Electrochemical Lithium Storage Performance of Molten Salt Derived V2SnC MAX Phase

Small size V2SnC MAX phase was prepared by the molten salt method. V2SnC MAX phase electrode is able to deliver high gravimetric capacity up to 490 mAh g−1 and volumetric capacity of 570 mAh cm−3 A charge storage mechanism with V2C-Li redox and Sn–Li alloying dual reactions was proposed Supplementary Information The online version contains supplementary material available at 10.1007/s40820-021-00684-6.


Introduction
The MAX phases are hexagonal ternary metal carbides and/ or nitrides with a general formula M n+1 AX n , where M is a transition metal, A is primarily a group 13-16 element, X is carbon and/or nitrogen, and n is typically 1, 2, or 3 [1]. MAX phases (including solid solutions and ordered quaternary phases) were synthesized by common synthesis methods such as hot pressing (HP) and spark plasma sintering (SPS) [2]. Compared to HP and SPS, molten salt method (MSM) is a simple and cost-effective route for preparing MAX phase powders, which has attracted broaden appeal in recent years. Molten salt synthesis is a modification of the powder metallurgical method by adding low melting point salts to the reactants for synthesizing at temperature above the melting point of the salts, where molten salts were used as the solvents and/or reacting species [3]. As a high-temperature ionic solvent, the molten salt bath offers high solvation power and liquid environment for reactants that facilitate the mass transport and nucleation processes. The synthesis temperature and time could be reduced to obtain more fine and uniform particles [4]. Some MAX phases (e.g., Cr 2 AlC, Ti 3 SiC 2 , V 2 AlC, Ti 3 AlC 2 , Ti 2 AlN) have been synthesized by molten salt method with small size at relatively low temperatures [3,[5][6][7][8].
Lithium-ion batteries (LIBs) as one of the key electrochemical energy storage power sources have witnessed tremendous progress with many energy storage materials are developed [9][10][11]. Considering the laminated structure, metallic conductivity, MAX phases are potentially excellent lithium storage hosts. Xu et al. [12] reported on the reversible electrochemical intercalation of Li + ions into Ti 2 SC and Ti 3 SiC 2 MAX phases. By reducing the particle size, the Ti 2 SC MAX phase delivered an initial capacity of 80 mAh g -1 (0.4 A g -1 ), increasing to 180 mAh g -1 after 1000 cycles. Chen et al. [13] demonstrated that the partially etched Ti 3 AlC 2 MAX phase is found to have a much higher capacity (160 mAh g −1 at 1C) than its corresponding Ti 3 C 2 T x -MXene thanks to the extra capacity originating from the formation of Li-Al alloy. An oxygen doped Ti 3 SiC 2 MAX was also reported with an initial capacity of 70 mAh g −1 at 1 A g −1 and subsequent increase to 180 mAh g −1 after 3000 cycles [14]. Metallic Sn, as one promising alloy anode, had attracted broaden appeal due to its high theoretical capacity [15][16][17]. Zhao et al. [18] reported the Sn-MAX phase (Nb 2 SnC) with a capacity of 234 mAh g −1 at 0.05 A g −1 and identified the additional capacity contribution as the Sn-Li alloying reaction. Therefore, these few works dedicated to investigating MAX phases as lithium storage hosts attest that MAX phases (especially Sn-MAX) show promise as electrode materials for Li-ion batteries (LIBs).
In 2020, a new V 2 SnC MAX phase was first reported by Xu et al. [19] via solid phase reaction of V, Sn, and C mixtures. Thus, further investigation on synthesizing this new V 2 SnC MAX phase in a facile way and exploring its potential application as electrode materials for Li-ion batteries is with great significance. In this work, we demonstrate that the V 2 SnC MAX phase particles can be prepared by the molten salt method. The prepared V 2 SnC MAX phase with uniform and refined particle size exhibit high lithium storage capacity with high-rate and excellent cyclability performance as lithium storage anode.

Preparation of V 2 SnC
The powders were mixed in a stoichiometric ratio of V:Sn:C = 2:1.1:1 (The melting point of Sn was relatively low, we increased the content of tin because of the weight loss of tin at a high-temperature, as in the preparation of V 2 (Sn,A)C MAX phases) [20]. The starting powders of V, Sn, and graphite are mixed with inorganic salt (NaCl + KCl). After grounding for 10 min, mixture powders were put into an aluminum oxide boat, and then the alumina boat was put into a tube furnace and heated to 1000 °C during 3 h with a heating rate of 10 °C min −1 under an argon atmosphere, 1 3 respectively. After the end of the reaction, the product is washed, filtered, and dried at 40 °C in vacuum.

Characterizations and Measurements
The phase composition of the samples was analyzed by X-ray diffraction (XRD, D8 Advance, Bruker AXS, Germany) with Cu Kα radiation. X-ray diffractograms were collected at a step size of 0.02° 2θ with a collection time of 1 s per step. The microstructure and chemical composition were observed by scanning electron microscopy (SEM, QUANTA 250 FEG, FEI, USA) equipped with an energy-dispersive spectrometer (EDS), and the EDS values were fitted by XPP (extended Puchou/Pichoir). Structural and chemical analysis was carried out by high-resolution high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM), and lattice resolved energy-dispersive X-ray spectroscopy (EDS) using the Linköping double corrected FEI G2 Titan 3 60-300 microscope operated at 300 kV, and STEM-EDS was recorded with the embedded high sensitivity Super-X EDX detector. The Rietveld refinement of powder XRD pattern of V 2 SnC was by Total Pattern Solution (TOPAS-Academic V6) software.

Electrochemical Tests
The electrochemical tests were carried out on CR2032 coin-type cells. The MAX phase electrodes were prepared by mixing 90 wt% active materials, 5 wt% acetylene black, and 5 wt% PVDF in N-methylpyrrolidinone (NMP), and the slurry mixture was then coated on Cu foil with a mass loading of 1.13 mg cm −2 . After coating, the electrodes were dried at 60 °C for 30 min to remove the solvent. The electrodes were then cut into disks with a diameter of 12 mm, and vacuum dried at 100 °C for 12 h and weighed before assembly. All cells were assembled in Argon atmosphere glovebox with oxygen and water content less than 0.01 ppm. The lithium metal foil was used as counter and reference electrode and 1 M Li-PF6 in EC: DMC (volume 1:1) solvent as the electrolyte, glass fiber membrane as the separator. Cyclic voltammetry and impedance spectroscopy tests were carried out using a Metrohm AutoLab M204 electrochemical workstation, and the charge-discharge experiments were performed on a Neware battery test system.

Theoretical Calculations
The theoretical calculations are performed in the framework of density functional theory with projector augmented wave method by implementing the Vienna Ab-initio Simulation Package (VASP). The generalized gradient approximation (GGA) proposed by Perdew, Burke and Ernzerhof (PBE) is selected as the exchange-correlation potential with the DFT-D3 van der Waals correction. Brillouin zone integrations are performed on 3 × 3 × 1 k-meshes for geometry optimization and electronic structure calculations. The cutoff energy of plane wave basis is set to 500 eV, and the Kohn-Sham equations is set to 10 -5 eV which make the results reliable. To simulate the real surface circumstance, a 2 × 2 supercell of MAX phases with six layers is built to investigate the adsorption of Li and vacuum layer of 2-nm-thickness is found to be sufficient to avoid unphysical interaction between images due to the periodic boundary conditions.

Characterization of V 2 SnC MAX Phase
The XRD patterns were shown in Fig. 1a (blue line). The characteristic peaks typical of M 2 AX phases located at 2θ ≈ 13°, 2θ ≈ 26°, and 2θ ≈ 41° indicate the synthesis of V 2 SnC is synthesized. In addition, small mass fraction of Sn metal and VC x can be identified. In comparison with experimental result, the simulated XRD pattern of V 2 SnC (red line) coincides well with the experimentally measured pattern, which further supports the phase identification of V 2 SnC MAX by molten salt method. Moreover, the Rietveld refinement of the XRD pattern of V 2 SnC is shown in Fig. S1, the reliability factor is R wp = 9.54% indicates the good agreement between fitting results and measured data. A small amount (2.39 wt%) of Sn metal was determined. The lattice parameters of the V 2 SnC are a = 2.98 Å and c = 13.46 Å. The atomic positions of the V 2 SnC determined form the Rietveld refinement are listed in Table S1. Figure 1b is an SEM image of the obtained V 2 SnC powders. The V 2 SnC particles are plate-like, with a smooth surface that is similar to Ti 3 SiC 2 synthesized by molten salt method [6]. The powder particles with length and thickness less than 0.2 µm are agglomerated in clusters of about 10 µm. As previously reported [12], the submicrometer size of the prepared MAX phase thin flakes may benefit the electrochemical performance as the LIB anode. EDS analysis (Fig. 1c) confirms the presence of V, Sn, and C elements, and the relative atomic composition of V 2 Sn 0.98 C 0.93 , that is, very close to the composition of M 2 AC (211) type MAX phases. In addition, the energy-dispersive X-ray spectrometry (EDS) elemental mapping for V 2 SnC in Fig. 1d indicates a homogeneous distribution of V, Sn, and C elements.
To further determine the crystal structure and elemental sites of the V 2 SnC MAX phase, the material was further identified by high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and lattice resolved energy-dispersive X-ray spectroscopy (EDS). STEM images of the V 2 SnC phase along the [11 Fig. 2a, b, respectively. Along the vertical direction of both images, it can be observed that single layers of atomic columns (the A layers) are interleaved by two adjacent layers of dark atomic columns (the M layers). Carbon is typically not visible because of its weak electron scattering nature as compared to the heavier M and A atoms [21]. Except for the different relative brightness between elements, owing to mass-enhanced contrast, the structure is equivalent to the configuration of other M 2 AX phases and exhibit the characteristic zig-zag stacking of M n+1 X n slabs along [11  zone axes, which was also observed for other MAX phases [22][23][24]. Furthermore, the STEM-EDS elemetal maps analysis in Fig. 2c indicates only V (red) and Sn (green) in the prepared MAX phase (C is not feasible to map out), suggesting that the M and A sites consist exclusively of V and Sn, respectively, which is consistent with XRD results, indicates that the 211 type V 2 SnC MAX phase is synthesized. Figure 2d further confirms this observation by integrating the elemental map in Fig. 2c to a line-scan of the V -K α and Sn -K α EDS peaks. Therefore, we have established the successful preparation of the V 2 SnC MAX phase by the molten salt method.

Electrochemical Performance of V 2 SnC MAX Phase
To investigate the electrochemical behavior, the V 2 SnC working electrodes were prepared with addition of 5 wt% acetylene black and 5 wt% PVDF, and lithium metal foil was used counter and reference electrode. Figure 3a shows the cyclic voltammetry profiles of the V 2 SnC MAX phase electrode at a scan rate of 0.1 mV s −1 within potential range from 0.01 to 3 V versus Li/Li + . During the first cathodic process, the irreversible capacity below 1.0 V versus Li/ Li + is observed for the first cycle that can be explained by the irreversible contribution of the solid electrolyte interface (SEI) layer formation as previously reported [18]. Galvano charge-discharge measurements at current densities ranging from 0.05 to 5 A g −1 were carried out after cycling tests. As presented in Fig. 3b, c, the V 2 SnC electrode shows the feasibility to deliver a maximum reversible capacity of 490 mAh g −1 at 0.05 A g −1 , corresponding to a reversible uptake of 4.25 Li + per V 2 SnC unit, which is much higher than the previous reported results [18]. It should be noted that the electrochemical performance of the V 2 SnC was obtained with an electrode contains that only 5 wt% acetylene black while previous reported studies were investigated with at least 10 wt% of conducting carbon [12,18]. That is, compared to other lithium storage anodes such as silicon or metal oxides [25][26][27], a lower amount of conducting carbon is needed for the metallic conducting V 2 SnC electrode. The decrease of conducting carbon may be important for increasing the volumetric capacity considering the low density of carbon as compare to MAX phases. A maximum volumetric capacity of 570 mAh cm −3 was achieved with a 10-um-thick electrode. In addition, the capacity retains 100 mAh g −1 at 5 A g −1 , indicating the high-power capability of the electrode. The high capacity and high-power could be ascribed to the small particle size (few hundreds nanometers, Fig. S3) of molten salt derived V 2 SnC MAX phase after ball milling treatment. The smaller size of the MAX phase particles enables more Sn atoms to be exposed to the electrolyte and induce alloy formation with the lithium ions. To test this hypothesis, microsize V 2 SnC particle (tens of micrometers) without ball milling was investigated for comparison. XRD patterns (Fig. S3a) of V 2 SnC before and after ball milling confirm that the samples retain the V 2 SnC phase after ball milling, but the peak intensities decreases and the beaks are broader, indicating the reduction of particle size after ball milling [28,29], which is confirmed by SEM (Fig. S3b, c) and particle size analyzer test (Fig. S3e). Figure S4 presents the electrochemical characterization of micro size V 2 SnC particle. Cyclic voltametric profiles are the same as the nano size V 2 SnC particle after ball milling as presented in Fig. 3a, however, a maximum capacity of 210 mAh g −1 (Fig. S4c) is much lower than the electrode with nano size V 2 SnC particle, which highlights the importance of decreasing particle size to achieve high electrochemical performance. The Galvano charge-discharge (GCD) cycling profiles in Fig. 3d with a current density of 1 A g −1 demonstrate the increase of the capacity from 100 mAh g −1 at the 100th cycle to 260 mAh g −1 at 1000th cycle. The deceasing size of MAX phases particles that causes by Sn-Li (de)alloying reaction during (de)lithiation as described by Zhao et al. [18] is believed to be responsible for the increase of the capacity during cycling. The maximum capacity of 280 mAh g −1 was achieved at the 800th cycle, however, further cycling tests witness a slightly capacity decrease that may cause by the abscission of Sn atom at the particle edge after repeating (de)alloying reaction.

Charge Storage Mechanism of V 2 SnC Electrode
To further confirm our speculations on the charge storage mechanism, operando and ex-situ XRD investigations of V 2 SnC electrode during charge-discharge process were performed. As shown in Figs. 4a and S5, the XRD diffraction peaks of V 2 SnC MAX phase do not show noticeable change during several charge-discharge cycles, evidencing the stable crystal structure of V 2 SnC MAX phase without phase transition or evolution of lattice constant [30]. This is consistent with the previous study of Nb 2 SnC explained by the Li-Sn alloying only at the edges of Nb 2 SnC particles, whereas Sn in the bulk was not involved in the redox process [18]. The residual Sn metal may contribute to the Li + storage capacity, however, the low content of 2.39 wt% results in limited Li + storage capacity contribution [16]. Figures S6-S8 present SEM, TEM images and EDS mapping results the V 2 SnC particles after electrochemical cycling tests. V, Sn, C elements are well and evenly distributed on the particle. The high-resolution TEM (HRTEM) image shows clear lattice fringe spacing of 0.67 nm (Fig. S8d), corresponding to the (002) lattice plane of V 2 SnC, further confirming the stable structure of V 2 SnC and agreeing well with the proposed charge storage mechanism, indicating the outstanding stability of V 2 SnC electrode during cycling [31]. Such a charge storage mechanism is quite different from the typical charge storage mechanism of battery electrodes with phase transition or lattice constant change. It is believed that the stable crystal structure without phase transition or lattice constant evolution is the origin of the cycling stability and high-rate performance of V 2 SnC electrode. The edge Sn alloy reaction mechanism explains that the lithium storage capacity of Snbased MAX phase could be further increase by reducing the particle size and exposing more Sn atom to the electrolyte.
To gain insight into the charge storage mechanism from the atomic scale, DFT calculations were performed. In order to evaluate the lithium storage mechanism of V 2 SnC, the preferred adsorption sites of Li ions have been calculated on the 2 × 2 × 1 supercell surface, as shown in Fig. 4b-e. The adsorption energies E ad on the top sites of C 1 (out-plane C), C 2 (second-plane C), Sn, and V are −0.754, −0.38, −0.448, and −1.255 eV, respectively. Noticeable, the Li adsorbed only in second-plane Sn and V indicating the potential energy surface is in line with the geometric construction. It is the special zig-zag edge structure that makes the adsorptions more preferable. To investigate the limitation of adsorptions, we locate the Li ions on the top of such sites, and the results show that the first-plane V will trap all the Li on the top of C due to the strong interaction between the Li and V. Besides, the Sn prefer to protruding out of surface which makes energic favorable adsorption.
To gain a deeper insight into the diverse adsorption behaviors, Bader charge and charge redistribution were performed. Compared to the inside of V 2 SnC, the change of Bader charge in surface elements shows the interaction between the host materials and guest cation. V 2 C layer of V 2 SnC gains charge ranges from 0.022 to 0.029 |e| with one Li cation adsorbed on V 2 C layer (C 1 , C 2 , V sites). On the other hand, Sn gains 0.026 |e| when one Li cation adsorbed on Sn adsorption site. These results indicate that the V 2 C-Li and Sn-Li reactions are both preferable to occur and contribute to the redox capacity. The top and side views of charge redistribution from C 1 , C 2 , Sn, and V at the same isosurface value (6 × 10 -3 electrons bohr −3 ) are shown in Fig. 4c-e. The yellow/blue color represents the charge accumulation/depletion, and the electron cloud represents the degree of interaction. It is obvious that a strong interaction between the Li and Sn, V, suggesting the dual redox reaction of V 2 C-Li and Sn-Li as speculated in the previous discussion (Table 1) Fig. 4 a Openrando XRD patterns of V 2 SnC electrode during the first three cycles. Charge redistributions due to the interaction with Li on b C 1 (out-plane C), c C 2 (second-plane C), d Sn, and e V adsorption sites. Yellow/blue color represents the charge accumulation/depletion, where the isosurfaces refer to an isovalue of 6 × 10 -3 eletrons/bohr 3 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.