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Redox mediators (RMs) have a substantial ability to govern oxygen reduction reaction (ORR) in Li-O 2 batteries, which can realize large capacity and high-rate capability. However, studies on understanding RM-assisted ORR mechanisms are still in their infancy. Herein, a quinone-based molecule, vitamin K1 (VK1), is first used as the ORR RM for Li-O 2 batteries, together with 2,5-di- tert -butyl-1,4-benzoquinone (DBBQ), to elucidate key factors on the catalytic activity of RMs. By combining experiments and first-principle computations, we demonstrate that the reduced VK1 has strong oxygen affinity and can effectively retard the deposition of Li 2 O 2 films on the electrode surface, thereby guaranteeing enough active sites for electron transfer. Besides, the low reaction free energy of disproportionation of the Li(VK1)O 2 intermediate into Li 2 O 2 also significantly accelerates the ORR process. Consequently, the catalytic activity of VK1 is significantly boosted, and the discharge capacity of VK1-assisted batteries is 3.2-4.5 times that of DBBQ-assisted batteries. This study provides new insight for better understanding the working roles of RMs in Li-O 2 batteries.

Structure determines properties, and properties determine applications, which is an important ideology of natural sciences. For optical materials, it is vital to lucubrate the corresponding relationship between the local crystal structure and luminescence properties for their design, synthesis, and application. This work reports a newly designed Y 2 Mg 2 Al 2 Si 2 O 12 (YMAS):Eu 3+ red phosphor, in which difunctional Eu 3+ ion is used as a red-light activator and spectroscopic probe. The qualitative and quantitative studies on the relationship between the local crystal structure and the luminescence properties of YMAS:Eu 3+ are performed experimentally and computationally, using the Y 3 Al 5 O 12 (YAG):Eu 3+ as contrast. Moreover, compared with YAG:Eu 3+ , the newly designed YMAS:Eu 3+ has stronger luminescence, superior Commission Internationale de L'Eclairage chromaticity coordinates, a lower optimal doping concentration, and equally excellent thermal stability. The satisfactory color-rendering index of packaged white-light-emitting diodes demonstrates its potential performance as a red phosphor. Briefly, this work provides not only a new case for the study of the local crystal structure and luminescence properties but also a new possibility for the application of a red phosphor in solid-state lighting.

Multiple negative factors, including the poor electronic conductivity of sulfur, dissolution and shuttling of lithium polysulfides (Li 2 S n ), and sluggish decomposition of solid Li 2 S, seriously hinder practical applications of lithium-sulfur (Li-S) batteries. To solve these problems, a general strategy was proposed for enhancing the electrochemical performance of Li-S batteries using surface-functionalized Ti 3 C 2 MXenes. Functionalized Ti 3 C 2 T 2 (T = N, O, F, S, and Cl) showed metallic conductivity, as bare Ti 3 C 2 . Among all Ti 3 C 2 T 2 investigated, Ti 3 C 2 S 2 , Ti 3 C 2 O 2 , and Ti 3 C 2 N 2 offered moderate adsorption strength, which effectively suppressed Li 2 S n dissolution and shuttling. This Ti 3 C 2 T 2 exhibited effective electrocatalytic ability for Li 2 S decomposition. The Li 2 S decomposition barrier was significantly decreased from 3.390 eV to ∼0.4 eV using Ti 3 C 2 S 2 and Ti 3 C 2 O 2 , with fast Li + diffusivity. Based on these results, O- and S-terminated Ti 3 C 2 were suggested as promising host materials for S cathodes. In addition, appropriate functional group vacancies could further promote anchoring and catalytic abilities of Ti 3 C 2 T 2 to boost the electrochemical performance of Li-S batteries. Moreover, the advantages of a Ti 3 C 2 T 2 host material could be well retained even at high S loading, suggesting the potential of surface-modified MXene for confining sulfur in Li-S battery cathodes.

Transition metal polysulfides with high S content, such as VS 4 , TiS 4 , and MoS 3 , have high specific Li + capacities, but their reaction mechanisms for lithium-ion batteries remain unclear due to unknown intermediate products. In this work, first-principles calculations based on the density functional theory were performed to reveal the electrochemical properties of VS 4 for lithium-ion batteries. The results demonstrated multiple phase transformations during Li + insertion, starting with nucleation transformation from VS 4 to Li 3 VS 4 and followed by gradual decomposition reactions. Enthalpy-driven long-range migration of Li 2 S molecules resulted in crystalline to amorphous transformation during decomposition. S and V successively behaved as redox centers for Li x VS 4 before and after x = 3. Moreover, low activation energy and high Li + diffusivity were observed at room temperature, revealing superior rate capability of the material.