Phenolic resin-based carbon microspheres for potassium ion storage

Journal Pre-proofs Full Length Article Phenolic Resin-based Carbon Microspheres for Potassium Ion Storage Shuo Wang, Yanyan Li, Fanteng Ma, Xiaozhong Wu, Pengfei Zhou, Zhichao Miao, Peibo Gao, Shuping Zhuo, Jin Zhou PII: DOI: Reference:

S0169-4332(19)33621-9 https://doi.org/10.1016/j.apsusc.2019.144805 APSUSC 144805

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Applied Surface Science

Received Date: Revised Date: Accepted Date:

17 September 2019 15 November 2019 19 November 2019

Please cite this article as: S. Wang, Y. Li, F. Ma, X. Wu, P. Zhou, Z. Miao, P. Gao, S. Zhuo, J. Zhou, Phenolic Resin-based Carbon Microspheres for Potassium Ion Storage, Applied Surface Science (2019), doi: https:// doi.org/10.1016/j.apsusc.2019.144805

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Phenolic Resin-based Carbon Microspheres for Potassium Ion Storage Shuo Wanga, Yanyan Lia, Fanteng Maa, Xiaozhong Wua, Pengfei Zhoua, Zhichao Miaoa, Peibo Gaob, Shuping Zhuoa, Jin Zhoua* a

School of Chemistry and Chemical Engineering, Shandong University of Technology, Zibo

255049, P. R. China b

School of Physics and Optoelectronic Engineering, Shandong University of Technology, Zibo

255049, PR China *corresponding author. E-mail: [email protected]

Abstract: Potassium ion batteries (PIBs) have received widespread attention nearly four years due to sufficient potassium resources and similar chemical properties of potassium to lithium. Several works reported that N-doping on the carbon electrodes could efficiently improve the storage capacity of potassium ions. Herein, we synthesized two series of carbon spheres with the main difference is with or without N-doping. It is found that the storage of potassium ions is mainly determined by the crystal structure instead of N-doping. N-doping seems to affect the ion diffusion process, but the total capacity is still determined by the (002) lateral size of the carbon materials (La). The specific capacity increases as La increases. The N-doped carbon sphere with larger La of 8.32 nm, delivers a higher reversible capacity of 241 mAh g-1 at a current density of 25 mA g-1. And it still remains 121 mAh g-1 after 100 cycles at a current density of 100 mA g-1. Keywords: carbon electrode; potassium ion batteries; potassium ions storage; N-

doping; crystallite size 1. Introduction The contradiction between fossil resources and environmental pollution is becoming more and more serious with the development of society and economy. Renewable energies (e.g., wind energy, solar energy) have attracted much attention, which requires low-cost and high-performance energy storage systems (EESs). Lithium-ion batteries (LIBs) have become the most promising candidates in EESs due to their high energy and power densities

[1, 2]

. But the continuous reduction and uneven distribution of

lithium resources have forced us to find the alternatives for LIBs. Potassium is a cheaper, abundant and uniformly distributed alkali metal resource compared to lithium [3-5]

. In addition, based on the more negative redox potential of K/K+ (-2.79 V for Li/Li+

and -2.88 V for K/K+ in propylene carbonate), potassium ion batteries (PIBs) may provide higher cell potentials

[6]

. More importantly, the smaller stokes radius of K+

improves the ionic conductivity of the electrolyte, which can enhance the batteries’ rate performance [7]. Thus, PIBs may be a new replacement in EESs in the future. Recently, carbonaceous materials have been introduced as anode electrodes for PIBs [8-12]

. Jian et al. found that potassium ion (K+) can electrochemically intercalate into

graphite by a three-stage reversible conversion, achieving a reversible capacity of 273 mAh g-1 at 0.1C [13]. However, because of the large radius of K+, graphite experiences a huge volume expansion during potassiation, resulting in 50% capacity fade after 50 cycles. In contrast, hard carbons display improved performance of rate and cycling stability owing to their larger interlayer spacing and stable structure [14, 15]. For example,

hard carbon microspheres reported by Jian et al. presented 262 mAh g-1 at 0.1C, and the capacity fade was only 17% after 100 cycles [16]. However, there is still much room to improve in the performance of carbonaceous materials, especially in reversible capacity. Heteroatoms doping is considered to be an effective strategy to improve the electrochemical properties of carbonaceous materials

[17, 18]

. N-, O-, F-, and P-doped

carbonaceous materials used in PIBs have been reported [19-22]. Among the heteroatoms, the radius of nitrogen atom is close to the carbon’s, making it easier to replace the carbon atoms in the lattice of the carbonaceous materials, thereby forming an N-doped carbon material [23]. Share at al. used In-situ Raman to illustrate the mechanism of the capacity enhancement due to distribute storage at local nitrogen sites in a staged KC8 compound

[24]

. Xu at al. reported N-doped carbon nanofibers with a capacity of 248

mAh g-1 at 25 mA g-1. They used the first-principles calculation to demonstrate that the pyrrolic and pyridinic N species lead to stronger K-adsorption than the graphitic N species [25]. These studies attributed the improvement of electrochemical performance to the high electron affinity of nitrogen atoms or vacancies and defects of the carbon structure resulted from N-doping. Actually, the crystal structure of the carbon materials is a very important factor for their storage performance of alkali metal ions [26, 27]. But few researchers consider the effect of the carbon crystal structure on potassium storage when studying the N-doped carbon electrode materials. In this work, we synthesized two series of carbon spheres with almost the same morphology which are used as anode electrode materials in PIBs. Although these

carbon spheres have been studied as Li/Na ion battery anode

[28-30]

, few studied as

potassium ion battery anode. And it is still a good material to detailed study of ion storage mechanisms due to its easy and controllable synthesis. The as-prepared Ndoped carbon sphere with a large La of 8.32 nm delivers a high reversible capacity of 241 mAh g-1 at a current density of 25 mA g-1. The main difference between the two series of carbon spheres we prepared here is whether N-doped or not, thus making us comparatively study the effect of crystal structure and N-doping on the K+ storage. It is found that the crystal structure has a more significant effect on improving potassium ion storage than N-doping. We further constructed the relationship between the crystalline lateral size (La) and the specific capacity and found that the specific capacities are roughly proportional to the value of La. Although high nitrogen content seems to induce more diffusion capacity, the total capacity is mainly determined by La. 2. Experimental section 2.1 Materials preparation All the raw materials, including resorcinol, 3-aminophenol, ammonia aqueous solution, aqueous formaldehyde solution (37 wt%) and so on, were purchased from Aladdin Ltd. (Shanghai, China), and were used as received without further purification. Resorcinolformaldehyde resin spheres (RF) and 3-aminophenol-formaldehyde resin spheres (APF) were synthesized based on an extension of the Stöber method

[31]

. In order to

synthesize RF, 1.25 mL of ammonia aqueous solution (NH3·H2O, 25 wt%) was mixed with a solvent containing 800 mL of deionized water (H2O) and 50 mL of absolute ethanol (EtOH). After stirring at 30 °C for 1 h, 3.5 mL of aqueous formaldehyde

solution (37 wt%) and 2.5 g of resorcinol were added into the above solution and further reacted for 24h, and subsequently heated for 24 h at 80 °C. The RF was separated from the reaction mixture by centrifugation and dried under vacuum at 50 °C for 24 h. APF was synthesized by a similar method. 2.5 g of 3-aminophenol and 1.25 mL ammonia aqueous solution were dissolved to a mixed solvent containing 350 mL H2O and 50 mL EtOH. The mixed solution was treated in the same procedure as RF. Then RF or APF is heated under N2 atmosphere at the target temperature for 4 h with a heating rate of 1 °C min-1. For convenience, the obtained carbon spheres and N-doped carbon spheres were denoted as CS-x and NCS-x, in which x represents for heating temperature (600, 700, 800 and 1000 °C). 2.2 Material characterizations The morphology and crystalline structure of as-prepared carbon spheres were characterized by using scanning electron microscopy (SEM, Sirion 200 FEI, Netherlands) and transmission electron microscope (TEM, JEM2100 JEOL, Japan). Surface chemical properties were characterized by energy dispersive spectroscopy (EDS, BRUKER AXS) and X-ray photoelectron spectroscopy (XPS, Escalab 250, USA). X-ray diffraction (XRD, Brucker D8 with advance diffractometer with Cu Kα radiation) and Raman spectra (LabRAM HR800 from JY Horiba with laser excitation at 532 nm) were used to characterize crystal structure. TGA (Discovery SDT 650) was performed at a heating rate of 10 °C min-1 at room temperature to 1000 °C under N2 flow. Nitrogen sorption measurements were conducted at 77K using ASAP 2020 analyzer (Micromeritics, USA). The carbon materials were degassed at 350 °C for 4 h

before sorption test. The values of specific surface area were determined according to the Brunauer-Emmett-Teller (BET) theory. The pore size distributions were obtained from the adsorption isotherms by using the nonlocal density functional theory (NLDFT) model and assuming a slit-shape pore. 2.3 Electrochemical measurements In order to investigate the effect of the nitrogen doping and the crystal size on potassium ion storage. The electrochemical tests were evaluated using CR2032 coin test cells. The working electrodes were made by active materials, acetylene black and polyvinylidene fluoride (PVDF) with a weight ratio of 8:1:1 in N-methyl-2-pyrrolidinone (NMP). Then the ground slurry was coated onto a copper foil and dried in a vacuum oven at 80 °C overnight. Each electrode contains about 1.5 mg of active materials. The half-cell was assembled with potassium metal as the counter and reference electrode. A glass microfiber filter (Whatman GF/D) was used as a separator, and 0.8M KPF6 in a mixture of ethylene carbonate (EC): Diethyl carbonate (DEC) in a volume ratio of 1:1 was used as the electrolyte. The cell was assembled in a glove box under an argon atmosphere with concentrations of moisture and O2 contents below 0.1 ppm. The cyclic voltammetry (CV) measurements were performed on CHI760E from 0.1 to 5 mV s-1. The galvanostatic charge-discharge test was performed on a Neware battery test system in the voltage range of 0.01-3 V at room temperature. 3. Results and discussions 3.1 Materials characterization Fig. 1 SEM images of (a) CS-600, (b) NCS-600, (c) CS-800, and (e) NCS-800; (e-f)

TEM images of NCS-1000; (g-h) EDS element mapping of NCS-1000 As shown in Fig. S1, the RF and APF spheres possess an average diameter of about 400 nm. In Fig. 1(a-d), both the CS and NCS carbons still remain a standard spherical shape with an average diameter of about 300 nm after carbonization. The shrink of diameter is due to the further polymerizing and thermal degrading of resin during carbonization. The NCS-1000 sample was further characterized by TEM observation (Fig. 1e), which highlights the integrity of NCS-1000 with a diameter of about 300 nm. As shown in the high-resolution TEM image (Fig. 1f), the NCS-1000 sample is a typical amorphous carbon. Element mapping images (Fig. 1(g-h)) demonstrate the incorporation of N atoms and their even distribution over the entire carbon spheres. Fig.2 (a) N2 adsorption-desorption isotherms for NCS; (b) pore size distribution of NCS; (c) pore size distribution of CS; (d) TG and DSC curves of RF and APF resins The N2 adsorption/desorption behaviors of the NCS (Fig. 2a) and CS (Fig. S2) samples were measured. All the adsorption curves show an isotherm of type I with an obvious adsorption increment at a higher relative pressure (P/P0>0.9). The adsorption quantity of the NCS samples decreases with increasing carbonization temperature, but the adsorption quantity of the CS samples has no clear relationship with the carbonization temperature. The BET surface areas of CS-600, CS-700, CS-800, and CS-1000 are 483.2, 515.7, 518.2, and 414.1 m2 g-1, while those of NCS-600, NCS-700, NCS-800, and NCS-1000 are 356.2, 322.1, 201.2, and 60.6 m2 g-1, respectively. Clearly, the CS samples show higher BET specific surface area than the NCS samples. As shown in Fig. 2(b-c), most of the pores are micropores with a size less than 2 nm. In order to

explain the changing trend of the porosity, TGA measurement was conducted under an N2 atmosphere. Fig. 2d shows the TG and DSC curves of the samples of RF and APF. According to the pyrolysis mechanism that Ouchi proposed [32], three stages of pyrolysis may occur in this process, including the formation of additional crosslinks, the scission of crosslinks, and stripping of hydrogen atoms from the aromatic structure. From the DSC curves, both the CS and NCS samples show an endothermic peak below 200 °C, which is attributed to the evaporation of water adsorbed by the resin spheres. Since RF resin has more phenolic hydroxyl groups, which allows it to bind more water, thus exhibiting a higher weight loss than the APF resin in this stage. In the range of 200400 °C, further crosslink may occur along with the production of water molecule resulting in a further weight loss. However, the pristine APF resin may possess a much higher degree of cross-linking than the RF resin due to the p-π conjugation effect between the amino groups and benzene rings

[33]

. That is the reason for the fact that

APF shows less weight loss in the range of 200-400 °C. This can further explain the observation that APF exhibits much lower weight loss (~20 wt%) than RF (~40 wt%) above 400 °C. In the high temperature, the methylene and ether bonds are broken to form methane, carbon monoxide, etc. Finally, hydrogen atoms are detached from the carbon skeleton to form hydrogen and some C-C bonds break. In this process, π-π stacking often occurs between the benzene rings. Due to the much more release of small molecule, the CS carbon derived from RF resin possesses much more pores and higher specific surface area. Fig. 3 XRD patterns of (a) NCS and (b) CS; Raman spectra of (c) NCS-600, (d) NCS-

700, (e) NCS-800, and (f) NCS-1000 XRD and Raman were used to investigate the crystal structure of the prepared carbon materials. As shown in Fig. 3(a-b), no sharp peaks are present in the XRD patterns, demonstrating that all the samples are amorphous carbons. The XRD patterns of CS600 and NCS-600 have a broad peak near 12°, which is due to the accommodation of various oxygen-containing groups in carbon materials [34]. Clearly, this peak completely disappeared as the carbonization temperature rises. The broad diffraction peaks at 2θ around 22° and 44° can be approximately indexed to the (002) and (100) planes of graphite, sharpen with the carbonization temperature rises up due to the crystallite grows. A slight right-shift of the (002) diffraction peak is observed for both CS and NCS, which corresponds to a diminution of interlayer distances. The interlayer distances calculated via Bragg’s equation (2dsinθ = nλ) are shown in Table 1. Both CS and NCS show amorphous carbon’s XRD patterns. Raman spectra could give more information about the structure of carbon materials, such as defects, crystal size [35, 36]. Two remarkable featured peaks of carbon materials, D-band (1360 cm-1) and G-band (1595 cm-1) are observed in Fig. 3(c-f). Generally, The D-band is arising from a fairly disordered or distorted structure at the edge of microcrystalline, and the G-band is characteristic for sp2-hybridized C-C bonds in graphene

[37]

. It is well known that the

R-value, the relative area ratio of the D-band to the G-band (ID/IG), relies on both the degree of graphitization and the arrangement of the graphitic planes, i.e. the lower the R-value, the higher is the amount of sp2 graphite clusters and fewer defects in the samples [38]. The crystallite diameter (La) could be calculated from the Tuinstra-Koenig

(TK) relationship equation, La = (2.4×10-10)λ4(ID/IG)-1 [39]. Accordingly, La is estimated to range from 6.20 to 8.32 nm for NCS, and the R-value and La are shown in Table 1. CS and NCS show a similar R-value and La at the same carbonization temperature, this indicating N-doping may have little effect on the crystal structure of carbon microspheres (Fig. S3). The CS-600 and NCS-600 having more sp2 graphite clusters may be attributed to π-π staking of the polyacene. When increasing the carbonization temperature, the carbonization degree increases along with the further removal of H, O, and C atoms, and the sp2-hybridized structure is partly damaged reflecting from the increased R-value. When the carbonization temperature reaches to 1000 °C, the graphite crystallite begins to grow, and then the R-value reduces and La increases. Table 1 R-value and Crystallite diameter of CS and NCS Sample

CS-600

CS-700

CS-800

CS-1000

NCS-600

NCS-700

NCS-800

NCS-1000

-

0.414

0.411

0.401

-

0.403

0.399

0.391

R

2.27

2.82

3.26

2.51

2.31

2.83

3.10

2.69

La (nm)

8.46

6.81

5.89

7.66

8.32

6.79

6.20

7.14

d002 (nm)

Fig. 4 N1s spectra of (a) NCS-600, (b) NCS-700, (c) NCS-800, and (d) NCS-1000 The elemental composition of NCS was investigated by element analysis and X-ray photoelectron spectroscopy (XPS). As shown in Table S1, the N contents determined by element analysis are 9.58, 8.33, 6.70, and 3.61 wt% for NCS-600, NCS-700, NCS800, and NCS-1000, respectively. Apparently, the carbonization temperature is a vital factor in determining the N content. The surface chemical properties of NCS can be further explored by XPS analysis and the spectra are displayed in Fig. S2. The XPS

spectra of NCS show three distinct peaks at about 285 (C1s), 399 (N1s) and 533 (O1s) eV. In the survey spectra of NCS, the binding energy intensities of N1s and O1s decrease as the increase of carbonization temperature, indicating the gradual decrease of these two elements, which is identical with the results of element analysis (Table S2). Fig. 4 shows the high resolution of N1s spectra and three types of nitrogen-containing groups could be observed. The peaks centered at 398.6, 400.2, and 401.2 eV could be attributed to pyridinic N (N-6), pyrrolic N (N-5) and quaternary N (N-Q), respectively [40]

. It should be pointed out that no N-Q group is observed in the samples prepared at

a temperature of < 1000 °C since the N-Q groups usually generate at a high carbonization temperature [41]. 3.2 Electrochemical performance in PIBs Fig. 5 CV curves of (a) CS-600 and NCS-600, (b) CS-700 and NCS-700, (c) CS-800 and NCS-800, (d) CS-1000 and NCS-1000 In order to investigate the individual effect of N-doping and carbon structure on K+ storage, the electrochemical performance of CS and NCS are systematically studied. Firstly, the electrochemical properties of the samples are examined by cyclic voltammetry (CV) at a scan rate of 0.1 mV s-1. Fig. 5 shows the first three consecutive CV curves of the electrodes made from NCS and CS. As shown in Fig. 5(a-d), during the 1st cathodic scan, a peak appears at around 0.5 V and disappears in the 2nd scan, which is attributed to the formation of solid electrolyte interface (SEI) film [42]. It should be noted that the peak position of the SEI layer in NCS-1000 moves to a high potential. This change may be caused by the higher graphitization degree of NCS-1000 than the

others [43]. The sharp cathodic peak centered at ~0.05 V is attributed to the intercalation of K+ into the carbon framework to form the K-intercalated compounds

[44]

. And the

broad anodic peak around 0.35 V is the extraction process of K+ from the carbonaceous materials. From the CV curves, it is seen that the anode peak sharpening as the carbonization temperature rises due to an increased graphitization degree. The CV curves of the 2nd and 3rd cycles in all cases are almost overlapped, indicating the good reversibility for all charge storage systems. We can also find that the shape of the CV curves for the CS and NCS samples prepared at the same temperature are approximately the same. Preliminarily, it is proposed that the N-doping doesn't produce a significant effect on K+ storage. Fig. 6 (a-d) Comparison of cycling performance of NCS and CS at a current density of 0.1 A g-1; (e-h) Comparison of rate performance of NCS and CS from 25 mA g-1 to 1 A g-1 We further study the galvanostatic charge-discharge (GCD) and rate performance of CS and NCS. As shown in Fig. 6(a-d), the NCS-600 displays a discharge capacity of 482.0 and 224.2 mAh·g-1 on the 1st and 2nd cycles at 0.1 A g-1, responding to an initial coulombic efficiency of 46.5%. After 100 cycles, this carbon still exhibits a capacity of 121 mAh g-1. The capacity loss could be attributed to the formation of an SEI layer and the decomposition of the electrolyte on the surfaces of the NCS samples. The NCS-600 shows better rate capacity with the reversible capacities of 241, 159, 140, 117, 93, and 66 mAh·g-1 at 25, 50, 100, 200, 500, and 1000 mA·g-1, respectively. As shown in Table S3, the NCS-600 prepared here shows a comparable capacity and cyclic performance

with some carbon materials, such as graphite, soft carbon [13, 42]. As shown in Fig. 6(ad), the specific capacity of the N-doped samples is only slightly higher than those of the non-doped ones. For example, the specific capacity of NCS-600 is only 9% higher than that of CS-600, although this sample possesses very high N-doping (9.58 wt%). However, when the nitrogen content decreases, the capacities of NCS-700 and NCS800 increase by 17% and 20% compared with CS-700 and CS-800, respectively. The results of the rate measurements exhibit the same regularity as the GCD cycle test, and the capacity gap between the CS and NCS samples increase gradually from 600 °C to 800 °C, but there is almost no difference in capacity between CS-1000 and NCS-1000. Compared with NCS-800, NCS-1000 has less nitrogen content, and some of them are N-Q with weaker adsorption for K+ [25]. If the nitrogen content is an important positive factor affecting the K+ storage, the specific capacity of NCS-800 should be higher than that of NCS-1000. However, the specific capacity of NCS-800 (101 mAh·g-1) is obviously lower than that of NCS-1000 (132 mAh·g-1). Fig. 7 (a) The relationship between nitrogen content and specific capacity for NCS, (b) The relation between La and specific capacity in NCS, (c) The relationship between La and specific capacity for CS and NCS We select the discharge capacities of the 15th (25 mA g-1), 25th (50 mA g-1), 35th (100 mA g-1) and 45th (200 mA g-1) charge/discharge cycles. The specific capacities first decrease and then increase as the nitrogen content (data determined by elemental analysis measurement) increases (Fig. 7a). We also correlated the absolute N-5 and N6 groups with the specific capacities. As shown in Fig. S4, K storage capacity still has

no clear relationship with the specific nitrogen species. This means that simply increasing N content may have little effect on K+ storage. The same result can be seen in Li+ storage calculated by Huang et al

[45]

. This confirms our proposal that the N-

doping does not produce a significant effect on K+ storage. In another word, there are other factors which determine the K+ storage, e.g. the carbon crystal structure. Based on the above hypothesis, we construct the relationship between the specific capacity and La. Interestingly, the specific capacity increases as the La of NCS at all the discharge current densities, and a good linear relationship is observed from Fig. 7b. This linearity still maintains well when we plot the specific capacities and La values for all the carbon materials prepared here (Fig. 7c). The above facts confirm that increasing the value of La may have a more obvious result on K+ storage than N-doping. Generally, there are three types of lithium storages in carbon materials, including insertion of lithium into the graphite layer to form intercalation compounds, storage at the edge of the carbon layer and storage on the surface of the carbon layer [46]. The storage of potassium ions in carbon materials is similar with that of lithium. From the K+ storage form in graphite (KC36 (0.3-0.2 V) -KC24-KC8 (below 0.2 v)), it can be seen that the lower potential K−graphite intercalation compound (KC8) can store more K+ [13]. As shown in Fig. S5, NCS-600 and NCS-1000 with larger La have more intercalated contributed capacities (about 60 mAh g-1), while NCS-700 and NCS-800 have less intercalated contributed capacities (about 30~40 mAh g-1). Thus, the total capacities increase as the La increases. Fig. 8 (a-d) Sweep rates from 0.1 to 5 mV s-1 and (e-h) The capacitive (blue) and diffusion (gray) current contribution to the charge storage in NCS (e-h) at different

scan rate (0.1-5 mV S-1) for the NCS samples In order to clarify the kinetic effects in the electrochemical process, the CV curves of the as-obtained CS and NCS electrodes in PIBs were analyzed at scanning rates from 0.1 to 5 mV s-1 (Fig. 8). Generally, the relationship between current (i) and scanning rate (v) can be expressed by Equation 1 [47] i=avb

(1)

where i is the peak current, v is the scanning rate, a and b are the adjustable values. When the value of b=0.5, the electrochemical reaction is dominated by the solid-state diffusion, while b=1 indicates it is controlled by a capacitive process. As shown in Fig. S6, the b values of all the samples range between 0.5 and 1, indicates a mixed potassium storage mechanism. According to Dunn’s method, the ion storage mechanism of the CS and NCS electrodes can be quantitatively quantized by separating the current response (i) at a particular voltage (V) into the capacitive contribution (K1v) and diffusioncontribution (K2v0.5) and can be expressed by Equation 2 [48]. i (V)=K1v+k2v0.5

(2)

The capacitive and diffusion contributions are calculated according to Equation 2. As the carbonization temperature rises, the contribution of capacitive capacities increases while the contribution of diffusion capacities decreases (Fig. 8e-h). The same trend is also seen for the CS electrodes (Fig. S7). The reason may be that the diffusion kinetics of K+ becomes slower due to the gradual reduction of the interlayer spacing with the increasing of carbonization temperature. Fig. 9 (a) The relationship between diffusion contributed capacity and La in CS; (b)

The relationship between diffusion contributed capacity and La in NCS We further calculated the absolute values of the diffusion-contributed capacity according to CV curves at 0.1 and 0.2 mV s-1, and study the relation between it and La. Interestingly, we find that for the CS electrodes without N-doping the diffusioncontributed capacity is in direct proportion to La (Fig. 9a). However, the situation of the NCS samples is more complicated, the diffusion-contributed capacities of these samples firstly decrease and then increase as the La increases (Fig. 9b). Considering the high similarity between CS and NCS series except for the N-doping, we propose that N-doping may be the reason for the difference in Fig. 9a and 9b. That is to say, Ndoping may affect the potassium ion storage by affecting diffusion contributed capacity. The N-doping changes the electronic structure and could increase the reactivity by producing locally accessible activity sites in the graphitic lattice. Due to the possible synergistic effect of high N-doping and large La, the NCS-600 exhibits better potassium storage capacity. Although we are not able to clarify the true effect mechanism of Ndoping still now, it is believed that N-doping has a slight contribution to the whole capacities through affect ion diffusion process, and the crystalline structure of the carbon electrodes is the more important factor. Conclusion In conclusion, we successfully prepared two series of carbon microspheres as the anode materials of potassium ion batteries. The only difference between these two series samples is with or without N-doping. It is demonstrated that the crystalline size has more effect than N-doping in potassium ion storage, and the specific capacity increases

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Highlights 1. Carbon microspheres are used as anode materials of potassium ion batteries. 2. The effects of N-doping and crystal structure on K+ storage are studied. 3. The potassium storage is determined by the (002) crystal size instead of N-doping. 4. NCS-600 gives a high reversible capacity of 241 mAh g-1 at 25 mA g-1.

CRediT author statement Shuo Wang: Investigation, Methodology, Writing – Original Draft; Yanyan Li: Investigation; Fanteng Ma: Data Curation; Xiaozhong Wu: Writing – Review & Editing; Pengfei Zhou: Writing – Review & Editing; Zhichao Miao: Resources; Peibo Zhou: Resources; Shuping Zhuo: Supervision; Jin Zhou: Conceptualization, Funding Acquisition

Declaration of interests ‫ ܈‬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.

‫܆‬The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: