Solid-State Battery Research: Prof. Huang Jianyu's Team Finds Stack Pressure Enhances Silicon Anode Fracture Resistance

First author: Li Menglin and Xue Dingchuan

Corresponding author: Tang Yongfu, Zhang Sulin, Huang Jianyu 

Corresponding Units: Yanshan University Pennsylvania State University Xiangtan University

Original link:https://doi.org/10.1002/adfm.202415696

Full text at a glance

This paper focuses on summarizing the new findings of Professor Huang Jianyu's team based on the size effect of silicon anode in the field of all-solid-state batteries. This research not only breaks through the inherent critical size ( 150 nm ) limitation of silicon anode in liquid lithium batteries, but also fills the research gap of the important basic scientific issue of whether silicon anode has size effect in the field of solid-state batteries. This discovery has far-reaching significance for the economical and efficient integration of micron-scale silicon anode into sulfide all-solid-state batteries.

Background

Silicon (Si) has long been considered as the best candidate material for anodes due to its extremely high specific capacity, which is ten times that of graphite commonly used in commercial lithium-ion batteries. However, the commercialization of Si anodes has been elusive due to their large volume fluctuations during cycling, which leads to Si anode fracture and premature battery failure. The critical size threshold of about 150 nm has become the benchmark for integrating Si anodes into liquid electrolyte-based LIBs (LELIBs) , laying the foundation for applications in this field. Compared with LELIBs , Si anodes are expected to provide better stability and safety in all-solid-state batteries (ASSBs) , thereby extending cycle life and increasing energy density. However, whether Si has a size effect in ASSBs and whether it has a fixed critical size threshold has been a gap in this field. Our findings in this work fill the gap in this important basic scientific issue. We demonstrate that the size effect of Si anode not only exists in ASSBs , but also that the size threshold of Si in ASSBs depends on the stack pressure, i.e., the higher the stack pressure, the larger the critical size threshold. Above the size threshold, Si particles will break and pulverize; below this size threshold, Si particles remain intact. Remarkably, under commonly applied stack pressures, the size threshold shifts from the nanoscale observed in LELIBs to the microscale in ASSBs . The main challenge in deploying Si anodes in ASSBs is to overcome chemical-mechanical degradation. Our study provides a solid scientific basis for preventing such degradation. This research will also serve as a cornerstone for the application of Si anodes in all-solid-state batteries.

Research starting point

We assembled a series of LiNi _0.8 Mn _0.1 Co _0.1 O ₂ (NMC811)/Li ₁₀ Si _0.3 PS _6.7 Cl _1.8 (LSPSCl)/Si-X ASSBs in a solid-state mold and performed electrochemical tests (where X represents Si particles of different sizes ). On the negative electrode side, we used pure Si powders with sizes ranging from nanometers to micrometers as negative electrode materials. Si powders with particle sizes of 50 nm , 100 nm , 300 nm , 1μm , 5μm and 44 μm were purchased from commercial products and named Si-0.05 , Si-0.1 , Si-0.3 , Si-1 , Si-5 , and Si-44 , respectively . It is worth noting that no LSPSCl was added to the Si negative electrode , which enabled us to eliminate the influence of factors such as electrolyte and conductive carbon materials on the results.

All batteries were kept in the same conditions except for the negative electrode, and long cycle tests were carried out at a stack pressure of 460 MPa at a rate of 1 C. The results show that ASSBs of 1 μm and below all maintain normal operation, and the overall Coulombic efficiency is close to 100% . In contrast, when the particle size of Si exceeds 1 μm (such as 5 μm and 44 μm ), after the initial normal operation, the Coulombic efficiency will decrease after a certain number of cycles. As the number of cycles increases, this downward trend will continue, resulting in the battery not being able to operate normally. After analysis, the decrease in Coulombic efficiency is caused by the penetration of lithium dendrites into the solid electrolyte. To further illustrate this strong size effect, we ball-milled the Si-5 sample to obtain a sample with a reduced particle size ( Si-5BM ). Subsequently, we assembled ASSBs under the same conditions , and the battery operated normally with a Coulombic efficiency close to 100% . This result further confirms that there is a strong size effect of Si negative electrode in ASSBs , and the size plays a crucial role in electrochemical performance.

We selected failed Si-5 ASSBs and normally functioning Si-1 ASSBs for anode side characterization analysis. The results showed that the 5μm Si anode in the delithiation state experienced severe cracking and pulverization after long-term cycling (Figure 3a ). In contrast, the 1μm Si anode did not experience particle cracking and pulverization after long-term cycling, in sharp contrast to the phenomenon observed in the 5 μm Si anode (Figure 3b ). It is worth noting that the size effect affects the battery performance in liquid batteries due to structural damage to Si . The SEM results presented here show that different sizes also affect the Si structure in solid-state batteries. The cracking and pulverization of Si leads to deteriorated contact between Si particles and between the Si anode and sulfide electrolyte interface. Cracks hinder the transport of lithium ions and promote the growth of lithium dendrites, resulting in the attenuation of capacity and coulombic efficiency. Therefore, lithium dendrites were found in the SEM image of the cross-section of the Si-5 anode (lithiation stage) (Figure 3c ). Correspondingly, there was no growth of lithium dendrites on the Si-1 anode side (Figure 3h ).

To further elucidate the potential mechanisms behind the experimental data, we developed a phase-field model to delve into the complex interplay between stacking pressure and Si particle size-dependent fracture in ASSBs . During lithiation, the volume expansion mismatch generates tensile hoop stresses within the shell. This hoop stress evolves from compressive to tensile as the lithiated / non-lithiated interface is swept, which is consistent with our previous findings. Our results show that the driving force ( J- integral) for fracture decreases with increasing applied pressure. This interesting phenomenon can be attributed to the fact that particles subjected to significant compression are intrinsically more resistant to cracking because the applied stack pressure counteracts the cracking tensile stress generated during lithiation. In Figure 4f , each curve plots the critical fraction of lithiation ( critical fraction of Si particle fracture) for different particle sizes and prescribed stack pressures. With increasing stack pressure, the critical curve shifts to the upper right quadrant, suggesting that increasing applied pressure may increase the critical size threshold in ASSBs .

The size effect is also verified by theoretical modeling: through numerical simulation, we predict a power-law relationship between the critical size of Si particles and the applied stacking pressure. In Figure 5 , at 460 MPa , the critical size reaches about 2 μm , below which the Si particles remain intact (e.g., 1 μm ), and above which the Si particles break after cycling (e.g., 5 μm ). This prediction of the simulation is in good agreement with our experimental results (Figure 2d , e and Figure 3a , b ). Our numerical simulation is also consistent with the basic physics of fracture: fracture is caused by tensile stress, while compressive stress inhibits fracture; the applied stacking pressure makes Si particles more compressible, so particles of the same size are less likely to break under the applied stacking pressure. To further evaluate the validity of our prediction, we performed additional experiments using Si with sizes of 300 nm and 1 μm at a constant stacking pressure of 200 MPa . The experimental results are consistent with the results of 1 μm and 5 μm Si at 460 MPa . These additional results reinforce our initial conclusions on the Si size effect in ASSBs and suggest a link between Si particle fracture and electrochemical performance. This finding helps pursue battery designs that can accommodate larger particle sizes while maintaining excellent performance, breaking through the limitations of nanosize selection in LELIBs .

Summary and Outlook

In summary, our study highlights that Si particles exhibit a significant size effect in sulfide ASSBs . Externally applied stacking pressure is crucial for enhancing interfacial contact and charge transfer in ASSBs , which lifts the critical size threshold of Si particles from 150 nm in LELIBs to micrometer scale in ASSBs . This finding has far-reaching implications for the seamless and cost-effective integration of micrometer-scale Si anodes into sulfide-based ASSBs . Such integration enables sustained cycling performance and durability, achievements that remain beyond the reach of conventional LELIBs . In essence, our findings pave the way for next-generation Si- based ASSBs with significantly enhanced performance .