| Sodium-ion batteries are abundantly investigated and expected to be used in large-scale energy storage for grid utilization because of their rich resources,low cost and environmental friendliness over lithium-ion batteries.For achieving higher capacity,conversion-type and alloy anodes with high theoretical capacities were extensively used in sodium-ion batteries.However,the high capacity is accompanied by severe volume change.Carbon coating is a common strategy to increase electronic conductivity and alleviates the bad influence of volume change to electrodes.Although,these as-prepared composites show improved cyclability,their rate capabilities are still limited.Considering that,the materials are tailored to possess pseudocapacitve behavior and achieve high pseudocapacitance contribution at high rate.First,the mesoporous graphitic carbon-encapsulating nano Fe2O3 composite was fabricated,combining the high electrode-electrolyte area,high electronic conductivity and shortened Na+diffusion length.It shows superior electrochemical performance,exhibiting a reversible capacity of 232 mAh g-1 at 8 A g-1.Through kinetic investigaton,we find that it shows predominant pseudocapacitive mechanism.A contribution ratio of 76%from pseudocapacitance can be pursued at a scan rate of 1.2 mV s-1.Second,Fe2O3-Bi@carbon nanofiber composites was prepared through electrospinning method,with high electronic conductivity of carbon nanofibers.Nanosized Fe2O3 and Bi has abundance grain boundaries which provide more active sites.Fe2O3 and Bi expand simultaneously buffering each other.Owing to these,the Fe2O3-Bi@carbon nanofiber composite with Fe:Bi=1:1 exhibites outstanding sodium and lithium storage properties with obvious pseudocapacitive behavior,which achieves85.5%pseudocapacitive contribution under a scan rate of 1 mV s-1.Third,the core-shell structure of doped polypyrrole（d-PPy）coated ZnS nanosphere was prepared.The d-PPy increases the electronic conductivity of the composite and accommodates the electrochemical expansion of ZnS.Meanwhile,d-PPy can also provide more redox active sites.And hence,it shows high rate capability but with a limited high-rate stability.Last,the ZnS/CNT composite was synthesized via a one-pot hydrothermal method.The coupling between ZnS and CNT is the key to the prominent pseudocapacitive behavior,which also helps maintain a good cyclability under high rate.The ZnS/CNT deliveres 313 mAh g-1 at 5 A g-1 over 500 cycles,and a high pseudocapacitance contribution ratio of 92.9%is also achieved at 1 mV s-1. |
