Ultrahigh “Relative Energy Density” and Mass Loading of Carbon Cloth Anodes for K-Ion Batteries

Abstract

Open AccessCCS ChemistryCOMMUNICATION1 Feb 2021Ultrahigh “Relative Energy Density” and Mass Loading of Carbon Cloth Anodes for K-Ion Batteries Junpeng Xie, Jinliang Li, Xiaodan Li, Hang Lei, Wenchen Zhuo, Xibo Li, Guo Hong, Kwun Nam Hui, Likun Pan and Wenjie Mai Junpeng Xie Siyuan Laboratory, Guangdong Provincial Engineering Technology Research Center of Vacuum Coating Technologies and New Materials, Department of Physics, Jinan University, Guangzhou 510632 Institute of Applied Physics and Materials Engineering (IAPME), University of Macau. Department of Physics and Chemistry, Faculty of Science and Technology, University of Macau, Macao SAR 999078 Google Scholar More articles by this author , Jinliang Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Siyuan Laboratory, Guangdong Provincial Engineering Technology Research Center of Vacuum Coating Technologies and New Materials, Department of Physics, Jinan University, Guangzhou 510632 Google Scholar More articles by this author , Xiaodan Li Siyuan Laboratory, Guangdong Provincial Engineering Technology Research Center of Vacuum Coating Technologies and New Materials, Department of Physics, Jinan University, Guangzhou 510632 Google Scholar More articles by this author , Hang Lei Siyuan Laboratory, Guangdong Provincial Engineering Technology Research Center of Vacuum Coating Technologies and New Materials, Department of Physics, Jinan University, Guangzhou 510632 Department of Chemistry, Jinan University, Guangzhou 510632 Google Scholar More articles by this author , Wenchen Zhuo Siyuan Laboratory, Guangdong Provincial Engineering Technology Research Center of Vacuum Coating Technologies and New Materials, Department of Physics, Jinan University, Guangzhou 510632 Department of Chemistry, Jinan University, Guangzhou 510632 Google Scholar More articles by this author , Xibo Li Siyuan Laboratory, Guangdong Provincial Engineering Technology Research Center of Vacuum Coating Technologies and New Materials, Department of Physics, Jinan University, Guangzhou 510632 Google Scholar More articles by this author , Guo Hong Institute of Applied Physics and Materials Engineering (IAPME), University of Macau. Department of Physics and Chemistry, Faculty of Science and Technology, University of Macau, Macao SAR 999078 Google Scholar More articles by this author , Kwun Nam Hui Institute of Applied Physics and Materials Engineering (IAPME), University of Macau. Department of Physics and Chemistry, Faculty of Science and Technology, University of Macau, Macao SAR 999078 Google Scholar More articles by this author , Likun Pan Shanghai Key Laboratory of Magnetic Resonance, School of Physics and Electronic Science, East China Normal University, Shanghai 200062. Google Scholar More articles by this author and Wenjie Mai *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Siyuan Laboratory, Guangdong Provincial Engineering Technology Research Center of Vacuum Coating Technologies and New Materials, Department of Physics, Jinan University, Guangzhou 510632 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.202000203 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Mass loading and potential plateau are the two most important issues of potassium (K)-ion batteries (KIBs), but they have long been ignored in previous studies. Herein, we report a simple and scalable method to fabricate acidized carbon clothes (A-CC) as high mass loading (13.1 mg cm−2) anode for KIBs, which achieved a reversible areal-specific capacity of 1.81 mAh cm−2 at 0.2 mA cm−2. Besides, we have proposed the concept of “relative energy density” to reasonably evaluate the electrochemical performance of the anode. According to our calculation method, the A-CC electrode exhibited an ultrahigh relative energy density of 46 Wh m−2 in the initial charge process and remained at 40 Wh m−2 after 50 cycles. Furthermore, we performed the operando Raman spectroscopy (ORS) to investigate the K-ion storage mechanism. We believe that our work might provide a new guideline for the evaluation of anode performance, thereby, opening an avenue for the development of commercial anode. Download figure Download PowerPoint Introduction Nowadays, lithium (Li)-ion batteries (LIBs) are being used extensively in portable electric products and electric vehicles.1–4 Driven by the challenges of limited and expensive Li resources, potassium (K)-ion batteries (KIBs) have taken the privilege over LIBs on account of their accessibility and low-cost K resources.5–7 Unfortunately, the oversized radius of K-ion compared with Li-ion hinders the plating–stripping process, which ultimately results in sluggish diffusion kinetics.8 Such behavior not only destroys the inherent material structure but also ruins the electrochemical performance for KIBs.9 Therefore, it is significant to explore compatible host materials for reversible K-ion intercalation and deintercalation10,11. Carbonaceous materials have aroused great concern in KIBs, due to their abundant resources, environmental friendliness, and accessibility.12–17 Considerable progress has been made in carbon-based KIB fields. Ji’s group14 found that K-ion could insert into graphite in a nonaqueous electrolyte, with the graphite exhibiting a reversible specific capacity of 273 mAh g−1 at 7 mA g−1, approaching the theoretical capacity for K-ion storage. Also, Pint’s group18 reported that N-doping could increase the K-ion storage capacity of graphene at 350 mAh g−1 at 50 mA g−1. Further, Xiong’s group19 prepared N/O dual-doped hierarchical porous hard carbon for KIBs, delivering high reversible capacities of 365 mAh g−1 at 25 mA g−1. Nevertheless, the hidden problems for further industrial applications should not be ignored. Firstly, electrochemical performance, in the most reported materials, is based only on the low mass loading evaluations (usually less than 1 mg cm−2), which is far from the practical industrial requirement (more than 10 mg cm−2). Besides, as the critical parameters in practical battery production, the areal and volumetric capacities are frequently overlooked in KIBs. Most importantly, some anode materials always exhibit high charging potential, resulting in lower energy density after fabrication of the full battery. Accordingly, numerous works need to be achieved to realize industrialization in KIBs. Herein, we present a two-step method for the preparation of acidized carbon clothes (A-CC) to obtain a desirable improvement of the kinetic progress and K-ion storage performance. After facile processing, a free-standing A-CC electrode, with its high mass loading (∼13.1 mg·cm−2), exhibited a high reversible areal-specific capacity of 1.81 mAh·cm−2 at a current density of 0.2 mA·cm−2, which could even be doubled by simple electrode stacking to achieve a two-fold areal capacity. In order to reflect the practical benefits of the A-CC electrode, a “relative energy density” (ER) assessment was presumed to evaluate the performance by considering the requirement of both low potential plateau and large anode capacity. Benefiting from the slight oxygen decorated, both the battery rate performance and ER of the carbon clothes were improved. Besides, we employed the operando Raman spectra (ORS) of the A-CC electrode during different potassiation–depotassiation process, used to investigate the K-ion storage mechanism. Materials and Methods Materials and synthesis Pristine carbon clothes (P-CC; WOS1009, CeTech Co., Taichung City, Taiwan) were cut into 2 × 10 cm pieces and immersed in a 120 mL mixture of concentrated sulfuric acid, concentrated nitric acid, and deionized water with a volumetric ratio of 1∶1∶1. Then the mixture was sealed and transferred into an electric oven at 70 °C for 24 h. The crude intermediate sample was washed and dried after cooling. Subsequently, the A-CC was synthesized successfully by heating in a muffle furnace at 500 °C for 2 h with a heating rate of 10 °C min−1. Characterization The samples were characterized by obtaining its morphology and structure information via the conduction of scanning electron microscopy (SEM; Zeiss Ultra 55, JEOL Ltd., Beijing, China), transmission electron microscope (TEM; JEOL 2100 F), X-ray diffraction (XRD; Rigaku, MiniFlex600, Beijing, China) with Cu Kα radiation and Raman spectrometer (T64000, Horiba Scientific, Eindhoven, Netherlands) with a wavelength of 532 nm, X-ray photoelectron spectrometry (Thermo Fisher Scientific, Shanghai, China) with Al Kα radiation, Fourier-transform infrared spectrometer (FTIR; NEXUS 670, Nicolet, Madison, WI, USA). The Brunauer–Emmett–Teller (BET) specific surface areas were obtained using a nitrogen adsorption apparatus (Biaode-Kubo X1000, Beijing, China). Electrochemical measurement Generally, the electrochemical performance was tested by CR2032 coin half-cell at room temperature. The samples were used directly as working electrodes after cutting into circles with a 14 mm diameter. The mass loading of A-CC electrodes was ∼ 13.1 mg cm−2. For the graphite electrode, we mixed with the natural graphite, super P carbon black and carboxymethyl cellulose at a ratio of 8∶1∶1 in water to form a slurry, then coated it onto a Cu foil and dried at 100 °C for 12 h in a vacuum oven. After that, the electrode was cut into a circle with a 14 mm diameter. The different mass loading of the graphite electrodes were obtained by the optimization of the thickness during the coating process. K metal, glass fiber (Whatman), and 1 M KPF6 in 1∶1 by volume of propylene carbonate (PC) and ethylene carbonate (EC) were utilized as counter electrodes, separators, and electrolyte, respectively. The half-cell was assembled in an argon (Ar)-filled glove box (Etelux Lab2000) with H2O and O2 < 0.1 ppm. Galvanostatic charge–discharge process was recorded by a battery testing system (BTS 4000; Neware Tech. Ltd., Kowloon Bay, Hong Kong) with a potential range of 0.01–3.0 V (vs K+/K). Cyclic voltammetry (CV) measurements were conducted on an electrochemical workstation (CHI 1030 C, Chenhua, Shanghai, China) in the potential window between 0.01 and 3.0 V at a sweep rate of 0.2 mV s−1. Electrochemical impedance spectroscopy (EIS) was recorded by an electrochemical workstation (Veras STAT3-400, Princeton, Beijing, China) with a frequency range of 0.01 Hz–0.1 MHz. Operando Raman spectra testing system consisting of Raman spectrometer (532 nm laser) with per-Raman spectrum in voltage internal of 0.1 V, electrochemical workstation (CV test at 0.6 mV s−1), a computer, and an Operando Raman cell (Tianjin Aida Hengsheng Sci. & Tech. Co. Ltd., China) was utilized with A-CC electrode, separator, and K metal with a hole in the middle. All the Operando Raman spectra information collected was raw, untreated data. Results and Discussion The typical A-CC synthesis method is shown in Figure 1a. After acidifying and annealing process, P-CC could be converted readily into A-CC. In Figure 1b, this facile two-step synthesis led to the achievement of a free-standing electrode after cutting into small circles without any current collector and electrode preparation process. This simple experimental design elevated the electric conductivity in the entire electrode and also avoided the complicated electrode preparation process, thereby, decreasing the liberation of harmful pollutants and minimizing the production cost. Figure 1c displays the SEM image of A-CC, which shows the braided-fabric structure inherited from the P-CC. Locally ordered structure and random orientation in graphene layers were observed in the TEM image, featuring a typical hard carbon (Figure 1d). Raman spectra of both P-CC and A-CC corroborate D band at ∼ 1339 cm−1 and G band at ∼ 1579 cm−1, as shown in Figure 1e. The peak intensity ratio of D and G band, ID/IG, was utilized to assess the defect level of carbon materials.20 The higher value of ID/IG in A-CC (1.04) indicated that the A-CC electrode exhibited higher distortion structure, compared with P-CC (1.00). The slight increase of disorder structure might be derived from the modification of the carbon cloth’s surface during acidization that could have damaged the original structure of carbon cloth, resulting in a higher distortion structure.21,22 This increase in disordered structure could facilitate the K-ion striping–plating process. In addition, 2D bands at 2680 cm−1 in both spectra were detected, related to a second-zone boundary phonon mode for graphene, and displayed low degrees of ordering in both electrodes. In Supporting Information Figure S1, the XRD patterns of both P-CC and A-CC exhibited similar amorphous structures with two broad peaks attributable to (002) and (100), thereby, demonstrating that the acidified treatment of the carbon cloth did not change its bulk property. To validate the surface property of P-CC and A-CC further, we obtained the XPS spectra of C 1s and O 1s, presented in Figure 1f-g. Both of the C 1 s spectra exhibited the characteristic peaks at 290.7, 286.8, 285.1, and 284.8 eV, attributable to COOH, C=O, C–OH, and C–C/C=C bond, respectively.19 Meanwhile, three peaks located at 532.0, 533.3, and 536.1 eV were decomposed in both O 1s spectra and were assigned to C–O, C=O and COOH bonds, respectively.16 However, the intensity of fingerprint peaks at C=O in A-CC was enhanced after the acid treatment, demonstrating that the oxygen content increased on the surface of A-CC, compared with the prototype P-CC. Besides, FTIR spectra were employed to verify the XPS results. The characteristic absorption peaks of A-CC at 1587 and 1637 cm−1, shown in Figure 1h indicate the stretching vibration of the functional group of COOH and C=O.23 Compared with P-CC, the signal intensity peaks of A-CC exhibited an increase of C=O (at 1637 cm−1), and decrease of COOH (at 1587 cm−1) and C–O (at 1087 cm−1), indicating that the COOH and C–O groups were converted into C=O group due to the slight oxidation in the acidification process.23,24 The slight oxygen decoration on the surface of the carbon cloth was helpful as an improvement of the electrochemical performance, especially, in the rate performance.25 Supporting Information Figure S2 shows the nitrogen adsorption–desorption isotherms of P-CC and A-CC, respectively. According to the calculation by the BET method, the specific surface areas of P-CC and A-CC were 1.42 and 3.00 m2 g−1, respectively, demonstrating slight carbon cloth improvement of the A-CC after acidification, which should be attributable to some created pores on the carbon surface. Indeed, such a slight upgrade in the specific surface area is not the key factor for the enhancement of the electrochemical performance. Figure 1 | (a) Scheme of the A-CC synthetic process, (b) photograph, (c) SEM image, and (d) TEM image of A-CC. Insets in (c) and (d) are the enlarged SEM and TEM images of A-CC. (e) Raman spectra, (f) C1s XPS spectra, (g) O1s XPS spectra, and (h) FTIR spectra of P-CC and A-CC. SEM, scanning electron microscopy; TEM, transmission electron microscopy; XPS, X-ray photoelectron spectroscopy; FTIR, Fourier-transform infrared spectroscopy. Download figure Download PowerPoint Mass loading is a crucial technical parameter for industrial production, which is often ignored in basic frontier researches. Our A-CC electrode exhibited a high mass loading of 13.1 mg cm−2, almost the same as P-CC electrode (13.9 mg cm−2). To evaluate the electrochemical performance of such high mass loading samples after acidification, cycling performance in mass, areal, and volumetric specific capacities at 0.2 mA cm−2 were achieved, as shown in Supporting Information Figure S3a and Figure 2a and 2b. The corresponding coulombic efficiencies of P-CC and A-CC electrodes are provided in Supporting Information Figure S4. We observed that the P-CC electrode delivered an initial reversible areal-specific capacity of 1.56 mAh cm−2 (volumetric specific capacity of 54.0 mAh cm−3) and remained at 1.47 mAh cm−2 (volumetric specific capacity of 50.9 mAh cm−3) after 50 cycles. After acidizing, the reversible areal-specific capacity of A-CC electrode was improved to 1.81 mAh cm−2 (volumetric specific capacity of 65.6 mAh cm−3 and mass-specific capacity of 136 mAh g−1) and maintained at 1.59 mAh cm−2 (volumetric specific capacity of 57.4 mAh cm−3 and mass-specific capacity of 119 mAh g−1), hence, displaying a better cycling performance, compared with P-CC electrode. Supporting Information Figure S5 shows the EIS spectra of P-CC and A-CC electrodes before and after the cycles. Although the Rct of the A-CC electrode was slightly higher than that of P-CC before the cycling procedure, the Rct of the A-CC after 50 cycles was lower than that of the P-CC electrode, indicating that the decorated oxygen could promote ionic migration during the potassiation and depotassiation process. These results proved that our free-standing A-CC electrode, with high mass loading, still exhibited desirable electrochemical performance for K-ion storage, although it was difficult to compare the P-CC electrode with a low mass loading with the performance of our A-CC electrode. Generally, the natural graphite exhibits excellent electrochemical performance for K-ion storage in a low mass loading, which has been proven in previous reports.14,26 Therefore, we also provide the different mass loading graphite electrode for comparison with our A-CC electrode, as shown in Supporting Information Figure S6. We found that all of the natural graphite electrodes still exhibited low mass-specific capacity, and their capacities decay more rapidly with the increase in mass loading. This result indicated that the high mass loading electrode was difficult to obtain for high-performance K-ion storage. Compared with the mass-specific capacity, our free-standing electrode exhibited better K-ion storage performance (including high mass loading and cyclic stability). To further measure the electrochemical performance, the rate performance in mass, areal, and volumetric specific capacities of A-CC and P-CC were measured, as shown in Supporting Information Figure S3b and Figure 2c and 2d, we found that A-CC electrode exhibited 1.93, 1.76, 1.51, and 0.98 mAh cm−2 (the corresponding volumetric specific capacities are 70.4, 64.2, 55.1, and 35.8 mAh cm−3) at a current density of 0.1, 0.2, 0.5, and 1.0 mA cm−2, respectively, which were higher than that of the P-CC electrode (the areal specific capacities are 1.83, 1.63, 1.22, 0.74 mAh cm−2 and the corresponding volumetric specific capacities are 63.8, 57.0, 42.7, 26.0 mAh cm−3. This result indicated that the slight oxygen decorated on the surface of A-CC could accelerate the K-ion diffusion kinetics, which helped to improve its rate performance. To analyze the effect of oxygen-decorated, carbon cloth surface further, we obtained SEM images of P-CC and A-CC electrodes before and after the cycles. As shown in Supporting Information Figure S7, and observed some by-products in P-CC after the cycles, presumably, contributed by the inevitable decomposition of electrolyte, which might comprise alkyl carbonates, alkoxides, aldehydes, and their oxidation products. However, compared with P-CC, the surface structure of A-CC remained the same before and after the cycles. This favorable cycling outcome of A-CC might be attributable to the effect of the decorated oxygen that facilitated uniform insertion reaction and restrained the generation of by-products. Figure 2 | (a) Areal and (b) volumetric specific capacities of P-CC and A-CC during different cycles at a current density of 0.2 mA cm−2; (c) areal and (d) volumetric specific capacities of P-CC and A-CC electrodes at a different current density from 0.1 to 1.0 mA cm−2. Download figure Download PowerPoint Except for mass loading, the high energy density in practical production is achieved by adopting anode materials with low charge–discharge plateau. To identify the charge–discharge plateau, CV curves in several initial cycles were obtained. In Figure 3a, a broad peak between 0.42 and 0.01 V is observed, contributed by the intercalation of K-ion and formation of solid electrolyte interphase (SEI) layer. An intense anodic peak at 0.62 V was detected after the initial CV cycle, indicating that there was a great charge plateau in this region. Different from the other carbonaceous-based materials with large specific surface area, our A-CC with low specific surface area demonstrated mainly interlayer insertion mechanism with the display of a sharp peak at low-voltage plateau.27 Supporting Information Figure S8 shows the CV curves at different sweep rates. We found that the anodic peaks shifted to a high potential region with an increased sweep rate due to the electrochemical polarization from an accumulation of charge on the electrode at high current density.28 Figure 3b displays galvanostatic charge–discharge curves of the A-CC electrode at 0.1 mA cm−2. We observed that the capacity contributed mainly at low potential (below 0.5 V vs K/K+). Such behavior would indeed play a significant role in improving energy density of a full battery in terms of choosing anode materials. We noted a discharge and charge potential in a capacity contribution below 0.5 V vs K/K+ reached 99.5% and 86.6%, respectively. Figure 3 | (a) Cyclic voltammetry (CV) curves of A-CC electrode at a sweep rate of 0.2 mV s−1; (b) galvanostatic charge–discharge curves of A-CC electrode at a current density of 0.1 mA cm−2; (c) areal ER of A-CC electrode at the charge and discharge process; (d) comparisons of areal/volumetric ER of P-CC and A-CC electrodes at the initial and 50th charge process; (e) cycling performance (areal-specific capacity) of a double-stacked electrode at a current density of 1 mA cm−2; (f) comparisons of areal/volumetric ER of A-CC single electrode and double-stacked electrode in the 50th charge process. Download figure Download PowerPoint We considered evaluating the electrochemical performance of the anode material more accurately and conveniently by proposing the concept of “relative energy density,” which not only included the specific capacity but also contained the charge–discharge potential plateau of the anode materials, and thus, aided an effective evaluation of the electrochemical performance in the light of working potential. The corresponding equations are as follows: E R = Δ U Q (1) Δ U = − P K − V A (2) V A = ∫ U d Q / Q (3)where ER, Q, VA, and PK are relative energy density, specific capacity, average potential, and relative standard hydrogen potential of K (−2.93 V vs K/K+), respectively. The VA is the average discharging potential in the discharge process, and again, the VA is also the average charging potential in the charging process. ΔU is the different value between −PK and VA. The corresponding typical legend is shown in Figure 4. Based on these formulas, the areal and volumetric ER of A-CC and P-CC electrode at the charge and discharge process were obtained, as shown in Figure 3c and Supporting Information Figures S9 and S10. The ER of A-CC electrode achieved 46 Wh m−2 (166 Wh L−1) in the initial charge process and sustained at 40 Wh m−2 (143 Wh L−1) after 50 cycles. In addition, the concept of “relative energy conversion efficiency” was also proposed, derived from the charging energy density divided by discharging energy density. The relative energy conversion efficiency could also be used to evaluate the conversion efficiency of the “relative input energy” and “relative output available energy.” Generally, high relative energy conversion efficiency could improve the energy utilization efficiency of KIBs. Supporting Information Figures S11 and S12 show the relative energy conversion efficiency of both A-CC and P-CC electrode. We showed that the electrode exhibited a stable relative energy conversion efficiency of ∼ 90% after the initial cycle. For comparison, we have provided the ER of P-CC and A-CC electrode in the initial and 50 cyclic charge processes, as shown in Figure 3d, which revealed that A-CC electrode exhibited higher relative energy density than that of P-CC electrode. Figure 4 | Typical legends for the determination of the relative energy density (ER) of (a) discharge ER and (b) charge ER. Download figure Download PowerPoint We assessed further the possibility of high mass loading electrode by examining the electrochemical performance of the double-stacked electrode, which was simply stacked by two A-CC electrodes, as shown in Figure 3e and Supporting Information Figure S13. The double-stacked electrode delivered an areal-specific capacity of 2.24 mAh cm−2 (a volumetric specific capacity of 40.8 mAh cm−3) after 50 cycles, and at a high current density of 1 mA cm−2 even with a mass loading of 26.1 mg cm−2. In Figure 3f, the ER of 53 Wh m−2 (97 Wh L−1) and in double-stacked A-CC electrode at 1.0 mA cm−2 was calculated at the 50th cyclic charge process, indicating that the A-CC electrode exhibited excellent scalability with high mass loading for KIBs. Supporting Information Figure S14 shows the EIS spectra of A-CC with a single- and double-stacked electrode (See the Supporting Information). The impedance of A-CC of the double-stacked electrode was larger than that of a single electrode, which was ascribed to the loose interface contact between the two electrodes. Finally, we investigated the practical K-ion storage status in low potential without an equilibrated time using an operando Raman spectrometry. Figure 5a shows the schematic representation of the operando Raman spectra testing system, and Supporting Information Figures S15a and S15b also show the photograph of the operando testing device and the practical testing system, while Figure 5b is a display of the operando Raman spectra A-CC electrode in the initial cycle. Our results showed that the A-CC electrodes of D and G band were located at 1353 and 1593 cm−1 at the open-circuit voltage, respectively. After discharge to 0.01 V, the D and G bands shifted to a lower wavenumber of 1331 and 1571 cm−1, respectively. The shift of D band might be related to biaxial strain induced by expansion of intercalation-induced in-plane C–C bond and some degradation of sp2 hybridized carbon.29 For the shift of the G band, the charge-transfer effects and the formation of the SEI layer of particular oxygenated groups were suggested.13,30 After the depotassiation process, the D band and the G band shifted back to 1356 and 1595 cm−1, implying that the A-CC electrode exhibited excellent reversibility. Figures 5c and 5d show the intensities of the D and G band at a different charge–discharge states from the operando Raman spectra, demonstrating that the ratio of ID/IG value also attenuated from 1.12 to 0.96 in the potassiation process. The decrease of intensity originated from the loss of resonance with K-ion intercalation, which was ascribed to the breathing motion of limited sp2 atoms by the absorbed K-ions.13,30 After the depotassiation process, the ID/IG value was augmented from 1.02 to 1.06, indicating that the electrode exhibited superior reversibility of K-ion striping and plating. An interesting phenomenon was that the structure of the carbon cloth tended to become a more ordered structure in the potassiation process and then inclined to disordered structure in the depotassiation process. This result indicated that the K-ion inserted into the interlayer or interspace after discharging, which, in turn, induced the disordered carbon layer to rearrange at a specific interlayer distance.27,31,32 On the contrary, the degree of graphitization also decreased with the process of depotassiation, the as-ordered graphite microcrystalline had been recovered.31 Figure 5 | (a) Schematic diagram of operando Raman testing system; (b) operando Raman spectra of A-CC electrode in the first cycle at a sweep rate of 0.6 mV s−1. The ID/IG value of operando Raman spectra in the (c) potassiation process and (d) depotassiation process. Download figure Download PowerPoint Conclusion We obtained the A-CC electrode by a facial acidified treatment as anode for KIBs. Benefiting from the free-standing structure and high mass loading (13.1 mg cm−2), the A-CC electrode exhibited a high areal reversible specific capacity of 1.81 mAh cm−2 and volumetric reversible specific capacity of 65.6 mAh cm−3 at a current density of 0.2 mA cm−2. The double-stacked electrode (26.1 mg cm−2) also delivered a high reversible areal-specific capacity of 2.24 mAh cm−2 at a high current density of 1.0 mA cm−2. Besides, equations for an effective calculation method to evaluate the contribution of the low potential plateau was proposed. In this principle, our fabricated sample delivered an ultrahigh relative energy density of 46 Wh m−2 (166 Wh L−1). Further, operando Raman spectrometry was conducted to investigate the K-ion storage mechanism, which revealed the evolution of graphene microcrystalline (re)arranged of A-CC electrode during K-ion (de)intercalation process. We believe that our work would open an avenue to access more practical KIB industrialization in terms of a low potential plateau and high mass loading. Supporting Information Supporting Information is available. Conflict of Interest There is no conflict of interest to report. Acknowledgments The authors thank the financial supports from the National Natural Science Foundation of China (51702056 and 51772135), the Ministry of Education of China (6141A02022516), China Postdoctoral Science Foundation (2017M622902 and 2019T120790), funding from the University of Macau (SRG2016-00092-IAPME, MYRG2018-00079-IAPME, and MYRG2019-00115-IAPME), and the Science and Technology Development Fund, Macau SAR (FDCT081/2017/A2, FDCT0059/2018/A2, and FDCT009/2017/AMJ). The authors also thank the Guangzhou Micro-Discovery Scientific Corporation Limited for the XPS testing service.