Using Nanoscale Domain Size To Control Charge Storage Kinetics in Pseudocapacitive Nanoporous LiMn₂O₄Powders

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 LiMn 2 O 4 with ∼15 nm domains as a pseudocapacitive cathode material. In this work, we use this insight to create nanoporous LiMn 2 O 4 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 LiMn 2 O 4 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.