A recent article published in NPG Asia Materials demonstrated the fabrication of composite electrodes with different cathode-material morphologies for all-solid-state batteries (ASSBs) using a scalable infiltration sheet-type process. The study specifically investigated the impact of the crystalline structure of Li[Ni0.8Co0.1Mn0.1]O2 (NCM811) cathodes on their solid electrolyte (SE) infiltration capacity.
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Background
Electronic devices such as wearables, the Internet of Things (IoT), and electric vehicles (EVs) demand rapid-charging lithium-ion batteries (LIBs) that are both safe and possess high energy density. However, commercial LIBs using flammable organic liquid electrolytes are unsafe, often causing fires or explosions in smartphones and EVs.
Flame-retardant ASSBs are considered promising alternatives due to their durability, high safety, energy density, and simple design. The choice of cathode material is crucial for ASSB performance, as it directly impacts rate capability, energy density, and stability.
While cathodes with higher nickel content yield batteries with increased energy density and capacity, they also face issues such as particle cracking and loss of contact between the SE and active material. Additionally, conventional wet-slurry methods for manufacturing large-scale electrodes are unsuitable for ASSB cathodes due to the high reactivity of sulfide SEs with polar solvents.
This study employed an infiltration approach to fabricate composite electrodes while addressing these challenges.
Learn More: Why Do Lithium-Ion Batteries Catch Fire?
Methods
The SE was synthesized using Li6PS5Cl (LPSCl) powder, which comprises Li2S, P2S5, and LiCl in a 42.8:41.4:15.8 weight ratio. Single- and polycrystalline NCM811 powders were commercially procured.
The traditional NCM-based composite electrodes were fabricated by casting a slurry of NCM811 powder, polyvinylidene fluoride (PVDF) binder, and Super P carbon additive in a 96:2:2 weight ratio in N-methyl pyrrolidinone on aluminum current collectors. These electrodes were then immersed in SE solution and recrystallized (RC) at 180 °C under a high vacuum for infiltration.
The prepared LPSCl-infiltrated NCM-based composite electrodes were characterized using field-emission scanning electron microscopy (FESEM) and energy-dispersive X-ray spectroscopy (EDS). X-ray diffraction (XRD) studies confirmed the composition and crystallinity of the SE and RC-SE specimens. Additional characterization involved Raman spectroscopy, thermogravimetric analysis, and Brunauer-Emmett-Teller analysis.
Pressed cells with a Li-Ag anode, Ni-based cathode, and LPSCl-pellet membrane (800 μm thick) were prepared to examine the electrochemical performance of the half cells comprising SE-infiltrated NCM-based composite cathodes. Galvanostatic intermittent titration technique and electrochemical impedance spectroscopy measurements were performed on these cells.
Results and Discussion
The infiltrated composite electrodes exhibited intimate contact between the electrolyte and cathode, regardless of SE particle size. Notably, the electrodes fabricated from LPSCl with both large and small particles demonstrated similar charge-discharge profiles.
The ethanol used in preparing the LPSCl solution facilitated successful infiltration due to its compatibility with the SE, electrode components, and aluminum current collector. This infiltration process could enable efficient manufacturing of ASSBs, eliminating the time and costs associated with SE pulverization.
XRD patterns of the RC-SE composite electrodes showed the characteristic argyrodite LPSCl peak without any impurity phases, confirming the successful infiltration and RC of LPSCl into the electrode. The Raman spectrum corroborated these findings.
FESEM images depicted homogeneous SE infiltration into the porous electrodes at the LPSCl-NCM interface. Notably, electrodes with polycrystalline NCM (poly-NCM) particles demonstrated more effective LPSCl infiltration than those with single-crystalline particles (single-NCM).
The poly-NCM electrode exhibited a higher initial discharge capacity (197 vs. 172 mAh/g) and greater Coulombic efficiency (80.02 % vs. 69.49 %) compared to the single-NCM electrode, attributed to the polycrystalline nature of the active material.
The particle morphology significantly influenced the electrochemical performance; poly-NCM structures with nanosized primary particles showed superior performance due to homogeneous SE infiltration between primary particles, facilitating excellent contact between the electrode and SE.
Conclusion
The researchers examined the relationship between the morphology of active materials fabricated using a scalable infiltration process and their electrochemical performance. The polycrystalline active material exhibited exceptional electrochemical performance, effectively interacting with the SE.
The optimized drying and heat treatment of the LPSCl-SE solution enhanced the initial discharge capacity of its battery systems. LPSCl-infiltrated polycrystalline NCM811 electrodes exhibited a capacity of ~196 mAh/g at 55 °C.
The proposed method using solution-based LPSCl SEs is promising for large-scale fabrication of sheet-type electrodes for ASSBs. It helps overcome the limitations of conventional methods involving SE pulverization.
The composite electrodes, with high active material content obtained through infiltration-induced dissolution of SE particles, exhibit high energy density, making them suitable for commercialization. The researchers suggest further developing these electrodes for high-energy-density, high-safety ASSBs for next-generation technologies.
Journal Reference
Sung, J., et al. (2024). Infiltration-driven performance enhancement of poly-crystalline cathodes in all-solid-state batteries. NPG Asia Materials. DOI: 10.1038/s41427-024-00555-7, https://www.nature.com/articles/s41427-024-00555-7
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