Importance of Selecting an Appropriate Li+ Concentration in Prolonging the Cyclability of a Solid-State Si Anode with an Organic Ionic Plastic Crystal
Authors: Ueda Hiroyuki , Huang Gongyue , Forsyth Maria , Howlett Patrick C. 
Published: December 2024

To address the ever-growing demands for battery-driven electrification, successful solid-state batteries (SSBs) are expected to deliver a higher energy density than lithium-ion batteries (LIBs). One of the drastic pathways to realize this is utilizing a high-capacity anode active material such as silicon and lithium metal. In particular, silicon electrodes can be made via a roll-to-roll process that has been used in producing LIB electrodes without worrying about the ambient moisture level and, therefore, their use in SSBs will be more cost-competitive and safer than those using lithium metal if the incorporation of solid electrolytes into the electrode layer can be achieved easily. In this respect, we previously developed an incorporation method of emergent soft solid electrolytes, i.e., Li+-containing organic ionic plastic crystals (OIPCs), into a silicon electrode and demonstrated their excellent applicability to doctor-blade coating, which can be scaled up to the roll-to-roll process.1

OIPCs, which are composed of organic cations and organic/inorganic anions, are unique ion-conduction media and have been actively employed in research of SSBs, especially since the discovery of a significant improvement of ionic conductivity by the addition of a Li+ salt to OIPCs.2 Incorporating additional components into OIPC structures alters their ion-conduction behaviors and the resulting ionic conductivities of the composites depend on various factors such as the chemical nature,3 particle size,4 and concentration of dopants.5 In the case of Li+ addition, it has been known that the physical states (i.e., softness) of Li+-containing OIPCs also depend on Li+ concentration.1 Therefore, both the electrochemical and mechanical properties of Li+-containing OIPCs need to be tailored to achieve the long-term cyclability of OIPC-containing electrodes for SSBs, but this has yet to be fully explored.

In the present study, we made solid-state silicon composite electrodes with Li+- containing triethylmethylphosphonium bis(fluorosulfonyl)imide (Li
x
[P1222]1−x
[FSI]) of various Li+ concentrations (x = 0.05, 0.10, 0.30, 0.50, 0.70, or 0.90) via the “all-in-one” method1 and evaluated their electrochemical performances in CR2032 half cells with Li0.50[P1222]0.50[FSI]-containing poly(diallyl dimethylammonium) bis(fluorosulfonyl)imide (PDADMA-FSI) interlayers at 50 °C, where the silicon electrode’s composition was 70 : 15 : 15 wt% and the weight ratio of the electrode to Li
x
[P1222]1−x
[FSI] was 85 : 15 wt%. The mechanical properties of Li
x
[P1222]1−x
[FSI] were evaluated by rheometry. We also performed cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic intermittent titration technique (GITT) to decipher the electrochemical properties of the electrodes with different Li+ concentrations. These results allowed a discussion about how the properties of Li
x
[P1222]1−x
[FSI] influenced the half-cell cyclability.

Fig. 1 shows the delithiation capacities of silicon–OIPC composite electrodes with different Li+ concentrations over the first ten cycles. From the first cycle, the delithiation capacity of the electrode with a relatively low or high Li+ concentration (i.e., 5, 70, or 90 mol%) was lower than those of the other electrodes with middle Li+ concentrations. As the cycle number increased, the dependence of the delithiation capacity on the Li+ concentration became obvious and, at the 10th cycle, the electrode with 50 mol% Li+ showed the highest delithiation capacity. The potential difference (ΔE) between dQ/dV peaks for the redox couple involving amorphous silicon (a-Si) also showed the same trend (Fig. 2), i.e., the 50 mol% sample provided the prolonged observation of the redox couple over the cycle test as well as the lowest ΔE value among various Li+ concentrations. This highlights the importance of controlling the interfacial Li+ concentration at a silicon surface to a middle range. In the presentation, we will show the results of other electrochemical and mechanical analyses to clarify the benefit of using the middle concentration further.

Acknowledgment

The authors acknowledge the Australian Research Council (ARC) for the financial support under the Linkage Project scheme [grant number: LP180100674]. H. Ueda would like to thank Deakin University for providing an Alfred Deakin Postdoctoral Research Fellowship.

References

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H. Ueda, N. Saito, A. Nakanishi, H. Zhu, R. Kerr, F. Mizuno, P. C. Howlett and M. Forsyth, Mater. Today Phys., 43, 101395 (2024).

Figure 1