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reduced graphene oxide by spray drying as cathode materials for sodium ion batteries
Solid State Sciences 94 (2019) 77–84
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Preparation of Na4Mn9O18/carbon nanotube/reduced graphene oxide by spray drying as cathode materials for sodium ion batteries
T
Zhenzhen Shana, Yusen Hea, Taizhe Tanb, Yongguang Zhanga,∗, Xin Wangc,∗∗ a
School of Materials Science and Engineering, Hebei University of Technology, Tianjin, 300130, China Synergy Innovation Institute of GDUT, Heyuan, Guangdong Province, China c International Academy of Optoelectronics at Zhaoqing, South China Normal University, Guangdong, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Na4Mn9O18/carbon nanotube/reduced graphene oxide Aqueous sodium-ion batteries Cathode Energy storage
In this work, we report a new type of aqueous sodium-ion batteries employing Na4Mn9O18/carbon nanotube/ reduced graphene oxide (NMO/CNT/RGO) composite as a cathode material while zinc metal as anode material. The NMO/CNT/RGO composites are prepared by a spray drying method and possess a microsphere structure. Addition of RGO and CNT significantly improve the electrical conductivity of NMO composite materials. A stable network skeleton structure was formed in the interior and was beneficial to relieve stress and strain caused due to sodium ion movement and significantly improve the electrochemical performance of cathode materials. The initial reversible discharge specific capacity of the ternary composite electrode is 96.2 mAh g−1 at 4 C, and retained 68% capacity after 150 cycles (discharge capacity 65.8 mAh g−1). NMO/CNT/RGO composites have an excellent rate performance and high cycle capacity.
1. Introduction Lithium-ion batteries (LIBs) have been widely used in the world due to their high energy density, long life and least environmental effect [1,2]. LIBs are used extensively in power electronics, electric vehicles and others portable power sources [3]. Due to increase in power demands every day, limited lithium availability in the earth's reserves, and high Li price intrigued researcher to look for alternatives electrochemical technologies [4–6]. Due to similar electrochemical properties of sodium and lithium, sodium-ion batteries have shown potential to an alternative energy storage system and in future substitute lithium-ion batteries. Sodium is widely distributed in the form of salt on the planet and significantly cheaper than lithium, usually 1/10 of the lithium salt [7]. All these advantages with proper technology improvement can make sodium-ion battery easily available for large-scale energy storage applications [8]. In sodium ion batteries (SIB), organic liquid electrolytes can be replaced with aqueous electrolytes. The use of non-flammable aqueous electrolytes can reduce the cost of the batteries, besides, the aqueous electrolytes could provide higher ionic conductivity than most organic electrolytes [9–22]. Due to all these merits, aqueous sodium ion batteries (ASIBs) have attracted a lot of attention. At present, various sodium-based materials such as Na3V2(PO4)3 [20], Na2/3Mn1-xAlxO2 [23], ∗
Na2FeP2O7 [24], Na2CuFe(CN)6 [25], P2-Na2/3Mn1/2Co1/3Cu1/6O2 [26], P2-Na2/3Ni1/3Mn7/12Fe1/12O2 [27], P2-Na2/3Ni1/3Mn5/9Al1/9O2 [28] and Na4Mn9O18 [6,7] are studied as active materials for ASIBs [29]. However, different from the well-known LIB counterparts, the sodium ion battery systems face a set of new challenges: the radius of Na+ is larger than that of Li+, which results in sluggish kinetics during the charge/discharge process [30]. Besides, the cathodes for SIBs usually suffer intrinsic low electrical, which limits their practical application. Numerous attempts have been made to overcome these challenges by constructing composite cathodes within a conductive framework, such as graphene, carbon nanotubes, and other carbon materials [31–39]. Herein, we will explore a new three dimensional (3D) ternary composite material, i.e. Na4Mn9O18/carbon nanotubes/reduced graphene oxide (NMO/CNT/RGO) prepared by spray dry method (Fig. 1). In the composite, a three-dimensional 3D CNT/RGO scaffolding material produced by connecting one-dimensional (1D) CNT and two-dimensional (2D) RGO provides large surface area and high interconnected electron pathways for active material NMO, and NMO homogeneously and tightly lies in this effective and robust 3D network, which significantly improves the structural stability and the conductivity of the whole composite. In a novel ASIB system, the NMO/ CNT/RGO composite works as cathode material together with zinc as
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Y. Zhang), [email protected] (X. Wang).
∗∗
https://doi.org/10.1016/j.solidstatesciences.2019.05.019 Received 18 February 2019; Received in revised form 8 May 2019; Accepted 27 May 2019 Available online 31 May 2019 1293-2558/ © 2019 Elsevier Masson SAS. All rights reserved.
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Fig. 1. Schematic of the experimental apparatus.
an anode material and Na+/Zn2 mixed ions as an electrolyte. By the synergistic effect of RGO and CNT, the NMO/CNT/RGO composite exhibits enhanced cyclic stability and rate capability.
CNT/RGO sample was identified by Raman spectroscopy (Horiba) at room temperature. The specific surface area were measured using a Tristar II 3020 instrument. High-resolution transmission electron microscopy (HR-TEM, JEOL 2100) and scanning electron microscopy (SEM, Hitachi S-4800) was performed to characterize the morphology and structure of the composite. Cathode was prepared by casting the slurry of NMO/CNT/RGO, polyvinyldiene fluoride and acetylene black in n-methyl-2-pyrrolidinone on carbon foil with a weight ratio of 8:1:1 respectively. Then, the NMO/CNT/RGO electrode material was placed in a 60 °C oven for 24 h. Finally, the electrodes with the active material mass loading of about 1.8 mg cm−2 and zinc thin slice were tailor circular electrodes by a slicer. The mixed electrolyte consisted of 0.5 mol L−1 Na2SO4 and 1 mol L−1 ZnSO4, and the pH was adjusted to 4 with 0.1 mol L−1 sulfuric acid titration. The galvanostatic charge/discharge tests were conducted at different current densities (1 C = 121.5 mA g−1) and voltage of 1.0–1.85 V (vs. Zn+/Zn). Absorbed glass fiber mats (NSG Corporation) serve as batteries separators [43]. The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were tested using the VersaSTAT electrochemical workstation (Princeton, VersaSTAT 4). CV was performed on coin cells at different scan rates between 1 and 2 V. The EIS measurement frequency is from 0.01 to 100 kHz.
2. Materials and methods 0.1 mol L−1 KMnO4 and 3.0 mol L−1 NaOH were mixed in 25 mL deionized water. Next, 25 mL of 0.28 mol L−1 MnSO4 was poured into the NaOH and KMnO4 solution and vigorously stirred till a dark brown precipitate was formed. The precipitate was allowed to stand at 25 °C for 24 h to get a Na-birnessite precursor. Na4Mn9O18 was synthesized by adding 4.38 g Na-birnessite precursor to 15 mol L−1 high concentration NaOH solution of 110 mL and stirred well until brown suspension was obtained. Precursor's solution was placed in 150 mL Teflon liner in a stainless steel autoclave, and incubated at 180 °C for 24 h. At last, final product was washed repeatedly with DI water to remove excess of NaOH and then further dried at 60 °C. 22.32 mL of Graphene oxide (GO, 2 mg mL−1, prepared by Hummer's method), 48 mg of carbon nanotubes (9 wt%, Timesnano, Chengdu) and 401.7 mg of Na4Mn9O18 mixture was poured in 100 mL of DI water and stirred for 30 min. The mixture was ultra-sonicated for an hour using an ultrasonic cell mill and the resulting solution was spray-dried. The spray drying [40–42] device adopts ordinary air pressure, air inlet rate of 6 m3 min−1, air inlet temperature of 200 °C and feed rate of 8 mL min−1 to obtain the NMO/ CNT/GO composites. The sample synthesized in the previous step was placed in a tube furnace (heat treatment at 350 °C for 2 h) and protected with N2 (or Ar) gas to prepare Na4Mn9O18/CNT/RGO composite. (Fig. 2). NMO/CNT/RGO component ratio analysis was conducted by thermogravimetric (TG, PerkinElmer, Series 7) analysis in the temperature range of 25–1000 °C at a heating rate of 5 °C min−1 under N2 atmosphere. The samples were characterized by X-ray diffraction (XRD, Bruker D8, Cu Kα radiation). The phase and structure of the NMO/
3. Results and discussions The X-ray diffraction patterns of Na4Mn9O18, Na4Mn9O18/CNT/ RGO, CNT, RGO are shown in Fig. 3. The diffraction peaks are consistent with JCPDS card number 27-0750 [44]. The diffraction peaks of the NMO/CNT/RGO samples were not significantly different from NMO, with only two insignificant carbon diffraction peaks at 25.2° and 44°. RGO has two broad peaks at 25.3° and 43.8°, which are characteristic diffraction peaks of graphene, corresponding to the (002) and (100) crystal faces of carbon [13]. The broad peaks to which the CNTs belong appear around 26° and 44.5°, corresponding to the (004) and (102) crystal faces of carbon [45]. In the XRD pattern of the NMO/ CNT/RGO composite, the carbon peak is weak, which may be due to the high content of NMO in the composite. Fig. 4a represents the Raman spectrum of NMO/CNT/RGO and pure NMO sample. There is a broad peak at 600-650 cm−1, which is a characteristic peak of NMO and is related to the stretching vibration of the Mn-O bond in NMO [46]. Different from NMO, the Raman spectrum of NMO/CNT/RGO composite shows two distinct broad peaks at 1352 cm−1 and 1594 cm−1 [47], which are typical D and G peaks for carbon materials. The D peak represents the lattice defect of carbon atoms in the sample, G peak belongs to the ordered graphitic carbon [48]. The high intensity ratio of D band and G band (ID/IG = 0.9389) is
Fig. 2. The schematic diagram of NMO/CNT/RGO. 78
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Fig. 3. XRD patterns of NMO/CNT/RGO, NMO, CNT and RGO.
RGO, rod-shaped CNT and Na4Mn9O18. Furthermore, TEM analysis was conducted on NMO/CNT/RGO to further investigate the structure and cross-linkage between individual components. TEM image shows that the diameter of the NMO/CNT/RGO particles is 3.65 μm, which is in close agreement with SEM results (Fig. 5d). The resulting network structure of CNT and RGO are shown in Fig. 5e and f. Morphology and cross-linked network assists in improving electrical conductivity and structural stability of the composite material. High-resolution TEM (HRTEM) and selected area (electron) diffraction (SAED) image analysis was performed on NMO/CNT/RGO and results are shown in Fig. 6. Fig. 6a is a HRTEM image of NMO/CNT/RGO. The lattice structure of NMO can be seen from the figure, and its lattice spacing is 0.45 nm, corresponding to the NMO (200) crystal plane. The diffraction spots in the SAED pattern were formed from single crystal NMO, thereby demonstrating that the crystallinity of NMO is high (Fig. 6b). The diffraction ring present in the SAED diagram is formed by CNT/RGO, and the diffraction spots are more ambiguous, which is consistent with the XRD results of the NMO/CNT/RGO composite. Fig. 7 shows the specific capacity of NMO/CNT/RGO composites and NMO,NMO/RGO materials after 150 cycles at 4 C, and the specific capacity of NMO/CNT materials after 100 cycles at 4 C. The first reversible discharge capacity of NMO/CNT/RGO composites was 96.2 mAh g−1, and there was still a retention of 65.8 mAh g−1 reversible capacity after 150 cycles. After 150 cycles at 4 C, NMO, NMO/ CNT and NMO/RGO discharge capacities were 30.9 mAh g−1, 45.8 mAh g−1 and 48.1 mAh g−1, respectively. The NMO/CNT/RGO electrode exhibits excellent cycle stability and high reversible specific capacity. Superior stability of NMO/CNT/RGO can be attributed to interlinked composite structure of electrode material and high capacity of material can related with presence of conductive CNT and RGO in the electrode material. CNT can provide an additional electron channel and boost charge transfer rate. Fig. 8a shows a cyclic voltammetry (CV) plot of a sodium ion battery at a scan rate of 0.1 mV s−1. During the scan of each cycle, the two oxidation peaks appear at 1.54–1.57 V and 1.60–1.65 V, respectively, corresponding to the deintercalation process of sodium ions in NMO/ CNT/RGO when the battery was charged. The two reduction peaks
a reflection of high ordered graphitic carbon. In the Raman spectrum of NMO/CNT/RGO, there are both NMO peaks and carbon peaks, and there are no other peaks, indicating that NMO and CNT/RGO are successfully combined. In Fig. 4b, the first weight loss of the composites was observed at 120 °C, which can be due to evaporation of water molecules in the composite material. The NMO content in NMO-RGO composite was estimated using TGA analysis in air. The results presented in Fig. 4b show that a first weight loss occurs in composites between room temperature and 200 °C, which is due to evaporation of water molecules in the composite material. The following major weight loss occurring from 200 to 800 °C is related to the CNT and RGO fastslow combustion in air. When the temperature increases from 800 °C to 900 °C, the NMO/CNT/RGO composites maintain their weights almost unchanged, and these data allows estimating the NMO content in sample as 74.45%. The results of nitrogen adsorption/desorption isotherms of NMO and NMO/CNT/RGO composites are shown in Fig. 4c. From the adsorption branch of the isotherm, the BET specific surface areas of NMO, NMO/CNT/RGO composites are 23.6 cm2 g−1, 38.2 cm2 g−1, respectively. The value of the NMO/CNT/RGO composite is higher than that the NMO sample. These results can be explained by rod-shaped Na4Mn9O18 and CNT are tightly packed together within the layered RGO to form a sphere like morphology. The total pore volumes of NMO and NMO/CNT/RGO composites are 0.216 cm3 g−1, 0.383 cm3 g−1, respectively. The relatively large specific surface area and pore volume provided by NMO/CNT/RGO composite would increase the contact area of electrolyte/electrode, which is beneficial for decreasing the areal current density when the composites are used as a cathode for sodium rechargeable hybrid aqueous battery. Fig. 5 shows SEM and TEM image of the NMO/CNT/RGO composite material to demonstrate the morphology and structure of synthesized material. Fig. 5a shows spherical/oval shape of NMO/CNT/RGO composite material with diameter of about 3–6 μm. While, Fig. 5b shows a zoomed-in image of individual NMO/CNT/RGO particles, which have a diameter of about 3.67 μm. The rod-shaped Na4Mn9O18 and CNT are tightly packed together within the layered RGO to form a sphere like morphology. Fig. 5c clearly shows the cross-linkage between layered 79
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Fig. 4. (a) Raman spectrum. (b) TGA curves of the NMO/CNT/RGO composites. (c) The nitrogen adsorption-desorption isotherm curves of NMO and NMO/CNT/ RGO composites.
appear at 1.13–1.16 V and 1.34–1.37 V, respectively, corresponding to the process of intercalating sodium ions into NMO/CNT/RGO when the battery was discharged. The peak current of oxidation peak in the first cycle is higher than the other cycle parts, which may be caused by the polyatomic phase transition of NMO/CNT/RGO during the first charge to accommodate the deintercalation of sodium ions [49]. It shows that the composite material has a certain degree of irreversibility in the first cycle. In order to prevent the occurrence of hydrolysis side reactions, the upper limit of the charge and discharge voltage of the sodium ion battery of the system cannot exceed 2 V [13]. The storage capacity of Na ions is determined by discharging/ charging the battery with a constant current. The charge and discharge curves for the voltage range from 1 to 1.85 V are shown in Fig. 8b. As shown, in the first cycle, the charge capacity is only 28.4 mAh g−1, which is due to the decomposition of interstitial water and low content of Na ion. And considerable corresponding discharge capacity is 118.3 mAh g−1, which is due to the sodium deintercalated from the NMO during charging and Na+ in the electrolyte are intercalated into the NMO/CNT/RGO. The electrode and total reactions are simply shown as follows:
Fig. 6. The morphology and structure of the NMO/CNT/RGO, (a) HRTEM image; (b) SAED pattern.
capacity of the NMO/CNT/RGO electrode was found to be 150, 106, 93, and 84 mAh g−1 at 1 C, 2 C, 3 C, and 4 C rates, respectively. When the rate returns to 1 C, the electrodes capacity comes back to 150.6 mAh g−1. Additionally, the reversible capacity increase (the activation process) in the first 10 cycles is possibly due to a slow infiltration of electrolyte into the interior of the NMO/CNT/RGO microsphere, which creates gradually the electrochemically active interface [50]. After the first 10 cycles, the potential plateaus were well-maintained upon further cycling. The reversible discharge capacities of other electrodes at each current rate are lower than NMO/CNT/RGO. These results show that the high conductivity and excellent stability of NMO/CNT/RGO electrodes. The excellent stability of the NMO/CNT/RGO electrode may be related to the microsphere structure of NMO/CNT/RGO. The curved CNTs are interspersed between the NMO to form a crosslinked network structure, and the RGO forms a layered network structure, and the three materials wrap together to form a three-dimensional structure, which can more effectively prevent the loss of sodium. The electrochemical impedance spectroscopy (EIS) of the NMO and NMO/CNT/RGO electrodes measured in the frequency range of 0.01–100 kHz are shown in Fig. 8d. It is not difficult to see that the charge transfer resistance (RCT) of NMO, NMO/CNT and NMO/RGO electrodes are 300, 218 and 178 Ω, respectively. The charge transfer resistance of the NMO/CNT/RGO electrode is only 45 Ω. The inset is a simple equivalent circuit diagram of the EIS. CPE represents the double
The anode electrode: Zn2+ + 2 e− ⇔ Zn The cathode electrode: Na4Mn9O18 ⇔ Na4-xMn9O18 + x Na+ + x eThe total reaction: 2Na4Mn9O18 + x Zn2+ ⇔ 2 Na4-xMn9O18 + 2× Na+ + x Zn From the 2nd to 4th cycles, the NMO/CNT/RGO cathode material exhibited a high specific capacity with discharge capacities of 96.2 mAh g−1, 87.6 mAh g−1, and 83.7 mAh g−1, respectively. The efficiencies are 105.3%, 98.5%, and 99.2%, respectively, indicating that as the cycle progresses, the amount of sodium ion intercalation and deintercalation tends to balance, and the electrochemical performance gradually become stable. The charging curve of the 2nd to 4th cycles has a voltage platform around 1.55 V and 1.63 V, and the discharge curve has a voltage platform around 1.16 V and 1.37 V, which is consistent with the CV curve result. Fig. 8c shows the electrochemical performance of aqueous sodium ion batteries at different current densities. Reversible discharge
Fig. 5. The morphology and structure of the NMO/CNT/RGO composite, (a–c) SEM images; (d–f) TEM images. 81
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Fig. 7. Cycling performances and coulombic efficiency of NMO, NMO/RGO, NMO/CNT and NMO/CNT/RGO electrodes at 4 C.
4. Conclusions
layer capacitance between the electrolyte and the electrode interface resistance, RS stands for the electrolyte resistance, RCT stands for the charge transfer resistance of the electrolyte and the electrode interface, and ZW represents the diffusion impedance of the ion, also known as the Warburg impedance [10,22]. The lower in charge transfer resistance of the NMO/CNT/RGO electrode is due to the addition of CNT and RGO, which reduces the batteries resistance and provides addition network and channels for charge transfer and improves the electrochemical performance of the battery. As shown in Table 1, it is apparent that comparing the performance of the sodium-based ion battery of different Na4Mn9O18 composite cathodes, we can obtain that the electrochemical performance of the ternary composite electrode obtained in this work is more stable. After circulating 150 times at 4 C, the sodium battery had a discharge capacity of 65.8 mAh g−1.
In conclusion, NMO/CNT/RGO composites were successfully prepared via spray drying method. The sodium ion battery with the metal zinc as an anode and the NMO/CNT/RGO composite as a cathode material exhibits superior rate performance. After circulating 150 cycles at 4 C, the discharge capacity of the sodium battery was still 65.8 mAh g−1. The superior electrochemical performance of NMO/ CNT/RGO composites material is attributed to the interconnected spherical structure of the composite material which was achieved due to spray drying method. This method is simple, and can effectively maintain the three-dimensional structure of the composites, therefore, improve the electrical conductivity of the materials and effectively prevent the loss of sodium, and enable the battery to have excellent cycle performance and a long life.
Fig. 8. Electrochemical performance of NMO/CNT/RGO electrodes. (a) CV behavior of NMO/CNT/RGO electrodes at a scan rate of 0.1 mV s−1; (b) Discharge/charge voltage profiles of NMO/CNT/RGO electrodes for the 1st, 2nd, 3rd, and 4th cycles at 4 C; (c) The rate performances of NMO, NMO/CNT, NMO/RGO and NMO/CNT/ RGO electrodes from 1 to 4 C; (d) EIS spectra of NMO, NMO/CNT, NMO/RGO and NMO/CNT/RGO electrodes. 82
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Table 1 Electrochemical Performance comparison of Na4Mn9O18 composite cathode for sodium-based battery. Materials
Current density (discharge)
Initial discharge capacity (mAh/g)
Discharge capacity (mAh/g) (after n th)
Reference
Na4Mn9O18 Na4Mn9O18/NR Na4Mn9O18/NF Na4Mn9O18/RGO Na4Mn9O18/CNT Na4Mn9O18/CNT/RGO
4C 0.42 C 0.42 C 4C 4C 4C
68.9 118 117 61.7 85.6 96.2
40 (100) 60 (100) 45 (50) 58.9 (150) 53.2 (150) 65.8 (150)
[51] [49] [49] [10] [22] This work
Acknowledgements
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This work was supported by the Program for the Outstanding Young Talents of Hebei Province; Scientific Research Foundation for Selected Overseas Chinese Scholars, Ministry of Human Resources and Social Security of China [Grant No. CG2015003002]; and Cultivation Project of National Engineering Technology Center [Grant No. 2017B090903008]. References [1] A. Zhamu, G.R. Chen, C.G. Liu, D. Neff, Q. Fang, Z.N. Yu, W. Xiong, Y.B. Wang, X.Q. Wang, B. Jang, Reviving rechargeable lithium metal batteries: enabling nextgeneration high-energy and high-power cells, Energy Environ. Sci. 5 (2012) 5701–5707. [2] H.P. Li, Y.Q. Wei, Y.G. Zhang, C.W. Zhang, G.K. Wang, Y. Zhao, F.X. Y, Z. Bakenov, In situ sol-gel synthesis of ultrafine ZnO nanocrystals anchored on graphene as anode material for lithium-ion batteries, Ceram. Int. 42 (2016) 12371–12377. [3] J.L. Shi, D.D. Xiao, M.Y. Ge, X.Q. Yu, Y. Chu, X.J. Huang, X.D. Zhang, Y.X. Yin, X.Q. Yang, Y.G. Guo, L. Gu, L.J. Wan, High-capacity cathode material with high voltage for Li-ion batteries[J], Adv. Mater. 30 (9) (2018) 1705575. [4] S. Kalluri, K.H. Seng, W.K. Pang, Z.P. Guo, Z.X. Chen, H.K. Liu, S.X. Dou, Electrospun P2-type Na(2/3)(Fe(1/2)Mn(1/2))O2 hierarchical nanofibers as cathode material for sodium-ion batteries, ACS Appl. Mater. Interfaces 6 (2014) 8953–8958. [5] N. Yabuuchi, K. Kubota, M. Dahbi, S. Komaba, Research development on sodiumion batteries, Chem. Rev. 114 (2014) 11636–11682. [6] H. Lim, H.J. Ji, Y.M. Park, H.N. Lee, H.J. Kim, High-performance aqueous rechargeable sulfate- and sodium-ion battery based on polypyrrole-MWCNT core-shell nanowires and Na0.44MnO2 nanorods, Appl. Surf. Sci. 446 (2018) 131–138. [7] S. Demirel, E. Oz, E. Altin, A. Bayri, P. Kaya, S. Turan, S. Avci, Growth mechanism, magnetic and electrochemical properties of Na0.44MnO2 nanorods as cathode material for Na-ion batteries, Mater. Char. 105 (2015) 104–112. [8] M.I. Jamesh, A.S. Prakash, Advancement of technology towards developing Na-ion batteries, J. Power Sources 378 (2018) 268–300. [9] H. Kim, J. Hong, K.Y. Park, H. Kim, S.W. Kim, K. Kang, Aqueous rechargeable Li and Na ion batteries, Chem. Rev. 114 (2014) 11788–11827. [10] F.X. Yin, Z.J. Liu, Y. Zhao, Y.T. Feng, Y.G. Zhang, Electrochemical properties of an Na4Mn9O18-reduced graphene oxide composite synthesized via spray drying for an aqueous sodium-ion battery, Nanomaterials 7 (2017) 253. [11] M. Pasta, C.D. Wessells, R.A. Huggins, Y. Cui, A high-rate and long cycle life aqueous electrolyte battery for grid-scale energy storage, Nat. Commun. 3 (2012) 1149. [12] C.D. Wessells, S.V. Peddada, R.A. Huggins, Y. Cui, Nickel hexacyanoferrate nanoparticle electrodes for aqueous sodium and potassium ion batteries, Nano Lett. 11 (2011) 5421–5425. [13] G.H. Yuan, J.M. Xiang, H.F. Jin, Y.Z. Jin, L.Z. Wu, Y.G. Zhang, A. Mentbayeva, Bakenov, Flexible free-standing Na4Mn9O18/reduced graphene oxide composite film as a cathode for sodium rechargeable hybrid aqueous battery, Electrochim. Acta 259 (2018) 647–654. [14] J. Rodríguez-García, I. Cameán, A. Ramos, E. Rodríguez, A. García, Graphitic carbon foams as anodes for sodium-ion batteries in glyme-based electrolytes, Electrochim. Acta 270 (2018) 236–244. [15] G. Yee, S. Shanbhag, W. Wu, K. Carlisle, J. Chang, TiP2O7 exhibiting reversible interaction with sodium ions in aqueous electrolytes, Electrochem. Commun. 86 (2017) 104–107. [16] A. Eguia-Barrio, E. Castillo-Martínez, F. Klein, R. Pinedo, L. Lezama, J. Janek, P. Adelhelm, T. Rojo, Electrochemical performance of CuNCN for sodium ion batteries and comparison with ZnNCN and lithium ion batteries, J. Power Sources 367 (2017) 130–137. [17] G. Longoni, J.E. Wang, Y.H. Jung, D.K. Kim, C. Mari, R. Ruffo, The Na2FeP2O7carbon nanotubes composite as high rate cathode material for sodium ion batteries, J. Power Sources 302 (2016) 61–69. [18] V.G. Pol, E. Lee, D. Zhou, F. Dogan, J. Calderon-Moreno, C. Johnson, Spherical carbon as a new high-rate anode for sodium-ion batteries, Electrochim. Acta 127 (2014) 61–67. [19] M. Vujković, S. Mentus, Potentiodynamic and galvanostatic testing of NaFe0.95V0.05PO4/C composite in aqueous NaNO3 solution, and the properties of aqueous Na1.2V3O8/NaNO3/NaFe0.95V0.05PO4/C battery, J. Power Sources 325 (2016) 185–193.
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Preparation of Na4Mn9O18/carbon nanotube/reduced graphene oxide by spray drying as cathode materials for sodium ion batteries
T
Zhenzhen Shana, Yusen Hea, Taizhe Tanb, Yongguang Zhanga,∗, Xin Wangc,∗∗ a
School of Materials Science and Engineering, Hebei University of Technology, Tianjin, 300130, China Synergy Innovation Institute of GDUT, Heyuan, Guangdong Province, China c International Academy of Optoelectronics at Zhaoqing, South China Normal University, Guangdong, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Na4Mn9O18/carbon nanotube/reduced graphene oxide Aqueous sodium-ion batteries Cathode Energy storage
In this work, we report a new type of aqueous sodium-ion batteries employing Na4Mn9O18/carbon nanotube/ reduced graphene oxide (NMO/CNT/RGO) composite as a cathode material while zinc metal as anode material. The NMO/CNT/RGO composites are prepared by a spray drying method and possess a microsphere structure. Addition of RGO and CNT significantly improve the electrical conductivity of NMO composite materials. A stable network skeleton structure was formed in the interior and was beneficial to relieve stress and strain caused due to sodium ion movement and significantly improve the electrochemical performance of cathode materials. The initial reversible discharge specific capacity of the ternary composite electrode is 96.2 mAh g−1 at 4 C, and retained 68% capacity after 150 cycles (discharge capacity 65.8 mAh g−1). NMO/CNT/RGO composites have an excellent rate performance and high cycle capacity.
1. Introduction Lithium-ion batteries (LIBs) have been widely used in the world due to their high energy density, long life and least environmental effect [1,2]. LIBs are used extensively in power electronics, electric vehicles and others portable power sources [3]. Due to increase in power demands every day, limited lithium availability in the earth's reserves, and high Li price intrigued researcher to look for alternatives electrochemical technologies [4–6]. Due to similar electrochemical properties of sodium and lithium, sodium-ion batteries have shown potential to an alternative energy storage system and in future substitute lithium-ion batteries. Sodium is widely distributed in the form of salt on the planet and significantly cheaper than lithium, usually 1/10 of the lithium salt [7]. All these advantages with proper technology improvement can make sodium-ion battery easily available for large-scale energy storage applications [8]. In sodium ion batteries (SIB), organic liquid electrolytes can be replaced with aqueous electrolytes. The use of non-flammable aqueous electrolytes can reduce the cost of the batteries, besides, the aqueous electrolytes could provide higher ionic conductivity than most organic electrolytes [9–22]. Due to all these merits, aqueous sodium ion batteries (ASIBs) have attracted a lot of attention. At present, various sodium-based materials such as Na3V2(PO4)3 [20], Na2/3Mn1-xAlxO2 [23], ∗
Na2FeP2O7 [24], Na2CuFe(CN)6 [25], P2-Na2/3Mn1/2Co1/3Cu1/6O2 [26], P2-Na2/3Ni1/3Mn7/12Fe1/12O2 [27], P2-Na2/3Ni1/3Mn5/9Al1/9O2 [28] and Na4Mn9O18 [6,7] are studied as active materials for ASIBs [29]. However, different from the well-known LIB counterparts, the sodium ion battery systems face a set of new challenges: the radius of Na+ is larger than that of Li+, which results in sluggish kinetics during the charge/discharge process [30]. Besides, the cathodes for SIBs usually suffer intrinsic low electrical, which limits their practical application. Numerous attempts have been made to overcome these challenges by constructing composite cathodes within a conductive framework, such as graphene, carbon nanotubes, and other carbon materials [31–39]. Herein, we will explore a new three dimensional (3D) ternary composite material, i.e. Na4Mn9O18/carbon nanotubes/reduced graphene oxide (NMO/CNT/RGO) prepared by spray dry method (Fig. 1). In the composite, a three-dimensional 3D CNT/RGO scaffolding material produced by connecting one-dimensional (1D) CNT and two-dimensional (2D) RGO provides large surface area and high interconnected electron pathways for active material NMO, and NMO homogeneously and tightly lies in this effective and robust 3D network, which significantly improves the structural stability and the conductivity of the whole composite. In a novel ASIB system, the NMO/ CNT/RGO composite works as cathode material together with zinc as
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Y. Zhang), [email protected] (X. Wang).
∗∗
https://doi.org/10.1016/j.solidstatesciences.2019.05.019 Received 18 February 2019; Received in revised form 8 May 2019; Accepted 27 May 2019 Available online 31 May 2019 1293-2558/ © 2019 Elsevier Masson SAS. All rights reserved.
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Fig. 1. Schematic of the experimental apparatus.
an anode material and Na+/Zn2 mixed ions as an electrolyte. By the synergistic effect of RGO and CNT, the NMO/CNT/RGO composite exhibits enhanced cyclic stability and rate capability.
CNT/RGO sample was identified by Raman spectroscopy (Horiba) at room temperature. The specific surface area were measured using a Tristar II 3020 instrument. High-resolution transmission electron microscopy (HR-TEM, JEOL 2100) and scanning electron microscopy (SEM, Hitachi S-4800) was performed to characterize the morphology and structure of the composite. Cathode was prepared by casting the slurry of NMO/CNT/RGO, polyvinyldiene fluoride and acetylene black in n-methyl-2-pyrrolidinone on carbon foil with a weight ratio of 8:1:1 respectively. Then, the NMO/CNT/RGO electrode material was placed in a 60 °C oven for 24 h. Finally, the electrodes with the active material mass loading of about 1.8 mg cm−2 and zinc thin slice were tailor circular electrodes by a slicer. The mixed electrolyte consisted of 0.5 mol L−1 Na2SO4 and 1 mol L−1 ZnSO4, and the pH was adjusted to 4 with 0.1 mol L−1 sulfuric acid titration. The galvanostatic charge/discharge tests were conducted at different current densities (1 C = 121.5 mA g−1) and voltage of 1.0–1.85 V (vs. Zn+/Zn). Absorbed glass fiber mats (NSG Corporation) serve as batteries separators [43]. The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were tested using the VersaSTAT electrochemical workstation (Princeton, VersaSTAT 4). CV was performed on coin cells at different scan rates between 1 and 2 V. The EIS measurement frequency is from 0.01 to 100 kHz.
2. Materials and methods 0.1 mol L−1 KMnO4 and 3.0 mol L−1 NaOH were mixed in 25 mL deionized water. Next, 25 mL of 0.28 mol L−1 MnSO4 was poured into the NaOH and KMnO4 solution and vigorously stirred till a dark brown precipitate was formed. The precipitate was allowed to stand at 25 °C for 24 h to get a Na-birnessite precursor. Na4Mn9O18 was synthesized by adding 4.38 g Na-birnessite precursor to 15 mol L−1 high concentration NaOH solution of 110 mL and stirred well until brown suspension was obtained. Precursor's solution was placed in 150 mL Teflon liner in a stainless steel autoclave, and incubated at 180 °C for 24 h. At last, final product was washed repeatedly with DI water to remove excess of NaOH and then further dried at 60 °C. 22.32 mL of Graphene oxide (GO, 2 mg mL−1, prepared by Hummer's method), 48 mg of carbon nanotubes (9 wt%, Timesnano, Chengdu) and 401.7 mg of Na4Mn9O18 mixture was poured in 100 mL of DI water and stirred for 30 min. The mixture was ultra-sonicated for an hour using an ultrasonic cell mill and the resulting solution was spray-dried. The spray drying [40–42] device adopts ordinary air pressure, air inlet rate of 6 m3 min−1, air inlet temperature of 200 °C and feed rate of 8 mL min−1 to obtain the NMO/ CNT/GO composites. The sample synthesized in the previous step was placed in a tube furnace (heat treatment at 350 °C for 2 h) and protected with N2 (or Ar) gas to prepare Na4Mn9O18/CNT/RGO composite. (Fig. 2). NMO/CNT/RGO component ratio analysis was conducted by thermogravimetric (TG, PerkinElmer, Series 7) analysis in the temperature range of 25–1000 °C at a heating rate of 5 °C min−1 under N2 atmosphere. The samples were characterized by X-ray diffraction (XRD, Bruker D8, Cu Kα radiation). The phase and structure of the NMO/
3. Results and discussions The X-ray diffraction patterns of Na4Mn9O18, Na4Mn9O18/CNT/ RGO, CNT, RGO are shown in Fig. 3. The diffraction peaks are consistent with JCPDS card number 27-0750 [44]. The diffraction peaks of the NMO/CNT/RGO samples were not significantly different from NMO, with only two insignificant carbon diffraction peaks at 25.2° and 44°. RGO has two broad peaks at 25.3° and 43.8°, which are characteristic diffraction peaks of graphene, corresponding to the (002) and (100) crystal faces of carbon [13]. The broad peaks to which the CNTs belong appear around 26° and 44.5°, corresponding to the (004) and (102) crystal faces of carbon [45]. In the XRD pattern of the NMO/ CNT/RGO composite, the carbon peak is weak, which may be due to the high content of NMO in the composite. Fig. 4a represents the Raman spectrum of NMO/CNT/RGO and pure NMO sample. There is a broad peak at 600-650 cm−1, which is a characteristic peak of NMO and is related to the stretching vibration of the Mn-O bond in NMO [46]. Different from NMO, the Raman spectrum of NMO/CNT/RGO composite shows two distinct broad peaks at 1352 cm−1 and 1594 cm−1 [47], which are typical D and G peaks for carbon materials. The D peak represents the lattice defect of carbon atoms in the sample, G peak belongs to the ordered graphitic carbon [48]. The high intensity ratio of D band and G band (ID/IG = 0.9389) is
Fig. 2. The schematic diagram of NMO/CNT/RGO. 78
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Fig. 3. XRD patterns of NMO/CNT/RGO, NMO, CNT and RGO.
RGO, rod-shaped CNT and Na4Mn9O18. Furthermore, TEM analysis was conducted on NMO/CNT/RGO to further investigate the structure and cross-linkage between individual components. TEM image shows that the diameter of the NMO/CNT/RGO particles is 3.65 μm, which is in close agreement with SEM results (Fig. 5d). The resulting network structure of CNT and RGO are shown in Fig. 5e and f. Morphology and cross-linked network assists in improving electrical conductivity and structural stability of the composite material. High-resolution TEM (HRTEM) and selected area (electron) diffraction (SAED) image analysis was performed on NMO/CNT/RGO and results are shown in Fig. 6. Fig. 6a is a HRTEM image of NMO/CNT/RGO. The lattice structure of NMO can be seen from the figure, and its lattice spacing is 0.45 nm, corresponding to the NMO (200) crystal plane. The diffraction spots in the SAED pattern were formed from single crystal NMO, thereby demonstrating that the crystallinity of NMO is high (Fig. 6b). The diffraction ring present in the SAED diagram is formed by CNT/RGO, and the diffraction spots are more ambiguous, which is consistent with the XRD results of the NMO/CNT/RGO composite. Fig. 7 shows the specific capacity of NMO/CNT/RGO composites and NMO,NMO/RGO materials after 150 cycles at 4 C, and the specific capacity of NMO/CNT materials after 100 cycles at 4 C. The first reversible discharge capacity of NMO/CNT/RGO composites was 96.2 mAh g−1, and there was still a retention of 65.8 mAh g−1 reversible capacity after 150 cycles. After 150 cycles at 4 C, NMO, NMO/ CNT and NMO/RGO discharge capacities were 30.9 mAh g−1, 45.8 mAh g−1 and 48.1 mAh g−1, respectively. The NMO/CNT/RGO electrode exhibits excellent cycle stability and high reversible specific capacity. Superior stability of NMO/CNT/RGO can be attributed to interlinked composite structure of electrode material and high capacity of material can related with presence of conductive CNT and RGO in the electrode material. CNT can provide an additional electron channel and boost charge transfer rate. Fig. 8a shows a cyclic voltammetry (CV) plot of a sodium ion battery at a scan rate of 0.1 mV s−1. During the scan of each cycle, the two oxidation peaks appear at 1.54–1.57 V and 1.60–1.65 V, respectively, corresponding to the deintercalation process of sodium ions in NMO/ CNT/RGO when the battery was charged. The two reduction peaks
a reflection of high ordered graphitic carbon. In the Raman spectrum of NMO/CNT/RGO, there are both NMO peaks and carbon peaks, and there are no other peaks, indicating that NMO and CNT/RGO are successfully combined. In Fig. 4b, the first weight loss of the composites was observed at 120 °C, which can be due to evaporation of water molecules in the composite material. The NMO content in NMO-RGO composite was estimated using TGA analysis in air. The results presented in Fig. 4b show that a first weight loss occurs in composites between room temperature and 200 °C, which is due to evaporation of water molecules in the composite material. The following major weight loss occurring from 200 to 800 °C is related to the CNT and RGO fastslow combustion in air. When the temperature increases from 800 °C to 900 °C, the NMO/CNT/RGO composites maintain their weights almost unchanged, and these data allows estimating the NMO content in sample as 74.45%. The results of nitrogen adsorption/desorption isotherms of NMO and NMO/CNT/RGO composites are shown in Fig. 4c. From the adsorption branch of the isotherm, the BET specific surface areas of NMO, NMO/CNT/RGO composites are 23.6 cm2 g−1, 38.2 cm2 g−1, respectively. The value of the NMO/CNT/RGO composite is higher than that the NMO sample. These results can be explained by rod-shaped Na4Mn9O18 and CNT are tightly packed together within the layered RGO to form a sphere like morphology. The total pore volumes of NMO and NMO/CNT/RGO composites are 0.216 cm3 g−1, 0.383 cm3 g−1, respectively. The relatively large specific surface area and pore volume provided by NMO/CNT/RGO composite would increase the contact area of electrolyte/electrode, which is beneficial for decreasing the areal current density when the composites are used as a cathode for sodium rechargeable hybrid aqueous battery. Fig. 5 shows SEM and TEM image of the NMO/CNT/RGO composite material to demonstrate the morphology and structure of synthesized material. Fig. 5a shows spherical/oval shape of NMO/CNT/RGO composite material with diameter of about 3–6 μm. While, Fig. 5b shows a zoomed-in image of individual NMO/CNT/RGO particles, which have a diameter of about 3.67 μm. The rod-shaped Na4Mn9O18 and CNT are tightly packed together within the layered RGO to form a sphere like morphology. Fig. 5c clearly shows the cross-linkage between layered 79
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Fig. 4. (a) Raman spectrum. (b) TGA curves of the NMO/CNT/RGO composites. (c) The nitrogen adsorption-desorption isotherm curves of NMO and NMO/CNT/ RGO composites.
appear at 1.13–1.16 V and 1.34–1.37 V, respectively, corresponding to the process of intercalating sodium ions into NMO/CNT/RGO when the battery was discharged. The peak current of oxidation peak in the first cycle is higher than the other cycle parts, which may be caused by the polyatomic phase transition of NMO/CNT/RGO during the first charge to accommodate the deintercalation of sodium ions [49]. It shows that the composite material has a certain degree of irreversibility in the first cycle. In order to prevent the occurrence of hydrolysis side reactions, the upper limit of the charge and discharge voltage of the sodium ion battery of the system cannot exceed 2 V [13]. The storage capacity of Na ions is determined by discharging/ charging the battery with a constant current. The charge and discharge curves for the voltage range from 1 to 1.85 V are shown in Fig. 8b. As shown, in the first cycle, the charge capacity is only 28.4 mAh g−1, which is due to the decomposition of interstitial water and low content of Na ion. And considerable corresponding discharge capacity is 118.3 mAh g−1, which is due to the sodium deintercalated from the NMO during charging and Na+ in the electrolyte are intercalated into the NMO/CNT/RGO. The electrode and total reactions are simply shown as follows:
Fig. 6. The morphology and structure of the NMO/CNT/RGO, (a) HRTEM image; (b) SAED pattern.
capacity of the NMO/CNT/RGO electrode was found to be 150, 106, 93, and 84 mAh g−1 at 1 C, 2 C, 3 C, and 4 C rates, respectively. When the rate returns to 1 C, the electrodes capacity comes back to 150.6 mAh g−1. Additionally, the reversible capacity increase (the activation process) in the first 10 cycles is possibly due to a slow infiltration of electrolyte into the interior of the NMO/CNT/RGO microsphere, which creates gradually the electrochemically active interface [50]. After the first 10 cycles, the potential plateaus were well-maintained upon further cycling. The reversible discharge capacities of other electrodes at each current rate are lower than NMO/CNT/RGO. These results show that the high conductivity and excellent stability of NMO/CNT/RGO electrodes. The excellent stability of the NMO/CNT/RGO electrode may be related to the microsphere structure of NMO/CNT/RGO. The curved CNTs are interspersed between the NMO to form a crosslinked network structure, and the RGO forms a layered network structure, and the three materials wrap together to form a three-dimensional structure, which can more effectively prevent the loss of sodium. The electrochemical impedance spectroscopy (EIS) of the NMO and NMO/CNT/RGO electrodes measured in the frequency range of 0.01–100 kHz are shown in Fig. 8d. It is not difficult to see that the charge transfer resistance (RCT) of NMO, NMO/CNT and NMO/RGO electrodes are 300, 218 and 178 Ω, respectively. The charge transfer resistance of the NMO/CNT/RGO electrode is only 45 Ω. The inset is a simple equivalent circuit diagram of the EIS. CPE represents the double
The anode electrode: Zn2+ + 2 e− ⇔ Zn The cathode electrode: Na4Mn9O18 ⇔ Na4-xMn9O18 + x Na+ + x eThe total reaction: 2Na4Mn9O18 + x Zn2+ ⇔ 2 Na4-xMn9O18 + 2× Na+ + x Zn From the 2nd to 4th cycles, the NMO/CNT/RGO cathode material exhibited a high specific capacity with discharge capacities of 96.2 mAh g−1, 87.6 mAh g−1, and 83.7 mAh g−1, respectively. The efficiencies are 105.3%, 98.5%, and 99.2%, respectively, indicating that as the cycle progresses, the amount of sodium ion intercalation and deintercalation tends to balance, and the electrochemical performance gradually become stable. The charging curve of the 2nd to 4th cycles has a voltage platform around 1.55 V and 1.63 V, and the discharge curve has a voltage platform around 1.16 V and 1.37 V, which is consistent with the CV curve result. Fig. 8c shows the electrochemical performance of aqueous sodium ion batteries at different current densities. Reversible discharge
Fig. 5. The morphology and structure of the NMO/CNT/RGO composite, (a–c) SEM images; (d–f) TEM images. 81
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Fig. 7. Cycling performances and coulombic efficiency of NMO, NMO/RGO, NMO/CNT and NMO/CNT/RGO electrodes at 4 C.
4. Conclusions
layer capacitance between the electrolyte and the electrode interface resistance, RS stands for the electrolyte resistance, RCT stands for the charge transfer resistance of the electrolyte and the electrode interface, and ZW represents the diffusion impedance of the ion, also known as the Warburg impedance [10,22]. The lower in charge transfer resistance of the NMO/CNT/RGO electrode is due to the addition of CNT and RGO, which reduces the batteries resistance and provides addition network and channels for charge transfer and improves the electrochemical performance of the battery. As shown in Table 1, it is apparent that comparing the performance of the sodium-based ion battery of different Na4Mn9O18 composite cathodes, we can obtain that the electrochemical performance of the ternary composite electrode obtained in this work is more stable. After circulating 150 times at 4 C, the sodium battery had a discharge capacity of 65.8 mAh g−1.
In conclusion, NMO/CNT/RGO composites were successfully prepared via spray drying method. The sodium ion battery with the metal zinc as an anode and the NMO/CNT/RGO composite as a cathode material exhibits superior rate performance. After circulating 150 cycles at 4 C, the discharge capacity of the sodium battery was still 65.8 mAh g−1. The superior electrochemical performance of NMO/ CNT/RGO composites material is attributed to the interconnected spherical structure of the composite material which was achieved due to spray drying method. This method is simple, and can effectively maintain the three-dimensional structure of the composites, therefore, improve the electrical conductivity of the materials and effectively prevent the loss of sodium, and enable the battery to have excellent cycle performance and a long life.
Fig. 8. Electrochemical performance of NMO/CNT/RGO electrodes. (a) CV behavior of NMO/CNT/RGO electrodes at a scan rate of 0.1 mV s−1; (b) Discharge/charge voltage profiles of NMO/CNT/RGO electrodes for the 1st, 2nd, 3rd, and 4th cycles at 4 C; (c) The rate performances of NMO, NMO/CNT, NMO/RGO and NMO/CNT/ RGO electrodes from 1 to 4 C; (d) EIS spectra of NMO, NMO/CNT, NMO/RGO and NMO/CNT/RGO electrodes. 82
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Table 1 Electrochemical Performance comparison of Na4Mn9O18 composite cathode for sodium-based battery. Materials
Current density (discharge)
Initial discharge capacity (mAh/g)
Discharge capacity (mAh/g) (after n th)
Reference
Na4Mn9O18 Na4Mn9O18/NR Na4Mn9O18/NF Na4Mn9O18/RGO Na4Mn9O18/CNT Na4Mn9O18/CNT/RGO
4C 0.42 C 0.42 C 4C 4C 4C
68.9 118 117 61.7 85.6 96.2
40 (100) 60 (100) 45 (50) 58.9 (150) 53.2 (150) 65.8 (150)
[51] [49] [49] [10] [22] This work
Acknowledgements
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