  A key cause of chemo-mechanical degradation in battery electrodes is that
they undergo abrupt phase transformation during the charging/discharging cycle.
This phase transformation is accompanied by lattice misfit strains that
nucleate microcracks, induce fracture and, in extreme cases, amorphize the
intercalation electrode. In this work, we propose a strategy to prevent the
chemo-mechanical degradation of intercalation electrodes: we show that by
engineering suitable film strains we can regulate the phase transformations in
thin-film intercalation electrodes and circumvent the large volume changes. We
test this strategy using a combination of theory and experiment: we first
analytically derive the effect of film strain on the electrochemical response
of a thin-film intercalation electrode and next apply our analytical model to a
representative example (LixV2O5 with multiple phase transformations). We then
test our theoretical predictions experimentally. Specifically, we
electrochemically cycle thin-film V2O5 electrodes with different film strains
and measure their structure, voltage, and stress responses. Our findings show
that tensile film strains lower the voltage for phase transformations in
thin-film V2O5 electrodes and facilitate their reversible cycling across a
wider voltage window without chemo-mechanical degradation. These results
suggest that film strain engineering is an alternative approach to preventing
chemo-mechanical degradation in intercalation electrodes. Beyond thin-film
electrodes, our findings from this study are applicable to the study of
stress-induced phase transformations in particle-based electrodes and the thin
surface layers forming on cathode particles.
