Metal-chelating organogel electrolyte for lithium ion batteries

Abstract

Lithium manganese oxide spinel (LiMn2O4), one of the popular cathode material for lithium ion batteries (LIBs), can be delithiated to Mn2O4 at ~4 V and lithiated to Li2Mn2O4 at ~3 V. In a practical LIB cells, the 4 V reaction has been used to deliver ~100 mA gactive-1. The 3 V reaction, one the other hand, providing additional ~100 mA gactive-1 has not been used because (1) it is unstable, (2) it proceeds at low potential and (3) it requires lithium source from anodes. In both reactions, LiMn2O4 suffers from (1) severe Mn2+ dissolution into electrolyte and (2) subsequent Mn0 deposition on anodes. The divalent ion dissolution is initiated by Mn3+ formation followed by disproportionation 2Mn3+ ＆agrave; Mn2+ + Mn4+ at 4 V or 3 V. To suppress the problems caused by Mn2+ dissolution, we introduced an organogel electrolyte characterized by (1) in situ thermal gelation, (2) high Li+ transference number and (3) Mn2+ chelating capability. The Mn2+-capturing organogel dramatically improved cyclability of LiMn2O4||Gr cells at practical 4 V reaction and both 4 V and 3 V reactions. Two roles of the organogel were found. First, it suppressed Mn0 deposition on anodes by capturing Mn2+ before the ions reach anodes (anode protection). Second, it suppressed the disproportionation reaction of cathode by keeping highly-concentrated Mn2+ around the surface of Li2Mn2O4 and Li2Mn2O4 (cathode protection by Le Chatelier＆rsquo;s principle).