A strategy and detailed explanations to the composites of Si/MWCNTs for lithium storage

Highlights

• Freeze-drying method is used to combine Si with MWCNTs to enhance Si-based material’s rate performance.

• We analyzed the reason why freeze drying brought beneficial effects on Si-MWCNTs composites.

• Precursor treated by freeze-drying method was encapsulated into porous carbon to enhance material’s high rate performance.

• The 500th reversible capacity of Si-MWCNTs-PVPC-FPC-SC-1 reaches 501 mAh g⁻¹ at 1 A g⁻¹.

Abstract

Nano-Si/MWCNTs composite was a representative solution to improve Si-based anode material’s rate performance in lithium-ion batteries (LIBs). However, the problems of easy agglomeration of silicon nanoparticles and carbon nanotubes hindered Si/MWCNT’s further development. In this study, we combine silicon nanoparticles with MWCNTs cleverly by utilizing freeze-drying method to solve the problems and enhance silicon-based material’s rate performance. Compared with Si-MWCNTs composite treated by electric blast-drying method, the rate performance of Si-MWCNTs treated by freeze-drying is significantly improved, especially at different current densities. When Si-MWCNTs are encapsulated in FPC (flour-derived porous carbon, FPC), the as-obtained Si-MWCNTs-PVPC-FPC-SC-1 (sucrose-derived carbon, SC) prepared by freeze-drying method delivers a reversible capacity of 1347.5 mAh g⁻¹ at 0.1 A g⁻¹ after cycling at 5 A g⁻¹ and a reversible capacity of 501 mAh g⁻¹ at 1 A g⁻¹ after 500 cycles. Our study demonstrates that the freeze-drying method can solve the problems of easy agglomeration of silicon nanoparticles and MWCNTs as well as improve Si-based anode’s rate performance for LIBs. The synthetic route presented in this paper is low-cost and easy to scale up for silicon-carbon (Si/C) composites with high rate performance and long cycle life.

Introduction

With the higher and higher demand of energy density and power density in electric traffic, portable devices and power grid storage, silicon-based materials are the development trend of the next generation commercial anode materials for lithium ion batteries. Silicon-based anode materials have highest theoretical capacity (3596 mAh g⁻¹ at room temperature) [1,2], low intercalated-lithium potential (～0.2 V vs. Li/Li⁺), low price and eco-friendly nature [3,4]. However, there exist several disadvantages, such as poor conductivity, high-volume expansion rate as well as poor cycle performance, which hinder its commercialization process to a large extent [5,6]. Only by dealing with these shortcomings correctly, silicon can be applied to anode material of LIBs. In previous work, we ever designed a kind of flour-derived porous carbon structure to improve Si-based anode’s cycle performance successfully [7]. In fact, rate performance of Si-based anode still needs to be further strengthen to meet LIB’s high energy density requirements. In LIBs, rate performance is a multiscale problem, we need to understand and improve the high current density performance from the perspective of atoms to nanoparticles [8], such as the rate of cathode and anode’s lithiation/delithiation [9,10], the diffusion coefficient of Li⁺ in the electrolyte and different interface [11].

As we all know, silicon-based materials are weak in transportation of electron and Li⁺ owing to its semiconductor characteristic. Combining silicon with multi-walled carbon nanotubes (MWCNTs) is an effective way to enhance Si-based materials’ electronic conductivity [12,13]. MWCNTs not only provides excellent conductive matrix, but also buffer the large volume change [14]. A large number of studies have shown that MWCNTs can be used as additive [10,15] for cathode material or as a binder free anode material [16,17]. Used as additive, carbon nanotubes will inevitably decrease material’s tap density, which determines the energy density of full battery. Applied in binder free silicon-carbon anode material, carbon nanotubes together with silicon nanowires require electrospinning technique, which is not very practical for large-scale application [16]. From some practical perspectives, more economical and practical methods need to be developed. What’s more, nanomaterials such as carbon nanotubes and silicon nanoparticles are easily agglomerated in practical applications, which will greatly reduce the excellent properties of nanomaterials.

Differently, in this work, we design a synthetic route that is low-cost and easy to scale up for Si/C composites with high rate performance. We found that highly dispersed MWCNTs and silicon nanoparticles would easily reaggregate in traditional way of drying. Freeze-drying method was put forward to solve this problem in our paper. Freeze-drying refers to the sublimation of solid solvents under low temperature and vacuum environment to dry samples and maintain their microstructures. The ice formed by the solvent would keep the dispersion system highly dispersed until the ice sublimed. In previous researches, freeze-drying strategy was applied to design 3D-structure porous carbon composites [[18], [19], [20], [21]]. We encapsulated Si-MWCNTs into porous carbon system and dried samples by freeze-drying method, the prepared Si-MWCNTs-FPC composite exhibited long cycle life in high current density. In addition, we analyzed the reason why freeze drying brought beneficial effects on Si-MWCNTs composites.