Solid-state batteries (SSBs) are promising energy storage alternatives that can achieve high energy densities by enabling Li metal anodes and high-voltage cathodes.
(1,2) When combined with long cycle life, improved safety, and low cost (<$100/kWh), the value proposition of solid-state lithium metal batteries becomes more and more relevant. There are, however, significant materials and processing challenges that disrupt the materialization of working SSBs at present.
(3−5) Recent reviews make it clear that the energy and power densities of the reported SSBs fall short in comparison with those of the state-of-the-art Li-ion batteries. The technical barriers to be addressed in the long term include achieving areal capacities in the range of 3–10 mAh cm
–2, with less than 10% excess Li anode and more than 70% active material loading, in solid-state composite cathodes that are assembled against thin electrolytes which can withstand current densities higher than 1 mA cm
–2 and yet enable higher Coulombic efficiencies. Cell design calculations show that solid electrolytes with <100 μm thicknesses and Li metal anodes with <50 μm thickness are necessary to achieve energy densities comparable to or higher than those of the state-of-the-art lithium-ion batteries.
(4−6) The processability and integration of thick composite cathodes, thin solid electrolytes, and thin lithium metal anodes into cell configurations in scalable fashions are of utmost significance to improve the value proposition of near-market SSBs.
An ideal SSB cell is a dense, thin, tri-layer assembly with conformal interfaces.
(1,3) Generating these dense, thin multi-layer systems required for practical SSBs is typically not possible with conventional processing approaches for a variety of reasons.
(7−9) Indeed, each component of the SSB has a multitude of issues that need to be tackled to engineer high-performance cells (
Figure 1a).
(4,10,11) The sheer variety of materials and their properties independent of the solid electrolytes make development of a consistent processing platform almost impossible to achieve.
(2,12,13) For example, garnet solid electrolytes typically undergo dual-step sintering for achieving the cubic phase and densification of the pellets along with an additional sintering step in Ar to remove the Li
2CO
3 impurities.
(14,15) The sintering temperatures for garnets are typically in excess of 1000 °C, with hold-times of 10 h or longer. In contrast, sulfide, argyrodites, and anti-perovskite materials are synthesized at lower temperatures of 100–400 °C and are generally processed into cells at room temperature and under uniaxial loading.
(16−19) Most of the processing routes described are strictly for small-scale pellet preparation. So far, a large-scale roll-to-roll kind of processing is typically only demonstrated on hybrid electrolytes that leverage the slot-die processing of polymer dispersions to make larger form factor materials.
(20,21,37)