Nanoporous LiMn2O4: An Extrinsically Pseudocapacitive Cathode Material for Li-ion Batteries

Seminar series
Physical Chemistry Seminar
Thu, May 18 12:00pm
2033 Young Hall
Speaker Ben Lesel
University of California, Los Angeles
Dept. of Chemistry and Biochemistry

Abstract: Batteries capable of charging and discharging at fast rates are of particular interest in today mobile world. When ionic diffusion processes in battery materials are fast due to intrinsic high diffusion coefficients or extrinsically designed short diffusion path lengths, the term pseudocapacitance is used to describe the fast insertion/desertion redox processes. To make full cell pseudocapacitive batteries a reality for Li-ion batteries, both anode (low voltage vs Li/Li+) and cathode (high voltage vs Li/Li+) pseudocapacitive materials must be identified. Thus far, many anode materials capable of pseudocapacitance have been recognized but the same is not true for the cathodes. The lack of intrinsically pseudocapacitive cathode materials has led to a great effort in nano-structuring known cathode materials in the hopes of attaining extrinsic pseudocapacitive behaviors within such materials.  LiMn2O4 is one of the easiest cathode materials to nanostructure due to the fact that it has a relatively low crystallization temperature among the common cathode materials. We have shown that mesoporous LiMn2O4 thin films with structures around 15 nm show dominantly pseudocapacitive behavior with both fast charge and discharge kinetics. Although these small-structured LiMn2O4 materials are capable of impressive kinetics, the capacity is reduced significantly compared to bulk due to an electrochemically inactive surface which is overexpressed in high surface area nanostructured materials. To address the inactive surface layer issue and create a more practical thick electrode system, nanoporous LiMn2O4 powders with various crystallite sizes have been fabricated and built into slurry electrodes for examination of their capacity and kinetics. We find that below a certain crystallite size kinetics and cycleability of LiMn2O4 improves significantly. To better understand these improvements, in-situ synchrotron X-ray diffraction was utilized to study the phase changes that occur in nanostructured LiMn2O4 during charge and discharge. Finding suggest that the suppression of a phase transition is responsible for both improved kinetics and cycleability below a certain crystallite size.