The development of all solid state lithium-ion batteries (ASSLB) is a key requirement in order to overcome safety issues related to the use of conventional liquid electrolytes and to enable the use of energy storage systems with high energy and power densities in the automotive sector. Over the past few decades, particular interest has been given to solid electrolytes with relatively high ionic conductivity at room temperature (10^-3S cm^-1) such as garnets due to their high thermodynamic and electrochemical stability and the possibility to use of lithium metal as anode.¹ Despite these advantages, one of the main drawback of solid state electrolytes is their short life-time due to the formation of lithium dendrites that leads to cell short-circuiting.² A better understanding of the mechanism of formation of dendrites and their chemical nature is therefore urgently required in order to engineer and develop solid electrolytes which could be employed in commercial systems.

In this talk I will highlight the current challenges in the development of solid state electrolytes, with particular focus on the problems which arise at the Li/electrolyte interface such as the formation of lithium dendrites. Here, we employed cubic Al- and Ga- doped Li₇La₃Zr₂O₁₂ (LLZO) as solid electrolyte in symmetrical cells and we investigated the formation of dendrites as a result of electrochemical cycling. We used surface analysis techniques such as FIB-SIMS, ToF-SIMS and LEIS in order to clarify the chemical composition of dendrites both in Al-LLZO and Ga-LLZO. Following electrochemical cycling, Al-LLZO displays a critical current density (CCD) of 0.10 mA cm^-2 sensibly lower than the CCD of Ga-LLZO (0.16 mA cm^-2), suggesting that the dopant used to stabilise the cubic phase of LLZO also plays a fundamental role on dendrites formation. Chemical analysis revealed that in Al-LLZO the dendritic features are composed of a mixture of Al and Li, whereas in Ga-LLZO are uniquely composed of Li. FIB-SIMS and ToF-SIMS higlighted that Al preferentially segregates at the grain boundaries, whereas Ga is uniformly distributed on the LLZO, suggesting that the chemical inhomogeneity in Al-LLZO is one of the factors influencing its lower CCD.³

 ¹ T. Thompson et al. ACS Energy Lett., 2017, 2 (2), 462–468

² A. Sharafi et al., J. Power Sources, 2016, 302, 135–139

3 F. M. Pesci et al., Manuscript in preparation