Skip to main content

Molecular Foundation of Structural and Dynamic Transformations in Novel Sodium-Ion Battery Materials

Periodic Reporting for period 1 - NAPANODE (Molecular Foundation of Structural and Dynamic Transformations in Novel Sodium-Ion Battery Materials)

Reporting period: 2017-03-01 to 2019-02-28

The future of widespread clean energy relies heavily on understanding, developing, and optimizing materials for electrochemical energy storage. The influence of short-range structure on macroscopic device properties during operation has hindered implementation of promising technologies such as Na-ion batteries. Na-ion batteries offer a more sustainable solution for energy storage compared to their Li-ion counterparts because Na is cheaper, more abundant, and more widespread in the Earth’s crust. Efforts to develop Na-ion batteries have led to the discovery of cathode materials for Na-ion batteries, but the identification of suitable anode materials has been more arduous. Many intercalation and alloying anode materials that work well in Li-ion batteries fail in Na-ion chemistries, such as graphite or Si. Phosphorus (P) is an exceptionally promising anode material for Na-ion batteries because it offers the highest theoretical capacity of any known anode material, with the end member composition Na3P corresponding to a capacity of 2596 mA h g-1. Unfortunately, P-based anodes suffer from performance degradation issues such as low conductivity and poor capacity retention over multiple cycles. Correlating changes in material structure with specific electrochemical signatures in the charge-discharge profiles allows us to understand which processes immediately precede and follow degradation in anodes for Na-ion batteries. Here, we used solid-state NMR in combination with powder X-ray diffraction (XRD) and theoretical calculations to monitor the evolution of NaxP phases that form on (de)sodiation in black P anodes in Na-ion batteries. We identified key structural units in the amorphous intermediates (P helices) as well as provided the first assignment of the final discharge product of the crystalline architecture, Na3P, in Na-ion batteries.
The structural transformations that occur during cycling in black P anodes for Na-ion batteries were tracked with solid-state NMR, XRD, and DFT calculations. We found new NaxP structures via a computational genetic algorithm that allowed the assignment of various P motifs present in amorphous NaxP (a-NaxP) intermediates. During the first sodiation, P atoms at the end of a chain are observed as early as 0.60 V (Na0.52P) showing that P−P cleavage begins at this composition. In a-NaxP, P motifs that correspond to both P helices and the terminal unit in phosphorus zig-zags are observed. Once the potential drops below 0.22 V, c-Na3P-P63cm, which was predicted to form a priori, appears and persists during desodiation until >0.60 V. We find that Na3P-P63cm is formed in both Na-ion batteries and solid-state synthesis, indicating that both routes access the thermodynamically favorable structure. Slight differences in P-containing environments in a-NaP during sodiation/desodiation may result from different pathways that form extended P structures from the pristine black P vs isolated P atoms in c-Na3P-P63cm, respectively. At the end of desodiation, we find that black P is not re-formed, which may contribute to the poor capacity retention in this material. The combined approach of analyzing both experimental and theoretical chemical shift anisotropy can be extended to understand other structural transformations on (de)lithiation/(de)sodiation in not only P-containing materials, but also systems, such as Si, where distinct structural motifs, such as clusters and chains, play an important role in the battery chemistry of these largely amorphous phases.

Dr. Marbella has also used her expertise to collaborate on other projects in Prof. Grey’s laboratory. One of the PhD students that Dr. Marbella supervises, Yanting Jin, studies the solid electrolyte interphase (SEI) that forms on silicon anodes for Li-ion batteries - anodes that show similar degradation issues to P, resulting in two publications. Dr. Marbella has measured 7Li diffusivities in novel niobium tungsten oxide electrode materials for use in high rate Li-ion batteries - materials that represent a new strategy to design electrodes, which was accepted for publication in Nature. Dr. Marbella used her expertise in pulse field gradient NMR to supervise PhD student Simon Engelke in Prof. Grey’s laboratory to study 1H and 7Li diffusion in anisotropic Si substrates. Dr. Marbella has also pushed the use of MRI techniques within the Grey group, and has supervised a PhD student, Anna Gunnarsdóttir, who focuses on using MRI to understand the role of the SEI in Li dendrite growth. In collaboration with Peter Bruce’s Group at Oxford, she has used MRI to monitor Li dendrite growth in garnet-type solid electrolytes for the first time – work that is currently being written up for publication.

Exploitation and dissemination:

1. Jin, Y.; Kneusels, N.-J. H.; Marbella, L. E.; Castillo-Martínez, E.; Weatherup, R. S.; Magusin, P. C. M. M.; Paul, S.; Grey, C. P. J. Am. Chem. Soc., 2018, DOI: 10.1021/jacs.8b03408.
2. Griffith, K. J.; Wiaderek, K. M.; Cibin, G.; Marbella, L. E.; Grey, C. P. Nature, 2018, DOI: 10.1038/s41586-018-0347-0.
3. Marbella, L. E.; Evans, M. L.; Groh, M. F.; Nelson, J.; Griffith, K. J.; Morris, A. J.; Grey, C. P. J. Am. Chem. Soc., 2018, 140, 7794.
4. Deringer, V. L.; Bernstein, N.; Bartók, A. P.; Cliffe, M. J.; Kerber, R. N.; Marbella, L. E.; Grey, C. P.; Elliott, S. R.; Csányi, G. J. Phys. Chem. Lett., 2018, 9, 2879.
5. Jin, Y.; Kneusels, N.-J. H.; Magusin, P. C. M. M.; Kim, G.; Castillo-Martinez, E.; Marbella, L. E.; Kerber, R. N.; Howe, D.; Paul, S.; Liu, T.; Wright, D.; Grey, C. P. J. Am. Chem. Soc., 2017, 139, 14992.

1. WMG Battery School, April 12, 2018, University of Warwick, Coventry, UK (invited talk)
2. Gordon Research Conference: Batteries: February 25-March 2, 2018, Ventura, CA (poster)
3. Lunchtime Research Seminar, Novemb
The work completed in this project is the first study that provides a detailed assignment of the amorphous NaxP intermediates that form during cycling black P anodes. Numerous studies have shown the tremendous promise of black P for use in Na- and Li-ion batteries, yet a detailed assignment of the chemistry in these systems had remained elusive. Our work reported the combined use of advanced solid-state NMR (specifically, 2D 31P phase-adjusted spinning sideband experiments to correlate 31P isotropic and anisotropic chemical shift) and density functional theory (genetic algorithm, ab initio random structure searching, and prototyping to sample a vast composition space) to determine the intermediate structures. Further, with the use of NMR, XRD, and DFT, we correctly assigned the crystalline architecture of the final discharge product, Na3P, for the first time. These results have the potential to impact how anodes for Na- and Li-ion batteries are designed in next generation batteries that offer higher capacity and longer cycle life. By improving our options for batteries that offer optimal performance, we can transition to clean energy sources more rapidly.