Exploring Electrode-Preparation Methods of Si Anodes with Li+-Containing Organic Ionic Plastic Crystals for High-Energy-Density Solid-State Batteries
Authors: Ueda Hiroyuki , Forsyth Maria , Howlett Patrick C. 
Published: December 2024

Silicon has an extremely high theoretical capacity (3,579 mAh/g for Si ⇄ Li15Si4), which is almost 10-fold greater than that of a commercialized anode active material in lithium-ion batteries (LIBs), i.e., graphite (371.9 mAh/g for C ⇄ Li6C). Therefore, the successful use of silicon is required for “beyond” LIBs to offer drastically higher energy density than current LIBs. In contrast to lithium metal, silicon is not sensitive to moisture, i.e., no risk of vigorous hydrolysis, which enables the manufacturing of silicon anodes in an ambient condition and provides safer rechargeable batteries than lithium−metal batteries.1 However, reported “beyond” LIBs using silicon anodes often show poor cycle performance primarily because of the large volume change of silicon particles during their alloying and dealloying reactions with lithium species. Prolonged cycling of silicon anodes is prone to particle cracking and new surfaces are exposed to the electrolyte solution, where further electrolyte decomposition (i.e., the formation of solid-electrolyte interphase, SEI) occurs, consuming both lithium inventory and electrolyte.1 To extend the cyclability of silicon anodes, the loss of lithium inventory needs to be minimized.
On the other hand, solid-state batteries (SSBs) are regarded as a promising battery format for silicon anodes because solid electrolytes are unlikely to flow through the electrode layer and, therefore, they would experience less aggressive SEI formation on silicon than liquid electrolyte solutions. This concept has been proven by an outstanding cycle performance of Si | solid electrolyte (Li6PS5Cl) | LiNi0.8Mn0.1Co0.1O2 cells.2 However, their cycle performance was strongly dependent on the external pressure on the cells and it had to be sufficiently high (i.e., ≥50 MPa) to ensure intimate electrode–electrolyte contacts.2 To realize the operation of silicon-based SSBs under moderate or no external pressure, replacing the solid electrolyte with a soft one will be beneficial, but silicon composite anodes with soft solid electrolytes have not been widely explored.1

Herein, we present solid-state silicon composite electrodes with an emerging class of soft solid electrolytes, i.e., Li+-containing organic ionic plastic crystals (OIPCs).3 We explored four methods of incorporating OIPC electrolytes into a silicon electrode (Si : carbon black : sodium carboxymethyl cellulose = 70 : 15 : 15 wt%) to provide insights into the process–structure–property relationship for silicon–OIPC composite electrodes (Fig. 1). Cross-section structures and elemental distributions of silicon–OIPC composite electrodes containing 15 wt% of the OIPC electrolyte, i.e., the equimolar mixture of N-ethyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide ([C2mpyr][FSI]) and LiFSI, were resolved by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX), respectively. The half-cell performances of silicon–OIPC composite electrodes were evaluated in a coin-cell (CR2032) configuration, where electrospun poly(vinylidene fluoride) (PVDF)–Li0.50[C2mpyr]0.50[FSI] composite membranes were used as solid-electrolyte interlayers.4

Fig. 2 shows the delithiation capacities of silicon–OIPC composite electrodes made by four different methods over the first five cycles. Except for the one made by the drop-cast method, all silicon–OIPC composite electrodes showed high delithiation capacities that were close to or even greater than that of a silicon electrode (without the OIPC electrolyte) in a liquid electrolyte solution. Elemental mapping of the electrode clearly suggested the localization of the OIPC electrolyte for the drop-cast sample, whereas in the other silicon–OIPC composite electrodes, it was homogeneously distributed, highlighting the importance of OIPC-electrolyte addition to a silicon composite slurry before coating. In the presentation, we will also present differences in Nyquist plots (i.e., bulk, interphase, and charge-transfer resistances) between various silicon–OIPC composite electrodes. The results unveiled the effect of electrolyte-preparation methods on electrode structures and consequent electrochemical performances in SSBs, which will lay a robust foundation for high-energy OIPC-based SSBs with practical cycle life.

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.

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Figure 1