Siva P.V. Nadimpalli

Measurement and modeling of the mechanical and electrochemical response of amorphous Si thin film electrodes during cyclic lithiation

A combination of experimental measurements and numerical simulations are used to characterize the mechanical and electrochemical response of thin film amorphous Si electrodes during cyclic lithiation. Parameters extracted from the experiment include the variation of elastic modulus and the flow stress as functions of Li concentration; the strain rate sensitivity; the diffusion coefficient for Li transport in the electrode; the free energy of mixing as a function of Li concentration in the electrode; the exchange current density for the Lithium insertion reaction; as well as reaction rates and diffusion coefficients characterizing the rate of formation of solid-electrolyte interphase layer at the electrode surface. Model predictions are compared with experimental measurements; and the implications for practical Si based electrodes are discussed.

Stress Response of Germanium Electrodes During Lithiation/Delithiation Cycling

An in situ study of stress evolution of germanium as a lithium-ion battery electrode material is presented. Thin films of germanium are cycled in a half-cell configuration with lithium metal foil as counter/reference electrode, with 1M lithium hexafluorophosphate in ethylene carbonate, diethylene carbonate, dimethyl carbonate solution (1:1:1, wt. %) as electrolyte. Real-time stress evolution in the germanium thin-film electrodes during electrochemical lithiation and delithiation is measured by monitoring the substrate curvature using the multi-beam optical sensing method. Germanium thin film undergoes extensive inelastic deformation during electrochemical lithiation and delithiation similar to silicon. The peak compressive stress during lithiation in germanium was 0.6 GPa, which is almost half of the peak compressive stress of lithiated silicon. The tensile stress of lithiated Ge on the other hand was almost same as that of lithiated silicon, 1 GPa of peak stress. The stress response of the first cycle was slightly different from that of the second cycle, which is an indication of irreversible structural changes in the Ge electrode during the first cycle. The Ge thin films survived stresses as high as 1 GPa of tensile stress without developing any cracks while the Si films under same conditions showed extensive cracking, consistent with previous observations which claimed lithiated Ge to be tougher than lithiated Si.Copyright

The impact of nozzle and bed temperatures on the fracture resistance of FDM printed materials

Additive manufacturing refers to a new technology in which physical parts are directly produced from a computer model by incremental addition of the constituent materials. Fused deposition modeling (FDM) is one of the most common types of additive manufacture processes. The ultimate mechanical performance of FDM printed parts is a function of the interlayer bond quality. Current literature however focuses only on the phenomenological evolution of standard mechanical properties (such as tensile and bending) as a function of printing conditions. Such studies do not provide direct information about the interlayer adhesion and in-practice failure characteristics. In this work, a fracturemechanics- based methodology was used to characterize the fracture resistance of FDM 3D printed Acrylonitrile Butadiene Styrene (ABS) samples as a function of nozzle and bed temperatures. Double cantilever beam (DCB) specimens was printed in such pattern that the applied load exerted only tensile opening stresses at the crack front. This facilitated the measurement of crack growth under pure mode-I condition. A finite element model was then used to obtain the J-integral strain energy release rate values, as a measure of the fracture resistance. Since the crack propagated at the interlayer in all the cases, the fracture resistance was a direct indication of the interlayer adhesion. The results revealed that the critical crack growth load, the actual fracture surface area (governed by printed mesostructure) and the apparent fracture resistance all increased when the nozzle or bed temperature was increased; the nozzle temperature showed a much stronger effect. The layer-to-layer adhesion, as reflected by the interlayer fracture resistance, did not show monotonous increase with the temperatures and appeared to level off at higher temperatures, indicating that complete interlayer fusion was achieved. This work provides insight into and characterizes the relationships between the 3D printing conditions, the resultant mesostructure, the apparent fracture resistance and the interlayer adhesion in FDM 3D printed materials.