V_2O_5·nH_2O nanosheets and multi-walled carbon nanotube composite as a negative electrode for sodium-ion batteries

Two dimensional(2D) transition metal oxides and chalcogenides demonstrate a promising performance in sodium-ion batteries(SIBs) application. In this study, we investigated the use of a composite of freeze dried V_2O_5·nH_2O nanosheets and multi-walled carbon nanotube(MWCNT) as a negative electrode material for SIBs. Cyclic voltammetry(CV) results indicated that a reversible sodium-ion insertion/deinsertion into the composite electrode can be obtained in the potential window of 0.1–2.5 V vs. Na^+/Na. The composite electrodes delivered sodium storage capacities of 140 and 45 m Ah g^(-1) under applied current densities of 20 and 100 m A g^(-1), respectively. The pause test during constant current measurement showed a raise in the open circuit potential(OCP) of about 0.46 V, and a charge capacity loss of ～10%. These values are comparable with those reported for hard carbon electrodes. For comparison, electrodes of freeze dried V_2O_5·nH_2O nanosheets were prepared and tested for SIBs application. The results showed that the MWCNT plays a significant role in the electrochemical performance of the composite material.

Driving the sodium-oxygen battery chemistry towards the efficient formation of discharge products: The importance of sodium superoxide quantification

Sodium-oxygen batteries(SOBs) have the potential to provide energy densities higher than the state-ofthe-art Li-ion batteries. However, controlling the formation of sodium superoxide(NaO_(2)) as the sole discharge product on the cathode side is crucial to achieve durable and efficient SOBs. In this work, the discharge efficiency of two graphene-based cathodes was evaluated and compared with that of a commercial gas diffusion layer. The discharge products formed at the surface of these cathodes in a glyme-based electrolyte were carefully studied using a range of characterization techniques. NaO_(2) was detected as the main discharge product regardless of the specific cathode material while small amounts of Na_(2)O_(2).2H_(2)O and carbonate-like side-products were detected by X-ray diffraction as well as by Raman and infrared spectroscopies. This work leverages the use of X-ray diffraction to determine the actual yield of NaO_(2)which is usually overlooked in this type of batteries. Thus, the proper quantification of the superoxide formed on the cathode surface is widely underestimated;even though is crucial for determining the efficiency of the battery while eliminating the parasitic chemistry in SOBs. Here, we develop an ex-situ analysis method to determine the amount of NaO_(2) generated upon discharge in SOBs by transmission X-ray diffraction and quantitative Rietveld analysis. This work unveils that the yield of NaO_(2) depends on the depth of discharge where high capacities lead to very low discharge efficiency, regardless of the used cathode. We anticipate that the methodology developed herein will provide a convenient diagnosis tool in future efforts to optimize the performance of the different cell components in SOBs.