Impact of Diglyme Cosolvent on Low-Temperature Microstructural Lithium Growth

The development of high-energy density batteries combining a high-voltage oxide cathode and a lithium metal anode hinges on the development of an electrolyte with a sufficiently large electrochemical voltage window and on preventing lithium (Li) microstructure formation. These microstructures pose serious challenges to battery safety and performance and are more likely to grow on Li metal during low-temperature operation. Carbonate-ether blends have emerged as promising and cost-effective electrolytes that combine the high-oxidative stability of carbonate solvents and reductive stability of ethers, making them suitable for high-voltage Li metal batteries. Yet, low-temperature Li electrodeposition in hybrid carbonate-ether electrolytes has not been studied. In this work, we use in-situ magnetic resonance imaging (MRI) to obtain semiquantitative insights into Li microstructure growth upon cycling between 20 and −10 °C and when varying the amount of ether cosolvent added to a carbonate electrolyte. 3D reconstructions from MRI images collected on Li/Li symmetric cells suggest that 20 vol % and 40 vol % diglyme cosolvent suppresses Li microstructural growth down to 0 °C, while 40 vol % diglyme cosolvent significantly reduces microstructural growth down to −10 °C. Our results from electrochemistry, nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy, and rheology, reveal that the diglyme cosolvent suppresses low-temperature Li microstructure growth in three ways: (1) it increases the Li-ion desolvation energy, leading to a lower exchange current density and more homogeneous Li nucleation; (2) it leads to the formation of a passivating solid electrolyte interphase; and (3) it reduces the electrolyte viscosity, enhancing ion transport properties over the 20 to −10 °C temperature range. These findings highlight the benefits of using a diglyme cosolvent in carbonate electrolytes for low-temperature Li metal battery applications.