Stacking sequence changes and Na ordering in layered intercalation materials

The performance of Na-ion batteries is sensitive to the nature of cation ordering and phase transformations that occur within the intercalation compounds used as electrodes. In order to elucidate these effects in layered Na intercalation compounds, we have carried out a first-principles statistical mechanics study of Na ordering and stacking-sequence preferences in the model compound NaxTiS2. Our calculations predict a series of structural phase transitions at room temperature between O3, P3, O1, O1–O3 staged hybrid, and O1–P3 staged hybrid. We further explore the ordering of Na ions in P3 and O3 and find that these host structures favor very distinct Na-vacancy patterns. Low energy orderings on the honeycomb lattice in P3 consist of triangular island domains with vacancies coalescing at antiphase boundaries. This results in a devil’s staircase of ground-state Na orderings within P3 that are unlike the orderings possible in the triangular lattice of Na sites in O3. We explore the role that antiphase boundaries play in mediating Na diffusion in the P3 host

First-principles study of layered transition-metal oxides and sulfides for battery applications

Renewable energy sources are generally abundant but intermittent, with peak production and peak demand occurring at different times. Therefore, storage and controlled distribution of energy is an important component of the shift away from on-demand fuels like petroleum and natural gas. Secondary electrochemical batteries are one convenient method for closing the gap between energy supply and demand. Battery performance and cost are largely restricted by the materials, especially those used for the electrodes. Part of the problem is material instability upon cycling. As the battery is charged (and discharged), the composition of the electrode changes, often resulting in reversible and irreversible phase transitions which result in material degradation. Therefore, battery capacity and lifetime can be limited by thermodynamic instabilities. We employ first-principles methods like density functional theory and Monte Carlo to study phase stability in layered electrode materials. We look at stacking-sequence changes, ion ordering, and atom migration to better understand bulk degradation mechanisms in Li- and Na-ion materials. We use NaxTiS2, NaxTiO2, and LixMO2 (M = Co, Ni, Mn) as model systems to explore phenomena present in a variety of layered transition-metal oxides and sulfides. Our work aids in interpreting experimental observations and in generating design rules for more robust electrode materials

First-principles study of layered transition-metal oxides and sulfides for battery applications

Renewable energy sources are generally abundant but intermittent, with peak production and peak demand occurring at different times. Therefore, storage and controlled distribution of energy is an important component of the shift away from on-demand fuels like petroleum and natural gas. Secondary electrochemical batteries are one convenient method for closing the gap between energy supply and demand. Battery performance and cost are largely restricted by the materials, especially those used for the electrodes. Part of the problem is material instability upon cycling. As the battery is charged (and discharged), the composition of the electrode changes, often resulting in reversible and irreversible phase transitions which result in material degradation. Therefore, battery capacity and lifetime can be limited by thermodynamic instabilities. We employ first-principles methods like density functional theory and Monte Carlo to study phase stability in layered electrode materials. We look at stacking-sequence changes, ion ordering, and atom migration to better understand bulk degradation mechanisms in Li- and Na-ion materials. We use NaxTiS2, NaxTiO2, and LixMO2 (M = Co, Ni, Mn) as model systems to explore phenomena present in a variety of layered transition-metal oxides and sulfides. Our work aids in interpreting experimental observations and in generating design rules for more robust electrode materials

Delocalized Metal-Oxygen π-Redox Is the Origin of Anomalous Non-Hysteretic Capacity in Li-Ion and Na-Ion Cathode Materials

The anomalous capacity of Li-excess cathode materials has ignited a vigorous debate over the nature of the underlying redox mechanism, which promises to substantially increase the energy density of rechargeable batteries. Unfortunately, nearly all materials exhibiting this anomalous capacity suffer from irreversible structural changes and voltage hysteresis. Non-hysteretic excess capacity has been demonstrated in Na2Mn3O7 and Li2IrO3, making these materials key to understanding the electronic, chemical and structural properties that are necessary to achieve reversible excess capacity. Here, we use high-fidelity random-phase-approximation (RPA) electronic structure calculations and group theory to derive the first fully consistent mechanism of non-hysteretic oxidation beyond the transition metal limit, explaining the electrochemical and structural evolution of the Na2Mn3O7 and Li2IrO3 model materials. We show that the source of anomalous non-hysteretic capacity is a network of pi-bonded metal-d and O-p orbitals, whose activity is enabled by a unique resistance to transition metal migration. The pi-network forms a collective, delocalized redox center. We show that the voltage, accessible capacity, and structural evolution upon oxidation are collective properties of the pi-network rather than that of any local bonding environment. Our results establish the first rigorous framework linking anomalous capacity to transition metal chemistry and long-range structure, laying the groundwork for engineering materials that exhibit truly reversible capacity exceeding that of transition metal redox

Comparing Charge Compensation Mechanisms of Li1.3Nb0.3Mn0.4O2 and Li1.3Nb0.3Fe0.4O2 As Novel Cathode Material

Li-rich oxides have generated widespread interest for their use as next generation high energy density Li-ion battery cathodes. Among Li-rich candidates, cation disordered phases are novel lithium intercalation compounds that warrant further fundamental characterization. Here, we compare Li1.3Nb0.3Mn0.4O2 and Li1.3Nb0.3Fe0.4O2 system to explore differences in charge compensation and electrochemical performance. Using Operando XANES/EXAFS measurements examine how the Mn/Fe/Nb local environments and oxidation states change and their effect on electrochemistry. Li1.3Nb0.3Mn0.4O2 displays capacities beyond the traditional transition metal redox (Mn3+/4+), which is considered accessible through oxygen redox. Li1.3Nb0.3Fe0.4O2 exhibits charge capacities at similar potentials to that of anionic redox but our investigations suggest the charge compensation mechanism displays more Fe3+/4+ redox in origin. By using a suite of characterization techniques, we explore the capacity contributions of cation and anionic components in these systems

Pushing the limit of 3d transition metal-based layered oxides that use both cation and anion redox for energy storage

Intercalation chemistry has dominated electrochemical energy storage for decades, and storage capacity worldwide has now reached the terawatt-hour level. State-of-the-art intercalation cathodes for Li-ion batteries operate within the limits of transition metal cation electrochemistry, but the discovery of anion-redox processes in recent decades suggests rich opportunities for substantially increasing stored energy densities. The diversity of compounds that exhibit anion redox in the solid state has inspired the exploration of new materials for next-generation cathodes. In this Review, we outline the mechanisms proposed to contribute to anion redox and the accompanying kinetic pathways that can occur in layered transition metal oxides. We discuss the crucial role of structural changes at both the atomic and mesoscopic scales with an emphasis on their impact on electrochemical performance. We emphasize the need for an integrated approach to studying the evolution of both the bulk structure and electrode–electrolyte interphase by combining characterization with computation. Building on the fundamental understanding of electrochemical reaction mechanisms, we discuss engineering strategies such as composition design, surface protection and structural control to achieve stable anion redox for next-generation energy storage devices