Black phosphorus (BP) has drawn growing
attention as the anode
material for lithium-ion batteries (LIBs) because of its high theoretical
lithium storage capacity. However, its electrochemical processes and
fundamental failure mechanisms have not been completely understood
due to the lack of direct evidence. Here, we report the direct visualization
of the electrochemical lithiation/delithiation behavior of the BP
anode in nano-LIBs using the <i>in situ</i> transmission
electron microscopy technique. Upon lithiation, the BP anode is found
to undergo obvious anisotropic size expansion and phase change from
orthorhombic BP to amorphous Li<sub><i>x</i></sub>P<sub><i>y</i></sub> compounds. Unexpectedly, the BP anode pulverizes
suddenly during discharging, resulting in irreversibility of the lithiated
product and thus poor electrochemical cycling performance. This finding
discloses that the failure mechanism of the BP anode is mainly correlated
with the delithiation process rather than the lithiation one, which
subverts the commonly accepted understanding. The new mechanism insights
would serve to provide viable solutions for eliminating rapid capacity
fading that plagues the bulk BP LIBs
