By Kent Griffith
November 17, 2021 | The race is on for a solid-state battery that could enhance the safety and performance of energy storage for applications such as electric vehicles, consumer electronics, and intermittent renewable energy utilization. Underlying a solid-state battery, fundamentally, is a solid electrolyte—a material with a very difficult job. An electrolyte is an electronically insulating phase that is designed to shuttle lithium ions between a positive and a negative electrode while providing chemical and mechanical stability. Meanwhile, said electrodes are undergoing large volume changes and are strong chemical oxidizers (cathodes) or reducing agents (anodes). Beyond these chemo-mechanical requirements, the electrolyte should ideally be rapid and single-ion conducting, selectively transporting lithium ions in the case of a lithium battery. For commercial applications, the electrolyte must be synthesizable with raw materials and manufacturing processes that are scalable and cost-efficient. Few materials are up to the task, and so, despite several billion dollars in academic and commercial investment, solid-state batteries are a work in progress.
Solid electrolyte candidates can be divided into two main categories: inorganic or (organic) polymeric. Inorganic electrolytes include oxide material families and sulfide material families. The field of solid polymer electrolytes has been led by poly(ethylene oxide), or PEO. As the only commercialized solid electrolyte in large-scale applications, PEO is used by Blue Solutions for electric vehicle and grid energy storage systems because it has the advantageous combination of low density, good processability, good contact with the electrodes—known as wetting—and good lithium-ion conductivity, albeit only at elevated temperatures. A room-temperature electrolyte with suitable properties is elusive.
Enter Liangbing Hu and the copper-coordinated cellulose solid electrolyte. In a simple Pyrex baking dish, the team modified cellulose nanofibrils with copper ions to separate the chains and then infused the polymer with lithium ions. The result? A lithium-ion solid polymer electrolyte capable of conducting lithium ions faster than any known polymer and on par with high-performance inorganic oxides such as the lithium lanthanum zirconium oxide (LLZO)-based garnet electrolyte. The conductivity of the new material is still about an order-of-magnitude below the best inorganic sulfide solid electrolytes. Moreover, the copper-coordinated cellulose boasts good conductivity down to low temperatures and appears to be stable against chemical reactivity while in contact with lithium metal. The work appeared recently in the journal Nature: Yang et al. Nature 2021, 598, 590–596.
Cellulose is a common organic polymer derived from biomass and is the primary component of plant matter such as cotton and wood. It is commonly encountered as the main ingredient in paper products and as dietary fiber. Within the hierarchy of a cellulose fiber, one finds nanofibrils, elementary fibrils, and molecular chains. At the molecular level, the chains are essentially polymers of sugar molecules, making cellulose a carbohydrate and a polysaccharide. The new ionically conductive medium was prepared by first dissolving copper wire in an alkaline solution and then soaking the cellulose nanofibrils over the course of weeks until they became blue and saturated with Cu2+ ions. Next, the water was removed by solvent exchange with dimethylformamide, a harsh organic solvent, which was subsequently evaporated. Water is generally to be avoided in high-energy lithium battery cells because it may react with the other cell components and/or undergo electrolysis to hydrogen and oxygen gases. After removing the water and organic solvent, the copper-coordinated cellulose nanofibrils were soaked in a standard liquid battery electrolyte comprising lithium hexafluorophosphate salt in an alkyl carbonate organic solvent.
Another challenge that the researchers sought to overcome with the new solid polymer electrolyte is the manufacturing of thick cathode composites. In a fully solid-state battery, the cathode is typically mixed with the solid electrolyte to facilitate lithium-ion conduction. This cathode composite layer must be thick and contain a minimal quantity of electrolyte to ensure high energy density for the resulting battery. With only 5 wt.% (15 vol.%), the copper-coordinated cellulose creates a percolating pathway that enabled the full theoretical capacity to be accessed in a 120 µm thick electrode of LiFePO4 (LFP), albeit at a slow rate of C/10. At more practical charge/discharge rates of C/2, solids-state batteries with cathodes of LiNi0.8Mn0.1Co0.1O2 (NMC811) or LiMn2O4 (LMO) were demonstrated to reach about 75% of their typical respective capacities. These initial data show promising results for the cycle life performance as well, particularly for LFP and LMO.
According to Professor Lauren Marbella from the Department of Chemical Engineering at Columbia University, who was not involved in the study, “The new, exciting finding in this work is that Li-ion transport can be decoupled from polymer dynamics to control ionic conductivity in polymeric materials. The ability to manipulate Li transport and transference number [a measure of the desired single-ion conducting nature of an electrolyte] shows that we have a lot more synthetic control than previously thought to turn poor ion conductors into materials that combine the most desirable properties from ceramics and polymer electrolytes into one.” Marbella, an expert in metal anode batteries, went on to note, “This approach also has implications in the design of stable, highly conductive interfaces and coatings for Li-based systems as well as more challenging chemistries, such as other alkali metals or multivalent batteries.”
The authors of the study are also hoping the new solid polymer electrolyte may prove useful in applications beyond lithium-ion batteries, with early results demonstrating rapid conduction of sodium and zinc ions. The biomass type is not limited to cellulose either, good results were demonstrated with chitosan (from shellfish), alginate acid (from seaweed), xanthan gum (from common bacteria), and cellulose derivatives.
“The engineering of ionically conductive cellulose is an interesting and creative advance that involves altering the molecular structure to enable fast ion conduction. The metrics they report, such as ionic conductivity, are quite good. This work potentially opens the door to simplified processing and manufacturing methods for batteries.” says Professor Matthew McDowell of Georgia Tech University, an expert in solid-state batteries who was not involved in the study. “However, there are a number of expected limitations as well—it is not clear that such materials would prevent dendritic growth of lithium in solid-state batteries, which is a major challenge facing the field.”
Polymer solid electrolytes, though not always receiving the attention of their ceramic oxide and sulfide counterparts, may have new roles to play in the future of solid-state batteries. Further studies will be needed on long-term stability, scalability and technoeconomics, and safety/dendrite suppression. With a new, general platform for ion conduction from biomass polymers, the door is open for exploration.
The project, led by Hu and his research group at the University of Maryland, was completed in collaboration with Brown University, Florida State University, Hunter College–City University of New York, the University of Delaware, the National Institute of Standards and Technology, the National High Magnetic Field Laboratory, the University of Tokyo, the University of Münster, and the Army Research Laboratory.