Browsing by Subject "Three-dimensional analysis"

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Advanced high-nickel layered oxide cathodes for lithium-ion batteries
(2018-07-17) Li, Wangda; Manthiram, Arumugam; Goodenough, John B.; Yu, Guihua; Henkelman, Graeme
The growing demand for rechargeable Li-ion batteries with higher performance metrics has spurred intensive research efforts. In the quest for safe and low-cost cathode materials with desirable energy/power capabilities, high-nickel layered oxides (LiNi [subscript 1- x] M [subscript x] O₂; x < 0.5, M = Co, Mn, Al) are among the most promising candidates. However, limited cycle/calendar life especially at elevated temperatures and poor thermal-abuse tolerance are serious challenges for their practical applications. This dissertation focuses on the fundamental understanding of electrode-electrolyte incompatibility for high-Ni LiNi [subscript 1-x] M [subscript x] O₂ with state-of-the-art nonaqueous electrolytes at deep charge during battery operation, and corresponding strategies for inhibiting the associated unwanted parasitic reactions and enabling excellent cyclability/safety in practical cell configurations. First, we reveal the dynamic behaviors of the CEI on LiNi [subscript 0.7] Co [subscript 0.15] Mn [subscript 0.15] O₂ driven by conductive carbon in composite electrodes. Secondary-ion mass spectrometry (SIMS) shows that the CEI, initially formed on carbon black from spontaneous reactions with the electrolyte prior to cell operation, passivates the cathode through a mutual exchange of surface species. By tuning the CEI thickness, we demonstrate its impact on the evolution of the electrode-electrolyte interface during cell operation at high voltages. Next, we study the evolution of the SEI on anodes, where metallic Li deposition causes capacity fade and safety issues. On graphite harvested from pouch cells paired with LiNi [subscript 0.61] Co [subscript 0.12] Mn [subscript 0.27] O₂ after 3,000 cycles, SIMS reveals large Li deposition in the SEI, triggered by transition-metal cations dissolved from the cathode and migrated to the anode. With Al doping (~1 mol %) in LiNi [subscript 0.61] Co [subscript 0.12] Mn [subscript 0.27] O₂, dissolution is effectively inhibited and superior long-term cyclability is achieved (> 80% after 3,000 cycles). With knowledge on both electrodes, we then conduct a comprehensive assessment on the long-term cyclability of high-Ni LiNi [subscript 0.7] Co [subscript 0.15] Mn [subscript 0.15] O₂ and commercially established LiNi [subscript 0.8] Co [subscript 0.15] Al [subscript 0.05] O₂ in pouch full cells (1,500 cycles). Various degradation processes leading to performance deterioration are carefully invesitgaeted. Based on the results, we identify key challenges, relative to NCA, for realizing a long service life of high-Ni NCM and corresponding mitigation strategies. Finally, we design tailored nonaqueous electrolytes based on exclusively aprotic acyclic carbonates free of ethylene carbonate (EC) and realize unusual thermal and electrochemical performance of an ultrahigh-nickel cathode (LiNi [subscript 0.94] Co [subscript 0.06] O₂), reaching a specific capacity of 235 mA h g⁻¹. By using two model electrolyte systems, we present assembled graphite |LiNi [subscript 0.94] Co [subscript 0.06] O₂ pouch full cells with exceptional thermal stability, energy/power capabilities, and long service life
Constitutive modeling of viscoelastic behavior of bituminous materials
(2012-12) Motamed, Arash; Bhasin, Amit
Asphalt mixtures are complex composites that comprise aggregate, asphalt binder, and air. Several research studies have shown that the mechanical behavior of the asphalt mixture is strongly influenced by the matrix, i.e. the asphalt binder. Therefore, accurate constitutive models for the asphalt binders are critical to ensure accurate performance predictions at a material and structural level. However, researchers who use computational methods to model the micromechanics of asphalt mixtures typically assume that (i) asphalt binders behave linearly in shear, and (ii) either bulk modulus or Poisson’s ratio of asphalt binders is not time dependent. This research develops an approach to measure and model the shear and bulk behavior of asphalt binders at intermediate temperatures. First, this research presents the findings from a systematic investigation into the nature of the linear and nonlinear response of asphalt binders subjected to shear using a Dynamic Shear Rheometer (DSR). The DSR test results showed that under certain conditions a compressive normal force was generated in an axially constrained specimen subjected to cyclic torque histories. This normal force could not be solely attributed to the Poynting effect and was also related to the tendency of the asphalt binder to dilate when subjected to shear loads. The generated normal force changed the state of stress and interacted with the shear behavior of asphalt binder. This effect was considered to be an “interaction nonlinearity” or “three dimensional effect”. A constitutive model was identified to accommodate this effect. The model was successfully validated for several different loading histories. Finally, this study investigated the time-dependence of the bulk modulus of asphalt binders. To this end, poker-chip geometries with high aspect ratios were used. The boundary value problem for the poker-chip geometry under step displacement loading was solved to determine the bulk modulus and Poisson’s ratio of asphalt binders as a function of time. The findings from this research not only improve the understanding of asphaltic materials behavior, but also provide tools required to accurately predict pavement performance.