieved via pre-intercalation of inorganic ions. The effects of carbon chainlength and positively charged nitrogen termini on the interlayer spacing and electrochemical stability is investigated, with two N-termini on the cation (DMO+) resulting in increased electrochemical stability of the preintercalated phase.
Chemical pre-intercalation is a soft chemistry synthesis approach that allows for the insertion of inorganic ions into the interlayer space of layered battery electrode materials prior to electrochemical cycling. Previously, we have demonstrated that chemical pre-intercalation of Na+ ions into the structure of bilayered vanadium oxide (δ-V2O5) 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. However, the effect of chemical pre-intercalation of δ-V2O5 has not been studied for other ion-based systems beyond sodium. In this work, we report the effect of the chemically preintercalated alkali ion size on the mechanism of charge storage of δ- MxV2O5 (M = Li, Na, K) in Li-ion, Na-ion, and K-ion batteries, respectively. The interlayer spacing of the δ-MxV2O5 varied depending on inserted ion, with 11.1 Å achieved for Li-preintercalated δ-V2O5, 11.4 Å for Na-preintercalated δ- V2O5, and 9.6 Å for K-preintercalated δ-V2O5. Electrochemical performance of each material has been studied in its respective ion-based system (δ-LixV2O5 in Li-ion cells, δ-NaxV2O5 in Na-ion cells, and δ-KxV2O5 in K-ion cells). All materials demonstrated high initial capacities above 200 mAh g-1. However, the mechanism of charge storage differed depending on the charge-carrying ion, with Li-ion cells demonstrating predominantly pseudocapacitive behavior and Naion and K-ion cells demonstrating a significant portion of capacity from diffusion-limited intercalation processes. In this study, the combination of increased ionic radii of the charge-carrying ions and decreased synthesized interlayer spacing of the bilayered vanadium oxide phase correlates to an increase in the portion of capacity attributed diffusion-limited charge-storage processes.
Vanadium oxide with bilayered crystal structure shows high specific capacity in intercalation-based energy storage systems, such as Li-ion and Na-ion batteries. The enhanced charge storage ability is attributed to the high oxidation state of vanadium enabling intercalation of more than one Li+ (or Na+) ion per V2O5 unit cell. In addition, large interlayer spacing of ∼10–13 Å, typical for the bilayered vanadium oxide, is believed to lead to the facilitated diffusion of charge carrying ions further improving specific capacity of this material. However, we found that initial high capacity of the bilayered V2O5 notably decreases only after a few cycles. In this work, we show results of the capacity stabilization strategy based on inclusion of inorganic ions, other than lithium ion, between the structural layers using chemical pre-intercalation approach. These ions are believed to form bonds with the V–O layered framework improving structural stability of the material during electrochemical cycling, and therefore they are often called stabilizing ions. In this paper we report how electrochemical stability of the AxV2O5 (A = Na, K, Mg, Ca) cathode materials is correlated with the size and charge of the stabilizing ions. Li-preintercalated vanadium oxide (LixV2O5) served as the reference material in this study. We found that chemical insertion of doubly charged, small (r = 0.86 Å) Mg2+ stabilizing ion results in the highest capacity retention.
Na-ion batteries have received considerable attention in recent years but still face performance challenges such as limited cycle lifetime and low capacities at high current rates. In this work, we propose novel combinations of preand post-synthesis treatments to modify known Na-ion battery electrode materials to achieve enhanced electrochemical performance. We work with two model metal oxide materials to demonstrate the effectiveness of the different treatments. First, wet chemical preintercalation is combined with post-synthesis aging, hydrothermal treatment, and annealing of α-V2O5, resulting in enhanced capacity retention in a Na-ion battery system. The hydrothermal treatment resulted in an increased specific capacity of nearly 300 mAh/g. Second, post-synthesis acid leaching is performed on α- MnO2, also resulting in improved electrochemical capacity. The chemical, structural, and morphological changes brought about by the modifications are fully characterized.
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