Transition Metal Migration Research Articles

Migration Mechanism of the B−H Activation of Carboranes

AbstractDensity functional theory was used to investigate the possible migration mechanism of the B−H activation on carboranes. The effects of inducing groups, mediating atoms and ligands were clarified, and the specific reaction process and driving force of transition metal migration in carboranes were explored. The study of thermodynamics, dynamics, wave function analysis and bond order showed that hydrogen atoms and ligands had remarkable effects on metal migration in carboranes. Results indicated that the transition metal complexes can migrate between B/B or C/B mediated by H‐atom, which contributes to the regioselective B−H activation of carboranes.

(Invited) Thermal Properties of NMC Cathode Materials

NMC (LiNixMnyCozO2; x+y+z»1) materials deliver high capacity and excellent performance when used as cathodes in lithium-ion batteries. Increasing demand for higher energy density and concerns about the cost and ethics of using cobalt have prompted battery manufacturers to use Ni-rich formulations such as LiNi0.6Mn0.2Co0.2O2 (NMC622) and LiNi0.8Mn0.1Co0.1O2 (NMC811). The thermal stability of (partially) delithiated NMCs is known to decrease with increasing Ni content, however, so that scenarios of fires and other safety events become more distinct possibilities. To investigate the thermal properties of Ni-rich NMCs, we prepared partially and fully delithiated Ni-rich NMCs corresponding to various states-of-charge by chemical methods. We then studied the bulk and surface properties of these materials as they were heated or after they were heated, using a variety of ex situ and in situ synchrotron methods to understand the chemical changes that occur. Of particular concern are reactions that result in release of oxygen. Heated materials progress through a series of bulk structural changes from layered to spinel to rock salt as temperatures rise. Transition temperatures depend on both lithium and nickel content, with bulk changes evident at temperatures as low as 150°C. Surface-sensitive techniques such as soft X-ray absorption spectroscopy (sXAS) indicate that subtle changes that lead to oxygen release can happen well below these temperatures. The thermal behavior of the Ni-rich materials is very complex, and involves lattice transformation, transition metal migration and valence change and lithium redistribution. Moreover, these changes are dependent upon primary particle size, with smaller particles reacting at lower temperatures than larger ones. These observations suggest that materials can be engineered (through primary particle size manipulation, for example) to improve thermal robustness, and therefore, safety and reliability of lithium-ion batteries.

Feasibility to Improve the Stability of Lithium-Rich Layered Oxides by Surface Doping.

Li-rich layer-structured oxides are considered promising cathode materials for their specific capacities above 250 mAh·g-1. However, the drawbacks such as poor rate performance, fast capacity fading, and the continuous transition metal (TM) migration into the Li layer hinder their commercial applications. To address these issues, surface doping of Ti and Zr was conducted to the Li- and Mn-rich layered oxide (LMR), Li1.2Mn0.54Ni0.13Co0.13O2. The drop of the average discharge potentials of the Ti- and Zr-doped LMR was reduced by 593 and 346 mV in 100 cycles, respectively. With aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy, we clarified that Ti4+ and Zr4+ ions are located near the surface of the material, anchor the surface oxygen, and stabilize the LMR structure. The difference in the strengths of the Ti-O and Zr-O bonds and the doping-resultant electronic structures were determined with density functional theory (DFT) calculations and soft X-ray absorption spectroscopy (SXAS), responsible for the electrochemical performance of surface-doped materials. These findings verify our modification strategies to enhance the cycling performances of the promising LMR cathode materials.

Destructive effects of transitional metal ions on interfacial film of carbon anode for lithium-ion batteries

The dissolution, migration, and deposition of transition metal (TM) ions are the main reasons for the capacity degradation of lithium-ion batteries, which significantly affect their comprehensive electrochemical performance and safety. In this work, the effects of different TM ions (Mn2+, Co2+, and Ni2+) on the electrochemical performance of lithium/mesocarbon microbeads half-cell were compared, and the mechanism was discussed. It is found that Mn2+ ions inhibit the decomposition of lithium salt and promote the decomposition of solvents, resulting in the crack of electrode material and solid electrolyte interface (SEI) film and the increase of capacity degradation. In contrast, Ni2+ ions promote the decomposition of lithium salts to a certain extent, thus inhibiting the excessive damage to the SEI film and electrode material. This study can provide theoretical support and research direction for the design of the electrolytes that can alleviate the damage of TM ions to the anode and the SEI film.

Understanding high-temperature cycling-induced crack evolution and associated atomic-scale structure in a Ni-rich LiNi0.8Co0.1Mn0.1O2 layered cathode material

X-ray nano-computed tomography (nano-CT) and deep learning combined with Cs-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy were employed to investigate the atomic to microscopic structural evolution of LiNi0.8Co0.1Mn0.1O2 (NCM) upon cycling at 55 °C. Two types of intergranular cracks were clearly distinguished by nano-CT for cycled cathode particles; denoted open and closed cracks depending on whether or not the cracks reach the surface of the NCM secondary particles. The volume of high-temperature cycling-induced cracks quantified by deep learning increased drastically, particularly for the open cracks, and this phenomenon was accompanied by rapid degradation of capacity retention. Further precise STEM analysis of the crack regions revealed that migration of transition metal (TM) ions to the Li layer forms a rocksalt-like structure, and the associated reduction of TM ions, e.g., Ni3+ to Ni2+, predominately occurred in the open crack regions in the presence of penetrated electrolyte, even for regions extending to the center of the secondary particle. In contrast, in the closed crack regions, no significant atomic-scale structure distortion and limited reduction of TM ions was observed. Thus, our results provide fundamental guidance for improving the electrochemical performance of Ni-rich layered cathode materials for cycling at elevated temperatures.

Structural Reconstruction Driven by Oxygen Vacancies in Layered Ni‐Rich Cathodes

AbstractStructure reconstruction induced by the migration of Li, O, and transition metal (TM) ions plays a key role in the performance of Ni‐based cathodes, yet their interactions are still poorly understood. This work investigates systematically the structure transformation of the high‐temperature lithiation in air and oxygen rich atmosphere, charging process, and long‐term storage. Structural and electrochemical characterization of Li‐free/Li‐containing phases (Ni0.92Co0.04Mn0.04(OH)2 and LixNi0.92Co0.04Mn0.04Oy) and a series of detailed analyses provide an in‐depth understanding of the structural reconstruction induced by the interaction of Li, O, and TM ions, i.e., the shift of Li/TM ions in the lattice leads to structural reconstruction via a layered structure with oxygen vacancies.

Improving the oxygen redox reversibility of Li-rich battery cathode materials via Coulombic repulsive interactions strategy

The oxygen redox reaction in lithium-rich layered oxide battery cathode materials generates extra capacity at high cell voltages (i.e., >4.5 V). However, the irreversible oxygen release causes transition metal (TM) dissolution, migration and cell voltage decay. To circumvent these issues, we introduce a strategy for tuning the Coulombic interactions in a model Li-rich positive electrode active material, i.e., Li1.2Mn0.6Ni0.2O2. In particular, we tune the Coulombic repulsive interactions to obtain an adaptable crystal structure that enables the reversible distortion of TMO6 octahedron and mitigates TM dissolution and migration. Moreover, this strategy hinders the irreversible release of oxygen and other parasitic reactions (e.g., electrolyte decomposition) commonly occurring at high voltages. When tested in non-aqueous coin cell configuration, the modified Li-rich cathode material, combined with a Li metal anode, enables a stable cell discharge capacity of about 240 mAh g−1 for 120 cycles at 50 mA g−1 and a slower voltage decay compared to the unmodified Li1.2Mn0.6Ni0.2O2.

Synergistic Effect of TMSPi and FEC in Regulating the Electrode/Electrolyte Interfaces in Nickel-Rich Lithium Metal Batteries.

Nickel-rich LiNi0.8Co0.1Mn0.1O2 (NCM811) with respect to Li metal can enhance the energy density of lithium batteries effectively. However, the unstable Li deposition, together with the dissolution and migration of transition metal (TM) ions toward the anode deteriorate the cycle performance of NCM811||Li battery, especially when commercial carbonate electrolyte is used. Herein, tris(trimethylsilyl)phosphite (TMSPi) and fluoroethylene carbonate (FEC) are used to construct a dual-additive electrolyte, by which both electrodes can be protected. It is found that TMSPi can be preferentially adsorbed on the cathode surface through its strong coordination with Ni4+, playing the role as a HF scavenger and suppressing TM ions dissolution, as well as mitigating the structural degradation of the cathode effectively. When it comes to the lithium anode, the presence of TMSPi may lead to side reactions with Li metal, accompanied by fast dendrite growth. The introduction of FEC could facilitate the formation of stable electrode/electrolyte interfaces on both sides. Particularly, reduce the direct contact between TMSPi and Li anode, thus ameliorate the incompatibility issue. Consequently, the NCM811||Li cell with dual-additive demonstrates excellent capacity retention of 81.2% after 500 cycles at 1 C rate. As a sharp contrast, it only retains 13.9% in the one with blank electrolyte. The findings of this work provide a new insight into enhancing the cycle performance of NCM811||Li system via the synergistic effect between additives.

Tuning Bulk O2 and Nonbonding Oxygen State for Reversible Anionic Redox Chemistry in P2-Layered Cathodes.

Improving the reversibility of oxygen redox is quite significant for layered oxides cathodes in sodium-ion batteries. Herein, we for the first time simultaneously tune bulk O2 and nonbonding oxygen state for reversible oxygen redox chemistry in P2-Na0.67 Mn0.5 Fe0.5 O2 through a synergy of Li2 TiO3 coating and Li/Ti co-doping. O2- is oxidized to molecular O2 and peroxide (O2 )n- (n<2) during charging. Molecular O2 derived from transition metal (TM) migration is related to the superstructure ordering induced by Li doping. The synergy mechanism of Li2 TiO3 coating and Li/Ti co-doping on the two O-redox modes is revealed. Firstly, Li2 TiO3 coating restrains the surface O2 and inhibits O2 loss. Secondly, nonbonding Li-O-Na enhances the reversibility of O2- →(O2 )n- . Thirdly, Ti doping strengthens the TM-O bond which fixes lattice oxygen. The cationic redox reversibility is also enhanced by Li/Ti co-doping. The proposed insights into the oxygen redox reversibility are insightful for other oxide cathodes.

Tuning Bulk O2 and Nonbonding Oxygen State for Reversible Anionic Redox Chemistry in P2‐Layered Cathodes

AbstractImproving the reversibility of oxygen redox is quite significant for layered oxides cathodes in sodium‐ion batteries. Herein, we for the first time simultaneously tune bulk O2 and nonbonding oxygen state for reversible oxygen redox chemistry in P2‐Na0.67Mn0.5Fe0.5O2 through a synergy of Li2TiO3 coating and Li/Ti co‐doping. O2− is oxidized to molecular O2 and peroxide (O2)n− (n&lt;2) during charging. Molecular O2 derived from transition metal (TM) migration is related to the superstructure ordering induced by Li doping. The synergy mechanism of Li2TiO3 coating and Li/Ti co‐doping on the two O‐redox modes is revealed. Firstly, Li2TiO3 coating restrains the surface O2 and inhibits O2 loss. Secondly, nonbonding Li−O−Na enhances the reversibility of O2−→(O2)n−. Thirdly, Ti doping strengthens the TM−O bond which fixes lattice oxygen. The cationic redox reversibility is also enhanced by Li/Ti co‐doping. The proposed insights into the oxygen redox reversibility are insightful for other oxide cathodes.

Importance of Chemical Distortion on the Hysteretic Oxygen Capacity in Li-Excess Layered Oxides.

Nonhysteretic redox capacity is a critical factor in achieving high energy density without energy loss during cycling for rechargeable battery electrodes, which has been considered a major challenge in oxygen redox (OR) for Li-excess layered oxide cathodes for lithium-ion batteries (LIBs). Until recently, transition metal migration into the Li metal layer and the formation of O-O dimers have been considered major factors affecting hysteretic oxygen capacity. However, Li-excess layered oxides, particularly Ru oxides, exhibit peculiar voltage hysteresis that cannot be sufficiently described by only these factors. Therefore, this study aims to unlock the critical impeding factors in restraining the non-polarizing oxygen capacity of Li-excess layered oxides (herein, Li2RuO3) that exhibit reversible OR reactions. First, Li2RuO3 undergoes an increase in the chemical potential fluctuation as both the thermodynamic material instability and vacancy content increase. Second, the chemical compression of O-O bonds occurs at the early stage of the OR reaction (0.5 ≤ x ≤ 0.75) for Li1-xRu0.5O1.5, leading to flexible voltage hysteresis. Finally, in the range of 0.75 ≤ x ≤ 1.0, for Li1-xRu0.5O1.5, the formation of an O(2p)-O(2p)* antibonding state derived from the structural distortion of the RuO6 octahedron leads to the irreversibility of the OR reaction and enhanced voltage hysteresis. Consequently, our study unlocks the new decisive factor, namely, the structural distortion inducing the O(2p)-O(2p)* antibonding state, of the hysteretic oxygen capacity and provides insights into enabling the full potential of the OR reaction for Li-excess layered oxides for advanced LIBs.

Correlating the Voltage Hysteresis in Li- and Mn-Rich Layered Oxides to Reversible Structural Changes by Using X-ray and Neutron Powder Diffraction

Li- and Mn-rich layered oxides (LMR-NCMs) are promising cathode active materials (CAMs) in future lithium-ion batteries (LIBs) due to their high energy density. However, the material undergoes a unique open circuit voltage (OCV) hysteresis between charge and discharge after activation, which compromises its roundtrip energy efficiency and affects the thermal management requirements for a LIB system. The hysteresis is believed to be caused by transition metal (TM) migration and/or by oxygen redox activities. Using in-situ X-ray powder diffraction (XPD), we monitor the lattice parameters of over-lithiated NCMs during the initial cycles and show that also the lattice parameters feature a distinct path dependence. When correlated to the OCV instead of the state of charge (SOC), this hysteresis vanishes for the unit cell volume and gives a linear correlation that is identical for different degrees of over-lithiation. We further aimed at elucidating the role of TM migration on the hysteresis phenomena by applying joint Rietveld refinements to a series of ex-situ XPD and neutron powder diffraction (NPD) samples. We critically discuss the limitations of this approach and compare the results with DFT simulations, showing that the quantification of TM migration in LMR-NCMs by diffraction is not as straightforward as often believed.

Structure design enables stable anionic and cationic redox chemistry in a T2-type Li-excess layered oxide cathode

Coupled with anionic and cationic redox chemistry, Li-rich/excess cathode materials are prospective high-energy-density candidates for the next-generation Li-ion batteries. However, irreversible lattice oxygen loss would exacerbate irreversible transition metal migration, resulting in a drastic voltage decay and capacity degeneration. Herein, a metastable layered Li-excess cathode material, T2-type Li0.72[Li0.12Ni0.36Mn0.52]O2, was developed, in which both oxygen stacking arrangement and Li coordination environment fundamentally differ from that in conventional O3-type layered structures. By means of the reversible Li migration processes and structural evolutions, not only can voltage decay be effectively restrained, but also excellent capacity retention can be achieved upon long-term cycling. Moreover, irreversible/reversible anionic/cationic redox activities have been well assigned and quantified by various in/ex-situ spectroscopic techniques, further clarifying the charge compensation mechanism associated with (de)lithiation. These findings of the novel T2 structure with the enhanced anionic redox stability will provide a new scope for the development of high-energy-density Li-rich cathode materials.

Unraveling transition-metal-mediated stability of spinel oxide via in situ neutron scattering

The energy materials performance is intrinsically determined by structures from the average lattice structure to the atom arrangement, valence, and distribution of the containing transition metal (TM) elements. Understanding the mechanism of the structure transition and atom rearrangement via synthesis or processing is key to expediting the exploration of excellent energy materials. In this work, in situ neutron scattering is employed to reveal the real-time structure evolution, including the TM-O bonds, lattice, TM valence and the migration of the high-voltage spinel cathode LiNi0.5Mn1.5O4. The transition-metal-mediated spinel destabilization under the annealing at the oxygen-deficient atmosphere is pinpointed. The formation of Mn3+ is correlated to the TM migration activation, TM disordered rearrangement in the spinel, and the transition to a layered-rocksalt phase. The further TM interdiffusion and Mn2+ reduction are also revealed with multi-stage thermodynamics and kinetics. The mechanisms of phase transition and atom migrations as functions of temperature, time and atmosphere present important guidance on the synthesis in various-valence element containing oxides.

A Baseline Kinetic Study of Co-Free Layered Li1+x(Ni0.5Mn0.5)1−xO2 Positive Electrode Materials for Lithium-Ion Batteries

Variations of Li chemical diffusion coefficient () with voltage in a series of Co-free Li1+x(Ni0.5Mn0.5)1−xO2, 0 ≤ x ≤ 0.12, materials were systematically investigated using the recently developed “Atlung Method for Intercalant Diffusion”. The effects of primary and secondary particle sizes, excess Li content, heating temperature, and synthesis atmospheres on were measured. Li-ion kinetics can be enhanced by an order of magnitude by lowering the amount of Ni atoms in the Li layers (cation mixing) from 10% to 4%. Decreasing cation mixing can be accomplished by either increasing the excess Li content or heating temperature. When cycled to 4.6 V, higher specific capacities were obtained, but with a penalty to due to transition metal migration to the Li layers. The primary particles control the Li diffusion length in these materials, regardless of the secondary particle size indicating that grain boundary diffusion must be very rapid. The general trends observed in this work are of great value for the development of higher Mn-containing, Co-free materials. It should be possible to increase energy/power density by making large secondary particles, composed of small primary particles to minimize the solid-state diffusion length while maximizing grain boundary diffusion.

(Invited) From Layered Oxides to Disordered Rocksalt Cathodes: The Role of Electronic Structure, Cation Order, and Structure on the Performance of Dense Cathode Materials

Layered cathode materials are the enabler of today’s Li-ion industry. I will review some of the fundamental relations between electronic structure, cation ordering, and electrochemical performance in layered Li, Na and K-compounds. Diffusion in layered cathodes occurs through a divacancy mechanism which enables the low-energy Li passage through the tetrahedral sites that connect the octahedral positions in the Li layers. This diffusion mechanism provides high rate capability to layered cathodes, but relies on the Li-slab spacing to remain large upon electrochemical cycling. Any migration of transition metals into the Li-layer tends to contract the slab spacing and reduce rate capability. This limits practical layered materials to the NMC chemistry as Ni, Co and Mn4+ are the only ions that have a strong enough octahedral ligand-field stabilization to remain in the transition metal layer upon cycling. This problem is less pronounced for Na-layered oxides as the larger slab spacing makes occupancy of the tetrahedral and octahedral site in the Na-layer much less favorable for transition metals. But layered oxides with large alkali ions such as Na+ and K+ suffer from more sloped voltage profiles with limits their usable energy density.Finally, I will show how the chemical diversity of Li-ion cathodes can be extended to a much broader set of elements by enabling Li-excess disordered rocksalts (DRX). These materials diffuse lithium ions through a statistical network of low-energy migration environments, and have recently been shown to have very high capacity and rate performance.

Vacancy Enabled O3 Phase Stabilization for Manganese‐Rich Layered Sodium Cathodes

Manganese‐rich layered oxide materials hold great potential as low‐cost and high‐capacity cathodes for Na‐ion batteries. However, they usually form a P2 phase and suffer from fast capacity fade. In this work, an O3 phase sodium cathode has been developed out of a Li and Mn‐rich layered material by leveraging the creation of transition metal (TM) and oxygen vacancies and the electrochemical exchange of Na and Li. The Mn‐rich layered cathode material remains primarily O3 phase during sodiation/desodiation and can have a full sodiation capacity of ca. 220 mAh g−1. It delivers ca. 160 mAh g−1 specific capacity between 2–3.8 V with >86 % retention over 250 cycles. The TM and oxygen vacancies pre‐formed in the sodiated material enables a reversible migration of TMs from the TM layer to the tetrahedral sites in the Na layer upon de‐sodiation and sodiation. The migration creates metastable states, leading to increased kinetic barrier that prohibits a complete O3‐P3 phase transition.

Lithium-rich manganese-based cathode materials with highly stable lattice and surface enabled by perovskite-type phase-compatible layer

Li-rich Mn-based cathode materials possess a high specific capacity, but their application is hindered by inherent anion activity and surface instability. In this regard, a perovskite-type LaMnO 3 modified layer was epitaxially grown on the surface of Li-rich Mn-based cathode through La reaction with Mn provided by bulk materials. XRD and XPS results confirmed the successful preparation of LaMnO 3 and revealed the reaction mechanism. SEM and TEM results further verified the epitaxial growth and uniform distribution of LaMnO 3 . The modified Li-rich Mn-based cathode materials showed outstanding rate capability and satisfactory cycle durability, which exhibited 155.4 mAh g −1 at 5 C and maintained 85.4% at 1 C after 200 cycles during 2.0–4.8 V of operation. The anion activity (O 2- , O 2 2- ) was effectively suppressed, resulting in more stable discharge voltage and enhanced initial coulomb efficiency. The surface of the cathode was protected by LaMnO 3 , leading to reduced surface by-products and fixed lattice O. The more stable lattice structures and high conductivity of LaMnO 3 led to easy charge transfer and excellent electrochemical performance. • The lattice O is fixed and the migration of transition metal is well restricted by the La-O‐Mn bond. • The perovskite LaMnO 3 help in improving the electron transfer of charged particles and prevent the attacking of electrolytes. • Slightly La-doping regulates the Li/Ni disorder and stabilizes the crystal lattice, resulting in more stable structure.

Cation/Anion Codoped and Cobalt-Free Li-Rich Layered Cathode for High-Performance Li-Ion Batteries.

Lithium-rich layered oxides have received great attention due to their high energy density as cathode material. However, the progressive structural transformation from layered to spinel phase triggered by transition-metal migration and the irreversible release of lattice oxygen leads to voltage fade and capacity decay. Here, we report a Fe, Cl codoped and Co-free Li-rich layered cathode with significantly improved structural stability. It is revealed that the Fe and Cl codoping can facilitate the Li-ion diffusion and improve the rate performance of the materials. Moreover, the calculations show that the structural stability is enhanced by Fe and Cl codoping. As a result, the Fe and Cl codopant reduces the irreversible release of lattice oxygen, mitigates voltage fade, and improves the first-cycle Coulombic efficiency. This work provides a low-cost, environmentally friendly, practical strategy for high-performance cathode materials.

Voltage-Dependent Li Kinetics Leads to Charge-Discharge Asymmetry in Co-Free Li-Rich Li1.12Ni0.44Mn0.44O2 under Conditions without Transition Metal Migration

Understanding factors that influence Li-ion kinetics in positive electrode materials is important for development of Li-ion batteries capable of operating at high rates and low temperatures. Herein, the impact of rate and operating temperature on Li diffusion was studied to explain charge-discharge asymmetry where lithiation is kinetically hindered compared to delithiation. Co-free, Li-rich Li1.12Ni0.44Mn0.44O2 materials with different secondary particle sizes of 4 and 16 μm, but nearly identical primary particle sizes, lattice parameters, and percentage of Ni atoms in the Li layers, were chosen to demonstrate how worsened Li kinetics during lithiation leads to charge-discharge asymmetry. Cells were cycled between 4.4–1.5 V to eliminate transition metal migration and to understand Li kinetics as a function of state of charge. Measurements were done on an Ultra-high precision charger to measure apparent capacity loss due to kinetic limitations. At low and moderate rates, the cell capacity was fully recovered by allowing the cell voltage to pass below 2 V where a voltage plateau corresponding to a Li2MO2-like phase was observed. Li ion diffusion measurements confirmed that Li diffusion is slower at low voltage. Taken together with the rate and temperature dependence, the charge-discharge asymmetry is fully accounted for solely from Li-ion kinetic limitations.