Pyrrolidinium Containing Ionic Liquid Electrolytes for Li-Based Batteries

Abstract

Ionic liquids are potential alternative electrolytes to the more conventional solid-state options under investigation for future energy storage solutions. This review addresses the utilization of IL electrolytes in energy storage devices, particularly pyrrolidinium-based ILs. These ILs offer favorable properties, such as high ionic conductivity and the potential for high power drain, low volatility and wide electrochemical stability windows (ESW). The cation/anion combination utilized significantly influences their physical and electrochemical properties, therefore a thorough discussion of different combinations is outlined. Compatibility with a wide array of cathode and anode materials such as LFP, V₂O₅, Ge and Sn is exhibited, whereby thin-films and nanostructured materials are investigated for micro energy applications. Polymer gel electrolytes suitable for layer-by-layer fabrication are discussed for the various pyrrolidinium cations, and their compatibility with electrode materials assessed. Recent advancements regarding the modification of typical cations such a 1-butyl-1-methylpyrrolidinium, to produce ether-functionalized or symmetrical cations is discussed.

Keywords: anode; cathode; electrolyte; energy storage; ionic liquids; lithium ion batteries; lithium metal batteries; polymer gel electrolyte; pyrrolidinium.

Figure 1

The performance of Li|SPE: 50:20:50 P(DADMA)-TFSI:LiTFSI: C₂mpyrFSI|LiFePO₄ cells: (a) discharge capacity and coulombic efficiency versus cycle number at 25 °C, 40 °C and 80 °C at a current rate of 0.2 C, (b) SEM images of the pristine lithium and lithium anodes after a rate test (0.2 C, 0.5 C, 1 C and 0.2 C). Adapted from Li et al. [140] with permission from The Royal Society of Chemistry.

Figure 2

SEM and EDX-mapping images of the sectional view of the NMC cathode. (a,b) are with no ionogels (control sample) and (c,d) are with ionogel respectively. Red, green and blue colors indicate cobalt (NMC), sulphur (ionogels) and aluminum (current collector), respectively. (e,f) are EDX-mapping images of cobalt and sulphur extracted from (d). Reprinted with permission from Ogawa et al. [32].

Figure 3

Cycling performance of Li|C₃mpyrFSI–LiTFSI (0.5 mol kg⁻¹) |LiFePO₄ cell at indicated discharge rates (at 50 °C) for: (a) cycles 1–55 and (b) cycles 130–250. Charging rate is constant at C/10. Reprinted with permission from Lewandowski et al. [155].

Figure 4

Cross-sectional SEM images of lithiated Si active material layers with a charge capacity limit of 1000 mAh g⁻¹. The blue colored area indicates Si, while the topmost layer is a carbon coating. Reprinted with permission from Domi et al. [164]. Copyright 2019 American Chemical Society.

Figure 5

SEM images of different IGPE-X membranes, where the X refers to the length of time (in minutes) the gel spent in the coagulation bath. Reprinted with permission from Yang et al. [172].

Figure 6

Cycling stability at 0.2 C of (a) LFP|Li cell, (b) NCM|Li cell, (c) LCO|Li cell and (d) cycling stability at 0.5 C of LFP|Li cell at 80 °C with liquid, polymer and BN-functionalized-polymer versions of C₃mpyrTFSI. Adapted with permission from Kim et al. [173].

Figure 7

Graphical summary of the range of ionic and neutral decomposition species of C₄mpyr-cation. The ionic species (orange box) were identified by Pyschik et al. [176,177] while Kroon et al. [178] confirmed the presence of the neutral species (green box). The neutral vinyl species was identified by Preibisch et al. [175] Reprinted with permission from Preibisch et al. [175]. Copyright 2020 American Chemical Society.

Figure 8

Schematic diagram representing the microstructure of the composite electrodes with polymer gel electrolyte (LCBIM), showing that LCBIM improves the interfacial contact area with conductive and adhesive properties. Carbon additives included in the composite electrode are not shown. Reprinted with permission from Cho et al. [183].

Figure 9

Li|IL|LFP cells galvanostatically cycled at (a) 25 mA g⁻¹ (0.12 mA cm⁻²), with inset reporting the magnification of the final part of the curves, and (b) 250 mA g⁻¹ (1.2 mA cm⁻²). (c) Cycling trend and columbic efficiency of the Li/IL/LFP cells at increasing currents. Voltage cut-offs were 2.2 and 4 V. Pyr₁₄TFSI–LiTFSI (green), Pyr₁₄FSI–LiTFSI (black), Pyr_1(2o1)TFSI–LiTFSI (blue) and DEME-TFSI–LiTFSI (red). All measurements were performed at 40 °C. Reprinted with permission from Elia et al. [39].

Figure 10

(a) Optical images of the 400 μm thick polymer gel synthesized, (b) long-term galvanostatic cycling data for V₂O₅/Li cells at indicated C-rates and (c,d) Ge galvanostatic profiles obtained at indicated C-rates. (a,c,d) Reprinted with permission from McGrath et al. [195].

Figure 11

(a,c) First discharge voltage profiles and (b,d) capacity vs. cycle number at indicated discharge current rates of (a,b) LFP|0.9:0.1 C₄mpyrFSI:LiTFSI|LTO at 20 °C (Li-ion battery) and (c,d) LFP|cl-PEO-C₄mpyrTFSI-LiTFSI|Li at 40 °C (Li metal polymer battery). Reprinted with permission from Balducci et al. [185].

Figure 12

(a): Low magnification image of free-standing Ge nanotubes after removal of the template. (b) Higher magnification image of the Ge nanotubes. (c) Cross sectional view of the Ge nanotubes. (d) Raman spectra of electrodeposited Ge thin films and nanotubes obtained by exposing the sample to 5 mW, 532 nm laser for 10 s. Reprinted with permission from Lahiri et al. [201].

Figure 13

(a–c) Performance of various electrolytes at room temperature with LFP/Li: (a) CV curves, (b) cycling performance at 0.2 C, (c) rate performance at indicated C-rates and (d) cycling performance of RT and high temperature (60 °C). Reprinted with permission from Zhang et al. [209].