Na MAS NMR spectra of sodium-oxygen (Na-O) cathodes reveals a combination of degradation species: newly observed sodium fluoride (NaF) and the expected sodium carbonate (NaCO), as well as the desired reaction product sodium peroxide (NaO). The initial reaction product, sodium superoxide (NaO), is not present in a measurable quantity in the Na NMR spectra of the cycled electrodes. The reactivity of solid NaO is probed further, and NaF is found to be formed through a reaction between the electrochemically generated NaO and the electrode binder, polyvinylidene fluoride (PVDF). The instability of cell components in the presence of desired electrochemical reaction products is clearly problematic and bears further investigation.

Sodium-oxygen batteries have received a significant amount of research attention as a low-overpotential alternative to lithium-oxygen. However, the critical factors governing the composition and morphology of the discharge products in Na-O cells are not thoroughly understood. Here we show that oxygen containing functional groups at the air electrode surface have a substantial role in the electrochemical reaction mechanisms in Na-O cells. Our results show that the presence of functional groups at the air-electrode surface conducts the growth mechanism of discharge products toward a surface-mediated mechanism, forming a conformal film of products at the electrode surface. In addition, oxygen reduction reaction at hydrophilic surfaces more likely passes through a peroxide pathway, which results in the formation of peroxide-based discharge products. Moreover, in-line X-ray diffraction combined with solid state Na NMR results indicate the instability of discharge products against carbonaceous electrodes. The findings of this study help to explain the inconsistency among various reports on composition and morphology of the discharge products in Na-O cells and allow the precise control over the discharge products.

The physiochemical properties of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in adiponitrile (ADN) electrolytes were explored as a function of concentration. The phase diagram and ionic conductivity plots show a distinct relationship between the eutectic composition of the electrolyte and the concentration of maximum ionic conductivity in the 25 °C isotherm. We propose a structure-based explanation for the variation of electrolyte ionic conductivity with LiTFSI concentration, where the eutectic concentration is a transitionary region at which the structure changes from solvated contact ion pairs to extended units of [Liz(ADN)xTFSIy]z−y aggregates. It is found through diffusion coefficient measurements using pulsed-field gradient (PFG) NMR that both D L i / D T F S I and D L i / D A D N increase with concentration until 2.9 M, where after Li+ becomes the fastest diffusing species, suggesting that ion hopping becomes the dominant transport mechanism for Li+. Variable diffusion-time (Δ) PFG NMR is used to track this evolution of the ion transport mechanism. A differentiation in Li+ transport between the micro and bulk levels that increases with concentration was observed. It is proposed that ion hopping within [Liz(ADN)xTFSIy]z−y aggregates dominates the micro-scale, while the bulk-scale is governed by vehicular transport. Lastly, we demonstrate that LiTFSI in ADN is a suitable electrolyte system for use in Li-O2 cells.

A rigid new monoanionic pincer ligand was used to prepare yttrium and scandium dialkyl complexes, and reactivity with CPh3+ is described.

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.