Due to the high abundance, low costs and very suitable redox potential, Na-ion batteries (NIBs) should open new avenues of research and engineering as complementary alternatives to Li-ion batteries (LIBs).^[1] This shift has to be accompanied with a deeper understanding of the chemical reactions involving the multiple cell components. The application of a non-invasive analysis tool that can follow the reactions in operando is therefore highly desired. A fundamental technique for this approach is in situ solid-state nuclear magnetic resonance (NMR) due to its high chemical specificity and sensitivity to crystalline, amorphous as well as metastable/short-lived phases.^[2] However, in situ NMR on LIBs/NIBs does not come without its challenges, e.g., significantly different shifts of the multi-component samples, changing sample conditions (such as the magnetic susceptibility and conductivity) during cycling, signal broadening due to paramagnetism as well as interferences between the NMR and external cycler circuit that might impair the real-time monitoring of the electrochemical processes.^[2–4]

In this matter, we are developing and exploring the use of a new Automatic Tuning Matching Cycler (ATMC) in situ NMR probe system that addresses many of these issues (Figure 1a).^[4] We applied the new in situ NMR methodology to paramagnetic Na₃V₂(PO₄)₂F₃ cathodes as well as Na—Na cells and Sn anodes to study local atomic changes and the formation of new/intermediate phases, respectively, during cycling of these materials while being used in electrochemical cells.

³¹P in situ NMR on Na₃V₂(PO₄)₂F₃ as a cathode in a NIB enabled the detection of different P signals within a huge frequency range of 4000 ppm (Figure 1b).^[4] Thereby, two NMR carrier frequencies with 4000 and 5900 ppm offset were applied and re-calibrated “on-the-fly” during the in situ NMR experiment (Figure 1b). The data shows a significant shift and changes in the number as well as intensities of ³¹P signals during desodiation of the cathode. Furthermore, the in situ experiments reveal changes of local P environments that in part have not been seen in ex situ NMR investigations.^[5] Furthermore, we applied ATMC ²³Na in situ NMR on symmetrical Na—Na cells during galvanostatic plating. An automatic adjustment of the NMR carrier frequency during the in situ experiment ensured on-resonance conditions for the Na metal and electrolyte peak, respectively. Thus, interleaved measurements with different optimal NMR set-ups for the metal and electrolyte, respectively, became possible. This allowed the formation of different Na metal species as well as a quantification of electrolyte consumption during the electrochemical experiment to be monitored.^[4,6] The use of Sn metal as an anode material for NIBs was investigated by ²³Na in situ NMR to study the formation of both crystalline and amorphous Na_xSn phases during sodiation.^[7] The new approach is likely to benefit a further understanding of Na-ion battery chemistries.

References

[1]    K. Kubota, S. Komaba, J. Electrochem. Soc. 2015, 162, A2538–A2550.

[2]    F. Blanc, M. Leskes, C. P. Grey, Acc. Chem. Res. 2013, 46, 1952–1963.

[3]    N. M. Trease, L. Zhou, H. J. Chang, B. Y. Zhu, C. P. Grey, Solid State Nucl. Magn. Reson. 2012, 42, 62–70.

[4]    O. Pecher, P. M. Bayley, H. Liu, Z. Liu, N. M. Trease, C. P. Grey, J. Magn. Reson. 2015, submitted.

[5]    Z. Liu, Y.-Y. Hu, M. T. Dunstan, H. Huo, X. Hao, H. Zou, G. Zhong, Y. Yang, C. P. Grey, Chem. Mater. 2014, 26, 2513–2521.

[6]    P. M. Bayley, N. M. Trease, C. P. Grey, J. Am. Chem. Soc. 2015, in press.

[7]    J. Stratford, P. K. Allan, O. Pecher, M. Mayo, A. Morris, C. P. Grey, Manuscript in preparation 2016.