Electrochemical Performance and Electrodes Surface Analysis of He-NCM¦LTO Cells – Ionic Liquid vs.Carbonate Based Electrolytes

The instability of the most commonly used carbonate-based electrolytes at high temperatures (> 50°C) and high potentials (> 4.5 V) is limiting the development of the next generation of high specific energy Li-ion batteries.[1] However, ionic liquids (ILs) has been proven promising under very demanding conditions. Their physical properties, such as thermal (> 120°C) and electrochemical (> 5.5 V) stability as well as low vapor pressure, make them promising for use as electrolytes. [2]

 In this context, we investigated the electrochemical performance of a full cell combining spinel lithium titanate Li₄Ti₅O₂ (LTO) vs. over lithiated layered Li(Ni, Co, Mn)O₂ (denoted HE-NCM) at room temperature (RT) and 50°C. The electrolytes considered for this study are the LP30 carbonate-based (1M LiPF₆ in 1:1 EC:DMC) and IL-based (1M LiBF₄ with BMIMBF₄).[3]

 Figure 1 confirms a stable electrochemical cycling at room temperature with LP30 electrolyte, however, with the IL 55% less specific charge is obtained on the 2^ndcharge and a continuous fading is observed with only 30 mAh/g after almost 130 cycles.

 Post mortemX-ray photoemission spectroscopy (XPS) measurements were carried out on both electrode surfaces after cycling at room temperature and 50°C. A detailed physico-chemical description of the (SEI/SPI) will be presented. In addition, the impact of the electrodes preparation on the SEI/SPI composition pre-XPS analysis is also investigated by comparing washed with dimethyl carbonate (DMC) and unwashed electrodes.

 Operando high resolution XRD and post mortemSEM analysis of the bulk and the surface morphology, respectively, will be discussed as well and correlated with the XPS measurements to better explain the SEI/SPI formation and the electrochemical behavior.

 The combination of the electrochemical, structural and physico-chemical characterization techniques offer new insights into the complex electrode/electrolyte interface reactions. A unified model describing the mechanism of the electrodes film formation as a result of electrolyte decomposition will be explored.

References:

[1] John B. Goodenough, Rechargeable batteries: challenges old and new, J. Solid State Electrochem, 2012 162019-2029.

[2] M. Armand, F. Endres, D. R. MacFarlane, H. Ohno, B. Scrosati, Ionic-liquid materials for the electrochemical challenges of the future, Nature Materials, 2009 8621-629.

[3] E. Markevich, V. Baranchugov, D. Aurbach, On the possibility of using ionic liquids as electrolyte solutions for rechargeable 5V Li ion batteries, Electrochem. Commun., 2006 8 1331-1334.