Reversibility Enhancement of Conversion Reaction for Lithium Storage in SnO2/Graphene Composite Enabled By Graphene Nanoperforation at the Contacts of SnO2
Authors: Choi Yeon Jun , Kim Young Hwan , Kim Kwang-Bum 
Published: May 2020

Lithium-ion batteries (LIBs) have been widely used in hybrid electric vehicles and portable electronic devices because of their high energy density and light weight. However, the energy density of the batteries is required to improve for practical applications. For LIB anode materials, commercially used graphite based on intercalation reaction is far from satisfying the requirements for high energy density due to its limited theoretical capacity (372 mAh g-1). Searching for alternative electrode materials with high specific capacity is vital for the development of next-generation Li-ion batteries with high energy densities. In the case of anode materials, electrode materials based on conversion reaction has attracted extensive attention due to its high specific capacities. For example, SnO2 have been studied as one of the promising candidates for anode materials of LIBs owing to their high theoretical capacities (783 mAh g-1) and natural abundance. However, SnO2 anodes suffer from three main drawbacks: 1) irreversible conversion reaction of Sn and Li2O to SnO2 during initial cycles; 2) large volume change (~300 %) during lithiation/delithiation process, which causes severe electrode pulverization and fast capacity fading; 3) low electrical conductivity, thereby low rate capability. Numerous studies tried to solve the issues 2) and 3) through the design of nanostructures and incorporation of conductive carbon materials to mitigate the internal stress from the volume expansion of the SnO2.
Previous studies show that SnO2 goes through an irreversible conversion reaction during the initial cycle, which leads to formation of metallic Sn and Li2O, and each Sn atom reversibly reacts with 4.4 Li atoms, corresponding to a reversible capacity of 783 mAh g-1. Therefore, the theoretical capacity of SnO2 was considered to be 783 mAh g-1. If the conversion reaction of Li2O is made reversible, the theoretical specific capacity of SnO2 would increase to 1494 mAh g-1. Accordingly, the irreversibility of conversion reaction of Li2O in SnO2-based anodes has been a critical challenge for achieving high specific capacity.
On the other hand, few works were reported about the causes of the irreversible conversion reaction and detailed mechanism of the reversible conversion reaction still remains unclear.
Recently, other researchers have reported that SnO2 shows a capacity beyond the above-mentioned theoretical value owing to the reversible conversion reaction between Sn and Li2O during the delithiation process. The increased capacity was mainly attributable to partial reversible conversion of Sn and Li2O to SnO2.
Design of nanostructured SnO2 is regarded as one of the effective ways to enhance reversibility of conversion reaction between Sn and Li2O during the delithiation process. The conversion of Sn and Li2O to SnO2 is known to be irreversible, but could be reversible in the case of nanostructured SnO2 through high fraction of Sn/Li2O interface. Thus, there have been various researches ranging from fabricating SnO2 nanomaterials to incorporating SnO2 with conductive carbon materials.
Furthermore, nano-sized transition metals (Co, Ni, Fe) can act as the catalyst to promote the decomposition of Li2O which is beneficial for the conversion reaction. Although partially reversible conversion reaction has occurred in these works, but they reported limited ICE and capacity decay upon cycling. Thus, to enhance the reversibility of the conversion reaction in SnO2-based anodes, increasing interface area between Sn and Li2O is of great importance.
In this work, we synthesized SnO2/nanoperforated graphene (NPG) microballs with nanoperforations at the contact of SnO2 and graphene which are novel design of SnO2/graphene structure to increase Sn/Li2O interface. SnO2 can act as nanocatalysts inducing catalytic carbon gasification, which is essential to introduce nanoperforations at the contact of SnO2 and graphene. Catalytic carbon gasification induces the selective decomposition of the carbon in contact with the nanocatalysts at a temperature lower than the normal combustion temperature of carbon. The SnO2/nanoperforated graphene (NPG) microballs with nanoperforations at the contact of SnO2 and graphene have several advantages in Li-ion storage performance. 1) Enhancement in reversibility of conversion reaction of Li2O through an increase in Sn/Li2O interface area by increasing interfacial area of SnO2 in contact with electrolyte, 2) improvement in rate capability by increasing interfacial area of SnO2 in contact with the electrolyte, 3) improved cross-plane diffusion of Li-ions through nanoperforations, 4) increased capacity through Li-ion insertion/extraction from the edge sites of nanoperforated graphene, and 5) improved cycle performance by accommodating the volume change of SnO2.