Electrode materials for Na-ion battery (NIB) have been extensively studied to realize a low-cost and high-energy battery.  Hard carbon, which delivers 300 mAh g^-1, is one of the promising candidates as negative electrode materials for NIB.[1]  However, its reversible capacity and energy density are still lower compared to those of graphite in Li-ion battery.  Therefore, material developments for higher capacity are required for the practical application.  We recently reported that nano-sized Sn delivers  ca. 600 mAh g^-1 reversible capacity with a good capacity retention in a voltage range of 0.00 − 0.65 V, in spite of ca. 420% volume change of the Sn during charge and discharge.[1, 2]  In this research, the impact of the Sn particle size of NIB electrochemical properties was examined.  The differences of the electrode surface depending on the Sn particle size are discussed based on the results of surface analyses; secondary ion mass spectroscopy and soft and hard photoelectron spectroscopy to elucidate mechanism of the capacity deterioration.  Furthermore we have reported that the FEC additive function as the formation of stable solid electrolyte interphase (SEI) on the surface of the active materials.[3] We also examined the impact of FEC concentrations.

To prepare the working electrode, 80 wt% Sn powder (<150 nm or 10 μm, Sigma-Aldrich Co., Ltd), 10 or 20 wt% sodium polyacrylate (PANa) and 10 wt% graphite were mixed with 10 vol% methanol aqueous solution, and the resultant slurry was pasted onto an Al foil.  Na metal was used as the counter electrode and 1 mol dm^-3 NaPF₆/EC:DEC:FEC (49:49:2 vol%) was used as the electrolyte.  The charge/discharge measurements were carried out using coin-type cells with a sodiation/desodiation current density of 25/25 or 10/25 mA g^-1 only for the 1^st cycle.  After that, the test was continued with the current density of 50/50 mA g^-1.

Nano-sized Sn composite electrode with 10 wt% PANa delivers a reversible capacity of ca. 600 mAh g^-1 over 100 cycles at 50 mA g^-1 (Figure 1(a)).  On the other hand, Figure 1(b) confirms that micro-sized Sn electrode with 10 wt% PANa delivers ca. 700 mAh g^-1 for only 10 cycles and the capacity rapidly decays after 20 cycles.  In this case, the desodiation capacity corresponding to a plateau region at ca. 0.18 V decreases as a function of the cycle number.  However, by increasing amount of PANa binder to 20 wt% and changing the 1^st sodiation rate from 25 mA g^-1 to 10 mA g^-1, micro-sized Sn delivers ca. 600 mAh g^-1 over 50 cycles at 50 mA g^-1 (Figure 1(c)).  Even if micro-sized Sn electrode was prepared with 20 wt% of PANa, higher capacity is not delivered at a current density of 25 mA g^-1for the initial sodiation.  These results suggest that the low initial sodiation rate influences the electrode performance and could be related to SEI formation.  Results of surface analysis will be also discussed.

References:

[1] M. Dahbi, S. Komaba et al., Phys. Chem. Chem. Phys., 16, 15007 (2014).

[2] M. Fukunishi, S. Komaba et al., 2014 ECS/SMEQ Joint Meeting, Mexico, Abs. #94 (2014)

[3] S. Komaba et al., ACS Applied Mater. Interfaces, 3 (11), 4165 (2011).