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Hard–Soft Composite Carbon as a Long-Cycling and High-Rate Anode for Potassium-Ion Batteries Zelang Jian, Sooyeon Hwang, Zhifei Li, Alexandre S. Hernandez, Xingfeng Wang, Zhenyu Xing, Dong Su,* and Xiulei Ji* remain underexplored. To minimize maintenance cost, cycling life of beyondlithium devices must be maximized. This can be challenging for KIBs as K-ions are much larger than Li-ions; thus, their insertion/extraction into/from the electrodes often undergo a large strain and may consequently suffer irreversibility. Therefore, to move forward the KIB technology, it is a top priority to improve its cycling life. K-ions as charge carriers are heavier than Na-ions and Li-ions; therefore, in terms of the specific capacities of individual electrodes, electrodes of KIBs are not highly competitive compared to those of LIBs or even NIBs. However, when considering the mass at the whole device scale for stationary storage purposes, the weight burden of K-ions is truly not significant.[5] Further+ more, K /K is of a lower redox potential in nonaqueous electrolyte compared to Na+/Na and even Li+/Li, which leaves an ample voltage range for the design of full-cell KIBs.[6] Namely, a cathode of NIBs with a modest potential, e.g., 3.0 V versus Na+/Na, will exhibit a more desirable potential near 3.5 V versus K+/K. When considering the electrodes for KIBs, for the cathode side, there have been studies on Prussian blue analogs, layered oxides, and organic solids or polymers.[7] As for the anodes, several carbon allotropes have been investigated for K-ion storage.[8] For graphite, even though it does not exhibit a meaningful capacity for Na-ion storage, i.e., 35 mAh g−1, unless solvent co-intercalation occurs, graphite hosts K-ions reversibly in the electrochemical insertion/extraction environment.[9] Graphite anode delivers a high reversible capacity of ≈273 mAh g−1, close to its theoretical value of 279 mAh g−1 when KC8 is formed during potassiation.[8a,10] However, likely due to the large volume expansion by ≈61% upon potassiation, graphite suffers poor rate capability and rapid capacity fading.[8a] To address the drawbacks of graphite KIB anode, we also conducted initial studies on soft carbon, hard carbon, and a new polynanocrystalline graphite as K-ion hosts.[8a,b] However, to date, it remains elusive about the comparative properties between hard carbon and soft carbon.[8a,b,11] This has caused some confusion when considering the choice of the anode for a full-cell device. The goal of the study is to further identify the desirable properties of nongraphitic carbons and their correlation to K-ion storage performance. We synthesized three nongraphitic carbons: hard carbon spheres (HCS), soft carbon (SC), and an HCS-SC composite
There exist tremendous needs for sustainable storage solutions for intermittent renewable energy sources, such as solar and wind energy. Thus, systems based on Earth-abundant elements deserve much attention. Potassium-ion batteries represent a promising candidate because of the abundance of potassium resources. As for the choices of anodes, graphite exhibits encouraging potassium-ion storage properties; however, it suffers limited rate capability and poor cycling stability. Here, nongraphitic carbons as K-ion anodes with sodium carboxymethyl cellulose as the binder are systematically investigated. Compared to hard carbon and soft carbon, a hard–soft composite carbon with 20 wt% soft carbon distributed in the matrix phase of hard carbon microspheres exhibits highly amenable performance: high capacity, high rate capability, and very stable long-term cycling. In contrast, pure hard carbon suffers limited rate capability, while the capacity of pure soft carbon fades more rapidly.
1. Introduction Metal-ion batteries beyond lithium-ion batteries (LIBs) have attracted tremendous interest in the past decade.[1] Along the line of pushing the limits of energy density, lithium-metal batteries, such as Li-O2 batteries and Li-S batteries, remain as the focus of attention.[2] Nevertheless, to address the demand of stationary energy storage, i.e., for the grid, energy density is no longer the highest priority of considerations, whereas the primary metric is the levelized cost, which includes the acquisition cost and the maintenance cost. To decrease the acquisition cost, usage of rare elements should be minimized in storage batteries, where lithium certainly suffers its poor abundance. In contrast, sodium and potassium are much more abundant than lithium by any standards.[3,4] To this end, potassium-ion batteries (KIBs) bear remarkable potential, which, unfortunately,
Dr. Z. Jian, Z. Li, A. S. Hernandez, X. Wang, Dr. Z. Xing, Prof. X. Ji Department of Chemistry Oregon State University Corvallis, OR 97331, USA E-mail:
[email protected] Dr. S. Hwang, Dr. D. Su Center for Functional Nanomaterials Brookhaven National Laboratory Upton, NY 11973, USA E-mail:
[email protected] DOI: 10.1002/adfm.201700324
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(HCS/SC: 80/20, mass ratio), and systematically studied their K-ion storage properties. We discover that when both hard carbon and soft carbon phases are vicinal on the same electrode particles, the HCS-SC composite carbon is able to integrate the advantages of soft carbon in high rate capability and of hard carbon in long cycling life. Furthermore, we employed sodium carboxymethyl cellulose (CMC) as the binder to replace the commonly used polyvinylidene fluoride (PVdF) in electrode fabrication, which greatly enhances the first-cycle coulombic efficiency, as it does for graphite KIB anode.[10c] More importantly, we found that different nongraphitic carbon structures can be differentiated by their performance in K-ion storage. Both HCS and composite HCS-SC present much more stable cycling performance than SC, while SC and HC-SC exhibit greater rate capability than HCS.
2. Results and Discussion HCS is obtained by a hydrothermal reaction of dissolved sucrose, followed by pyrolysis.[12] Scanning electron microscope (SEM) image in Figure 1a shows the typical morphology of HCS, which are spherical particles sized from ≈5 to 10 µm with smooth surface. SC is synthesized by pyrolysis of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA, C24H8O6).[13] Figure 1b displays the morphology of SC, which comprises micrometer-sized rods as well as scattered small particles. As for the HCS-SC composite, it is prepared by ball milling the mixture of HCS (1.6 g) and PTCDA (0.8 g), where the mixture is annealed afterward at 900 °C. The HCS/SC ratio in the resulting composite carbon is about 80:20 in mass, as the carbon yield of PTCDA pyrolysis is ≈50%, as revealed in the thermogravimetric analysis (TGA) in Figure S1 (Supporting Information). In the composite carbon, SC small particles, sized around 50 nm, are decorated in the matrices of HCS larger spherical particles, thereby serving as “plugs” into the surface pores of HCS, and there are also some SC “needles” tethered onto the surface of HCS particles. PTCDA-derived carbon is a soft carbon defined by its graphitizable nature. When being annealed at 1600 °C, the X-ray diffraction (XRD) pattern of the carbon displays a sharp (002) peak, as shown in Figure S2 (Supporting Information), which confirms its soft carbon nature.
In fact, the reason that SC small particles can find spots to be implanted into the HCS particles is that hydrothermally formed HCS exhibits a relatively large Brunauer–Emmett–Teller (BET) surface area of 175 m2 g−1, where the PTCDA phase could find pores to be smeared on during ball-milling. Interestingly, the addition of SC phase significantly reduces the surface area of HCS to 20 m2 g−1 of HCS-SC, comparable to 13 m2 g−1 of pure SC (Figure 2a). The pore volume decreases from 0.075 to 0.004 cm3 g−1 after the addition of the SC phase. XRD patterns shown in Figure 2b reveal the modest graphitic degrees of different samples. SC presents a relatively sharp (002) peak centered at 2Θ of 25.0°; HCS shows a broad peak centered at 2Θ of 22.3°; HCS-SC composite displays a broad peak with the similar shape as HCS, but this peak is slightly right-shifted to 2Θ 23.8°, compared to HCS. It is expected that the large average d-spacing of HCS and HCS-SC of 0.40 and 0.37 nm, respectively, may be beneficial to host large K-ions. We tested the K-ion storage properties of all samples with potassium metal as the counter/reference electrode. Figure 3 shows their potassiation/depotassiation curves at 0.1 C (1 C rate is defined as 279 mA g−1). The first potassiation/depotassiation capacity values of HCS are 344/260 mAh g−1, where the initial coulombic efficiency is 76%, much higher than that of the same HCS electrode with PVdF as the binder (57%).[8b] The galvanostatic curves present two slopes, indicative of K-ion storage inside hard carbon’s different substructures, which is suggested by the Franklin’s House-of-Cards structural model.[14] As for SC, the first potassiation/depotassiation capacities are 392/246 mAh g−1 with an initial coulombic efficiency of 63%, which is lower than that of HCS, but still much greater than the same SC electrode when using PVdF binder (≈49%).[8a] Considering that the surface area of SC (13 m2 g−1) is much smaller than that of HCS (175 m2 g−1), the lower coulombic efficiency of SC cannot originate from the formation of surface-area-derived solid electrolyte interphase (SEI), but from a unique K-ion trapping mechanism in soft carbon. In the first potassiation process of SC, there is a short plateau at ≈0.8 V versus K+/K, which disappears in the following depotassiation. The low initial coulombic efficiency of SC is certainly associated with this irreversible plateau. The galvanostatic curves of HCS-SC resemble those of HCS, where the first potassiation/depotassiation capacity values are 389/261 mAh g−1, and the coulombic efficiency is 67%, which is between those of HCS and SC.
Figure 1. SEM images of a) HCS, b) SC, and c) HCS-SC.
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Figure 2. a) BET results, and b) XRD patterns of HCS, SC, and HCS-SC.
At low current rates of 0.1 and 0.2 C, all the materials show the similar reversible capacity, as shown in Figure 4a. However, when the current rate increases, the capacity of HCS drops faster than the other two materials. At the intermediate current rates of 0.5, 1, and 2 C, SC and HCS-SC exhibit the capacities of around 230, 210, and 190 mAh g−1, respectively, compared to HCS with 193, 157, and 135 mAh g−1 at the same rates. It is worth noting that in HCS-SC, only 20 wt% of SC could improve the rate performance as such. At high current rates of 5 and 10 C, SC shows the highest capacity values, retaining 167 and 121 mAh g−1, respectively, compared to 121 and 81 mAh g−1 of HCS-SC, and 81 and 45 mAh g−1 of
HCS. Figure S3 (Supporting Information) shows the potassiation/depotassiation curves at 10 C rate, where SC shows the lowest level of polarization. To understand the disparity of rate capability among samples, we tested the electronic conductivity of HCS and SC pellets, which are 1.7 and 5.7 S m−1, respectively. The greater electronic conductivity of SC certainly plays a role for its superior rate capability. Importantly, the capacity contribution above 0.2 V versus K+/K from the SC electrode is greater than that from HC, which correlates to their difference in capacity values obtained at high current rates. Thus, with the same levels of overpotential, HC is subject to more capacity loss from the low-potential region than soft carbon.
Figure 3. The first three galvanostatic cycles of a) HCS, b) SC, and c) HCS-SC.
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Figure 4. a) Rate cycling performance, b) long-term cycling performance at 0.2 °C of HCS, c) corresponding potassiation/depotassiation curves at different cycles in (b), and d) long cycling performance at 1 C rate of HCS, SC, and HCS-SC.
Figure 4b shows the excellent cycling performance of HCS with the CMC binder at 0.2 C, which does not fade in the first 200 cycles and exhibits an excellent capacity retention of 89% even after 440 cycles. The galvanostatic curves from different cycles over the long-term cycling are almost identical, and the polarization degree increases only slightly after 440 cycles, which is quite surprising considering that K-metal counter/reference electrode may contribute to some ohmic resistance after long cycling as well (Figure 4c). The cycling performance of the three carbon materials at 1 C rate is compared in Figure 4d, where both HCS and HCS-SC exhibit excellent cycling stability. HCS-SC delivers a high capacity of ≈200 mAh g−1 with impressive capacity retention of 93% after 200 cycles; HCS delivers a lower capacity of ≈160 mAh g−1 with good capacity retention of ≈90% after 200 cycles. In the case of SC, which is known to have a structure more graphite-like than hard carbon, the capacity decreases from 215 to 118 mAh g−1 after 200 cycles at 1 C. Note that graphite electrodes with CMC binder also exhibit poor longterm cycling performance, whose capacity fades to nearly nil after 145 cycles (see Figure S4, Supporting Information). Therefore, it is considered that the highly stable cycling performance of HCS-SC and HCS is attributed to the unique nongraphitic structure with much disorder along the c-axis. Overall, the composite HCS-SC delivers not only the highest capacity at 0.1 C, good rate capability, but also stable cycling performance. It is intriguing why soft carbon fades much faster than hard-carbon-based counterparts for K-ion storage. In order to elucidate the correlation between carbon structures and their potassiation properties, we acquired transmission electron microscopy (TEM) images of pristine and potassiated HCS
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and SC samples, as shown in Figure 5. In the hard carbon, the short turbostratic nanodomains with large (002) d-spacing are randomly oriented and cross-linked, forming a “molecular sieve” framework with a relatively low density of 1.6 g cc−1 (measured by the method of Archimedes), in contrast to ≈2.2 g/cc of bulk graphite. After potassiation, the fringes of the short turbostratic nanodomains are still visible, where it appears nearly the same as the pristine structure (Figure 5b). The insets of Figure 5a,b show the corresponding selected area electron diffraction (SAED) patterns before and after potassiation, respectively. No big difference is observed, indicating the negligible structural change after potassiation for hard carbon. The intact local structure of hard carbon after potassiation may explain the good cycling performance. As for the pristine soft carbon, TEM images and the corresponding SAED pattern reveal a typical turbostratic structure with a great number of graphenic layers stacked and aligned, which is essentially a quasigraphitic structure (Figure 5d). However, the graphenic fringes of the soft carbon’s structure completely vanish under TEM after potassiation (Figure 5e), as further evidenced in the comparison of their SAED patterns (insets in Figure 5d,e). In their radial intensity profiles, as for HCS, there is an absence of the (002) peak (Figure 5c), and this (002) peak is also weak for the SC (Figure 5f), where it is difficult to draw a conclusion on its shift upon potassiation. Interestingly, the insertion of K-ions in SC causes the (100) peak to vanish, whereas potassiation does not have an impact on the (100) peak of HCS. The results indicate that major local structural transformation takes place in SC electrodes upon potassiation, which may lead to its poor cycling performance.
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5 h, which generated black powder. After drying, the obtained powder was annealed at 1100 °C for 5 h in a tube furnace under Ar. Soft carbon was obtained by pyrolysis of PTCDA in a tube furnace at 900 °C for 5 h under Ar. HCS-SC composite was prepared by pyrolysis of a balled milled mixture of HCS (1.6 g) and PTCDA (0.8 g) at 900 °C for 5 h under Ar. Material Characterization: Powder XRD patterns were collected with Cu Kα radiation at 40 kV and 40 mA (Rigaku). TGA was performed on a TA Instrument SDT-Q600 thermal analyzer, where PTCDA was heated under Ar from room temperature to 1100 °C at a heating rate of 5 °C min−1. The morphology and structures of samples were examined by an FEI NOVA 230 highresolution SEM and a JEOL JEM-2100F TEM. The specific surface area was calculated by the BET method using the adsorption branch of N2 sorption isotherms. Electrochemical Tests: The working electrodes were prepared by mixing 80 wt% carbon active mass, 10 wt% carbon additive, and 10 wt% sodium CMC as the binder in deionized water. The obtained homogeneous slurry was evenly pasted onto Cu foil and dried at 100 °C for 8 h under vacuum. The active mass loading was around 2 mg cm−2. Potassium metal foil was used as the Figure 5. TEM images of a) pristine and b) potassiated HCS samples with insets showing the counter and reference electrode, and glass fiber corresponding SAED patterns, respectively, c) comparison of the radical intensity profiles of these SAEDs. TEM images of d) pristine and e) potassiated SC samples with insets showing the corre- paper was−1used as a separator. The electrolyte was 0.8 mol L KPF6 in ethylene carbonate and diethyl sponding SAED patterns, respectively, f) comparison of the radical intensity profiles of these SAEDs. carbonate (EC: DEC = 1:1 by volume). CR2032 coin cells were fabricated in an Ar-filled glove box. The electrochemical measurements were performed on 3. Conclusion an Arbin BT2000 system at room temperature, where the voltage range was from 0.01 to 2 V versus K+/K.
Hard carbon spheres, soft carbon, and hard carbon–soft carbon composite were prepared to understand the structure– property correlations of carbon anodes in K-ion storage. An important insight from this study is that the local structures of soft carbon and hard carbon response very differently to K-ion intercalation. The long-term cycling performance can well differentiate the structures of hard carbon from soft carbon, where the former is much more stable than the latter in K-ion storage. This reveals that nongraphitic structure with a great level of disorder along the c axis is critical to avoid capacity fading of carbonaceous KIB anodes. Interestingly, soft carbon is a superior high-rate anode to hard carbon, where even at 10 C, soft carbon anode can exhibit a high capacity of 121 mAh g−1 in contrast to 45 mAh g−1 of hard carbon. To integrate advantages of both hard and soft carbons, we designed a composite carbon, HCS-SC, that comprises both phases of hard carbon and soft carbon well mixed, where the latter is smeared into the pores of the former by ball milling. With only 20% of soft carbon in this composite, the rate capability is increased by a great deal compared to hard carbon. Most importantly, HCS-SC keeps the ability of hard carbon in stable long-term cycling.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements X.J. thanks the financial supports from U.S. National Science Foundation, Award Number: 1551693. Electron Microscopy work was performed in the Center for Functional Nanomaterials, Brookhaven National Laboratory, which was supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, under Contract No. DE-SC-00112704.
Conflict of Interest The authors declare no conflict of interest.
Keywords composite carbon, hard carbon, nongraphitic carbon, potassium-ion batteries, soft carbon
4. Experimental Section Material Synthesis: Hard carbon spheres were synthesized by a hydrothermal reaction, where an aqueous solution (40 mL) with 6.4 g sugar was filled into an autoclave (45 mL), and was heated at 195 °C for
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Received: January 19, 2017 Revised: March 2, 2017 Published online:
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