Measuring Strain In Operando By X-Ray Diffraction in Bicontinuous Si and Nisn Inverse Opal Anodes Under Rapid Cycling Conditions
One effective route towards reversibly accommodating these large volume changes, improving capacity retention and at the same time increasing power density is to take advantage of lower stress and strain values and gradients at smaller length scales using nanostructures such as the inverse opal structure. While many different nanostructures and morphologies have been explored, the rational design and optimization of these structures has been hindered by a dearth of experimentally measured, quantitative strain data for nanostructured NiSn and Si anodes. The amorphous nature of lithiation in nanostructured Si anodes and the unclear lithiation mechanism(s) in nanostructured NiSn anodes has greatly hindered in operando strain measurement to date, especially on microscale and larger format cells. Additionally, transient effects or mechanistic changes that may occur when cycling these anodes at higher rates have not been very well explored in the literature.
Using synchrotron-based X-ray diffraction techniques at the Advanced Photon Source, lattice strains in Si and NiSn coated Ni nanostructured inverse opal scaffolds were measured in operando at a variety of rates in order to deduce mismatch stresses and strain evolution during (dis)charging in the active anode material thin film and the nickel scaffold. Since both the active anode materials form strong bonds with the inverse opal nickel scaffold, the elastic strains measured in the nickel are similar to those present in the anode material, allowing stress and strain states present in the Si or NiSn to be indirectly measured. These inverse opal anodes were cycled at rates between 1C and 20C, and 1C and 500C for Si and NiSn based anodes respectively, with charge and discharge current densities held constant and equivalent at each cycling rate. Additionally, asymmetric cycling parameters were utilized to explore, in operando, a fast charge, slow discharge behavior that may be more representative of current and emerging applications, where lithiation rates of up to 115C for NiSn and 60C for Si and fixed delithiation current densities corresponding to a symmetric 1C rate were utilized. Strains measured in the Ni scaffold were directly correlated with the electrochemical cycling of the anode. These strains are discussed in terms of elasto-plastic deformation mechanisms in the scaffold, cracking of the active materials and other potential stress relief mechanisms. As observed through in operando strain measurements, potential changes in lithiation mechanisms and possible mechanical failure modes at various (dis)charge rates are also discussed.