First-principles and Continuum Modeling of Charge Transport in Li-O2 Batteries.

Li-O2 batteries are a very attractive energy storage technology due to their high theoretical specific energy density. However, several critical challenges impede the development of a practical Li-O2 battery. One of these challenges is the sluggish transport of ions and/or electrons through the Li2O2 discharge product. The purpose of this work is to develop a physics-based picture of transport phenomena within the Li-O2 discharge product and to elucidate how different characteristics of the discharge product influence its apparent transport properties. To this end we employ density functional theory calculations in conjunction with continuum-scale transport models. Our calculations indicate that charge transport in bulk Li2O2 is mediated by hole polarons and Li-ion vacancies, and that a low concentration of these species results in poor intrinsic ionic and electronic conduction. However, structural disorder, the presence of impurities, and the formation of space-charge layers are predicted to significantly enhance charge transport. These results suggest several design strategies for improving Li-O2 cell performance: promoting the formation of amorphous Li2O2, introducing impurities into the discharge product, controlling crystallite orientation in the discharge product, and increasing the operating temperature.PhDPhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/110343/1/maxradin_1.pd

Electronic structure of Li 2 O 2 {0001} surfaces

Abstract The surface properties of the Li 2 O 2 discharge phase are expected to impact strongly the capacity, rate capability, and rechargeability of Li-oxygen batteries. Prior calculations have suggested that the presence of halfmetallic surface states in Li 2 O 2 may mitigate electrical passivation resulting from the growth of Li 2 O 2 , which is a bulk insulator. Here we revisit the electronic structure of bulk Li 2 O 2 and the dominant Li 2 O 2 {0001} surface by comparing results obtained with the PBE GGA functional, the HSE06 hybrid functional, and quasiparticle GW methods. Our results suggest that the bulk band gap lies between the value predicted by the G 0 W 0 method, 5.15 eV, and the value predicted by the self-consistent quasiparticle GW (scGW) approximation, 6.37 eV. The PBE, HSE06, and scGW methods agree that the most stable surface, an oxygen-rich {0001} termination, is indeed half-metallic. This result supports the notion that the electronic structure of surfaces may play an important role in understanding performance limitations in Li-oxygen batteries

Stacking sequence changes and Na ordering in layered intercalation materials

The performance of Na-ion batteries is sensitive to the nature of cation ordering and phase transformations that occur within the intercalation compounds used as electrodes. In order to elucidate these effects in layered Na intercalation compounds, we have carried out a first-principles statistical mechanics study of Na ordering and stacking-sequence preferences in the model compound NaxTiS2. Our calculations predict a series of structural phase transitions at room temperature between O3, P3, O1, O1–O3 staged hybrid, and O1–P3 staged hybrid. We further explore the ordering of Na ions in P3 and O3 and find that these host structures favor very distinct Na-vacancy patterns. Low energy orderings on the honeycomb lattice in P3 consist of triangular island domains with vacancies coalescing at antiphase boundaries. This results in a devil’s staircase of ground-state Na orderings within P3 that are unlike the orderings possible in the triangular lattice of Na sites in O3. We explore the role that antiphase boundaries play in mediating Na diffusion in the P3 host

Identifying challenges towards practical quantum advantage through resource estimation: the measurement roadblock in the variational quantum eigensolver

Recent advances in Noisy Intermediate-Scale Quantum (NISQ) devices have brought much attention to the potential of the Variational Quantum Eigensolver (VQE) and related techniques to provide practical quantum advantage in computational chemistry. However, it is not yet clear whether such algorithms, even in the absence of device error, could achieve quantum advantage for systems of practical interest and how large such an advantage might be. To address these questions, we have performed an exhaustive set of benchmarks to estimate number of qubits and number of measurements required to compute the combustion energies of small organic molecules to within chemical accuracy using VQE as well as state-of-the-art classical algorithms. We consider several key modifications to VQE, including the use of Frozen Natural Orbitals, various Hamiltonian decomposition techniques, and the application of fermionic marginal constraints. Our results indicate that although Frozen Natural Orbitals and low-rank factorizations of the Hamiltonian significantly reduce the qubit and measurement requirements, these techniques are not sufficient to achieve practical quantum computational advantage in the calculation of organic molecule combustion energies. This suggests that new approaches to estimation leveraging quantum coherence, such as Bayesian amplitude estimation [arxiv:2006.09350, arxiv:2006.09349], may be required in order to achieve practical quantum advantage with near-term devices. Our work also highlights the crucial role that resource and performance assessments of quantum algorithms play in identifying quantum advantage and guiding quantum algorithm design.Comment: 27 pages, 18 figure

OpenFermion: The Electronic Structure Package for Quantum Computers

Quantum simulation of chemistry and materials is predicted to be an important application for both near-term and fault-tolerant quantum devices. However, at present, developing and studying algorithms for these problems can be difficult due to the prohibitive amount of domain knowledge required in both the area of chemistry and quantum algorithms. To help bridge this gap and open the field to more researchers, we have developed the OpenFermion software package (www.openfermion.org). OpenFermion is an open-source software library written largely in Python under an Apache 2.0 license, aimed at enabling the simulation of fermionic models and quantum chemistry problems on quantum hardware. Beginning with an interface to common electronic structure packages, it simplifies the translation between a molecular specification and a quantum circuit for solving or studying the electronic structure problem on a quantum computer, minimizing the amount of domain expertise required to enter the field. The package is designed to be extensible and robust, maintaining high software standards in documentation and testing. This release paper outlines the key motivations behind design choices in OpenFermion and discusses some basic OpenFermion functionality which we believe will aid the community in the development of better quantum algorithms and tools for this exciting area of research.Comment: 22 page