Metal-Organic Framework-Derived Co3 V2 O8 @CuV2 O6 Hybrid Architecture as a Multifunctional Binder-Free Electrode for Li-Ion Batteries and Hybrid Supercapacitors

Metal-organic frameworks (MOFs) are promising materials in diverse fields because of their constructive traits of varied structural topologies, high porosity, and high surface area. MOFs are also an ideal precursor/template to derive porous and functional morphologies. Herein, Co₃ V₂ O₈ nanohexagonal prisms are grafted on CuV₂ O₆ nanorod arrays (CuV-CoV)-grown copper foam (CF) using solution-processing methods, followed by thermal treatment. Direct preparation of active material on CF can potentially eliminate electrochemically inactive and non-conductive binders, leading to improved charge-transfer rate. Furthermore, solution-processing methods are simple and cost-effective. Owing to versatile valence states and good redox activity, the vanadium-incorporated mixed metal oxides (CuV-CoV) exhibited superior electrochemical performance in lithium (Li)-ion battery and supercapacitor (SC) studies. Furthermore, hollow carbon particles (HCPs) derived from MOF particles (MOF-HCPs) are used as the anode material in SCs. A hybrid SC (HSC) fabricated with CuV-CoV and MOF-HCP materials exhibited noteworthy electrochemical properties. Moreover, a solid-state HSC (SSHSC) is constructed and its real-time feasibility is investigated by harvesting the dynamic energy of a bicycle with the help of a direct current generator. The charged SSHSCs potentially powered various electronic components.

A Surface Se-Substituted LiCo[O2- δ Seδ ] Cathode with Ultrastable High-Voltage Cycling in Pouch Full-Cells

Cycling LiCoO₂ to above 4.5 V for higher capacity is enticing; however, hybrid O anion- and Co cation-redox (HACR) at high voltages facilitates intrinsic O^{α -} (α < 2) migration, causing oxygen loss, phase collapse, and electrolyte decomposition that severely degrade the battery cyclability. Hereby, commercial LiCoO₂ particles are operando treated with selenium, a well-known anti-aging element to capture oxygen-radicals in the human body, showing an "anti-aging" effect in high-voltage battery cycling and successfully stopping the escape of oxygen from LiCoO₂ even when the cathode is cycled to 4.62 V. Ab initio calculation and soft X-ray absorption spectroscopy analysis suggest that during deep charging, the precoated Se will initially substitute some mobile O^{α -} at the charged LiCoO₂ surface, transplanting the pumped charges from O^{α -} and reducing it back to O^2- to stabilize the oxygen lattice in prolonged cycling. As a result, the material retains 80% and 77% of its capacity after 450 and 550 cycles under 100 mA g^-1 in 4.57 V pouch full-cells matched with a graphite anode and an ultralean electrolyte (2 g Ah^-1 ).

Scalable One-Pot Synthesis of Hierarchical Bi@C Bulk with Superior Lithium-Ion Storage Performances

Lithium-ion batteries (LIBs), the most successful commercial energy storage devices, are now widespread in our daily life. However, the lack of appropriate electrode materials with long lifespan and superior rate capability is the urgent bottleneck for the development of high-performance LIBs. Herein, a hierarchical Bi@C bulk is developed via a scalable pyrolysis method. Due to the ultrafine size of Bi nanoparticles and in situ generated porous carbon framework, this Bi@C anode evidently facilitates the diffusion of Li⁺/electron, availably inhibits the agglomeration of active nano-Bi, and effectively mitigates the volume fluctuation. This hierarchical Bi@C bulk exhibits stable cycling performance for both LIBs (256 mAh g^-1 at 1.0 A g^-1 over 1400 cycles) and potassium-ion batteries (271 mAh g^-1 at 0.1 A g^-1 for 200 cycles). More importantly, when coupled with a commercial LiCoO₂ cathode, the assembled LiCoO₂//Bi@C cells provide an output voltage of 2.9 V and retain a capacity of 202 mAh g^-1 at 0.15 A g^-1. Moreover, kinetic analysis and in situ X-ray diffraction characterization reveal that the Bi@C anode displays a dominated pseudocapacitance behavior and a typical alloying storage mechanism during the cycling process.

Free-standing SnS/carbonized cellulose film as durable anode for lithium-ion batteries

Metal sulfides have recently attracted broad attention for lithium-ion batteries (LIB) owing to their high theoretical capacity and long lifetime. However, the inferior structural integrity and low electron conductivity of metal sulfides limit their practical applications. A feasible strategy is to distribute these materials in conductive carbonaceous substrates with shapeable morphology. Here we report the design of free-standing films of tin sulfide (SnS) nanosheets distributed uniformly on carbonized bacterial cellulose (CBC) nanofibers. The SnS/CBC composites possess three dimensional interconnected nanostructures, which is crucial for the high conductivity and high lithium storage capacity. LIB using SnS/CBC as anode exhibits a reversible capacity of 872 mA h g^-1 at 100 mA g^-1 after 100 cycles, and the capacity remains as high as 527 mA h g^-1 at 2000 mA g^-1 after 1000 cycles. The free-standing sulfide-based nanocomposites with unique nanostructure composition and flexibility could be utilized as promising electrode materials for future LIB systems.

Three-Dimensional Hierarchical Porous Structures Constructed by Two-Stage MXene-Wrapped Si Nanoparticles for Li-Ion Batteries

As the demand for batteries increases with the development of electric vehicles, the energy density of lithium-ion batteries (LIBs) should be continuously enhanced. Due to the excellent theoretical specific capacity, silicon (Si) is the most promising anode material for LIBs. Nevertheless, the application of Si-based anodes is constrained by critical problems such as low conductivity and extreme volume change. Herein, we demonstrate an effective strategy for the fabrication of a three-dimensional (3D) hierarchical porous-structured Si-based anode with dual MXene protection (namely, SiNP@MX1/MX2). By electrostatic force induced self-assembly between modified Si with a positive charge and MXene nanosheets with a negative charge on the surface, Si nanoparticles are riveted to the MXene nanosheets (namely, SiNP@MX1), and then embedded into the 3D MXene skeleton (MX2) via a hydrothermal reaction and freeze-drying. Through the tailored and reasonable design, the internal MX1 coating can accommodate the volume expansion and avoid particle aggregation. The external MX2 allows for rapid electron transport and ion transfer while further buffering volume changes. Most importantly, by preventing Si from directly contacting the electrolyte, the double MXene-wrapped protection design benefits from the formation of a stable solid electrolyte interphase (SEI) film. The SiNP@MX1/MX2 anode material has a high capacity of 1422 mA h g^-1 at a current density of 0.5 A g^-1 after 200 cycles, excellent cycle stability, and good rate performance. At the same time, the method proposed in this study is expected to be applied to the preparation of other alloy anodes/MXene hybrids for storage batteries.

Application of precise neutron focusing mirrors for neutron reflectometry: latest results and future prospects

A large-area focusing supermirror manufactured with ultra-precision machining has been employed at the SOFIA reflectometer at the J-PARC Materials and Life Science Experimental Facility, and a gain of approximately 100% in the neutron flux was achieved. For future upgrade, optics using the focusing mirror for multi-incident-angle neutron reflectometry are proposed, in order to reveal evolutions of interfacial structures for operando measurements with a wide reciprocal space.

Neutron reflectometry (NR) is a powerful tool for providing insight into the evolution of interfacial structures, for example via operando measurements for electrode–electrolyte interfaces, with a spatial resolution of nanometres. The time resolution of NR, which ranges from seconds to minutes depending on the reflection intensity, unfortunately remains low, particularly for small samples made of state-of-the-art materials even with the latest neutron reflectometers. To overcome this problem, a large-area focusing supermirror manufactured with ultra-precision machining has been employed to enhance the neutron flux at the sample, and a gain of approximately 100% in the neutron flux was achieved. Using this mirror, a reflectivity measurement was performed on a thin cathode film on an SrTiO₃ substrate in contact with an electrolyte with a small area of 15 × 15 mm. The reflectivity data obtained with the focusing mirror were consistent with those without the mirror, but the acquisition time was shortened to half that of the original, which is an important milestone for rapid measurements with a limited reciprocal space. Furthermore, a method for further upgrades that will reveal the structural evolution with a wide reciprocal space is proposed, by applying this mirror for multi-incident-angle neutron reflectometry.

Polymer-Derived SiOC Integrated with a Graphene Aerogel As a Highly Stable Li-Ion Battery Anode

Amorphous polymer-derived silicon oxycarbide (SiOC) is an attractive candidate for Li-ion battery anodes, as an alternative to graphite, which is limited to a theoretical capacity of 372 mAh/g. However, SiOC tends to exhibit poor transport properties and cycling performance as a result of sparsely distributed carbon clusters and inefficient active sites. To overcome these limitations, we designed and fabricated a layered graphene/SiOC heterostructure by solvent-assisted infiltration of a polymeric precursor into a modified three-dimensional (3D) graphene aerogel skeleton. The use of a high-melting-point solvent facilitated the precursor's freeze drying, which following pyrolysis yielded SiOC as a layer supported on the surface of nitrogen-doped reduced graphene oxide aerogels. The fabrication method employed here modifies the composition and microstructure of the SiOC phase. Among the studied materials, the highest levels of performance were obtained for a sample of moderate SiOC content, in which the graphene network constituted 19.8 wt % of the system. In these materials, a stable reversible charge capacity of 751 mAh/g was achieved at low charge rates. At high charge rates of 1480 mA/g, the capacity retention was ∼95% (352 mAh/g) after 1000 consecutive cycles. At all rates, Coulombic efficiencies >99% were maintained following the first cycle. Performance across all indicators was majorly improved in the graphene aerogel/SiOC nanocomposites, compared with unsupported SiOC. The performance was attributed to mechanisms across multiple length scales. The presence of oxygen-rich SiO_4-xC_x tetrahedral units and a continuous free-carbon network within the SiOC provides sites for reversible lithiation, while high ionic and electronic transport is provided by the layered graphene/SiOC heterostructure.

Fire-Preventing LiPF6 and Ethylene Carbonate-Based Organic Liquid Electrolyte System for Safer and Outperforming Lithium-Ion Batteries

Battery safety is an ever-increasing significance to guarantee consumer's safety. Reducing or preventing the risk of battery fire and explosion is a must for battery manufacturers. Major reason for the occurrence of fire in commercial lithium-ion batteries is the flammability of conventional organic liquid electrolyte, which is typically composed of 1 M LiPF₆ salt and ethylene carbonate (EC)-based organic solvents. Herein, we report the designed 1 M LiPF₆ and EC-based nonflammable electrolyte including methyl(2,2,2-trifluoroethyl)carbonate, which breaks the conventional perception that EC-based liquid electrolyte is always flammable. The designed electrolyte also provides high anodic stability beyond the conventional charge cut-off voltage of 4.2 V. A graphite∥LiNi_0.6Co_0.2Mn_0.2O₂ lithium-ion full cell with our designed EC-based nonflammable electrolyte with a small fraction of vinylene carbonate additive under an aggressive condition of 4.5 V charge cut-off voltage, 0.5C rate, and 45 °C exhibits increased capacity, reduced interfacial resistance, and improved performance and rate capability. A basic understanding of how a high-voltage cathode-electrolyte interface and anode-electrolyte interface are stabilized and how failure modes are mitigated by fire-preventing electrolyte is discussed.

Feasibility of Chemically Modified Cellulose Nanofiber Membranes as Lithium-Ion Battery Separators

Chemical modification of cellulose is beneficial to produce highly porous lithium-ion battery (LIB) separators, but introduction of high charge density adversely affects its electrochemical stability in a LiNi_1/3Mn_1/3Co_1/3O₂ (NMC)/graphite full cell. In this study, the influence of carboxylate functional groups in 2,2,6,6-tetramethylpiperidine-1-oxyl-mediated oxidized cellulose nanofibers (TOCNs) on the electrochemical performances of the LIB separator was investigated. X-ray photoelectron spectroscopy and in operando mass spectrometry measurements were used to elucidate the cause of failure of the batteries containing TOCN separators in the presence and absence of sodium counterions in the carboxylate groups and additives. For the TOCN separator with sodium carboxylate functional groups, it seems that Na deposition is the dominant reason for poor electrochemical stability of the cell thereof. The poor performance of the protonated TOCN separator, attributed to a high amount of gas evolution, is dramatically improved by adding 2 wt % of vinylene carbonate (VC) because of suppressed gas evolution. Unveiling the failure mechanism of the TOCN separators and successively implementing the strategies to improve performance, for example, removing Na, adding VC, and adjusting cycling rates, enable a remarkable cycling performance in the NMC/graphite full cell at ≈2 C (3 mA/cm²) of a fast discharging rate. Despite the aforementioned efforts and compromises required, an increased charge density of the TOCN is beneficial to acquire a mechanically stronger separator. In conclusion, the manufacturing process of cellulose nanofibers needs to be carefully adjusted to acquire a desired separator property. To the best of our knowledge, it is first reported to perform operando gas evolution measurements to systematically investigate the electrochemical stability of nanocellulose as an LIB separator material. The results elucidate not only the challenges for extensive applications of hygroscopic biomaterials for commercial LIBs but also the practical solutions to achieve high electrochemical stability of the materials.

Boosting reaction kinetics and reversibility in Mott-Schottky VS2/MoS2 heterojunctions for enhanced lithium storage

Heterostructures have lately been recognized as a viable implement to achieve high-energy Li-ion batteries (LIBs) because the as-formed built-in electric field can greatly accelerate the charge transfer kinetics. Herein, we have constructed the Mott-Schottky heterostructured VS₂/MoS₂ hybrids with tailorable 1T/2H phase based on their matchable formation energy, which are made of metallic and few-layered VS₂ vertically grown on MoS₂ surface. The density functional theory (DFT) calculations unveil that such heterojunctions drive the rearrangement of energy band with a facilitated reaction kinetics and enhance the Li adsorption energy more than twice compared to the MoS₂ surface. Furthermore, the VS₂ catalytically expedites the Li-S bond fracture and meantime the enriched Mo⁶⁺ enables the sulfur anchoring toward the oriented reaction with Li⁺ to form Li₂S, synergistically enhancing the reversibility of electrochemical redox. Consequently, the as-obtained VS₂/MoS₂ hybrids deliver a very large specific capacity of 1273 mAh g^-1 at 0.1 A g^-1 with 61% retention even at 5 A g^-1. It can also stabilize 100 cycles at 0.5 A g^-1 and 500 cycles at 1 A g^-1. The findings provide in-depth insights into engineering heterojunctions towards the enhancement of reaction kinetics and reversibility for LIBs.

Enhancing Lithium Storage Performances of the Li4Ti5O12 Anode by Introducing the CuV2O6 Phase

The low electronic conductivity of spinel-structured Li₄Ti₅O₁₂ could be improved by introducing CuV₂O₆. Herein, several Li₄Ti₅O₁₂/CuV₂O₆ composites with different CuV₂O₆ contents have been successfully prepared by a facile liquid-phase dispersion technique. The amount of CuV₂O₆ in composites is shown to affect the particle size and electrochemical performances of Li₄Ti₅O₁₂. The Li₄Ti₅O₁₂/CuV₂O₆ composite prepared with a 5 wt % CuV₂O₆ content (referred to as 5 wt % Li₄Ti₅O₁₂/CuV₂O₆) exhibits the best electrochemical performances among all the Li₄Ti₅O₁₂/CuV₂O₆ composites. The initial discharge/charge capacities of the 5 wt % Li₄Ti₅O₁₂/CuV₂O₆ composite reach 241.1/199.8 mAh g^-1 and retain at 136.8/135.7 mAh g^-1 over 500 cycles at 30 mA g^-1 between 1.0 and 3.0 V. In addition, initial discharge/charge capacities of the 5 wt % Li₄Ti₅O₁₂/CuV₂O₆ composite amount to 129.8/90.5 mAh g^-1 even at 1200 mA g^-1 with maintained discharge/charge capacities of 71.1/71.1 mAh g^-1 over 2500 cycles, which are superior to the pristine Li₄Ti₅O₁₂ in all cases. The detailed electrode kinetic analysis reveals that the introduction of the CuV₂O₆ phase can enhance the lithium-ion transferring rate and cycling stability of Li₄Ti₅O₁₂. The enhanced lithium-storage mechanism of the 5 wt % Li₄Ti₅O₁₂/CuV₂O₆ composite is clarified by in situ X-ray diffraction (XRD) analysis. The acquired data confirms that in situ formation of small amounts of metallic Cu during discharge/charge processes highly enhance the electronic conductivity and decreases the charge-transfer resistance of Li₄Ti₅O₁₂. In sum, the as-obtained 5 wt % Li₄Ti₅O₁₂/CuV₂O₆ composite has potential for future construction of high-rate and long-lifespan anode materials for Li-ion batteries. The work also provides an innovative route to improve electrochemical performances of Li₄Ti₅O₁₂.

High rate capable and high energy supercapacitor performance of reduced graphene oxide/Al(OH)3/polyaniline nanocomposite

The high rate capable, high energy (higher than the lead acid batteries & Nickel-cadmium batteries and comparable with Li-ion batteries) and long lasting supercapacitive performance was achieved from a ternary nanocomposite of rGO/Al(OH)₃/PANI (5.88%:11.77%:82.85%) (GAlP82). The GAlP82 exhibited high cyclic stability till 47,500 cycles at 400 mV s^-1, with increasing trend of specific capacitance (C_s) with increase in No. of energy storage/delivery cycles. After 41,500 cycles the GAlP82 exhibited a C_s of 490.19 F g^-1, an energy density (E) of 98.03 W h kg^-1 and a power density (P) of 2.2829 kW kg^-1 at 1 A g^-1. The GAlP82 exhibited a good rate capability by retaining 73% of C_s up to 10 A g^-1 before cyclic stability study and 33% of C_s up to 23 A g^-1 after 41,500 cycles; and all these impressive performances are achieved from the symmetric supercapacitor cell of GAlP82.

Surface Modification of the LiNi0.8Co0.1Mn0.1O2 Cathode Material by Coating with FePO4 with a Yolk-Shell Structure for Improved Electrochemical Performance

Coating with FePO₄ with the size of 20-30 nm on the surface of a LiNi_0.8Co_0.10Mn_0.1O₂ (NCM811) cathode produces an LFP3@NCM811 cathode via a sol-gel method, which markedly reduces secondary crystal cracking. A stable particle structure greatly improves the cycling stability of the LFP3@NCM811cathode, which retains 97% of its initial discharge capacity compared to NCM811 (78%) after 100 cycles at 2.7-4.5 V. Furthermore, it retains 86 and 63% of its initial discharge capacity after 400 cycles for LFP3@NCM811 and NCM811, respectively. The initial discharge capacity of the LFP3@NCM811 cathode is 218.8 mAh g^-1 at 0.1 C, and the discharge capacity of the LFP3@NCM811 cathode is achieved to be 151.4 mAh g^-1 at 5 C, which is 15 mAh g^-1 higher than that of the NCM811 cathode. These are due to the reduction of cation mixing for a certain amount of Fe²⁺/Fe³⁺ or PO₄^3- doped into the NCM811 surface, and the yolk-shell structure formed by coating with FePO₄ helps improve the electronic conductivity and accelerate the Li⁺ transport. The cycling stability is mainly due to the secondary cleavage inhibition, which maintains the structural integrity of the cathode particles during the long cycle process and protects the inside of the particle from harmful electrolytes.

Hierarchical Design of Mn2P Nanoparticles Embedded in N,P-Codoped Porous Carbon Nanosheets Enables Highly Durable Lithium Storage

Although transition metal phosphide anodes possess high theoretical capacities, their inferior electronic conductivities and drastic volume variations during cycling lead to poor rate capability and rapid capacity fading. To simultaneously overcome these issues, we report a hierarchical heterostructure consisting of isolated Mn₂P nanoparticles embedded into nitrogen- and phosphorus-codoped porous carbon nanosheets (denoted as Mn₂P@NPC) as a viable anode for lithium-ion batteries (LIBs). The resulting Mn₂P@NPC design manifests outstanding electrochemical performances, namely, high reversible capacity (598 mA h g^-1 after 300 cycles at 0.1 A g^-1 ), exceptional rate capability (347 mA h g^-1 at 4 A g^-1), and excellent cycling stability (99% capacity retention at 4 A g^-1 after 2000 cycles). The robust structure stability of Mn₂P@NPC electrode during cycling has been revealed by the in situ and ex situ transmission electron microscopy (TEM) characterizations, giving rise to long-term cyclability. Using in situ selected area electron diffraction and ex situ high-resolution TEM studies, we have unraveled the dominant lithium storage mechanism and confirmed that the superior lithium storage performance of Mn₂P@NPC originated from the reversible conversion reaction. Furthermore, the prelithiated Mn₂P@NPC∥LiFePO₄ full cell exhibits impressive rate capability and cycling stability. This work introduces the potential for engineering high-performance anodes for next-generation high-energy-density LIBs.

Evaluation of Amorphous Oxide Coatings for High-Voltage Li-Ion Battery Applications Using a First-Principles Framework

Cathode surface coatings are widely used industrially as a means to suppress degradation and improve electrochemical performance of lithium-ion batteries. However, developing an optimal coating is challenging, as different coating materials may enhance one aspect of performance while hindering another. To elucidate the fundamental thermodynamic and transport properties of amorphous cathode coating materials, here, we present a framework for calculating and analyzing the Li⁺ and O^2- transport and the stability against delithiation in such materials. Our framework includes systematic workflows of ab-initio molecular dynamics calculations to obtain amorphous structures and diffusion trajectories coupled with an analysis of critical changes of the active-ion local environment during diffusion. Based on these data, we provide an estimate of room-temperature diffusivities, including statistical error bars, and the evaluation of the coating suitability in terms of its ability to facilitate Li⁺ transport while blocking O^2- transport. Finally, we add the thermodynamic stability analysis of the coating chemistry within the operating voltage of common Li-ion cathodes. We apply this framework to two commonly used amorphous coating materials, Al₂O₃ and ZnO. We find that (1) in general, a higher Li⁺ content increases both Li⁺ and O^2- diffusivities in both Al₂O₃ and ZnO. Also, Li⁺ and O^2- diffuse much faster in ZnO than in Al₂O₃. (2) However, neither Al₂O₃ nor ZnO is expected to retain a significant concentration of Li⁺ at high charge. (3) ZnO performs much more poorly in terms of O^2- blocking, and hence, Al₂O₃ is preferred for high-voltage cathode applications. These results will help to quantitatively evaluate amorphous materials, such as metal oxides and fluorides, for different performance metrics and facilitate the development of optimal cathode coatings.

An Atomic View of Cation Diffusion Pathways from Single-Crystal Topochemical Transformations

The diffusion pathways of Li-ions as they traverse cathode structures in the course of insertion reactions underpin many questions fundamental to the functionality of Li-ion batteries. Much current knowledge derives from computational models or the imaging of lithiation behavior at larger length scales; however, it remains difficult to experimentally image Li-ion diffusion at the atomistic level. Here, by using topochemical Li-ion insertion and extraction to induce single-crystal-to-single-crystal transformations in a tunnel-structured V₂ O₅ polymorph, coupled with operando powder X-ray diffraction, we leverage single-crystal X-ray diffraction to identify the sequence of lattice interstitial sites preferred by Li-ions to high depths of discharge, and use electron density maps to create a snapshot of ion diffusion in a metastable phase. Our methods enable the atomistic imaging of Li-ions in this cathode material in kinetic states and provide an experimentally validated angstrom-level 3D picture of atomic pathways thus far only conjectured through DFT calculations.

Lithiation Mechanism Change Driven by Thermally Induced Grain Fining and Its Impact on the Performance of LiMn2 O4 in Lithium-Ion Batteries

The nature of precursors employed in the synthesis of lithium-ion battery cathode materials is a crucial performance-dictating factor. Therefore, it is of great importance to establish a way to manipulate the precursor and seek a comprehensive understanding of its influence on the electrochemical behavior of a targeted electrode material. A thermal route is herein demonstrated for the synthesis of lithium-excess LiMn₂ O₄ (LMO) by exploiting an intriguing thermal phenomenon, thermally induced grain fining, and sheds light on how it affects the mechanism and kinetics of lithiation, and, furthermore, the electrochemical behavior of LMO. Detailed insights into the lithiation mechanism and kinetics reveal that the use of a finely grained, porous Mn₃ O₄ , which possesses an open crystal structure, is a key to the success of incorporating excess Li. In addition, this in-depth electrochemical investigation verifies a very recent theoretical prediction of faster Li diffusion kinetics enabled by excess Li.

Graphene Matrix Sheathed Metal Vanadate Porous Nanospheres for Enhanced Longevity and High-Rate Energy Storage Devices

Rational design of anode materials comprising rich benefits of high capacity, superior rate capability, and exalted lifetime is of considerable significance in the progress of high-performance Li-ion batteries (LIBs) and supercapatteries. Herein, highly porous cobalt vanadate (Co₂VO₄) nanospheres encapsulated with reduced graphene oxide (rGO) nanosheets (rGO@CoV PNSs) were prepared by a facile hydrothermal method and employed as a hybrid composite-based anode material for energy storage devices. The nanocavities and porous features of CoV nanospheres, and the laminated rGO nanosheets over CoV PNSs can significantly surpass the volume changes and enhance the surface electrokinetics, respectively. With benefits of rich redox activity and constructive traits, the rGO@CoV PNSs as an electrode material in LIBs exhibited superior reversible capacity of 780.6 mAh/g after 100 cycles with remarkable rate performance. Moreover, the hybrid composite displayed an excellent reversible capacity of 531.8 mAh/g even after 1000 cycles performed at 1000 mA/g. Utilizing the synergistic features, the rGO@CoV PNSs composite was also explored as a battery-type electrode for supercapatteries. The fabricated supercapattery device with rGO@CoV PNSs and rGO demonstrated good rate performance including superior areal energy (0.048 mWh/cm²) and power (9.96 mW/cm²) densities. Therefore, the graphene sheathed metal vanadates would be an ultrahigh rate electrode candidates for energy storage devices.

Insights into Structural Transformations in the Local Structure of Li2VO2F Using Operando X-ray Diffraction and Total Scattering: Amorphization and Recrystallization

Disordered rock salt Li₂VO₂F cathode material for lithium-ion batteries was investigated using operando X-ray diffraction and total scattering to gain insight into the structural changes of the short-range and long-range orders during electrochemical cycling. The X-ray powder diffraction data show the well-known pattern of the disordered rock salt cubic structure, whereas the pair distribution function (PDF) analysis reveals significant deviations from the ideal cubic structure. During battery operation, a reversible rock salt-to-amorphous phase transformation is observed, upon Li extraction and reinsertion. The X-ray total scattering data show strong indications of the formation of tetrahedrally coordinated V in a nondisordered rock salt phase of the charged electrode material. The results show that the disordered rock salt Li₂VO₂F material undergoes a hidden structural rearrangement during battery operation.

Lithium-Ion-Based Electrochemical Energy Storage in a Layered Vanadium Formate Coordination Polymer

A vanadium formate (VF) coordination polymer and its composite with partially reduced graphene oxide (prGO), namely VF-prGO, can be applied as anode materials for Li-ion based electrochemical energy storage (EcES) systems in the potential range of 0-3 V (vs Li⁺ /Li). This study shows that a reversible capacity of 329 mAh g^-1 at a current density of 50 mA g^-1 after 50 cycles can be realized for VF along with a high rate capability. The composite exhibits even a higher capacity of 504 mAh g^-1 at 50 mA g^-1 . A good capacity retention is observed even after 140 cycles for both VF and the composite. An ex-situ X-ray photoelectron spectroscopy study indicates the involvement of V³⁺ /V⁴⁺ redox couple in the charge storage mechanism. A significant contribution of this reversible capacity is attributed to the pseudocapacitive behavior of the system.

A Ternary Hybrid-Cation Room-Temperature Liquid Metal Battery and Interfacial Selection Mechanism Study

The dendrite-free sodium-potassium (Na-K) liquid alloy composed of two alkali metals is one of the ideal alternatives for Li metal as an anode material while maintaining large capacity, low potential, and high abundance. However, Na- or K-ion batteries have limited cathode materials that can deliver stably large capacity. Combining advantages of both, a hybrid-cation liquid metal battery is designed for a Li-ion-insertion-based cathode to deliver stable high capacity using a Na-K liquid anode to avoid dendrites. The mechanical property of the Na-K alloy is confirmed by simulation and experimental characterization, which leads to stable cycling performance. The charge carrier selection principle in this ternary hybrid-cation system is investigated, showing consistency with the proposed interfacial layer formation and ion distribution mechanism for the electrochemical process as well as the good stability. With Li ions contributing stable cycling as the cathode charge carrier, the K ion working as charge carrier on the anode, and Na as the medium to liquefy K metal, such a ternary hybrid battery system not only inherits the rich battery chemistry of Li-insertion cathodes but also broadens the understanding of alkali metal alloys and hybrid-ion battery chemistry.

Solvated Ion Intercalation in Graphite: Sodium and Beyond

Reversible intercalation of guest ions in graphite is the key feature utilized in modern battery technology. In particular, the capability of Li-ion insertion into graphite enabled the successful launch of commercial Li-ion batteries 30 years ago. On the road to explore graphite as a universal anode for post Li-ion batteries, the conventional intercalation chemistry is being revisited, and recent findings indicate that an alternative intercalation chemistry involving the insertion of solvated ions, designated as co-intercalation, could overcome some of the obstacles presented by the conventional intercalation of graphite. As an example, the intercalation of Na ions into graphite for Na-ion batteries has been perceived as being thermodynamically impossible; however, recent work has revealed that a large amount of Na ions can be reversibly inserted in graphite through solvated-Na-ion co-intercalation reactions. More recently, it has been extensively demonstrated that with appropriate electrolyte selection, not only Na ions but also other ions such as Li, K, Mg, and Ca ions can be co-intercalated into a graphite electrode, resulting in high capacities and power capabilities. The co-intercalation reaction shares a lot in common with the conventional intercalation chemistry but also differs in many respects, which has attracted tremendous research efforts in terms of both fundamentals and practical applications. Herein, we aim to review the progress made in understanding the solvated-ion intercalation mechanisms in graphite and to comprehensively summarize the state-of-the-art achievements by surveying the correlations among the guest ions, co-intercalation conditions, and electrochemical performance of batteries. In addition, the advantages and challenges related to the practical application of graphite undergoing co-intercalation reactions are presented.

Effects of Graphene Nanosheets with Different Lateral Sizes as Conductive Additives on the Electrochemical Performance of LiNi0.5Co0.2Mn0.3O2 Cathode Materials for Li Ion Batteries

In this study, we focus on lateral size effects of graphene nanosheets as conductive additives for LiNi_0.5Co_0.2Mn_0.3O₂ (NCM) cathode materials for Li-ion batteries. We used two different lateral sizes of graphene, 13 (GN-13) and 28 µm (GN-28). It can be found that the larger sheet sizes of graphene nanosheets give a poorer rate capability. The electrochemical measurements indicate that GN-13 delivers an average capacity of 189.8 mAh/g at 0.1 C and 114.2 mAh/g at 2 C and GN-28 exhibits an average capacity of 179.4 mAh/g at 0.1 C and only 6 mAh/g at 2 C. Moreover, according to the results of alternating current (AC) impedance, it can be found that the GN-28 sample has much higher resistance than that of GN-13. The reason might be attributed to that GN-28 has a longer diffusion distance of ion transfer and the mismatch of particle size between NCM and GN-28. The corresponding characterization might provide important reference for Li-ion battery applications.

Mellitic Triimides Showing Three One-Electron Redox Reactions with Increased Redox Potential as New Electrode Materials for Li-Ion Batteries

The mellitic triimide (MTI) bearing three imide groups on a benzene core with C3 symmetry is proposed as a new building block for organic electrode materials in lithium-ion batteries. MTI was anticipated to deliver a higher theoretical specific capacity of up to 282 mAh g^-1 with increased reduction potentials compared with the well-known pyromellitic diimide building block bearing two imide groups because the additional imide group can accept one more electron and provide an electron-withdrawing effect. A model compound, ethyl-substituted mellitic triimide (ETTI), shows three well distinguished and reversible one-electron redox reactions at -0.97, -1.62, and -2.34 V versus Ag/Ag⁺ in 0.1 m tetrabutylammonium hexafluorophosphate electrolyte, but the redox potentials were increased in 2 m lithium bis(trifluoromethanesulfonyl)imide electrolyte: -0.60 V, -0.86 V, and -1.42 V vs. Ag/Ag⁺ . The DFT calculations revealed that the unique C3 symmetric structural design leads to the higher reduction potential of MTI in the Li-based electrolyte by formation of a stable 7-membered ring with a Li ion and the two carbonyl oxygen atoms from the adjacent imide groups. In a Li-ion coin cell, the ETTI electrode delivered a specific capacity of 176 mAh g^-1 , corresponding to 81 % of capacity utilization, with three clear voltage plateaus. The higher average discharge voltage (2.41 V vs. Li/Li⁺ ) of ETTI allows it to deliver one of the highest specific energies (421 Wh kg^-1 ) among reported diimide-based electrode materials. Finally, its redox mechanism was investigated by ex situ FTIR measurements and DFT calculations.