Weixin
Song
a,
Xiaobo
Ji
*a,
Zhengping
Wu
a,
Yirong
Zhu
a,
Yingchang
Yang
a,
Jun
Chen
a,
Mingjun
Jing
a,
Fangqian
Li
a and
Craig E.
Banks
*b
aKey Laboratory of Resources Chemistry of Nonferrous Metals, Ministry of Education, College of Chemistry and Chemical Engineering, Central South University, Changsha, 410083, China. E-mail: xji@csu.edu.cn; Fax: +(86)731 88879616
bFaculty of Science and Engineering, School of Science and the Environment, Division of Chemistry and Environmental Science, Manchester Metropolitan University, Chester Street, Manchester M1 5GD, Lancs, UK. E-mail: c.banks@mmu.ac.uk; Fax: +44(0)1612476831
First published on 5th March 2014
Ion occupation and migration pathways are investigated to explore the ion-migration mechanism of Na3V2(PO4)3 with the help of first principles calculations. Na3V2(PO4)3 with a NASICON framework generates high performances as a cathode material in sodium-ion batteries.
Notably, the number of migrated Na ions from the Na sites of Na3V2(PO4)3 should directly influence the capacity performances,8,20 though there are multiple possibilities for the occupation of the interstitial sites in this compound referred to the reported literature. Consequently it is of significant importance to the field to explore the ion migrated mechanism and the probable ion pathways, which would be of great importance to understand electrochemical behaviours of Na3V2(PO4)3.
In this work, Na3V2(PO4)3 has been synthesized by a novel solution-based carbothermal reduction (S-CTR) method and used as a cathode material to construct Na3V2(PO4)3/NaClO4/Na sodium-ion batteries for investigation. Additionally, Density Functional Theory (DFT) calculations on Na3V2(PO4)3 have been performed to obtain insight into the structural characteristics and formation energy along the three-dimensional (3D) migration paths in conjunction with the crystal structure (for Experimental sections, see the ESI†); this is the first report on a dual experimental and mechanistic approach. This work is of great significance and of guidance to those working in and constructing such batteries (both industry and academia) to understand the 3D character for ion transport by exploration of the internal diffusion ways of Na3V2(PO4)3 combined the experiments with first principles calculations.
Fig. 1 displays the powder X-ray diffraction pattern (XRD) of the as-prepared Na3V2(PO4)3 as well as the XRD calculated by DFT. All the diffraction peaks of the observed XRD can be indexed to the rhombohedral NASICON structure with the Rc space group (2 Na in the 18e position; 1 Na in the 6b position), which are in good agreement with previous literature12,18,31 and JCPDS card no. 53-0018. The lattice parameters are a = b = 8.72 Å and c = 21.764 Å, and these sharp peaks of the experimental results match the DFT calculations very well including the peak positions and intensities, for which they could indicate the good crystallinity of the as-prepared Na3V2(PO4)3 when the simulated models were optimized based on the first principles calculations.18
Fig. 2 shows the Na3V2(PO4)3 crystal structure with a NASICON framework. The octahedral VO6 interlinks via corners with tetrahedral PO4 to establish a three-dimensional (3D) V2(PO4)3 framework which is interconnected through PO4 with the neighboring units. These created lantern units could feature a highly covalent open structure that generates large interstitial spaces through which Na ions can diffuse. Two different oxygen-environment interstitial sites, Na(1) (one position per formula unit) and Na(2) (three positions per formula unit), in this rhombohedral form are usually filled (fully or partially) by mobile Na ions of which the Na(1) site has six fold coordination situated between two adjacent V2(PO4)3 units along the z axis and the Na(2) site has eight fold coordination located at the same z value as the phosphorus atoms between two PO4 tetrahedra. Houria17 and Soo et al.7 have reported that if all of the Na(1) and Na(2) sites were occupied by Na ions, four cations could be totally hosted in the voids/channels per formula unit of Na4V2(PO4)3, one for the Na(1) site and three for Na(2) sites. However to the best of our knowledge, very few literature reports have published the theoretical investigation towards the ion occupations in the Na sites of Na3V2(PO4)3. According to the sparse literature, a high theoretical capacity of 236 mA h g−1 is reported to be capable to be delivered with a corresponding extraction of 4 Na ions per formula unit which depends on the applied potential range and the variation between V3+/4+ and V3+/2+ redox states.5 However, as a consequence of the unstable state of V2+ in Na4V2(PO4)3, Na3V2(PO4)3 is more favourable to be synthesized with the relatively stable state of V3+, while it is possible to generate V2+ from Na3V2(PO4)3 by electrochemical reduction methods.8 Accompanied by the redox reactions of Na3V2(PO4)3, the ion migration would take place where the ion number could play an important role in the capacity performances. By comparison with the bond populations between Na and O atoms (Table S1–S6†), it was found that the Na ions at Na(2) sites can be extracted more easily than those at Na(1) sites due to the smaller bond populations of Na(2) sites which are associated with the relatively weak limited environments, and thus the transported behaviours of ions at Na(2) sites should be responsible for the exhibited electrochemical properties.
In order to explore the ion-migrated mechanism of Na3V2(PO4)3, studies on ion occupations are of significance though only Soo et al.7 have defined the ion occupations that one Na ion occupies the Na(1) site (1 occupancy) and two Na ions occupy the Na(2) site (0.67 occupancy) in the crystal structure of Na3V2(PO4)3. According to their conclusions, this could be used to demonstrate the extracted ion number of nearly two from Na(2) sites, but correspond to a primary unit cell of Na3.01V2(PO4)3 when six Na(1)-site and two Na(2)-site ions exist in the DFT model, the same as those used in this work. However, the calculated results based on the atom coordinates of Na3V2(PO4)3 and NaV2(PO4)3 originating from Landolt–Börnstein Database in Springer Materials (Table S7 and S8†) seem successful to explain the ion-migration mechanism for Na3V2(PO4)3 with the scheme of possible ion migration shown in Fig. 3. The ion occupations with 0.75 in Fig. 3a are capable of contributing to the primary cell of Na3V2(PO4)3 which involves eight ions in the DFT model, two ions for Na(1) sites and six for Na(2) sites. This arrangement would change to one-Na-extracted Na2V2(PO4)3 with 0.5 occupation for all Na ions as shown in Fig. 3b after a structural reorganization and for the further extraction, the six Na ions at Na(2) sites would be migrated to produce a configuration leaving two Na(1)-site ions with 0.5 occupation as shown in Fig. 3c. In this way, it is reasonable to explain the theoretical capacity of 117 mA h g−1 (ref. 5 and 7) corresponding to the extraction of two ions which are generated by a two-phase reaction from Na3V2(PO4)3 to NaV2(PO4)3.
Fig. 4 shows a typical cyclic voltammogram (CV) of a Na3V2(PO4)3 sodium-ion battery at a scan rate of 0.2 mV s−1, from which the obvious redox peaks are due to the extraction/insertion of two Na ions during the phase transformation by the V3+/4+ reaction. These small peaks circled in red could be attributed to the structural reorganization related to the change of ion occupations. Moreover, the hysteretic voltage (ΔV) between the two redox peaks is 0.66 V of which the minimal value could show good reversibility of Na3V2(PO4)3 due to its open 3D framework. It is interesting to explore the ion migrated pathways in this NASICON structure since it is helpful to understand the ion-transport characteristics.
The simulation methods based on the first principles calculations can be used to enhance the comprehension of migration pathways by evaluating the activation energies for various possible mechanisms at the atomic level. Three main migrated mechanisms are considered within this 3D NASICON structure involving conventional vacancy hopping between neighbouring Na positions similar to the previous discussion.32Fig. 5a shows mechanism A where a Na ion would migrate through the channel between two PO4 tetrahedra along the x direction and Fig. 5b displays mechanism B where a Na ion could pass through the voids between a PO4 tetrahedron and a VO6 octahedron along the y direction. Migration energies (E) for these mechanisms can be calculated along the diffusion path, and it is found that the values of E for mechanisms A and B are similar to 0.0904 and 0.11774 eV, respectively. While for mechanism C where the ions migrate across the channels between adjacent octahedra along the z direction, the corresponding high migration energy (>200 eV) shows that this method is not an appropriate diffusion path. However, when the ions are transported in a curved pathway to bypass the octahedron and go into the voids/channels between the adjacent PO4 tetrahedron and VO6 octahedron, the calculated E with ca. 2.438 eV would demonstrate that this curved course in Fig. 5c may be also feasible for ion migration. Examination of the calculated results has revealed two favoured pathways along the x and y directions and one possible curved route for ion migration in this NASICON structure, which could be utilized to confirm the 3D transport characteristic of Na3V2(PO4)3. Here, this conclusion is consistent with the literature, but in this work it has been expounded with the combination of DFT calculations with experimental results. Reasonably, this open framework with 3D dimensions for ion transport would contribute to a fast chemical diffusion and probably be able to achieve a high rate capability.
Fig. 5 Possible Na ion migration paths in Na3V2(PO4)3 along (a) x, (b) y and (c) curved z directions. |
Fig. 6a depicts the CV curves of the sodium-ion battery with Na3V2(PO4)3 at different scan rates of 0.1, 0.2, 0.5, 0.8 and 1 mV s−1 in a voltage range of 2–4.6 V vs. Na+/Na, respectively. One couple of redox CV peaks still exists, even at a high scan rate, but the peak differences seem to augment with the increment of scan rate which indicates an enlarged irreversibility at high current densities. Furthermore, the linear relationship between the peak current ip and the square root of the scan rate v1/2 as presented in Fig. 6b illustrates that the whole electrode reaction would be favoured to be a diffusion-controllable process, while the chemical diffusion of Na ions DNa+ could be determined using the Randles–Sevcik equation (eqn (1)).
ip/m = 0.4463(F3/RT)1/2n3/2AD1/2Cv1/2 | (1) |
Fig. 6 (a) CV curves at different scan rates of Na3V2(PO4)3 in a voltage range of 2–4.6 V vs. Na+/Na and (b) the relationship between the square root of the scan rate v1/2 and peak current ip. |
The first charge/discharge profiles of Na3V2(PO4)3 sodium-ion batteries at different current densities of 0.1C, 0.2C, 0.5C and 1C (note that 1C refers to two Na extraction from the crystal structure per formula unit in 1 h) are displayed in Fig. 7a, with corresponding specific capacities of 113, 107, 99 and 84 mA h g−1, respectively, in a voltage range of 2–4.6 V vs. Na+/Na. It could be found that the corresponding Coulombic efficiencies are as high as 97.4%, 98.1%, 98.4% and 99.5%, respectively, of which the efficiencies would also be ascribed to the characteristic of the NASICON framework. Additionally, an electrode polarization could reach 0.03 V for this Na3V2(PO4)3 sodium-ion battery under a current density of 0.1C, and this lower value when compared with the result of Jian's work (0.07 V) could be a result of the good electronic and ionic conduction.7,9Fig. 7b exhibits the C-rate and cycling performances of the Na3V2(PO4)3 sodium-ion battery. For the 50th cycle, the specific capacity is 107 mA h g−1 with a capacity retention of 95% related to the initial capacity at 0.1C, and the corresponding Coulombic efficiency is also as high as 95.6% when the battery was cycled for 50 times in a sequence of C-rates. All these electrochemical properties demonstrate that Na3V2(PO4)3 could be a promising cathode material for utilisation in sodium-ion batteries.
Footnote |
† Electronic supplementary information (ESI) available: Experimental sections and the calculated results. See DOI: 10.1039/c4ta00230j |
This journal is © The Royal Society of Chemistry 2014 |