Lithium Insoluble Interlayer Suppress Void Induced Dendrite Growth in Garnet (LLZTO) Electrolyte Based Li-Solid State Batteries

Abstract

Lithium ion batteries (LIBs) have been in prevalence for over two decades. They find use in portable electronics, electric vehicles and also grid storage. The conventional Li-ion battery technology employs electrolytes that have flammable organic solvents as key constituents. In addition, these electrolytes are thermodynamically unstable at voltages beyond 4 V; stability at voltages beyond 4 V is essential for the development of high voltage and therefore, high energy density Li-ion batteries. Moreover, state-of-the-art lithium ion batteries use a graphitic anode, which has a low theoretical specific capacity of 372 mAh/g. Due to this low specific capacity, conventional LIBs are limited in scope with respect to their energy and power density. Lithium metal anodes with a theoretical specific capacity of 3860 mAh/g are ideal for the development of high energy density Li-ion batteries. However, Lithium metal when used with liquid electrolytes tends to form filament like structures called dendrites due to uneven deposition of Li metal leading to short battery lifetime and also possible battery fires.Solid state batteries (SSB’s) have been considered as a promising battery system to overcome the flaws of current LIB’s. The solid electrolyte (SE) employed in these battery systems are inflammable, eliminating any concerns regarding battery safety while also increasing battery’s operating temperature. A wide electrochemical stability of 0 to 5-6 V against Li+/Li and high sheer modulus opens up the window for use of high voltage cathodes and metallic lithium anode which otherwise cannot be employed in LIB’s. At cell level researchers have demonstrated an energy density of 400 Wh/kg which can further be increased to 800 Wh/kg, with a cycle life of 10,000 cycles, making them more than ideal candidate for EV’s, portable electronics and grid storage applications. LLZTO (Li 7 - x La 3 Zr 2 - x Ta x O 12 ) an inorganic ceramic electrolyte has attracted a lot of attention owing to its high ionic conductivity (10 - 3 S/cm) and stability against Li metal up to 6 V. However, dendrite growth at current densities as low as 100 µA/cm2 has been observed which has plagued their integration into all solid-state batteries. Improvements to the interface between lithium anode and the solid electrolyte via the use of metal interlayers which alloys with lithium and surface modification techniques, have been shown to mitigate dendrite growth at current densities exceeding 100 µA/cm2. When such interfacial improvements have been coupled with an external mechanical pressure of several hundred kilopascal, cells were shown to sustain higher current densities without any dendrite growth. Although such high stack pressures are impractical for battery development.In this work we have synthesized high ion conductivity pure cubic phase LLZTO and studied dendrite growth mechanism. We observe that voids at Li/LLZTO interface act as field concentrators and have prime role to play in nucleation of dendrites. We suggest that these voids might be inevitable owing to nature of this battery system and propose the use of electron conducting interlayers which have no solubility with lithium metal as way of suppressing voids and mitigating dendrite growth. Figure 1