The theory of transport phenomena in multicomponent electrolyte solutions is presented here through the integration of continuum mechanics, electromagnetism, and nonequilibrium thermodynamics. The governing equations of irreversible thermodynamics, including balance laws, Maxwell's equations, internal entropy production, and linear laws relating the thermodynamic forces and fluxes, are derived. Green-Kubo relations for the transport coefficients connecting electrochemical potential gradients and diffusive fluxes are obtained in terms of the flux-flux time correlations. The relationship between the derived transport coefficients and those of the Stefan-Maxwell and infinitely dilute frameworks are presented, and the connection between the transport matrix and experimentally measurable quantities is described. To exemplify the application of the derived Green-Kubo relations in molecular simulations, the matrix of transport coefficients for lithium and chloride ions in dimethyl sulfoxide is computed using classical molecular dynamics and compared with experimental measurements. K E Y W O R D S electrochemistry, nonequilibrium thermodynamics, thermodynamics/statistical, transport where c i is the concentration of species i and K ij are the Stefan-Maxwell transport coefficients. These equations may be interpreted

While Cu is the only electrocatalyst that converts CO 2 into meaningful quantities of CH 4 fuel, it requires significant overpotentials (onset potential of ∼−0.80 V vs RHE), decreasing energy conversion efficiencies. We report that Mo 2 C is capable of catalyzing CO 2 into CH 4 at low potentials (onset potential of ∼−0.55 V vs RHE), where Cu electrocatalysts do not convert CO 2 . This lowoverpotential catalyst was first identified as a candidate by electronic structure calculations, which indicated the free energetics of CO hydrogenation to be more favorable than that on conventional transition metals such as Cu. Despite the low onset potential for CH 4 , the CH 4 has a steep Tafel slope (∼−280 mV/dec), resulting in most of the current passing through the Mo 2 C electrocatalysts being utilized for the competitive hydrogen evolution reaction. We conducted a detailed theoretical analysis on the basis of density functional theory calculations, microkinetic analysis, and simulated Pourbaix diagrams to suggest the reasons for these characteristics. These analyses suggest that the potential-limiting step in CH 4 evolution is the clearing of OH from the surface, while the rate-limiting step is the nonelectrochemical C−O bond scission, resulting in a high OH coverage and a high Tafel slope. Our calculations suggest that this high coverage weakens H binding, causing enhancement of the H 2 evolution reaction in comparison to that under CO 2 -free conditions. This analysis shows that the detailed interaction of theory and experiment can be used to design and analyze operational electrocatalysts for CO 2 reduction and other complicated electrocatalytic reactions.

Lithium-ion batteries face low temperature performance issues, limiting the adoption of technologies ranging from electric vehicles to stationary grid storage. This problem is thought to be exacerbated by slow transport within the electrolyte, which in turn may be influenced by ion association, solvent viscosity, and cation transference number. How these factors collectively impact low temperature transport phenomena, however, remains poorly understood. Here we show using all-atom classical molecular dynamics (MD) simulations that the dominant factor influencing low temperature transport in LP57 (1 M LiPF 6 in 3:7 ethylene carbonate (EC)/ethyl methyl carbonate (EMC)) is solvent viscosity, rather than ion aggregation or cation transference number. We find that ion association decreases with decreasing temperature, while the cation transference number is positive and roughly independent of temperature. In an effort to improve low temperature performance, we introduce γ-butyrolactone (GBL) as a low viscosity co-solvent to explore two alternative formulations: 1 M LiPF 6 in 15:15:70 EC/GBL/EMC and 3:7 GBL/EMC. While GBL reduces solution viscosity, its low dielectric constant results in increased ion pairing, yielding neither improved bulk ionic conductivity nor appreciably altered ion transport mechanisms. We expect that these results will enhance understanding of low temperature transport and inform the development of superior electrolytes.

Development of Li + -containing electrolytes with improved transport properties requires reliable, reproducible, and ideally low volume techniques to rigorously understand ion-transport with varying composition. Precisely measuring the complete set of transport coefficients in liquid electrolytes under battery-relevant operating conditions is difficult and the reliability of these methods are sparsely described in electrolyte transport literature. In this work, we apply the Balsara-Newman transport characterization approach typically used for polymer electrolytes to liquid electrolyte systems in an attempt to fully measure all transport coefficients (conductivity, total salt diffusion coefficient, thermodynamic factor and transference number) for the model system of LiPF 6 in an ethylene carbonate -ethyl methyl carbonate (EC:EMC) mixture. Using systematic timescale and statistical analyses, we find that transport coefficients measured using potentiostatic polarization of Li-Li symmetric cells exhibit strong correlation to Li electrode interfacial resistance, indicating that such methods are probing both bulk and interfacial phenomena. This reveals a major roadblock in characterizing electrolyte systems where the interfacial resistance is significantly larger than ohmic electrolyte resistance. As a result, we find that methods that rely on potentiostatic Li metal stripping/plating do not readily result in reliable liquid electrolyte transport coefficients, unlike similar methods for solid polymer electrolytes, where interfacial resistances are typically smaller than electrolyte resistances at the elevated temperatures typically of interest for such electrolytes.

Fast charging of most commercial lithium-ion batteries is limited due to fear of lithium plating on the graphite anode, which is difficult to detect and poses significant safety risk. Here we demonstrate the power of simple, accessible, and high-throughput cycling techniques to quantify irreversible Li plating spanning data from over 200 cells. We first observe the effects of energy density, charge rate, temperature, and State-of-Charge (SOC) on lithium plating, use the results to refine mature physicsbased electrochemical models, and provide an interpretable empirical equation for predicting the plating onset SOC. We then explore the reversibility of lithium plating for varied deposition rates, amounts, and electrolyte compositions, applying our understanding towards development of electrolytes that reduce irreversible Li formation. Finally, we provide the first quantitative comparison of lithium plating in the experimentally convenient Graphite|Li cell configuration compared with commercially relevant Graphite|LiNi0.5Mn0.3Co0.2O2 (NMC). The hypotheses and abundant data herein were generated primarily with equipment universal to the battery researcher, encouraging further development of innovative testing methods and data processing that enable rapid battery IntroductionThe urgent need to combat climate change has sparked extreme growth in demand for lithium-ion batteries (LIB). Rapid innovation in battery materials and cell design is critical to meet this demand for diverse applications from electronics to vehicles and utility-scale energy storage. Composite graphite electrodes remain a universal component of the LIB and are expected to dominate anode market share through 2030 despite the introduction of silicon and lithium-based materials 1 . The design space for graphite electrodes is immense, with parameters such as the loading, porosity, particle size, binder composition, and electrolyte being carefully selected to meet requirements for lifetime, operating temperature, charge time, and manufacturing. Regardless of design and application, the lithium plating reaction on graphite is a performance and safety concern due to the formation of noncyclable 'dead' lithium metal and salts. While recent studies have focused on Li plating during fast charging, the phenomenon is also pertinent to other operating extremes such as low temperature 2 , overcharge 3 , or system malfunction 4 . Electrochemical (EChem) modeling is an important tool for understanding design tradeoffs that improve graphite performance while avoiding plating. Over decades, Newman-based models that relate cell current density, voltage, temperature, and material properties to graphite intercalation have been enhanced to also estimate lithium plating. [5][6][7][8][9][10] This has led to initial insight into the effect of charge rate, electrode loading, and temperature on lithium plating onset/amount, but simulations rely on debated parameters such as the plating exchange current density or reversibility and are frequently not verified with direct ex...

Selective removal of oxygen is the key challenge in the upgrading of biomass-derived molecules, and reducible metal oxides have shown the ability to catalytically remove oxygen even at low exogenous H2 pressures.

The solid-electrolyte interphase (SEI) enables the remarkable capacity retention of lithium-ion batteries, yet a comprehensive quantitative description of the SEI composition remains elusive. Using a combination of differential electrochemical mass spectrometry and mass spectrometry titration, we quantify graphite SEI components formed under electrolytes of varying salt concentrations. We find that, regardless of salt concentration, a conversion of initially deposited lithium ethylene dicarbonate to monocarbonates (likely lithium ethylene monocarbonate) and noncarbonate species occurs, and the extent of this conversion increases with electrolyte aging. We additionally demonstrate that as the concentration increases (up to 2.0 M LiPF 6 ), the SEI becomes progressively thinner with more LiF and less solid carbonates deposited. Finally, we reveal that less dead lithium formation and less solid carbonate deposition occur during prolonged fast charging for higher-concentration electrolytes. Because of the advantages imparted by a thinner SEI, the onset state of charge for lithium plating for the 2.0 M electrolyte is later than that predicted by a standard electrochemical model, underscoring the importance of explicit SEI effects in future electrochemical models.

Reliable prediction of freezing point depression in liquid electrolytes will accelerate the development of improved Li-ion batteries which can operate in low temperature environments. In this work we establish a computational methodology to calculate activity coefficients and liquidus lines for battery-relevant liquid electrolytes. Electronic structure methods are used in conjunction with classical molecular dynamics simulations and theoretical expressions for Born solvation energy, ion-atmosphere effects from Debye-Hückel theory and solvent entropic effects. The framework uses no a priori knowledge beyond neat solvent properties and the concentration of salt. LiPF 6 in propylene carbonate (PC), LiPF 6 in dimethyl carbonate (DMC) and LiClO4 in DMC are investigated up to 1 molal with accuracy better than 3◦C when compared to experimental freezing point measurements. We find that the difference in freezing point depression between the propylene carbonate-based electrolyte and the dimethyl cabonate electrolytes originates from the difference in the solvent dielectric constant.