Acid-Scavenging Separators: A Novel Route for Improving the Li-Ion Batteries’ Durability
Authors: Banerjee Anjan , Ziv Baruch , Shilina Yuliya A. , Luski Shalom , Aurbach Doron , Halalay Ion C. 
Published: February 2020

The present day state-of-art electrolyte solutions for Li ion batteries consist of LiPF6 salt solutions in mixed alkyl carbonate solvents. Autocatalytic decomposition of LiPF6, with generation of HF and other Lewis acids is the root of causing performance degradation of Li-ion batteries (LIB). While offering the best balance between performance benefits and drawbacks, these electrolyte solutions are neither electrochemically stable in contact with the electrochemically active materials in the electrodes, nor chemically stable at elevated temperatures, when operating with most relevant electrodes’ materials. The presence of acidic species promotes various parasitic reactions, among which transition metal ions’ dissolution and the loss of electroactive Li+ ions (due to destruction of the passivation of the graphite anodes) have the most detrimental consequences for LIB performance.

Several means for minimizing the Mn ions’ dissolution and its consequences were explored over the years: cationic and anionic substitutions into the active material lattice,1 surface coating,2 and the use of various additives.2,3 Nevertheless, none of them were proved completely successful all alone and only a combination thereof is likely to maximize performance improvements. Although many details of LIB degradation reaction mechanisms are still not fully elucidated, there is no doubt that the interaction of HF and other acidic species with positive electrode materials is the major factor driving the TM ions’ dissolution in Li-ion batteries with LiPF6 based solutions, which has a detrimental effect of the stability of graphite anodes in LIBs.4-6 Recently, we proposed a different mitigation route: multifunctional separators placed in the inter-electrode space.7,8 We demonstrate herein the benefits for LIB performance enabled by active separators which can trap acidic species and transition metal cations.

Herein we report on the performance improvements enabled by an acid-scavenging separator in cells with graphite negative and LiMn2O4 or LiNi0.6Mn0.2Co0.2O2 positive electrodes. After 4 weeks of cycling at 55°C, LiMn2O4║graphite and LiNi0.6Mn0.2Co0.2O2 ║graphite cells with functional separators retain 100% and 43% more capacity, respectively, than cells with plain polypropylene separators. Furthermore, cells with functional separators have half the interfacial impedances of cells with baseline separators, irrespective of positive electrode. The benefits afforded by acid scavenging separators thus extend to broader classes of cell chemistries, beyond those affected mainly by manganese dissolution and loss of electroactive Li+ ions.

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