Nucleation and Conversion Transformations of the Transition Metal

Jun 3, 2019 - While in the conversion reaction, V acted as the redox center, ... by the projector-augmented wave(30) potentials with a plane-wave cuto...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 22307−22313

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Nucleation and Conversion Transformations of the Transition Metal Polysulfide VS4 in Lithium-Ion Batteries Ruqian Lian,† Jianrui Feng,‡ Dashuai Wang,† Qifeng Yang,† Dongxiao Kan,† Muhammad Mamoor,† Gang Chen,† and Yingjin Wei*,† †

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Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education), College of Physics, Jilin University, Changchun 130012, China ‡ Department of Chemistry, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: Transition metal polysulfides with high S content, such as VS4, TiS4, and MoS3, have high specific Li+ capacities, but their reaction mechanisms for lithium-ion batteries remain unclear due to unknown intermediate products. In this work, first-principles calculations based on the density functional theory were performed to reveal the electrochemical properties of VS4 for lithium-ion batteries. The results demonstrated multiple phase transformations during Li+ insertion, starting with nucleation transformation from VS4 to Li3VS4 and followed by gradual decomposition reactions. Enthalpy-driven long-range migration of Li2S molecules resulted in crystalline to amorphous transformation during decomposition. S and V successively behaved as redox centers for LixVS4 before and after x = 3. Moreover, low activation energy and high Li+ diffusivity were observed at room temperature, revealing superior rate capability of the material. KEYWORDS: transition metal polysulfide, vanadium tetra-sulfide, structure prediction, conversion reaction, structure transformation

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mA·h·g−1 was retained. The electrochemical performance was further improved by different groups via preparation of VS4/ graphene composites and using conductive polymer coatings.21−23 The unique long-chain structure of VS4 composed of dimerized S22− primary units motivated intensive experimental research on its reaction mechanisms.24 The material is generally accepted to undergo a VS4 to Li3VS4 transition during initial lithiation, followed by a conversion reaction forming V and Li2S. Zhang et al. have shown that the twophase transition and conversion reactions are partially reversible during the following charge.25 However, because of the low crystallinity of VS4 samples and amorphous nature of the intermediate products, the structural properties of lithiated VS4 have been difficult to reveal using spectroscopic techniques. To date, the material’s local structure evolution remains unclear as it converts from dimerized S22− in VS4 to the S2− in Li3VS4 and Li2S. In addition, the charge-transfer properties and ionic diffusion kinetics of VS4 have not been experimentally and theoretically clarified. Deep understanding of these questions relied on systematic prediction of the intermediate structures of lithiated VS4 and reasonable description of crystalline to amorphous transitions derived at the atomic scale.

INTRODUCTION The rapid growth in large-scale energy storage applications, such as electric vehicles and stationary grids, has motivated urgent research for high-capacity electrode materials for lithium-ion batteries (LIBs).1,2 To a large extent, the specific capacities of LIB materials depend on their reaction mechanisms. Intercalation materials maintain a stable framework during Li+ insertion that ensures good cycle reversibility.3−5 However, restricted transition metal redox (typically 1 e− per reaction) and limited Li+ accommodation sites result in a low specific capacity. Alloying- and conversion-type materials6−11 have larger theoretical capacities because their free volume and nontopotactic structural changes provide more sites for accommodating Li+. Various transition metal sulfides have been studied as conversion-type anode materials. For a given transition metal, the larger the S content, the larger the theoretical capacity. Inspired by this, transition metal polysulfides with high S content, such as VS4,12−14 TiS4,15,16 and MoS3,17 have attracted particular attention as highcapacity LIB anode materials. Current research on transition metal polysulfides has mainly focused on VS4. In the VS4 structure, S22− dimers are connected to two adjacent V4+ cations, forming a linearchain structure.18,19 VS4 was first studied as a LIB anode by Shin et al.,20 which showed a specific capacity of 1105 mA·h· g−1 at 0.1 C rate with 95% capacity retention after 100 cycles. With the current rate increase to 4.5 C, a large capacity of 766 © 2019 American Chemical Society

Received: March 5, 2019 Accepted: June 3, 2019 Published: June 3, 2019 22307

DOI: 10.1021/acsami.9b03975 ACS Appl. Mater. Interfaces 2019, 11, 22307−22313

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Vertical- and lateral-view of the linear-chain structure of VS4. The numbers 1, 2, 3 denote the different channels. (b) Predicted structures of LixVS4 (1 ≤ x ≤ 3). conjunction with first-principles calculations. About 10 000 Li−V−S structures were generated by USPEX. Each generation compared the enthalpy of 35 configurations. The 50% energetically worst structures were discarded and a new generation was created from the remaining structures through heredity and mutation of structures. The best structure of a generation was carried over into the next generation. The ground state was determined after a 25-generation screen. Simulation of Amorphous States. The “melt-and-quench” technique35−37 was employed based on AIMD simulation to obtain model structures of the amorphous phase. A suitable supercell was employed for each phase to ensure lattice constants along each axis >10 Å. First, appropriate supercells started with crystalline character were established to minimize long-range ordering. To obtain randomized structures, these supercells were annealed at high temperatures (3000 K) for 300 ps, which were enough to eliminate the memory effects from the initial configuration. Then, the final structures after annealing were rapidly quenched from 3000 to 300 K at a rate of 0.6 K/fs; the time step was set at 1 fs. The obtained structures were further optimized by DFT calculations to obtain the amorphous structures and their total energies. Diffusion Kinetics Properties. AIMD simulations were performed to examine the Li diffusion properties in VS4. All structures were assigned an initial temperature of 100 K in AIMD simulations. Then, the temperature was elevated to desired temperatures, including 300, 400, 600, and 800 K, over 2000 time steps in 2 ps and then equilibrated at the equilibrium temperature for 5000 time steps in 5 ps. Statistical analysis of the AIMD simulations was performed for 40 ps. A suitable supercell was employed in this section to ensure lattice constants along each axis >10 Å.

In this study, detailed first-principles calculations were performed within the density functional theory (DFT) framework to investigate the structural transitions, redox reactions, and diffusion kinetics of VS4 during lithiation. According to previous experimental findings,23 the lithiation process of VS4 was divided into two parts before and after the composition reached Li3VS4, corresponding to the two-phase reaction and conversion reaction, respectively. Our DFT calculations showed anionic redox of S during the two-phase reaction. A peculiar nucleation from VS4 to Li3VS4 was defined in this part for the first time. While in the conversion reaction, V acted as the redox center, resulting in the conversion from V5+ to V0+. The structural evolution and the formation mechanism of amorphous phase during the conversion process were elucidated based on the calculated formation enthalpy. Finally, the facile Li conduction in LixVS4 was evaluated by ab initio molecular dynamics (AIMD), resulting in promising high rate capability.

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COMPUTATIONAL METHODS

First-Principles Calculations. First-principles calculations were performed in the framework of DFT using the Vienna ab initio simulation package (VASP)26,27 with the generalized gradient approximation (GGA)28,29 in the scheme proposed by the Perdew− Burke−Ernzerhof. Ion−electron interactions were described by the projector-augmented wave30 potentials with a plane-wave cutoff energy of 550 eV. K-spacing was set to 0.4 for all structures to allow the smallest spacing between k-points in units of 0.4 Å−1. All atoms were allowed to move with a convergence threshold of 0.01 eV·Å−1 per atom in force and 10−6 eV in energy. In addition, a newly developed DFT-D3 method was incorporated to accurately simulate the van der Waals correction. Considering the strong correlation in V elements, the GGA plus Hubbard U (GGA + U) method with Ueff = 3.25 eV31 was used in all calculations. Structure Prediction. Bond length and electron density analysis of S−S bonds suggested that VS4 linear chain structures were broken after 1.5 Li+ intercalations, and thus, it became unreasonable to add further Li atoms into the linear chain framework. Therefore, structural predictions for further Li insertion and possible products were performed using the evolutionary algorithm in USPEX32−34 in

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RESULTS AND DISCUSSION The structure of pristine VS4 was obtained from the Material Project Database (https://www.materialsproject.org/ materials/mp-541155) with the C2/c space group. The structure of VS4 contained three nonequivalent channels for Li+ accommodation (Figure 1a). Each channel possessed four possible Li+ intercalation sites based on the repeating unit (V4S16). Therefore, initial VS4 lithiation was simulated by adding Li+ into the different possible sites, which was a general and effective method for obtaining reasonable structures in different concentrations.38−41 The ground state structures were 22308

DOI: 10.1021/acsami.9b03975 ACS Appl. Mater. Interfaces 2019, 11, 22307−22313

Research Article

ACS Applied Materials & Interfaces

Figure 2. (a) Average Li+ binding energy and S−S bond length in LixVS4. (b,c) Structure and electron density distribution of VS4 and Li1.5VS4, respectively. Li, V, and S atoms are denoted by green, red, and yellow sphere, respectively. (d) Schematic diagram for the nucleation of Li3VS4 in VS4 framework.

determined after comparing the energies for Li+ interstitials at various positions within the VS4 structure. Based on the obtained ground state of LixVS4 (Figure 1b), the average Li+ binding energy, which reflected the ability of LixVS4 for Li+ incorporation, was calculated by the equation ΔE(x ̅ ) =

Then, structural predictions for LixVS4 (x > 1.5) were performed using the evolutionary algorithm in USPEX. The predicted structures were much different from the linear chain structure of VS4. The average S−S bond length was >3 Å at high lithiation content. The numbers of S−S bonds in the predicted Li2VS4 and Li2.5VS4 structures were less than 2 due to linear chain fragmentation. In addition, with the lithiation content of LixVS4 increased to x = 0.5, 1, 1.5, 2, 2.5, and 3, the average Bader charge of S decreased from −0.33 (VS4) to −0.44, −0.53, −0.68, −0.78, −0.89, and −0.95 e, respectively. While the Bader charge of V was maintained at ∼+1.30 e. These indicated that the S−S bond was broken during 0 < x ≤ 3. After insertion of the first Li+, the unsaturated S with higher activity caused the aggregation of Li+. The Li+ binding energy stopped increasing at x = 3.0 (Figure 1b), which indicated that Li+ aggregation stopped in Li3VS4 when the linear chains were completely broken into separated VS4 tetrahedrons.42 According to the above analysis, Li+ intercalation into VS4 started with Li3VS4 nucleation (Figure 2d). Grey et al. showed that Li3VS4 was formed immediately after the electrode was lithiated to x = 0.66, which indicated that the electrode underwent a VS4 to Li3VS4 transformation during initial lithiation.24 This nucleation transformation most likely occurred at VS4 particle surfaces, where the unsaturated S−S bonds were more reactive for Li+ aggregation. As the formed Li3VS4 nuclei had higher Li+ binding energies than the surrounding VS4 phase, the subsequent intercalated Li+ aggregated at the Li3VS4/VS4 boundary, leading to Li3VS4 phase growth. The structural transformations of LixVS4 (x > 3) were also studied using USPEX (Figure S2). LixVS4 maintained a singlephase structure at x = 3 and 4, with the Li+ in Li3VS4 acting as a pillar between VS4 tetrahedrons. However, with further lithiation, strong Li−Li electrostatic repulsion among Li4VS4 structures changed the VS4 tetrahedron stacking. The singlephase structure was destroyed when the lithiation content increased to x = 5, with a portion of S observed to separate from VS4 tetrahedron, forming Li−S bonds with inserted Li+. To maintain local structural stability, VS4 tetrahedron

Ex1 − Ex2 − (x1 − x 2)E Li (x1 − x 2)

(1)

where Ex is the total energy of LixVS4 (x1 > x2). The average binding energy decreased in the lithiation range of 0 < x < 3, which indicated that intercalated Li+ tended to locally aggregate in the material. This Li+ aggregation phenomenon was verified by manual insertion of two Li atoms into a 2 × 2 × 2 VS4 supercell to form different configurations. In one case, two Li atoms were aggregated in a single unit cell (Figure S1a), while, in other cases, Li atoms were dispersed in different unit cells (Figure S1b−h). Energy calculations showed that the aggregation model was indeed energetically favored (Figure S1i). To gain more insight into Li+ aggregation behavior, the next focus was on the structures of LixVS4, which showed the average S−S bond length of S22− dimers in LixVS4 (Figure 2a). The steep increase in bond length with Li+ insertion indicated that the bond strength of S22− dimers monotonously weakened. After 1.5 Li+ insertion, the S−S bond length increased to 2.52 Å, which was much larger than the initial bond length of 2.03 Å for pristine VS4. The breakage of S22− dimers ruled out topological Li+ intercalation in VS4, although a large cavity existed between VS4 chains. The electron density distribution for V−S bonds in VS4 chains was reduced from 0.24 (VS4) to 0.06 (Li1.5VS4) e/Å3 (Figures 2b,c), indicating linear chain breakage. In addition, the electron density between two S atoms in the adjacent chains increased from 0.06 (VS4) to 0.88 (Li1.5VS4) e/Å3, while the corresponding length between them shortened from 3.29 (VS4) to 2.13 Å (Li1.5VS4). This evidence suggested that the linear chain structure of VS4 was broken after 1.5 Li+ intercalation. 22309

DOI: 10.1021/acsami.9b03975 ACS Appl. Mater. Interfaces 2019, 11, 22307−22313

Research Article

ACS Applied Materials & Interfaces

Figure 3. (a) Formation energy of possible decomposition phases for Li+ inserted VS4. (b) Average Bader charge of V and S for LixVS4, the minimum (Δ) and maximum (∇) charges of S are indicated. (c) Relative energies for the possible crystalline and amorphous phases. The crystal LixVS4 phase is chosen as the reference state for all compositions. (d) Schematic diagram for the decomposition and amorphization transformations of LixVS4.

of S separated from VS(