水熱法合成磷酸鈉鈦及其作為水系鈉離子電池負極之性能分析

Hydrothermal Synthesis of Sodium Titanium Phosphate and its Electrochemical Properties as a Negative Electrode of the Aqueous Sodium Ion Batteries

本研究以水熱法合成鈉超離子導體（sodium super-ionic conductor, NASICON）-磷酸鈉鈦奈米粉末，經碳包覆增進其導電性，以掃描式電子顯微鏡觀察顯示，磷酸鈉鈦粉末之平均粒徑於100 ~ 500 nm範圍內。經碳包覆後，以拉曼光譜分析偵測出碳之訊號並以熱重分析測得碳包覆量約為3 wt%。並將此粉末作為鈉離子電池負極，量測其鈉離子電池性能。在不同充放電速率下均展現出優異電容量，隨充放電速率由0.2增加至0.5、1、2、5 C時其電容量由110下降至103、97、90、67 mAh/g、至於庫倫效率除首圈外，皆高達99%以上、放電電容量則均維持在95%以上；經200次充放電循環測試後，放電電容量仍保持約70%，顯示採用本研究水熱法合成之鈉超離子導體（NASICON）-磷酸鈉鈦粉末作為水系鈉離子電池負極材料極具潛力。

Nano powders of sodium titanium phosphate, belonging to sodium super-ionic conductors (NASICONs), were successfully prepared by hydrothermal method in this work. Examination by field-emission scanning electron microscope revealed that the diameter of the powders was in the range from 100 to 500 nm depending upon the experimental conditions. After coating with carbon, the powders increased their electrical conductivity indicating that the coating was more suitable for preparation of negative electrodes for sodium-ion batteries. Raman spectroscopy of the C- coated powders demonstrated the characteristic D and G-bands carbon. The result of thermal gravimetric analysis displayed that the carbon content was estimated at around 3 wt%. Excellent performance was regardless of the charge/discharge rates of the C-coated nanoparticles NaTi2(PO4)3/C. The specific capacities decreased from 110 to 103, 97, 90 and 67 mAh/g with increased the charge/discharge rates from 0.2 to 0.5, 1, 2 and 5 C. The coulomb efficiency was up high to 99% except for the first cycle; the discharge capacity retained higher than 95% at any charge/discharge rate. The discharge capacity maintained a high delivery (around 70%) even after 200 cycles. The characters indicated that the carbon coated sodium titanium phosphate nano powders provided a potential candidate of the negative electrode (i.e., anode) used in aqueous sodium ion battery.