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Lithium batteries have become very desirable, and the battery of choice for many a year, especially in mobile devices and energy efficient transportation vehicles. To keep up with current demand, two of the ever-advancing properties of lithium ion batteries are rate capability and power performance.
Qian Cheng, a researcher from NEC Corporation in Japan, has developed a porous graphene sponge additive, known as Magic G, that can be used in both the anode and the cathode of a lithium-ion battery to increase its rate and power performance.
In the last 20 years, technology has experienced a profound increase towards the use of lithium-ion (Li-ion) batteries, whether it be for common everyday items such as mobile devices, or more specialist technology such as electric vehicles (EVs), hybrid electric vehicles (HEVs) or in smart energy systems.
Lithium batteries come in many forms, but the commercially available devices generally consist of a graphite anode and a LiMO2 layered cathode structure, where M is commonly a representative of a binary or ternary system composed of cobalt, nickel or manganese.
Despite years of research and development, and although Li-ion batteries show some great properties (including a gravimetric energy density more than 160Wh/kg at the cell level), they still suffer from low power performances as a result of poor charge and discharge rate capabilities and high-rate cyclabilities.
Many cells nowadays suffer from poor performances due to the energy density focused cell design they employ. The way in which the cells are structured requires a high mass loading on both the anode and the cathode, low electrolyte coefficient, a low porosity of both the anode and cathode, little conductive additive usage and the active material containing a low surface area. The need to balance so many parameters has inevitably resulted in high energy density Li-ion batteries that exhibit a poor power performance.
As the consumption demands for Li-ion batteries increase, most notably from small devices to implementation in electric vehicles, the need to improve not only the power performance but the cycling ability, has become much greater. This rings especially true in cars, where drivers want both the longest battery life possible coupled with short recharging times.
There have been many approaches suggested and tested to increase the performance of Li-ion batteries, especially through cell design engineering approaches. Unfortunately, despite some methods increasing the performance of the cell, the produced batteries have not been feasible from a commercial standpoint.
This has been mainly associated with low densities in the cell and high costs. Qian Cheng has developed a honeycomb-like porous graphene sponge, also known as “Magic G” (MG), with a high electric conductivity, high specific surface area and a high ability of electrolyte absorption. The sponge has been incorporated into both the anode and cathode of Li-ion cells, as an additive, to increase the rate capability and high rate cyclability.
Magic G was produced through a series of methods and initially started out as graphite. From there, the researcher oxidised the graphite through a modified Hummer’s method to produce graphite oxide. This was further followed by a thermal shock and air oxidation treatment which formed a material coined pre-Magic G (pre-MG). Magic G was then realised through thermally treating the precursor at 1000 °C.
Both the precursor and the final Magic G product were characterised through many commercially available machines and methods, including field-emission scanning electron microscopy (FE-SEM) (Hitachi SU8000), transmission electron microscopy (TEM) (Hitachi H-90000UHR), atomic force microscopy (AFM) (Bruker Nano Scope V Dimension Icon), Fourier transform infrared (FT-IR) (Varian 7000FT-IR), Raman spectroscopy (NRS-7000), gas adsorption (BELSORP18PLUSUS-HT) and temperature programmed desorption–mass spectrometry (TPD-MS) (Shimadzu GC/MS- QP2010 Plus).
Both the anode and cathode of the cell showed a much greater performance and charge rate after the incorporation of Magic G.
When compared to other non-Magic G Li-ion batteries, the addition of 0.5 %wt addition into the anode improved the charge capacity retention from 56% to 77% at 6 cycles and from 7% to 45% at 10 cycles.
In the cathode, the same amount of Magic G was incorporated and showed an increase in the discharge capacity rate from 43% to 76% at 6 cycles and an increase from 16% to 40% at 10 cycles.
The cyclability for both electrodes, at high rates, was improved through the addition of the graphene sponge.
Alongside more noticeable improvements, the addition of Magic G into the electrodes also increased the electronic conductivity, the adsorption of electrolytes and decreased the active materials charge transfer resistance.
The produced honeycomb-like material shows great potential as an additive in next-generation Li-ion batteries as they solve many challenges through possessing both a high charge density and good rate capabilities.
The combination of properties that the additive can introduce into the electrode(s) could be essential in getting the most out of Li-ion batteries for electrical vehicle applications. Qian also looks to further optimise the structure in the future to obtain even greater performances.
Sources and Further Reading
“Porous Graphene Sponge Additives for Lithium Ion Batteries with Excellent Rate Capability”- Q. Cheng, Scientific Reports, 2017, DOI:10.1038/s41598-017-01025-7