Si-Based Anode Materials for Li-Ion Batteries: A Mini Review

Abstract

Si has been considered as one of the most attractive anode materials for Li-ion batteries (LIBs) because of its high gravimetric and volumetric capacity. Importantly, it is also abundant, cheap, and environmentally benign. In this review, we summarized the recent progress in developments of Si anode materials. First, the electrochemical reaction and failure are outlined, and then, we summarized various methods for improving the battery performance, including those of nanostructuring, alloying, forming hierarchic structures, and using suitable binders. We hope that this review can be of benefit to more intensive investigation of Si-based anode materials.

1 Introduction

In the last two decades, the Li-ion batteries (LIBs) have successfully captured the portable electronic market. However, when it is proposed to conquer the upcoming markets of electric vehicles, storage of energy from renewable energy sources, such as photovoltaic plants and/or wind turbines and other KWh levels load, great improvements in storage capacity, which is currently mainly limited by their electrode materials, are urgently needed [1–5]. It is well known that the commercial graphite anode cannot meet these challenges due to its low theoretical capacity (372 mAh g⁻¹). There is a consensus that the breakthrough in capacity can be achieved by moving from classical intercalation reaction to alloying reaction because the alloying reaction can store more Li compared with intercalation reaction. For example, Li can react with Si to form Li₂₂Si₅ alloy, but with graphite only, to form LiC₆ alloy. Since Dey demonstrated that Li metal can electrochemically alloy with other metals (Sn, Pb, Al, Au, Pt, Zn, Ag, Mg, and Cd) at room temperature [6], Li-alloying reactions with metallic or semi-metallic elements and various compounds have been investigated during the past few decades, such as Sn, P, Ge, Pb, and Sb. Wen et al. showed that Sn reacted with lithium to yield different Li–Sn phases: Li₂Sn₅, LiSn, Li₇Sn₃, Li₅Sn₂, Li₁₃Sn₅, Li₇Sn₂, and Li₂₂Sn₅. A black P/C nanocomposite also showed high capacity (about 2,000 mAh g⁻¹) [3]. Among the various Li alloy elements, Si has been considered as one of the most attractive anode materials for LIBs, not only because of its high gravimetric (4,200 mAh g⁻¹) and volumetric capacity (2,400 mAh cm⁻³), but also due to its abundance, cheapness, and environmentally benign property, as shown in Table 1. However, it suffers from fast capacity fading, which greatly hampers the application of Si anode materials.

Table 1 Property of Li alloy elements [3]

1.1 The Mechanism of Electrochemical Lithiation

LIBs are mainly composed of anode (generally graphite), a carbonate-based organic electrolyte, and a cathode (generally LiCoO₂). Li ions are intercalated and deintercalated between graphite and LiCoO₂ through the electrolyte during discharge and charge. The theoretical capacities of anode and cathode are 372 mAh g⁻¹ (graphite) and less than 160 mAh g⁻¹ (LiCoO₂), respectively, which are too low, especially for anode material. Si anode is very attractive because of its high theoretical capacity of 4,200 mAh g⁻¹ which is 10 times more than that of commercial graphite [3]. Moreover, the discharging potential is about 0.2 V with respect to Li/Li⁺, which is lower than most of other alloy-type and metal oxide anodes [7]. Furthermore, it is safer and stabler than graphite (lithiated silicon is more stable in typical electrolytes than lithiated graphite) [8].

The mechanism of electrochemical lithiation of Si is critical to improve the performance of Si anode, which has been investigated by several groups [9–16]. It is found that the reactions follow the equilibrium Li–Si binary phase diagram at high temperature, forming different intermetallic compounds and showing distinct voltage plateaus for each two-phase region [17]. However, there is only a two-phase region at about 0.1 V at room temperature during first discharge process [18], as shown in Fig. 1. It should be noted that the two-phase region disappears after first cycle. In order to find out the lithiation mechanism, X-ray diffraction (XRD) analysis was performed to investigate the phase transition [13–15], and the reaction mechanism is explained as follows:

Fig. 1

Voltage profiles of Si powder electrode for the first and second discharge/charge cycles [18]

During discharge

$$\text{Si}(\text{crystalline})+x{\text{Li}}^{+}+x{\text{e}}^{-}\to {\text{Li}}_x\text{Si}\text{(amorphous)}+(3.75-x){\text{Li}}^{+}+(3.75-x){\text{e}}^{-}$$

(1)

$$\to {\text{Li}}_{15}{\text{Si}}_4(\text{crystalline)}$$

(2)

During charge

$${\text{Li}}_{15}{\text{Si}}_4(\text{crystalline)}\to \text{Si(amorphous) +}y{\text{Li}}^{+}+y{\text{e}}^{-}+{\text{Li}}_{15}{\text{Si}}_4(\text{residual)}$$

(3)

In the two-phase region, crystalline Si becomes amorphous Li–Si alloy during the first lithiation (1), and the highly lithiated amorphous Li_xSi phase is suddenly found to crystallize into Li₁₅Si₄ phase around 60 mV (vs. Li/Li⁺) (2). Another two-phase region appears during the first delithiation process, and the final product is amorphous Si (3). There are also amounts of residual Li₁₅Si₄ phase after the first delithiation, which can be avoided if the potential of the Si electrode is controlled above 70 mV during cycling. When Li ions react with the amorphous Si during the second cycle, the two-phase region disappears, and slo** voltage plateaus are observed, which indicates single-phase region. After the second cycle, reactions (2) and (3) were repeated, show the above features repeatedly, and reversible capacity faded quickly.

1.2 The Failure Mechanism

Although Si has the highest theoretical capacity, its cycling performance is very poor. Figure 2 shows the charge–discharge profiles of Si powder anode at a current density of 100 mA g⁻¹. It could be found that a large amount of irreversible capacity appears in the first cycle. The first discharging capacity is about 3,260 mAh g⁻¹ but that of the charging is only 1,170 mAh g⁻¹. After 10 cycles, only very low capacity (about 200 mAh g⁻¹) can be retained. To understand the reasons for the poor cycling stability of Si anode, the failure mechanism has been investigated by several groups [16, 19]. The conclusions can be drawn as follows:

Fig. 2

Charge–discharge voltage profiles of Si powder anode [16]

• a. The large change in the volume of Si anodes, which increases internal resistance and loss of contact area between Si and conductive materials, is considered to be the main reason for their rapid capacity loss. Figure 3 shows the schematic of morphologic changes that occur in Si during electrochemical cycling [20]. The volume of Si anodes changes by about 400 % during cycling. As a result, Si films and particles tend to undergo pulverization during cycling. Most of the material loses contact with the current collector, resulting in poor transport of electrons.

  Fig. 3

  

  Schematic of morphologic changes that occur in Si during electrochemical cycling [20]

• b. Solid electrolyte interphase (SEI) stability at the interface between the silicon and the liquid electrolyte is also responsible for the failure of the Si anode. The SEI layer is formed during battery discharging, due to electrolyte decomposition on the surface of anode at the low potential. As shown in Fig. 4, a thin layer of SEI is formed in the lithiated and expanded state [21]. During delithiation, the Si particle shrinks, and the SEI layer breaks down into separate pieces, and fresh Si surface is exposed to the electrolyte. In later cycles, new SEI layer continues to be formed on the newly exposed silicon surfaces. The SEI is an electronic insulator but a Li–ion conductor, and so the growth of the SEI layer is eventually terminated at a certain thickness. The thick SEI layer is harmful for the cycle life, because it can cause a rise of the electrode impedance/polarization and decrease of the electrode’s electrochemical reactivity. As discussed above, a large volume change and unstable formation of SEI film are the main issues for the failure of Si anode.

  Fig. 4

  

  Schematic of SEI formation on silicon surfaces [21]

2 The Methods to Improve the Battery Performance

2.1 Si Nanostructures

Tremendous efforts have been made to improve the batteries performance of Si anode. In order to overcome the volume change during electrochemical reaction, many researches are focused on accommodating the volume changes in the earlier studies. Nanomaterials have the genuine potential to make a significant impact on the electrochemical performance of Si anode [22], as their reduced dimensions enable far higher intercalation/deintercalation rates. In addition, the volume change can be also buffered after downsizing the Si particle to nano-size. The significance of nano-sized Si on battery performance was demonstrated by several groups. Li et al. reported that a nano-Si (78 nm) powder anode showed better capacity retention than bulk Si powder [23]. Kim et al. also synthesized Si nanoparticles with various sizes (5, 10, and 20 nm) and studied their battery performance [24]. The results indicated that 10-nm-sized Si showed the highest capacity retention among the samples, as shown in Fig. 5.

Fig. 5

a Voltage profiles of 5, 10, and 20-nm-sized Si and 10-nm-sized Si after carbon coating during the first cycle in coin-type half-cells at a rate of 0.2 C between 0 and 1.5 V. b Plot of charge capacity versus cycle number (red: 5 nm, blue: 10 nm, orange: 10 nm after carbon coating, black circles: 20 nm) [24]. (Color figure online)

One-dimensional (1D) nanowires and nanotubes are also intriguing structures with good cycle stability. Cui et al. synthesized Si nanowires which were grown directly on the metallic current collector substrate [20]. The limited nanowire diameter allows for better accommodation of the large volume change and provides 1D electronic pathways allowing for efficient charge transport. The Si nanowires display high capacities at higher current density. Even at the 1 C rate, the capacities remain 2,100 mAh g⁻¹, and a reversible capacity of over 3,000 mAh g⁻¹ is maintained after 10 cycles. An array of sealed Si nanotubes is also prepared by CVD of Si on to ZnO nanorods and selective removal of ZnO (see Fig. 6) [73, 74].

In addition, the electrolyte offers a promising field for more extensive research efforts. It is found that the electrolyte containing VC or FEC has been recognized to favor the formation of more stable SEI film. Furthermore, the fabrication cost of nano-structured Si remains high and needs to be reduced for its practical applications.