Infiltration process to improve polycrystalline cathodes

A recent article published in Materials NPG Asia demonstrated the fabrication of composite electrodes with different cathode material morphologies for solid-state batteries (ASSBs) using a scalable infiltration sheet process. The study specifically investigated the impact of the crystal structure of Li (Ni_0.8co_0.1Mr_0.1)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) require fast-charging lithium-ion batteries (LIBs) that are safe and have high energy density. However, commercial LIBs using flammable organic liquid electrolytes are unsafe, often causing fires or explosions in smartphones and electric vehicles.

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 to ASSB performance, as it directly affects rate capability, energy density, and stability.

Although cathodes with higher nickel content produce batteries with higher energy density and capacity, they also face problems such as particle breakage and loss of contact between the SE and the active material. Furthermore, conventional wet slurry methods for large-scale electrode fabrication are not suitable for ASSB cathodes due to the high reactivity of sulfide SEs with polar solvents.

This study used an infiltration approach to fabricate composite electrodes while addressing these challenges.

More information: Why do lithium-ion batteries catch fire?

methods

The SE was synthesized using Li₆P.S₅Cl (LPSCl) powder, which includes Li₂S, P₂S₅and LiCl in a weight ratio of 42.8:41.4:15.8. Single and polycrystalline NCM811 powders were purchased commercially.

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 weight ratio of 96:2:2 in N-methyl pyrrolidinone in aluminum current collectors. These electrodes were then immersed in SE solution and recrystallized (RC) at 180 °C under high vacuum for infiltration.

The prepared LPSCl-infiltrated NCM-based composite electrodes were characterized by 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. Further characterization included 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 half-cells comprising composite cathodes based on infiltrated NCMs in SE Measurements of the galvanostatic intermittent titration technique and electrochemical impedance spectroscopy were carried out in these cells.

Results and discussion

The infiltrated composite electrodes showed intimate contact between the electrolyte and the cathode, regardless of the SE particle size. In particular, electrodes fabricated from LPSCl with large and small particles demonstrated similar charge–discharge profiles.

The ethanol used in the preparation of the LPSCl solution facilitated successful infiltration due to its compatibility with the SE, electrode components, and aluminum current collector. This infiltration process could enable the efficient fabrication of ASSB, eliminating the time and costs associated with SE sputtering.

The XRD patterns of the RC-SE composite electrodes showed the characteristic peak of argirodite LPSCl without any impurity phase, confirming the successful infiltration and RC of LPSCl into the electrode. The Raman spectrum corroborated these findings.

FESEM images depicted a homogeneous infiltration of SE into the porous electrodes at the LPSCl–NCM interface. In particular, electrodes with polycrystalline NCM particles (poly-NCM) demonstrated more effective LPSCl infiltration than those with monocrystalline particles (single NCM).

The poly-NCM electrode showed higher initial discharge capacity (197 vs. 172 mAh/g) and higher coulombic efficiency (80.02% vs. 69.49%) compared to the single NCM electrode , attributed to the polycrystalline nature of the active material.

The morphology of the particles significantly influenced the electrochemical performance; the poly-NCM structures with nanometric primary particles showed superior performance due to the homogeneous infiltration of SE between the 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 showed outstanding electrochemical performance, effectively interacting with the SE.

Optimized drying and heat treatment of the LPSCl-SE solution improved the initial discharge capacity of their battery systems. Polycrystalline NCM811 electrodes infiltrated with LPSCl exhibited a capacity of ~196 mAh/ga ​​55 °C.

The proposed method using solution-based LPSCl SEs is promising for the large-scale fabrication of sheet electrodes for ASSB. It helps overcome the limitations of conventional methods involving SE spraying.

The composite electrodes, with a high content of active material obtained through the infiltration-induced dissolution of SE particles, have a high energy density, which makes them suitable for commercialization. The researchers suggest further developing these electrodes for high energy density and high safety ASSB for next-generation technologies.

Journal reference

Sung, J., et al. (2024). Infiltration-driven performance enhancement of polycrystalline cathodes in solid-state batteries. Materials NPG Asia. DOI: 10.1038/s41427-024-00555-7, https://www.nature.com/articles/s41427-024-00555-7

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