Understanding Rechargeable Battery Function Using In Operando Neutron Powder Diffraction

The performance of rechargeable batteries is influenced by the structural and phase changes of components during cycling. Neutron powder diffraction (NPD) provides unique and useful information concerning the structure–function relation of battery components and can be used to study the changes to component phase and structure during battery cycling, known as in operando measurement studies. The development and use of NPD for in operando measurements of batteries is summarized along with detailed experimental approaches that impact the insights gained by these. A summary of the information gained concerning battery function using in operando NPD measurements is provided, including the structural and phase evolution of electrode materials and charge-carrying ion diffusion pathways through these, which are critical to the development of battery technology.     

Tuning the Solid Electrolyte Interphase for Selective Li‐ and Na‐Ion Storage in Hard Carbon

Solid‐electrolyte interphase (SEI) films with controllable properties are highly desirable for improving battery performance. In this paper, a combined experimental and theoretical approach is used to study SEI films formed on hard carbon in Li‐ and Na‐ion batteries. It is shown that a stable SEI layer can be designed by precycling an electrode in a desired Li‐ or Na‐based electrolyte, and that ionic transport can be kinetically controlled. Selective Li‐ and Na‐based SEI membranes are produced using Li‐ or Na‐based electrolytes, respectively. The Na‐based SEI allows easy transport of Li ions, while the Li‐based SEI shuts off Na‐ion transport. Na‐ion storage can be manipulated by tuning the SEI layer with film‐forming electrolyte additives, or by preforming an SEI layer on the electrode surface. The Na specific capacity can be controlled to < 25 mAh g^?1; ≈ 1/10 of the normal capacity (250 mAh g^?1). Unusual selective/preferential transport of Li ions is demonstrated by preforming an SEI layer on the electrode surface and corroborated with a mixed electrolyte. This work may provide new guidance for preparing good ion‐selective conductors using electrochemical approaches.     

Ionic conductive polymers as artificial solid electrolyte interphase films in Li metal batteries – A review

Lithium (Li) metal has been considered as the ultimate anode material for next-generation rechargeable batteries due to its ultra-high theoretical specific capacity (3860 mAh g⁻¹) and the lowest reduction voltage (−3.04 V vs the standard hydrogen electrode). However, the dendritic Li formation, uncontrolled interfacial reactions, and huge volume variations lead to unstable solid electrolyte interphase (SEI) layer, low Coulombic efficiency and hence short cycling lifetime. Designing artificial solid electrolyte interphase (artificial SEI) films on the Li metal electrode exhibits great potential to solve the aforementioned problems and enable Li–metal batteries with prolonged lifetime. Polymer materials with good ionic conductivity, superior processability and high flexibility are considered as ideal artificial SEI film materials. In this review, according to the ionic conductive groups, recent advances in polymeric artificial SEI films are summarized to afford a deep understanding of Li ion plating/stripping behavior and present design principles of high-performance artificial SEI films in achieving stable Li metal electrodes. Perspectives regarding to the future research directions of polymeric artificial SEI films for Li–metal electrode are also discussed. The insights and design principles of polymeric artificial SEI films gained in the current review will be definitely useful in achieving the Li–metal batteries with improved energy density, high safety and long cycling lifetime toward next-generation energy storage devices.     

Probing Sodium Storage Mechanism in Hollow Carbon Nanospheres Using Liquid Phase Transmission Electron Microscopy

Carbonaceous materials are promising sodium-ion battery anodes. Improving their performance requires a detailed understanding of the ion transport in these materials, some important aspects of which are still under debate. In this work, nitrogen-doped porous hollow carbon spheres (N-PHCSs) are employed as a model system for operando analysis of sodium storage behavior in a commercial liquid electrolyte at the nanoscale. By combining the ex situ characterization at different states of charge with operando transmission electron microscopy experiments, it is found that a solvated ionic layer forms on the surface of N-PHCSs at the beginning of sodiation, followed by the irreversible shell expansion due to the solid-electrolyte interphase (SEI) formation and subsequent storage of Na(0) within the porous carbon shell. This shows that binding between Na(0) and C creates a Schottky junction making Na deposition inside the spheres more energetically favorable at low current densities. During sodiation, the SEI fills the gap between N-PHCSs, binding spheres together and facilitating the sodium ions' transport toward the current collector and subsequent plating underneath the electrode. The N-PHCSs layer acts as a protective layer between the electrolyte and the current collector, suppressing the possible growth of dendrites at the anode.     

Water‐Soluble Sericin Protein Enabling Stable Solid–Electrolyte Interphase for Fast Charging High Voltage Battery Electrode

Spinel LiNi_0.5Mn_1.5O₄ (LNMO) is the most promising cathode material for achieving high energy density lithium‐ion batteries attributed to its high operating voltage (≈4.75 V). However, at such high voltage, the commonly used battery electrolyte is suffered from severe oxidation, forming unstable solid–electrolyte interphase (SEI) layers. This would induce capacity fading, self‐discharge, as well as inferior rate capabilities for the electrode during cycling. This work first time discovers that the electrolyte oxidation is effectively negated by introducing an electrochemically stable silk sericin protein, which is capable to stabilize the SEI layer and suppress the self‐discharge behavior for LNMO. In addition, robust mechanical support of sericin coating maintains the structural integrity during the fast charging/discharging process. Benefited from these merits, the sericin‐based LNMO electrode possesses a much lower Li‐ion diffusion energy barrier (26.1 kJ mol⁻¹) for than that of polyvinylidene fluoride‐based LNMO electrode (37.5 kJ mol⁻¹), delivering a remarkable high‐rate performance. This work heralds a new paradigm for manipulating interfacial chemistry of electrode to solve the key obstacle for LNMO commercialization, opening a powerful avenue for unlocking the current challenges for a wide family of high operating voltage cathode materials (>4.5 V) toward practical applications.     

Highly Improved Cycling Stability of Anion De‐/Intercalation in the Graphite Cathode for Dual‐Ion Batteries

Conventional ion batteries utilizing metallic ions as the single charge carriers are limited by the insufficient abundance of metal resources. Although supercapacitors apply both cations and anions to store energy through absorption and/or Faradic reactions occurring at the interfaces of the electrode/electrolyte, the inherent low energy density hinders its application. The graphite‐cathode‐based dual‐ion battery possesses a higher energy density due to its high working potential of nearly 5 V. However, such a battery configuration suffers from severe electrolyte decomposition and exfoliation of the graphite cathode, rendering an inferior cycle life. Herein, a new surface‐modification strategy is developed to protect the graphite cathode from the anion salvation effect and the deposition derived from electrolyte decomposition by generating an artificial solid electrolyte interphase (SEI). Such SEI‐modified graphite exhibits superior cycling stability with 96% capacity retention after 500 cycles under 200 mA g^?1 at the upper cutoff voltage of 5.0 V, which is much improved compared with the pristine graphite electrode. Through several ex situ studies, it is revealed that the artificial SEI greatly stabilizes the interfaces of the electrode/electrolyte after reconstruction and gradual establishment of the optimal anion‐transport path. The findings shed light on a new avenue toward promoting the performance of the dual‐ion battery (DIB) and hence to make it practical finally.     

Direct observation of inhomogeneous solid electrolyte interphase on MnO anode with atomic force microscopy and spectroscopy

Solid electrolyte interphase (SEI) is an in situ formed thin coating on lithium ion battery (LIB) electrodes. The mechanical property of SEI largely defines the cycling performance and the safety of LIBs but has been rarely investigated. Here, we report quantitatively the Young's modulus of SEI films on MnO anodes. The inhomogeneity of SEI film in morphology, structure, and mechanical properties provides new insights to the evolution of SEI on electrodes. Furthermore, the quantitative methodology established in this study opens a new approach to direct investigation of SEI properties in various electrode materials systems.     

Emerging characterization techniques for delving polyanion-type cathode materials of sodium-ion batteries

Polyanion-type cathode materials have grown in leaps and bounds and become one of the promising candidates for metal-ion batteries since the successful case of LiFePO₄ in lithium-ion batteries, which own stable crystal structure, high thermal stability, good ionic conductivity, adjustable voltage and chemical composition. However, further exploration is requisite, such as, the change of crystal/electronic structure, reaction mechanism, and structure evolution during charge/discharge processes, which results from variety of crystal types and redox centers, anion and cationic doping/substitution, as well as transition metal ion migration in polyanion-type materials. In this review, we focus on the advanced characterization techniques referred in polyanion-type cathode materials of sodium-ion batteries, mainly consist of the structure-related, morphology-related, composition-related techniques and in-situ/operando techniques during charge/discharge processes. The respective detection mechanisms, scope of application, information available and limitations of each technique are discussed in detail, and the latest developments of these characterization techniques used in polyanion-type materials are summarized. Advanced characterization techniques play a crucial role in understanding the reaction mechanisms of electrode materials, and can provide an important guiding principle for designing high-performance polyanion-type cathode materials and further optimizing the battery systems of sodium-ion batteries.     

Defect Engineering on Electrode Materials for Rechargeable Batteries

The reasonable design of electrode materials for rechargeable batteries plays an important role in promoting the development of renewable energy technology. With the in-depth understanding of the mechanisms underlying electrode reactions and the rapid development of advanced technology, the performance of batteries has significantly been optimized through the introduction of defect engineering on electrode materials. A large number of coordination unsaturated sites can be exposed by defect construction in electrode materials, which play a crucial role in electrochemical reactions. Herein, recent advances regarding defect engineering in electrode materials for rechargeable batteries are systematically summarized, with a special focus on the application of metal-ion batteries, lithium–sulfur batteries, and metal–air batteries. The defects can not only effectively promote ion diffusion and charge transfer but also provide more storage/adsorption/active sites for guest ions and intermediate species, thus improving the performance of batteries. Moreover, the existing challenges and future development prospects are forecast, and the electrode materials are further optimized through defect engineering to promote the development of the battery industry.     

In Operando Probing of Lithium‐Ion Storage on Single‐Layer Graphene

Despite high‐surface area carbons, e.g., graphene‐based materials, being investigated as anodes for lithium (Li)‐ion batteries, the fundamental mechanism of Li‐ion storage on such carbons is insufficiently understood. In this work, the evolution of the electrode/electrolyte interface is probed on a single‐layer graphene (SLG) film by performing Raman spectroscopy and Fourier transform infrared spectroscopy when the SLG film is electrochemically cycled as the anode in a half cell. The utilization of SLG eliminates the inevitable intercalation of Li ions in graphite or few‐layer graphene, which may have complicated the discussion in previous work. Combining the in situ studies with ex situ observations and ab initio simulations, the formation of solid electrolyte interphase and the structural evolution of SLG are discussed when the SLG is biased in an electrolyte. This study provides new insights into the understanding of Li‐ion storage on SLG and suggests how high‐surface‐area carbons could play proper roles in anodes for Li‐ion batteries.     

In Situ Electrochemistry of Rechargeable Battery Materials: Status Report and Perspectives

The development of rechargeable batteries with high performance is considered to be a feasible way to satisfy the increasing needs of electric vehicles and portable devices. It is of vital importance to design electrodes with high electrochemical performance and to understand the nature of the electrode/electrolyte interfaces during battery operation, which allows a direct observation of the complicated chemical and physical processes within the electrodes and electrolyte, and thus provides real‐time information for further design and optimization of the battery performance. Here, the recent progress in in situ techniques employed for the investigations of material structural evolutions is described, including characterization using neutrons, X‐ray diffraction, and nuclear magnetic resonance. In situ techniques utilized for in‐depth uncovering the electrode/electrolyte phase/interface change mechanisms are then highlighted, including transmission electron microscopy, atomic force microscopy, X‐ray spectroscopy, and Raman spectroscopy. The real‐time monitoring of lithium dendrite growth and in situ detection of gas evolution during charge/discharge processes are also discussed. Finally, the major challenges and opportunities of in situ characterization techniques are outlined toward new developments of rechargeable batteries, including innovation in the design of compatible in situ cells, applications of dynamic analysis, and in situ electrochemistry under multi‐stimuli. A clear and in‐depth understanding of in situ technique applications and the mechanisms of structural evolutions, surface/interface changes, and gas generations within rechargeable batteries is given here.     

Origin of Surface Coating Effect for MgO on LiCoO2 to Improve the Interfacial Reaction between Electrode and Electrolyte

Surface coating on lithium‐ion battery cathodes improves their durability at high potentials, which is a well‐known practical application. However, the mechanism is still unclear because the coating influences the electrode/electrolyte interface at a few nanometer‐scale and direct observation of the interface under real operating conditions of a battery is challenging. This study reveals the mechanism of the surface coating effect on lithium‐ion battery cathodes by using in operando X‐ray absorption spectroscopy (XAS) on well‐defined MgO‐coated LiCoO₂ thin‐film electrodes prepared via pulsed laser deposition. Total‐reflection in operando XAS measurements reveal that LiCoO₂ forms a reductive phase at the interface between the uncoated‐LiCoO₂ electrode and the electrolyte, while the MgO coating layer inhibits the redox process, leading to an improvement in the cycle performance of the battery. Depth‐resolved in operando XAS measurements indicate that a solid solution of the magnesium phase forms at the LiCoO₂ surface upon MgO coating. Magnesium ions function as pillars to stabilize the layered structure at the interface between the LiCoO₂ electrode and the electrolyte for delithiated states upon cycling at potentials.     

The ionic interphases of the lithium anode in solid state batteries

Solid state battery (SSB) performance is largely governed by processes occurring at electrolyte–electrode interfaces. At the Li metal anode, where the overwhelming majority of solid electrolyte (SE) are unstable against Li metal, the interface can readily react to form emergent Li-solid electrolyte interphases (SEI) with ionic, electronic, chemical, mechanical, and electrochemical properties substantially distinct from the parent phase. Facing similar challenges with liquid electrolytes, the Li battery community underwent a half century-long effort, still in progress, to illuminate fundamental properties of the Li SEI—including chemistry, morphology, transport, and sources of Li loss upon cycling—from which guiding principles have emerged to drive improvement in electrolyte and interface design. The Li metal SEI with solid electrolytes presents both similarities and differences to that in liquid electrolytes, with differences defining unique research needs. Here, we examine current understanding of the Li-SE interface as well as learnings from the liquid electrolyte community that we propose might be adopted to help rationalize and improve SE integration with Li anodes. Through this lens, we inspect current state-of-understanding of Li SEI composition, structure, and properties, along with Coulombic efficiency values reported so far for Li cycling with SE. We also highlight potential Li modification strategies for SSB, which are informed by and exploit understanding of the ionic SEI phases; in some instances, engineering strategies utilize a liquid electrolyte SEI directly, making liquid-derived SEI knowledge of immediate relevance.     

Porous Materials Applied in Nonaqueous Li–O2 Batteries: Status and Perspectives

Porous materials possessing high surface area, large pore volume, tunable pore structure, superior tailorability, and dimensional effect have been widely applied as components of lithium–oxygen (Li–O₂) batteries. Herein, the theoretical foundation of the porous materials applied in Li–O₂ batteries is provided, based on the present understanding of the battery mechanism and the challenges and advantageous qualities of porous materials. Furthermore, recent progress in porous materials applied as the cathode, anode, separator, and electrolyte in Li–O₂ batteries is summarized, together with corresponding approaches to address the critical issues that remain at present. Particular emphasis is placed on the importance of the correlation between the function-orientated design of porous materials and key challenges of Li–O₂ batteries in accelerating oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) kinetics, improving the electrode stability, controlling lithium deposition, suppressing the shuttle effect of the dissolved redox mediators, and alleviating electrolyte decomposition. Finally, the rational design and innovative directions of porous materials are provided for their development and application in Li–O₂ battery systems.     

The Absence and Importance of Operando Techniques for Metal‐Free Catalysts

Operando characterization techniques have played a crucial role in modern technological developments. In contrast to the experimental uncertainties introduced by ex situ techniques, the simultaneous measurement of desired sample characteristics and near‐realistic electrochemical testing provides a representative picture of the underlying physics. From Li‐ion batteries to metal‐based electrocatalysts, the insights offered by real‐time characterization data have enabled more efficient research programs. As an emerging class of catalyst, much of the mechanistic understanding of metal‐free electrocatalysts continues to be elusive in comparison to their metal‐based counterparts. However, there is a clear absence of operando characterization performed on metal‐free catalysts. Through the proper execution of operando techniques, it can be expected that metal‐free catalysts can achieve exceptional technological progress. Here, the motivation of using operando characterization techniques for metal‐free carbon‐based catalyst system is considered, followed by a discussion of the possibilities, difficulties and benefits of their applications.     

Recent Progress in the Design of Advanced Cathode Materials and Battery Models for High‐Performance Lithium‐X (X = O2, S,Se, Te,I2, Br2) Batteries

Recent advances and achievements in emerging Li‐X (X = O₂, S, Se, Te, I₂, Br₂) batteries with promising cathode materials open up new opportunities for the development of high‐performance lithium‐ion battery alternatives. In this review, we focus on an overview of recent important progress in the design of advanced cathode materials and battery models for developing high‐performance Li‐X (X = O₂, S, Se, Te, I₂, Br₂) batteries. We start with a brief introduction to explain why Li‐X batteries are important for future renewable energy devices. Then, we summarize the existing drawbacks, major progress and emerging challenges in the development of cathode materials for Li‐O₂ (S) batteries. In terms of the emerging Li‐X (Se, Te, I₂, Br₂) batteries, we systematically summarize their advantages/disadvantages and recent progress. Specifically, we review the electrochemical performance of Li‐Se (Te) batteries using carbonate‐/ether‐based electrolytes, made with different electrode fabrication techniques, and of Li‐I₂ (Br₂) batteries with various cell designs (e.g., dual electrolyte, all‐organic electrolyte, with/without cathode‐flow mode, and fuel cell/solar cell integration). Finally, the perspective on and challenges for the development of cathode materials for the promising Li‐X (X = O₂, S, Se, Te, I₂, Br₂) batteries is presented.     

The Regulating Role of Carbon Nanotubes and Graphene in Lithium‐Ion and Lithium–Sulfur Batteries

The ever‐increasing demands for batteries with high energy densities to power the portable electronics with increased power consumption and to advance vehicle electrification and grid energy storage have propelled lithium battery technology to a position of tremendous importance. Carbon nanotubes (CNTs) and graphene, known with many appealing properties, are investigated intensely for improving the performance of lithium‐ion (Li‐ion) and lithium–sulfur (Li–S) batteries. However, a general and objective understanding of their actual role in Li‐ion and Li–S batteries is lacking. It is recognized that CNTs and graphene are not appropriate active lithium storage materials, but are more like a regulator: they do not electrochemically react with lithium ions and electrons, but serve to regulate the lithium storage behavior of a specific electroactive material and increase the range of applications of a lithium battery. First, metrics for the evaluation of lithium batteries are discussed, based on which the regulating role of CNTs and graphene in Li‐ion and Li–S batteries is comprehensively considered from fundamental electrochemical reactions to electrode structure and integral cell design. Finally, perspectives on how CNTs and graphene can further contribute to the development of lithium batteries are presented.     

Chemical,Structural, and Electronic Aspects of Formation and Degradation Behavior on Different Length Scales of Ni‐Rich NCM and Li‐Rich HE‐NCM Cathode Materials in Li‐Ion Batteries

In order to satisfy the energy demands of the electromobility market, both Ni‐rich and Li‐rich layered oxides of NCM type are receiving much attention as high‐energy‐density cathode materials for application in Li‐ion batteries. However, due to different stability issues, their longevity is limited. During formation and continuous cycling, especially the electronic and crystal structure suffers from various changes, eventually leading to fatigue and mechanical degradation. In recent years, comprehensive battery research has been conducted at Karlsruhe Institute of Technology, mainly aiming at better understanding the primary degradation processes occurring in these layered transition metal oxides. The characteristic process of formation and mechanisms of fatigue are fundamentally characterized and the effect of chemical composition on cell chemistry, electrochemistry, and cycling stability is addressed on different length scales by use of state‐of‐the‐art analytical techniques, ranging from “standard” characterization tools to combinations of advanced in situ and operando methods. Here, the results are presented and discussed within a broader scientific context.     

Shielding Polysulfide Intermediates by an Organosulfur-Containing Solid Electrolyte Interphase on the Lithium Anode in Lithium–Sulfur Batteries

The lithium–sulfur (Li–S) battery is regarded as a promising high-energy-density battery system, in which the dissolution–precipitation redox reactions of the S cathode are critical. However, soluble Li polysulfides (LiPSs), as the indispensable intermediates, easily diffuse to the Li anode and react with the Li metal severely, thus depleting the active materials and inducing the rapid failure of the battery, especially under practical conditions. Herein, an organosulfur-containing solid electrolyte interphase (SEI) is tailored for the stabilizaiton of the Li anode in Li–S batteries by employing 3,5-bis(trifluoromethyl)thiophenol as an electrolyte additive. The organosulfur-containing SEI protects the Li anode from the detrimental reactions with LiPSs and decreases its corrosion. Under practical conditions with a high-loading S cathode (4.5 mg_S cm⁻²), a low electrolyte/S ratio (5.0 µL mg_S⁻¹), and an ultrathin Li anode (50 µm), a Li–S battery delivers 82 cycles with an organosulfur-containing SEI in comparison to 42 cycles with a routine SEI. This work provokes the vital insights into the role of the organic components of SEI in the protection of the Li anode in practical Li–S batteries.     

Solid-state lithium–sulfur batteries: Advances,challenges and perspectives

Secondary batteries with high energy density, high specific energy and long cycle life have attracted increasing research attention as required for ground and aerial electric vehicles and large-scale stationary energy-storage. Lithium–sulfur (Li–S) batteries are considered as a particularly promising candidate because of their high theoretical performance and low cost of active materials. In spite of the recent progress in both fundamental understanding and developments of electrode and electrolyte materials, the practical use of liquid electrolyte-based Li–S batteries is still hindered by their poor cycling performance and safety concerns. Solid-state Li–S batteries have the potential to overcome these challenges. In this review, the mechanisms of Li ion transport and the basic requirements of solid-state electrolytes are discussed. We focus on recent advances in various solid-state Li–S battery systems, from quasi-solid-state to all-solid-state Li–S batteries. We also describe the remaining challenges and plausible solutions, including improved designs and compositions of electrode materials, solid-state electrolytes and the electrode/electrolyte interfaces. Though many fundamental and technological issues still need to be resolved to develop commercially viable technologies, solid-state Li–S batteries offer an attractive opportunity to address the present limitations.