arXiv subjects

Gopalakrishnan Sai Gautam

Publications and source records attributed to Gopalakrishnan Sai Gautam.

At least 37 records · Page 2Linked to original sources

arXiv2024

Thermodynamics of Sodium-Lead Alloys for Negative Electrodes from First-Principles

Metals, such as tin, antimony, and lead (Pb) have garnered renewed attention for their potential use as alloyant-negative electrode materials in sodium (Na)-ion batteries (NIBs). Despite Pb's toxicity and its high molecular weight, lead is one of the most commonly recycled metals, positioning Pb as a promising candidate for a cost-effective, high-capacity anode material. Understanding the miscibility of Na into Pb is crucial for the development of high-energy density negative electrode materials for NIBs. Using a first-principles multiscale approach, we analyze the thermodynamic properties and estimate the Na-alloying voltage of the Na-Pb system by constructing the compositional phase diagram. In the Pb-Na system, we elucidate the phase boundaries of important phases, such as Pb-rich face-centered cubic and $β$-NaPb$_3$, thereby improving our understanding of the phase diagram of the Na-Pb alloy. Due to the strong ordering tendencies of the Na-Pb intermetallics (such as NaPb, Na$_5$Pb$_2$, and Na$_{15}$Pb$_4$), we do not observe any solid-solution behavior at intermediate and high Na concentrations.

cond-mat.mtrl-sci↗

arXiv2024

Exploration of oxyfluoride frameworks as Na-ion cathodes

Na-ion batteries (NIBs) are increasingly looked at as a viable alternative to Li-ion batteries due to the abundance, low cost, and thermal stability of Na-based systems. To improve the practical utilization of NIBs in applications, it is important to boost the energy and power densities of the electrodes being used, via discovery of novel candidate materials. Thus, we explore the chemical space of transition metal containing oxyfluorides (TMOFs) that adopt the perovskite structure as possible NIB electrodes. Our choice of the perovskite structure is motivated by the `large' cationic tunnels that can accommodate Na$^+$, while the chemistry of TMOFs is motivated by the high electronegativity and inductive effect of F$^-$, which can possibly lead to higher voltages. We use density functional theory based calculations to estimate the ground state polymorphs, average Na (de)intercalation voltages, thermodynamic stabilities and Na$^+$ mobility on two distinct sets of compositions: the F-rich Na$_{x}$MOF$_{2}$, and the O-rich Na$_{1+x}$MO$_{2}$F where $x$ = 0--1 and M~=~Ti, V, Cr, Mn, Fe, Co, or Ni. Upon identifying the ground state polymorphs in the charged compositions (i.e., MOF$_2$ and NaMO$_2$F), we show that F-rich perovskites exhibit higher average voltages compared to O-rich perovskites. Also, we find six stable/metastable perovskites in the F-rich space, while all O-rich perovskites (except NaTiO$_2$F) are unstable. Finally, our Na-ion mobility calculations indicate that TiOF$_{2}$-NaTiOF$_2$, VOF$_{2}$-NaVOF$_2$, CrOF$_{2}$, and NaMnOF$_{2}$ can be promising compositions for experimental exploration as NIB cathodes, primarily if used in a strained electrode configuration and/or thin film batteries. Our computational approach and findings provide insights into developing practical NIBs involving fluorine-containing intercalation frameworks.

cond-mat.mtrl-sci↗

arXiv2024

Fluoride Frameworks as Potential Calcium Battery Cathodes

Calcium batteries (CBs) are potential next-generation energy storage devices, offering a promising alternative to lithium-ion batteries due to their theoretically high energy density, better safety, and lower costs associated with the natural abundance of calcium. However, the limited availability of positive electrode (cathode) materials has constrained the development of CBs so far. Given the similar ionic radii of Na$^+$ and Ca$^{2+}$, structures that are effective at reversibly intercalating Na$^+$ may be able to reversibly intercalate Ca$^{2+}$ as well. In this context, transition metal fluorides (TMFs) exhibiting weberite and perovskite structures that are known for intercalating Na$^+$ form an interesting set of possible CB cathode frameworks. Thus, we use first principles calculations to explore weberite and perovskite TMFs as CB cathodes, of compositions Ca\textsubscript{x}M\textsubscript{2}F\textsubscript{7} and Ca\textsubscrpt{x}MF\textsubscript{3}, respectively, where M = Ti, V, Cr, Mn, Fe, Co, or Ni. We systematically evaluate key cathode properties, including ground state structure, average Ca-intercalation voltage, thermodynamic stability (at 0 K), theoretical capacity, and Ca$^{2+}$ migration barriers. Importantly, we identify Ca\textsubscript{x}Cr\textsubscript{2}F\textsubscript{7} and Ca\textsubscript{x}Mn\textsubscript{2}F\textsubscript{7} weberite frameworks as promising Ca-cathodes. Our study not only unveils potential CB cathodes but also paves the way for further advancement in TMF-based intercalation cathodes, diversifying the chemical space for next-generation energy storage systems.

cond-mat.mtrl-sci↗

arXiv2024

Accuracy of metaGGA functionals in describing transition metal fluorides

Accurate predictions of material properties within the chemical space of transition metal fluorides (TMFs), using density functional theory (DFT) is important for advancing several technological applications. The state-of-the-art semi-local exchange-correlation functionals within DFT include the strongly constrained and appropriately normed (SCAN) and the restored regularized SCAN (r$^2$SCAN), both of which are meta generalized gradient approximation (metaGGA) functionals. Both SCAN and r$^2$SCAN are susceptible to self-interaction errors (SIEs) while modelling correlated $d$ electrons of transition metals. Hence, in this work, we evaluate the accuracy of both functionals in estimating properties of TMFs, including redox enthalpies, lattice geometries, on-site magnetic moments, and band gaps. We observe both SCAN and r$^2$SCAN to exhibit poor accuracy in estimating fluorination enthalpies among TMFs, attributable to SIEs among the $d$ electrons. Thus, we derive optimal Hubbard $U$ corrections for both functionals based on experimental fluorination enthalpies of binary TMFs. Note that the linear response theory yielded unphysical $U$ values for V, Fe, and Ni fluorides. While adding the optimal $U$ to the metaGGA functionals does not significantly affect the lattice volumes and magnetic moments, it does significantly increase the calculated band gaps. Also, we calculate the average Na intercalation voltage in Mn, Fe, Co, and Ni fluorides as a transferability check of our optimal $U$ values. Overall, we recommend using the Hubbard $U$ correction to improve predictions of redox enthalpies in other TMFs, while for band gap predictions, we suggest using the non-corrected functionals. Finally, our study should advance the accuracy of DFT-based screening studies to unearth novel TMFs, which can be used in various applications, including energy storage, catalysis, and magnetic devices.

cond-mat.mtrl-sci↗

arXiv2023

Towards a comprehensive understanding of the low energy luminescence peak in 2D materials

An intense low-energy broad luminescence peak (L-peak) is usually observed in 2D transition metal dichalcogenides (TMDs) at low temperatures. L-peak has earlier been attributed to bound excitons, but its origins are widely debated with direct consequences on optoelectronic properties. To decouple the contributions of physisorbed and chemisorbed oxygen, organic adsorbates, and strain on L-peak, we measured a series of monolayer (ML) MoS2 samples (mechanically exfoliated (ME), synthesized by oxygen-assisted chemical vapour deposition (O-CVD), hexagonal boron nitride (hBN) covered and hBN encapsulated). Emergence of L-peak below 150 K and saturation of photoluminescence (PL) intensity with laser power confirm bound nature of L-peak. Anomalously at room temperature, O-CVD samples show high A-exciton PL (c.f. ME), but reduced PL at low temperatures, which is attributed to strain-induced direct-to-indirect bandgap change in low defect O-CVD MoS2. Further, L-peak redshifts dramatically ~ 130 meV for O-CVD samples (c.f. ME). These observations are fully consistent with our predictions from density functional theory (DFT) calculations, considering effects of both strain and defects, and supported by Raman spectroscopy. In ME samples, charged oxygen adatoms are identified as thermodynamically favourable defects which can create in-gap states, and contribute to the L-peak. The useful effect of hBN is found to originate from reduction of charged oxygen adatoms and hydrocarbon complexes. This combined experimental-theoretical study allows an enriched understanding of L-peak and beneficial impact of hBN, and motivates collective studies of strain and defects with direct impact on optoelectronics and quantum technologies.

cond-mat.mtrl-sci↗

arXiv2023

Kinetic Monte Carlo Simulations of Sodium Ion Transport in NaSICON Electrodes

The development of high-performance sodium (Na) ion batteries requires improved electrode materials. The energy and power densities of Na superionic conductor (NaSICON) electrode materials are promising for large-scale energy storage applications. However, several practical issues limit the full utilization of the theoretical energy densities of NaSICON electrodes. A pressing challenge lies in the limited sodium extraction in low Na content NaSICONs, e.g., $\rm Na_1V^{IV}V^{IV}(PO_4)_3 \leftrightarrow V^{V}V^{IV}(PO_4)_3 + 1e^- + 1Na^+$. Hence, it is important to quantify the Na-ion mobility in a broad range of NaSICON electrodes. Using a kinetic Monte Carlo approach bearing the accuracy of first-principles calculations, we elucidate the variability of Na-ion transport vs. Na content in three important NaSICON electrodes, Na$_{\rm x}$Ti$_{2}$(PO$_{4}$)$_{3}$, Na$_{\rm x}$V$_{2}$(PO$_{4}$)$_{3}$, and Na$_{\rm x}$Cr$_{2}$(PO$_{4}$)$_{3}$. Our study suggests that Na$^+$ transport in NaSICON electrodes is almost entirely determined by the local electrostatic and chemical environment set by the transition metal and the polyanionic scaffold. The competition with the ordering-disordering phenomena of Na-vacancies also plays a role in influencing Na-transport. We link the variations in the Na$^+$ kinetic properties by analyzing the competition of ligand field stabilization transition metal ions and their ionic radii. We interpret the limited Na-extraction at $x = 1$ observed experimentally by gaining insights into the local Na-vacancy interplay. We propose that targeted chemical substitutions of transition metals disrupting local charge arrangements will be critical to reducing the occurrence of strong Na$^+$-vacancy orderings at low Na concentrations, thus, expanding the accessible capacities of these electrode materials.

cond-mat.mtrl-sci↗

arXiv2023

Evaluation of P3-type layered oxides as K-ion battery cathodes

Given increasing energy storage demands and limited natural resources of Li, K-ion batteries (KIBs) could be promising next-generation systems having natural abundance, similar chemistry and energy density. Here, we have investigated the P3-type K$_{0.5}$TMO$_2$ (where TM = Ti, V, Cr, Mn, Co, or Ni) systems using density functional theory calculations, as potential positive intercalation electrodes (or cathodes) for KIBs. Specifically, we have identified the ground state configurations, and calculated the average topotactic voltages, electronic structures, on-site magnetic moments, and thermodynamic stabilities of all P3-K$_{0.5}$TMO$_2$ compositions and their corresponding depotassiated P3-TMO$_2$ frameworks. We find that K adopts the honeycomb or zig-zag configuration within each K-layer of all P3 structures considered, irrespective of the TM. In terms of voltages, we find the Co- and Ti-based compositions to exhibit the highest (4.59 V vs. K) and lowest (2.24 V) voltages, respectively, with the TM contributing to the redox behavior upon K (de-)intercalation. We observe all P3-K$_{0.5}$TMO$_2$ to be (meta)stable, and hence experimentally synthesizeable according to our 0 K convex hull calculations, while all depotassiated P3-TMO$_2$ configurations are unstable and may appear during electrochemical cycling. Also, we verified the stability of the prismatic coordination environment of K compared to octahedral coordination at the K$_{0.5}$TMO$_2$ compositions using Rouxel and cationic potential models. Finally, combining our voltage and stability calculations, we find P3-K$_x$CoO$_2$ to be the most promising cathode composition, while P3-K$_x$NiO$_2$ is worth exploring. Our work should contribute to the exploration of strategies and materials required to make practical KIBs.

cond-mat.mtrl-sci↗

arXiv2023

Constructing and evaluating machine-learned interatomic potentials for Li-based disordered rocksalts

Lithium-based disordered rocksalts (LDRs), which are an important class of cathodes for advanced Li-ion batteries, represent a complex chemical and configurational space for conventional density functional theory (DFT)-based high-throughput screening approaches. Notably, atom-centered machine-learned interatomic potentials (MLIPs) are a promising pathway to accurately model the potential energy surface of highly-disordered systems, such as LDRs, where the performance of such MLIPs have not been rigorously explored yet. Here, we represent a comprehensive evaluation of the accuracy, transferability, and ease of training of five MLIPs in modelling LDRs, including artificial neural network potential developed by the atomic energy network (AENET), Gaussian approximation potential (GAP), spectral neighbor analysis potential (SNAP) and its quadratic extension (qSNAP), and moment tensor potential (MTP). Specifically, we generate a DFT-calculated dataset of 10842 disordered LiTMO$_2$ and TMO$_2$ configurations, where TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and/or Cu. Importantly, we find AENET to be the best in terms of accuracy and transferability for energy predictions, while MTP is the best for atomic forces. While AENET is the fastest to train at low number of epochs, the training time increases significantly as epochs increase, with a corresponding reduction in training errors. Note that AENET and GAP tend to overfit in small datasets, with the extent of overfitting reducing with larger datasets. Finally, we observe AENET to provide reasonable predictions of average Li-intercalation voltages in layered, single-TM LiTMO$_2$ frameworks, compared to DFT ($\sim$10% error on average). Our study should pave the way both for discovering novel LDR electrodes and for modelling other configurationally complex systems, such as high-entropy ceramics and alloys.

cond-mat.mtrl-sci↗

arXiv2023

Performance of the r$^{2}$SCAN functional in transition metal oxides

We assess the accuracy and computational efficiency of the recently developed meta-generalized gradient approximation (metaGGA) functional, the restored regularized strongly constrained and appropriately normed (r$^2$SCAN), in transition metal oxide (TMO) systems and compare its performance against SCAN. Specifically, we benchmark the r$^2$SCAN-calculated oxidation enthalpies, lattice parameters, on-site magnetic moments, and band gaps of binary 3\textit{d} TMOs against the SCAN-calculated and experimental values. Additionally, we evaluate the optimal Hubbard \emph{U} correction required for each transition metal (TM) to improve the accuracy of the r$^2$SCAN functional, based on experimental oxidation enthalpies, and verify the transferability of the \emph{U} values by comparing against experimental properties on other TM-containing oxides. Notably, including the \textit{U}-correction to r$^2$SCAN increases the lattice parameters, on-site magnetic moments and band gaps of TMOs, apart from an improved description of the ground state electronic state in narrow band gap TMOs. The r$^2$SCAN and r$^2$SCAN+\textit{U} calculated oxidation enthalpies follow the qualitative trends of SCAN and SCAN+\emph{U}, with r$^2$SCAN and r$^2$SCAN+\textit{U} predicting marginally larger lattice parameters, smaller magnetic moments, and lower band gaps compared to SCAN and SCAN+\textit{U}, respectively. We observe that the overall computational time (i.e., for all ionic+electronic steps) required for r$^2$SCAN(+\textit{U}) to be lower than SCAN(+\textit{U}). Thus, the r$^2$SCAN(+\textit{U}) framework can offer a reasonably accurate description of the ground state properties of TMOs with better computational efficiency than SCAN(+\textit{U}).

cond-mat.mtrl-sci↗

arXiv2022

Study of pnictides for photovoltaic applications

For the transition into a sustainable mode of energy usage, it is important to develop photovoltaic materials that exhibit better solar-to-electricity conversion efficiencies, a direct optimal band gap, and made of non-toxic, earth abundant elements compared to the state-of-the-art silicon photovoltaics. Here, we explore the non-redox-active pnictide chemical space, including binary A$_3$B$_2$, ternary AA'$_2$B$_2$, and quaternary AA'A"B$_2$ compounds (A, A', A" = Ca, Sr, or Zn; B = N or P), as candidate beyond-Si photovoltaics using density functional theory calculations. Specifically, we evaluate the ground state configurations, band gaps, and 0 K thermodynamic stability for all 20 pnictide compositions considered, besides computing the formation energy of cation vacancies, anion vacancies, and cation anti-sites in a subset of candidate compounds. Importantly, we identify SrZn$_2$N$_2$, SrZn$_2$P$_2$, and CaZn$_2$P$_2$ to be promising candidates, exhibiting optimal (1.1-1.5 eV) hybrid-functional-calculated band gaps, stability at 0 K, and high resistance to point defects (formation energies $>$1 eV), while other possible candidates include ZnCa$_2$N$_2$ and ZnSr$_2$N$_2$, which may be susceptible to N-vacancy formation. We hope that our study will contribute to the practical development of pnictide semiconductors as beyond-silicon light absorbers.

cond-mat.mtrl-sci↗

arXiv2022

The Resistive Nature of Decomposing Interfaces of Solid Electrolytes with Alkali Metal Electrodes

A crucial ingredient in lithium (Li) and sodium (Na)-ion batteries (LIBs and NIBs) is the electrolytes. The use of Li-metal (Na-metal) as anode in liquid electrolyte LIBs (NIBs) is constrained by several issues including thermal runway and flammability, electrolyte leakage, and limited chemical stability. Considerable effort has been devoted toward the development of solid electrolytes (SEs) and all-solid-state batteries, which are presumed to mitigate some of the issues of Li-metal(Na-metal) in contact with flammable liquid electrolytes. However, most SEs, such as Li$_3$PS$_4$, Li$_6$PS$_5$Cl and Na$_3$PS$_4$ readily decompose against the highly reducing Li-metal and Na-metal anodes. Using first-principles calculations we elucidate the stability of more than 20 solid$||$solid interfaces formed between the decomposition products of Li$_3$PS$_4$, Li$_6$PS$_5$Cl (and Na$_3$PS$_4$) against the Li-metal (Na-metal) electrode. We suggest that the work of adhesion needed to form a hetereogenous interfaces is an important descriptor to quantify the stability of interfaces. Subsequently, we clarify the atomistic origins of the resistance to Li-ion transport at interfaces of the Li-metal anode and selected decomposition products (Li$_3$P, Li$_2$S} and LiCl) of SEs, via a high-fidelity machine learned potential (MLP). Utilising an MLP enables nano-second-long molecular dynamics simulations on `large' interface models (here with 8320 atoms), but with similar accuracy to first-principles approaches. Our simulations demonstrate that the interfaces formed between Li-metal and argyrodite (e.g., Li$_6$PS$_5$Cl) decomposition products are resistive to Li-ion transport. The implications of this study are important since binary compounds are commonly found in the vicinity of Li(Na)-metal upon chemical and/or electrochemical decomposition of ternary and quaternary SEs.

cond-mat.mtrl-sci↗

arXiv2022

First principles investigation of anionic redox in bisulfate lithium battery cathodes

The search for an alternative high-voltage polyanionic cathode material for Li-ion batteries is vital to improve the energy densities beyond the state-of-the-art, where sulfate frameworks form an important class of high-voltage cathode materials due to the strong inductive effect of the S$^{6+}$ ion. Here, we have investigated the mechanism of cationic and/or anionic redox in Li$_x$M(SO$_4$)$_2$ frameworks (M = Mn, Fe, Co, and Ni and 0 $\leq$ x $\leq$ 2) using density functional calculations. Specifically, we have used a combination of Hubbard $U$ corrected strongly constrained and appropriately normed (SCAN+$U$) and generalized gradient approximation (GGA+$U$) functionals to explore the thermodynamic (polymorph stability), electrochemical (intercalation voltage), geometric (bond lengths), and electronic (band gaps, magnetic moments, charge populations, etc.) properties of the bisulfate frameworks considered. Importantly, we find that the anionic (cationic) redox process is dominant throughout delithiation in the Ni (Mn) bisulfate, as verified using our calculated projected density of states, bond lengths, and on-site magnetic moments. On the other hand, in Fe and Co bisulfates, cationic redox dominates the initial delithiation (1 $\leq$ x $\leq$ 2), while anionic redox dominates subsequent delithiation (0 $\leq$ x $\leq$ 2). In addition, evaluation of the crystal overlap Hamilton population reveals insignificant bonding between oxidizing O atoms throughout the delithiation process in the Ni bisulfate, indicating robust battery performance that is resistant to irreversible oxygen evolution. Finally, we observe both GGA+$U$ and SCAN+$U$ predictions are in qualitative agreement for the various properties predicted. Our work should open new avenues for exploring lattice oxygen redox in novel high voltage polyanionic cathodes, especially using the SCAN+$U$ functional.

cond-mat.mtrl-sci↗

arXiv2021

Phase Stability and Sodium-Vacancy Orderings in a NaSICON Electrode

We elucidate the thermodynamics of sodium (Na) intercalation into the sodium super-ionic conductor (NaSICON)-type electrode, Na$_x$V$_2$(PO$_4$)$_3$, for promising Na-ion batteries with high-power density. This is the first report of a computational temperature-composition phase diagram of the NaSICON-type electrode Na$_x$V$_2$(PO$_4$)$_3$. We identify two thermodynamically stable phases at the compositions Na$_2$V$_2$(PO$_4$)$_3$ and Na$_{3.5}$V$_2$(PO$_4$)$_3$, and their structural features are described for the first time based on our computational analysis. We unveil the crystal-structure and the electronic-structure origins of the ground-state compositions associated with specific Na/vacancy arrangements, which are driven by charge orderings on the vanadium sites. These results are significant for the optimization of high-energy and power densities electrodes for sustainable Na-ion batteries

cond-mat.mtrl-sci↗

arXiv2021

Searching Ternary Oxides and Chalcogenides as Positive Electrodes for Calcium Batteries

The identification of alternatives to the Lithium-ion battery architecture remains a crucial priority in the diversification of energy storage technologies. Accompanied by the low reduction potential of $\mathrm{Ca^{2+}/Ca}$, -2.87 V vs. SHE, metal-anode-based rechargeable Calcium (Ca) batteries appear competitive in terms of energy densities. However, the development of Ca-batteries lacks high-energy density intercalation cathode materials. Using first-principles methodologies, we screen a large chemical space for potential Ca-based cathode chemistries, with composition of $\mathrm{Ca_iTM_jZ_k}$, where TM is a 1$^{st}$ or 2$^{nd}$ row transition metal and $\mathrm{Z}$ is oxygen, sulfur, selenium or tellurium. 10 materials are selected and their Ca intercalation properties are investigated. We identify two previously unreported promising electrode compositions: the post-spinel $\mathrm{CaV_2O_4}$ and the layered $\mathrm{CaNb_2O_4}$, with Ca migration barriers of $\sim$654 meV and $\sim$785 meV, respectively. Finally, we analyse the geometrical features of the Ca migration pathways across the 10 materials studied and provide an updated set of design rules for the identification of good ionic conductors, especially with large mobile cations.

cond-mat.mtrl-sci↗

arXiv2019

Ionic Transport in Potential Coating Materials for Mg Batteries

A major bottleneck for the development of Mg batteries is the identification of liquid electrolytes that are simultaneously compatible with the Mg-metal anode and high-voltage cathodes. One strategy to widen the stability windows of current non-aqueous electrolytes is to introduce protective coating materials at the electrodes, where coating materials are required to exhibit swift Mg transport. In this work, we use a combination of first-principles calculations and ion-transport theory to evaluate the migration barriers for nearly 27 Mg-containing binary, ternary, and quaternary compounds spanning a wide chemical space. Combining mobility, electronic band gaps, and stability requirements, we identify MgSiN$_2$, MgI$_2$, MgBr$_2$, MgSe, and MgS as potential coating materials against the highly reductive Mg metal anode, and we find MgAl$_2$O$_4$ and Mg(PO$_3$)$_2$ to be promising materials against high-voltage oxide cathodes (up to $\sim$3~V).

cond-mat.mtrl-sci↗

arXiv2018

Evaluating transition-metal oxides within DFT-SCAN and SCAN+U frameworks for solar thermochemical applications

Using the strongly constrained and appropriately normed (SCAN) and SCAN+U approximations for describing electron exchange-correlation (XC) within density functional theory, we investigate the oxidation energetics, lattice constants, and electronic structure of binary Ce-, Mn-, and Fe-oxides, which are crucial ingredients for generating renewable fuels using two-step, oxide-based, solar thermochemical reactors. Unlike other common XC functionals, we find that SCAN does not over-bind the O2 molecule, based on direct calculations of its bond energy and robust agreement between calculated formation enthalpies of main group oxides versus experiments. However, in the case of transition-metal oxides (TMOs), SCAN systematically overestimates (i.e., yields too negative) oxidation enthalpies due to remaining self-interaction errors in the description of their ground-state electronic structure. Adding a Hubbard U term to the transition-metal centers, where the magnitude of U is determined from experimental oxidation enthalpies, significantly improves the qualitative agreement and marginally improves the quantitative agreement of SCAN+U-calculated electronic structure and lattice parameters, respectively, with experiments. Importantly, SCAN predicts the wrong ground-state structure for a few oxides, namely, Ce2O3, Mn2O3, and Fe3O4, while SCAN+U predicts the right polymorph for all systems considered in this work. Hence, the SCAN+U framework, with an appropriately determined U, will be required to accurately describe ground-state properties and yield qualitatively consistent electronic properties for most transition- metal and rare earth oxides.

cond-mat.mtrl-sci↗

arXiv2018

On the Balance of Intercalation and Conversion Reactions in Battery Cathodes

We present a thermodynamic analysis of the driving forces for intercalation and conversion reactions in battery cathodes across a range of possible working ion, transition metal, and anion chemistries. Using this body of results, we analyze the importance of polymorph selection as well as chemical composition on the ability of a host cathode to support intercalation reactions. We find that the accessibility of high energy charged polymorphs in oxides generally leads to larger intercalation voltages favoring intercalation reactions, whereas sulfides and selenides tend to favor conversion reactions. Furthermore, we observe that Cr-containing cathodes favor intercalation more strongly than those with other transition metals. Finally, we conclude that two-electron reduction of transition metals (as is possible with the intercalation of a $2+$ ion) will favor conversion reactions in the compositions we studied.

cond-mat.mtrl-sci↗

arXiv2018

Particle Morphology and Lithium Segregation to Surfaces of the Li$_7$La$_3$Zr$_2$O$_{12}$ Solid Electrolyte

Solid electrolytes for solid-state Li-ion batteries are stimulating considerable interest for next-generation energy storage applications. The Li$_7$La$_3$Zr$_2$O$_{12}$ garnet-type solid electrolyte has received appreciable attention as a result of its high ionic conductivity. However, several challenges for the successful application of solid-state devices based on Li$_7$La$_3$Zr$_2$O$_{12}$ remain, such as dendrite formation and maintaining physical contact at interfaces over many Li intercalation/extraction cycles. Here, we apply first-principles density functional theory to provide insights into the Li$_7$La$_3$Zr$_2$O$_{12}$ particle morphology under various physical and chemical conditions. Our findings indicate Li segregation at the surfaces, suggesting Li-rich grain boundaries at typical synthesis and sintering conditions. On the basis of our results, we propose practical strategies to curb Li segregation at the Li$_7$La$_3$Zr$_2$O$_{12}$ interfaces. This approach can be extended to other Li-ion conductors for the design of practical energy storage devices.

cond-mat.mtrl-sci↗