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Low-cost carbon materials as anode for high-performance potassium-ion batteries
Materials Letters 262 (2020) 127147
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Low-cost carbon materials as anode for high-performance potassium-ion batteries Chunxia Zhao 1, Hang Li 1, Yujie Zou, Yanyuan Qi, Zelang Jian ⇑, Wen Chen State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, PR China
a r t i c l e
i n f o
Article history: Received 5 August 2019 Received in revised form 3 November 2019 Accepted 8 December 2019 Available online 16 December 2019 Keywords: Low-cost Carbon materials Anode Potassium-ion batteries Energy storage and conversion
a b s t r a c t Energy storage plays an important role in modern society. However, the cost limits its large-scale application. We design a low-cost carbon material as anode for the low-cost system of potassium-ion batteries. The batteries assembled with the obtained carbon as anode exhibit brilliant electrochemical performance, including encouraging capacities and good cycling stability. This work provides a promising strategy for large-scale energy storage applications. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction In recent years, lithium-ion batteries (LIBs) have achieved unprecedented success in consumer electronics and electric vehicles. However, the poor abundance of lithium resource in the earth’s crust limits its large-scale applications [1]. Therefore, sodium/potassium-ion batteries (SIBs/PIBs) have received more and more attention as promising energy storage technology due to their low-cost, abundant resources, and similar working principle with LIBs [2]. The potassium has a similar standard hydrogen potential (2.93 V) with that of lithium (3.04 V), which is lower than that of sodium (2.7 V), meaning a higher working voltage in the full PIBs than that of SIBs [3]. Carbon as anode of PIBs has been widely investigated. The volume change during the potassiation and depotassiation processes greatly affects the cycle stability due to the large radius of K+ ion (1.38 Å) [4]. Therefore, to alleviate the volume expansion, various carbon materials with different structures, such as N/P/S-doped carbons [5,6], porous carbons [7], hard/soft carbons [8] and other amorphous carbons [9,10], have been studied in recent years. However, most of these carbon materials are pyrolyzed from organic materials or polymers with the ⇑ Corresponding author at: State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, PR China. E-mail address: [email protected] (Z. Jian). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.matlet.2019.127147 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
high cost but low carbon yield, which limit their industrial applications. So far, few carbon products combining with low-cost and highcarbon-yield have been reported. Anthracite is an attractive raw material with the high carbon yield, low cost, rich resources and low impurity content (Fig. S1). Herein, we demonstrate the carbon materials by a simple acid- and heat-treatment at 800, 1000, 1200, 1400, 1600 °C with HNO3 (denoted as AN-800, AN-1000, AN-1200, AN-1400, AN-1600, respectively). The defects and graphitic layer distance of samples depend on the heat-treatment temperature. Among all the samples, AN-1000 delivers the highest specific capacity of 206 mAh g1 with a retention of 82.7% over 400 cycles. It also shows the best rate performance.
2. Results and discussion 2.1. Structural and morphology characterization The samples were prepared by a facile method and the details are described in ESIy. The structure of samples was characterized by XRD patterns and Raman spectra. As the XRD patterns shown in Fig. 1a, the broaden peaks at around 24° and 44° correspond to the diffraction of the (0 0 2) and (1 0 1) planes in graphite. The position of the (0 0 2) peak shifts to high diffraction angle with the temperature increasing, which indicates the d-space of the samples decreases. The parameters obtained from the XRD
2
C. Zhao et al. / Materials Letters 262 (2020) 127147
Fig. 1. (a) XRD patterns, (b) Raman spectra of the samples. Fig. 2. (a) SEM, (b) HRTEM image and SAED pattern of the AN-1000 sample.
patterns are shown in Table S1, where the La and Lc (average length of the graphite sheets along a-axis and c-axis) values gradually increase, indicating a graphitization tendency with the treatment temperature increasing. The d-space shows the decreasing tendency with the temperature increasing except AN-800 sample, which d-space is about 3.69 Å, slightly lower than that of AN-1000 (3.71Å). This abnormal phenomenon is due to the existence of C–H bond in AN-800 (Fig. S2). The XRD pattern of AN-1600 shows a sharp peak at 25.40°, comparing to 23.97° of AN-1000, corresponding to a small d-spacing. The result indicates that the carbon obtained from anthracite can be assigned to the soft carbon, which will graphitize at the high temperature. The Raman spectra of the samples (Fig. 1b) present two typical characteristic bands located at 1343 cm1 and 1589 cm1, where the two broad bands indicating the amorphous structure for these samples. The integral area ratios of D-band and G-band of AN-800 to AN-1600 are 2.18, 2.40, 2.29, 1.86, and 1.35, respectively. When the heat treatment temperature is above 1000 °C, the decrease of ratio of I(D)/I(G) is contributed to the high graphitization state, which is consistent with the XRD results. However, the ratio of I(D)/I(G) of AN-1000 is higher than that of AN-800, which could be related to the defects caused by the uncompleted decomposition of C–H and C–N bonds (Fig. S2, the C–H and C–N bonds exist in AN-800 but disappear in other samples). To detect the morphology of samples, the SEM and TEM are carried out. Fig. 2a shows the SEM image of AN-1000, the particle size is in the range of 1–10 lm. The TEM image in Fig. 2b demonstrates that the carbon obtained from anthracite has unintelligible lattice fringe. Furthermore, the SAED pattern shown in the inset of Fig. 2b reveals the characteristic of the amorphous carbon. The d-spacing could be calculated by the clear diffraction rings of 3.72 and 2.08 Å, which is in accordance with the XRD results (23.97°, 3.71 Å and 43.51°, 2.08 Å, Table S1). The d-spacing is a little higher than graphene layer.
2.2. Electrochemical measurements The potassium storage properties of the samples were measured in coin half-cells with potassium metal as the counter electrodes. Fig. 3a shows the initial discharge/charge curves at 0.1C (1C rate defined as 300 mA g1) in a voltage range of 0.01–2 V. AN-800, AN-1000, AN-1200, AN-1400, and AN-1600 display the charge capability of 176.6, 206.4, 209.3, 181.6, 181.8 mAh g1, respectively. The discharge profiles of AN-800 and AN-1000 can be clearly delivered into two regions, the high one located at 1.5–0.9 V, corresponds to the potassiation and depotassiation in/ on the defects and surfaces, and the low one at around 0.1 V is attributed to the intercalation/deintercalation of potassium ion in the interlayer structure [2]. The potential drop is investigated with the temperature raising. The AN-1400 and AN-1600 samples show the sharp decline of the high slope, owing to the sharp decrease of defects. The PA-1000 (anthracite pyrolyzed at 1000 °C without acid treatment, Fig. S3) shows the lower specific charge capacity (187 mAh g1) than that of AN-1000 at 0.1C. This could be attributed to the acid treatment removing the impurities and providing more defects, confirmed by the Raman spectra of PA-1000 (I(D)/I (G) = 2.32, Fig. S4). Due to the more defects of AN-1000, which could provide much active sites, it shows a high capacity of 206 mAh/g. For all the samples, the second CV curve of AN-1000 is most similar to the initial one (Fig. S5), which indicates the less SEI film formation of AN-1000, leading to the highest initial Coulombic efficiency (67%, Table S2). Fig. 3b shows the rate performance of the samples obtained at different temperatures. AN-1000 displays the best charge capability of 109.1 mAh g1 and 75.3 mAh g1 at 1C and 2C, respectively. While AN-1200, AN-1400, and AN-1600 only deliver the specific capacity of 65.8, 28.5 and 16.9 mAh g1 at 2C, respectively. The low capacity of the samples obtained at high temperature is
C. Zhao et al. / Materials Letters 262 (2020) 127147
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formance at 1C. Fig. 3c shows the capacity retention of AN-800, AN-1000, AN-1200, AN-1400, AN-1600 is 76.3%, 82.7%, 70.6%, 67.9%, and 62.8%, respectively. AN-1000 sample shows the best rate performance and the highest capacity retention, mainly due to their high defects and large d-spacing. This special structure can accommodate much K+ ions, allow fast K+ ions transfer and tolerate large volume expansion. Thus, the low-cost and great cycling performance of AN-1000 is comparable with other materials in recent publications (Table S3). 3. Conclusions In summary, a low-cost carbon is prepared by a facile method from anthracite. The sample of AN-1000 exhibits the high capacity of 206 mAh g1, the best rate ability of 76 mAh g1 at 2C and longcycling performance with capacity retention of 82.7% over 400 cycles. The excellent electrochemical performance is proposed due to its high defects (I(D)/I(G) = 2.40) and large d-spacing (3.71 Å). Therefore, carbon obtained from anthracite is a promising anode material for PIBs with low cost and excellent electrochemical performance. It would push PIBs to become the competitive choice for large-scale energy storage. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This research was supported by the National Natural Science Foundation of China (51972258) and Fundamental Research Funds for the Central Universities (WUT: 2019-zy-018 and WUT: 2019IVA007). Appendix A. Supplementary data
Fig. 3. (a) The first discharge/charge curves at 0.1C, (b) rate capability and (c) cycling performance at 1C-rate of 400 cycles for the samples.
Supplementary data to this article can be found online at https://doi.org/10.1016/j.matlet.2019.127147. References
related to their less defects. The rate capability of samples decreases with the treatment temperature increase, except the sample of AN-800. AN-800 shows poor rate capability due to its uncompleted decomposition of C–H group. Actually, the K+ ion storage capability depends on their ratios of I(D)/I(G) importantly (Fig. 1b). Furthermore, the capacities of all samples can be restored to their original one after undergoing the high-rate cycles, which indicates that they can tolerate the fast K+ ion insertion/extraction and the structure is not destroyed during the high-rate test. The Rct of AN-800, AN-1000, AN-1200, AN-1400, and AN-1600 is 382, 375, 406, 846, and 907 X, respectively (Fig. S6). The Rct of AN-1000 is smaller than others, indicating its faster charge transfer. To evaluate the intrinsic rate performance of the AN anode, an asymmetric discharge/charge test was adopted to AN-1000. As Fig. S7a shown, it delivers a specific capacity of 170 mAh g1 even at 20C, which remains 82.8% capacity of that at 0.1C. The resistances, such as ohmic, charge transfer, and mass transport, can push the discharge curve under to the 0 V, especially in high C-rate (Fig. S7b). Thus, the poor rate performance for symmetric test mostly attributes to the high polarization, which can be enhanced by reducing the particle size and carbon coating. They also present great long-cycling per-
[1] J. Han, Y. Niu, S.J. Bao, Y.N. Yu, S.Y. Lu, M. Xu, Nanocubic KTi2(PO4)3 electrodes for potassium-ion batteries, Chem. Commun. 25 (2016) 11661–11664. [2] Z. Jian, W. Luo, X. Ji, Carbon electrodes for K-ion batteries, J. Am. Chem. Soc. 137 (2015) 11566–11569. [3] K. Lei, C. Wang, L. Liu, Y. Luo, C. Mu, F. Li, J. Chen, A porous network of bismuth used as the anode material for high-energy-density potassium-ion batteries, Angew. Chem. Int. Ed. 57 (2018) 4687–4691. [4] H. Kim, J.C. Kim, M. Bianchini, D.H. Seo, J. Rodriguez-Garcia, G. Ceder, Recent progress and perspective in electrode materials for K-ion batteries, Adv. Energy Mater. 8 (2018) 1702384. [5] M. Chen, W. Wang, X. Liang, S. Gong, J. Liu, Q. Wang, S. Guo, H. Yang, Sulfur/ oxygen codoped porous hard carbon microspheres for high-performance potassium-ion batteries, Adv. Energy Mater. 8 (2018) 1800171. [6] Q. Gan, J. Xie, Y. Zhu, F. Zhang, P. Zhang, Z. He, S. Liu, Sub-20 nm carbon nanoparticles with expanded interlayer spacing for high-performance potassium storage, ACS Appl. Mater. Interfaces 11 (2019) 930–939. [7] W. Wang, J.H. Zhou, Z.P. Wang, L.Y. Zhao, P.H. Li, Y. Yang, C. Yang, H.X. Huang, S. J. Guo, Short-range order in mesoporous carbon boosts potassium-ion battery performance, Adv. Energy Mater. 8 (2018) 1701648. [8] Z. Jian, Z. Xing, C. Bommier, Z. Li, X. Ji, Hard carbon microspheres: potassiumion anode versus sodium-ion anode, Adv. Energy Mater. 6 (2016) 1501874. [9] Z. Chen, M. Zhang, K+ storage in porous red blood cell-like hollow carbon, J. Alloys Compd. 779 (2019) 505–510. [10] Y. Wang, Z. Wang, Y. Chen, H. Zhang, M. Yousaf, H. Wu, M. Zou, A. Cao, R.P.S. Han, Hyperporous sponge interconnected by hierarchical carbon nanotubes as a high-performance potassium-ion battery anode, Adv. Mater. 30 (2018) 1802074.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Low-cost carbon materials as anode for high-performance potassium-ion batteries Chunxia Zhao 1, Hang Li 1, Yujie Zou, Yanyuan Qi, Zelang Jian ⇑, Wen Chen State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, PR China
a r t i c l e
i n f o
Article history: Received 5 August 2019 Received in revised form 3 November 2019 Accepted 8 December 2019 Available online 16 December 2019 Keywords: Low-cost Carbon materials Anode Potassium-ion batteries Energy storage and conversion
a b s t r a c t Energy storage plays an important role in modern society. However, the cost limits its large-scale application. We design a low-cost carbon material as anode for the low-cost system of potassium-ion batteries. The batteries assembled with the obtained carbon as anode exhibit brilliant electrochemical performance, including encouraging capacities and good cycling stability. This work provides a promising strategy for large-scale energy storage applications. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction In recent years, lithium-ion batteries (LIBs) have achieved unprecedented success in consumer electronics and electric vehicles. However, the poor abundance of lithium resource in the earth’s crust limits its large-scale applications [1]. Therefore, sodium/potassium-ion batteries (SIBs/PIBs) have received more and more attention as promising energy storage technology due to their low-cost, abundant resources, and similar working principle with LIBs [2]. The potassium has a similar standard hydrogen potential (2.93 V) with that of lithium (3.04 V), which is lower than that of sodium (2.7 V), meaning a higher working voltage in the full PIBs than that of SIBs [3]. Carbon as anode of PIBs has been widely investigated. The volume change during the potassiation and depotassiation processes greatly affects the cycle stability due to the large radius of K+ ion (1.38 Å) [4]. Therefore, to alleviate the volume expansion, various carbon materials with different structures, such as N/P/S-doped carbons [5,6], porous carbons [7], hard/soft carbons [8] and other amorphous carbons [9,10], have been studied in recent years. However, most of these carbon materials are pyrolyzed from organic materials or polymers with the ⇑ Corresponding author at: State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, PR China. E-mail address: [email protected] (Z. Jian). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.matlet.2019.127147 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
high cost but low carbon yield, which limit their industrial applications. So far, few carbon products combining with low-cost and highcarbon-yield have been reported. Anthracite is an attractive raw material with the high carbon yield, low cost, rich resources and low impurity content (Fig. S1). Herein, we demonstrate the carbon materials by a simple acid- and heat-treatment at 800, 1000, 1200, 1400, 1600 °C with HNO3 (denoted as AN-800, AN-1000, AN-1200, AN-1400, AN-1600, respectively). The defects and graphitic layer distance of samples depend on the heat-treatment temperature. Among all the samples, AN-1000 delivers the highest specific capacity of 206 mAh g1 with a retention of 82.7% over 400 cycles. It also shows the best rate performance.
2. Results and discussion 2.1. Structural and morphology characterization The samples were prepared by a facile method and the details are described in ESIy. The structure of samples was characterized by XRD patterns and Raman spectra. As the XRD patterns shown in Fig. 1a, the broaden peaks at around 24° and 44° correspond to the diffraction of the (0 0 2) and (1 0 1) planes in graphite. The position of the (0 0 2) peak shifts to high diffraction angle with the temperature increasing, which indicates the d-space of the samples decreases. The parameters obtained from the XRD
2
C. Zhao et al. / Materials Letters 262 (2020) 127147
Fig. 1. (a) XRD patterns, (b) Raman spectra of the samples. Fig. 2. (a) SEM, (b) HRTEM image and SAED pattern of the AN-1000 sample.
patterns are shown in Table S1, where the La and Lc (average length of the graphite sheets along a-axis and c-axis) values gradually increase, indicating a graphitization tendency with the treatment temperature increasing. The d-space shows the decreasing tendency with the temperature increasing except AN-800 sample, which d-space is about 3.69 Å, slightly lower than that of AN-1000 (3.71Å). This abnormal phenomenon is due to the existence of C–H bond in AN-800 (Fig. S2). The XRD pattern of AN-1600 shows a sharp peak at 25.40°, comparing to 23.97° of AN-1000, corresponding to a small d-spacing. The result indicates that the carbon obtained from anthracite can be assigned to the soft carbon, which will graphitize at the high temperature. The Raman spectra of the samples (Fig. 1b) present two typical characteristic bands located at 1343 cm1 and 1589 cm1, where the two broad bands indicating the amorphous structure for these samples. The integral area ratios of D-band and G-band of AN-800 to AN-1600 are 2.18, 2.40, 2.29, 1.86, and 1.35, respectively. When the heat treatment temperature is above 1000 °C, the decrease of ratio of I(D)/I(G) is contributed to the high graphitization state, which is consistent with the XRD results. However, the ratio of I(D)/I(G) of AN-1000 is higher than that of AN-800, which could be related to the defects caused by the uncompleted decomposition of C–H and C–N bonds (Fig. S2, the C–H and C–N bonds exist in AN-800 but disappear in other samples). To detect the morphology of samples, the SEM and TEM are carried out. Fig. 2a shows the SEM image of AN-1000, the particle size is in the range of 1–10 lm. The TEM image in Fig. 2b demonstrates that the carbon obtained from anthracite has unintelligible lattice fringe. Furthermore, the SAED pattern shown in the inset of Fig. 2b reveals the characteristic of the amorphous carbon. The d-spacing could be calculated by the clear diffraction rings of 3.72 and 2.08 Å, which is in accordance with the XRD results (23.97°, 3.71 Å and 43.51°, 2.08 Å, Table S1). The d-spacing is a little higher than graphene layer.
2.2. Electrochemical measurements The potassium storage properties of the samples were measured in coin half-cells with potassium metal as the counter electrodes. Fig. 3a shows the initial discharge/charge curves at 0.1C (1C rate defined as 300 mA g1) in a voltage range of 0.01–2 V. AN-800, AN-1000, AN-1200, AN-1400, and AN-1600 display the charge capability of 176.6, 206.4, 209.3, 181.6, 181.8 mAh g1, respectively. The discharge profiles of AN-800 and AN-1000 can be clearly delivered into two regions, the high one located at 1.5–0.9 V, corresponds to the potassiation and depotassiation in/ on the defects and surfaces, and the low one at around 0.1 V is attributed to the intercalation/deintercalation of potassium ion in the interlayer structure [2]. The potential drop is investigated with the temperature raising. The AN-1400 and AN-1600 samples show the sharp decline of the high slope, owing to the sharp decrease of defects. The PA-1000 (anthracite pyrolyzed at 1000 °C without acid treatment, Fig. S3) shows the lower specific charge capacity (187 mAh g1) than that of AN-1000 at 0.1C. This could be attributed to the acid treatment removing the impurities and providing more defects, confirmed by the Raman spectra of PA-1000 (I(D)/I (G) = 2.32, Fig. S4). Due to the more defects of AN-1000, which could provide much active sites, it shows a high capacity of 206 mAh/g. For all the samples, the second CV curve of AN-1000 is most similar to the initial one (Fig. S5), which indicates the less SEI film formation of AN-1000, leading to the highest initial Coulombic efficiency (67%, Table S2). Fig. 3b shows the rate performance of the samples obtained at different temperatures. AN-1000 displays the best charge capability of 109.1 mAh g1 and 75.3 mAh g1 at 1C and 2C, respectively. While AN-1200, AN-1400, and AN-1600 only deliver the specific capacity of 65.8, 28.5 and 16.9 mAh g1 at 2C, respectively. The low capacity of the samples obtained at high temperature is
C. Zhao et al. / Materials Letters 262 (2020) 127147
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formance at 1C. Fig. 3c shows the capacity retention of AN-800, AN-1000, AN-1200, AN-1400, AN-1600 is 76.3%, 82.7%, 70.6%, 67.9%, and 62.8%, respectively. AN-1000 sample shows the best rate performance and the highest capacity retention, mainly due to their high defects and large d-spacing. This special structure can accommodate much K+ ions, allow fast K+ ions transfer and tolerate large volume expansion. Thus, the low-cost and great cycling performance of AN-1000 is comparable with other materials in recent publications (Table S3). 3. Conclusions In summary, a low-cost carbon is prepared by a facile method from anthracite. The sample of AN-1000 exhibits the high capacity of 206 mAh g1, the best rate ability of 76 mAh g1 at 2C and longcycling performance with capacity retention of 82.7% over 400 cycles. The excellent electrochemical performance is proposed due to its high defects (I(D)/I(G) = 2.40) and large d-spacing (3.71 Å). Therefore, carbon obtained from anthracite is a promising anode material for PIBs with low cost and excellent electrochemical performance. It would push PIBs to become the competitive choice for large-scale energy storage. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This research was supported by the National Natural Science Foundation of China (51972258) and Fundamental Research Funds for the Central Universities (WUT: 2019-zy-018 and WUT: 2019IVA007). Appendix A. Supplementary data
Fig. 3. (a) The first discharge/charge curves at 0.1C, (b) rate capability and (c) cycling performance at 1C-rate of 400 cycles for the samples.
Supplementary data to this article can be found online at https://doi.org/10.1016/j.matlet.2019.127147. References
related to their less defects. The rate capability of samples decreases with the treatment temperature increase, except the sample of AN-800. AN-800 shows poor rate capability due to its uncompleted decomposition of C–H group. Actually, the K+ ion storage capability depends on their ratios of I(D)/I(G) importantly (Fig. 1b). Furthermore, the capacities of all samples can be restored to their original one after undergoing the high-rate cycles, which indicates that they can tolerate the fast K+ ion insertion/extraction and the structure is not destroyed during the high-rate test. The Rct of AN-800, AN-1000, AN-1200, AN-1400, and AN-1600 is 382, 375, 406, 846, and 907 X, respectively (Fig. S6). The Rct of AN-1000 is smaller than others, indicating its faster charge transfer. To evaluate the intrinsic rate performance of the AN anode, an asymmetric discharge/charge test was adopted to AN-1000. As Fig. S7a shown, it delivers a specific capacity of 170 mAh g1 even at 20C, which remains 82.8% capacity of that at 0.1C. The resistances, such as ohmic, charge transfer, and mass transport, can push the discharge curve under to the 0 V, especially in high C-rate (Fig. S7b). Thus, the poor rate performance for symmetric test mostly attributes to the high polarization, which can be enhanced by reducing the particle size and carbon coating. They also present great long-cycling per-
[1] J. Han, Y. Niu, S.J. Bao, Y.N. Yu, S.Y. Lu, M. Xu, Nanocubic KTi2(PO4)3 electrodes for potassium-ion batteries, Chem. Commun. 25 (2016) 11661–11664. [2] Z. Jian, W. Luo, X. Ji, Carbon electrodes for K-ion batteries, J. Am. Chem. Soc. 137 (2015) 11566–11569. [3] K. Lei, C. Wang, L. Liu, Y. Luo, C. Mu, F. Li, J. Chen, A porous network of bismuth used as the anode material for high-energy-density potassium-ion batteries, Angew. Chem. Int. Ed. 57 (2018) 4687–4691. [4] H. Kim, J.C. Kim, M. Bianchini, D.H. Seo, J. Rodriguez-Garcia, G. Ceder, Recent progress and perspective in electrode materials for K-ion batteries, Adv. Energy Mater. 8 (2018) 1702384. [5] M. Chen, W. Wang, X. Liang, S. Gong, J. Liu, Q. Wang, S. Guo, H. Yang, Sulfur/ oxygen codoped porous hard carbon microspheres for high-performance potassium-ion batteries, Adv. Energy Mater. 8 (2018) 1800171. [6] Q. Gan, J. Xie, Y. Zhu, F. Zhang, P. Zhang, Z. He, S. Liu, Sub-20 nm carbon nanoparticles with expanded interlayer spacing for high-performance potassium storage, ACS Appl. Mater. Interfaces 11 (2019) 930–939. [7] W. Wang, J.H. Zhou, Z.P. Wang, L.Y. Zhao, P.H. Li, Y. Yang, C. Yang, H.X. Huang, S. J. Guo, Short-range order in mesoporous carbon boosts potassium-ion battery performance, Adv. Energy Mater. 8 (2018) 1701648. [8] Z. Jian, Z. Xing, C. Bommier, Z. Li, X. Ji, Hard carbon microspheres: potassiumion anode versus sodium-ion anode, Adv. Energy Mater. 6 (2016) 1501874. [9] Z. Chen, M. Zhang, K+ storage in porous red blood cell-like hollow carbon, J. Alloys Compd. 779 (2019) 505–510. [10] Y. Wang, Z. Wang, Y. Chen, H. Zhang, M. Yousaf, H. Wu, M. Zou, A. Cao, R.P.S. Han, Hyperporous sponge interconnected by hierarchical carbon nanotubes as a high-performance potassium-ion battery anode, Adv. Mater. 30 (2018) 1802074.