- Kramer, Denis

# Kramer, Denis

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- PublicationMetadata onlyPushing the boundaries of lithium battery research with atomistic modelling on dfferent scales(Institute of Physics Publishing (IOP), 2021-12-07)
;Morgan, Lucy ;Mercer, Michael ;Bhandari, Arihant ;Peng, Chao ;Islam, Mazharul M. ;Yang, Hui ;Holland, Julian Oliver ;Coles, Samuel William ;Sharpe, Ryan ;Walsh, Aron ;Morgan, Benjamin J.; ;Islam, Saiful M. ;Hoster, Harry ;Edge, Jacqueline SophieSkylaris, Chris-KritonShow more Computational modelling is a vital tool in the research of batteries and their component materials. Atomistic models are key to building truly physics-based models of batteries and form the foundation of the multiscale modelling chain, leading to more robust and predictive models. These models can be applied to fundamental research questions with high predictive accuracy. For example, they can be used to predict new behaviour not currently accessible by experiment, for reasons of cost, safety, or throughput. Atomistic models are useful for quantifying and evaluating trends in experimental data, explaining structure-property relationships, and informing materials design strategies and libraries. In this review, we showcase the most prominent atomistic modelling methods and their application to electrode materials, liquid and solid electrolyte materials, and their interfaces, highlighting the diverse range of battery properties that can be investigated. Furthermore, we link atomistic modelling to experimental data and higher scale models such as continuum and control models. We also provide a critical discussion on the outlook of these materials and the main challenges for future battery research.Show more - PublicationMetadata onlyMechanism of Li nucleation at graphite anodes and mitigation strategies(Royal Society of Chemistry, 2021-07-20)
;Peng, Chao ;Bhandari, Arihant ;Dziedzic, Jacek ;Owen, John R. ;Skylaris, Chris-KritonShow more Lithium metal plating is a critical safety issue in Li-ion cells with graphite anodes, and contributes significantly to ageing, drastically limiting the lifetime and inducing capacity loss. Nonetheless, the nucleation mechanism of metallic Li on graphite anodes is still poorly understood. But in-depth understanding is needed to rationally design mitigation measures. In this work, we conducted First-Principles studies to elucidate the Li nucleation mechanism on graphite surfaces. These large-scale density-functional-theory (DFT) calculations indicate that nano-particulate Li forms much more readily than classical nucleation theory predicts. Further, our calculations indicate a crucial role of topological surface states near the zigzag edge, lowering the nucleation barrier by a further 1.32 eV relative to nucleation on the basal plane. Li nucleation, therefore, is likely to initiate at or near the zigzag edges of graphitic particles. Finally, we suggest that chemical doping with a view to reducing the effect of the topological surface states might be a potential mitigation strategy to increase nucleation barriers and reduce the propensity to plate Li near the zigzag edge.Show more - PublicationMetadata onlyElectrochemistry from first-principles in the grand canonical ensemble(American Inst. of Physics, 2021-07-12)
;Bhandari, Arihant ;Peng, Chao ;Dziedzic, Jacek ;Anton, Lucian ;Owen, John R.; Skylaris, Chris-KritonShow more Progress in electrochemical technologies, such as automotive batteries, supercapacitors, and fuel cells, depends greatly on developing improved charged interfaces between electrodes and electrolytes. The rational development of such interfaces can benefit from the atomistic understanding of the materials involved by first-principles quantum mechanical simulations with Density Functional Theory (DFT). However, such simulations are typically performed on the electrode surface in the absence of its electrolyte environment and at constant charge. We have developed a new hybrid computational method combining DFT and the Poisson-Boltzmann equation (P-BE) capable of simulating experimental electrochemistry under potential control in the presence of a solvent and an electrolyte. The charged electrode is represented quantum-mechanically via linear-scaling DFT, which can model nanoscale systems with thousands of atoms and is neutralized by a counter electrolyte charge via the solution of a modified P-BE. Our approach works with the total free energy of the combined multiscale system in a grand canonical ensemble of electrons subject to a constant electrochemical potential. It is calibrated with respect to the reduction potential of common reference electrodes, such as the standard hydrogen electrode and the Li metal electrode, which is used as a reference electrode in Li-ion batteries. Our new method can be used to predict electrochemical properties under constant potential, and we demonstrate this in exemplar simulations of the differential capacitance of few-layer graphene electrodes and the charging of a graphene electrode coupled to a Li metal electrode at different voltages.Show more - PublicationMetadata onlyVoltage hysteresis during lithiation/delithiation of graphite associated with meta-stable carbon stackings(Royal Society of Chemistry, 2020-11-27)
;Mercer, Michael ;Peng, Chao ;Soares, Cindy ;Hoster, HarryShow more Cell voltage is a fundamental quantity used to monitor and control Li-ion batteries. The open cir- cuit voltage (OCV) is of particular interest as it is believed to be a thermodynamic quantity, free of kinetic effects and history and, therefore, “simple” to interpret. Here we show that the OCV characteristics of graphite show hysteresis between charge and discharge that do not solely orig- inate from Li dynamics and that the OCV is in fact history dependent. Combining First-Principles calculations with temperature-controlled electrochemical measurements, we identify a residual hysteresis that persists even at elevated temperatures of greater than 50 degC due to differences in the phase succession between charge and discharge. Experimental entropy profiling, as well as energies and volume changes determined from First-Principles calculations, suggest that the residual hysteresis is associated with different host lattice stackings of carbon and is related to Li disorder across planes in stage II configurations.Show more - PublicationMetadata onlyPractical approach to large-scale electronic structure calculations in electrolyte solutions via continuum-embedded linear-scaling density functional theory(American Chemical Society, 2020-04-09)
;Dziedzic, Jacek ;Bhandari, Arihant ;Anton, Lucian ;Peng, Chao ;Womack, James C. ;Famili, Marjan; Skylaris, Chris-KritonShow more We present the implementation of a hybrid continuum-atomistic model for including the effects of a surrounding electrolyte in large-scale density functional theory (DFT) calculations within the Order-N Electronic Total Energy Package (ONETEP) linear-scaling DFT code, which allows the simulation of large complex systems such as electrochemical interfaces. The model represents the electrolyte ions as a scalar field and the solvent as a polarizable dielectric continuum, both surrounding the quantum solute. The overall energy expression is a grand canonical functional incorporating the electron kinetic and exchange-correlation energies, the total electrostatic energy, entropy, and chemical potentials of the surrounding electrolyte, osmotic pressure, and the effects of cavitation, dispersion, and repulsion. The DFT calculation is performed fully self-consistently in the electrolyte model, allowing the quantum-mechanical system and the surrounding continuum environment to interact and mutually polarize. A bespoke highly parallel multigrid Poisson-Boltzmann solver library, DL-MG, deals with the electrostatic problem, solving a generalized Poisson-Boltzmann equation. Our model supports open boundary conditions, which allows the treatment of molecules, entire biomolecules, or larger nanoparticle assemblies in the electrolyte. We have also implemented the model for periodic boundary conditions, allowing the treatment of extended systems such as electrode surfaces in contact with the electrolyte. A key feature of the model is the use of solute size and solvation-shell-aware accessibility functions that prevent the unphysical accumulation of electrolyte charge near the quantum solute boundary. The model has a small number of parameters - here we demonstrate their calibration against experimental mean activity coefficients. We also present an exemplar simulation of an 1634-atom model of the interface between a graphite anode and LiPF 6 electrolyte in an ethylene carbonate solvent. We compare the cases where Li atoms are intercalated at opposite edges of the graphite slab and in solution, demonstrating a potential application of the model in simulations of fundamental processes in Li-ion batteries.Show more - PublicationMetadata onlyElectronic structure calculations in electrolyte solutions: Methods for neutralization of extended charged interfaces(American Inst. of Physics, 2020)
;Bhandari, Arihant ;Anton, Lucian ;Dziedzic, Jacek ;Peng, Chao; Skylaris, Chris-KritonShow more Density functional theory (DFT) is often used for simulating extended materials such as infinite crystals or surfaces, under periodic boundary conditions (PBCs). In such calculations, when the simulation cell has non-zero charge, electrical neutrality has to be imposed, and this is often done via a uniform background charge of opposite sign ("jellium"). This artificial neutralization does not occur in reality, where a different mechanism is followed as in the example of a charged electrode in electrolyte solution, where the surrounding electrolyte screens the local charge at the interface. The neutralizing effect of the surrounding electrolyte can be incorporated within a hybrid quantum-continuum model based on a modified Poisson-Boltzmann equation, where the concentrations of electrolyte ions are modified to achieve electroneutrality. Among the infinite possible ways of modifying the electrolyte charge, we propose here a physically optimal solution, which minimizes the deviation of concentrations of electrolyte ions from those in open boundary conditions (OBCs). This principle of correspondence of PBCs with OBCs leads to the correct concentration profiles of electrolyte ions, and electroneutrality within the simulation cell and in the bulk electrolyte is maintained simultaneously, as observed in experiments. This approach, which we call the Neutralization by Electrolyte Concentration Shift (NECS), is implemented in our electrolyte model in the Order-N Electronic Total Energy Package (ONETEP) linear-scaling DFT code, which makes use of a bespoke highly parallel Poisson-Boltzmann solver, DL_MG. We further propose another neutralization scheme ("accessible jellium"), which is a simplification of NECS. We demonstrate and compare the different neutralization schemes on several examples.Show more