Solid-Solution Li Intercalation as a Function of Cation Order/Disorder in the High-Voltage Li_<i>x</i>Ni_0.5Mn_1.5O₄ Spinel

Many Li-ion cathode materials transform via two-phase reactions, which can lead to long-term structural damage and limited cyclability. To elucidate the coupling between favorable solid-solution Li intercalation and the underlying cation ordering, we take the high-voltage spinel, Li_<i>x</i>Ni_0.5­Mn_1.5O₄ (0 ≤ <i>x</i> ≤ 1), as a case example. Through grand canonical Monte Carlo (MC) simulations based on the ab initio cluster expansion model, we show a striking dependence between the solid-solution phase domain and the Ni–Mn cation ordering. The perfectly ordered Li_<i>x</i>Ni_0.5­Mn_1.5O₄ spinel resists solid solution until very high temperatures, but introducing various degrees of Ni–Mn cation disorder results in a dramatic increase in stability for a single-phase reaction, particularly at high Li contents. This opens up the possibility of designing single-phase reaction materials via targeted cation ordering, and to this end, we show that a uniformly distributed cation high-voltage spinel has access to solid solution throughout the entire Li composition range at room temperature

Mitigating the High-Charge Detrimental Phase Transformation in LiNiO₂ Using Doping Engineering

Cobalt-free layered LiNiO2 has gained increased interest due to the scarcity and high cost of cobalt. However, LiNiO2 suffers from poor cycling stability, which is mainly due to oxygen loss and structural instability, especially when operating at high voltages. Herein, we present a doping strategy to mitigate the detrimental O3-to-O1 phase transformation in LiNiO2 from first-principles calculations. Temperature–composition phase diagrams of pristine and doped Li1–xNiO2 are obtained using a cluster-expansion and Monte Carlo simulation approach. We investigate the effects of dopant oxidation states, sizes, and concentrations on the dopant distribution in LiNi1–yMyO2 (M = Sb, Ti, Si, Al, and Mg) as well as the phase transitions during delithiation. We find that introducing high-valence dopants with ionic radii similar to that of Ni3+ into LiNiO2 stabilizes the O3-phase cathode bulk structure at high charge. Our results provide a general guidance on using doping engineering to realize Ni-rich, Co-free cathodes for lithium-ion batteries

Revealing the Intrinsic Li Mobility in the Li₂MnO₃ Lithium-Excess Material

One of the most promising avenues for future high energy Li-ion batteries originate from the family of Li-rich layered cathodes. However, while exhibiting excellent initial capacity, these materials also suffer from voltage fade, high impedance, and poor rate capability, particularly in the Mn-rich, high Li excess concentration regime. Though it is clear that the Li₂MnO₃ component contributes to the high capacity as well as the chemical and structural degradation of the material, the inherent ionic conductivity of the material has not been clarified. In this work, we investigate the delithiation mechanism, involving coherent Li migration from two layers by first-principles density functional theory. Surprisingly, and contrary to expectations from available experimental results, we find that the pristine material exhibits excellent Li mobility enabling facile Li extraction from both the transition metal layer and Li-layer. Generally, the Li-extractions are highly accelerated by di- and trivacancy clusters, which stabilize the saddle point tetrahedral sites. Hence, we deduce that the observed inferior rate behavior of this class of Li cathode materials is not due to intrinsic poor bulk ionic mobility, but more likely due to surface-passivation, structural deterioration, and/or particle–particle electrode-level transport limitations

Theory-Guided Exploration of the Sr₂Nb₂O₇ System for Increased Dielectric and Piezoelectric Properties and Synthesis of Vanadium-Alloyed Sr₂Nb₂O₇

Ab initio methods provide a powerful tool in the search for novel polar materials. In particular, there has been a surge to identify lead-free piezoelectric materials to replace PbZr0.52Ti0.48O3. This study examines a computational strategy to identify increased piezoelectric and dielectric responses of alloy systems based on the linear interpolation of force constants, Born effective charges, and internal strain tensors from their end-point compounds. We choose the ferroelectric layered perovskite Sr2Nb2O7 as a parent structure and employ this alloying strategy for 19 potential cation substitutions, targeting thermodynamically metastable alloys with high piezoelectric response. From this screening, we identify Sr2Nb2–2xV2xO7 as a promising polar system. We conduct large-unit-cell calculations of Sr2Nb2–2xV2xO7 at x = 0.0625, 0.125 for multiple cation orderings and find a significant 184% enhanced piezoelectric response. The solid solution system is synthesized as single-crystalline thin-film heterostructures using pulsed-laser deposition, and an enhanced dielectric response is observed at x = 0.05 and at x = 0.1. We present the Sr2Nb2–2xV2xO7 alloy system designed through high-throughput computational screening methods with a large calculated piezoelectric response and experimentally verified increased dielectric response. Our methodology is provided as a high-throughput screening tool for novel materials with enhanced polarizability and alloy systems with potential morphotropic phase boundaries

Theory-Guided Exploration of the Sr₂Nb₂O₇ System for Increased Dielectric and Piezoelectric Properties and Synthesis of Vanadium-Alloyed Sr₂Nb₂O₇

Ab initio methods provide a powerful tool in the search for novel polar materials. In particular, there has been a surge to identify lead-free piezoelectric materials to replace PbZr0.52Ti0.48O3. This study examines a computational strategy to identify increased piezoelectric and dielectric responses of alloy systems based on the linear interpolation of force constants, Born effective charges, and internal strain tensors from their end-point compounds. We choose the ferroelectric layered perovskite Sr2Nb2O7 as a parent structure and employ this alloying strategy for 19 potential cation substitutions, targeting thermodynamically metastable alloys with high piezoelectric response. From this screening, we identify Sr2Nb2–2xV2xO7 as a promising polar system. We conduct large-unit-cell calculations of Sr2Nb2–2xV2xO7 at x = 0.0625, 0.125 for multiple cation orderings and find a significant 184% enhanced piezoelectric response. The solid solution system is synthesized as single-crystalline thin-film heterostructures using pulsed-laser deposition, and an enhanced dielectric response is observed at x = 0.05 and at x = 0.1. We present the Sr2Nb2–2xV2xO7 alloy system designed through high-throughput computational screening methods with a large calculated piezoelectric response and experimentally verified increased dielectric response. Our methodology is provided as a high-throughput screening tool for novel materials with enhanced polarizability and alloy systems with potential morphotropic phase boundaries

The Interplay between Salt Association and the Dielectric Properties of Low Permittivity Electrolytes: The Case of LiPF₆ and LiAsF₆ in Dimethyl Carbonate

In this article, we present evidence that the dielectric constant of an electrolyte solution can be effectively used to infer the association regime of the salt species from computational methods. As case studies, we consider the low dielectric constant solvent dimethyl carbonate with LiAsF₆ and LiPF₆ salts at low concentrations. Using both quantum “<i>ab initio</i>” methods as well classical molecular dynamics simulations, we elucidate the salt’s contribution to the dielectric constant as well as the dipolar relaxation times, which act as quantitative signatures. By comparing to previously published measurements, we provide strong evidence for the presence of contact-ion pairs at these low concentrations. Interestingly, these ion pairs increase the dielectric constant of the solution, allowing for significantly improved ionic conductivity as a function of salt concentrations. We also discuss the role of multimeric equilibrium species as contributors to the functional properties of designer electrolytes, such as dielectric properties of the solution and ionic conductivity

The Coupling between Stability and Ion Pair Formation in Magnesium Electrolytes from First-Principles Quantum Mechanics and Classical Molecular Dynamics

In this work we uncover a novel effect between concentration dependent ion pair formation and anion stability at reducing potentials, e.g., at the metal anode. Through comprehensive calculations using both first-principles as well as well-benchmarked classical molecular dynamics over a matrix of electrolytes, covering solvents and salt anions with a broad range in chemistry, we elucidate systematic correlations between molecular level interactions and composite electrolyte properties, such as electrochemical stability, solvation structure, and dynamics. We find that Mg electrolytes are highly prone to ion pair formation, even at modest concentrations, for a wide range of solvents with different dielectric constants, which have implications for dynamics as well as charge transfer. Specifically, we observe that, at Mg metal potentials, the ion pair undergoes partial reduction at the Mg cation center (Mg²⁺ → Mg⁺), which competes with the charge transfer mechanism and can activate the anion to render it susceptible to decomposition. Specifically, TFSI^– exhibits a significant bond weakening while paired with the transient, partially reduced Mg⁺. In contrast, BH₄^– and BF₄^– are shown to be chemically stable in a reduced ion pair configuration. Furthermore, we observe that higher order glymes as well as DMSO improve the solubility of Mg salts, but only the longer glyme chains reduce the dynamics of the ions in solution. This information provides critical design metrics for future electrolytes as it elucidates a close connection between bulk solvation and cathodic stability as well as the dynamics of the salt

Computational Design of New Magnesium Electrolytes with Improved Properties

In this work, we use computational design to examine 15 new electrolyte salt anions by performing chemical variations and mutations on the bis­(trifluoromethane)­sulfonamide (TFSI) anion. On the basis of our calculations, we propose two new anions as potential candidates for magnesium energy-storage systems, which are evolved from TFSI with the substitution of sulfur atoms in TFSI and the modification of functional groups. The applicability of these new anion salts is examined through comprehensive calculations using both first-principles as well as benchmarked classical molecular dynamics. We elucidate the important properties of these anions, including the electrochemical stability window, chemical decomposition, preferred solvation structure, diffusion coefficient, and other dynamical properties for 15 rationally designed molecules. Two of the designed anions are found to successfully avoid the vulnerability of TFSI during ion-pair charge-transfer reactions while retaining comparable or better performance of other properties. As such, our work provides, to our knowledge, the first theoretically designed electrolyte salt for contemporary multivalent batteries and provides guidance for the synthesis and testing of novel liquid electrochemical systems

Data for "Elementary Decomposition Mechanisms of Lithium Hexafluorophosphate in Battery Electrolytes and Interphases"

Contained here is a JavaScript Object Notation (JSON)-formatted file called pfx_named_data.json. This file contains the structures (as serialized Pymatgen Molecule objects) and thermochemical properties of the reaction endpoints and TS reported in the article "Elementary Decomposition Mechanisms of Lithium Hexafluorophosphate in Battery Electrolytes and Interphases" (see DOI:10.26434/chemrxiv-2022-4bd1p-v2). The key for each key-value pair in pfx_named_data.json is the name of the species as reported in the main text or the Supporting Information. For instance, the data for TS11 would be found under the key "TS11". For reactions where species, namely LiF, HF, and CO2, are removed, two entries for the relevant endpoint are provided. The species with LiF, HF, and/or CO2 present are named "Mn", where n is the appropriate index; the species with the species removed are named "Mn-x", where x is the species that is removed. Where multiple species are removed, the name takes the form "Mn-x-y", where x and y are the species removed. All structures were optimized in Jaguar using the range-separated hybrid generalized gradient approximation (GGA) density functional ωB97X-D, def2-SVPD basis set, and Conductor-like Screening Model (COSMO) implementation of the polarizable continuum model (PCM) with water as the solvent. In Jaguar, all basis functions representing f and higher orbitals were removed to further reduce cost, making the basis more precisely def2-SVPD(-f). All TS were confirmed to have one imaginary frequency and to connect to the expected endpoints. The electronic energies of all TS and reaction endpoints (reactants and products) were corrected with single-point energy evaluations in Jaguar using range-separated hybrid meta-GGA functional ωB97M-V with the def2-TZVPD basis set in COSMO. To load this data in Python, use monty (https://github.com/materialsvirtuallab/monty): from monty.serialization import loadfn data = loadfn("pfx_named_data.json")</p

Inputs/Outputs of each Surface Calculation run in VASP

For the convenience of the readers, we have included the relevant inputs of each surface calculation run using the Vienna Ab intio Simulation Package (VASP). The relevant outputs generated from these calculations have also been included in this file