  Phosphorus is one of the most promising anodes for sodium ion batteries
(SIBs). Little is known about the structural mechanism of Na/P cycling due to
the fact that only one of the structures involved (Na3P) is crystalline. Using
operando X-ray diffraction computed tomography (XRD-CT) analysed by Rietveld
and pair distribution (PDF) methods combined with density functional theory
(DFT) calculations we show that the sodiation and desodiation mechanisms of
phosphorus are very different. Sodiation follows the thermodynamic path of
lowest energy from P via NaP to Na3P while desodiation follows a kinetically
controlled deintercalation mechanism in which the layered Na3-xP type structure
is maintained until P nanoclusters form. Using XRD-CT allows analysis of the 3D
structure of the anode, but most importantly, removes the contributions from
the sample container and other battery components, particularly important for
PDF analysis.
