Impact of Electrode Nature on Lithium-Ion Battery Performances with Ionic Liquids or Carbonate Electrolytes

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)// LiNi_1/3Mn_1/3Co_1/3O₂ (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 ([C₁C_nIm][NTf₂] and [C₁C₁C_nIm][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 (C_2-H → C_2-CH3) in [C₁C₁C_nIm][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.

References:

1. J.-M. Tarascon, M. Armand, Nature, 414, 2001; J.-K. Park, Wiley-VCH, Weinheim, 2012;  B. Scrosati et al., Wiley-VCH, 2013
2. A. Lewandowski, A. Swiderska-Mocek, J. Power Sources, 194, 2009
3. K. Xu, Chem. Rev., 2014
4. S. Tan et al., ChemPhysChem, 15, 1956 – 1969, 2014
5. S. Seki, Electrochemical and Solid-State Letters, 10, A237-A240, 2007 and S. Seki, J. Phys. Chem. B, 110, No. 21, 2006
6. M. Kirchhöfer et al., Int. J. Mol. Sci., 15, 14868-14890, 2014