Graphene is constantly being employed into various battery and energy storage technologies and this has become one of the largest growing areas of graphene research commercially.
A team composed of academics and industry scientists from Korea have created a graphene-silica assembly, called a graphene ball, for use as an anodic material and a cathodic coating material in lithium-ion battery batteries.
As technology advances (and the energy required in technological systems increases), batteries and other components need to advance. Areas of rapid charge and holding charge for longer periods of time, whilst ensuring the battery maintains its integrity in terms of safety, are areas of great interest for both academics and battery manufacturers.
An extensive team of researchers from Korea have created a novel approach by using a ‘graphene ball’ as the cathodic material in a lithium-ion cell. The Korean team who produced the research included researchers from many academic institutions and Samsung Electronics.
The graphene ball produced by the researchers is a 3D hierarchical graphene–silica (SiOx) assembly and is said to be ‘popcorn-like’. The graphene balls are a core-shell particle, where the inner core is a SiOx nanoparticle and the ‘shell’ is composed of graphene layers.
The team grew the graphene layers onto the silicon dioxide nanoparticle (20-30 nm) using chemical vapor deposition (CVD) methods. Methane gas was fed into the reaction and was subsequently decomposed into hydrogen atoms under high temperatures.
The hydrogen atoms then reduced the SiO2 nanoparticles into SiOx and the graphene was grown around the nanoparticle core.
The graphene balls have a multifunctional purpose and can serve as an anode by themselves, or they can be coated onto cathodes in lithium-ion batteries. In this instance, the researchers chose a nickel rich cathode and coated it using a scalable Nobilta milling process.
A combination of X-ray diffraction (XRD, D8 Advance, Bruker Inc), X-ray photoelectron spectroscopy (XPS, (Quantera II, ULVAC-PHI, Inc), thermogravimetric analysis (TGA, METTLER TOLEDO TGA/DSC1), high-resolution transmission electron microscopy (HRTEM, FEI Titan Cubed 60–30), field emission scanning electron microscopy (FE-SEM, Nova NanoSEM 450s, FEI) and Raman spectroscopy (Renishaw InVia), inductively coupled plasma-atomic emission spectroscopy (ICP-AES, IPS-8100, Shimadzu) was used to characterize the graphene balls. A two-probe DC method (MCP-PD511, Mitsubishi) was also employed to measure the electrical conductivity of the cell.
The formation of SiOx nanoparticles at the core is a crucial part of the graphene ball. Not only does it prevent a silicon carbide layer from forming at the interface of the nanoparticle and graphene layers during growth, it also ensures that the coating is even when applied to a cathode.
The presence of a silicon nanoparticle also allows for a high specific capacity when used as an anode. The hierarchical nature of the graphene ball’s structure allows for up to 1 wt% of graphene to be incorporated around the silicon-based core.
When applied as a coating to a cathode, the graphene balls were found to enhance the interfacial stability with the electrolyte and the electronic conductivity across the electrode. In turn, the cyclability and charging capabilities of the cathode were dramatically increased by suppressing the unfavorable side reactions and providing efficient conductive pathways.
As an anodic material, the graphene ball exhibited a specific capacity of 716.2 mAh g-1. A fuel cell composed of a graphene ball-coated cathode and a graphene anode was also constructed. The fuel cell was found to have the potential of possessing a high volumetric energy density near 800 Wh L-1.
The fuel cell containing the graphene balls was also found to increase the volumetric energy density by 27.6% against a control cell and possessed a cyclability of 78.6% capacity retention after 500 cycles at 5°C and 60 °C
The material is of an interesting composition compared to most graphene-based electrode materials in lithium-ion batteries. Whether the graphene balls have commercial potential remains to be seen, but the constructed fuel cell shows promise in terms of its properties and performance.
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“Graphene balls for lithium rechargeable batteries with fast charging and high volumetric energy densities”- Son I. H., et al, Nature Communications, 2017, DOI: 10.1038/s41467-017-01823-7