Ionic Liquid Enabled High Energy-Density Lithium-Ion Batteries
Authors: Evans Tyler , Molina Piper Daniela , Kim Seul Cham , Han Sangsub , Doeff Marca , Ban Chunmei , Cho Sung-Jin , Oh Kyu Hwan , Lee Sehee 
Published: February 2020

The automotive segment of the global lithium-ion battery market is slated for exceedingly high growth in the near future, stoking a demand for next-generation electrode materials capable of delivering the 400 Wh/kg benchmark at low-costs. While most of the recent research efforts to enable such materials focus on complex material modifications and nano-architectured electrode design, developing alternative electrolyte chemistries presents another avenue towards creating the next generation Li-ion battery. Previous work at the University of Colorado Boulder demonstrated the viability of this approach. By pairing a simple, scalable, yet robust Si electrode architecture with an imide-based room temperature ionic liquid (RTIL) electrolyte, this work enabled a Si/L333 full-cell capable of long-term cycling (>1000 cycles) at a charge-discharge rate of 1C.1

Recently an initial feasibility study has revealed the impressive compatibility of the same imide-based RTIL electrolyte with lithium-manganese-rich (LMR) layered oxides. The newly developed LMR material had the potential to revolutionize the transportation industry, especially if the material could be paired with a high capacity anode material such as silicon (Si). The resulting full-cell was proposed to truly enable the electric vehicle (EV), driving down battery costs to less than $200/kWh while supplying double the drive range of state-of-the-art Li-ion technology. The material, formulated as xLi2MnO3(1-x)LiMO2 or Li[LixM1-x]O2 (M = Ni, Mn, Co), is known as the lithium-manganese-rich (LMR) oxide. The beauty of the LMR material lies in the activation process undergone at >4.4 V vs. Li/Li+ during initial charging, resulting in an unprecedentedly high operating voltage and capacities of ~260 mAh g-1.2-8 Despite the potential for massive technological impact, worldwide research has struggled to enable the LMR material. Early work impressively laid the foundation for widespread efforts targeting this material and its signature drawback: the gradual lowering of cell operating voltage over cycling life as the originally layered crystal structure transforms to a spinel phase, accompanied by oxygen evolution during activation of the Li2MnO3 component and transition metal dissolution.

In this work, we have focused our efforts on the electrode-electrolyte interactions known to accelerate phase change in the LMR system. Leveraging the understandings of LMR interfacial behavior built by decades of research, we employ a unique electrolyte composition to form a cathode-electrolyte interface (CEI) that allows for the improved long-term voltage stability of the LMR cathode. Our novel CEI is formed in situ through the oxidative decomposition of a room temperature ionic liquid (RTIL) electrolyte doped with a sacrificial fluorinated salt additive. For the first time, we demonstrate an LMR system capable of 1000 high capacity cycles with minimal voltage decay, shedding light on the importance of the LMR CEI and elucidating the complex interplay between the electrolyte and the atomic scale transformations of an unstable crystal lattice. In this talk, these results will be presented in more detail.

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