Currently, Li-ion batteries generally use graphite as the anode material, with a theoretical capacity of 372 mAh/g and excellent capacity retention over extended cycling. However, the Purdue team noted, with its low operating voltage (<0.3 V vs. Li/Li+), graphite is subject to lithium plating and subsequently lithium dendrite formation when cycled at fast rates or low temperature. Significant research efforts have been directed toward developingnext-generation anode materials for LIBs that can address those issues and offer higher capacities.

One candidate is hard carbon which typically allows faster lithiation due to larger interlayer spacing, and higher cycling capacity than graphite due to the additional nanopore filling Li storage mechanism. Hard carbon materials of various morphologies are commonly prepared by direct pyrolysis or hydrothermal decomposition of biomass. Biomass sources can be carefully selected to fine tune the morphologies.

A vast biomass resource with wide selectivity of morphologies is pollen. The significance of the varied pollen morphologies are seldom noticed by the general public due to their miniature sizes and their antagonistic role in allergy symptoms. Pollen grains typically have a tough outer layer made of sporopollenin biopolymer that is capable of deriving very divergent structures. During the pollination season, plants can be thought to be minifactories that replicate and generate species-specific pollen grains. Due to the micrometer particle size of pollens (above 6 μm), considering their shrinkage after pyrolysis they could fall in the range of commerical carbon anode particle sizes. Solid, dense carbon particles provides high energy density to the rechargeable batteries.

Herein, we report the conversion of allergenic pollen grains into carbon microstructures through a facile, one-step, solid-state thermochemical decomposition in an inert atmosphere at elevated temperatures. For this study, carbon derived from both cattail pollens and bee pollens were evaluated for their potential application in LIB anodes. Cattail pollens was chosen to represent pollens with selective morphology but with limited commercial supply, while bee pollens represent pollens with unselective/diverse morphologies but are commercially available in large quantities.

—Tang and Pol

Whereas bee pollen is a mixture of different pollen types collected by honeybees, the cattail pollens all have the same shape. Tang and Pol (2016). Click to enlarge.

The research showed the pollen anodes could be charged at various rates. While charging for 10 hours resulted in a full charge, charging them for only one hour resulted in more than half of a full charge, Pol said—200 milliamp hours per gram.

Our findings have demonstrated that renewable pollens could produce carbon architectures for anode applications in energy storage devices.

—Vilas Pol

The work is ongoing. Whereas the current work studied the pollen in only anodes, future research will include work to study them in a full-cell battery with a commercial cathode.

Electron microscopy studies were performed at the Birck Nanotechnology Center in Purdue's Discovery Park.

Resources

• Jialiang Tang & Vilas G. Pol (2016) “From Allergens to Battery Anodes: Nature-Inspired, Pollen Derived Carbon Architectures for Room-and Elevated-Temperature Li-ion Storage” Scientific Reports 6, Article number: 20290 doi: 10.1038/srep20290