Understanding the Structural Phase Transitions in Na3 V2 (PO4 )3 Symmetrical Sodium-Ion Batteries Using Synchrotron-Based X-Ray Techniques.

Sodium-ion batteries (SIBs) hold great potential for use in large-scale grid storage applications owing to their low energy cost compared to lithium analogs. The symmetrical SIBs employing Na3 V2 (PO4 )3 (NVP) as both the cathode and anode are considered very promising due to negligible volume changes and longer cycle life. However, the structural changes associated with the electrochemical reactions of symmetrical SIBs employing NVP have not been widely studied. Previous studies on symmetrical SIBs employing NVP are believed to undergo one mole of Na+ storage during the electrochemical reaction. However, in this study, it is shown that there are significant differences during the electrochemical reaction of the symmetrical NVP system. The symmetrical sodium-ion cell undergoes ≈2 moles of Na+ reaction (intercalation and deintercalation) instead of 1 mole of Na+ . A simultaneous formation of Na5 V2 (PO4 )3 phase in the anode and NaV2 (PO4 )3 phase in the cathode is revealed by synchrotron-based X-ray diffraction and X-ray absorption spectroscopy. A symmetrical NVP cell can deliver a stable capacity of ≈99 mAh g-1 , (based on the mass of the cathode) by simultaneously utilizing V3+ /V2+ redox in anode and V3+ /V4+ redox in cathode. The current study provides new insights for the development of high-energy symmetrical NIBs for future use.

Tin sulfide modified separator as an efficient polysulfide trapper for stable cycling performance in Li-S batteries.

Lithium-sulfur batteries (Li-S) are considered the most promising systems for next-generation energy storage devices due to their high theoretical energy density and relatively low cost. However, the practical applications of Li-S batteries are hindered by the poor electronic conductivity of sulfur and capacity degradation resulting from the shuttle effect of lithium polysulfides (LiPSs). Herein, we demonstrate use of a tin-sulfide (SnS2) modified separator to facilitate the redox reaction involving LiPS intermediates and realize improved electrochemical performance in a Li-S battery. Density functional theory (DFT) calculations revealed that SnS2 exhibits a strong affinity with LiPSs and induces a rapid conversion of trapped polysulfides. As a result, Li-S batteries with a SnS2-modified separator exhibited an enhanced specific capacity of 1300 mA h g-1 at 0.2C (corresponding to a high areal capacity of 4.03 mA h cm-2), which was maintained at 1040 mA h g-1 after 150 cycles. Furthermore, an excellent rate capability is achieved with a capacity of 700 mA h g-1 (2.17 mA h cm-2) at 5C. Additionally, the modified separator exhibited excellent cycling performance up to 500 cycles at 2C, with a low capacity decay rate of 0.0710% per cycle. The excellent performance of the sulfur electrode is mainly attributed to the incorporation of the SnS2 coating layer on the separator, which effectively confines polysulfides via both chemical and physical interaction and rapidly improves lithium ion diffusion. Moreover, the SnS2 coating layer greatly improves sulfur utilization and efficiently accelerates the kinetic conversion of trapped polysulfides.