Ti4O7-Enhanced Carbon Supports to Stabilize NaO2 in Sodium-Oxygen Batteries

Sodium-oxygen batteries (NaOBs) have been investigated extensively over the past decade as a high-energy-density alternative to the Li-ion battery system. However, the instability of the main discharge product, sodium superoxide (NaO2), toward the carbon cathode has limited the rechargeability and cycle life of the cell. Here, Magnéli-phase Ti4O7 is studied as a stable coating for carbon paper cathodes in NaOBs. Ti4O7 is shown to be stable toward NaO2 attack via 23Na magic angle spinning (MAS) solid-state nuclear magnetic resonance (ssNMR). Subsequently, NaOB coin cells are constructed with cathodes made from slurries of various Ti4O7 wt % cast onto carbon paper substrates. It is found that cycle life increases dramatically with Ti4O7 content. Characterization of the discharged cathodes by 23Na ssNMR shows that NaO2 is the main electrochemical product in both pure carbon and Ti4O7-coated systems, although the degradation of NaO2 is significantly hindered in Ti4O7-containing cells. Scanning electron microscopy (SEM) data acquired for the discharged cathodes demonstrates that NaO2 is indeed formed electrochemically on the Ti4O7 surface, confirming that the stability of Ti4O7 is able to contribute to the observed extended cell lifetime. A discharge/charge model is proposed where NaO2 precipitates and reversibly dissociates on the Ti4O7 surface through the previously reported solution-based formation and decomposition mechanisms. Characterization of lifetime-cycled cathodes using the two-dimensional 23Na-1H dipolar heteronuclear multiple quantum correlation (23Na{1H} D-HMQC) and 23Na triple quantum magic angle spinning (3QMAS) experiments shows that eventual cell death is caused by the buildup of carbonaceous NaO2 degradation products such as sodium carbonate (Na2CO3) and sodium formate (NaHCO2, NaFormate), but is delayed in Ti4O7 systems as a higher proportion of NaO2 is formed on the stable Ti4O7 surface, which agrees with the proposed formation–decomposition mechanism.