Dual heteroatom-doped biomass-derived porous carbon anode for superior sodium storage

The green preparation of anode materials is desirable for high-performance sodium-ion batteries. In this paper, a N/S co-doped porous carbon was prepared from longan shell via a facile and low-cost route. Heteroatom doping increases the defect sites to facilitate Na+ storage and improves the electronic conductivity of the carbon material. The N/S co-doped porous carbon prepared using CS(NH2)2 with a ratio of 1:2 displays the highest specific capacity of 314.7 mAh g−1 after 200 cycles at 0.1 A g−1. In addition, a reversible specific capacity of 202.5 mAh g−1 is maintained after 1000 cycles at 1 A g−1. This work offers a new strategy for designing high-performance anode materials for sodium energy storage applications.


Introduction
Renewable and cleaner energy resources are widely utilised due to the depletion of fossil fuel reserves and environmental concerns.However, these resources require the development of efficient, low-cost, and largescale energy-storage systems (ESSs) [1,2].Sodium-ion batteries (SIBs) are novel secondary batteries that have the characteristics of rich sodium resources and low cost.Consequently, SIBs have attracted widespread research attention and are a promising potential alternative for lithium-ion batteries (LIBs) in large-scale ESSs [3][4][5].
Even though the energy storage mechanisms of SIBs and LIBs are similar, the different characteristics of Na + and Li + mean that the electrode materials used in LIBs are unsuitable for SIBs [1,2,6].To promote the development of SIBs, exploiting high-performance and green electrode materials is essential and urgent.
Carbon materials have been widely used as anode materials for LIBs and SIBs due to their advantages of rich resources, stability, safety, and eco-friendliness [7,8].However, frequently used carbon materials such as hard carbon, carbon black, and carbon nanofibers show poor performance in SIBs.This is because the larger ionic radius of Na + (1.02 Å for Na + versus 0.76 Å for Li + ) hinders its insertion/desertion into carbonaceous materials with an interlayer distance smaller than 0.37 nm [1,9].Moreover, the thermodynamic instability of sodiumgraphite systems has further limited their utilization in SIBs.As a result, strategies such as heteroatom doping, decreasing the particle size, and constructing a porous structure have been used to prepare high-performance anode materials [10].Benefiting from their specific microstructures, many carbonaceous anode materials have been reported with outstanding electrochemical performance.However, these materials are mainly prepared from special precursors with complex procedures, leading to high costs and low yields.This has hindered the commercial application of SIBs [4].
Over the past few years, biomass materials have been widely used to prepare carbonaceous materials for ESSs [6,11].Biomass materials are cheap and eco-friendly precursors developed from plants (including land-and water-based vegetation) and organic waste.In battery applications, the natural structures of these precursors, such as their porous structures, can be maintained after pyrolysis, which enables the penetration of the electrolyte into the structure and shortens the ion diffusion route.Thus, biomass-derived carbons can provide a higher capacity than graphite.Furthermore, the utilization of biomass for carbon production enables the recycling of different waste materials, leading to cost-effectiveness and high yields [4].
Longan shell, a type of agricultural waste widely generated around the world, was used in this work to prepare a sponge-like nitrogen/sulphur co-doped porous carbon with a high specific surface area (SSA).When used as an anode material for SIBs, this carbon material displays a high capacity of 314.7 mA h g −1 at a current density of 0.1 A g −1 .Even at a high current density of 1 A g −1 , the prepared anode still maintains a capacity of 202.5 mA h g −1 .This study provides a facile and feasible strategy for designing high-performance anode materials for SIBs.

Material synthesis
First, the clean longan shell was continuously ground into a powder.The powder-like longan shell and KOH were added to deionised water and stirred for 30 min.Next, the mixture was transferred to a stainless-steel autoclave for hydrothermal treatment at 180 °C for 12 h, followed by freeze-drying for 24 h to obtain precarbonated O-NPC.The pre-carbonated O-NPC was pyrolyzed in a N 2 atmosphere at 800 °C for 2 h.After pyrolysis, the product was washed with 1 M HCl and then deionised water.The washed solid was dried to obtain porous carbon (denoted as O-NPC).Next, the prepared O-NPC was mixed with thiourea using different ratios (1:1, 1:2, and 1:3) in deionised water and sealed in stainless-steel autoclaves for hydrothermal treatment at 180 °C for 24 h.Each product was then carbonised in a N 2 atmosphere at 800 °C for 2 h, and the obtained carbon materials were denoted as NPC/NS-x (where x represents the ratio of O-NPC to thiourea).

Material characterisation
The structure and phase purity of the samples were characterised by x-ray diffraction (XRD, Empyrean, Holland, λ = 0.154 nm) using Cu-K ɑ radiation at room temperature.The morphology of the materials was studied by scanning electron microscopy (SEM, JSM-7800F, Japan) operated at 5 kV and transmission electron microscopy (TEM, JEM-2100, Japan) operated at 200 kV.Material structures were also studied by Raman spectroscopy (Raman, inVia, UK).X-ray photoelectron spectroscopy (XPS, K-Alpha+, USA) was performed to probe the chemical composition of the sample surfaces.The SSA and pore size distribution of the products were obtained using N 2 adsorption/desorption measurements (BELSORP-Max, Japan).Fourier transform infrared spectrometer (FTIR, Nicolet iS50, USA) was used to analyse the molecular structure and the chemical composition of the materials.

Electrochemical measurements
Polyvinylidene difluoride (PVDF), acetylene black, and NPC/NS-x were mixed with a ratio of 8:1:1 and homogeneously dispersed in N-methyl-2-pyrrolidone to form a slurry.The slurry was coated onto Al foil and dried at 90 °C for 12 h in a vacuum oven.The loading of active material on each electrode was maintained at 0.8 mg cm −2 .CR2032 coin-type cells were assembled using a prepared electrode, Na foil counter electrode, carbonate-based electrolyte (1 M NaClO 4 in EC/FEC, volume ratio 1:1), and glass fibre separator (Whatman GF/F) in an Ar-filled glovebox with sub-0.1 ppm water and oxygen content.Cyclic voltammetry (CV) was carried out using a CHI760E electrochemical workstation at a scan rate of 0.1 mV s −1 in the voltage range of 0.01-3.0V. Galvanostatic charging/discharging (GCD) profiles were measured using a Land battery tester CT2001A at a current density of 0.1 A g −1 in the voltage range of 0.01-3.0V. Electrochemical impedance spectroscopy (EIS) measurements were also obtained using the CHI760E electrochemical workstation in the frequency range of 10 -2 -10 5 Hz.

Results and discussion
The preparation procedure of NPC/NS-x using longan shell is shown in figure 1.The porous structure was obtained by an activation process.According to previous reports, KOH is widely utilised to obtain uniform micropores, as described in the following equations [4,12]: During the activation process, chemical reactions (equations ( 1)-( 3)) occurred and generated pore networks, causing the formation of O-NPC.Later, the introduction of CS(NH 2 ) 2 caused the doping of N and S into the carbon framework, which was expected to enhance the electrochemical performance of NPC/NS-x [13].(figure 3(f)) show a hierarchical pore distribution comprised of mesopores around 2-4 nm and micropores around 1-2 nm.This hierarchical porous nanostructure can provide a more favourable transportation route, while the large surface area affords more adsorption sites for Na + .Therefore, these carbon materials show good promise for being used as Na + storage reservoirs.
Figure 4(a) shows the XRD patterns of O-NPC and NPC/NS-x.Two peaks at about 25°and 45°correspond to the (002) and (100) crystal planes of graphite, respectively, indicating that all samples contain amorphous natural carbon [14,15].A slight peak shift to lower angles can be observed with increasing CS(NH 2 ) 2 ratio, indicating that the interlayer space is enlarged [9].showing a non-linear relationship with the CS(NH 2 ) 2 ratio.This is because the carbon nitride (g-C 3 N 4 ) formed by self-condensation removes some reducing gases (such as H 2 S) when the pyrolysis temperature of CS(NH 2 ) 2 is less than 500 °C.With increasing temperature, the carbon nitride is partially decomposed to produce C x H y N z gas, which provides more nitrogen sources to a certain extent.As a result, the heteroatoms at the edge of the carbon materials provide more reactive sites for the storage of Na + ions.
Electrochemical performance was evaluated by obtaining CV curves of the O-NPC and NPC/NS-x cells at a low scanning rate of 0.1 mV s −1 in the voltage range of 0.01 V to 3.0 V, as shown in figure 5.The CV curve of the first cycle is different from those of subsequent cycles.During the first cycle, two reduction peaks at 0.75 V and 0.91 V corresponding to the decomposition of the electrolyte and the formation of the solid electrolyte interface (SEI) film can be observed, and these factors result in a large irreversible capacity loss.This phenomenon explains why the initial Coulombic efficiency of carbon materials is generally lower than that of other materials.A pair of redox peaks near 0.01-0.10V can be observed in the CV curves, demonstrating the reversible  insertion/desertion of Na + ions in O-NPC [9,24].The CV curves of NPC/NS-x show a broad peak near 0.01 V, which indicates that Na ions undergo 'pseudoadsorption' behaviour [6,25].The redox peaks of O-NPC shift to lower potentials after the first cycle, while those of NPC/NS-x do not significantly change.This reflects the excellent stability of NPC/NS-x.
Figure 5(e) shows the GCD profiles of NPC/NS-2 as the anode at a current density of 0.1 A g −1 .The platform between 0.6 V and 1.2 V represents the formation of the SEI film, which is consistent with the CV analysis.The first discharge and charge specific capacities of NPC/NS-2 are 753.8 and 416.8 mAh g −1 , respectively, indicating a Coulombic efficiency of 55.3%.This low Coulombic efficiency is attributed to electrolyte decomposition and the formation of the SEI film.After the first cycle, the GCD curves of NPC/NS-2 are almost overlapped, and only a slope can be observed.As shown in TEM images (figure 3), the lattice spacing of NPC/NS-x (0.42 nm) is larger than that of O-NPC (0.37 nm).This indicates that S doping increases the lattice distance and results in 'pseudoadsorption' behaviour, which is beneficial for achieving high rate capability and outstanding cyclic stability.
The specific capacities and long-term cycling stability of NPC/NS-x were evaluated, as shown in figure 6(a).O-NPC, NPC/NS-1, NPC/NS-2, and NPC/NS-3 have reversible capacities of 252.5, 212.7, 314.7, and 243.7 mA h g −1 after 200 cycles at 0.1 A g −1 , respectively.Notably, the S content of NPC/NS-3 is higher than that of NPC/NS-2, but the proportion of oxidised S in NPC/NS-3 is also higher than that in NPC/NS-2.This leads to a larger irreversible specific capacity.Due to the high CS(NH 2 ) 2 doping content of NPC/NS-3, some macroporous structures collapse, and the particles of this sample are refined and smaller.Although this could increase the SSA of the material to a certain extent, the contact resistance between the particles is also increased, resulting in higher internal resistance between the electrode and some particles.Poor contact with the electrolyte and shedding from the particle surfaces of the electrode reduce the reversible specific capacity of this material.The cyclic stability of NPC/NS-2 at a highrate of 1 A g −1 is shown in figure 6(b).NPC/NS-2 still maintains a high reversible specific capacity of 202.5 mAh g −1 after 1000 cycles, which reflects its outstanding cyclic stability (figure 6(b)).
The rate performance, another important aspect for characterising SIBs, was also evaluated via a repetitive GCD process at different current densities (0.05, 0.1, 0.5, 0.8, 1, and 2 A g −1 ), as shown in figure 6(c).After 10 cycles at each step, the NPC/NS-2 anode shows reversible specific capacities of 324. 5, 251.7, 228.6, 207.3, 195.7, and 142.3 mA h g −1 , respectively.When the current density is restored to 0.05 A g −1 , the specific capacity returns to 325.4 mA h g −1 , demonstrating that the rate performance of NPC/NS-2 is better than that of O-NPC.These values are also comparable or even superior to the leading results of other reported carbonaceous anodes for SIBs (figure 6(d)), such as AHC-800, PLHC-N-1000, HPNDC, NCT-1000, and LS1200 [26][27][28][29][30]. Figure 6(e) shows the Nyquist plots of O-NPC and NPC/NS-x, which display very similar shapes.NPC/NS-2 has a smaller semicircle and the largest slope, indicating that its overall impedance is the smallest among the four materials [31].Therefore, the NPC/NS-2 anode exhibits good kinetics for sodium ion diffusion, explaining its excellent electrochemical performance.

Conclusion
In summary, a two-step pyrolysis route was designed to prepare N/S co-doped porous carbon from longan shell.The N/S co-doped porous carbon was first activated by KOH.Next, CS(NH 2 ) 2 was employed as a N/S source to dope the porous carbon material.Electrochemical performance tests showed that the NPC/NS-2 electrode exhibits the highest reversible specific capacity of the prepared samples.This electrode still maintains a high reversible specific capacity of 202.5 mA h g −1 at a high rate of 1 A g −1 , and its cycle efficiency reaches more than 98%.NPC/NS-2 shows excellent electrochemical performance and compares favourably to other reported materials.This work provides a facile and cheap route to prepare high-performance anode materials for SIBs.

FESEM
images of O-NPC are shown in figures 2(a)-(b), which displays an irregularly shaped and porous structure.The cross-linked and interconnected structure is conducive to achieving full contact between the electrode and the electrolyte.After the doping process, the number of macropores in the carbon material declines with increasing CS(NH 2 ) 2 ratio, as shown in figures 2(c)-(e).Some broken particles can be observed, increasing the number of edges and the availability of some sodium storage sites.However, broken particles may also lead to poor contact with the electrolyte and thus cause electrode failure.NPC/NS-3 was selected as a model for analysing the distribution of elements, and EDS mapping is shown in figures 2(f)-(i).C, N, O, and S elements are uniformly distributed in NPC/NS-3, indicating the uniform incorporation of N and S from CS(NH 2 ) 2 into O-NPC.TEM images of O-NPC and NPC/NS-x are shown in figures 3(a)-(d).Both O-NPC and NPC/NS-x have many pores, and their graphite-like carbon crystallites are arranged in a 'helical structure' due to the strong cross-linking between the crystallites.This structure could facilitate the transfer of electrons.The interlayer spacing of NPC/NS-x is 0.42 nm, which is larger than that of O-NPC (0.37 nm).This large interlayer distance is wide enough for Na ions to undergo 'pseudoadsorption' behaviour.The N 2 adsorption/desorption isotherms of NPC/NS-x and O-NPC are displayed in figure 3(e).The isotherms are horizontal over a wide pressure range and display slight hysteresis loops, indicating that they can be categorised as type I.The SSAs of O-NPC, NPC/NS-1, NPC/NS-2, and NPC/NS-3 are 428, 230, 638, and 1522 m 2 g −1 , respectively.The pore size distributions

Figure 1 .
Figure 1.Schematic illustration of the preparation of NPC/NS-x using longan shell.

Figure 4 (
b) shows the Raman spectra of O-NPC and NPC/ NS-x.The peaks located at 1348 cm −1 and 1588 cm −1 respectively correspond to the D-band (ascribed to marginal, defective, and disordered carbon) and the G-band (attributed to sp 2 -hybridised carbon).These spectra indicate that O-NPC and NPC/NS-x are hard carbons[16,17].The intensity ratios of the D-band and G-band (I D /I G ) were obtained to reflect the degree of crystallization and disorder of the carbon structures.With increasing CS(NH 2 ) 2 content, the I D /I G ratio increases because the higher degree of N and S doping enhances the breathing mode of the sixfold aromatic ring near the basal edge.FTIR spectra were used to characterise the functional groups present in NPC/NS-x (figure3(c)).Three peaks located at 3463 cm −1 , 1730 cm −1 , and 1611 cm −1 correspond to the stretching vibrations of O-H (C-H), aromatic C=C, and C=O (C=N) band, respectively[18,19].The intensity of the peak at 3463 cm −1 decreases with increasing CS(NH 2 ) 2 ratio.This is because increasing the amount of CS(NH 2 ) 2 during synthesis increases the quantity of doped heteroatoms at the edges of the carbon structure, leading to reduced C-H bond content.XPS was performed to further analyse the surface components and chemical states of the samples, as shown in figure 4. The XPS spectra in figure 4(d) indicate the presence of C, N, O, and S in O-NPC and NPC/NS-x.The high-resolution C 1s spectra of NPC/NS-x are shown in figure 4(e).Three peaks corresponding to C-S (287.0 eV), C-O/C-N (286.5 eV), and C-C(284.8 eV), are observed, demonstrating the successful doping of N and S into the carbon framework [20, 21].The high-resolution N 1 s spectra shown in figure 4(f) are deconvoluted into four peaks ascribed to pyridinic N (N-6, 398.0 eV), pyrrole N (N-5, 401.0 eV), graphite N (N-Q, 403.0 eV), and nitrogen oxide (N-O, 404.0 eV) [14, 22].The high-resolution S 2p spectra (figure 4(g)) are deconvoluted into peaks at 166.5 eV (S 2p 3/2 ), 167.0 eV (S 2p 1/2 ), and sulphate.These peaks are associated with C-S x -C (x = 1-2) functional groups and the oxidation of sulphur during the pyrolysis process [23].Notably, O-NPC, NPC/NS-1, NPC/NS-2, and NPC/NS-3 contain 1.37, 1.83, 3.53, and 5.05 wt% N content, respectively (figure 4(h)).Thus, the N element of NPC/NS-x increases with increasing CS(NH 2 ) 2 ratio.In contrast, O-NPC, NPC/NS-1, NPC/NS-2, and NPC/NS-3 contain 0.19, 0.57, 0.62, and 0.32 wt% sulphur content, respectively,

Figure 6 .
Figure 6.Cyclic performance tests of (a) NPC/NS-x at 0.1 A g −1 and (b) O-NPC and NPC/NS-2 at 1 A g −1 .(c) Rate performance of NPC/NS-2 and O-NPC.(d) Comparison of this work with previously studied carbon-based anodes for SIBs with respect to rate capacity.(e) EIS curves of NPC/NS-x and O-NPC.