Reacquainting the Electrochemical Conversion Mechanism of FeS2 Sodium-Ion Batteries by Operando Magnetometry

Inducing weak and negative Jahn-Teller distortions to alleviate structural deformations for stable sodium storage

In the quest for efficient supercapacitor materials, manganese-based layered oxide cathodes stand out for their cost-effectiveness and high theoretical capacity. However, their progress is hindered by the Jahn-Teller (J-T) distortion due to the unavoidable Mn4+ to Mn3+ reduction during ion storage processes. Our study addresses this challenge by stabilizing the K0.5MnO2 cathode through strategic Mg2+ substitution. This substitution leads to an altered Mn3+ electronic configuration, effectively mitigating the strong J-T distortion during ion storage processes. We provide a comprehensive analysis combining experimental evidence and theoretical insights, highlighting the emergence of the weak and negative J-T effects with reduced structural deformation during electrochemical cycling. Our findings reveal that the K0.5Mn0.85Mg0.15O2 cathode exhibits remarkable durability, retaining 96.0% of initial capacitance after 8000 cycles. This improvement is attributed to the specific electronic configurations of Mn3+ ions, which play a crucial role in minimizing volumetric changes and counteracting structural deformation typically induced by the strong J-T distortion. Our study not only advances the understanding of managing J-T distortion in manganese-based cathodes but also opens new avenues for designing high-stability supercapacitors and other energy storage devices by tailoring electrode materials based on their electronic configurations.

2D Se-Rich ZnSe/CoSe2@C Heterostructured Composite as Ultrastable Anodes for Alkaline-Ion Batteries

2D transitional metal selenide heterostructures are promising electrode materials for potassium-ion batteries (PIBs) owing to the large surface area, high mechanical strength, and short diffusion pathways. However, the cycling performance remains a significant challenge, particularly concerning the electrochemical conversion reaction. Herein, 2D Se-rich ZnSe/CoSe2@C heterostructured composite is fabricated via a convenient hydrothermal approach followed by selenization process, and then applied as high-performance anodes for PIBs. For example, the capacity delivered by the heterostructured composite is mainly contributed to the synergistic effect of conversion and alloy/de-alloy processes aroused by K+, where K+ may highly insert or de-insert into Se-rich ZnSe/CoSe2@C. The obtained electrode delivers an outstanding reversible charge capacity of 214 mA h g-1 at 1 A g-1 after 4000 cycles for PIBs, and achieves 262 mAh g-1 when coupled with a PTCDA cathode in the full cell. The electrochemical conversion mechanism of the optimized electrode during cycling is investigated through in situ XRD, Raman, and ex situ HRTEM. In addition, the heterostructured composite as anodes also displays excellent electrochemical performances for sodium-ion batteries (SIBs) and lithium-ion batteries (LIBs). This work opens up a new window for investigating novel electrode materials with excellent capacity and long durability.

Powering the Future by Iron Sulfide Type Material (FexSy) Based Electrochemical Materials for Water Splitting and Energy Storage Applications: A Review

Water electrolysis is among the recent alternatives for generating clean fuels (hydrogen). It is an efficient way to produce pure hydrogen at a rapid pace with no unwanted by-products. Effective and cheap water-splitting electrocatalysts with enhanced activity, specificity, and stability are currently widely studied. In this regard, noble metal-free transition metal-based catalysts are of high interest. Iron sulfide (FeS) is one of the essential electrocatalysts for water splitting because of its unique structural and electrochemical features. This article discusses the significance of FeS and its nanocomposites as efficient electrocatalysts for oxygen evolution reaction (OER), hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), and overall water splitting. FeS and its nanocomposites have been studied also for energy storage in the form of electrode materials in supercapacitors and lithium- (LIBs) and sodium-ion batteries (SIBs). The structural and electrochemical characteristics of FeS and its nanocomposites, as well as the synthesis processes, are discussed in this work. This discussion correlates these features with the requirements for electrocatalysts in overall water splitting and its associated reactions. As a result, this study provides a road map for researchers seeking economically viable, environmentally friendly, and efficient electrochemical materials in the fields of green energy production and storage.

Voltage Control of Multiple Electrochemical Processes during Lithium Ion Migration in NiFe2O4 Ferrite

Li-ion-based electric field control has been attracting significant attention, since it is able to penetrate deep into materials to exhibit diverse and controllable electrochemical processes, which offer more degrees of freedom to design multifunctional devices with low power consumption. As opposed to previous studies that mainly focused on single lithiation/delithiation mechanisms, we reveal three Li-ion modulation mechanisms in the same NiFe2O4 spinel ferrite by in situ magnetometry, i.e., intercalation, conversion, and space charge, which are respectively demonstrated in high, medium, and low voltage range. During the intercalation stage, the spinel structure is preserved, and a reversible modulation of magnetization arises from the charge transfer-induced variation of Fe valence states (Fe2+/Fe3+). Conversion-driven change in magnetization is the largest up to 89 emu g-1, due to the structural and magnetic phase transitions. Although both intercalation and conversion exhibit sluggish kinetics and long response times, the space charge manifests a faster switching speed and superior durability due to its interface electrostatic effect. These results not only provide a clear and comprehensive understanding on Li-based modulation mechanisms but also facilitate multifunctional and multiscenario applications, such as multistate memory, micromagnetic actuation, artificial synapse, and energy storage.

Advanced Insights on MXenes: Categories, Properties, Synthesis, and Applications in Alkali Metal Ion Batteries

The development and optimization of promising anode material for next-generation alkali metal ion batteries are significant for clean energy evolution. 2D MXenes have drawn extensive attention in electrochemical energy storage applications, due to their multiple advantages including excellent conductivity, robust mechanical properties, hydrophilicity of its functional terminations, and outstanding electrochemical storage capability. In this review, the categories, properties, and synthesis methods of MXenes are first outlined. Furthermore, the latest research and progress of MXenes and their composites in alkali metal ion storage are also summarized comprehensively. A special emphasis is placed on MXenes and their hybrids, ranging from material design and fabrication to fundamental understanding of the alkali ion storage mechanisms to battery performance optimization strategies. Lastly, the challenges and personal perspectives of the future research of MXenes and their composites for energy storage are presented.

Dual-salt strategy tuning the solvation structure to achieve high cycling stability for FeS2 cathodes

Pyrite FeS2, as a promising conversion-type cathode material, faces rapid capacity degradation due to challenges such as polysulfide shuttle and massive volume changes. Herein, a localized high-concentration electrolyte (LHCE) based on dual-salt lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethanesulphonyl)imide (LiTFSI) is designed to address the challenges. By the dual-salt strategy, we tailor a more desirable solvation structure than that in the single-salt system. Specifically, the solvation structure involving FSI- and TFSI- enables milder electrolyte decomposition, which reduces initial capacity loss. Meanwhile, it facilitates the formation of a stable and flexible cathode/electrolyte interphase (CEI), effectively mitigating side effects and accommodating volume changes. Consequently, the micro-sized FeS2 realizes a capacity of 641 mAh g-1 after 600 cycles with a retention rate of 90%, significantly improving the cycling stability of the FeS2 cathode. This work underscores the pivotal role of solvation structure in modulating electrochemical performances and provides a simple and effective electrolyte design concept for conversion-type cathodes.

Deciphering the Performance Enhancement, Cell Failure Mechanism, and Amelioration Strategy of Sodium Storage in Metal Chalcogenides-Based Andes

Transition metal chalcogenides (TMCs) emerge as promising anode materials for sodium-ion batteries (SIBs), heralding a new era of energy storage solutions. Despite their potential, the mechanisms underlying their performance enhancement and susceptibility to failure in ether-based electrolytes remain elusive. This study delves into these aspects, employing CoS2 electrodes as a case in point to elucidate the phenomena. The investigation reveals that CoS2 undergoes a unique irreversible and progressive solid-liquid-solid phase transition from its native state to sodium polysulfides (NaPSs), and ultimately to a Cu1.8S/Co composite, accompanied by a gradual morphological transformation from microspheres to a stable 3D porous architecture. This reconstructed 3D porous structure is pivotal for its exceptional Na+ diffusion kinetics and resilience to cycling-induced stress, being the main reason for ultrastable cycling and ultrahigh rate capability. Nonetheless, the CoS2 electrode suffers from an inevitable cycle life termination due to the microshort-circuit induced by Na metal corrosion and separator degradation. Through a comparative analysis of various TMCs, a predictive framework linking electrode longevity is established to electrode potential and Gibbs free energy. Finally, the cell failure issue is significantly mitigated at a material level (graphene encapsulation) and cell level (polypropylene membrane incorporation) by alleviating the NaPSs shuttling and microshort-circuit.

Reinterpreting the Intercalation-Conversion Mechanism of FeP Anodes in Lithium/Sodium-Ion Batteries from Evolution of the Magnetic Phase

Batteries with intercalation-conversion-type electrodes tend to achieve high-capacity storage, but the complicated reaction process often suffers from confusing electrochemical mechanisms. Here, we reinterpreted the essential issue about the potential of the conversion reaction and whether there is an intercalation reaction in a lithium/sodium-ion battery (LIB/SIB) with the FeP anode based on the evolution of the magnetic phase. Especially, the ever-present intercalation process in a large voltage range followed by the conversion reaction with extremely low potential was confirmed in FeP LIB, while it is mainly the conversion reaction for the sodium storage mechanism in FeP SIB. The insufficient conversion reaction profoundly limits the actual capacity to the expectedly respectable value. Accordingly, a graphene oxide modification strategy was proposed to increase the reversible capacity of FeP LIB/SIB by 99% and 132%, respectively. The results facilitate the development of anode materials with a high capacity and low operating potential.

Real-time tracking of electron transfer at catalytically active interfaces in lithium-ion batteries

Transition metals and related compounds are known to exhibit high catalytic activities in various electrochemical reactions thanks to their intriguing electronic structures. What is lesser known is their unique role in storing and transferring electrons in battery electrodes which undergo additional solid-state conversion reactions and exhibit substantially large extra capacities. Here, a full dynamic picture depicting the generation and evolution of electrochemical interfaces in the presence of metallic nanoparticles is revealed in a model CoCO3/Li battery via an in situ magnetometry technique. Beyond the conventional reduction to a Li2CO3/Co mixture under battery operation, further decomposition of Li2CO3 is realized by releasing interfacially stored electrons from its adjacent Co nanoparticles, whose subtle variation in the electronic structure during this charge transfer process has been monitored in real time. The findings in this work may not only inspire future development of advanced electrode materials for next-generation energy storage devices but also open up opportunities in achieving in situ monitoring of important electrocatalytic processes in many energy conversion and storage systems.

Spin-Polarized Surface Capacitance Effects Enable Fe3 O4 Anode Superior Wide Operation-Temperature Sodium Storage

Fe3 O4 is widely investigated as an anode for ambient sodium-ion batteries (SIBs), but its electrochemical properties in the wide operation-temperature range have rarely been studied. Herein, the Fe3 O4 nanoparticles, which are well encapsulated by carbon nanolayers, are uniformly dispersed on the graphene basal plane (named Fe3 O4 /C@G) to be used as the anode for SIBs. The existence of graphene can reduce the size of Fe3 O4 /C nanoparticles from 150 to 80 nm and greatly boost charge transport capability of electrode, resulting in an obvious size decrease of superparamagnetic Fe nanoparticles generated from the conversion reaction from 5 to 2 nm. Importantly, the ultra-small superparamagnetic Fe nanoparticles (≈2 nm) can induce a strong spin-polarized surface capacitance effect at operating temperatures ranging from -40 to 60 °C, thus achieving highly efficient Na-ion transport and storage in a wide operation-temperature range. Consequently, the Fe3 O4 /C@G anode shows high capacity, excellent fast-charging capability, and cycling stability ranging from -40 to 60 °C in half/full cells. This work demonstrates the viability of Fe3 O4 as anode for wide operation-temperature SIBs and reveals that spin-polarized surface capacitance effects can promote Na-ion storage over a wide operation temperature range.

Boosting Potassium Storage via Multifunctional Interface with High Lattice-Matching

Atomic-scale interface engineering is a prominent strategy to address the large volume expansions and sluggish redox kinetics for reinforcing K-storage. Here, to accelerate charge transport and lower the activation energy, dual carbon-modified interfacial regions are synthesized with high lattice-matching degree, which is formed from a CoSe2 /FeSe2 heterostructure coated onto hollow carbon fibers. State-of-the-art characterization techniques and theoretical analysis, including ex-situ soft X-ray absorption spectroscopy, synchrotron X-ray tomography, ultrasonic transmission mapping, and density functional theory, are conducted to probe local atomic structure evolution, mechanical degradation mechanisms, and ion/electron migration pathways. The results suggest that the heterostructure composed of the same crystal system and space group can sharply regulate the redox kinetics of transition metal selenium and dual carbon-modified approach can tailor physicochemical degradation. Overall, this work presents the design of a stable heterojunction synergistic superior hollow carbon substrate, inspiring a pathway of interface engineering strategy toward high-performance electrode.

Interfacial engineering of Si anodes by confined doping of Co toward high initial coulombic efficiency

A Si/Si-Co multilayer film, with Co confined doping in the silicon anode, was successfully fabricated by alternating magnetron sputtering, achieving both metal doping and surface coating. Operando magnetometry revealed the stability of the Si-Co layers during cycling. The symmetrical Si-Co layers can protect the overall structure of the Si anodes and facilitate electron conduction. Consequently, the resultant Si anode delivers an impressive initial coulombic efficiency of 93.4% with large capacity retention of 85.07% after 100 cycles.

Revealing the effect of LiOH on forming a SEI using a Co magnetic "probe"

The solid-electrolyte-interphase (SEI) plays a critical role in lithium-ion batteries (LIBs) because of its important influence on electrochemical performance, such as cycle stability, coulombic efficiency, etc. Although LiOH has been recognized as a key component of the SEI, its influence on the SEI and electrochemical performance has not been well clarified due to the difficulty in precisely controlling the LiOH content and characterize the detailed interface reactions. Here, a gradual change of LiOH content is realized by different reduction schemes among Co(OH)2, CoOOH and CoO. With reduced Co nanoparticles as magnetic "probes", SEI characterization is achieved by operando magnetometry. By combining comprehensive characterization and theoretical calculations, it is verified that LiOH leads to a composition transformation from lithium ethylene di-carbonate (LEDC) to lithium ethylene mono-carbonate (LEMC) in the SEI and ultimately results in capacity decay. This work unfolds the detailed SEI reaction scenario involving LiOH, provides new insights into the influence of SEI composition, and has value for the co-development between the electrode materials and electrolyte.

Self-Assembled 2D VS2 /Ti3 C2 Tx MXene Nanostructures with Ultrafast Kinetics for Superior Electrochemical Sodium-Ion Storage

Constructing nanostructures with high structural stability and ultrafast electrochemical reaction kinetics as anodes for sodium-ion batteries (SIBs) is a big challenge. Herein, the robust 2D VS2 / Ti3 C2 Tx MXene nanostructures with the strong Ti─S covalent bond synthesized by a one-pot self-assembly approach are developed. The strong interfacial interaction renders the material of good structural durability and enhanced reaction kinetics. Meanwhile, the enlarged and few-layered MXene nanosheets can be easily obtained according to this interaction, providing a conductive network for sufficient electrolyte penetration and rapid charge transfer. As predicted, the VS2 /MXene nanostructures exhibit an extremely low sodium diffusion barrier confirmed by DFT calculations and small charge transfer impedance evidenced by electrochemical impedance spectroscopy (EIS) analysis. Therefore, the SIBs based on the VS2 /MXene electrode present first-class electrochemical performance with the ultrahigh average initial columbic efficiency of 95.08% and excellent sodium-ion storage capacity of 424.6 mAh g-1 even at 10 A g-1 . It also shows an outstanding sodium-ion storage capacity of 514.2 mAh g-1 at 1 A g-1 with a capacity retention of nearly 100% within 500 times high-rate cycling. Such impressive performance demonstrates the successful synthesis strategy and the great potential of interfacial interactions for high-performance energy storage devices.

Preparation of CuO@humic acid@carbon nanotube composite material using humic acid as a coupling agent and its lithium-ion storage performance

The conventional Li-ion battery composite electrode material composed of CuO and carbon nanotubes (CNTs) suffer from poor contact between CuO and CNTs. This results in high electrode resistance and poor electrochemical performance. To solve this problem, CuO@humic acid (HA) @CNT anode material with cross-linked network structure was generated by linking CuO and CNT with HA as a coupling agent. For comparison, CuO@HA or CuO@CNT were also prepared in the absence of CNT or HA, respectively. The results showed that CuO@HA@CNT had lower charge transfer resistance, higher conductivity, lithium-ion diffusion coefficient, specific capacity, and rate capability than CuO@HA and CuO@CNT. The specific capacity of the CuO@HA@CNT electrode was significantly better than that of the composite electrode materials of CuO and CNT, which have been prepared by scientists using various methods. Due to the introduction of HA, not only was the uniformly distributed flower-like CuO obtained, but also the specific capacity and rate capability of the electrode material were substantially improved. This study thus provides a good strategy to optimize the capability of transition metal oxide lithium-ion anode materials.

The Review of Hybridization of Transition Metal-Based Chalcogenides for Lithium-Ion Battery Anodes

Transition metal chalcogenides as potential anodes for lithium-ion batteries have been widely investigated. For practical application, the drawbacks of low conductivity and volume expansion should be further overcome. Besides the two conventional methods of nanostructure design and the doping of carbon-based materials, the component hybridization of transition metal-based chalcogenides can effectively enhance the electrochemical performance owing to the synergetic effect. Hybridization could promote the advantages of each chalcogenide and suppress the disadvantages of each chalcogenide to some extent. In this review, we focus on the four different types of component hybridization and the excellent electrochemical performance that originated from hybridization. The exciting problems of hybridization and the possibility of studying structural hybridization were also discussed. The binary and ternary transition metal-based chalcogenides are more promising to be used as future anodes of lithium-ion batteries for their excellent electrochemical performance originating from the synergetic effect.

Pressure-Stabilized High-Entropy (FeCoNiCuRu)S2 Sulfide Anode toward Simultaneously Fast and Durable Lithium/Sodium Ion Storage

Pressure-stabilized high-entropy sulfide (FeCoNiCuRu)S₂ (HES) is proposed as an anode material for fast and long-term stable lithium/sodium storage performance (over 85% retention after 15 000 cycles @10 A g^-1 ). Its superior electrochemical performance is strongly related to the increased electrical conductivity and slow diffusion characteristics of entropy-stabilized HES. The reversible conversion reaction mechanism, investigated by ex-situ XRD, XPS, TEM, and NMR, further confirms the stability of the host matrix of HES after the completion of the whole conversion process. A practical demonstration of assembled lithium/sodium capacitors also confirms the high energy/power density and long-term stability (retention of 92% over 15 000 cycles @5 A g^-1 ) of this material. The findings point to a feasible high-pressure route to realize new high-entropy materials for optimized energy storage performance.

Tuning Local Structural Configurations to Improve Oxygen-Redox Reversibility of Li-Rich Layered Oxides

Li-rich layered oxides (LLOs) are regarded as one of the most desirable cathode materials due to their high specific capacity. Nevertheless, the irreversible oxygen release associated with low oxygen stability prevents their widespread application. Herein, an improved oxygen redox reversibility was achieved by constructing Ni²⁺-O^2--Ni²⁺ configurations. Superconducting Quantum Interference Device (SQUID) magnetometry measurements are used to track the evolution of the Ni²⁺-O^2--Ni²⁺ configuration during the electrochemical process. The strongest 180° superexchange interaction in the Ni²⁺-O^2--Ni²⁺ configuration, derived from the inevitable Li/Ni mixing in LLOs, regulates the local structure to form the ferrimagnetic (FiM) structural units. Consequently, the FiM structural units prevent the irreversible oxygen release and endow LLOs with high initial Coulombic efficiency (ICE). This work emphasizes the importance of the Ni²⁺-O^2--Ni²⁺ configuration for LLOs with high reversible capacity and proposes a synthesis approach to modulate the amount of FiM structural units.

Enhanced Interfacial Magnetization is Responsible for the Negative Capacity Fading of Cobalt Ditelluride Anodes for Lithium Storage

In lithium-ion batteries (LIBs), the stabilized capacities of transition metal compound anodes usually exhibit higher values than their theoretical values due to the interfacial charge storage, the formation of reversible electrolyte-derived surface layer, or interfacial magnetization. But the effectively utilizing the mechanisms to achieve novel anodes is rarely explored. Herein, a novel nanosized cobalt ditelluride (CoTe₂ ) anodes with ultra-high capacity and long term stability is reported. Electrochemical tests show that the lithium storage capacity of the best sample reaches 1194.7 mA h g^-1 after 150 cycles at 0.12 A g^-1 , which increases by 57.8% compared to that after 20 cycles. In addition, the sample offers capacities of 546.6 and 492.1 mA h g^-1 at 0.6 and 1.8 A g^-1 , respectively. During cycles, CoTe₂ particles (average size 20 nm) are gradually pulverized into the smaller nanoparticles (<3 nm), making the magnetization more fully due to the larger contact area of Co/Li₂ Te interface, yielding an increased capacity. The negative capacity fading is observed, and verified by ex situ structural characterizations and in situ electrochemical measurements. The proposed strategy can be further extended to obtain other high-performance ferromagnetic metal based electrodes for energy storage applications.

Prussian Blue Analogue-Derived Fe-Doped CoS2 Nanoparticles Confined in Bayberry-like N-Doped Carbon Spheres as Anodes for Sodium-Ion Batteries

Obvious volume change and the dissolution of polysulfide as well as sluggish kinetics are serious issues for the development of high performance metal sulfide anodes for sodium-ion batteries (SIBs), which usually result in fast capacity fading during continuous sodiation and desodiation processes. In this work, by utilizing a Prussian blue analogue as functional precursors, small Fe-doped CoS2 nanoparticles spatially confined in N-doped carbon spheres with rich porosity were synthesized through facile successive precipitation, carbonization, and sulfurization processes, leading to the formation of bayberry-like Fe-doped CoS2/N-doped carbon spheres (Fe-CoS2/NC). By introducing a suitable amount of FeCl3 in the starting materials, the optimal Fe-CoS2/NC hybrid spheres with the designed composition and pore structure exhibited superior cycling stability (621 mA h g−1 after 400 cycles at 1 A g−1) and improved the rate capability (493 mA h g−1 at 5 A g−1). This work provides a new avenue for the rational design and synthesis of high performance metal sulfide-based anode materials toward SIBs.

Ultrathin CuF2 -Rich Solid-Electrolyte Interphase Induced by Cation-Tailored Double Electrical Layer toward Durable Sodium Storage

Solid-electrolyte interphase (SEI) seriously affects battery's cycling life, especially for high-capacity anode due to excessive electrolyte decomposition from particle fracture. Herein, we report an ultrathin SEI (3-4 nm) induced by Cu⁺ -tailored double electrical layer (EDL) to suppress electrolyte consumption and enhance cycling stability of CuS anode in sodium-ion batteries. Unique EDL with SO₃ CF₃ -Cu complex absorbing on CuS in NaSO₃ CF₃ /diglyme electrolyte is demonstrated by in situ surface-enhanced Raman, Cyro-TEM and theoretical calculation, in which SO₃ CF₃ -Cu could be reduced to CuF₂ -rich SEI. Dispersed CuF₂ and F-containing compound can provide good interfacial contact for formation of ultrathin and stable SEI film to minimize electrolyte consumption and reduce activation energy of Na⁺ transport. As a result, the modified CuS delivers high capacity of 402.8 mAh g^-1 after 7000 cycles without capacity decay. The insights of SEI construction pave a way for high-stability electrode.

Improved Lithium Storage Performance of a TiO2 Anode Material Doped by Co

TiO₂ is a promising anode material for lithium-ion batteries (LIBs) due to its low cost, suitable operating voltage, and excellent structural stability. The inherent poor electron conductivity and low ion diffusion coefficient, however, severely limit its application in lithium storage. Here, Co-doped TiO₂ is synthesized by a hydrothermal method as an anode material since Co@TiO₂ possesses a large specific surface area and high electronic conductivity. Thanks to the Co dopants, the ion diffusion and electron transport are both greatly improved, which is very beneficial for cycle stability, coulombic efficiency (CE), reversible capacity, and rate performance. As a result, Co@TiO₂ shows a high reversible capacity of 227 mAh g^-1 at 3 C, excellent rate performance, and cycling stability with a capacity of about 125 mAh g^-1 at 10C after 600 cycles (1 C = 170 mA g^-1).

Understanding the Predominant Potassium-Ion Intercalation Mechanism of Single-Phased Bimetal Oxides by in Situ Magnetometry

The electrochemical performance of electrode materials is largely dependent on the structural and chemical evolutions during the charge-discharge processes. Hence, revealing ion storage chemistry could enlighten mechanistic understanding and offer guidance for rational design for energy storage materials. Here, we investigate the mechanisms of potassium (K)-ion storage in the promising bimetal oxide materials by in situ magnetometry. We focus on a single-phased hollow FeTiO₃ (SPH-FTO) hexagonal prism synthesized through a complexing-reagent assisted approach and find that the K-ion storage in this compound occurs predominantly with an intercalation mechanism and fractionally a conversion mechanism. We also demonstrate a K-ion hybrid capacitor assembled with the prepared SPH-FTO hexagonal prism anode and activated carbon cathode, delivering a high energy density and high power density as well as extraordinary cycling stability. This new understanding is used to showcase the inherently high K-ion storage properties from the earth-abundant FeTiO₃.

Fe2(MoO4)3 assembled by cross-stacking of porous nanosheets enables a high-performance aluminum-ion battery

Rechargeable aluminum-ion batteries have attracted increasing attention owing to the advantageous multivalent ion storage mechanism thus high theoretical capacity as well as inherent safety and low cost of using aluminum. However, their development has been largely impeded by the lack of suitable positive electrodes to provide both sufficient energy density and satisfactory rate capability. Here we report a candidate positive electrode based on ternary metal oxides, Fe2(MoO4)3, which was assembled by cross-stacking of porous nanosheets, featuring superior rate performance and cycle stability, and most importantly a well-defined discharge voltage plateau near 1.9 V. Specifically, the positive electrode is able to deliver reversible capacities of 239.3 mA h g-1 at 0.2 A g-1 and 73.4 mA h g-1 at 8.0 A g-1, and retains 126.5 mA h g-1 at 1.0 A g-1 impressively, after 2000 cycles. Furthermore, the aluminum-storage mechanism operating on Al3+ intercalation in this positive electrode is demonstrated for the first time via combined in situ and ex situ characterization studies and density functional theory calculations. This work not only explores potential positive electrodes for aluminum-based batteries but also sheds light on the fundamental charge storage mechanism within the electrode.

Pseudocapacitance-Enhanced Storage Kinetics of 3D Anhydrous Iron (III) Fluoride as a Cathode for Li/Na-Ion Batteries

Transition metal fluoride (TMF) conversion cathodes, with high energy density, are recognized as promising candidates for next-generation high-energy Li/Na-ion batteries (LIBs/SIBs). Unfortunately, the poor electronic conductivity and detrimental active material dissolution of TMFs seriously limit the performance of TMF-LIBs/SIBs. A variety of FeF3-based composites are designed to improve their electrochemical characteristics. However, the storage mechanism of the conversion-type cathode for Li+ and Na+ co-storage is still unclear. Here, the storage mechanism of honeycomb iron (III) fluoride and carbon (FeF3@C) as a general cathode for LIBs/SIBs is analyzed by kinetics. In addition, the FeF3@C cathode shows high electrochemical performance in a full-cell system. The results show that the honeycomb FeF3@C shows excellent long-term cycle stability in LIBs (208.3 mA h g-1 at 1.0 C after 100 cycles with a capacity retention of 98.1%). As a cathode of SIBs, the rate performance is unexpectedly stable. The kinetic analysis reveals that the FeF3@C cathode exhibit distinct ion-dependent charge storage mechanisms and exceptional long-durability cyclic performance in the storage of Li+/Na+, benefiting from the synergistic contribution of pseudocapacitive and reversible redox behavior. The work deepens the understanding of the conversion-type cathode in Li+/Na+ storage.

Revealing An Intercalation-Conversion-Heterogeneity Hybrid Lithium-Ion Storage Mechanism in Transition Metal Nitrides Electrodes with Jointly Fast Charging Capability and High Energy Output

The performance of electrode materials depends intensively on the lithium (Li)-ion storage mechanisms correlating ultimately with the Coulombic efficiency, reversible capacity, and morphology variation of electrode material upon cycling. Transition metal nitrides anode materials have exhibited high-energy density and superior rate capability; however, the intrinsic mechanism is largely unexplored and still unclear. Here, a typical 3D porous Fe2 N micro-coral anode is prepared and, an intercalation-conversion-heterogeneity hybrid Li-ion storage mechanism that is beyond the conventional intercalation or conversion reaction is revealed through various characterization techniques and thermodynamic analysis. Interestingly, using advanced in situ magnetometry, the ratio (ca. 24.4%) of the part where conversion reaction occurs to the entire Fe2 N can further be quantified. By rationally constructing a Li-ion capacitor comprising 3D porous Fe2 N micro-corals anode and commercial AC cathode, the hybrid full device delivers a high energy-density (157 Wh kg-1 ) and high power-density (20 000 W kg-1 ), as well as outstanding cycling stability (93.5% capacitance retention after 5000 cycles). This research provides an original and insightful method to confirm the reaction mechanism of material related to transition metals and a fundamental basis for emerging fast charging electrode materials to be efficiently explored for a next-generation battery.

Elastic Buffering Layer on CuS Enabling High-Rate and Long-Life Sodium-Ion Storage

The latest view suggests the inactive core, surface pulverization, and polysulfide shuttling effect of metal sulfides are responsible for their low capacity and poor cycling performance in sodium-ion batteries (SIBs). Whereas overcoming the above problems based on conventional nanoengineering is not efficient enough. In this work, erythrocyte-like CuS microspheres with an elastic buffering layer of ultrathin polyaniline (PANI) were synthesized through one-step self-assembly growth, followed by in situ polymerization of aniline. When CuS@PANI is used as anode electrode in SIBs, it delivers high capacity, ultrahigh rate capability (500 mAh g^-1 at 0.1 A g^-1, and 214.5 mAh g^-1 at 40 A g^-1), and superior cycling life of over 7500 cycles at 20 A g^-1. A series of in/ex situ characterization techniques were applied to investigate the structural evolution and sodium-ion storage mechanism. The PANI swollen with electrolyte can stabilize solid electrolyte interface layer, benefit the ion transport/charge transfer at the PANI/electrolyte interface, and restrain the size growth of Cu particles in confined space. Moreover, finite element analyses and density functional simulations confirm that the PANI film effectively buffers the volume expansion, suppresses the surface pulverization, and traps the polysulfide.

Strain-regulated Gibbs free energy enables reversible redox chemistry of chalcogenides for sodium ion batteries

Manipulating the reversible redox chemistry of transition metal dichalcogenides for energy storage often faces great challenges as it is difficult to regulate the discharged products directly. Herein we report that tensile-strained MoSe₂ (TS-MoSe₂) can act as a host to transfer its strain to corresponding discharged product Mo, thus contributing to the regulation of Gibbs free energy change (ΔG) and enabling a reversible sodium storage mechanism. The inherited strain results in lattice distortion of Mo, which adjusts the d-band center upshifted closer to the Fermi level to enhance the adsorbability of Na₂Se, thereby leading to a decreased ΔG of the redox chemistry between Mo/Na₂Se and MoSe₂. Ex situ and in situ experiments revealed that, unlike the unstrained MoSe₂, TS-MoSe₂ shows a highly reversible sodium storage, along with an evidently improved reaction kinetics. This work sheds light on the study on electrochemical energy storage mechanism of other electrode materials.

Manipulating the redox chemistry of transition metal dichalcogenides still faces challenges. Here the authors report that tensile-strained MoSe2 can pass on the strain to its sodiated product Mo, and thus regulate the Gibbs free energy in the charging process to enable the reversible sodium storage.

A comparative study of electrochemical and electrostatic doping modulation of magnetism in Fe3O4via ultracapacitor structure

Electric field control of magnetism can boost energy efficiency and have brought revolutionary breakthroughs in the development of widespread applications in spintronics. Electrolyte gating plays an important role in magnetism modulation. In this work, reversible room-temperature electric field control of saturation magnetization in Fe₃O₄via a supercapacitor structure is demonstrated with three types of traditional gate electrolytes for comparison. Different magnetization response and responsible mechanisms are revealed by Operando magnetometry PPMS/VSM and XPS characterization. The main mechanism in Na₂SO₄, KOH aqueous electrolytes is electrochemical effect, while both electrochemical and electrostatic effects were found in LiPF₆organic electrolyte. This work offers a kind of reference basis for selecting appropriate electrolyte in magnetism modulation by electrolyte-gating in the future, meanwhile, paves its way towards practical use in magneto-electric actuation, voltage-assisted magnetic storage, facilitating the development of high-performance spintronic devices.

Hollow CoS/C Structures for High-Performance Li, Na, K Ion Batteries

Alkali ion (Li, Na, and K) batteries as a new generation of energy storage devices are widely applied in portable electronic devices and large-scale energy storage equipment. The recent focus has been devoted to develop universal anodes for these alkali ion batteries with superior performance. Transition metal sulfides can accommodate alkaline ions with large radius to travel freely between layers due to its large interlayer spacing. Moreover, the composite with carbon material can further improve electrical conductivity of transition metal sulfides and reduce the electron transfer resistance, which is beneficial for the transport of alkali ions. Herein, we designed zeolitic imidazolate framework (ZIF)–derived hollow structures CoS/C for excellent alkali ion (Li, Na, and K) battery anodes. The porous carbon framework can improve the conductivity and effectively buffer the stress-induced structural damage. The ZIF-derived CoS/C anodes maintain a reversible capacity of 648.9, and 373.2, 224.8 mAh g⁻¹ for Li, Na, and K ion batteries after 100 cycles, respectively. Its outstanding electrochemical performance is considered as a universal anode material for Li, Na, and K ion batteries.

Hydrothermal Preparation and High Electrochemical Performance of NiS Nanospheres as Anode for Lithium-Ion Batteries

Nickel sulfide has been widely studied as an anode material for lithium-ion batteries due to its environmental friendliness, low cost, high conductivity, and high theoretical capacity. A simple hydrothermal method was used to prepare NiS nanospheres materials with the size in the range of 100–500 nm. The NiS nanospheres electrodes exhibited a high reversible capacity of 1402.3 mAh g−1 at 200 mA g−1 after 280 cycles and a strong rate capability of 814.8 mAh g−1 at 0.8 A g−1 and 1130.5 mAh g−1 when back to 0.1 A g−1. Excellent electrochemical properties and the simple preparation method of the NiS nanospheres make it possible to prepare NiS on a large scale as the anode of lithium-ion batteries.

One-Pot Synthesized Amorphous Cobalt Sulfide With Enhanced Electrochemical Performance as Anodes for Lithium-Ion Batteries

In order to solve the poor cycle stability and the pulverization of cobalt sulfides electrodes, a series of amorphous and crystalline cobalt sulfides were prepared by one-pot solvothermal synthesis through controlling the reaction temperatures. Compared to the crystalline cobalt sulfide electrodes, the amorphous cobalt sulfide electrodes exhibited superior electrochemical performance. The high initial discharge and charge capacities of 2,132 mAh/g and 1,443 mAh/g at 200 mA/g were obtained. The reversible capacity was 1,245 mAh/g after 200 cycles, which is much higher than the theoretical capacity. The specific capability was 815 mAh/g at 800 mA/g and increased to 1,047 mAh/g when back to 100 mA/g, indicating the excellent rate capability. The outstanding electrochemical performance of the amorphous cobalt sulfide electrodes could result from the unique characteristics of more defects, isotropic nature, and the absence of grain boundaries for amorphous nanostructures, indicating the potential application of amorphous cobalt sulfide as anodes for lithium-ion batteries.

Growing Co–Ni–Se nanosheets on 3D carbon frameworks as advanced dual functional electrodes for supercapacitors and sodium ion batteries

The reasonable design of electrode materials is crucial for tuning the electrochemical performances of advanced energy storage systems.

The Synergetic Effect Induced High Electrochemical Performance of CuO/Cu2O/Cu Nanocomposites as Lithium-Ion Battery Anodes

Due to the high theoretical capability, copper-based oxides were widely investigated. A facile water bath method was used to synthesis CuO nanowires and CuO/Cu₂O/Cu nanocomposites. Owing to the synergetic effect, the CuO/Cu₂O/Cu nanocomposites exhibit superior electrochemical performance compared to the CuO nanowires. The initial discharge and charge capacities are 2,660.4 mAh/g and 2,107.8 mAh/g, and the reversible capacity is 1,265.7 mAh/g after 200 cycles at 200 mA/g. Moreover, the reversible capacity is 1,180 mAh/g at 800 mA/g and 1,750 mAh/g when back to 100 mA/g, indicating the excellent rate capability. The CuO/Cu₂O/Cu nanocomposites also exhibit relatively high electric conductivity and lithium-ion diffusion coefficient, especially after cycling. For the energy storage mechanism, the capacitive controlled mechanism is predominance at the high scan rates, which is consistent with the excellent rate capability. The outstanding electrochemical performance of the CuO/Cu₂O/Cu nanocomposites indicates the potential application of copper-based oxides nanomaterials in future lithium-ion batteries.

Ultrahigh Rate and Ultralong Life Span Sodium Storage of FePS3 Enabled by the Space Confinement Effect of Layered Expanded Graphite

Metal phosphorus trichalcogenides have been regarded as promising high-capacity anode materials for sodium-ion batteries (SIBs) owing to their high reversible capacity. Nevertheless, their practical application is plagued by poor diffusion kinetics and dramatic volume fluctuations during the charge-discharge process, resulting in no satisfactory rate and life span so far. Herein, we propose a space-confinement strategy to remarkably promote the cycling stability and rate capacity by embedding FePS₃ particles in the interlayer of expanded graphite (EG), which are derived from in situ transformation of graphite intercalation compounds. The layered EG not only greatly alleviates the volume fluctuations of FePS₃ by the space confinement effect so as to maintain the stability of the electrode microstructure, but it also ensures rapid Na⁺ and electron transfer during cycling. When acting as an anode for SIBs, the hybrid electrode delivers a highly reversible capacity of 312.5 mAh g^-1 at an ultrahigh rate of 50 A g^-1 while retaining an ultralong life span of 1300 cycles with a retention of 82.4% at 10 A g^-1. Moreover, the excellent performance of the assembled full battery indicates the practical application potential of FPS/EG.

Recent Progress on In Situ/Operando Characterization of Rechargeable Alkali Ion Batteries

The specific chemical and physical evolutions of electrode materials under operating conditions should be understood to optimize their electrochemical performances. The in-situ/operando techniques including Raman spectrum, transmission electron microscope, X-ray diffraction, X-ray absorption spectrum, and magnetization are powerful tools, which can provide the real-time surficial/interfacial changes of electrodes, the transformation of crystal lattice structures, the adjustment of electronic states and even the influence of magnetic properties under operating conditions. In this Review, the advantages and limitations of these in-situ/operando techniques in investigating the inner energy storage mechanisms of various type electrode materials are analyzed. The representative research results such as the ion dependent storage mechanism, step-alloying processes and space charge storage theory are highlighted. In addition, the challenges and opportunities of in-situ/operando characterizations are proposed as well.