Inhomogeneous volume expansion in phosphorus anodes for sodium-ion batteries revealed by X-ray computed tomography

Black phosphorus is a promising high-capacity anode for sodium-ion batteries, but its volume expansion during sodiation can trigger mechanical failure. Here, pure black phosphorus electrodes are compared with black phosphorus carbon composite (1:1) electrodes to connect electrochemical behavior with electrode-scale deformation and loss of electrical contact. The pure black phosphorus electrode delivers an initial sodiation capacity of 2210 mAh g −1 but fails after the first cycle, showing delamination from the current collector, increased cell resistance, and negligible reversible capacity (∼13 mAh g −1 ). In contrast, the composite shows initial sodiation and desodiation capacities of 1010 and 847 mAh g −1 , remains attached, and sustains reversible cycling. Using X-ray computed tomography in a tomography cell, the electrode-scale volume expansion and spatial distribution after one cycle were quantified in detail. The pure black phosphorus electrode expands by 65.5% and forms a blunt-hill shape, while the composite expands by 33% with a doughnut-like pattern that redistributes volume expansion more uniformly across the electrode. These results clearly show that carbon composites mitigate mechanical degradation and reduce the mechanical burden on the counter electrode, highlighting electrode-scale volume changes as a key design constraint for alloy anodes and offering a pathway towards more dimensionally stable sodium-ion batteries.