C (A= Li, Li0.5Na0.5 and Na) for High Rate

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C: Energy Conversion and Storage; Energy and Charge Transport 2

7

0.5

0.5

Exploration of AVPO/C (A= Li, Li Na and Na) for High Rate Sodium-Ion Battery Applications Mani Vellaisamy, Mogalahalli Venkatashamy Reddy, Bobba Venkateshwara Rao Chowdari, and Kalaiselvi Nallathamby J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09451 • Publication Date (Web): 12 Oct 2018 Downloaded from http://pubs.acs.org on October 13, 2018

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The Journal of Physical Chemistry

Exploration of AVP2O7/C (A= Li, Li0.5Na0.5 and Na) for High Rate Sodium-Ion Battery Applications Vellaisamy Mani1, Mogalahalli Venkatashamy Reddy2, B. V. R. Chowdari2 and Nallathamby Kalaiselvi1* 1

ECPS Division, CSIR- Central Electrochemical Research Institute, Tamilnadu, India.

2

Department of Physics, National University of Singapore-117 542.

ABSTRACT: In this communication, we demonstrate the electrochemical activity of AVP2O7/C (A= Li, Li0.5Na0.5 and Na) prepared by a scalable and an easy-to-adopt oxalic dihydrazide (ODH) assisted solution combustion method for application in sodium-ion batteries (SIBs). Series of compounds thus prepared are found to possess monoclinic structure consisting of 3-D porous network and sufficient void volume to facilitate facile intercalation/deintercalation of sodium ions. However, among the corresponding composites obtained with super P carbon, the lithium analog viz., LiVP2O7/C, demonstrates better electrochemical performance than the rest of the compositions viz., Li0.5Na0.5VP2O7/C and NaVP2O7/C, when explored as anode in SIBs. In other words, LiVP2O7/C anode exhibits a stable reversible capacity of 125 mA h g-1 at 1 C rate (115 mA g-1) up to 100 cycles. Further, at 1 C rate condition, an acceptable capacity of 90 mA h g-1 has been observed up to 1000 cycles with a coulombic efficiency of 99 %, which is noteworthy. The currently prepared LiVP2O7/C anode validates its suitability for high capacity, long cycle life and versatile rate capability by way of displaying significant capacity values of 118, 97, 70, 58, 50 and 40 mA h g−1, under the influence of 2, 3, 5, 10, 20 and 40 C rate, respectively. The study reports for the first time about the possibility of exploiting LiVP2O7/C as a potential insertion anode for high rate SIB applications.

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1. INTRODUCTION Sodium-ion batteries (SIBs) receive significant attention as a potential substitute to LIBs due to the plentiful availability of resources and the low cost benefits of sodium.1 Furthermore, due to the physical and chemical resemblance of lithium and sodium ions, especially with reference to the redox behaviour associated with the

intercalation/de-intercalation mechanism which is responsible for the

electrochemical properties of SIBs and LIBs, developments in the field of SIBs are taking place at a faster rate.2 Despite the widespread improvements realized with respect to the invention of high performance cathode materials for SIBs that include transition metal phosphates, oxides, and ferrocyanides, development of anode materials for SIBs has not seen much progress till date.3-4 Apart from few carbonaceous materials, validated for their electrochemical activity in SIBs as anode5, relatively lesser focus has been paid in general to develop SIB anodes. Hereagain, the performance of such carbonaceous electrodes in terms of cycleability and storage capability is not comparable with that of the corresponding lithium counterpart, due to reasons such as larger size of Na+ (than Li+)

and

the

resultant

difficulties

involved

in

the

electrochemical

intercalation/deintercalation of Na+ from/to the electrode materials.6 Apart from such a drawback due to the higher ionic radius, volume variations, observed upon progressive cycling with few other SIB anodes leads to structural degradation, resulting in the poor cycleability and rate capability behaviour.7 Hence, there arises a critical demand to identify suitable and better performing anode materials for SIBs with appreciable storage performance, long cycle life and excellent rate capability. Unlike conversion reaction and alloying mechanism, responsible for inadmissible volume changes, intercalation-type anode materials deliver stable cycling capacity.7-8 Based on these


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reasons, anodes capable of exhibiting electrochemical performance in SIBs through intercalation/deintercalation mechanism receive paramount importance amongst researchers as well from the manufacturing point of view. Towards this direction, lithium, sodium and potassium counterparts of intercalation compounds are being investigated as anodes in SIBs, especially in the recent days. 9-13 For example, Na+ ion superionic conductor (NASICON) family compounds have been studied extensively as both Li-ion and Na-ion battery cathode and anode individually14-21 and become the ubiquitous choice of alkali metal ion battery electrodes. However, safety aspects prefer polyanionic materials (such as borates, phosphates, sulphates, silicates, etc.) containing strong covalent bonds X−O (X = B, P, S, Si), because covalent bonds of polyanionic derivatives are susceptible to oxygen evolution and further ensure hazardous free safety upon usage as electrodes in batteries.22 As value addition to this series, pyrophosphates of AMP2O7 type, wherein A = Li, Li0.5Na0.5 and Na; M = 3d transition metals form yet another class of polyanionic compounds, bestowed with a structure consisting of diffusion channels, desirable for the facile transport of alkali metal ion. It is well known that pyrophosphate anion is more stable than phosphate anion especially at high temperatures, thus renders value addition by providing a platform to design thermally stable electrode materials.23-24 Further, the large interstitial channels, capable of accommodating alkali cations ensure facile transport kinetics. Based on these advantages, we have chosen AVP2O7/C (A= Li, Li0.5Na0.5 and Na) compounds as alternative anode material for use in SIBs. Herein, it is worth mentioning that we have recently reported LiVP2O7/C as anode material for LIBs25, whereas no literature is found to be available till date on AVP2O7/C (A= Li, Li0.5Na0.5 and Na) as anode for SIBs, which is the motivation



behind this work and endorses the novelty of this study. Further, our idea to explore LiVP2O7 for SIBs encompass an extended interest to understand the structurestability-electrochemical activity correlation of pyrophosphates, governed mainly by the presence of P2O7 group and due to the involvement of intercalation/deintercalation mechanism in SIBs. Therefore, with a view to gain more insights on the synthesisstructure-property relationship, we have synthesized series of AVP2O7/C (A= Li, Li0.5Na0.5 and Na) and investigated for the first time as anode material for SIBs. Our results clearly demonstrate the suitability of LiVP2O7/C anode for SIBs with good cycleability and rate capability behaviour. In short, the study recommends LiVP2O7/C anode for futuristic SIB applications. 2. EXPERIMENTAL PROCEDURE AVP2O7 (A= Li, Li0.5Na0.5 and Na) samples were prepared by oxalic dihyrazide (ODH) solution combustion method. The precursors, viz., V2O3 (Alfa Aesar), NH4H2PO4 (Merck), and LiNO3 (or) NaNO3 (Alfa Aesar) were used without further purification. Initially, ODH and stoichiometric amount of precursors were dissolved in de-ionised water with magnetic stirring and heated at 80 °C to get a homogeneous solution. The process of heating and stirring was continued to get a thick mass, which was initially dried in hot air oven at 120 °C and heated further to 350 °C for 4 h in a tubular furnace under Argon atmosphere. Subsequently, the powder was heated to 800 °C for 8 h in Ar atmosphere with an intermittent grinding to obtain AVP2O7 compounds. The AVP2O7/C composite was prepared by ball milling (290 rpm for 3h) the synthesized powder with 5 wt. % of Super P carbon. The mixture was heated in Ar atmosphere up to 700 °C, to ensure the perfect adherence of added carbon on the


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surface of porous LiVP2O7, Li0.5Na0.5VP2O7 and NaVP2O7, and the final products were named as LiVP2O7/C, Li0.5Na0.5VP2O7/C and NaVP2O7/C respectively.

3. RESULTS AND DISCUSSION Figure 1a displays the XRD pattern of as-prepared samples at 850˚C. Using Rietveld refinement (Figure1b, c and d), perfect similarity with the standard pattern is found with respect to the position and intensity of the Bragg peaks of crystalline LiVP2O7 and NaVP2O7, belonging to the monoclinic crystal structure with P2 1 and P21/C space group respectively. From the absence of impurities, phase purity of currently synthesized LiVP2O7 and NaVP2O7 compounds for their monoclinic structure is understood. Further, the calculated lattice parameter values (shown in Table 1) which are in good agreement with those of the reported ones on LiVP2O7 25 and NaVP2O7

26-27

,

confirm the desired purity and monoclinic structure of title

compounds. The calculated Rietveld parameters are Rwp=11.57; Rp=9.49; GOF=6.71 for LiVP2O7 and Rwp= 10.54; Rp=7.13; GOF=4.96 for NaVP2O7. On the other hand, Li0.5Na0.5VP2O7 falls under the category of solid solution of LiVP 2O7 and NaVP2O7 and hence the currently observed similarity in XRD peak position, pattern and intensity is felt to be sufficient to substantiate the desired crystal structure. Upon careful investigation using Rietveld refinement, we understood that the XRD pattern of Li0.5Na0.5VP2O7 possesses a combination of 50.42 % of LiVP2O7 and 49.57 % of NaVP2O7.



Figure 1. Powder X-ray diffraction pattern of a) indexed cumulative pattern b) LiVP2O7, c) Li0.5Na0.5VP2O7 and d) NaVP2O7 powder The morphology of LiVP2O7, Li0.5Na0.5VP2O7 and NaVP2O7 has been examined by field-emission scanning electron microscopy (FESEM), and the obtained images are shown in Figure 2. From the figure, one can visualise the presence of interconnected 3-D porous channels with a honey bee hive morphology, made out of micron sized particles, which is clearly seen from all the images, irrespective of the magnification. It is interesting to note that the porous nature of LiVP2O7 has been obtained without deploying any porogen or template, which in turn testimonies the vital role played by ODH as combustible fuel to offer the desired porosity to the final product. The as-obtained porous structure offers beneficial effect, especially when used as an electrode material in battery applications, due to its ability to encourage the electrolyte percolation and to facilitate the rapid shuttling of diffusing sodium ions.


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Table 1 Comparison of XRD parameters Compound

Lattice Parameters (Å)

Space Group

Cell Volume (Å^3)

LiVP2O7

a=4.814, b=8.126, c=6.949

P21

257

NaVP2O7

a= 7.319, b=7.857, c=9.542

P21/c

514

Figure 2. (a and b) FESEM images of pristine LiVP2O7, (d and e) pristine Li0.5Na0.5VP2O7, (g and h) pristine NaVP2O7 and (c, f and i) EDS results of porous LiVP2O7, Li0.5Na0.5VP2O7 and NaVP2O7



Figure 3. a) FT-IR spectrum of LiVP2O7/C, Li0.5Na0.5VP2O7/C and NaVP2O7/C b) Raman spectrum of LiVP2O7/C, Li0.5Na0.5VP2O7/C and NaVP2O7/C c) TGA results of LiVP2O7/C, Li0.5Na0.5VP2O7/C and NaVP2O7/C and d) N2 adsorption/desorption isotherm and Inset: LiVP2O7/C pore size distribution curve It is also expected to enhances the electrode-electrolyte contact area and to improve the electrochemical performance of the electrode material, particularly at high rates, as per literature reports.22 Figure 2(d, e, g and h) indicates the presence of micron sized particles of Li0.5Na0.5VP2O7 and NaVP2O7 with irregular morphology, but with the desired porosity. Further, Energy Dispersive Spectroscopy (EDS) displayed in Figure 2(c, f and i) evidences the signature peaks corresponding to V, Na, P and O. Absence of impurities confirms the purity of as synthesized samples and substantiates the XRD results. 8 ACS Paragon Plus Environment

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Figure 3a shows Fourier Transform Infrared (FTIR) spectra of porous LiVP2O7/C, Li0.5Na0.5VP2O7/C and NaVP2O7/C compounds obtained using ODHAC method. In the infrared spectrum, the intra molecular vibration of VO6 unit is found to be in the mid-region, i.e., > 400 cm-1. Similarly, peaks located at 748 and 935 cm-1 are attributed to the symmetric and antisymmetric P-O-P vibrations corresponding to the presence of pyrophosphate group. The triplet at 637, 545 and 452 cm-1 is due to the VO related stretching frequencies and the O-P-O bending and P-O stretching vibrational frequencies associated with the PO4 groups are found in the 1000-1300 cm-1 region respectively. The currently observed results are consistent with similar category alkali pyrophosphate compounds. 25, 27 Raman spectroscopy, used generally to characterize sp2 bonded carbons has been exploited to gain more insights about the chosen pyrophosphates. Figure 3b shows the Raman spectrum of porous LiVP2O7/C, Li0.5Na0.5VP2O7/C and NaVP2O7/C composites wherein two peaks at 1337 (D-band) and 1608 cm-1 (G-band) are observed, which are matching with the peaks due to amorphous carbon

28

. Further,

ID/IG ratio of the composites (0.73, 0.75 and 0.79) has been calculated individually, which in turn evidences the slightly dominant sp2 character of super P amorphous carbon. TGA experiments were carried out to quantify the amount of carbon present in LiVP2O7/C, Li0.5Na0.5VP2O7/C and NaVP2O7/C composites and the results are shown in Figure 3c. There is only 5 wt. % weight loss observed until 400 °C, corresponding to the removal of physically adsorbed water and air.29-30 The subsequent weight loss observed in the 400-600 °C region results from the oxidation of carbon (to form CO2) present in the composites. Hence, the estimated amount of carbon in all the composites is 5 wt. %, which is exactly matching with the actual concentration of super P used to prepare AVP2O7 (Li, Li0.5Na0.5 and Na)/C composites.



Nitrogen adsorption/desorption isotherm was recorded in order to understand the porous structure and the surface area of LiVP2O7/C (Figure 3d), Li0.5Na0.5VP2O7/C and NaVP2O7/C composites and the results are appended in Figure S1. The recorded isotherm exhibits a type IV hysteresis loop at a relative pressure between 0.2 and 0.9, which is the characteristic property of mesoporous compound31 consisting of larger amount of mesopores (3-50 nm) (inset of Figure 3d) and lesser amount of macropores (50-180 nm).25 The Barrett-Joyner-Halenda (BJH) pore size distribution obtained from the isotherm of the present study suggests that the samples contain broadly distributed pores with sizes in the 3-180 nm region. Further, Brunauer-Emmett-Teller (BET) specific surface area and pore volume are estimated to be 20. 9, 16.4 and 10.0 m2 g-1 and 0.08, 0.06 and 0.02 cm3 g-1 for LiVP2O7/C Li0.5Na0.5VP2O7/C and NaVP2O7/C composites respectively, and the values are displayed in Table S1. The table evidences the surface area and pore volume associated with LiVP 2O7/C are appreciable in comparison with the rest of the composites and hence it is believed that LiVP2O7/C might demonstrate superior electrochemical performance amongst the chosen composites. The currently observed combination of meso and macro pores is reported to deliver short and low-resistant ion transport paths for Na+ ions through effective electrolyte percolation32-33 and therefore holds the promise for enhanced Na+ ion storage performance in SIBs. The high-resolution transmission electron microscopy (HR-TEM) images are shown in Figure 4. Presence of interconnected porous channels in the pristine LiVP 2O7 particles is visible from the HR-TEM images and the results are in line with the recorded FESEM images. Selected area electron diffraction (SAED) pattern shown in Figure 4c confirms the single-crystalline nature and indexed with the (111) and (011) planes of pristine LiVP2O7 product, obtained from ODHAC method. Further, Figure


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4d reveals the presence of lattice fringes with an inter-planar spacing of 0.402 nm, which is consistent with the (120) plane of monoclinic LiVP 2O7 phase. HR-TEM images, SAED pattern pertinent to Li0.5Na0.5VP2O7 and NaVP2O7 are shown in Figure S2, which evidences the porous and the crystalline nature of the compounds respectively. The spotted SAED pattern indicating its correlation with the (110), (022) and (022) planes indexed in XRD is visible from the inset of Figure S2 (a & b). Lattice fringes corresponding to the Li0.5Na0.5VP2O7 and NaVP2O7 (Figure S2 c and d) also endorse the correlation of d spacing calculated from the fringes with the indexed h, k, l values of XRD related to (011) (022) and (022) planes. Further, TEM based cumulative and individual elemental mapping results endorse the uniform distribution of V, P, O and C, as shown in Figure S3.

Figure 4. (a-b) HRTEM image of LiVP2O7 recorded at two different magnifications, c) SAED pattern of LiVP2O7 and d) lattice fringes related to LiVP2O7 XPS is recorded to confirm the oxidation state of the individual elements present in the LiVP2O7/C composite and the result is shown in Figure S4. The survey spectrum of LiVP2O7/C electrode contains signature peaks of V, P, O, and C, as shown in Figure 11 ACS Paragon Plus Environment


S4 a. Further, it endorses the binding energy values of V2p at 515 and 522 eV, corresponding to the V2p3/2 and V2p1/2 respectively, which is in favour of the presence of vanadium in the +3 oxidation state.25, 34 Similarly, the O1s spectrum consists of a peak at 530.33 eV, which is in favour of the O2-oxidation state of oxygen,35-36 Figure S4 c exhibits a peak at 284.35 eV, corresponding to that of C1s signal, which is in agreement with the literature values.37 Figure 5 illustrates the typical cyclic voltammogram (CV) of LiVP2O7/C, Li0.5Na0.5VP2O7/C and NaVP2O7/C composite electrodes, wherein CV was recorded in the potential range between 0.01-3.0 V under the influence of 0.05 mVs-1 condition. In Figure 5a, CV of the initial cycle is quite different from the subsequent cycles and the peak at 0.5 V is owing to the solid electrolyte interphase (SEI) layer formation.38 From the second cycle onwards, during cathodic scan, LiVP2O7/C anode displays a peak at 0.98 V due to the presence of Na+ insertion. During the anodic process, a peak at 1.72 V is seen, due to the de-insertion of Na+ ion from the monoclinic crystal structure of LiVP2O7/C electrode. The complete CV curve endorses the occurrence of perfectly reversible oxidation-reduction (V3+/V2+) process that leads to a stable structure. Similarly, CV behaviour of Li0.5Na0.5VP2O7/C and NaVP2O7/C electrodes is shown in Figure 5b and 5c. Herein, the oxidation-reduction peaks are not predominant, thereby indicating the inferior cycling reversibility of these pyrophosphates compared with that of LiVP2O7/C anode, which is believed to be due to the sluggish reaction kinetics resulting from the large ionic size of sodium ions, especially within the specified matrix. Galvanostatic discharge-charge cycling was carried out in the voltage range of 0.01 to 3.0 V, under 115 mA g-1 (1C rate) current density and the result is shown in Figure 5d. The porous LiVP2O7/C composite anode exhibits an acceptable initial


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discharge capacity of 350 mA h g-1 and a corresponding charge capacity of 160 mA h g-1 with a Coulombic efficiency of 46 %. The irreversible capacity loss of 54 % is due to the tapping of fraction of sodium from the electrode material. The solid electrolyte interface (SEI) formation and electrolyte decomposition triggered initial irreversible loss of sodium ion is responsible for the observed capacity loss in the first cycle, which is a commonly observed behaviour in sodium and lithium based electrode materials.39-40 However, an appreciable progressive capacity of 125 mA h g-1 is found to get continued up to 100 cycles, which is noteworthy.

Figure 5. a) Cyclic voltammogram of LiVP2O7/C, Cyclic voltammogram of Li0.5Na0.5VP2O7/C, c) Cyclic voltammogram of NaVP2O7/C, and d) Cycling performance of LiVP2O7/C, Li0.5Na0.5VP2O7/C,

and NaVP2O7/C anode up to 100

cycles under the influence of 1 C rate



On the contrary, Li0.5Na0.5VP2O7/C and NaVP2O7/C anodes deliver a nominal initial capacity of 110 and 100 mA h g-1, which upon prolonged cycles decreases to insignificant values, viz., to 10 and 5 mA h g -1 respectively. The currently observed rapid fade in the capacity upon cycling is likely to be due to factors such as change in crystal structure and /or morphology of the electroactive material upon cycling. With a view to understand the effect of possible change in the crystal structure of electrodes after completing 100 charge/discharge cycles, we have recorded the post cycled XRD of LiVP2O7/C, Li0.5Na0.5VP2O7/C and NaVP2O7/C anodes. (Figure 6a)

Figure 6. a) Ex-situ XRD of LiVP2O7/C,

and NaVP2O7/C cycled electrode b)

Voltage vs. capacity profile of LiVP2O7/C composite anode @ 115 mA g-1 condition, c) Composition vs. voltage profile of LiVP2O7/C (versus sodium) recorded at 1 C rate, and d) dQ/dV curve of LiVP2O7/C composite anode @ 115 mA g-1 current density


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From the Figure 6a, one can observe that LiVP2O7/C electrode maintains good crystallinity and structural stability even after the completion of 100 charge/discharge cycles whereas Li0.5Na0.5VP2O7/C and NaVP2O7/C electrodes, seem to undergo changes in terms of crystal structure and reduced crystallinity. This observation has further been substantiated from the view point of Density Functional Theory (DFT) calculations shown in Figure S5. With a view to understand the effect of sodiation in LiVP2O7 and NaVP2O7 matrices we have incorporated sodium in to the crystal structure of LiVP2O7 and NaVP2O7 individually. Interestingly, LiVP2O7 readily accepts one sodium per formula unit without showing any visible change in the structure. Because sodiation in LiVP2O7 is found to be thermodynamically feasible due to the weak binding energy value of 0.60 eV. On the other hand, it is very difficult to introduce even 0.25 sodium per formula unit in the NaVP2O7 matrix, as the structure does not allow excess sodium concentration due to the higher binding energy driven restrictions. As a result, perfect reversibility and maintenance of capacity pose difficulties with respect NaVP2O7 anode. The learning from DFT calculations infer that the structure of NaVP2O7 becomes less stable in the excess sodium environment. As a result, the disappearance of 100% intensity peak @ 2 =17 and 30˚ of NaVP2O7 indicating the change in the monoclinic crystal structure due to prolonged charge discharge process could be understood. Due to the said reasons, capacity fade is observed with respect to Li0.5Na0.5VP2O7/C and NaVP2O7/C anodes, as indicated by CV and charge-discharge studies. Considering all these results, we confirm that LiVP2O7/C falls under the category of intercalating anode and demonstrates appreciable performance in sodium ion battery applications. Based on the comparison of charge discharge cycling performance of LiVP2O7/C, Li0.5Na0.5VP2O7/C and NaVP2O7/C composites, we



conclude that LiVP2O7/C anode could be recommended for sodium battery applications. As a result, further detailed studies to understand the electrochemical behaviour have been restricted to LiVP2O7/C anode only. Charge/discharge profile displayed in Figure 6b reveals the electrochemical stability of LiVP2O7/C composite anode upon cycling. The observed initial discharge/charge capacity values are 350 and 160 mA h g-1 respectively, followed by 140, 135, 130 and 125 mA h g-1 at the end of 10, 50 and100 cycles. Figure 6c indicates the number of sodium ion participated in the charge/discharge process. Participation of ∼1 sodium ion per formula unit is noticeable from the figure, which in turn demonstrates the superior electrochemical performance exhibited by LiVP2O7/C composite anode.

Figure 7. a) Charge-discharge performance of porous LiVP2O7/C composite anode up to 1000 cycle, b) Rate capability test of LiVP2O7/C anode and c) Impedance performance of the as fabricated cell consisting of LiVP2O7/C anode and the cell after completing 1000 cycles


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Figure 6d shows the derivative curves (dQ/dV) vs. cell voltage. This graph exhibits the distinct presence of single CV peak, corresponding to the (V 3+/2+ redox couple) reversible shuttling of Na+ ion obtained from/into the LiVP2O7/C host matrix. This peak matches well with the CV results. Based on these encouraging results, the suitability of LiVP2O7/C anode for extended cycle life has been checked especially under the influence of 115 mA g-1 (1 C rate)

up to 1000 cycles (Figure 7a).

Interestingly, LiVP2O7/C anode exhibits an average progressive capacity of 90 mA h g-1 at the end of 1000 cycles. Subsequent to such a demonstration of suitability of LiVP2O7/C anode for better cycleability, it was subjected further to rate capability test, (Figure 7b). Herein, acceptable specific capacity values of 118, 97, 70, 58, 50, and 40 mA h g-1 have been observed after 15, 30, 40, 50, 100 and 150 cycles corresponding to the current density values of 230 (2 C), 345 (3 C), 575 (5 C), 1150 (10 C), 2300 (20 C) and 4600 (40 C) mA g-1 respectively. Notably, the title anode, even after tested under a high current density of 4600 mA g-1 (40 C rate) is capable of exhibiting an appreciable capacity of 90 mA h g-1 when switched back to 2 C rate condition, thereby evidencing the better retention of capacity (Figure 7b). Hence, the suitability of currently synthesized LiVP2O7/C anode for high rate sodium battery applications has been demonstrated. Further, to substantiate our results, we have compared the electrochemical performance of LiVP2O7/C with few other intercalation anodes of SIBs, which is furnished in Table 2. From the table, it is obvious that LiVP2O7/C of the current study exhibits superior electrochemical behaviour as SIB anode.



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Table 2. Comparison of electrochemical performance of LiVP2O7/C with similar category anodes exhibiting intercalation/deintercalation mechanism in SIBs

Compounds

Capacity/current/ Cycles (mAh g -1/mA g -1/n)

LiVP2O7/C

125/115/100

Cycling stability& rate Capacity/ Current/Cycles (mAh g-1/mA g -1/n)

Ref.

This work 90/115/1000 50/2300/100 40/4600/50

Na2/3Ni1/6Mg1/6Ti2/3O2

80/9.6/100

---

41

Na2.65Ti3.35Fe0.65O9

110/40/100

----

42

NaAlTi3O3

62/25/100

65/250

43

Na2Ti3O7

125/35.4/50

71/885

44

Na3V2(PO4)3/C

136/12/50

103/117/5

45

Na2Ti3O7/C

75/100/200

50/200/10

46

Na3V2(PO4)3/HHC

95/50/100

50/1000/5

47

Li4Ti5O12

62/350/24

----

9

Na3V2(PO4)3/C

170/20/30

60/2000/3000

48


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Figure 8. Ex-situ XRD pattern of LiVP2O7/C composite anode a) As-fabricated cell, b) After discharged to 0.01 V and c) After the completion of 1000 cycles Electrochemical impedance (EIS) measurements were recorded by applying an AC voltage of 5 mV amplitude in the 100 kHz to 10 mHz frequency range to understand the Na+ ion transfer behaviour in LiVP2O7/C composite electrode. The Nyquist plots shown in Figure 7c indicates the results of EIS analysis and fitted with an equivalent circuit model of the as fabricated cell and the one after 1000 cycles (charged to 3.0 V vs. Na+/Na) respectively. The proposed equivalent circuit model for the currently observed impedance spectra (inset of Figure 7c) involves Rs that represents the combination of electrolyte solution resistance and ohmic resistance of the cell components. Rf and Rct indicate the resistance pertinent to solid electrolyte interface (SEI) film and the charge transfer resistance of the electrochemical reaction, respectively. Further, parameters such as surface-passivating layer capacitance (Cf), double layer capacitance (Cdl,), and the diffusion controlled Warburg impedance (Zw) involved in the equivalent circuit model endorse the observed electrochemical



behavior. As expected, a semicircle and an inclined straight line in the high and low frequency region are observed. The high frequency region intersect is associated with the electrolyte resistance (Rs) and the semicircle in the medium frequency region corresponds to the charge transfer resistance (Rct). The observed Warburg (Zw) behaviour indicates the involvement of diffusion controlled intercalation behaviour of sodium ions in the chosen LiVP2O7/C anode. Before cycling, the electrolyte resistance and charge transfer resistance values are found to be 27 and 950 Ω, respectively, which are further reduced to 26 and 430 Ω after 1000 cycles. The observed decrease in the charge-transfer resistance value can be attributed to the reversible sharing of more number of effective Na+ ions and electron transfer taking place at the interface of the electrolyte and active material. As a result, LiVP2O7/C anode exhibits enhanced electrode reaction kinetics and ensures better cycling behaviour of the cell.

Figure 9. (a-c) Ex-situ TEM image with different magnifications and d) SAED pattern of LiVP2O7/C composite anode captured after completing 1000 cycles With a view to understand and to exhibit the structural integrity and stability of the crystalline and porous LiVP2O7/C anode upon extended cycles, ex-situ XRD 20 ACS Paragon Plus Environment

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analysis was implemented for the electrodes under different conditions, viz., asfabricated electrode, after completing one discharge, and the electrode after completing 1000 cycles. The results obtained are given in Figure 8. All the diffractions peaks exactly match with the monoclinic phase without any impurities, signifying the desired and perfect maintenance of structural stability during the prolonged cycles. As a sequel to post cycled XRD, it is much more intriguing to investigate the post cycled morphology changes using TEM results. From Figure 9, it is confirmed that the ex-situ TEM images recorded after completing 1000 cycles evidence the retention of surface morphology and structural integrity. Further, SAED pattern (Figure 9d) evidences the combination of crystalline (LiVP2O7/C) and amorphous nature of carbon coating found on the surface of LiVP2O7. While the ring pattern corresponds to the presence of amorphous carbon coating, then the indication of crystalline LiVP2O7 is substantiated by correlating the indexed spots with the h, k, l plane of the XRD pattern of LiVP2O7. Hence, the ex-situ XRD and TEM analyses substantiate the structural integrity and absence of morphology changes pertinent to LiVP2O7/C active material upon extended cycling, viz., 1000 cycles.

4. CONCLUSIONS In summary, AVP2O7/C (A= Li, Li0.5Na0.5 and Na) containing interconnected 3D porous channels with honey bee hive morphology, made out of micron sized particles has been prepared by a simple and cost effective solution combustion method. The currently observed combination of mesopores and micropores is found to be advantageous in ensuring short and low-resistant ion transport paths for the diffusion of Na+ ions through effective electrolyte percolation and hence holds promise for Na+ ion storage performance in SIBs. Among the chosen pyrophosphates,



LiVP2O7/C demonstrates better electrochemical performance than Li0.5Na0.5VP2O7/C and NaVP2O7/C anodes. The ex-situ XRD results reveal that LiVP2O7/C electrode is bestowed with good structural stability and exhibits good reversibility. Further, LiVP2O7/C anode exhibits a stable reversible capacity of 125 mA h g-1 at 1 C rate (115 mA g-1) up to 100 cycles. Similarly, when subjected to 1 C rate for 1000 cycles, a nominal capacity of 90 mA h g-1 is observed with a coulombic efficiency of 99 %. The LiVP2O7/C anode validates its suitability for high capacity and high rate applications by way of displaying significant capacity values such as 118, 97, 70, 58, 50 and 40 mA h g−1, under the influence of 2, 3, 5, 10, 20 and 40 C rates, respectively. Even after tolerating a high current rate of 40 C, LiVP2O7/C anode is capable of exhibiting 90 mA h g-1 capacity, when switched back to 2C rate condition. Such a high rate (40 C) tolerance capability and extended cycleability (1000 cycles) qualify LiVP2O7/C as a superior electrode amongst AVP2O7/C family anodes. This study reveals the possibility of exploiting LiVP2O7/C as yet another potential insertion anode in rechargeable sodium-ion batteries.

ASSOCIATED CONTENT Supporting Information: The Supporting Information is available free of charge on the ACS Publications website. Electrode preparation and coin cell assembly, physical and electrochemical chemical characterization, BET, HR-TEM, elemental mapping and XPS. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]


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Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT DST-SERB, New Delhi is acknowledged for financial support through GAP-14/16 Projects respectively. V. M. is thankful to NUS for financial support through India research initiative (NUS-IRI) fund and Council of Scientific and Industrial Research (CSIR), India for providing Senior Research Fellowship (SRF). The authors thank Dr. P. Murugan and Dr. J. Karthikeyan FMD Division, CSIR-CECRI for DFT Calculations.

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TOC Graphic


C (A= Li, Li0.5Na0.5 and Na) for High Rate

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