Red Phosphorus Single-Walled Carbon Nanotube Composite as a Superior Anode for Sodium Ion Batteries

vaporization-condensation method.Bene?ting from the nondestructive preparation process,

enhances the conductivity of the composite and stabilizes the solid overall sodium storage capacity(~700mAh/g composite at50mA/g composite),

long-term cycling performance with80%capacity retention after2000

vaporizationàcondensation method signi?cantly extends the cycling nanotubes

between red P and SWCNTs,which increase the electrical conductivity of the composite and decrease the charge transfer resistance.Moreover,electroche-mical impedance spectroscopy study indicates that the SWCNTs enable a stable interface between elec-trolyte and composite,realizing stable long-term electrochemical cycling.

RESULTS AND DISCUSSION

Scheme1schematically illustrates the synthesis pro-cess of red PàSWCNT composite.The red PàSWCNT composite is prepared by a modi?ed vaporizationàcondensation method.Red P and SWCNTs are?rst sealed in a glass tube under vacuum condition.Then, the glass tube is heated to600°C and held at this temperature.After the glass tube naturally cools down to room temperature during which P vapor condenses between the SWCNT bundles,the red PàSWCNT com-posite is obtained by washing and drying the collected powder(see Methods section for more details). Figure1a shows the XRD patterns for SWCNTs,and~43°correspond to the re?ection of graphitic planes of SWCNTs.40The XRD pattern of commercial red P is similar to the one reported in the literature with the?rst sharp peak centered at2θ~15°,indicating medium-range ordered structure.41,42The XRD pattern of red P/SWCNT mixture is simply the combination of XRD patterns of SWCNTs and commercial red P.The XRD pattern of the red PàSWCNT composite resem-bles that of the red P/SWCNT mixture without any detectable new peak,implying that no crystalline impurity is generated during the preparation process. Di?erent from the XRD patterns of Pàcarbon compo-sites prepared by high energy ball milling,in which the di?raction peaks corresponding to red P completely disappeared,27,32the red P peak centered at2θ~15 can still be clearly observed in our red PàSWCNT composite,indicating the crystalline structure of red P is not completely destroyed during the preparation process.Figure1b shows the Raman spectra for the samples.SWCNTs show a weak D band at1345cmàand a strong G band at1582cmà1,indicating a highly

1.Schematic illustration of the synthesis process for red PàSWCNT composite. Figure1.(a)XRD and(b)Raman spectra for di?erent materials.

àSWCNT composite and red P/SWCNT mixture, signals corresponding to red P and SWCNTs are clearly detected.The intensity ratio of the D band to the G is similar between red PàSWCNT composite and P/SWCNT mixture,and no peak shift is observed both SWCNTs and red P in the red PàSWCNT composite compared with red P,SWCNTs,and red P/SWCNT mixture,which might indicate that there is surface interaction between red P and SWCNTs in composite.32,33The peak intensity of red P is decreased in the red PàSWCNT composite compared that in the red P/SWCNT mixture,probably because red P is partially covered by SWCNTs as evidenced by scanning electron microscopy(SEM) images(Figure2).

Supporting Information Figure S1shows the SEM transmission electron microscopy(TEM)images pristine SWCNTs.At low magni?cation,pristine high magni?cation,individual SWCNT bundles can observed and these bundles are loosely entangled empty space between them(Supporting Information Figure S1b).As shown by the TEM images(Supporting Information Figure S1c,d),the pristine SWCNTs dles make contact with each other to form a network. Figure2shows the SEM images of the red PàSWCNT composite.After the vaporizationàcondensation cess,the average particle size of red PàSWCNT posite is around5.0μm(Figure2a).In contrast loose structure of pristine SWCNTs,the red PàSWCNT composite appears much denser and several SWCNT bundles can be clearly observed on the surface composite(Figure2b),implying that red P is?lled the empty space between SWCNT bundles suggesting a high tap density.Elemental mapping by the SEM energy dispersive X-ray spectrometry (EDS)(Figure2dàf)demonstrates that carbon

2.(aàc)SEM images of red PàSWCNT composite.(dàf)EDS elemental mapping of(d)phosphorus and carbon, phosphorus,and(f)carbon for the red PàSWCNT composite particles shown in(c).

composite.For the red P/SWCNT mixture prepared by hand-grinding,EDS elemental mapping in Supporting Information Figure S2shows that microsized P particle isolated from SWCNTs.TEM is used to investigate the interior structure of red PàSWCNT composite.As shown by the high resolution TEM image in Figure3a, crystalline lattices of SWCNTs and red P can be observed,and SWCNTs are randomly distributed around red P domains to form a conductive network.

TEM EDS line scanning in Figure3b demonstrates existence and continuous distribution of SWCNTs across a composite particle.To investigate the distribu-of P inside the SWCNT bundles,the TEM EDS line scanning across a bundle of SWCNTs is performed and result(Figure3c)shows very weak P signal,imply-that very limited amount of P is stored inside the SWCNT bundles.

Figure4shows the electrochemical performance of erent samples.Unless otherwise speci?ed,both capacity and current density reported in this work calculated on the basis of the overall weight of of the initial?ve scans for red PàSWCNT composite at a scan rate of0.05mV/s between0.001and (vs Na/Nat).A broad peak around1.0V(vs Na/Na observed during the?rst cathodic scan,which attributed to the reduction of electrolyte to form SEI layer.In the subsequent cathodic scans,instead the peak around1.0V(vs Na/Nat),a weak peak around 0.92V(vs Na/Nat),which overlapped with the SEI

in the?rst cathodic scan,is observed and

be ascribed to the initial sodiation of red P.Starting from the third cathodic scan,a peak around0.35 (vs Na/Nat)is developed and the peak intensity gradually increases from the third scan to the scan,indicating gradually enhanced sodiation netics.When the voltage is scanned close to0.001 (vs Na/Nat),the current increases sharply and incomplete peak is developed around0.001 (vs Na/Nat),corresponding to further sodiation red P.During all?ve anodic scans,a strong around0.63V(vs Na/Nat)with a shoulder around 0.71V(vs Na/Nat)and a weak peak around

t

3.(a)High resolution TEM image of red PàSWCNT with P and carbon nanotubes(CNTs)marked according to

erent crystalline lattices.EDS line scan of(b)one red PàSWCNT composite particle and(c)a bundle of SWCNTs composite(P signal is red and carbon signal is blue).

that,for the peak located at0.63V(vs Na/Nat), peak current increases and the peak potential shifts to lower voltages from the?rst scan to fth scan,implying that the desodiation kinetics initial?ve cycles for red PàSWCNT composite

a constant current density of50mA/g composite

for the?rst sodiation curve which is di?erent others due to the SEI formation,the general sodiation

Electrochemical performance of red PàSWCNT composite.(a)Cyclic voltammetry of red PàSWCNT between

vs Na/Nat)at a scanning rate of0.05mV sà1.(b)Galvanostatic chargeàdischarge voltage pro?les of red Pàcomposite during the?rst5cycles at a current density of50mA/g composite.(c)Cycling performance and(d)Coulombic ciency of red PàSWCNT composite,red P/SWCNT mixture hand-milled for15min,red P,and SWCNTs at a current 50mA/g.(e)Sodiation/desodiation capacity and(f)voltage pro?les of red PàSWCNT composite at

densities.(g)Cycling performance of red PàSWCNT composite at500mA/g composite with initial10cycles performed mA/g composite.(h)Cycling performance of red PàSWCNT composite at2000mA/g composite with desodiation capacities 25th,50th,75th,...,2000th cycle are plotted.Note:for red PàSWCNT composite and red P/SWCNT mixture,both densities and speci?c capacities are calculated on the basis of the total weight of red PàSWCNT composite or red P/SWCNT For red P and SWCNTs,both current densities and speci?c capacities are calculated on the basis of the weight SWCNTs.

Figure5.Nyquist plot of the red PàSWCNT composite cell before and after cycling with the inset showing the equiva-lent circuit used for data?tting(symbols,experimental data;dashed lines,?tting results).

variations during the electrochemical test,which was also reported in the literature.44,45It is worth mention-ing previous studies which showed that binders,such as PAA or carboxymethyl cellulose(CMC),need to be used to achieve the stable cycling performance of amorphous Pàcarbon materials because of the three-dimensional interconnection structure and high adhesive strength of those binders.28,29When polyvinylidene?uoride(PVDF)was used as the binder, the electrochemical performance of amorphous Pàcarbon composite was unsatisfactory.28,29In our case,conventional PVDF is used for the electrode preparation and the cycling performance of red PàSWCNT composite is comparable to the one reported in the literature using PAA or CMC binder, manifesting the intrinsic structure fortitude of the composite.

Supporting Information

Red Phosphorus-Single-Walled Carbon Nanotube Composite as a Superior Anode for Sodium Ion Batteries

Yujie Zhu, ? Yang Wen, ? Xiulin Fan, ? Tao Gao, ? Fudong Han, ? Chao Luo, ? Sz-Chian Liou, ? and Chunsheng Wang?,*

?Department of Chemical and Biomolecular Engineering, University of Maryland, College Park,

Maryland 20742, USA

?Maryland Nanocenter, University of Maryland, College Park, Maryland 20742, USA

*Corresponding author e-mail: cswang@https://www.360docs.net/doc/f718632041.html,

Figure S1. (a,b) SEM and (c,d) TEM images of pristine SWCNTs.

Figure S2.(a) SEM image and EDS elemental mapping of (b) phosphorus + carbon, (c) phosphorus, and (d) carbon for red P/SWCNT mixture prepared by hand-grinding.

Figure S3. Galvanostatic sodiation-desodiation voltage profiles at 50 mA/g for (a) pristine red P, (b) red P/SWCNT mixture prepared through hand-grinding for 15 min, and (c) SWCNTs. (d) Electrochemical performance of red P/SWCNT mixture as a function of hand-grinding time at 50 mA/g. Note: for red P/SWCNT mixture, both current densities and specific capacities are calculated on the basis of the total weight of red P/SWCNT mixture. For red P and SWCNTs, both current densities and specific capacities are calculated on the basis of the weight of red P or SWCNTs.

Figure S4. Thermogravimetric analysis (TGA) of red P-SWCNT composite in argon from room temperature to 800 °C.

Figure S5. (a)Cycling stability and (b) galvanostatic sodiation-desodiation voltage profiles of red P-SWCNT composite tested in the electrolyte without FEC (1 M NaClO4in ethylene carbonate/dimethyl carbonate, 1:1 by volume) under a current density of 50 mA/g composite.

Figure S6. SEM image of red P-SWCNT composite showing that some SWCNTs are not incorporated into the composite.

Table S1. R ele and R int obtained by fitting experimental data in Figure 5 using equivalent circuit (inset in Figure 5) for red P-SWCNT cell before and after cycling.

Fresh cell 5th cycle 10th cycle 50th cycle 100th cycle 150th cycle 200th cycle

R ele (?) 5 8 8 6 9 11 6

R int (?) 202 79 51 65 83 70 66

Figure S7.(a-c) SEM images, and EDS elemental mapping of (d) phosphorus + carbon, (e) phosphorus, and (f) carbon for red P-SWCNT composite electrode after 2000 sodiation-desodiation cycles. Note: the cell is charged to fully desodiated state before dissassembling and the electrode is rinsed by propylene carbonate.

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