Zinc-ion batteries are emerging as next-generation rechargeable batteries that can operate in aqueous electrolytes. We first examine the feasibility of open-structured VO 2 (B) as a Zn 2+ intercalation host. A bond-valence sum energy map predicts that four Zn 2+ -ion sites (Zn C , Zn A1 , Zn A2 , and Zn C′ ) can exist in the structure. Using first-principles calculations, we verified that 0.5 mol of Zn 2+ ions can be reversibly (de)intercalated with an average voltage of ∼0.61 V (vs Zn 2+ /Zn), which is comparable with the experimental results. The specific capacity of VO 2 (B) at 50 mA g −1 is maintained up to ∼365 mAh g −1 corresponding to the storage capacity of ∼0.57 mol of Zn 2+ ions in the framework of VO 2 (B), and its redox reaction occurs at ∼0.61 V. The high capacity is maintained for 200 cycles, with capacity retention of 80% (288 mAh g −1 ). Moreover, the capacity delivered by the VO 2 (B) electrode is stable even with cycling at a rate of 5C (1750 mA g −1 ) at approximately 110 mAh g −1 . This high-power capability of VO 2 is supported by the theoretical approach based on first-principles calculation, which shows the activation barrier for Zn 2+ diffusion in the VO 2 (B) structure. These findings demonstrate the potential of open-structured VO 2 (B) as a new candidate material.
intensively investigated and developed at a global level. Among several possible energy storage and power sources, lithium-ion batteries (LIBs) have been successfully used in applications ranging from portable electronic devices to electric vehicles and large-scale energy storage systems (ESSs) owing to their high energy density. However, some critical concerns have arose regarding LIBs such as the cost of lithium resources and safety issues from mobile to ESS applications. [1-6] Thus, rechargeable divalent-ion (e.g., Zn 2+ , Mg 2+ , and Ca 2+) batteries operated in aqueous electrolytes have recently received significant attention because the use of water results in low cost, improved safety, cell fabrication in ambient environment, and reasonable environmental impact. [7-9] Furthermore, the ionic conductivity of aqueous electrolytes for rechargeable divalent-ion batteries is several-ordersof-magnitude higher than that of organic electrolytes. [7,10-12] Among divalent-ion batteries, aqueous zincion batteries (ZIBs) have been intensively investigated owing to the high gravimetric capacity (820 mAh g −1) and sufficient earth abundance of Zn metal. Moreover, the high overpotential of Herein, the promising properties of open-structured NaV 3 O 8 as a cathode material for Zn-ion batteries (ZIBs) are investigated. First-principles calculations predict the insertion of Zn 2+ (0.74 Å) in NaV 3 O 8 with an interlayer distance of ≈7 Å, enabling delivery of a high discharge capacity of 353 mAh g −1 at 70 mA g −1 (0.2 C) for 300 cycles in the operating window of 0.3−1.5 V in 1 m Zn(CF 3 SO 3) 2 aqueous solution. Operando synchrotron X-ray diffraction, X-ray absorption near edge structure spectroscopy, and first-principles calculations validate the insertion of Zn 2+ into the NaV 3 O 8 structure within the operation range. Moreover, operando synchrotron X-ray diffraction and operando Raman spectroscopy reveal the formation of layered zinc hydroxytriflate (Zn 5 (OH) 8 (CF 3 SO 3) 2 •xH 2 O) as a side reaction below 0.8 V on discharge (reduction) and its dissolution into the electrolyte above 0.8 V on charge (oxidation). The formation of the Zn hydroxytriflate interfacial layer increases the charge-transfer activation energy from 15.5 to 48 kJ mol −1 , leading to kinetics fade below 0.8 V. The findings reveal the charge-storage mechanism for NaV 3 O 8 , which may also be applicable to other vanadate cathodes, providing new insights for the investigation and design of ZIBs.
Electrochemistry of quantum dots (QDs) can provide useful information on their redox behavior and energy states. Here, we present a study of the surface-limited electrochemical deposition of cadmium adatoms (underpotential deposition or upd) on electrophoretically formed films of CdSe QDs of different sizes (2.4–6.3 nm) capped with different ligands (oleate and sulfide). Oleate-capped QD films were shown to have a weak electrochemical response in upd reaction, whereas sulfide-capped QDs demonstrated a significant increase in cathodic current that was attributed to enhanced surface accessibility and improved interparticle electron transfer. Cadmium upd onset potential in sulfide-treated QD films was found to be markedly size dependent. The increase of QD size results in the positive shift of upd onset potential which is in agreement with the change in lowest unoccupied molecular orbital position. The results imply that the proposed method utilizing electrochemical surface-limited reaction on QDs can be applied to probe energy states of chalcogenide semiconductor nanoparticles.
Electrophoretically deposited (EPD) quantum dots (QDs) can be charged electrochemically via electron injection from a conducting substrate, leading to pronounced changes in their electrical and optical properties. The 180–550 nm thick EPD films composed of CdSe QDs with different diameters (2.8–6.3 nm) demonstrate a strong and reversible electrochromic response due to bleaching of excitonic transitions. The number of injected electrons was found to increase with QD size from 1.3 (QD diameter of 2.8 nm) to 6 (QD diameter of 6.3 nm) electrons per nanoparticle. As a result for 3.4 nm, 4.5 nm, and 6.3 nm QDs a complete 1Se level filling was observed, while the smallest studied QDs (2.8 nm) exhibited only a partial 1Se level population. In addition, 4.5 and 6.3 nm QDs also showed partial 1Pe level filling with electrons. The data from both cyclic voltammetry measurements and electrochemically driven spectral bleaching enabled determining the electrochemically derived 1Se level energies. Additionally, we demonstrate fast charging–discharging kinetics for EPD CdSe QD films with complete absorption bleaching and recovery in a sub-100 ms time scale, which opens prospects for utilizing such films in various applications such as electrochromic displays, smart windows or tunable color filters for photography.
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