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The valance state of vanadium- key factor in the flexibility of potassium vanadates structure as cathode materials in li-ion batteries
Potassium hexavanadate (K2V6O16·nH2O) nanobelts have been synthesized by the LPE-IonEx method, which is dedicated to synthesis of transition metal oxide bronzes with controlled morphology and structure. The electrochemical performance of K2V6O16·nH2O as a cathode material for lithium-ion batteries has been evaluated. The KVO nanobelts demonstrated a high discharge capacity of 260 mAh g-1, and long-term cyclic stability up to 100 cycles at 1 A g-1. The effect of the vanadium valence state and unusual construction of the nanobelts, composed of crystalline and amorphous domains arranged alternately were also discussed in this work. The ex-situ measurements of discharged electrode materials by XRD, MP-AES, XAS and XPS show that during the subsequent charge/discharge cycle the potassium in the K2V6O16·nH2O structure are replacing by lithium. The structural stability of the potassium hexavandate during cycling depends on the initial vanadium valence state on the sample surface and the presence of the “fringe free” domains in the K2V6O16·nH2O nanobelts.
The electrochemical performance, as well as the structural flexibility of K2V6O16·nH2O, strongly depends on the vanadium valence state. The charge on hydrated potassium vanadate is stored via redox reaction mainly at the surface. Thus, via the presence of the V4+ on the surface, the electronic transfer is facilitated, and higher capacity is achievement. Moreover, the higher V4+ surface concertation leads to faster structural changes during subsequent charge/discharge cycles. The ex-situ characterization by XRD and MP-AES shows that during the subsequent charge/discharge cycle, the potassium ions in the K2V6O16·nH2O structure are replacing by lithium. However, the higher initial concentration of V4+ leads to increased vacancies gradually during cycling, both on the surface and in the bulk as confirmed by the ex-situ XPS and XANES analysis. Therefore, the structural damage occur slowly. The unusual construction of the nanobelts, composed of crystalline and amorphous domains arranged alternately, probably also prevents the crystal structure from collapsing during this exchange.
V K-edge the XANES for samples KVO-20 (left) and KVO-40 (right), respectively. The insets show the relation between vanadium oxidation state and the edge position
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Written by: Marta Prześniak-Welenc
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