High theoretical capacity, availability abundance, non-toxicity, and environmental benignity make Silicon (Si) the preferred choice of anode for next-generation Lithium-ion batteries. Large volumetric expansion (~300%), achievement of Si nanostructures (characteristic dimension below 150 nm), and pragmatic fabrication process are critical obstacles to overcome to build Lithium-Silicon batteries for real-world applications. Electrospinning of polymers, dissolved in organic solvents along with active material such as Si, is a feasible fabrication process. However, current electrospinning methods:
- Require lengthy, thermal oxidative stabilization and carbonization steps.
- Build electrodes with reduced capacity because the weight percentage of active material can be less than 50%.
Prof. Cengiz Ozkan and his research team have developed a patented, binderless, freestanding, Silicon Nanofiber (SF) paper with a-Si weight percentage in excess of 80%. SF paper is synthesized via magnesiothermic reduction of SiO2 nanofiber (SiO2NF) paper which is produced by an in situ acid-catalyzed polymerizations of tetraethyl orthosilicate (TEOS) in flight. To enhance the surface conductivity of the electrode, a 4 nm carbon coating is applied to the SF paper.
Schematic illustration of the electrospinning and the subsequent reduction process.
Cycling data for carbon-coated SF compared to uncoated SF at C/10 rate.
The novel aspects and benefits of their invention are:
- The existence of a 1-2 nm thickness, native SiO2 shell on all the Silicon nanoparticles (SNP) serves to mitigate volume expansion effects during lithiation.
- The diameter of the SiNPs is 8 – 25 nm, well below the critical dimension.
- Reduced bulk diffusion length for Lithium.
- Excellent electrochemical stability and a high degree of scalability.
- No carbon black, metallic current collectors, or polymer binders.
- The magnesiothermic reduction process requires lower operating temperatures (700 deg. C).
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