The Science/Achievement: This work investigates the hypothesis that glass solid ionic electrolyte stable performance with lithium metal is achieved in large part due to absence of grain boundaries. Lipon (Lithium phosphorous oxynitride) solid electrolyte for lithium batteries was chosen as a model system to test this hypothesis. An artificial “grain boundary” was created by deposition of a bilayer of Lipon. Subsequent cycling with metallic lithium revealed preferential deposition of Li within this artificial boundary, thus confirming the hypothesis. Such controlled Li plating has been demonstrated for the first time and reveals the importance of grain boundaries and defects in general for stable performance of Li all solid-state batteries.
Significance and Impact:
Lipon has been known for its ability to resist penetration of metallic lithium upon battery cycling. However, Lipon has rather modest ionic conductivity and can only be made by PVD (RF sputtering), which is rather costly technique with low production speed. Understanding what makes this electrolyte so robust will help to design other electrolytes that can be manufactured by less time and cost demanding methods. Demonstration of Li plating confined to one planar interface between two Lipon layers reveals importance of defects in Li penetration prevention and mechanics of Li penetration.
- A thin film battery was made by depositing layers of Au, LiCoO2, and Lipon using RF magnetron sputtering.
- Copper electrode was deposited on top of the first Lipon layer in form 2mm wide fingers. A second layer of Lipon was deposited on top of this structure to create an artificial interface.
- Upon cycling, Li deposited at this interface forming dendrite-like structures which eventually resulted in shorting the cell.
- FEA was performed to reveal current focusing on the edge of Cu current collector.
Summary: In this work we were able to successfully fabricate interface consisting of two layers of Lipon ionic conductor and observe formation of metallic lithium as tree-like structures confined to this interface. This work signifies that in order to achieve robust solid electrolytes for lithium cells, knowledge of how to create defect- and interface-free electrolyte surface is essential. Therefore, in design of solid electrolytes engineering the proper composition for high ionic conductivity is as critical as engineering electrolyte with suitable mechanical properties.