Progress on graphitic carbon materials for potassium-based energy storage

WANG Deng-ke,ZHANG Jia-peng,DONG Yue,CAO Bin,LI Ang,CHEN Xiao-hong,YANG Ru,SONG Huai-he

（State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Electrochemical Process and Technology for Materials,Beijing University of Chemical Technology, Beijing 100029, China）

Abstract：Potassium ion batteries (KIBs) and potassium-based dual ion batteries (KDIBs) are newly-emerging energy storage devices that have attracted considerable attention owing to the low-cost of potassium resources and their comparable performance to lithium-ion batteries (LIBs).Graphite materials,as the successful commercialized anode materials of LIBs,can also be used as anodic and cathodic host materials for the intercalation of the large potassium cations and other anions,respectively.However,there are still some challenges hindering the practical application of graphite materials in the anode for KIBs and the cathode for KDIBs.The huge volume changes after intercalation (61% for K and 130% for anions) result in graphite interlayer slipping and structural collapse,causing capacity fade and a short cycle life.Moreover,the intercalation of large K+ and anions have poor kinetics due to the small graphite interlayer spacing,restricting the rate capability.To solve these issues of the use of graphite materials,this review attempts to provide a better understanding of the intercalation mechanisms for K+ and anions,and to correlate the electrochemical performance of KIBs and KDIBs to the microstructure of graphite,and the physicochemical properties of electrolytes and binders.Finally,research prospects are provided to guide the future development of graphite materials for potassium-based energy storage.

Key words:Graphite；Potassium ion battery；Potassium-based dual ion battery

1 Introduction

To date,the rapid economic development relies on the overexploitation of natural resources,especially fossil fuels.The combustion of fossil fuels produces lots of harmful gases,which damage the environment and cause many problems to the development of contemporary society[1].Therefore,clean energy is a good choice as a candidate to relieve fossil-fuel dependence.Due to the intermittent nature of clean energy resources,energy storage systems are required[2].In addition,with the widespread applications of lithium-ion batteries (LIBs) since 1991,the lithium resources and precious metals in cathodes like Co and Ni are quickly consumed in the earth[3–5].Simultaneously,the recycle system is barely built,expensive prices with the shortage of resources begin to restrict applications of LIBs[6,7].Therefore,novel high-energy-density and low-cost batteries should be developed.

Potassium (K)[8],sodium (Na)[9],magnesium(Mg)[10]and other novel batteries have attracted attention to replace LIBs.These ions all can commute between the anode and the cathode to achieve the energy storage[11].But due to the strong solvation and large polarization of Mg2+,the two-electron transfer in the magnesium ion batteries to intercalate into the inorganic host is difficult[12].The existing electrode materials for LIBs cannot be well applied in magnesium ion batteries.More efforts are still needed to promote the application of magnesium ion batteries.Na+and K+have the similar property to Li+.However,due to the thermodynamic instability of Na-graphite intercalation compounds,low-cost graphite cannot be used as the anode material of sodium ion batteries.On the contrary,the K+can intercalate into graphite to deliver a high theoretical capacity,which makes potassium ion batteries (KIBs) become a promising energy storage device.

However,the large ionic radius of K+is a challenge to develop KIBs.Exploring suitable anode materials is key to realize the high-performance of KIBs.Currently,the alloy-based[13–16],conversion[17],organics[18],polyanionics[19]and carbon materials[20,21]are applied as the anode materials of KIBs.Carbon materials[22]such as graphite[23],graphene[24,25],and amorphous carbon[26]have aroused increasing attention owing to their low-cost and environmental friendliness.Among them,graphite is extremely advantageous for KIBs (Fig.1) because K+reversibly inserts into graphite,delivering a theoretical capacity of 279 mAh g?1[20].Moreover,the intercalation potential platform with K+is low (～0.3 V),which is beneficial to obtain a high voltage full-battery[27].As we know,graphite as the anode material of LIBs has developed more than 20 years.There are mature production technology and application experience with prosperity of LIBs,which is helpful for the development of KIBs[28].Lastly and more importantly,graphite resources are relatively abundant and inexpensive,which is promising in future for scale-up energy storage station in practical applications.However,despite the existence of some advantages,there are some challenges limiting its applications.The intercalation of large K+leads to a volume expansion of about 61%[29]along the c-axis of graphite.The huge volume change easily results in the graphite layer slipping and structural collapse,causing a capacity decay.Additionally,the slow intercalation reaction kinetics also restricts its electrochemical performance due to a limited graphite layer spacing[30].This is harmful to the ability of fast charging and discharging of a battery and is hard to satisfy the requirement of high-power density in modern society.In addition,the solvent in the electrolyte of KIBs is easily reduced on the graphite electrode surface because of a low electrochemical potential of K+/K[31],meaning that there are more side reactions especially in half-cell tests.

Fig.1 The advantages,challenges and the designing strategies of graphite for potassium storage.

In addition,graphite not only can be used as an anode material,but also can be use in the cathode with alternative energy storage styles emerging like potassium-based dual ion batteries (KDIBs)[32].KDIBs store energy by a new working mechanism[33],cations and anions can insert into graphite in the anode and cathode,respectively,delivering a high capacity.Graphite cathode replaces the high cost and toxic metal compound-based cathodes,which makes KDIBs ecofriendly and more cost-effective.A high work potential enables KDIBs to become a promising device for energy storage.While graphite will face the intercalation of bulk anions generating more huge volume change (＞130%) when it is used in the cathode of KDIBs (Table 1).A deep understanding of working mechanism,designing of the graphite structure and the development of high-stable electrolytes are also important research fields of KDIBs[34].

Table 1 The overview of three ions (cation and anion) for potassium-based energy storage.

For the whole battery system,the electrolyte plays an essential role for electrochemical performance of the graphite electrode.Types of potassiumsalts and solvent,additives and the electrolyte concentration all have effects on the electrochemical performance[18].Exploring electrolyte that can generate a uniform and stable SEI layer to suppress excessive side-reactions is important for the applications of graphite electrodes.Moreover,binder also influences the properties of electrodes and is required to relieve the volume expansion of graphite to a certain extent[35].

Aiming at the problems of graphite for potassium-based energy storage,its structural design and morphology control are extremely critical[22],and optimizing of the electrolyte and the binder is another important challenge to get high power density,excellent cyclic stability and long cycle life of batteries.In this review,the intercalation mechanisms of K+and anions into graphite are discussed and the research progress of graphite as the anode material for KIBs and the cathode material for KDIBs are summarized to guide the structural design of graphite.Finally,the perspectives are provided to demonstrate the developmental direction of graphite electrodes for potassiumbased energy storage.

2 Intercalation mechanisms

To comprehensively understand the intercalation mechanisms,the recent advance in this aspect is reviewed in this section.Many advanced characterization methods can help to reveal the mechanisms of cation and anion intercalation[23].Here,we summarize mainly the research work to employ XRD and theoretical calculation to explain the intercalation mechanisms.

2.1 Cation intercalation

Ji et al.first found electrochemical potassium intercalated compounds in a non-aqueous electrolyte in 2015[36].Furthermore,commercial graphite was used in the anode of KIBs,then different stages of intercalated compounds (KC36,KC24and KC8) were confirmed by ex-situ XRD study of K-graphite battery(Fig.2a,2b).

Due to limitations of ex-situ XRD,whole potassiation and depotassiation processes can be hardly characterized fully,and the intercalation mechanism is not fully explained.Fan et al.further explained the intercalation mechanism through Operando XRD.The peaks of KC48located at 23°/29° are firstly discovered in an electrochemical process.Therefore,four-step phase transformations of graphite are certified in potassiation process (Fig.2c).Furthermore,Liu et al.studied the electrochemical K+intercalation/deintercalation process through a combination of density-functional theory simulation and in situ XRD characterization.Five regions are divided from the entire discharge process as shown in Fig.2e.The five regions correspond to the five-step K-GIC staging transitions.The understanding of intercalation mechanism of K+helps the researchers to know the failure mechanism of the graphite anode and to design a reasonable structure of graphite.

2.2 Anion intercalation

Previously,the intercalation of various anions(ClO4?,TFSI?,FSI?,PF6?and BF4?) into the graphite cathode has been extensively studied,especially in lithium/sodium-based dual-ion batteries (DIBs)[39,40].With the emergence of potassium-based dual-ion batteries,the anions (PF6?and FSI?) are widely applied as the electrolytes due to their small sizes.The intercalation mechanism also is investigated further in KDIBs[41].

Ji et al.initially explained the intercalation mechanism of PF6?through the ex-situ XRD patterns(Fig.3a),which show the different electrochemical states of the graphite cathode during the initial charge/discharge process at 1 C.In the intercalation process,the sharp peak (002) of graphite at 26.5°splits into two peaks,indicating an intercalation compound (C24PF6) is formed as shown in Fig.3b.At the deintercalation process,these two peaks merge into a peak at about 26.5°,meaning the gradual deintercalation of the anion (PF6?).But the intensity of peak is lower and peak width is wider than the original graphite peak because the partial intercalation of PF6?is irreversible.

Moreover,the intercalation mechanism of FSI?into graphite is analyzed[44].Four different stages(Fig.3c) with varying gallery heights and periodic repeat distances are modelled to simulate the staging mechanism.In-situ XRD patterns (Fig.3d) further proved the four stage intercalation process.To date,the mechanisms of cationic intercalation have been understood to some extent,which is significant to design the suitable graphite structure to obtain highperformance KDIBs.More advanced technologies still should be explored to further reveal the intercalation mechanisms of anions.

Fig.3 (a) The ex-situ XRD patterns of an expanded graphite cathode with the different intercalation/de intercalation stages (Reproduced with permission,Copyright 2017,Willey[42]),(b) the in-plane structure of C24PF6 (Reproduced with permission,Copyright 2014,Willey[43]) and (c-d) schematics of the staging mechanism of intercalation of FSI- anion into the graphite and the in-situ XRD of the graphite electrode during the first two cycles[44].

3 Graphite for KIBs

In this section,the progress of graphite anode in KIBs is summarized (Fig.4 and Table 2).We will give an analysis and introduction based on the following two aspects:(a) the design of graphite structure and (b) the optimization of the electrolyte and the binder.

Table 2 Electrochemical performance of different graphite anodes in KIBs (1 C=279 mAh g?1).

Fig.4 Schematic diagram of various structured graphite electrodes that have been reported for the potassium-based energy storage.

3.1 The structural design of graphite

Commercial graphite was used in the anodes of KIBs in a non-aqueous electrolyte for the first time[36].The anode showed high capacities of 273 and 475 mAh g?1at C/40 in initial charge and discharge processes,respectively,giving an initial coulombic efficiency (ICE) of 57.4%.The battery underwent a capacity decay of about 100 mAh g?1at C/2 after 50 cycles due to the graphite layer slipping.Simultaneously,it also exhibits ordinary rate performance due to sluggish intercalation kinetics (capacities of 263,234,172 and 80 mAh g?1at C/10,C/5,C/2 and 1 C,respectively).

Expanding,activating,ball-milling and strengthening the connection between layers of flake graphite are adopted to modify the traditional flake graphite structure,which can improve the electrochemical performance.Feng et al.reported an expanded graphite(EG) with the worm–like structure[47].Due to the increase of the number of oxygen-containing functional groups after expanding,the interlayer spacing is enlarged to 0.387 nm.Thus,large potassium ion transport channels are offered.Compared with the limited interlayer spacing (0.34 nm) of original commercial graphite,the enlarged interlayer spacing can well accommodate the large K+.Both rate performance and cycle stability have a great improvement.Very recently,Kang et al.proposed a wet chemical oxidation route,which can effectively adjust the interlayer spacing of graphite.And as-obtained mildly-expanded graphite exhibits the improved electrochemical performance.In addition,activated carbon from the graphite was fabricated by etching through potassium hydroxide[46].After activation,well-crystalline graphite layers are smashed into many small pieces.Compared with the original graphite,the electrochemical performance is improved due to the nanosized carbon sheets with large interlayer spacing after etching treatment.Thus,the expanding and activating are very effective methods to modify flake graphite to make easy diffusion of K+.

Ball-milling is also selected as the graphite modification method to improve its electrochemical performance for KIBs[49].After mild ball-milling,the size of graphite reduces and the defects on the particle surface increases,which is helpful for improving the electrochemical performance to a certain extent.Very recently,Rahman et al.reported a synthetic graphite modified by low-energy liquid phase ball milling[54].After milled,the thin flake graphite is obtained,which exhibits the enhanced electrochemical performance due to an increased surface area.In addition,Jiang et al.constructed a conducting and buffering CNT layer between graphite flakes[50].The existence of interweaved CNTs in the layer can not only accelerate the diffusion of K+but also stabilize the graphite structure.Specifically,a reversible capacity of 234 mAh g?1can be achieved after 1 500 cycles at a current density of 2 A g?1.The electrochemical performance is significantly better than the corresponding performance of counterpart graphite flakes.The existence of the CNTinterweaved layer greatly improves the cyclic stability and rate performance.But this method is barely meaningful for large-scale applications of graphite due to the tedious synthesis.

Spherical graphite has a high tap density,which is significant for practical applications.Cao et al.compared the electrochemical performance of graphitized carbon nanocage (CNC) and mesophase graphite(MG)[48].These two types of spherical graphite are prepared by graphitization of a polycondensation product from coal tar pitch and Ketjen carbon black(EC300J),respectively.The morphology of CNC is about 50 nm hollow cage with 5 nm shell.The MG exhibits a spherical morphology with the size of about 10 μm and an open-layered structure.When used as the anode materials of KIBs,the CNC and MG electrodes deliver a similar electrochemical capacity at a small current density.More interestingly,the CNC exhibits a good cycling stability (195 mAh g?1after 100 cycles) and outstanding high-rate depotassiation capability (a high capacity of 175 mAh g?1at a ultrahigh rate of 35 C) as shown in Fig.5a.The best performance is benefited from the hollow cage structure,which can buffer volume change during charge/discharge process and reduce the ion diffusion length of solid phase.But the MG is stripped into pieces due to a 61% volume expansion after repeated intercalation of K+(Fig.5b).Thus,the enclosed construction of the cage-like structure is more conducive to getting the better electrochemical performance than the openlayered structure.

Fig.5 (a) The rate retentions and capacities of the CNC anode at different current densities,(b) schematic illustration of structural variations of CNC and MG electrodes during potassium storage (Reproduced with permission,Copyright 2019,Willey[48]).

3.2 Graphite-like materials

With the development of graphite materials,many kinds of graphite-like materials are found,which have a high degree of crystallinity and a graphite-like structure without ultrahigh temperature graphitization.These graphitic carbons have various defects or heteroatom doping,which offer more active sites to store potassium ions and help to increase the theoretical capacity of graphite.

Xing et al.prepared polynanocrystalline graphite(PG)[52]with the short range ordered carbon layer,which is different from long range ordered graphite and amorphous carbon.But the interlayer spacing(0.345 nm) is closed to that of graphite (0.34 nm).As the anode material of KIBs,PG delivers potassiation/depotassiation capacities of 414/224 mAh g?1,and a CE of 54.1% in first time at 20 mA g?1.After 240 cycles at 100 mA g?1,50% of its original capacity is maintained,which is superior to 6% capacity retention of graphite after 140 cycles.The PG with a nanometric scale disorder benefits for retaining the structural integrity of the electrode.

Zhang et al.prepared a graphitic nanocarbon(GNC) by introducing C―C sp3defects and nitrogendoping into the GNC to improve the electrochemical performance[53].The authors find that C―C sp3defects in graphitic carbon can be used as channels for the efficient diffusion of K+and the nitrogen-doping-induced defects can help to increase the capacity of graphite.The GNC has hollow interconnected spherical shells,and the interlayer spacing (002) of the graphitic regions is closed to the that of graphite(0.335 nm).When used in the anode of KIBs,it displays an excellent cycle stability and a high capacity of 189 mAh g?1at 200 mA g?1after 200 cycles.Thus,designing new graphite-like materials is of significant

importance for their applications in KIBs.

3.3 Electrolyte

Electrolyte is an important part of KIBs[55].A qualified electrolyte must have a wide potential window,thermal and chemical stability,high ionic conductivity,low-cost and low-toxicity.More importantly,it must help to form a thin and uniform SEI layer and can prevent side reactions.Specifically,the electrolyte is composed of potassium salt,solvent and electrolyte additives.The types and concentrations of salts,kinds of solvent and electrolyte additives all have significant impacts on the performance of KIBs[18].The work on the influence of additives on the performance of KIBs is less.Zhang et al.found that fluoroethylene carbonate (FEC) had no obvious positive effect on forming a stable SEI on electrode surface in the potassium-based electrolyte based on the DFT calculation[56].This conclusion is not consistent with other battery systems[57,58].There are relatively few articles in this area for KIBs,more efforts should be made to explore and enrich this field.

Zhao et al.[45]compared the effects of three electrolytes,1 mol L?1KPF6in EC∶DMC,EC∶DEC and EC∶PC.The almost same reversible capacities are obtained in first cycle.But the KPF6in EC∶PC showed the highest ICE of 66.5% compared with 47.0% of KPF6in EC∶DEC and 42.7% of KPF6in EC∶DMC.The low ICEs of KPF6in EC∶DEC and KPF6in EC∶DMC are caused by the decomposition of DEC and DMC at low voltage.After 10 cycles,the CE of KPF6in EC∶PC increases to 95%,while KPF6in EC∶DEC needs 30 cycles to increase to 95% and KPF6in EC∶DMC is consistently below 90% for all cycles.The continued sluggish efficiency is caused by the decomposition of DEC and DMC.Thus,it is verified that the electrolyte of KPF6in EC∶PC is beneficial to the graphite anode.

Wang et al.discussed the electrochemical performance of the carbonate-based electrolytes (KPF6-EC/DMC) and the ether-based electrolytes (KPF6-DME)[51].Interestingly,the operational potential in the DME electrolyte is about 0.7 V (versus 0.2 V in EC/DMC-based one) because K+-ether co-intercalates into the graphite.A higher operational potential leads to a smaller potential polarization,making it easy for K+intercalation.Simultaneously,the lowest unoccupied molecular orbital level (LUML) of K+-DME solvent is higher than that of graphite so that there are no electrochemical reactions to form SEI[59].This means that the [K-solvent]+molecule passes through SEI easily and the final intercalated compound displays a small volume expansion.Finally,a lower volume expansion (＜10%) and a higher K+diffusion rate of 10?8cm2s?1are achieved in the DME electrolyte as compared with the EC/DEC electrolyte.

Komaba et al.found that the solubility of potassium bis(fluorosulfonyl)imide (KFSI) in organic solvents is higher than that of KPF6[31].And Zhang et al.discovered the KFSI salt dissolved in the carbonate-based electrolytes shows less side reactions and higher conductivity compared with KPF6salt.Therefore,a more stable solid electrolyte interphase layer is generated,resulting in notably enhanced electrochemical performance[60].In addition,a high-concentration electrolyte has been reached due to unique electrochemical and physicochemical properties.Niu et al.discussed the effect of the concentration of KFSI in DME solvent on the performance of KIBs[61].High specific capacity,high CE,small electrochemical polarization and low operational voltage are achieved in the highly concentrated electrolyte (7 mol kg?1).Furtherly,Fan et al.developed another concentrated electrolyte with the KFSI/ EMC molar ratio of 1∶2.5 for the graphite anode of KIBs[37].The KIB displays ultralong cycle life because a more robust and stable inorganic-rich SEI layer has been generated on the surface of graphite.Very recently,Liu et al.reported a nonflammable,moderate-concentration electrolyte of trimethyl phosphate (TMP) and KFSI at a molar ratio of 8∶3.The decomposition of the electrolyte is suppressed because the nearly 100% of TMP molecules are tightly integrated with K+cations.A stable FSI?anion derived F-rich SEI is formed at the surface of graphite,therefore,the super-long cycling life is achieved.

However,a highly concentrated electrolyte also faces the problems of a low ionic conductivity,a high viscosity and an increased cost.Qin et al.tried to add a low-polarity cosolvent to dilute the highly concentrated electrolyte[62].Then the localized high-concentration electrolyte will form to solve the above disadvantages,which not only overcomes the low ionic conductivity of a high-concentration electrolyte but also possesses a low flammability and high oxidation resistance.Simultaneously,a durable potassium fluoride-rich SEI layer is formed to help to eliminate the solvent co-intercalation and to enable highly reversible K+intercalation into graphite interlayers.

3.4 Binder

Binder also greatly influences the physicochemical properties of the graphite anode.Komaba et al.[31]discussed the effect of kinds (carboxymethylcellulose sodium (CMCNa),polyvinylidene fluoride (PVDF),polyacrylate sodium (PANa)) on the performance of KIBs.The PANa binder showed the best cyclic stability among three binders due to its excellent cohesiveness.Wu et al.[35]furtherly quantified the effect of the three binders on the electrochemical performance,swelling properties,charge-transfer resistance (Rct)and the potassium-ion diffusion coefficients[35].The battery using PANa exhibited excellent cycle stability,low charge transfer resistance,low swelling property of 1.31 and high potassium-ion diffusion coefficients during first 50 cycles among the three binders.Therefore,it is concluded that PANa is the appropriate binder to improve the performance of graphite anodes.

4 Graphite for KDIBs

Before the appearance of KDIBs,the sodiumbased and lithium-based dual ion batteries have been adequately researched and developed[63].The similar work mechanism and roughly the same electrode requirements make the KDIBs develop rapidly.The graphite is a frequently-used cathode of KDIBs.While the exfoliation of the graphite and hard anion intercalation limit its electrochemical performance.Moreover,the intercalation/deintercalation occurs at a high voltage of about 5.0 V.The electrolyte easily decomposes and some other irreversible side reactions happen,which leads to a CE lower than 90%.Therefore,recent advance of KDIBs are reviewed here(Table 3).

Table 3 Electrochemical performance of KDIBs.

Beltrop et al.reported a concept of a KDIB for the first time,using commercial available flake-type graphite (KS6L) as both the cathode and the anode[32]and ionic liquid as the electrolyte.42 mAh g?1capacity is obtained with CE exceeding 99% at 250 mA g?1and the capacity retention is about 95% after 1 500 cycles.But the interlayer distance is enlarged to 0.821 nm after the cycling.Then,nanographite and expanded graphite are used in cathodes in carbonate solvent by Fan et al.[64]and Ji et al.[65],respectively.Nanocrystallization and increasing interlayer spacing are good strategies to increase reactivity and enhance the intercalation extent of anions.The nanographite cathode in KDIBs exhibits a 62 mAh g?1reversible capacity at 100 mA g?1.And the expanded graphite cathode shows a reversible capacity of 61 mAh g?1at a current density of 100 mA g?1and the capacity has no decay after 100 cycles.Thus,the morphology and size of the graphite have great impacts on the performance of KDIBs.The graphite with accessible edges is a prerequisite for insertion of large anions.Simultaneously,the structure stability should be considered due to the huge volume change.

Beltrop et al.used the ionic liquid electrolyte firstly[32],which is a high-voltage electrolyte with a wide electrochemical stable window.But the price of ionic liquid is high,which constrains a widespread application.Different solvent combinations are explored,such as 0.8 mol L?1KPF6in EC:DEC (1∶1,v/v),EC:DMC (1∶1,v/v),and 1 mol L?1KPF6in EC:DMC:EMC (4∶3∶2v/v/v)[64–66].An ether-based electrolyte is more favorable because it exhibits a high cycling stability.A high-concentration electrolyte offers a possible route to the safe operation[67].However,more efforts should be made to explore and enrich this field.

In addition,the size and anodic stability of anions are crucial for limited space of host graphite.Meister et al.compared the electrochemical intercalation performance of anions with different sizes (0.8 ×0.39 nm of TFSI?and 0.65 × 0.39 nm of FTSI?) to obtain better comprehension into the effect of the anion size[68].The capacity for the smaller sized FTFSI?is higher than that for TFSI?,which means the small anion can intercalate into graphite easily.But the CE for the smaller sized FTFSI?is lower due to its poor stability.While the influence of the anion size is not completely clear,the other factors (such as ionic conductivity,solubility,degree of ionization and viscosity of electrolytes) maybe have impacts.More detailed research should be conducted to deeply understand their impacts.

5 Conclusion and perspectives

KIBs and KDIBs have drawn wide attention and become the promising energy storage devices due to cheap potassium resources.Owing to abundant resources,mature applications and high theoretical capacity,graphite has great application potential in KIBs and KDIBs.Herein,we summarize the development of graphite electrode used in the anode of KIBs and the cathode of KDIBs.Additionally,the effects of kinds of electrolytes and binders also are discussed to achieve better electrochemical performance.

A thorough understanding of the intercalation mechanism is essential to overcome the problems of graphite electrode.With the progress of research,it is revealed that the large K+inserts into graphite layers by a five-stage phase transformation,which are graphite ? stage 5 (KC60) ? stage 4 (KC48) ? stage 3(KC36) ? stage 2 (KC24/KC16) ? stage 1 (KC8).The formation of the final intercalation compound causes a 61% volume expansion along the c-axis of graphite.The huge volume change will damage the fragile interlayer bonding of graphite layers,causing the layer slipping and structural collapse.The capacity of a battery will decay with the destruction of the graphite structure,which is fatal for the applications of graphite electrodes.On the other hand,the well-crystalline structure of graphite makes the intercalation of large ions hard.The sluggish intercalation dynamics leads to poor rate performance and hardly gets high power density of batteries.

To solve these problems,morphological control and structural design of graphite are employed as good strategies.The expanded graphite with an enlarged layer spacing and the activated graphite with a reduced size both can facilitate the intercalation of K+.When used in the anode of KIBs,they display the significant improvement of electrochemical performance.However,these structures have limited effect to solve the graphite interlayer slipping caused by repeated intercalation in the long cycle of batteries.Owing to the stable interconnected cage-like morphology,the graphite nanocages show excellent structural stability and remarkable depotassiation capability.This is an excellent strategy to solve the existing problems of graphite.Therefore,a reasonable graphite structure should take into account not only a strong structure stability but also a fast ion transmission path.Therefore,to optimize the structure of graphite,there are some crucial factors to be considered.First,a controllable layer spacing is used to accommodate the large K+intercalation/deintercalation.Second,a good crystallinity can maximize storage of K+to deliver a high capacity.When a good crystallinity is considered,carbon layers are needed to be connected to each other as much as possible to ensure structural stability.

Additionally,when graphite is used in the cathode the much larger anions should be accommodated in the interlayers. As a result,the huge volume changes of 136% for PF6?and 134% for FSI?put forward higher requirements for graphite cathode.Currently,flake graphite,nanographite and expanded graphite have been used in the cathode of KDIBs.It is found that the structure,morphology and size of graphite all have a great influence on the intercalation of anions.Therefore,the structure of cathode graphite also should both have an excellent structural stability and a rapid ion diffusion coefficient.Quick ion transmission paths should facilitate bulk anion intercalation.

Electrolyte and binder have impacts on electrochemical performance for both KIBs and KDIBs.For KIBs,the potassium salts,solvents,additives and concentration of electrolytes all have influences on the electrochemical performance.Suppressing the side-reactions of electrolytes and formation of a thin and uniform SEI layer are critical.Current researches show that the use of KFSI salt with highly concentrated electrolyte can improve the performance of graphite electrode.But the high price of the salt is not conducive to reduce the cost of KIBs.Thus,the research on the electrolyte system should be strengthened to promote the development of the graphite applications in KIBs.Besides mentioned above,the sizes of anions affect the electrochemical performance of KDIBs.The researchers found that a smaller anion can insert into a host material easily.But the other properties such as the solubility,degree of ionization,viscosity of electrolytes,and ionic conductivity should be considered to optimize the electrolyte in KDIBs.As to the binder,PANa is considered as a suitable binder for graphite electrode due to its excellent cohesiveness.More excellent binders should be developed to improve the electrochemical performance of graphite electrode.

Overall,there are some developments in the applications of graphite materials for KIBs and KDIBs.Continuing to understand the intercalation mechanism deeply,optimizing the configuration and exploring a better graphite structure to accommodate the large K+and bulk anions are effective strategies to obtain potassium-based energy storage devices with superb performance.Combining the advantages of graphite and the potassium-based energy storage devices can significantly push the development of energy storage to large scale applications.

Acknowledgement

This work was supported by the National Natural Science Foundation of China (U1610252).