1. Introduction
Batteries are essential device to support the apparent growth of portable electric devices, electric vehicles, and energy storage for new and renewable energy power plants. Until recently, lithium-ion batteries (LIBs) dominated the battery market; however, nowadays there is a substantial concern with the limited lithium abundance, which is only 0.01% on Earth, and the price increases fast up to $5000/ton [1]. Following the abundance, the supply, the lower price, and the comparable electrochemical performance to LIBs, sodium-ion batteries (SIBs) are becoming a feasible alternative for LIBs [2,3]. Some previous research on sodium batteries used amorphous or hard carbon (HCs) as anode material due to its high electronic conductivity, good thermal stability, environmental safety [4], and low-cost production [5]. Hard carbon also has a sizeable interlayer spacing of 0.36–0.40 nm, a tortuous structure, and contains micropores, which are essential for a potential host material in a secondary battery [6]. However, hard carbon as a host electrode still faces deficiencies for sodium batteries because Na+ has a larger ionic radius (1.02 Å) than Li+ (0.76 Å) [7], which causes hard Na+ to intercalate into the carbon microstructure. Therefore, some efforts to increase Na+ intercalation have been developed, such as by doping N, P, O into nanoporous carbon [8], and N, B co-doping carbon nanosheet [9].
Some researchers have successfully prepared HCs from biomass, such as wood [10], banana peels [11], cotton [12], corn stalk [13], cellulose [14,15], rice husk [16], and sunflower seed shells [17]. Bagasse is a solid waste of sugarcane production with high carbon content consisting of 50% cellulose, 25% hemicellulose, and 25% lignin [18]. The world bagasse production reached almost 100000 metric tons/year, spread over Brazil, India, China, Thailand, and some countries by 3500 metric tons/year average, i.e., USA, Pakistan, Mexico, Russia, France and Germany [19]. Meanwhile, Indonesia has a huge potential bagasse production of 9.9–11.2 million tons/year [20]. It became our background to investigate the carbon active preparation from bagasse [21–23]. Activation with water vapor was preferred to replace chemical activators such as KOH [24], ZnCl2 [25], and H3PO4 [26], considering the lower price and safety reasons for the environment.
In this research, the activated carbon that has been investigated previously; was further treated by submitting N and S into the C networks. Thiourea (CH4N2S) was used as an N/S precursor. This research aims to increase the electrochemical performance of the bagasse-activated carbon and projects the result as a functional electrode material for secondary battery. Some characterizations investigated the diffraction peaks, the functional groups available within, the indication of structure defects because of N/S doping, and the chemical structure elucidation based on the binding energies revealed from the material. The feasibility of the prepared material as a functional electrode was tested by combining the N/S-doped carbon with Na metal anode, and NaClO4 electrolyte in a CR2023 cell.
2. Experimental
2.1 Preparation of N/S-doped carbon and Its characterization
Carbon powder was prepared from bagasse, a solid waste of sugarcane production. The bagasse was washed with water, followed by carbonation at 600°C and then activated at 700°C under water vapor flows, as previously mentioned in our previous publication [23]. The activated carbon from bagasse (ACB) was then mashed and filtered at 100 mesh to get a small, homogenous carbon particle. Next, the produced powder was mixed with thiourea for dual N/S doping at a mass ratio 1:1 of carbon to the precursor with ethanol as solvent. 30 mL of ethanol was added to the mixture of carbon and precursor, stirred for 2 h at room temperature, and then heated in the oven at 80°C for 6 h. After heating in the oven, the mixture was dried, resulting in powder sediment at the flask’s bottom. Finally, the powder was heated at 850°C under N2 gas flow for 2 h to produce a black powder a black powder, named by NSCE.
The powder was then characterized to understand its crystallinity and the specific diffraction peaks through X-ray Diffraction (Rigaku Miniflex 600), the functional groups content by FT-IR analysis (Shimadzu IRrestige-21), XPS (JEOL JPS-9200, MgKa source, 10 kV, 10 mA), and impedance analysis at 20 Hz–200 kHz (LCR meter EUCOL 20 Hz–5 MHz) was conducted to understand the resistance (W) and the electrical conductivity (S cm−1). The undoped carbon was also analysed for comparison.
2.2 Preparation of the electrode film
A 0.7 g of the prepared powder, NSCE, was mixed with 0.1 g acetylene black (AB) and then crushed until homogenously mixed. The mixture was then dried at 60°C for 45 min. A 0.1 g of PVDF was dissolved within 4 mL NMP and stirred until completely dissolved. The NSCE-AB mixture was added gradually under a stirring condition for 3 h until it formed a homogenous slurry. The slurry was then cast on aluminium foil and dried under vacuum at 80°C for 12 h in a vacuum oven (B-One Vacuum Oven VOV-50, 220 V/1400 W).
2.3 Sodium battery fabrication and Battery test
The electrode film was assembled with NaClO4 electrolytes and a Na metal anode in a coin cell CR2032. The NaClO4 solution was fed into the PTFE membrane by dipping the membrane in 1 M NaClO4 solution in propylene carbonate for a night. All the assembly was conducted within the Argon glove box (KF-40 VGB-1). The fabricated sodium battery was then analyzed by cyclic voltammetry (Corrtest Electrochemical Workstation CS150) under a voltage range of −1 to 4 V vs. Na/Na+ at 1 mV s−1 and 3 mV s−1 of scan rate. Galvanostatic charge-discharge test was conducted with NEWARE Battery Testing System (5V1A) at 0.5 to 3 V under various current rates of 0.2C, 0.5C, and 1C (1C= 374 mAh g−1) at Room Temperature. The cycling performance test was conducted by running a charge-discharge test for 50 cycles. An impedance analysis (EUCOL LCR meter 20 Hz–5 MHz) was also applied to the prepared-sodium battery before and after the charge-discharge test. Impedance before and after 50 cycles was measured with LCR meter (EUCOL, 20 Hz–5 MHz frequencies) to understand the changes after cycling.
3. Results and Discussion
XRD analysis of the prepared carbon resulted in patterns, as shown in Fig. 1. The patterns of N/S-doped carbon are like the undoped activated carbon, in which a broaden-peak lying at 2θ at 24° corresponds to the [002] crystal plane of carbon (JCPDS #41-1487). The broad peak confirms amorphous carbon, and the similar patterns demonstrate that the carbon characteristic still dominates the after-doping material. Raman analysis of the prepared carbon reveals broad Raman shifting at 1350 cm−1 and 1590 cm−1.
Raman spectra (Fig. 2) show two broad peaks at 1350 cm−1 and 1590 cm−1, confirming the D and G bands [27,28]. The D band reveals that N/S doping caused structure defects within the single C6 layer networks. Meanwhile, the G band confirms the sp2 hybridization of C orbitals, representing the carbon material’s graphitic degree [29]. The D and G Intensity (ID/IG) ratio expresses the degree of carbon defect, in which a high value indicates more defect within the crystal structure. The N/S doping increased the ID/IG ratio from 0.92 before doping to 1.15, indicating more defects available after doping. The Kröger–Vink notation in equations (1) theoretically explains the vacancy formation due to the N/S doping.
Eq. 1 shows that two N atoms replaced C (NC), producing two negative charges (′) relative to its initial state, and an atom S replaced C (SC), creating two negative charges (″) close to its initial state. As a result, carbon vacancies (VC) formed to bring the network back into neutral charge by compensating with 4 positive charges (•••• ).
EDX mapping (Fig. 3) found that NSCE consist of C, O, S, Si, and Cl with composition as listed in Table 1. The result shows that NSCE consist mainly of C with 91.35 at.%, meanwhile S available at 1.61%. EDX mapping failed to detect light element like nitrogen, N, because of its low photon energy [30]. However, the presence of N in carbon network proven by FTIR analysis as described in Fig. 4.
FTIR analysis confirms some specific vibrations of carbon, as shown in Fig. 4. The FT-IR spectrum of undoped-carbon or activated-carbon shows characteristic peaks at 3446 cm−1 refers to O–H stretching [31,32], C–H stretching at 2884 cm−1 [33,34], C–O stretching at 1092 cm−1 [32,35], and C=C aromatic at 1570 cm−1 [32,34]. The C=C vibrations shift to about 1587 cm−1 after N and N/S doping, almost piled up with C=N stretching at around 1534 cm−1 [36,37]. The C–N vibration was revealed at about 1048 cm−1 and 1066 cm−1 [38], and the C–N vibration piled up with C–S at around 1084 cm−1 after N/S doping [38]. A broad peak at 3446 cm−1 refers to O–H stretching, representing H2O adsorption onto carbonaceous material, and an O–H group attached to the carbon network around 3418 cm−1 [31,32].
XPS analysis furthermore explains the presence of N and S within the carbon network. The XPS spectrum as shown by Fig. 5 shows C1s peak at 288.83 eV, 289.6 eV, 293.3 eV, and 304.86 eV, corresponding to C–C/C=C, C–S, C–N, and C=O, respectively. The result is like N, P, and S-doped hard carbon prepared from the Camphor tree [7]. Sulfur (S2p) can be observed at the binding energy of 168.26 eV and 169.55 eV, corresponding to S2p3/2 and S2p1/2 [7], and 171. 95 eV corresponds to C–SO3–C [39], indicating the presence of S within the thiol group attached to the carbon. A peak at 165 eV also confirms the presence of C–S and S–S bonds. The doped sulfur existed as thiophene sulfur (C4S). N1s reveal 402.4 eV and 405.5 eV, confirming pyrrolic nitrogen (N-5) and quaternary nitrogen (N-Q). N-5 can increase the diffusivity of Na+, and N-Q can increase the conductivity of the carbon [31]. O1s revealed at 527.84 eV and 536.97 eV correspond to C=O and C–O [39]. Fig. 6 describes the double N/S doped carbon’s predicted structure and its surface morphology. The structure is drawn based on the XPS result and the doping reaction as indicated in Eq. 1.
N/S doping to the activated carbon (ACB) has increased the specific conductivity from 2.42×10−1 S cm−1 to 23.74×10−1 S cm−1 at 20 Hz of frequency for the ACB and NSCE, respectively. Meanwhile, at a high frequency of 200 kHz, the conductivity increased from 3.06×10−1 S cm−1 for the ACB to 23.7×10−1 S cm−1 for the NSCE. The increasing is higher than a single N doping to the activated carbon, i.e., 5.41×10−1 S cm−1, as found by our previous research [40].
CV analysis to NSCE|NaClO4| Na full cell is described in Fig. 7, confirming quasi-reversible oxidation-reduction, attributed to solid-electrolyte Interface (SEI) formation [41,42], which lies between 0.764 V–0.9 V. Fig. 7 shows that the electrochemical reaction within the NSCE-NaClO4-Na is reversible and consists of an onset potential at 0.1 V vs. Na/Na+, representing Na oxidation and intercalation to the carbon network. The oxidation reached a maximum peak at 1.4 V vs. Na/Na+ and then started to be reduced at an onset potential of 0.1 V vs. Na/Na+, then peaked at −0.98 V vs. Na/Na+. Another research on carbon preparation from Camphor wood by HCl activation found that the oxidation peak started at 0.01 V and reached a peak at 0.15 V vs. Na/Na+ at reduction started at around 0.7 V vs. Na/Na+ up to 0.01 V [43]. Previous research on biomass-derived carbon also found anodic-cathodic activity at 0–0.2 V vs. Na/Na+ as insertion and extraction of Na+ in the carbon framework [44].
The sodium battery NSCE-NaClO4-Na shows an initial charging capacity of 720 mAh g−1 and an initial discharge capacity of 570 mAh g−1 at 0.2C, equal to 0.07 Ag−1 (1C = 372 mAh g−1). The discharge capacity after the 4th cycle is 325 mAh g−1, and the charging capacity remains constant at 385 mAh g−1 (Fig. 8), resulting in a Coulombic efficiency of 84.4%. The result is close to a sodium battery which was built from mango kernel-derived carbon with a discharge capacity of 328 mAh g−1 and Coulombic efficiency of 86% [45], and also close to a full sodium cell of NVP@C with biomass-hard carbon with 324.6 mAh g−1 of initial charge capacity [43]. This result is even higher than sodium full cell with Na0.8[Cu0.22Fe0.30Mn0.48]O2 as cathode and seaweed-derived carbon as the anode, which produced ~180 mAh g−1 at 0.025 A g−1 of current rate [46]. The initial charging capacity is 400 mAh g−1, and the discharge capacity is 200 mAh g−1 at 0.5 C (equal to 0.19 A g−1) current drawn. Meanwhile, the sodium battery with activated-bagasse carbon (ACB), the activated carbon before doped by N/S, shows an initial charging capacity of 270 mAh g−1 and the initial discharge capacity of 387 mAh g−1 at 0.07 A g−1 (Fig. 8b), and at 0.19 A g−1, the charging capacity is 159 mAh g−1 and the discharge capacity is 265 mAh g−1 and drop to 170 mAh g−1 after 4th cycle (Fig. 8d). It seems that 0.07 A g−1 and 0.19 A g−1 are high enough for the ACB sodium battery, and it caused an oscillation in voltage profile and prolong the discharge time. This happen because the voltages reached the upper limit (3 V) in a time longer than normal time [47]. Some comparisons are listed in Table 2.
The cycle ability (Fig. 9) was done after the first 5 cycles of 0.5C current drawn. The battery shows a consistent capacity of ~200 mAh g−1 with a Coulombic efficiency of around 84.4%, and capacity retention of around 57% after 50 cycles. The result is even higher than a sodium full cell developed from mango kernel-derived carbon as anode and NVP (Na3V2(PO4)3) as cathode, which provides ~123 mAh g−1 and 113 mAh g−1 of charge and discharge capacity, respectively, at the end of 50 cycles [45].
Impedance analysis after 50 cycles (Fig. 10) reveals two semi-circles as of 2800 Ω of ionic resistance and 2200 Ω of charge transfer electrode-electrolyte resistance. The resistance type is confirmed by 9.248×10−9 F and 1.44×10−7 F of ionic resistance and charge transfer resistance, respectively. The ionic resistance increases around 10 times after 50 cycles, indicating the formation of some resistive phases. Meanwhile, the charge transfer at electrode-electrolyte resistance rises to twice after 50 cycles, indicating the presence of an SEI layer between the electrode-electrolyte (separator).
4. Conclusions
The N/S doping to the prepared carbon from bagasse, a solid biomass produced by sugarcane production, initiates defect formation in the carbon network, as proven by the increasing Raman ID/IG ratio. The doped sulfur existed as thiophene sulfur (C4S). Meanwhile, N1s reveal 402.4 eV and 405.5 eV, confirming pyrrolic nitrogen (N-5) and quaternary nitrogen (N-Q). N-5 can increase the diffusivity of Na+, and N-Q can increase the conductivity of the carbon. The produced N/S-doped carbon (NSCE) is a suitable electrode for sodium-ion batteries by delivering the initial charge-discharge capacity of 720 mAh g−1 and 570 mAh g−1, respectively, at 0.2 C (0.07 A g−1). The battery has a capacity retention of 57% after 50 cycles, a stable 200 mAh g−1 capacity, and a Coulombic efficiency of 84.4%.
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