Na-storage Anode Materials And Their Structural Design For Na-ion Batteries

Sodium-ion batteries are now actively revisited as one of the most promising alternatives to lithium-ion batteries and attracted great attention for large-scale energy storage applications such as electric vehicles and renewable electric power stations, because of the low cost and wide availability of sodium resources. Developing advanced Na-storage anode materials with substantially enhanced capacity and cycling capability is a great challenge for energy and materials chemists in the pursuit of efficient energy storage systems. To search for high performance Na-storage anode, we have successfully synthesized various nanocomposites with special structures. The main results and new findings in this work are summarized as follows:1. Among the known Na-storable metals, Sn is the most attractive anode material for Na-ion batteries because of its high theoretical capacity (Na15Sn4:847mAh g-1) and relatively low cost. However, it is difficult to prepare uniformly dispersed Sn nanoparticles owing to its very low melting point, whereas usually prepared larger Sn aggregates suffer from huge volume change during Na insertion and extraction, leading to poor capacity retention. To solve this problem, we attempted to embed the Sn particles into a SnS matrix so as to buffer the volumetric change and thus to maintain the structural stability by simple mechanical ball-milling method. The as prepared SnS-Sn-C nanocomposite exhibited a high Na-storage capacity of 664 mAh g-1 at 20 mA g-1 and a long-term cycling stability. These excellent electrochemical performances of the SnS-Sn-C nanocomposite can be attributed to a continuous self-supporting matrix during charge and discharge cycles to provide a stable structure due to the introduction of S in the preparation process. SnS-C nanocomposite was prepared by a simple high-energy mechanical milling method. The SnS-C electrode exhibits a high Na storage capacity of 568 mAh g-1 at 20 mA g-1 and excellent cycling stability as well as high-rate capability. In addition, we also synthesized SnS@RGO composite by a simple precipitation method. The electrochemical measurements show that the Na storage reaction in the SnS@RGO composite took part through a conversion reaction of SnS to Sn and NaxS and followed by an alloying reaction of Sn to NaxSn. The SnS@RGO electrode exhibits a reversible capacity of 457 mAh g-1 at 20 mA g'1, excellent cycling stability (94% of capacity retention over 100 cycles at 100 mA g-1) and good rate performance. Compared to the blank SnS nanoparticles, the improved electrochemical performance of the SnS@RGO composite benefits from the highly conductive, flexible as well as large surface area scaffold of RGO, which provides a good electronic contact between active materials, suppress the aggregation of intermediate products and alleviate the volume change during sodiation and desodiation reactions.2. To search for high performance Na-storage anode, we have extended our work to metal Sb. SiC-Cu-Sb-C nanocomposite with core-shell structure was synthesized by simple mechanical ball-milling method. The SiC-Cu-Sb-C electrode can deliver a high reversible capacity of 595 mAh g-1 after 100 cycles at 100 mA g-1 and 511 mAh g"1 at 800 mA g-1, showing excellent cycling capacity and rate capability. The core-shell structure of SiC-Cu-Sb-C nanocomposite can effectively buffer the volume change and remain structural stability. Sb-C nanofibers were synthesized successfully through a single-nozzle electrospinning technique followed by subsequent calcination. Electrochemical characterization show that the Sb-C nanofiber electrode can deliver large reversible capacity of 631 mAh g"1 at C/15 (1C=600 mA g-1) and excellent cycling stability (90% capacity retention after 400 cycles). The superior electrochemical performances of the Sb-C nanofibers are due to the unique nanofiber structure and uniform distribution of Sb nanoparticles in carbon matrix, which provides a conductive and buffering matrix for effective release of mechanical stress caused by Na ion insertion/extraction and prevent the aggregation of the Sb nanoparticles. The pitaya-like Sb@C electrode can exhibit high a Na storage capacity of 655 mAh g-1 at C/15 (1C=600 mAg-1) with excellent cyclability (93% of capacity retention over 100 cycles) as well as remarkable rate capability. The structural stability guarantees tight contact of Sb with carbon buffer, as well as uniform distribution of Sb to balance the localized mechanical stress, ensuring excellent electrochemical performance. The structural design and synthetic method reported in this work may provide an effective way to improve electrochemical performance of Na-storable alloy materials and therefore provide a new prospect for creation of cycle-stable alloy anodes for high capacity Na-ion batteries.3. To identify suitable electrolyte for Na-ion chemistry we compared various electrolytes containing diverse solvent mixtures (EMC?DMC?DEC) and additives (VEC?VC?FEC). We also studied the effect of different polymer binders (PVDF? CMC-SBR?PAA) on electrochemical performance of the metal alloy electrode in rechargeable sodium-ion batteries. The EC:DEC solvent mixture with FEC additive has been proved to be the best solvent composition. In addition, PAA binder can remarkably improve the electrochemical performance of the SiC-Sb-C electrode. FEC in the electrolytes can prevent electrolyte decomposition and lead to the formation of a thin, uniform, flexible SEI films on the cycled electrode surfaces, which can enhance comsiderable the cycling stability of the electrode.Overall, the results in this thesis provide basic information about Na-alloying reaction, which may help to develop high capacity and long life anodes for sodium-ion batteries.