Understanding the Air-Exposure Degradation Chemistry at a Nanoscale of Layered Oxide Cathodes for Sodium-Ion Batteries.
basic_science · Level V
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- Record sourced from PubMed, PMID 30517790.
- Also identified by DOI 10.1021/acs.nanolett.8b03637.
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Abstract
Undesired reactions between layered sodium transition-metal oxide cathodes and air impede their utilization in practical sodium-ion batteries. Consequently, a fundamental understanding of how layered oxide cathodes degrade in air is of paramount importance, but it has not been fully understood yet. Here a comprehensive study on a model material NaNi<sub>0.7</sub>Mn<sub>0.15</sub>Co<sub>0.15</sub>O<sub>2</sub> reveals its reaction chemistry with air and the dynamic evolution of the degradation species upon air exposure. We find that besides the extraction of Na<sup>+</sup> ions from the crystal lattice to form NaOH, Na<sub>2</sub>CO<sub>3</sub>, and Na<sub>2</sub>CO<sub>3</sub>·H<sub>2</sub>O in contact with air, nickel ions gradually dissolve from the bulk to form NiO and accumulate on the particle surface as revealed by subnanometer surface-sensitive time-of-flight secondary ion mass spectroscopy. The degradation species on the surface are insulating, leading to an increase in interfacial resistance and declined electrochemical performance. We also demonstrate a feasible surface coating strategy for suppressing the unfavorable degradation process. Understanding the degradation mechanism at a nanoscale can facilitate the future development of high-energy cathodes for sodium-ion batteries.