Unravelling the origin of capacity fade in Prussian white hard carbon full cells through X-ray diffraction

• Published in: Journal of materials chemistry. A, Materials for energy and sustainability Materials for energy and sustainability, 2024-07, Vol.12 (28), p.17413-17421
• Main Authors: Nielsen, Ida, Hall, Charles Aram, Mattsson, Agnes-Matilda, Younesi, Reza, Buckel, Alexander, Ek, Gustav, Brant, William R
• Format: Article
• ISSN: 2050-7488; 2050-7496
• DOI: 10.1039/d4ta02325k

Prussian white (PW), Na 2− x Fe[Fe(CN) 6 ], is an attractive cathode material for sodium-ion batteries due to its porous framework enabling fast sodium-ion extraction and insertion, environmentally safe elements, scalable synthesis, and performance comparable to current lithium-ion technologies. However, PW suffers from large volume changes between rhombohedral and cubic phases during cycling which is suggested to be detrimental over time because of structural degradation and increased ion insertion resistance. In particular, studies on PW hard carbon full cells revealed that most of the capacity is lost from the lower potential plateau, where this phase transition occurs. It is proposed that cycling in a restricted potential range, where the phase transition is avoided, could benefit the cycle lifetime and capacity retention. Here, we show an operando X-ray diffraction study aiming at determining how the structure evolves after prolonged cycling in different restricted potential ranges and how this impacts the cycling stability and capacity fade in PW. No signs of structural degradation were observed independently of the pre-cycling conditions used. In addition, more of the rhombohedral phase and capacity were recovered in the discharged state when a more restricted potential range had been applied. Thus, it was shown that the phase transition and corresponding volume changes have little impact on the capacity fade. Instead, the main source for capacity fade was proved to be sodium inventory loss, especially during the initial cycles, in combination with, to a lesser extent, polarization. This study gives a new perspective on PW-based batteries in that neither volume changes nor phase transitions are detrimental to battery performance. These results aid the development of improved cycling protocols and battery systems comprised of PW where the lifetime of the material is prolonged. The capacity loss during long-term cycling of Prussian white hard carbon full cells is due to sodium inventory loss in combination with polarization and not because of structural degradation.