Northwestern study finds Li3PO4 promising coating to limit dissolution of transition metals from Li-ion cathodes
The dissolution of transition metals (TMs) from Li-ion battery cathodes is a major contributor to cell degradation during cycling and aging. First, such dissolution decreases the amount of cathode material, directly contributing to loss of capacity. Secondly, dissolved TMs can migrate through the electrolyte to the anode, causing chaneges to the Solid Electrolyte Interphase (SEI), resulting in increased impedance, decreased cell capacity, and decreased lifespan.
A study by a team at Northwestern University has found that Li3PO4 is a promising candidate as a stable coating on oxide materials to limit such dissolution of transition metals into the Li-ion electrolyte. An open access paper on their work is published in the Journal of The Electrochemical Society.
A variety of strategies have been used to limit degradation by TMs. Electrolyte additives have been used to control SEI formation and limit the impact of TMs in the electrolyte. Molecules have been attached to the polymer separator to sequester TMs from the electrolyte before they reach the anode. In this work, we consider upstream strategies that limit TM dissolution from the cathode into the electrolyte. These upstream strategies are complementary to other strategies, including SEI formation and TM sequestration.
There are several distinct categories of strategies for limiting TM dissolution from the cathode. Electrolytes can be tailored to reduce reactivity with the cathode. Cathode materials can be doped to control the oxidation states of transition metals. This doping can be applied to the entire cathode particle or just near the surface. Cathode materials can also be covered with surface coatings to limit TM dissolution. Surface coatings can perform a variety of functions for different cathode materials. In this work, we evaluate the ability of coating materials to contain TMs in the cathode and thereby prevent TM dissolution into the electrolyte.—Snydacker et al.
In their study, the researchers performed density functional theory calculations to evaluate cathode/coating pairs for TM containment, specifically focusing on reactive stability of coating/cathode pairs as well as TM solubility in the coating materials.
They found that many cathode/coating pairs are reactive when lithiated, while other cathode-coating pairs are stable when lithiated but become reactive following delithiation.
Li3PO4 was unique, in that its coatings on oxide cathode materials maintained equilibrium under both lithiated and delithiated conditions.
Further, for oxide cathode materials, the Li3PO4 coatings exhibit low TM solubilities across all cathode states of charge.
Our finding of exceptional stability for Li3PO4 can be understood in terms of underlying physics. Li3PO4 has the strongest (lowest) formation energy out of all the phases contained within the Li-M-P-O (M = Mn, Fe, Ni) subset of our database. Li3PO4 has a polyanion with covalent bonds that impart stability with respect to oxygen evolution at elevated temperatures. Also, the only ionic cation in Li3PO4 is Li, which exhibits only the 1+ oxidation state in oxides and thereby avoids redox reactions.
Our findings suggest reexamination of certain cathode coating strategies that have been published in the literature. … Our findings also suggest reexamination of MgO and ZnO coatings on layered cathodes to confirm coating reactivity during battery operation. The unanticipated phases that form due to coating reactivity during heat-treatment and battery operation may be harmful or helpful for TM containment and other cathode performance criteria. In any case, consideration of our findings can help target coating compositions for improved performance.—Snydacker et al.
David H. Snydacker, Muratahan Aykol, Scott Kirklin and C. Wolverton (2016) “Lithium-Ion Cathode/Coating Pairs for Transition Metal Containment”Journal of The Electrochemical Society volume 163, issue 9, A2054-A2064 doi: 10.1149/2.1101609jes