Lithium batteries provide high energy density storage with applications ranging from consumer electronics to electric vehicles. However, they have a limited lifespan and experience capacity loss with aging. Multiple mechanisms contribute to battery aging. The battery binder plays two important roles in the electrodes, and the damage it sustains during cycling may play a role in the degradation of the overall battery performance. Mechanical stress during battery operations occurs as a result of the swelling and shrinking of the electrodes because of the movement of lithium with cycling. The yield stress of the swollen polyvinylidene fluoride carbon black (PVDFCB) binder was measured at approximately 4MPa for PVDF with carbon black CB weight fractions between 10-30% swollen in propylene carbonate. This is far less stress than is typically experienced in an electrode during cycling. The effects of this permanent damage to the binder were explored by measuring the conductivity loss with strains in excess of the binder yield.
Nanosecond pulsed discharges provide versatile experimental and computational testbeds for the exploration of fundamental plasma physics. In particular, the fast rise time and short duration produce plasmas which are both spatially diffuse and uniform enough to probe experimentally and confine the kinetics of interest to sufficiently short time scales to be computationally tractable. This work will focus on validation of particle-in-cell with Monte Carlo collisions (PIC-MCC) modeling and analysis of plasma phenomenon during and after formation of the conductive plasma channel of a nanosecond pulse discharge in helium at 200 Torr and 300 K over a 1 cm gap. The validation will compare results of the simulation to measurements of electron number density, temperature, 1D electron energy distribution function, and Townsend ionization coefficient, as well as ion mobility. Analysis of the stochastic nature of the electron avalanche ahead of the ionization wave front and of significant ionization overshoot in the presheath region is also performed.
Glass-ceramic seals may be the future of hermetic connectors at Sandia National Laboratories. They have been shown capable of surviving higher temperatures and pressures than amorphous glass seals. More advanced finite-element material models are required to enable model-based design and provide evidence that the hermetic connectors can meet design requirements. Glass-ceramics are composite materials with both crystalline and amorphous phases. The latter gives rise to (non-linearly) viscoelastic behavior. Given their complex microstructures, glass-ceramics may be thermorheologically complex, a behavior outside the scope of currently implemented constitutive models at Sandia. However, it was desired to assess if the Simplified Potential Energy Clock (SPEC) model is capable of capturing the material response. Available data for SL 16.8 glass-ceramic was used to calibrate the SPEC model. Model accuracy was assessed by comparing model predictions with shear moduli temperature dependence and high temperature 3-point bend creep data. It is shown that the model can predict the temperature dependence of the shear moduli and 3- point bend creep data. Analysis of the results is presented. Suggestions for future experiments and model development are presented. Though further calibration is likely necessary, SPEC has been shown capable of modeling glass-ceramic behavior in the glass transition region but requires further analysis below the transition region.
We present selected results from a series of Open Stack thermal battery tests performed in FY14 and FY15 and discuss our findings. These tests were meant to provide validation data for the comprehensive thermal battery simulation tools currently under development in Sierra/Aria under known conditions compared with as-manufactured batteries. We are able to satisfy this original objective in the present study for some test conditions. Measurements from each test include: nominal stack pressure (axial stress) vs. time in the cold state and during battery ignition, battery voltage vs. time against a prescribed current draw with periodic pulses, and images transverse to the battery axis from which cell displacements are computed. Six battery configurations were evaluated: 3, 5, and 10 cell stacks sandwiched between 4 layers of the materials used for axial thermal insulation, either Fiberfrax Board or MinK. In addition to the results from 3, 5, and 10 cell stacks with either in-line Fiberfrax Board or MinK insulation, a series of cell-free “control” tests were performed that show the inherent settling and stress relaxation based on the interaction between the insulation and heat pellets alone.
Flows of complex materials are not only scientifically intriguing, but can also be visually striking. At the SOR Annual Meeting in Denver this fall, you will have the opportunity to have your work recognized as the most engaging image or movie of rheology for 2017.
