In-situ construction of porous Si@C composites with LiCl template to provide silicon anode expansion buffer

Highlights

• Porous Si@C composites is constructed by an environmentally friendly method.

• LiCl template can be removed easily by washing and recycled by recrystallization.

• The porous structure provides a buffer for the expansion of silicon.

• The porous structure inhibits the damage to the solid electrolyte interphase film.

• Differential potential EIS was used to analyze the formation of SEI film.

Abstract

Silicon-based anode material is one of the most promising alternative anodes for graphite due to its advantages of abundant reserves, environmental protection and high-energy-density. However, silicon still cannot be directly applied commercially as anode materials owing to the existences of huge volume change during the alloying and dealloying process. Herein, porous Si@C composites are prepared by employment of nano silicon as the active substance particles, lithium chloride as the template and pitch powder as the carbon precursor. Results of electrochemical performance tests show that the cell based on the in-situ prepared porous Si@C composites deliver a reversible good cycle stability. Besides, porous Si@C composites show a lower thickness expansion rate of electrode (15.38%), compared with pure nano silicon (162.46%) and conventional Si@C anodes (40.24%), respectively. It is believed that the improved performance is benefitted from the porous structure, which provides a buffer for the expansion of silicon, reduces the volume expansion of the electrode during the charge and discharge process, and thus inhibits the damage to the solid electrolyte interphase film outside the Si@C composites. This work provides an environmentally friendly method to prepare porous Si@C composite anode materials, which is suitable for large-scale preparation and industrialized production.

Introduction

With the rapid growth of new energy vehicles with long endurance mileage, photovoltaic energy storage, and portable electronic devices, people are having an increasingly high demand for the energy density of energy storage devices [[1], [2], [3], [4]]. The traditional high-performance graphite anode materials of lithium ion battery are unable to satisfy the needs of applications requiring high-energy-density caused by its limited specific capacity and low-energy-density, and researchers now focus on seeking the next generation of anode materials with high-energy-density [5,6]. Among the high-energy-density anode materials have been studied yet, silicon is one of the most promising alternative anode materials for graphite because of its advantages of abundant reserves, environmental protection, and high-energy-density [7]. However, silicon still cannot be directly applied commercially as anode materials owing to the existence of serious technical barriers, which are mainly caused by huge volume change about 300% of silicon during the alloying and dealloying. The expansion and contraction of silicon will lead to electrode pulverization, making it difficult to form a stable passivation “solid electrolyte interphase (SEI)” film on the electrode surface. Meanwhile, the repeated formation and destruction of the SEI film causes the continuous consumption of the electrolyte, resulting in low coulombic efficiency and rapid capacity degradation in the cycle process [8,9].

To improve the structural stability and cycling performance of silicon materials, recent studies have made great improvements by nanosizing of silicon particles or optimising the morphology of silicon materials. The nano silicon anode materials reported presently mainly include silicon nanoparticles [[10], [11], [12], [13], [14]], nano silicon films [[15], [16], [17]], silicon nanowires, silicon nanotubes [[18], [19], [20], [21]], 3D porous silicon, hollow porous silicon, etc [19,[22], [23], [24]]. Nanosizing of silicon particles can reduce the expansion stress in the electrochemical process to partly avoid the collapse of the structure of silicon. The modification of the morphology of silicon particles is mainly aimed at improving the stability of the electrode interface structure in the electrochemical process, and often achieved by introducing other materials. Carbonaceous materials are the optimal substrates for silicon materials to prepare composites because of their small volume change, high conductivity, and good cycling stability in the process of charging and discharging. According to the structure of composites, they are mainly classified into the coated structure and the embedded structure. The coated structure is to coat carbon layers on the surface of active substance silicon, which can enhance the conductivity of silicon and alleviate the volume effect. The composites with coated structure can be subdivided into the core-shell type [[25], [26], [27]], the yolk-shell type [28,29], and the porous type [19,23,30] according to the morphology.

With deepening the understanding of silicon carbon (Si@C) anode composites, the improvement scheme of such composites is also constantly updated. Among them, structural design strategy of porous Si@C anode composites not only provide buffer space for the change of silicon volume but also shorten the transport path of electrolyte ions on the condition of improving the conductivity of silicon materials. Moreover, porous Si@C anode composites can prevent the repeated formation of SEI films by reducing silicon’s direct contact with the electrolyte [31,32]. Usually, porous silicon carbon (porous Si@C) anode composites are prepared by intermediate templates, such as etching the silicon and the silicon dioxide with hydrofluoric acid (HF) [[33], [34], [35]]. However, the presence of HF etchant with strong corrosiveness property increases the difficult of operation and the cost of preparation. As another method, it has been reported that a gap between silicon and carbon layers can be constructed through the volatilization of low-melting-point elemental sulfur [36]. However, elemental sulfur will cause pollution to the environment. Therefore, to develop an economic and environmentally friendly preparation method that is easy to realize large-scale production is warranted for porous Si@C anode composites.

In this paper, porous Si@C composites are prepared by employment of nano silicon as the active substance particles, lithium chloride (LiCl) as the template and pitch power as the carbon precursor [37]. This in-situ construction method of structural system has at least five advantages. (1) The preferred template of LiCl is an environment-friendly salt with low toxicity. (2) The template of LiCl has low melting point, which will keep molten state during the carbonization process at high temperature. That will effectively inhibit the caking and agglomeration during the carbonization process of organic gel compounds in pitch. (3) The addition of LiCl will reduce the viscosity of pitch-based system, which is conducive to the uniform dispersion of nano silicon. (4) The template of LiCl which is highly soluble in water is easy to remove by washing. This will construct a 3D porous structure which can provide an expansion buffer for silicon anode and a short transmission distance for Li⁺ ions. (5) The separated LiCl can be recycled by recrystallization [38]. As expected, the as-prepared composites delivered a reversible capacity of 642 mA h g^-1 after 50 cycles at the current density of 0.2 C, suggesting good cycle stability. Besides, porous Si@C composites showed a lower thickness expansion rate of electrode (15.38%), compared with pure nano silicon (162.46%) and conventional Si@C anode (40.24%), respectively.