Nanoscale compositional changes during first delithiation of Si negative electrodes

The local composition of negative silicon electrodes is studied by ex situ electron energy-loss spectroscopy along the first delithiation in a lithium battery. By measuring dozens of sample areas for over a dozen compositions, the local and overall inhomogeneities in these practical electrodes are evaluated. The statistical treatment of the data highlights the existence of larger inhomogeneities at the beginning of the delithiation as well as at a 100 nm scale. It is also shown that, even if incremental capacity curves are different, the compositional changes during delithiation are identical for nano- and micro-Si. Namely, an initial Li15Si4 phase is replaced by a Li2±0.3Si amorphous phase in a biphasic process, the latter compound being further delithiated to amorphous silicon in single phase process. Results also show that the electrochemical irreversibility associated with the liquid electrolyte reduction/degradation is generated during the lithiation process, not the delithiation process.

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► A biphasic process for x > 2 followed by a solid solution is proven for delithiation.
► Both nano- and micro-Si present the same behaviour.
► In electrodes prepared with nano-silicon, nanoscale inhomogeneities can be large.
► These inhomogeneities decrease as delithiation increases.
► Inhomogeneities through the electrode thickness are relatively small.