Today’s mobile applications demand batteries with high energy density and high cycle life. A promising material for high capacity batteries is metallic lithium. It has a very high specific energy and a very low electrochemical potential.
The use of metallic lithium (Li) results in batteries with little cyclic and calendrical stability. The main problems of Li-metal-batteries are the growth of dendrites during charging and discharging of the batteries, as well as, parasitic reactions of the electrolyte with the Li-metal. These reactions lead to a massive consumption of electrolyte. Since Li is chemically unstable in the electrolyte, the decomposition even occurs without cycling. The reactions of the electrolyte with Li take place at the electrolyte/Li interphase, leading to a very thick layer of decomposition produucts on the surface.
To inhibit the electrolyte decomposition, additives might boost the electrolyte. The in-situ study of these additives and their effects on the chemical stability of Li-metal in the electrolyte provides information to design new electrolytes that increase the lifespan of Li-metal-batteries.
A powerful in-situ method for the analysis of the Li/electrolyte interphase is the electrochemical scanning microscopy (SECM). The method uses a platinum microelectrode that is embedded in glass and a redox mediator that is added to the electrolyte. The redox mediator is oxidized at the microelectrode and reduced at a counter electrode. The current between the microelectrode and the counter electrode depends on the distance from the sample and its surface conductivity. The use of a high precision positioning system allows to measure the surface conductivity space resolved on the micrometer scale and the microscopy may be used to observe the lithium/electrolyte interphase.
Through this microscopy, we were able to detect changes on the lithium surface over time and observe the growth of the native SEI on metallic Li. Further experiments using GC-MS showed an ongoing electrolyte decomposition of the baseline electrolyte for several days. The decomposition products lead to an increased internal resistance of symmetrical coin cells. By adding 5%wt vinylene carbonate to the electrolyte, the decomposition reaction could be inhibited, proving an effective VC based passivation layer on the metallic lithium.