(669g) High Energy Density Solid State Li Batteries Using a Trilayer Oxide Architecture

Li-ion battery technology has improved consistently since its introduction to the market in the early 1990s. At the same time, however, demand for better energy storage has outpaced what Li-ion batteries can provide. New battery chemistries that incorporate next generation electrodes and electrolytes will be the step change in technology that is needed to meet these demands. The best anode in terms of specific energy density is lithium metal—it has the most negative redox potential (-3.05 V vs SHE) and has 10x the capacity of today’s standard graphite anode (3860 mAh/g vs 330 mAh/g). On the cathode side, sulfur also 10x the capacity of state-of-the-art oxide cathodes at 1680 mAh/g, which offsets the lower voltage. The combination of lithium metal and sulfur electrodes is inherently capable of extremely high energy and power density batteries, but there has yet been no electrolyte system capable of supporting this chemistry for prolonged cycling.

Researchers have sought to use lithium metal and sulfur in batteries years but have been unable to overcome several issues. First, lithium metal is highly reactive and when in contact with an electrolyte forms a solid-electrolyte interface (SEI). When recharging the battery, lithium metal does not plate uniformly, and instead forms dendrites, creating new surface area and hence more SEI on every cycle. This consumes electrolyte and causes volume expansion, decreasing cycle life. The dendrites can also break off from the rest of the lithium metal, losing capacity or crossing the cell to cause short circuit resulting in catastrophic failure.

Sulfur electrodes have their own obstacles. The volume expansion of sulfur upon lithiation to the discharge produce Li₂S is up to ~80%, causing extreme stresses on the cell structure that ultimately leads to capacity fade. Moreover, intermediate lithium polysulfides form during the charge/discharge process that are soluble in most liquid electrolytes. Dissolved polysulfides cross the cell and react with the lithium metal, termed the polysulfide shuttle.

Solid state batteries composed of appropriate solid electrolytes can resolve these issues. Lithium garnets, specifically Li₇La₃Zr₂O₁₂ (LLZ) and its variants, are electrochemically stable to both lithium metal and sulfur. Eliminating liquid electrolyte between the electrodes precludes the formation of any polysulfide shuttle. Critically, dendrites cannot pass through garnets when appropriately manufactured. However, solid state batteries with lithium garnets typically suffer from high resistance at the electrode/electrolyte interfaces. Using a newly developed porous-dense-porous trilayer LLZO microstructure and interfacial treatments, the high resistance that solid electrolytes are known for can be overcome. The trilayer structure also addresses the volume expansion of the lithium metal and sulfur electrodes—by incorporating the electrodes within the pores of the solid electrolyte, all volume expansion occurs within the electrolyte structure and external expansion can be greatly reduced or eliminated.

The LLZ trilayer structure is produced via scalable tapecasting methods and reduces the amount of the electrolyte needed in the cell while increasing electrode loading. The thin dense layer (< 20 µm) at the center of the structure separates the electrodes and effectively blocks dendrites at an exceptionally high lithium cycling current density of 10 mA/cm² (1). The dense layer is mechanically supported by porous layers also composed of LLZ. The pores create open volume for electrode loading and provide high surface area for electrode-electrolyte contact. Li-S batteries based on the trilayer design are shown to cycle for > 500 cycles with 80% capacity retention and energy density of 250 Wh/kg at room temperature is demonstrated. With optimized cell design and improved fabrication methods, energy densities of > 500 Wh/kg are on the horizon. The LLZ trilayer cell design represents a significant advancement in solid-state battery technology and demonstrates how lithium metal and sulfur electrodes can be used in practical battery configurations for next generation energy storage.

1. G. T. Hitz, D. W. McOwen, L. Zhang, Z. Ma, Z. Fu, Y. Wen, Y. Gong, J. Dai, T. R. Hamann, L. Hu, E. D. Wachsman, Mater. Today, 2018, in press.