Low-cost route to porous silicon is key to improving Li-ion batteries

Lithium-ion batteries are commonly found in portable electronics and electric or hybrid cars and have traditionally contained a graphite anode. Silicon has since emerged as a promising anode substitute because it is the second most abundant element on earth and has a theoretical capacity of 3,600 milliamp hours per gram (mAh/g), almost 10 times the capacity of graphite.

The capacity of a lithium-ion battery is determined by how many lithium ions can be stored in the cathode and anode. Using silicon in the anode increases the battery’s capacity because one silicon atom can bond up to 3.75 lithium ions, whereas with a graphite anode six carbon atoms are needed for every lithium atom.

The USC Viterbi team developed a commercially viable silicon anode with a stable capacity above 1100mAh/g for extended 600 cycles, making their anode nearly three times more powerful and longer lasting than a typical commercial anode.

Up until recently, the successful implementation of silicon anodes in lithium-ion batteries was hampered by the severe pulverisation of the electrode due to the volume expansion and retraction that occurs with the use of silicon.

Last year, the same team led by USC Viterbi electrical engineering professor Chongwu Zhou developed an anode design using porous silicon nanowires that allowed the material to expand and contract without breaking, effectively solving the pulverisation problem.

According to USC Viterbi, this solution yielded a new problem as the method of producing nanostructured silicon was prohibitively expensive for commercial adoption. Graduate student Mingyuan Ge and other members of Zhou’s team built on their previous work to develop a cost-efficient method of producing porous silicon particles through ball-milling and stain-etching.

’Our method of producing nanoporous silicon anodes is low-cost and scalable for mass production in industrial manufacturing, which makes silicon a promising anode material for the next generation of lithium-ion batteries,’ Zhou said in a statement. ’We believe it is the most promising approach to applying silicon anodes in lithium-ion batteries to improve capacity and performance.’

In addition, graduate student Jiepeng Rong and other team members developed a method of coating sulphur powder with graphene oxide to improve performance in lithium-sulphur batteries.

Sulphur has been a promising cathode candidate for many years owing to its high theoretical capacity, which is over 10 times greater than that of traditional metal oxide or phosphate cathodes. Elemental sulphur is also abundant, cheap, and has low toxicity. However, the practical application of sulphur has been hindered by challenges including poor conductivity and poor cyclability.

Their research proved that a graphene oxide coating over sulphur can solve both problems. Graphene oxide has unique properties such as high surface area, chemical stability, mechanical strength and flexibility, and is therefore commonly used to coat core materials in products like sensors or solar cells to improve their performance. The team’s graphene oxide coating improved the sulphur cathode’s capacity to 800mAh/g for 1000 cycles of charge/discharge, which is over 5 times the capacity of commercial cathodes.

Zhou and his team recently published their results on silicon anodes in Nano Letters.