Electrochemomechanical failure in layered oxide cathodes caused by rotational stacking faults.

Electrochemomechanical degradation is one of the most common causes of capacity deterioration in high-energy-density cathodes, particularly intercalation-based layered oxides. Here we reveal the presence of rotational stacking faults (RSFs) in layered lithium transition-metal oxides, arising from specific stacking sequences at different angles, and demonstrate their critical role in determining structural/electrochemical stability. Our combined experiments and calculations show that RSFs facilitate oxygen dimerization and transition-metal migration in layered oxides, fostering microcrack nucleation/propagation concurrently with cumulative electrochemomechanical degradation on cycling. We further show that thermal defect annihilation as a potential solution can suppress RSFs, reducing microcracks and enhancing cyclability in lithium-rich layered cathodes. The common but previously overlooked occurrence of RSFs suggests a new synthesis guideline of high-energy-density layered oxide cathodes.

Structurally robust lithium-rich layered oxides for high-energy and long-lasting cathodes.

O2-type lithium-rich layered oxides, known for mitigating irreversible transition metal migration and voltage decay, provide suitable framework for exploring the inherent properties of oxygen redox. Here, we present a series of O2-type lithium-rich layered oxides exhibiting minimal structural disordering and stable voltage retention even with high anionic redox participation based on the nominal composition. Notably, we observe a distinct asymmetric lattice breathing phenomenon within the layered framework driven by excessive oxygen redox, which includes substantial particle-level mechanical stress and the microcracks formation during cycling. This chemo-mechanical degradation can be effectively mitigated by balancing the anionic and cationic redox capabilities, securing both high discharge voltage (~ 3.43 V vs. Li/Li+) and capacity (~ 200 mAh g-1) over extended cycles. The observed correlation between the oxygen redox capability and the structural evolution of the layered framework suggests the distinct intrinsic capacity fading mechanism that differs from the previously proposed voltage fading mode.

Design of a trigonal halide superionic conductor by regulating cation order-disorder.

Lithium-metal-halides have emerged as a class of solid electrolytes that can deliver superionic conductivity comparable to that of state-of-the-art sulfide electrolytes, as well as electrochemical stability that is suitable for high-voltage (>4 volt) operations. We show that the superionic conduction in a trigonal halide, such as Li3MCl6 [where metal (M) is Y or Er], is governed by the in-plane lithium percolation paths and stacking interlayer distance. These two factors are inversely correlated with each other by the partial occupancy of M, serving as both a diffusion inhibitor and pillar for maintaining interlayer distance. These findings suggest that a critical range or ordering of M exists in trigonal halides, and we showcase the achievement of high ionic conductivity by adjusting the simple M ratio (per Cl or Li). We provide general design criteria for superionic trigonal halide electrolytes.

All-Graphene Quantum Dot-Derived Battery: Regulating Redox Activity Through Localized Subdomains.

In the quest for materials sustainability for grid-scale applications, graphene quantum dot (GQD), prepared via eco-efficient processes, is one of the promising graphitic-organic matters that have the potential to provide greener solutions for replacing metal-based battery electrodes. However, the utilization of GQDs as electroactive materials has been limited; their redox behaviors associated with the electronic bandgap property from the sp2 carbon subdomains, surrounded by functional groups, are yet to be understood. Here, the experimental realization of a subdomained GQD-based anode with stable cyclability over 1000 cycles, combined with theoretical calculations, enables a better understanding of the decisive impact of controlled redox site distributions on battery performance. The GQDs are further employed in cathode as a platform for full utilization of inherent electrochemical activity of bio-inspired redox-active organic motifs, phenoxazine. Using the GQD-derived anode and cathode, an all-GQD battery achieves a high energy density of 290 Wh kgcathode -1 (160 Wh kgcathode+anode -1 ), demonstrating an effective way to improve reaction reversibility and energy density of sustainable, metal-free batteries.

Slab gliding, a hidden factor that induces irreversibility and redox asymmetry of lithium-rich layered oxide cathodes.

Lithium-rich layered oxides, despite their potential as high-energy-density cathode materials, are impeded by electrochemical performance deterioration upon anionic redox. Although this deterioration is believed to primarily result from structural disordering, our understanding of how it is triggered and/or occurs remains incomplete. Herein, we propose a theoretical picture that clarifies the irreversible transformation and redox asymmetry of lithium-rich layered oxides by introducing a series of global and local dynamic structural evolution processes involving slab gliding and transition-metal migration. We show that slab gliding plays a key role in trigger/initiating the structural disordering and consequent degradation of the anionic redox reaction. We further reveal that the 'concerted disordering mechanism' of slab gliding and transition-metal migration produces spontaneously irreversible/asymmetric lithiation and de-lithiation pathways, causing irreversible structural deterioration and the asymmetry of the anionic redox reaction. Our findings suggest slab gliding as a crucial, yet underexplored, method for achieving a reversible anionic redox reaction.

Aging Property of Halide Solid Electrolyte at the Cathode Interface.

Halide solid electrolytes have recently emerged as a promising option for cathode-compatible catholytes in solid-state batteries (SSBs), owing to their superior oxidation stability at high voltage and their interfacial stability. However, their day- to month-scale aging at the cathode interface has remained unexplored until now, while its elucidation is indispensable for practical deployment. Herein, the stability of halide solid electrolytes (e.g., Li3 InCl6 ) when used with conventional layered oxide cathodes during extended calendar aging is investigated. It is found that, contrary to their well-known oxidation stability, halide solid electrolytes can be vulnerable to reductive side reactions with oxide cathodes (e.g., LiNi0.8 Co0.1 Mn0.1 O2 ) in the long term. More importantly, the calendar aging at a low state of charge or as-fabricated state causes more significant degradation than at a high state of charge, in contrast to typical lithium-ion batteries, which are more susceptible to high-state-of-charge calendar aging. This unique characteristic of halide-based SSBs is related to the reduction propensity of metal ions in halide solid electrolytes and correlated to the formation of an interphase due to the reductive decomposition triggered by the oxide cathode in a lithiated state. This understanding of the long-term aging properties provides new guidelines for the development of cathode-compatible halide solid electrolytes.

Two-Terminal Lithium-Mediated Artificial Synapses with Enhanced Weight Modulation for Feasible Hardware Neural Networks.

Recently, artificial synapses involving an electrochemical reaction of Li-ion have been attributed to have remarkable synaptic properties. Three-terminal synaptic transistors utilizing Li-ion intercalation exhibits reliable synaptic characteristics by exploiting the advantage of non-distributed weight updates owing to stable ion migrations. However, the three-terminal configurations with large and complex structures impede the crossbar array implementation required for hardware neuromorphic systems. Meanwhile, achieving adequate synaptic performances through effective Li-ion intercalation in vertical two-terminal synaptic devices for array integration remains challenging. Here, two-terminal Au/LixCoO2/Pt artificial synapses are proposed with the potential for practical implementation of hardware neural networks. The Au/LixCoO2/Pt devices demonstrated extraordinary neuromorphic behaviors based on a progressive dearth of Li in LixCoO2 films. The intercalation and deintercalation of Li-ion inside the films are precisely controlled over the weight control spike, resulting in improved weight control functionality. Various types of synaptic plasticity were imitated and assessed in terms of key factors such as nonlinearity, symmetricity, and dynamic range. Notably, the LixCoO2-based neuromorphic system outperformed three-terminal synaptic transistors in simulations of convolutional neural networks and multilayer perceptrons due to the high linearity and low programming error. These impressive performances suggest the vertical two-terminal Au/LixCoO2/Pt artificial synapses as promising candidates for hardware neural networks.

Revisiting Lithium- and Sodium-Ion Storage in Hard Carbon Anodes.

The galvanostatic lithiation/sodiation voltage profiles of hard carbon anodes are simple, with a sloping drop followed by a plateau. However, a precise understanding of the corresponding redox sites and storage mechanisms is still elusive, which hinders further development in commercial applications. Here, a comprehensive comparison of the lithium- and sodium-ion storage behaviors of hard carbon is conducted, yielding the following key findings: 1) the sloping voltage section is presented by the lithium-ion intercalation in the graphitic lattices of hard carbons, whereas it mainly arises from the chemisorption of sodium ions on their inner surfaces constituting closed pores, even if the graphitic lattices are unoccupied; 2) the redox sites for the plateau capacities are the same as those for the closed pores regardless of the alkali ions; 3) the sodiation plateau capacities are mostly determined by the volume of the available closed pore, whereas the lithiation plateau capacities are primarily affected by the intercalation propensity; and 4) the intercalation preference and the plateau capacity have an inverse correlation. These findings from extensive characterizations and theoretical investigations provide a relatively clear elucidation of the electrochemical footprint of hard carbon anodes in relation to the redox mechanisms and storage sites for lithium and sodium ions, thereby providing a more rational design strategy for constructing better hard carbon anodes.

Design of a lithiophilic and electron-blocking interlayer for dendrite-free lithium-metal solid-state batteries.

All-solid-state batteries are a potential game changer in the energy storage market; however, their practical employment has been hampered by premature short circuits caused by the lithium dendritic growth through the solid electrolyte. Here, we demonstrate that a rational layer-by-layer strategy using a lithiophilic and electron-blocking multilayer can substantially enhance the performance/stability of the system by effectively blocking the electron leakage and maintaining low electronic conductivity even at high temperature (60°C) or under high electric field (3 V) while sustaining low interfacial resistance (13.4 ohm cm2). It subsequently results in a homogeneous lithium plating/stripping, thereby aiding in achieving one of the highest critical current densities (~3.1 mA cm-2) at 60°C in a symmetric cell. A full cell paired with a commercial-level cathode exhibits exceptionally long durability (>3000 cycles) and coulombic efficiency (99.96%) at a high current density (2 C; ~1.0 mA cm-2), which records the highest performance among all-solid-state lithium metal batteries reported to date.

In situ multiscale probing of the synthesis of a Ni-rich layered oxide cathode reveals reaction heterogeneity driven by competing kinetic pathways.

Nickel-rich layered oxides are envisaged as key near-future cathode materials for high-energy lithium-ion batteries. However, their practical application has been hindered by their inferior cycle stability, which originates from chemo-mechanical failures. Here we probe the solid-state synthesis of LiNi0.6Co0.2Mn0.2O2 in real time to better understand the structural and/or morphological changes during phase evolution. Multi-length-scale observations-using aberration-corrected transmission electron microscopy, in situ heating transmission electron microscopy and in situ X-ray diffraction-reveal that the overall synthesis is governed by the kinetic competition between the intrinsic thermal decomposition of the precursor at the core and the topotactic lithiation near the interface, which results in spatially heterogeneous intermediates. The thermal decomposition leads to the formation of intergranular voids and intragranular nanopores that are detrimental to cycling stability. Furthermore, we demonstrate that promoting topotactic lithiation during synthesis can mitigate the generation of defective structures and effectively suppress the chemo-mechanical failures.

High-energy and durable lithium metal batteries using garnet-type solid electrolytes with tailored lithium-metal compatibility.

Lithium metal batteries using solid electrolytes are considered to be the next-generation lithium batteries due to their enhanced energy density and safety. However, interfacial instabilities between Li-metal and solid electrolytes limit their implementation in practical batteries. Herein, Li-metal batteries using tailored garnet-type Li7-xLa3-aZr2-bO12 (LLZO) solid electrolytes is reported, which shows remarkable stability and energy density, meeting the lifespan requirements of commercial applications. We demonstrate that the compatibility between LLZO and lithium metal is crucial for long-term stability, which is accomplished by bulk dopant regulating and dopant-specific interfacial treatment using protonation/etching. An all-solid-state with 5 mAh cm-2 cathode delivers a cumulative capacity of over 4000 mAh cm-2 at 3 mA cm-2, which to the best of our knowledge, is the highest cycling parameter reported for Li-metal batteries with LLZOs. These findings are expected to promote the development of solid-state Li-metal batteries by highlighting the efficacy of the coupled bulk and interface doping of solid electrolytes.

Coupling structural evolution and oxygen-redox electrochemistry in layered transition metal oxides.

Lattice oxygen redox offers an unexplored way to access superior electrochemical properties of transition metal oxides (TMOs) for rechargeable batteries. However, the reaction is often accompanied by unfavourable structural transformations and persistent electrochemical degradation, thereby precluding the practical application of this strategy. Here we explore the close interplay between the local structural change and oxygen electrochemistry during short- and long-term battery operation for layered TMOs. The substantially distinct evolution of the oxygen-redox activity and reversibility are demonstrated to stem from the different cation-migration mechanisms during the dynamic de/intercalation process. We show that the π stabilization on the oxygen oxidation initially aids in the reversibility of the oxygen redox and is predominant in the absence of cation migrations; however, the π-interacting oxygen is gradually replaced by σ-interacting oxygen that triggers the formation of O-O dimers and structural destabilization as cycling progresses. More importantly, it is revealed that the distinct cation-migration paths available in the layered TMOs govern the conversion kinetics from π to σ interactions. These findings constitute a step forward in unravelling the correlation between the local structural evolution and the reversibility of oxygen electrochemistry and provide guidance for further development of oxygen-redox layered electrode materials.

In operando visualization of redox flow battery in membrane-free microfluidic platform.

Redox flow batteries (RFBs) are attractive large-scale energy storage techniques, achieving remarkable progress in performance enhancement for the last decades. Nevertheless, an in-depth understanding of the reaction mechanism still remains challenging due to its unique operation mechanism, where electrochemistry and hydrodynamics simultaneously govern battery performance. Thus, to elucidate the precise reactions occurring in RFB systems, an appropriate analysis technique that enables the real-time observation of electrokinetic phenomena is indispensable. Herein, we report in operando visualization and analytical study of RFBs by employing a membrane-free microfluidic platform, that is, a membrane-free microfluidic RFB. Using this platform, the electrokinetic investigations were carried out for the 5,10-bis(2-methoxyethyl)-5,10-dihydrophenazine (BMEPZ) catholyte, which has been recently proposed as a high-performance multiredox organic molecule. Taking advantage of the inherent colorimetric property of BMEPZ, we unravel the intrinsic electrochemical properties in terms of charge and mass transfer kinetics during the multiredox reaction through in operando visualization, which enables theoretical study of physicochemical hydrodynamics in electrochemical systems. Based on insights on the electrokinetic limitations in RFBs, we verify the validity of electrode geometry design that can suppress the range of the depletion region, leading to enhanced cell performance.

Challenges and Strategies towards Practically Feasible Solid-State Lithium Metal Batteries.

Remarkable improvement of the ionic conductivity of inorganic solid electrolytes (SEs) exceeding 10 mS cm-1 at room temperature has opened up the opportunities to realize the commercialization of solid-state batteries (SSBs). The transition to the intrinsically inflammable SEs also promises that SSBs would successfully utilize lithium metal anode thus achieving the high-energy-density lithium metal batteries without the risk of a safety hazard. However, the practical operation of solid-state lithium metal batteries (SSLMBs) still faces the challenges of the poor cycle stability and the low energy efficiency, which are coupled with the interface stability and even with the dendrite growth of lithium metal. This article overviews current understandings regarding the underlying origins of the issues in employing the lithium metal anode in SSLMBs from the five main standpoints: i) the chemical/electrochemical interfacial stability, ii) the microscopic evolution of interfacial morphology, iii) the intrinsic diffusivity of lithium atom/vacancy at the interface, iv) imperfections (defect/pores), and v) non-negligible electronic conductivity of SEs. The discussions are followed on the state-of-the-art efforts and strategies to overcome these respective challenges. Finally, the authors provide their perspectives for the future research directions toward achieving the commercial level of high-energy SSLMBs.

Non-Electrode Components for Rechargeable Aqueous Zinc Batteries: Electrolytes, Solid-Electrolyte-Interphase, Current Collectors, Binders, and Separators.

Rechargeable aqueous zinc batteries (AZBs) are one of the promising options for large-scale electrical energy storage owing to their safety, affordability and environmental friendliness. During the past decade, there have been remarkable advancements in the AZBs technology, which are achieved through intensive efforts not only in the area of electrode materials but also in the fundamental understandings of non-electrode components such as electrolytes, solid electrolyte interphase (SEI), current collectors, binders, and separators. In particular, the breakthroughs in the non-electrode components should not be underestimated in having enabled the AZBs to attain a higher energy and power density beyond that of the conventional AZBs, proving their critical role. In this article, the recent research progress is comprehensively reviewed with respect to non-electrode components in AZBs, covering the new-type of electrolytes that have been introduced, attempts for the tailoring of SEI, and the design efforts for multi-functional current collectors, binders and separators, along with the remaining challenges associated with these non-electrode components. Finally, perspectives are discussed toward future research directions in this field. This extensive overview on the non-electrode components is expected to guide and spur further development of high-performance AZBs.

Versatile Redox-Active Organic Materials for Rechargeable Energy Storage.

With the ever-increasing demand on energy storage systems and subsequent mass production, there is an urgent need for the development of batteries with not only improved electrochemical performance but also better sustainability-related features such as environmental friendliness and low production cost. To date, transition metals that are sparse have been centrally employed in energy storage devices ranging from portable lithium ion batteries (e.g., cobalt and nickel) to large-scale redox flow batteries (e.g., vanadium). Toward the sustainable battery chemistry, there are ongoing efforts to replace the transition metal-based electrode materials in these systems to redox-active organic materials (ROMs). Most ROMs are composed of the earth abundant elements (e.g., carbon, nitrogen, oxygen, sulfur), thus are less restrained by the resource, and their production does not require high-energy consuming processes. Furthermore, the structural diversity and chemical tunability of organic compounds make them more attractive for the versatile design of future energy storage systems. Accordingly, the timely development of high-performance ROM-based electrodes would expedite the shift from the current resource-limited battery chemistry to more sustainable energy solutions.In this Account, we provide an overview of the endeavors to employ and develop ROMs as high-performance active materials for various battery systems. Diverse approaches will be introduced starting from the new ROM design mimicking the energy carrying molecules in biological metabolism to the chemical modifications to tailor the properties for specific battery systems. The molecular redesign of ROM, for example, can be carried out by substituting heteroatoms in the redox center, which leads to the enhancement of the redox potential by the inductive effect. Or, tailoring the ROM molecule by removing redox-inactive functionals results in a reduced molecular weight, thereby an increased specific capacity. The intrinsic limitations of ROMs, such as the low electrical conductivity and the dissolving nature, have been under extensive scrutiny; however, they can be partly addressed through efforts including intermolecular fusion and/or nanoscale hybridization with a conducting scaffold. On the other hand, this problematic dissolving nature of ROMs makes them appealing for some new battery configurations such as redox flow batteries that employ the liquid-state active materials. The high solubility and the stability of the ROM were found to be beneficial in attaining the enhanced energy density and the cycle stability of flow batteries, which could be further optimized by the chemical modifications of ROMs. Besides the role of active materials, the redox activity of ROMs has also enabled their use as catalysts to promote the electrode reaction in metal-air batteries. The redox capability of the ROM was often proven to be effective in the solution-based redox mediation that facilitates both the charging and discharging reaction in metal-air batteries. Finally, we conclude this account by proposing the future research directions regarding the fundamental electrochemistry and the further practical development of ROMs for the sustainable rechargeable energy storage.

Pyrrolinium-Substituted Persistent Zwitterionic Ferrocenate Derivative Enabling the Application of Ferrocene Anolyte.

Here, we report the imidazolium-/pyrrolinium-substituted persistent zwitterionic ferrocenate derivatives, which were characterized by electron paramagnetic resonance (EPR) and 57Fe Mössbauer spectroscopy. Additional theoretical studies on these zwitterionic ferrocenate derivatives clearly explain the origin of their thermal stability and the orbital interactions between iron and imidazolium-/pyrrolinium-substituted zwitterionic cyclopentadienyl ligand. Exploiting the facile Fe(II/I) redox chemistry, we successfully demonstrated that the pyrrolinium-substituted ferrocene derivative can be applied as an example of derivatized ferrocene anolyte for redox-flow batteries. These zwitterionic ferrocenate derivatives will not only deepen our understanding of the intrinsic chemistry of ferrocenate but have the potential to open the way for the rational design of metallocenate derivatives for various applications.

A new high-voltage calcium intercalation host for ultra-stable and high-power calcium rechargeable batteries.

Rechargeable calcium batteries have attracted increasing attention as promising multivalent ion battery systems due to the high abundance of calcium. However, the development has been hampered by the lack of suitable cathodes to accommodate the large and divalent Ca2+ ions at a high redox potential with sufficiently fast ionic conduction. Herein, we report a new intercalation host which presents 500 cycles with a capacity retention of 90% and a remarkable power capability at ~3.2 V (vs. Ca/Ca2+) in a calcium battery. The cathode material derived from Na0.5VPO4.8F0.7 is demonstrated to reversibly accommodate a large amount of Ca2+ ions, forming a series of CaxNa0.5VPO4.8F0.7 (0 < x < 0.5) phases without any noticeable structural degradation. The robust framework enables one of the smallest volume changes (1.4%) and the lowest diffusion barriers for Ca2+ among the cathodes reported to date, offering the basis for the outstanding cycle life and power capability.

A Biodegradable Secondary Battery and its Biodegradation Mechanism for Eco-Friendly Energy-Storage Systems.

The production of rechargeable batteries is rapidly expanding, and there are going to be new challenges in the near future about how the potential environmental impact caused by the disposal of the large volume of the used batteries can be minimized. Herein, a novel strategy is proposed to address these concerns by applying biodegradable device technology. An eco-friendly and biodegradable sodium-ion secondary battery (SIB) is developed through extensive material screening followed by the synthesis of biodegradable electrodes and their seamless assembly with an unconventional biodegradable separator, electrolyte, and package. Each battery component decomposes in nature into non-toxic compounds or elements via hydrolysis and/or fungal degradation, with all of the biodegradation products naturally abundant and eco-friendly. Detailed biodegradation mechanisms and toxicity influence of each component on living organisms are determined. In addition, this new SIB delivers performance comparable to that of conventional non-degradable SIBs. The strategy and findings suggest a novel eco-friendly biodegradable paradigm for large-scale rechargeable battery systems.

Permselective metal-organic framework gel membrane enables long-life cycling of rechargeable organic batteries.

Rechargeable organic batteries show great potential as a low-cost, sustainable and mass-producible alternatives to current transition-metal-based cells; however, serious electrode dissolution issues and solubilization of organic redox intermediates (shuttle effect) have plagued the capacity retention and cyclability of these cells. Here we report on the use of a metal-organic framework (MOF) gel membrane as a separator for organic batteries. The homogeneous micropores, intrinsic of the MOF-gel separator, act as permselective channels for targeted organic intermediates, thereby mitigating the shuttling problem without sacrificing power. A battery using a MOF-gel separator and 5,5'-dimethyl-2,2'-bis-p-benzoquinone (Me2BBQ) as the electrode displays high cycle stability with capacity retention of 82.9% after 2,000 cycles, corresponding to a capacity decay of ~0.008% per cycle, with a discharge capacity of ~171 mA h g-1 at a current density of 300 mA g-1. The molecular and ionic sieving capabilities of MOF-gel separators promise general applicability, as pore size can be tuned to specific organic electrode materials. The use of MOF-gel separators to prevent side reactions of soluble organic redox intermediates could lead to the development of rechargeable organic batteries with high energy density and long cycling life.