(433e) Modeling of Nature-Inspired Hierarchical Porous Materials for Energy Storage

Climate change and energy are two of the major challenges facing modern society, with the development of greener and more efficient energy conversion and storage technologies critical for the mitigation of greenhouse gas emissions and meeting the global energy demand in the future^1-4. The Lithium ion rechargeable battery provided a breakthrough in modern energy storage; however they are reaching their maximum predicted energy capacity^1-6. As a result, scientists are keen to explore rechargeable batteries with higher specific (gravimetric) energy. The lithium-air battery is predicted to have the highest theoretical specific energy density among all rechargeable battery devices (3500 Wh kg^-1)^5-10. A typical Li-air battery consists of a Li anode (negative electrode), carbon cathode (positive electrode), an electrolyte and an O₂ diffusion membrane on the cathode surface^5-8. During discharge, O₂ is reduced at the cathode/electrolyte interface by reacting with Li⁺ ions produced from the Li anode to form lithium peroxide (Li₂O₂)^5-8,10. Li₂O₂ is an insulating solid that is oxidized during charging of the battery to release O₂ as per the following ideal cathode reaction^5,6:

2Li⁺+ 2e^-+O₂ <-> Li₂O₂

The exact mechanism through which O₂ reacts with Li ions at the electrode/electrolyte interface during discharge is a matter of debate and is critical for the future development of the Li-air battery^5-8,10-11. Electrode passivation as a result of the insulating Li₂O₂ layer is the leading cause for early cell death due to the diminishing electrode/electrolyte interface^5-7,10-11. A simple solution to this issue would be to use high donor number electrolytes like dimethyl sulfoxide (DMSO). However, such electrolytes are generally unstable^5,6. A more genuine solution is the design of hierarchically porous materials like 3DOM porous carbon and hierarchical graphene as cathodes^6-8,11.

Hierarchically structured porous materials are nature inspired structures in which the porosity is engineered such that the resulting material composes various porosity scales which are namely micropores (˂ 2 nm), mesopores (2 – 50 nm) and macropores (˃ 50 nm)^3,9,12. The utilization of hierarchical porous materials as cathodes for the Li-air battery offers several advantages^3,6,7,10,12. The porosity offers a higher interfacial contact between the electrode and the electrolyte resulting in higher capacity⁶. Moreover, the presence of wider voids in the form of mesopores and macropores offers transport “highways” for faster charge and molecular transport^3,7,10-12. Most importantly however, the mesopores and micropores offer the possibility of discharge product (Li₂O₂) storage without clogging the macropores ensuring continuous oxygen transport to the reaction centres and reducing passivation^6-8,11.

In a field where a fundamental understanding of electrochemical mechanisms is of great significance, we will present first principles molecular simulations to model the phenomena occurring at the electrode/electrolyte interface of a Li-air battery. In addition, classical molecular simulations have been utilized to model and characterize the hierarchical porous materials such as 3DOM carbon and hierarchical graphene as cathodes for Li-air batteries. Molecular simulations helped provide insights into the possible reaction mechanism occurring on hierarchical cathodes. We have explored the feasibility of the hierarchical cathodes in improving the efficiency of the Li-air battery, through the proposed improved diffusion and discharge product storage and results will be discussed here. We believe that our work will be of great relevance to the fields of hierarchical materials and energy storage.

References

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