Improving the Performance of Li-Ion Batteries with Multifunctional Separators - the Present State-of-the-Art
Authors: Halalay Ion C. , Aurbach Doron , Banerjee Anjan , Ziv Baruch , Shilina Yuliya A. , Luski Shalom , Ziegelbauer Joseph M. 
Published: February 2020

Transition metal (TM) ions dissolution from positive electrodes, migration to and deposition on negative electrodes, followed by Mn-catalyzed reactions of the solvent and anions, with loss of electroactive Li+ ions, is major degradation (DMDCR) mechanism in Li-ion batteries (LIBs). Work on various mitigation measures for the DMDCR mechanism spans now more than two decades. While details of the DMDCR mechanism and the relative contributions of different causes to TM ions dissolution are still under debate, it is clear that HF and other acid species’ attack is the main cause in solutions with LiPF6 electrolyte.
Several measures for mitigating manganese dissolution or its consequences have been reported over the years in the literature,1,2 including elemental substitutions (doping) in the lattice of the positive electrode active material,3 surface coatings4 and the application an inorganic barrier coatings onto electrodes by atomic layer deposition (ALD),5 passivating additives in the electrolyte solution,6 and the reduction of the state-of-charge swing during battery operation. Unfortunately, no single mitigation measure turned out 100% successfully so far, i.e., without negatively affecting other properties of the LIB such as energy density and internal resistance.
A different approach, that of using a separator containing multifunctional (manganese ion chelating, HF scavenging and alkali metal dispensing) material, may avoid the above described previously mentioned drawbacks.7-9 Herein we review recent progress on our understanding of Mn species in electrolyte solutions and on a mitigation measure first proposed by Tarascon and coworkers in 1999,7 namely chelation of TM cations. Our focus is a practicable, drop-in technical solution, based on multi-functional materials placed in the inter-electrode space. Such materials can trap Mn cations, scavenge HF and dispense sacrificial Li+ ions,8-10 with significant benefits for LIBs performance: increased capacity retention during operation at room and above-ambient temperatures, robust (more electronically insulating) solid-electrolyte interfaces, as well as reduced charge transfer and film resistances at both negative and positive electrode surfaces.

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