Heqin Li

A conductive carbon interlayer modified by magnetron sputtering for improved-performance lithium–sulfur batteries

Cathode materials (S–AC) for lithium–sulfur (Li–S) batteries were synthesised with elemental sulfur (S) and activated carbon (AC). Conductive carbon films (CF1) were prepared with filter paper and aluminum (Al) thin films were plated onto the surface of the filter paper by the method of magnetron sputtering to fabricate modified carbon films (CF2). The as-prepared carbon films were applied as conductive interlayers inserted between the cathode and the separator for Li–S batteries S/AC/CF1 and S/AC/CF2. The properties of the cathode materials and the carbon interlayers were characterized by XRD and FESEM. Electrochemical performances of three Li–S batteries with and without interlayers (S/AC/CF1, S/AC/CF2 and S/AC) were determined by alternating-current impedance, cyclic voltammetry and constant-current charge and discharge. The assessment results show that S/AC/CF2 is superior to the others with an initial discharge specific capacity of 1273 mA h g−1 at a current rate of 1C. It delivered a reversible capacity of 924 mA h g−1 after 100 cycles and the coulombic efficiency after 200 cycles is still over 95%.

Improved-performance lithium–sulfur batteries modified by magnetron sputtering

The cathode material S/AC for lithium–sulfur batteries was synthesized with elemental sulfur as the active material and activated carbon (AC) as the conductive matrix. Al and Ti were respectively deposited onto the surface of S/AC electrodes by the method of radio-frequency magnetron sputtering to modify the electrodes and improve the battery performance. The properties of S/AC and the sputtered cathode materials, labeled S/AC/Ti and S/AC/Al, were characterized by XRD and FESEM. Electrochemical performances of the Li/S batteries with the three cathode materials were determined by alternating-current impedance, cyclic voltammetry (CV) and constant-current charge and discharge. Experiments showed that S/AC, S/AC/Ti and S/AC/Al delivered initial specific capacity of 1197 mA h g−1, 1255 mA h g−1 and 1257 mA h g−1 respectively under the current rate of 0.5C. And the modified batteries operated reversibly over 100 cycles and maintained a discharge specific capacity of 722 mA h g−1 and 977 mA h g−1 after 100 cycles, superior to 634 mA h g−1 of S/AC. Besides, the coulombic efficiencies of the sputtered electrodes were over 0.97 after 100 cycles.

Formation of the amorphous phase in the carbothermal reduction and nitridation route to SrSi2O2N2 : Eu2+: a new understanding of the catalytic effect of carbon in the synthesis of Sr2Si5N8 : Eu2+ for white LEDs

This study provides an explanation for the mechanism of carbothermal reduction contributing to the synthesis of a Sr2Si5N8 : Eu2+ nitride phosphor, in terms of the heat released and propagated within the local region, which makes the actual temperature far higher than that determined in the reaction. Two pathways of carbon mixing were designed for synthesizing SrSi2O2N2 : Eu2+ at relatively lower temperatures than that required for synthesis of Sr2Si5N8: Eu2+. However, formation of the nitride, Sr2Si5N8 : Eu2+, was observed by mixing carbon with the raw materials, rather than placing a layer of carbon underneath for heat conduction, indicating that close contact of carbon with the raw materials was a precondition of nitride formation. In addition, the presence of an amorphous layer on the surface of the phosphor particles, identified using a high-resolution transmission electron microscope, provided a clue to understanding the mechanism of the nitridation reaction at relatively low temperatures.

A dual-faced interlayer prepared by electron beam evaporation for enhanced-performance lithium–sulfur batteries

In this work, a dual-faced carbon paper was prepared by depositing Al2O3 on one side of carbonized filter paper via the technique of electron beam evaporation. Assembled into Li–S batteries, the Al2O3-deposited carbon paper served as a multifunctional interlayer. The cathode of 70% sulfur content with this interlayer presented notable enhancements in electrochemical performance in contrast to that with a single carbon interlayer. At a current rate of 0.5C, the battery with the Al2O3-deposited carbon interlayer delivered a high initial capacity of 1253 mA h g−1 and retention of 700 mA h g−1 over 120 cycles, while the battery with the carbon interlayer delivered an approximate initial capacity of 1117 mA h g−1 but much poorer retention of 441 mA h g−1 over 87 cycles. The carbon side of the interlayer was placed toward the cathode as the upper current collector, which ensured the conductivity of the contact interface with the active material and promoted activation of sulfur. At the same, Al2O3 as a polar material facing toward the separator impeded the leakage of soluble polysulfides and mitigated the shuttle effect by chemical adsorption of soluble polysulfides. Combined with the interstitial structure of the interlayer acting as a physical container, Li–S batteries with this novel interlayer demonstrate superiorities in capacity, variable discharge/charge rate, Electrochemical Impedance Spectroscopy (EIS) and Cyclic Voltammetry (CV) characteristics.