Weak Interaction Between Cations and Anions in Electrolyte Enabling Fast Dual‐Ion Storage for Potassium‐Ion Hybrid Capacitors

Identifying an effective electrolyte is a primary challenge for hybrid ion capacitors, due to the intricacy of dual‐ion storage. Here, this study demonstrates that the electrochemical behavior of graphite oxide in ether‐solvent electrolyte outperforms those in ester‐solvent electrolytes for the cathode of potassium‐ion hybrid capacitor. The experimental and theoretical assessments verify that the anion and cation are isolated effectively in dimethyl ether, endowing a weaker interaction between cations and anions compared to that of ester‐solvent electrolytes, which facilitates the dual‐ion diffusion and thus enhances the electrochemical performance. This result provides a rational strategy to realize high‐rate cations and anions storage on the carbon cathode. Furthermore, a new low‐cost and high‐performance capacitor prototype, modified graphite oxide (MGO) cathode versus pristine graphite (PG) in ether‐solvent electrolyte (MGOǁDMEǁPG), is proposed. It exhibits a high energy density of 150 Wh kg−1cathode at a high power density of 21443 W kg−1cathode (calculation based on total mass: 60 Wh kg−1 at 8577 W kg−1).


DOI: 10.1002/adfm.202304086
bridge the performance of supercapacitors with high-power output and conventional secondary-ion batteries with high-energy output. [1,2]In 1999, the first lithium-ion hybrid capacitor was configurated by an activated carbon (AC) cathode and a graphite anode, since then many lithium-ion hybrid capacitors with different configurations have been investigated and some even been deployed into the market. [3]However, it has been predicted that the existing lithium-ion battery (LIB) technologies are facing challenges for future applications mainly because of the scarce and unevenly distributed resources of lithium.Due to the potassium's abundant and well-distributed reserves, potassium-ionbased energy storage devices shall be a promising supplementary.6][7][8] As a major cathode material of hybrid-ion capacitors, oxygencontaining functionalized carbon has been widely studied.[11][12][13][14][15][16][17][18][19][20][21] Investigating the effect of electrolytes on ion storage is still lacking for carbon cathodes. [2,22]As known, electrolyte is an important component of ion storage devices both for batteries and capacitors.[24] For PIHCs which mostly consists of capacitor-type cathodes and battery-type anodes, the ion storage feature of PIHC cathodes is different from PIB and dual-ion battery.][27][28] Therefore, although PIHCs and PIBs could basically utilize the same electrolytes, implications that can be derived from the electrolyte research of PIBs are not highly referable for researching PIHC electrolytes.Thus, intensive research to understand the role of electrolytes in PIHCs shall be extremely important.Among all the electrolytes utilized in PIBs, KPF 6 is the most common solute, ethylene carbonate and diethyl carbonate (ECˆDEC), ethylene carbonate and propylene carbonate (ECˆPC) are the most common ester solvents, and dimethyl glycol (DME) is the most common ether-based solvent. [29,30]A systematic understanding of the electrochemical characteristics of these three electrolytes is urgently needed to reveal the relationship between the electrolytes and performance for PIHCs.
Another important issue for the research of PIHCs is to reduce the cost, which is the prerequisite to promote the commercialization of PIHCs.Replacing lithium with potassium in a HIC system is definitely helpful to reduce cost, but it is not enough to fulfill the overall task of reducing cost so as to realize a commercialized PIHC, and it does need other strategies such as applying mature and low-cost materials of commercial LIBs in PIHC systems to further reduce the cost.Graphite has been applied as a commercial LIB anode for decades, with low-cost and highly sustainable manufacturing processes of graphite anode.Thus, selecting graphite as the anode of PIHCs should be a very good choice.Graphite has been demonstrated that it can reversibly store potassium ions for an anode of PIBs.However, so far graphite doesn't enable high-rate charge and discharge for potassium ion storage in ester electrolytes, which results in a very low output power density and hence rigorously limits the application of graphite in PIHCs where high-rate performance is a must and is much more required than the PIBs. [31,32]On the other hand for the cathode, modified graphite oxide (MGO) is a mature material and its industrial-scale fabrication has been demonstrated. [33]Therefore, if the challenge of applying graphite in PIHCs as an efficient anode could be resolved, and in the meantime graphite oxide is used as a cathode and also shows superior performances within a well-selected electrolyte, a solid step to promote the commercialization of PIHCs with low-cost and high-performance anode and cathode is achievable.
In this work, we firstly investigated the effects of different electrolytes on the performance of the MGO cathode in PIHCs.It is found that the electrochemical behavior in DME shows high charge storage capacity, long cyclability, and especially excellent rate capability (it exhibits high specific capacity of 97 and 83 mAh g −1 at 5.0 and 10 A g −1 ), which significantly outperforms those in ECˆDEC and ECˆPC.On that basis, it is exper-imentally and theoretically verified that the anions and cations in DME are more isolated from each other and hence giving more freedom to these ions for diffusion.Overall, the weak interaction between cation and anion in DME is the key to facilitating ion transportation, and hence realizing fast dual-ion storage kinetics.Furthermore, a full PIHC was configured using pristine graphite (PG) as the anode and MGO as the cathode in DME electrolyte.The full MGOǁDMEǁPG PIHC exhibits a superb energy density of 150 Wh kg −1 cathode at a high power density of 21 443 W kg −1 cathode (calculation based on total mass: 60 Wh kg −1 at 8577 W kg −1 ), which shall be one of the best electrochemical performances among almost all the PIHCs reported so far. [22]The MGOǁDMEǁPG presented in this work demonstrates a promising prototype with high performance (weak cation/anion interaction in DME) and cost-effectiveness (low-cost MGO and PG electrodes), which could be a key step to the commercialization of this type of capacitors.

Synthesis and Characterization of MGO
The MGO nanosheets were obtained through a modified Hummers' method. [34]The high-magnification scanning electron microscopy (SEM) image in Figure S1a (Supporting Information) shows the typical morphology of GO 2D nanosheets.The high-resolution transmission electron microscopy (HRTEM) image (Figure S1b, Supporting Information) indicates that the graphite powders are exfoliated to multiply graphene layers.The exfoliation process generates meso-and micropores on the graphene layers, which suggests that there are many carbon defects on the surface of the MGO nanosheets. [35]Both X-ray diffraction (XRD) pattern (Figure S2a, Supporting Information) and Raman spectra (Figure S2b, Supporting Information) show that the MGO nanosheets possess ordered and disordered carbon lattice structures, being consistent with the TEM measurement. [36]According to the energy dispersive Xray spectrometry (EDS) analysis (Figure S3, Supporting Information), MGO nanosheets are doped with a high content of oxygen-containing functional groups.The element mapping further displays a homogeneous distribution of C and O elements in the MGO nanosheets (Figures S1c-e, Supporting Information).The MGO nanosheet cathode with abundant defects and oxygen-containing functional groups is confirmed, the defects and oxygen-containing functional groups both are effective hosts for the absorption of K + and PF 6 − during ion-storage processes.

Electrochemical Performance Assessments
To investigate the electrochemical behaviors of as-prepared MGO nanosheets in different electrolytes, we configurated a series of half cells of PIHCs using MGO as the cathode against potassium metal as the anode in DME, ECˆDEC, and ECˆPC electrolytes, respectively (labeled as MGO-DME, MGO-ECˆDEC, and MGO-ECˆPC).The cyclic voltammetry (CV) curves in Figure S4 (Supporting Information) show MGO-DME possesses quick access Charge and discharge profiles of d) MGO-DME, e) MGO-ECˆDEC, and f) MGO-ECˆPC at current densities from 0.5 to 10 A g −1 .g) Cycling stability of MGO-DME at 2.0 A g −1 (inset: initial charge and discharge at 0.5 A g −1 for 10 cycles).h) Cycling performance under charging at high current density and discharge at low current density.
for the infiltration of electrolytes, benefitting that it reaches the full-discharge state with only two cycles.Furthermore, a box-like shape of the CV curves also indicates a typical pseudocapacitive behavior. [37]Figures 1a and b show the cycling stability and rate capability at various current densities, where MGO-DME exhibits the highest performance.It delivers capacities of 120, 111, 106, 97, and 83 mAh g −1 at 0.5, 1.0, 2.0, 5.0, and 10 A g −1 , respectively.Moreover, it maintains excellent cyclability during the next 900 cycles.However, MGO-ECˆDEC and MGO-ECˆPC, respectively, deliver lower capacities of 75/62, 66/52, 59/35, 44/11, and 31/8 mAh g −1 at the same rates.The high rate performance of MGO-DME configuration is among the best, as compared with previously reported cathodes in PIHCs (Table S1).The initial electrochemical performances for MGO-DME, MGO-ECˆDEC, and MGO-ECˆPC shown in Figure 1c reveal that MGO-DME possesses better electrolyte accessibility, ensuring that the electrode material quickly reaches the fully charging and discharg-ing state, which are consistent with the corresponding CV results.As shown in Figures 1d-f, the charging and discharging sloping plots of MGO-DME are relatively symmetrical, further demonstrating its superior pseudocapacitive property.It is worth noting that the polarization of MGO-DME is much weaker than those of MGO-ECˆDEC and MGO-ECˆPC.This phenomenon is attributed to the better ion-storage kinetics in the DME electrolyte.As shown in Figure 1g, MGO-DME can undergo charging and discharging over 2500 cycles at 2.0 A g −1 without obvious capacity decay, maintaining a capacity of 99 mAh g −1 .Encouragingly, MGO-DME outputs almost twice the energy densities of MGO-ECˆDEC and MGO-ECˆPC at the same power density (calculation based on MGO mass), which provides a basis for estimating its high energy density and power density in the full PI-HCs (Figure S5, Supporting Information).
To further evaluate its practical application, MGO-DME was tested under a condition of charging at a high current density of 2.0 A g −1 and discharging at a low current density of 0.5 A g −1 , delivering a capacity of 98 mAh g −1 over 500 cycles without observable decay (Figure 1h).The charging process only takes 184 s, but it can maintain a high-capacity output for 987 s (Figure S6, Supporting Information), which shows a great potential for superior fast charging capability for MGO-DME.The electrochemical differences of the active carbon at the different electrolytes were also verified (Figure S7, Supporting Information).The outstanding rate performances without much degradation mainly benefit from the fast kinetics of hybrid-ion storage reactions which occurs on or near the surface of MGO nanosheets.In addition, the electrochemical performances of MGO in DEGDMEand TEGDME-based electrolytes show that they both possess similar rate capability as compared with that in DME-based electrolytes (Figure S8, Supporting Information), indicating the ether-based electrolytes play a significant role in fast ion storage capability.

Ion-Storage Kinetics of GO in Different Electrolytes
To investigate the kinetics of the electrodes, a series of CV measurements were conducted at scan rates from 0.5 to 10 mV s −1 .As shown in Figures 2a-c, in contrast to MGO-ECˆDEC and MGO-ECˆPC, the broad cathodic and anodic curves are maintained at high scan rates for MGO-DME.In the cases of MGO-ECˆDEC and MGO-ECˆPC, their curves become steeper and show large polarization.To determine the different levels of their capacitive contributions, further quantitative analysis was performed according to the relationship between the current response, i, and scan rates, v, based on the following Equation ( 1): where k 1 v indicates the response of surface-controlled behavior, k 2 v 1/2 indicates the diffusion-controlled behavior. [38,39]By determining the k 1 constant, the surface-controlled contributions of the electrodes were clarified.As shown in Figure 2d, comparably large contributions were obtained for all MGO-DME, MGO-ECˆDEC, and MGO-ECˆPC.Thereby, a surface-dominated pseudocapacitance was identified as the major charge storage mechanism for MGO electrodes.Moreover, MGO-DME possesses the largest contributions compared to MGO-ECˆDEC and MGO-ECˆPC at each scan rate, suggesting the fastest kinetics dominated by the charge storage of MGO-DME.Figures 2e and f shows the fractions of the currents from capacitive domination and diffusion domination.The capacitive contribution enlarges from 83% to 92% with increasing scan rates, which complies with the general characteristic of electrochemical reaction kinetics for electrode materials.Therefore, the fastest in-storage kinetics for MGO-DME is confirmed, which is in accordance with its highest rate capability.The ex situ Raman spectra in Figure S9 (Supporting Information) show that there is no obvious change for the I G /I D of the MGO-DME at different states, which further demonstrates the position of the reaction active site is at or near the electrode interfaces.
The enhanced pseudocapacitance of MGO-DME indicates fast ion diffusion and electron transportation in the DME electrolyte.To further understand the fast charge and electron transfers, we performed electrochemical impedance spectroscopy (EIS).Figures 2g-i shows the EIS measurements of MGO-DME, MGO-ECˆDEC, and MGO-ECˆPC, respectively, after the 1st, 10th, and 50th cycle at the current density of 0.5 A g −1 .The semicircles represent the interface resistances of electron transfer, corresponding to the ionic diffusion at the electrode interface.They become smaller during the acceleration of cycle number, meaning the interface resistances for all samples decrease after cycling.This phenomenon is mainly caused by the more electrolyte diffusion within the electrode, which increases the interfacial area of the ion transfer at electrode interfaces. [40]More importantly, in the case of MGO-DME, the interface resistances corresponding to each cycle are exponentially lower than those of MGO-ECˆDEC and MGO-ECˆPC.It indicates that the ion diffusion from the electrolyte to the electrode in DME is much easier than those in ECˆDEC and ECˆPC electrolytes.Additionally, we performed the galvanostatic intermittent titration (GITT) technique to understand the ion-storage kinetics (Figure S10, Supporting Information).The voltage change during each relaxation period represents overpotential at the corresponding ion storage stage.The MGO electrode in DME electrolyte exhibits the smallest overpotential as compared with that in ECˆDEC-and ECˆPC electrolytes.The difference is more obvious in the lower voltage range.This implies better ion-storage kinetic properties in DME-based electrolyte.The ion diffusion coefficient in DME-based electrolyte shows a progressive decrease, whereas it dramatically decreases in ECˆDEC-and ECˆPC-based electrolytes.This phenomenon may result from the additional potassium-ion storage between potassium and oxygen-containing functional groups in the relatively lower voltage range.Therefore, the kinetic property of dualion storage at a low voltage range more relies on the electrolytes, further suggesting the weak interaction can enable fast dual-ion storage in DME-based electrolyte.

Solvation Structures of Different Electrolytes
Considering the obvious difference in the electrochemical behavior between the ether-based and ester-based electrolytes, we believe that there is an intrinsic reason which can interpret this phenomenon.As we know, many works have demonstrated that oxygen-functionalized carbons can reversibly store anions and cations within a high voltage window for hybrid-ion capacitors.Recently, it was also verified in PIHCs. [8,25,34]To further validate the ion storage mechanism in the MGO cathode, ex situ X-ray photoelectron spectroscopy (XPS) was measured at three different cycling states (Figure S11, Supporting Information).It provides direct evidence of dual-ion storage.As illustrated in Figure 3a, unlike PIBs and dual-ion batteries, the graphite oxide electrode with abundant oxygen-containing functional groups and defects can store both cations and anions at different active sites. [41,42]Therefore, the interaction between the cation and anion plays a crucial role in the ionic transfer at the electrode interface, especially for the HICs.To further demonstrate this conjecture, we mainly focused on investigating the solvation environment of anions and cations in different electrolytes using Fouriertransform infrared (FTIR) spectroscopy, Raman spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy.The infrared spectra of the different electrolytes in Figure 3b suggest that two different kinds of anion states exist in the electrodes, one is free PF 6 − which is assigned at the wavenumber of 839 cm −1 , another one is the K + PF 6 − contact ion pair which is assigned at the wavenumber of 857 and 876 cm −1 . [43]It is found that the K + and PF 6 − in the ECˆDEC (10.4%) and ECˆPC (11.8%) are easy to form the K + PF 6 − contact ion pairs than that in DME (6.8%), as evaluated by the partition of ions pairs/free ions ratio (Figure 3c).This result demonstrates that the anions and cations in DME are easier to be separated, as compared with ECˆDEC and ECˆPC. [44]Figure 3d shows the Raman spectra of the different electrolytes and the pure KPF 6 salt.It is observed that there are similar peaks at about 740 cm −1 in the series of Raman spectra for different electrolytes. [43]They are the P-F Raman vibrations in PF 6 − anions which provide information on the distance between PF 6 − and K + , indicating the solvation structure of the electrolytes.Inset displays a blue shift at the P-F peak position (comparing DME electrolytes with ECˆDEC and ECˆPC electrolytes), suggesting that the PF 6 − anions are positioned farther from K + cations in DME electrolyte.The P-F Raman vibration in pure KPF 6 salt appears at a high wavenumber of 752 cm −1 , corresponding to the shortest distance between PF 6 − and K + among all samples.Moreover, it demonstrates that the K + [DME] x clusters are much denser than the K + [EC] x [DEC] x and K + [EC] x [DEC] x clusters, resulting in the weaker interaction that K + makes contact with PF 6 − in DME-based electrolyte. [44]The 19 F-NMR peaks of the PF 6 − in DME-based electrolyte shift toward a lower magnetic field (Figure 3e), which further confirms the denser K + [DME] x clusters. [45]− contact ion pairs.e) 19 F-NMR (nuclear magnetic resonance) spectra of the anion PF 6 − in the different electrolytes.f) Proposed coordination structure of solvating K + in the different solvents.
On that basis, we assume that each K + coordinates with four oxygen-containing groups, thereby we propose the universal coordination structures of the [solvent-K + ] in the different electrolytes.Note that this work utilized the same content of the different electrolytes, the proposed electrolytes are shown in Figure 3f.The EC-, PC-, and DEC-based solvation structures are loose because of the branched structures of the carbonyl groups and carbon rings.The bidentate chelation of K + [DME] 2 exhibits much condensed, therefore the PF 6 − is not easy to contact with the K + to form the K + PF 6 − contact ion pair.The DME solvent can separate the PF 6 − and K + effectively, contributing to adequate freedom of movement of the PF 6 − anions and K + cations.These viewpoints are verified by the aforementioned analysis of the FTIR, Raman, and NMR spectra.Furthermore, as shown in Table S2 (Supporting Information), DME-based electrolyte possesses a lower viscosity and higher ionic conductivity which are consistent with the molecular and ionic interaction in a fluid at a molecular level.In this way, the PF 6 − anions possess different opportunities to contact with K + cations in the different solvents, which is accordant with the ion diffusion efficiency.Thus, the more isolated cations and anions in electrolytes lead to much fast ion diffusion.It further suggests that the solvent structure plays a crucial role in determining the fast kinetics of the anions and cations storage in the DME-based electrolyte.

Theoretical Calculation on Different Solvation Structures
To further verify the difference in the interaction between anions and cations in the different electrolytes, a density-functional theory (DFT) calculation was conducted to simulate the solvation structures and energy values.− ] complexes show that K + is easier to be isolated with the DME solvents in ether-based electrolyte, as compared with those in ester-based electrolytes.Figure 4b shows the solvation energies of the K + -solvents and K + -solvents-PF 6 − complexes according to the optimized solvation structures. [46]The calculated solvation energy values of [K + -DME 4 ], [K + -EC 2 -DEC 2 ], and [K + -EC 2 -PC 2 ] are −4.37,−4.09, and −3.76 eV, respectively.It ].Among those solvents, the DME shows a stronger ability in solvating K + .The solvation energy differences between the K + -solvents structure and K + -solvents-PF 6 − structure respectively are 1.61, 1.81, and 1.95 eV, indicating that the interaction between cations and anions in the ether-based electrolyte is weaker than those in the ester-based electrolytes.As illustrated in Figure 4c, the computational results may be ascribed to two aspects: The first one is the steric hindrance effect.The K + is isolated by DME molecules more efficiently because of the compact solvation structures, giving the cations with lower opportunity to contact with the anions.The second one is the electromagnetic shielding effect.Due to the bidentate chelation of DME molecules, the twining DME molecules serve as the barriers that weaken the positivity of the K + , thus reducing the Coulomb interaction between the cations and anions in ether-based electrolyte.However, the loose solvation structures in ECˆDEC and ECˆPC expose more surface of K + to interact with the anions, giving a stronger Coulomb interaction be-tween the cations and anions in ester-based electrolytes.A similar result can be explained with the [Ion-2 solvent molecules] simulated mode, as shown in Table S3 (Supporting Information).The above theoretical computational results coincide well with the electrolyte characterizations, which are consistent with the difference in electrochemical properties in the different electrolytes.
In principle, as illustrated in Scheme 1, the oxygen-containing carbon cathode stores anions and releases cations during the charging process.During the discharging process, it stores cations and releases anions. [8,25]Considering the fact that the cations and anions always diffuse in the opposite direction from each other during charging and discharging processes, the interaction between the cation and anion plays a critical role in determining the ion-storage kinetics of the carbon cathodes.In addition to the ion-storage mechanism, for the capacitor-type carbon cathode in which interfacial ion storage dominates the charge and discharge processes, the interfacial ion transfer plays a crucial role in determining the ion-storage kinetics.In ester-based electrolyte, the interactions between the cations and anions are stronger, displaying a crowded solvated-ion environment, which results in sluggish ion-storage kinetics at the electrode interfaces.On the contrary, the ether solvents can isolate the cation and anion efficiently, endowing weaker interactions between the cations Scheme 1. Illustrations of the dual-ion storage.Dual-ion storage mechanism for the graphite oxide cathode (left) and the influence of interaction between cations and cations on the ion storage in different types of electrolytes (right).
and anions, which benefits the fast ion-storage kinetics at electrode interfaces.

Full Capacitor Configuration
Finally, we configurated a full potassium-ion capacitor using MGO as the cathode, commercialized PG as the anode, and ether as the electrolyte solvent.We proposed this prototype as MGOǁDMEǁPG.The working mechanism of this prototype in the ether-based electrolyte is that the cathode reversibly stores the cations and anions through the electrode interface adsorption, and the anode reversibly stores the cations through the cointercalation (Figure 5a). [47]PG has been demonstrated as the anode for storing solvated-K + in the DME electrolyte, which shows a great rate capability (Figure S12, Supporting Information).In this case, it was pre-potassiated before assembling the full cell.Both adsorption and co-intercalation possess fast ion-storage kinetics, which enables an ultrafast potassium-ion capacitor.According to the working voltage of MGO and PG in half cells (details are discussed in Figure S13, Supporting Information), we operated the capacitor within the voltage window of 0.5-4.5 V.The active mass ratio for the anode to cathode is ≈1.5. Figure 5b shows the excellent rate performance of MGOǁDMEǁPG capacitor at intermediate and high current densities, it delivers high capacities of 89, 85, 81, 76, and 72 mAh g −1 at 0.5, 1, 2, 5, and 10 A g −1 .The profiles in Figure 5c display the symmetric characteristic of capacitor devices without obvious polarization.Furthermore, as shown in Figure 5d, the capacitor exhibits a long-term cyclability that cycle for up to 2500 cycles with 84% capacity retention.The MGOǁDMEǁPG without the pre-potassiation process shows poor cycling stability (Figure S14, Supporting Information).The energy/power densities of this capacitor are evaluated by multiplying the voltage and specific capacity factors.][18] A superb energy density of 190 Wh kg −1 can be obtained at a power density of 969 W kg −1 , even at a high power density of 21 443 W kg −1 , an impressive energy density of 150 Wh kg −1 maintains (calculation based on total mass: 76 Wh kg −1 at 388 W kg −1 , 60 Wh kg −1 at 8577 W kg −1 ).These values are also highly competitive with the reported LICs and SICs. [37]Here, considering that the anode and electrolyte sources are commercialized, and MGO fabrication can be industrialized too, once again it highlights the attractiveness of our proposed prototype.We believe that MGOǁDMEǁPG could be a priority prototype to be further improved with industrial-style engineering optimization.

Conclusion
In summary, a systematic investigation of the relationship between the electrochemical performances and electrolytes demonstrates that the interaction between the cations and anions in electrolytes is an important factor in determining the ion-storage behavior of the carbon-based cathode for PIHCs.The DME solvent can separate cations and anions effectively compared to the ECˆDEC and ECˆPC solvents, which contribute to more freedom of ion diffusion in PIHCs.On that basis, a low-cost and highperformance capacitor prototype, MGOǁDMEǁPG was proposed.The fast ion-storage kinetics of the ion adsorption on the cathode and the co-intercalation on the anode enable a great performance of the prototype which exhibits a high energy density of 190 Wh kg −1 at 969 W kg −1 cathode , 150 Wh kg −1 at 21 443 W kg −1 cathode (calculation based on total mass: 60 Wh kg −1 at 8577 W kg −1 ).More broadly, this work suggests a new idea of identifying electrolytes that can achieve enhanced high-rate performance for carbon-based HICs.

Experimental Section
Synthesis of MGO: The MGO nanosheets were prepared using a modified Hummer's method. [31]Briefly, the PG (1 g) and concentrated H 2 SO 4 (23 mL, 95 wt% in water) were mixed and stirred in an ice bath.After the mixture was completely dispersed, add NaNO 3 (0.5 g) to the mixture and stir for 30 min.Then, potassium permanganate (3 g) was added to the mixture and stirred for another 1 h.The color of the mixture becomes dark green.After that, transfer the mixture to an oil bath and kept it at 32-35 °C for 30 min while stirring.After the color of the mixture became brown, add distilled water (46 mL) slowly into the mixture within 15 min, well controlling the temperature around 90 °C.Afterward, add additional warm distilled water (150 mL) to the mixture, followed by adding H 2 O 2 (10 mL, 30 wt% in H 2 O).Finally, the golden mixture was formed.The warm mixture was filtered and washed with distilled water until the supernatant was pH neutral.The dark brown slurry was transferred into a flask (50 mL) and annealed in a furnace with ambient air at 250 °C for 6 h.
Material Characterization: The microstructures and morphology of MGO were observed using Hitachi 434 S4800 SEM and JEOL JEM-435 2100F TEM.The XRD pattern was obtained by an 18 KW D/MAX2500V PC diffractometer at a scan rate of 0.2°min −1 .The XPS was conducted by XPS Thermo Kalpha, Thermo ESCALAB 250XI.The FTIR spectra were collected through Nicolet 6700 FT-IR spectrometer (Thermo), and the 1 m electrolyte samples were tested with the assistance of an attachment for liquid sample testing.Raman spectra were collected on a LabRAM HR Evolution Raman spectrometer with the incident laser light of 532 nm wavelength.Another attachment for liquid sample testing also was employed for the Raman spectra measurements of the 1 m electrolytes. 19F NMR spectra of the electrolytes were collected by Bruker AV-III 600 MHz, using a co-axial internal insert filled with 0.05 m KPF 6 in the deteriorated water (D 2 O) as the reference.Viscosity data was obtained from a viscosity measurement instrument (MEP Instruments, Pty Ltd.).Ion conductivities were obtained by AC impedance.
Electrochemical Characterization: The MGO electrode was prepared by mixing the MGO nanosheets (90 wt%) and carboxymethyl cellulose (10 wt%) in distilled water.After fully grinding, the slurry was coated on the Al foil and then dried in a vacuum oven at 110 °C for 24 h.The elec-trode film was punched into circular pieces, and the active mass on each piece was controlled to about 1.5 mg.The coin cells were assembled in an N 2 -filled glove box.The electrolytes including 1 m KPF 6 in DME, 1 m KPF 6 in ethylene carbonate and diethyl carbonate (vol, EC:DEC = 1:1), 1 m KPF 6 in ethylene carbonate and propylene carbonate (vol, EC:PC = 1:1) were also prepared in N 2 -filled glove box.The glass microfiber filter (Whatman, Grade GF/B) was used as the separator.The CV and EIS tests were performed on a VSP electrochemical workstation (Bio-Logic, France).Galvanostatic charging/discharging tests were performed on a Land CT 2001A 449 battery testing system (Land, China).For the full cells, the PG electrode consists of commercial graphite (90 wt%) and carboxymethyl cellulose (10 wt%).The anodes were pre-potassiated by cycling in half cells.
First-Principles Calculation: Density functional theory calculations were applied to calculate the solvation energy E s for the [K + -solvents] and [K +solvents-PF 6 − ] complexes.The solvation energy (E s ) values of the K +solvents and K-solvents-PF 6 − complexes were calculated according to the following Equations ( 2) and (3), respectively: (2) where , and E [PF − 6 ] are the Gibbs free energies of the K + -solvents complex, K-solvents-PF 6 − complex, solvent molecule, K + , and PF 6 − , respectively.x means a K + is solvated by x solvent molecules [46] .
Optimization and frequency calculations of solvation complexes were performed using the Vienna ab initio simulation package (VASP).The exchange-correlation energy was described using the Perdew-Burke-Ernzerh (PBE) of exchange-correlation density functional within the generalized-gradient approximation (GGA).Atomic positions and cell vectors were fully optimized until all force components were less than 0.01 eV Å −1 .

Figure 1 .
Figure 1.Electrochemical performance of modified graphite oxide (MGO) in half-cell configurations.a) Cycling performance and b) rate performance of MGO-DME (dimethyl glycol), MGO-ECˆDEC (ethylene carbonate and diethyl carbonate), and MGO-ECˆPC at various current densities.c) Initial cycling behaviors at a current density of 0.5 A g −1 .Charge and discharge profiles of d) MGO-DME, e) MGO-ECˆDEC, and f) MGO-ECˆPC at current densities from 0.5 to 10 A g −1 .g) Cycling stability of MGO-DME at 2.0 A g −1 (inset: initial charge and discharge at 0.5 A g −1 for 10 cycles).h) Cycling performance under charging at high current density and discharge at low current density.

Figure 3 .
Figure 3.The characterization of the solvent structures in different electrolytes.a) Illustration of the anion and cation storage on modified graphite oxide (MGO) for potassium-ion hybrid capacitors (PIHCs).b) Fourier-transform infrared (FTIR) spectroscopy of KPF 6 and 1.0 m KPF 6 dissolved in different solvents.c) Relative content of free PF 6 − and K + PF 6 − contact ion pairs in different electrolytes.d) Raman spectra of the different electrolytes and KPF 6 electrolyte salt; inset: the peak of K + PF 6− contact ion pairs.e)19 F-NMR (nuclear magnetic resonance) spectra of the anion PF 6 − in the different electrolytes.f) Proposed coordination structure of solvating K + in the different solvents.
Considering that the concentrations of the different electrolytes are 1.0 m and the ester-based electrolytes have two different solvents, we mainly simulated two different solvation structures: [Ion-4 solvent molecules] and [Ion-2 solvent molecules].As shown in Figure 4a, the coordination structures between the complexes of K + -ether solvents and K +ester solvents are different.In the ether electrolyte, DME solvents can keep a close distance from K + because of the bidentate chelation, displaying a compact structure.In the ester electrolytes, the complexes display loose structures because of the carbonyls and carbon rings.The different features of the solvation structures of K + -solvents complexes endow them with different capabilities of contacting PF 6 − anions.The simulated solvation structures of [K + -solvents-PF 6

Figure 4 .
Figure 4. Theoretical simulations of the solvation structures in the different electrolytes.a) Optimized structures of K + -solvent complexes (upper) and K + -solvent-PF 6 − complexes (bottom) for KPF 6 salt in dimethyl glycol (DME), ethylene carbonate and diethyl carbonate (ECˆDEC), and ECˆPC (propylene carbonate) solvents.b) The solvation energy of the corresponding K + -solvent complexes and K + -solvent-PF 6 − complexes, inset: the energy difference between them.c) Schematic of the interaction between anions and cations in different electrolytes.

Figure 5 .
Figure 5. Electrochemical performances of the MGOǁDMEǁPG (where MGO is modified graphite oxide, DME is dimethyl glycol, and PG is pristine graphite) capacitor.a) Schematic illustration of the charging and discharging process in the full potassium-ion capacitor.b) Rate performance and c) rate profiles at various rates.d) Cycling performance at 2 A g −1 .e) Ragone plots in comparison with other previously reported potassium-ion hybrid capacitors (PIHCs) (calculated based on cathode active mass).