SnO2 is one of the high capacity (782 mAh/g) anode materials used in lithium ion batteries (LIBs) with a tetragonal rutile structure and it alloys at voltage of 0.5V vs Li. However cyclability for SnO2/Sn based materials is very poor due to high volume expansion during alloying with Li ions (charging) and disintegration of structure during de-alloying (discharging) besides the formation of solid electrolyte interface (SEI) at lower operating voltage of the anode. Many attempts have been made to improve the cyclability and minimize the capacity losses of these materials by nanostructuring, making nano composites with graphene and CNT. Even though the results are promising, reproducibility and the scaling up of the electrode material still remains a challenge. Here we present a new way of improving the cyclability with minimum capacity loss using a composite electrode of SnO2 and lithium titanate (LTO). LTO with a cubic spinal structure can intercalate reversibly with Li ions delivering a capacity of 175 mAh/g, theoretically. Low crystal strains during charging –discharging makes the material work even at high charging rates. The combination of SnO2 and LTO can reduce the volume expansion experienced by bare SnO2 during alloying de-alloying reaction as LTO itself is a zero-strain material.
Experimental:
We have synthesized SnO2/LTO composite using combinational approach of sol-gel and electrospinning. Sol-gel synthesis of nanomaterials is known for obtaining materials with high purity. Electrospinning is a simple and continuous method of nanomaterial fabrication which involves application of high voltage to a viscous solution to yield nanofibers. This combinational approach of sol-gel and electrospinning opens the possibilities to obtain pure inorganic nanomaterial. Further sol-gel/ electrospun based inorganic nanomaterials may be promising as electrodes for LIBs as diffusion path or volume expansion of electrode can be reduced to a greater extent due to the presence of nano grains in porous structures. In this work we present the fabrication of SnO2/LTO composite dimpled spheres prepared by sol-gel/electrospinning followed by measuring their electrochemical performance. LTO and SnO2 sols were prepared in ethanol and mixed in 1:1 ratio followed by ageing prior to electrospinning. As ageing time increased, it resulted in higher viscosity which changed the morphology of SnO2/LTO composite to particles to dimpled spheres to fibers.
Electrospun SnO2/LTO dimpled spheres were then calcined in argon and air at different temperature for the required phase formation and to obtain SnO2/LTO and SnO2/LTO/Carbon based dimpled spheres with variation in particle sizes. The synthesized composite structures could minimize the grain growth by creating smaller grains ~40 nm in the sub-micron sized electrospun structures.
Results and discussion:
FESEM and XRD analysis were carried out to understand their morphology and phase formation respectively. The average grain size was measured to be 25 nm. BET and SAXS were performed for surface area analysis. Electrochemical measurements such as Galvanostatic charge-discharge studies, cyclic voltammetry and impedance spectroscopy were performed. Electrospun SnO2/LTO based composite sub-micron structures have shown excellent electrochemical performance (Figure 1) as compared to only SnO2 fibers with improved cycle stability and capacity retention even at high charging rates.
References
[1] Haridas, A. K.; Sharma, C. S.; Rao, T. N., Electrochimica Acta, 212, 500 (2016).
[2] Haridas, A. K.; Hebalkar, N.Y.; Sharma, C. S.; Rao, T. N., Manuscript under review.
[3] Haridas, A. K.; Sharma, C. S.; Rao, T. N., Small, 11, 290 (2015).
[4] Haridas, A. K.; Sharma, C. S.; Rao, T. N., Electroanaysis, 26, 2315 (2014).