Lithium metal is appealing as an anode in rechargeable batteries for its high capacity and high energy density. However, electrochemical plating of lithium is accompanied by the formation of lithium dendrites, which have a negative impact on the efficiency and safe operation of the cell.
In order to shed light on how dendrite formation occurs, Yuegang Zhang and co-workers from Suzhou Institute of Nano-Tech and Nano-Bionics at the Chinese Academy of Sciences turned to electrochemical scanning electron microscopy (or SEM). Compared to tunneling electron microscopy and optical microscopy, SEM is the intermediate option for resolution while allowing for sufficient cell dimensions and amount of electrolyte. Their study is the first in situ SEM investigation into the cycling process occurring in a lithium battery.
After several seconds of lithium plating, dendrites are visible and rapidly grow, reaching a length of ≈60 µm after 300 seconds. When lithium nitrate is added to the ether-based electrolyte, dendrite length is reduced to ≈18 µm after 350 seconds of plating. During the stripping process, some of these dendrites dissolve in the electrolyte while others remain as electrochemically inactive or “dead” lithium.
When lithium polysulfide is used as an additive, dendrites become even shorter: ≈10 µm after 600 seconds of plating. Density functional theory calculations suggest that polysulfides inhibit dendrite growth via an “etching” reaction due to the lower energy of a lithium atom in lithium polysulfide clusters than that in lithium metal. The authors find that the co-addition of lithium nitrate and lithium polysulfide to the electrolyte give the best results. In this case, lithium dendrite lengths are observed to be well below 3 μm—significantly shorter than the dendrites that formed in the absence of an additive.
Overall, the in situ electrochemical SEM technique provides valuable insight into the mechanism of lithium dendrite formation. This technique could be used in the future to elucidate the mechanisms of other electrochemical systems.
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