稀有金属(英文版) 2020,39(09),970-988

Recent advances in alloy-based anode materials for potassium ion batteries

Shi-Han Qi Ji-Wei Deng Wen-Chao Zhang Yue-Zhan Feng Jian-Min Ma

State Key Laboratory of High-Performance Complex Manufacturing,College of Mechanical and Electrical Engineering,Central South University

School of Physics and Electronics,Hunan University

Institute for Superconducting and Electronic Materials (ISEM),School of Mechanical,Materials,Mechatronics & Biomedical Engineering,Faculty of Engineering and Information Sciences,University of Wollongong

Key Laboratory of Materials Processing and Mold(Zhengzhou University),Ministry of Education,Zhengzhou University

作者简介：*Ji-Wei Deng e-mail:dengjw@csu.edu.cn;*Wen-Chao Zhang e-mail:wenchao@uow.edu.au;*Jian-Min Ma is an associate professor in Hunan University, China.He received his BSc degree in chemistry from Shanxi Normal University in 2003 and his Ph.D.degree in materials physics and chemistry from Nankai University in 2011. During 2011-2015,he also conducted research in several overseas universities as a postdoctoral research associate.His research interests are focused on energy storage and conversion, green catalytic technologies and theoretic calculations.e-mail:nanoelechem@hnu.edu.cn;

收稿日期：11 December 2019

基金：financially supported by the National Natural Science Foundation of China (Nos.51302079 and 51702138);the Natural Science Foundation of Hunan Province (No. 2017JJ1008);the Key Research and Development Program of Hunan Province of China (No.2018GK2031);

Recent advances in alloy-based anode materials for potassium ion batteries

Shi-Han Qi Ji-Wei Deng Wen-Chao Zhang Yue-Zhan Feng Jian-Min Ma

State Key Laboratory of High-Performance Complex Manufacturing,College of Mechanical and Electrical Engineering,Central South University

School of Physics and Electronics,Hunan University

Institute for Superconducting and Electronic Materials (ISEM),School of Mechanical,Materials,Mechatronics & Biomedical Engineering,Faculty of Engineering and Information Sciences,University of Wollongong

Key Laboratory of Materials Processing and Mold(Zhengzhou University),Ministry of Education,Zhengzhou University

Abstract：

Potassium ion batteries(PIBs) are regarded as one of promising low-cost energy storage technologies.Achieving long cycle life and high energy density has been considered as important tasks for developing high-performance PIBs.The alloy-based anodes for PIBs have attracted great attentions because of their high theoretical capacity and relatively low operating voltage.In this review,the latest advance in the related alloy-based anodes was overviewed.Specifically,the correlations among the morphology and potassium storage performance,phase transition mechanisms,the formation of solid electrolyte interphases and ionic transport kinetics are critically discussed.It is expected that this review will provide meaningful guidance and possible pathways for the developments of alloy-based anodes for PIBs.

Keyword：

Potassium ion batteries; Anode; Alloy-based alloys; Performance; Nanostructures;

Received： 11 December 2019

1 Introduction

Owing to the staggering developments of green renewable energy,such as hydroelectric power,solar power,tide power,there is need for scalable energy storage systems with low cost and high reliability [ 1, 2, 3, 4] .In the past few decades,the public was impressed by the remarkable development in electrochemical energy storage systems,such as lithium ion batteries (LIBs) [ 5] ,sodium ion batteries [ 6, 7, 8, 7] and supercapacitors (SCs) [ 9, 10, 11, 12] .Specifically,LIBs have been successfully commercialized in portable devices such as cell phones and laptops.Indeed,the recent progress in replacing combustion energy vehicles to electric vehicles motivated the increasing demand for high-energy and safe LIBs [ 13] .However,the limited reserve and uneven distribution of lithium resources portend a gloomy outlook for the future application of LIBs.Concerns surrounding the increasing cost of LIBs have resulted in the development of new-type electrical energy storage technology [ 14] .In recent years,sodium ion batteries (SIBs) have been one of the most active research topics worldwide because of the high abundance of sodium resources and similar”rocking-chair”type mechanism with LIBs [ 15, 16, 17] .Unfortunately,the high standard redox potential of sodium causes SIBs to only operate at lower potentials than that of LIBs (Li:-3.04 V and Na:-2.71 V vs.the standard hydrogen electrode (SHE)),which leads to relatively low energy density [ 18] .Recently,potassium ion batteries (PIBs) have shown a strong competition with SIBs rivals [ 19] ,which is due to the abundant potassium resources in the Earth's crust and a lower reduction potential than that of sodium (K:-2.93 V vs.SHE) which lead to a higher energy density than that of SIBs.Furthermore,due to the weak Lewis acidity,the solvated K⁺ions possess the small Stokes radius,which brings the small desolvation activation energy [ 20, 21] .Therefore,potassium-based electrolyte shows higher ionic mobility than those of lithium-based and sodium-based electrolytes.In terms of these above advantages,PIBs have been regarded as a promising electrochemical energy storage device [ 22, 23, 24, 25, 26] .

Nevertheless,it is a major challenge to search potential electrode materials for PIBs due to the large ionic radius of K⁺.Benefited by the smaller solvated K⁺than those of Na⁺,graphite,the primary commercial anode material for LIBs,can offer a potassium storage capacity of about 270mAh·g^-1 [ 27] .Besides,plenty of efforts have been paid to develop amorphous carbons (hard carbon and soft carbon)as anode materials for PIBs,which show the reasonable reversible capacity and cycling stability.However,due to the low density,the carbon-based anodes for PIBs only present limited volumetric capacities,hindering their practical application [ 28] .

Along with the research history of LIBs and SIBs,alloybased anodes have attracted much attention due to their relatively low operating voltage and high theoretical capacity [ 29] .For instance,Si,Ge,Sn and Sb possess the remarkable theoretical capacities of 4200,1600,994 and660 mAh·g^-1 for LIBs and P and Sn can form alloy with sodium to present the theoretical capacities of 2596 and847 mAh·g^-1,respectively [ 30, 31] .Therefore,it is expected that enough attentions should be paid on the aspects of alloy-based anodes for high-energy-density PIBs.According to the theoretical calculation reported by Kim et al.,alloy-based anodes including Si,P,Ge,Sn,Sb and Pb can be alloyed with potassium in various ratio [ 28] .However,the large volume variations and the sluggish kinetics of K-alloying reaction are still great challenges.Past few years have witnessed the growing research on the alloy-based anode materials for PIBs [ 29, 32, 33] .Among them,several effective strategies have been developed to overcome those problems,including employing carbonaceous matrix,coating protective layer and employing suitable electrolytes.At this right time,it is necessary to give a timely review of the recent advances and provide some valuable outlooks to the future research of the alloybased anodes for PIBs.

2 Tin anode

It is well known that tin (Sn) is an attractive alloy-based anode material because of the high theoretical capacities of990 mAh·g-1 (based on the Li₂₂Sn₅ phase as the end product) and low operating voltage for LIBs and 847mAh·g^-1 for SIBs (based on the Na₁₅Sn₄ phase as the end product),respectively [ 34] .However,in both LIBs and SIBs,Sn anodes suffer from severe volume expansions during the lithiation/sodiation processes (420%for Na₁₅Sn₄and 260%for Li₂₂Sn₅).Similarity,this large volume change could also be observed in PIBs during potassiation process [ 35, 36] .Xu and co-workers studied the volume change in Sn nanoparticle as anode for PIBs by in situ transmission electron microscopy (TEM) [ 37] .As shown in Fig.1a,the pristine Sn particle had a diameter of 226 nm.According to the in situ TEM measurements,the potassiation process of Sn could be pided into two steps.As displayed in Fig.1b,c,during the first step,K⁺diffused from the solid electrolyte consisting of K₂O and KOH to the Sn particle.The Sn crystal turned into K-poor phases,K₂Sn₅ or K₄Sn₂₃.A sharp boundary could be observed between the K-poor phases and the unreacted Sn phase.Then,during the second step,with the further potassiation,the phase boundary gradually disappeared,indicating the formation of a single phase,as exhibited in Fig.1e,f.After the first-step potassiation,the volume expansion rate was113%for the K-poor phase,whereas a volume expansion of～197%after the second step.It should also be noted that a thick KOH layer was observed on the surface of potassiated Sn nanoparticle,as shown in Fig.1j.This layer acted as the solid electrolyte interphases (SEI) layer which would bring low Columbic efficiency and high impedance.Confirmed by the electron diffraction pattern given in Fig.1k,the fully potassiated product was KSn with a tetragonal structure.This result was also proved by some other works [ 36] .Figure lg-i shows the dealloying process.The volume of Sn nanoparticle shrunk and some nanopores emerged.According to the electron diffraction pattern presented in Fig.11,the KSn phase transformed back to theβ-Sn phase.Above results suggested that the significant volume change during potassiation/depotassiation is the main reason for the rapid capacity decay for Sn anode [ 38] .

A common strategy to improve the structural stability of Sn anode is employing suitable carbonaceous materials as a protective layer or flexible substrate [ 39] .For example,Huang et al. [ 40] prepared a 3D-HPCS as anode material for PIBs with NaCl serving as hierarchical templates.The morphology and structure of the as-prepared sample were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).As shown in Fig.2a,main Sn nanoparticles homogeneously anchored on the surface of the hierarchically porous carbon matrix.Figure 2b shows a TEM image of two typical Sn nanoparticles with sizes of～65 nm.It could be observed that an ultra-thin carbon layer was coated on those nanoparticles.The porous carbon matrix can provide enough space for potassium storage,and the thin coating layer can prevent the pulverization of Sn nanoparticles.Therefore,as anode material for PIBs,3D-HPCS delivered good potassium storage performance.As exhibited in Fig.2c,at a current density of 50 mA.g-1,3D-HPCS showed an initial discharge capacity of 847.9 mAh.g-1.Moreover,in the subsequent cycles,reversible capacities of about 270 mAh·g^-1 could be exhibited.Figure 2d shows the cycling performance of this composite.After 100cycles,the Columbic efficiencies were maintained over96%.Moreover,employing graphene as a conductive matrix is another effective method to enhance the structural stability and electrode integrity of Sn anode.Wang et al. [ 41] encapsulated Sn submicron particles into porous graphene networks.For the introduction of the 3D porous network,the volume expansion was alleviated.Hence,the Sn@graphene anode exhibited a reversible capacity of123.6 mAh·g^-1 over 500 cycles at 500 mA·g^-1.

Fig.1 Morphology changes in Sn particles during the first potassiation/depotassiation processes:a pristine Sn nanoparticle;b,c first-step potassiation process,where arrows represent direction of K⁺diffusion;d-f second-step potassiation process,without a clear boundary;g-i depotassiation process;j thick KOH layer (～10 nm) on surface of potassiated Sn particle;electron diffraction pattern of k fully potassiated Sn particle and I fully depotassiated Sn particle [37] (Reproduced with permission,Copyright 2017 American Chemical Society)

The existing works of Sn anode for PIBs focused on the strategies of employing carbon substrate and nanocrystallization to meet the challenge of volume change.However,there is a lack of the investigation of coulombic efficiency and interphase issue.To improve the initial coulombic efficiency and build senior SEI layer are the potential research directions for Sn anodes.

3 Antimony anode

Antimony (Sb) has been regarded as a promising anode for high-energy-density alkaline metal ion batteries for high volumetric capacities.For LIBs and SIBs,these high capacities were originated from the formation of Li₃Sb and Na₃Sb phases as the end products,respectively [ 42] .To determine the mechanism and the end product of potassiation reaction for Sb anode,several research works have been engaged [ 43, 44] .Zheng and co-workers [ 45] investigated the phase evolution of Sb anode during the potassiation process by density functional theory (DFT)calculation.As shown in Fig.3a,four intermediate phases,including KSb₂,KSb,K₅Sb₄ and K₃Sb with the potential decreasing,emerge during the potassiation process.The theoretical equilibrium potentials are located at 0.89,0.85,0.44 and 0.40 V,respectively.Based on the end product of K₃Sb,the theoretical capacity of 660 mAh·g^-1 can be obtained.The CV measurement can prove this mechanism.As displayed in Fig.3b,two pairs of redox peaks were located at～0.9 and 0.4 V.In detail,the alloying peak at0.78 V could be attributed to the phase transformation from Sb to KSb₂ and KSb.And then,the reduction peak at～0.23 V was resulted from the formation of K₅Sb₄ and K₃Sb phases.During the depotassiation process,the peak representing the dealloying of K₃Sb and K₅Sb₄ located at0.64 V and the inconspicuous peak caused by the dealloying of KSb and KSb₂ shifted to 1.12 V because of the polarization.Qian et al.also studied this issue and reported the same mechanism by using in situ XRD and ex situ Raman spectroscopy [ 46] .The in situ XRD patterns are shown in Fig.3c.It can be observed that the characteristic peaks of hexagonal Sb gradually disappeared,while characteristic peaks of the cubic K₃Sb phase emerged during the potassiation process.It should be noticed that after fully depotassiation,although the characteristic peaks of the K₃Sb phase disappeared completely,the characteristic peaks of Sb still could not be detected.It is indicated that amorphous Sb was obtained after the first depotassiation process.Ex situ Raman spectroscopy was also carried out to verify the potassiation mechanism.As shown in Fig.3d,the fully potassiated Sb anode showed a broad peak at145 cm^-1,suggesting the formation of the cubic K₃Sb phase.Furthermore,after the depotassiation process,the Raman pattern was different from that of the pristine state,further indicating the formation of amorphous Sb.

Fig.2 a SEM image and b TEM image of 3D hierarchically porous carbon/Sn composite (3D-HPCS);c galvanostatic charge-discharge curves;and d cycling performance for 3D-HPCS at 50 mA·g^-1 [34] (Reproduced with permission,Copyright 2018 Royal Society of Chemistry)

Similar to other alloy-based anode materials,Sb also experiences large volume variation during the alloying/dealloying reactions [ 47, 48] .Besides,the terrible polarization caused by the limited K⁺diffusion rate in Sb crystalline is also a severe problem [ 49] .To date,some strategies to solve these problems have been reported,including nanosizing particles, [ 50] incorporating in carbon component, [ 51] optimizing binders [ 52] and building open architecture [ 53] .Mai's group prepared an Sb@HCT composite as anode for PIBs by pyrolyzing resin coated Sb₂S₃ [ 54] .As shown in Fig.4a,the Sb@HCT nanorods had a diameter of～100 nm and the length of several micrometers.According to the TEM image exhibited in Fig.4b,the smaller Sb nanorods were encapsulated in carbon tubes with some void space.The thickness of the carbon shell was～11 nm.This hollow structure can alleviate volume change of Sb effectively.Besides,the N-doped hollow carbon shell will also enhance the ionic permission,Copyright2018 Royal Society of Chemistry)conductivity of the Sb@HCT electrode.The impact of different electrolytes was also investigated.Figure 4c displays the cycling performances of Sb@HCT in potassium hexafluorophosphate (KPF₆) or KFSI salt in EC/dimethyl carbonate (DMC)(volume ratio of 1:1).Over 80 cycles,in KPF₆ electrolyte,this composite only delivered very limited cycling stability.On the contrary,in the KFSI electrolyte,most capacity was maintained at last.As demonstrated in Fig.4d,the coulombic efficiency obtained in KFSI electrolyte was more stable than that obtained in the KPF₆ electrolyte.It has been proved that FSI^-anion can prevent the decomposition of electrolyte and generate a more durable SEI layer.As a result,the Sb anode exhibited better cycling performance in KFSI electrolyte than that of KPF₆ electrolyte.Benefited from the hollow structure,N-doping carbon coating layer and optimized electrolyte,Sb@HCT delivered a stable long-term cycling life.As exhibited in Fig.4e,a relatively high capacity of 300.1mAh·g^-1 at 2 A·g^-1 was obtained after 120 cycles.

Fig.3 a DFT calculated equilibrium voltages (vs.K/K⁺) forpotassiation process and b cyclic voltammetry (CV) curves for Sb@carbon sphere network electrode at a scan rate of 0.05 mV·s^-1and in an electrolyte of 4 mol·L^-1 KTFSI/ethylene carbon (EC)+diethyl carbonate (DEC)[45](Reproduced with permission,Copyright2019,Royal Society of Chemistry);potassium storage mechanism of Sb nanoparticles:c in situ XRD patterns;and d ex situ Raman spectra obtained by testing fresh,fully potassiated and depotassiated electrode,respectively[46](Reproduced with permission,Copyright2018 Royal Society of Chemistry)

4 Bismuth anode

In recent years,among various alloy-based anode materials,bismuth (Bi) has been considered as the promising one because of the following numerous advantages,such as non-toxic property,large lattice spacing along c-axis for ions insertion,and high theoretical volumetric capacity(3800 mAh·L^-1) [ 55] .Li and co-workers studied the alloying reaction mechanism of Bi anode in PIBs [ 56] .To determine the intermediates,the seven states marked in Fig.5a were subjected to XRD measurements,and the corresponding results are shown in Fig.5b.During the first potassiation process,after discharging to 0.9 V,the characteristic peaks of fresh Bi anode disappeared and those of THE cubic KBi₂ phase with the space group of Fd-3m emerged.With continued discharging to 0.45 V,the diffraction peaks at 18.1°,30.3°,31.5°and 40.2°appeared,suggesting the formation of monoclinic K₃Bi₂ phase.After fully discharged to 0.1 V,the characteristic peaks assigned to the end product,hexagonal K₃Bi,emerge at 18.7°,29.1°,29.9°,33.4°,41.9°and 51.8°.The corresponding crystal structures of KBi₂,K₃Bi₂ and K₃Bi are exhibited in Fig.5c-e,respectively.Based on these results,it can be predicted that the volume expansion of Bi will reach 409%for the formation of the K₃Bi phase.This severe volume expansion can cause the morphology change during the alloying/dealloying process.Li et al.also characterized the morphology transformation of bulk Bi particles in an etherbase electrolyte [ 56] .SEM image of the pristine Bi particle is shown in Fig.5f.After 10 cycles in Fig.5g,it became porous with lots of holes.Then,the porous structure gradually became stable as displayed in Fig.5h.Similar results can also be proved by other studies.This porous and stable structure can provide the channels for K⁺diffusion,which brings good rate capability and withstands long-term cycling tests.Thus,some previous works reported that bulk Bi particles can also deliver superior potassium storage performance.However,in carbonate electrolyte,the structure of bulk Bi particle will be pulverized during the alloying/dealloying process.

Recently,it was reported that high concentration electrolyte could enhance the stability of electrode material in both aqueous and non-aqueous systems [ 57, 58, 59] .This strategy can also be applied to Bi anode for PIBs.Sun and co-workers proved that various electrolyte concentrations have a huge impact on the electrochemical cyclability of Bi anode [ 60] .As shown in Fig.6a,only in the concentration of 5 mol·L^-1,the Bi@C electrode delivered stable cyclability with high coulombic efficiency.In other concentrations,the cyclabilities were very limited.To determine the reason for this difference,the cycled Bi@C electrodes tested in different electrolyte concentrations were characterized by ex situ SEM.As can be seen from Fig.6b,in1 mol·L^-1 electrolyte concentration,a thick SEI film was formed on the surface of Bi@C particles.On the contrary,in high electrolyte concentrations of 5 and 7 mol.L^-1,the SEI films were much thinner.Moreover,compared with that in 7 mol·L^-1 electrolyte,the Bi@C particles were more porous with smaller sizes in 5 mol·L^-1 electrolyte.That is why the best electrochemical cyclability was obtained in 5 mol-L^-1 electrolyte.The chemical composition of SEI film on the potassiated Bi electrode was confirmed by X-ray photoelectron spectra (XPS) [ 61] .Figure 6e-f shows the C 1s and O 1s spectra,indicating that the SEI film consists of two parts,organic and inorganic compounds.The main components of the inorganic parts were K₂O_x and KOH.According to the characteristic peaks of C-O,O=C-O,C=O,O-C-O,C-C and O-K species,the organic compounds consisted of potassium oligomers ((CH₂CH₂-O-)_nK,(CH₂CH₂-OCH₂-O-)_nK).

Fig.4 Morphology characterizations of Sb@hollow carbon tube (Sb@HCT) composite:a SEM image and b TEM image;electrochemical performances of Sb@HCT as anode for PIB s:c cycling performance and d corresponding coulombic efficiency in KFSI and KPF6 electrolyte at0.5 A·g^-1;e long-term cycling performance of Sb@HCT in KFSI electrolyte at 2 A·g^-1 [54] (Reproduced with permission,Copyright 2019Springer)

Other works focused on the strategy of coating carbon on Bi as a protective layer to alleviate the volume change [ 61, 62, 63, 64] .Yu et al.synthesized a multicore-shell-structured Bi@N-doped carbon composite as anode for PIBs [ 65] .Benefited from the protection and high conductivity of N-doped carbon shell,this composite showed unprecedented cycling life and rate capability for PIBs.(203mAh·g^-1 at 10 A·g^-1 over 1000 cycles and 152 mAh·g^-1at 100 A·g^-1).

Fig.5 a Discharge-charge curve of 5th cycle at 0.1C;b XRD patterns at different states (1-7) as indicated in a;crystal structures of intermediates in a discharging process:c KBi₂,d K₃Bi₂ and e K₃Bi;SEM images of Bi electrodes after selected cycles:f pristine state,g after 10cycles and h after 100 cycles [56] (Reproduced with permission,Copyright 2018 Wiley)

Based on the above summary about Sb and Bi anodes,it can be known that those two materials have been studied systematically,including reaction mechanism,morphology construction and formation of SEI layer.If the industrially scalable production methods can be developed,the Sb and Bi electrodes will become the promising candidates of anodes for PIBs.

5 Phosphorus anode

Phosphorus has various allo tropes,including red phosphorus (red P),black phosphorus (black P),white phosphorus and phosphorene [ 66] .Among them,red P and black P have attracted many attentions for their high theoretical capacity of 2596 mAh·g^-1 [ 67] .Thus,some researchers devoted their efforts to exploring the application of P anodes for PIBs [ 68, 69] .Yang and co-workers reported the first-principles study for the alloying reaction mechanism of black P in PIBs [ 70] .As shown in Fig.7a,during the potassiation process,the formation energies of various possible K-P alloy phases were computed,including K₃P₁₁,K₃P₇,K₂P₃,KP,K₄P₃,K₃P.For the relatively smaller formation energy in magnitude,the point of K₃P₇deviated from the energy curve,implying the absence of this phase during the potassiation process.The equilibrium potentials of each intermediate phase were also computed,as displayed in Fig.7b.Unlike the three-electron alloying mechanism in LIBs and SIBs,K₃P cannot be formed in PIBs for the much negative potential.This will limit the potassiation capacity of black P for PIBs.The experimental result proved this theoretical mechanism.Glushenkov's group prepared a black P/carbon composite anode for PIBs[71]As exhibited in Fig.7c,some black P particles embedded into bulk graphite particles.The charge-discharge profiles of this composite anode are given in Fig.7d.Even at the optimized condition,it only displayed a reversible capacity of～500 mAh·g^-1.After 50 cycles,about 40%capacity decayed.Up to now,to use black P as anode for KIBs is still challenging.In addition,the high price of black P may hinder its application.To develop cheap synthesis method of black P is one of the key directions in future.

Fig.6 a Depotassiation capacity and coulombic efficiency at 100 mA·g^-1 of Bi@C anode in different electrolyte concentrations;SEM images of cycled Bi@C anode in different concentrations of KTFSI-DEGDME electrolyte;b 1 mol·L^-1,c 5 mol·L^-1 and d 7 mol·L^-1 [60] (Reproduced with permission,Copyright 2018,Royal Society of Chemistry);XPS spectra of cycled Bi electrodes in 1 mol-L^-1 KPF₆/diglyme including e C 1s and f O 1s [61] (Reproduced with permission,Copyright 2019 Wiley)

To address the bottlenecks of poor electronic conductivity and large volume variations upon cycling of phosphorus-based anodes,employing carbon as soft matrix is the common strategy [ 72] .Ball milling and vaporization condensation methods are the most common synthesis methods to prepare red P-carbon anode material.For example,Tuan and co-workers dispersed red P particles into multiwall carbon nanotubes by the ball-milling method,as shown in Fig.8 a [ 73] .The structural scheme of this composite anode is shown in Fig.8b.The P-C bond may decrease the electronic conductivity.In this RP/C anode,the formation of P-C bonds was avoided.The interconnected carbon nanotube can play the role as electron transport channel,and the open structure will not hinder the diffusion of K⁺.Therefore,the RP/C anode delivered a good cycling performance at 1000 mA·g^-1,as exhibited in Fig.8c.Over 60 cycles,a high reversible capacity of～300 mAh·g^-1 was still maintained.Furthermore,Wang and co-workers prepared an RP@RGO composite through the vaporization-condensation method [ 74] .The synthesis was carried out in an autoclave,as shown in Fig.8d.To anchor red P tiny particles on the RGO sheet can not only enhance its electronic conductivity,but also improve its structural stability during alloying/dealloying process.As can be seen from Fig.8e,at the current density of 100 mA·g^-1,the RP@RGO anode showed a reversible capacity of～600 mAh·g^-1 during the second and third cycles.Because of the synergy effect,the RP@RGO composite delivered better cycling performance than pure RGO and graphite electrodes in PIBs.As exhibited in Fig.8f,the RP@RGO still held a potassiation capacity of 366.6 mAh·g^-1 with the coulombs efficiency maintained over 98%.

Fig.7 a Calculated formation energies and b equilibrium potentials of possible phase during potassiation of black P anode [70] (Reproduced with permission,Copyright 2019,Springer);c TEM image and d charge-discharge profiles at 50 mA·g^-1 of black P-graphite anode[71](Reproduced with permission,Copyright 2017 Royal Society of Chemistry)

Although much attention has been paid to develop P-based anodes,there are still some challenges to meet.Firstly,there is a lack of the green,cheap and safe synthesis methods to synthesize P.Secondly,the flammability of some phosphorus allotrope makes them difficult to store.It also brings the worries about safety of P-based anodes.At last,the coulombic efficiencies of P-based anodes reported by previous works are not satisfying.Thus,the green synthesis method,improved safety and high coulombic efficiencies are the future research directions for P-based anodes.

6 Binary alloy-based composite

Except for the elementary metallic alloy-based anode as mentioned before,binary alloy-based composite is another class of anode for PIBs.Simply,binary alloy-based anode can be pided into two binary alloy (SnSb) [ 75, 76, 77] and phosphides (GeP₅ [ 78] and Sn_xP_y [ 21, 40, 79, 80, 81] ).Compared with the elementary anode,some different potassiation mechanisms are revealed for binary alloy-based composites,leading to the huge impact on their electrochemical performance [ 82] .Stievano and co-workers studied the difference between SnSb and Sn as anodes for PIBs [ 75] .The discharge-charge profiles of theβ-Sn electrode of selected cycles are exhibited in Fig.9a.Because of the catalytic effect of Sn,a narrow voltage window (0-1.1 V) was used to avoid the decomposition of electrolyte.During the first potassiation process,a potential plateau at～0.1 V was observed,suggesting the one-step alloying reaction with a capacity of 283 mAh.g-.In the subsequent potassiation processes,this plateau was shifted to higher potential.During the depotassiation processes,three distinct phenomena were revealed.The slope region between 0.20 and 0.65 V could be related to the carbon additive.The two potential plateaus in the range of0.65-0.92 and 0.92-1.10 V were related to the formation of intermediate phase.When charged to 0.92 V,0.42 mol K were extracted from per mole Sn,suggesting the existence of K₄Sn₉,which was established on the basis of DFT.Figure 9b shows the galvanostatic curves of SnSb electrode in the voltage range of 0-2 V.During the initial discharge,a potential plateau began at 0.3 V and decreased to 0 V gradually.The initial discharge capacity was 478 mAh·g^-1,revealing that 5.3 mol K alloyed with per mole SnSb.Similar potential plateaus with lower capacities occurred in the following cycles.Obviously,there exists significant difference between the potassium storage reaction mechanism of SnSb and that ofβ-Sn.The comparison of the two materials and Sb electrode is presented in Fig.9c.It could be known that the potassiation mechanism of SnSb went through a hybrid reaction pathway of Sb andβ-Sn.In detail,the first alloying process of SnSb could be pided into two steps,a first one located at 0.25,corresponding to the reduction peak of Sb,and a second step at～0.1 V,similar to that ofβ-Sn.Furthermore,ex situ low-temperature Sn Mossbauer spectra were carried out to study the reaction mechanism of SnSb.After charged to 0 V (end of discharge state,EOD state),the spectrum could be fitted to the doublet at 2.38 mm·s^-1 with a quadrupole splitting of0.31 mm·s^-,indicating the formation of KSn.After charged to 2 V (end of charge state,EOC state),the spectrum presented an asymmetric broad peak which can be fitted by two different spectral components:SnSb (unresolved doublet centered ofδ=2.81 mm·s^-1 and quadrupole splitting of⊿=0.49 mm·s^-1) andα-Sn(δ=2.01 mm·s^-1 and⊿=0.32 mm·s^-1).Based on the above results,the potassiation mechanism of SnSb can be concluded as follows:

Fig.8 a Image of red P/C (RP/C) after a ball milling process in a stainless steel jar;b schematic illustration of structural configuration of activated RP/C-based PIB anode;c cycling performance of the RP/C electrode at a current density of 1000 mA.g-1 over 60 cycles [73] (Reproduced with permission,Copyright 2019,Wiley);d schematic illustration of red RP@reduced graphene oxide (RP@RGO) fabrication process;e galvanostatic charge-discharge curves of RP@RGO;f cycling performance at 100 mA·g^-1 of graphite,RGO and P@RGO electrodes [74] (Reproduced with permission,Copyright 2018 Wiley)

As a result,compared withβ-Sn,Sn in SnSb can store more K.This example indicates that the presence of antimony atoms changes the potassiation mechanism to offer the higher capacity.Because of this advantage,some advanced SnSb-based anodes for PIBs have already been developed.For example,Wang et al. [ 76] used template method to prepare a SnSb-3D N-doped porous carbon composite (3D SnSb@NC) and test it in both ether-based and ester-based electrolytes.TEM images of 3D SnSb@NC are shown in Fig.9e,f.It can be seen that a large number of ultra-thin particles were confined in the porous carbon matrix.The diameter of a SnSb particle is about a dozen of nanometers with the lattice spacing of0.306 nm,which was corresponding to the (200) plane of SnSb (Fm3m group).Benefited from the synergy effect of ultra-thin size and protection of carbon matrix,the 3D SnSb@NC showed good potassium storage property,especially in ether-based electrolyte.As shown in Fig.9h,in dimethoxyethane (DME) electrolyte,it shows a high reversible capacity of 357.2 mAh·g^-1 at 50 mA·g^-1.When the current density increased to 2 A·g-1,a capacity of 116.6 mAh·g^-1 still maintained.The 3D SnSb@NC also provided good cycling performance.After 200 cycles at 0.5A·g^-1 in DME electrolyte,a capacity of 185.8 mAh·g^-1was still reserved,as displayed in Fig.9i.

Fig.9 a Galvanostatic profiles ofβ-Sn electrode at 0.1C between 0 and 1.1 V;b galvanostatic profiles of SnSb electrode at 0.2C between 0 and2 V;c derivatives of the first cycle of galvanostatic profiles for SnSb,Sn and Sb electrodes;d ex situ ¹¹⁹Sn Mossbauer spectra measured at low temperature [75] (Reproduced with permission,Copyright 2019 The Royal Society of Chemistry);e low-magnification and f high-magnification TEM images of 3D SnSb@NC;g average interspaces of a single particle calculated by TEM diffraction;h rate performance and i cycling performance at 0.5 A.g-1 of 3D SnSb@NC as anode for PIBs in DME electrolyte [76] (Reproduced with permission,Copyright 2019 The Royal Society of Chemistry)

Tin phosphides are another popular material as anode for PIBs recently.Guo and co-workers paid much effort in developing Sn₃P₄ as advanced anode for KIBs [ 21, 40] .To determine the electrochemical reaction mechanism of Sn₃P₄,they used in operando synchrotron XRD measurement,and the result is presented in Fig.10a [ 21] .During the potassiation reaction,a peak,which was assigned to the(222) plane of K₃P₁₁ phase,emerged at～0.21 V and vanished at～0.17 V.It indicates that K₃P₁₁ is one of the intermediate products of potassiation reaction.Subsequently,two new reflections for the (321) plane of KSn phase and the (103) plane of K₃P phase appeared,suggesting the end products.During the following depotassiation reaction,the two reflections disappeared with the increase in voltage,demonstrating the good reversibility of the potassiation and depotassiation reactions.Accordingly,the electrochemical reaction mechanism of S1n₃P₄ in PIBs can be schematically shown in Fig.10b [ 40] .Firstly,the Sn₃P₄ converts into K_3-xP (such as K₃P₁₁) and nano-Snparticles.As the potassiation went on,K inserted into Sn to form KSn through the alloying reaction.The K_3-xP was converted to K₃P finally.During the depotassiation process,the K-Sn alloy and K-P alloy were dealloyed and reacted with each other to return to Sn₃P₄.These processes can be described by the following equations: [ 21, 79, 80]

Fig.10 a Galvanostatic profiles ofβ-Sn electrode at 0,1 C between 0 and 1.1 V and b SnSb electrode at 0.2C between 0 and 2 V;c derivatives of the first cycle of galvanostatic profiles for SnSb,Sn and Sb electrodes;d ex situ ¹¹⁹Sn Mossbauer spectra measured at low temperature [75] (Reproduced with permission,Copyright 2019 The Royal Society of Chemistry);e low-magnification and f high-magnification TEM images of3D SnSb@NC;g average interspaces of a single particle calculated by TEM diffraction;h rate performance and i cycling performance at 0.5A-g-1 of 3D SnSb@NC as anode for PIBs in DME electrolyte [76] (Reproduced with permission,Copyright 2019 The Royal Society of Chemistry)

Based on this mechanism,Sn₃P₄ shows a high theoretical capacity of 620 mAh·g^-1,which is more than2 times that of pure Sn anode.Besides,the K-P alloy can also play the role of matrix to buffer the volume changes in Sn particles during potassiation/depotassiation.Therefore,Sn₃P₄ delivered a better cycling performance than that of Sn anode generally.Zhang et al.'s work [ 40] can prove this point well;they prepared a Sn₃P₄/C composite as anode for KIBs and compared its cycling performance with Sn/C and P/C electrodes.As exhibited in Fig.10c,many Sn₃P₄crystallines with sizes of 20-50 nm were distributed in the amorphous carbon matrix.The discharge/charge profiles of Sn₃P₄/C electrode are presented in Fig.10d.During the initial discharge process,it delivered a high capacity which was more than 600 mAh·g^-1,due to some irreversible side reaction (such as formation of SEI film).During the following cycles,there existed the discharge slopes in the voltage range of 0.1-0.6 V,indicating the alloying process of Sn₃P₄.During the charge process,the dealloying reaction mainly occurred below 1.2 V.The cycling performance comparison of Sn₃P₄/C,Sn/C and P/C electrodes is displayed in Fig.10e.Although the P/C and Sn/c electrodes showed the considerable first discharge capacities,they lost most of capacities rapidly in the initial20 cycles.In contrast,Sn₃P₄ electrode delivered a stable cycling performance.Over 50 cycles,a capacity of307.2 mAh·g^-1 was retained,which was almost 80%of its reversible capacity.Furthermore,the rate performance of the Sn₄P₃/C electrode was characterized.As presented in Fig.1 0f,with the current densities increasing from 50 to1000 mA·g^-1,the discharge capacities were decreased from 399.4 to 221.9 mAh·g^-1.It should be noticed that the coulombic efficiency was relatively low and should be improved.In addition to Sn₃P₄,SnP_0.94 has also been studied as anode material recently [ 81] .

Recently,GeP₅ has been reported as a novel anode for PIBs for the following reasons.To begin with,the specific gravimetric capacity is ultrahigh.As shown in Fig.11a,among hard carbon anode,metallic anodes and binary phosphides anodes,GeP₅ shows the highest volumetric capacity of 6865 mAh·cm^-3,which is beneficial for the application [ 79] .Then,although the cost of Ge is high,the low content (at%) of Ge in GeP₅ brings a relatively low price to GeP_5.At last,GeP₅ has a layered structure,which is similar to black P and graphite,and a high electronic conductivity,which is closed to that of graphite.This high electronic conductivity is favorable for potassium storage.Zhang et al.synthesized a GeP_s compound and measured its potassium storage property [ 78] .As can be seen from TEM image shown in Fig.11b,the as-prepared GeP₅compound consisted of many irregular agglomerated nanoparticles.Through the in operando synchrotron XRD measurement,the potassium storage mechanism of GeP₅was confirmed.During the potassiation process,the electrochemical reaction can be pided into two steps as following:

Fig.11 a Theoretical gravimetric and volumetric capacities for various anode materials in K ion batteries;b TEM image of GeP₅powder;c schematic illustration of the potassiation/depotassiation process in GeP₅ electrode;d long-term cycling performance of GeP₅anode with various electrolytes at 500 mA·g^-1[78](Reproduced with permission,Copyright 2018 Elsevier)

This reaction mechanism is schematically shown in Fig.11c.The K-Ge alloy and K-P phase can buffer the volume expansion of each other,leading to a good cycling stability.Figure 11d presents the cycling performance of GeP₅ in various electrolyte.In the optimized electrolyte(KFSI in EC/DEC),the GeP₅ delivered a stable long-term cycling life.After 2000 cycles,a capacity of 213.7mAh·g-1was still maintained.However,in KPF_6-based electrolyte,the performance of the cell became anomalous.Besides,the fluoroethylene carbonate (FEC) additive would deteriorate the performance of GeP₅ anode in PIBs.

Among various binary alloy-type anodes,metal phosphides show the most application potential,especially Sn₃P₄,for its high reversible capacity and good rate capability.However,as mentioned above,the lack of environmental-friendly and safe synthesis method is the main handicap of large-scale application.Furthermore,there has been few reports of metal phosphides applied in potassium ion full cell.The large-scale application of metal phosphides still needs further efforts.

7 Other alloy-based anodes

Other potential candidates such as silicon (Si),lead (Pb)and germanium (Ge) might also attract researchers'attentions.In particular,although theoretical calculation pointed out that silicon (Si) could alloy with potassium to form KSi phase [ 83] ;to the best of our knowledge,satisfying work has been rarely reported up till now.Glushenkov and coworkers attempted to investigate the electrochemistry of Si anode in PIBs [ 32] .The pure Si anode could show barely discharged capacity,indicating it was inert to the alloying reaction with potassium.To the best of our knowledge,there are few works trying to seek the reason of the contradiction between the theoretical prediction and the experimental result.It should be placed an emphasis on this issue.

Owing to the relatively large atomic weight and toxicity,Pb might not be suitable as anodes for alkaline metal ion batteries.Tang's group used Pb foil as the anode in alkali ion-based dual ion batteries,especially potassium ionbased dual ion batteries [ 84, 85, 86, 87] .Their work indicated the feasibility of using Pb as an alloy-based anode in PIBs.Berthelot et al.investigated the alloying electrochemistry of Pb anode in PIBs [ 88] .During the potassiation process,the Pb would transform into the poor-K phases of K₁₀Pb₄₈and K₄Pb₉ and turn into KPb at last with the volume expansion rate of 259%.Based on the end product of KPb,a limited theoretical capacity of 129 mAh.g-1 can be calculated.The discharge capacity decreased from 108 to 75mAh.g-1 over only 20 cycles.Furthermore,it was proved that germanium (Ge) anode could be applied to PIBs by the experimental result recently.He's group prepared a porous Ge sample as anode for PIBs [ 89] .Over 400 cycles at20 mA.g-1,the Ge anode delivered a capacity of～120mAh·g-1.The electrochemical mechanism in Ge anode needs to be further studied in the follow-up works.

8 Electrolyte engineering

Electrolyte engineering is very important to address the challenges of cycling stability and rate capability in alloybased anode materials for PIBs [ 20] .Up to now,there have already some works focusing on developing better organic liquid electrolyte in this field,including seeking better electrolyte salt and solvent,confirming optimized concentration and building stable SEI film.

Guo and co-workers studied the impact of different potassium salts (KFSI and KPF₆ in EC/DEC) on the alloybased anode in PIBs (Sn₃P₄@carbon fiber,Bi/rGO,Sn/C and Sb/C as examples) [ 21, 61] .For both at the low current density and high current density,the cycling stability in KFSI electrolyte was much better than that in KPF₆ electrolyte,as shown in Fig.12a,b.This is because various electrolyte s alts bring different surface chemistries.KPF₆ is easy to hydrolyze and decompose to form strong Lewis acids (KPF₆→KF+PF₅ and KPF₆+H₂O→KF+2HF+POF₃).Those Lewis acids would cause the polymerization of carbonate solvent,and even react with alloy-based anode,which are harmful to building stable SEI film.On the contrary,KFSI is more stable with moistness and more easily reducible [ 61] .Accordingly,it can be known that in KFSI electrolyte,the formation of SEI film consumes less electrolyte than that in KPF6 electrolyte and the KFSI-derived SEI film is stable than the one derived from KPF₆ electrolyte.

The different solvents also have huge impacts on the potassium storage properties of alloy-based anode [ 61] .The commonly used organic solvent can be pided into ester solvent (dimethyl carbonate:DMC,diethyl carbonate:DEC,ethylene carbonate:EC,propylene carbonate:PC,and ethyl methyl carbonate:EMC) and ether solvent(dimethoxyethane:DME,and diethylene glycol dimethyl ether:DEGDME).In terms of the high chemical stability and wide potential window,commercial LIBs employed ester-based electrolyte.However,alloy-based anodes in ester-based electrolytes often show a low initial coulombic efficiency,due to the irreversible decomposition of electrolyte to form SEI film and severe side reactions.To solve this problem,an effective method is to use alternative ether-based electrolyte.It is believed that ether-based electrolyte is favorable to help form thin and uniform SEI film.Generally,the SEI film formed in ester-based electrolyte could be illustrated as"mosaic"model.The exterior surface of this kind of SEI film consists of non-uniform mixture organic and inorganic compounds [ 90] .On the contrary,a thin organic layer consisting of poly ethers forms on the surface of SEI film in ether-based electrolyte.This thin organic film can cover the inorganic part and prevent the further decomposition of the electrolyte on its surface.Therefore,a thin and uniform SEI film forms in ether-based electrolyte could reduce the consumption of electrolyte which leads to a high initial coulombic efficiency [ 91] .Besides,some previous work pointed out that cycled alloy-based electrode shows better wettability with ether-based electrolyte than that in the ester-based electrolyte.This is beneficial to the transition of potassium ions from liquid electrolyte to solid anode crystalline [ 55] .However,it should be noticed that the potential window of ether-based electrolyte is narrower than that of ester-based electrolyte.This will hinder the application of high-voltage cathode materials in ester-based electrolyte,leading to low energy density.

Fig.12 a Cycling performance of Sn₃P₄@carbon fiber with various electrolytes at 50 mA·g-1;b long-term cycling performance of cells with various electrolytes at 500 mA·g^-1 [21] (Reproduced with permission,Copyright 2018 Elsevier);c cycling performance of Sb@carbon sphere network composite anode (Sb@CSN) at 100 mA·g^-1 in 1 mol·L^-1 KTFSI and 4 mol·L^-1 KTFSI electrolytes with inset of lighting an electronic candle with a coin cell in the 4 mo1·L^-1 KTFSI electrolyte after 100 cycles;d illustration of influence of dilute and concentrated electrolytes on formation of SEI layer [42] (Reproduced with permission,Copyright 2019 The Royal Society of Chemistry)

Wang's group reported that high salt concentration electrolyte can significantly improve the performance of batteries through widening potential window of electrolyte,stabilizing both cathode and anode electrodes with stable SEI and CEI films [ 60] .They used this strategy to enhance the performance of alloy-based anodes [ 42] .As can be seen from Fig.12c,the cycling performances of Sb@CSN anode tested in 1 mol·L^-1KTFSI and 4 mol·L^-1 KTFSI electrolytes (EC:DEC as solvents) were presented.In the high salt concentration electrolyte,a much more stable cycling performance was obtained with the stable coulombic efficiency.As illustrated in Fig.12d,it is believed that in 4 mol·L^-1 KTFSI electrolyte,the formed SEI film on the surface of Sb@CSN was rich in KF.The high content of KF can make the SEI film more robust.Furthermore,the high concentration of TFSI^-can also reduce the dissolution of KF from the SEI film.Based on these advantages,the good long-term cycling property were provided.

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Table 1 Potassium ion storage performances of various alloy-based electrodes

FEC is extensively studied additive in LIBs and SIBs systems,which is favorable to form high-quality SEI film [ 92, 93] .However,Guo's works reported that in PIBs system,rather than improving the potassium storage property,the FEC additive will cause the rapid capacity decay of alloy-based anode [ 21] .Through DFT calculations,the reason of this negative effect of FEC was possibly explained.The addition of FEC will increase the solvation energy of K⁺from 0.305 to 1.281 eV (in the KFSI-EC/DEC electrolyte), [ 79] which boosted the side reaction to some extent and then decreased the coulombic efficiency.Therefore,FEC additive might not be suitable for alloy-based anodes for PIBs.

Table 1 exhibits a summary of the potassium storage properties of some above-mentioned alloy-based anodes for PIBs.

9 Summary and outlook

In this progress report,recent advances in alloy-based anode for KIBs are comprehensively reviewed.Some of the effective strategies,including employing carbonaceous matrix,coating protective layer and building stable SEI film,have been developed to overcome the problems of volume expansion and sluggish kinetics.For the large-scale application of alloy-based anodes for PIBs still continuous efforts are needed.Here,we outline several possible research directions for advanced alloy-based anodes for PIBs as following.

9.1 Increasing initial Coulombic efficiency

The low initial coulombic efficiency (ICE) is the current bottleneck for the PIBs,which will cause the energy wasting and low energy density [ 94] .Generally,for alloybased anodes,the ICEs are lower than 80%,which limit their practical application [ 32] .The origins of the low ICE of alloy-based anodes in PIBs are complex.Up to now,the proven reasons are the decomposition of electrolyte to form SEI film and the irreversible transformation of crystalline and morphology [ 90] .Some previous works pointed that the concentrated electrolyte can increase the ICE through forming thin SEI film with less electrolyte consumption [ 20] .However,it was still not sufficient to address this problem.To tackle this issue,the following strategies could be taken into considerations.(1) Binder optimization:In LIBs and SIBs,it has been proved that advanced binder can increase the ICE of anode electrodes,such as sodium alginate [ 95] .Developing binder with high Young's modulus and rich carboxylic group might also improve the low ICE of alloy-based anode in PIBs.(2) Interfacial engineering:Building artificial SEI film is an effective strategy to protect the metallic lithium anode [ 96] .Similarly,interfacial engineering,such as coating organic/inorganic hybrid film on alloy-based electrodes as the artificial SEI film,can also prevent the consumption of electrolyte to obtain a high ICE.(3) Finding proper additives:Up to now,there are less work focusing on the additives in this field.A traditional additive,FEC,has been proved to be ineffective for PIBs.To the best of our knowledge,other additives,such as vinylene carbonate (VC),vinyl ethylene carbonate(VEC),trimethylene sulfite (PS),have not been discussed to alloy-based anodes for PIBs yet.Moreover,developing a novel additives,like ionic liquid additive or fluorine-rich alkali salts,is also important.(4) Balancing the particle size and surface area:Although the strategy of nanocrystallization brings large surface area of alloy-based anodes,which is a two-edged sword,the large surface area can offer more active sites to storage potassium and consume too much electrolyte to form SEI film as well.Designing the structure of alloy-based anode should be based on the thoughtful consideration to balance the particle size and surface area.

9.2 Developing solid electrolyte

In terms of the huge safety concerns of commercial battery,the all-solid-state battery was considered as an ultimate solution.To develop all-solid-state battery,one of the key issues is to fabricate solid electrolyte.Compared with traditional organic liquid electrolyte,the solid electrolyte shows the advantages of high modulus,inflammability and lightweight [ 97] .In this regard,the fabricated PIBs with alloy-based anode and solid electrolyte can feature high safety and energy density.As reported,Sb,Bi and Sn anodes have been practically used in solid-state LIBs,which shows considerable reversible capacity [ 98, 99, 100] .It indicates the possibility that the alloy-based anode can also serve as the promising electrode for all-solid-state PIBs.In addition,some alloy-based metal,like Sb,can play the role of dopant of crystalline solid electrolyte.This might be favorable to build a senior interphase between alloy-based anode and solid electrolyte to facilitate the diffusion of K⁺.Although fabricating solid electrolyte with enough ionic transference number at room temperature is challenging,we believe that developing all-solid-state PIBs with alloybased anode is a promising direction.

9.3 Full-cell design

For the practical application and commercialization,the exploration of prototyped full cell configurations of PIBs with alloy-based anode materials is an important task in the future.To achieve the high energy density,assembling the full-cell PIBs needs deep analysis on the aspects of electrochemical reaction mechanism for both cathodes and anodes as well.Only few pioneering works reported the performance of alloy-based anode in full-cell PIBs,for instance,Sb@C//KFeHCF, [ 49] Bi//K_0.72Fe[Fe(CN)6], [ 55] and RP/C//KMnHCF [ 72] .Those full-cell PIBs showed limited capacity and energy density,which make the alloybased anodes far away from applying into future practical application.Further research on alloy-based anode should pay more attention to the full-cell fabrication technology.

Acknowledgements This study was financially supported by the National Natural Science Foundation of China (Nos.51302079 and51702138),the Natural Science Foundation of Hunan Province (No.2017JJ1008) and the Key Research and Development Program of Hunan Province of China (No.2018GK2031).

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