Interface Failure and Strategy of Silicon based Solid State Batteries (Part 1): Mechanism

Silicon based solid-state batteries can draw on the design experience of traditional silicon-based lithium-ion batteries, but there are some phenomena in solid-state batteries that are different from traditional lithium-ion batteries. External pressure is often applied to solid-state batteries, which is transmitted to silicon-based materials through solid-state electrolytes with high mechanical strength. This method limits the volume expansion rate of silicon-based materials to a certain extent and enhances the structural stability of electrode materials. Therefore, in solid-state batteries, the silicon negative electrode exhibits less particle pulverization, which to some extent alleviates the problem of electronic path failure caused by alloy material cracking, and is expected to achieve better cycling stability. However, the low fluidity and low wettability of solid electrolytes increase the risk of electrolyte interface failure in electrode materials.

This article mainly provides SSB performance optimization strategies from the perspectives of controlling the volume expansion of silicon-based materials and regulating the electrode electrolyte interface (Figure 1). Firstly, the reasons for interface failure between electrode materials and electrolytes were discussed based on the basic properties of silicon-based materials; Subsequently, the latest research on the intrinsic properties of silicon-based materials was summarized, including the changes in electronic conductivity, ion diffusion coefficient, and Young's modulus of silicon-based materials with the degree of lithiation; Finally, strategies to alleviate interface failure were introduced from the aspects of adhesive, electrode structure design, buffer layer, electrode material, and electrolyte particle size matching, while emphasizing the potential impact of equal and constant cycling pressure on solid-state battery performance testing.
Mechanism study of silicon-based negative electrode
Due to the differences in wettability and fluidity between solid electrolytes and liquid electrolytes, the solid electrolyte interface (SEI), interface contact, and ion transport processes generated between electrode materials and electrolytes also differ. Meanwhile, the importance of mechanical stability between the silicon-based negative electrode and electrolyte in solid-state batteries cannot be ignored. This section reviews recent research on the crystal structure, critical diameter, and electrochemical sintering properties of silicon both domestically and internationally. It fundamentally explores the causes of interface failure in silicon-based solid-state batteries, as well as the research results on the electronic conductivity, ion diffusion coefficient, Young's modulus, and volume of silicon-based materials during lithiation, which can provide a theoretical basis for solving the problem of interface failure in silicon-based solid-state batteries.
1.1 Crystal Structure and Critical Diameter
Based on the differences in material crystal structures, silicon-based materials can be divided into two categories: crystalline silicon (c-Si) and amorphous silicon (a-Si). C-Si has an ordered lattice structure and obvious grain anisotropy. In contrast, due to the presence of numerous dangling bonds inside a-Si, Si atoms deviate from their orbitals, forming isotropy [Figure 2 (a), (b)]. In addition, the presence of dangling bonds also reduces the potential barrier that needs to be overcome in the a-Si alloying reaction, which is more conducive to the progress of the alloying reaction. Due to the different crystal structures, the lithiation processes of c-Si and aSi exhibit differences. In the lithiation process of a-LixSi (x<3.75) formed by c-Si, the movement speed of phase boundaries on different crystal planes is different, resulting in uneven expansion rates and stress concentration in the direction of faster expansion rates. Exceeding the material's bearing limit may lead to structural fracture. In contrast, the isotropic lithiation rate of a-Si makes stress release more uniform and avoids stress concentration in a certain direction [Figure 2 (c)]. This stress release mechanism widens the critical diameter of a-Si to 870nm, significantly larger than the 150nm of c-Si
[Figure 2 (d), (e)]. Meanwhile, due to the absence of grain orientation and boundaries, a-Si has a lower lithiation potential barrier and lower lithiation overpotential, which is more conducive to the progress of lithiation reactions.

1.2 Electrochemical sintering
During the lithiation and delithiation processes, accompanied by periodic breakage and reconstruction of Si Si bonds, different Si particles undergo chemical bond reformation and merge into larger particles, a phenomenon known as electrochemical sintering. The originally tightly arranged Si particles with pores were transformed into structurally dense large particle blocks through electrochemical sintering after multiple cycles, losing their small volume advantage and increasing local expansion, resulting in a decrease in their electrochemical performance, as shown in Figure 2 (f). In addition, in solid-state batteries, in order to maintain close contact between electrode materials and electrolytes during the cycling process, a large stacking pressure (2-250MPa) is usually applied. High stacking pressure makes the contact between active materials tighter, thus reducing the difficulty of forming large particles through Si Si bonding between adjacent small particles and making the electrochemical sintering phenomenon more pronounced.
1.3 Lithium ion properties of silicon
C-Si/a-Si will transform into a-Si after the first lithium removal. There are two types of volume expansion during the lithiation process of Si anode: one is the formation of LixSi (x<3.75), which leads to an increase in volume, and the volume expansion caused by this phase transition is closely related to the release of capacity of silicon-based materials. Strictly limiting this volume expansion will reduce the storage capacity of lithium ions and decrease the capacity of electrode materials, so it is necessary to find a balance between volume expansion and capacity release. Another type is irreversible expansion caused by defect accumulation during the cycling process [Figure 3 (a)], which is independent of the lithium storage capacity, but the accumulation of defects increases the risk of silicon-based material breakage. Therefore, controlling this irreversible volume expansion is crucial for improving the mechanical integrity and cycling stability of silicon-based materials.

As a semiconductor material, pure silicon has a conductivity of 10-4S/cm. During the charging and discharging process, its lower electronic conductivity will form a large overpotential on Si, limiting the release of capacity. To solve this problem, people in the past usually chose to add carbon or other conductive materials to prepare composite electrodes to enhance the electronic conductivity of silicon electrodes. However, the contribution of the added conductive material to the capacity is relatively low, which reduces the overall energy density of the battery. Worse still, the added carbon material can cause the decomposition of sulfide electrolytes, not only reducing the Coulombic efficiency of the battery, but also affecting its cycling stability.
The recent research results of Meng Ying's team indicate that Li itself can also serve as a high-performance silicon conductive agent. As the Li content increases, the electronic conductivity of LixSi increases from 10-4S/cm (Li0Si) to 10S/cm (Li2Si) [Figure 3 (b) (c)]. Huo et al. used constant current intermittent titration technique to measure the diffusion coefficient of Li in Si electrode without SE, and confirmed that the ion diffusion coefficient of LixSi also improved with the increase of Li [Figure 3 (d)] [DLi (Li0.188Si)=5.7 × 10-10cm2/s, DLi (Li3.656Si)=6.9 × 10-8cm2/s, average DLi]
At a rate of 1.0 × 10-8cm2/s, the cycling curves of InLi | LPSCl | Si/LPSCl and InLi | LPSCl | Si half cells at 0.1C show that as the lithiation process progresses, the overpotential of the Si negative electrode without SE gradually decreases [Figure 3 (e)], which is consistent with the improvement of ion and electron conductivity by Li insertion.

Some argue that the local mechanical stress caused by the phase transition of silicon from crystalline to amorphous structure during lithium insertion and extraction is the main factor leading to rapid capacity decay. Iwasa et al. studied the Young's modulus of the first lithiation of nanocrystalline silicon electrodes into Li0.6Si, Li1.08Si, Li2.06Si, and Li3.75Si phases. They found that when x ≤ 0.375, the Young's modulus of LixSi alloy was linearly related to the Li content, while the Young's modulus of LixSi alloy remained constant in the range of 0.52<x<0.67. Zeng et al. measured the bulk modulus of metastable polycrystalline Li15Si4 phase using in-situ high-pressure synchrotron X-ray diffraction (XRD) experiments at room temperature, and found that the bulk moduli of crystals Li12Si7 and Li7Si3, as well as the ratio of Li to Si, exhibited nonlinear behavior. The non in situ characterization results are affected by voltage relaxation during sample transfer and sample non-uniformity interference. Recently, Putra et al. used a bimodal atomic force microscope with Young's modulus as the mechanical mapping index to measure the real-time morphology and modulus changes of amorphous Si thin film electrodes during the first lithium insertion and extraction process in solid-state batteries. In the early stage of lithiation (x=0-0.37), the modulus of Si thin film electrode sharply decreases, and as x in LixSi continues to increase, the Young's modulus slowly decreases [Figure 3 (f)]; During the lithium removal process, the modulus of LixSi electrode shows an approximately linear relationship with x [Figure 3 (g)]. More testing methods are needed to directly obtain local mechanical stress changes in LixSi during continuous lithium insertion and extraction processes, especially in the two-phase region. This is crucial for mitigating mechanical degradation and electrode failures in silicon-based solid-state batteries.

Along with the alloying phase transition process of silicon-based materials, the volume, modulus, electronic conductivity, and Li+diffusion coefficient of electrode materials will also undergo corresponding changes. The changes in electrode material properties inevitably affect the mechanical stability, electrochemical performance, and thermal stability of solid-state batteries. Therefore, considering the coupling effects of thermal mechanical electrochemical multiple physical fields, establishing a physical and chemical model of solid-state batteries under real operating conditions is of great significance for understanding the interface failure mechanism of solid-state batteries and promoting their original innovation.