Nanostructured bimetallic sulfides as Na ion battery anodes

Sodium ion batteries (SIBs) with major advantage of cost effectiveness has been regarded as promising energy storage alternatives to lithium ion battery (LIB). However, the commercial LIB anode graphite is unable to effectively intercalate Na+. Metal sulfides with conversion-based mechanism exhibit higher theoretical capacity than intercalation-based anode materials, and smaller volume change than alloying-based anode materials. Nonetheless, the capacity fading issues caused by volume change of metal sulfides during electrochemical testing are still unsolved problems. This thesis hypothesized that by employing nanostructured bimetallic sulfides with open frameworks and carbon incorporation, the degradation in capacity can be alleviated. In order to achieve this goal, NiMn layered double hydroxides (LDHs) with layered structure and CoFe Prussian blue analogues (PBs) with large interstitial sites are selected as the bimetallic precursors for subsequent sulfidation, resulting in the formation of bimetallic sulfides Mn-doped multiphase Ni sulfides (NMS) and sulfidized PBs (PBS), respectively. In another approach, bimetallic sulfides Cu2MoS4 (CMS) with layered structure were directly prepared. After sulfidation, the open frameworks of bimetallic sulfides NiMn LDHs and CoFe PBs are no longer maintained. However, improved electrochemical performance of bimetallic sulfides over monometallic sulfides have been confirmed. With necessary structure design, size reduction and carbon incorporation, nanostructured bimetallic sulfide-based composites including NMS nanoparticles/reduced graphene oxide (NMGS), carbon coated PBS hollow nanocubes (PBC1-1S) and CMS nanospheres (CMS1)/reduced graphene oxide (CMS1-rGO) can be obtained with improved electrochemical performance. By employing ether electrolyte, bimetallic sulfide-based composites anode further enhanced the rate performance and cycling stability, thus delivering a fast and stable SIB full cell by employing CMS1-rGO anode. In addition, the electrochemical mechanism of NMGS and PBC1-1S have been investigated, confirming the conversion reaction in carbonate-based electrolyte while intercalation is dominant in the ether electrolyte. By combining ex-situ X-ray photoelectron microscopy and in-situ X-ray absorption spectroscopy, the electrochemical mechanism of CMS1 was also studied, revealing the intercalation dominant electrochemical reaction in ether-electrolyte while conversion occurred in carbonate-based electrolyte. These findings provide insights on the governing factors that explain the structural-electrochemical performance interplay of the bimetallic sulfides.Doctor of Philosoph