High-Power Lithium Vanadate / Carbon Composite for Hybrid Supercapacitors

1. Introduction The next step in the improvement of the high-energy-density Li4Ti5O12(LTO) / activated carbon hybrid supercapacitor1 is to substitute the negative electrode (LTO) with other potential candidates with higher voltage and capacity. Here, we selected Li3VO4 (LVO) because of its low redox potential (1.0 V – 0.1 V vs. Li+/Li) and reversible insertion mechanism with high theoretical capacity (394 mAh g-1 in case of 2-electron reaction and 591 mAh g-1 in case of the 3-electron).2-4 To overcome the well-known drawbacks of LVO such as low electronic conductivity (<10-10 S m-1) and large voltage hysteresis (>500 mV) which prevent to achieve high rate performances5-7, we investigated the potentialities of a nanosized LVO (10-50 nm) hyper-dispersed within multi-walled carbon nanotube (MWCNT) matrix prepared by our original ultracentrifugation (UC) process. As expected, the rate performances are considerably enhanced for the uc-LVO/MWCNT, and in addition, the structural mechanism involved during the reversible insertion/extraction of Li is clarified thanks to a deep investigation using in operando XRD. The performances of the obtained composite show that it can be used as a LTO alternative for hybrid supercapacitors. 2. Experimental NH4VO3, citric acid, ethylene glycol and lithium hydroxide were dissolved into deionized water to form a clear orange solution. After the addition under stirring of MWCNT, the solution was subjected to the UC treatment at 80 ˚C for 5min. Then, the mixture was dried at 130˚C under vacuum for 12 h to accelerate the polymerization of ethylene glycol. A calcination stage at 300°C for 3h under air was applied to remove the polymer and a final sintering at 800˚C under N2 atmosphere allows obtaining the Li3VO4/MWCNT composite. Physicochemical characterizations were performed on the synthesized composites by XRD, XPS and TEM observation. Relation between electrochemical behavior and structure change were investigated by in operando XAFS and XRD measurements.   3. Results and discussion Successful synthesis of nanocrystalline LVO particles of ca. 10~50 nm on the surface of MWCNT was confirmed by the combination of HRTEM observation and XRD measurements. The LVO/MWCNT composite shows excellent electrochemical behavior such as high charge (delithiation) rate capability with 170 mAh g-1 per composite at a high rate of 20 A g-1, when charged and discharged down to 0.1 V vs. Li. In operando XRD patterns on the composites show the existence of irreversible structure changes acting as an activation process, where the reaction mechanism of LVO switches from a “two-phase” to a “solid-solution” reaction. The relationship between the hysteresis and Li insertion mechanism was further investigated by charge-discharge tests at different cut-off voltages. It was found that the “activated” LVO/MWCNT composites within the limited voltage window (0.76 -2.5 V vs. Li) can achieve higher reversibility and energy efficiency (low voltage hysteresis below 100mV) with ultrafast Li insertion kinetics and long-term cycle life for the needed hybrid supercapacitor applications. References 1) K. Naoi, W. Naoi, S. Aoyagi, J. Miyamoto and T. Kamino, Acc. Chem. Res., 46, 1075 (2013). 2) S. Ni, J. Zhang, J. Ma, X. Yang, L. Zhang and H. Zeng, Adv. Mater. Interfaces, 3, 1500340 (2016). 3) J. Zhang, S. Ni, J. Ma, X. Yang and L. Zhang, J. Power Sources, 301, 41 (2015). 4) Z. Liang, Y. Zhao, Y. Dong, Q. Kuang, X. Lin, X. Liu, D. Yan, J. Power Sources, 274, 345 (2015). 5) Y. Shi, J. Gao, H.-D. Abruna, H.-J. Li, H.-K. Liu, D. Wexler, J.-Z. Wang and Y. Wu, Chem. Eur. J., 20, 5608 (2014). 6) Y. Shi, J.-Z. Wang, S.-L. Chou, D. Wexler, H.-J. Li, K. Ozawa, H.-k. Liu and Y.-P. Wu, Nano lett., 13, 4715 (2013). 7) H. Li, X. Liu, T. Zhai, D. Li and H. Zhou, Adv. Energy Mater., 3, 428 (2012).