Solid-state sodium batteries with P2-type Mn-based layered oxides by utilizing anionic redox

Abstract

Recently, P2-type manganese-based sodium layered cathodes have gained considerable attention as a viable option for grid-scale energy storage due to their high natural abundance, low cost, and high specific capacity from both cationic and anionic redox activity. We report here a solid-state battery with a cobalt/nickel-free layered Na_0.72Li_0.24Mn_0.75Si_0.01O₂ cathode, sodium bis(fluorosulfonyl)imide (NaFSI)-based polymer composite solid electrolyte (CSE), and a sodium-metal anode. Electrochemical measurements confirm both reversible cationic (Mn-redox) and anionic (O-redox) can be achieved in solid-state sodium metal batteries (SSNMBs) with a CSE and layered Na_0.72Li_0.24Mn_0.75Si_0.01O₂ cathode. Solid-state cells were found to achieve a maximum specific capacity of 180 mA h g⁻¹ with capacity retention of 72% after 50 cycles at C/2 rate at 1.50–4.50 V at 60 °C. Post mortem analysis reveals capacity fade can be primarily attributed to an increase in cell polarization at the cathode-electrolyte interphase (CEI), specifically oxidation/degradation of the infiltrated solid polymer electrolyte. To improve the cycle performance, the oxidative stability of a solid-electrolyte with the high-voltage cathode needs to be considered to minimize the formation of resistive CEI layers, which limit capacity utilization. Altogether, this work provides a promising strategy to utilize anionic redox-based cathodes in solid-state batteries, which in turn can aid the development of practically viable SSNMBs.