Ultrafast Sodium Storage through Capacitive Behaviors inCarbon Nanosheets with Enhanced Ion TransportBiao Jiang, Yu Zhang, Dong Yan, Ji-Li Xia, and Wen-Cui Li*[a]
Sodium-ion batteries are regarded as a promising alternative forscalable electricity storage, but the rate capability is still ascientific challenge to be solved, owing to the poor Na+ kineticsin electrodes. Herein, nitrogen-doped carbon nanosheets withrich defects, nanopores and various interlayer spacings areprepared by pre-carbonized polymer sheets at different temper-atures followed by sodium amide treatment. Combination ofthin nanosheets, nanopores and large interlayer spacing canboost the rapid Na+ diffusion, as proven by the calculation of
the diffusion coefficient. Meanwhile, the kinetics analysisdemonstrates the capacitive behaviors, which favor fast Na+
storage. When applied as an anode, the material exhibitssuperior rate capability (111.1 mAhg� 1 even at 20 Ag� 1),remarkable cycle stability (over 2000 cycles) and ultrahigh initialcoulombic efficiency (81.4%). Furthermore, this work demon-strates a new way to develop attractive high-rate carbon anodematerials for sodium-ion batteries.
1. Introduction
Over the last several years, sodium-ion batteries (SIBs) havecaptured ever-growing interest due to the similarities ofphysicochemical nature between lithium and sodium as well asworking principle between lithium-ion batteries and SIBs.Furthermore, SIBs are regarded as one of the most promisinglarge-scale energy storage devices, because of the abundantreserves, widespread distribution and low cost of sodium.[1–6]
Nevertheless, the development of SIBs is greatly impeded bythe lack of suitable anode materials.[7,8] Among potential anodematerials, metal alloys/oxides/sulfides exhibit extremely poorcycling performance owing to exaggerative volume expansionduring sodiation/desodiation process and titanium-based ox-ides display low specific capacity, while amorphous carbon is asuitable candidate because of low cost, natural abundance,thermal stability, controllable structure and high electrochem-ical activity.[9] Hence, a variety of carbonaceous materials, likeexpanded graphite,[10] biomass derived carbon[11–16] and heter-oatom doped carbon,[17–21] have been developed, which showseffective sodium storage owing to wide interlayer distance anddefects. Nonetheless, to explore the high rate materials for SIBsstill remains a major challenge.
According to previous studies, the mechanism of sodium-ion storage involves intercalation/deintercalation reaction andadsorption mechanism. For the former, the sluggish reactionkinetics usually leads to nearly no capacity at large currentcharge/discharge.[22] Besides, the low voltage plateau is vicinalto the sodium-plating potential,[23] which easily triggers safety
accidents. Fortunately, the latter has monotonously slopingvoltage curves, high sodiation/desodiation potential and fastreaction kinetics,[24] which can resolve these drawbacks. There-fore, the rate capacity can be improved by introducingadsorption mechanism for sodium storage, which alwayscorresponds to the rich defects in materials. For example, Ji’sgroup synthesized defective hard carbon anode with adsorptionmechanism for sodium-ion storage, delivering the reversiblecapacity of 100.0 mAhg� 1 at 2 Ag� 1.[25] Nevertheless, thespecific capacity and applied current density are still unsatisfied.This fact suggests that storing sodium only via adsorptionmechanism is not sufficient to achieve excellent rate perform-ance.
It’s acknowledged that the whole electrode process ofsodium storage involves reaction process and diffusion process.From this perspective, the anode materials still need fast iontransport besides adsorption mechanism for sodium storage tofurther improve the rate capability as well.[26] Generally, toaccelerate the ion transport, decreasing Na+ transport distanceis a significant method in the light of “t ¼ L2=D” (here τ, L andD represent diffusion time, diffusion distance and diffusioncoefficient respectively).[27] Meanwhile, introducing porosity canpromote ion migration as well.[22] From here we see that thinmicroporous nanosheets and nanofibers are a good choice torealize fast ion diffusion in comparison with bulk or carbonspheres, which have been confirmed by some reports. Forinstance, Lu’s group demonstrates that microporous carbonnanosheets exhibit improved kinetics compared with micro-porous carbon spheres by direct experiment data and theoret-ical calculations.[28] In addition, nitrogen-doping can enhanceelectron conductivity, reactivity and reduce energy barrierbecause of the extrinsic defects and high electronegativity ofnitrogen, which further improves electrochemicalperformance.[17,29–31] Hence, constructing a material like thinmicroporous nitrogen-doped carbon nanosheets as an anodefor SIBs transports sodium ions quickly to the active sites.Subsequently, adsorption process for sodium storage can be
[a] B. Jiang, Dr. Y. Zhang, D. Yan, J.-L. Xia, Prof. W.-C. LiState Key Laboratory of Fine ChemicalsSchool of Chemical EngineeringDalian University of TechnologyDalian 116024, P. R. ChinaE-mail: [email protected] information for this article is available on the WWW underhttps://doi.org/10.1002/celc.201900552
completed quickly. Eventually, ultrahigh rate capability isachieved.[32] In order to obtain the desired structure, sodiumamide (NaNH2) treatment is a good choice due to the functionof nitrogen-doping and nanopore.[33]
Guided by those viewpoints, herein we fabricated thenitrogen-doped carbon nanosheets with rich defects, producednanopores, larger interlayer spacing by pre-carbonized poly-mer-sheets at different temperatures followed by NaNH2 treat-ment. When used in ether electrolyte as a negative electrodefor SIBs, the treated carbon nanosheets realize an applicableinitial Coulombic efficiency (ICE=81.4%) and ultrahigh ratecapability (140.7 mAhg� 1 at 2 Ag� 1, 111.1 mAhg� 1 even at20 Ag� 1). Electrochemical kinetics analysis confirms the capaci-tive behaviors without diffusion control for capacity contribu-tion and the impedance verifies higher diffusion velocity.
2. Results and Discussion
The morphologies and local microstructure were observed byscanning electron microscopy (SEM) and high-resolution trans-mission electron microscopy (HRTEM) respectively. The SEMimages show that the integrity of carbon nanosheets remains,even after NaNH2 treatment and the thickness of carbonnanosheets is about 50 nm for all samples (Figure 1a, b and
Figure S1, Supporting Information), which effectively shortensthe ion transport distance. HRTEM images display that both CS-800 and NCS-800 have typically turbostratic nanodomains ofhard carbon (Figure 1c, d). Nevertheless, NCS-800 was com-posed of shorter and more curved graphitic layers and itsinterlayer spacing is larger in comparison with CS-800. Thesevariations exhibit the impact of NaNH2-treatment on increaseddisorder degree and expanded interlayer spacing, whichfacilitate the transport and intercalation of sodium ions.[34]
The microstructure of all specimens was further investigatedby X-ray diffraction, laser Raman and nitrogen sorption as
shown in Figure 2. The obviously broadened peaks of (002) and(100) plane in all patterns locate at �23° and �43° respectively,typically for disordered carbon (Figure 2a). In sequence of NCS-600, NCS-700 or CS and NCS-800, the (002) peak slightly shiftsto a lower diffraction angle and the intensity decreases,corresponding to a decrease in crystallinity. Based on the data,the average interlayer spacing (d) is calculated and listed inTable 1. As can be seen, NCS-800 has the largest d-value
(3.90 Å) which is wider than that of CS-800 (3.85 Å), while NCS-600 shows the smallest one (3.67 Å). These facts are consistentwith the HRTEM images (Figure 1c, d). From the point of largerionic radius of sodium ions, the enlarged interlayer spacing ofNCS-800 can benefit the transport and intercalation of sodiumions.
The specific surface area and porosity distribution werecalculated from nitrogen sorption isotherms (Figure 2b, c). Allsamples present type I absorption isotherms. The resultantBrunauer-Emmett-Teller surface area is 991.2 m2g� 1 for NCS-800and 742.3 m2g� 1 for CS-800 respectively (Table 1). The incre-mental part attributes to the new etching nanopores via NaNH2
treatment, which is more beneficial for the infiltration ofelectrolyte and the fast diffusion of sodium ions, further
Figure 1. a,b) SEM images and c,d) HRTEM images of CS-800 and NCS-800,respectively.
Figure 2. a) X-ray diffraction patterns, b) nitrogen sorption isotherms, c)pore-size distributions, and d) Raman spectra. The isotherms of NCS-600,NCS-700 and NCS-800 are offset vertically by 100 cm3g� 1, 200 cm3g� 1,300 cm3g� 1 STP, respectively.
testified by electrochemical impedance spectroscopy. What’smore, the ultramicropore (<0.7 nm) helps to improve ICE.[35]
Raman spectra of nanosheets were deconvoluted into threeLorentzian bands and one Gaussian band (Figure 2d, Figure S2).For all specimens, two prominent bands at 1342 cm� 1 and1592 cm� 1 are ascribed to the D1 and G bands respectively. Twoshoulders at 1520 cm� 1 and 1200 cm� 1 are clearly visible as well.Generally, D1 band corresponds to the breathing vibration ofcarbon aromatic rings with A1g symmetry, typically for disor-dered and defective structure. The G band stems from thestretching vibration of sp2 carbon with E2g symmetry. Therefore,the intensity (band area) ratio ID1/IG usually quantifies the defectdegree of carbon materials.[36,37] The value of CS-800 equals to2.39, while values for the NaNH2 treated samples of NCS-600 toNCS-800 are 2.52, 2.65 and 2.96 respectively, which proves thata large number of defects can be introduced into carbonmaterials by the NaNH2 treatment. The plentiful defects canadsorb a large number of sodium ions, contributing to theenhancement of reversible specific capacity particularly at alarge current charge/discharge.
The variation of element contents was investigated byelemental analysis and listed in Table 2. After NaNH2 treatment,
the content of nitrogen rises, which certainly proves thefunction of nitrogen-doping by NaNH2 treatment. According toJin’s group, the main component of doped nitrogen ispyridinic-N[33]. It is worth noting that oxygen content decreasesas the pre-treatment temperatures increase from 600 to 800 °C.Excess oxygen can occupy the active sites, which reduces thesodium storage and slows the sodium-ion transport.[10] Addi-tionally, the tap density of NCS-800 increases to 0.132 gcm� 3
compared with 0.077 gcm� 3 of CS-800. Compact overlapbetween nanosheets is beneficial to unblocked electron trans-port. The electronic conductivity of NCS-800 is 12.50 Sm� 1
which is higher than that of CS-800 (6.25 Sm� 1) measured by afour-probe resistivity tester.
The electrochemical performance was investigated over thevoltage range of 0.01–2.8 V (Figure 3 and 4). Figure 3a showsthe impressive discharge-charge profiles with almost slopinglines for all samples at the current density of 0.1 Ag� 1. Thesecurves reflect surface adsorption/desorption mechanism, involv-ing defects, like capacitive behaviors of supercapacitors, whichcontributes to high rate capability and long-term cyclability.[24]
The initial discharge/charge capacities of CS-800, NCS-600, NCS-700 and NCS-800 are equal to 220.6/150.7, 169.3/124.9, 216.5/159.7 and 235.5/191.7 mAhg� 1 separately, suggesting that NCS-800 possesses the excellent ICE up to 81.4% in ether electrolyte.
Additionally, the ICE reaches 45.7% in ester electrolyte which isstill attractive (Figure 4a). The outstanding ICE can be attributedthat a large amount of specific surface area is derived fromultra-micropore, whose pore size is less than 0.7 nm. Such ultra-micropore can achieve the low contact between materialsurface and electrolyte and inhibit the side reaction from SEIformation.[35] Further consideration, if a soft carbon coating canbe adopted in the future, it is possible to continue improvingthe ICE.[38] Meanwhile, this consequence enlightens us aboutthe selection of electrolyte to improve the ICE for theapplication as well.
In order to meet the demand of large-scale and sustainableenergy storage, high power density and energy density for SIBs
Table 2. Chemical composition of all samples obtained from elementalanalysis.
Figure 3. Electrochemical characterization of CS-800, NCS-600, NCS-700 andNCS-800 as the anodes of SIBs in ether electrolyte. a) Discharge/chargevoltage profiles at 0.1 Ag� 1 for the initial cycle. b) Reversible capacity cycledat various rates from 0.1 to 20 Ag� 1. c) Cyclability at 5 Ag� 1.
Figure 4. Electrochemical characterization of NCS-800 as the anode of SIBs inester electrolyte. (a) Sodiation/desodiation capacity cycled at various ratesfrom 0.1 to 20 Ag� 1. (b) Discharge/charge voltage profile at 0.1 A g� 1 for theinitial cycle. (c) Long-term cyclability at 0.5, 1, 2, 5 Ag� 1.
are significant. As shown in Figure 3b and 3c, the NCS-800electrode exhibits excellent reversible capacity (here referringto desodiation capacity) especially at high current densities anddelivers 191.7, 149.8, 140.7, 126.4, 113.9 and 111.1 mAhg� 1 atcurrent densities of 0.1, 1, 2, 5, 10 and 20 Ag� 1 respectively,which is higher than that of other samples. When the currentdensity turns back to 0.1 Ag� 1, 184.7 mAhg� 1 is recovered,exhibiting strong structural integrity. On the contrary, the NCS-600 electrode delivers the lowest reversible capacity at each ofthe same current densities and the rate capacities of CS-800and NCS-700 are middle. In brief, these results originate fromthe consequent discrepancies in ion diffusion, electronicconductivity and sodium storage mechanism, involving crystal-linity degree, interlayer spacing, pore structure, nitrogencontent and defect degree, which are well in line with thestructural characterization. Additionally, NCS-800 delivers ex-cellent cycle stability and maintains 108.3 mAhg� 1 in etherelectrolyte even at 5 Ag� 1 after 400 cycles, which is higher thanthat of CS-800 (75.0 mAhg� 1, Figure 3c). More encouragingly,the reversible capacity and ICE of NCS-800 lay at a significantlevel in comparison with previously reported carbon nano-sheets as well (Table S1).
On the other hand, NCS-800 still exhibits encouraging ratecapacity and displays 179.6, 161.9, 141.2, 127.4, 114.6, 97.4, 80.6and 55.5 mAhg� 1 at 0.1, 0.2, 0.5, 1, 2, 5, 10 and 20 Ag� 1 in esterelectrolyte with nearly 100% Coulombic efficiency, respectively(Figure 4a). Long-term cyclability are also outstanding and thecapacity decay is as low as 0.014% per cycle at 5 Ag� 1
(Figure 4c). Interestingly, the charge-discharge profile is stillsloping and different from other reported intercalation-typecarbon materials with obvious plateau in ester electrolyte(Figure 4b). This mechanism is beneficial to excellent perform-ance.
To further understand the storage mechanism, cyclicvoltammetry curves of NCS-800 at various sweep rates from 0.2to 2.0 mVs� 1 were investigated as shown in Figure 5a. Quasi-rectangle can be clearly observed at a low sweep rate, whichpreliminarily illustrates the adsorption mechanism for sodiumstorage. This result is consistent with the charge-dischargeprofile. It is acknowledged that the process of Na+ intercalationinto graphite-like interlayer is identified as a diffusion-controlledintercalation process. The behaviors of Na+ adsorption onsurface defects are termed as the surface-induced capacitiveprocess. Their differences can be distinguished by a power-lawfunction between peak current (i) and scan rate (v). Relativeequation and deformation are as follows [Eqs. (1) and (2)]:[39–42]
i ¼ avb (1)
lni ¼ lnaþ blnv (2)
Here, a and b are adjustable constants, which can beobtained from the intercept and slope of the Eq. (2). Generally,a value of b close to 1 indicates that the contribution ofcapacity is totally attributed to capacitive effects, whereas b=
0.5 infers totally diffusion-controlled intercalation process.Linear relationship between ln i and ln ν of cathodic peaks is
fitted in Figure 5b and Figure S3b. Afterwards, the b-value ofNCS-800 and CS-800 are calculated to be 0.84 and 0.78respectively, certifying the capacitive process. The b-value ofCS-800 is lower than that of NCS-800, certifying that NaNH2
treatment effectively strengthens capacitive behaviors forsodium storage, which is beneficial to excellent performance.Furthermore, the percentage of capacity from capacitivebehaviors and diffusion-controlled process at a fixed scan ratecan be further calculated on the basis of the followingequations [Eqs. (3) and (4)].[43,44]
i Vð Þ ¼ k1v þ k2v1=2 (3)
i Vð Þ=v1=2 ¼ k1v1=2 þ k2 (4)
Where k1v and k2v1/2 represent corresponding current
originated from capacitive effect and intercalation effectrespectively. To obtain the value of k1 and k2 from slope andintercept, the reformulation and linear plots of i/v1/2 as afunction of v1/2 are presented (Eq. (4), Figure 5c). Then, wecalculate the contribution proportion at a fixed potential forvarying scan rates and the results are depicted in Figure 5d. Aswe can see, the proportion of capacitive contribution graduallyrises as the scan rate increasing and 79% is reached outclassingthe value of diffusion-controlled intercalation contribution at2 mVs� 1. Additionally, the ratio of capacitive contribution ofNCS-800 are much higher than those of CS-800 under the sameconditions (Figure S3d). These facts prove that capacitivebehaviors dominate for NCS-800 as an anode in SIBs to storesodium ions. This result is encouraging for the reason thatcapacitive behaviors facilitate rapid charge and discharge,particularly even at ultrahigh current density.
Figure 5. Kinetics analysis for the NCS-800 electrode: a) cyclic voltammetrycurves at various sweep rates from 0.2 to 2.0 mVs� 1. b) Linear fittingbetween ln i and ln v of cathodic peaks. c) The plot of i/v1/2 vs v1/2 of cathodicpeaks. d) Normalized capacity contribution ratio at different scan rates.
To better understand remarkable sodium-storage propertyof NCS-800, electrochemical impedance spectroscopy measure-ments were performed after the discharge/charge for 5 cyclesat 0.05 Ag� 1 for evaluating Na+ diffusion properties as shown inFigure 6a. The Warburg factor (σ) is obtained from the slope of
linear fit (Figure 6b) between Z (Z, the real part of Nyquist plot)and ω� 0.5 (ω, angular frequency) at low-frequency region (Eq. 5).Subsequently, the diffusion coefficient of sodium ions (D) iscalculated according the following equation (Eq. 6).[45–47]
Z ¼ sw� 1=2 þ Re þ Rct (5)
D ¼ R2T2=2A2n4F4C2s2 (6)
Where R, T, A, n, F and C are the gas constant, absolutetemperature, electrode geometric area, number of electronsduring redox reaction, Faraday constant and solid-state concen-tration of sodium ions respectively. The Na+ diffusion coef-ficient of NCS-800 (2.11×10� 11 cm2S� 1) is much larger than thatof CS (5.85×10� 12 cm2S� 1), which attests that etching nanoporeby NaNH2 treatment can facilitate fast Na+ transport. Combina-tion of fast diffusion and adsorption mechanism for storagesodium contributes to ultrahigh rate performance.
3. Conclusions
In summary, nitrogen-doped carbon nanosheets with richdefects, produced nanopores and large interlayer spacing aredesigned by pre-carbonized polymer-sheets at different tem-peratures followed by NaNH2 treatment, which simultaneouslyboosts fast ion transport. In coordination with adsorptionmechanism for sodium storage revealed by cyclic voltammetrytechniques, NCS-800 delivers remarkable electrochemical per-formance such as ultrahigh rate capability (the reversiblecapacity of 111.1 mAhg� 1 even at 20 Ag� 1), remarkable cyclingstability (108.3 mAhg� 1 after 400 cycles at 5 Ag� 1), andpromising ICE (81.4%) for application in ether electrolyte. Whenassembled in ester electrolyte, it still displays superior reversiblecapacity particularly for ultrafast charge storage and unforget-table long-term cyclability for over 2000 cycles. These factsimply not only a facile method to enhance ion transport, butalso an outstanding anode material for SIBs that may be
applicable for supercapacitors, lithium-ion batteries and adsorb-ents.
Experimental SectionMaterial synthesis: Thin and flat polymer nanosheets weresynthesized as a precursor. Subsequently, NaNH2 treatment aims fornitrogen-doping, etching nanopores and introducing defects at onestep. Firstly, resorcinol, formaldehyde and propylamine werepolymerized around stearic acid sheets catalyzed by ammonia inaqueous solution at 80 °C accompanied with stirring 4 h, accordingto our previous work.[48] Afterwards, the precursor was pre-annealedat a target temperature for 2 h. Subsequently, the homogenousmixtures of the carbonized product and NaNH2 (1 : 7 in mass ratio)were placed in a tube furnace with annealing treatment at 320 °Cfor 10 h under Ar flow. Finally, after washing with ethanol anddeionized water in turn and drying at 50 °C for 24 h, the collectedproduct was carbonized at 1000 °C for 2 h, correspondingly namedas NCS� X for finished sample, where X represents the pre-annealedtemperature which was varied as 600 °C, 700 °C and 800 °C. Thepolymer nanosheets were pre-annealed at 800 °C. Subsequently,implementing the same process without NaNH2 treatment aims toobtain the comparison sample (CS-800).
Material Characterization: The material morphologies were ob-served by field-emission scanning electron microscopy (FEI NovaNanoSEM 450) and high-resolution transmission electron micro-scopy (FEI Tecnai F30). The material microstructures were analyzedby X-ray diffraction (PANalytical X’Pert3 Powder) with Cu Kαradiation, laser Raman spectra (DXR Smart Raman) with a wave-length of 532 nm, elemental analysis (Vario EL) and nitrogenadsorption� desorption measurements (ASAP 2020, Micromeritics),respectively. Porosity distributions were analyzed according todensity functional theory.
Electrochemical Tests: The electrochemical measurements wereconducted in 2025-type coin cells with metal Na as counterelectrode and glass fiber membrane (Whatman, GF/F) as separator.The anode electrode consisted of 80 wt% active material, 10 wt%carbon nanotubes, 6 wt% carboxymethylcellulose and 4 wt%acrylonitrile multicomponent copolymer (LA133). The loading masswas 1.2–1.4 mgcm� 2 on Cu foil of 12 mm in diameter. The etherelectrolyte was 1 M NaCF3SO3 in diglyme. The ester electrolyte was1 M NaClO4 in ethylene carbonate, ethyl methyl carbonate anddimethyl carbonate (1 : 1 : 1, v : v : v) with 1 wt% fluoroethylenecarbonate. The galvanostatic charge/discharge cycling was per-formed by Land CT2001 A in the voltage interval of 0.01–2.8 V.Cyclic voltammetry curves were recorded at various sweep ratesand electrochemical impedance tests were carried out in afrequency range from 100 kHz to 0.01 Hz on an electrochemicalworkstation (Chen-Hua Instrument Company, Shanghai, China,CHI660D). The batteries were first discharged/charged for 5 cyclesat 0.05 Ag� 1 and the EIS measurements are all tested at full chargestate.
Acknowledgements
This work was supported by the National Science Fund for theNational Natural Science Foundation of China (No. 21776041 andNo. 21875028), and Cheung Kong Scholars Programme of China(T2015036).
Figure 6. a) Nyquist plots for CS-800 and NCS-800 obtained after 5 cycles at0.05 A g� 1. b) Relationship between Z and ω� 0.5 in the low-frequency region.
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Manuscript received: April 3, 2019Revised manuscript received: May 9, 2019Accepted manuscript online: May 10, 2019
