Investigating the effect of a fluoroethylene carbonate additive on lithium deposition and the solid electrolyte interphase in lithium metal batteries using in situ NMR spectroscopy†
Using lithium metal as the negative electrode in a rechargeable lithium battery can increase the energy density, but to date, its use is limited due to uncontrolled and inhomogeneous electrodeposition upon cycling, leading to both low coulombic efficiencies and safety issues. The solid electrolyte interphase (SEI) has been identified as a key component in controlling microstructural growth but its role is still not well-understood. Here we explore the effect that fluoroethylene carbonate (FEC), a common electrolyte additive, along with pulse plating, has on the SEI on lithium metal and the electrodeposition of lithium. In situ NMR techniques, which are both non-invasive and quantitative, are used to monitor the microstructural growth during lithium deposition. We show how lithium whisker growth in a commercial carbonate electrolyte leads to increased SEI formation and low current efficiency, whereas using an FEC additive leads to denser lithium metal electrodeposits. We use 6,7Li isotopic labelling to monitor the exchange between lithium metal and the electrolyte and develop a numerical model to describe the process, which is discussed in the context of the standard model of electrochemical kinetics. The model allows us both to extract an exchange current density at the open circuit voltage, which takes into account the growth of the SEI and allows the extent of Li metal corrosion to be quantified. The results demonstrate that the isotope exchange rate depends significantly on the electrolyte and the corresponding SEI. The numerical simulations show that with an FEC additive the exchange is twice as fast as without, which is attributed to faster lithium ion transport in the SEI. Furthermore, the simulations indicate that FEC results in an accelerated SEI formation rate, more than four times faster than without the additive. These beneficial SEI properties, namely the fast lithium transport and faster SEI formation, help to explain why the fluorinated FEC additive results in a more uniform lithium deposition. The fast lithium ion transport will lead to a more homogeneous current distribution at the electrode surface. In the event that the SEI layer is ruptured, passivation of the freshly exposed lithium will occur more rapidly further leading to more homogeneous deposition.