Functional electrolytes to improve electrochemical properties of cathodes and anodes in Na ion batteries

Abstract

As increasing importance of environment and demand of energy resources, energy devices have attracted a great deal of attention as facilitating smart grid system in society. Especially, large-scale electric energy storage (EES) which can store the electricity during times of excess production and release the electrical energy to the grid is reasonable way to save the energy resources. The Li-ion batteries which are one of successful energy devices show the possibility to application. However, the cost in large-scale ESS is the key issue for application because the cost of Li materials continuously increased. Recently, Na-ion batteries have been introduced as a promising candidate for next-generation battery systems because the natural abundance of Na resources. The Na materials have high benefit of the cost and it seem to suitable for large-scale ESS. Moreover, Na ion can easily apply Li-battery system due to the similarities with Li. Although Na-ion battery has many advantages for ESS, the large ionic size of sodium make it difficult to application. In this work, I demonstrate key aspects of the electrolyte formulation and additives to afford high electrochemical performances of Na4Fe3(PO4)2(P2O7) cathode and Sn4P3 anodes for Na-ion batteries. In the case of cathode, electrolytes are usually decomposed on electrode due to the high voltage and directly affect the electrochemical performance of cathode. However, I proposed that EC/PC-based electrolyte not only endure electrochemical decomposition in high voltage but also stable against the highly reactive Na metal. In case of anode, severe volume changes of the metallic anode by Na insertion and extraction cause cracking of the anode particles and lead to a continuous solid electrolyte interphase (SEI)-filming process on the exposed active surface of the anode. In Sn4P3, my investigation revealed that the FEC additive makes a resistive NaF-based SEI controlling the formation of Na15Sn4 phase with severe volume expansion and the TMSP additive effectively eliminated HF from the FEC decomposition to mitigate the formation of a large fraction of NaF and build up more stable and robust SEI layers. Furthermore, I demonstrate the surface chemistry of cathode and anode by using various analyses. This research can contribute to understand the interfacial characteristics and electrochemical reactions for Na-ion batteries.