Layer-Structured Ti Doped O3-Na1−xCr1−xTixO2(x=0,0.03,0.05)with Excellent Electrochemical Performance as Cathode Materials for Sodium Ion Batteries

Yu ShaoZhong-feng TangJia-ying LiaoChun-hua Chen

CAS Key Laboratory of Materials for Energy Conversion,Department of Materials Science and Engineering&Collaborative Innovation Center of Suzhou Nano Science and Technology,University of Science and Technology of China,Hefei 230026,China

Key words:Sodium chromium oxide,Doping,Sodium ion battery,Layered oxide,Cathode

I.INTRODUCTION

Over the years,Na-ion batteries are believed to replace Li-ion batteries for the applications in large-scale energy storage systems because of the abundant Na resources and relatively low cost[1–3].The electrode materials for Na-ion batteries include layered oxides[4,5],prussian blue analogues[6,7],polyanion compounds[8,9]etc.,among which the transition metal oxides,such as NaxMnO2,NaxCoO2,NaxVO2are particularly attractive for their highly reversible structure.In 1982,Braconnieret al.[10] first reported NaCrO2could be used as sodium ion battery electrode.NaCrO2withα-NaFeO2layered structure shows attractive potential as positive electrode for Na-ion batteries due to its high coulombic efficiency and superior cyclability.According to Delmas[11],the crystal structure of NaCrO2belongs to O3-type structure,in which“O”means the coordination environment of octahedral sites between Cr and Na while“3” stands for the number of MO2(M=transition metals)slabs in a hexagonal unit cell.The theoretical capacity of NaCrO2is 250 mAh/g,unfortunately,it only achieves 110 mAh/g which is similar to LiCoO2owing to the limited structure stability.

Recently,Myunget al.[12]reported a carbon-coated NaCrO2as a cathode material with excellent cycling stability(90%capacity rentention at 300th cycle),rate performance(106 mAh/g at 50 C)and coulombic efficiency(nearly 100%).Fuet al. [13]reported another carbon-coated NaCrO2using citric acid as a carbon source and its capacity reached 110 mAh/g after 40th cycles.Yabuuchiet al.[14]reported P2-type Na2/3Cr2/3Ti1/3O2that shows good performances as a positive electrode with a capacity retention of more than 95%after 20th cycle.Komabaet al.[15]reported O3-type NaCrO2remained 85%of the original capacity after 50th cycles at 1 C rate.

In this work,we attempt to improve the electrochemical properties of this material by element doping.It turns out that Ti-doping leads to very excellent cycle and rate performances,which are important for the use of energy storage and fast energy conversion.

II.EXPERIMENTS

A.Material preparation

All the chemical reagents used in the experiment were purchased from Sinopharm Chemical Reagent Company.The Na1−xCr1−xTixO2(x=0,0.03,0.05)powders were fabricated by a thermo-polymerization method.In a typical experiment(x=0.03),9.7 mmol sodium nitrate(NaNO3,AR),9.7 mmol chromium nitrate(Cr(NO3)3·9H2O,AR)and 0.3 mmol tetrabutyl titanate(TBOT,AR)were dissolved in 100 mL DIH2O to get a mixed solution.Then,50 mL acrylic acid was added to the mixed solution.The solution was then kept in an oven at 180◦C overnight to obtain the precursor.The precursor was first sintered at 500◦C for 6 h to get rid of the organics,followed by another grinding and then calcined at 900◦C for 10 h to get the final products.Finally,the electrode materials were stored in a glove box.

B.Characterization

XRD patterns of the powders were examined by using Philips X’pert Pro Super diffracrometer with Cu Kαradiation in the 2θrange of 10◦to 70◦.Scanning electron microscope(SEM,JSM-6390LA)was used to investigate the morphologies of the powders.

C.Electrochemical measurements

The charge and discharge tests were conducted in CR-2032 type coin half cells.The as-synthesized material(80%)was mixed with acetylene black(10%),PVDF binder(10%),andN-methyl-2-pyrrolidone(NMP)to form uniform slurry,and then coated on an aluminum foil.The electrodes were kept in an oven overnight to remove NMP.The amount of active material loaded was approximately 2 mg/cm2.The electrolyte was 1 mol/L NaClO4in PC.The assembled cells were finally cycled between 2.0 and 3.6 V to investigate the electrochemical properties.

III.RESULTS AND DISCUSSION

A.Materials characterization

As can be seen in FIG.1(a),all diffraction peaks of Na1−xCr1−xTixO2can be well indexed to the O3-type NaFeO2structure(R-3m space group)without any impurity phase,which is consistent with the standard card(JCPDS No.25-0819).As shown in the XRD patterns(FIG.1(b)),Ti doping causes a shift of(003)peak to a lower angle direction,indicating that the layer distance increases,which is bene ficial to the rate capability.A Rietvield re finement indicates that there is a slight increase inc-axis after the Ti-doping(Table I).

As shown in FIG.2,the SEM images illustrate that the samples appear as partial agglomerates and the particle size decreases with increasing the Ti content.Also,the color of the sample changed from cadmium green light to bottle green with the content of Ti increasing.This also proves indirectly the successful doping of Ti.The pristine NaCrO2powder(1−5 µm in particle size)consists of irregular particles,while the Ti doped samples have smaller particles(1−2 µm in particle size)with a more uniform distribution.

B.Electrochemical performance of Na-ion batteries

FIG.1 (a)The XRD patterns of Na1−xCr1−xTixO2(x=0,0.03,0.05),(b)themagnified (003)peak of Na1−xCr1−xTixO2(x=0,0.03,0.05).

TABLE IThe XRD re finement data of NaCrO2andNa0.97Cr0.97Ti0.03O2.

FIG.3 shows the rate performances ofthe Na1−xCr1−xTixO2samples.It is worth noting that the initial coulombic efficiency is very high(all above 96%)(FIG.3(a)).After 100 cycles at 0.2 C,the NaCrO2electrode suffers a capacity loss of about 15%while it is only 6%for the Na0.97Cr0.97Ti0.03O2sample(FIG.3(b)).Clearly,Na0.97Cr0.97Ti0.03O2shows the best rate capability with a discharge capacity of 110 mAh/g even at 32 C while the pristine one only delivers 45 mAh/g,which is extremely high compared to other works shown in Table S1 in the supplementary materials.The superior rate performance is likely due to the doping of Ti,which decreases the particle size and enlarges the distance ofc-axis so that Na+can diffuse easily at ultrafast rates.On the other hand,in a typical O3 layered structure cathode materials,when a sodium ion migrates to the nearest sodium vacancy,it has to pass through the adjacent tetrahedron sites,in which the neighboring transition metal ion hinders the Na+diffusion because of the strong electrostatic repulsion.According to Cederet al.[16],higher valence metal ions in the transition metal layer(Co3+or Mn4+vs.Cu2+or Ni2+)have a stronger repulsion to the active Li+/Na+.Thus,the increasing content of Ti4+would increase the resistance to the Li/Na diffusion.When this resistance surpasses the bene fit of the increased layer distance,the diffusion of Li+/Na+will be obstructed,resulting in the worst rate capability ofx=0.05 sample.

FIG.2 SEM images of the prepared Na1−xCr1−xTixO2(a)x=0,(b)x=0.03,and(c)x=0.05.

FIG.3(a)The first charge-discharge curves at 0.2 C of Na1−xCr1−xTixO2,(b)the cycling performances,(c)charge-discharge curves at different C-rates of Na0.97Cr0.97Ti0.03O2,(d)the rate performances.

Myunget al.suggested that carbon coating can also improve the rate performance[12],while in this work,without carbon coating,it has proven that the doping with Ti can stabilize the crystal lattice during the phase transition in charge/discharge process,bringing about the enhanced cycle stability and rate capability.

IV.CONCLUSION

In summary,a layer-structured O3 type cathode materials Na1−xCr1−xTixO2(x=0,0.03,0.05)are fabricated by an acrylic acid assist thermal polymerization method to study the effect of Ti doping on the electrochemical properties.The results show that Na0.97Cr0.97Ti0.03O2exhibits the best cycle stability and rate capability,which results from the stabilized crystal structure and enlarges the distance ofc-axis.

Supplementary materials:The comparison of rate performance and cycle performance of NaCrO2between this work and other work was shown in Table S1.

V.ACKNOWLEDGEMENTS

This work was supported by the National Natural Science Foundation of China Academy of Engineering Physics(No.U1630106),the National Natural Science Foundation of China(No.51577175)and Education Ministry of Anhui Province(No.KJ2014ZD36).The authors also thank Elementec Ltd.in Suzhou.

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