Attainable Gravimetric and Volumetric Energy Density of Li–S and Li Ion Battery Cells with Solid Separator-Protected Li Metal Anodes

As
a result of sulfur’s high electrochemical capacity (1675
mA h/gs), lithium–sulfur batteries have received
significant attention as a potential high-specific-energy alternative
to current state-of-the-art rechargeable Li ion batteries. For Li–S
batteries to compete with commercially available Li ion batteries,
high-capacity anodes, such as those that use Li metal, will need to
be enabled to fully exploit sulfur’s high capacity. The development
of Li metal anodes has focused on eliminating Coulombically inefficient
and dendritic Li cycling, and to this end, an interesting direction
of research is to protect Li metal by employing mechanically stiff
solid-state Li+ conductors, such as garnet phase Li7La3Zr2O12 (LLZO), NASICON-type
Li1+xAlxTi2–x(PO4)3 (LATP),
and Li2S–P2S5 glasses (LPS),
as electrode separators. Basic calculations are used to quantify useful
targets for solid Li metal protective separator thickness and cost
to enable Li metal batteries in general and Li–S batteries
specifically. Furthermore, maximum electrolyte-to-sulfur ratios that
allow Li–S batteries to compete with Li ion batteries are calculated.
The results presented here suggest that controlling the complex polysulfide
speciation chemistry in Li–S cells with realistic, minimal
electrolyte loading presents a meaningful opportunity to develop Li–S
batteries that are competitive on a specific energy basis with current
state-of-the-art Li ion batteries.