Bohu Deng,Ning Yue,Hoyng Dong,Qiuyue Gui,Ling Xio,Jinping Liu,b,c,*

a School of Chemistry, Chemical Engineering and Life Science, Wuhan University of Technology, Wuhan 430070, China

b State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China

c Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education, School of Physics and Electronic Engineering, Harbin Normal University, Harbin 150025, China

ABSTRACT Flexible Na-ion storage cathodes are still very few due to the challenge in achieving both reliable mechanical flexibility and excellent electrochemical performances.Herein, a new type of flexible Na3(VOPO4)2F cathode with nanocubes tightly assembled on carbon cloth is fabricated by a facile solvothermal method for the first time.The cathode is able to exhibit superior rate capability and stable cycling performance up to 1000 cycles,due to the surface-assembling of crystalline nanocubes on carbon fibers.In addition, it shows good mechanical flexibility, nearly no capacity decay is observed after continuous bending of 500 times.With this novel cathode and a directly-grown Na2Ti2O5anode,a fully binder-free Na-ion battery is assembled.It can deliver a high working voltage and increased gravimetric energy/power densities(maximum values:220.2 Wh/kg;5674.7 W/kg),and can power a LED indicator at bending angles from 0to 180.

Keywords:Flexible cathode Na3(VOPO4)2F Nanocube Surface-assembling Sodium-ion battery

Flexible energy-storage systems have attracted increasing research interest for the development of flexible electronic devices such as wearable electronics,roll-up displays and bendable phones 1–3].Lots of efforts have been dedicated to flexible Li-ion batteries(LIBs) due to their high working voltage, high energy density and cycling stability.However, the commercial expansion of LIBs is limited by the ever-increasing cost and shortage of lithium salts and costly transition metal oxides.Therefore, considerable academic attentions have been transferred to flexible sodium ion batteries(SIBs),due to the natural abundance(and low cost)of Na resources as well as similar energy-storage mechanisms between LIBs and SIBs [3–12].It is noticed that previous investigations for flexible Na-ion storage have been mainly concentrated on anode materials,and those for cathode materials are very few due to the challenge in attaining both reliable flexibility and satisfied electrochemical performances [7, 9].

Herein,we report a novel binder-free configuration for NVOPF nanocube cathodes directly assembled on flexible carbon cloth(2.0–2.5 mg/cm2).It exhibits superior high-rate capability (90%and 86% of the 1 C capacity retained at 10 and 20 C, respectively)and excellent cycling stability(88%capacity retention after 1000 cycles at 5 C).Moreover, this NVOPF cathode demonstrates good flexibility (nearly no capacity decay after continuous bending for 500 times).With this cathode and a flexible Na2Ti2O5anode,model flexible SIBs were assembled, which show increased gravimetric energy/power density compared with previously reported SIBs,and a maximum energy density of ca.220.2 Wh/kg can be achieved.

The SEM morphologies show that nanocubes of ca.300 nm in average size are tightly and uniformly decorated on the carbon fibers(Figs.1a and b).The elemental mapping of an individual fiber indicates the presence and uniform distribution of Na, V, P and F elements from NVOPF (Fig.1c).Based on the characteristic XRD diffraction peaks(Fig.1d),the nanocubes can be indexed to a wellcrystallized phase of tetragonal space group (P42/mnm, PDF#89-8485) [16].Furthermore, the TEM images and selected area electron diffraction (SAED) pattern of NVOPF nanocubes suggest that the particles are highly crystalline, which is beneficial to the facile charge-transfer between carbon fibers and nanocubes(Figs.1e-g).Moreover, the lattice spacing of ca.0.522 nm is in good agreement with the(002)plane of P42/mnm phase of NVOPF.

Fig.1.(a,b)SEM images,(c)the elemental mapping images,(d)XRD patterns,(e-g)TEM images and SAED pattern of NVOPF nanocubes.

Fig.2.(a) FTIR spectrum; (b-d) XPS spectra of NVOPF on the carbon cloth.

Fig.3.(a) Typical charge/discharge curves in the initial 2 cycles.(b) Rate performance.(c) Cycling performance at 5 C.(d) CV curves at different scan rates.

The Na-ion storage performances of the flexible NVOPF electrode were examined by galvanostatic testing.Fig.3a shows the typical charge/discharge profiles of the initial two cycles at 0.2 C between 2.5 V and 4.3 V,consistent with Na3(VOPO4)2F materials in previous reports [16,21].Two potential plateaus at about 3.61 and 4.02 V (vs.Na+/Na), as well as a reversible capacity of ca.mAh/g are observed.The NVOPF cathode was also tested at various C rates(Fig.S1a in Supporting information),and about 90%(112.mAh/g)and 86%(106.71 mAh/g)of the 1 C capacity are retained at 10 and 20 C,respectively(Fig.3b).By comparing with the excellent performances of other reported NVOPF materials (Table S1 in Supporting information)[22,24–27],this high-rate performance is superior based on a mass loading>2.0 mg/cm2.Furthermore,the NVOPF cathode exhibits excellent cycling stability, and88% of the discharge capacity is retained after 1000 cycles while cycled at 5 C (Fig.3c).

The NVOPF electrode was further characterized by cyclic voltammetry.The CV profiles at various sweeping rates are presented in Fig.3d, two redox couples can be seen at about 3.65 and 4.10 V (corresponding to the two voltage plateaus in the charge/discharge processes).The Ipof the redox peaks at ca.3.6 and 4.0 V are plotted against ?1/2,and the data in scattered dots are well fitted by a linear dependence of Ipon ?1/2(Fig.S1b in Supporting information).Therefore,the Na-ion intercalation processes at both 3.6 and 4.0 V are attributed to be diffusion-controlled [21].This implies that the superior high-rate performance of flexible NVOPF cathode is mainly attributed to the nano-scaled Na-ion diffusion path and enhanced electronic transport owing to the surfaceassembling of crystalline nanocubes on carbon fibers [28].

To demonstrate the advantages of the flexible NVOPF cathode,model SIB was assembled by combining it with a flexible Na2Ti2O5anode (Fig.4a).NTO anode materials (including Na2Ti2O5or Na2Ti3O7) show merits including high practical capacity (ca.mAh/g), low potential (down to 0.2 V vs.Na+/Na) and natural abundance.Moreover, flexible NTO anodes can be constructed with nanostructured NTO on Ti foil by facile hydrothermal methods [29,30].Recently,superior rate performance and cycling stability could be attained with Na2Ti2O5nanosheet-array anodes by replacing the ester-based electrolytes with ether-based electrolytes[31].Accordingly,a Na2Ti2O5nano-array anode is selected to pair with the flexible NVOPF cathode in the lab-made flexible SIBs.

Fig.4.(a) Schematic illustration for flexible SIB.(b) Typical charge/discharge profiles of the SIB.(c) Rate performance.(d) Cycling performance at 5 C.Inset photographs for powering a while LED indicator.(e)The discharge/charge curves.(f)Ragone plot of gravimetric energy density versus power density.

The NTO anode is constructed with nanowires of ca.100 nm in diameter uniformly grown on the Ti foil (Fig.S2a in Supporting information), which can be assigned to an orthorhombic phase(H2Ti2O5, PDF#47-0124) based on the powder XRD characterization (Fig.S2b in Supporting information) [29].As a reference, the NTO anode was galvanostatic tested in an ether-based electrolyte(1 mol/L NaPF6in diglyme).It delivers a high coulombic efficiency of ca.80% in the initial cycle and a reversible capacity of ca.mAh/g while cycled at 0.2 C between 2.5 V and 0.2 V (Fig.S2c in Supporting information).Moreover, it exhibits excellent rate performance (and 69% of the 1 C capacity retained at 10 C and 20 C, respectively) and cycling stability (ca.97% of the reversible capacity maintained after 2000 cycles at 5 C)(Figs.S2d-f in Supporting information).The NVOPF film cathode was also examined in the ether-based electrolyte, and Na-ion storage performances comparable to those in the ester-based electrolyte are observed (Fig.S3 in Supporting information).Therefore,flexible SIBs were assembled with the ether-based electrolyte for optimized energy-storage performances.

The electrochemical behaviors of the flexible SIBs were characterized between 1.5 V and 4.1 V.As for a typical battery,the mass ratio of the NTO to NVOPF is about 1:2.5 and the initial coulombic efficiency is ca.62%.Due to the combination of highpotential NVOPF and low-potential NTO electrodes, the highvoltage plateau with a medium voltage of ca.3.6 V (contributing nearly half of the reversible capacity)is observed at 0.2 C(Fig.4b),outperforming most SIB systems in previous reports[21,24,32–35].Moreover,our SIB can also exhibit excellent rate performance and good cycling stability.The SIB was tested at various rates(Fig.4b),ca.73%and 69%of the 1 C discharge capacity are retained at 10 and 20 C,respectively(Fig.4c).While cycled at 5 C,capacity retention of ca.86% is obtained after 500 cycles.Meanwhile, the good flexibility of this NVOPF cathode was manifested by continuously bending for 500 times (Fig.S1c in Supporting information), and nearly no capacity loss is observed for the SIB assembled with bending-treated electrode (Fig.4e).In addition, a model flexible SIB in soft package was assembled, and used to power a LED indicator at different bending angles (Fig.4d inset).The good flexibility can be attributed to the strong adhesion of NVOPF nanocubes to the flexible carbon cloth.

In summary, a novel flexible cathode with NVOPF nanocubes directlygrownoncarbonclothwaspreparedbyafacilesolvothermal method.This cathode shows superior high-rate performance,excellent cycling stability and good mechanical performance.Flexible SIB assembled with this cathode and a flexible NTO anode delivers high working voltage and increased energy/power density,and it can power a LED indicator at bending angles fromtoOur study presents a new and facile approach to fabricate flexible Na3(VO1-xPO4)2F1+2xpositive electrodes,which may be extendable to other cathodes for future LIBs and SIBs.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China(Nos.51972257,51672205 and 21673169) and the National Key R&D Program of China (No.2016YFA0202602).

Appendix A.Supplementary data

Supplementarymaterialrelatedtothisarticlecanbefound,inthe online version,at doi:https://doi.org/10.1016/j.cclet.2020.04.054.