Electrochemical Na-Insertion/Extraction Properties of Sno Thick-Film Electrodes Prepared by Gas-Deposition

Sn is one of the most attractive materials for Na-ion battery anode due to its high theoretical capacity of 847 mA h g^－1 compared with carbon-based anodes. The volume of Sn is, however significantly changed during alloying/dealloying reaction with Na, which leads to disintegration of Sn anode and a poor cycle performance. To overcome the problem, we used tin monoxide (SnO) as active material, and investigated their electrochemical Na-insertion/extraction properties. In addition, we tried to enhance the cycle performance by adding fluoroethylene carbonate to an electrolyte.

SnO and Sn thick-film electrodes were prepared by a gas-deposition method to clarify an original anode performance of its active materials. Anode performances of SnO electrodes were investigated by constant current charge-discharge tests using binder-free thick-film electrodes. We assembled 2032-type coin cells consisted of the thick-film electrodes as working electrode, Na foil as counter electrode, electrolyte, and propylene-based separator. The electrolytes used in this study were 1 M NaClO₄-dissolved in propylene carbonate without and with addition of 20 vol.% fluoroethylene carbonate (FEC). The surface morphologies of the thick-film electrodes were observed by using a field-emission scanning electron microscope (FE-SEM).

SnO thick-film electrode delivered the initial discharge (Na-extraction) capacity of 580 mA h g^－1, which is approximately two times larger than that obtained from a conventional carbon-based anode. The initial charge (Na-insertion) capacity and coulombic efficiency were 1040 mA h g^－1 and 56%, respectively. In the first charge-discharge curve for SnO thick-film electrode, two plateaus at 0.41 V and 0.015 V were clearly observed. The plateau at 0.41 V is attributed to a reductive reaction of SnO to form Na₂O and metallic Sn. We could not confirm the plateau at 0.41 V in the subsequent charge-discharge cycles, indicating that the reductive reaction is irreversible. The plateau at 0.015 V is assigned to alloying reaction of Sn with Na such as a crystalline Na₁₅Sn₄ phase.

   Figure 1 represents cycle performances and coulombic efficiencies of the SnO and Sn electrodes in 1 M NaClO₄/PC without and with 20 vol.% FEC. Although the Sn electrode showed a comparatively high discharge capacity of 560 mA h g^－1 at the first cycle, the capacity was quickly decreased to 30 mA h g^－1 by the sixth cycle, resulting in a very poor cycle performance. The reason for the capacity decay is that mechanical stress induced by repeating alloying/dealloying reaction leads to the deterioration of Sn electrode. In contrast, the SnO electrode in the additive-free electrolyte maintained the discharge capacity of 260 mA h g^－1 at the 20th cycle, which was superior cycle performance to that of the Sn electrode. It is considered that Na₂O formed in the first cycle plays a role as a matrix to release the stress, leading to the better cyclability of SnO electrode. When we used FEC as an additive, the SnO electrode exhibited the initial capacity of 570 mA h g^－1. The first coulombic efficiency of 58% was observed. There was no significant difference in the first coulombic efficiencies between each SnO electrode in both the electrolytes. Although the coulombic efficiency of SnO electrode in the additive-free electrolyte was temporarily increased to 92% by the third cycle, the coulombic efficiency was decreased to 84% at the tenth cycle and was remained below 90% until 40th cycle. We consider that these behaviors indicate not only the disintegration of the SnO electrode but also the continuous decomposition of an electrolyte. In contrast, the SnO electrode in the electrolyte with FEC constantly exhibited higher coulombic efficiencies than those obtained in the additive-free electrolyte, and the coulombic efficiency was improved to 98% after the sixth cycle. This enhanced reversibility means that the further decomposition of the electrolyte was suppressed by introducing FEC additive. It was noteworthy that the SnO electrode achieved a high capacity of 250 mA h g^－1even at the 50th cycle. As a result, the capacity retention was remarkably improved to 44%, which was five times higher than that in the electrolyte without FEC.

   Figure 2 displays FE-SEM images of the SnO and Sn electrodes as prepared and after charge–discharge cycling, respectively, in the electrolytes without and with 20 vol.% FEC. The Sn and SnO electrodes before cycling showed smooth surface, respectively, as shown in Figs. 2(a), 2(c). In Fig. 2(b), we can clearly observe the disintegration of the Sn electrode related to the rapid capacity fading. In both the SnO electrodes after cycling, we confirmed changes in surface morphology such as the formation of cracks. It is notable point that degrees of disintegration for both the SnO electrodes after cycling in the electrolytes without and with FEC additive were mostly similar (see Figs. 2(d), 2(e)). These results indicate that the capacity decay originates from not only disintegration of SnO electrodes but also deteriorated Na-ion transfer between the electrode and the electrolyte. It was demonstrated that SnO electrode exhibited the initial high discharge capacity and the good cycle performance because FEC additive effectively enhanced the utilization of active material by suppressing deterioration of the Na-ion transfer.