1,2-Dimethoxyethane (DME) is a common
electrolyte solvent for lithium
metal batteries. Various DME-based electrolyte designs have improved
long-term cyclability of high-voltage full cells. However, insufficient
Coulombic efficiency at the Li anode and poor high-voltage stability
remain a challenge for DME electrolytes. Here, we report a molecular
design principle that utilizes a steric hindrance effect to tune the
solvation structures of Li+ ions. We hypothesized that
by substituting the methoxy groups on DME with larger-sized ethoxy
groups, the resulting 1,2-diethoxyethane (DEE) should have a weaker
solvation ability and consequently more anion-rich inner solvation
shells, both of which enhance interfacial stability at the cathode
and anode. Experimental and computational evidence indicates such
steric-effect-based design leads to an appreciable improvement in
electrochemical stability of lithium bis(fluorosulfonyl)imide
(LiFSI)/DEE electrolytes. Under stringent full-cell conditions of
4.8 mAh cm–2 NMC811, 50 μm thin Li, and high
cutoff voltage at 4.4 V, 4 M LiFSI/DEE enabled 182 cycles until 80%
capacity retention while 4 M LiFSI/DME only achieved 94 cycles. This
work points out a promising path toward the molecular design of non-fluorinated
ether-based electrolyte solvents for practical high-voltage Li metal
batteries.
Fully understanding the mechanism of lithium metal deposition is critical for the development of rechargeable lithium battery anodes. The heterogeneous electron transfer kinetics are an important aspect of lithium electrodeposition, but they have been difficult to measure and understand. Here, we use transient voltammetry with ultramicroelectrodes to explicitly investigate the electron transfer kinetics of lithium electrodeposition. The results deviate from the Butler−Volmer model of electrode kinetics; instead, a Marcus model accurately describes the electron transfer. Measuring the kinetics in a series of electrolytes shows the mechanism of lithium deposition under electron transfer control is consistent with the general framework of Marcus theory. Comparison of the transient voltammetry results to electrochemical impedance spectra provides a strategy for understanding how the interplay of the electron transfer and mass transport resistances affect the morphology of lithium.
A major hurdle to the successful deployment of high‐energy‐density lithium metal based batteries is dendrite growth during battery cycling, which raises safety and cycle life concerns. Coating the Li metal anode with a soft polymer layer has been previously shown to be effective in suppressing dendrite growth, leading to uniform lithium deposition even at high current densities. A 3D coarse‐grained molecular model to study the mechanism of dendrite suppression is presented. It is found that the most effective coatings delay or even prevent dendrites from penetrating the polymer layer during deposition. The optimal deposition can be achieved by jointly tuning the polymer stiffness and relaxation time. Higher polymer dielectric permittivity and coating thickness are also effective, but the deposition rate and, therefore, the charging current density is reduced. These findings provide the basis for rational design of soft polymer coatings for stable lithium deposition.
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