Impact of Electrode Nature on Lithium-Ion Battery Performances with Ionic Liquids or Carbonate Electrolytes
Authors: Bolimowska Ewelina , Chancelier Léa , Srour Hassan , Gutel Thibaut , Rouault Helene , Santini Catherine C. 
Published: February 2020

Lithium-ion batteries have rapidly become the most common power source for portable electronic devices, power tools and electric vehicles due to their high energy density and good cycling stability. However, before implementation of the Li-ion batteries in EVs and HEVs, many features still must be resolved, like low cost, long calendar life, safety and high power capability (1). Replacement of the organic carbonates with more thermally and electrochemically stable ionic liquids (ILs) could increase the security (2). A lot of work was already reported in the literature about the ILs as electrolytes, especially in comparison of cycling with organic electrolyte, but still with different electrodes combination. By following the nature of the electrodes couples, the power capacity could be tuned (1). Nevertheless, a few systematic comparison of these different systems are available (3-4).

In this work, we report the performance of three full cell configurations: graphite (Cgr)// LiFePO4 (LFP), Li4Ti5O12 (LTO)// LiFePO4 (LFP) and Li4Ti5O12 (LTO)// LiNi1/3Mn1/3Co1/3O2 (NMC), with five different ionic liquids and one organic carbonate electrolytes. Their cycling results, at 333 K, with a pyrrolidinium [PYR14][NTf2], series of imidazolium ionic liquids ([C1CnIm][NTf2] and [C1C1CnIm][NTf2]/ n= 4 and 6) associated with LiNTf2 (1mol.L-1)  then an organic electrolyte (LiPF6 (1mol.L-1) DEC: EC), are depicted in Figure 1 (a-c).

Some trends have been found. For Cgr//LFP system cycling is observed only in the presence of organic additive, vinylene carbonate (VC) (Fig. 1a). Carbonates, except for LTO//NMC cell, show lower capacity then ILs based electrolytes (Fig. 1). The highest capacity is observed for LTO//NMC system with ([C1C4Im][NTf2], [PYR14][NTf2] and carbonate electrolyte, ~180 mAh.g-1, ~140 mAh.g-1 and ~180 mAh.g-1, respectively (Fig. 1c). The introduction of a donating group (-CH3) on position two of the imidazolium ring (C2-H → C2-CH3) in [C1C1CnIm][NTf2] highly improve the performance of Cgr//LFP cells (Fig. 1) (5). Besides LTO//NMC, the charge/discharge performance is improved by extending the alkyl chain length of the imidazolium ring. Finally, in LTO//LFP and LTO//NMC, higher performances were obtained with ([C1C4Im][NTf2] compare to [PYR14][NTf2].

From these results, we highlight the tremendous impact of each element of the system (electrode, electrolyte, and separator (6)) onto the charge discharge capacity, especially with ionic liquids based ones.

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Figure 1