Xi
Jiang
,
Liuqing
Yang
,
Bo
Ding‡
,
Baihua
Qu§
,
Ge
Ji
and
Jim Yang
Lee
*
Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore. E-mail: cheleejy@nus.edu.sg; Fax: +65 6779 1936; Tel: +65 6516 2899
First published on 26th August 2016
The increasing interest in Na-ion batteries is based on their lower projected cost relative to Li-ion batteries and hence are more economically viable for the large-scale storage of electrical energy. Similar to Li-ion batteries, the capacity of Na-ion batteries is cathode-limited. Na3V2(PO4)3 (NVP), a prevalent cathode candidate and one of the most stable Na-ion host materials, still exhibits capacity losses in prolonged cycling. We report herein a method which can improve the durability of NVP in extended use. This is done by using a carbon scaffold to constrain the movement of NVP during charge and discharge reactions. The procedure consists of the sol–gel synthesis of densely aligned dense NVP nanofibers under hydrothermal conditions, followed by sucrose infiltration into the interstices of these fibers to form an interdigitated carbon scaffold after calcination. The NVP-carbon nanocomposite fabricated as such shows ultra-stable cycling performance at very high C-rates, 99.9% capacity retention at 20C for more than 10000 cycles, thereby demonstrating the effectiveness of the materials design principles behind this modification strategy.
Na3V2(PO4)3 (NVP) is a prominent NIB cathode material that has been researched for some time. It features a flat voltage plateau at 3.4 V vs. Na+/Na and a theoretical capacity of 117 mA h g−1 based on the V4+/V3+ redox reaction.2 The research done to date has focused on overcoming the low electronic conductivity of NVP in order to improve its rate performance in battery applications. Nanosizing and carbon coating,3–11 which have been successfully used to address a similar material issue in LiFePO4,12–14 were also applied to NVP. By comparison the cyclability of NVP has received less attention and as such has not improved much over the years. For example, Saravanan et al.7 observed a 50% loss of the initial capacity after cycling at 40C for 30000 cycles. Fang et al.4 reported 54% capacity retention after 20000 cycles at 30C. These numbers imply that half of the initial capacity would be lost in less than two months of operation at these rates. This is also an indication that the NVP modification techniques developed to date have not been optimized for cycle stability.
Capacity loss upon cycling is commonly caused by irreversible side reactions with the electrolyte15 or the dissolution of the active material in the electrolyte.16 For NVP which operates in the moderate voltage window of 2.6 V to 3.8 V (narrower than that of LiFePO4), side reactions are less likely to occur in the common battery electrolyte.15 The stability of the NVP structure also categorically dismisses dissolution in the electrolyte as the major cause for capacity fading. Hence capacity loss is more likely caused by the deterioration of the electrical contact between the active material (NVP) and carbon conducting additive leading to an increase in cell internal resistance with time. The smaller the NVP nanoparticles, the more susceptible they are to the connectivity loss since there are fewer points of contact with the conducting additive and a minor shift in the particle location has a higher probability of losing electrical contact with the surrounding conductive material totally.
With these considerations we have developed a method which can effectively inhibit the movement of the active material by building an interdigitated carbon scaffold around it. A hydrothermal synthesis is first used to form the active material as high aspect ratio nanofibers (Scheme 1). The ease of high aspect-ratio particles to form arrayed structures is then used to restrain relative particle movement without compromising the nanoscale advantage. This is followed by sol–gel processing with a sucrose solution, and calcination thereafter, to form a connected conducting carbon network which not only facilitates electron transport but also further inhibits the movements of the NVP nanofibers in the bundled structure. The NVP–carbon nanocomposite fabricated as such displays ultra-stable cycling performance at high rates, 99.9% capacity retention at 20C for more than 10000 cycles, and validates the materials design principles behind this modification strategy.
Fig. 1 (a and b) TEM and SEM images of the as-synthesized NVP nanofibers; (c) carbon-coated secondary particles after calcination (C8) and (d) a magnified view of a typical secondary particle (C8). |
Two Na3V2(PO4)3/C samples with different carbon contents, denoted as C2 and C8 respectively, were produced by varying the amount of sucrose added to the NVP suspension (200 and 800 mg per 1824 mg Na3V2(PO4)3). They were used to measure the impact of carbon content on the rate performance and cycle life. Rietveld refinement of the XRD patterns was used to determine the product phase purity and the crystal structure of NVP. The XRD patterns in Fig. 2a could be indexed to a trigonal lattice in the rhombohedral Rc space group without any other crystalline impurity. The calculated lattice parameters in Table S1† correspond well with the results from a previous report.17 On the whole the XRD data confirm the synthesis of phase pure NASICON Na3V2(PO4)3. The carbon content was measured by TGA in flowing air. Fig. 2b shows that Na3V2(PO4)3/C is stable up to 330 °C, indicating that Na3V2(PO4)3 is safe to use as a sodium-ion cathode material up to a reasonably high temperature. Weight loss began at 330 °C and was maximum at about 460 °C. The carbon contents calculated from these measurements were 1.74 wt% for C2 and 6.95 wt% for C8. A separate measurement indicated that about 17 wt% of carbon was converted from the sucrose source under the prevailing preparation conditions. The Brunauer–Emmett–Teller (BET) surface area and the pore structure of C8 were measured by using the nitrogen adsorption–desorption isotherms. The results in Fig. S7a† were used to derive a surface area of 84.25 m2 g−1 and from Fig. S7b† a pore size distribution dominated by <2 nm micropores. The large surface area and microporous structure are beneficial to support an extensive electrode–electrolyte contact.
Fig. 2 (a) XRD pattern of C8 and its Rietveld refinement. (b) TGA profiles of C2 and C8 between 150° and 550°. |
The existence of an extensive interconnected carbon network in the composite was exposed after equilibrating C2 and C8 in 1 mol L−1 HCl solution for 3 days to etch away the Na3V2(PO4)3. The TEM images in Fig. 3a, c and b, d show the residual carbon skeletons in C2 and C8 respectively. The carbon coatings in both cases formed a consolidated network with a similar bundled aligned nanofiber appearance and size as the secondary NVP particles (even after the ultra-sonication in the preparation of TEM samples). The close-up TEM images in Fig. 3c and d confirm the majority of carbon as aligned nanotubes. Independent of the carbon content, the carbon nanofibers were all ∼4 nm in thickness. These observations indicate that the carbon coating filled not only the interstitial space between the neighboring NVP nanofibers but also the free volume in the stacked structure. This formed an interdigitated structure where the NVP nanofibers and the carbon coating were interlocked. Sucrose was essential to retaining the NVP stacked structure as it held the nanofibers in position and prevented their amalgamation during calcination. In the absence of sucrose, the nanofibers amalgamated during calcination to form irregularly shaped particles (Fig. S1†). The carbon content changed mostly the density of the carbon assembly but not the individual fiber-like carbon morphology. The low carbon content in C2 resulted in a looser assembly than in C8 (Fig. 3a–c). The close-up images in Fig. 3b and d confirm the dense packing of carbon surrounding the NVP fibers. The use of excess sucrose in C8 also resulted in the formation of some discrete carbon nanoparticles. These nanoparticles could provide extra electron transport pathways. The width of the carbon-encapsulated Na3V2(PO4)3 was around 12 nm, smaller than that of the nanofibers assembled by using sucrose (∼30 nm) before carbonization.
The electrochemical performance of NVP/C was evaluated in half cells using sodium metal as the counter electrode. Fig. 4a shows the cyclic voltammogram (CV) of C2 and C8 in the 2.6–3.8 V voltage window at 0.1 mV s−1. The sharper and more intense redox peaks in C8 (the sample with the higher carbon content) suggest a facile Na+ insertion/extraction process. The narrower separation between the oxidation and reduction peaks is yet another indication of a small electrode polarization. In contrast, the broader and more drawn out voltammetric response of C2 is typical of overall more sluggish kinetics. The oxidation peak at ∼3.45 V and the two distinct reduction peaks at ∼3.20 V and 3.33 V can be assigned to the de-intercalation/intercalation of Na+ as a result of the V4+/V3+ redox reaction.20 In particular the reduction peak at 3.2 V may be associated with the rearrangement in the local redox environment caused by the transfer from Na(1) to Na(2), as has been found in other NASICON Na3M2(PO4)3 (M: Sc, Cr, Fe) structured frameworks.21 In the constant current discharge/charge curves, a peak appeared at 3.2 V initially and shifted progressively towards 3.33 V before it disappeared completely after a few cycles. This was observed for both C2 and C8 samples (Fig. S2a and b†). Interestingly this phenomenon did not affect the cell capacity, as shown in Fig. S3.† There was no similar observation during oxidation. The first cycle efficiencies calculated from the charge and discharge curves in Fig. S4† are 95.3% for C2 and 94% for C8. The first cycle activation process in Fig. S2b† may have contributed to the slightly lower first cycle efficiency of C8.
Rate capability is another important material performance indicator for grid electricity storage. The rate capabilities of C2 and C8 as shown by the discharge curves of both samples at different C rates (1C = 117 mA g−1) are compared in Fig. 4b and c. The rate capability of pure NVP without the carbon scaffold was also measured (Fig. S6a†). The latter delivered ∼90 mA h g−1 at 0.2C and showed large polarization at higher C-rates. The specific capacity of C2 is ∼100 mA h g−1 at the 0.1C rate, indicating that the carbon coating on C2 is inadequate to activate all NVP to deliver the theoretical capacity of 117 mA h g−1. By comparison C8 could deliver 15% more capacity (114 mA h g−1) at twice the C-rate (0.2C). C8 distances even more from C2 in terms of performance at higher rates – delivering 76 mA h g−1 at 40C. Hence the performance of C2 is most likely limited by its low carbon content. The higher carbon content in C8 gave rise to a more extensive carbon surface coating of NVP; and the bridging of neighboring carbon coatings through intervening carbon nanoparticles also formed contiguous networks to facilitate electron and ion transport. Fig. 4d shows the specific capacities of C2 and C8 cycled at different C rates. C2 could only be used at rates lower than 20C while C8 could sustain a 40C operation. The capacity could be restored to its initial value even after cycling at different C rates, which is another indication of the good stability of the material.
The long-term cycling performance of C2 and C8 was also measured. Fig. 5 shows the results for C8 at the 1C, 2C, 5C and 20C rates. Impressively C8 showed very little capacity difference at these rates. At least 96.1% of the initial capacity was still available after 1000 cycles of operation at the 1C, 2C and 5C (Fig. 5a–c) rates; and 99.9% of the initial capacity was available after 10000 cycles at 20C (Fig. 5d). The average coulombic efficiency was close to 100%. It can therefore be concluded that the NVP/C made this way is highly cyclable. The good capacity retention performance represents a notable improvement over previous research in the open literature (Table 1). We have also tested the cells with an alternative electrolyte of 1 M NaClO4/EC:DEC at 10C. 99.6% of the initial capacity was retained after 1700 cycles (Fig. 6), and hence the different electrolytes used (1 M NaClO4/EC:DEC vs. 1 M NaClO4/PC:FEC) are not the reason for the performance difference. When a C2 sample was tested likewise at the 1C, 2C and 5C rates, Fig. S5† also shows a similar excellent capacity retention; albeit at lower specific capacities. The capacity loss of the NVP without the carbon scaffold was ∼10% at 5C (from 76 mA h g−1 to 68 mA h g−1). Hence the important function of the carbon scaffold to provide a high capacity retention and a long cycle life could be realized even with a low carbon content (1.74 wt%, C2). The combination of stacked raft-like nanofibers and an interdigitated carbon scaffold is therefore an effective means of preserving the active material and extracting the best performance from it. The random drops in efficiency in Fig. 5 and 6 could be caused by some temporary short-circuiting arising from microscopic Na dendrites which penetrated through the glass fiber separator.
Reference | Active material:carbon black:binder (by weight) | Carbon content | Electrolyte | Capacity retention |
---|---|---|---|---|
7 | 75:15:10 | 6 wt% | 1 M NaClO4/PC:EC (1:1 v/v) | 1000 cycles at 1C, 86.7%; 1000 cycles at 10C, 84.9%; 3500 cycles at 20C, 89.7%; 30000 cycles at 40C, 50% |
4 | 70:20:10 | 6.41 wt% | 1 M NaClO4/EC:DEC (1:1 v/v) | 10000 cycles at 20C, 70%; 20000 cycles at 30C, 54%; 10000 cycles at 50C, 57% |
5 | 80:10:10 | 12.5 wt% | 1 M NaClO4/PC | 700 cycles at 5C, 96.1% |
This work | 80:10:10 | 6.95 wt% | 1 M NaClO4/PC:FEC (95:5 v/v) | 1000 cycles at 1C, 96.1%; 1000 cycles at 2C, 97.5%; 1000 cycles at 5C, 101%; 1700 cycles at 10C, 99.62%; 10000 cycles at 20C, 99.9% |
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ta05030a |
‡ Present address: School of Materials Science and Engineering, Nanyang Technological University, Block N4.1, Nanyang Avenue, Singapore 639798, Singapore. |
§ Present address: Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, No. 422, Siming South Road, Xiamen 361005, PR China. |
This journal is © The Royal Society of Chemistry 2016 |