Computational investigation of silicene/nickel anode for lithium-ion battery

Highlights

• A computer model of the functioning of a silicene anode for LIB is presented.

• The anode element was a two-layer silicene on a Ni(111) substrate.

• The influence of defects on the stability of the silicene channel was investigated.

• Monovacancies in silicene stimulate better filling of the channel with lithium.

• The greatest shear stress is achieved in silicene sheets with trivacancies.

Abstract

Silicon-based anodes are distinguished by an exceptionally high energy capacity, which is an order of magnitude superior to that of a graphite anode. The thin-film design of the anode creates the conditions for increasing the specific energy, energy density and power density of LIB. Bilayer silicene on a nickel substrate is the anode material that can increase LIB performance. In the present work, using the molecular dynamic method, a detailed analysis of the packings of lithium atoms in silicene channels with different types of walls is performed. The occupancy of the channels with lithium depends on the type of defects present in its walls. Lithiation and delithiation are carried out in the presence of a constant electric field. The type of substrate affects the packing of lithium atoms in the channel and consequently on battery capacity and performance. The maximum filling of the lithium channel is achieved when monovacancies are present in silicene sheets. It is shown that the most preferable location of Li atoms in a channel is their location over hexagonal Si cells. The most significant stresses in silicene σ_zz are maximal in the presence of trivacancies in silicene sheets.

Introduction

The development of stable anode materials, providing a high energy and power density during long-term cycling is a priority when creating lithium-ion batteries (LIBs) of a new generation [1]. When improving LIBs, complex synthesis procedures and the use of expensive materials should be excluded [2]. In general, the use of silicon materials, meets this requirement. In particular, the relative cheapness of silicon is due to its wide distribution in the Earth's crust, as well as the ability of silicon atom to bind to four lithium atoms, which makes it possible to achieve an extremely high electrode capacity.

Silicon has a high theoretical gravimetric capacity (~4200 mA h g⁻¹), which is significantly higher than the capacity of graphite (372 mA h g⁻¹) [3,4]. Although bulk silicon has a very high capacity, it experiences a rapid degradation with each cycle due to a substantial increase in the electrode volume (up to 300%) during lithiation [2]. Such an electrode swelling creates a large load on the material [5,6]. Unlike graphite, lithium is introduced into silicon in the form of neutral atoms, not ions [5].

Fundamental research aimed at finding reliable, high-performance Si anodes allows a detailed study of the lithization mechanism, knowledge of which is important for the construction of Si anodes. A number of works are devoted to the study of changes in the mechanical properties during lithiation of silicon [[7], [8], [9], [10]] and silicene [[11], [12], [13]].

Silicene can be considered as a silicon analog of graphene. However, there are some fundamental differences in the structure of these two-dimensional materials. Unlike graphene, the Si atoms in silicene are not in the same plane. If graphene can be obtained from graphite by exfoliating its layers, then such a possibility is not foreseen for obtaining silicene, because there is no natural silicon multilayer material like graphite. The two-dimensional form of silicon is prone to bending, more specifically buckles. The silicene structure is formed not only on the basis of sp² hybridization, but also with some participation of sp³ hybridization. The epitaxially grown silicene sheets are not detached from the substrate.

Silicene having an atomic thickness can serve as a high-capacity host of Li in lithium-ion rechargeable batteries [14,15]. During the adsorption of alkali metals with silicene, a significant charge transfer occurs from the metals Li, Na, and K to two-dimensional silicon, as a result of which the metallization of silicene occurs [16]. The maximum energy barrier for the migration of Li/Li⁺ adatoms along the sides of a silicene is only 1.70/1.75 eV [17]. A low-energy barrier means that Li adatoms can easily penetrate into two-layer or multi-layer silicone [18]. Greater charge storage capacity and better energy density of silicene compared with graphene are the basis for improving the performance of LIBs.

The presence of defects has a great effect on mechanical, thermodynamic and electronic properties of two-dimensional materials. Conducted experiments have provided clear evidence of the presence of larger multivacancies in silicene [19,20]. The structure of the main defects, their stability, and mechanism of formation to a certain extent, they were considered in [19]. Sometimes small defects are introduced purposefully for specific applications [20]. In particular, it is known that the creation and elimination of point defects provides a simple way for the targeted adjustment of the local structure, thermal stability and the band gap of low-dimensional materials [21].

The proposal to use Ni as an inactive material in the construction of the LIB anode was made in [22]. This is due to the fact that Ni has a high electrical conductivity, low cost and good adhesion to Si. It is extremely important to take into account the influence of the substrate on the functioning of 2D materials in the LIB. The work [23] emphasizes the significant influence of “inert” materials included in the construction of a thin-film silicon anode on the functioning of this electrode. Based on computer simulation [[24], [25], [26]], we investigated the ability of a pair of “two-layer silicene on an Ag(111) substrate” to be represented as an anode material of a lithium-ion battery (LIB). It has been shown that this pair is not the best option when using silicene for this purpose [26]. It turns out that a nickel substrate can constitute a worthy competition for a silver substrate in an electrochemical device.

The capacity of the film electrode on the nickel substrate will depend on how the Li atoms are packed in a silicene channel. Since the channel is rather narrow, the boundary conditions, i.e. the channel walls will affect the nature of the packing of lithium atoms in it. We used the channel walls formed by both perfect and defective silicene. Defects were formed by removing one, two, three, and six compactly positioned Si atoms. In certain cases, the capacity of the electrode can be increased due to the high concentration of defects [[11], [12], [13], [14], [15]]. A more detailed study of the processes occurring in the LIB can be performed using computer simulation.

The purpose of this work is to study the effect of the Ni(111) substrate on the physical properties of the silicene channel supported by it, as well as on the determination of the effect of the type of vacancy defects in silicene on the completeness of filling the channel with lithium and on the stresses generated in it.