Dynamics of Spatiotemporal Heterogeneities in Particulate Intercalation Electrodes

Electrochemical energy systems rely on particulate porous electrodes to store or convert energies. While the three-dimensional porous structures of the electrodes were introduced to maximize the interfacial area for better overall performance of the system, spatiotemporal heterogeneities arising from materials thermodynamics localize the charge transfer processes onto a limited portion of the available interfaces. These reaction heterogeneities may cause local hot and cold spots, and early battery failures. This dissertation focuses on the following three aspects of the dynamic reaction heterogeneities in the particulate cathodes and anodes in the lithium-ion batteries: (i) the real-time evolution of reaction heterogeneities in graphite anodes, (ii) the origin of reaction heterogeneities and their interplay with the phase transformation mechanisms in graphite anodes, and (iii) the quantification method of reaction heterogeneities in solid-solution cathodes. The dissertation also concentrates on the systematic electrochemical investigation of the graphite cathodes in aluminum-ion batteries for their coherent design.A simple but precision method has been developed that can directly track and analyze the operando (i.e. local and reacting) interfaces at the mesoscale in a practical graphite porous electrode to obtain the true local current density. The seemingly random reaction heterogeneities are actually controlled by the interplay between the non-equilibrium material thermodynamics and the electrochemical kinetics. The combined theoretical and experimental analyses revealed that unlike other phase-transforming porous electrodes, not all phase separation processes in graphite electrodes can be suppressed by high currents. The results shed light on the long-standing discrepancies in kinetics parameters derived from electroanalytical measurements and from first principles predictions and highlight the necessity to examine the concentration-dependent exchange current density for intercalation electrodes undergoing complex phase transformation processes. While optical microscopy revealed the subtleties of spatiotemporal heterogeneities in graphite electrodes, their identification in solid-solution materials posed challenges. A Raman spectroscopy tool has been developed to map and quantify the spatiotemporal heterogeneities in Ni-rich layered oxide cathode materials (NMC532). The results revealed a significantly high true current density than the widely-accepted globally-averaged one. Incorporating nonequilibrium thermodynamics into classical electrochemical models and electroanalytical techniques will ensure self-consistent understandings of practical porous electrodes toward precision design and management. Lithium-ion batteries rule the energy storage market owing to their overall high performance, which, however, deteriorate severely at temperatures below -10°C. Emerging aluminum-ion batteries (AIBs) can deliver higher reversible capacities at low temperatures down to even -30°C. A systematic electrochemical characterization of the AIBs using classical electroanalytical methods at five temperatures selected between -20°C and room temperature, has been performed to assess the fundamental kinetics. The temperature-insensitive fast kinetics could be attributed to the high availability and easy access of active species at the inner Helmholtz plane near the electrode surface. The results revealed the governing mechanisms facilitating the high performance of AIBs in a wide temperature range and demonstrated the necessity of electrolyte optimization with a focus on the inner Helmholtz plane of the electric double layer structure to ensure high-rate electrode performance at low temperatures

Integer Factoring Using Small Algebraic Dependencies

Integer factoring is a curious number theory problem with wide applications in complexity and cryptography. The best known algorithm to factor a number n takes time, roughly, exp(2*log^{1/3}(n)*log^{2/3}(log(n))) (number field sieve, 1989). One basic idea used is to find two squares, possibly in a number field, that are congruent modulo n. Several variants of this idea have been utilized to get other factoring algorithms in the last century. In this work we intend to explore new ideas towards integer factoring. In particular, we adapt the AKS primality test (2004) ideas for integer factoring. In the motivating case of semiprimes n=pq, i.e. p<q are primes, we exploit the difference in the two Frobenius morphisms (one over F_p and the other over F_q) to factor n in special cases. Specifically, our algorithm is polynomial time (on number theoretic conjectures) if we know a small algebraic dependence between p,q. We discuss families of n where our algorithm is significantly faster than the algorithms based on known techniques

Seismic Design Coefficients for SpeedCore or Composite Plate Shear Walls - Concrete Filled (C-PSW/CF)

This report summarizes the results from FEMA P695 analytical studies conducted to verify the seismic design factors for composite plate shear walls – concrete filled (C-PSW/CF), also referred to as Speedcore. ASCE 7-16 provides the seismic design factors, which include the seismic response modification factor, R, deflection amplification factor, Cd, and overstrength factor, Ωo, for various approved seismic systems. C-PSW/CFs are assigned a response modification factor of 6.5, a deflection amplification factor of 5.5, and an overstrength factor of 2.5 for C-PSW/CFs. These seismic design factors were selected based on the seismic performance of similar structural systems and engineering judgment of the committee. This analytical study investigated and verified the appropriateness of these seismic design factors for walls with flange plates as boundary elements. Four planar (3-story, 6-story, 9-story, and 12-story) and three C-shaped (15-story, 18-story, and 22-story) C-PSW/CF walls were analyzed following the FEMA P695 procedure. This procedure included development of representative planar and C-shaped C-PSW/CF archetypes, calibration of numerical models for these archetypes, and evaluation of nonlinear static (pushover) and incremental dynamic (time history) analyses. The results indicate that seismic design coefficients of R = 6.5, Cd = 5.5, and Ωo = 2.5 appropriately quantify the seismic performance of C-PSW/CF with boundary elements. Walls without any boundary elements or closure plates are not recommended for seismic design based on supplementary analytical studies