Realizing Rapid Kinetics of Na Ions in Tin‐Antimony Bimetallic Sulfide Anode with Engineered Porous Structure

Metallic sulfide anodes show great promise for sodium‐ion batteries due to their high theoretic capacities. However, their practical application is greatly hampered by poor electrochemical performance because of the large volume expansion of the sulfides and the sluggish kinetics of the Na+ ions. Herein, a porous bimetallic sulfide of the SnS/Sb2S3 heterostructure is constructed that is encapsulated in the sulfur and nitrogen codoped carbon matrix (SnS/Sb2S3@SNC) by a facile and scalable method. The porous structure can provide void space to alleviate the volume expansion upon cycling, guaranteeing excellent structural stability. The unique heterostructure and the S, N codoped carbon matrix together facilitate fast‐charge transport to improve reaction kinetics. Benefitting from these merits, the SnS/Sb2S3@SNC electrode exhibits high capacities of 425 mA h g−1 at 200 mA g−1 after 100 cycles, and 302 mA h g−1 at 500 mA g−1 after 400 cycles. Moreover, the SnS/Sb2S3@SNC anode shows an outstanding rate performance with a capacity of over 200 mA h g−1 at a high current density of 5000 mA g−1. This study provides a new strategy and insight into the design of electrode materials with the potential for the practical realization and applications of next‐generation batteries.


Introduction
In the carbon-neutral future, a huge demand will exist for largescale energy storage (LSES) to regulate renewable energy. [1]thium-ion batteries (LIBs) show great success in portable electronics and electrical vehicles (EVs), [2] but their application in LSES is hindered by their high cost and limited lithium resources. [3]Sodium-ion batteries (SIBs) have similar energy storage mechanisms compared to LIBs, while sodium is in natural abundance and is cheaper than lithium. [4]Thus, in LSES, SIBs are considered a potential replacement for LIBs. [5]However, the Na þ ion has a larger ionic radius than the Li þ ion (1.02 Å of Na þ vs 0.76 Å of Li þ ), which makes the advanced anode materials for LIBs failure in the SIBs system. [6]raphite, for example, which is widely used in LIBs as an anode, forms NaC 64 compound in SIBs with a low capacity of 64 mA h g À1 . [7]Therefore, developing anode materials for SIBs with high capacity and long cycling stability is a must to meet the ever-growing demand for LSES devices. [8]onsiderable materials have been investigated as anodes for SIBs, including carbon-based materials (hard carbon, [9] graphene [10] ), alloy-based materials (tin, [11] antimony [12] ), metal oxides, [13] metallic sulfides, [8b,14] etc.The metallic sulfides have been widely studied in LIBs as advanced anodes.For example, Lee et al. reported the hierarchical sulfurdoped carbon Bi 2 S 3 (Bi 2 S 3 @SC) hollow nanotubes as anode materials for LIBs, exhibiting favorable rate capability and enhanced cycling stability.2c] Lee et al. employed a one-pot hydrothermal method to synthesize Sb 2 S 3 nanorods enveloped in graphene sheets, which serve as anode materials for LIBs.The Sb 2 S 3 nanorods contribute to a shorter diffusion path for lithium ions and promote greater contact area between the electrode and electrolyte during cycling. [15]Besides LIBs, metallic sulfides have demonstrated many advantages in SIBs too.The metallic sulfides can deliver high theoretical capacity since the Na þ ions can be stored via mechanisms of conversion reaction and/or alloying reaction. [16]They also show good reversibility upon cycling because of the weak M─S bonds. [17]For example, tin sulfide (SnS) with cubic phase and antimony sulfide (Sb 2 S 3 ) [15,18] with DOI: 10.1002/sstr.202300100Metallic sulfide anodes show great promise for sodium-ion batteries due to their high theoretic capacities.However, their practical application is greatly hampered by poor electrochemical performance because of the large volume expansion of the sulfides and the sluggish kinetics of the Na þ ions.Herein, a porous bimetallic sulfide of the SnS/Sb 2 S 3 heterostructure is constructed that is encapsulated in the sulfur and nitrogen codoped carbon matrix (SnS/Sb 2 S 3 @SNC) by a facile and scalable method.The porous structure can provide void space to alleviate the volume expansion upon cycling, guaranteeing excellent structural stability.The unique heterostructure and the S, N codoped carbon matrix together facilitate fast-charge transport to improve reaction kinetics.Benefitting from these merits, the SnS/Sb 2 S 3 @SNC electrode exhibits high capacities of 425 mA h g À1 at 200 mA g À1 after 100 cycles, and 302 mA h g À1 at 500 mA g À1 after 400 cycles.Moreover, the SnS/Sb 2 S 3 @SNC anode shows an outstanding rate performance with a capacity of over 200 mA h g À1 at a high current density of 5000 mA g À1 .This study provides a new strategy and insight into the design of electrode materials with the potential for the practical realization and applications of nextgeneration batteries.
an orthorhombic crystal structure are two promising anodes for SIBs, which have large theoretical specific capacities of 1022 and 947 mA h g À1 , respectively. [19]19c,20] Ou et al. used the wet-chemical method to develop the heterostructure of SnS 2 /Mn 2 SnS 4 .20a] Wang's group reported an advanced anode of SnO 2 /SnSe 2 @C and demonstrated that the construction of the SnO 2 /SnSe 2 heterostructure can boost the localized charge transfer kinetics. [21]espite the heterojunctions formed between metallic sulfides can improve the conversion-alloying reaction reversibility, they cannot maintain the structure stable in a long cycle life or under a high current density. [22]Thus, it is necessary to build a novel structure together with the heterojunctions to mitigate the volume expansion of the metallic sulfides upon an extensive sodiation/desodiation process.
Here, we come up with a "killing three birds with one stone" method to build bimetallic sulfide of the SnS/Sb 2 S 3 heterostructure with an engineered porous structure.The precursor of the composite SnSb x /PAN/S was first obtained by ball milling SnSb x nanoparticles, polyacrylonitrile (PAN) polymer, and sulfur (S) together.During the following calcination process, the S evaporated and reacted with the SnSb x alloy to form the bimetallic SnS/Sb 2 S 3 heterostructure, and the original position of sulfur became void, generating a porous structure.Meanwhile, the PAN polymer was carbonized to form a highly disordered carbon matrix with sulfur and nitrogen codoping.The advantages of this novel and facile process are threefold.First, the SnS and Sb 2 S 3 with bandgap difference form the heterostructures, which enhances the surface reaction kinetics and facilitates charge transport due to the internal electric field at the heterointerface.Second, the porous structure provides space to mitigate the volume expansion of the metallic sulfides upon cycling.In addition, the S, N codoped carbon matrix can boost the conductivity of the composite material, where abundant defects can provide fruitful sites to store sodium ions, leading to a high pseudocapacitance.Because of the synergistic effect of the unique structure design, the SnS/Sb 2 S 3 @SNC electrode shows excellent performance as an advanced anode for SIBs.The SnS/Sb 2 S 3 @SNC electrode delivers high capacities of 425 mA h g À1 at 200 mA g À1 after 100 cycles, and 302 mA h g À1 at 500 mA g À1 after 400 cycles.In the rate test, the SnS/Sb 2 S 3 @SNC anode shows a capacity of over 200 mA h g À1 at a high current density of 5000 mA g À1 .

Results and Analysis
Figure 1a illustrates the synthesis process of the SnS/ Sb 2 S 3 @SNC anode material.Briefly, the SnSb x alloy nanoparticles were synthesized by a coprecipitation method by mixing aqueous solutions containing NaBH 4 and salts of SnCl 2 and SbCl 3 in the presence of trisodium citrate dihydrate.The SnSb x nanoparticles were further dispersed with S and PAN by a simple ball-milling process.The desired product of SnS/Sb 2 S 3 @SNC was obtained after being calcined in the nitrogen atmosphere.During the calcination process, the sulfur evaporated from the mixture to form a porous structure and reacted with the SnSb x alloy to form bimetallic sulfide of the SnS/Sb 2 S 3 heterostructure.The PAN was simultaneously carbonized into a robust S, N codoped carbon matrix.
The SEM images of the as-prepared SnS/Sb 2 S 3 @SNC particles are shown in Figure 1b-f.As shown in Figure 1b,c, the SnS/Sb 2 S 3 @SNC particles show an average particle size from hundreds of nanometers to a few microns.The high-resolution SEM images of the particle, as shown in Figure 1d,e, exhibit fruitful pores on its surface, which act as the void space to alleviate volume expansion of the SnS/Sb 2 S 3 heterostructure.To verify the distribution of the elements, the SnS/Sb 2 S 3 @SNC material was investigated by the EDS mapping test (Figure 1f ).The results show that the elements of C, S, Sn, and Sb are homogeneously distributed in the SnS/Sb 2 S 3 @SNC particle.The C element presents a similar distribution pattern compared to the other elements, indicating the SnS/Sb 2 S 3 composite is well coated with the carbon layer on its surface.The STEM mappings in Figure S4, Supporting Information, also confirm the existence of Sn and Sb elements in the as-prepared sample.The porous structure of the SnS/Sb 2 S 3 @SNC material was further characterized by BET tests.The nitrogen adsorption-desorption isotherm is shown in Figure 1g, and the result fits the type III adsorption isotherm well. [23]Meanwhile, the pore-size distribution (Figure 1h) reveals a hierarchical pore structure of the as-prepared composite material, which consists of the micro-, meso-, and macropores.The formation of the hierarchical pore structure facilitates the infiltration of the electrolyte and provides void space to mitigate the large volume change of the electrode material during cycling. [24]he TEM was used to investigate the microstructure of the SnS/Sb 2 S 3 @SNC particles.The results are shown in Figure 2. According to Figure 2a,b, the SnS/Sb 2 S 3 @SNC particle shows a porous structure that many mesopores (2-50 nm) are inside the particle.In the enlarged TEM images in Figure 2c,d, the mesopores and the carbon layer are labeled.The thickness of the carbon layer coated on the SnS/Sb 2 S 3 @SNC particle is around 10 nm.From the high-resolution TEM (HRTEM) images in Figure 2e, the lattice structures of SnS and Sb 2 S 3 are observed, which are next to each other and form a heterostructure interface.The mesopores are also detected in the HRTEM image in Figure 2f.The TEM data agree well with the results of BET and SEM.
XRD measurement was performed to study the phases of the as-prepared material.The XRD pattern in Figure 3a shows that the intense and instinctive peaks of SnS/Sb 2 S 3 @SNC can be attributed to a cubic phase of SnS (JCPDS no.43-0304), and some small peaks can be indexed to the orthorhombic phase of Sb 2 S 3 (JCPDS no.42-1393). [25]To optimize the dosage of PAN, different samples were prepared by ball milling different amounts of the PAN polymer with the percentages in the precursors of 40%, 33.3%, 25%, 20%, and 18.3%, and the XRD results are provided in Figure S5, Supporting Information.The electrochemical tests in Figure S6, Supporting Information, show that the 33.3% PAN in the precursor delivers the best cycling stability among all the samples, while too much (40%) or too little (18.3%)PAN shows poor cycling performance.In that case, the 33.3% in the precursor is the optimized dosage of PAN studied in our manuscript.To confirm the carbon content in the SnS/ Sb 2 S 3 @SNC material, TGA is conducted in the air with a temperature range from 25 to 800 °C (Figure 3b).The slight weight increase at around 500 °C in the TGA curve can be ascribed to the oxidation of functional groups in the carbon matrix, which was also reported by others. [26]According to Equation (1)-(3), the weight fractions of carbon, SnS, and Sb 2 S 3 in the composite are 0.376, 0.442, and 0.182, respectively.
Figure 1.a) Schematic illustration of the synthesis of SnS/Sb 2 S 3 @SNC.b-e) SEM images of SnS/Sb 2 S 3 @SNC, f ) selected SnS/Sb 2 S 3 @SNC particle for the EDS mapping and corresponding EDS elemental mappings.BET analysis of g) nitrogen adsorption-desorption isotherm and h) pore-size distribution curves.
The chemical bonding information of SnS/Sb 2 S 3 @SNC was analyzed by XPS. Figure 3d shows the existence of the Sn, Sb, O, S, C, and N elements in the survey of the XPS spectrum.The high-resolution Sn 3d spectrum (Figure 3e) displays a pair of characteristic peaks at 495.2 and 483.7 eV from Sn 3d 3/2 and Sn 3d 5/2 of Sn 2þ , respectively. [27]The peak of O 1s overlaps with the peak of Sb 3d 5/2 , as shown in Figure 3d.Thus, the peak at around 530.8 eV in Sb 3d can be fitted into Sb 3d 5/2 at 530.5 eV and O 1s at 531.9 eV (Figure 3f ), where the peak located at 539.8 eV belongs to the Sb 3d 3/2 . [28]The high-resolution S 2p spectrum can be fitted into four peaks, as shown in Figure 3g.19b] Another two peaks with the binding energy of 164.7 and 169.1 eV correspond to the S─C and S─O bonds, respectively.It indicates the successful S-doping of the carbon matrix and some of the surface sulfide ions are oxidized during the XPS measurements. [29]In Figure 3h, the high-resolution C 1s spectrum can be fitted to the peaks at 284.7, 285.3, 286.1, and 289.7 eV, which are related to the C─C, C─S, C─N, and C═O, respectively. [30]The C═N and C-S peaks show that the carbon matrix is successfully doped by N and S atoms.Furthermore, three peaks can be observed in the high-resolution N 1s spectrum (Figure 3i), and they correspond to pyridinic N (398.5 eV), pyrrolic N (400.2eV), and graphitic N (401.8eV).This phenomenon also confirms the successful N-doping in the carbon matrix.The amorphous feature of the as-prepared carbon matrix is further investigated by the Raman spectrum.In Figure 3c, two characteristic peaks located at 1,334.7 and 1,553.1 cm À1 are D and G bands, respectively.The intensity ratio of the D band and G band (I D /I G ) is 1.49, demonstrating the dominant defective and disordered nature of the S, N codoped carbon layer. [30,31]The highly disordered carbon matrix has been proven to enhance the electrochemical performance of the carbon-based composite by improving conductivity. [32]The enriched defects can provide Na ions adsorption sites to achieve a dominant pseudocapacitive contribution. [33]he electrochemical performance of the SnS/Sb 2 S 3 @SNC electrode as an anode for SIBs is explored and presented in Figure 4. Figure 4a shows the CV curves of SnS/Sb 2 S 3 @SNC for the first three cycles, which were tested between 0.01 and 2.5 V at a scan rate of 0.1 mV s À1 .In the first cathodic sweep, the sharp and board peak at around 0.6 V corresponds to the conversion of SnS and Sb 2 S 3 to metallic Sn and Sb, the alloying process of Sn and Sb with Na þ , and the formation of the solid electrolyte interphase (SEI) layer. [34]After that, three small broad anodic peaks around 0.3, 0.7, and 1 V are observed, corresponding to the desodiation of the Na x Sn and Na x Sb phases. [35]The CV curves of the second and third cycles overlap very well, demonstrating good reversibility of the electrode.The basic reversible reactions of SnS/Sb 2 S 3 @SNC can be described as follows: Figure 4b displays the charge/discharge curves of the SnS/Sb 2 S 3 @SNC electrode at 200 mA g À1 .The first discharge and charge capacities of SnS/Sb 2 S 3 @SNC are calculated as 995 and 456 mA h g À1 , respectively.It should be noted that the relatively low initial Coulombic efficiency (CE) can be attributed to the formation of an SEI layer on the surface of the electrode, and the trapping of sodium ions in the carbon matrix. [36]The charge/discharge curves of the SnSb x and SnS/Sb 2 S 3 electrodes are provided in Figure S7 and S8, Supporting Information.The electrochemical cycling performance of the SnS/Sb 2 S 3 @SNC electrode was tested at 200 mA g À1 and compared with that of the SnS/Sb 2 S 3 electrode.As shown in Figure 4c, the SnS/ Sb 2 S 3 electrode shows poor cycling stability with a rapid performance degradation that the capacity decay to just 130 mA h g À1 after 100 cycles.This is because the solid structure of the SnS/ Sb 2 S 3 material cannot fully resist the stress derived from the large volume exchange upon cycling, resulting in the particle pulverization and turning into inactive components.In contrast, the SnS/Sb 2 S 3 @SNC electrode reveals much better cycling stability than that of SnS/Sb 2 S 3 , delivering a capacity of 426 mA h g À1 at 200 mA g À1 after 100 cycles.The CE of the SnS/Sb 2 S 3 @SNC electrode rises quickly to over 98% after only 5 cycles and then remains a stable CE larger than 99% throughout the whole cycling process.In contrast, the CE of the SnS/Sb 2 S 3 anode is quite lower than that of SnS/Sb 2 S 3 @SNC with apparent fluctuation.
The good cycling performance of the SnS/Sb 2 S 3 @SNC electrode can be ascribed to the unique structure design.First, the porous structure can provide buffer space to alleviate large volume changes upon cycling.The structure also facilitates electrolyte infiltration to boost charge transfer.Second, the SnS/Sb 2 S 3 heterostructure can create a microelectric field at heterointerfaces as illustrated in Figure 4h, which can improve the iondiffusion efficiency and promote interfacial electron transport.Third, the S, N codoped carbon matrix with abundant defects can improve the conductivity.The SnS/Sb 2 S 3 @SNC electrode was taken out from the cell after 100 cycles at 200 mA g À1 to examine the structure stability by TEM test.As shown in Figure S9a-c, Supporting Information, the structure of SnS/Sb 2 S 3 @SNC can  and c) Raman spectra of SnS/Sb 2 S 3 @SNC.XPS spectra of SnS/Sb 2 S 3 @SNC, d) survey, e) Sn 3d, f ) Sb 3d, g) S 2p, h) C 1s, and i) N 1s.
be preserved to some extent after 100 cycles and fully charged to 2.5 V.The engineered porous structure with the efficient buffering effect is still observed in the TEM images.HRTEM image in Figure S9d, Supporting Information, shows that the phase of SnS and Sb 2 S 3 can be formed again when the electrode was fully charged to 2.5 V after 100 cycles, demonstrating the good reversibility of the SnS/Sb 2 S 3 @SNC anode.
The synergistic effect of the porous structure, SnS/Sb 2 S 3 heterostructure, and S, N codoped carbon matrix is expected to significantly enhance ionic and electronic transport, which is further investigated by the EIS tests.As shown in Figure 4d, the semicircle in the EIS spectra of the SnS/Sb 2 S 3 @SNC electrode, which represents the charge transfer resistance, is much smaller than that of SnS/Sb 2 S 3 before the cycle.The SnS/Sb 2 S 3 @SNC electrode shows a smaller charge transfer resistance after 10 cycles compared with the fresh cell (Figure 4e).The improved conductivity benefits the rate performance of the SnS/Sb 2 S 3 @SNC electrode.As shown in Figure 4f, the cells were tested at various current densities from 50 to 5000 mA g À1 .The SnS/Sb 2 S 3 @SNC electrode delivers specifically capacities of 457, 404, 357, 301, and 252 mA h g À1 at the current densities of 50, 200, 500, 1,000, and 2000 mA g À1 , respectively.Even at a very high current density of 5000 mA g À1 , the cell with the SnS/Sb 2 S 3 @SNC electrode can provide a capacity of 200 mA h g À1 .When the current density dropped back to 50 mA g À1 after various current densities, the capacity rebounded to 440 mA h g À1 , indicating an excellent rate performance and cycling stability of the SnS/Sb 2 S 3 @SNC electrode.In contrast, because of the poor conductivity, the SnS/Sb 2 S 3 electrode shows lower capacities at various current densities compared to the SnS/Sb 2 S 3 @SNC electrode.The long-term cycling stability of the SnS/Sb 2 S 3 @SNC electrode was further examined at a high current density of 500 mA g À1 (Figure 4g).The SnS/Sb 2 S 3 @SNC electrode delivers a reversible capacity of 322 mA h g À1 after 400 cycles, and the CE reaches more than 99% after the 5th cycle and remains stable throughout the whole cycling process.The capacity retention is 89% from the third cycle to the 400th cycle, showing a 0.027% capacity decay per cycle.
The CV test of the SnS/Sb 2 S 3 @SNC electrode at various scan rates from 0.1 to 1 mV s À1 was performed to further examine electrochemical kinetic properties.The same peaks can be found in the CV curves at different scan rates, as shown in Figure 5a.The intensities of those peaks grow gradually with the increase of the scan rates.The capacity of SnS/Sb 2 S 3 @SNC originates from two different parts, diffusion-controlled behavior and capacitivecontrolled behavior.The contribution of the capacitive-controlled behavior, which is also known as pseudocapacitance, can be clarified by the relationship between the peak current (i) and the sweep rate (v).Equation (8) suggests that i and v follow the power law, where both a and b are adjustable values [22b,37] i ¼ av b (8) When the b value is close to 1, it indicates that the capacitivecontrolled behavior dominates the contribution of the capacity.In contrast, the domination of the diffusion-controlled behavior in the capacity contribution makes the b value approach 0.5.The b value can be calculated by plotting log (v)-log (i).According to Figure 5b, the b values of three cathodic peaks are 0.839, 0.854, and 0.769.In addition, the b values of the anodic peaks are 0.841, 0.897, and 0.864, respectively.The results of the b value of the SnS/Sb 2 S 3 @SNC electrode show that the pseudocapacitance plays an important role in contributing capacity.The capacity contribution from the capacitive-controlled process is derived from the S, N codoped carbon matrix, which can improve the surface redox kinetics of the SnS/Sb 2 S 3 @SNC electrode.The improved kinetics further leads to a better rate capability, which has been demonstrated in Figure 4f.The quantitative ratio of the capacitive contribution at different scan rates can be calculated from the CV curves according to Equations ( 9) and (10).In Equation ( 9), the k 1 v is the capacitive contribution and the k 2 v 1/2 is the contribution from the diffusion-controlled process, where k 1 and k 2 are variable constants. [38] According to Equation ( 10), the specific values of k 1 and k 2 were determined from the slope at a fixed voltage by plotting the v 1/2 versus i/v 1/2 at different scan rates.At a high scan rate of CV at 0.8 mV s À1 , the capacitive contribution is 79.5% as shown in the shaded area (Figure 5c).The high capacitive contribution reveals the predomination of pseudocapacitance at this scan rate, which benefits the fast-charging capability and high-power density of the assembled cell.It is also found that the SnS/Sb 2 S 3 @SNC electrode shows a large capacitive contribution at various CV scan rates, as shown in Figure 5d.Moreover, the capacitive contribution has a rising trend as the scan rate increases from 57.9% at 0.1 mV s À1 up to 82.3% at 1 mV s À1 .The dominant pseudocapacitive contribution of the SnS/Sb 2 S 3 @SNC anode can be ascribed to the unique porous SnS/Sb 2 S 3 heterostructure and the S, N codoped carbon matrix.Since the enhanced capacitive contribution avoids slow sodium ions diffusion and prevents structure damage, it can be concluded that the SnS/Sb 2 S 3 @SNC electrode has a better rate performance to deliver a higher reversible capacity at a high current density.

Conclusion
In summary, a bimetallic sulfide of the SnS/Sb 2 S 3 heterostructure with an engineered porous structure has been prepared and encapsulated in the S, N codoped carbon matrix.In the calcination process, the sulfur in the precursor plays three roles, evaporating to generate a porous structure, reacting with SnSb x alloy to form SnS/Sb 2 S 3 heterostructure, and doping the carbon matrix.The internal electric field at the SnS/Sb 2 S 3 heterointerface can enhance charge transfer and promote reaction kinetics.The S, N codoping can greatly improve the conductivity of the carbon matrix, which addresses the issue of the sluggish kinetics of the Na þ ions.Meanwhile, the porous structure can provide void space to mitigate the volume expansion upon cycling, guaranteeing long-term cycling stability.The SnS/Sb 2 S 3 @SNC anode delivers high capacities of 425 mA h g À1 at 200 mA g À1 after 100 cycles, and 302 mA h g À1 at 500 mA g À1 after 400 cycles.The SnS/Sb 2 S 3 @SNC anode also demonstrates good rate capability, and it shows a capacity of over 200 mA h g À1 at a high current density of 5000 mA g À1 .The electrochemical kinetic test has confirmed that pseudocapacitance plays a key role in the contribution of capacity.

Figure 2 .
Figure 2. a-d) TEM images and e-f ) HRTEM images of the SnS/Sb 2 S 3 @SNC particles.

Figure 4 .
Figure 4. Electrochemical performance of the SnS/Sb 2 S 3 @SNC electrode.a) CV curves.b) Galvanostatic discharge/charge profiles of the initial five cycles.c) Cycling performance at 200 mA g À1 .d,e) Nyquist plots.f ) Rate performance.g) Long-term cycling performance at 500 mA g À1 .h) Schematic illustration of the microelectric field at the SnS/Sb 2 S 3 heterointerfaces.

Figure 5 .
Figure 5.The electrochemical kinetic properties of SnS/Sb 2 S 3 @SNC.a) CV curves.b) The corresponding curves log i versus log v. c) Capacitive contribution at 0.8 mV s À1 .d) The capacitive contribution e at different scan rates.