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.

An anode composed of tin-core, graphitic-carbon-shell nanoparticles distributed on graphene nanosheets, Sn@C-GNs, is studied during the lithiation process. 7 Li NMR provides an accurate measure of the stepwise reduction of metallic Sn to lithium-tin alloys and reduction of the graphitic carbon. The metallic nanoparticle cores are observed to form ordered, crystalline phases at each step of the lithiation process. The 7 Li 2D experiments presented provide insight into the proximity of the various phases, reflecting the mechanism of the electrochemical reaction. In particular, a sequential model of nanoparticle lithiation, rather than a simultaneous process, is suggested. Movement of lithium ions between two elements of the nanostructured Sn@C-GNsmaterial-the metallic core and carbon shell-is also observed. Conventional 13 C SSNMR experiments on <5 mg of active material from electrochemical cells were found to be impossible, but signal enhancements (up to 18 fold) via the use of extended echo trains in conjunction with magic-angle spinning enabled NMR characterization of the carbon. We demonstrate that the 13 C data is extremely sensitive to the added electron density when the graphitic carbon is reduced. We also investigate ex situ carbon electrodes from cycled Li-O 2 cells, where we find no evidence of charge sharing between the electrochemically active species and the graphitic carbon in the 13 C NMR.

Solid-state 17O NMR was used to compare the stability of two potential Li–O2 electrolytestetraethylene glycol dimethyl ether (TEGDME) and trimethyl phosphate (TMP). The TEGDME electrolyte demonstrated superior stability to the TMP electrolyte. Li2O2 and evidence of electrolyte breakdown was observed in the TEGDME cell, whereas only electrolyte breakdown products were discovered within the TMP cell. Potential decomposition pathways of TMP are proposed here that account for the formation of the discharge species observed in the 17O, 7Li, 1H, and 31P solid-state NMR of the cycled cathodes.

In the Li-O 2 battery system, it is has been shown to be challenging to differentiate the discharge products or determine the electrolyte stability with direct 7 Li NMR. Defined 7 Li quadrupole lineshapes are not observed for cycled cathodes. Here, 7 Li nutation NMR is demonstrated to be an effective method for the identification of Li 2 O 2 in cycled cathodes. The 7 Li quadrupole interaction of Li 2 O 2 (35 kHz) and Li 2 CO 3 (120 kHz) are of similar magnitude to typically radiofrequency fields (ranging from 40 to 60 kHz). The 7 Li nutation frequency will therefore be influenced by both interactions. The discharge products of the cycled cathodes were determined by comparing the 7 Li nutation frequencies of the cycled cathodes to the 7 Li nutation frequency of the pristine materials when the applied radiofrequency field was 30 kHz. Li 2 CO 3 was determined to be the main discharge product in the propylene carbonate/dimethyl carbonate and trimethyl phosphate electrolyte systems, since the 7 Li nutation frequencies of the cathodes corresponded to the 7 Li nutation frequency of pristine Li 2 CO 3 . The 7 Li nutation frequency of the tetraethylene glycol dimethyl ether cathode was between the 7 Li nutation frequencies of both pristine Li 2 O 2 and pristine Li 2 CO 3 , indicating that both Li 2 O 2 and Li 2 CO 3 were discharge products influencing the observed nutation frequency. From 7 Li nutation NMR the novel trimethyl phosphate electrolyte was determined to be an unsuitable Li-O 2 electrolyte, as the fast 7 Li nutation frequency indicated that Li 2 O 2 was not a primary discharge species. With 17 O NMR, Li 2 CO 3 was confirmed to be a main discharge product formed with the trimethyl phosphate electrolyte.Résumé : Dans les systèmes de batteries Li-O 2 , on sait qu'il est difficile de distinguer les produits de décharge ou de déterminer la stabilité électrolytique par observation directe en RMN 7 Li. Dans le cas des cathodes cyclées, les bandes du quadrupole du 7 Li n'ont pas une forme définie. Dans ces travaux, nous démontrons que la RMN 7 Li de nutation peut être une méthode efficace pour l'identification du Li 2 O 2 dans les cathodes cyclées. L'interaction quadrupolaire du 7 Li dans le Li 2 O 2 (35 kHz) et le Li 2 CO 3 (120 kHz) est d'intensité similaire à celle qui est typique des champs radiofréquences (de 40 kHz à 60 kHz). La fréquence de nutation du 7 Li devrait donc être influencée par ces deux interactions. Nous avons identifié les produits de décharge des cathodes cyclées en comparant les fréquences de nutation du 7 Li dans les cathodes cyclées à celles du 7 Li dans les substances pures sous un champ de radiofréquences appliqué de 30 kHz. Comme les fréquences de nutation du 7 Li dans les cathodes correspondaient à celles du 7 Li dans le Li 2 CO 3 pur, nous avons pu identifier le Li 2 CO 3 comme étant le principal produit de décharge dans des systèmes électrolytiques composés de carbonate de propylène / carbonate de diméthyle ou de phosphate de triméthyle. La fréquence de nutation du 7 Li de la cathode dans...

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