简介概要

Hierarchical porous hard carbon enables integral solid electrolyte interphase as robust anode for sodium-ion batteries

来源期刊：Rare Metals2020年第9期

论文作者：Xu-Kun Wang Juan Shi Li-Wei Mi Yun-Pu Zhai Ji-Yu Zhang Xiang-Ming Feng Zi-Jie Wu Wei-Hua Chen

文章页码：1053 - 1062

摘要：Hard carbon is the most promising anode for sodium-ion battery applications due to the wide availability and low work voltage.However,it often delivers worse electrochemical performance in ester-based electrolytes.Herein,a hierarchically porous loose sponge-like hard carbon with a highly disordered phase,prepared from the biomass of platanus bark,exhibits superior rate performance with a capacity of 165 mAh·g^-1 at a high current of1 A·g^-1,and high retention of 71.5% after 2000 cycles in an ester-based electrolyte.The effect of the hierarchically porous loose sponge-like structure on the formation dynamics of solid electrolyte interphase（SEI）,and related properties,was studied via cyclic voltammetry（CV）,galvanostatic intermittent titration technique（GITT）,X-ray photoelectron spectroscope（XPS）,Fourier transform infrared spectroscopy（FTIR） and electrochemical impedance spectroscopy（EIS） analysis.These results reveal that the hierarchically porous structure can construct continued connecting channels and accelerate the electrolyte transport,which is beneficial to the reaction kinetics of SEI.Moreover,the mesoporous structure is conducive to good contact between electrolyte and materials and shortens the Na⁺ diffusion path,which in turn facilitates the charge transfer kinetics in the material.

详情信息展示

稀有金属(英文版) 2020,39(09),1053-1062

Hierarchical porous hard carbon enables integral solid electrolyte interphase as robust anode for sodium-ion batteries

Xu-Kun Wang Juan Shi Li-Wei Mi Yun-Pu Zhai Ji-Yu Zhang Xiang-Ming Feng Zi-Jie Wu Wei-Hua Chen

Green Catalysis Center,College of Chemistry,Zhengzhou University

Center of Advanced Materials Research,Zhongyuan University of Technology

Northwest Composites Center,Department of Materials,University of Manchester

作者简介：*Wei-Hua Chen e-mail:chenweih@zzu.edu.cn;

收稿日期：17 March 2020

基金：financially supported by the National Natural Science Foundation of China (Nos.U1804129, 21771164,21671205 and U1804126);Zhongyuan Youth Talent Support Program of Henan Province and Zhengzhou University Youth Innovation Program;

Hierarchical porous hard carbon enables integral solid electrolyte interphase as robust anode for sodium-ion batteries

Xu-Kun Wang Juan Shi Li-Wei Mi Yun-Pu Zhai Ji-Yu Zhang Xiang-Ming Feng Zi-Jie Wu Wei-Hua Chen

Green Catalysis Center,College of Chemistry,Zhengzhou University

Center of Advanced Materials Research,Zhongyuan University of Technology

Northwest Composites Center,Department of Materials,University of Manchester

Abstract：

Hard carbon is the most promising anode for sodium-ion battery applications due to the wide availability and low work voltage.However,it often delivers worse electrochemical performance in ester-based electrolytes.Herein,a hierarchically porous loose sponge-like hard carbon with a highly disordered phase,prepared from the biomass of platanus bark,exhibits superior rate performance with a capacity of 165 mAh·g^-1 at a high current of1 A·g^-1,and high retention of 71.5% after 2000 cycles in an ester-based electrolyte.The effect of the hierarchically porous loose sponge-like structure on the formation dynamics of solid electrolyte interphase(SEI),and related properties,was studied via cyclic voltammetry(CV),galvanostatic intermittent titration technique(GITT),X-ray photoelectron spectroscope(XPS),Fourier transform infrared spectroscopy(FTIR) and electrochemical impedance spectroscopy(EIS) analysis.These results reveal that the hierarchically porous structure can construct continued connecting channels and accelerate the electrolyte transport,which is beneficial to the reaction kinetics of SEI.Moreover,the mesoporous structure is conducive to good contact between electrolyte and materials and shortens the Na⁺ diffusion path,which in turn facilitates the charge transfer kinetics in the material.

Keyword：

Sodium-ion battery; Hard carbon; Hierarchically porous structure; SEI; Ester-based electrolyte;

Received： 17 March 2020

1 Introduction

Sodium-ion batteries have become one of the most promising energy storage systems in the coming generation owing to wide resource distribution and low cost [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11] .Hard carbon with abundant resources has a lower sodiumembedded plateau region,ergo high energy density.Therefore,it is considered as the most promising anode material for sodium-ion batteries [ 12, 13, 14, 15, 16, 17] .However,rate performance and cycling stability of hard carbon are poor and are limiting factors in their wider uptake in applications.

Solid electrolyte interphase (SEI) layer plays an important role in maintaining cycling and rate performance of anode materials [ 18, 19, 20, 21] .Usually,the ether-based electrolytes are employed to improve the performance of the hard carbon anode due to the formation of thinner but more stable SEI induced by ether solvent [ 22] .It has been reported that the SEI layer containing organic and inorganic species formed in the ether-based electrolyte favors sodiation dynamics [ 23, 24] .However,an ester-based electrolyte,which has a higher voltage window,benefits the assembly of full cells with positive electrodes,tending to form thick SEI layers that originate from the reductive products of ester molecules,leading to poor rate performance.Many strategies have been reported to improve the performance of hard carbon through constructing superior SEI in an ester-based electrolyte.Xu and co-authors reported a new strategy to stabilize the sucrose-derived amorphous hard carbon by pre-engineering a foreign esteroriginated SEI [ 15] .They demonstrated improved electrochemical performance,but the electrode pretreatment was first pre-cycled in an electrolyte system,which is complicated to implement in practical applications.Chang et al.put forward the concept that a moderate concentration ester-based electrolyte can induce a robust SEI with fast Na⁺transport and excellent durability,but the cost of the electrolyte was too high for scale-up in industry settings [ 25] .Cao et al.proposed although some additives can form a compact SEI film,they also increase the polarization and the impedance of the electrode simultaneously [ 26] .So,the problem of constructing an effective SEI on hard carbon in an ester-based electrolyte remains a challenge.A deep understanding of the reaction kinetics of SEI on hard carbon in an ester-based electrolyte is required to help construct an effective SEI and solve the challenges associated with hard carbon in an ester-based electrolyte.

Material structure design not only is an effective strategy to improve the structure stability and reaction kinetics,but also can help modify the SEI layer.The structure of hard carbon is mainly determined by the precursors because hard carbon is usually prepared by pyrolysis.The precursors include polymers,fossil fuels and biomass [ 27, 28, 29] .Among them,biomass-derived hard carbon is highly attractive due to its wide availability,low cost,environmental friendliness and sustainability [ 30, 31] .During the past few years,various biomass methods have been widely adopted to prepare hard carbon [ 32, 33, 34, 35] .More importantly,biomasses usually have unique and inartificial structures [ 36] ,which are hardly synthesized by the traditional chemical method.The unique structures directly determine the phase and composition of the resulting hard carbon,which has a significant influence on the electrochemical storage performances.However,the effect of structure on the formation dynamics of SEI and related properties has not yet been further studied.

Biomass platanus bark has many advantages including high density,a high carbon content,porous structure and wide availability.Herein,a hierarchically porous loose sponge-like hard carbon with a highly disordered phase is prepared at a low temperature of 700℃from concentrated sulfuric acid treatment of platanus bark,which has a main pore diameter of～4 nm which accounts for 27.29%of the total pore volume and is mesoporous at around 50 nm which accounts for 72.71%and a specific surface area of602.1 m²·g^-1.The as-prepared material exhibits excellent rate performance in an ester-based electrolyte.Moreover,the effect of the hierarchically porous structure on the formation dynamics of SEI in ester-based electrolyte,and related properties,was studied via cyclic voltammetry(CV),galvanostatic intermittent titration technique (GITT),X-ray photoelectron spectroscope (XPS),Fourier transform-infrared spectroscopy (FTIR) and electrochemical impedance spectroscopy (EIS) analysis.

2 Experimental

2.1 Material synthesis

The raw material was obtained from the platanus on our campus (Zhengzhou University).It was crushed into small particles and washed with water/ethanol before drying at60℃for 12 h to form precursor.Then,2 g precursor was immersed in 20 ml concentrated H₂SO₄ (AR,≥98%,Sigma Aldrich),which was mixed in 50℃thermostatic water baths at 365 r·min^-1.After centrifuging and filtration,the obtained black sample was washed with distilled water several times to obtain pH neutral.The product was dried by freeze drying for 24 h.Finally,the dried product was placed in a tube furnace with flowing Ar gas so it could be carbonized.It was heated to 550℃at a heating rate of1℃·min^-1 before stabilized at 550℃for 1 h,and then heated to 700℃for 2 h at a heating rate of 10℃·min^-1.The obtained sample was labeled as NHC-7.For comparison,the sample,labeled as HC-7,was also prepared using the same steps without sulfuric acid treatment.

2.2 Material characterization

The morphology and elemental distribution of samples were characterized by scanning electron microscope (SEM,Merlin Compact,ZEISS) equipped with an energy-dispersive X-ray spectroscopy (EDS,Oxford X-Max) system.Transmission electron microscopy (TEM) images and corresponding SAED patterns were recorded using a transmission electron microscope (TEM,Tecnai G2 F30,FEI).The crystal structures of samples were analyzed through X-ray power diffraction (XRD,PANalytical X'Pert PRO) with Cu Kαradiation (λ=0.15416 nm) at a scan rate of 10 (°)·min^-1.Raman spectra were obtained using a Laser Raman Spectrometer with a laser wavelength of 532 nm.Brunauer-Emmett-Teller surface area (BET,Micromeritics ASAP 2420),adsorption and desorption under a N₂ atmosphere were conducted to measure the surface area.The functional groups on the surface of the material were characterized by Fourier transform infrared spectroscopy (FTIR,PerkinElmer Spectrum Two).The XPS measurements were performed by X-ray photoelectron spectroscope (XPS,Thermo Escalab 250Xi).

2.3 Electrochemical testing

The hard carbon electrodes were prepared by mixing 70wt%of sample,20 wt%of acetylene black and 10 wt%sodium carboxymethyl cellulose (CMC) solution to form a slurry,which was uniformly spread onto a Cu foil and dried at 60℃for 24 h.The mass loading of active material in electrodes was approximately 1.0-1.5 mg·cm^-2.The electrochemical performance of the hard carbon materials was tested using a CR2025 coin cells with an Na metal foil as a counter electrode,1 mol·L^-1 NaClO₄ in ethylene carbonate (EC) and diethyl carbonate (DEC)(1:1,by volume) as electrolyte,and glass fibers as separator.The coin cells were assembled in an Ar filled glove box with water/oxygen content lower than 0.1 ppm.The galvanostatic charge-discharge performances were evaluated on a NEW ARE battery test system at a voltage range of 0.01-2.80 V.Electrochemical impedance spectroscopy (EIS)was conducted on an CHI 604e electrochemical work station in the frequency range from 0.01 Hz to 100 kHz with an alternative current (AC) amplitude of 5 mV,and CV was performed in a voltage range of 0.01-2.80 V at a scan rate of 0.1 mV·s^-1.All electrochemical measurements were conducted at room temperature.

For full-cell assembly,the NaFe_1/3Ni_1/3Mn_1/3O₂ (labeled as NFM) commercial materials were produced by Shanghai Zijian Chemical Technology Co.,Ltd [ 37] .The cathode electrode was prepared by mixing 80 wt%NFM,10 wt%super P and 10 wt%polyvinylidene difluoride (PVDF)dissolved in N-methyl-2-pyrrolidone (NMP) to form a slurry,which was uniformly spread onto an Al foil,and the foil was then dried in a vacuum oven at 120℃overnight.To improve the coulombic efficiency before the fabrication of the full cell,the hard carbon anode was pre-cycled for ten cycles at a current density of 0.05 A·g^-1.The full cells were assembled with the NHC-7 as the anode,NFM as the cathode and 1 mol·L^-1 NaClO₄ EC/DEC as the electrolyte.The galvanostatic charge-discharge performances of full cells were evaluated in the voltage range of 0.5-3.9 V at a current density of 0.05 A·g^-1.

3 Results and discussion

As illustrated in Fig.1a,the obtained carbon material HC-7inherits the intrinsic structure of platanus bark,which contains large sheets with small micropores (Fig.1b,c).The concentrated sulfuric acid treatment can dissolve the cellulose and remove many impurities and trace elements to form micropores (Support Information,Figures S1,S2);herefore,the obtained hard carbon material NHC-7 consists of small sheets of various sizes,which are heaped up to construct a hierarchically porous structure like loose sponge (Fig.1d,e) after pyrolyzing for 2 h under a steady stream of argon.According to HRTEM and SAED images(Fig.1f,g),the prepared NHC-7 has a highly disordered structure with a low degree of graphitization.

After acid treatment,the precursor shows only the (002)crystal plane,indicating an amorphous structure is formed.After pyrolysis,the XRD pattern of NHC-7 (Fig.1h)exhibits two broad peaks attributed to the crystal plane(002) and a graphitized characteristic crystal plane (101),confirming the amorphous structure [ 38] .According to the Bragg equation,the interlayer spacing of NHC-7 is calculated to be 0.3941 nm,which is greater than that of HC-7(0.3786 nm).Therefore,acid treatment can enhance the interlayer spacing which is of benefit to the insertion/extraction of sodium ions.However,HC-7 has a strong CaCO₃ diffraction peak,indicating that the acid treatment can dissolve the metal elements in the biomass and avoid the formation of CaCO₃ through the reaction of metal elements and CO₂ in the biomass.

Characteristic peaks D located at 1326 cm^-1 and G at1590 cm^-1 are shown in the Raman spectrum of NHC-7,respectively (Fig.1i).The ratio value ofI_G/I_D increases from 0.37 to 0.47 after acid treatment,demonstrating that the order degree of the sample increases.In addition,the defect ratio of the graphitized structure in NHC-7 reflected byI_D/(I_D+I_G) is reduced to 0.67,corresponding to the reduced post-defects of NHC-7 from treatment with acid [ 39] .

BET results (Fig.1j,k) indicate that the NHC-7 has micropores at around 4 nm and mesopores at around 50 nm formed by the stacked small carbon sheet.The specific surface area of NHC-7 is measured to be 602.1 m²·g^-1,while it is only 307.2 m²·g^-1 for HC-7 with a main pore diameter near 0.7 nm.Micropores account for 27.29%of the total pore volume,and the mesoporous area accounts for 72.71%,indicating that acid treatment can increase the mesoporous ratio.The large carbon layers are broken into small sheets in the acid treatment process,where various carbon sheets are stacked to construct a hierarchically porous structure which consists of a loose sponge-like structure.All these results correspond well with SEM images.

FTIR spectrum (Fig.11) shows rich functional groups of NHC-7;the peaks located at about 3040,1800,1310,915 cm^-1 correspond to C=C vibrations,while the peaks at2959,1486 cm^-1 are related to C-H bonds,respectively.The peak intensities of C-H,C-O and C=O in NHC-7gradually decreased relative to biomass,precursors and HC-7.This is related to the oxidation of concentrated sulfuric acid and the volatilization of CO,CO₂,CH₄ and other volatile organic compounds during pyrolysis.It can be seen from Table S1 that in the EDS and XPS results,the NHC-7 has a higher C/O ratio than the HC-7 material,which corroborates the FTIR spectrum results.To further understand the functional groups on the surface of the carbon layer,XPS was tested.The peaks located at 284.5,285.2,285.9,286.7 and 288.4 eV (Fig.1m) correspond to sp2-C,sp3-C,C-O,C=O and COOR [ 40] ,respectively.The content of sp2-C increases from 51.53%to 60.93%after sulfuric acid treatment,and sp3-C content is reduced from 23.49%to 13.78%,indicating that the carbon plane is more complete and the defect is reduced [ 41] .A small amount of sp3-C at the edge of the graphite layer can help to improve the structural stability of NHC-7 by connecting adjacent graphite layers,while XPS full spectrum of HC-7(Figure S3) suggests there are metal elements of Ca 2p,K2p in HC-7,further confirming XRD results.Thus,FTIR and XPS results indicate that there are some O-containing functional groups on the surface of NHC-7,which hold benefit for enhancing its electrochemical reactivity.

Fig.1 a Process of hard carbon prepared from platanus bark;b SEM image of biomass platanus bark;c SEM image of HC-7;d low-magnification and e high-magnification SEM images of NHC-7;f HRTEM and g SAED images of NHC-7;structure characterizations of NHC-7and HC-7:h XRD pattern,i Raman spectrum,j pore size distribution,k N₂ adsorption/desorption isotherms,and I FTIR spectrum of biomass platanus bark (labeled as B),sulfuric acid-treated precursor (labeled as P),HC-7 and NHC-7;m for NHC-7 and HC-7

Fig.2 Electrochemical characterization of NHC-7:a CV curve,b rate performance,c charge and discharge curves at different rates,d cycle performance at 0.05 and 1.00 A·g-1,and e a rate performance image of NHC-7 compared to other studies;electrochemical characterization of NFM//NHC-7 sodium-ion full cell:f schematic diagram,g cycle performance at 0.05 A·g^-1,and h charge and discharge curves

The CV curves of NHC-7 electrode (Fig.2a) show that there are two reduction peaks during the first discharge.The obvious sharp peak near 0.5 V is mainly due to the formation of SEI film.Another peak in the vicinity of 0.01 V may be attributed to the insertion of Na⁺in the carbon layer [ 42] .During the two subsequent cycles,one reduction peak disappears,indicating that the formation process of SEI film in the first cycle is irreversible.NHC-7 electrode presents discharge capacities of 324.7,269.1,236.0,214.5,193.2,167.6and 146.1 mAh·g^-1 at the current densities of 0.02,0.05,0.10,0.20,0.50,1.00 and 2.00 A·g-1,respectively (Fig.2b,c).When the current densities decreased back to 1.00,0.50,0.20,0.10,0.05,0.02 A·g^-1,the discharge capacity can separately reach 167.9,184.2,206.5,215.5,233.6 and257.1 mAh·g^-1,which are 100.2%,95.3%,96.3%,913%,86.8%and 79.2%of the initial value,respectively.Compared with other biomass-derived hard carbon reported in the literature,NHC-7 shows a better rate performance at high current densities (Fig.2e,Table S3) [ 43, 44, 45, 46, 47, 48] .At 0.05 A·g^-1,the reversible capacity reached 223.3 mAh·g^-1 with a retention of 80.7%after 400 cycles (Fig.2d).When cycled at high current density of 1 A·g^-1,the reversible capacity is118.3 mAh·g^-1 with a retention of 71.5%after 2000 cycles(Fig.2d).Benefiting from the stable sponge-like structure,the NHC-7 electrode maintains the same charge/discharge curves during 2000 cycles (Figure S4) at a high current density of 1 A·g^-1.The rate and cycle performance of HC-7(Figure S5) demonstrated that the electrochemical performance can be greatly improved by acid activation,inducing a hierarchically porous structure with a higher specific surface area,more pores,edges and active sites.For better comparison,the hard c arbon materials were also prepared with other acids,including phosphoric and hydrochloric acid.As shown in Figure S6,the reversible capacities of hard carbon materials treated with concentrated sulfuric acid,phosphoric acid and hydrochloric acid are 165,36,and 49 mAh·g^-1 at a current density of 1 A·g^-1,respectively.As a result,concentrated sulfuric acid-treated biomass-derived hard carbon has the best electrochemical performance,which may be ascribed to the strong oxidizability of concentrated sulfuric acid,leading to an abundantly porous structure.The low defect and highly graphitized layered hierarchical pore structure prepared by sulfuric acid pretreatment can improve specific capacity.In addition,the effects of pyrolysis temperature and pyrolysis rate on the cycle performance of hard carbon material were further investigated.As shown in Figure S7,the capacity of hard carbon material increases from 188 to 215 mAh·g^-1 as the carbonization temperature rises from 700 to 1000℃,and the cycle stability of the specific capacity is improved significantly when the pyrolysis rate decreases.At higher temperatures,the defect content in carbon layers is lower and more short-range ordered graphitic layers are formed,which are conducive to improving the reversible capacity of hard carbon materials.As the temperature ramp decreases,it provides enough time for gas molecules to escape,which promotes the formation of integrated hexagonal carbon rings and decreases the defect concentration in the basal planes.The slower the pyrolysis rate is,the more the growth of carbon planes completes,which can lead to better cycle stability with a well-ordered structure.

Unfortunately,the initial coulombic efficiency (ICE) of NHC-7 electrode is only 34%,which is lower than that of an HC-7 electrode (40%) due to a large specific surface area.In the subsequent cycles,the coulombic efficiency(CE) of NHC-7 electrode rises to nearly 100%in ten cycles and maintains stability.By comparison,the CE of an HC-7electrode takes more time to become stable.The phenomenon can be ascribed to NHC-7 being more mesoporous,which improves the wettability of the ester-based electrolyte,promoting better contact between the electrolyte and materials,thereby facilitating the reaction kinetics of SEI.The electrochemical discharge/charge curves in ether-based electrolyte have a similar profile to the ester-based electrolyte which has a long slope and short plateau region (Figure S8a).And in an ether-based electrolyte,NHC-7 shows a reversible capacity of189 mAh·gat 1 A·g^-1 with a high initial coulombic efficiency of 90%,while the capacity retention is about82%after 800 cycles (Figure S8b).Although an NHC-7electrode has a high initial coulombic efficiency and high specific capacity in ether-based electrolyte,the capacity decays more quickly and overcharge is prone to occur after long cycling with unstable coulombic efficiencies,which may be the result of rapid growth of sodium dendrite in the ether-based electrolyte and can lead to a failed battery.According to the discharge/charge profiles of an NHC-7electrode,it is considered to be an adsorption-intercalation mechanism [ 12, 45] .The slope capacity is provided by sodium ions adsorbed on the surface of the carbon layer,with the large specific surface area and defects (such as edges,nanopores and heteroatoms) providing more adsorption sites.In the plateau region,sodium ions intercalate in the carbon layer to form NaC₈.Owing to the low degree of graphitization and high degree of disorder,the plateau region is short and the capacity is low.Therefore,the major capacity contribution of NHC-7 lies in the slope region with adsorption/desorption of Na⁺on the surface of the carbon layer,leading to superior electrochemical performance in ester-based electrolyte because of its hierarchical porous structure.

To evaluate the practical application value of the hard carbon material,electrochemical performance of a full cell was also investigated.The NFM//NHC-7 full cell delivers good cycle performance and a discharge capacity of155.8 mAh·g^-1 after 120 cycles at 0.05 A·g^-1 based on the mass loading of the anode,corresponding to 69.9%of its initial capacity (Fig.2f-h).Therefore,full cells assembled using an NHC-7 anode can be a potential choice for practical applications.

The kinetics characterization of NHC-7 was analyzed by CV curves at different scan rates (Fig.3a) [ 49] .The curve(Fig.3b) shows a well-defined linear relationship,and the b values corresponding to B and D in Fig.3a are equal to0.84 and 0.93,respectively,indicating that the kinetics are fast and subject to surface process control.In addition,the calculated capacitive contribution ratio of NHC-7 gradually increases from 53%,57%,59%,63%,75%and 81%with the increase in scan rate,respectively (Fig.3c,Figure S9).The high capacitive contribution of NHC-7 is due to its faster charge transfer.

Galvanostatic intermittent titration technique (GITT)measurement (Fig.3d,e) was used to further analyze the solid-phase diffusion of sodium (D_Na+) in NHC-7 electrode [ 50] .In Fig.3f,the calculated value of D_Na+is as high as1×10^-12 cm^-2·s^-1 and varies with the charging and discharging process.In detail,the value of D_Na+decreases with the voltage dropping during discharge.The slope capacity is provided by the sodium ions adsorbed on the surface of carbon layers at the beginning of discharging with larger specific surface area,providing more adsorption sites.In this process,Na⁺is easy to transfer,which is reflected in a high value of D_Na+initially.Then,D_Na+in the voltage range of 1.0-0.5 V appears approximately the same,indicating that in this range,the Na⁺diffusion process reaches an equilibrium.Finally,the value of D_Na+is low,which is related to the sluggish intercalation of Na⁺in the carbon layer [ 51] .The D_Na+value decreases sharply with a further voltage drop,which indicates that as the voltage decreases,it becomes more difficult for the sodium ions to intercalate into the carbon layer.In contrast,as the reverse reaction of sodiation in the charge process,the D_Na+reaches the highest value at the beginning of the charge process,but gradually decreases in the following desodiation process.The variation tendency of D_Na+in HC-7 electrode is similar to that of NHC-7 (Figure S10).While the average values of D_Na+in HC-7 and NHC-7 electrodes in desodiation process are 4.30×10^-13 and 4.49×10^-13 cm^-2·s^-1,the average values are 6.26×10^-13 and7.04×10^-13 cm^-2·s^-1 in the sodiation process,respectively.In comparison,the D_Na+value in an NHC-7 electrode is significantly higher than that in HC-7 electrode,which may be ascribed to the favorable accommodation energy of sodium ions to the hierarchically porous structure.According to the CV and GITT analysis,it is concluded that the hierarchically porous structure of NHC-7improves the interface contact between the materials and electrolyte,which can shorten the diffusion path of Na⁺and accelerate the charge transfer reaction.

Fig.3 Kinetics characterizations of NHC-7:a CV curves at various scan rates (B,the lowest point of reduction curve;D,the highest point of oxidation curve);b linear relationship between current and scan rates in logarithmic format;c contribution ratio of adsorption capacity at different scan rates;d GITT potential profiles during sodiation and desodiation in fourth cycle at 0.01-2.80 V;e typical single-step GITT curves used to determineΔE_s andΔE_t (start and end of discharge pulse);f sodium-ion diffusion coefficients calculated from GITT curves as a function of cell voltage during discharge/charge processes;g EIS spectra of symmetric cells with cycled NHC-7 electrode and HC-7 electrode;h EIS plot of symmetric cells with cycled NHC-7 and HC-7 electrodes at different temperatures;i activation energies of Na⁺transport derived in Nyquist plots of symmetric cells with cycled NHC-7 electrode and HC-7 electrode

EIS measurements and activation energy were further conducted to understand the impedance of the SEI layer and the kinetics of Na⁺transfer process.In order to avoid the impedance contributions of the sodium metal,symmetric cells of cycling NHC-7 electrodes were used in this measurement [ 52] .In the EIS spectra of symmetric cells(Fig.3g),R_SEI in the equivalent circuit (Figure S11,Table S2) represents the impedance of the SEI film.R_SEI of cycled NHC-7 electrode is 4.3Ω,which is smaller than that of HC-7 electrode (7.2Ω),indicating that the Na⁺transfer is faster in the SEI film formed on the surface of NHC-7.The activation energy (E_a) of Na⁺transport process(Fig.3h,i) was measured to verify the conclusion.E_a is in line with the law of Arrhenius [ 53] ,so E_a is calculated to be27.91 kJ·mol^-1 in NHC-7,while 62.02 kJ·mol^-1 in HC-7.It can be attributed to that the mesopores allow for easier penetration of electrolytes and shorten the Na⁺diffusion path,which facilitates the charge transfer kinetics.

To further investigate the effect of porous structure on the surface reaction,the sodiated NHC-7 electrode in the first cycle was tested by XPS measurements to reveal the SEI surface composition [ 54] .The C 1s spectrum can be fitted into six peaks at 284.6,285.5,286.1,287.4,289.1 and290.3 eV,respectively,attributed to C-C/H,C-O,C-O-C,C=O,O-C=O,Na₂CO₃,and the O 1s spectra can be deconvoluted into two peaks at 531.3 and 532.6 eV,corresponding to C=O and C-O bonds (Fig.4a).After the first discharge cycle,the surface of the carbon layer is covered by an SEI layer,leading to the reduction of C-C/H species in the first discharge cycle.However,organic species of the SEI layer containing RONa,(CH_2-CH_2-O-)_n,ROCO₂Na and inorganic component Na₂CO₃ increase.In FTIR spectroscopy (Fig.4b),two strong absorption peaks at1063 and 1289 cm^-1,characterizing the C-O stretching vibration,originated from the reduction of electrolyte solvent EC and DEC.The pronounced peaks located at 1405and 1631 cm^-1 are characteristic peaks of RCOO,which is one of the main components of the SEI layer.XPS and FTIR results indicate that the content of SEI surface composition at the NHC-7 electrode is more than that found at the HC-7 electrode;therefore,more electrolytes are in contact with the active material decomposed,leading to a lower ICE.Although the ICE of the NHC-7 electrode is low,a more integral SEI layer can protect the material and prevent further reaction between the material and the electrolyte.What,s more,the hierarchically porous structure of NHC-7 can accelerate Na⁺surface diffusion kinetics and the formation dynamics of SEI.

Fig.4 a XPS spectra of pristine and first discharged NHC-7 and HC-7 electrodes;b FTIR spectra of pristine and first discharged NHC-7 and HC-7 electrodes;c schematic diagram of Na⁺diffusion path in NHC-7 during cycle

Thus,it can be seen the hard carbon structure has significant influences on the formation of an SEI layer.The mesoporous structure allows the electrolyte and materials to contact well,and easier penetration of the electrolyte shortens the Na⁺diffusion path,which facilitates the charge transfer kinetics.Furthermore,the hierarchically porous structure can construct continued connecting channels and accelerate the electrolyte transport,leading to the rapid formation of an integral SEI film,facilitating the reaction kinetics of SEI (Fig.4c).Therefore,tuning the porous structure of hard carbon can promote the formation of an integral SEI,resulting in the excellent rate performance of an ester-based electrolyte.

4 Conclusion

In summary,a hierarchically porous loose sponge-like hard carbon (NHC-7) with a highly disordered phase was prepared at a low temperature of 700℃from concentrated sulfuric acid treatment of platanus bark,which has a main pore diameter of～4 nm and mesopores at around 50 nm,with a specific surface area of 602.1 m²·g^-1.Most importantly,XPS,FTIR and EIS analyses demonstrated that the hierarchically porous structure has a significant influence on the formation dynamics of SEI and Na⁺and their transfer kinetics.The mesoporous structure in NHC-7 can promote better contact between the electrolyte and materials,consequently improving the availability of the surface Na⁺,shortening the diffusion path,which in turn facilitates the charge transfer kinetics.What's more,the hierarchically porous structure can construct continued connecting channels and accelerate the electrolyte transport,leading to the rapid formation of an integral SEI film and facilitating the reaction kinetics of SEI.As aresult,the as-synthesized NHC-.,y7 exhibits superior rate performance with a capacity of 165 mAh·g^-1at a high current of 1 A·g^-1,and high retention of 71.5 % after 2000 cycles in an ester-based electrolyte. Therefore,tuning the material structure is also an important way to help construct effective and sustainable SEI film rapidly in an ester-based electrolyte.