Fabricating Highly Functional Li4Ti5O12 Composite Anodes Using Li+-Containing Organic Ionic Plastic Crystals
Authors: Ueda Hiroyuki , Quattrocchi Zak , Forsyth Maria , Howlett Patrick C. 
Published: December 2024

Solid-state batteries (SSBs) are one of the next-generation batteries that are expected to overcome some limitations (e.g., inherent safety issues) of current lithium-ion batteries (LIBs). The key components of SSBs are solid electrolytes that are generally more stable against temperature rise than conventional flammable liquid electrolyte solutions in LIBs, thereby reducing the risk of catching a fire.1 Although high ionic conductivities of solid electrolytes are regarded as the prerequisite for high-performing SSBs, excellent applicability of solid electrolytes to high-throughput processes is also a considerable requirement to make the production of SSBs commercially viable.2 In this respect, exploring soft solid electrolytes will be a worthwhile approach as some of them (e.g., polyether-based solid polymer electrolytes, SPEs) have been successfully employed in the roll-to-roll processes.3

In the last two decades, organic ionic plastic crystals (OIPCs) have gained researchers’ attention as an emerging class of soft solid electrolytes.4 OIPCs are regarded as solid-state cousins of ionic liquids (ILs). Hence, they inherit IL's beneficial physicochemical properties such as low flammability, negligible vapor pressure, design flexibility, and high thermal and electrochemical stability. So far, OIPC-based interlayers have been successfully employed in SSBs,4 but the use of OIPC electrolytes (e.g., OIPC + Li salt) in electrodes for SSBs had yet to be explored until recently.5, 6 The incorporation of OIPC electrolytes in electrodes has been demonstrated as an effective method to improve the electrochemical performances of both cathode and anode active materials for SSBs.5, 6 However, for an anode side (i.e., using graphite or Si), realizing the continuous access to the full capacity of active material is still challenging, which is partly due to the irreversible reduction of OIPC electrolytes and poor electron- and ion-conduction pathways in solid-state electrodes.
In this study, we created Li4Ti5O12 (LTO) composite anodes with OIPC electrolytes, i.e., Li+-containing N-methyl-N-ethylpyrrolidinium bis(fluorosulfonyl)imide (Li
x
[C2mpyr]1−x
[FSI]) to address the aforementioned issue. The typical lithiation/delithiation reactions of LTO occur at a higher electrode potential (ca. 1.55 V Li/Li+) than graphite (ca. 0−0.25 V Li/Li+) and Si (ca. 0−0.50 V Li/Li+), and, therefore, unwanted side reactions below this potential and any consequent adverse effects are expected to be minimized. LTO−Li
x
[C2mpyr]1−x
[FSI] composite electrode disks, SPE membranes comprising Li
x
[C2mpyr]1−x
[FSI]-containing poly(diallyl dimethylammonium) bis(fluorosulfonyl)imide (PDADMA-FSI), and Li metal disks were used to assemble half cells for battery tests at 50 °C.
Fig. 1 shows the half-cell performance of LTO−Li0.10[C2mpyr]0.90[FSI] (LTO loading: 1.17 mg/cm2) at 0.1C over the initial 30 cycles. The typical lithiation/delithiation plateaus of LTO were identified from the first cycle (Fig. 1a), but the initial lithiation and delithiation capacities (76.9 and 50.3 mAh/g, respectively) were lower than the theoretical capacity of LTO (175 mAh/g for Li4Ti5O12 ⇄ Li7Ti5O12). The capacities increased with cycle number up to ≈12 and the Coulombic efficiency also increased gradually from 65.4% at the first cycle to 98.9% at the 30th cycle (Fig. 1b), indicating “pre-conditioning” of the electrode to form better electron-/ion-conduction pathways over cycles. The electrode also showed excellent rate capability for both directions; Its lithiation and delithiation capacity ratios (i.e., 2C capacity vs. 0.1C capacity) were 89.4% and 93.8%, respectively. The results demonstrate the promising performance of LTO−Li
x
[C2mpyr]1−x
[FSI] electrodes. In this presentation, the long-term cycling test results, LTO−Li
x
[C2mpyr]1−x
[FSI] anodes, and current-collector-induced degradation will also be presented.

Acknowledgment

H. Ueda would like to thank Deakin University for providing an Alfred Deakin Postdoctoral Research Fellowship.

References

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H. Ueda, “Interphase-driven ion conduction in organic ionic plastic crystal-based solid electrolytes: A review of symmetric cell studies.” Encyclopedia of Solid-Liquid Interfaces, ed. K. Wandelt and G. Bussetti, (Oxford) (Elsevier) 1st ed., p. 743, (2024).

H. Ueda, F. Mizuno, M. Forsyth and P. C. Howlett, J. Electrochem. Soc., 171, 020556 (2024).

P. Howlett, M. Forsyth, R. Kerr, T. Mendes, H. Ueda and Y. Liang (Deakin University), Ionic Binders for Solid State Electrodes, WO 2023/019322 A1, (2023).

Figure 1