Recent progress of flexible aqueous multivalent ion batteries

Flexible aqueous batteries have been thriving with the growing demand for wearable and portable electrical devices.In particular,flexible aqueous mul tivalent ion batteries(FAMIBs),the charge carriers of which include Zn^(2+),Al^(3+),Mg^(2+),and Ca^(2+),have great potential for development owing to their high safety,high elemental abundance in the Earth's crust,and a multi-electron redox mechanism with a high theoretical specific capacity.Therefore,for a comprehensive understanding of this developing field,it is necessary to summarize the recent research progress of FAMIBs in a timely manner.Herein,the advancements of the state-of-the-art FAMIBs are reviewed,and the prospects toward this field are also proposed.This study focuses on the rational material and configuration design for FAMIBs in recent studies to achieve high battery performances under deformation conditions,which is elaborated on by classification of the anode,cathode,hydrogel electrolyte,and configurations of FAMIBs.Besides,the electrochemical performance of FAMIBs under flexible conditions is also reviewed from the perspective of their working voltage,specific capacity,and cycling stability.Finally,the ap proaches to improve the performance of FAMIBs are comprehensively eval uated,followed by the outlook on the challenges and opportunities in future development of FAMIBs.

Asymmetric Electrolytes Design for Aqueous Multivalent Metal Ion Batteries

With the rapid development of portable electronics and electric road vehicles,high-energy-density batteries have been becoming front-burner issues.Traditionally,homogeneous electrolyte cannot simultaneously meet diametrically opposed demands of high-potential cathode and low-potential anode,which are essential for high-voltage batteries.Meanwhile,homogeneous electrolyte is difficult to achieve bi-or multi-functions to meet different requirements of electrodes.In comparison,the asymmetric electrolyte with bi-or multi-layer disparate components can satisfy distinct requirements by playing different roles of each electrolyte layer and meanwhile compensates weakness of individual electrolyte.Consequently,the asymmetric electrolyte can not only suppress by-product sedimentation and continuous electrolyte decomposition at the anode while preserving active substances at the cathode for high-voltage batteries with long cyclic lifespan.In this review,we comprehensively divide asymmetric electrolytes into three categories:decoupled liquid-state electrolytes,bi-phase solid/liquid electrolytes and decoupled asymmetric solid-state electrolytes.The design principles,reaction mechanism and mutual compatibility are also studied,respectively.Finally,we provide a comprehensive vision for the simplification of structure to reduce costs and increase device energy density,and the optimization of solvation structure at anolyte/catholyte interface to realize fast ion transport kinetics.

Potassium ion pre‑intercalated MnO_(2)for aqueous multivalent ion batteries

Manganese dioxide(MnO_(2)),as a cathode material for multivalent ion(such as Mg^(2+)and Al^(3+))storage,is investigated due to its high initial capacity.However,during multivalent ion insertion/extraction,the crystal structure of MnO_(2)partially collapses,leading to fast capacity decay in few charge/discharge cycles.Here,through pre-intercalating potassium-ion(K+)intoδ-MnO_(2),we synthesize a potassium ion pre-intercalated MnO_(2),K_(0.21)MnO_(2)·0.31H_(2)O(KMO),as a reliable cathode material for multivalent ion batteries.The as-prepared KMO exhibits a high reversible capacity of 185 mAh/g at 1 A/g,with considerable rate performance and improved cycling stability in 1 mol/L MgSO_(4)electrolyte.In addition,we observe that aluminum-ion(Al^(3+))can also insert into a KMO cathode.This work provides a valid method for modifcation of manganesebased oxides for aqueous multivalent ion batteries.

Ultrafiltration recovery of alginate: Membrane fouling mitigation by multivalent metal ions and properties of recycled materials

Recovery of alginate extracted from aerobic granular sludge(AGS)has given rise to a novel research direction.However,these extracted alginate solutions have a water content of nearly 100%.Alternately,ultrafiltration(UF)is generally used for concentration of polymers.Furthermore,the introduction of multivalent metal ions into alginate may provide a promising method for the development of novel nanomaterials.In this study,membrane fouling mitigation by multivalent metal ions,both individually and in combination,and properties of recycled materials were investigated for UF recovery of sodium alginate(SA).The filtration resistance showed a significantly negative correlation with the concentration of metal ions,arranged in the order of Mg^2+<Ca^2+<Fe^3+<Al^3+(filtration resistance mitigation),and the moisture content of recycled filter cake showed a marked decrease.For Ca^2+,Mg^2+,Fe^3+,and Ca^2++Fe^3+,the filtration resistances were almost the same when the total charge concentration was less than 5 mmol·L^–1.However,when the total charge concentration was greater than 5 mmol·L^–1,membrane fouling mitigation increased significantly in the presence of Ca^2+or Fe^3+and remained constant for Mg^2+with the increase of total charge concentration.The filtration resistance mitigation was arranged in the order of Fe^3+>Fe^3++Ca^2+>Ca^2+>Mg^2+.Three mechanisms were proposed in the presence of Fe^3+,such as the decrease of SA concentration,change in p H,and production of hydroxide iron colloids from hydrolysis.The properties of recycled materials(filter cake)were investigated via optical microscope observation,dynamic light scattering,Fourier transform infrared,X-ray photoelectron spectroscopy(XPS),and scanning electron microscopy.The results provide further insight into UF recoveries of alginate extracted from AGS.

Printable Zinc‑Ion Hybrid Micro‑Capacitors for Flexible Self‑Powered Integrated Units

Wearable self-powered systems integrated with energy conversion and storage devices such as solar-charging power units arouse widespread concerns in scientific and industrial realms.However,their applications are hampered by the restrictions of unbefitting size matching between integrated modules,limited tolerance to the variation of input current,reliability,and safety issues.Herein,flexible solar-charging self-powered units based on printed Zn-ion hybrid micro-capacitor as the energy storage module is developed.Unique 3D micro-/nano-architecture of the biomass kelp-carbon combined with multivalent ion(Zn2+)storage endows the aqueous Zn-ion hybrid capacitor with high specific capacity(196.7 mAh g^−1 at 0.1 A g^−1).By employing an in-plane asymmetric printing technique,the fabricated quasi-solid-state Zn-ion hybrid microcapacitors exhibit high rate,long life and energy density up to 8.2μWh cm^−2.After integrating the micro-capacitor with organic solar cells,the derived self-powered system presents outstanding energy conversion/storage efficiency(ηoverall=17.8%),solar-charging cyclic stability(95%after 100 cycles),wide current tolerance,and good mechanical flexibility.Such portable,wearable,and green integrated units offer new insights into design of advanced self-powered systems toward the goal of developing highly safe,economic,stable,and long-life smart wearable electronics.

Critical factors to inhibit water-splitting side reaction in carbon-based electrode materials for zinc metal anodes

Zinc metal anodes(ZMA)have high theoretical capacities(820 mAh g−1 and 5855 mAh cm−3)and redox potential(−0.76 V vs.standard hydrogen electrode),similar to the electrochemical voltage window of the hydrogen evolution reaction(HER)in a mild acidic electrolyte system,facilitating aqueous zinc batteries competitive in next-generation energy storage devices.However,the HER and byproduct formation effectuated by water-splitting deteriorate the electrochemical performance of ZMA,limiting their application.In this study,a key factor in promoting the HER in carbon-based electrode materials(CEMs),which can provide a larger active surface area and guide uniform zinc metal deposition,was investigated using a series of threedimensional structured templating carbon electrodes(3D-TCEs)with different local graphitic orderings,pore structures,and surface properties.The ultramicropores of CEMs are the determining critical factors in initiating HER and clogging active surfaces by Zn(OH)2 byproduct formation,through a systematic comparative study based on the 3D-TCE series samples.When the 3D-TCEs had a proper graphitic structure with few ultramicropores,they showed highly stable cycling performances over 2000 cycles with average Coulombic efficiencies of≥99%.These results suggest that a well-designed CEM can lead to high-performance ZMA in aqueous zinc batteries.

Collapse of Four-Arm Stars Polyelectrolyte Brushes Under An Electric Field in the Presence of Trivalent Salt Coions

Langevin dynamics simulations were conducted to study the collapse of grafted partially charged 4-arm star chains onto the oppositely charged grafting electrode in the presence of trivalent salt coions.Simulation results reveal that the average charge fraction of the grafted star chains and the salt concentration play critical roles in the competitive adsorption of charged monomers and trivalent salt coions onto the oppositely charged electrode.For grafted star chains with relatively high charge fraction,charged monomers are the dominant species collapsing on the oppositely charged electrode with the emergence of charge reversal on the grafting electrode.At a low charge fraction such that the total amount of charges on a grafted star molecule is comparable to that of a trivalent salt coion,trivalent salt coions absorb more strongly onto the electrode than grafted stars even at very low salt concentration.It is found that at relatively low charge fraction of star chains,the addition of trivalent salt coions does not lead to charge overcompensation of the surface charges on the grafting electrode.The stretching of star brushes under an electric field in the presence of trivalent salt coions was also briefly investigated.

Progress in 3D‑MXene Electrodes for Lithium/Sodium/Potassium/Magnesium/Zinc/Aluminum‑Ion Batteries

MXenes have attracted increasing attention because of their rich surface functional groups,high electrical conductivity,and outstanding dispersibility in many solvents,and have demonstrated competitive efficiency in energy storage and conversion applications.However,the restacking nature of MXene nanosheets like other two-dimensional(2D)materials through van der Waals forces results in sluggish ionic kinetics,restricted number of active sites,and ultimate deterioration of MXene mate-rial/device performance.The strategy of raising 2D MXenes into three-dimensional(3D)structures has been considered an efficient way for reducing restacking,providing greater porosity,higher surface area,and shorter distances for mass transport of ions,surpassing standard one-dimensional(1D)and 2D structures.In multivalent ion batteries,the positive multivalent ions combine with two or more electrons at the same time,so their capacities are two or three times that of lithium-ion batteries(LIBs)under the same conditions,e.g.,a magnesium ion battery has a high theoretical specific capacity of 2205 mAh g^(−1)and a high volumetric capacity of 3833 mAh cm^(−3).In this review,we summarize the most recent strategies for fabricating 3D MXene architectures,such as assembly,template,3D printing,electrospinning,aerogel,and gas foaming methods.Special consideration has been given to the applications of highly porous 3D MXenes in energy storage devices beyond LIBs,such as sodium ion batteries(SIBs),potassium ion batteries(KIBs),magnesium ion batteries(MIBs),zinc ion batteries(ZIBs),and aluminum ion batteries(AIBs).Finally,the authors provide a summary of the future opportunities and challenges for the construction of 3D MXenes and MXene-based electrodes for applications beyond LIBs.

Progress in electrolytes for beyond-lithium-ion batteries

The constant increase in global energy demand and stricter environmental standards are calling for advanced energy storage technologies that can store electricity from intermittent renewable sources such as wind,solar,and tidal power,to allow the broader implementation of the renewables.The gridoriented sodium-ion batteries,potassium ion batteries and multivalent ion batteries are cheaper and more sustainable alternatives to Li-ion,although they are still in the early stages of development.Additional optimisation of these battery systems is required,to improve the energy and power density,and to solve the safety issues caused by dendrites growth in anodes.Electrolyte,one of the most critical components in these batteries,could significantly influence the electrochemical performances and operations of batteries.In this review,the definitions and influences of three critical components(salts,solvents,and additives)in electrolytes are discussed.The significant advantages,challenges,recent progress and future optimisation directions of various electrolytes for monovalent and multivalent ions batteries(i.e.organic,ionic liquid and aqueous liquid electrolytes,polymer and inorganic solid electrolytes)are summarised to guide the practical application for grid-oriented batteries.

Ultrahigh-energy and-power aqueous rechargeable zinc-ion microbatteries based on highly cation-compatible vanadium oxides

Aqueous multivalent-metal-ion intercalation chemistries hold genuine promise to develop safe and powerful microbatteries for potential use in many miniaturized electronics.However,their development is beset by state-of-the-art electrode materials having practical capacities far below their theoretical values.Here we demonstrate that high compatibility between layered transition-metal oxide hosts and hydrated cation guests substantially boost their multi-electron-redox reactions to offer higher capacities and rate capability,based on typical bipolar vanadium oxides preintercalated with hydrated cations(M_(x)V_(2)O_(5)).When seamlessly integrated on Au current microcollectors with a three-dimensional bicontinuous nanoporous architecture that offers high pathways of electron transfer and ion transport,the constituent Zn_(x)V_(2)O_(5) exhibits specific capacity of as high as∼527 mAh g^(−1) at 5 mV s^(−1) and retains∼300 mAh g^(−1) at 200 mV s^(−1) in 1 M ZnSO_(4) aqueous electrolyte,outperforming the M_(x)V_(2)O_(5)(M=Li,Na,K,Mg).This allows aqueous rechargeable zinc-ion microbatteries constructed with symmetric nanoporous Zn_(x)V_(2)O_(5)/Au interdigital microelectrodes as anode and cathode to show high-density energy of∼358 mWh cm^(−3)(a value that is forty-fold higher than that of 4 V/500μAh Li thin film battery)at high levels of power delivery.

Flexible aqueous Ca-ion full battery with super-flat discharge voltage plateau

Recently,multivalent metal-ion batteries have attracted considerable interests on the merits of their natural abundance and multielectron redox property.However,the development of Ca-ion battery is still in their preliminary stage because of the lack of suitable electrode material.The Ca-storage performance of the existing materials is still unsatisfactory with low capacity,poor cyclic stability,as well as sloping discharge profiles,which cannot provide stable energy output.In this work,transition metal oxide Sn-doped In2O3(ITO)has been explored as the aqueous Ca-ion battery anode,which could deliver a high discharge capacity of 71.2 mAh·g^(-1) with an ultra-flat discharge voltage plateau.The Ca storage mechanism was revealed to be reversible conversion reaction based on ex-situ X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),and transmission electron microscopy(TEM)characterizations.A flexible aqueous Ca-ion battery was subsequently assembled with zinc hexacyanoferrate(ZnHCF)cathode and ITO anode sandwiched by hydrogel electrolyte,which could deliver a high specific capacity of 75.3 mAh·g^(-1) at 0.4 A·g^(-1) with a flat output voltage plateau at around 0.8 V.The bendable and flexible Ca-ion battery with decent voltage output will pave the way for the energy storage devices towards practical applications in flexible and wearable electronics.

Prussian-blue materials:Revealing new opportunities for rechargeable batteries

The demand to increase energy density of rechargeable batteries for portable electronic devices and electric vehicles and to reduce the cost for grid-scale energy storage necessitates the exploration of new chemistries of electrode materials for rechargeable batteries.The open framework-structure of Prussian-blue materials has recently been demonstrated as a promising cathode host for a variety of monovalent and multivalent cations with the tunable working voltage and discharge capacities.Recent progress toward the application of Prussian-blue cathode materials for rechargeable batteries is reviewed,with special emphasis on charge-storage mechanisms of different insertion species,factors influencing electrochemical performances,and possible approaches to overcome their intrinsic limitations.

High energy superstable hybrid capacitor with a self-regulated Zn/electrolyte interface and 3D graphene-like carbon cathode

Rechargeable aqueous zinc ion hybrid capacitors(ZIHCs),as an up-and-comer aqueous electrochemical energy storage system,endure in their infancy because of the substandard reversibility of Zn anodes,structural deterioration of cathode materials,and narrow electrochemical stability window.Herein,a scalable approach is described that addresses Zn-anode/electrolyte interface and cathode materials associated deficiencies and boosts the electrochemical properties of ZIHCs.The Zn-anode/electrolyte interface is self-regulated by alteration of the traditional Zn2+electrolyte with Na-based supporting salt without surrendering the cost,safety,and green features of the Zn-based system which further validates the excellent reversibility over 1100 h with suppressed hydrogen evolution.The deficits of cathode materials were overcome by using a high-mass loaded,oxygen-rich,3D,multiscaled graphene-like carbon(3D MGC)cathode.Due to the multiscaled texture,high electronic conductivity,and oxygen-rich functional groups of 3D MGC,reversible redox capacitance was obtained with a traditional adsorption/desorption mechanism.Prototype ZIHCs containing the modified electrolyte and an oxygen-rich 3D MGC cathode resulted in battery-like specific energy(203 Wh kg1 at 1.6 A g^(-1))and supercapacitor-type power capability(4.9 kW kg1 at 8 A g^(-1))with outstanding cycling durability(96.75%retention over 30000 cycles at 10 A g^(-1)).These findings pave the way toward the utilization of highly efficient ZIHCs for practical applications.