Achieving a Deeply Desodiated Stabilized Cathode Material by the High Entropy Strategy for Sodium-ion Batteries.

Manganese-based layered oxides are currently of significant interest as cathode materials for sodium-ion batteries due to their low toxicity and high specific capacity. However, the practical applications are impeded by sluggish intrinsic Na+ migration and poor structure stability as a result of Jahn-Teller distortion and complicated phase transition. In this study, a high-entropy strategy is proposed to enhance the high-voltage capacity and cycling stability. The designed P2-Na0.67Mn0.6Cu0.08Ni0.09Fe0.18Ti0.05O2 achieves a deeply desodiation and delivers charging capacity of 158.1 mAh g-1 corresponding to 0.61 Na with a high initial Coulombic efficiency of 98.2 %. The charge compensation is attributed to the cationic and anionic redox reactions conjunctively. Moreover, the crystal structure is effectively stabilized, leading to a slight variation of lattice parameters. This research carries implications for the expedited development of low-cost, high-energy-density cathode materials for sodium-ion batteries.

Revealing the Origin of Transition-Metal Migration in Layered Sodium-Ion Battery Cathodes: Random Na Extraction and Na-Free Layer Formation.

Cation migration often occurs in layered oxide cathodes of lithium-ion batteries due to the similar ion radius of Li and transition metals (TMs). Although Na and TM show a big difference of ion radius, TMs in layered cathodes of sodium-ion batteries (SIBs) can still migrate to Na layer, leading to serious electrochemical degeneration. To elucidate the origin of TM migration in layered SIB cathodes, we choose NaCrO2 , a typical layered cathode suffering from serious TM migration, as a model material and find that the TM migration is derived from the random desodiation and subsequent formation of Na-free layer at high charge potential. A Ru/Ti co-doping strategy is developed to address the issue, where the doped active Ru is first oxidized to create a selective desodiation and the doped inactive Ti can function as a pillar to avoid complete desodiation in Ru-contained TM layers, leading to the suppression of the Na-free layer formation and subsequent enhanced electrochemical performance.

Advanced cobalt-free cathode materials for sodium-ion batteries.

Attempts to utilize lithium-ion batteries (LIBs) in large-scale electrochemical energy storage systems have achieved initial success, and solid-state LIBs using metallic lithium as the anode have also been well developed. However, the sharply increased demands/costs and the limited reserves of the two most important metal elements (Li & Co) for LIBs have raised concerns about future development. Sodium-ion batteries (SIBs) equipped with advanced cobalt-free cathodes show great potential in solving both "lithium panic" and "cobalt panic", and have made remarkable progress in recent years. In this review, we comprehensively summarize the recent advances of high-performance cobalt-free cathode materials for advanced SIBs, systematically analyze the conflicts of structural/electrochemical stability with intrinsic insufficiencies of cobalt-free cathode materials, and extensively discuss the strategies of constructing stable cobalt-free cathode materials by making full use of non-cobalt transition-metal elements and suitable crystal structures, all of which aim to provide insights into the key factors (e.g., phase transformation, particle cracks, crystal defects, lattice distortion, lattice oxygen oxidation, morphology, transition-metal migration/dissolution, and the synergistic effects of composite structures) that can determine the stability of cobalt-free cathode materials, provide guidelines for future research, and stimulate more interest on constructing high-performance cobalt-free cathode materials.

Dual Honeycomb-Superlattice Enables Double-High Activity and Reversibility of Anion Redox for Sodium-Ion Battery Layered Cathodes.

Anion redox contributes to the anomalous capacity exceeding the theoretical limit of layered oxides. However, double-high activity and reversibility is challenging due to the structural rearrangement and potential oxygen loss. Here, we propose a strategy for constructing a dual honeycomb-superlattice structure in Na2/3 [Li1/7 Mn5/14 ][Mg1/7 Mn5/14 ]O2 to simultaneously realize high activity and reversibility of lattice O redox. Theoretical simulation and electrochemical tests show that [Li1/7 Mn5/14 ] superlattice units remarkably trigger the anion redox activity and enable the delivery of a record capacity of 285.9 mA g-1 in layered sodium-ion battery cathodes. Nuclear magnetic resonance and in situ X-ray diffraction reveal that [Mg1/7 Mn5/14 ] superlattice units are beneficial to the structure and anion redox reversibility, where Li+ reversibly shuttles between Na layers and transition-metal slabs in contrast to the absence of [Mg1/7 Mn5/14 ] units. Our findings underline the importance of multifunctional units and provide a path to advanced battery materials.

Pinning Effect Enhanced Structural Stability toward a Zero-Strain Layered Cathode for Sodium-Ion Batteries.

Layered oxides as the cathode materials of sodium-ion batteries are receiving extensive attention due to their high capacity and flexible composition. However, the layered cathode tends to be thermodynamically and electrochemically unstable during (de)sodiation. Herein, we propose the pinning effect and controllable pinning point in sodium storage layered cathodes to enhance the structural stability and achieve optimal electrochemical performance. 0 %, 2.5 % and 7.3 % transition-metal occupancies in Na-site as pinning points are obtained in Na0.67 Mn0.5 Co0.5-x Fex O2 . 2.5 % Na-site pinned by Fe3+ is beneficial to restrain the potential slab sliding and enhance the structural stability, resulting in an ultra-low volume variation of 0.6 % and maintaining the smooth two-dimensional channel for Na-ion transfer. The Na0.67 Mn0.5 Co0.4 Fe0.1 O2 cathode with the optimal Fe3+ pinning delivers outstanding cycle performance of over 1000 cycles and superior rate capability up to 10 C.

Mesoporous and Nanostructured TiO2 layer with Ultra-High Loading on Nitrogen-Doped Carbon Foams as Flexible and Free-Standing Electrodes for Lithium-Ion Batteries.

A simple and green method is developed for the preparation of nanostructured TiO2 supported on nitrogen-doped carbon foams (NCFs) as a free-standing and flexible electrode for lithium-ion batteries (LIBs), in which the TiO2 with 2.5-4 times higher loading than the conventional TiO2 -based flexible electrodes acts as the active material. In addition, the NCFs act as a flexible substrate and efficient conductive networks. The nanocrystalline TiO2 with a uniform size of ≈10 nm form a mesoporous layer covering the wall of the carbon foam. When used directly as a flexible electrode in a LIB, a capacity of 188 mA h g-1 is achieved at a current density of 200 mA g-1 for a potential window of 1.0-3.0 V, and a specific capacity of 149 mA h g-1 after 100 cycles at a current density of 1000 mA g-1 is maintained. The highly conductive NCF and flexible network, the mesoporous structure and nanocrystalline size of the TiO2 phase, the firm adhesion of TiO2 over the wall of the NCFs, the small volume change in the TiO2 during the charge/discharge processes, and the high cut-off potential contribute to the excellent capacity, rate capability, and cycling stability of the TiO2 /NCFs flexible electrode.

High Entropy-Induced Kinetics Improvement and Phase Transition Suppression in K-Ion Battery Layered Cathodes.

Layered oxides are widely accepted to be promising cathode candidate materials for K-ion batteries (KIBs) in terms of their rich raw materials and low price, while their further applications are restricted by sluggish kinetics and poor structural stability. Here, the high-entropy design concept is introduced into layered KIB cathodes to address the above issues, and an example of high-entropy layered K0.45Mn0.60Ni0.075Fe0.075Co0.075Ti0.10Cu0.05Mg0.025O2 (HE-KMO) is successfully prepared. Benefiting from the high-entropy oxide with multielement doping, the developed HE-KMO exhibits half-metallic oxide features with a narrow bandgap of 0.19 eV. Increased entropy can also reduce the surface energy of the {010} active facets, resulting in about 2.6 times more exposure of the {010} active facets of HE-KMO than the low-entropy K0.45MnO2 (KMO). Both can effectively improve the kinetics in terms of electron conduction and K+ diffusion. Furthermore, high entropy can inhibit space charge ordering during K+ (de)insertion, and the transition metal-oxygen covalent interaction of HE-KMO is also enhanced, leading to suppressed phase transition of HE-KMO in 1.5-4.2 V and better electrochemical stability of HE-KMO (average capacity drop of 0.20%, 200 cycles) than the low-entropy KMO (average capacity drop of 0.41%, 200 cycles) in the wide voltage window.

Recent Progress on Honeycomb Layered Oxides as a Durable Cathode Material for Sodium-Ion Batteries.

Sodium-ion batteries (SIBs) are becoming promising candidates for energy storage devices due to the low cost, abundant reserves, and excellent electrochemical performance. As the most important unit, layered cathodes attract much attention, where honeycomb-layered-oxides (HLOs) manifest outstanding structural stability, high redox potential, and long-life electrochemistry. Here, recent progress on HLOs as well as Na3 Ni2 SbO6 and Na3 Ni2 BiO6 as two representative materials are introduced, and the crystal and electronic structure, electrochemical performance, and modification strategies are summarized. The advanced high nickel HLOs are highlighted toward development of state-of-the-art sodium-ion batteries. This review would deepen the understanding of superstructure in layered oxides, as well as structure-property relationship, and inspire more interest in high output voltage, long lifespan sodium-ion batteries.

A new Li-rich layered cathode with low lattice strain for lithium-ion batteries.

A new layered C2/m oxide, Li2Ni0.2Mn0.4Ru0.4O3 (LNMR), is introduced as a cathode for lithium-ion batteries, which undergoes a low volume variation of 1.5% in the voltage window of 2.0-4.6 V studied via in situ X-ray diffraction. Compared with the contrast sample Li2MnO3, LNMR displays superior capacity, a more stable capacity retention rate, and higher energy density and average discharge voltage.

A CNT-Toughened Strategy for In-Situ Repair of Aircraft Composite Structures.

This study aimed to develop an in-situ field-repair approach, especially for aircraft composite structures, to enhance the interlaminar toughness of plain-woven composites (PWCs) by adding multi-walled carbon nanotubes (MWCNTs). MWCNTs were dispersed at each interface between prepreg layers by means of solvent spraying, with a density of 1.58 g/m2. Then, the layers were stacked with the predefined sequence and cured at 120 °C and 1 bar pressure, using a heat-repairing instrument. A standard double cantilever beam (DCB) test was used to investigate the interlaminar toughening effect that was due to the MWCNTs. For comparison, original samples were also prepared. The results indicated that the introduction of MWCNTs can favorably enhance the interlaminar toughness of PWCs in a field-repair approach and the Mode I fracture energy release rate, GIC, increased by 102.92%. Based on the finite element method (FEM) of continuum damage mechanics, the original samples and the MWCNTs toughening specimen under DCB Mode I fracture were modeled and analyzed. The simulation and the experiment were in good agreement. Finally, when the toughening mechanism of MWCNTs was explored with a scanning electron microscope (SEM), we found that a large amount of fiber-matrix (F-M) interface debonding and matrix cracking in mountain shape were the major modes of fracture, accompanied by a small amount of fiber breakage and matrix peeling for the MWCNTs-toughening specimens.

Enhancing the Reversibility of Lattice Oxygen Redox Through Modulated Transition Metal-Oxygen Covalency for Layered Battery Electrodes.

Utilizing reversible lattice oxygen redox (OR) in battery electrodes is an essential strategy to overcome the capacity limitation set by conventional transition metal redox. However, lattice OR reactions are often accompanied with irreversible oxygen oxidation, leading to local structural transformations and voltage/capacity fading. Herein, it is proposed that the reversibility of lattice OR can be remarkably improved through modulating transition metal-oxygen covalency for layered electrode of Na-ion batteries. By developing a novel layered P2-Na0.6 Mg0.15 Mn0.7 Cu0.15 O2 electrode, it is demonstrated that the highly electronegative Cu dopants can improve the lattice OR reversibility to 95% compared to 73% for Cu-free counterpart, as directly quantified through high-efficiency mapping of resonant inelastic X-ray scattering. Crucially, the large energetic overlap between Cu 3d and O 2p states dictates the rigidity of oxygen framework, which effectively mitigates the structural distortion of local oxygen environment upon (de)sodiation and leads to the enhanced lattice OR reversibility. The electrode also exhibits a completely solid-solution reaction with an ultralow volume change of only 0.45% and a reversible metal migration upon cycling, which together ensure the improved electrochemical performance. These results emphasize the critical role of transition metal-oxygen covalency for enhancing the reversibility of lattice OR toward high-capacity electrodes employing OR chemistry.

Design of Block-Copolymer Nanoporous Membranes for Robust and Safer Lithium-Ion Battery Separators.

Lithium-ion batteries (LIBs) suffer from unsatisfied performance and safety risks mainly because of the separators. Herein, a block copolymer (BCP) composed of robust and electrolyte-affinitive polysulfone (PSF) and Li+-affinitive polyethylene glycol (PEG) is rationally designed to prepare a new type of LIB separator. The copolymer is subjected to selective swelling, producing nanoporous membranes with PEG chains enriched along the pore walls. Intriguingly, when used as LIB separators, thus-produced BCP membranes efficiently integrate the merits of both PSF and PEG chains, endowing the separators thermal resistance as high as 150 °C and excellent wettability. Importantly, the nanoporous separator is able to close the pores with a temperature of 125 °C, offering the battery a thermal shutdown function. The membrane exhibits ultrahigh electrolyte uptake up to 501% and a prominent ionic conductivity of 10.1 mS cm-1 at room temperature. Batteries assembled with these membranes show excellent discharge capacity and C-rate performance, outperforming batteries assembled from other separators including the extensively used Celgard 2400. This study demonstrates a facile strategy, selective swelling of block copolymer, to engineer high-performance and safer LIB separators, which is also applicable to produce advanced copolymer-based separators for other types of batteries.

P2-Type Layered Na0.75Ni1/3Ru1/6Mn1/2O2 Cathode Material with Excellent Rate Performance for Sodium-Ion Batteries.

Layered oxides acting as sodium hosts have attracted extensive attention due to their structural flexibility and large theoretical capacity. However, the diffusion of Na ions always presents sluggish kinetics due to the larger ionic radius sand mass of Na compared to Li. Herein, we report a P2-type layered cathode material, namely, Na0.75Ni1/3Ru1/6Mn1/2O2 with superfast ion transport, where the Na+ diffusion coefficient is calculated mainly in the region of 10-10 to 10-11 cm2 s-1 during the charge and discharge process. The electrochemical tests also show that this cathode material exhibits a high capacity of 161.5 mAh g-1, excellent rate performance (when the rate increases from 0.2C-10C, the capacity retention is 74%), and outstanding cyclic performance (maintaining 79.5% for 500 cycles even at a high rate of 10C). Our findings provide new insights for the design of the open framework for fast transport of Na and promote the high-power performance of sodium-ion batteries (SIBs).

Rationally designed Water-Insertable Layered Oxides with Synergistic Effect of Transition-Metal Elements for High-Performance Oxygen Evolution Reaction.

Oxygen evolution reaction (OER) is a key step in many energy conversion and storage processes. Here, by rationally adding an appropriate amount of Mn into the lattice of a layered NaxCoO2 parent oxide, high solubility of iron into the NaxCoO2 oxide lattice is realized without the use of an extremely air-sensitive Na2O2 raw material, and the synergy created between the Co and Fe can boost the catalytic activity of the layered oxide for OER. Moreover, the water intercalation capability of the layered oxides can be utilized to make the oxide resemble mixed metal hydroxides, which will also bring a beneficial effect for OER. As a result, the as-developed Na0.67Mn0.5Co0.3Fe0.2O2 (CF-32) layered oxide with an optimal Co/Fe ratio and water intercalation shows high OER performance in alkaline media, overperforming the benchmark IrO2 catalyst. In 0.1 M KOH solution, the novel catalyst shows 0.39 V overpotential at 10 mA cm-2 and favorable stability. The excellent OER performance of CF-32 is due to the synergistic effect of transition-metal elements (Co and Fe) and water intercalation, leading to little charge transfer resistance, large amounts of exposed catalytic active sites, plenty of surface high oxidation state O22-/O- oxygen species, and hydroxide-rich surface. The facile synthesis and high OER performance of CF-32 enriches the non-noble metal family of OER catalysts and boosts the practical application of non-noble metal catalysts.