Spherical Li1.95Na0.05FeSiO4/C composite as nanoporous cathode material exhibiting high rate capability
Introduction
Li2FeSiO4 has been identified as a promising cathode material for the next generation of lithium-ion batteries owing to its high theoretical capacity (166 mAh g−1 for one Li+ ion exchange, and 332 mAh g−1 for two Li+ ions exchange), low cost, high safety, environment-friendly [1], [2]. Unfortunately, as a polyanion cathode material, Li2FeSiO4 also suffers from the slow lithium-ion diffusion rate and low electronic conductivity [3], which becomes a huge barrier to its extensive application as cathode materials for LIBs of hybrid and electric vehicles. Until now, great efforts including carbon coating [4], nano-architecturing [5] and metal or nonmetal ion doping [6], have been proposed to solve this issue.
In recent years, Na doping at Li-site has been considered as an effective way to improve the electrochemical property of cathode material such as LiFePO4 [7], LiNi0.5Mn1.5O4 [8] and Li3V2(PO4)3 [9]. It has been verified that the electronic conductive properties were enhanced, and the ionic transport feature was favorable for high rate performance via Na doping at Li site [7]. With these considerations, in our present research, the doping of Na+ in the form of Li1.95Na0.05FeSiO4/C via a spray drying-assisted method has been reported in detail.
Section snippets
Experimental
Li1.95Na0.05FeSiO4/C composite was prepared by a spray drying-assisted method. Firstly, Lithium hydroxide (3.3059 g, Aladdin), sodium carbonate (0.1062 g, Aladdin), citric acid (12.6718 g, Aladdin) and polyvinylpyrrolidone (PVP) (0.6 g, Aladdin) were dissolved in 500 ml deionized water,and then ferrous oxalate dihydrate (7.2177 g, Shenzheng China) was added into the solution. Secondly, the pH of solution was maintained at 7.0–8.0 by ammonia water. After stirring at 90 °C until a dark green solution
Results and discussion
The X-ray diffraction patterns of Li2FeSiO4/C and Li1.95Na0.05FeSiO4/C composites are described in Fig. 1. As shown in Fig. 1, all the diffraction peaks of both samples can be indexed to a monoclinic structure in space group P21 and no impurity phase peaks are detected, indicating doping a low amount of Na has no effect on the structure of Li2FeSiO4. The calculated lattice parameters are a=8.2176 Å, b=5.0277 Å, c=8.2447 Å, β=99.2° for Li1.95Na0.05FeSiO4/C and a=8.1907 Å, b=5.0075 Å, c=8.2262 Å,
Conclusions
In this current study, porous spherical Li1.95Na0.05FeSiO4/C composite is successfully prepared by a spray drying-assisted method. The results verify that Li1.95Na0.05FeSiO4/C composite exhibits better rate property and cyclic performance than Li2FeSiO4/C composite prepared by the similar process. It can be attributed to its higher electronic conductivity and smaller charge transfer resistance compared to Li2FeSiO4/C composite, demonstrating Na doping play an important role to improve the
Acknowledgements
This project was supported by National Natural Science Foundation of China (Nos. 21576030, 51574090), and 51304077 the Hunan Provincial Natural Science Foundation of China (No. 13JJ4100 and 2016JJ2092), State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources (No. LAPS15001), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (No. 15KJA150002), the Changzhou City Science and Technology Plan Projects (CE20150084), the Priority
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