In-Situ Studies toward the Occurrence of “Pseudoelasticity” in Confined Nanostructured NiTi Films and Its Implications toward “Stress Buffering” during Electrochemical Li-Alloying/De-Alloying of Si

The combination of in-situ synchrotron X-ray diffraction and in-situ stress measurements performed during electrochemical Li-alloying/-dealloying of amorphous Si (a-Si) films, having ∼150 nm thick NiTi film as an interlayer between it and the current collector, has confirmed the occurrence of reversible B2↔B19′ phase transformation and associated pseudoelastic deformation of the nanoscaled NiTi film in response to dimensional changes of a-Si. The in-situ stress results also indicate that pseudoelastic deformation of the NiTi interlayer helps “buffer” the stress development in a-Si during lithiation/delithiation. Top-view and cross-section imaging at different stages of electrochemical cycling confirm the concomitant improvement in integrity of a-Si film electrode over multiple lithiation/delithiation cycles, in the presence of the NiTi interlayer. This, in turn, contributes toward significant improvement in the cyclic stability of a-Si. In more specific terms, the presence of NiTi interlayer improved the capacity retention of a-Si to ∼78% after 50 cycles, as compared to just ∼10% retention in the absence of any buffer interlayer. This is expected to allow moving ahead from the usually investigated material as a buffer interlayer, viz., graphenic carbon; as also confirmed by our comparative studies. However, the synchrotron X-ray diffraction results also indicate the presence of some retained B19′ phase in the NiTi interlayer after each lithiation–delithiation “full” cycle, presumably due to straining of the NiTi interlayer beyond the 8–10% limit for pseudoelasticity. This reduces the effectiveness of the interlayer with cycling, as also evidenced with the in-situ stress results. Hence, sustaining the effectiveness of the NiTi interlayer over multiple cycles necessitates addressing this issue via careful designing/engineering of the electrode and/or the electrochemical cycling parameters.