The quest for more efficient and effective rechargeable batteries is ongoing, especially given the limitations of conventional technologies. Now, Kisuk Kang of Seoul National University and colleagues there and at the Hyundai Motor Company in Korea have developed a new approach that could lead to a high-voltage battery based on sodium rather than lithium at low cost. Their approach would side-step the limited supply of lithium as well as avoid some of the risks of batteries based on that element as well as the need for highly flammable organic electrolytes.

A new high-voltage battery might be used in grid-scale energy storage systems particularly if it couples high energy density with compactness. In addition a system with low maintenance requirements and high round-trip efficiency might also be highly versatile and find use in electric vehicles and other applications too.

Writing in the journal Materials Today [Lee, M.H. et al. Mater. Today (2019); DOI: 10.1016/j.mattod.2019.02.004] the team explains how the recent discovery of high-concentration electrolyte systems could help them roll out a new type of rechargeable sodium battery. They have revisited what they refer to as the commonly used low-cost solutes for high-concentration systems. In their study they have found that sodium perchlorate as solute effectively gives a wide electrochemical stability by inhibiting water decomposition as well as inducing stable solid-electrolyte interphase (SEI) layer formation without lowering the oxidation state of the salt anions.

The team used a cathode made from a sodium-iron-phosphate compound, Na4Fe3(PO4)2(P2O7) cathode and an anode comprising NaTi2(PO4)3. With the SEI layer containing sodium carbonate and Na-O compounds, such as sodium hydroxide, the team was able to guarantee excellent electrochemical storage stability of the battery. The system outperforms state of the art experimental sodium batteries based on NaCF3SO3, the team reports.

This is, the team says, the first time that the feasibility of using a high-concentration electrolyte system has been demonstrated with low-cost inorganic solutes. The team was able to operate their full cell with a high energy density of 36 Watt-hours per kilogram, Wh kg-1 and a coulombic efficiency of up to 99% for more than 200 cycles. Voltage up to 2.7 volts was achieved and storage stability for up to about 900 hours was observed.

"Our findings on the new inorganic solutes suitable for the high-concentration aqueous electrolytes broaden our understanding on this new class of electrolyte systems and provide important guidance for the realization of low-cost high-voltage aqueous batteries," the team concludes.

"We are now trying to understand further what aspects of the solute properties determines the efficacy of the high-concentration aqueous electrolyte, which can give us more general insights in exploring diverse candidates of cost-effective salts," Kang told Materials Today.