We report on the performance of density functional theory (DFT) with the Tran–Blaha modified Becke–Johnson exchange potential and the random phase approximation dielectric function for optical constants of semiconductors in the ultraviolet–visible (UV-Vis) light region. We calculate optical bandgaps Eg, refractive indices n, and extinction coefficients k of 70 semiconductors listed in the Handbook of Optical Constants of Solids [(Academic Press, 1985), Vol. 1; (Academic Press, 1991), Vol. 2; and (Academic Press, 1998), Vol. 3] and compare the results with experimental values. The results show that the calculated bandgaps and optical constants agree well with the experimental values to within 0.440 eV for Eg, 0.246–0.299 for n, and 0.207–0.598 for k in root mean squared error (RMSE). The small values of the RMSEs indicate that the optical constants of semiconductors in the UV-Vis region can be quantitatively predicted even by a low-cost DFT calculation of this type.

Assessing the performance of the Tran-Blaha modified Becke-Johnson exchange potential for optical constants of semiconductors in the ultraviolet-visible light region

Organic/Inorganic Anions Coupling Enabled Reversible High-Valent Redox in Vanadium-based Polyanionic Compound

The low-cost and profusion of sodium reserves make Na-ion batteries (NIBs) a potential candidate to lithium-ion batteries for grid-scale energy storage applications. NaVOPO4 has been recognized as one of the most promising cathodes for high-energy NIBs, owing to their high theoretical capacity and energy density. However, their further application is hindered by the multiphase transition and conductivity confinement. Herein, we proposed a feasible, one-step hydrothermal synthesis to regulate the synthesis of α-NaVOPO4 with controlled morphologies. The electrochemical properties of the NaVOPO4 electrode can be significantly enhanced taking Ketjen black (KB) as the optimized conductive carbon. Besides, combining with the nanocrystallization and construction of the conductive framework via high-energy ball milling, taking KB as the conductive carbon, the as-prepared NaVOPO4/5%KB exhibits superior Na-storage performance (140.2 mA h g-1 at 0.1 C and a capacity retention of 84.8% over 1000 cycles at 10 C) to the original NaVOPO4 (128.5 mA h g-1 at 0.1 C and a capacity retention of 83.1% over 1000 cycles at 10 C). Moreover, the aqueous full cell with NaTi2(PO4)3 as the anode delivers a capacity of 114.7 mA h g-1 at 0.2 C (141 W h kg-1 energy density) and 80.6% capacity retention over 300 cycles at 5 C. The excellent electrochemical performance can be attributed to the nanosized structural and enhanced interfacial effect, which could be rewarding to construct electron transportation tunnels, thus speeding up the Na+-diffusion kinetics. The modified strategy provides an efficient approach to intensify the electrochemical performance, which exhibits potential application of the NaVOPO4 cathode for NIBs.

Regulated Synthesis of α-NaVOPO4 with an Enhanced Conductive Network as a High-Performance Cathode for Aqueous Na-Ion Batteries.

Na-ion batteries (NIBs) are an important technological alternative to Li-ion batteries (LIBs) for developing energy storage systems that are cost-effective and less constrained by geographical supply chains, with similar energy densities. Analogous to LIBs, cathodes play a critical role in determining the energy density of NIBs, and layered transition metal oxide compounds are widely used as NIB cathodes. However, the instability-driven irreversible phase transitions in layered frameworks, especially at large degrees of Na removal, has necessitated the exploration of rigid structural frameworks that are resistant to structural changes with Na exchange. Thus, polyanionic frameworks, which primarily consist of transition metal polyhedra interconnected by PO_4, SiO_4, and/or SO_4 units, with or without the presence of fluorine, have been investigated as NIB cathodes. This review provides a critical overview of recent studies using polyanion cathodes, which will be useful in guiding the scientific community towards the development of better NIB cathodes. Graphical abstract

https://link.springer.com/content/pdf/10.1557/s43578-022-00646-7.pdf

Critical overview of polyanionic frameworks as positive electrodes for Na-ion batteries

The rational design of the unique morphology of particles has been considered as the key to improving the structural stability of spinel LiMn2O4 cathode materials for Li-ion batteries. Herein, a facile solid-state combustion process, combined with a Cr and Al co-doping approach, is proposed to prepare various LiCr0.01AlxMn1.99−xO4 (x ≤ 0.10) cathode materials with a good crystallinity. Cr and Al co-doping facilitates the formation of a single crystal truncated octahedral morphology. This endows the as-prepared LiCr0.01AlxMn1.99−xO4 with abundant {111} planes for Mn dissolution reduction and a few {100} and {110} planes for Li+ ion fast diffusion channels. Moreover, the introduction of Cr and Al elements with a stable electronic configuration further boosts the structural stability of the spinel LiMn2O4 owing to the relatively robust Al–O and Cr–O bonds compared with the Mn–O bond. Owing to these advantages, the optimal LiCr0.01Al0.05Mn1.94O4 delivers a good electrochemical performance with a high first discharge capacity of 118.5 mA h g−1 and a capacity retention of 70.8% after 1000 cycles at 1 C. Even at relatively high current rates of 15 and 20 C, a durable and prolonged cycling performance of up to 3000 cycles can be achieved. In addition, a high-temperature capacity retention of 72.1% is also maintained after 200 cycles at 5 C under 55 °C. This work provides potential candidates for developing long-life Li-ion batteries with a simultaneously high capacity.

Yanxia Wang

Papers

Monoclinic α‐NaVOPO4 as cathode materials for sodium‐ions batteries: Experimental and DFT investigation

Molten salt synthesis of LiMn1.2Ni0.3Cr0.1Co0.15Al0.23La0.02O4 as a positive electrode for lithium‐ion batteries