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Surface modification of active cathode particles is commonly observed in battery research as either a surface phase evolving during the cycling process, or intentionally engineered to improve capacity retention, rate capability, and/or thermal stability of the cathode material. Here, a continuum-scale model is developed to simulate the galvanostatic charge/discharge of a cathode particle with core-shell heterostructure. The particle is assumed to be comprised of a core material encapsulated by a thin layer of a second phase that has a different open-circuit voltage. The effect of the potential difference between the surface and bulk phases ( Ω ) on the kinetics of lithium intercalation and the galvanostatic charge/discharge profiles is studied at different values of Ω , C-rates, and exchange current densities. The difference between the Li chemical potential in the surface and bulk phases of the cathode particle results in a concentration difference between these two phases. This leads to a charge/discharge asymmetry in the galvanostatic voltage profiles, causing a decrease in the accessible capacity of the particle. These effects are more significant at higher magnitudes of surface-bulk potential difference. The proposed model provides detailed insight into the kinetics and voltage behavior of the intercalation/de-intercalation processes in core-shell heterostructure cathode particles.

Compared to oxides, the nitrides are relatively unexplored, making them a promising chemical space for novel materials discovery. Of particular interest are nitrogen-rich nitrides, which often possess useful semiconducting properties for electronic and optoelectronic applications. However, such nitrogen-rich compounds are generally metastable, and the lack of a guiding theory for their synthesis has limited their exploration. Here, we review the remarkable metastability of observed nitrides, and examine the thermodynamics of how reactive nitrogen precursors can stabilize metastable nitrogen-rich compositions during materials synthesis. We map these thermodynamic strategies onto a predictive computational search, training a data-mined ionic substitution algorithm specifically for nitride discovery, which we combine with grand-canonical DFT-SCAN phase stability calculations to compute stabilizing nitrogen chemical potentials. We identify several new nitrogen-rich binary nitrides for experimental investigati...

Li–Fe antisite defects are commonly found in LiFePO4 particles and can impede or block Li diffusion in the single-file Li diffusion channels. However, due to their low concentration (∼1%), the effect of antisite defects on Li diffusion has only been systematically investigated by theoretical approaches. In this work, the exchange between Li in solid LiFePO4 (92.5% enriched with 6Li) and Li in the liquid Li electrolyte solution (containing natural abundance Li, 7.6% 6Li and 92.4% 7Li) was measured as a function of time by both ex situ and in situ solid-state nuclear magnetic resonance experiments. The experimental data reveal that the time dependence of the isotope exchange cannot be modeled by a simple single-file diffusion process and that defects must play a role in the mobility of ions in the LiFePO4 particles. By performing kinetic Monte Carlo simulations that explicitly consider antisite defects, which allow Li to cross over between adjacent channels, we show that the observed tracer exchange behavio...

Exploratory synthesis in new chemical spaces is the essence of solid-state chemistry. However, uncharted chemical spaces can be difficult to navigate, especially when materials synthesis is challenging. Nitrides represent one such space, where stringent synthesis constraints have limited the exploration of this important class of functional materials. Here, we employ a suite of computational materials discovery and informatics tools to construct a large stability map of the inorganic ternary metal nitrides. Our map clusters the ternary nitrides into chemical families with distinct stability and metastability, and highlights hundreds of promising new ternary nitride spaces for experimental investigation-from which we experimentally realized seven new Zn- and Mg-based ternary nitrides. By extracting the mixed metallicity, ionicity and covalency of solid-state bonding from the density functional theory (DFT)-computed electron density, we reveal the complex interplay between chemistry, composition and electronic structure in governing large-scale stability trends in ternary nitride materials.

Nanoparticles with a tendency to phase separate interact with each other during a process of either intraparticle phase separation (occurring inside the particle) or interparticle phase separation (occurring between particles). In this paper, we examine a half-cell consisting of two particles to systematically analyze the particle interactions and their resulting voltage response at different insertion currents. The kinetics of the interactions between nanoparticles is studied for cases in which particles undergo interparticle and intraparticle phase separation. Our results indicate that the interactions between particles in a cell containing only particles that undergo intraparticle phase separation are similar to those in a cell containing only particles that undergo interparticle phase separation. In both cases, sequential transformation occurs at low currents, whereas a simultaneous transformation occurs at high currents. However, such transformation dynamics changes in cases where the cell contains a mixture of particles that undergo interparticle and intraparticle phase separations, which exhibits more complex dynamics., United States. Dept. of Energy. Office of Basic Energy Sciences. NorthEast Center for Chemical Energy Storage (Award DE-SC0001294), United States. Dept. of Energy. Office of Basic Energy Sciences. NorthEast Center for Chemical Energy Storage (Award DE-SC0012583)

Inhomogeneous Li intercalation and localized concentration reversal in nanoparticles are investigated on a nanometer scale., Nanoparticulate electrodes, such as LixFePO4, have unique advantages over their microparticulate counterparts for the applications in Li-ion batteries because of the shortened diffusion path and access to nonequilibrium routes for fast Li incorporation, thus radically boosting power density of the electrodes. However, how Li intercalation occurs locally in a single nanoparticle of such materials remains unresolved because real-time observation at such a fine scale is still lacking. We report visualization of local Li intercalation via solid-solution transformation in individual LixFePO4 nanoparticles, enabled by probing sub-angstrom changes in the lattice spacing in situ. The real-time observation reveals inhomogeneous intercalation, accompanied with an unexpected reversal of Li concentration at the nanometer scale. The origin of the reversal phenomenon is elucidated through phase-field simulations, and it is attributed to the presence of structurally different regions that have distinct chemical potential functions. The findings from this study provide a new perspective on the local intercalation dynamics in battery electrodes.

Elucidating the role of interparticle Li transport and multi-particle (de)lithiation kinetics in nanoparticulate two-phase electrode materials such as LiFePO4 is a challenging task because of the small temporal and spatial scale associated with the process. Often, the relevant processes that determine the kinetics of (dis)charging an electrode are assumed to be exclusively those associated with Li transport to and from the counter-electrode, without a consideration of interactions between particles. However, the redistribution of Li between nanoparticles can have a strong influence on the overall cell rate performance. Using a continuum model to simulate the lithiation kinetics of a porous aggregate of LiFePO4 nanoparticles, we demonstrate the impact of cell architecture (in terms of ionic and electronic connectivities between active particles) and cycling rate on the multi-particle (de)lithiation kinetics. Specifically, the connectivity between particles is shown to have a strong effect on “interparticle phase separation,” a process by which active particles undergo additional cycling (charge during the overall discharge) and amplified reaction rates. We show that interparticle phase separation can be reduced or eliminated by improving (“homogenizing”) the connectivity between particles. Extensive comparisons to experimental literature and insights toward improving the performance of nanoparticulate electrodes are also provided.

© 2016 Author(s). Recent advances in theoretical structure prediction methods and high-throughput computational techniques are revolutionizing experimental discovery of the thermodynamically stable inorganic materials. Metastable materials represent a new frontier for these studies, since even simple binary non-ground state compounds of common elements may be awaiting discovery. However, there are significant research challenges related to non-equilibrium thin film synthesis and crystal structure predictions, such as small strained crystals in the experimental samples and energy minimization based theoretical algorithms. Here, we report on experimental synthesis and characterization, as well as theoretical first-principles calculations of a previously unreported mixed-valent binary tin nitride. Thin film experiments indicate that this novel material is N-deficient SnN with tin in the mixed ii/iv valence state and a small low-symmetry unit cell. Theoretical calculations suggest that the most likely crystal structure has the space group 2 (SG2) related to the distorted delafossite (SG166), which is nearly 0.1 eV/atom above the ground state SnN polymorph. This observation is rationalized by the structural similarity of the SnN distorted delafossite to the chemically related Sn3N4spinel compound, which provides a fresh scientific insight into the reasons for growth of polymorphs of metastable materials. In addition to reporting on the discovery of the simple binary SnN compound, this paper illustrates a possible way of combining a wide range of advanced characterization techniques with the first-principle property calculation methods, to elucidate the most likely crystal structure of the previously unreported metastable materials.

In nanoparticulate phase-separating electrodes, phase separation inside the particles can be hindered during their charge/discharge cycles even when a thermodynamic driving force for phase separation exists. In such cases, particles may (de)lithiate discretely in a process referred to as mosaic instability. This instability could be the key to elucidating the complex charge/discharge dynamics in nanoparticulate phase-separating electrodes. In this paper, the dynamics of the mosaic instability is studied using Smoothed Boundary Method simulations at the particle level, where the concentration and electrostatic potential fields are spatially resolved around individual particles. Two sets of configurations consisting of spherical particles with an identical radius are employed to study the instability in detail. The effect of an activity-dependent exchange current density on the mosaic instability, which leads to asymmetric charge/discharge, is also studied. While we show that our model reproduces the results of a porous-electrode model for the simple setup studied here, it is a powerful framework with the capability to predict the detailed dynamics in three-dimensional complex electrodes and provides further insights into the complex dynamics that result from the coupling of electrochemistry, thermodynamics, and transport kinetics., National Science Foundation (U.S.) (Contract No. DMS-0842504), National Science Foundation (U.S.) (grant number OCI-1053575, under allocation No. TG-DMR110007), National Science Foundation (U.S.) (Contract No. DMS-0948071), Samsung (Firm) (Samsung-MIT Program for Materials Design and Energy Applications)

Olivine LiFePO4 is believed to form two-phase equilibrium between the FePO4 (FP) and LiFePO4(LFP) phases in large particles. However, in rechargeable batteries, LFP is nanosized to overcome the low electric conductivity. The phase transformation behavior of nanosized LFP remains poorly understood. To address this issue, we have developed models and an electrochemical simulation framework for Li intercalation dynamics at particle/microstructural level to examine the phase transformation dynamics of LFP nanoparticles. The charge/discharge processes of Li-ion batteries involve several coupled physical mechanisms that occur within complex electrode microstructures. To simulate the electrochemical processes at particle/microstructural level, we developed an innovative technique that allows us to efficiently simulate electrochemical (de)intercalation processes at length scales ranging from electrode particle to complex electrode microstructures. In this presentation, we will discuss the smoothed-boundary-method electrochemical simulation framework, which straightforwardly couples the multiple physical mechanisms in the different phases of a composite electrode and incorporates the complex electrode geometries, as well as its applications. For demonstration, we used an experimentally obtained LiCoO2 cathode microstructure and corresponding material properties as simulation input [1]. We applied this simulation framework to investigate the phase-separation dynamics of LFP-nanoparticle electrodes. We first simulated a two-particle system to elucidate the detailed interparticle interactions upon (de)lithiation [2], and identified a unique process of interparticle phase separation. The simulation was extended to a collection of a modest number of LFP nanoparticles, where the particles were not contacting one another and formed a dilute agglomerate [3]. The simulation revealed a group-by-group interparticle phase separation process at low currents. In a further extension, we investigated the dynamics in a densely packed LFP-nanoparticle agglomerate [3], in which direct contact between particles was taken into account. In the dense agglomerate, we found that interparticle phase separation can still prevail at high currents due to the direct Li redistribution across particle interfaces. Finally, we included the consideration of particle-size dependent equilibrium potential into the model [4]. The simulation showed an asymmetry of phase transformation between lithiation and delithiation of an electrode at low currents. Acknowledgement: This material is based upon work supported as part of NECCES, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Numbers DE-SC0001294 and DE-SC0012583. [1] H.-C. Yu et al., in preparation. [2] B. Orvanonos et al., J Electrochem Soc, 162 (2015) A965 [3] B. Orvananos et al., Electrochimca Acta, 137 (2014) 245 [4] B. Orvananos et al., submitted to J Electrochem Soc.

Lithium iron phosphate (LFP) has been a successful material for Li-ion battery cathodes due to its enhanced safety and rate capability. For cathodes consisting of LFP nanoparticles, the current-voltage response in macroscopic electrochemical measurements exhibits a feature indicative of solid-solid phase transformation during the charge-discharge processes. However, the Li concentration evolution within individual nanoparticles remains to be elucidated. In this work, we used phase field modeling to investigate the effects of a variety of material parameters and their anisotropies on the morphological evolution of the lithiated and delithiated phases in cathode particles. An elliptical particle of 45 and 35 nm in diameters along the longest and shortest axes was used in the simulations. This particle was assumed to be immersed in electrolyte and to be placed in contact with a current collector. To simulate the lithium concentration evolution during the electrochemical process, six coupled governing equations were solved self-consistently. These equations describe lithium transport in (1) the particle and (2) the electrolyte, electrostatic potential in (3) the particle and (4) the electrolyte, (5) the electrochemical reaction rate at the particle surface, and (6) mechanical equilibrium in the particle. The highly anisotropic material properties of LFP reported in the literature were adopted as the simulation parameters, including lithium diffusivities, interfacial energies, surface insertion rate, electronic conductivity, misfit strain, and elastic moduli. To examine the individual and combined effects from these parameters, the simulation parameters were altered from the reported values in our parametric studies. By exploring the material parameter space, we found that the anisotropy of electronic conductivity is the dominant factor in determining phase evolution in a bare LFP particle (without conductive coating). In contrast, for a carbon-coated particle, the anisotropy in electronic conductivity loses its significance and the lithiation process becomes more isotropic. This work was supported as part of the NorthEast Center for Chemical Energy Storage (NECCES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0012583. Figure 1

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