Current Progress and Future Perspectives of Electrolytes for Rechargeable Aluminum-Ion Batteries

Aluminum-ion batteries(AIBs)with Al metal anode are attracting increasing research interest on account of their high safety,low cost,large volumetric energy density(≈8046 mA h cm−3),and environmental friendliness.Specifically,the reversible Al electrostripping/deposition is achieved with the rapid development of room temperature ionic liquids,and rapid progress has been made in fabricating high-performance and durable AIBs during the past decade.This review provides an integrated comprehension of the evolution of AIBs and highlights the development of various non-aqueous and aqueous electrolytes including high-temperature molten salts,room temperature ionic liquids,and gel–polymer electrolytes.The critical issues on the interplay of electrolytes are outlined in terms of the voltage window span,the effective ion species during charge storage(Al 3+or Al x Cl?y)and their underlying charge transfer(e.g.,interfacial transfer and diffusion),and the solid electrolyte interface formation and its role.Following the critical insight,future perspectives on how to practically design feasible AIBs are given.

Defect Engineering:Can it Mitigate Strong Coulomb Effect of Mg^(2+)in Cathode Materials for Rechargeable Magnesium Batteries?

Rechargeable magnesium batteries(RMBs)have been considered a promising“post lithium-ion battery”system to meet the rapidly increasing demand of the emerging electric vehicle and grid energy storage market.However,the sluggish diffusion kinetics of bivalent Mg^(2+)in the host material,related to the strong Coulomb effect between Mg^(2+)and host anion lattices,hinders their further development toward practical applications.Defect engineering,regarded as an effective strategy to break through the slow migration puzzle,has been validated in various cathode materials for RMBs.In this review,we first thoroughly understand the intrinsic mechanism of Mg^(2+)diffusion in cathode materials,from which the key factors affecting ion diffusion are further presented.Then,the positive effects of purposely introduced defects,including vacancy and doping,and the corresponding strategies for introducing various defects are discussed.The applications of defect engineering in cathode materials for RMBs with advanced electrochemical properties are also summarized.Finally,the existing challenges and future perspectives of defect engineering in cathode materials for the overall high-performance RMBs are described.

Dual-Defect Engineering Strategy Enables High-Durability Rechargeable Magnesium-Metal Batteries

Rechargeable magnesium-metal batteries(RMMBs)are promising next-generation secondary batteries;however,their development is inhibited by the low capacity and short cycle lifespan of cathodes.Although various strategies have been devised to enhance the Mg^(2+)migration kinetics and structural stability of cathodes,they fail to improve electronic conductivity,rendering the cathodes incompatible with magnesium-metal anodes.Herein,we propose a dual-defect engineering strategy,namely,the incorporation of Mg^(2+)pre-intercalation defect(P-Mgd)and oxygen defect(Od),to simultaneously improve the Mg^(2+)migration kinetics,structural stability,and electronic conductivity of the cathodes of RMMBs.Using lamellar V_(2)O_(5)·nH_(2)O as a demo cathode material,we prepare a cathode comprising Mg_(0.07)V_(2)O_(5)·1.4H_(2)O nanobelts composited with reduced graphene oxide(MVOH/rGO)with P-Mgd and Od.The Od enlarges interlayer spacing,accelerates Mg^(2+)migration kinetics,and prevents structural collapse,while the P-Mgd stabilizes the lamellar structure and increases electronic conductivity.Consequently,the MVOH/rGO cathode exhibits a high capacity of 197 mAh g^(−1),and the developed Mg foil//MVOH/rGO full cell demonstrates an incredible lifespan of 850 cycles at 0.1 A g^(−1),capable of powering a light-emitting diode.The proposed dual-defect engineering strategy provides new insights into developing high-durability,high-capacity cathodes,advancing the practical application of RMMBs,and other new secondary batteries.

A perspective on the key factors of safety for rechargeable magnesium batteries

Rechargeable Mg batteries(RMBs)have become one of the best subsitutes for lithium-ion batteries due to the high volumetric capacity,abundant resources,and uniform plating behavior of Mg metal anode.However,the safety hazard induced by the formation of high-modulue Mg dendrites under a high current density(10 mA cm^(-1))was still revealed in recent years.It has forced researchers to re-examine the safety of RMBs.In this review,the intrinsic safety factors of key components in RMBs,such as uneven plating,pitting and flammability of Mg anode,heat release and crystalline water decomposition of cathode,strong corrosion,low oxidition stability and flammability of electrolytes,and soforth,are systematacially summarized.Their origins,formation mechanisms,and possible safety hazards are deeply discussed.To develop high-performance Mg anode,current strategies including designing artificial SEI,three-dimensional substrates,and Mg alloys are summarized.For practical electrolytes,the configurations of boron-centered anions and simple Mg salts and the functionalized solvent with high boiling point and low flammability are suggested to comprehensively design.In addition,the future study should more focus on the investigation on the thermal runaway and decomposition of cathode materials and separa-tors.This review aims to provide fundamental insights into the relationship between electrochemistry and safety,further promoting the sustainable development of RMBs.

Heteroatom anchors Fe-Mn dual-atom catalysts with bi-functional oxygen catalytic activity for low-temperature rechargeable flexible Zn-air batteries

M-N-C(M=Fe,Co,Ni,etc.) catalyst owns high catalytic activity in the oxygen catalytic reaction which is the most likely to replace the Pt-based catalysts.But it is still a challenge to further increase the active site density.This article constructs the high-efficiency FeMn-N/S-C-1000 catalyst to realize ORR/OER bifunctional catalysis by hetero-atom,bimetal(Fe,Mn) doped simultaneously strategy.When evaluated it as bi-functional electro-catalysts,FeMn-N/S-C-1000 exhibits excellent catalytic activity(E_(1/2)=0.924 V,E_(j=10)=1.617 V) in alkaline media,outperforms conventional Pt/C,RuO_(2) and most non-precious-metal catalysts reported recently,Such outstanding performance is owing to N,S co-coordinated with metal to form multi-types of single atom,dual atom active sites to carry out bi-catalysis.Importantly,nitrite poison test provides the proof that the active sites of FeMn-N/S-C are more than that of single-atom catalysts to promote catalytic reactions directly.To better understand the local structure of Fe and Mn active sites,XAS and DFT were employed to reveal that FeMn-N_5/S-C site plays the key role during catalysis.Notably,the FeMn-N/S-C-1000 based low-temperature rechargeable flexible Zn-air also exhibits superior discharge performance and extraordinary durability at-40℃.This work will provide a new idea to design diatomic catalysts applied in low-temperature rechargeable batteries.

Functionally gradient materials for sustainable and high-energy rechargeable lithium batteries:Design principles,progress,and perspectives

Rechargeable lithium batteries with high-capacity cathodes/anodes promise high energy densities for nextgeneration electrochemical energy storage.However,the associated limitations at various scales greatly hinder their practical applications.Functional gradient material(FGM)design endows the electrode materials with property gradient,thus providing great opportunities to address the kinetics and stability obstacles.To date,still no review or perspective has covered recent advancements in gradient design at multiple scales for boosting lithium battery performances.To fill this void,this work provides a timely and comprehensive overview of this exciting and sustainable research field.We begin by overviewing the fundamental features of FGM and the rationales of gradient design for improved electrochemical performance.Then,we comprehensively review FGM design for rechargeable lithium batteries at various scales,including natural or artificial solid electrolyte interphase(SEI)at the nanoscale,micrometer-scale electrode particles,and macroscale electrode films.The link between gradient structure design and improved electrochemical performance is particularly highlighted.The most recent research into constructing novel functional gradients,such as valence and temperature gradients,has also been explored.Finally,we discussed the current constraints and future scope of FGM in rechargeable lithium batteries,aiming to inspire the development of novel FGM for next-generation high-performance lithium batteries.

A non-nucleophilic electrolyte based on all-inorganic salts with conditioning-free characteristic for rechargeable magnesium batteries

Conditioning-free electrolytes with high reversibility of Mg plating/stripping are of vital importance for the commercialization of the superior rechargeable magnesium batteries(RMBs).In the present work,a non-nucleophilic electrolyte(denoted as MLCH)based on all-inorganic salts of MgCl_(2),LiCl and CrCl_(3) for RMBs is prepared by a straightforward one-step reaction.As a result,the MLCH electrolyte shows the noticeable performance of high ionic conductivity(3.40 mS cm^(−1)),low overpotential(∼46 mV vs Mg/Mg^(2+)),high Coulombic efficiency(∼93%),high anodic stability(SS,∼2.56 V vs Mg/Mg^(2+))and long-term(more than 500 h)cycling stability,especially the conditioning-free characteristic.The main equilibrium species in the MLCH electrolyte are confirmed to be the tetracoordinated anions of[LiCl2(THF)2]−and solvated dimers of[Mg_(2)(μ-Cl)3(THF)6]+.The addition of LiCl can assist the dissolution of MgCl_(2) and activation of the electrode/electrolyte interface,resulting in a superior Mg plating/stripping efficiency.The synergistic effect of LiCl,CrCl_(3),a small amount of HpMS and the absence of polymerization THF enable the conditioning-free characteristic of the MLCH electrolyte.Moreover,the MLCH electrolyte exhibits decent compatibility with the cathodic materials of CuS.The Mg/CuS full cell using the MLCH electrolyte presents a discharge specific capacity of 215 mAh g^(−1)at 0.1 C and the capacity can retain∼72%after 40 cycles.Notably,the MLCH electrolyte has other superiorities such as the broad sources of materials,low-cost and easy-preparation,leading to the potential prospect of commercial application.

Engineering single-atom Mn on nitrogen-doped carbon to regulate lithium-peroxide reaction kinetics for rechargeable lithium-oxygen batteries

Precision engineering of catalytic sites to guide more favorable pathways for Li_(2)O_(2) nucleation and decom-position represents an enticing kinetic strategy for mitigating overpotential,enhancing discharge capac-ity,and improving recycling stability of Li-O_(2) batteries.In this work,we employ metal-organic frameworks(MOFs)derivation and ion substitution strategies to construct atomically dispersed Mn-N_(4) moieties on hierarchical porous nitrogen-doped carbon(Mn SAs-NC)with the aim of reducing the over-potential and improving the cycling stability of Li-O_(2) batteries.The porous structure provides more chan-nels for mass transfer and exposes more highly active sites for electrocatalytic reactions,thus promoting the formation and decomposition of Li_(2)O_(2).The Li-O_(2) batteries with Mn SAs-NC cathode achieve lower overpotential,higher specific capacity(14290 mA h g^(-1) at 100 mAg^(-1)),and superior cycle stability(>100 cycles at 200 mA g^(-1))compared with the Mn NPs-NC and NC.Density functional theory(DFT)cal-culations reveal that the construction of Mn-N_(4) moiety tunes the charge distribution of the pyridinic N-rich vacancy and balances the affinity of the intermediates(LiO_(2) and Li_(2)O_(2)).The initial nucleation of Li_(2)O_(2) on Mn SAs-NC favors the O_(2)-→LiO_(2)→Li_(2)O_(2) surface-adsorption pathway,which mitigates the overpoten-tials of the oxygen reduction(ORR)and oxygen evolution reaction(OER).As a result,Mn SAs-NC with Mn-N_(4) moiety effectively facilitates the Li_(2)O_(2) nucleation and enables its reversible decomposition.This work establishes a methodology for constructing carbon-based electrocatalysts with high activity and selectivity for Li-O_(2)batteries.

Interfacial chemistry of anode/electrolyte interface for rechargeable magnesium batteries

Rechargeable magnesium batteries(RMBs),as a low-cost,high-safety and high-energy storage technology,have attracted tremendous attention in large-scale energy storage applications.However,the key anode/electrolyte interfacial issues,including surface passivation,uneven Mg plating/stripping,and pulverization after cycling still result in a large overpotential,short cycling life,poor power density,and possible safety hazards of cells,severely impeding the commercial development of RMBs.In this review,a concise overview of recently advanced strategies to address these anode/electroyte interfacial issues is systematically classified and summarized.The design of magnesiophilic substrates,construction of artificial SEI layers,and modification of electrolyte are important and effective strategies to improve the uniformity/kinetics of Mg plating/stripping and achieve the stable anode/electrolyte interface.The key opportunities and challenges in this field are advisedly put forward,and the insights into future directions for stabilizing Mg metal anodes and the anode/electrolyte interface are highlighted.This review provides important references fordeveloping the high-performance and high-safety RMBs.

3D porous reduced graphene cathode and non-corrosive electrolyte for long-life rechargeable aluminum batteries

Owing to their high volumetric capacity,low cost and high safety,rechargeable aluminum batteries have become promising candidates for energy applications.However,the high charge density of Al^(3+)leads to strong coulombic interactions between anions and the cathode,resulting in sluggish diffusion kinetics and irreversible collapse of the cathode structure.Furthermore,AlCl_(3)-based ionic liquids,which are commonly used as electrolytes in such batteries,corrode battery components and are prone to side reactions.The above problems lead to low capacity and poor cycling stability.Herein,we propose a reduced graphene oxide(rGO)cathode with a three-dimensional porous structure prepared using a simple and scalable method.The lamellar edges and oxygen-containing group defects of rGO synergistically provide abundant ion storage sites and enhance ion transfer kinetics.We matched the prepared rGO cathode with noncorrosive electrolyte 0.5 mol·L^(-1) Al(OTF)_(3)/[BMIM]OTF and Al metal to construct a high-performance battery,Al||rGO-150,with good cycling stability for 2700 cycles.Quasi-in-situ physicochemical characterization results show that the ion storage mechanism is codominated by diffusion and capacitance.The capacity consists of the insertion of Al-based species cations as well as synergistic adsorption of Al(OTF)_(x)^((3-x)+)(x<3)and[BMIM]+.The present study promotes the fundamental and applied research on rechargeable aluminum batteries.

Organopolysulfides as high-performance cathode materials for rechargeable aluminum-ion batteries

The development of aluminum-ion batteries(AIBs)is significantly confined by the limited high-performance cathode materials.Herein,organopolysulfides are investigated as active cathode materials to fabricate AIBs.A liquid-phase phenyl tetrasulfide(PTS)can deliver a capacity above 600 mAh g−1 after activation,with the maintenance of 253 mAh g−1 after 100 cycles.Owing to the different S locations,PTS shows several voltage plateaus and an average voltage of∼0.7 V vs.Al3+/Al with no decay upon cycling.More importantly,the liquid PTS can serve as a high-Coulombic-efficiency cathode(∼99.88%±0.57%after stabilization),enlighting the design of high-efficiency and low-resistance conversion-type cathode materials for AIBs.By experimental characterizations accompanied by theoretical calculations,it is found that PTS undergoes stepwise reaction procedures during discharge with final products of Al2S3 and AlCl2-coordinated phenyl sulfide,and partially reforms with elemental S and other organopolysulfides during charge.This study demonstrates new opportunities for the design of high-efficiency conversion-type cathode materials for advanced AIBs.

Template synthesis of MnO_2/CNT nanocomposite and its application in rechargeable lithium batteries

Nanostructured MnO2/CNT composite was synthesized by a soft template approach in the presence of Pluronic P123 surfactant. The product was characterized by X-ray diffraction, thermogravimetric and differential thermal analyses, Fourier transformed infrared spectroscopy and high-resolution transmission electron microscopy. The results show that the sample consists of poor crystalline α-MnO2 nanorods with a diameter of about 10 nm and a length of 30-50 nm, which absorb on the carbon nanotubes. The electrochemical properties of the product as cathode material for Li-MnO2 cell are evaluated by galvanostatic charge-discharge and electrochemical impedance spectroscopy （EIS）. Compared with pure MnO2 electrode, the MnO2/CNT composite delivers a much larger initial capacity of 275.3 mA-h/g and better rate and cycling performance.

Progress in aqueous rechargeable batteries

Over the past decades, a series of aqueous rechargeable batteries(ARBs) were explored, investigated and demonstrated. Among them,aqueous rechargeable alkali-metal ion(Li^+Na^+, K^+) batteries, aqueous rechargeable-metal ion(Zn^(2+),Mg^(2+), Ca^(2+), Al^(3+)) batteries and aqueous rechargeable hybrid batteries are standing out due to peculiar properties. In this review, we focus on the fundamental basics of these batteries, and discuss the scientific and/or technological achievements and challenges. By critically reviewing state-of-the-art technologies and the most promising results so far, we aim to analyze the benefits of ARBs and the critical issues to be addressed, and to promote better development of ARBs.

Toward better electrode/electrolyte interfaces in the ionic-liquid-based rechargeable aluminum batteries

The past decade has witnessed the germination of rechargeable aluminum batteries(RABs)with the colossal potential to enact as a device for the large scale energy storage and conversion.The Majority of investigations are dedicated to the exploration of suitable cathode materials,while less is known about the electrode/electrolyte interfaces that determine the electrochemistry of batteries.In this perspective,we will highlight the significance of electrode/electrolyte interface for RABs,in overall kinetics and capacity retention.Emphasis will be laid on the complicated yet basic understandings of the phenomena at the interfaces,including the dendrite growth,surface Al2O3 and solid–electrolyte-interphase(SEI).And we will summarize the reported practice in effort to build better electrode/electrolyte interfaces in RAB.In the end,outlook regarding to the challenges,opportunities and directions is presented.

Ionic liquid/tetraglyme hybrid Mg[TFSI]_2 electrolytes for rechargeable Mg batteries

The electrochemical reversibility of Mg in hybrid electrolytes based on mixtures of ionic liquid and glyme based organic solvents was investigated for applications in rechargeable magnesium batteries(RMBs). The electrolytes demonstrate reversible reduction and oxidation of Mg only after being pre-treated with the dehydrating agent, magnesium borohydride, Mg[BH_4]_2, highlighting the importance of removing water in Mg based electrolytes. The addition magnesium di[bis(trifluoromethanesulfonyl)imide](Mg[TFSI]_2)(0.3 M) to N-butyl-n-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide [C4 mpyr][TFSI]/tetraglyme at a mole ratio of 1:2 showed stable CV cycling over almost 300 cycles while scanning electron microscopy(SEM) and X-ray diffraction(XRD) confirmed Mg deposition, showing non-dendritic morphology and a well-aligned growth. Further thermogravimetric analysis(TGA) demonstrated a mass retention of 79% at 250℃ for this electrolyte suggesting that the presence of the ionic liquid increases thermal stability substantially making these hybrid electrolytes compatible for RMBs.

The application of synchrotron X-ray techniques to the study ofrechargeable batteries

The increased use of rechargeable batteries in portable electronic devices and the continuous develop-ment of novel applications （e.g. transportation and large scale energy storage）, have raised a strong de-mand for high performance batteries with increased energy density, cycle and calendar life, safety andlower costs. This triggers significant efforts to reveal the fundamental mechanism determining batteryperformance with the use of advanced analytical techniques. However, the inherently complex character-istics of battery systems make the mechanism analysis sophisticated and difficult. Synchrotron radiationis an advanced collimated light source with high intensity and tunable energies. It has particular ad-vantages in electronic structure and geometric structure （both the short-range and long-range structure）analysis of materials on different length and time scales. In the past decades, synchrotron X-ray tech-niques have been widely used to understand the fundamental mechanism and guide the technologicaloptimization of batteries. In particular, in situ and operando techniques with high spatial and temporalresolution, enable the nondestructive, real time dynamic investigation of the electrochemical reaction,and lead to significant deep insights into the battery operation mechanism. This review gives a brief introduction of the application of synchrotron X-ray techniques to the inves-tigation of battery systems. The five widely implicated techniques, including X-ray diffraction （XRD）, PairDistribution Function （PDF）, Hard and Soft X-ray absorption spectroscopy （XAS） and X-ray photoelectronspectroscopy （XPS） will be reviewed, with the emphasis on their in situ studies of battery systems during cycling.

Mesoporous Graphene Hosts for Dendrite-Free Lithium Metal Anode in Working Rechargeable Batteries

Lithium(Li) metal anode has received extensive attentions due to its ultrahigh theoretical capacity and the most negative electrode potential. However, dendrite growth severely impedes the practical applications of the Li metal anode in rechargeable batteries. In this contribution, a mesoporous graphene with a high specific surface area was synthesized to host the Li metal anode. The mesoporous graphene host(MGH) has a high specific surface area(2090 m^2/g), which affords free space and an interconnected conductive pathway for Li plating and stripping, thus alleviating the volume variation and reducing the generation of dead Li during repeated cycles. More importantly, the high specific surface area of MGH efficiently reduces the local current density of the electrode, which favors a uniform Li nucleation and plating behavior, rendering a dendritefree deposition morphology at a low overpotential. These factors synergistically boost the Li utilization(90.1% vs. 70.1% for Cu foil) and life span(150 cycles vs. 100 cycles for Cu foil) with a low polarization of MGH electrode at an ultrahigh current of 15.0 mA/cm^2. The as-prepared MGH can provide fresh insights into the electrode design of the Li metal anode operating at high rates.

Development of Magnesium-Insertion Positive Electrode for Rechargeable Magnesium Batteries

Magnesium-based rechargeable batteries might be an interesting future alternative to lithium-based batteries. It is so far well known that Mg2+ ion insertion into ion-transfer hosts proceeds slowly compared with Li+, so it is necessary to realize fast Mg2+ transport in the host in addition to other requirements as practical cathode materials for magnesium batteries. Positive electrode materials based on inorganic transition-metal oxides, sulfides, and borides are the only ones used up to now to insert magnesium ions. In this paper, the available results of research on materials suitable as possible, for secondary magnesium batteries, are reviewed.

Interface Engineering of CoS/CoO@N‑Doped Graphene Nanocomposite for High‑Performance Rechargeable Zn–Air Batteries

Low cost and green fabrication of high-performance electrocatalysts with earth-abundant resources for oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)are crucial for the large-scale application of rechargeable Zn-air batteries(ZABs).In this work,our density functional theory calculations on the electrocatalyst suggest that the rational construction of interfacial structure can induce local charge redistribution,improve the electronic conductivity and enhance the catalyst stability.In order to realize such a structure,we spatially immobilize heterogeneous CoS/CoO nanocrystals onto N-doped graphene to synthesize a bifunctional electrocatalyst(CoS/CoO@NGNs).The optimization of the composition,interfacial structure and conductivity of the electrocatalyst is conducted to achieve bifunctional catalytic activity and deliver outstanding efficiency and stability for both ORR and OER.The aqueous ZAB with the as-prepared CoS/CoO@NGNs cathode displays a high maximum power density of 137.8 mW cm^−2,a specific capacity of 723.9 mAh g^−1 and excellent cycling stability(continuous operating for 100 h)with a high round-trip efficiency.In addition,the assembled quasi-solid-state ZAB also exhibits outstanding mechanical flexibility besides high battery performances,showing great potential for applications in flexible and wearable electronic devices.

Electrochemical characteristics of CoSn_x alloy composite anode for lithium-ion rechargeable batteries

Nano/micro-scaled CoSnx alloy powders synthesized via carbothermal reduction at 800 ℃ with different compositions were characterized for anode materials in Li-ion battery. The synthesized spherical CoSnx particles show a loose nano/micro sized particle structural characteristic, which is apparently favorable for the improvement of cycling stability. The prepared CoSn3 alloy composite electrode exhibits a low initial irreversible capacity of ca.130 mAh·g-1 and a high specific capacity of ca.440 mAh·g-1 at constant current density of 100 mA·g-1. The relatively large particle size is considered to be the main reason for the lower irreversible capacity of CoSn3 electrode.