An Aqueous Symmetrical Sodium-Ion Battery Using New Concentrated Sodium Trifluoroacetate Electrolyte
Authors: Nakamoto Kosuke , Sakamoto Ryo , Nishimura Yuki , Xia Jingyu , Ito Masato , Okada Shigeto 
Published: December 2020

Aqueous Na-ion batteries have gained increasing attention as alternative devices to conventional non-aqueous Li-ion batteries for larger-scale energy storage, because they face formidable problems on manufacturing cost, rate capability, and safety. Although thermodynamic theoretical electrochemical window of water (1.23 V) has imposed a restriction on aqueous batteries in the selection of cathode and anode active materials, recent studies have revealed that some highly concentrated electrolytes effectively prevent electrochemical decomposition of water, and that a judicious choice of electrode active materials with flat potential plateaus within the range of a kinetic practical electrochemical window successfully leads to the expansion of working voltage in aqueous cell [1-3]. Nevertheless, the solute for reported highly concentrated electrolytes has been limited to either highly expensive perfluoroalkylsulfonylates or perchlorates that require special care in handling under dry condition. We have explored more beneficial solutes that contribute in the reduction of manufacturing cost and found that a concentrated aqueous solution of NaCO2CF3 (NaTFA, sodium trifluoroacetate) serves as a suitable electrolyte when NASICON-type Na2VTi(PO4)3 (NVTP) is employed both as cathode (V3+/V4+) and anode (Ti4+/Ti3+ and V3+/V2+). Herein we discuss on the symmetrical operation of NVTP with very flat plateaus in this aqueous Na-ion battery, whose capacity retention is highly dependent on the choice of electrolytes.
Our initial study focused on the electrochemical performance of Na3V2(PO4)3 (NVP) and NVTP as an electrode in the symmetrical cell using 26 mol/kg (m) NaTFA aqueous electrolyte. NVP and NVTP were prepared by conventional solid-state reaction at 900℃ for 24 h under Ar + H2 atmosphere. The cathode/anode disk pellets were fabricated with acetylene black (AB) as a conductive additive and polytetrafluoroethylene (PTFE) as a binder in a weight ratio of active material/AB/PTFE = 70/25/5, respectively, and then sandwiched by titanium (Ti) or aluminum (Al) mesh as a current collector. As is clear from Figures 1a and 1b, NVTP//NVTP was superior to NVP//NVP in terms of overpotential. Another point to note is the reversible and 2 flat plateaus observed with NVTP//NVTP. These notable differences may arise from the marked robustness, insolubility and ionic conductivity of NVTP, which indicates the importance of partial substitution of V in NVP with Ti.
Next, we identified the importance of 26 m NaTFA aqueous electrolyte by comparing it with other representative highly concentrated aqueous electrolytes under otherwise identical conditions. Figure 1c illustrates the capacity retentions of NVTP//NVTP symmetrical full cells when 17 m NaClO4 (purple) and 26 m NaTFA (green) were used as an electrolyte. The purple lines declined prominently in earlier cycles and the dissipated capacity after 50 cycles reached to 10 mAh/g. By contrast, the green line was reluctant to decline even after 300 cycles. We believe that 26 m NaTFA may form anion-derived SEI on the anode, which could possibly suppress reductive decomposition of water.
Acknowledgement
This work was supported by Element Strategy Initiative of MEXT, Grant Number JPMXP0112101003.
References
[1] L. Suo, O. Borodin, T. Gao, M. Olguin, J. Ho, X. Fan, C. Luo, C. Wang, and K. Xu, Science, 350 (2015) 938.
[2] Y. Yamada, K. Usui, K. Sodeyama, S. Ko, Y. Tateyama, and A. Yamada, Nat. Energy, 1 (2016) 16129.
[3] K. Nakamoto, R. Sakamoto, Y. Sawada, M. Ito, and S. Okada, Small Methods, 3 (2019) 1800220.

Figure 1