Understanding of the charge storage mechanism of MnO_(2)-based aqueous zinc-ion batteries:Reaction processes and regulation strategies

Though secondary aqueous Zn ion batteries(AZIBs)have been received broad concern in recent years,the development of suitable cathode materials of AZIBs is still a big challenge.The MnO_(2) has been deemed as one of most hopeful cathode materials of AZIBs on account of some extraordinary merits,such as richly natural resources,low toxicity,high discharge potential,and large theoretical capacity.However,the crystal structure diversity of MnO_(2) results in an obvious various of charge storage mechanisms,which can cause great differences in electrochemical performance.Furthermore,several challenges,including intrinsic poor conductivity,dissolution of manganese and sluggish ion transport dynamics should be conquered before real practice.This work focuses on the reaction mechanisms and recent progress of MnO_(2)-based materials of AZIBs.In this review,a detailed review of the reaction mechanisms and optimal ways for enhancing electrochemical performance for MnO_(2)-based materials is proposed.At last,a number of viewpoints on challenges,future development direction,and foreground of MnO_(2)-based materials of aqueous zinc ions batteries are put forward.This review clarifies reaction mechanism of MnO_(2)-based materials of AZIBs,and offers a new perspective for the future invention in MnO_(2)-based cathode materials,thus accelerate the extensive development and commercialization practice of aqueous zinc ions batteries.

High-Resolution Mass Spectroscopy for Revealing the Charge Storage Mechanism in Batteries: Oxamide Materials as an Example

The pursuit of high-performance electrode materials is highly desired to meet the demand of batteries with high energy and power density.However,a deep understanding of the charge storage mechanism is always challenging,which limits the development of advanced electrode materials.Herein,high-resolution mass spectroscopy(HR-MS)is employed to detect the evolution of organic electrode materials during the redox process and reveal the charge storage mechanism,by using small molecular oxamides as an example,which have ortho-carbonyls and are therefore potential electrochemical active materials for batteries.The HR-MS results adequately proved that the oxamides could reversibly store lithium ions in the voltage window of 1.5–3.8 V.Upon deeper reduction,the oxamides would decompose due to the cleavage of the C–N bonds in oxamide structures,which could be proved by the fragments detected by HR-MS,^(1)H NMR,and the generation of NH_(3)after the reduction of oxamide by Li.This work provides a strategy to deeply understand the charge storage mechanism of organic electrode materials and will stimulate the further development of characterization techniques to reveal the charge storage mechanism for developing high-performance electrode materials.

Charge storage mechanism of MOF-derived Mn2O3 as high performance cathode of aqueous zinc-ion batteries

Aqueous Zinc-ion batteries(ZIB) are attracting immense attention because of their merits of excellent safety and quite cheap properties compared with lithium-ion batteries(LIB).Manganese oxide is one of the most important cathode materials of ZIB.In this paper,α-Mn2O3 used as cathode of ZIB is synthesized via Metal-Organic Framework(MOF)-derived method,which delivers a high specific capacity of225 mAh g^(-1) at 0.05 A g^(-1) and 92.7 mAh g^(-1) after 1700 cycles at 2 A g^(-1).The charge storage mechanism of α-Mn2O3 cathode is found to greatly depend on the discharge current density.At lower current density discharging,the H+ and Zn2+ are successively intercalated into the α-Mn2O3 before and after the "turning point" of discharge voltage and their discharging products present obviously different morphologies changing from flower-like to large plate-like products.At a higher current density,the low-voltage plateau after the turning point disappears due to the decrease of amount of Zn2+ intercalation and the H+intercalation is dominated in α-Mn2 O3.This study provides significant understanding for future design and research of high-performance Mn-based cathodes of ZIB.

Computational Insights into Charge Storage Mechanisms of Supercapacitors

Computational modeling methods,including molecular dynamics(MD)and Monte Carlo(MC)simulations,and density functional theory(DFT),are receiving booming interests for exploring charge storage mechanisms of electrochemical energy storage devices.These methods can effectively be used to obtain molecular scale local information or provide clear explanations for novel experimental findings that cannot be directly interpreted through experimental investigations.This short review is dedicated to emphasizing recent advances in computational simulation methods for exploring the charge storage mechanisms in typical nanoscale materials,such as nanoporous carbon materials,2 D MXene materials,and metal-organic framework electrodes.Beyond a better understanding of charge storage mechanisms and experimental observations,fast and accurate enough models would be helpful to provide theoretical guidance and experimental basis for the design of new high-performance electrochemical energy storage devices.

Covalency competition induced selective bond breakage and surface reconstruction in manganese cobaltite towards enhanced electrochemical charge storage

Manganese cobaltite(MnCo_(2)_(4))is a promising electrode material because of its attractive redox chemistry and excellent charge storage capability.Our previous work demonstrated that the octahedrally-coordinated Mn are prone to react with the hydroxyl ions in alkaline electrolyte upon electrochemical cycling and separates on the surface of spinel to reconstruct into d-MnO_(2) nanosheets irreversibly,thus results in a change of the reaction mechanism with Kþion intercalation.However,the low capacity has greatly limited its practical application.Herein,we found that the tetrahedrally-coordinated Co_(2) þions were leached when MnCo_(2)_(4) was equilibrated in 1 mol L^(-1) HCl solution,leading to the formation of layered CoOOH on MnCo_(2)_(4) surface which is originated from the covalency competition induced selective breakage of the CoT–O bond in CoT–O–CoO and subsequent rearrangement of free Co_(6) octahedra.The as-formed CoOOH is stable upon cycling in alkaline electrolyte,exhibits conversion reaction mechanism with facile proton diffusion and is free of massive structural evolution,thus enables utilization of the bulk electrode material and realizes enhanced specific capacity as well as facilitated charge transfer and ion diffusion.In general,our work not only offers a feasible approach to deliberate modification of MnCo_(2)_(4)'s surface structure,but also provides an in-depth understanding of its charge storage mechanism,which enables rational design of the spinel oxides with promising charge storage properties.

Disclosure of charge storage mechanisms in molybdenum oxide nanobelts with enhanced supercapacitive performance induced by oxygen deficiency

Molybdenum oxide(MoO_(3)), with superior features of multi-electrochemical states, high theoretical capacitance, and low cost, is a desirable supercapacitor electrode material but suffers from low conductivity and insufficient active sites. The MoO_(3) capacitance can be largely amplified by introducing oxygen(O) vacancies, but the mechanisms at the atomic scale are still ambiguous.Herein, O vacancies are created at the O2 and O3 sites in the MoO_(3) nanobelts by carbonization to maximize the supercapacitance in the MoO_(2.39). The supercapacitive storage is mainly ascribed to the proton adsorption at the O1 sites to create Mo–OH, leading to an expansion of the interlayer spacing along the lattice B-axis. Roughly 98% of the initial supercapacitance is retained after 1000 cycles,due to the reversible change in the interlayer spacing. Our results provide an insight into the oxygen deficiency-related mechanisms of the supercapacitive performance at the atomic scale and devise a facile method to enhance the supercapacitance for energy storage and conversion.

Charge storage mechanisms of cathode materials in rechargeable aluminum batteries

Rechargeable aluminum batteries(RABs)have attracted great interest as one of the most promising candidates for large-scale energy storage because of their high volumetric capacity,low cost,high safety and the abundance of aluminum.However,compared with the aluminum anodes,the cathode materials face more problems including low specific capacity,relatively sluggish kinetics in most host structures and/or limited cycle lifespan,which pose the major challenge for RABs in further practical applications.During the past years,intensive efforts have been devoted to developing new cathode materials and/or designing engineered nanostructures to greatly improve RABs’electrochemical performances.In addition to nanotechnologybased electrode structure designs,the intrinsic chemical structures and charge storage mechanisms of cathode materials play an equally crucial role,if not more,in revolutionizing the battery performances.This review,here,focuses on current understandings into the charge storage mechanisms of cathode materials in RABs from a chemical reaction point of view.First,the fundamental chemistry,charge storage mechanisms and design principles of RAB cathode materials are highlighted.Based on different ion charge carriers,the current cathode materials are classified into four groups,including Al^(3+)-hosting,Al Cl_(4)^(-)-hosting,Al Cl_(2)^(+)/Al Cl_(2)^(+)-hosting,and Cl^(-)-hosting cathode materials.Next,the respective typical electrode structures,optimization strategies,electrochemical performances and charge storage mechanisms are discussed in detail to establish their chemistry-structure-property relationships.This review on current understandings of the cathode charge storage mechanisms will lay the ground and hopefully set new directions into the rational design of high-performance cathode materials in RABs,and open up new opportunities for designing new electrolyte systems with respect to the targeted cathode systems.

Charge storage mechanism of copper hexacyanoferrate nanocubes for supercapacitors

The widely accepted theory concerning the electrochemical energy storage mechanism of copper hexacyanoferrate(CuHCF)for supercapacitors is that CuHCF stores charge by the reversible redox processes of Fe^3+/Fe2+couple and Cu cations are electrochemically inactive.In this work,CuHCF nanocubes(CuHCF-NC)were synthesized in the presence of potassium citrate and its electrochemical properties were tentatively studied in 1 mol/L Na2 SO4 aqueous electrolyte.Good supercapacitive performance was exhibited.The combined analyses of cyclic voltammogram(CV)and X-ray photoelectron spectroscopy(XPS)disclosed that the CuHCF nanocubes underwent the redox reactions of Fe^3+/Fe2+and Cu^2+/Cu+couples to store charges.The Cu^2+/Cu+redox couple was activated due to the strong coordination interaction between the carboxylate groups of citrate ions and surface Cu cations.

Unraveling the mechanism of potassium metal capacitor for highly efficient charge storage

Although hybrid metal ion capacitors(MICs) are highly desired to achieve both high power density of supercapacitors and high energy density of rechargeable batteries, the mismatch problem of electrochemical kinetics of negative and positive electrodes in MICs hampers the realization of this goal. Here, a new hybrid capacitor concept-potassium metal capacitor(PMC) is proposed for the first time, where potassium metal and commercial activated carbon(AC) without any modification are applied as negative and positive electrodes, respectively, and the electrolyte is the same as that of non-aqueous potassium ion batteries. The simplest PMC prototype exhibits a good combination of high energy density(184.9 Wh kg^(-1)) and power density(12.4 kW kg^(-1)), which benefits from the synergistic effect of potassium metal and AC electrode. The former experiences fast potassium plating/striping during charging and discharging, and the later possesses complex multiple charge behaviors driven by low potential of potassium metal. Specifically, below open-circuit voltage, transportation of solvated cations in AC pores plays an important role;beyond this voltage, synergy actions of cations and anions, including adsorption/desorption of solvated cations and anions, and ions exchange between them, dominate the capacitance contribution. This work enriches the types of MICs, and deepens the understanding of the energy storage mechanism of non-aqueous hybrid metal capacitors.

Molecular Understanding of Charge Storage in MoS_(2)Supercapacitors with Ionic Liquids

Owing to high electrical conductivity and ability to reversibly host a variety of inserted ions,2D metallic molybdenum disulfide(1 T-MoS_(2))has demonstrated promising energy storage performance when used as a supercapacitor electrode.However,its charge storage mechanism is still not fully understood,in particular,how the interlayer spacing of 1 T-MoS_(2)would affect its capacitive performance.In this work,molecular dynamics simulations of 1 T-MoS_(2)with interlayer spacing ranging from 0.615 to 1.615 nm have been performed to investigate the resulting charge storage capacity in ionic liquids.Simulations reveal a camel-like capacitance-potential relation,and MoS_(2)with an interlayer spacing of 1.115 nm has the highest volumetric and gravimetric capacitance of118 F cm^(-3)and 42 F g^(-1),respectively.Although ions in MoS_(2)with an interlayer spacing of 1.115 nm diffuse much faster than with interlayer spacings of 1.365 and 1.615 nm,the MoS_(2)with larger interlayer spacing has a much faster-charging process.Our analyses reveal that the ion number density and its charging speed,as well as ion motion paths,have significant impacts on the charging response.This work helps to understand how the interlayer spacing affects the interlayer ion structures and the capacitive performance of MoS_(2),which is important for revealing the charge storage mechanism and designing MoS_(2)supercapacitor.

Mechanistic understanding of the charge storage processes in FeF_(2) aggregates assembled with cylindrical nanoparticles as a cathode material for lithium-ion batteries by in situ magnetometry

Transition metal fluorides(TMFs)cathode materials have shown extraordinary promises for electrochemical energy storage,but the understanding of their electrochemical reaction mechanisms is still a matter of debate due to the complicated and continuous changing in the battery internal environment.Here,we design a novel iron fluoride(FeF_(2))aggregate assembled with cylindrical nanoparticles as cathode material to build FeF_(2) lithium-ion batteries(LIBs)and employ advanced in situ magnetometry to detect their intrinsic electronic structure during cycling in real time.The results show that FeF_(2) cannot be involved in complete conversion reactions when the FeF_(2) LIBs operate between the conventional voltage range of 1.0–4.0 V,and that the corresponding conversion ratio of FeF_(2) can be further estimated.Importantly,we first demonstrate that the spin-polarized surface capacitance exists in the FeF_(2) cathode by monitoring the magnetic responses over various voltage ranges.The research presents an original and insightful method to examine the conversion mechanism of TMFs and significantly provides an important reference for the future artificial design of energy systems based on spinpolarized surface capacitance.

Organic cathode materials for rechargeable magnesium-ion batteries:Fundamentals, recent advances, and approaches to optimization

Rechargeable magnesium-ion batteries(MIBs) are favorable substitutes for conventional lithium-ion batteries(LIBs) because of abundant magnesium reserves, a high theoretical energy density, and great inherent safety. Organic electrode materials with excellent structural tunability,unique coordination reaction mechanisms, and environmental friendliness offer great potential to promote the electrochemical performance of MIBs. However, research on organic magnesium battery cathode materials is still preliminary with many significant challenges to be resolved including low electrical conductivity and unwanted but severe dissolution in useful electrolytes. Herein, we provide a detailed overview of reported organic cathode materials for MIBs. We begin with basic properties such as charge storage mechanisms(e.g., n-, p-, and bipolartype), moving to recent advances in various types of organic cathodes including carbonyl-, nitrogen-, and sulfur-based materials. To shed light on the diverse strategies targeting high-performance Mg-organic batteries, elaborate summaries of various approaches are presented.Generally, these strategies include molecular design, polymerization, mixing with carbon, nanosizing and electrolyte/separator optimization.This review provides insights on exploring high-performance organic cathodes in rechargeable MIBs.

Electrochemical Proton Storage:From Fundamental Understanding to Materials to Devices

Simultaneously improving the energy density and power density of electrochemical energy storage systems is the ultimate goal of electrochemical energy storage technology.An effective strategy to achieve this goal is to take advantage of the high capacity and rapid kinetics of electrochemical proton storage to break through the power limit of batteries and the energy limit of capacitors.This article aims to review the research progress on the physicochemical properties,electrochemical performance,and reaction mechanisms of electrode materials for electrochemical proton storage.According to the different charge storage mechanisms,the surface redox,intercalation,and conversion materials are classified and introduced in detail,where the influence of crystal water and other nanostructures on the migration kinetics of protons is clarified.Several reported advanced full cell devices are summarized to promote the commercialization of electrochemical proton storage.Finally,this review provides a framework for research directions of charge storage mechanism,basic principles of material structure design,construction strategies of full cell device,and goals of practical application for electrochemical proton storage.

Enhancing charge storage of O-doped perovskite fluorides via in-situ electrochemical oxidation for high-performance potassium-ion capacitors

Potassium ion capacitors(PICs)are regarded as promising large-scale aqueous energy storage systems.However,due to the poor K^(+)transport kinetics and the structural instability of the cathode materials,the key issues of limited energy density and poor cyclic stability are obstacles to the in-depth growth of PICs.Herein,a novel O-doped perovskite fluoride is demonstrated via an in-situ electrochemical oxidation strategy as the cathode for PICs,introducing additional defects that improve the capacitance and facilitate the reaction kinetics of the electrode.During the electrochemical oxidation process,it is discovered that the perovskite fluoride crystal tends to transform into disordered O-doped KMnF 3(K_(x)MnF_(y)O_(z)),realizing a structural reconstruction at the electrode material/electrolyte interface.The First-principles calculations based on density functional theory(DFT)are performed to confirm that the improved electrical conduc-tivity and low ionic adsorption energy may be ascribed to the substitution of oxygen for fluorine.The obtained K_(1.14)MnF_(1.17)O_(1.26) cathode achieves a high specific capacitance of 694 F g^(-1) at 1 A g^(-1),as well as high capacitance retention of 91.3%after 10,000 charge/discharge cycles in mild K_(2)SO_(4) electrolyte.This study provides an effective strategy to improve the capacitive performance of perovskite fluoride cathode materials in electrochemical energy storage.

Advances in in-situ characterizations of electrode materials for better supercapacitors

In past decades,the performance of supercapacitors has been greatly improved by rationalizing the electrode materials at the nanoscale.However,there is still a lack of understanding on how the charges are efficiently stored in the electrodes or transported across the electrolyte/electrode interface.As it is very challenging to investigate the ion-involved physical and chemical processes with single experiment or computation,combining advanced analytic techniques with electrochemical measurements,i.e.,developing in-situ characterizations,have shown considerable prospect for the better understanding of behaviors of ions in electrodes for supercapacitors.Herein,we briefly review several typical in-situ techniques and the mechanisms these techniques reveal in charge storage mechanisms specifically in supercapacitors.Possible strategies for designing better electrode materials are also discussed.

Identifying Heteroatomic and Defective Sites in Carbon with Dual-Ion Adsorption Capability for High Energy and Power Zinc Ion Capacitor

Aqueous zinc-based batteries(AZB s)attract tremendous attention due to the abundant and rechargeable zinc anode.Nonetheless,the requirement of high energy and power densities raises great challenge for the cathode development.Herein we construct an aqueous zinc ion capacitor possessing an unrivaled combination of high energy and power characteristics by employing a unique dual-ion adsorption mechanism in the cathode side.Through a templating/activating co-assisted carbonization procedure,a routine protein-rich biomass transforms into defect-rich carbon with immense surface area of 3657.5 m^(2) g^(-1) and electrochemically active heteroatom content of 8.0 at%.Comprehensive characterization and DFT calculations reveal that the obtained carbon cathode exhibits capacitive charge adsorptions toward both the cations and anions,which regularly occur at the specific sites of heteroatom moieties and lattice defects upon different depths of discharge/charge.The dual-ion adsorption mechanism endows the assembled cells with maximum capacity of 257 mAh g^(-1) and retention of72 mAh g^(-1) at ultrahigh current density of 100 A g^(-1)(400 C),corresponding to the outstanding energy and power of 168 Wh kg^(-1)and 61,700 W kg^(-1).Furthermore,practical battery configurations of solid-state pouch and cable-type cells display excellent reliability in electrochemistry as flexible and knittable power sources.

Symmetrizing cathode-anode response to speed up charging of nanoporous supercapacitors

Asymmetric behaviors of capacitance and charging dynamics in the cathode and anode are general for nanoporous supercapacitors.Understanding this behavior is essential for the optimal design of supercapacitors.Herein,we perform constant-potential molecular dynamics simulations to reveal asymmetric features of porous supercapacitors and their effects on capacitance and charging dynamics.Our simulations show that,counterintuitively,charging dynamics can be fast in pores providing slow ion diffusion and vice versa.Unlike electrodes with singlesize pores,multi-pore electrodes show overcharging and accelerated co-ion desorption,which can be attributed to the subtle interplay between the dynamics and charging mechanisms.We find that capacitance and charging dynamics correlate with how the ions respond to an applied cell voltage in the cathode and anode.We demonstrate that symmetrizing this response can help boost power density,which may find practical applications in supercapacitor optimization.

Ultrathin Zn-free anode based on Ti_(3)C_(2)T_(x) and nanocellulose enabling high-durability aqueous hybrid Zn-Na battery with Zn2+/Na+co-intercalation mechanism

With low cost and high safety,aqueous zinc-based batteries have received considerable interest.Nevertheless,the excess utilization of zinc metal in the anodes of these batteries reduces energy density and increases costs.Herein,an ultrathin electrode of approximately 6.2μm thick is constructed by coating Ti_(3)C_(2)T_(x)/nanocellulose hybrid onto a stainless steel foil.This electrode is used as the Zn-free anode for aqueous hybrid Zn-Na battery,in which,a concentrated electrolyte is used to improve electrochemical reversibility.The Ti_(3)C_(2)T_(x)/nanocellulose coating is found to improve the electrolyte wettability,facilitate desolvation process of hydrated Zn^(2+) ions,lower nucleation overpotential,improve zinc plating kinetics,guide horizontal zinc plating along the Zn(002)facet,and inhibit parasitic side reactions.It is also found that the Na_(3)V_(2)(PO_(4))_(3) cathode material adopts a highly reversible Zn^(2+)/Na^(+)co-intercalation charge storage mechanism in this system.Thanks to these benefits,the assembled hybrid Zn-Na battery exhibits excellent rate capability,superior cyclability,and good anti-freezing ability.This work provides a new concept of electrode design for electrochemical energy storage.

Iron anode-based aqueous electrochemical energy storage devices: Recent advances and future perspectives

The ever-growing demands for green and sustainable power sources for ap-plications in grid-scale energy storage and portable/wearable devices have enabled the continual development of advanced aqueous electrochemical energy storage(EES)systems.Aqueous batteries and supercapacitors made of iron-based anodes are one of the most promising options due to the remark-able electrochemical features and natural abundance,pretty low cost and good environmental friendliness of ferruginous species.Though impressive ad-vances in developing the state-of-the-art ferruginous anodes and designing various full-cell aqueous devices have been made,there still remain key issues and challenges on the way to practical applications,which urgently need discussing to put forwards possible solutions.In this review,rather than focusing on the detailed methods to optimize the iron anode,electrolyte,and device performance,we first give a comprehensive review on the charge storage mechanisms for ferruginous anodes in different electrolyte systems,as well as the newly developed iron-based aqueous EES devices.The deep in-sights,involving the inherent failure mechanisms and corresponding modification/optimization strategies toward iron anodes for the development of high-performance aqueous EES devices,will then be discussed.The advances in applying iron-based aqueous EES devices for emerging fields such as flexible/wearable electronics and functionalized building materials will be further outlined.Last,future research trends and perspectives for maximizing the potential of current iron anodes and devices as well as exploiting brand-new iron-based aqueous EES systems are put forward.

MXenes:A comprehensive review of synthesis,properties,and progress in supercapacitor applications

MXenes,a class of two-dimensional materials,have garnered significant attention due to their unique properties and versatile applications in various fields.This review provides a comprehensive overview of MXene synthesis methods,highlighting their distinctive layered structure and tunable properties through surface functionalization.The focus then shifts to their remarkable role in supercapacitor technology.MXenes exhibit high electrical conductivity,large surface areas,and tailored surface chemistry,making them promising candidates for energy storage in supercapacitors.The paper discusses the interplay of electric double-layer capacitance and pseudocapacitance mechanisms within MXenebased electrodes,detailing recent research efforts aimed at optimizing their energy storage performance.Through a combination of theoretical insights and experimental findings,the potential of MXenes to revolutionize supercapacitor technology emerges,offering prospects for high-energy-density and long-lasting energy storage solutions.Additionally,this review highlights recent advances in MXenebased supercapacitors,including novel electrode designs,electrolyte engineering,and hybrid materials,showcasing the dynamic evolution of MXene-based supercapacitor research.This comprehensive overview aims to provide a thorough understanding of MXenes'synthesis,properties,and their pivotal role in advancing supercapacitor technology,while also encompassing the latest breakthroughs in this rapidly evolving field.