Energy Storage and Distributed Resources Division, Lawrence Berkeley National Laboratory Conventional Li-ion battery electrolytes have been designed to optimize numerous desirable properties, including interfacial and thermal stability, conductivity, and low flammability. However, all Li+-bearing electrolytes still possess low Li+ transference (t+) numbers, where current passed through them is primarily carried by the counteranion, resulting in large concentration gradients that limit battery performance, particularly at high discharge and charging rates. The development of high t+ electrolytes—those in which most (or all) current is carried by the Li+ ion—could enable safer battery cycling, faster charging rates, and thicker, more energy-dense cathode designs in Li-ion batteries.
Bryan McCloskey,
University of California, Berkeley
This presentation will outline our attempts to develop high t+ electrolytes using two strategies. In the first, Li-neutralized polyanions are used as a salt in nonaqueous liquid electrolytes (so-called nonaqueous polyelectrolyte solutions). In this configuration, Li ions, when appropriately solvated, have hydrodynamic radii much smaller than the polymer chain’s size, ostensibly allowing them to diffuse or migrate faster than their appended counteranions, and hence enable high t+ electrolytes. Ultimately, I show this picture to be oversimplified, and that anion-anion and cation-anion correlations severely limit the t+ of high conductivity polyelectrolyte solutions.
In the second, we suspend Li-ion conducting inorganic particles, which have both high conductivity and unity Li+ transference numbers, in organic electrolytes. The development of these organic-inorganic composite electrolytes could enable solid state batteries, an important emerging energy storage technology that has been hindered by the poor processability of thin-film pure inorganic ion conductors. Although the composite electrolyte field is highly active due to the processability advantages composite electrolytes possess, researchers are still puzzled about why, in most cases, no significant improvement in the electrolyte conductivity is observed after incorporating inorganic particles, whose conductivity is orders of magnitude larger than that of polymer electrolytes at room temperature. I will present our efforts to quantify phase contributions to ion transport in model inorganic-organic systems, ultimately showing that three critical factors govern the conductivity of composite electrolytes: Li+-desolvation dynamics, Li+-transference number in the organic phase, and the ceramic particle size. Using this knowledge, show that certain composite configurations have enhanced conductivity and substantially higher transference numbers than the pure model organic electrolyte alone.
Bryan McCloskey is the Department Chair and Warren & Katharine Schlinger Distinguished Professor in Chemical Engineering in the Department of Chemical and Biomolecular Engineering at the University of California, Berkeley. He also holds a joint appointment as a Faculty Chemical Engineer in the Energy Storage and Distributed Resources Division at Lawrence Berkeley National Laboratory. His laboratory explores numerous applications of electrochemistry to energy sustainability, conversion, and storage.
Current projects focus on elucidating the fundamental electrochemistry of metal-air batteries and understanding a variety of challenges facing Li-ion and Na-ion batteries, including high voltage cathode-electrolyte interfacial stability and organic-inorganic composites for solid-state batteries. He has co-authored more than 175 articles and has won numerous awards for his research, including The Electrochemical Society Charles Tobias Award, The International Society of Electrochemistry Tajima Prize, and the VW/BASF Science Award- Electrochemistry. More information about the McCloskey Lab can be found at the Lab’s website: www.mccloskeylab.com.