Molecular Origin of Capacity Fade in Sodium Ion Batteries
Lauren E. Marbella,1 Kent J. Griffith,1 Matthias F. Groh,1 Joseph Nelson,2 Matthew Evans,2 Andrew J. Morris,2 and Clare P. Grey1,*
1University of Cambridge, Department of Chemistry, Lensfield Road, Cambridge CB2 1EW, United Kingdom
2University of Cambridge, Theory of Condensed Matter Group, Cavendish Laboratory, J. J. Thomson Avenue, CB3 0HE, United Kingdom
As the demand for batteries for portable electronics, electric vehicles, and large-scale energy storage continues to increase, improvements in capacity, safety, lifetime, and particularly cost, to the current Li-ion standard are crucial. To address these needs, Na-ion batteries are a promising alternative for long-term energy storage sustainability in terms of both cost and natural abundance. For example, highly competitive layered Na-transition metal phosphate and oxide intercalation cathode materials offer a cost-effective alternative to their Li-ion counterparts. Further, Na-ion systems allow the replacement of expensive Cu current collectors with Al. However, robust candidates for anode materials in Na systems that offer equivalent capacities are lacking. As a result, progress in the development of suitable Na-ion batteries has been substantially stalled. Typical anode materials that are high performing for Li-ion systems, such as Si and graphite, do not reversibly store Na ions or suffer from low capacities, respectively. Otherwise, the high theoretical capacity for the formation of Na3P (2596 mAh/g) makes phosphorus-based materials promising candidates for anodes in Na-ion systems.
Indeed, by combining elemental phosphorus with conductive carbon, we can produce high capacity (2510 mAh/g) in Na-ion batteries. However, while we find that performance near that of theoretical capacity is reached in the first cycle, the capacity retention in phosphorus anodes is poor. Here, we use advanced nuclear magnetic resonance (NMR) techniques (ultrafast magic-angle spinning, variable temperature quadrupolar NMR, and two dimensional phase adjusted spinning sidebands experiments) to probe the phase chemistry and structural transformations that occur during electrochemical cycling to begin to understand the processes that are responsible for capacity fade in phosphorus anodes in Na-ion batteries. The insights gained from this work should help to guide the design and formulation of electrode materials used in next generation electrochemical energy storage devices.