Revealing alkali metal ions transport mechanism in the atomic channels of Au@a-MnO_(2)

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 a-MnO_(2)nanowires as cathodes,the transport kinetics of Li^(+)/Na^(+)/K^(+)in the 2×2 channels of a-MnO_(2)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 a-MnO_(2),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-O_(2)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.

In situ observation of electrochemical Ostwald ripening during sodium deposition

Sodium(Na)metal batteries(SMBs)using Na anode are potential“beyond lithium”electrochemical technology for future energy storage applications.However,uncontrollable Na dendrite growth has plagued the application of SMBs.Understanding Na deposition mechanisms,particularly the early stage of Na deposition kinetics,is critical to enable the SMBs.In this context,we conducted in situ observations of the early stage of electrochemical Na deposition.We revealed an important electrochemical Ostwald ripening(EOR)phenomenon which dictated the early stage of Na deposition.Namely,small Na nanocrystals were nucleated randomly,which then grew.During growth,smaller Na nanocrystals were contained by bigger ones via EOR.We observed two types of EOR with one involving only electrochemical reaction driven by electrochemical potential difference between bigger and smaller nanocrystals;while the other being dominated by mass transport governed by surface energy minimization.The results provide new understanding to the Na deposition mechanism,which may be useful for the development of SMB for energy storage applications.