Scalable synthesis of Na3V2(PO4)3/C with high safety and ultrahigh-rate performance for sodium-ion batteries

Guijia Cui,Hong Wang,*,Fengping Yu,Haiying Che,3,Xiaozhen Liao,Linsen Li,Weimin Yang,Zifeng Ma,3,*

1 Department of Chemical Engineering,Shanghai Electrochemical Energy Devices Research Center,Shanghai Jiao Tong University,Shanghai 200240,China

2 SINOPEC Shanghai Research Institute of Petrochemical Technology,Shanghai 201208,China

3 Zhejiang NaTRIUM Energy Co.Ltd.,Shaoxing 312000,China

Keywords: Na3V2(PO4)3/C Sodium-ion batteries Symmetrical battery Accelerating rate calorimetry Battery thermal safety

ABSTRACT NASICON-type Na3V2(PO4)3 is a promising electrode material for developing advanced sodium-ion batteries.Preparing Na3V2(PO4)3 with good performance by a cost-effective and large-scale method is significant for industrial applications.In this work,a porous Na3V2(PO4)3/C cathode material with excellent electrochemical performance is successfully prepared by an agar-gel combined with freeze-drying method.The Na3V2(PO4)3/C cathode displayed specific capacities of 113.4 mAh?g-1,107.0 mAh?g-1 and 87.1 mAh?g-1 at 0.1 C,1 C and 10 C,respectively.For the first time,the 500-mAh soft-packed symmetrical sodium-ion batteries based on Na3V2(PO4)3/C electrodes are successfully fabricated.The 500-mAh symmetrical batteries exhibit outstanding low temperature performance with a capacity retention of 83% at 0°C owing to the rapid sodium ion migration ability and structural stability of Na3V2(PO4)3/C.Moreover,the thermal runaway features are revealed by accelerating rate calorimetry (ARC) test for the first time.Thermal stability and safety of the symmetrical batteries are demonstrated to be better than lithium-ion batteries and some reported sodium-ion batteries.Our work makes it clear that the soft-packed symmetrical sodium ion batteries based on Na3V2(PO4)3/C have a prospect of practical application in high safety requirement fields.

1.Introduction

Sodium-ion batteries (SIBs) have attracted interest in recent years due to the outstanding advantages in high safety and low cost[1].Cathode materials play an important role in batteries,people have made a lot of efforts to develop high-performance cathode materials for SIBs [2–4].Among,the NASICON-type (Na super ion conductor)Na3V2(PO4)3is considered as one of the most promising cathode material due to its excellent electrochemical performance[5–8].In Na3V2(PO4)3,two redox couples of V3+/V4+and V2+/V3+are accessed with two voltage plateaus located at 3.4 V and 1.6 Vvs.Na+/Na and the theoretical discharge capacity is 117.6 mAh?g-1.However,the low intrinsic electronic conductivity owing to the large energy difference between V 3d and O 2p orbitals significantly limits its electrochemical performance and further practical application [9].

In order to overcome this problem,Huet al.first reported carbon coating as an effective way to improve the conductivity of Na3-V2(PO4)3[1].Yuet al.have paid much attention on designing Na3V2(PO4)3nanoparticles combined with highly conductive carbon framework [10].Their results demonstrated that the electrochemical properties of electrode materials are strongly related to their structures,surface properties,and crystal particle size.Thus,effective strategies to improve electrochemical performance include:1)decreasing the Na3V2(PO4)3particle to nano-scale level and overcoming volume strain [11];2) embedding Na3V2(PO4)3particles in carbon matrix to improve electronic conductivity and prevent aggregation[12,13].The Na3V2(PO4)3/C has been prepared into special morphologies like micro spheres [14–16],nanorods[10],nanowires [17],nanoflakes [18],nanofibers [19],hollow spheres [12],core-sheath mesostructure [20],porous structures[21].However,the synthesis of the nano-size Na3V2(PO4)3/C material usually requires complicated steps,making the synthesis process expensive and inefficient [22];Coating high-quality carbon onto Na3V2(PO4)3nanocrystal is hardly achieved through traditional solid state reaction methods [23–25].The large-scale synthesis of Na3V2(PO4)3/C with good performance by greener and feasible route is still a difficult task.In order to promote the Na3-V2(PO4)3/C electrode materials into industrial application,it is necessary to develop more feasible and scalable technique in synthesizing the Na3V2(PO4)3/C materials [26–29].Last but not least,in contrast to the electrochemical performance of the batteries,the thermal safety has been paid much less attention.Thermal safety is a critical consideration for sodium-ion batteries.

In this paper,a agar-gel combined freeze-drying method was reported to achieve the scalable synthesis of Na3V2(PO4)3/C.For the first time,500-mAh soft-packed symmetrical batteries were assembled based on the Na3V2(PO4)3/C,which exhibited high rate and long cycle performance even at -20 °C.Meanwhile,for the first time,the thermal runaway behavior of the Na3V2(PO4)3//Na3-V2(PO4)3soft-packed symmetrical battery and Na3V2(PO4)3//hard carbon battery were measured by accelerating rate calorimetry(ARC).It is extremely interesting that the symmetrical battery showed much higher thermal stability than the hard carbon battery and most of the reported lithium-ion batteries.We expect this new finding in our work can be a helpful guide to develop high performance sodium-ion battery for practical application.

2.Experimental

2.1.Material synthesis

All chemical regents were purchased from Aladdin and used without further treatment.Carbon coated Na3V2(PO4)3(Na3V2(-PO4)3/C) was synthesized using both agar and glucose as carbon source as following:Firstly,312.4 g of NH4VO3,1029.6 g of NaH2PO4-?2H2O and 35.2 g of Na2CO3were completely dissolved in solution A.Then,the solution B containing 97.0 g of agar and 176.0 g of glucose was added into solution A carefully.The mixture solution turns green-blue after being kept for 30 min under heating.After cooling down to room temperature,the solution quickly turned into a gel.Then,the dried gray-blue gel can be obtained by freeze-drying treatment.Finally,the dried gel was calcinated at 750 °C for 8 h in Ar atmosphere to obtain the Na3V2(PO4)3/C material.

2.2.Material characterization

The SEM and TEM analysis were carried out on a Nova NanoSEM 450(FEI Company,USA)and a TALOS F200X transmission electron microscope (FEI Company,USA),respectively.The TGA was operated on Q5000IR(TA Instruments,USA),and measurement temperature was set in the range of 30–800 °C with a heating rate of 10°C?min-1in air atmosphere.The total carbon content in samples was analysized on G4 ICARUS (Germany).The X-ray powder diffraction patterns were obtained on a Poly-functional X-Ray Diffractometer with CuKα (D8 ADVANCE Da Vinci,Germany).The diffraction data were recorded in the 2θ range of 10°–90° with a scan rate of 2(°)?min-1.The analysis of elemental ratio in NVP/C was conducted on Inductively Coupled Plasma Optical (iCAP6300,USA).The thermal stability of large format prismatic battery was tested by Accelerating Rate Calorimeter with a temperature range of 50 °C to 305 °C (Thermal Hazard Technology,UK).

2.3.Electrochemical measurements

The electronchemical performance of Na3V2(PO4)3/C was carried out with half batteries and 500-mAh soft-packed batteries.For a typical fabrication of coin cell,the electrodes were prepared by mixing 80% (mass) active material,10% (mass) super P and 10% (mass) polyvinylidene fluoride (PVDF) inN-methyl-2-pyrrolidone (NMP),and then coating the mixture onto an aluminum foil.After drying at 80 °C for 2 h,the electrode disks (with diameter of 14 mm)were punched.With further drying under vacuum at 120 °C for 12 h,the cathodes were weighed and incorporated into coin cells (R2016) with sodium metal as anode and 0.8 mol?L-1NaPF6/EMC+PC+FEC (50:48:2,V/V/V) electrolyte in an argon filled glove box.The mass loading of active materials on cathodes was 3–4 m?cm-2.For preparing the 500 mAh softpacked full sodium ion batteries,hard carbon (Sumitomo Bakelite Co.,Ltd.) was used as anode and 0.8 mol?L-1NaPF6/EMC+PC+FE C (50:48:2,V/V/V) as electrolyte.The Na3V2(PO4)3/C cathode consisted of 90% (mass)NVP/C,5% (mass)super P,and 5% (mass)PVDF.The hard carbon anode consisted of 90% (mass) hard carbon,5% (mass) super P,and 5% (mass) PVDF.In Na3V2(PO4)3//Hard Carbon battery,the mass loadings of active materials on anode and cathode were～15 mg?cm-2and～30 mg?cm-2,respectively.The total capacity loading of hard anode:cathode was 1.15:1 and then packaged with aluminum plastic film.For preparing the 500 mAh softpacked symmetrical full batteries,the NVP was used both as anode and cathode.The NVP electrodes consisted of 90% (mass) NVP,5% (mass) super P,and 5% (mass) PVDF.And these batteries had the same electrolyte as the above batteries.The mass loadings of active materials on anode and cathode were～30 mg?cm-2and～15 mg?cm-2,respectively.The total capacity loading of anode:-cathode was 1:1 and then packaged with aluminum plastic film.The electrochemical measurements of half cells and soft-packed batteries were obtained on Land CT2001A and Neware CT-4008-5V10A-FA.

3.Results and Discussion

3.1.Structural characterization

The synthesis procedure of Na3V2(PO4)3/C is presented in Fig.1.Na3V2(PO4)3/C was obtained with a series of processes including gel,freeze-drying and calcination.The agar plays an important role in the synthesis of high-performance Na3V2(PO4)3/C material.Benefiting from the agar-gel polymer,gel blocks containing abundant micron-sized pores could be obtained after treatment of freezedrying.During the process of carbon pyrolysis,agar acts not only as carbon source,but also as supporting formwork to form the 3D interconnected porous carbon network.

Fig.2(a)shows the morphology of Na3V2(PO4)3/C sample.It can be seen that the secondary particles are interconnected to form a sponge-like structure with abundant pores,which is beneficial to shortening the diffusion path of sodium ion between electrode particles and electrolyte.In addition,abundant pores are beneficial to expose more reactive sites,which is very important for high-rate performance.Fig.2(b) shows the high-resolution transmission electron microscope image (HRTEM) of the Na3V2(PO4)3/C.The Na3V2(PO4)3crystals are uniformly coated by carbon layer,which effectively enhance the electron conductivity[30,31].The total carbon content of about 5% in the sample was revealed by thermogravimetric analysis (TGA) in Fig.2(d).The crystallographic structure and phase of the materials were characterized by X-ray diffraction (XRD) analysis.As shown in Fig.2(c),all diffraction peaks can be ascribed to the standard spectrum of Na3V2(PO4)3(ICSD#98-024-8140) [1,32],which indicates the pureNa3V2(-PO4)3phase in the as-synthesized Na3V2(PO4)3/C sample.

3.2.Electrochemical performance in half cell

Fig.2.Characterization of structure and composition.(a) The SEM,(b) HRTEM images,(c) XRD pattern,and (d) TGA curve of the Na3V2(PO4)3/C sample.

The electrochemical behavior of the Na3V2(PO4)3/C material as the cathode and anode was tested in coin batteries by employing sodium metal as the counter electrode.Fig.3(a) shows the cyclic voltammetry(CV)curve of Na3V2(PO4)3/C electrode.The discharge plateau at 3.4 V corresponds to a two-phase transformation between Na3V2(PO4)3and NaV2(PO4)3(V4+/V3+) while the low potential plateau at 1.6 V presents the transformation between Na3V2(PO4)3and Na4V2(PO4)3(V3+/V2+) [32].These reactions can be described as follows:

(redox at 1.6 V) Na3V2(PO4)3+Na++e-?Na4V2(PO4)3

(redox at 3.4 V) Na3V2(PO4)3?Na1V2(PO4)3+2Na++2e-

Fig.3(b) shows the corresponding crystal structure of Na4V2(-PO4)3and Na1V2(PO4)3containing 4Na and 1Na,respectively.The Na3V2(PO4)3/C cathode displayed discharge capacities of 113.4 mAh?g-1,107.0 mAh?g-1and 87.1 mAh?g-1at the rates of 0.1 C,1 C and 10 C (1 C=118 mA?g-1),showing outstanding rate performance (Fig.3(c)).In addition,the Na3V2(PO4)3as anode displayed capacity of 58.1 mAh?g-1,57.0 mAh?g-1and 45.8 mAh?g-1at the rates of 0.1 C,1 C and 10 C (1 C=59 mA?g-1).Fig.3(d) displays the cycling performance of Na3V2(PO4)3/C at 1 C with a capacity retention of 88.2% after 1000 cycles.Excellent rate and cycling performance benefit from the high sodium ion migration and high electron conductivity in 3D carbon coated nano-size Na3V2(PO4)3sample.

Fig.3.The electrochemical performance of the Na3V2(PO4)3/C in half cell.(a) The CV curves of Na3V2(PO4)3 in half battery at 0.1 mV?s-1;(b) The corresponding crystal structure of Na4V2(PO4)3 and Na1V2(PO4)3;(c)The cycling performance of Na3V2(PO4)3/C at 3.4 V and 1.6 V plateaus,respectively;(d)Rate performance of Na3V2(PO4)3/C at 3.4 V and 1.6 V plateaus,respectively.

3.3.Electrochemical performance of 500-mAh soft-packed batteries

For the first time,500-mAh soft-packed Na3V2(PO4)3//Na3V2(PO4)3symmetrical sodium-ion batteries (NNB) and 500-mAh Na3V2(PO4)3//hard carbon soft-packed sodium-ion batteries(NHB) were assembled in this work.The first charge/discharge curves of the 500-mAh soft-packed battery in a voltage range of 1.0–2.5 V for NNB and 1.5–3.8 V for NHB at 25 °C were shown in Fig.4(a).Symmetrical soft-packed NIB has an initial output voltage at～1.73 V,which is in agreement with previous work [33,34].Figs.4(b) and 5(a) display the capacity retention of two batteries during 500 cycles.It can be seen that the discharge capacity of NHB decreased from 501 mAh to 360 mAh after 500 cycles with capacity retention of 71.9% .Compared with NHB,the NNB had much better cycling stability (capacity retention of 87.2% after 500 cycles)and more stable coulombic efficiency during prolonged cycles.Fig.S1 displayed the surface state of these two kinds of anode electrodes after 500 cycles.It is clear to see that the surface of the Na3V2(PO4)3anodes from NNB was smooth without any impurities.However,the hard carbon anodes from NHB has amount of gray matters on the surface,which has been believed to be a mixture of the dead sodium and the product of side reaction with the electrolyte [35–37].

These results can also be further proved by EDX mapping analysis in Fig.S2.Compared with hard carbon anode,no detectable gray byproduct was observed on the Na3V2(PO4)3anode,and the surface state was still similar to the initial.The better kinetics of NNB can be illustrated by high-rate charge-discharge performance,as is shown in Fig.4(c).Although NNB and NHB have the same discharge capacity about 500 mAh at 1C(1C=500 mA),the NHB displays a capacity of only 180 mAh when the current rate increased to 20 C.However,the NNB could exhibit a satisfactory capacity retention (300 mAh at 20 C) owing to the better Na+diffusion kinetics in Na3V2(PO4)3anode than hard carbon.As shown in Fig.4(d) the NNB displays discharge capacities of 416 mAh at 0 °C and 312 mAh at -20 °C.However,the NHB displays the discharge capacities of 222 mAh at 0 °C and 51 mAh at -20 °C.The detailed charge-discharge curves of NNB from 45°C to-20°C were exhibited in Fig.5(b).It can be clearly seen that the discharge voltage platform of Na3V2(PO4)3//Na3V2(PO4)3symmetrical sodiumion batteries decreased by less than 0.1 V even at-20°C.As shown in Fig.5(c),the NNB displayed outstanding cycling performance at different temperatures.It is worth noting that the NNB has the highest capacity retention of 95.5% after 500 cycles at 0 °C.These results indicate the symmetrical soft-packed sodium-ion battery would be a promising battery to meet the application in cold zone.To evaluate the practical application of the symmetrical battery,we calculated its energy density and power density.As shown in Fig.5(d),the NNB battery exhibited energy density of 20.2 Wh?kg-1at 1 C.Note that,we did not optimize the manufacturing process of the battery,such as the amount of electrolyte injection,the consumption of aluminum plastic shell and other components in this experiment.The high power performance of NNB means that this kind of battery has application prospect in some special fields,such as military.To the best of our knowledge,the soft-packed Na3V2(-PO4)3//Na3V2(PO4)3symmetric battery with long life and high rate performance has rarely been reported [38,39].

3.4.Thermal runaway features of the soft-packed Na-ion batteries

Fig.4.Comparison of the electrochemical performance of the two types of full batteries.(a) The discharge-charge curves in the first cycle;(b) Long-term cycling performances during 500 cycles;(c) Rate performances at 25 °C;(d) The discharge capacity at different operation temperatures of Na3V2(PO4)3//Na3V2(PO4)3 symmetrical battery (NVP//NVP battery) and Na3V2(PO4)3//hard carbon full battery (NVP//HC battery).

Fig.5.Electrochemical performance of the symmetrical battery.(a) The cycling performance of the 500-mAh Na3V2(PO4)3//Na3V2(PO4)3 symmetrical battery at ambient temperature;(b)The discharge-charge curves at different operation temperatures at 1 C;(c) The cycling stability during 500 cycles at different operation temperatures;(d)The Ragone plots of the 500-mAh soft-packed Na3V2(PO4)3//Na3V2(PO4)3 symmetrical battery (the relevance of energy and power densities).

Thermal runaway is one of the most critical problems in lithium-ion battery[40,41].In contrast to the electrochemical performance of the batteries,safety about sodium-ion batteries has been paid much less attention.Accelerated rate calorimeter(ARC) is a new type of thermal analysis instrument recommended by the United Nations for the evaluation of dangerous goods like batteries[42,43].It can provide accurate data of time and temperature of chemical reactions in a heating battery under adiabatic conditions.However,to the best of our knowledge,the research about thermal safety of Na3V2(PO4)3batteries has not been reported till now.In this work,the thermal safety behavior of 500-mAh soft-packed batteries composed of Na3V2(PO4)3electrodes were evaluated by extended volume ARC(EV+)for the first time,and the testing state was displayed in Fig.S3.In order to better understand the thermal runaway phenomenon,three characteristic temperaturesT1,T2,andT3are proposed in precious report [42].T1is the onset temperature of the battery selfheating process.The exothermic reaction at this temperature may be caused by the decomposition of the solid electrolyte interface (SEI) on anodes.

Fig.6.Thermal runaway test.(a)Temperature vs time plot and(b)heating rate vs temperature plot of the charged Na3V2(PO4)3//hard carbon battery;(c)Temperature vs time plot and (d) heating rate vs temperature plot of the charged Na3V2(PO4)3//Na3V2(PO4)3 symmetrical battery.

As shown in Fig.6(a) and (c),T1of the NNB and NHB locates at 188.511 °C and 158.026 °C,respectively.TheT1of NNB is higher than that of NHB,which may be owning to the absence of SEI on Na3V2(PO4)3anodes.During this period,a variety of reactions including internal short circuits,decomposition of electrode and electrolyte will occur,resulting in a chain reaction.As a result,the chain exothermic reactions will cause the battery temperature to rise continuously until a catastrophic thermal runaway occurs.The higher initiation point of exothermic of NNB implies outstanding capability of heat tolerance,which is of great significance in safety.As a result of chain reaction,the battery temperature increases exponentially.Thus,T2is defined as the point that temperature increase rate reaches 1 °C?min,representing the onset temperature of the thermal runaway [44].It is interesting that theT2of 500-mAh Na3V2(PO4)3//Na3V2(PO4)3symmetrical battery(254.400 °C) is much higher than that of the Na3V2(PO4)3//hard carbon battery (243.129 °C),which is similar to the result of NaNi1/3Fe1/3Mn1/3O2//hard carbon battery reported by our group recently [45].In addition,to the best of knowledge,theT2of NNB in our work shows higher value than lithium-ion batteries with various cathodes,such as LiFePO4,LiCoO2,LiMnO2,Li(Ni0.1-Co0.8Mn0.1)O2,NaxNi1/3Fe1/3Mn1/3O2,etc.[43–47].AfterT2,the battery temperature increased dramatically and reached the maximum temperatureT3at 268.504°C.This temperature is much lower than that of layer cathode materials for lithium-ion battery,which can go up to 800 °C in few seconds [47,48],indicating that the Na3V2(PO4)3//Na3V2(PO4)3symmetrical battery presents much better thermal stability and could be a reliable choice for sodiumion battery.From a safety perspective,the Na3V2(PO4)3-based sodium-ion batteries could be valuable in guiding battery engineers and researchers to select promising battery development.

4.Conclusions

In summary,a feasibly scalable synthesis of Na3V2(PO4)3/C electrode material was successfully achieved by green agar-gel combined with freeze-drying in this paper.The agar plays an extremely significant role in constructing the conductive carbon network with porous structure.The as-synthesized Na3V2(PO4)3/C displayed outstanding cycling performance with a retention of 88.2% after 1000 cycles at 1 C.For the first time,the Na3V2(PO4)3/C was employed as both cathode and anode active materials to fabricate 500-mAh soft-packed symmetrical sodium ion batteries.The symmetrical battery showed satisfactory cycling stability and high-rate performance with capacity retention over 60% at the current rate of 20 C.Accelerating rate calorimeter measurement of the 500-mAh soft-packed symmetrical sodium ion batteries revealed that the onset thermal runaway temperature was 254.400 °C.It is higher than Na3V2(PO4)3//hard carbon battery,NaNi1/3Fe1/3-Mn1/3O2//hard carbon battery and lithium-ion batteries,which indicated the Na3V2(PO4)3//Na3V2(PO4)3symmetrical battery is reliable for special fields with high safety requirements.We believe this study has a guiding significance for the development of advanced sodium-ion batteries with high performance,long life and high safety.

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.

Acknowledgements

This work was supported by the National Key Research and Development Program (2016YFB0901505),the Natural Science Foundation of China(22005190,21938005),the Science&Technology Commission of Shanghai Municipality (19DZ1205500),Zhejiang Key Research and Development Program (2020C01128).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.06.008.