Using Nanoscale Domain Size To Control Charge Storage Kinetics in Pseudocapacitive Nanoporous LiMn2O4 Powders

Pseudocapacitive materials can produce charge storage devices that have both high energy and power density. Although many pseudocapacitive anode materials have been identified, there is a lack of equivalently fast charging cathode materials necessary to create full-cell devices. Recently, thin-film studies from our group have identified nanoporous LiMn2O4 with similar to 15 nm domains as a pseudocapacitive cathode material. In this work, we use this insight to create nanoporous LiMn2O4 powders that can be used in practical, slurry-type thick electrode systems. Using these materials, we specifically examine the role of crystalline domain size in controlling charge storage kinetics. Four nanoporous LiMn2O4 powders were synthesized with crystallite sizes of 10, 20, 40, and 70 nm, and their charge/discharge kinetics were studied. Smaller crystallite sizes showed lower capacity but faster charge/discharge speeds, longer cycle life, and higher capacitive contribution based on kinetic analysis, whereas larger crystallite sizes showed the opposite trend. Importantly, there appeared to be a critical size above which the charge/discharge kinetics and cycle life deteriorated significantly; materials with domain sizes just below this critical size showed the best combination of electrochemical performance characteristics.