Enhanced Electrochemical Performances of Mn3O4/Heteroatom-Doped Reduced Graphene Oxide Aerogels as an Anode for Sodium-Ion Batteries

Owing to their high theoretical capacity, transition-metal oxides have received a considerable amount of attention as potential anode materials in sodium-ion (Na-ion) batteries. Among them, Mn3O4 has gained interest due to the low cost of raw materials and the environmental compatibility. However, during the insertion/de-insertion process, Mn3O4 suffers from particle aggregation, poor conductivity, and low-rate capability, which, in turn, limits its practical application. To overcome these obstacles, we have successfully prepared Mn3O4 nanoparticles distributed on the nitrogen (N)-doped and nitrogen, sulphur (N,S)-doped reduced graphene oxide (rGO) aerogels, respectively. The highly crystalline Mn3O4 nanoparticles, with an average size of 15–20 nm, are homogeneously dispersed on both sides of the N-rGO and N,S-rGO aerogels. The results indicate that the N-rGO and N,S-rGO aerogels could provide an efficient ion transport channel for electrolyte ion stability in the Mn3O4 electrode. The Mn3O4/N- and Mn3O4/N,S-doped rGO aerogels exhibit outstanding electrochemical performances, with a reversible specific capacity of 374 and 281 mAh g−1, respectively, after 100 cycles, with Coulombic efficiency of almost 99%. The interconnected structure of heteroatom-doped rGO with Mn3O4 nanoparticles is believed to facilitate fast ion diffusion and electron transfer by lowering the energy barrier, which favours the complete utilisation of the active material and improvement of the structure’s stability.


Introduction
Because of the rising demand for lithium-ion (Li-ion) batteries, the scarcity of lithium sources, and the expected steep rise in lithium prices, there is an urgent need for innovative and low-cost battery systems [1]. The most researched new battery technologies use the same insertion and extraction chemistry as Li-ion batteries, such as potassium-ion (K-ion) and sodium-ion (Na-ion) batteries, despite the fact that the electrode materials needed to be reconfigured [2]. In large-scale energy storage, Na-ion batteries have gained considerable interest owing to the availability and natural resources of sodium [3][4][5][6][7][8]; moreover, they appear to be a better alternative to Li-ion batteries. Unfortunately, because of the large radius (1.02 Å), high atomic mass (23 g mol −1 ), and low redox potentials (2.71 V vs. SHE) for Na-ion, most examined electrodes, especially the anode, are not ideal hosts for Naion insertion [9][10][11][12]. In addition, because Na has a higher chemical activity than Li, the

Physical Characterisation
The structural phases of the obtained samples were determined via X-ray diffraction (XRD; Rigaku Miniflex II, Tokyo, Japan). The amount of Mn 3 O 4 in the heteroatom-doped rGO aerogel was confirmed using a thermogravimetric analyser (TGA, Perkin Elmer, Waltham, MA, USA) within the temperature range of 30 • C-800 • C at a heating rate of 10 • C min −1 in air. The morphology of these samples was observed via scanning electron microscopy (SEM; JOEL, Akishima, Tokyo, Japan) (JSM-6360L) and transmission electron microscopy (TEM; TECNAI G2 20 S-TWIN FEI, FEI Company, Lincoln, NE, USA). Fourier transform infrared spectroscopy (FTIR) was recorded on IR Tracer-100, Shimadzu, Kyoto, Japan. Raman spectra were collected via Raman spectroscopy (Renishaw, Gloucestershire, UK (532 nm radiation)) extended with 0.1% power laser measurement. Surface composition analysis was further conducted via X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD XPS, Kratos, Manchester, UK).

Results and Discussion
Scheme 1 presents the synthesis of the Mn 3 O 4 /heteroatom-doped rGO aerogel using GO through hydrothermal, followed by freeze-drying. Upon heating, GO was converted to rGO, and simultaneously, through the π-π interactions, hydrogen bonding, coordination, and electrostatic interactions, the GO layers could self-assemble into three-dimensional (3D) networks. Concurrently, the presence of NH 3 and L-cysteine introduced the doping of N-atom and N,S-atoms on the rGO layer, respectively. Strong cross-links, which are the building blocks of the 3D rGO network, were produced as a result of this process and acted as an effective conductive network for ion and electron transportation [30]. coordination, and electrostatic interactions, the GO layers could self-assemble into threedimensional (3D) networks. Concurrently, the presence of NH3 and L-cysteine introduced the doping of N-atom and N,S-atoms on the rGO layer, respectively. Strong cross-links, which are the building blocks of the 3D rGO network, were produced as a result of this process and acted as an effective conductive network for ion and electron transportation [30]. Scheme 1. Schematic representation of the synthesis procedure of the Mn3O4/heteroatom-doped rGO aerogel.
The thermal stability (in air) of all samples was determined via TGA. The TGA curve ( Figure 1) shows two weight loss stages for all samples, except for pristine Mn3O4, and the evaporation of physically and chemically adsorbed water was adequately attributed to the weight loss at temperatures above 100 °C. Between 150 °C and 600 °C, the N-rGO and N,S-rGO aerogels demonstrated the decomposition and disintegration of nitrogen and sulphur-containing functional groups, followed by decarboxylation and elimination of hydroxyl functionalities, respectively [50]. For a temperature less than 500 °C in air, rGO is often entirely burnt to CO2 [51]. For the Mn3O4 nanoparticles, no weight loss was observed in the temperature range of 100 °C to 500 °C. As the temperature approached 540 °C, the weight began to increase, which could be attributed to the transformation of Mn3O4 to Mn2O3 [30,33]. Therefore, the amounts of Mn3O4 in the Mn3O4/N-rGO and Mn3O4/N,S-rGO aerogels were estimated to be 63 and 60 wt.%, respectively. The thermal stability (in air) of all samples was determined via TGA. The TGA curve ( Figure 1) shows two weight loss stages for all samples, except for pristine Mn 3 O 4 , and the evaporation of physically and chemically adsorbed water was adequately attributed to the weight loss at temperatures above 100 • C. Between 150 • C and 600 • C, the N-rGO and N,S-rGO aerogels demonstrated the decomposition and disintegration of nitrogen and sulphur-containing functional groups, followed by decarboxylation and elimination of hydroxyl functionalities, respectively [50]. For a temperature less than 500 • C in air, rGO is often entirely burnt to CO 2 [51]. For the Mn 3 O 4 nanoparticles, no weight loss was observed in the temperature range of 100 • C to 500 • C. As the temperature approached 540 • C, the weight began to increase, which could be attributed to the transformation of Mn 3 O 4 to Mn 2 O 3 [30,33]. Therefore, the amounts of The phase purity and structure of the synthesised N-rGO aerogel, N,S-rGO aerogel, pure Mn3O4, Mn3O4/N-rGO aerogel, and Mn3O4/N,S-rGO aerogel were analysed via XRD. The disordered configuration of loosely packed graphene sheets of the N-rGO and N,S-rGO aerogels was disclosed by the enormous broad peak at approximately 24°-25°, corresponding to the graphite (002) plane ( Figure 2) [52,53]. Because of the relatively low diffraction intensity of the N-rGO and N,S-rGO aerogels compared with Mn3O4 prominent peaks, the diffraction peaks of these aerogels were less evident in the XRD patterns of the Mn3O4/N-rGO and Mn3O4/N,S-rGO aerogels [54]. Both Mn3O4/N-rGO and Mn3O4/N,S-rGO aerogels corresponded to the planes of tetragonal crystallinity in Mn3O4 (JCPDS card no 240734), hence indicating the presence of pure Mn3O4 without any noticeable impurities. The crystallite size of Mn3O4/N-rGO and Mn3O4/N,S-rGO aerogels were 12.7 and 17.4 nm, respectively, calculated using Scherrer's Equation. These results supported the formation and dispersion of Mn3O4 nanoparticles on the network surface of the N-rGO and N,S-rGO aerogels.   Figure 3b) aerogel samples exhibited a typical well-defined and interconnected 3D network structure of rGO aerogel with a pore structure smaller than 1 µm. Such structures could provide an open channel for the access of electrolytes and minimise volume changes during the charge and discharge processes [55]. It is clearly demonstrated that Mn 3 O 4 agglomerated in a size of 0.5-1.1 µm, which is an aggregate of individual Mn 3 O 4 nanoparticles, anchored uniformly on the porous N-rGO ( Figure 3c) and N,S-rGO ( Figure 3d) aerogel layers. This suggests an effective assembly between the Mn 3 O 4 nanoparticles and rGO aerogel sheets during the hydrothermal treatment. Moreover, pristine Mn 3 O 4 nanoparticles with a particle size of 0.5-1.0 µm are beneficial in that they provide more active sites for the electrochemical reaction [56,57]. Therefore, the synergistic effect between the small-sized Mn 3 O 4 and heteroatom-doped rGO aerogel could have a tremendous effect on the electrochemical properties, especially the cyclability and rate capability of the batteries.   Figure 3b) aerogel samples exhibited a typical well-defined and interconnected 3D network structure of rGO aerogel with a pore structure smaller than 1 µ m. Such structures could provide an open channel for the access of electrolytes and minimise volume changes during the charge and discharge processes [55]. It is clearly demonstrated that Mn3O4 agglomerated in a size of 0.5-1.1 µ m, which is an aggregate of individual Mn3O4 nanoparticles, anchored uniformly on the porous N-rGO ( Figure 3c) and N,S-rGO ( Figure 3d) aerogel layers. This suggests an effective assembly between the Mn3O4 nanoparticles and rGO aerogel sheets during the hydrothermal treatment. Moreover, pristine Mn3O4 nanoparticles with a particle size of 0.5-1.0 µ m are beneficial in that they provide more active sites for the electrochemical reaction [56,57]. Therefore, the synergistic effect between the smallsized Mn3O4 and heteroatom-doped rGO aerogel could have a tremendous effect on the electrochemical properties, especially the cyclability and rate capability of the batteries. Further characterisation of the morphology and structure of the Mn3O4/N-rGO and Mn3O4/N,S-rGO aerogels was carried out using high-resolution transmission electron microscopy (HRTEM). A typical crumpled structure of rGO and interconnected and crosslinked random rGO layers construct a 3D framework with open-pore structures ( Figure  4a,b). The heteroatom-doped rGO aerogels exhibited a thin lamellar structure with dis- Further characterisation of the morphology and structure of the Mn 3 O 4 /N-rGO and Mn 3 O 4 /N,S-rGO aerogels was carried out using high-resolution transmission electron microscopy (HRTEM). A typical crumpled structure of rGO and interconnected and crosslinked random rGO layers construct a 3D framework with open-pore structures (Figure 4a,b). The heteroatom-doped rGO aerogels exhibited a thin lamellar structure with distinct edges, overlaps, and curve profiles. Furthermore, the N-rGO and N,S-rGO aerogels had more wrinkled surfaces because of structural defects caused by heteroatom doping. The disordered degree of the heteroatom-doped rGO aerogels was characterised via Raman spectroscopy and is presented in Figure 5. Typical broad peaks corresponding to the D and G bands at 1359 and 1600 cm −1 , respectively, were observed in the heteroatomdoped rGO aerogels. The G band reflected the radial C-C stretching of ordered sp 2 -linked carbon atoms, whereas the D band indicated the defects or irregularities on the graphene edges [58,59]. Furthermore, the intensity ratio of the D and G bands (I D /I G ) for the N-rGO aerogel was 0.95, whereas it was 0.98 for the N,S-rGO aerogel. After the nitrogen and sulphur doping, defects were introduced to the rGO aerogel layers [60,61]  The disordered degree of the heteroatom-doped rGO aerogels was characterised vi Raman spectroscopy and is presented in Figure 5. Typical broad peaks corresponding t the D and G bands at 1359 and 1600 cm −1 , respectively, were observed in the heteroatom doped rGO aerogels. The G band reflected the radial C-C stretching of ordered sp 2 −linked carbon atoms, whereas the D band indicated the defects or irregularities on the graphen edges [58,59]. Furthermore, the intensity ratio of the D and G bands (ID/IG) for the N-rGO aerogel was 0.95, whereas it was 0.98 for the N,S-rGO aerogel. After the nitrogen and sul phur doping, defects were introduced to the rGO aerogel layers [60,61], where the ID/I Mn3O4/N,S rGO aerogels, respectively [62,63]. These strong peaks corresponded to the Mn−O breathing vibration of Mn 2+ ions and thus demonstrated that Mn3O4 is successfully attached to the rGO layer [64,65]. Additionally, the peaks at 2450 cm −1 were associated with the second-order two-phonon mode 2D band. It is worth noting that the Raman spectrum associated with Mn3O4 in the Mn3O4/N-rGO and Mn3O4/N,S-rGO aerogels was shifted to a low wavenumber in comparison with the pristine Mn3O4, indicating the electronic coupling between Mn3O4 and heteroatom rGO aerogel.  Figure 6 presents the FTIR spectra of the Mn3O4/N-rGO and Mn3O4/N,S-rGO aerogels to further support the presence of heteroatom in the rGO aerogel, as well as the Mn3O4 nanoparticles in the nanocomposites. The peak positioned at 1730, 1363, and 1215 cm −1 corresponded to the stretching vibration C=O of carboxylic groups, O−H deformation, and C-O stretching vibration from epoxy groups, respectively, indicating that GO was successfully converted into rGO [66,67]. The peak located at 1099 and 2451 cm −1 associated with the absorption band of C=S and S-H stretching vibration, respectively, was observed in the N,S-rGO and Mn3O4/N,S-rGO aerogels and, thus, confirmed the presence of S atom on the surface of N,S-rGO aerogel samples [68]. In addition, the C-N stretching vibration band located at 1416 cm −1 could be assigned to the characteristic band of nitrogen doping. For the Mn3O4 nanoparticles, the strong peaks at 528 and 621 cm −1 were attributed to the Mn-O stretching of the tetrahedral and octahedral sites in Mn3O4 [30]. As a result, Mn-O, C-N, C=S, and N-H linkages confirmed that Mn3O4 nanoparticles were successfully integrated into the heteroatom rGO layers. The XPS technique was used to obtain further insight into the chemical states of elements on the surface of the samples. Figure 7 presents the XPS spectra of the Mn3O4/N,S-rGO and Mn3O4/N-rGO aerogels, respectively. From the survey scan XPS spectra, the presence of nitrogen (Figure 7a) and nitrogen-sulphur (Figure 7b) is noticeable, which agrees well with the FTIR results. For the Mn3O4/N-rGO aerogel, the atomic percentage of The XPS technique was used to obtain further insight into the chemical states of elements on the surface of the samples. Figure 7 presents the XPS spectra of the Mn 3 O 4 /N,S-rGO and Mn 3 O 4 /N-rGO aerogels, respectively. From the survey scan XPS spectra, the presence of nitrogen (Figure 7a) and nitrogen-sulphur (Figure 7b) is noticeable, which agrees well with the FTIR results. For the Mn 3 O 4 /N-rGO aerogel, the atomic percentage of the N was 11.54%, whereas, for the Mn 3 O 4 /N,S-rGO aerogel, the atomic percentages of the N and S were 4.12% and 2.76%, respectively. Both nanocomposites exhibited an Mn 2p 3/2 at 642 eV, Mn 2p 1/2 at 653 eV, Mn 3s at 771 eV, an O 1s peak at 531.6 eV, a C 1s peak at 284.5 eV, and N 1s peak at 399.3 eV. For O 1s (Figure 7c,d), the XPS spectra could be deconvoluted into four peaks located at 527, 531, 533, and 534 eV, which correspond to Mn-O, C-O-Mn, C=O, and surface adsorbed oxygen, respectively. The C 1s (Figure 7e [24]. Figure 7g,h present the N 1s region, where the binding energies located at 397, 399, and 403 eV were assigned to pyridinic N, pyrrolic N, and graphitic N, respectively. The presence of pyridinic N, graphitic N, and pyrrolic N at the defect or edge sites was favourable to improving the Na-ion transport and sodium storage capacity [69]. As can be seen from Figure 7i, the S 2p spectra positioned at 163.8, 164.9, and 168.9 eV are assigned to S 2p 3/2 , S p 1/2 , and SO x , respectively, indicating the existence of Mn-S [70,71]. The curve fitting of the high-resolution Mn 2p spectrum for the Mn 3 O 4 /N-rGO and Mn 3 O 4 /N,S-rGO aerogels is presented in Figure 7j   To evaluate the sodium storage performances of the samples, the CV and galvanostatic charge/discharge testing have been conducted in a half-cell within the potential range of 0.01-3.00 V. As can be seen from Figure 8a, all samples exhibit a strong cathodic peak at 0.85 V in the first cycle and could be ascribed to the reductions of Mn 3 O 4 to MnO as well as the formation of Na 2 O, which is attributed to the decomposition of the electrolyte according to Equation (1) pair of small redox peaks at 1.6 and 1.8 V, which could be attributed to the sulphur embedded in the porous N,S-rGO aerogel during the Na-ion insertion/de-insertion process [77]. The CV curves exhibited good reversibility, leading to good cycling stability for longer cycles, and the overall sodium storage mechanism between Mn3O4 and Na + is expressed in Equation (3).
Mn3O4 + 8Na + + 8e −  3Mn + 4Na2O (3)  The peaks at 0.45 to 0.01 V could be attributed to the reduction of MnO to metallic Mn (Equation (2)) and solid electrolyte interphase (SEI) layer formation on the electrode surface of the electroactive material.
MnO + 2Na + + 2e − → Mn + Na 2 O During subsequent cycles, the CV curves nearly overlapped, indicating that the Mn 3 O 4 /heteroatom rGO aerogel was reversible during the insertion/de-insertion process of Na + ions [74]. The tiny peaks at 0.1 V and the broad peak at 0.8 V in the anodic process corresponded to Na + extraction into the graphitic carbon layers, which was the inverse process of Na + intercalation. When compared with Li-ion batteries, the CV peaks in Na-ion batteries were broader and weaker. This may be due to the larger radius of Na + than Li + and the slower Na + intercalation between graphitic carbon layers [75,76]. Contrary to the Mn 3 O 4 /N-rGO aerogel (Figure 8b), the Mn 3 O 4 /N,S-rGO aerogel (Figure 8c) exhibited a pair of small redox peaks at 1.6 and 1.8 V, which could be attributed to the sulphur embedded in the porous N,S-rGO aerogel during the Na-ion insertion/de-insertion process [77]. The CV curves exhibited good reversibility, leading to good cycling stability for longer cycles, and the overall sodium storage mechanism between Mn 3 O 4 and Na + is expressed in Equation (3). Figure 9 presents the typical discharge/charge profiles of Mn 3 O 4 and the Mn 3 O 4 /N-rGO and Mn 3 O 4 /N,S-rGO aerogels electrodes for selected cycles at a current density of 0.1 A g −1 . The subsequent CV curves are different from the first sodiation, and the discharge plateau for the Mn 3 O 4 /N-rGO and Mn 3 O 4 /N,S-rGO aerogels is much longer than the pristine one, indicating that more Na-ions can be inserted into these nanocomposites [78]. Furthermore, no distinct plateau is observed, which is consistent with the CV results. Figure 9 presents the typical discharge/charge profiles of Mn3O4 and the Mn3O4/N-rGO and Mn3O4/N,S-rGO aerogels electrodes for selected cycles at a current density of 0.1 A g −1 . The subsequent CV curves are different from the first sodiation, and the discharge plateau for the Mn3O4/N-rGO and Mn3O4/N,S-rGO aerogels is much longer than the pristine one, indicating that more Na-ions can be inserted into these nanocomposites [78]. Furthermore, no distinct plateau is observed, which is consistent with the CV results. The cycling stability of all electrodes is presented in Figure 10a. The initial discharge capacities of Mn3O4, Mn3O4/N-rGO aerogel and Mn3O4/N,S-rGO aerogel were measured to be 522, 1950, and 884 mAh g −1 , respectively. In the second cycle, the discharge capacities were 336, 470, and 425 mAh g −1 for Mn3O4, Mn3O4/N-rGO aerogel and Mn3O4/N,S-rGO aerogel, respectively. The irreversible capacity loss was mainly due to the irreversible formation of the SEI layer. Interestingly, the Mn3O4/N-rGO aerogel maintained its discharge capacity at 374 mAh g −1 after 100 cycles with an 80% retention rate. Contrarily, the Mn3O4/N,S-rGO aerogel and Mn3O4 exhibited much lower discharge capacities of 281 mAh g −1 (68% retention rate) and 185 mAh g −1 (55% retention rate) after 100 cycles. For the few initial cycles, the porous structure of rGO aerogel promoted the formation of excessive SEI layers, resulting in a lower initial Coulombic efficiency [79]. Overall, the average Coulombic efficiency of all electrodes was almost 99%. Nevertheless, the Mn3O4/N,S-rGO aerogel electrodes demonstrated much lower discharge capacity, presumably because of the large atomic radius of S, as well as the large crystallite size than that of Mn3O4/N-rGO aerogel electrodes.
In addition to their high discharge capacity and good cycling stability, the Mn3O4/N-rGO and Mn3O4/N,S-rGO aerogel electrodes exhibited remarkable rate performance, as presented in Figure 10b. The Mn3O4/N-rGO aerogel electrode exhibited discharge The cycling stability of all electrodes is presented in Figure 10a. The initial discharge capacities of Mn 3 O 4 , Mn 3 O 4 /N-rGO aerogel and Mn 3 O 4 /N,S-rGO aerogel were measured to be 522, 1950, and 884 mAh g −1 , respectively. In the second cycle, the discharge capacities were 336, 470, and 425 mAh g −1 for Mn 3 O 4 , Mn 3 O 4 /N-rGO aerogel and Mn 3 O 4 /N,S-rGO aerogel, respectively. The irreversible capacity loss was mainly due to the irreversible formation of the SEI layer. Interestingly, the Mn 3 O 4 /N-rGO aerogel maintained its discharge capacity at 374 mAh g −1 after 100 cycles with an 80% retention rate. Contrarily, the Mn 3 O 4 /N,S-rGO aerogel and Mn 3 O 4 exhibited much lower discharge capacities of 281 mAh g −1 (68% retention rate) and 185 mAh g −1 (55% retention rate) after 100 cycles. For the few initial cycles, the porous structure of rGO aerogel promoted the formation of excessive SEI layers, resulting in a lower initial Coulombic efficiency [79]. Overall, the average Coulombic efficiency of all electrodes was almost 99%. Nevertheless, the Mn 3 O 4 /N,S-rGO aerogel electrodes demonstrated much lower discharge capacity, presumably because of the large atomic radius of S, as well as the large crystallite size than that of Mn 3 O 4 /N-rGO aerogel electrodes.
capacities of 203, 166, 145, 135, 128, and 156 mAh g −1 at various current densities of 0.2, 0.4, 0.6, 0.8, 1.0, and returning to 0.2 A g −1 . The minimal drop in capacities with increased current densities indicated a higher degree of reversible Na-ion insertion/de-insertion owing to their expanded interlayer spacing [80]. The improved cycle and rate performance of Mn3O4/heteroatom-doped rGO aerogels may be due, in part, to the 3D porous structure, which may minimise the electron and ion transport path. The specific capacity and cyclability of the Mn3O4/N-rGO aerogel and Mn3O4/N,S-rGO aerogel were improved, which benefitted from the synergistic effect of the heteroatom doping and porous structure of the rGO, as well as the small particle sizes of Mn3O4. The rGO sheets as well as the porous structure in the conductive network of the Mn3O4/heteroatom rGO aerogels provided an efficient electron and ion transport path, thereby decreasing the internal resistance to enhance the reaction kinetics and resulting in In addition to their high discharge capacity and good cycling stability, the Mn 3 O 4 /N-rGO and Mn 3 O 4 /N,S-rGO aerogel electrodes exhibited remarkable rate performance, as presented in Figure 10b. The Mn 3 O 4 /N-rGO aerogel electrode exhibited discharge capacities of 203, 166, 145, 135, 128, and 156 mAh g −1 at various current densities of 0.2, 0.4, 0.6, 0.8, 1.0, and returning to 0.2 A g −1 . The minimal drop in capacities with increased current densities indicated a higher degree of reversible Na-ion insertion/de-insertion owing to their expanded interlayer spacing [80]. The improved cycle and rate performance of Mn 3 O 4 /heteroatom-doped rGO aerogels may be due, in part, to the 3D porous structure, which may minimise the electron and ion transport path.
The specific capacity and cyclability of the Mn 3 O 4 /N-rGO aerogel and Mn 3 O 4 /N,S-rGO aerogel were improved, which benefitted from the synergistic effect of the heteroatom doping and porous structure of the rGO, as well as the small particle sizes of Mn 3 O 4 . The rGO sheets as well as the porous structure in the conductive network of the Mn 3 O 4 /heteroatom rGO aerogels provided an efficient electron and ion transport path, thereby decreasing the internal resistance to enhance the reaction kinetics and resulting in a high specific capacity and rate capability [81][82][83]. Poor electrical conductivity and large volume expansion in transition-metal oxide electrodes during the Na-ion insertion/de-insertion processes are among the constraints in the development of these materials for Na-ion battery applications. Here, the rGO sheets are more likely to provide sufficient elastic buffer space for the transition-metal oxide to accommodate the volume expansion/contraction and prevent the electrode from cracking or crumbling during the charge/discharge processes. In addition, the presence of the rGO aerogel could effectively enhance the electrical conductivity of the nanocomposites [84]. Meanwhile, Mn 3 O 4 nanoparticles anchored on rGO can prevent the restacking of the rGO layers, preserve their high active surface area and maintain the channels for Na-ion diffusion, which is advantageous for increasing the Na storage within the nanocomposites [85]. Doping the rGO aerogels with nitrogen and codoping nitrogen/sulphur improves the physicochemical properties of the rGO component [43]. The incorporation of these heteroatoms into the rGO aerogels could facilitate the charge transfer between adjacent carbon atoms [86][87][88], thus improving the electrical conductivity and electrochemical activity of the rGO itself. The defects created by N-doping and functionalised groups may increase the electrical conductivity, and the larger covalent radius of S compared with Na may increase the interlayer spacing to facilitate Na-ion insertion/de-insertion within the electrode. All the aforementioned characteristics contributed to the improvements in the specific capacity and cycling ability of the Mn 3 O 4 /heteroatom-doped rGO aerogels. This strategy can be used as one of the approaches for mitigating the large volume change and poor electrical conductivity, which is associated with bare transition-metal oxide anodes.

Conclusions
In summary, the Mn 3 O 4 /heteroatom-doped rGO aerogels have been successfully synthesised via a hydrothermal route, followed by a freeze-drying process using NH 3 and L-cysteine as nitrogen and nitrogen-sulphur sources, respectively. The aerogel structure built well-interconnected heteroatom-doped rGO layers, and the Mn 3 O 4 nanoparticles distributed on the rGO layers prevented the graphene layers from restacking again. The 3D structure provides a large active surface area and eases electron diffusion and Na-ion transportation. Both the N-and N,S-doped rGO aerogels with Mn 3 O 4 exhibited high specific capacity, excellent cycling stability, and rate capability than the pristine Mn 3 O 4 . The heteroatom-doped rGO aerogel acts as a robust structure to accommodate the volume expansion of Mn 3 O 4 nanoparticles and enables reversible Na-ion insertion/de-insertion. Our work demonstrates that N-and N,S-doped rGO aerogels can efficiently improve the Na storage capacity of Mn 3 O 4 and offer a useful strategy for synthesising high-yield anode materials. Considering the simple step of the preparation process and the excellent cycling stability of the samples, the Mn 3 O 4 /heteroatom-doped rGO aerogel can be considered a potential candidate and provide an opportunity to explore these materials for the next generation of Na-ion batteries.