稀有金属(英文版) 2020,39(09),989-1004

Recent progresses on alloy-based anodes for potassium-ion batteries

Kai-Xiang Lei Jing Wang Cong Chen Si-Yuan Li Shi-Wen Wang Shi-Jian Zheng Fu-Jun Li

Tianjin Key Laboratory of Materials Laminating Fabrication and Interface Control Technology,School of Materials Science and Engineering,Hebei University of Technology

Henan Provincial Key Laboratory of Surface and Interface Science,College of Materials and Chemical Engineering,Zhengzhou University of Light Industry

Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education),Renewable Energy Conversion and Storage Center(RECAST),College of Chemistry,Nankai University

作者简介：*Kai-Xiang Lei is a lecturer at School of Materials Science and Engineering,Hebei University of Technology.He received his Ph.D.degree from Nankai University in 2019.His research interests mainly focus on electrocatalytic oxygen reduction and oxygen evolution and rechargeable alkaline metal ion batteries.e-mail:kaixinglei@hebut.edu.cn;*Shi-Wen Wang is currently an associate professor at College of Materials and Chemical Engineering,Zhengzhou University of Light Industry.He received his Ph.D.at Nankai University under the supervision of Professor Jun Chen in 2014.His research focuses on green and cheap energy storage materials for advanced battery applications.e-mail:wshwory@zzuli.edu.cn;*Fu-Jun Li is a professor at Key Laboratory of Advanced Energy Materials Chemistry (KLAEMC,Ministry of Education),Nankai University.He obtained his Ph.D.degree from The University of Hong Kong in 2011 and then worked as a postdoc fellow at the University of Tokyo and National Institute of Advanced Industrial Science and Technology (AIST,Tsukuba),Japan,till 2015.He has published more than 40 peerreviewed journal papers as the first or corresponding author,including Nature Communications, Angewandte Chemie,Advanced Materials,Energy & Environmental Science,Advanced Energy Materials,Nano Letters,and Advanced Functional Materials.His research interests include energy materials chemistry,Li-O_2(air) batteries,and Li-ion batteries.e-mail:fujunli@nankai.edu.cn;

收稿日期：19 January 2020

基金：financially supported by the National Natural Science Foundation of China (Nos.21822506, 51761165025,51671107 and 21603108);

Recent progresses on alloy-based anodes for potassium-ion batteries

Kai-Xiang Lei Jing Wang Cong Chen Si-Yuan Li Shi-Wen Wang Shi-Jian Zheng Fu-Jun Li

Tianjin Key Laboratory of Materials Laminating Fabrication and Interface Control Technology,School of Materials Science and Engineering,Hebei University of Technology

Henan Provincial Key Laboratory of Surface and Interface Science,College of Materials and Chemical Engineering,Zhengzhou University of Light Industry

Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education),Renewable Energy Conversion and Storage Center(RECAST),College of Chemistry,Nankai University

Abstract：

Potassium-ion batteries(KIBs) are one of the most promising large-scale electric energy storage systems due to the high abundance and low redox potential of K.As the key component,anode determines their energy density and safety.Alloy-based anodes,such as P,Sn,Sb,and Bi,have attracted extensive attention due to their abundant resources,suitable working potentials,and large theoretical capacities.However,the dramatic volume variation upon(de)potassiation results in pulverization of particles and their detaching from the current collector accompanied with performance decay.Various strategies,including designing micro-/nanostructures,introducing carbon substrates,and optimizing electrode/electrolyte interface,have been demonstrated to effectively alleviate these issues.Herein,we summarize the recent research progresses on alloy-based materials in KIBs.The synthesis methods,electrochemical performance,reaction mechanisms,and structure-activity relationships of these materials are considered,and challenges and perspectives are provided.This review provides new insight into designing of high-activity electrode materials for KIBs and beyond.

Keyword：

Potassium-ion batteries; Alloys; Reaction mechanism; (De)potassiation; Electrochemical performance;

Received： 19 January 2020

1 Introduction

With the rapid development of battery technologies,lithium-ion batteries (LIBs) have dominated many aspects in our life,such as portable electronic devices,electric vehicles,and smart grid [ 1, 2, 3, 4] .However,the growing cost of LIBs derived from the scare lithium resources restricts their applications in the field of large-scale energy storage [ 5, 6, 7, 8] .Sodium-ion batteries (SIBs) show distinct resource advantage,but they suffer from low output voltages caused by the high redox potential of Na⁺/Na (-2.71 V vs.standard hydrogen electrode (SHE)) [ 9, 10, 11] .Unlike Na⁺/Na,the redox potential of K⁺/K (-2.93 V vs.SHE) is close to that of Li⁺/Li (-3.04 V vs.SHE) [ 12, 13, 14] .Particularly in propylene carbonate (PC),the value is-0.09 V vs.Li⁺/Li [ 15, 16, 17] .Furthermore,the natural abundance of potassium is comparable to that of sodium [ 18, 19, 20, 21] .Therefore,potassium-ion batteries (KIBs) have potential as a low-cost and high-energy-density battery technology for large-scale electric energy storage.

KIBs work by the shuttling of K⁺between the cathode and anode during cycles.However,the large-sized K⁺(0.138 nm) could result in the structural damage and capacity fading of electrodes upon extraction/insertion [ 22, 23, 24] .Moreover,metal K is very active and will cause severe risks once the dendrites generate.How to construct a high-performance and high-safety KIB largely depends on the anode.Although graphite offers reversible (de)intercalation of K⁺and a high capacity of 230 mAh·g^-1,it suffers from an output voltage that is close to the deposition potential of K [ 25] .The other intercalation-type anodes are subjected to limited capacities [ 26, 27, 28] .Transition metal oxides/sulfides based on conversion reaction can provide large capacities due to multi-electron transfer.Unfortunately,these materials usually present high working potentials,large voltage hysteresis,and dramatic volume change and trade off the output voltages and stabilities of full batteries [ 29, 30, 31, 32, 33] .Organic materials withadjustable voltages and flexible molecular structures yield poor electronic conductivity and high solubility in organic electrolytes [ 34, 35, 36, 37] .Recently,alloying materials,such as Sn,Bi,and Sb,are of great interest because of their high theoretical capacities,good electronic conductivity,and safe potentials [ 38, 39, 40] .Li's group reported that metallic Bi can alloy with K to form K₃Bi and offer a large capacity of385 mAh·g^-1 [ 41] .However,a huge challenge that impedes the development of alloying anodes is the pulverization of active materials in the traditional carbonatebased electrolytes,which is ascribed to severe volume variation upon (de)potassiation [ 42, 43, 44] .Currently,numerous efforts,including designing micro-/nanostructures,introducing carbon substrates,and using effectivebinders and electrolytes,have been devoted to enhancing the potassium storage activity of alloying anodes,and significant progresses have been achieved.

Fig.1 a Relationship between volume of A_xSi (per mole of Si) and stored charge (reprinted with permission [51] ,Copyright 2011Electrochemical Society);selected discharge/charge profiles of b Si/graphene composite and c pure Si (reprinted with permission [38] .Copyright2011 John Wiley and Sons);d SEM image of Si/graphene electrode after 100 cycles (reprinted with permission [38] ,Copyright 2011 John Wiley and Sons)

Fig.2 a Initial three discharge/charge profiles of P/C composite at 50 mA·g^-1 (reprinted with permission [58] ,Copyright 2017 American Chemical Society);b XRD patterns of P/C electrodes at different states(reprinted with permission [58] ,Copyright 2017 American Chemical Society);c calculated voltages for formation of various K-P alloys (reprinted with permission [59] ,Copyright 2019,Springer Nature);d selected discharge/charge profiles of BP-C at 50 mA·g^-1 (reprinted with permission [60] ,Copyright 2017 Royal Society of Chemistry);e XRD patterns of discharged and charged products of BP-C (reprinted with permission [60] ,Copyright 2017 Royal Society of Chemistry);f contour plots of in-operando synchrotron XRD with superimposed voltage curves displayed between 13.7°and 13.9°(reprinted with permission [61] ,Copyright2 0 1 8 Elsevier);g schematic illustration of reaction mechanism in GeP₅ electrode (reprinted with permission [61] ,Copyright 2 0 1 8 Elsevier)

In this review,we summarize the research progresses of alloying anodes for KIBs.The reaction and deactivation mechanisms of alloying anodes in KIBs are analyzed,the applied strategies are discussed,and significant progresses are highlighted.Finally,this article offers perspectives and discusses future challenges to promote the applications of alloying anodes in KIBs.

2 Si-based anodes

Silicon (Si) is a very attractive anode material for the nextgeneration LIBs due to its high abundance,large theoretical capacity (4200 mAh·g-1 based on the formed Li₂₂Si₅),and environmental friendliness [ 45, 46, 47, 48, 49] .And many companies are dedicating its commercial application.Si can form a variety of alloy phases with K,such as KSi,K₁₂Si₁₇,K₇Si₄₆,and K₈Si₄₆ [ 50] ,but its electrochemical property in KIBs is rarely reported.Tran et al. [ 51] first proposed that Si could provide a high capacity of 957 mAh·g-1 based on the theoretical calculations (Fig.1a).The average voltage for the simulated KSi phase is 0.15 V.To verify the theoretical predicts,Sultana et al. [ 38] synthesized a Si/graphene composite and then tested its K-storage performance.As exhibited in Fig.1b,c,the achievable capacity of about 100 mAh·g^-1 is supplied by graphene,suggesting that Si does not react with K.Figure 1d displays the scanning electron microscopy (SEM) image of the Si/graphene electrode after 100 cycles.No distinct cracks and pulverizations are observed,which also confirms that Si has no activity in KIBs.Therefore,researchers are switched to the other alloy anodes,such as P,Sn,Sb,and Bi.

Fig.3 Schematic illustrations of synthesis process for a red P@CN composite (reprinted with permission [64] ,Copyright 2018 John Wiley and Sons),b red P@TBMC (reprinted with permission [65] ,copyright (2018) Elsevier),and c red P@N-PHCNFs and red P/N-PHCNFs electrodes(reprinted with permission [66] ,Copyright 2019 American Chemical Society);d-g in situ TEM images of a single red P@N-PHCNFs upon potassiation (reprinted with permission [66] ,Copyright 2019 American Chemical Society)

3 P-based anodes

Phosphorus (P) is one of the most promising anodes in alkaline ion batteries.It has three allo tropes,including white P,red P,and black P [ 52] .Among them,white P is highly toxic,unstable,and flammable in the air,making it unsuitable as anode.Amorphous red P is nontoxic and relatively stable,but limited by low electrical conductivity(10^-14 S·cm^-1).Black P is converted from white P under high temperature and pressure and possesses high electrical conductivity (300 S·cm^-1) and two-dimensional layered structure.In LIBs and SIBs,P owns a high theoretical capacity of 2596 mAh·g^-1 through three-electron transfer to form Li₃P and Na₃P [ 53, 54, 55, 56] .Theoretically,P can also form the binary K₃P alloy with K [ 57] .Therefore,it is of great significance to understand its development tendency and reaction mechanism in KIBs.

Fig.4 a XRD patterns of commercial Sn powder and Sn-C composite (reprinted with permission [76] ,Copyright 2016 Royal Society of Chemistry);b selected discharge-charge profiles of Sn-C composite (reprinted with permission [76] ,Copyright 2016 Royal Society of Chemistry);c XRD patterns of charged and discharged Sn-C electrodes (reprinted with permission [76] ,Copyright 2016 Royal Society of Chemistry);d discharge/charge curves of initial two cycles for Sn film (reprinted with permission [77] ,Copyright 2017 Electrochemical Society);e in situ XRD patterns of Sn film recorded in the first cycle (reprinted with permission [77] ,Copyright 2017 Electrochemical Society);f enlarged XRD patterns from e (reprinted with permission [77] ,Copyright 2017 Electrochemical Society)

Guo’s group first introduced P-based materials into KIBs [ 58] .The authors prepared a P/C composite which is consisted of agglomerated microparticles by the ball milling method.In the electrolyte of 0.8 mol·L^-1 KPF₆ in ethylene carbonate/diethyl carbonate (0.8 mol·L^-1 KPF₆/EC/DEC),it delivers an initial capacity of 2171.7 mAh·g^-1at 50 mA·g-1 (Fig.2a),which is close to the theoretical capacity of P.A nonstoichiometric K_3-xP compound is monitored at 25.38°and 65.91°by ex situ X-ray diffraction(XRD) when discharged to 0.01 V due to the partial potassiation (Fig.2b).However,Yang et al. [ 59] considered that the K₃P phase cannot be obtained upon potassiation because the calculated working potential is less than 0 V (Fig.2c).This viewpoint was immediately supported by Sultana et al. [ 60] .The as-synthesized black P/graphite (BP-C) composite only presents a capacity of～617 mAh·g-1 with an initial Coulombic efficiency(ICE) of 67%in Fig.2d,corresponding to the formation of KP alloy (Fig.2e).After 50 cycles,the capacity retention is only 4.86%.Zhang et al. [ 61] used the in-operando synchrotron XRD technology (Fig.2f) to deeply characterize the reaction process of GeP₅ electrode.It undergoes two reactions upon (de)potassiation (Fig.2g).Namely,K fist alloy with P to form K₄P₃ and then react with Ge to obtain KGe alloy.Different reaction mechanisms are attributed to different physical-chemical properties,such as structure,composition,size,morphology,crystal phase,and crystallinity.Additionally,Zhang et al. [ 61] also investigated the effect of the electrolytes on the stability of GeP_5.The capacity remains 213.7 mAh·g^-1 over 2000 cycles at500 mA.g-1 in potassium bis(fluorosulfonyl)imide (KFSI)-based electrolyte,while severe fluctuations appear in the traditional KPF_6-based electrolyte.The stabilities of the resulted solid electrolyte interphase (SEI) layers in KFSI and KPF_6-based electrolytes result in the huge difference of the electrochemical performance.Stable SEI layer can prevent the electrolyte eroding the active material and reduce the side reactions at the electrode/electrolyte interface [ 62] .As an electrolyte additive,fluoroethylene carbonate (FEC) has the positive effect in LIBs and SIBs,but its addition deteriorates the K-storage stability of GeP₅.The same conclusion was also obtained by Tuan's group [ 63] .The reversible capacity of the electrode is<3 mAh·g^-1 at 100 mA·g^-1 in FEC-based electrolytes.Density functional theory (DFT) calculations suggest that the interaction between K⁺and the solvent molecules is relatively weak without FEC,which is in favor of the diffusion and desolvation of K⁺.In addition to the optimization of electrolytes,various micro-/nanostructures and carbon substrates have been introduced to enhance the electrochemical activities of P-based materials.Xiong et al. [ 64] impregnated red P nanoparticles into a three-dimensional (3D) porous carbon nanosheet (CN) framework by a vaporization-condensation strategy in Fig.3a.Transmission electron microscopy(TEM) image displays that red P particles with a size of10-20 nm are uniformly distributed in the 3D porous CN framework.The chemical bonds between red P and the CN host,such as P-O-C and P=O-C bonds,are detected by Raman spectra,X-ray photoelectron spectroscopy (XPS),and Fourier transform infrared (FTIR) spectra.Benefited from the strong interaction between red P and CN framework and the unique 3D porous structure,the composite exhibits outstanding electrochemical activity.A high reversible capacity of 715.2 mAh·g^-1 is achieved at100 mA·g^-1.Even at a large current density of2000 mA·g^-1,the electrode still maintains a capacity of323.7 mAh·g^-1.The potassiation reaction evolution is simulated by DFT calculations.The formation energies of K₂P₃,KP,and K₃P are-0.148,-0.421,and-0.218 eV,respectively.Therefore,a one-electron reaction is proposed based on the formation of KP,which is in accordance with the experimental characterization.Liu et al. [ 65] used a carbon nanotube-backboned mesoporous carbon (TBMC)as the carbon host to confine the red P in Fig.3b.In the red P@TBMC composite,the multi-walled carbon nanotubes with high-content sp² carbon can accelerate the electron transfer,and the mesoporous structure can accommodate the dramatic volume variation during cycles.Accordingly,the electrode possesses superior cycle and rate performance.A high reversible capacity of 244 mAh·g-1 is obtained at 500 mA·g^-1 after 200 cycles in the electrolyte of 0.6 mol·L^-1 KPF₆ in EC and propylene carbonate(0.6 mol·L^-1 KPF₆/EC/PC).Similarly,Yu's group encapsulated red P nanoparticles into electrospun freestanding and porous nitrogen-doped hollow carbon nanofibers (denoted as red P@N-PHCNFs,Fig.3c) [ 66] .The formed P-C chemical bonds and N-doping can facilitate the contact between red P nanoparticles and carbon matrix as well as the adsorption of P atoms.The structure evolution of the electrode during charging was characterized by in situ TEM in Fig.3d-g.The initial thickness of a single red P@N-PHCNFs is 74 nm.After 160-s potassiation,this value keeps 93 nm,indicating that the volume expansion is only 26%.Based on the unique design,the red P@N-PHCNFs electrode exhibits superior electrochemical performance.High reversible capacities of 650 mAh·g^-1 at100 mA·g^-1 after 100 cycles and 465 mAh·g^-1 at2000 mA·g^-1 after 800 cycles are achieved,respectively.In contrast,the red P loaded on N-PHCNFs (red P/N-PHCNFs) presents poor cycle stability and rate capability due to the dramatic volume expansion.The discharged product of red P@N-PHCNFs is K₄P₃ that is demonstrated by in situ Raman and ex situ XRD.

Fig.5 (De)potassiation processes of Sn nanoparticles in the first cycle:a morphology of pristine Sn particles;b,c first-step potassiation;d-f second-step potassiation;g-i (de)potassiation-induced nanopores in Sn nanoparticles;j A KOH layer with thickness of～10 nm observed on surface of discharged Sn nanoparticles;k electron diffraction pattern of discharged sample;I electron diffraction pattern of charged sample(reprinted with permission [81] ,Copyright 2017 American Chemical Society)

Fig.6 a XRD patterns of pristine and discharged Sb-C electrodes (reprinted with permission [87] ,Copyright 2015 American Chemical Society);b CV curves of Sb-C composite in initial six cycles at 0.1 mV-s^-1 between 0.05 and 2.00 V (reprinted with permission [87] ,Copyright2015 American Chemical Society);c,d schematic illustration of the synthesis of nanoporous Sb (reprinted with permission [88] ,Copyright 2018American Chemical Society)

P-based materials can alloy with K and offer the highest capacity among the alloy-based anodes.However,the reaction mechanism of P in KIBs is still controversial,and the large volume change upon potassiation and depotassiation is the main problem for its application.Developing in situ technology to monitor the phase transition of the P-based anodes is highly desirable.

Fig.7 a Schematic illustration of synthesis of Sb@CSN;b TEM image of Sb@CSN;c size distribution of Sb from b;d SEM image of Sb@CSN;e simulated potentials for potassiation process based on DFT calculations;f crystal structures and corresponding lattice parameters of Sb,KSb₂,KSb,K₅Sb₄,and K₃Sb (reprinted with permission [89] ,Copyright 2019 Royal Society of Chemistry)

Fig.8 a CV curves of Bi electrode in initial three cycles at 0.1 mV·s^-1 between 0.1 and 1.5 V;b selected discharge-charge profiles at 0.5C(1C=400 mA·g^-1);c discharge-charge profile at 0.1C in the fifth cycle;d XRD patterns of different states as recorded in c;e fitted lattice parameters of Bi,KBi₂,K₃Bi₂,and K₃Bi based on Rietveld refinement (reprinted with permission [41] ,Copyright 2018 John Wiley and Sons)

4 Sn-based anodes

Tin (Sn)-based materials have been widely investigated in LIBs and SIBs because of their large capacities,high abundance'and nontoxicity [ 67, 68, 69, 70, 71] .However,Sn suffers from dramatic volume expansion (260%for Li₂₂Sn₅ and420%for Na₁₅Sn₄) upon lithiation or sodiation [ 72, 73, 74, 75] .The volume change of Sn may be severer in KIBs than that in LIBs and SIBs because of the large-sized K⁺.To relieve this issue,a lot of strategies have been devoted to confining the Sn-based alloys into a carbon matrix.

Sn-based anodes for K storage were firstly reported by Glushenkov's group in 2016 [ 76] .A Sn-C composite was synthesized by a ball milling method under Ar atmosphere with the commercial Sn powder and graphite as the rawmaterials.XRD patterns in Fig.4a show that the commercial Sn powder is indexed to the tetragonal system with a space group of I41/amnd.No impurity is observed after ball milling,indicating that the structure of pristine Sn is well maintained.At a current density of 25 mA·g^-1,the reversible capacity is between 150 and 170 mAh·g^-1 in the initial cycles (Fig.4b) and then decreases to 110 mAh·g^-1after 30 cycles.Figure 4c displays the ex situ XRD patterns of the discharged and charged products.After discharged to0.01 V,the diffraction peaks of the pristine Sn disappear,and new phases are formed.But these new diffraction peaks are not clearly attributed.They may be assigned to K₂Sn₅ or K4Sn₂₃.Ramireddy et al. [ 77] employed in situ technologies to have an insight into the reaction mechanism of Sn in KIBs.A layer ofβ-Sn film with a thickness of 1μm was deposited on the surface of a Cu substrate through the e-beam evaporation and then directly used as the electrode.The initial two discharge and charge profiles at 25 mA.g-1 are presented in Fig.4d.A flat plateau is observed at 0.2 V in the first discharging process,which is ascribed to the potassiation of Sn.Inversely,there are two plateaus in the following charging process.The sloped one is within 0.60-0.87 V and the flat one is at 0.88 V,suggesting that the depotassiation process undergoes via two steps.Unlike the first cycle,a high-voltage hump and a large irreversible capacity are found in the second cycle,which is resulted from the decomposition of the electrolyte and formation of the SEI film [ 78, 79, 80] .Figure 4e records the structure evolution ofβ-Sn during cycles through the in situ synchrotron XRD.Upon potassiation,the signals of Sn gradually decrease and disappear,as confirmed by the enlarged XRD patterns in Fig.4f.After discharging to0.01 V,Sn is completely converted into the tetragonal K₄Sn₄ phase,corresponding to the reversible capacity of245 mAh·g^-1 in the first cycle.During depotassiation,K₄Sn₄ directly transformed intoβ-Sn,and no other K-Sn intermediates are detected,which are inconsistent with the discharge/charge curves.The possible reason is that the signals of the intermediates are too weak,which result in difficulty of the data collection.It is necessary to developin situ technology with high sensitivity to capture these weak signals.

Fig.9 a-f SEM images of Bi electrodes at different cycles and inset image being contact angle between Bi electrode and DME-based electrolyte;three adsorptions of DME molecules surface of Bi:g bridge,h top and i hollow (reprinted with permission [41] ,Copyright 2018 John Wiley and Sons)

Fig.10 a K-storage mechanism of Bi during the first discharge and following cycles (reprinted with permission [101] ,Copyright 2018 John Wiley and Sons);b schematic illustration of morphological evolution and SEI formation (reprinted with permission [101] ,Copyright 2018 John Wiley and Sons)

Although it is difficult to identify the amorphous intermediates through XRD,the K content can be estimated via volume expansions.In situ TEM is a very useful tool to investigate the structure evolution of the electrode during cycles.Wang's group [ 81] assembled a K-Sn nanobattery for the in situ test,where Sn nanoparticles on the Pt rod and K metal on the W rod are used as the working electrode and anode,respectively.The solid electrolyte is K₂O and KOH in situ generated on the surface of the anode due to the contact of K metal with air.Figure 5 shows the (de)alloying processes of a single Sn nanoparticle in KIBs.Before discharging,an irregular Sn particle with a size of 226 nm is clearly observed in Fig.5a.At the beginning of potassiation,a K-poor phase is generated and has an obvious boundary with Sn(Fig.5b).Subsequently,the boundary gradually spreads into Sn and disappears,indicating that a single-phase reaction occurs (Fig.5c-f).The corresponding volume expansion is about 113%,which is close to the theoretical value of K₄Sn_9.A tetragonal KSn phase is detected after fully discharged in Fig.5k,corresponding to the initial charge capacity of 197 mAh·g^-1 and a volume expansion of 194%.Although the KSn phase is gradually converted intoβ-Sn during depotassiation,the volume shrinkage is not obvious (Fig.5g-i,Fig.51).Thus,the authors conclude that the reaction mechanism of Sn nanoparticles isβ-Sn→amorphous K₄Sn₉→KSn→crystalβ-Sn.To mitigate the volume expansion,Zhang et al. [ 58] prepared a Sn₄P₃/C composite through the ball milling method.The Sn₄P₃ particles with the size of 20-50 nm are evenly distributed in the amorphous carbon host.It delivers a reversible capacity of 307.2 mAh·g^-1 with a capacity retention of 79.8%at 50 mA·g^-1 after 50 cycles.In contrast,the capacity retention of the pure Sn electrode is only 20%after 20 cycles.Obviously,how to accommodate the volume variation is important for the Sn anodes for KIBs.

Unlike multiple electron transfer in LIBs and SIBs,Sn alloy with K to form the KSn phase is based on a oneelectron reaction mechanism,leading to a limited capacity.Therefore,further improving the capacities of Sn-based anodes can be achieved from its binary or multiple alloys,such as Sn-P,Sn-Sb,and Sn-Bi alloys.Also,it is necessary to develop novel characterization technologies to detect the amorphous phases and reveal the (de)potassiation mechanisms of Sn-based materials.

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Table 1 Summary on synthetic methods and electrochemical performance of different alloy-based anodes in KIBs

5 Sb-based anodes

Antimony (Sb) is widely studied for Li and Na storage due to its high abundance and low cost.In LIBs and SIBs,it offers a theoretical capacity of 660 mAh·g^-1 based on the three-electron reaction [ 82, 83, 84, 85] .According to the K-Sb phase diagram,Sb can be alloyed with K to form the K₃Sb phase [ 86] .However,the inevitable volume variation upon(de)potassiation hinders its practical application.A lot of researches have been concentrating on designing novel micro-/nanostructures to stabilize the (de)alloying processes of Sb-based materials.

Wu's group first attempted the application of Sb in KIBs [ 87] .They synthesized a Sb-C composite composed of70 wt%Sb and 30 wt%super P by mechanical milling.In Fig.6a,the XRD pattern of the Sb-C composite matches well with the standard.The weakened and broadened diffraction peaks imply reduced particle size.Beyond the first cycle,the composite displays two couples of redox peaks at 0.75/0.08 and 1.20/0.38 V in Fig.6b,indicating two two-phase reactions.After 10 cycles,the reversible capacity maintains 650 mAh·g^-1,which is close to the theoretical value of three-electron transfer.The reaction mechanism based on the corresponding capacities is considered to be Sb←→KSb←→K₃Sb.However,only the cubic K₃Sb phase is detected by ex situ XRD upon potassiation because of the amorphous nature of the intermediates (Fig.6a).As illustrated in Fig.6c,d,An et al. [ 88] designed a 3D nanoporous Sb by removing the Zn atoms from the commercial Zn-Sb alloy using a vacuum distillation method.The morphology and porosity can be adjusted through tailoring the ratio of Zn and Sb and the reaction temperature.Such unique porous structure can alleviate the volume expansion and accelerate the diffusion of K⁺.The optimized sample delivers a high reversible capacity of 510 mAh.g-1 at 100 mA.g刈in the second cycle.After 50 cycles,the capacity retention is 62.35%.Interestingly,the crystalline Sb transforms into the amorphous Sb after the first cycle in this work.

Wang's group enhanced the electrochemical activity of Sb by simultaneously tuning the structure and interface [ 89] .In Fig.7a,an electrospray technology is employed to encapsulate Sb nanoparticles into the carbon spheres network (CSN).The Sb nanoparticles with an average size of14 nm are uniformly confined in a carbon sphere in Fig.7b,c.The carbon spheres are interconnected together to form a 3D conductive network in Fig.7d,which is beneficial to shorten the transfer paths of ions and electrons and accommodate the volume variation.It exhibits a high reversible capacity of 504 mAh·g^-1 at 200 mA·g^-1 over200 cycles with a high Coulombic efficiency of 98%and a high capacity retention of 80.5%in 4.0 mol·L^-1 KFSI/EC/DEC.A thin and dense KF-rich SEI film is formed due to the decomposition of FSI^-,preventing the electrolyte from further eroding the active material and alleviating the volume change.A reversible and sequential phase transition of Sb,KSb₂,KSb,K₅Sb₄,and K₃Sb is demonstrated by DFT calculations in Fig.7e and electrochemical characterizations.Figure 7f shows their crystal structures and the corresponding lattice parameters.The values of relative volume expansions from Sb to KSb₂,KSb,K₅Sb₄,and K₃Sb are 73.7%,147.6%,191.7%,and 456.6%,respectively.The research promotes the in-depth understanding of potassium storage mechanism of alloy-based anodes and provides experimental and theoretical insights into the rational design of electrode structure and electrolyte system for KIBs.

6 Bi-based anodes

Metalic bismuth (Bi) has attracted extensive attention in many fields due to its unique two-dimensional (2D) layered structure and high electronic conductivity [ 90, 91, 92, 93, 94] .It delivers a high theoretical capacity of 385 mAh·g^-1 based on three-electron reaction [ 95, 96, 97] .According to the K-Bi phase diagram,it can provide such a high capacity for the formation of the K₃Bi phase [ 98] .The investigations on its Na and K storage are active recently.

Li's group directly used the commercial Bi powder as the electrode material and explored the effect of the interface interaction between the Bi electrode and etherbased electrolytes on its electrochemical performance [ 41] .In the electrolyte of 1.0 mol·L^-1 KPF₆ in dimethoxyethane(DME),it presents three couples of redox peaks at 1.15/0.93,0.45/0.67,and 0.30/0.57 V,respectively,in the CV curves beyond the first cycle in Fig.8a,implying that three two-phase reactions occurred during cycles.Its discharge and charge profiles at 0.5C (1C=400 mA·g^-1) are shown in Fig.8b.A high charge capacity of 397.8 mAh.g^-1 with a Coulombic efficiency of 80.2%is achieved in the first cycle.After 10 cycles,it maintains 394.2 mAh·g^-1 with the Coulombic efficiency close to 100%.The calculated discharge capacity ratio of 1:3:6 for the three plateaus in Fig.8b suggests the successive formation of KBi₂,K₃Bi₂,and K₃Bi.The ex situ XRD patterns in Fig.8d also confirm the three reversible two-phase reactions of Bi→KBi₂K₃Bi₂←→K₃Bi.Their cell parameters fitted by Rietveld refinement are shown in Fig.8e.The volume expansion gradually increases with the continuous discharging,and it is high up to 409%for the formation of K₃Bi.Surprisingly,the bulk Bi electrode still exhibits excellent cycle and rate performance in the DME-based electrolyte.A high reversible capacity of 322.7 mAh.g-1 with a capacity retention of 86.9%and a Coulombic efficiency of 99.6%is obtained at 2C after 300 cycles.This research highlights the synergistic effect of the Bi electrode and DME-based electrolyte and provides new insights into the alloy-based electrodes for KIBs.

To clarify the mechanism,the morphology evolution of Bi is monitored in cycles through SEM.A bulk Bi particle is clearly presented before potassiation in Fig.9a.After the first cycle,some small holes are generated on its surface(Fig.9b).With the continuous cycling,these pores become larger and are linked together to form a 3D porous network in Fig.9c.After 70 and 100 cycles,this porous structure gradually tends to stabilize (Fig.9d-f).Such unique 3D porous feature is related to the movement of surface Bi atoms induced by the strong chemical adsorption of DME molecules,as confirmed by DFT calculations in Fig.9g-i.Its formation is not only beneficial to wettability of the electrolytes and transport of K⁺and electrons,but also can accommodate the volume change and realize the long cycle.Moreover,the SEI film is also an important factor for the interfacial stability.It consists of polyether,RCH₂OK,KOH,KF,and KPF₆ in DME-based electrolyte,which is different from the carbonate-based electrolyte containing unstable ROCO₂K [ 99, 100] .Such superior performance is attributed to the synergistic effect between Bi and the ether-based electrolyte,which provides new insights into the alloy-based anodes for KIBs.

In the meantime,Huang et al. [ 101] also reported the electrochemical properties of microsized Bi particles in diglyme (G2)-based electrolytes.In agreement with Li's group [ 41] ,they revealed conversion of Bi into K₃Bi with three reversible two-phase reactions of Bi←→KBi₂←→K₃Bi₂←→K₃Bi upon discharging,as confirmed by theoretical calculations and in situ XRD (Fig.10a).The volume expansion is about 406%from Bi to K₃Bi,which is almost twice of Na₃Bi.The superior electrochemical stability is attributed to the stable and flexible SEI film,which is composed of organic and inorganic compounds,such as(CH_2-CH_2-O-)_nK,(CH₂CH_2-OCH_2-O-)_nK,RCO₂K,and K₂O_x.The oligomer-containing SEI layer possesses good mechanical flexibility and can alleviate the huge volume variation upon potassiation and depotassiation (Fig.l0b) [ 102] ,highlighting the synergistic effect between the Bi electrode and the ether-based electrolyte.Various modification methods have been attempted to accommodate the volume change upon (de)potassiation and minimize the inferior effect on the stability of the alloy-based anodes:(1)rational design of micro-/nanostructures;(2) introduction of carbon hosts;(3) interface optimization of the electrode and electrolyte.The synthetic methods and electrochemical performance are listed in Table 1.Introducing carbon hosts and designing micro-/nanostructures can effectively buffer the internal stress and volume variation and shorten the ion diffusion pathways,which lead to the enhanced cycle and rate performance.But micro-/nanomaterials have low active material loads and tap densities that will reduce volumetric energy densities of batteries.Furthermore,high surface areas of these micro-/nanomaterials can increase the contact area between the electrode and the electrolyte,resulting in many unexpected side reactions and large irreversible capacities.Bulk material with high tap density also exhibits excellent performance by optimizing the electrolyte to form the effective interfaces.

7 Conclusions and perspectives

The recent progress of alloy-based anodes for KIBs is summarized.The crystal structure,composition,morphology,size,and crystallinity of the materials have significant effect on K-storage performance and mechanism.Currently,in situ characterization methods,such as in situ SEM,in situ scanning transmission electron microscopy (STEM),and in situ X-ray absorption near edge structure (XANES),are desirable to explore the K-storage and inactivation mechanisms of alloy-based anodes.Electrolyte also affects the electrochemical property through the formation of effective SEI film and the adsorption.The formation of SEI is important for the ionic and electronic conductivity and electric field distributions at interfaces.But the construction of effective SEI films is still challenging.Therefore,the relationship between the compositions and concentrations of electrolytes and the formation process and components of SEI film needs to be systematically studied.Furthermore,the mechanism of atom migration induced by the adsorption of electrolyte is unclear.The adsorption models between the electrolytes and electrodes are worthy of further explorations.

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