| Sodium ion batteries are now actively pursued as the most attractive alternativeto Li-ion batteries for electric vehicle propulsion and renewable electric power storage,because of their potential advantages of low cost and widespread availability ofsodium resources. To realize Na-ion technology, a critical issue is to find out suitablehost materials that can accommodate sufficient Na ions for reversible electrochemicalinsertion reaction. In this thesis, we were aimed at exploring new anode and cathodematerials and optimize their performance to realize their potentionally high redoxcapacity for reversible Na-storage。The main results and new findings in this work aresummarized as follows:1. A new family of sodium transition metal cyanides, such as hexacyanoferratesNa4Fe(CN)6and Prussian blue NaxMyFe(CN)6(M=Fe,Co,Ni,Mn) were proposed ascathode materials for sodium ion batteries. Firstly, a Na4Fe(CN)6/C nanocompositewere prepared simply by mechanical ball-milling Na4Fe(CN)6and conductive carbonpowders. The as-prepared Na4Fe(CN)6/C composite displays a full utilization of itsredox capacity of87mAh g-1at a high potential of3.4V, an excellent cyclingstability with a88%capacity retention over500cycles and a superior high ratecapability with45%capacity delivery at a10C rate. Secondly, four types of Prussianblue NaxMyFe(CN)6(M=Fe, Co, Ni, Mn) compounds were prepared simply bysolution precipitation method and tested as cathode materials for sodium-ion batteries.Electrochemical tests revealed very different electrochemical behaviors of these foursamples of NaxMyFe(CN)6(M=Fe, Co, Ni, Mn). When M was Fe, Co and Mn, thespecific capacities of NaxFeyFe(CN)6, NaxCoyFe(CN)6and NaxMnyFe(CN)6can reach113,120and113mAh g-1, respectively, indicating that both of the Fe(CN)64-and M+2ions in the Prussian blue lattices were electrochemically activated. When M was Ni,Ni ions in the Prussian blue lattice was found to be electrochemically inactive and theNaxNiyFe(CN)6delivered a specific capacity of only64mAh g-1but with quite stablecyclability. These results suggest a possible use of the transition metal cyanides as lowcost and environmentally benign cathode materials for sodium-ion batteries.2. Two typical conductive polymers: Polyaniline (PAn) and Polytriphenylamine(PTPAn) were chemically synthesized and suggested as promising cathode materials for sodium ion batteries, due to their good conductivity and electrochemical redoxproperty. It was found Na/PAn cell displays a quite high capacity of163mAh g-1at anaverage discharge voltage of3.16V, an excellent cycling stability with118mAh g-1reserved after100cycles and a superior high rate capability with60mAh g-1deliveryat300mA g-1current density, exceeding most of the current inorganic cathodes;Na/PTPAn cells also give a high capacity of93mAh g-1and a quite high averagedischarge voltage of3.56V. Even cycled at a very high rate of500mA g-1, thepolymer can still deliver a capacity of72mAh g-1, that is78%of initial capacity.Overall, these conductive polymers can realize their potential high redox capacitieswith sufficient cycleability and superior high power capability, suggesting a promisingcathode for high energy density and power density sodium ion batteries.3. In the search for suitable Na host materials for sodium ion batteries, weinvestigated the electrochemical sodium insertion behavior of several types ofcarbonaceous materials. Hard carbon materials were found to possess the best sodiuminsertion performance, as they contain random stacked carbon layers and significantquantities of nanosized pores, which can serve as active sites for sodium insertion.Base on these results, two kinds of polymers: polyvinyl chloride (PVC andpolyaniline (PAn, were selected as the precursors to make pyrolyzed hard carbon,and their performance were optimized by adjusting proper calcination condition. Itwas found that the PVC electrode can deliver quite a high reversible capacity of200mAh g-1, corresponding to the formation of Na0.54C6, an excellent cycling stabilitywith85%reserved after100cycles and a high rate capability with117mAh g-1delivered at a current density of200mA g-1. The PAn-pyrolysed carbon can also givea high capacity of194mAh g-1, and97%capacity reserved at200th cycle, and ahigh rate capability with103mAh g-1delivery at500mA g-1. In principle, highperformance carbon anodes can be achieved by adjusting the structure of carbonmaterials, including crystal lattice, surface texture, and structural defect, so as to meetthe practical requirements of the sodium ion batteries.4. Sodium alloying reactions with metals or semi-metals would be an effective wayto provide larger specific capacity and suitable thermodynamic potentials than carbonbased materials. For example, Na can alloy with Sn, P, Sb elements to produceNa15Sn4(847mAh g-1, Na3P (2560mAh g-1, and Na3Sb (660mAh g-1, respectively.In order to suppress enormous volume changes (i.e., a525%volume increase ongoing from Sn to Na15Sn4during sodium alloying reaction, several strategies, such as nanosizing, core-shell structure, amorphisizing and carbon coating, were adopted tooptimize their performances. Firstly, core-shell structured Sn@Cu nanocomposite wassynthesized by a simple substitutional reaction between Sn and Cu2+. The as preparedSn@Cu material can give an enhanced capacity of290mAh g-1, with200mAh g-1reserved after20cycles. Secondly, an amorphous P/C nanocomposite was simplyprepared by mechanical ball-milling red P and conductive carbon powders. The a-P/Ccomposite displays an quite high capacity of1764mAh g-1,an excellent cyclingstability with96.7%reserved after40cycles, and a superior fast sodiumdeintercalation kinetics with1725mAh g-1achieved at a current density of4000mAg-1. Thirdly, a Sn4P3/C nanocomposite was synthesized by ball-milling method, andcan exhibit a reversible capacity of816mAh g-1, approaching to its theoretical24Na-storage capacity and retain89%of initial capacity after100cycles. Finally, weprepared a novel Sb/C nanocomposite, simply by mechanical ball-milling commercialSb powder with conductive carbon, which demonstrates a nearly full utilization of itstheoretical3Na storage capacity, a strong rate capability with50%capacity realizedat a very a very high current of2000mA g-1. Particularly, in the optimized electrolytewith a SEI film-forming additive, the Sb/C anode demonstrates a long-term cyclingstability with94%capacity retention over100cycles. Overall, these excellentelectrochemical performances of four samples represent the highest level of the Nastorage materials, offering a practical feasibility as a high capacity and cycling-stableanode for room temperature Na-ion batteries. |
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