In Situ Lithiated ALD Niobium Oxide for Improved Long Term Cycling of Layered Oxide Cathodes: A Thin-Film Model Study

Abdessalem Aribia,Xubin Chen,Ayodhya N Tiwari,Moritz H Futscher,Yaroslav E Romanyuk,Evgeniia Gilshtein,Jordi Sastre

doi:10.1149/1945-7111/abf215

Abstract

Protective coatings applied to cathodes help to overcome interface stability issues and extend the cycle life of Li-ion batteries. However, within 3D cathode composites it is difficult to isolate the effect of the coating because of additives and non-ideal interfaces. In this study we investigate niobium oxide (NbOx) as cathode coating in a thin-film model system, which provides simple access to the cathode-coating-electrolyte interface. The conformal NbOx coating was applied by atomic layer deposition (ALD) onto thin-film LiCoO2 cathodes. The cathode/coating stacks were annealed to lithiate the NbOx and ensure sufficient ionic conductivity. A range of different coating thicknesses were investigated to improve the electrochemical cycling with respect to the uncoated cathode. At a NbOx thickness of 30 nm, the cells retained 80% of the initial capacity after 493 cycles at 10 C, more than doubling the cycle life of the uncoated cathode film. Elemental analysis using TOF-SIMS and XPS revealed a bulk and surface contribution of the NbOx coating. These results show that in situ lithiated ALD NbOx can significantly improve the performance of layered oxide cathodes by enhancing interfacial charge transfer and inhibiting surface degradation of the cathode, resulting in better rate performance and cycle life.

Highlights

Lithiated niobium oxide (NbOx) processed by atomic layer deposition (ALD) possesses an ionic conductivity of 10−9 S cm−112 and is electrochemically stable in combination with layered oxide cathode materials, such as LiCoO2 (LCO).[15,16]
Atomic layer deposition of NbOx.—NbOx layers were deposited by ALD on amorphous sputtered LCO thin films at a deposition temperature of 175 °C
The impedance of the cell decreases from about 40 kΩ cm[2] to below 200 Ω cm[2] when the NbOx coating is annealed together with the LCO film compared to the as-deposited NbOx on already crystallized LCO films (Fig. S2)

Summary

Introduction

Engineering of artificial solid-electrolyte interfaces in lithium ion batteries is a common strategy to improve a wide span of characteristics, including capacity, rate capability and cycle life.[1,2,3] By using an artificial solid-electrolyte interface for layered intercalation cathodes such as LiCoO2 and LiNi1-y-zMnyCozO2, the decay of the electrochemical performance can be decelerated and the power performance enhanced.[4,5] This is possible due to the suppression of redox-active metal dissolution from the cathode into the electrolyte and prevention of the formation of a resistive native layer at the cathode-electrolyte interface.[6,7] The employed coatings for this purpose are usually either organic polymers (e.g. polypyrrole8) or inorganic materials Inorganic coatings, such as metal oxides, are thermally and mechanically stable, nonflammable and normally display little to no reactivity with the cathode itself.[9] The function of the coating depends on the type: Li+ conducting materials elevate the ionic diffusion coefficient and enhance the rate performance, electronic conductors like graphene allow for improved electronic conductivity across the interface and lastly, oxides, fluorides and other inert materials construct a protective layer and reduce interactions between cathode and electrolyte.[10]. LCO was employed as a representative layered oxide cathode material as it is a well-studied cathode commonly used in Li-ion batteries and in thin-film configuration it possesses comparable performance to the widely employed bulk cathodes made from powders.[19]

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