Bioinspired Redox‐Active Catechol‐Bearing Polymers as Ultrarobust Organic Cathodes for Lithium Storage

Redox‐active catechols are bioinspired precursors for ortho ‐quinones that are characterized by higher discharge potentials than para ‐quinones, the latter being extensively used as organic cathode materials for lithium ion batteries (LIBs). Here, this study demonstrates that the rational molecular design of copolymers bearing catechol‐ and Li⁺ ion‐conducting anionic pendants endow redox‐active polymers (RAPs) with ultrarobust electrochemical energy storage features when combined to carbon nanotubes as a flexible, binder‐, and metal current collector‐free buckypaper electrode. The importance of the structure and functionality of the RAPs on the battery performances in LIBs is discussed. The structure‐optimized RAPs can store high‐capacities of 360 mA h g^?1 at 5C and 320 mA h g^?1 at 30C in LIBs. The high ion and electron mobilities within the buckypaper also enable to register 96 mA h g^?1 (24% capacity retention) at an extreme C‐rate of 600C (6 s for total discharge). Moreover, excellent cyclability is noted with a capacity retention of 98% over 3400 cycles at 30C. The high capacity, superior active‐material utilization, ultralong cyclability, and excellent rate performances of RAPs‐based electrode clearly rival most of the state‐of‐the‐art Li⁺ ion organic cathodes, and opens up new horizons for large‐scalable fabrication of electrode materials for ultrarobust Li storage.     

Tuning Oxygen Redox Chemistry in Li‐Rich Mn‐Based Layered Oxide Cathodes by Modulating Cation Arrangement

Li‐rich Mn‐based oxides (LRMO) are promising cathode materials to build next‐generation lithium‐ion batteries with high energy density exceeding 400 W h kg^?1. However, due to a lack of in‐depth understanding of oxygen redox chemistry in LRMO, voltage decay is not resolved thoroughly. Here, it is demonstrated that the oxygen redox chemistry could be tuned by modulating cation arrangement. It declares that the materials with Li/Ni disorder and Li vacancies can inhibit the formation of O? O dimers. Because of the high chemical activity, O? O dimers could accelerate lattice oxygen release and NiO/spinel formation. The samples without forming O? O dimers show improved performance in suppressing oxygen overoxidation and mitigating cation dissolution. As a result, the optimized cathode exhibits a high capacity over 280 mA h g^?1 at 0.1 C and a high plateau voltage of 3.58 V with a very low voltage decay of 1.6% after 150 cycles at 1 C. This study opens an attractive path in designing Li‐rich electrodes with stabilized redox chemistry.     

Nanosize Cation‐Disordered Rocksalt Oxides: Na2TiO3–NaMnO2 Binary System

To realize the development of rechargeable sodium batteries, new positive electrode materials without less abundant elements are explored. Enrichment of sodium contents in host structures is required to increase the theoretical capacity as electrode materials, and therefore Na‐excess compounds are systematically examined in a binary system of Na₂TiO₃–NaMnO₂. After several trials, synthesis of Na‐excess compounds with a cation disordered rocksalt structure is successful by adapting a mechanical milling method. Among the tested electrode materials, Na_1.14Mn_0.57Ti_0.29O₂ in this binary system delivers a large reversible capacity of ≈200 mA h g^?1, originating from reversible redox reactions of cationic Mn³⁺/Mn⁴⁺ and anionic O^2?/O^n? redox confirmed by X‐ray absorption spectroscopy. Holes in oxygen 2p orbitals, which are formed by electrochemical oxidation, are energetically stabilized by electron donation from Mn ions. Moreover, reversibility of anionic redox is significantly improved compared with a former study on a binary system of Na₃NbO₃–NaMnO₂ tested as model electrode materials.     

Regulating Surface and Grain‐Boundary Structures of Ni‐Rich Layered Cathodes for Ultrahigh Cycle Stability

The wide applications of Ni‐rich LiNi_1‐x‐yCo_xMn_yO₂ cathodes are severely limited by capacity fading and voltage fading during the cycling process resulting from the pulverization of particles, interfacial side reactions, and phase transformation. The canonical surface modification approach can improve the stability to a certain extent; however, it fails to resolve the key bottlenecks. The preparation of Li(Ni_0.4Co_0.2Mn_0.4)_1‐xTi_xO₂ on the surface of LiNi_0.8Co_0.1Mn_0.1O₂ particles with a coprecipitation method is reported. After sintering, Ti diffuses into the interior and mainly distributes along surface and grain boundaries. A strong surface and grain boundary strengthening are simultaneously achieved. The pristine particles are fully pulverized into first particles due to mechanical instability and high strains, which results in serious capacity fading. In contrast, the strong surface and the grain boundary strengthening can maintain the structural integrity, and therefore significantly improve the cycle stability. A general and simple strategy for the design of high‐performance Ni‐rich LiNi_1‐x‐yCo_xMn_yO₂ cathode is provided and is applicable to surface modification and grain‐boundary regulation of other advanced cathodes for batteries.     

Recent Progress of Layered Transition Metal Oxide Cathodes for Sodium‐Ion Batteries

Sodium‐ion batteries (SIBs) are attracting increasing attention and considered to be a low‐cost complement or an alternative to lithium‐ion batteries (LIBs), especially for large‐scale energy storage. Their application, however, is limited because of the lack of suitable host materials to reversibly intercalate Na⁺ ions. Layered transition metal oxides (Na_xMO₂, M = Fe, Mn, Ni, Co, Cr, Ti, V, and their combinations) appear to be promising cathode candidates for SIBs due to their simple structure, ease of synthesis, high operating potential, and feasibility for commercial production. In the present work, the structural evolution, electrochemical performance, and recent progress of Na_xMO₂ as cathode materials for SIBs are reviewed and summarized. Moreover, the existing drawbacks are discussed and several strategies are proposed to help alleviate these issues. In addition, the exploration of full cells based on Na_xMO₂ cathodes and future perspectives are discussed to provide guidance for the future commercialization of such systems.     

Paving the Path toward Reliable Cathode Materials for Aluminum‐Ion Batteries

Aluminum metal is a high‐energy‐density carrier with low cost, and thus endows rechargeable aluminum batteries (RABs) with the potential to act as an inexpensive and efficient electrochemical device, so as to supplement the increasing demand for energy storage and conversion. Despite the enticing aspects regarding cost and energy density, the poor reversibility of electrodes has limited the pursuit of RABs for a long time. Fortunately, ionic‐liquid electrolytes enable reversible aluminum plating/stripping at room temperature, and they lay the very foundation of RABs. In order to integrate with the aluminum‐metal anode, the selection of the cathode is pivotal, but is limited at present. The scant option of a reliable cathode can be accounted for by the intrinsic high charge density of Al³⁺ ions, which results in sluggish diffusion. Hence, reliable cathode materials are a key challenge of burgeoning RABs. Herein, the main focus is on the insertion cathodes for RABs also termed aluminum‐ion batteries, and the recent progress and optimization methods are summarized. Finally, an outlook is presented to navigate the possible future work.     

A Universal Strategy for Hollow Metal Oxide Nanoparticles Encapsulated into B/N Co‐Doped Graphitic Nanotubes as High‐Performance Lithium‐Ion Battery Anodes

Yolk–shell nanostructures have received great attention for boosting the performance of lithium‐ion batteries because of their obvious advantages in solving the problems associated with large volume change, low conductivity, and short diffusion path for Li⁺ ion transport. A universal strategy for making hollow transition metal oxide (TMO) nanoparticles (NPs) encapsulated into B, N co‐doped graphitic nanotubes (TMO@BNG (TMO = CoO, Ni₂O₃, Mn₃O₄) through combining pyrolysis with an oxidation method is reported herein. The as‐made TMO@BNG exhibits the TMO‐dependent lithium‐ion storage ability, in which CoO@BNG nanotubes exhibit highest lithium‐ion storage capacity of 1554 mA h g^?1 at the current density of 96 mA g^?1, good rate ability (410 mA h g^?1 at 1.75 A g^?1), and high stability (almost 96% storage capacity retention after 480 cycles). The present work highlights the importance of introducing hollow TMO NPs with thin wall into BNG with large surface area for boosting LIBs in the terms of storage capacity, rate capability, and cycling stability.     

Synthetic Control of Kinetic Reaction Pathway and Cationic Ordering in High‐Ni Layered Oxide Cathodes

Nickel‐rich layered transition metal oxides, LiNi_1?x (MnCo)_x O₂ (1?x ≥ 0.5), are appealing candidates for cathodes in next‐generation lithium‐ion batteries (LIBs) for electric vehicles and other large‐scale applications, due to their high capacity and low cost. However, synthetic control of the structural ordering in such a complex quaternary system has been a great challenge, especially in the presence of high Ni content. Herein, synthesis reactions for preparing layered LiNi_0.7Mn_0.15Co_0.15O₂ (NMC71515) by solid‐state methods are investigated through a combination of time‐resolved in situ high‐energy X‐ray diffraction and absorption spectroscopy measurements. The real‐time observation reveals a strong temperature dependence of the kinetics of cationic ordering in NMC71515 as a result of thermal‐driven oxidation of transition metals and lithium/oxygen loss that concomitantly occur during heat treatment. Through synthetic control of the kinetic reaction pathway, a layered NMC71515 with low cationic disordering and a high reversible capacity is prepared in air. The findings may help to pave the way for designing high‐Ni layered oxide cathodes for LIBs.     

Few Atomic Layered Lithium Cathode Materials to Achieve Ultrahigh Rate Capability in Lithium‐Ion Batteries

The most promising cathode materials, including LiCoO₂ (layered), LiMn₂O₄ (spinel), and LiFePO₄ (olivine), have been the focus of intense research to develop rechargeable lithium‐ion batteries (LIBs) for portable electronic devices. Sluggish lithium diffusion, however, and unsatisfactory long‐term cycling performance still limit the development of present LIBs for several applications, such as plug‐in/hybrid electric vehicles. Motivated by the success of graphene and novel 2D materials with unique physical and chemical properties, herein, a simple shear‐assisted mechanical exfoliation method to synthesize few‐layered nanosheets of LiCoO₂, LiMn₂O₄, and LiFePO₄ is used. Importantly, these as‐prepared nanosheets with preferred orientations and optimized stable structures exhibit excellent C‐rate capability and long‐term cycling performance with much reduced volume expansion during cycling. In particular, the zero‐strain insertion phenomenon could be achieved in 2–3 such layers of LiCoO₂ electrode materials, which could open up a new way to the further development of next‐generation long‐life and high‐rate batteries.     

Countering the Segregation of Transition‐Metal Ions in LiMn1/3Co1/3Ni1/3O2 Cathode for Ultralong Life and High‐Energy Li‐Ion Batteries

High‐voltage layered lithium transition‐metal oxides are very promising cathodes for high‐energy Li‐ion batteries. However, these materials often suffer from a fast degradation of cycling stability due to structural evolutions. It seriously impedes the large‐scale application of layered lithium transition‐metal oxides. In this work, an ultralong life LiMn_1/3Co_1/3Ni_1/3O₂ microspherical cathode is prepared by constructing an Mn‐rich surface. Its capacity retention ratio at 700 mA g^?1 is as large as 92.9% after 600 cycles. The energy dispersive X‐ray maps of electrodes after numerous cycles demonstrate that the ultralong life of the as‐prepared cathode is attributed to the mitigation of TM‐ions segregation. Additionally, it is discovered that layered lithium transition‐metal oxide cathodes with an Mn‐rich surface can mitigate the segregation of TM ions and the corrosion of active materials. This study provides a new strategy to counter the segregation of TM ions in layered lithium transition‐metal oxides and will help to the design and development of high‐energy cathodes with ultralong life.     

Ti‐Substituted NaNi0.5Mn0.5‐xTixO2 Cathodes with Reversible O3−P3 Phase Transition for High‐Performance Sodium‐Ion Batteries

Sodium‐ion batteries (SIBs) have been considered as potential candidates for stationary energy storage because of the low cost and wide availability of Na sources. O3‐type layered oxides have been considered as one of the most promising cathodes for SIBs. However, they commonly show inevitable complicated phase transitions and sluggish kinetics, incurring rapid capacity decline and poor rate capability. Here, a series of sodium‐sufficient O3‐type NaNi_0.5Mn_{0.5‐ x} Ti _x O₂ (0 ≤ x ≤ 0.5) cathodes for SIBs is reported and the mechanisms behind their excellent electrochemical performance are studied in comparison to those of their respective end‐members. The combined analysis of in situ X‐ray diffraction, ex situ X‐ray absorption spectroscopy, and scanning transmission electron microscopy for NaNi_0.5Mn_0.2Ti_0.3O₂ reveals that the O3‐type phase transforms reversibly into a P3‐type phase upon Na+ deintercalation/intercalation. The substitution of Ti for Mn enlarges interslab distance and could restrain the unfavorable and irreversible multiphase transformation in the high voltage regions that is usually observed in O3‐type NaNi_0.5Mn_0.5O₂, resulting in improved Na cell performance. This integration of macroscale and atomicscale engineering strategy might open up the modulation of the chemical and physical properties in layered oxides and grasp new insight into the optimal design of high‐performance cathode materials for SIBs.     

Unlocking the Potential of Disordered Rocksalts for Aqueous Zinc‐Ion Batteries

Owing to the intense charge repulsion of multivalent ions and intrinsic slugggish kenetics, vast and fast storage of zinc ions into electrode materials has remained unattainable. Here, an efficient strategy to unlock the electrochemical activity of rocksalt vanadium oxynitride is developed via the substitution of low‐valent oxygen for high‐valent nitrogen, forming disordered rocksalt with abundant vacancies/defects due to the charge‐compensating function. Unexpectedly, the disordered rocksalt not only provides plentiful active sites for zinc ions but is also beneficial for the rapid diffusion of zinc ions, owing to the large presence of vacancies/defects in the matrix. Hence, a very high reversible capacity (603 mAh g^?1, 0.2C) and high rate capability (124 mAh g^?1at 600C) are achieved for zinc storage. This should open a new and efficient avenue for the design of electrode materials with both high energy and power densities for aqueous zinc‐ion batteries.     

Revealing the Atomic Origin of Heterogeneous Li‐Ion Diffusion by Probing Na

Tracing the dynamic process of Li‐ion transport at the atomic scale has long been attempted in solid state ionics and is essential for battery material engineering. Approaches via phase change, strain, and valence states of redox species have been developed to circumvent the technical challenge of direct imaging Li; however, all are limited by poor spatial resolution and weak correlation with state‐of‐charge (SOC). An ion‐exchange approach is adopted by sodiating the delithiated cathode and probing Na distribution to trace the Li deintercalation, which enables the visualization of heterogeneous Li‐ion diffusion down to the atomic level. In a model LiNi_1/3Mn_1/3Co_1/3O₂ cathode, dislocation‐mediated ion diffusion is kinetically favorable at low SOC and planar diffusion along (003) layers dominates at high SOC. These processes work synergistically to determine the overall ion‐diffusion dynamics. The heterogeneous nature of ion diffusion in battery materials is unveiled and the role of defect engineering in tailoring ion‐transport kinetics is stressed.     

Metal Cation‐Assisted Synthesis of Amorphous B,N Co‐Doped Carbon Nanotubes for Superior Sodium Storage

Nearly inexhaustible sodium sources on earth make sodium ion batteries (SIBs) the best candidate for large‐scale energy storage. However, the main obstacles faced by SIBs are the low rate performance and poor cycle stability caused by the large size of Na⁺ ions. Herein, a universal strategy for synthesizing amorphous metals encapsulated into amorphous B, N co‐doped carbon (a‐M@a‐BCN; M = Co, Ni, Mn) nanotubes by metal cation‐assisted carbonization is explored. The methodology allows tailoring the structures (e.g., length, wall thickness, and metals doping) of a‐M@a‐BCN nannotubes at the molecular level. Furthermore, the amorphous metal sulfide encapsulated into a‐BCN (a‐MS_x@a‐BCN; MS_x: CoS, Ni₃S₂, MnS) nanotubes are obtained by one‐step sulfidation process. The a‐M@a‐BCN and a‐MS_x@a‐BCN possess the larger interlayer spacing (0.40 nm) amorphous carbon nanotube rich in heteroatoms active sites, making them exhibit excellent Na⁺ ions diffusion kinetics and capacitive storage behavior. As SIBs anodes, they show high capacity, excellent rate performance, and long cycle stability.     

Manganese‐Based Na‐Rich Materials Boost Anionic Redox in High‐Performance Layered Cathodes for Sodium‐Ion Batteries

To improve the energy and power density of Na‐ion batteries, an increasing number of researchers have focused their attention on activation of the anionic redox process. Although several materials have been proposed, few studies have focused on the Na‐rich materials compared with Li‐rich materials. A key aspect is sufficient utilization of anionic species. Herein, a comprehensive study of Mn‐based Na_1.2Mn_0.4Ir_0.4O₂ (NMI) O3‐type Na‐rich materials is presented, which involves both cationic and anionic contributions during the redox process. The single‐cation redox step relies on the Mn³⁺/Mn⁴⁺, whereas Ir atoms build a strong covalent bond with O and effectively suppress the O₂ release. In situ Raman, ex situ X‐ray photoelectron spectroscopy, and soft‐X‐ray absorption spectroscopy are employed to unequivocally confirm the reversibility of O₂^2? species formation and suggest a high degree of anionic reaction in this NMI Na‐rich material. In operando X‐ray diffraction study discloses the asymmetric structure evolution between the initial and subsequent cycles, which also explains the effect of the charge compensation mechanism on the electrochemical performance. The research provides a novel insight on Na‐rich materials and a new perspective in materials design towards future applications.     

Graphene Oxide Sieving Membrane for Improved Cycle Life in High‐Efficiency Redox‐Mediated Li–O2 batteries

As soluble catalysts, redox‐mediators (RMs) endow mobility to catalysts for unconstrained access to tethered solid discharge products, lowering the energy barrier for Li₂O₂ formation/decomposition; however, this desired mobility is accompanied by the undesirable side effect of RM migration to the Li metal anode. The reaction between RMs and Li metal degrades both the Li metal and the RMs, leading to cell deterioration within a few cycles. To extend the cycle life of redox‐mediated Li–O₂ batteries, herein graphene oxide (GO) membranes are reported as RM‐blocking separators. It is revealed that the size of GO nanochannels is narrow enough to reject 5,10‐dihydro‐5,10‐dimethylphenazine (DMPZ) while selectively allowing the transport of smaller Li⁺ ions. The negative surface charges of GO further repel negative ions via Donnan exclusion, greatly improving the lithium ion transference number. The Li–O₂ cells with GO membranes efficiently harness the redox‐mediation activity of DMPZ for improved performance, achieving energy efficiency of above 80% for more than 25 cycles, and 90% for 78 cycles when the capacity limits were 0.75 and 0.5 mAh cm^‐2, respectively. Considering the facile preparation of GO membranes, RM‐sieving GO membranes can be cost‐effective and processable functional separators in Li–O₂ batteries.     

Considering Critical Factors of Li‐rich Cathode and Si Anode Materials for Practical Li‐ion Cell Applications

In order to keep pace with increasing energy demands for advanced electronic devices and to achieve commercialization of electric vehicles and energy‐storage systems, improvements in high‐energy battery technologies are required. Among the various types of batteries, lithium ion batteries (LIBs) are among the most well‐developed and commercialized of energy‐storage systems. LIBs with Si anodes and Li‐rich cathodes are one of the most promising alternative electrode materials for next‐generation, high‐energy batteries. Si and Li‐rich materials exhibit high reversible capacities of <2000 mAh g^?1 and >240 mAh g^‐1, respectively. However, both materials have intrinsic drawbacks and practical limitations that prevent them from being utilized directly as active materials in high‐energy LIBs. Examples for Li‐rich materials include phase distortion during cycling and side reactions caused by the electrolyte at the surface, and for Si, large volume changes during cycling and low conductivity are observed. Recent progress and important approaches adopted for overcoming and alleviating these drawbacks are described in this article. A perspective on these matters is suggested and the requirements for each material are delineated, in addition to introducing a full‐cell prototype utilizing a Li‐rich cathode and Si anode.     

Interfacial Lattice‐Strain‐Driven Generation of Oxygen Vacancies in an Aerobic‐Annealed TiO2(B) Electrode

Oxygen vacancies play crucial roles in defining physical and chemical properties of materials to enhance the performances in electronics, solar cells, catalysis, sensors, and energy conversion and storage. Conventional approaches to incorporate oxygen defects mainly rely on reducing the oxygen partial pressure for the removal of product to change the equilibrium position. However, directly affecting reactants to shift the reaction toward generating oxygen vacancies is lacking and to fill this blank in synthetic methodology is very challenging. Here, a strategy is demonstrated to create oxygen vacancies through making the reaction energetically more favorable via applying interfacial strain on reactants by coating, using TiO₂(B) as a model system. Geometrical phase analysis and density functional theory simulations verify that the formation energy of oxygen vacancies is largely decreased under external strain. Benefiting from these, the obtained oxygen‐deficient TiO₂(B) exhibits impressively high level of capacitive charge storage, e.g., ≈53% at 0.5 mV s^?1, far surpassing the ≈31% of the unmodified counterpart. Meanwhile, the modified electrode shows significantly enhanced rate capability delivering a capacity of 112 mAh g^?1 at 20 C (≈6.7 A g^?1), ≈30% higher than air‐annealed TiO₂ and comparable to vacuum‐calcined TiO₂. This work heralds a new paradigm of mechanical manipulation of materials through interfacial control for rational defect engineering.     

Fading Mechanisms and Voltage Hysteresis in FeF2–NiF2 Solid Solution Cathodes for Lithium and Lithium‐Ion Batteries

The rapid development of ultrahigh‐capacity alloying or conversion‐type anodes in rechargeable lithium (Li)‐ion batteries calls for matching cathodes for next‐generation energy storage devices. The high volumetric and gravimetric capacities, low cost, and abundance of iron (Fe) make conversion‐type iron fluoride (FeF₂ and FeF₃)‐based cathodes extremely promising candidates for high specific energy cells. Here, the substantial boost in the capacity of FeF₂ achieved with the addition of NiF₂ is reported. A systematic study of a series of FeF₂–NiF₂ solid solution cathodes with precisely controlled morphology and composition reveals that the presence of Ni may undesirably accelerate capacity fading. Using a powerful combination of state‐of‐the‐art analytical techniques in combination with the density functional theory calculations, fundamental mechanisms responsible for such a behavior are uncovered. The unique insights reported in this study highlight the importance of careful selection of metals and electrolytes for optimizing electrochemical properties of metal fluoride cathodes.