Laser in-situ synthesis of SnO₂/N-doped graphene nanocomposite with enhanced lithium storage properties based on both alloying and insertion reactions

Highlights

• The SnO₂/N-Gr electrode is prepared by a laser in-situ synthesis method.

• The SnO₂/N-Gr shows improved lithium storage capacities and rate performance.

• Mechanisms of alloying and insertion are related to performance enhancement.

Abstract

This paper reported a SnO₂/N-doped graphene nanocomposite (SnO₂/N-Gr) electrode which was prepared by a laser in-situ synthesis method. When demonstrated as anodes for lithium storage, the SnO₂/N-Gr electrode showed improved lithium storage capacities and rate performance. In details, a reversible capacity of 830 mAh g⁻¹ was obtained after 300 cycles at a current density of 300 mA g⁻¹, and when the current density increased up to 3 A g⁻¹, the SnO₂/N-Gr electrode revealed a high reversible capacity of 600 mAh g⁻¹. It was proven that the excellent electrochemical performance mainly related to a hybrid lithium storage mechanism which combined with alloying and insertion reactions. By introducing huge numbers of micropores and defects on graphene sheets, N-doping increased the number of hosts for lithium insertion and enhanced the Li⁺ diffusion rate in graphene sheets, so both of lithium storage capacities and rate performance were effectively improved. The SnO₂/N-Gr electrode had a short preparing procedure and good electrochemical performance, which hold potential for development of next generation lithium ion batteries with high specific capacities and good rate performance.

Introduction

SnO₂-based materials have attracted great attention as promising anodes for the next-generation Li-ion batteries [1], [2]. With properties of natural abundance, less toxicity, higher electrode potential and an attractive specific capacity of 782 mAh g⁻¹ (cycling between Sn and Li_4.4Sn), SnO₂-based materials have been considered as low costs and high safety replacements for graphitic materials [2], [3], [4]. However, SnO₂ tends to pulverization after several reaction cycles, due to substantial volume expansion increases up to 260% [4], [5], [6], which causes continual consumption of electrolyte, excessive growth of unstable solid electrolyte interface (SEI) layer and capacity fading [7], [8], [9], [10]. To address these problems, conductive buffer materials have been used as host materials to cooperate with SnO₂ [4], [11], [12]. Due to their high electrical conductivity and volume buffer properties, the stability of SnO₂-based anodes has been significantly improved.

Using polymer binders, such as poly vinylidene fluoride (PVDF) [13], polyacrylic acid (PAA) [14], carboxymethyl cellulose (CMC) [15], [16] and etc. to cooperate with SnO₂ are another feasible approach to resolve these issues. Polymer binders can combine active materials and conducting parts together to maintain the structural integrity, thus the active materials show enhanced cycling performance during lithiation and delithiation [14], [17]. But polymer binders can reduce battery performances in aspects of increasing the electronic resistance and occupying weight in active materials, which inevitably compromise the electrochemical performance of electrode [18], [19], [20]. Moreover, nanomaterials normally have a larger surface area, which make them difficult to mix well with polymer binders [21]. Recently, in-situ synthesis methods have been carried out to grow nanocomposite on charge collector foils, benefit from no addition of any polymer binders, the as-prepared sample presents improved electrochemical performances in lithium storage application [17], [21], [22], [23], [24]. For example, Ui et al. [18] used an electrophoretic deposition (EPD) method to prepare a nano-SnO₂ and acetylene black co-deposited film, which exhibited a reversible capacity of 504 mAh g⁻¹ after 50 cycles for lithium storage. Followed by freeze-drying mixtures of graphene oxide and tin sol with a heat treatment, Botas et al. [25] prepared a SnO₂-based nanocomposite foam with a reversible capacity of 1010 mAh g⁻¹ after 50 cycles. Shen et al. [26] reported a porous carbon-nanofiber membrane (Sn-SnO₂-CNF@C) and showed an enhanced specific capacity of 712.2 mA h g⁻¹ at a high current density of 800 mA g⁻¹ after 200 cycles. Liang et al. [27] deposited SnO₂/nitrogen-doped graphene nanocomposites on a fiberglass membrane by using a hydrothermal method, the as-prepared binder-free electrodes also exhibited excellent lithium storage capacity.

Inspired by researches mentioned above, an efficient and environmentally friendly method was carried out to fabricate SnO₂/nitrogen-doped graphene nanocomposite (SnO₂/N-Gr) electrode, which combined processes of coating reactive precursors to Cu foils and followed by laser irradiation to trigger in-situ conversion of precursors. When the obtained electrode used as anode for lithium storage, it revealed both of high reversible capacities and good rate performance. Mechanisms of performance enhancement were investigated and discussed in this paper.