Sodium‐Ion batteries (SIB) are a promising drop‐in technology that can potentially replace the state of the art lithium‐ion batteries (LIB) in a wide range of applications. In this article, we investigate the performance characteristics, cycling ageing and the microstructural design of a newly commercialised 18650 SIB comprising a Na a Ni x Fe y Mn z O 2 ( X : Y : Z ratio of 1:1:1, NFM111) cathode and a hard carbon (HC) anode. Detailed analyses of the cell chemistry and electrode morphology are presented. The cycling data reveal a distinct knee‐point in the ageing trajectory, separating a linear ageing stage (Phase I) with moderate capacity loss from a rapid degradation phase (Phase II). Post‐mortem analyses show that the NFM111 cathode maintains structural integrity with minor capacity loss, whereas the HC anode undergoes significant microstructural and chemical changes that reduce its (de‐)sodiation capability. We found evidence that the underlying degradation path is similar to established two‐phase ageing models for LIBs. In the linear phase, continuous solid electrolyte interface (SEI) formation, resulting in loss of sodium inventory, is the dominant ageing mechanism. In the second phase, the pronounced capacity fade is caused by an increasing impairment of the ion transport on different scales, which makes the active material less and less accessible.
Auch et al. (Sun,) studied this question.