Engineering of nanonetwork-structured carbon to enable high-performance potassium-ion storage

Journal Pre-proofs Engineering of Nanonetwork-Structured Carbon to Enable High-Performance Potassium-Ion Storage Weicai Zhang, Yinjia Yan, Zhuohao Xie, Yinghan Yang, Yong Xiao, Mingtao Zheng, Hang Hu, Hanwu Dong, Yingliang Liu, Yeru Liang PII: DOI: Reference:

S0021-9797(19)31361-X https://doi.org/10.1016/j.jcis.2019.11.042 YJCIS 25659

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Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

15 October 2019 12 November 2019 12 November 2019

Please cite this article as: W. Zhang, Y. Yan, Z. Xie, Y. Yang, Y. Xiao, M. Zheng, H. Hu, H. Dong, Y. Liu, Y. Liang, Engineering of Nanonetwork-Structured Carbon to Enable High-Performance Potassium-Ion Storage, Journal of Colloid and Interface Science (2019), doi: https://doi.org/10.1016/j.jcis.2019.11.042

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Engineering of Nanonetwork-Structured Carbon to Enable High-Performance Potassium-Ion Storage

Weicai Zhang1, Yinjia Yan1, Zhuohao Xie, Yinghan Yang, Yong Xiao, Mingtao Zheng, Hang Hu, Hanwu Dong, Yingliang Liu*, Yeru Liang* College of Materials and Energy, South China Agricultural University, Guangzhou 510642, P. R. China

Correspondence information: Yeru Liang, South China Agricultural University, Guangzhou, [email protected],

Yingliang

Liu,

South

China

[email protected] a

1

Agricultural

University,

Guangzhou,

Engineering of Nanonetwork-Structured Carbon to Enable High-Performance Potassium-Ion Storage Weicai Zhang1, Yinjia Yan1, Zhuohao Xie, Yinghan Yang, Yong Xiao, Mingtao Zheng, Hang Hu, Hanwu Dong, Yingliang Liu*, Yeru Liang* College of Materials and Energy, South China Agricultural University, Guangzhou 510642, P. R. China

*

Corresponding authors. College of Materials and Energy, South China Agricultural University, Guangzhou 510642, China

E-mail address: [email protected]; [email protected] 1

These authors contributed equally to this work. They should thus be considered co-first authors.

2

KEYWORDS: potassium-ion battery, hollow carbon, nanosphere-interconnected network, local graphitized carbon, excellent performance ABSTRACT: Potassium-ion batteries (KIBs) have been developed as an emerging electrochemical energy storage device due to the low cost of potassium and resource-abundance. However, they suffer insufficient cyclability and poor rate capability caused by the large K+, severely limits their further applications. Herein, a nanonetwork-structured carbon (NNSC) is reported to address the issue, cycling stability with very low decay rate of 0.004% per cycle over 2000 cycles and excellent rate capability of 261 mAh g-1 at 100 mA g-1 and 108 mAh g-1 at 5000 mA g-1 are achieved. The superior performance is attributed to the unique structure of NNSC, in which the three-dimensional interconnected hierarchical porous structure with hollow nanosphere as network units not only can effectively alleviate the volume expansion induced by the insertion of large K+, but also can offer fast pathways for potassium ion diffusion. In addition, the local graphitized carbon shell of NNSC can promote conductivity of material and reduce the resistance to K+ transportation. Thus, the nanonetwork-structured carbon has great potential in developing stable-structure and high-rate electrodes for next generation KIBs. 1. Introduction The growing demand for lithium-ion battery has aroused concerns about the depletion of lithium resources.[1, 2] To address this issue, significant efforts have been devoted to development of new electrochemical energy storage systems beyond lithium-ion battery.[3-5] Among the various options, potassium-ion battery (KIB) is emerging as one of the most competitive candidates to replace lithium-ion battery because of its relatively low cost, abundant natural potassium resources and similar low redox potential to lithium.[6-9] Despite these attractive properties, one of the most bottlenecks for KIB is its

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severe capacity decay, especially at high charge/discharge rates, mainly arising from the poor structural stability of the anode during the rapid K+ intercalation/deintercalation process.[10-13] To address this issue, considerable efforts have been devoted to construction of suitable anode materials with various nanostructures.[14-19] Diverse materials including metal oxide/sulfide/selenide,[20-25] metal/alloy,[26-29] organic matter,[30, 31] graphite[32-37] and amorphous carbon materials[38-42] have been explored as anodes for KIB and shown remarkable electrochemical performances. Among them, amorphous carbon has been demonstrated to be the most promising candidate for KIBs due to their large interlayer spacing, abundant active sites for storing K+ and excellent physicochemical stability.[43-45] For example, Bin et al. developed a semi-hollow microrod soft carbon anode for KIBs, which exhibited a capacity of 172 mAh g−1 at a high rate of 500 mA g-1 with average capacity decay less than 0.04 % per cycle after 500 cycles.[46] Wang et al. used mesoporous carbon as an anode, achieving a reversible capacity of 146.5 mAh g-1 at a current density of 1000 mA g-1 with average capacity decay of 0.03 % per cycle after 1000 cycles.[13] Despite these advances, the high rate and long cycling performance of the current carbon-based anodes are still unsatisfactory. The main reason is probably ascribed to the anode kinetic problems that are related to the diffusion resistance of large-sized K+.[47, 48] A high diffusion resistance inevitably slows down the K+ transport from electrolyte to the deep of anode material, thus severely reducing charge/discharge storage capability and durability, especially at large current densities.[49] Herein, we propose a new class of amorphous carbon, i.e., nanonetwork-structured carbon (NNSC) as a promising anode material for high-performance KIBs. Our NNSC has a three-dimensional (3D) interconnected hierarchical porous carbon structure with hollow nanosphere as network units (Figure 1a), and thus provides significant advantages when compared with traditional amorphous carbon anodes. First of all, the well-developed meso-/macropores in NNSC offer rapid pathways for K+ diffusion from the electrolyte solution to the hollow carbon nanosphere. It signifies that the carbon framework is directly connected to those meso-/macropores among the nanonetwork in different directions, achieving increased 4

available and easily accessible active sites. In addition, the network unit with a hollow carbon nanosphere structure provides sufficient void space to relieve the volume change during charge/discharge process. Meanwhile, the hollow carbon nanosphere would shorten the K+ diffusion distance in the nanospheres (i.e., half of the shell thickness), which tremendously improves the rate performance and the K+ diffusion efficiency. Moreover, the large interlayer spacing of 0.407 nm is beneficial for rapid K+ intercalation/deintercalation in the carbon layer without obvious volume expansion. The above-mentioned benefits give rise to rapid kinetics behaviors and ultrastable lifespan in KIBs. Therefore, the NNSC anode demonstrates unusual long-cycling with an extremely low average capacity decay of 0.004 % per cycle after 2000 cycles at 5000 mA g-1 and excellent rate performances (i.e., 261 mAh g-1 at 100 mA g-1 and 108 mAh g-1 at 5000 mA g-1), outperforming most recently reported carbon-based anodes. 2. Experimental 2.1 Preparation of NNSC Typically, 10 g of the chelating resin was impregnated in 90 mL of 0.1 mol L-1 NiSO4 solution for 8 h. 2 g of the impregnated product was swollen in 50 mL of dichloroethane. At the same time, 3 g of anhydrous ferric chloride was mixed with 50 mL of dichloroethane in an oil bath at 70 ℃, and the swollen product was poured into it 12 h later. After 6 h, 2 mL of methylal was added and reacted for 24 h. The reaction product was stopped by adding water and ethanol to form a yellow-free supernatant. The solid product was heated to 800 oC at 5 oC min-1 for 4 h under N2 flow. Finally, the carbonized product was etched with an appropriate amount of 4 mol of L-1 HCl to obtain NNSC. 2.2 Structure characterization The microstructure of NNSC was observed by HITACHI SU8220 scanning electron microscope (SEM) and FEI Talos F200S transmission electron microscope (TEM). X-ray diffraction (XRD) data were obtained using a Rigaku-Ultima IV powder X-ray diffractometer (Cu Kα, λ = 0.15405 nm). Raman spectra

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were gained by a Jobin-Yvon HR800 micro Roman spectrophotometer (λ=457.9 nm). The specifc surface areas and pore size distributions were evaluated by a MicroActive for 3Flex 4.02 analyzer at 77 K. 2.3 Electrochemical tests The anodes were composed of 80% active material, 10% polyvinylidene fluoride binder and 10% Super-P. The slurry was coated onto Cu foil and desiccated at 100 ℃ under vacuum for 8 h. The electrodes were cut into circular disks with a diameter of 12 mm and a total mass loading density of about 0.5 mg/cm2. Then the cells were assembled in an Ar-filled glove box, where both steam and oxygen levels were kept at less than 0.1 ppm. Coin cells (CR2032) were assembled with potassium foil as the counter/reference electrode, a glass fiber separator, and the electrolyte was composed of 0.8 M KPF6 in a mixture of ethylene carbonate, diethyl carbonate (1:1 v/v). All electrochemical performances were measured at room temperature. Cycling voltammetry (CV) measurements were carried out between 0.01 and 2.80 V on a electrochemical CHI 660D electrochemical workstation at different scan rates. Charge/discharge measurements were performed between 0.01 and 2.80 V vs. K/K+ on a Neware battery test system (BTS 7.6.x, Neware, Shenzhen, China). Electrochemical impedance spectroscopy (EIS) were performed on a Modulab XM DSSC photoelectrochemical test system in the frequency range from 100 kHz to 10 mHz. 3.Results and discussion NNSC is fabricated by utilizing commercial chelating resin and metal oxides as carbon source and hollow core templates, respectively. Details can be found in the experimental section. The uniqueness of the hierarchical NNSC structure is evaluated by using SEM and TEM. As shown in SEM image of Figure 1b, the NNSC exhibits a well-defined 3D interconnected network hierarchical porous structure and a uniform nanospherical morphology with a diameter of about 18 nm (Figure S1). The tight and loose aggregates of these nanospheres form the mesopores and macropores, respectively. These well-

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interconnected mesopores and macroporesin the 3D network give rise to interlaced transmission channels.[50] TEM image of Figure 1c further demonstrates the characteristic of interconnected networkstructured in NNSC. Moreover, high-resolution TEM (HRTEM) image of Figure 1d visibly confirms the network unit is a unique hollow nanosphere with microporous thin shell and mesoporous core. The shell thickness and core size are about 2 and 12 nm, respectively. Meanwhile, HRTEM image reveals a disordered pattern with localized short-range order and the interlayer spacing of d002 was measured to be 0.407 nm.

Figure 1. (a) Schematic diagram of NNSC. (b) SEM, (c) TEM and (d) HRTEM images of NNSC.

N2 adsorption/desorption measurement is used to quantitatively appraise the pore characteristic of NNSC. N2 adsorption/desorption isotherm (Figure 2a) shows the rapid increase of adsorption amount of 7

N2 at low relative pressure (P/P0), giving the evidence about micropores. Meanwhile, a weak hysteresis loop in the P/P0 range of ca. 0.40 ~ 0.90 and a slightly upswept rear edge at the P/P0 close to 1.0 can confirm the existence of the mesoporous and macroporous structures, respectively.[51-53] These results are consistent with those observed by SEM and TEM. According to the pore size distribution curve (Figure 2b), the size of the micropores of NNSC is mainly centered at 1.1 nm. The mesopores among the 3D network ranges from 2 to 50 nm with a maximum peak centered at 12 nm. The size of macropores is distributed in the range of 50~60 nm. The calculated BET surface area and total pore volume of NNSC is 644 m2 g-1 and 0.512 cm3 g-1, respectively. The micropore surface area and the external (i.e., meso/macroporous) surface area are accounting for 52.3% and 47.7% of the total surface area, respectively. Such a hierarchical pore structure ensures a large number of K+ is able to diffuse rapidly within the NNSC anode. Raman and XRD spectra are conducted to study the microcrystalline structure of NNSC’s carbon framework. It is found that NNSC exhibits two characteristic peaks located at around 1335 cm-1 (i.e., the defect-induced band) and 1572 cm-1 (i.e., the crystalline graphite band) in the Raman spectroscopy (Figure 2c). The integral area ratio of the defect-induced band and crystalline graphite band (ID/IG) and corresponding stack width (La) are calculated to be 1.60 and 2.72 nm, respectively, implying a typical amorphous carbon framework in NNSC.[54] As shown in the XRD patterns of Figure 2d, NNSC displays two broad peaks centered at 2θ ≈ 22o and 43o, which can be assigned to the crystallographic planes of (002) and (100) in amorphous carbon with a low graphitization degree. The (002) crystallographic plane demonstrates a large interlayer spacing of 0.407 nm for NNSC, which is consistent with the HRTEM result, and significantly larger than that of graphite (i.e., 0.335 nm).[55] Such a large interlayer spacing of the carbon framework is able to facilitate rapid K+ intercalation/deintercalation.

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Figure 2. (a) N2 adsorption/desorption isotherm, (b) pore size distribution, (c) Raman spectra and (d) XRD pattern of NNSC.

The unique characteristics, including the 3D interconnected macro-mesoporous structure, hollow nanospherical network unit, and large interlayer spacing of the carbon framework, enable NNSC to be a promise anode material for KIBs. To demonstrate the dominance of NNSC, its electrochemical performance is compared to that of a traditional amorphous carbon (e.g., microporous carbon, MC) and graphite. The BET surface area of MC and graphite is 1659 and 10 m2 g-1, respectively (Table S1). As shown in Figure 3a, capacity loss is observed in the initial cycles for all the samples. But their capacities become more and more stable in the following cycles. In comparison, the NNSC anode delivers far higher capacity than the MC and graphite anodes. For example, a high discharge capacity of 353 mAh g-1 can be achieved for NNSC anode after 3 cycles. This value is obviously higher than that of the MC anode (i.e., 9

128 mAh g-1) and the graphite anode (i.e., 30 mAh g-1). In addition, the capacity of the NNSC anode maintains about 213 mAh g-1 after 100 cycles, which is 2.4 and 20.2 times than that of the MC and graphite anodes, respectively. Similar high K+ storage capability in NNSC is also observed at high charge/discharge rates. It is noteworthy that a capacity retention ratio of about 92.3 % with a coulombic efficiency of ~100 % (Figure S2) at a high current density of 5000 mA g-1 can be obtained after 2000 cycles, giving a capacity decrease of only 0.004 % per cycle (Figure 3b). As far as known, such a low attenuation rate is one of the lowest values among all the reported anodes for KIBs (Figure 3c and Table S2), which demonstrates ultra-longterm cycle performance and makes it a competitive choice in KIBs. For gathering more evidence of cyclic stability, the morphology of the NNSC anode after cycling test was characterized. According to the SEM image of the NNSC anode after cycling test, the 3D nanonetwork structure is still retained, confirming the remarkable structural stability of the NNSC anode during charge-discharge process (Figure S3).

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8. Carbon nanotube (Ref. 49) 9. Potato biomass carbon (Ref. 38) 10. Hard-soft composite carbon (Ref. 12) 11. Oak-based hard-carbon (Ref. 42) 12. Highly disordered hard carbon (Ref. 3) 13. Hollow carbon nanospheres (Ref. 44)

Figure 3. (a) Cycling performance of NNSC at 100 mA g-1 compared with MC and graphite. (b) Capacity retention ratios and capacity of NNSC at a current density of 5000 mA g-1 for 2000 cycles. (c) Comparisons of average capacity decay and cycle number of carbon-based anodes of KIBs.

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Besides the long cycling life, the NNSC anode also holds an exceptional rate capability at different current densities. The average reversible capacities of the NNSC anode are 261, 225, 179, 165, 160, 142, 133, 122 and 108 mAh g-1 with increasing the current densities from 100 to 5000 mA g-1 (Figure 4a). The capability retention exceeds 40% with the 50-fold increase of the current density, exhibiting the excellent rate performance for the NNSC anode. After cycling at various current densities, the capacity is recovered to about 200 mAh g-1 when the current density is back to 100 mA g-1, which further proves its remarkable rate performance. In stark contrast, both the MC and graphite anodes show much inferior capacities at various current densities. Such a difference becomes more evident at large current densities, demonstrating the superiority of NNSC in K+ storage, especially at high rates. The rate performance of the NNSC is also better than that of most reported carbon-based anode materials (Figure 4b and Table S3). To better gain insight into the reason of the superior rate capability in NNSC, the K+ diffusion behaviors is investigated by CV and EIS. A diffusion coefficient (D) is usually obtained by means of the RandlesSevcik equation (Equation (1)).[32, 56] Ip = 0.4463nFAC

𝑛𝐹𝑣𝐷 𝑅𝑇

(1)

where Ip is the peak current, n is the transferring electron numbers, F is the Faraday constant, A is the connecting area of electrolyte, C is the concentration of K+ in the electrolyte, ν is the scan rate, R is the gas constant, T is the temperature. Equation (1) can be simplified to obtain Equation (2) when the R, T and F are equal to 8.314 J mol-1 K-1, 298 K and 96485.3 C mol-1, respectively. Ip = 268600 n 1.5ACD 0.5v 0.5 (2) From Equation (2) we can know that the Ip is a positive proportional function of ν0.5, and the D is proportional to the square of the slope of the ν0.5-Ip curve. Based on the CV curves tested at different scan rates from 0.1 to 0.9 mV s-1 (Figure S4-S6), the v0.5-Ip curve can be obtained. As shown in Figure 4c, the v0.5-Ip curve slope of the NNSC anode is calculated to be 0.4175, which is significantly higher than that

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of AC (i.e., 0.1408) and graphite (i.e., 0.1002). Then according to Equation (2), the calculated K+ diffusion coefficient of the NNSC anode is 8.8 and 17.3 times higher than that of the MC and graphite anodes, respectively, demonstrating faster K+ diffusion rates in the hollow nanosphere-interconnected network structure of NNSC. Figure 4d shows the Nyquist plots obtained from EIS, and the inset is the corresponding equivalent circuit model. Generally, the intercept impedance on the Z′ axis represents the ohmic resistance (Rs), which is related to the resistance of the electrolyte and electrode. The diameter of the semicircle usually represents charge transform resistance (Rct).[57, 58] As shown in Figure S7, the Rs of all the anodes are similar, which demonstrates that the effect of the electrolyte and electrode can be negligible. The Rct of the NNSC anode (i.e., 453.4 Ω) is obviously smaller than that of the MC (i.e., 754.5 Ω) and graphite (i.e., 887.2 Ω) anodes, indicative of higher charge transfer efficiency in NNSC. Meanwhile, the slope of the NNSC anode is steeper than that of the MC anode in the low-frequency region, further confirming faster diffusion rates at the electrode-electrolyte interface.[59] Based on the above analysis and discussion, the NNSC demonstrates much superior lifespan and K+ diffusion rates to the MC and graphite, which is mainly attributed to the advanced structure in NNSC. In general, micron/millimeter-scaled microporous carbon particles structure for MC makes a long ion diffusion distance (i.e., > 5 μm) and a strong ion-transport resistance.[60] As for graphite, its small interlayer

spacing

and

poor

cycling

stability

causes

it

difficult

for

the

process

of

intercalation/deintercalation with large amounts of K+. As a comparison, NNSC with hollow carbon nanospheres structure can reduce the ion diffusion distance (i.e., half of the shell thickness) to improve the ion diffusion rate, and the hierarchical pore structure with nanosphere-interconnected network provides sufficient space and synergistic effect for the large amount of K+ migration at high rate.[61] Furthermore, the large interlayer spacing of the carbon framework can accommodate rapid K+ intercalation/deintercalation in the carbon layer without obvious fracture (Figure 4e). The synergistic effect of three structural advantages of NNSC results in the significant improvement of KIB performance. 13

NNSC MC Graphite

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Figure 4. (a) The rate capability of NNSC, MC and graphite anode at current densities from 100 to 5000 mA g-1. (b) Comparisons of rate performance of carbon-based anodes of KIBs. (c) v0.5-Ip curves and the corresponding linear fits. (d) Electrochemical impedance spectra and the corresponding equivalent circuit of NNSC, MC and graphite. The inset shows the corresponding equivalent circuit model. (e) Schematic diagram of the reversible K+ and electron transport process of NNSC anodes. 14

4.Conclusions To summarize, exceptional potassium storage performance was achieved by using nanonetworkstructured carbon material as KIB anodes. A ultra-stability with a low capacity decay rate of 0.004 % per cycle over 2000 cycles was delivered by the NNSC anodes. Furthermore, remarkable rate performance with a capacity of 261 mAh g-1 at 100 mA g-1 and 108 mAh g-1 at 5000 mA g-1 was demonstrated. These performances are superior to most of previously reported carbon-based anodes in KIBs. The excellent electrochemical performance is mainly attributed to the 3D hollow carbon nanosphere-interconnected network structure and the enlarged interlayer spacing, which are conducive to accommodate the volume change and improve the K+ diffusion efficiency.[62-64] In short, we demonstrate a new strategy to improve undesirable structural stability and kinetics in carbon nanomaterials for promoting the KIBs performance, which is crucial to the potential cost-effective and high-performance energy storage system of KIBs.

ACKNOWLEDGMENT: The authors gratefully acknowledge financial support from the project of the National Natural Science Foundation of China (51972121, 51602107, U1501242, 21571066 and 21671069), Guangdong Basic and Applied Basic Research Foundation (2019A1515011502), Tip-top Scientific and Technical Innovative Youth Talents of Guangdong Special Support Program (2017TQ04C419) and Program for Pearl River New Star of Science and Technology in Guangzhou (201710010104).

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Graphical abstract

Engineering of Nanonetwork-Structured Carbon to Enable High-Performance Potassium-Ion Storage Weicai Zhang1, Yinjia Yan1, Zhuohao Xie, Yinghan Yang, Mingtao Zheng, Hang Hu, Yong Xiao, Hanwu Dong, Yingliang Liu*, Yeru Liang*

Benefiting from the well-defined three-dimensional interconnected hierarchical porous nanonetwork, hollow carbon nanosphere and large interlayer spacing, our nanonetwork-structured carbon shows surprising performance of potassium ion battery.

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CRediT author statement Weicai Zhang: Methodology, Formal analysis, Investigation, Data Curation, Writing-Original draft preparation, Writing-Reviewing and Editing. Yinjia Yan: Formal analysis, Investigation, Data curation, Writing-Original draft preparation, WritingReviewing and Editing. Zhuohao Xie: Formal analysis, Investigation, Writing-Original draft preparation. Yinghan Yang: Formal analysis, Investigation. Yong Xiao: Writing-Reviewing and Editing, Funding acquisition. Mingtao Zheng: Writing-Reviewing and Editing, Funding acquisition. Hang Hu: Writing-Reviewing and Editing. Hanwu Dong: Writing-Reviewing and Editing. Yingliang Liu: Conceptualization, Writing-Reviewing and Editing, Supervision, Funding acquisition. Yeru Liang: Conceptualization, Validation, Resources, Writing-Reviewing and Editing, Supervision, Project administration, Funding acquisition.

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