A highly reversible lithium metal anode

Abstract

Lithium metal has shown a lot of promise for use as an anode material in rechargeable batteries owing to its high theoretical capacity. However, it does not meet the cycle life and safety requirements of rechargeable batteries owing to electrolyte decomposition and dendrite formation on the surfaces of the lithium anodes during electrochemical cycling. Here, we propose a novel electrolyte system that is relatively stable against lithium metal and mitigates dendritic growth. Systematic design methods that combined simulations, model-based experiments, and in situ analyses were employed to design the system. The reduction potential of the solvent, the size of the salt anions, and the viscosity of the electrolyte were found to be critical parameters determining the rate of dendritic growth. A lithium metal anode in contact with the designed electrolyte exhibited remarkable cyclability (more than 100 cycles) at a high areal capacity of 12 mAh cm(-2).

Figure 1. Dependence of the energy density of a battery cell on the areal capacity of the electrode for Li–air, Li–S, and Li-ion batteries, and the estimated driving distance of an electric vehicle with respect to the energy density of the battery cell used.

The energy densities of the battery cells were calculated assuming that they all had the same cell structure, namely, one comprising a “current collector—cathode—electrolyte—separator—protective layer—anode—current collector”, with no cell packing components used. The driving distances were estimated on the basis of the car Nissan Leaf, which uses Li-ion battery cells with an energy density of 140 Wh kg⁻¹ and has a driving range of 160 km (for details, see Supplementary Table S1).

Figure 2. Measurement of the short-circuit time.

(a), The Li symmetric cells used to evaluate the effect of the choice of liquid electrolyte on the growth of lithium dendrites. (b), Lithium deposition profiles of the Li symmetric cells for a current density of 1 mA cm⁻², showing the effect of the molecular weight of the solvent used. The dependence of t_s of the Li symmetric cells on (c), the viscosities and (d), the ionic conductivities of various screened solvents. The same salt (LiTFSI) was used with all the solvents.

Figure 3. Images of lithium dendrites taken by a microscope during the in situ observation of lithium deposition.

The electrolytes used were a, (a) 1 M solution of LiTFSI in DME, (b), a 1 M solution of LiTFSI in tetraglyme, and (c), a 1 M solution of LiI in tetraglyme at an areal capacity of 1.67 mAh cm⁻².

Figure 4. Statistical DLA model for the simulation of dendritic growth.

(a), (b), The mean collision time for Li ions for (a) was nine times greater than that for (b). (a) and (b) correspond to Figs. 3a and 3b, respectively, with the viscosity related to Fig. 3b being nine times greater than that for Fig. 3a. It was determined from equation (3) that the mean collision time of Li ions was inversely proportional to the viscosity of the electrolyte used.

Figure 5. Cycling profiles of Li symmetric cells that used different electrolytes.

The electrolytes used were (a), PEO, PEO + SiO₂, tetraglyme + PEO + SiO₂, and polyglyme + PEO + SiO₂, (b), PC + PEO + SiO₂, and (c), DME + PEO + SiO₂. The same salt (LiTFSI) was used in all of the cells.