Enabling Lithium Metal Anode for Garnet Electrolyte based Solid State Batteries
Solid-state lithium metal batteries (SSLMB) employing inorganic solid electrolytes (ISE) in conjunction with lithium metal anode and intercalation cathode are considered among the most promising alternatives for Li-ion batteries1. Li-metal is an optimal choice of anode because of its high gravimetric and volumetric energy density. ISE, along with being flame-retardant, has a superior tolerance for a wide operating temperature range (−30 to 100 °C) while delivering a reliable performance2. The high shear modulus of ISE is also expected to mechanically suppress lithium dendrite growth3, thus enabling high energy density batteries. However, lithium dendrite penetration at current densities as low as 50 µA/cm2 was observed in SSLMBs4, while current densities of ≥1mA/cm2 are desired for practical use. The microscopic mechanisms that lead to lithium dendrite growth in SSLMBs are still unclear. Furthermore, the poor electrode/electrolyte interface coupled with processing challenges of ISEs have thwarted their realization as a practical battery system5. In the present work, we investigate dendrite growth through ISE, one of the most fundamental challenges with SSLMBs. First, for the choice of ISE, we synthesized Li6.4La3Zr1.4Ta0.4O12 (LLZTO), a garnet-type fast Li-ion conducting oxide. We interface lithium metal to this ISE via a well-known approach of employing a lithium alloying interlayer in between lithium metal and ISE. However, cells fabricated using aluminium interlayer show signs of dendrite growth at a current density of <300 µA/cm2. Through a set of electrochemical and scanning electron microscopy (SEM) techniques, we observed that interfacial voids at Li/LLZTO interface precede dendrite nucleation and growth. We believe that the edges of these voids can act as a favourable nucleation site for dendrites. Using a simple electrostatic model, we show that current density at the edges of the voids could be amplified by as much as four orders of magnitude, making the cells highly susceptible to dendrite growth after void formation. By employing standard pattering to induce controlled discontinuities, we further confirm our hypothesis by showing selective dendrite growth along the edges of discontinuities6. Based on the above observations, we developed strategies to increase the tolerance for dendrite growth in these battery systems. We note that the aluminium interlayer will dissolve with lithium metal over time, making the interface susceptible to discontinuities, as observed in cells without interlayers. Hence, if a material that doesn’t alloy with lithium while also fulfilling the criteria of an interlayer is used, it can enhance the current densities for dendrite nucleation. Based on this, we employed tungsten (W) as an interlayer material. We observed that current densities for dendrite nucleation in W interlayer cells were ≥ 530 µA/cm2, nearly twice that of Al-interlayer cells. Computational calculations showed that lithium vacancy migration energies, which is the first step for void formation, were 2-5 times higher for W surface than for Al surface, further confirming our observation6. Increasing the structural density of LLZTO ISE further improved the current density to 1 mA/cm2. Finally, we explored the anode-free battery configuration, which employs in-situ generated metallic lithium anode without a need for lithium handling, a significant processing advantage for wide-scale production of SSLMBs. Our work is an essential step in realizing a practical SSLMB whilst gaining mechanistic insights into dendrite growth in these energy storage systems.