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Depth‐of‐Discharge Dependent Capacity Decay Induced by the Accumulation of Oxidized Lattice Oxygen in Li‐Rich Layered Oxide Cathode
Abstract
More and more basic practical application scenarios have been gradually ignored/disregarded, in fundamental research on rechargeable batteries, e.g. assessing cycle life under various depths‐of‐discharge (DODs). Herein, although benefit from the additional energy density introduced by anionic redox, we critically revealed that lithium‐rich layered oxide (LRLO) cathodes present anomalously poor capacity retention at low‐DOD cycling, which is essentially different from typical layered cathodes (e.g. NCM), and pose a formidable impediment to the practical application of LRLO. We systemically demonstrated that DOD‐dependent capacity decay is induced by the anionic redox and accumulation of oxidized lattice oxygen (On‐). Upon low‐DOD cycling, the accumulation of On‐ and the persistent presence of vacancies in the transition metal (TM) layer intensified the in‐plane migration of TM, exacerbating the expansion of vacancy clusters, which further facilitated detrimental out‐of‐plane TM migration. As a result, the aggravated structural degradation of LRLO at low‐DOD impeded reversible Li+ intercalation, resulting in rapid capacity decay. Furthermore, prolonged accumulation of On‐ persistently corroded the electrode‐electrolyte interface, especially negative for pouch‐type full‐cells with the shuttle effect. Once the “double‐edged sword” effect of anionic redox being elucidated under practical condition, corresponding modification strategies/routes would become distinct for accelerating the practical application of LRLO.
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Na-ion batteries (NIBs) are emerging as a promising alternative to Li-ion batteries (LIBs). To align with sustainability principles, the design of electrode materials must incorporate considerations for abundant and environmentally...
Compensating for the irreversible loss of limited active sodium (Na) is crucial for enhancing the energy density of practical sodium‐ion batteries (SIBs) full‐cell, especially when employing hard carbon anode with initially lower coulombic efficiency. Introducing sacrificial cathode presodiation agents, particularly those that own potential anionic oxidation activity with a high theoretical capacity, can provide additional sodium sources for compensating Na loss. Herein, Ni atoms are precisely implanted at the Na sites within Na2O framework, obtaining a (Na0.89Ni0.05□0.06)2O (Ni–Na2O) presodiation agent. The synergistic interaction between Na vacancies and Ni catalyst effectively tunes the band structure, forming moderate Ni–O covalent bonds, activating the oxidation activity of oxygen anion, reducing the decomposition overpotential to 2.8 V (vs Na/Na⁺), and achieving a high presodiation capacity of 710 mAh/g≈Na2O (Na2O decomposition rate >80%). Incorporating currently‐modified presodiation agent with Na3V2(PO4)3 and Na2/3Ni2/3Mn1/3O2 cathodes, the energy density of corresponding Na‐ion full‐cells presents an essential improvement of 23.9% and 19.3%, respectively. Further, not limited to Ni–Na2O, the structure–function relationship between the anionic oxidation mechanism and electrode–electrolyte interface fabrication is revealed as a paradigm for the development of sacrificial cathode presodiation agent.
Cathode materials are the core components of lithium‐ion batteries owing to the determination of the practical voltage and effective energy of the battery system. However, advanced cathodes have faced challenges related to cation migration and cation intermixing. In this review, the study summarizes the structural failure mechanisms due to the cation mixing of advanced cathodes, including Ni‐rich and Li‐rich layered cathodes, spinel, olivine, and disordered rock‐salt materials. This review starts by discussing the structural degradation mechanisms caused by cation intermixing in different cathodes, focusing on the electronic structure, crystal structure, and electrode structure. Furthermore, the optimization strategies for effective inhibition of cation migration and rational utilization of cation mixing are systematically encapsulated. Last but not least, the remaining challenges and proposed perspectives are highlighted for the future development of advanced cathodes. The accurate analysis of cation migration using advanced characterization, precise control of material synthesis, and multi‐dimensional synergistic modification will be the key research areas for cation migration in cathodes. This review provides a comprehensive understanding of cation migration and intermixing in advanced cathodes. The effective inhibition of cation migration and the rational utilization of cation intermixing will emerge as pivotal and controllable factors for the further development of advanced cathodes.
The undesirable capacity loss after first cycle is universal among layered cathode materials, which results in the capacity and energy decay. The key to resolving this obstacle lies in understanding the effect and origin of specific active Li sites during discharge process. In this study, focusing on Ah‐level pouch cells for reliability, an ultrahigh initial Coulombic efficiency (96.1%) is achieved in an archetypical Li‐rich layered oxide material. Combining the structure and electrochemistry analysis, we demonstrate that the achievement of high‐capacity reversibility is a kinetic effect, primarily related to the sluggish Li mobility during oxygen reduction. Activating oxygen reduction through small density would induce the oxygen framework contraction, which, according to Pauli repulsion, imposes a great repulsive force to hinder the transport of tetrahedral Li. The tetrahedral Li storage upon deep oxygen reduction is experimentally visualized and, more importantly, contributes to 6% Coulombic efficiency enhancement as well as 10% energy density improvement for pouch cells, which shows great potentials breaking through the capacity and energy limitation imposed by intercalation chemistry.
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.
Surface reconstruction and the associated severe strain propagation have long been reported as the major cause of cathode failure during fast charging and long-term cycling. Despite tremendous attempts, no known strategies can simultaneously address the electro-chemomechanical instability without sacrificing energy and power density. Here we report an epitaxial entropy-assisted coating strategy for ultrahigh-Ni LiNixCoyMn1−x−yO2 (x ≥ 0.9) cathodes via an oriented attachment-driven reaction between Wadsley–Roth phase-based oxides and the layered-oxide cathodes. The high anti-cracking and anti-corrosion tolerances as well as the fast ionic transport of the entropy-assisted surface effectively improved the fast charging/discharging capability, wide temperature tolerance and thermal stability of the ultrahigh-Ni cathodes. Comprehensive analysis from the primary and secondary particle level to the electrode level using multi-scale in situ synchrotron X-ray probes reveals greatly reduced lattice dislocations, anisotropic lattice strain and oxygen release as well as improved bulk/local structural stability, even when charging beyond the threshold state of charge (75%) of layered cathodes.
Lithium- and manganese-rich layered oxide cathode materials have attracted extensive interest because of their high energy density. However, the rapid capacity fading and serve voltage decay over cycling make the waste management and recycling of key components indispensable. Herein, we report a facile concentrated solar radiation strategy for the direct recycling of Lithium- and manganese-rich cathodes, which enables the recovery of capacity and effectively improves its electrochemical stability. The phase change from layered to spinel on the particle surface and metastable state structure of cycled material provides the precondition for photocatalytic reaction and thermal reconstruction during concentrated solar radiation processing. The inducement of partial inverse spinel phase is identified after concentrated solar radiation treatment, which strongly enhances the redox activity of transition metal cations and oxygen anion, and reversibility of lattice structure. This study sheds new light on the reparation of spent cathode materials and designing high-performance compositions to mitigate structural degradation.
Gas evolution from high‐nickel layered oxide cathodes (>90% Ni) remains a major issue for their practical application. Gaseous species, such as CO2, O2, and CO, that are evolved at high states of charge (SOC) worsen the overall safety of batteries, as pressure build‐up within the cell may lead to cell rupture. Since these gasses are produced during cathode degradation, tracking the formation of gasses is also important in diagnosing cathode failure. Online electrochemical mass spectrometry (OEMS) is a powerful in situ technique to study gas evolution from the cathode during high‐voltage charge. However, the differences in the OEMS experimental setups between different groups make it challenging to compare results between groups. In this perspective, the various factors that influence gas evolution based on the OEMS results collected in this group are presented. The focus is on the conditions that lead to gas release, with a particular emphasis on reactive oxygen formation and subsequent chemical reactions with the electrolyte. Promising strategies, such as electrolytes, compositional tuning, and surface coatings that are effective at suppressing gas evolution from the cathode are highlighted. Critical insights into mitigating cathode degradation and gas evolution are provided to guide the development of safer, high‐energy batteries.
Electrolyte engineering is a fascinating choice to improve the performance of Li‐rich layered oxide cathodes (LRLO) for high‐energy lithium‐ion batteries. However, many existing electrolyte designs and adjustment principles tend to overlook the unique challenges posed by LRLO, particularly the nucleophilic attack. Here, we introduce an electrolyte modification by locally replacing carbonate solvents in traditional electrolytes with a fluoro‐ether. By benefit of the decomposition of fluoro‐ether under nucleophilic O‐related attacks, which delivers an excellent passivation layer with LiF and polymers, possessing rigidity and flexibility on the LRLO surface. More importantly, the fluoro‐ether acts as “sutures”, ensuring the integrity and stability of both interfacial and bulk structures, which contributed to suppressing severe polarization and enhancing the cycling capacity retention from 39 % to 78 % after 300 cycles for the 4.8 V‐class LRLO. This key electrolyte strategy with comprehensive analysis, provides new insights into addressing nucleophilic challenge for high‐energy anionic redox related cathode systems.
Solid‐state batteries (SSBs) based on Li‐rich Mn‐based oxide (LRMO) cathodes attract much attention because of their high energy density as well as high safety. But their development was seriously hindered by the interfacial instability and inferior electrochemical performance. Herein, we design a three‐dimensional foam‐structured GaN−Li composite anode and successfully construct a high‐performance SSB based on Co‐free Li1.2Ni0.2Mn0.6O2 cathode and Li6.5La3Zr1.5Ta0.5O12 (LLZTO) solid electrolyte. The interfacial resistance is considerably reduced to only 1.53 Ω cm² and the assembled Li symmetric cell is stably cycled more than 10,000 h at 0.1–0.2 mA cm⁻². The full battery shows a high initial capacity of 245 mAh g⁻¹ at 0.1 C and does not show any capacity degradation after 200 cycles at 0.2 C (≈100 %). The voltage decay is well suppressed and it is significantly decreased from 2.96 mV/cycle to only 0.66 mV/cycle. The SSB also shows a very high rate capability (≈170 mAh g⁻¹ at 1 C) comparable to a liquid electrolyte‐based battery. Moreover, the oxygen anion redox (OAR) reversibility of LRMO in SSB is much higher than that in liquid electrolyte‐based cells. This study offers a distinct strategy for constructing high‐performance LRMO‐based SSBs and sheds light on the development and application of high‐energy density SSBs.
While it is widely recognized that the operating temperature significantly affects the energy density and cycle life of lithium‐ion batteries, the consequence of electrode‐electrolyte interphase chemistry to sudden environmental temperature changes remains inadequately understood. Here, we systematically investigate the effects of a temperature pulse (T pulse) on the electrochemical performance of LiNi0.8Mn0.1Co0.1O2 (NMC811) pouch full cells. By utilizing advanced characterization tools, such as time‐of‐flight secondary‐ion mass spectrometry, we reveal that the T pulse can lead to an irreversible degradation of cathode‐electrolyte interphase chemistry and architecture. Despite negligible immediate impacts on the solid‐electrolyte interphase (SEI) on graphite anode, aggregated cathode‐to‐anode chemical crossover gradually degrades the SEI by catalyzing electrolyte reduction decomposition and inducing metallic dead Li formation because of insufficient cathode passivation after the T pulse. Consequently, pouch cells subjected to the T pulse show an inferior cycle stability to those free of the T pulse. This work unveils the effects of sudden temperature changes on the interphase chemistry and cell performance, emphasizing the importance of a proper temperature management in assessing performance.
Li[LixNiyMnzCo1−x−y−z]O2 (lithium-rich NMCs) are benchmark cathode materials receiving considerable attention due to the abnormally high capacities resulting from their anionic redox chemistry. Although their anionic redox mechanisms have been much investigated, the roles of cationic redox processes remain underexplored, hindering further performance improvement. Here we decoupled the effects of nickel and cobalt in lithium-rich NMCs via a comprehensive study of two typical compounds, Li1.2Ni0.2Mn0.6O2 and Li1.2Co0.4Mn0.4O2. We discovered that both Ni3+/4+ and Co⁴⁺, generated during cationic redox processes, are actually intermediate species for triggering oxygen redox through a ligand-to-metal charge-transfer process. However, cobalt is better than nickel in mediating the kinetics of ligand-to-metal charge transfer by favouring more transition metal migration, leading to less cationic redox but more oxygen redox, more O2 release, poorer cycling performance and more severe voltage decay. Our work highlights a compositional optimization pathway for lithium-rich NMCs by deviating from using cobalt to using nickel, providing valuable guidelines for future high-capacity cathode design.
LiNiO2‐based high‐nickel layered oxide cathodes are regarded as promising cathode materials for high‐energy‐density automotive lithium batteries. Most of the attention thus far has been paid towards addressing their surface and structural instability issues brought by the increase of Ni content (>90 %) with an aim to enhance the cycle stability. However, the poor safety performance remains an intractable problem for their commercialization in the market, yet it has not received appropriate attention. In this review, we focus on the gas generation and thermal degradation behaviors of high‐Ni cathodes, which are critical factors in determining their overall safety performance. A comprehensive overview of the mechanisms of outgassing and thermal runaway reactions is presented and analyzed from a chemistry perspective. Finally, we discuss the challenges and the insights into developing robust, safe high‐Ni cathodes.
Although oxygen redox in Li‐rich layered cathodes can boost the available capacity over 250 mAh g⁻¹, it also brings a rapid capacity fade upon long‐term cycling and serious safety issue during thermal abuse. To circumvent these problems, an integrated strategy via interlayer regulation at surface and the delocalization of Li2MnO3‐like domain on bulk is proposed. The controllable interlayer by atomic layer deposition can maximize the coating effects on elimination of the lattice mismatch to inhibit the structural degradation during cycling. And the delocalized Li2MnO3‐like domain through compositional control can fully prohibit lattice oxygen release from the bulk to improve the thermal stability of electrode. The optimized cathode material exhibits a capacity retention of 94.0% after 200 cycles. A 1.25 Ah multilayer pouch cell with the cathode and graphite anode delivers an outstanding cycling performance that retains 80.4% of its capacity at 0.5 C after 710 cycles. More importantly, the distinguished safety features derived from the method are verified after successfully passing practical‐level thermal safety and nail penetration test.
Li‐rich layered oxides (LLOs) have been considered as the most promising cathode materials for achieving high energy density Li‐ion batteries. However, they suffer from continuous voltage decay during cycling, which seriously shortens the lifespan of the battery in practical applications. This review comprehensively elaborates and summarizes the state‐of‐the‐art of the research in this field. It is started from the proposed mechanism of voltage decay that refers to the phase transition, microscopic defects, and oxygen redox or release. Furthermore, several strategies to mitigate the voltage decay of LLOs from different scales, such as surface modification, elemental doping, regulation of components, control of defect, and morphology design are summarized. Finally, a systematic outlook on the real root of voltage decay is provided, and more importantly, a potential solution to voltage recovery from electrochemistry. Based on this progress, some effective strategies with multiple scales will be feasible to create the conditions for their commercialization in the future.
LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811) is a promising cathode material for lithium-ion batteries due to its high energy density and low cost. However, NCM811 suffers from poor cycling stability and storage sensitivity to air and moisture. This study introduces an AlF 3 protective layer onto the surface of NCM811 (NCM811-AlF 3) by atomic layer deposition (ALD). After the AlF 3 layer protection, the initial capacity of the quasi-solid-state pouch cell with the NCM811 cathode was significantly increased from 148 to 180 mAh g −1. In addition, NCM811-AlF 3 maintained a capacity of 167 mAh g −1 , which exceeded that of pristine NCM811 (126 mAh g −1) after 500 cycles. This excellent electrochemical performance is attributed to the conformal AlF 3 protective layer that prevents the NCM811 from coming into direct contact with the electrolyte. In addition, the AlF 3 protective layer can prevent the Li/Ni mixture and Li loss during cycling by limiting the lattice expansion. Moreover, it can suppress the generation of residual alkali on the NCM811 surface during storage, improving the interfacial stability between NCM811 and the electrolytes. These results indicate that AlF 3 protective layer by ALD can be an effective method for improving the performance of high-energy-density cathode materials.
Typical Li‐rich layered oxides are widely regarded as promising cathode candidates for high‐energy‐density Li‐ion batteries because of additional output capacities boosted by oxygen redox activities. However, its commercialized applications are hindered by serious capacity loss and voltage decay related to structural degradation upon cycling. Herein, a Co/Ni‐free biphasic O2/O3‐type layered cathode material is proposed, Li0.9[Li0.3Mn0.7]O2, which has been successfully prepared by the Li⁺/Na⁺ ion‐exchange strategy and characterized by the XRD Rietveld refinement and SAED as well as HRTEM analyses. O2/O3‐type layered cathode material with an approximate composition of 81% O2 and 19% O3 phases are confirmed. Furthermore, the biphasic cathode exhibits a high discharge capacity of 232 mAh g⁻¹ with capacity retention of 88.1% after 500 cycles at a current density of 200 mA g⁻¹. That is, volume changes of the O3‐type phase are effectively restricted during Li⁺ (de)intercalations, further enhancing the structural stability and suppressing the formation of spinel phase due to the biphasic structural design. Altogether, these findings prove the biphasic structural design is a feasible strategy to achieve Li‐rich cathode materials with high capacity and long‐term cycle stability.
Although the two active redox centers in Li-rich cathodes, including the anionic and cationic contributions, can enable Li-ion batteries to achieve outstanding specific energy, their behaviors at different current densities...
Li‐rich layered oxides (LLOs) suffer from rapid voltage decay and capacity fading, greatly hindering their applications as high‐energy cathode materials for Li‐ion batteries (LIBs), which are closely associated with irreversible structural transformation and lattice oxygen loss upon electrochemical cycling. A strategy of Al/Ti synergistic bulk doping is proposed to stabilize LLOs against structural degradations, yielding remarkable electrochemical performance including a minor voltage decay of 0.34 mV cycle⁻¹ and a superior capacity retention of 89.7% over 500 cycles at 1C. The improved structural stability of Al/Ti codoped LLOs is well maintained after not only initial but also long cycles. This effect can be attributed to the cooperation of Al and Ti, with the former stabilizing the lattice oxygen by strong Al–O bonds and the latter improving the Li⁺ conductivity by expanding the lattice. The facile bulk synergistic doping of LLOs paves avenues toward their application for high‐energy LIBs.
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.
Lithium‐rich layered oxides (LRLO) have attracted great interest for high‐energy Li‐ion batteries due to their high theoretical capacity. However, capacity decay and voltage fade during the cycling impede the practical application of LRLO. Herein, the use of lithium bis‐(oxalate)borate (LiBOB) as an electrolyte additive is reported to improve the cycling stability in high voltage LRLO/graphite full cells. The cell with LiBOB‐containing electrolyte delivers 248 mAh g⁻¹ initial capacity and shows no capacity decay after 70 cycles as well as 95.5% retention after 150 cycles over 4.5 V cycling. A systematic mechanism study for the LiBOB‐enabled cycling performance improvement is conducted. Analytical electron microscopy under cryo‐condition confirms the formation of a uniform interphase and less phase transformation on the LRLO particle, accompanied by less voltage decay in the cathode. The formation of B‐F species is identified in the cycled electrolyte, elucidating the HF scavenger effect of LiBOB. Due to less HF corrosion on both electrode interphases, a reduced amount of transition metal dissolution and redeposition on the graphite is proved, thereby mitigating the capacity decay in LRLO/graphite full cells. These findings suggest that the borate additive is a promising strategy to optimize high voltage electrolyte for the industrialization of LRLO.
As one of the most promising cathodes for Li‐ion batteries, Li‐rich layered oxides suffer from low Coulombic efficiency, severe capacity fading, and voltage decay, which are related to the aggregated Li@Mn6 superstructure units. Herein, a Co‐free Li‐rich oxide Li[Li1/4Mn1/2Ni1/6Al1/12]O2 through Al substitution of Co in Li[Li1/4Mn1/2Ni1/6Co1/12]O2, is designed. Combining the average structural refinement with the detailed local structural/chemical analysis, it is found that the introduced Al ions occupy the Mn sites in Li@Mn6 superstructure units, which further induces the partial replacement of the central Li ions in Li@Mn6 units by Ni²⁺. The modified superstructure units stabilize the anionic framework and suppress structural degradation during long‐term cycling. A superior cyclability (a capacity retention of 91.4% after 500 cycles at 1 C) is achieved. This work not only deepens the understanding into the mechanism of Al substitution, but also provides a novel route to design high‐performance Li‐rich cathodes by modifying the local functional units.
Fast charging is considered to be a key requirement for widespread economic success of electric vehicles. Current lithium‐ion batteries (LIBs) offer high energy density enabling sufficient driving range, but take considerably longer to recharge than traditional vehicles. Multiple properties of the applied anode, cathode, and electrolyte materials influence the fast‐charging ability of a battery cell. In this review, the physicochemical basics of different material combinations are considered in detail, identifying the transport of lithium inside the electrodes as the crucial rate‐limiting steps for fast‐charging. Lithium diffusion within the active materials inherently slows down the charging process and causes high overpotentials. In addition, concentration polarization by slow lithium‐ion transport within the electrolyte phase in the porous electrodes also limits the charging rate. Both kinetic effects are responsible for lithium plating observed on graphite anodes. Conclusions drawn from potential and concentration profiles within LIB cells are complemented by extensive literature surveys on anode, cathode, and electrolyte materials—including solid‐state batteries. The advantages and disadvantages of typical LIB materials are analyzed, resulting in suggestions for optimum properties on the material and electrode level for fast‐charging applications. Finally, limitations on the cell level are discussed briefly as well.
Oxygen release and irreversible cation migration are the main causes of voltage fade in Li-rich transition metal oxide cathode. But their correlation is not very clear and voltage decay is still a bottleneck. Herein, we modulate the oxygen anionic redox chemistry by constructing Li2ZrO3 slabs into Li2MnO3 domain in Li1.21Ni0.28Mn0.51O2, which induces the lattice strain, tunes the chemical environment for redox-active oxygen and enlarges the gap between metallic and anionic bands. This modulation expands the region in which lattice oxygen contributes capacity by oxidation to oxygen holes and relieves the charge transfer from anionic band to antibonding metal–oxygen band under a deep delithiation. This restrains cation reduction, metal–oxygen bond fracture, and the formation of localized O2 molecule, which fundamentally inhibits lattice oxygen escape and cation migration. The modulated cathode demonstrates a low voltage decay rate (0.45 millivolt per cycle) and a long cyclic stability. Voltage fade is a critical issue for Li-rich transition metal oxide cathode. Here, the authors modulate the oxygen anionic redox chemistry and enlarges the gap between metallic and anionic states by constructing Li2ZrO3 slabs into Li2MnO3 domain in Li1.21Ni0.28Mn0.51O2 which fundamentally suppresses the voltage decay.
Layered Li-rich transition metal oxides undergo O-redox, involving the oxidation of the O2− ions charge compensated by extraction of Li+ ions. Recent results have shown that for 3d transition metal oxides the oxidized O2− forms molecular O2 trapped in the bulk particles. Other forms of oxidised O2− such as O22− or (O–O)n− with long bonds have been proposed, based especially on work on 4 and 5d transition metal oxides, where TM–O bonding is more covalent. Here, we show, using high resolution RIXS that molecular O2 is formed in the bulk particles on O2‒ oxidation in the archetypal Li-rich ruthenates and iridate compounds, Li2RuO3, Li2Ru0.5Sn0.5O3 and Li2Ir0.5Sn0.5O3. The results indicate that O-redox occurs across 3, 4, and 5d transition metal oxides, forming O2, i.e. the greater covalency of the 4d and 5d compounds still favours O2. RIXS and XAS data for Li2IrO3 are consistent with a charge compensation mechanism associated primarily with Ir redox up to and beyond the 5+ oxidation state, with no evidence of O–O dimerization. In this work, authors show that O-redox in 4d and 5d transition metal oxides involves the formation of molecular oxygen trapped in the particles. These results are in accord with observations in 3d oxides and show that the greater covalency of the 4d and 5d oxides does not stabilise peroxo-like species.
Li‐rich and Li‐excess oxides have been regarded as a promising category of cathode materials for next‐generation Li‐ion batteries due to their high energy density on basis of anionic/cationic redox chemistry. However, the application of Li‐excess oxides suffers from significant problems, such as irreversible lattice oxygen loss and structural distortion. A Li/Na‐ion exchange strategy can be regarded as an effective way to address these obstacles by tuning oxygen stacking arrangements and the coordination environment of alkali metal within emerging structures. Herein, the state‐of‐the‐art progress made within conventional and Li‐excess layered cathodes generated by ion exchange strategy at first is reviewed. Moreover, the mechanisms of oxygen redox, the migrations of Li/transition metals, and structural evolutions within Li‐excess oxides have been further clarified by advanced characterizations, which play an important role in understanding lattice oxygen release, irreversible transition metal migration, phase transition, and the root of capacity/voltage decay. In addition, the theory calculations based on density of states, oxygen release energy, and migration energy barriers of Li/transition metals are systematically summarized, providing an essential guidance to harvest stable oxygen‐related redox reactions within Li‐excess materials. Altogether, this review offers fundamental understanding and future perspectives toward the rational design of high‐energy‐density Li‐excess oxides with reversible anionic/cationic redox chemistry.
Lithium‐rich layered oxides (LLOs) are prospective cathode materials for next‐generation lithium‐ion batteries (LIBs), but severe voltage decay and energy attenuation with cycling still hinder their practical applications. Herein, a series of full concentration gradient‐tailored agglomerated‐sphere LLOs are designed with linearly decreasing Mn and linearly increasing Ni and Co from the particle center to the surface. The gradient‐tailored LLOs exhibit noticeably reduced voltage decay, enhanced rate performance, improved cycle stability, and thermal stability. Without any material modifications or electrolyte optimizations, the gradient‐tailored LLO with medium‐slope shows the best electrochemical performance, with a very low average voltage decay of 0.8 mV per cycle as well as a capacity retention of 88.4% within 200 cycles at 200 mA g⁻¹. These excellent findings are due to spinel structure suppression, electrochemical stress optimization, and Jahn‐Teller effect inhibition. Further investigation shows that the gradient‐tailored LLO reduces the thermal release percentage by as much as about 41% when the battery is charged to 4.4 V. This study provides an effective method to suppress the voltage decay of LLOs for further practical utilization in LIBs and also puts forward a bulk‐structure design strategy to prepare better electrode materials for different rechargeable batteries.
Li-rich layered oxides (LLOs) are fascinating high-energy cathodes for lithium-ion batteries (LIBs), but still suffer from critical drawbacks that retard their practical applications. Although surface modification is effective to protect LLOs from structural deterioration, the delicate design of structures on a grain surface with promising scalability for industrial application is still challenging. Herein, using the atomic layer deposition (ALD) technique, a composite nanostructure comprising a uniform LiTaO3 coating layer (≈3 nm) and a spinel interlayer structure (≈1 nm) is constructed on the grain surface of industrial LLO (Li1.13 Mn0.517 Ni0.256 Co0.097 O2 ) agglomerated spheres. The surface composite nanostructure can not only enhance the structural/interfacial stability of the LLO, but also facilitates Li+ diffusion, thereby significantly improving its cycle stability, rate performance, thermal stability, and voltage maintenance. Specifically, the LLO coated with 10 ALD cycles exhibits a small voltage decay rate of 0.9 mV per cycle, a reversible capacity of 272.8 mAh g-1 at 0.1 C, and a capacity retention of 85% after 200 cycles at 1 C, suggesting the important role of surface composite nanostructure for improving the electrochemical performance. This work provides new insights into the composite nanostructure design on the grain surface of cathode materials for high-performance LIBs.
Li-rich layered oxide cathode materials show high capacities in lithium-ion batteries owing to the contribution of the oxygen redox reaction. However, structural accommodation of this reaction usually results in O-O dimerization, leading to oxygen release and poor electro-chemical performance. In this study, we propose a new structural response mechanism inhibiting O-O dimerization for the oxygen redox reaction by tuning the local symmetry around the oxygen ions. Compared with regular Li 2 RuO 3 , the structural response of the as-prepared local-symmetry-tuned Li 2 RuO 3 to the oxygen redox reaction involves the telescopic O-Ru-O configuration rather than O-O dimerization, which inhibits oxygen release, enabling significantly enhanced cycling stability and negligible voltage decay. This discovery of the new structural response mechanism for the oxygen redox reaction will provide a new scope for the strategy of enhancing the anionic redox stability, paving unexplored pathways toward further development of high capacity Li-rich layered oxides.
High-energy-density lithium-rich materials are of significant interest for advanced lithium-ion batteries, provided that several roadblocks, such as voltage fade and poor energy efficiency are removed. However, this remains challenging as their functioning mechanisms during first cycle are not fully understood. Here we enlarge the cycling potential window for Li1.2Ni0.13Mn0.54Co0.13O2 electrode, identifying novel structural evolution mechanism involving a structurally-densified single-phase A’ formed under harsh oxidizing conditions throughout the crystallites and not only at the surface, in contrast to previous beliefs. We also recover a majority of first-cycle capacity loss by applying a constant-voltage step on discharge. Using highly reducing conditions we obtain additional capacity via a new low-potential P” phase, which is involved into triggering oxygen redox on charge. Altogether, these results provide deeper insights into the structural-composition evolution of Li1.2Ni0.13Mn0.54Co0.13O2 and will help to find measures to cure voltage fade and improve energy efficiency in this class of material.
Despite the high energy density of lithium-rich layered-oxide electrodes, their real-world implementation in batteries is hindered by the substantial voltage decay on cycling. This voltage decay is widely accepted to mainly originate from progressive structural rearrangements involving irreversible transition-metal migration. As prevention of this spontaneous cation migration has proven difficult, a paradigm shift toward management of its reversibility is needed. Herein, we demonstrate that the reversibility of the cation migration of lithium-rich nickel manganese oxides can be remarkably improved by altering the oxygen stacking sequences in the layered structure and thereby dramatically reducing the voltage decay. The preeminent intra-cycle reversibility of the cation migration is experimentally visualized, and first-principles calculations reveal that an O2-type structure restricts the movements of transition metals within the Li layer, which effectively streamlines the returning migration path of the transition metals. Furthermore, we propose that the enhanced reversibility mitigates the asymmetry of the anionic redox in conventional lithium-rich electrodes, promoting the high-potential anionic reduction, thereby reducing the subsequent voltage hysteresis. Our findings demonstrate that regulating the reversibility of the cation migration is a practical strategy to reduce voltage decay and hysteresis in lithium-rich layered materials. The use of high-energy-density lithium-rich layered-oxide electrodes in batteries is hindered by voltage decay on cycling. Improving the reversible cation migration by altering oxygen stacking is shown to suppress voltage decay and redox asymmetry in lithium-rich nickel manganese oxides.
The performance of the recently commissioned spectrometer PEAXIS for resonant inelastic soft X-ray scattering (RIXS) and X-ray photoelectron spectroscopy and its hosting beamline U41-PEAXIS at the BESSY II synchrotron are characterized. The beamline provides linearly polarized light from 180 eV to 1600 eV allowing for RIXS measurements in the range 200–1200 eV. The monochromator optics can be operated in different configurations to provide either high flux with up to 10¹² photons s⁻¹ within the focal spot at the sample or high energy resolution with a full width at half maximum of <40 meV at an incident photon energy of ∼400 eV. The measured total energy resolution of the RIXS spectrometer is in very good agreement with theoretically predicted values obtained by ray-tracing simulations. PEAXIS features a 5 m-long RIXS spectrometer arm that can be continuously rotated about the sample position by 106° within the horizontal photon scattering plane, thus enabling the study of momentum-transfer-dependent excitations. Selected scientific examples are presented to demonstrate the instrument capabilities, including measurements of excitations in single-crystalline NiO and in liquid acetone employing a fluid cell sample manipulator. Planned upgrades of the beamline and the RIXS spectrometer to further increase the energy resolution to ∼100 meV at 1000 eV incident photon energy are discussed.
Alkali-rich transition metal oxide intercalation cathodes can increase the energy density of Li and Na-ion batteries by storing charge on the oxide and transition metal (TM) ions1–10. The high voltage associated with oxidation of O²⁻ on the first charge is not recovered on discharge, resulting in significantly reduced energy density¹¹. Displacement of TM ions from the TM to alkali metal layers has been proposed to explain the first cycle voltage loss9,12–16. By comparing two closely related intercalation cathodes, Na0.75[Li0.25Mn0.75]O2 and Na0.6[Li0.2Mn0.8]O2, we show that the first cycle voltage hysteresis is determined by the superstructure, specifically the local ordering of Li/TM ions in the TM layers. The honeycomb superstructure of Na0.75[Li0.25Mn0.75]O2, present in almost all O-redox compounds, is lost on charging, driven, in part, by formation of molecular O2 inside the solid. The O2 molecules are cleaved on discharge reforming O²⁻, but the Mn ions have migrated in the plane changing the coordination around O²⁻ significantly lowering the voltage on discharge (hysteresis). The ribbon superstructure in Na0.6[Li0.2Mn0.8]O2 inhibits Mn disorder and hence O2 formation, suppressing hysteresis and promoting stable electron holes on O²⁻ which are revealed by XAS. The results show that voltage hysteresis can be avoided in O-redox cathodes by forming materials with a superstructure (cation ordering) in the TM layers that suppresses TM migration.
0.5Li2MnO3∙0.5LiNi0.5Mn0.3Co0.2O2 cathode materials with nano‐MgO and nano‐TiO2 coating layers are synthesized via a facile approach. These nanocoating layers with thickness of approximately 20 nm are homogeneously coated on the surface of the materials. With the nano‐MgO and nano‐TiO2 coating layers, the modified materials deliver excellent electrochemical performance. Nano‐MgO‐coated and nano‐TiO2‐coated Li‐rich materials exhibit 147.56 and 137.96 mAh g−1 after 100 cycles at 1 C rate and deliver 156.13 and 130.69 mAh g−1 at 2 C rate, respectively, in the voltage range of 2.0–4.8 V. The materials with nano‐MgO and nano‐TiO2 coating layers also show high discharge voltage retention of more than 90% after 50 cycles, low impedance for Li+ migration, and high diffusion coefficients of the lithium ions. Nano‐MgO and nano‐TiO2 coating layers can help decrease the content of Li2CO3 on the surface of the materials and suppress side reaction. Thus, these materials play a significant role in strengthening the electrochemical performance of Li‐rich cathode materials.
Recent debates on the oxygen redox behaviors in battery electrodes have triggered a pressing demand for the reliable detection and understanding of non-divalent oxygen states beyond conventional absorption spectroscopy. Here, enabled by high-efficiency mapping of resonant inelastic X-ray scattering (mRIXS) coupled with first-principles calculations, we report distinct mRIXS features of the oxygen states in Li2O, Li2CO3, and especially, Li2O2, which are successfully reproduced and interpreted theoretically. mRIXS signals are dominated by valence-band decays in Li2O and Li2CO3. However, the oxidized oxygen in Li2O2 leads to partially unoccupied O-2p states that yield a specific intra-band excitonic feature in mRIXS. Such a feature displays a specific emission energy in mRIXS, which disentangles the oxidized oxygen states from the dominating transition-metal/oxygen hybridization features in absorption spectroscopy, thus providing critical hints for both detecting and understanding the oxygen redox reactions in transition-metal oxide based battery materials.
Voltage fade is a major problem in battery applications for high-energy lithium- and manganese-rich (LMR) layered materials. As a result of the complexity of the LMR structure, the voltage fade mechanism is not well understood. Here we conduct both in situ and ex situ studies on a typical LMR material (Li1.2Ni0.15Co0.1Mn0.55O2) during charge–discharge cycling, using multi-length-scale X-ray spectroscopic and three-dimensional electron microscopic imaging techniques. Through probing from the surface to the bulk, and from individual to whole ensembles of particles, we show that the average valence state of each type of transition metal cation is continuously reduced, which is attributed to oxygen release from the LMR material. Such reductions activate the lower-voltage Mn³⁺/Mn⁴⁺ and Co²⁺/Co³⁺ redox couples in addition to the original redox couples including Ni²⁺/Ni³⁺, Ni³⁺/Ni⁴⁺ and O²⁻/O⁻, directly leading to the voltage fade. We also show that the oxygen release causes microstructural defects such as the formation of large pores within particles, which also contributes to the voltage fade. Surface coating and modification methods are suggested to be effective in suppressing the voltage fade through reducing the oxygen release.
Reversible anionic redox has rejuvenated the search for high-capacity lithium-ion battery cathodes. Real-world success necessitates the holistic mastering of this electrochemistry's kinetics, thermodynamics, and stability. Here we prove oxygen redox reactivity in the archetypical lithium- and manganese-rich layered cathodes through bulk-sensitive synchrotron-based spectroscopies, and elucidate their complete anionic/cationic charge-compensation mechanism. Furthermore, via various electroanalytical methods, we answer how the anionic/cationic interplay governs application-wise important issues-namely sluggish kinetics, large hysteresis, and voltage fade-that afflict these promising cathodes despite widespread industrial and academic efforts. We find that cationic redox is kinetically fast and without hysteresis unlike sluggish anions, which furthermore show different oxidation vs. reduction potentials. Additionally, more time spent with fully oxidized oxygen promotes voltage fade. These fundamental insights about anionic redox are indispensable for improving lithium-rich cathodes. Moreover, our methodology provides guidelines for assessing the merits of existing and future anionic redox-based high-energy cathodes, which are being discovered rapidly.
Employing lithium-rich oxides as cathode materials offers an ideal option for achieving high-energy all-solid-state lithium batteries (ASSLBs), while lithium-rich cathode materials are usually hard to directly match with solid-state electrolytes...
Element doping/substitution has been recognized as an effective strategy to enhance the structural stability of layered cathodes. However, abundant substitution studies not only lack a clear identification of the substitution sites in the material lattice, but the rigid interpretation of the transition metal (TM)-O covalent theory is also not sufficiently convincing, resulting in the doping/substitution proposals being dragged into design blindness. In this work, taking Li1.2Ni0.2Mn0.6O2 as a prototype, the intense correlation between the "disordered degree" (Li/Ni mixing) and interface-structure stability (e.g., TM-O environment, slab/lattice, and Li+ reversibility) is revealed. Specifically, the degree of disorder induced by the Mg/Ti substitution extends in the opposite direction, conducive to sharp differences in the stability of TM-O, Li+ diffusion, and anion redox reversibility, delivering fairly distinct electrochemical performance. Based on the established paradigm of systematic characterization/analysis, the "degree of disorder" has been shown to be a powerful indicator of material modification by element substitution/doping.
The prominent problem with graphite anodes in practical applications is the detrimental Li plating, resulting in rapid capacity fade and safety hazards. Herein, secondary gas evolution behavior during the Li-plating process was monitored by online electrochemical mass spectrometry (OEMS), and the onset of local microscale Li plating on the graphite anode was precisely/explicitly detected in situ/operando for early safety warnings. The distribution of irreversible capacity loss (e.g., primary and secondary solid electrolyte interface (SEI), dead Li, etc.) under Li-plating conditions was accurately quantified by titration mass spectroscopy (TMS). Based on OEMS/TMS results, the effect of typical VC/FEC additives was recognized at the level of Li plating. The nature of vinylene carbonate (VC)/fluoroethylene carbonate (FEC) additive modification is to enhance the elasticity of primary and secondary SEI by adjusting organic carbonates and/or LiF components, leading to less "dead Li" capacity loss. Though VC-containing electrolyte greatly suppresses the H2/C2H4 (flammable/explosive) evolution during Li plating, more H2 is released from the reductive decomposition of FEC.
Layered Li-rich oxides (LROs) that exhibit anionic and cationic redox are extensively studied due to their high energy storage capacities. However, voltage hysteresis, which reduces the energy conversion efficiency of the battery, is a critical limitation in the commercial application of LROs. Herein, using two Li2RuO3 (LRO) model materials with C2/c and P21/m symmetries, we explored the relationship between voltage hysteresis and the electronic structure of Li2RuO3 by neutron diffraction, in situ X-ray powder diffraction, X-ray absorption spectroscopy, macro magnetic study, and electron paramagnetic resonance (EPR) spectroscopy. The charge-transfer band gap of the LRO cathode material with isolated eg electron filling decreases, reducing the oxidation potential of anion redox and thus displaying a reduced voltage hysteresis. We further synthesized Mn-based Li-rich cathode materials with practical significance and different electron spin states. Low-spin Li1.15Ni0.377Mn0.473O2 with isolated eg electron filling exhibited a reduced voltage hysteresis and high energy conversion efficiency. We rationalized this finding via density functional theory calculations. This discovery should provide critical guidance in designing and preparing high-energy layered Li-rich cathode materials for use in next-generation high-energy-density Li-ion batteries based on anion redox activity.
Layered oxide cathodes with high Ni content promise high energy density and competitive cost for Li-ion batteries (LIBs). However, Ni-rich cathodes suffer from irreversible interface reconstruction and undesirable cracking with severe performance degradation upon long-term operation, especially at elevated temperatures. Herein, we demonstrate in situ surface engineering of Ni-rich cathodes to construct a dual ion/electron-conductive NiTiO3 coating layer and Ti gradient doping ([email protected]) in parallel. The dual-modification synergy helps to build a thin, robust cathode–electrolyte interface with rapid Li-ion transport and enhanced reaction kinetics, and effectively prevents unfavorable crystalline phase transformation during long-term cycling under harsh environments. The optimized [email protected] delivers a high reversible capacity of 221.0 mAh g–1 at 0.1 C and 158.9 mAh g–1 at 10 C. Impressively, it exhibits a capacity retention of 88.4% at 25 °C after 500 cycles and 90.7% at 55 °C after 300 cycles in a pouch-type full battery. This finding provides viable clues for stabilizing the lattice and interfacial chemistry of Ni-rich cathodes to achieve durable LIBs with high energy density.
In the practical operations of Li-ion batteries, inevitable deep charge/discharge happens locally due to the intrinsic (de)lithiation inhomogeneity at the electrode and particle level, which would damage the health of batteries and even cause the safety concern. It is essential to develop the stable cathodes operating in a wide voltage window to ensure the health and safety of Li-ion batteries. Herein, we comprehensively investigate the charge/discharge behaviors of a representative Li-rich cathode Li1.2Mn0.54Ni0.13Co0.13O2 in a wide voltage window of 1.0–4.8 V, and reveal that, deep-lithiation would drive violent TM migration and severe Li/TM mixing, thereby leading to the irreversible structural transformation from layered to spinel then to rock salt, eventually causing the fast decay in electrochemical performance. Based on these understandings, a novel Li-rich cathode Li[Li1/4Mn1/2Ni1/6Sb1/12]O2 is successfully synthesized through introducing aromatic [email protected]6 superstructure units in the TM layers. The introduced [email protected]6 superstructure units can effectively tune the local oxygen environment, suppress TM migration, and stabilize the layered framework under deep lithiation. Finally, a stable charge/discharge is achieved in 1.0–4.8 V. This work deepens the understanding into the structural stability of Li-rich cathodes in a wide voltage window, and benefits the development of high-energy-density and safe cathodes.
The search for batteries with high energy density has highlighted lithium-rich manganese-based layered oxides due to their excep-tionally high capacity. Although it is clear that both cationic and anionic redox are present in the charge compensation mechanism, the microstructural evolution of the Li2MnO3-like phase during anionic redox and its role in battery performance and structural sta-bility are still not fully understood. Here, we systematically probe microstructural evolution using spatially resolved synchrotron X-ray measurements, and reveal an underlying interaction between the Li2MnO3-like domains and bulk rhombohedral structure. Mn ion activation and a previously unobserved structural distortion are discovered at high voltages, and can be related to structural strain present in the Li2MnO3-like phase upon substantial lithium ion extraction. Moreover, we elucidate a correlation between this structural distortion and irreversible phase transitions by thermally perturbing delithiated samples. These insights highlight a path-way towards achieving high capacity cathode materials required for future commercial applications.
Lithium-rich layered oxides (LLO), as the most attractive cathode materials for high-energy lithium-ion batteries (LIBs), are plagued by poor cyclability due to structural and electrode/electrolyte interface instability. Herein, we report the synthesis of LLO and its performance enhancement by using boron-containing electrolyte additives. In a formulated 1.0 M LiPF6 ethylene carbonate/ethyl methyl carbonate electrolyte with 0.1 M lithium bis(oxalato)borate (LiBOB), the battery assembled with Li1.2Mn0.54Co0.13Ni0.13O2 microspheres presents a stable specific capacity of 202 mAh g-1 at 0.5 C and a remarkable capacity retention of 96.4% after 100 cycles, significantly outperforming the cathode in baseline electrolyte without LiBOB. The combination of voltammetry, impedance, microscopy, spectroscopic analysis and density functional theory (DFT) calculations corroborates the beneficial effect of LiBOB on stabilizing the LLO/electrolyte interface. Reactions between LiBOB and activated oxygen radicals result in the formation of a dense cathode electrolyte interface (CEI) film (~15 nm) containing oxalate, lithium fluoride and alkyl borate species, which contributes to suppression of capacity/voltage decay of LLO. These results would provide insight in understanding the effect of boron-containing electrolyte additive on upgrading high-capacity Li-rich cathode materials.
Conventional intercalation cathodes for lithium batteries store charge in redox reactions associated with the transition metal cations, e.g. Mn3+/4+ in LiMn2O4, and this limits the energy storage of Li-ion batteries. Compounds such as Li[Li0.2Ni0.2Mn0.6]O2 exhibit a capacity to store charge in excess of the transition metal redox reactions. The additional capacity occurs at and above 4.5 V vs. Li+/Li. The capacity at 4.5 V is dominated by oxidation of the O2- anions accounting for ~0.43 e-/formula unit, with an additional 0.06 e-/formula unit being associated with O loss from the lattice. In contrast, the capacity above 4.5 V, is mainly O loss, ~ 0.08 e-/formula. The O redox reaction involves the formation of localized hole states on O during charge, which are located on O coordinated by (Mn4+/Li+). The results have been obtained by combining operando electrochemical mass spec on 18O labelled Li[Li0.2Ni0.2Mn0.6]O2 with XANES, soft X-ray spectroscopy, Resonant Inelastic X-ray spectroscopy and Raman spectroscopy. Finally the general features of O-redox are described with discussion about the role of comparatively ionic (less covalent) 3d metal-oxygen interaction on anion redox in lithium rich cathode materials.