Nanostructure Transformation as a Signature of Oxygen Redox in Li-Rich 3d and 4d Cathodes

Trapped O2 and the origin of voltage fade in layered Li-rich cathodes

Oxygen redox cathodes, such as Li1.2Ni0.13Co0.13Mn0.54O2, deliver higher energy densities than those based on transition metal redox alone. However, they commonly exhibit voltage fade, a gradually diminishing discharge voltage on extended cycling. Recent research has shown that, on the first charge, oxidation of O2- ions forms O2 molecules trapped in nano-sized voids within the structure, which can be fully reduced to O2- on the subsequent discharge. Here we show that the loss of O-redox capacity on cycling and therefore voltage fade arises from a combination of a reduction in the reversibility of the O2-/O2 redox process and O2 loss. The closed voids that trap O2 grow on cycling, rendering more of the trapped O2 electrochemically inactive. The size and density of voids leads to cracking of the particles and open voids at the surfaces, releasing O2. Our findings implicate the thermodynamic driving force to form O2 as the root cause of transition metal migration, void formation and consequently voltage fade in Li-rich cathodes.

Phase segregation and nanoconfined fluid O2 in a lithium-rich oxide cathode

Lithium-rich oxide cathodes lose energy density during cycling due to atomic disordering and nanoscale structural rearrangements, which are both challenging to characterize. Here we resolve the kinetics and thermodynamics of these processes in an exemplar layered Li-rich (Li1.2-xMn0.8O2) cathode using a combined approach of ab initio molecular dynamics and cluster expansion-based Monte Carlo simulations. We identify a kinetically accessible and thermodynamically favourable mechanism to form O2 molecules in the bulk, involving Mn migration and driven by interlayer oxygen dimerization. At the top of charge, the bulk structure locally phase segregates into MnO2-rich regions and Mn-deficient nanovoids, which contain O2 molecules as a nanoconfined fluid. These nanovoids are connected in a percolating network, potentially allowing long-range oxygen transport and linking bulk O2 formation to surface O2 loss. These insights highlight the importance of developing strategies to kinetically stabilize the bulk structure of Li-rich O-redox cathodes to maintain their high energy densities.

Engineering Reversible Lattice Structure for High-Capacity Co-Free Li-Rich Cathodes with Negligible Capacity Degradation

Co-free Li-rich Mn-based cathode materials are garnering great interest because of high capacity and low cost. However, their practical application is seriously hampered by the irreversible oxygen escape and the poor cycling stability. Herein, a reversible lattice adjustment strategy is proposed by integrating O vacancies and B doping. B incorporation increases TM─O (TM: transition metal) bonding orbitals whereas decreases the antibonding orbitals. Moreover, B doping and O vacancies synergistically increase the crystal orbital bond index values enhancing the overall covalent bonding strength, which makes TM─O octahedron more resistant to damage and enables the lattice to better accommodate the deformation and reaction without irreversible fracture. Furthermore, Mott-Hubbard splitting energy is decreased due to O vacancies, facilitating electron leaps, and enhancing the lattice reactivity and capacity. Such a reversible lattice, more amenable to deformation and forestalling fracturing, markedly improves the reversibility of lattice reactions and mitigates TM migration and the irreversible oxygen redox which enables the high cycling stability and high rate capability. The modified cathode demonstrates a specific capacity of 200 mAh g-1 at 1C, amazingly sustaining the capacity for 200 cycles without capacity degradation. This finding presents a promising avenue for solving the long-term cycling issue of Li-rich cathode.

Influence of Synthesis Parameters on the Short-Range Structure and Electrochemical Performances of Li2MnO2F

In the last years, disordered rocksalt structure (DRS) materials were proposed as a positive electrode for lithium-ion batteries. In particular, the fluorinated DRS materials were proposed to be more stable upon cycling than pure oxide counterparts. These materials are mainly obtained by mechanosynthesis in order to incorporate a significant number of F ions and maintain a disordered structure. Since the local structural arrangement is crucial for battery application, we aim to monitor its evolution upon the synthesis of Li2MnO2F from two sets of precursors: Mn2O3, Li2O, and LiF or LiMnO2 and LiF. The synthesis progress was thus followed, by 7Li and 19F MAS NMR coupled to XRD to probe the structure at different scales. This allowed us to identify an optimal milling time to reach the final compounds. We show that they exhibit similar morphology (by SEM), medium- and short-range orders (by XRD, 7Li and 19F NMR, EXAFS), and average Mn oxidation degree (by XANES). The electrochemical performances of the two compounds are almost similar, with high specific capacities of 319 mAh·g-1 ("from LiMnO2") and 304 mAh·g-1 ("from Mn2O3") for the first charge to 4.8 V vs Li+/Li, proving their interest as post-NMC candidates as positive electrode materials.

A Critical Analysis of Chemical and Electrochemical Oxidation Mechanisms in Li-Ion Batteries

Electrolyte decomposition limits the lifetime of commercial lithium-ion batteries (LIBs) and slows the adoption of next-generation energy storage technologies. A fundamental understanding of electrolyte degradation is critical to rationally design stable and energy-dense LIBs. To date, most explanations for electrolyte decomposition at LIB positive electrodes have relied on ethylene carbonate (EC) being chemically oxidized by evolved singlet oxygen (1O2) or electrochemically oxidized. In this work, we apply density functional theory to assess the feasibility of these mechanisms. We find that electrochemical oxidation is unfavorable at any potential reached during normal LIB operation, and we predict that previously reported reactions between the EC and 1O2 are kinetically limited at room temperature. Our calculations suggest an alternative mechanism in which EC reacts with superoxide (O2-) and/or peroxide (O22-) anions. This work provides a new perspective on LIB electrolyte decomposition and motivates further studies to understand the reactivity at positive electrodes.

Stabilized Anionic Redox by Rational Structural Design from Surface to Bulk for Long-Life Fast-Charging Li-Rich Oxide Cathodes

On account of high capacity and high voltage resulting from anionic redox, Li-rich layered oxides (LLOs) have become the most promising cathode candidate for the next-generation high-energy-density lithium-ion batteries (LIBs). Unfortunately, the participation of oxygen anion in charge compensation causes lattice oxygen evolution and accompanying structural degradation, voltage decay, capacity attenuation, low initial columbic efficiency, poor kinetics, and other problems. To resolve these challenges, a rational structural design strategy from surface to bulk by a facile pretreatment method for LLOs is provided to stabilize oxygen redox. On the surface, an integrated structure is constructed to suppress oxygen release, electrolyte attack, and consequent transition metals dissolution, accelerate lithium ions transport on the cathode-electrolyte interface, and alleviate the undesired phase transformation. While in the bulk, B doping into Li and Mn layer tetrahedron is introduced to increase the formation energy of O vacancy and decrease the lithium ions immigration barrier energy, bringing about the high stability of surrounding lattice oxygen and outstanding ions transport ability. Benefiting from the specific structure, the designed material with the enhanced structural integrity and stabilized anionic redox performs an excellent electrochemical performance and fast-charging property..

A Mechanistic Insight into the Oxygen Redox of Li-Rich Layered Cathodes and their Related Electronic/Atomic Behaviors Upon Cycling

Li-rich cathodes are extensively investigated as their energy density is superior to Li stoichiometric cathode materials. In addition to the transition metal redox, this intriguing electrochemical performance originates from the redox reaction of the anionic sublattice. This new redox process, the so-called anionic redox or, more directly, oxygen redox in the case of oxides, almost doubles the energy density of Li-rich cathodes compared to conventional cathodes. Numerous theoretical and experimental investigations have thoroughly established the current understanding of the oxygen redox of Li-rich cathodes. However, different reports are occasionally contradictory, indicating that current knowledge remains incomplete. Moreover, several practical issues still hinder the real-world application of Li-rich cathodes. As these issues are related to phenomena resulting from the electronic to atomic evolution induced by unstable oxygen redox, a fundamental multiscale understanding is essential for solving the problem. In this review, the current mechanistic understanding of oxygen redox, the origin of the practical problems, and how current studies tackle the issues are summarized.

Thermodynamically Stable Dual-Modified LiF&FeF3 layer Empowering Ni-Rich Cathodes with Superior Cyclabilities

Pushing the limit of cutoff potentials allows nickel-rich layered oxides to provide greater energy density and specific capacity whereas reducing thermodynamic and kinetic stability. Herein, a one-step dual-modified method is proposed for in situ synthesizing thermodynamically stable LiF&FeF₃ coating on LiNi_0.8 Co_0.1 Mn_0.1 O₂ surfaces by capturing lithium impurity on the surface to overcome the challenges suffered. The thermodynamically stabilized LiF&FeF₃ coating can effectively suppress the nanoscale structural degradation and the intergranular cracks. Meanwhile, the LiF&FeF₃ coating alleviates the outward migration of O^α- (α<2), increases oxygen vacancy formation energies, and accelerates interfacial Li⁺ diffusion. Benefited from these, the electrochemical performance of LiF&FeF₃ modified materials is improved (83.1% capacity retention after 1000 cycles at 1C), even under exertive operational conditions of elevated temperature (91.3% capacity retention after 150 cycles at 1C). This work demonstrates that the dual-modified strategy can simultaneously address the problems of interfacial instability and bulk structural degradation and represents significant progress in developing high-performance lithium-ion batteries (LIBs).

Accumulated Lattice Strain as an Intrinsic Trigger for the First-Cycle Voltage Decay in Li-Rich 3d Layered Oxides

Li- and Mn-rich layered oxides (LMLOs) are promising cathode materials for Li-ion batteries (LIBs) owing to their high discharge capacity of above 250 mA h g^-1. A high voltage plateau related to the oxidation of lattice oxygen appears upon the first charge, but it cannot be recovered during discharge, resulting in the so-called voltage decay. Disappearance of the honeycomb superstructure of the layered structure at a slow C-rate (e.g., 0.1 C) has been proposed to cause the first-cycle voltage decay. By comparing the structural evolution of Li[Li_0.2Ni_0.2Mn_0.6]O₂ (LLNMO) at various current densities, the operando synchrotron-based X-ray diffraction results show that the lattice strain in bulk LLNMO is continuously increased over cycling, resulting in the first-cycle voltage loss upon Li-ion insertion. Unlike the LLNMO, the accumulated average lattice strain of LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) and LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622) from the open-circuit voltage to 4.8 V could be released on discharge. These findings help to gain a deep understanding of the voltage decay in LMLOs.

Manipulating Charge-Transfer Kinetics of Lithium-Rich Layered Oxide Cathodes in Halide All-Solid-State Batteries

Employing lithium-rich layered oxide (LLO) as the cathode of all-solid-state batteries (ASSBs) is highly desired for realizing high energy density. However, the poor kinetics of LLO, caused by its low electronic conductivity and significant oxygen-redox-induced structural degradation, has impeded its application in ASSBs. Here, the charge transfer kinetics of LLO is enhanced by constructing high-efficiency electron transport networks within solid-state electrodes, which considerably minimizes electron transfer resistance. In addition, an infusion-plus-coating strategy is introduced to stabilize the lattice oxygen of LLO, successfully suppressing the interfacial oxidation of solid electrolyte (Li₃ InCl₆ ) and structural degradation of LLO. As a result, LLO-based ASSBs exhibit a high discharge capacity of 230.7 mAh g^-1 at 0.1 C and ultra-long cycle stability over 400 cycles. This work provides an in-depth understanding of the kinetics of LLO in solid-state electrodes, and affords a practically feasible strategy to obtain high-energy-density ASSBs.

Quantifying the Anomalous Local and Nanostructure Evolutions Induced by Lattice Oxygen Redox in Lithium-Rich Cathodes

Due to their accessible lattice oxygen redox (l-OR) at high voltages, Li-rich layered transition metal (TM) oxides have shown promising potential as candidate cathodes for high-energy-density Li-ion batteries. However, this l-OR process is also associated with unusual electrochemical issues such as voltage hysteresis and long-term voltage decay. The structure response mechanism to the l-OR behavior also remains unclear, hindering rational structure optimizations that would enable practical Li-rich cathodes. Here, this study reveals a strong coupling between l-OR and structure dynamic evolutions, as well as their effects on the electrochemical properties. Using the technique of neutron total scattering with pair distribution function analysis and small-angle neutron scattering, this study quantifies the local TM migration and formation of nanopores that accompany the l-OR. These experiments demonstrate the causal relationships among l-OR, the local/nanostructure evolutions, and the unusual electrochemistry. The TM migration triggered by the l-OR can change local oxygen coordination environments, which results in voltage hysteresis. Coupled with formed oxygen vacancies, it will accelerate the formation of nanopores, inducing a phase transition, and leading to irreversible capacity and long-cycling voltage fade.

Boosting performance of Co-free Li-rich cathode material through regulating the anionic activity by means of the strong TaO bonding

Benefiting from the extra contribution of O redox, Co-free Li-rich layered oxides (LRNMO) can satisfy the requirement of high specific capacities. However, during the high-voltage charging process, lattice oxygen being oxidized to O^- or O₂ leads to a gradual transition of the structure from layered to spinel phase, capacity and voltage decline, hindering the practical application of LRNMO in the lithium-ion battery. Here, a surface modification strategy of Li_1.2Ni_0.32Mn_0.48O_2-δ doped with Ta⁵⁺ ions is proposed, in which the Ta⁵⁺ ions occupy the lithium sites of the lattice structure on the surface layer of LRNMO and form a Ta₂O₅ coating layer. The modified electrode exhibits excellent rate performance and cycling stability, with 94.9% and 85.5% capacity retention rate and voltage retention rate, respectively, after 200 cycles at 1C. Moreover, the initial coulomb efficiency and ionic conductivity of the modified electrode are also apparently enhanced. Simultaneously, the decreased Li/Ni mixing degree of the modified electrode reflects the improvement of the structural stability. Therefore, the modification strategy through strong metal-oxygen bonding to integrate the surface structure to regulate the oxygen activity provides a new direction for the design of high energy density Co-free Li-rich cathode materials.

Superstructure Control of Anionic Redox Behavior in Manganese-Based Cathode Materials for Li-Ion Batteries

Anionic charge compensation creates conditions for realizing high capacity and energy density of Li-ion batteries cathode materials. However, the issues of voltage hysteresis, capacity attenuation, and structure transformation caused by the labile anionic redox are still difficult to solve fundamentally. The superstructure formed by a Li-Mn ordered arrangement is the intrinsic reason to trigger the anionic charge compensation. In this work, manganese-based cathode materials with series of Li-Mn ordered superstructure types have been prepared by an ion exchange method, and superstructure control of the anionic redox behavior has been synthetically investigated. With the dispersion of a LiMn₆ superstructure unit, the aggregation of Li vacancies in Mn slab is gradually inhibited, which eliminates the production of O-O dimers and improves the reversibility of oxygen redox. Therefore, the voltage hysteresis and capacity fading have been significantly improved. Meanwhile, the amount of reactive oxygen species and their capacity contribution is reduced, and the sluggish electrochemical reaction kinetics of anion requires a low current density to boost the high-capacity advantage. This paper provides effective ideas for the design of various superstructures and the rational utilization of anionic redox.

Structural Origin of Suppressed Voltage Decay in Single-Crystalline Li-Rich Layered Li[Li0.2 Ni0.2 Mn0.6 ]O2 Cathodes

Lithium- and manganese-rich layered oxides (LMLOs, ≥ 250 mAh g^-1 ) with polycrystalline morphology always suffer from severe voltage decay upon cycling because of the anisotropic lattice strain and oxygen release induced chemo-mechanical breakdown. Herein, a Co-free single-crystalline LMLO, that is, Li[Li_0.2 Ni_0.2 Mn_0.6 ]O₂ (LLNMO-SC), is prepared via a Li⁺ /Na⁺ ion-exchange reaction. In situ synchrotron-based X-ray diffraction (sXRD) results demonstrate that relatively small changes in lattice parameters and reduced average micro-strain are observed in LLNMO-SC compared to its polycrystalline counterpart (LLNMO-PC) during the charge-discharge process. Specifically, the as-synthesized LLNMO-SC exhibits a unit cell volume change as low as 1.1% during electrochemical cycling. Such low strain characteristics ensure a stable framework for Li-ion insertion/extraction, which considerably enhances the structural stability of LLNMO during long-term cycling. Due to these peculiar benefits, the average discharge voltage of LLNMO-SC decreases by only ≈0.2 V after 100 cycles at 28 mA g^-1 between 2.0 and 4.8 V, which is much lower than that of LLNMO-PC (≈0.5 V). Such a single-crystalline strategy offers a promising solution to constructing stable high-energy lithium-ion batteries (LIBs).

A multimodal analytical toolkit to resolve correlated reaction pathways: the case of nanoparticle formation in zeolites

Unraveling the complex, competing pathways that can govern reactions in multicomponent systems is an experimental and technical challenge. We outline and apply a novel analytical toolkit that fully leverages the synchronicity of multimodal experiments to deconvolute causal from correlative relationships and resolve structural and chemical changes in complex materials. Here, simultaneous multimodal measurements combined diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and angular dispersive X-ray scattering suitable for pair distribution function (PDF), X-ray diffraction (XRD) and small angle X-ray scattering (SAXS) analyses. The multimodal experimental data was interpreted via multi-level analysis; conventional analyses of each data series were integrated through meta-analysis involving non-negative matrix factorization (NMF) as a dimensional reduction algorithm and correlation analysis. We apply this toolkit to build a cohesive mechanistic picture of the pathways governing silver nanoparticle formation in zeolite A (LTA), which is key to designing catalytic and separations-based applications. For this Ag-LTA system, the mechanisms of zeolite dehydration, framework flexing, ion reduction, and cluster and nanoparticle formation and transport through the zeolite are elucidated. We note that the advanced analytical approach outline here can be applied generally to multimodal experiments, to take full advantage of the efficiencies and self-consistencies in understanding complex materials and go beyond what can be achieved by conventional approaches to data analysis.

Multimodal in situ experimental data probing a complex reaction have been integrated via a multi-level analysis involving non-negative matrix factorization and correlation analysis. This strategy can be applied generally to multimodal experiments.