In Situ Strain Distributions in 3D Solid-State Battery Electrodes

One of the main hurdles that solid state batteries are facing is the mechanical breakdown, best observed at the solid electrolyte-electrode interface, and particularly for those utilizing high energy density electrodes such as Si. Measuring stress and strain in high energy density electrodes in the course of charging/discharging is crucial for understanding the behavior at the interface and thus improve the robustness of SSBs. In our study, we used spatially resolved in-situ Raman spectroscopy to measure the lattice strain in micropatterned single c-Si pillars during electrochemical cycling. We are able to measure Raman peak shifts down to 0.2 cm^-1, corresponding to a strain of 0.05%. These pillars on the order of 10 µm scale provide sharp corners and small planar areas, enabling a systematic understanding of the mesoscale phenomena. Additionally, we coated the Si electrodes with a LiPON layer of varying thicknesses up to 150 nm to understand the impact of mechanical constraint imposed by the solid electrolyte at the interface.<br/>Upon lithiation/delithiation cycling, free-standing Si material is known to contract and expand up to three times of the initial volume. However, our Raman results suggest much smaller strain levels in the pillar shaped Si electrode than predicted for cycling at 0.2C rate. The full strain mapping also indicates that a sharp contrast in strain levels on and off the pillars. Presence of the LiPON coating produces a different strain pattern. We compare the Raman mapping results to finite element modeling of the strain in the coated and uncoated pillars, seeking a comprehensive understanding of the electrochemo-mechanical behavior. In general, our finite element model predictions agree with our Raman results: the strain level in the Si pillar is greatest near the edge and lower at the pillar center. The models also explain beneficial impact of a LiPON coating on the interfacial stability.