Revealing alkali metal ions transport mechanism in the atomic channels of Au@α-MnO2

Abstract

Understanding alkali metal ions' (e.g., Li+/Na+/K+) transport mechanism is challenging but critical to improving the performance of alkali metal batteries. Herein using α-MnO2 nanowires as cathodes, the transport kinetics of Li+/Na+/K+ in the 2 × 2 channels of α-MnO2 with a growth direction of [001] is revealed. We show that ion radius plays a decisive role in determining the ion transport and electrochemistry. Regardless of the ion radii, Li+/Na+/K+ can all go through the 2 × 2 channels of α-MnO2, generating large stress and causing channel merging or opening. However, smaller ions such as Li+ and Na+ cannot only transport along the [001] direction but also migrate along the < 110 > direction to the nanowire surface; for large ion such as K+, diffusion along the < 110 > direction is prohibited. The different ion transport behavior has grand consequences in the electrochemistry of metal oxygen batteries (MOBs). For Li-O2 battery, Li+ transports uniformly to the nanowire surface, forming a uniform layer of oxide; Na+ also transports to the nanowire surface but may be clogged locally due to its larger radius, therefore sporadic pearl-like oxides form on the nanowire surface; K+ cannot transport to the nanowire surface due to its large radius, instead, it breaks the nanowire locally, causing local deposition of potassium oxides. The study provides atomic scale understanding of the alkali metal ion transport mechanism which may be harnessed to improve the performance of MOBs.