Robust battery interphases from dilute fluorinated cations

Abstract

Controlling solid electrolyte interphase (SEI) in batteries is crucial for their efficient cycling. Herein, we demonstrate an approach to enable robust battery performance that does not rely on high fractions of fluorinated species in electrolytes, thus substantially decreasing the environmental footprint and cost of high-energy batteries. In this approach, we use very low fractions of readily reducible fluorinated cations in electrolyte (∼0.1 wt%) and employ electrostatic attraction to generate a substantial population of these cations at the anode surface. As a result, we can form a robust fluorine-rich SEI that allows for dendrite-free deposition of dense Li and stable cycling of Li-metal full cells with high-voltage cathodes. Our approach represents a general strategy for delivering desired chemical species to battery anodes through electrostatic attraction while using minute amounts of additive.

Fig. 1. (a) Schematic diagram of SEI formation from a fluorinated cationic additive on a Li-metal anode. (b) First and second (inset) cycle CV profiles collected in DME + 1 M LiFSI electrolytes with and without fluorinated cations (TFP) and fluorinated neutral analogue (TFN) using a 1 mm Cu disk working electrode at a 0.5 mV s⁻¹ scan rate and a voltage window of 0.3–2.5 V vs. Li/Li⁺. (c) Galvanostatic cycling of a Li⁰–Li⁰ symmetric cell at 10 mA cm⁻² using DME + 1 M LiFSI as an electrolyte with and without fluorinated cations (TFP). (d) and (e) Zoomed-in voltage profile of (c). (f) and (g) Cross-sectional SEM images of the cycled Li metal (f – after 1852 cycles in DME + 1 M LiFSI, g – after 2000 cycles in DME + 1 M LiFSI + 12 mM TFP) showing Li metal deposit morphologies. Scale bar: 5 μm. Before SEM, a lamella (5 μm deep) was cut out using cryo-FIB.

Fig. 2. (a) F : C atomic ratio in the SEI layer formed on a Cu electrode as a function of depth. (b) XPS F 1s spectrum of a SEI layer formed on a Cu electrode after cycling in DME + 1 M LiFSI + 18 mM TFP with Ar⁺ sputtered time of 72 s (estimated depth: 2.0 nm). (c) Voltage, charge, frequency, and dissipation change versus time using EQCM-D analysis for DME + 1 M LiFSI and DME + 1 M LiFSI + 18 mM TFP. (d) Products of reduction at the negative electrode obtained from DFT calculations, see Fig. S11 (ESI†) for further details.

Fig. 3. NCM811‖Li full-cell performance. (a) Long-term cycling of NCM811‖50 μm Li full cells in DME + 0.96 M LiFSI + 12 mM TFP ClO₄ (0.1C-rate for three cycles and 1C-rate for 50 cycles in a loop). (b) Voltage profile of NCM811‖Li full-cells at 0.1C-rate (top) and 1C-rate (bottom). (c) XPS F 1s spectra of CEI layer formed on NCM811 electrodes cycled in DME + 0.96 M LiFSI + 12 mM TFP ClO₄. (d) DFT results predicting H-transfer from DME to LiNiO₂ positive electrode followed by DME*_(−H) reaction with TFP⁺ that leads to the F-enriched CEI formation in the presence of additive.

Fig. 4. (a) A snapshot of the MD simulation cell for 0.91 M LiFSI and 0.27 M [TFP][ClO₄] in DME. (b) Radial distribution functions for 0.91 M LiFSI and 0.27 M [TFP⁺][ClO₄⁻] in DME at 298 K and (c) representative cation solvates for 0.91 M LiFSI and 0.27 M [TFP⁺][ClO₄⁻] in DME at 298 K.