Pressure-Induced Stable Li5P for High-Performance Lithium-Ion

Article pubs.acs.org/JPCC

Pressure-Induced Stable Li5P for High-Performance Lithium-Ion Batteries Ziyuan Zhao,† Lulu Liu,† Tong Yu,† Guochun Yang,*,† and Aitor Bergara*,‡,§,∥ †

Centre for Advanced Optoelectronic Functional Materials Research and Laboratory for UV Light-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, Changchun 130024, China ‡ Departamento de Física de la Materia Condensada, Universidad del País Vasco-Euskal Herriko Unibertsitatea, UPV/EHU, 48080 Bilbao, Spain § Donostia International Physics Center (DIPC), 20018 Donostia, Spain ∥ Centro de Física de Materiales CFM, Centro Mixto CSIC-UPV/EHU, 20018 Donostia, Spain S Supporting Information *

ABSTRACT: Black phosphorus, the result of white P under high pressure, has received much attention as a promising anode material for Li-ion batteries (LIBs). However, the final product of lithiation, P63/mmc Li3P, is not satisfactory due to its poor conductivity. In this article we explore the highpressure phase diagram of the Li−P system through firstprinciples swarm-intelligence structural search and present two hitherto unknown stable Li-rich compounds, Fm-3m Li3P at 4.2 GPa and P6/mmm Li5P at 10.3 GPa. Metallic Li5P exhibits interesting structural features, including graphene-like Li layers and P-centered octadecahedrons, where P is 14-fold coordinated with Li. Interestingly, both compounds exhibit good dynamical and thermal stability properties at ambient pressure, and the theoretical capacity of P6/mmm Li5P reaches 4326 mAhg−1, the highest among the already known Li−P compounds. Additionally, their mechanical properties are also favorable for electrode materials. Our work represents a significant step toward the performance improvement of Li−P batteries and understanding Li−P compounds. practical LIB applications.9 On the other hand, black P (BP), the product of WP under high pressure,10 which shows a graphite-like layered structure, is the least reactive of its allotropes, practically nonflammable, and insoluble in most solvents. Therefore, BP has attracted great attention as an anode material for LIBs.9,11,12 Its final product of lithiation is P63/mmc Li3P, with a theoretical capacity of 2596 mAhg−1 and volume expansion, ΔV/V, of 203% relative to BP.13 However, P63/mmc Li3P is insulating and electrochemically inactive, restricting its cyclability and performance rate. In order to improve the reversibility and efficiency of charge/discharge processes, carbon (C) is usually incorporated into the P anode material.14−17 As a result, a high capacity (2355 mAhg−1) and cyclability (90% capacity retention over 100 cycles) are achieved.14 On the other hand, as it is well-known, pressure has become a very useful tool to discover new materials with excellent chemical and physical properties.18,19 Many unusual stoichiometric compounds that are not accessible at ambient conditions

1. INTRODUCTION Due to the depletion of nonrenewable resources, such as oil and coal, the development of alternative energy sources becomes rather urgent. Up to now, lithium-ion batteries (LIBs), one of the most successful renewable energy storages, have been widely used in portable electronics devices.1 However, the performance of LIBs cannot meet the demand of large-scale energy storage equipment.2−4 Overall, current anode materials have become one of the key factors restricting the performance improvement of LIBs. Specifically, graphite is commonly used as a commercial anode material of LIBs, although it suffers from a low specific capacity (372 mAhg−1) due to the limited intercalation sites available for Li ions in the host lattice.1,5,6 On the other hand, although silicon (Si) acting as an anode presents a much higher capacity (4200 mAhg−1),7 its volume changes by about 400% during the lithiation process,8 resulting in pulverization. As a consequence, finding an excellent anode material, with high capacity, good conductivity, small volume expansion, and long life, is still a huge challenge. Phosphorus (P) was proposed as an interesting alternative to Si or graphite anodes. However, white P (WP), its main allotrope, is volatile and unstable, with high safety risks in © XXXX American Chemical Society

Received: July 20, 2017 Revised: August 28, 2017 Published: September 13, 2017 A

DOI: 10.1021/acs.jpcc.7b07161 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C have been found under high pressure.20−25 In particular, light element compounds can be synthesized under high pressure and usually survive at ambient conditions.26,27 For example, a diamond-like cubic phase, c-BC3, synthesized at 39 GPa and 2200 K,26 remains stable and superhard at ambient pressure. Moreover, mechanical and chemical properties of materials can be further improved under pressure.28 For example, quenched high-pressure phases in LIBs (e.g., Li15Si4) exhibit better performances, as they provide more accessible voids during the lithiation/delithiation process.29,30 Whether Li−P compounds under high pressure share similar properties remains elusive, but it is a subject of high interest. In this article we have explored the high-pressure phase diagram of the Li−P system up to 20 GPa via the firstprinciples swarm structural search. According to our results, as pressure increases, stable P-rich compounds at ambient conditions become unstable, while Li-rich ones become stable. Here we present two hitherto unknown Li-rich stable phases, Fm-3m Li3P and P6/mmm Li5P, above 4.2 and 10.3 GPa, respectively. Although both exhibit excellent mechanical properties compared to the already known P63/mmc Li3P, metallic P6/mmm Li5P shows the highest theoretical capacity among all known Li−P compounds.

calculated at each considered pressure according to the equation below ΔH(LixPy) = [H(LixPy) − xH(Li) − yH(P)]/(x + y) (1)

where H = U + PV is the enthalpy of each composition, and ΔH is the enthalpy of formation per atom. Here, U, P, and V are the internal energy, pressure, and volume, respectively. Figure 1a shows the relative thermodynamic stability of the Li−

2. COMPUTATIONAL METHODS To search the best structural candidates of Li−P alloys under pressure, we have employed the swarm-intelligence-based CALYPSO structure prediction method, which can efficiently find the stable structures just depending on the given chemical compositions.31,32 The CALYPSO method has demonstrated its great success in predicting many new compounds.33−37 Structural optimizations and electronic calculations were performed in the framework of density functional theory (DFT) within the Perdew−Burke−Ernzerhof (PBE) of the generalized gradient approximation (GGA)38 as implemented in the VASP5.3 code.39 The electron−ion interaction is described by pseudopotentials built within the scalar relativistic projector-augmented wave (PAW)40 approximation with 3s23p3 valence electrons for P and 1s22s12p0 valence electrons for Li. A cutoff energy of 500 eV and Monkhorst−Pack k-meshes41 with a grid spacing of 2π × 0.025 Å−1 were used to yield a good convergence for the enthalpy. In order to determine the dynamical stability of the predicted structures, phonon calculations were performed by using the finite displacement approach as implemented in the Phonopy code.42 Thermal stabilities of the predicted Li3P and Li5P structures were analyzed doing molecular dynamics (MD) simulations. Bulk, shear, and Young’s moduli were further estimated using the Voigt−Reuss−Hill approximation.43 Detailed descriptions of structural predictions and computational details can be found in the Supporting Information.

Figure 1. Relative thermodynamic stability of the Li−P system at 0 K and different pressures (0, 10, and 20 GPa). (a) The calculated enthalpy difference per atom of Li−P compounds with respect to elemental Li and P solids. The thermodynamically stable compounds are shown by solid symbols, connected by the convex hull line (solid lines). The fcc structure of solid Li45 and Cmca (from 0 to 3.6 GPa) or Pm-3m (above 3.6 GPa) structures of solid P46 were used to calculate the formation enthalpies. (b) Pressure−composition phase diagram of the Li−P system.

P system at different pressures. The stable phases lie on the global stability line of the convex hull, whereas compounds lying on the dotted lines are metastable with respect to decomposition into other Li−P compounds or elemental Li and P solids. It is interesting to mention that the already known phases at ambient pressure (P212121 Li3P7, P21/c LiP, and P63/ mmc Li3P) were readily reproduced by our structural search,13 and the calculated structural parameters at ambient pressure are in excellent agreement with the experimental ones. For example, the calculated lattice parameters of Li3P (space group P63/mmc, 2 formula units per cell) a = b = 4.22 Å and c = 7.54 Å agree with the ambient pressure experimental values a = b = 4.22 Å and c = 7.55 Å.44 These results validate our methodology when applied to the Li−P system. As pressure increases P-rich compounds gradually deviate from the hull line. Specifically, while Li3P7 and LiP are stable at ambient conditions, they decompose into Li3P plus P under high pressure. Their enthalpy difference curves can be found in Figures S1a and S1b in the Supporting Information. In contrast, Li-rich Li−P compounds become more stable with increasing

3. RESULTS AND DISCUSSION In order to find high-capacity anode materials for Li−P batteries, we have mainly paid attention to Li-rich compounds. Herein, we have performed structural searches at 0 K and selected pressures of 0, 10, and 20 GPa of Li−P compounds with various LixPy (x = 1−7 and y = 1; x = 1, 3, 5, 7, 9, 11, 13 and y = 2; x = 1, 4, 5, 7, 8, 11 and y = 3; x = 1, 3, 5, 7, 9 and y = 4, and x = 3 and y = 7) compositions. The formation enthalpy (ΔH), relative to the elemental solids (Li and P), was B


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The Journal of Physical Chemistry C pressure. Interestingly, the Li-rich composition, Li5P, appears to be stable at 10.3 GPa and remains stable until 20 GPa. In addition, at high pressures several Li-rich compositions (Li7P2, Li11P3, Li4P, Li9P2, Li11P2, Li6P, Li13P2, and Li7P) are very close to the convex hull line. Notably, Li3P is the most stable stoichiometry in the whole pressure range considered. Under compression P63/mmc Li3P transforms into a cubic Fm-3m structure (Figures S1c and S1d in the Supporting Information). The pressure−composition phase diagram of the Li−P system is shown in Figure 1b. To determine the dynamical stability of the new pressure induced stable structures (Li3P and Li5P), we have calculated their phonon dispersion curves at 0 and 20 GPa. As illustrated in Figure S3 in the Supporting Information, their lattice dynamical stabilities, even at 0 GPa, are clearly evidenced by the absence of any imaginary frequency mode. This allows for the possibility of stabilizing them even at ambient pressure. In order to analyze their thermal stability, we have performed MD simulations at 300 K. Snapshots at the end of 10 ps simulation for both structures are shown in Figure S5 in the Supporting Information, which clearly indicates they keep the original configuration at 300 K. Thus, Fm-3m Li3P and P6/mmm Li5P might be recoverable under ambient conditions. Additionally, we have also confirmed the dynamical stabilities of other metastable Li-rich phases (Figure S4 in the Supporting Information). Pressure-induced stable Li3P crystallizes into a BiF3-type structure (space group Fm-3m, 4 formula units, Figure 2a). The basic building blocks of the structure are square Li cages with alternating P/Li atoms at the center (Figure 2b). Notably, at 0 GPa the distance between the nearest-neighbor P and Li atoms is 2.48 Å, which is just slightly longer than in P63/mmc Li3P (2.44 Å). On the other hand, the predicted Li5P phase stabilizes into a hexagonal structure (space group P6/mmm, 1 formula unit, Figure 2c). Each P atom has 14 nearest-neighbor Li atoms, forming a P−Li octadecahedron (Figure 2d). The octadecahedra are connected by sharing both corner and faces in the ab plane and vertexes along the c-axis (Figure S2b). Li atoms in this structure occupy two nonequivalent positions. Those in the 4h sites (0.333, 0.667, 0.2706) form graphene-like layered structures in the ab plane (Figure 2e), as the ones observed in Li2Au.24 The distance between the nearest adjacent Li atoms is 2.44 Å, which is much shorter than in bcc Li.45 These short Li− Li distances result in strong interatomic interactions, playing an important role in stabilizing the structure. The other Li atoms, at 1a (0.00, 0.00, 0.00) positions, symmetrically distribute on both sides of graphene-like layered structure along the c-axis. As a result, P6/mmm Li5P presents an ABAB-like layered structure. Interestingly, layered structures, in which Li ions can insert without a significant rearrangement of the original structure, have served as electrodes for rechargeable LIBs.47 Although at ambient condition P commonly holds less than a 6-fold coordination in its compounds,48−54 as far as we know, Li5P with P6/mmm structure presents the highest coordination (14-fold) for P, as is also the case of the metastable P-3m1 Li13P2. As hypercoordination is usually associated with hypervalence, we have determined the valence of P in this compound based on the Bader charge analysis. According to our calculations, each P atom obtains 2.68 electrons from surrounding Li atoms (averaging 0.54 electrons per Li atom), indicating that P shows its usual valence state of P3+. However, as is well-known, the Li atom easily loses the 2s electron, but we have calculated that on average just 0.54 electrons per Li

Figure 2. Structural features of Fm-3m Li3P and P6/mmm Li5P at 0 GPa. (a) Li3P in the Fm-3m structure. (b) Coordination environment of P and Li atoms in Li3P. (c) Li5P in the P6/mmm structure. (d) Coordination environment of the P atom in Li5P. (e) P6/mmm structure of Li5P viewed along the c-axis. In all the structures, green and pink spheres represent Li and P atoms, respectively.

atom are transferred to P; so, where are the other electrons going? Electron localization function (ELF) analysis indicates they are gathered at the interspaces between Li-atom layers (Figures 3e and 3f), supporting the graphene-like electride model for this compound (Figure 3f). In electride compounds interstitial electrons act as anions,55,56 so that there is a strong interaction between Li ions and interstitial electrons, allowing its structural stabilization. It is well-known that the electrochemical performance of LIBs strongly correlates with the conductivity of electrode materials.57,58 Therefore, the poor conductivity of P63/mmc Li3P (Figure 3a) limits its cyclability and performance rate. On the other hand, the predicted Fm-3m Li3P is a direct semiconductor with a band gap of 1.57 eV at 0 GPa (Figure 3b) which also presents poor conductivity. In order to analyze potential applications of P6/mmm Li5P, we have calculated its electronic band structures and projected density of states (PDOS). Notably, P6/mmm Li5P exhibits a metallic character C


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The Journal of Physical Chemistry C

3d), indicating a charge transfer occurs between them, which is consistent with the Bader charge analysis presented above. Another bottleneck for electrodes is the great damage and fracture due to their elastic softening.59 P anodes also present this problem. In order to analyze the mechanical properties of the compounds we are predicting in this article, we have calculated the bulk modulus (B), shear modulus (G), Young’s modulus (E), and Poisson’s ratio (v) of Fm-3m Li3P and P6/ mmm Li5P (Figure 4). For comparison, P63/mmc Li3P was also included. Intriguingly, the compounds we are predicting exhibit good mechanical properties at atmospheric pressure. Actually, Fm-3m Li3P presents better mechanical properties compared to the already known P63/mmc Li3P (Figure 4a), and the elastic constants of P6/mmm Li5P are comparable to those of P63/ mmc Li3P. Interestingly, the Poisson’s ratio of Fm-3m Li3P increases with pressure, being in the range of 0.168−0.178.60 In addition, pressure dependence of mechanical properties (e.g., B, G, and E) indicates that the considered compounds exhibit better mechanical performance under pressure. Volume expansion is also an important parameter in determining the performance of anode materials.61,62 In general, the smaller the volume expansion of an electrode material, the better the battery performance.61,63 The calculated volume expansions of the considered phases were shown in Table 1. In comparison with the original volume of BP, the volume expansion of Fm-3m Li3P increases by 122% at ambient pressure, which is much smaller than that of P63/mmc Li3P (174%). However, the coefficient of volume expansion of P6/ mmm Li5P exhibits a larger value relative to BP (303%). Overall, compared to the volume expansion (∼400%) of silicon electrode,64 the two predicted phases are acceptable. Moreover, the volume expansions of both Fm-3m Li3P and P6/mmm Li5P are further limited under high pressure and increase by 87% and 185% at 20 GPa, respectively. Overall, the application of pressure can efficiently improve the mechanical properties and reduce the volume expansion, which are beneficial for making a safer and long life battery. At last, open-circuit voltages (OCVs) of Fm-3m Li3P and P6/mmm Li5P were also calculated (Table

Figure 3. Electronic properties of P63/mmc Li3P, Fm-3m Li3P, and P6/ mmm Li5P. (a) Projected density of states (PDOS) of P63/mmc Li3P. (b) PDOS of Fm-3m Li3P. (c) Electronic band structures of P6/mmm Li5P. (d) PDOS of P6/mmm Li5P. Calculated ELF for Li5P on the (110) (e) and (001) planes (f) in the P6/mmm structure. For clarity, Li atoms, located at the red dots, are deleted in Figure 3f (see also Figure S6 in the Supporting Information).

(Figure 3c), which is required for a Li-ion battery.57 Its metallicity mainly comes from the contribution of P 3p orbitals. The analysis of PDOS below the Fermi level shows that there is a strong overlap between P 3p and Li 2s and 2p orbitals (Figure

Figure 4. Bulk modulus (B), shear modulus (G), Young’s modulus (E), and Poisson’s ratio (v) of P63/mmc Li3P, Fm-3m Li3P, and P6/mmm Li5P between 0 and 20 GPa. D


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(FIS2016-76617-P) and the Department of Education, Universities and Research of the Basque Government and the University of the Basque Country (IT756-13).

Table 1. Calculated Volume Change and Theoretical Capacity of P63/mmc Li3P, Fm-3m Li3P, and P6/mmm Li5P Relative to BP

phase P63/mmc Li3P Fm-3m Li3P P6/mmm Li5P

pressure (GPa)

colume per P atom (Å3)

volume change (%)

theoretical capacity (mAhg−1)

opencircuit voltage (V)

0

58.19

174%

2596

0.93

0

47.25

122%

2596

0.78

20 0

39.75 85.55

87% 303%

− 4326

− 0.48

20

60.48

185%

−

−

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1). Their OCVs are 0.78 and 0.48 V at ambient pressure, which are much lower than that (0.93 V) of P63/mmc Li3P.13

4. CONCLUSION In summary, we have analyzed Li−P compounds by employing an unbiased structural search and found two novel phases, Fm3m Li3P and P6/mmm Li5P, are stable above 4.2 and 10.3 GPa, respectively. Analyzing their dynamical and thermal stability, we have concluded they might be quenching recoverable to ambient conditions. Li5P is metallic and exhibits interesting mechanical and structural features, including graphene-like Li layers and P-centered octadecahedrons (14-fold coordinated with Li). The theoretical capacity of P6/mmm Li5P reaches 4326 mAhg−1, the highest among the already known Li−P compounds. Compared with P63/mmc Li3P, the predicted Fm3m Li3P phase exhibits a smaller volume expansion at both ambient and high pressure. These results might provide some useful information for optimizing the performance of the P anode material. Our work is also important for understanding the phase diagram and physical and chemical properties of Li− P binary compounds.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b07161. Computational details, enthalpy difference curves of stable Li−P phases, phonon dispersion curves, and MD simulation snapshots of Li3P and Li5P at 300 K (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Guochun Yang: 0000-0003-3083-472X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Natural Science Foundation of China under No. 21573037, the Postdoctoral Science Foundation of China under grant 2013M541283, and the Natural Science Foundation of Jilin Province (20150101042JC). A.B. acknowledges financial support from the Spanish Ministry of Economy and Competitiveness E


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