Understanding Performance Degradation in Cation‐Disordered Rock‐Salt Oxide Cathodes

The collective redox activities of transition‐metal (TM) cations and oxygen anions have been shown to increase charge storage capacity in both Li‐rich layered and cation‐disordered rock‐salt cathodes. Repeated cycling involving anionic redox is known to trigger TM migration and phase transformation in layered Li‐ and Mn‐rich (LMR) oxides, however, detailed mechanistic understanding on the recently discovered Li‐rich rock‐salt cathodes is largely missing. The present study systematically investigates the effect of oxygen redox on a Li_1.3Nb_0.3Mn_0.4O₂ cathode and demonstrates that performance deterioration is directly correlated to the extent of oxygen redox. It is shown that voltage fade and hysteresis begin only after initiating anionic redox at high voltages, which grows progressively with either deeper oxidation of oxygen at higher potential or extended cycling. In contrast to what is reported on layered LMR oxides, extensive TM reduction is observed but phase transition is not detected in the cycled oxide. A densification/degradation mechanism is proposed accordingly which elucidates how a unique combination of extensive chemical reduction of TM and reduced quality of the Li percolation network in cation‐disordered rock‐salts can lead to performance degradation in these newer cathodes with 3D Li migration pathways. Design strategies to achieve balanced capacity and stability are also discussed.     

Ethylene Carbonate‐Free Electrolytes for High‐Nickel Layered Oxide Cathodes in Lithium‐Ion Batteries

Layered lithium nickel oxide (LiNiO₂) can provide very high energy density among intercalation cathode materials for lithium‐ion batteries, but suffers from poor cycle life and thermal‐abuse tolerance with large lithium utilization. In addition to stabilization of the active cathode material, a concurrent development of electrolyte systems of better compatibility is critical to overcome these limitations for practical applications. Here, with nonaqueous electrolytes based on exclusively aprotic acyclic carbonates free of ethylene carbonate (EC), superior electrochemical and thermal characteristics are obtained with an ultrahigh‐nickel cathode (LiNi_0.94Co_0.06O₂), capable of reaching a 235 mA h g^?1 specific capacity. Pouch‐type graphite|LiNi_0.94Co_0.06O₂ cells in EC‐free electrolytes withstand several hundred charge–discharge cycles with minor degradation at both ambient and elevated temperatures. In thermal‐abuse tests, the cathode at full charge, while reacting aggressively with EC‐based electrolytes below 200 °C, shows suppressed self‐heating without EC. Through 3D chemical and structural analyses, the intriguing impact of EC is visualized in aggravating unwanted surface parasitic reactions and irreversible bulk structural degradation of the cathode at high voltages. These results provide important insights in designing high‐energy electrodes for long‐lasting and reliable lithium‐ion batteries.     

Nickel‐Rich and Lithium‐Rich Layered Oxide Cathodes: Progress and Perspectives

Ni‐rich layered oxides and Li‐rich layered oxides are topics of much research interest as cathodes for Li‐ion batteries due to their low cost and higher discharge capacities compared to those of LiCoO₂ and LiMn₂O₄. However, Ni‐rich layered oxides have several pitfalls, including difficulty in synthesizing a well‐ordered material with all Ni³⁺ ions, poor cyclability, moisture sensitivity, a thermal runaway reaction, and formation of a harmful surface layer caused by side reactions with the electrolyte. Recent efforts towards Ni‐rich layered oxides have centered on optimizing the composition and processing conditions to obtain controlled bulk and surface compositions to overcome the capacity fade. Li‐rich layered oxides also have negative aspects, including oxygen loss from the lattice during first charge, a large first cycle irreversible capacity loss, poor rate capability, side reactions with the electrolyte, low tap density, and voltage decay during extended cycling. Recent work on Li‐rich layered oxides has focused on understanding the surface and bulk structures and eliminating the undesirable properties. Followed by a brief introduction, an account of recent developments on the understanding and performance gains of Ni‐rich and Li‐rich layered oxide cathodes is provided, along with future research directions.     

Insights into the Enhanced Cycle and Rate Performances of the F‐Substituted P2‐Type Oxide Cathodes for Sodium‐Ion Batteries

A series of F‐substituted Na_2/3Ni_1/3Mn_2/3O_2?xF_x (x = 0, 0.03, 0.05, 0.07) cathode materials have been synthesized and characterized by solid‐state ¹⁹F and ²³Na NMR, X‐ray photoelectron spectroscopy, and neutron diffraction. The underlying charge compensation mechanism is systematically unraveled by X‐ray absorption spectroscopy and electron energy loss spectroscopy (EELS) techniques, revealing partial reduction from Mn⁴⁺ to Mn³⁺ upon F‐substitution. It is revealed that not only Ni but also Mn participates in the redox reaction process, which is confirmed for the first time by EELS techniques, contributing to an increase in discharge specific capacity. The detailed structural transformations are also revealed by operando X‐ray diffraction experiments during the intercalation and deintercalation process of Na⁺, demonstrating that the biphasic reaction is obviously suppressed in the low voltage region via F‐substitution. Hence, the optimized sample with 0.05 mol f.u.^?1 fluorine substitution delivers an ultrahigh specific capacity of 61 mAh g^?1 at 10 C after 2000 cycles at 30 °C, an extraordinary cycling stability with a capacity retention of 75.6% after 2000 cycles at 10 C and 55 °C, an outstanding full battery performance with 89.5% capacity retention after 300 cycles at 1 C. This research provides a crucial understanding of the influence of F‐substitution on the crystal structure of the P2‐type materials and opens a new avenue for sodium‐ion batteries.     

Suppressing Manganese Dissolution from Lithium Manganese Oxide Spinel Cathodes with Single‐Layer Graphene

Spinel‐structured LiMn₂O₄ (LMO) is a desirable cathode material for Li‐ion batteries due to its low cost, abundance, and high power capability. However, LMO suffers from limited cycle life that is triggered by manganese dissolution into the electrolyte during electrochemical cycling. Here, it is shown that single‐layer graphene coatings suppress manganese dissolution, thus enhancing the performance and lifetime of LMO cathodes. Relative to lithium cells with uncoated LMO cathodes, cells with graphene‐coated LMO cathodes provide improved capacity retention with enhanced cycling stability. X‐ray photoelectron spectroscopy reveals that graphene coatings inhibit manganese depletion from the LMO surface. Additionally, transmission electron microscopy demonstrates that a stable solid electrolyte interphase is formed on graphene, which screens the LMO from direct contact with the electrolyte. Density functional theory calculations provide two mechanisms for the role of graphene in the suppression of manganese dissolution. First, common defects in single‐layer graphene are found to allow the transport of lithium while concurrently acting as barriers for manganese diffusion. Second, graphene can chemically interact with Mn³⁺ at the LMO electrode surface, promoting an oxidation state change to Mn⁴⁺, which suppresses dissolution.     

Mn versus Al in Layered Oxide Cathodes in Lithium‐Ion Batteries: A Comprehensive Evaluation on Long‐Term Cyclability

Nickel‐rich layered oxide cathodes with the composition LiNi_1?x?yCo_xMn_yO₂ (NCM, (1?x?y) ≥ 0.6) are under intense scrutiny recently to contend with commercial LiNi_0.8Co_0.15Al_0.05O₂ (NCA) for high‐energy‐density batteries for electric vehicles. However, a comprehensive assessment of their electrochemical durability is currently lacking. Herein, two in‐house cathodes, LiNi_0.8Co_0.15Al_0.05O₂ and LiNi_0.7Co_0.15Mn_0.15O₂, are investigated in a high‐voltage graphite full cell over 1500 charge‐discharge cycles (≈5–10 year service life in vehicles). Despite a lower nickel content, NCM shows more performance deterioration than NCA. Critical underlying degradation processes, including chemical, structural, and mechanical aspects, are analyzed via an arsenal of characterization techniques. Overall, Mn substitution appears far less effective than Al in suppressing active mass dissolution and irreversible phase transitions of the layered oxide cathodes. The active mass dissolution (and crossover) accelerates capacity decline with sustained parasitic reactions on the graphite anode, while the phase transitions are primarily responsible for cell resistance increase and voltage fade. With Al doping, on the other hand, secondary particle pulverization is the more limiting factor for long‐term cyclability compared to Mn. These results establish a fundamental guideline for designing high‐performing Ni‐rich NCM cathodes as a compelling alternative to NCA and other compositions for electric vehicle applications.     

Synergy Between Metal Oxide Nanofibers and Graphene Nanoribbons for Rechargeable Lithium‐Oxygen Battery Cathodes

The challenges for rechargeable lithium‐oxygen batteries of low practical capacity and poor round‐trip efficiency urgently demand effective cathode materials to overcome the limitations. However, the synergy between the multiple active materials is not well understood. Here, findings of the synergistic effect between electrospun zinc oxide (ZnO) nanofibers and graphene nanoribbons (GNRs) unzipped from carbon nanotubes (CNTs) as cathode materials in rechargeable lithium‐oxygen batteries are described. Furthermore, the overpotentials and discharge capacities are tuned by the surface defect states of ZnO nanofibers and Pt nanocrytals in GNRs. It is observed that the optimized zinc oxide nanofibers hybridized with GNRs achieved a high reversible capacity of 6300 mAh g^‐1_carbon and enhanced stable cyclability under specific 50% of full discharge capacities. This report demonstrates that the ZnO nanofibers with a high degree of defects and hydrophilicity of the surface may be a promising cathode component for rechargeable lithium‐oxygen batteries and the optimum synergy between ZnO nanofibers and GNRs can balance the discharge capacity and cycle life.     

Degradation of High‐Nickel‐Layered Oxide Cathodes from Surface to Bulk: A Comprehensive Structural,Chemical, and Electrical Analysis

Multiple applications of lithium‐ion batteries in energy storage systems and electric vehicles require highly stable electrode materials for long‐term battery operation. Among the various cathode materials, high‐Ni cathode materials enable a high energy density. However, cathode degradation accompanied by complex chemical and structural changes results in capacity and voltage fading in batteries. Cathode degradation remains poorly understood; the majority of studies have only explored the oxidation states of transition‐metal ions in localized areas and have rarely evaluated chemical degradation in complete particles after prolonged cycling. This study systematically investigates the degradation of a high‐Ni cathode by comparing the chemical, structural, and electrical changes in pristine and 500 times‐cycled cathodes. Electron probe micro‐analysis and X‐ray energy dispersive spectroscopy reveal changes in the Ni:O ratio from 1:2 to 1:1 over a large area inside the secondary particle. Electron energy loss spectroscopy analysis related to structural changes is performed for the entire primary particle area to visualize the oxidation state of transition‐metal ions in two dimensions. The results imply that the observed monotonic capacity fade without unusual changes is due to the continuous formation of the Ni²⁺ phase from the surface to the bulk through chemical and structural degradation.     

Ni and Co Segregations on Selective Surface Facets and Rational Design of Layered Lithium Transition‐Metal Oxide Cathodes

The chemical processes occurring on the surface of cathode materials during battery cycling play a crucial role in determining battery's performance. However, the understanding of such surface chemistry is far from clear due to the complexity of redox chemistry during battery charge/discharge. Through intensive aberration corrected STEM investigation on ten layered oxide cathode materials, two important findings on the pristine oxides are reported. First, Ni and Co show strong plane selectivity when building up their respective surface segregation layers (SSLs). Specifically, Ni‐SSL is exclusively developed on (200)_m facet in Li–Mn‐rich oxides (monoclinic C2/m symmetry) and on (012)_h facet in Mn–Ni equally rich oxides (hexagonal R‐3m symmetry), while Co‐SSL has a strong preference to (20?2)_m plane with minimal Co‐SSL also developed on some other planes in Li–Mn‐rich cathodes. Structurally, Ni‐SSLs tend to form spinel‐like lattice while Co‐SSLs are in a rock‐salt‐like structure. Second, by increasing Ni concentration in these layered oxides, Ni and Co SSLs can be suppressed and even eliminated. The findings indicate that Ni and Co SSLs are tunable through controlling particle morphology and oxide composition, which opens up a new way for future rational design and synthesis of cathode materials.     

In Situ Probing and Synthetic Control of Cationic Ordering in Ni‐Rich Layered Oxide Cathodes

Ni‐rich layered oxides (LiNi_1–x M_x O₂; M = Co, Mn, …) are appealing alternatives to conventional LiCoO₂ as cathodes in Li‐ion batteries for automobile and other large‐scale applications due to their high theoretical capacity and low cost. However, preparing stoichiometric LiNi_1–x M_x O₂ with ordered layer structure and high reversible capacity, has proven difficult due to cation mixing in octahedral sites. Herein, in situ studies of synthesis reactions and the associated structural ordering in preparing LiNiO₂ and the Co‐substituted variant, LiNi_0.8Co_0.2O₂, are made, to gain insights into synthetic control of the structure and electrochemical properties of Ni‐rich layered oxides. Results from this study indicate a direct transformation of the intermediate from the rock salt structure into hexagonal phase, and during the process, Co substitution facilities the nucleation of a Co‐rich layered phase at low temperatures and subsequent growth and stabilization of solid solution Li(Ni, Co)O₂ upon further heat treatment. Optimal conditions are identified from the in situ studies and utilized to obtain stoichiometric LiNi_0.8Co_0.2O₂ that exhibits high capacity (up to 200 mA h g^?1 ) with excellent retention. The findings shed light on designing high performance Ni‐rich layered oxide cathodes through synthetic control of the structural ordering in the materials.     

Layered Oxide Cathodes for Sodium‐Ion Batteries: Phase Transition,Air Stability,and Performance

The increasing demand for replacing conventional fossil fuels with clean energy or economical and sustainable energy storage drives better battery research today. Sodium‐ion batteries (SIBs) are considered as a promising alternative for grid‐scale storage applications due to their similar “rocking‐chair” sodium storage mechanism to lithium‐ion batteries, the natural abundance, and the low cost of Na resources. Searching for appropriate electrode materials with acceptable electrochemical performance is the key point for development of SIBs. Layered transition metal oxides represent one of the most fascinating electrode materials owing to their superior specific capacity, environmental benignity, and facile synthesis. However, three major challenges (irreversible phase transition, storage instability, and insufficient battery performance) are known for cathodes in SIBs. Herein, a comprehensive review on the latest advances and progresses in the exploration of layered oxides for SIBs is presented, and a detailed and deep understanding of the relationship of phase transition, air stability, and electrochemical performance in layered oxide cathodes is provided in terms of refining the structure–function–property relationship to design improved battery materials. Layered oxides will be a competitive and attractive choice as cathodes for SIBs in next‐generation energy storage devices.     

Solid Oxide Fuel Cells: Microstructure of Nanoscaled La0.6Sr0.4CoO3‐δ Cathodes for Intermediate‐Temperature Solid Oxide Fuel Cells (Adv. Energy Mater. 2/2011)

Nanocrystalline La_1‐xSr_xCoO_3‐δ (LSC) thin films with a nominal Sr‐content of x = 0.4 were deposited on Ce_0.9Gd_0.1O_1.95 electrolyte substrates using a low temperature sol‐gel process. The structural and chemical properties of the LSC thin films were studied after thermal treatment, which included a calcination step and a variable, extended annealing time at 700 °C or 800 °C. Transmission electron microscopy combined with selected‐area electron diffraction, energy‐dispersive X‐ray spectrometry, and scanning transmission electron microscopy tomography was applied for the investigation of grain size, porosity, microstructure, and analysis of the local chemical composition and element distribution on the nanoscale. The area specific resistance (ASR) values of the thin film LSC cathodes, which include the lowest ASR value reported so far (ASR_chem = 0.023 Ωcm² at 600 °C) can be interpreted on the basis of the structural and chemical characterization.     

Low‐Temperature Micro‐Solid Oxide Fuel Cells with Partially Amorphous La0.6Sr0.4CoO3‐δ Cathodes

Partially amorphous La_0.6Sr_0.4CoO_3‐δ (LSC) thin‐film cathodes are fabricated using pulsed laser deposition and are integrated in free‐standing micro‐solid oxide fuel cells (micro‐SOFC) with a 3YSZ electrolyte and a Pt anode. A low degree of crystallinity of the LSC layers is achieved by taking advantage of the miniaturization of the cells, which permits low‐temperature operation (300–450 °C). Thermomechanically stable micro‐SOFC are obtained with strongly buckled electrolyte membranes. The nanoporous columnar microstructure of the LSC layers provides a large surface area for oxygen incorporation and is also believed to reduce the amount of stress at the cathode/electrolyte interface. With a high rate of failure‐free micro‐SOFC membranes, it is possible to avoid gas cross‐over and open‐circuit voltages of 1.06 V are attained. First power densities as high as 200–262 mW cm^?2 at 400–450 °C are achieved. The area‐specific resistance of the oxygen reduction reaction is lower than 0.3 Ω cm² at 400 °C around the peak power density. These outstanding findings demonstrate that partially amorphous oxides are promising electrode candidates for the next‐generation of solid oxide fuel cells working at low‐temperatures.     

Microstructure of Nanoscaled La0.6Sr0.4CoO3‐δ Cathodes for Intermediate‐Temperature Solid Oxide Fuel Cells

Nanocrystalline La_1‐xSr_xCoO_3‐δ (LSC) thin films with a nominal Sr‐content of x = 0.4 were deposited on Ce_0.9Gd_0.1O_1.95 electrolyte substrates using a low temperature sol‐gel process. The structural and chemical properties of the LSC thin films were studied after thermal treatment, which included a calcination step and a variable, extended annealing time at 700 °C or 800 °C. Transmission electron microscopy combined with selected‐area electron diffraction, energy‐dispersive X‐ray spectrometry, and scanning transmission electron microscopy tomography was applied for the investigation of grain size, porosity, microstructure, and analysis of the local chemical composition and element distribution on the nanoscale. The area specific resistance (ASR) values of the thin film LSC cathodes, which include the lowest ASR value reported so far (ASR_chem = 0.023 Ωcm² at 600 °C) can be interpreted on the basis of the structural and chemical characterization.     

Tethered Molecular Sorbents: Enabling Metal‐Sulfur Battery Cathodes

A rechargeable battery that uses sulfur at the cathode and a metal (e.g., Li, Na, Mg, or Al) at the anode provides perhaps the most promising path to a solid‐state, rechargeable electrochemical storage device capable of high charge storage capacity. It is understood that solubilization in the electrolyte and loss of sulfur in the form of long‐chain lithium polysulfides (Li₂S_x, 2 < x < 8) has hindered development of the most studied of these devices, the rechargeable Li‐S battery. Beginning with density‐functional calculations of the structure and interactions of a generic lithium polysulfide species with nitrile containing molecules, it is shown that it is possible to design nitrile‐rich molecular sorbents that anchor to other components in a sulfur cathode and which exert high‐enough binding affinity to Li₂S_x to limit its loss to the electrolyte. It is found that sorbents based on amines and imidazolium chloride present barriers to dissolution of long‐chain Li₂S_x and that introduction of as little as 2 wt% of these molecules to a physical sulfur‐carbon blend leads to Li‐S battery cathodes that exhibit stable long‐term cycling behaviors at high and low charge/discharge rates.     

An Ultrafast Rechargeable Hybrid Sodium‐Based Dual‐Ion Capacitor Based on Hard Carbon Cathodes

Hybrid sodium‐based dual‐ion capacitors (NDICs), which integrate the advantages of supercapacitors and sodium‐ion batteries, have attracted tremendous attention recently. In this work, hybrid sodium‐based dual‐ion capacitors are successfully developed with nitrogen‐doped microporous hard carbon as the cathode and soft carbon as the anode. N‐doping is beneficial to the functional groups, porous structure, and electric conductivity of hard carbon. Hybrid NDICs possess a wide voltage range (0.01–4.7 V), high‐energy density of 245.7 W h kg^?1 at a power density of 1626 W kg^?1, long cycle life (1000 cycles), and outstanding rate performance.