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Recently, a large family of 2D materials called MXenes have attracted much attention in the field of supercapacitors, thanks to the excellent performance demonstrated by Ti3C2 MXene-based electrodes. However, research on MXenes for supercapacitor applications has been primarily focused on Ti3C2, even though there are more than 20 other members of this large family of materials already available. Studies on other MXenes are emerging, with promising results already achieved by Ti2C, Mo2C, and Mo1.33C in aqueous electrolytes. Yet, many other MXenes remain unexplored in aqueous supercapacitor applications. In this work, we report on the electrochemical behavior of a vanadium carbide MXene, V2C, in three aqueous electrolytes. Excellent specific capacitances were achieved, specifically 487 F/g in 1 M H2SO4, 225 F/g in 1 M MgSO4, and 184 F/g in 1 M KOH, which are higher than previously reported values for few micrometer-thick delaminated MXene electrodes. This work shows the promise of V2C MXene for energy storage using aqueous electrolytes. Keywords: MXene, Supercapacitor, Vanadium carbide, Two-dimensional material, Aqueous electrolyte, Pseudocapacitance

Journal of materials chemistry / A 8(19), 9567 - 9578 (2020). doi:10.1039/D0TA00858C, Monoclinic VO$_2$(B) nanoflakes prepared by a hydrothermal method displayed superior electrochemical performance in 1 M ZnSO$_4$ electrolyte. The reaction mechanisms of VO$_2$(B) and the essential causes of byproduct formation in aqueous rechargeable zinc batteries (ARZBs) were comprehensively studied by electrochemical measurements combined with in operando synchrotron techniques and first-principles calculations. During the electrochemical processes, the electrode underwent a reversible solid-solution reaction between VO$_2$(B) and Zn$_{0.44}$VO$_2$ with the simultaneous formation/decomposition of the (Zn(OH)$_2$)$_3$(ZnSO$_4$)��5H$_2$O byproduct. Importantly, the formation of the byproduct was attributed to [Zn(H$_2$O)$_6$]$^{2+}$ dehydration, where the byproduct could protect the electrode material from the corrosion of H$_3$O$^+$ and facilitate the dehydration process of Zn$^{2+}$ on the electrode���electrolyte interface. The byproducts could facilitate the migration of Zn$^{2+}$ on the electrode surface due to their three-dimensional pathways. In addition, the electrochemical performance of VO$_2$(B) and the byproduct in ZnSO$_4$ electrolyte were compared with those in Zn(CF$_3$SO$_3$)$_2$ and Zn(NO$_3$)$_2$. An appropriate electrolyte (1 M Zn(CF$_3$SO$_3$)$_2$) to form a byproduct with largely expanded ionic pathways was proven to further improve the electrochemical performance of VO$_2$(B). This work not only provides a deep understanding of the Zn$^{2+}$ storage mechanism in VO$_2$(B) but also establishes a clear relationship between the byproducts and electrochemical performance of vanadium-based electrode materials in ARZBs., Published by RSC, London ��[u.a.]��

Sodium-ion batteries operating at ambient temperature hold great promise for use in grid energy storage owing to their significant cost advantages. However, challenges remain in the development of suitable electrode materials to enable long lifespan and high rate capability. Here we report a sodium super-ionic conductor structured electrode, sodium vanadium titanium phosphate, which delivers a high specific capacity of 147 mA h g−1 at a rate of 0.1 C and excellent capacity retentions at high rates. A symmetric sodium-ion full cell demonstrates a superior rate capability with a specific capacity of about 49 mA h g−1 at 20 C rate and ultralong lifetime over 10,000 cycles. Furthermore, in situ synchrotron diffraction and X-ray absorption spectroscopy measurement are carried out to unravel the underlying sodium storage mechanism and charge compensation behaviour. Our results suggest the potential application of symmetric batteries for electrochemical energy storage given the superior rate capability and long cycle life., Discovering suitable electrodes is a challenge for the development of sodium-ion batteries. Here the authors demonstrate a high-performance symmetric battery based on Na2VTi(PO4)3, highlighting its potential application in a smart grid.

The electrochemical properties of Li[Li0.2Co0.4Mn0.4]O2 were studied at room temperature and −20 °C. EIS study showed that the electrochemical kinetics of the first charge was more sluggish than subsequent cycles, which became even worse at −20 °C. This made it difficult for the material to achieve a sufficient oxygen loss reaction at low temperature. XPS analysis showed that almost all of the Mn4+ ions in the material were activated at room temperature, whereas only a few of them were activated at −20 °C. As a result, the material showed a high discharge capacity of 246 mAh g−1 at room temperature, comparing to the 155 mAh g−1 at −20 °C. However, the inactive Mn4+ ions in the electrode suppressed the dissolution of manganese and the Jahn–Teller distortion of the material lattice, both of which resulted in excellent cycle life at low temperature. Keywords: Li-ion battery, Li-riched layered material, Electrochemical kinetics, Low temperature

Li[Li0.23Co0.3Mn0.47]O2 cathode material was prepared by a sol–gel method. The material had a primary particle size of about 100 nm, covered by a 30 Å of Li2CO3 layer. The material showed promising electrochemical performance when cycled up to 3C rate. The electrochemical kinetics of the first charge was much slower than that of the second charge, due to the complex electrochemical process which involved not only Li+ diffusion but also release of oxygen. By taking account of this, the material was pre-charged very slowly (∼C/50) in the first cycle. This led to excellent electrochemical performance in the following cycles. For instance, the 1C-rate capacity increased to 168 mA h g−1 after 50 cycles, comparing with the 145 mA h g−1 obtained without pre-charging. Keywords: Lithium ion battery, Cathode material, Nano material, Electrochemical kinetics, Cycling performance