The recent escalation in the demand for long-range electric vehicles (EVs) has rekindled research on lithium–air and lithium–sulfur batteries in which Li metal anodes are an essential component. … [one] important requisite for fabricating cells with high energy densities is that the capacity per unit electrode area (areal capacity of the electrodes) should also be high. … It is obvious that Li–S and Li–air batteries have the potential to considerably improve the driving ranges of EVs. However, long-range driving is feasible only when lithium anodes with significantly large areal capacities, preferably 10 mAh cm⁻² or higher, are used.

In rechargeable lithium batteries, an areal capacity of 10 mAh cm⁻² corresponds to the repeated stripping and plating of a lithium layer that measures about 50 μm in thickness. This large change in the volume of the Li electrode can cause two serious problems: (1) continuous decomposition of the electrolyte owing to contact with the fresh Li surface through cracks in the solid–electrolyte interphase (SEI); (2) acceleration of dendritic growth because of the presence of spatially inhomogeneous SEIs and non-uniform morphology of the Li surface. These two factors can seriously affect the performance, cycle life, and safety of lithium batteries. Hence, the prevention of electrolyte decomposition and the suppression of dendritic growth are necessary if Li anodes are to be employed in batteries.

—Park et al.

The role of areal capacity. The chart illustrates the dependence of cell energy density on areal capacity for Li-ion, Li–S, and Li–air batteries. The energy density of a Li-ion battery is almost saturated after 6 mAh cm−2. By contrast, the energy densities of Li–S and Li–air batteries increase without saturation. The energy densities of the battery cells were calculated assuming that they all had the same cell structure, with no cell packing components used. The driving distances were estimated on the basis of the 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. Park et al. 2014. Click to enlarge.

The Samsung study systematically examined the effect of electrolyte composition on the cycling of Li metal anodes by combining simulations, model-based experiments, and in situ analyses. The researchers screened electrochemically stable liquid solvents against Li metal anodes using density functional theory (DFT) calculations and investigated the interactions between these screened solvents and the Li surfaces using ab initio molecular dynamics (AIMD) simulations.

They prepared Li symmetric cells using the designed electrolytes and cycled them at an areal capacity of 12 mAh cm⁻²—the first instance of cycling tests being performed under such severe conditions, according to the team.

They initially screened solvents with calculated reduction potentials against Li⁺/Li of less than −0.5 V, implying that the probability of a direct reaction between these solvents and lithium metal would be low. The screened solvents included six linear ethers, five cyclic ethers, two aromatic ethers, two carbonates, two amides, an amine, and a urea. Based on the results, they performed experiments on the solubility of Li salt for further screening; as a result, 11 solvents exhibiting solubilities of more than 1.0 M for lithium tetrafluoroborate (LiBF₄) salt were screened for further experiments.

Seven solvents—1,2-dimethoxyethane (DME); tetraglyme; polyglyme; tetrahydrofuran (THF); 2-methyl THF; 2,2-dimethyl THF; 2,5-dimethyl THF—were then selected from the 11. These were then winnowed further. Ultimately they found that DME was more effective for mitigating dendritic growth.

Images of lithium dendrites taken by a microscope during the in situ observation of lithium deposition. The electrolytes are (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. Click to enlarge.

We found that the following rules were applicable for designing electrolytes suitable with Li anodes: the solvent should have a low reduction potential, the salt anions should be large in size, and the resulting electrolyte should have low viscosity. By combining a screened liquid electrolyte with a polymer electrolyte in a pressurized cell, we could achieve an excellent charging/discharging durability of around 100 cycles for an anode with an extremely high areal capacity of 12 mAh cm⁻². Although the practical use of this electrolyte composition might be limited owing to the low boiling point of DME, the results of this study clearly suggest that proper electrolyte design involving the control of electrolyte reactivity with respect to lithium, as well as the viscosity of the electrolyte and its anions’ sizes, can increase the longevity of lithium anodes.

—Park et al.

Resources

• Min Sik Park, Sang Bok Ma, Dong Joon Lee, Dongmin Im, Seok-Gwang Doo & Osamu Yamamoto (2014) “A Highly Reversible Lithium Metal Anode,” Scientific Reports 4, Article number: 3815 doi: 10.1038/srep03815