Electrochemical and photoelectrochemical CO2 reduction technologies offer the promise of zero-carbon-emission renewable fuels needed for heavy-duty transportation. However, the inert nature of the CO2 molecule poses a fundamental challenge that must be overcome before efficient (photo)­electrochemical CO2 reduction at scale will be achieved. Optimal catalysts exhibit enduring stability, fast kinetics, high selectivity, and low manufacturing cost. Identifying catalytic mechanisms of CO2 reduction in (photo)­electrochemical systems could accelerate design of efficient catalysts. In recent decades, numerous theoretical studies have contributed to our understanding of CO2 reduction pathways and identifying rate-limiting steps. Although a significant body of work exists regarding homogeneous electrocatalysis for CO2 reduction, this review focuses specifically on the theory of heterogeneous (photo)­electrochemical reduction. We first give an overview of the relevant thermodynamics and semiconductor physics. We then introduce important, widely used theoretical techniques and modeling approaches to catalysis. Recent progress in elucidating mechanisms of heterogeneous (photo)­electrochemical CO2 reduction is discussed through the lens of two experimental systems: pyridine (Py)-catalyzed CO2 (photo)­electrochemical reduction at p-GaP photoelectrodes and electrochemical CO2 reduction at Cu electrodes. We close by proposing strategies and principles for the future design of (photo)­electrochemical catalysts to improve the selectivity and reaction kinetics of CO2 reduction.

Electrolyte decomposition reactions on Li-ion battery electrodes contribute to the formation of solid electrolyte interphase (SEI) layers. These SEI layers are one of the known causes for the loss in battery voltage and capacity over repeated charge/discharge cycles. In this work, density functional theory (DFT)-based ab-initio calculations are applied to study the initial steps of the decomposition of the organic electrolyte component ethylene carbonate (EC) on the (10 4) surface of a layered Li(Ni x ,Mn y ,Co 1-xy )O 2 (NMC) cathode crystal, which is commonly used in commercial Li-ion batteries. The effects on the EC reaction pathway due to dissolved Li + ions in the electrolyte solution and different NMC cathode surface terminations containing adsorbed hydroxyl -OH or fluorine -F species are explicitly considered. We predict a very fast chemical reaction consisting of an EC ring-opening process on the bare cathode surface, the rate of which is independent of the battery operation voltage. This EC ring-opening reaction is unavoidable once the cathode material contacts with the electrolyte because this process is purely chemical rather than electrochemical in nature. The -OH and -F adsorbed species display a passivation effect on the surface against the reaction with EC, but the 1 2 extent is limited except for the case of -OH bonded to a surface transition metal atom. Our work implies that the possible rate-limiting steps of the electrolyte molecule decomposition are the reactions on the decomposed organic products on the cathode surface rather than on the bare cathode surface.

The surface coating of cathodes using insulator films has proven to be a promising method for high-voltage 10 cathode stabilization in Li-ion batteries, but there is still substantial uncertainty about how these films function. More 11 specifically, there is limited knowledge of lithium solubility and transport through the films, which is important for coating 12 design and development. This study uses first-principles calculations based on Density Functional Theory to examine the 13 diffusivity of interstitial lithium in the crystals of α-AlF3, α-Al2O3, m-ZrO2, c-MgO, and α-quartz SiO2, which provide benchmark 14 cases for further understanding of insulator coatings in general. In addition, we propose an Ohmic electrolyte model to predict 15 resistivities and overpotential contributions under battery operating conditions. For the crystalline materials considered we 16 predict that Li + diffuses quite slowly, with a migration barrier larger than 0.9 eV in all crystalline materials except α-quartz SiO2, 17 which is predicted to have a migration barrier of 0.276 eV along <001>. These results suggest that the stable crystalline forms of 18 these insulator materials, except for oriented α-quartz SiO2, are not practical for conformal cathode coatings. Amorphous Al2O3 19 and AlF3 have higher Li + diffusivities than their crystalline counterparts. Our predicted amorphous Al2O3 resistivity (1789 MΩm) is 20 close to the top of the range of the fitted resistivities extracted from previous experiments on nominal Al2O3 coatings (7.8 to 913 21 MΩm) while our predicted amorphous AlF3 resistivity (114 MΩm) is very close to the middle of the range. These comparisons 22 support our framework for modeling and understanding the impact on overpotential of conformal coatings in terms of their 23 fundamental thermodynamic and kinetic properties, and support that these materials can provide practical conformal coatings 24 in their amorphous form.25 I. Introduction 26 27 Surface modification of the cathode by artificial coating is an effective strategy to stabilize Li-ion batteries (LIBs) 28 operating at high voltages.[1-10] Nevertheless, the coating functionalities and the stabilizing mechanisms are still not 29 fully understood and currently a subject of intensive research in the development of next-generation LIBs. Several 30 roles for the coating have been proposed to account for its positive impacts on the cathode performance, including: (i) 31 electrical conduction medium that facilitates electron transport between cathode active particles,[11] (ii) modifier of 32 cathode surface chemistry that changes chemical properties of the cathode surface to improve stability and 33 performance,[12] (iii) HF scavenger that locally reduces the acidity of the electrolyte near the cathode surface, thereby 34 reducing electrolyte degradation,[13-15] and (iv) physical protection barrier that suppresses electrolyte oxidation and 35 cathode corrosion. [8,[16][17][18][19][20] In general, role (iii) and, particularly, role (iv) are the most widely cl...