Elsevier

Computational Materials Science

Volume 94, November 2014, Pages 214-217
Computational Materials Science

Comparative computational study of the energetics of Li, Na, and Mg storage in amorphous and crystalline silicon

https://doi.org/10.1016/j.commatsci.2014.04.010Get rights and content

Highlights

  • Ab initio comparative study of Li, Na, Mg storage in amorphous (a-Si) and crystalline Si (c-Si).

  • Binding energy of Li, Na, Mg stronger in a-Si than in c-Si.

  • Similar volumetric energy densities of a-Si and c-Si anodes.

  • Preamorphization could enable non-Li atom storage in Si.

Abstract

To assess the potential of amorphous Si (a-Si) as an anode for Li, Na, and Mg-ion batteries, the energetics of Li, Na, and Mg atoms in a-Si are computed from first-principles and compared to those in crystalline Si (c-Si). It is shown that Si preamorphization increases the average anode voltage and slightly reduces the volume expansion of the anode during the insertion of the metal atoms. Analysis of computed formation energies of Li, Na, and Mg defects in a-Si and c-Si suggests that the insertion energetics of single atoms into a-Si are thermodynamically more favorable. For instance, the lowest defect formation energies of Li, Na, and Mg defects in a-Si are respectively 0.43, 1.42, and 1.52 eV lower compared to those in c-Si. Moreover, the defect formation energies of Li, Na, and Mg defects (vs. vacuum reference states) in a-Si are comparable with the metal cohesive energies and consequently the insertion of the metal atoms might be possible with appropriate control of charging process. This is in contrast to c-Si, where the storage of Na and Mg atoms is limited due to high energy cost of Na and Mg insertion into c-Si.

Introduction

The development of high energy density and/or high-rate electrochemical batteries is the key to sustainable development, as they will enable large-scale storage of electricity derived from intermittent sources (such as wind and solar). The size and geographic distribution of Li resources indicate that Li-ion batteries alone, which are the most widely used metal-ion batteries today, cannot satisfy the increased needs in energy storage [1], [2]. Therefore, interest in non-Li-ion batteries is increasing. Among the alternative technologies, Na and Mg-ion batteries are considered as the most attractive. So far these batteries have shown worse performance compared to that of Li-ion batteries [3], [4], [5]. Nevertheless, since Na and Mg resources are abundant, the use of both Na and Mg-ion batteries is attractive from the economic point of view.

Recent investigations of cathode materials for both Na [6], [7], [8] and Mg-ion [9], [10], [11] batteries reported promising results. This puts the onus on the development of other battery components including electrolytes and negative electrodes, to achieve commercialization. Specifically, the design of anode materials for these batteries is a big challenge. Similar to metallic Li anodes, the use of metallic Na [1] and Mg [5], [12] is disadvantageous because of dendrite formation and undesired reactions with electrolyte species (especially for Mg-ion batteries), leading to capacity fading as well as safety problems. Replacing metallic Mg with an insertion/intercalation type anode materials would allow a degree of control over the insertion voltage that could mitigate this problem. Hence, the development of insertion/intercalation type anode materials for both Na and Mg-ion batteries is important. Unfortunately, anode materials which are attractive for Li-ion batteries are often not suitable for Na and Mg-ion batteries. As an illustration, it was reported that Na and Mg do not intercalate into graphite [13], [14], [15] and crystalline Si (c-Si) [16], [17]. The difference in the behavior of Li and Na/Mg atoms comes from both the sizes of the metal atoms and their electronegativities. Hence, design of alternative anode materials is needed and requires detailed theoretical and experimental studies.

Recently, it has been shown that Sn can be used as an attractive anode material for both Na [17], [18], [19] and Mg-ion [20], [21] batteries. However, Sn is significantly heavier compared to Si, therefore, the theoretical specific capacities of Sn-based materials are smaller compared to that of Si [3], [21]. Several experimental studies showed that preamorphization of an anode material can significantly improve (make the insertion of metal atoms more thermodynamically favorable) the energetics of inserted atoms [19], [22], and in some cases, it could enable reversible insertion/deinsertion of metal atoms. For instance, it has been shown experimentally that amorphous phosphorus [22] materials can be used as anode materials for Na-ion batteries. In contrast, the insertion process into the bulk crystalline materials is inhibited. Recently, Kaxiras’s group reported first-principles investigation of the behavior of Li atoms in c-Si and amorphous Si (a-Si), and they found that the preamorphization of Si leads to improved insertion energetics [23]. How does Si preamorphization change the energetics of inserted non-Li metal atoms? To the best of our knowledge, this question is still open. Therefore, in this paper, we present a comparative computational study of the energetics of Li, Na, and Mg atoms in c-Si and a-Si structures to explain how Si preamorphization changes the performance of anode materials for different metal-ion batteries.

Section snippets

Methods

All calculations were carried out using the density functional theory (DFT) [24] and the SIESTA code [25]. The Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional [26] and the double-polarized orbitals (DZP) basis set were used. The basis set was tuned to reproduce cohesive energies of Li, Na, Mg, and c-Si (see Supporting Information for details). A cutoff of 100 Ry was used for the Fourier expansion of the density, and Brillouin-zone integrations were done with a 3 × 3 × 3 k-point

Voltages and volumetric energy densities

a-Si is a metastable structure under normal conditions. The computed cohesive energy of a-Si is 0.13 eV per atom larger (a-Si is less stable) than that of c-Si. This indicates that an a-Si anode will have a larger average anode voltage compared to that of c-Si. For instance, if we assume that the charging of the anode material goes according to Eq. (1)Si+xMSiMx,then, using the well-defined methodology (see Supporting Information) [4], [31], the average anode voltages (V) vs. metallic reference

Conclusions

In conclusion, using first principle calculations, we have studied the energetics of Li, Na, and Mg atoms in a-Si and c-Si. We found that Si preamorphization increases the average anode voltage (by up to 0.12 eV for Na) and reduces the anode volume expansion (by up to 10% for Li3.75Si). Despite this, at the same volume expansion of the anode material, metal-ion batteries with c-Si and a-Si will have the same volumetric energy densities. We found that Si preamorphization reduces the defect

Acknowledgment

This work was supported by the Tier 1 AcRF Grant by the Ministry of Education of Singapore (R-265-000-430-133).

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