Enabling High-Voltage Stability in Solid-State Battery Cathodes with ALD Coatings
Authors: Jangid Manoj K. , Cho Tae H. , Ma Tao , Liao Daniel W. , Kim Hwangsun , Kim Younggyu , Chi Miaofang , Dasgupta Neil P. 
Published: December 2024

Solid-state batteries (SSBs) promise improved safety and energy density by using solid electrolytes (SEs) and high-voltage cathodes. However, the SEs are unstable at high voltages, leading to oxidative chemical decomposition at the SE/CAM (cathode active material) interface, resulting in the formation of an insulating cathode-electrolyte interface (CEI) on CAM particles [1]. The layered oxide cathodes (LiNixMnyCozO2; NMC), when cycled at elevated voltages (>4.3V), undergo irreversible structural and chemical deformations, including the formation of kinetically less active phases (spinel and rock-salt), oxygen evolution, and mechanical degradation (intergranular or intraparticle cracking) [2]. The instability of the SE, CAM particles, and SE/CAM interface, collectively, results in inferior electrochemical performance, including low initial Coulombic efficiency (CE), increased cell polarization and impedance, reduced rate capability, and capacity fade during extended cycling.
Therefore, it is crucial to stabilize the SE/CAM interface and preserve the CAM structure in SSB cathodes, particularly during high-voltage cycling. This is primarily achieved by developing a protective coating on CAM particles. Niobium (Nb)-based coatings are promising to protect the SE/CAM interface, mostly developed by wet chemical and solid-state processing routes that involve high-temperature annealing. On the other hand, atomic layer deposition (ALD) is a powerful technique for coatings, providing precise thickness and composition control [3]. Unfortunately, these coatings may have ‘hot spots’ for local current focusing (crystalline defects, i.e., grain boundaries) and oxygen release (pinholes at particle-particle contacts) [4].
In the present study, we developed an Nb-based thin conformal amorphous coating on single crystal NMC CAM particles using ALD equipped with a rotary bed attachment. Unlike traditional stationary ALD reactors, ALD with a rotary bed attachment ensures a pinhole-free continuous coating on CAM powders. The low-temperature ALD process results in an amorphous coating (chemically and structurally isotropic), which is expected to effectively withstand the volumetric changes occurring in CAM particles during repetitive charge/discharge cycling. Comparing the performance of uncoated and coated samples tested at high voltages (≥4.5V) shows that the coated cathodes display significant improvements, including high initial Coulombic efficiency (91% vs. 83%), enhanced rate capability (10x higher accessible capacity at a 2C rate), and improved capacity retention during extended cycling (99.4% after 500 cycles). These enhancements are attributed to reduced cell polarization and interfacial impedance in coated samples. Post-mortem electron microscopy and XRD analysis confirm that the coating remains intact, inhibiting oxygen release from the CAM structure and the formation of inactive phases (spinel and rock-salt), thereby preventing CAM particle cracking. These findings pave the way for stable and high-performance SSBs with high-voltage cathodes.
Keywords: Solid-state batteries, composite cathodes, Nb-based coating, atomic layer deposition (ALD), high voltage stability.
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