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Influence of P doping on Na and K storage properties of N-rich carbon nanosheets
Materials Chemistry and Physics 236 (2019) 121809
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Influence of P doping on Na and K storage properties of N-rich carbon nanosheets Yu Zhang a, Lin Li a, Wanwan Hong a, Tianyun Qiu a, Laiqiang Xu a, Guoqiang Zou a, Hongshuai Hou a, Xiaobo Ji a, *, Song Li b, ** a b
Hunan Provincial Key Laboratory of Chemical Power Sources, College of Chemistry and Chemical Engineering, Central South University, Changsha, 410083, China State Key Laboratory for Powder Metallurgy, Central South University, Changsha, 410083, China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Carbon nanosheets with high P and N contents were prepared. � P-doped N-rich carbon nanosheets exhibited higher sodiation capacity. � N-rich carbon nanosheets performed better in K-storage than Na-storage. � The electrode with large ion diffusion coefficient delivers a large capacity.
A R T I C L E I N F O
A B S T R A C T
Keywords: Heteroatom doping Sodium-ion batteries Potassium-ion batteries Carbon nanosheets Diffusion coefficient
Nitrogen or phosphorus doping is a commonly used method to ameliorate the electrochemical properties of carbonaceous material. In this study, nitrogen-rich carbon nanosheets are first prepared and then annealed with NaH2PO4 to synthesize phosphorus doping nitrogen-rich carbon nanosheets. Two obtained materials are utilized as anodes of sodium-ion batteries and potassium-ion batteries, revealing that phosphorus doping can improve sodiation capacity (247.9 vs. 175.1 mA h g 1) and lower potassiation capacity (207.2 vs. 242.6 mA h g 1). To explore the inner reason for this phenomenon, Naþ and Kþ diffusion coefficients are surveyed through galva nostatic intermittent titration technique, showing that the electrode with large ion diffusion coefficient delivers a large capacity. This work provides some use for reference in introduction of heteroatoms to carbonaceous anode materials of sodium-ion batteries and potassium-ion batteries.
1. Introduction Energy storage devices such as batteries and supercapacitors are extensively studied to meet the rising demand of energy [1,2]. Devel opment of lithium-ion batteries (LIBs) will be overshadowed by the
restricted lithium resources and growing cost in the future [3]. Sodium-ion batteries (SIBs), a prospective candidate to store large-scale energy, came into the attention of researchers on account of the widely distributed sodium resources [4,5]. Recently, potassium-ion batteries (PIBs) have also aroused lots of interests in virtue of low-cost, rich
* Corresponding author. ** Corresponding author.. E-mail addresses: [email protected] (X. Ji), [email protected] (S. Li). https://doi.org/10.1016/j.matchemphys.2019.121809 Received 17 May 2019; Received in revised form 12 June 2019; Accepted 1 July 2019 Available online 2 July 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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Materials Chemistry and Physics 236 (2019) 121809
Fig. 1. Schematic illustration of the preparation of NCNs and PNCNs.
potassium resource and analogous operation principle to that of LIBs and SIBs [6,7]. Moreover, potassium possesses a lower reduction po tential to sodium in nonaqueous electrolyte ( 2.93 V vs. 2.71 V), furnishing PIBs higher working voltage and energy density [8]. Graphite, the commercialized anode material of LIBs, is frustrated by low sodiation capacity as a result of the formation of NaC64 [9,10]. Note that the ionic radius of Naþ is larger than that of Liþ and the energy cost in the formation process of Na-graphite intercalation compounds (GICs) is relatively high, making it difficult for Naþ to be inserted into graphite [11]. Although the radius of Kþ is larger that of Naþ, the formation energy cost of KC8 is lower than that of Na-GICs, the graphite exhibits a high theoretical potassiation capacity of 279 mA h g 1 as anode of PIBs [12]. However, graphite suffers from fast capacity fading due to huge volume expansion of ~61% when stage-one K-graphite intercalation compound (KC8) forms [12]. It was found that the formation energy cost is decreased with the increase of the interlayer distance of graphite. To facilitate the insertion of Naþ or Kþ, the expanded graphite with larger interlayer distance was utilized to store sodium or potassium, presenting high specific capacity and long cyclicity [11,13]. Nevertheless, the complicated fabrication process and high cost limited the application of expanded graphite, leaving it essential to find suitable anode material for SIBs or PIBs. Non-graphitic hard carbon, derived from polymer or biomass by pyrolysis, was most widely investigated as anode material of SIBs, which was also considered to be appropriate for potassium storage [14]. Hard carbon with randomly oriented graphite crystallite can create numerous nanopores for absorption of Naþ or Kþ and simultaneously supply more space to buffer volume expansion, bringing SIBs or PIBs enhanced cycling life [15,16]. However, hard carbon at low graphitization degree is constrained by low electronic conductivity owning to its disordered microstructure [17,18]. Heteroatoms (such as boron, nitrogen, sulfur and phosphorus) doping is an effective method to alter the electro chemical behavior of hard carbon [19]. By nitrogen-doping, the con ductivity of hard carbon can be enhanced and more defects in hard carbon can be generated to absorb ions [20]. In addition to producing more active sites, doping phosphorus with larger radius and weaker electronegativity would also enlarge the interlayer distance of hard carbon, leading to additional anode capacity [21]. Therefore, it will be worth exploring electrochemical performances of nitrogen, phosphorus co-doped hard carbon anode to facilitate application of SIBs and PIBs. The fabrication procedure is illustrated in Fig. 1. In this work, nitrogen-rich carbon nanosheets (NCNs) were prepared from urea and citric acid through NaCl molten salt. Phosphorus doped nitrogen-rich carbon nanosheets (PNCNs) were obtained by annealing NaH2PO4 and NCNs at argon atmosphere. Na-storage and K-storage properties of these two synthesized materials were studied systematically. It was found that PNCNs are more applicable as anode of SIBs and NCNs perform better on K-storage, where PNCNs deliver a sodiation capacity of 247.9 mA h g 1 and NCNs exhibit 242.6 mA h g 1 when storing potassium at 0.1 A g 1 after 50 cycles. Cyclic voltammetry test and galvanostatic intermittent
titration technique were exploited to reveal the correlation between specific capacity and ion diffusion coefficient. 2. Experimental 2.1. Preparation of NCNs and PNCNs NCNs were synthetized by molten salt method. Sodium chloride (NaCl, 10 g), urea (CH4N2O, 5 g) and citric acid (C6H8O7, 1 g) were added into 50 mL deionized water to form a clear solution after agita tion. Then, water was evaporated from the solution by vacuum freezedrying at 56 � C, leaving a uniform solid mixture of three solutes. The mixture was heated to 800 � C under argon atmosphere with a ramping speed of 5 � C min 1 and annealed for 2 h. After cooling to room tem perature, the obtained product was washed by deionized water to remove sodium chloride and the NCNs were obtained after filtration and drying. NCNs (0.1 g) and sodium dihydrogen phosphate (NaH2PO4, 2.0 g) were mixed and calcined at 800 � C for 2 h with a ramping speed of 5 � C min 1 under argon atmosphere. The obtained sample was washed with diluted hydrochloric acid and deionized water successively. After filtration and drying, the PNCNs were obtained. 2.2. Materials characterization The microscopic morphologies of two samples were analyzed by utilizing TEM (JEOL JEM 2100F) at 200 kV and SEM (TESCAN MIRA3) at 20.0 kV. The crystal structures were characterized on a Rigaku Ultima IV X-ray diffractometer at a CuKα radiation (λ ¼ 1.54 Å). Raman spectra were obtained via a Renishaw Invia Raman spectrometer. A Micro meritics ASAP 2460 analyzer was used at 77 K to probe the surface area and distribution of pore size. XPS spectra were measured by X-ray photoelectron spectrometer (Thermo Scientific EscaLab 250Xi). 2.3. Electrochemical measurements 70 wt% PNCNs or NCNs, 15 wt% carboxymethyl cellulose (CMC) and 15 wt% conductive carbon were mixed in deionized water and stirred for 24 h. To prepare the working electrodes, the mixture was coated on a copper foil and dried in vacuum at 80 � C for 10 h. The loading amounts of NCNs and PNCNs in round electrode with a diameter of 8 mm are around 0.2 mg and 0.3 mg, respectively. The round electrode, Celgard 2400 membrane, electrolyte and metal sodium or potassium were assembled in the CR2016-type cells under argon atmosphere. The electrolytes of SIBs and PIBs were 1 M NaClO4 in propylene carbonate (PC) with 5% fluoroethylene (FEC) and 0.8 M KPF6 in ethylene car bonate (EC) and diethyl carbonate (DEC) (1:1, v/v), respectively. The galvanostatic charge/discharge and galvanostatic intermittent titration technique (GITT) tests were performed on LANHE CT2100A battery test system. Cyclic voltammetry analysis was tested on Metrohm Autolab 2
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Materials Chemistry and Physics 236 (2019) 121809
Fig. 2. SEM, TEM and HRTEM images of NCNs (a-d) and PNCNs (e-h).
Fig. 3. (a) XRD patterns, (b) Raman spectra, (c) N2 adsorption-desorption isotherms and (d) XPS survey spectrum of PNCNs and NCNs; high resolution (e) N 1s and (f) P 2p spectrum of PNCNs.
M204. Before GITT tests, the half cells were discharged and charged for four cycles at a current density of 0.05 A g 1. In the fifth cycle, the batteries were discharged at a current pulse of 0.02 A g 1 for 20 min and then left to rest for 1 h. The tests would continue until the working po tential of batteries reached 0.01 V.
microscope (TEM) images (Fig. 2c and g) further verify the fold flake structure of NCNs or PNCNs. High resolution TEM (HRTEM) images (Fig. 2d and h) show clearly both amorphous carbon structures of NCNs and PNCNs. The crystal structure of NCNs and PNCNs was characterized by X-ray diffraction (XRD) patterns (Fig. 3a). The broad peak centered at ~25� is ascribed to (002) plane of carbon material, suggesting a typical amor phous structure of NCNs or PNCNs, which is agreeable to HRTEM re sults. The values of 2θ degree for NCNs and PNCNs are 26.34� and 24.88� , respectively, demonstrating interlayer distances (d002) of 0.336 nm and 0.355 nm on the basis of the Bragg’s law. The value of d002 for PNCNs is larger than NCNs, which is attributed to phosphorus doping. Raman spectra of PNCNs and NCNs are demonstrated in Fig. 3b. Two characteristic peaks located around 1350 cm 1 and 1590 cm 1, which
3. Results and discussion Scanning electron microscopy (SEM) images of NCNs and PNCNs are displayed in Fig. 2a and e, respectively. NCNs and PNCNs are both composed of wrinkled and crooked carbon nanosheets, indicating that the micromorphology of prepared material didn’t transform massively after phosphorus-doping. SEM images on the scale of 1 μm are shown in Fig. 2b and f, where surface of PNCNs is rougher than that of NCNs, inferring a lager specific surface area of PNCNs. Transmission electron 3
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Materials Chemistry and Physics 236 (2019) 121809
Fig. 4. Electrochemical performances of PNCNs and NCNs for SIBs: CV curves of (a) PNCNs/Na half-cell and (b) PNCNs/Na half-cell at 0.1 mV s 1 with voltage range from 0.01 to 3V; (c) First charge and second discharge profiles, (d) cycle performances at 0.1 A g 1, (e) Rate capacities of PNCNs and NCNs; (f) Long cyclicities at large current densities of PNCNs.
are consistent with the defect-induced graphite band (D band) and crystalline graphite band (G band), respectively [15,22,23]. The in tensity ratio of G band to D band (IG/ID) is a measure to evaluate the graphitization degree [2,24]. IG/ID ¼ 1.03 of PNCNs is higher than IG/ID ¼ 0.97 of NCNs, indicating that an increase in graphitization de gree of PNCNs occurs after calcination of NCNs and NaH2PO4. N2 adsorption-desorption isotherms measurements were performed to estimate the surface area and pore size distribution of PNCNs and NCNs. As shown in Fig. 3c, two samples both reveal type IV isotherms with hysteresis loops [25]. The Brunauer-Emmett-Teller (BET) specific surface area (SBET) of NCNs is calculated to be 87.5 m2 g 1, while PNCNs hold a much larger SBET of 254.5 m2 g 1. In addition to possessing microporous around 0.8 nm, PNCNs share a similar pore diameter dis tribution with NCNs, whose microporous is centered at 1 nm and mes oporous is clustered around 17 and 34 nm. To inquire the surface chemical compositions of PNCNs and NCNs, Xray photoelectron spectroscopy (XPS) was conducted and the survey spectrum are revealed in Fig. 3d. The atom contents of C, N and O in
NCNs are 80.24%, 15.17% and 4.59%, severally. Besides containing C, N and O with atom contents of 74.81%, 13.03% and 9.99%, PNCNs contain up to 2.17% P. The high resolution N 1s spectrum of PNCNs (Fig. 3e) can be resolved into three peaks of pyridinic N (N-6, 398.3 eV), pyrrolic N (N-5, 399.8 eV) and graphitic N (N-Q, 401.1 eV) [24,26], whose proportion are in sequence of 30.7%, 26.5% and 42.8%. With regard to N 1s of NCNs (Fig. S2), three kinds of N account for 33.7%, 21.0% and 45.3%, respectively. Pyridinic N is believed as an effective one for facilitating the absorption of Naþ or Kþ at the anode, resulting from more detects and active sites that it can bring on [14,27]. Fig. 3f illustrates the high resolution P 2p spectrum of PNCNs, which is divided into peaks of P-C bond (132.7 eV) and P-O bond (133.8 eV) [28]. Considering their great properties, two obtained materials were first used as SIBs anode. Cyclic voltammetry (CV) was tested to investigate the Na-storage behaviors and the first four CV sweeps of PNCNs at 0.1 mV s 1 is illustrated in Fig. 4a. During the initial cathodic sweep, one broad peak locates around 0.7 V and disappears in the subsequent three scans, which should be caused by the formation of solid electrolyte 4
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Materials Chemistry and Physics 236 (2019) 121809
Fig. 5. Electrochemical performances of PNCNs and NCNs for PIBs: First five CV curves of (a) PNCNs/K half-cell and (b) NCNs/K half-cell at 0.1 mV s performances at 0.1 A g 1, (d) First charge and second discharge profiles, (e) Rate capacities of PNCNs and NCNs.
interface (SEI) layer [25]. The sharp cathodic peak at 0.01 V is accredited to the insertion of Naþ into PNCNs. During charge process, a broad low peak centered near 0.9 V corresponds to the extraction of Naþ from host material. According to Fig. 4b, CV curves during the initial four sweeps of NCNs share a similar shape with those of PNCNs and exhibit a lower current than those of PNCNs. To further explore the capacity difference between PNCNs and NCNs, galvanostatic charge-discharge technique was measured (Fig. 4d). The initial discharge/charge specific capacities of PNCNs and NCNs are 784.8/277.2 and 546.6/181.4 mA h g 1 at 0.1 A g 1 with working po tential between 0.01 and 3V, providing low initial coulombic effi ciencies (ICE) of 35.32% and 33.19%. After 100 cycles, PNCNs deliver a charge capacity of 246.5 mA h g 1 with a capacity retention of 88.9%, while NCNs exhibit 177.6 mA h g 1. As Fig. 4c displayed, PNCNs and NCNs both possess a great proportion of sloping capacities, which is contributed by the absorption of Naþ on the active sites on hard carbon surface [15]. Phosphorus doping bring PNCNs more active sites, which may be responsible for larger specific capacity [29]. The rate capacities
1
; (c) cycle
of two materials are diagrammed in Fig. 4e. PNCNs exhibit capacities of 235, 198, 166, 139 and 119 mA h g 1 at 0.2, 0.4, 0.8, 1.6 and 3.2 A g 1, respectively. Despite the rate performance of PNCNs is better than that of NCNs, the differences of rate capacities decrease with the increase in current density. The long cyclicity of PNCNs at high rate was also ana lysed (Fig. 4f). PNCNs present capacities of 236 mA h g 1 at 0.2 A g 1 after 400 cycles, 184 mA h g 1 at 0.5 A g 1 after 800 cycles and 158 mA h g 1 at 1.0 A g 1 after 1000 cycles, demonstrating remarkable electrochemical Na-storage properties. The electrochemical K-storage of PNCNs and NCNs were also researched. Fig. 5a shows the initial five CV profiles of PNCNs/K halfcell at 0.1 mV s 1. The irreversible cathodic peak near 0.55 V, cathodic peak situated around 0.05 V and weak anodic peak located around 1.1 V should be attributed to the SEI film formation, Kþ’s inserting/extracting into/from disorder turbostratic domains of PNCNs [16], respectively. The decrease in the height of cathodic peak at 0.01 V from 1st to 5th CV sweep represents the irreversible potassiation exists, resulting in low coulombic efficiencies during initial discharge and charge process. In 5
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Materials Chemistry and Physics 236 (2019) 121809
Fig. 6. CV curves of (a) PNCNs/Na cell, (b) NCNs/Na cell, (d) PNCNs/K cell and (e) NCNs/K cell; The linear relationship of log(i) vs. log(v) in (c) SIBs and (f) PIBs.
the light of CV curves of NCNs electrode in PIBs (Fig. 5b), there is no obvious differences between CV shapes of NCNs and PNCNs except that the cathodic peak at 0.01 V and anodic peak around 0.5 V of NCNs are sharper than those of PNCNs. The K-storage properties of NCNs and PNCNs at a current density of 0.1 A g 1 are shown in Fig. 5c. NCNs and PNCNs deliver initial specific capacities of 831.3/277.2 mA h g 1 and 859.7/267.0 mA h g 1, providing ICE of 33.35% and 31.06%, severally. After 250 cycles, the charge capacities of NCNs and PNCNs are 237.7 and 179.3 mA h g 1, with capacity retentions of 85.8% and 67.2%. Contrary to the afore mentioned Na-storage research result that PNCNs reveal greater sodia tion properties than NCNs, NCNs possesse larger potassiation capacity than PNCNs. As depicted in Fig. 5e, NCNs demonstrate greater rate performance than PNCNs, exhibiting specific capacities of 245, 202, 160, 132, 108 mA h g 1 at current densities of 0.2, 0.4, 0.8, 1.6, 3.2 A g 1, respectively. CV curves of two samples in SIBs and PIBs at various scan rates from 0.2 to 1.0 mV s 1 are pictured in Fig. 6a and b and 6d, e. With the in crease of sweep speed, the desodiation peaks of PNCNs and NCNs become weaker, while the depotassiation peaks of PNCNs and NCNs get steeper. To estimate the kinetic performance at anodic peaks of the tested electrode, the linear relationship of log(i) versus log(ν) was plotted on Fig. 6c,f on the basis of the equation: i ¼ aνb :, where i rep resents the peak current, ν is the scan rate [30]. The value of b approaching 1 betokens that the charge storage is controlled by capac itive process, which is reckoned from the slope of the fitted line, the b-value close to 0.5 bespeaks that the electrochemical behavior is pri marily diffusion-controlled process [24]. The b-values of PNCNs/Na, NCNs/Na, PNCNs/K and NCNs/K cells are 0.63, 0.56, 0.65 and 0.96, indicating that at anodic peaks the first and the third are dominated by a mixed mechanism of capacitive process and diffusion process, the sec ond is chiefly diffusion-controlled process and the last is mainly capacitive process. To further investigate the difference between K-storage and Nastorage mechanism in two obtained samples, galvanostatic intermit tent titration (GITT) technique was conducted (Fig. 7a and b). If there is a linear relationship between cell potential and the square root of pulse time (Fig. S3), the Naþ and Kþ diffusion coefficient can be calculated
from the following equation based on Fick’s second law, � �2 � �2 � � 4 mB VM ΔEs L2 τ≪ D¼ πτ MB S ΔEτ D where τ is the pulse current duration, mB, VM and MB are the mass, molar volume and molar mass of electrode active materials, S and L are the area and average thickness of the tested electrode [31,32]. ΔEs and ΔEτ can be evaluated from GITT profiles according to Fig. S4. As Fig. 7c diagrammed, Naþ diffusion coefficient (DNaþ) in NCNs electrode is slightly larger than that in PNCNs electrode when the potential is above 1.0 V. However, DNaþ of PNCNs holds a higher value than that of NCNs at low voltage domains. Considering that discharge capacities of PNCNs and NCNs are mainly exhibited below 1.0 V, the higher DNaþ makes it advantageous for PNCNs to store sodium, which is in accordance with the galvanostatic charge-discharge test results (Table 1). NCNs with a larger K-storage capacity delivers a higher Kþ diffusion coefficient (DKþ) than PNCNs at whole test voltage range (Fig. 7d). When comparing DNaþ and DKþ in one sample (PNCNs or NCNs, Fig. 7e and f and Table 1), there is also a correlation between ion diffusion coefficients and capacities, where PNCNs with higher DNaþ than DKþ performs better on Na-storage rather than K-storage and NCNs with larger DKþ is more suitable for K-storage. 4. Conclusions In summary, we have synthesized high nitrogen content carbon nanosheets and further introduced phosphorus to nanosheets by means of molten salt method. Electrochemical Na-storage and K-storage properties of two kinds of carbon nanosheets were studied, where the nanosheets without phosphorus doping are more appropriate for anode of PIBs with a specific capacity of 237.7 mA h g 1 at 0.1 A g 1 after 250 cycles and phosphorus doped nanosheets provides higher Na-storage capacity of 246.5 mA h g 1 after 100 cycles at 0.1 A g 1. Naþ and Kþ diffusion coefficients of two samples were measured, demonstrating that the introduction of phosphorus to nitrogen-rich carbon nanosheets im proves the Naþ diffusion coefficient and lowers the Kþ diffusion coeffi cient of the host material. The electrode with larger ion diffusion 6
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Materials Chemistry and Physics 236 (2019) 121809
Fig. 7. GITT profiles of PNCNs and NCNs electrodes during discharge process in (a) SIBs and (b) PIBs; (c) Na-ion and (d) K-ion diffusion coefficients as a function of state of discharge process; Na-ion and K-ion diffusion coefficients in (e) PNCNs and (f) NCNs. Table 1 Structural information, chemical compositions and electrochemical properties of NCNs and PNCNs. Sample NCNs PNCNs a b
D002 (nm) 0.336 0.355
IG/ID 0.97 1.03
SBET (m2 g 254.5 87.5
1
)
Element content (at %)
C (mA h g
1 a
)
D (10
11
, cm2 s 1)b
C
N
O
P
SIBs
PIBs
Naþ
Kþ
80.24 74.81
15.17 13.03
4.59 9.99
2.17
175.1 247.9
242.6 207.2
0.21 0.55
0.54 0.10
The discharge capacities are obtained from the 50th cycle at a current density of 0.1 A g 1. The ion diffusion coefficients are calculated from GITT at near 0.2 V during discharge processes.
coefficient delivers a larger specific capacity, implying that heteroatom doping can been utilized to ameliorate the ion diffusion coefficient and potassium/sodium storage performance of the carbon material. This work may offer a reference for designing carbon anode materials of SIBs and PIBs.
China (51622406, 21673298), Young Elite Scientists Sponsorship Pro gram by CAST (2017QNRC001), Project of Postdoctoral Innovative Talents (BX201600192) and Project of Innovation Driven Plan in Central South University (2017CX004, 2018CX005). Appendix A. Supplementary data
Acknowledgments
Supplementary data to this article can be found online at https://doi. org/10.1016/j.matchemphys.2019.121809.
This work was supported by National Natural Science Foundation of 7
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Materials Chemistry and Physics 236 (2019) 121809
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Influence of P doping on Na and K storage properties of N-rich carbon nanosheets Yu Zhang a, Lin Li a, Wanwan Hong a, Tianyun Qiu a, Laiqiang Xu a, Guoqiang Zou a, Hongshuai Hou a, Xiaobo Ji a, *, Song Li b, ** a b
Hunan Provincial Key Laboratory of Chemical Power Sources, College of Chemistry and Chemical Engineering, Central South University, Changsha, 410083, China State Key Laboratory for Powder Metallurgy, Central South University, Changsha, 410083, China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Carbon nanosheets with high P and N contents were prepared. � P-doped N-rich carbon nanosheets exhibited higher sodiation capacity. � N-rich carbon nanosheets performed better in K-storage than Na-storage. � The electrode with large ion diffusion coefficient delivers a large capacity.
A R T I C L E I N F O
A B S T R A C T
Keywords: Heteroatom doping Sodium-ion batteries Potassium-ion batteries Carbon nanosheets Diffusion coefficient
Nitrogen or phosphorus doping is a commonly used method to ameliorate the electrochemical properties of carbonaceous material. In this study, nitrogen-rich carbon nanosheets are first prepared and then annealed with NaH2PO4 to synthesize phosphorus doping nitrogen-rich carbon nanosheets. Two obtained materials are utilized as anodes of sodium-ion batteries and potassium-ion batteries, revealing that phosphorus doping can improve sodiation capacity (247.9 vs. 175.1 mA h g 1) and lower potassiation capacity (207.2 vs. 242.6 mA h g 1). To explore the inner reason for this phenomenon, Naþ and Kþ diffusion coefficients are surveyed through galva nostatic intermittent titration technique, showing that the electrode with large ion diffusion coefficient delivers a large capacity. This work provides some use for reference in introduction of heteroatoms to carbonaceous anode materials of sodium-ion batteries and potassium-ion batteries.
1. Introduction Energy storage devices such as batteries and supercapacitors are extensively studied to meet the rising demand of energy [1,2]. Devel opment of lithium-ion batteries (LIBs) will be overshadowed by the
restricted lithium resources and growing cost in the future [3]. Sodium-ion batteries (SIBs), a prospective candidate to store large-scale energy, came into the attention of researchers on account of the widely distributed sodium resources [4,5]. Recently, potassium-ion batteries (PIBs) have also aroused lots of interests in virtue of low-cost, rich
* Corresponding author. ** Corresponding author.. E-mail addresses: [email protected] (X. Ji), [email protected] (S. Li). https://doi.org/10.1016/j.matchemphys.2019.121809 Received 17 May 2019; Received in revised form 12 June 2019; Accepted 1 July 2019 Available online 2 July 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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Materials Chemistry and Physics 236 (2019) 121809
Fig. 1. Schematic illustration of the preparation of NCNs and PNCNs.
potassium resource and analogous operation principle to that of LIBs and SIBs [6,7]. Moreover, potassium possesses a lower reduction po tential to sodium in nonaqueous electrolyte ( 2.93 V vs. 2.71 V), furnishing PIBs higher working voltage and energy density [8]. Graphite, the commercialized anode material of LIBs, is frustrated by low sodiation capacity as a result of the formation of NaC64 [9,10]. Note that the ionic radius of Naþ is larger than that of Liþ and the energy cost in the formation process of Na-graphite intercalation compounds (GICs) is relatively high, making it difficult for Naþ to be inserted into graphite [11]. Although the radius of Kþ is larger that of Naþ, the formation energy cost of KC8 is lower than that of Na-GICs, the graphite exhibits a high theoretical potassiation capacity of 279 mA h g 1 as anode of PIBs [12]. However, graphite suffers from fast capacity fading due to huge volume expansion of ~61% when stage-one K-graphite intercalation compound (KC8) forms [12]. It was found that the formation energy cost is decreased with the increase of the interlayer distance of graphite. To facilitate the insertion of Naþ or Kþ, the expanded graphite with larger interlayer distance was utilized to store sodium or potassium, presenting high specific capacity and long cyclicity [11,13]. Nevertheless, the complicated fabrication process and high cost limited the application of expanded graphite, leaving it essential to find suitable anode material for SIBs or PIBs. Non-graphitic hard carbon, derived from polymer or biomass by pyrolysis, was most widely investigated as anode material of SIBs, which was also considered to be appropriate for potassium storage [14]. Hard carbon with randomly oriented graphite crystallite can create numerous nanopores for absorption of Naþ or Kþ and simultaneously supply more space to buffer volume expansion, bringing SIBs or PIBs enhanced cycling life [15,16]. However, hard carbon at low graphitization degree is constrained by low electronic conductivity owning to its disordered microstructure [17,18]. Heteroatoms (such as boron, nitrogen, sulfur and phosphorus) doping is an effective method to alter the electro chemical behavior of hard carbon [19]. By nitrogen-doping, the con ductivity of hard carbon can be enhanced and more defects in hard carbon can be generated to absorb ions [20]. In addition to producing more active sites, doping phosphorus with larger radius and weaker electronegativity would also enlarge the interlayer distance of hard carbon, leading to additional anode capacity [21]. Therefore, it will be worth exploring electrochemical performances of nitrogen, phosphorus co-doped hard carbon anode to facilitate application of SIBs and PIBs. The fabrication procedure is illustrated in Fig. 1. In this work, nitrogen-rich carbon nanosheets (NCNs) were prepared from urea and citric acid through NaCl molten salt. Phosphorus doped nitrogen-rich carbon nanosheets (PNCNs) were obtained by annealing NaH2PO4 and NCNs at argon atmosphere. Na-storage and K-storage properties of these two synthesized materials were studied systematically. It was found that PNCNs are more applicable as anode of SIBs and NCNs perform better on K-storage, where PNCNs deliver a sodiation capacity of 247.9 mA h g 1 and NCNs exhibit 242.6 mA h g 1 when storing potassium at 0.1 A g 1 after 50 cycles. Cyclic voltammetry test and galvanostatic intermittent
titration technique were exploited to reveal the correlation between specific capacity and ion diffusion coefficient. 2. Experimental 2.1. Preparation of NCNs and PNCNs NCNs were synthetized by molten salt method. Sodium chloride (NaCl, 10 g), urea (CH4N2O, 5 g) and citric acid (C6H8O7, 1 g) were added into 50 mL deionized water to form a clear solution after agita tion. Then, water was evaporated from the solution by vacuum freezedrying at 56 � C, leaving a uniform solid mixture of three solutes. The mixture was heated to 800 � C under argon atmosphere with a ramping speed of 5 � C min 1 and annealed for 2 h. After cooling to room tem perature, the obtained product was washed by deionized water to remove sodium chloride and the NCNs were obtained after filtration and drying. NCNs (0.1 g) and sodium dihydrogen phosphate (NaH2PO4, 2.0 g) were mixed and calcined at 800 � C for 2 h with a ramping speed of 5 � C min 1 under argon atmosphere. The obtained sample was washed with diluted hydrochloric acid and deionized water successively. After filtration and drying, the PNCNs were obtained. 2.2. Materials characterization The microscopic morphologies of two samples were analyzed by utilizing TEM (JEOL JEM 2100F) at 200 kV and SEM (TESCAN MIRA3) at 20.0 kV. The crystal structures were characterized on a Rigaku Ultima IV X-ray diffractometer at a CuKα radiation (λ ¼ 1.54 Å). Raman spectra were obtained via a Renishaw Invia Raman spectrometer. A Micro meritics ASAP 2460 analyzer was used at 77 K to probe the surface area and distribution of pore size. XPS spectra were measured by X-ray photoelectron spectrometer (Thermo Scientific EscaLab 250Xi). 2.3. Electrochemical measurements 70 wt% PNCNs or NCNs, 15 wt% carboxymethyl cellulose (CMC) and 15 wt% conductive carbon were mixed in deionized water and stirred for 24 h. To prepare the working electrodes, the mixture was coated on a copper foil and dried in vacuum at 80 � C for 10 h. The loading amounts of NCNs and PNCNs in round electrode with a diameter of 8 mm are around 0.2 mg and 0.3 mg, respectively. The round electrode, Celgard 2400 membrane, electrolyte and metal sodium or potassium were assembled in the CR2016-type cells under argon atmosphere. The electrolytes of SIBs and PIBs were 1 M NaClO4 in propylene carbonate (PC) with 5% fluoroethylene (FEC) and 0.8 M KPF6 in ethylene car bonate (EC) and diethyl carbonate (DEC) (1:1, v/v), respectively. The galvanostatic charge/discharge and galvanostatic intermittent titration technique (GITT) tests were performed on LANHE CT2100A battery test system. Cyclic voltammetry analysis was tested on Metrohm Autolab 2
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Materials Chemistry and Physics 236 (2019) 121809
Fig. 2. SEM, TEM and HRTEM images of NCNs (a-d) and PNCNs (e-h).
Fig. 3. (a) XRD patterns, (b) Raman spectra, (c) N2 adsorption-desorption isotherms and (d) XPS survey spectrum of PNCNs and NCNs; high resolution (e) N 1s and (f) P 2p spectrum of PNCNs.
M204. Before GITT tests, the half cells were discharged and charged for four cycles at a current density of 0.05 A g 1. In the fifth cycle, the batteries were discharged at a current pulse of 0.02 A g 1 for 20 min and then left to rest for 1 h. The tests would continue until the working po tential of batteries reached 0.01 V.
microscope (TEM) images (Fig. 2c and g) further verify the fold flake structure of NCNs or PNCNs. High resolution TEM (HRTEM) images (Fig. 2d and h) show clearly both amorphous carbon structures of NCNs and PNCNs. The crystal structure of NCNs and PNCNs was characterized by X-ray diffraction (XRD) patterns (Fig. 3a). The broad peak centered at ~25� is ascribed to (002) plane of carbon material, suggesting a typical amor phous structure of NCNs or PNCNs, which is agreeable to HRTEM re sults. The values of 2θ degree for NCNs and PNCNs are 26.34� and 24.88� , respectively, demonstrating interlayer distances (d002) of 0.336 nm and 0.355 nm on the basis of the Bragg’s law. The value of d002 for PNCNs is larger than NCNs, which is attributed to phosphorus doping. Raman spectra of PNCNs and NCNs are demonstrated in Fig. 3b. Two characteristic peaks located around 1350 cm 1 and 1590 cm 1, which
3. Results and discussion Scanning electron microscopy (SEM) images of NCNs and PNCNs are displayed in Fig. 2a and e, respectively. NCNs and PNCNs are both composed of wrinkled and crooked carbon nanosheets, indicating that the micromorphology of prepared material didn’t transform massively after phosphorus-doping. SEM images on the scale of 1 μm are shown in Fig. 2b and f, where surface of PNCNs is rougher than that of NCNs, inferring a lager specific surface area of PNCNs. Transmission electron 3
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Fig. 4. Electrochemical performances of PNCNs and NCNs for SIBs: CV curves of (a) PNCNs/Na half-cell and (b) PNCNs/Na half-cell at 0.1 mV s 1 with voltage range from 0.01 to 3V; (c) First charge and second discharge profiles, (d) cycle performances at 0.1 A g 1, (e) Rate capacities of PNCNs and NCNs; (f) Long cyclicities at large current densities of PNCNs.
are consistent with the defect-induced graphite band (D band) and crystalline graphite band (G band), respectively [15,22,23]. The in tensity ratio of G band to D band (IG/ID) is a measure to evaluate the graphitization degree [2,24]. IG/ID ¼ 1.03 of PNCNs is higher than IG/ID ¼ 0.97 of NCNs, indicating that an increase in graphitization de gree of PNCNs occurs after calcination of NCNs and NaH2PO4. N2 adsorption-desorption isotherms measurements were performed to estimate the surface area and pore size distribution of PNCNs and NCNs. As shown in Fig. 3c, two samples both reveal type IV isotherms with hysteresis loops [25]. The Brunauer-Emmett-Teller (BET) specific surface area (SBET) of NCNs is calculated to be 87.5 m2 g 1, while PNCNs hold a much larger SBET of 254.5 m2 g 1. In addition to possessing microporous around 0.8 nm, PNCNs share a similar pore diameter dis tribution with NCNs, whose microporous is centered at 1 nm and mes oporous is clustered around 17 and 34 nm. To inquire the surface chemical compositions of PNCNs and NCNs, Xray photoelectron spectroscopy (XPS) was conducted and the survey spectrum are revealed in Fig. 3d. The atom contents of C, N and O in
NCNs are 80.24%, 15.17% and 4.59%, severally. Besides containing C, N and O with atom contents of 74.81%, 13.03% and 9.99%, PNCNs contain up to 2.17% P. The high resolution N 1s spectrum of PNCNs (Fig. 3e) can be resolved into three peaks of pyridinic N (N-6, 398.3 eV), pyrrolic N (N-5, 399.8 eV) and graphitic N (N-Q, 401.1 eV) [24,26], whose proportion are in sequence of 30.7%, 26.5% and 42.8%. With regard to N 1s of NCNs (Fig. S2), three kinds of N account for 33.7%, 21.0% and 45.3%, respectively. Pyridinic N is believed as an effective one for facilitating the absorption of Naþ or Kþ at the anode, resulting from more detects and active sites that it can bring on [14,27]. Fig. 3f illustrates the high resolution P 2p spectrum of PNCNs, which is divided into peaks of P-C bond (132.7 eV) and P-O bond (133.8 eV) [28]. Considering their great properties, two obtained materials were first used as SIBs anode. Cyclic voltammetry (CV) was tested to investigate the Na-storage behaviors and the first four CV sweeps of PNCNs at 0.1 mV s 1 is illustrated in Fig. 4a. During the initial cathodic sweep, one broad peak locates around 0.7 V and disappears in the subsequent three scans, which should be caused by the formation of solid electrolyte 4
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Materials Chemistry and Physics 236 (2019) 121809
Fig. 5. Electrochemical performances of PNCNs and NCNs for PIBs: First five CV curves of (a) PNCNs/K half-cell and (b) NCNs/K half-cell at 0.1 mV s performances at 0.1 A g 1, (d) First charge and second discharge profiles, (e) Rate capacities of PNCNs and NCNs.
interface (SEI) layer [25]. The sharp cathodic peak at 0.01 V is accredited to the insertion of Naþ into PNCNs. During charge process, a broad low peak centered near 0.9 V corresponds to the extraction of Naþ from host material. According to Fig. 4b, CV curves during the initial four sweeps of NCNs share a similar shape with those of PNCNs and exhibit a lower current than those of PNCNs. To further explore the capacity difference between PNCNs and NCNs, galvanostatic charge-discharge technique was measured (Fig. 4d). The initial discharge/charge specific capacities of PNCNs and NCNs are 784.8/277.2 and 546.6/181.4 mA h g 1 at 0.1 A g 1 with working po tential between 0.01 and 3V, providing low initial coulombic effi ciencies (ICE) of 35.32% and 33.19%. After 100 cycles, PNCNs deliver a charge capacity of 246.5 mA h g 1 with a capacity retention of 88.9%, while NCNs exhibit 177.6 mA h g 1. As Fig. 4c displayed, PNCNs and NCNs both possess a great proportion of sloping capacities, which is contributed by the absorption of Naþ on the active sites on hard carbon surface [15]. Phosphorus doping bring PNCNs more active sites, which may be responsible for larger specific capacity [29]. The rate capacities
1
; (c) cycle
of two materials are diagrammed in Fig. 4e. PNCNs exhibit capacities of 235, 198, 166, 139 and 119 mA h g 1 at 0.2, 0.4, 0.8, 1.6 and 3.2 A g 1, respectively. Despite the rate performance of PNCNs is better than that of NCNs, the differences of rate capacities decrease with the increase in current density. The long cyclicity of PNCNs at high rate was also ana lysed (Fig. 4f). PNCNs present capacities of 236 mA h g 1 at 0.2 A g 1 after 400 cycles, 184 mA h g 1 at 0.5 A g 1 after 800 cycles and 158 mA h g 1 at 1.0 A g 1 after 1000 cycles, demonstrating remarkable electrochemical Na-storage properties. The electrochemical K-storage of PNCNs and NCNs were also researched. Fig. 5a shows the initial five CV profiles of PNCNs/K halfcell at 0.1 mV s 1. The irreversible cathodic peak near 0.55 V, cathodic peak situated around 0.05 V and weak anodic peak located around 1.1 V should be attributed to the SEI film formation, Kþ’s inserting/extracting into/from disorder turbostratic domains of PNCNs [16], respectively. The decrease in the height of cathodic peak at 0.01 V from 1st to 5th CV sweep represents the irreversible potassiation exists, resulting in low coulombic efficiencies during initial discharge and charge process. In 5
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Materials Chemistry and Physics 236 (2019) 121809
Fig. 6. CV curves of (a) PNCNs/Na cell, (b) NCNs/Na cell, (d) PNCNs/K cell and (e) NCNs/K cell; The linear relationship of log(i) vs. log(v) in (c) SIBs and (f) PIBs.
the light of CV curves of NCNs electrode in PIBs (Fig. 5b), there is no obvious differences between CV shapes of NCNs and PNCNs except that the cathodic peak at 0.01 V and anodic peak around 0.5 V of NCNs are sharper than those of PNCNs. The K-storage properties of NCNs and PNCNs at a current density of 0.1 A g 1 are shown in Fig. 5c. NCNs and PNCNs deliver initial specific capacities of 831.3/277.2 mA h g 1 and 859.7/267.0 mA h g 1, providing ICE of 33.35% and 31.06%, severally. After 250 cycles, the charge capacities of NCNs and PNCNs are 237.7 and 179.3 mA h g 1, with capacity retentions of 85.8% and 67.2%. Contrary to the afore mentioned Na-storage research result that PNCNs reveal greater sodia tion properties than NCNs, NCNs possesse larger potassiation capacity than PNCNs. As depicted in Fig. 5e, NCNs demonstrate greater rate performance than PNCNs, exhibiting specific capacities of 245, 202, 160, 132, 108 mA h g 1 at current densities of 0.2, 0.4, 0.8, 1.6, 3.2 A g 1, respectively. CV curves of two samples in SIBs and PIBs at various scan rates from 0.2 to 1.0 mV s 1 are pictured in Fig. 6a and b and 6d, e. With the in crease of sweep speed, the desodiation peaks of PNCNs and NCNs become weaker, while the depotassiation peaks of PNCNs and NCNs get steeper. To estimate the kinetic performance at anodic peaks of the tested electrode, the linear relationship of log(i) versus log(ν) was plotted on Fig. 6c,f on the basis of the equation: i ¼ aνb :, where i rep resents the peak current, ν is the scan rate [30]. The value of b approaching 1 betokens that the charge storage is controlled by capac itive process, which is reckoned from the slope of the fitted line, the b-value close to 0.5 bespeaks that the electrochemical behavior is pri marily diffusion-controlled process [24]. The b-values of PNCNs/Na, NCNs/Na, PNCNs/K and NCNs/K cells are 0.63, 0.56, 0.65 and 0.96, indicating that at anodic peaks the first and the third are dominated by a mixed mechanism of capacitive process and diffusion process, the sec ond is chiefly diffusion-controlled process and the last is mainly capacitive process. To further investigate the difference between K-storage and Nastorage mechanism in two obtained samples, galvanostatic intermit tent titration (GITT) technique was conducted (Fig. 7a and b). If there is a linear relationship between cell potential and the square root of pulse time (Fig. S3), the Naþ and Kþ diffusion coefficient can be calculated
from the following equation based on Fick’s second law, � �2 � �2 � � 4 mB VM ΔEs L2 τ≪ D¼ πτ MB S ΔEτ D where τ is the pulse current duration, mB, VM and MB are the mass, molar volume and molar mass of electrode active materials, S and L are the area and average thickness of the tested electrode [31,32]. ΔEs and ΔEτ can be evaluated from GITT profiles according to Fig. S4. As Fig. 7c diagrammed, Naþ diffusion coefficient (DNaþ) in NCNs electrode is slightly larger than that in PNCNs electrode when the potential is above 1.0 V. However, DNaþ of PNCNs holds a higher value than that of NCNs at low voltage domains. Considering that discharge capacities of PNCNs and NCNs are mainly exhibited below 1.0 V, the higher DNaþ makes it advantageous for PNCNs to store sodium, which is in accordance with the galvanostatic charge-discharge test results (Table 1). NCNs with a larger K-storage capacity delivers a higher Kþ diffusion coefficient (DKþ) than PNCNs at whole test voltage range (Fig. 7d). When comparing DNaþ and DKþ in one sample (PNCNs or NCNs, Fig. 7e and f and Table 1), there is also a correlation between ion diffusion coefficients and capacities, where PNCNs with higher DNaþ than DKþ performs better on Na-storage rather than K-storage and NCNs with larger DKþ is more suitable for K-storage. 4. Conclusions In summary, we have synthesized high nitrogen content carbon nanosheets and further introduced phosphorus to nanosheets by means of molten salt method. Electrochemical Na-storage and K-storage properties of two kinds of carbon nanosheets were studied, where the nanosheets without phosphorus doping are more appropriate for anode of PIBs with a specific capacity of 237.7 mA h g 1 at 0.1 A g 1 after 250 cycles and phosphorus doped nanosheets provides higher Na-storage capacity of 246.5 mA h g 1 after 100 cycles at 0.1 A g 1. Naþ and Kþ diffusion coefficients of two samples were measured, demonstrating that the introduction of phosphorus to nitrogen-rich carbon nanosheets im proves the Naþ diffusion coefficient and lowers the Kþ diffusion coeffi cient of the host material. The electrode with larger ion diffusion 6
Y. Zhang et al.
Materials Chemistry and Physics 236 (2019) 121809
Fig. 7. GITT profiles of PNCNs and NCNs electrodes during discharge process in (a) SIBs and (b) PIBs; (c) Na-ion and (d) K-ion diffusion coefficients as a function of state of discharge process; Na-ion and K-ion diffusion coefficients in (e) PNCNs and (f) NCNs. Table 1 Structural information, chemical compositions and electrochemical properties of NCNs and PNCNs. Sample NCNs PNCNs a b
D002 (nm) 0.336 0.355
IG/ID 0.97 1.03
SBET (m2 g 254.5 87.5
1
)
Element content (at %)
C (mA h g
1 a
)
D (10
11
, cm2 s 1)b
C
N
O
P
SIBs
PIBs
Naþ
Kþ
80.24 74.81
15.17 13.03
4.59 9.99
2.17
175.1 247.9
242.6 207.2
0.21 0.55
0.54 0.10
The discharge capacities are obtained from the 50th cycle at a current density of 0.1 A g 1. The ion diffusion coefficients are calculated from GITT at near 0.2 V during discharge processes.
coefficient delivers a larger specific capacity, implying that heteroatom doping can been utilized to ameliorate the ion diffusion coefficient and potassium/sodium storage performance of the carbon material. This work may offer a reference for designing carbon anode materials of SIBs and PIBs.
China (51622406, 21673298), Young Elite Scientists Sponsorship Pro gram by CAST (2017QNRC001), Project of Postdoctoral Innovative Talents (BX201600192) and Project of Innovation Driven Plan in Central South University (2017CX004, 2018CX005). Appendix A. Supplementary data
Acknowledgments
Supplementary data to this article can be found online at https://doi. org/10.1016/j.matchemphys.2019.121809.
This work was supported by National Natural Science Foundation of 7
Y. Zhang et al.
Materials Chemistry and Physics 236 (2019) 121809
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