Solid electrolyte-based Li-ion batteries can enable energy storage devices with high energy and power densities due to their compatibility with high voltage cathodes (>5 V vs Li/Li⁺) and Li metal anode [1]. Among numerous solid electrolytes, doped Lithium Lanthanum Zirconium Oxide (LLZO) has a high room temperature ionic conductivity (≈ 10^-3 mScm^-1) and suitable electrochemical stability [2]. LLZO when cycled with Li metal in a symmetrical cell configuration (Li-LLZO-Li), continuous stripping and plating results in the formation of Li metal filaments (dendrites) which nucleate on the cathode, propagate through the solid electrolyte, and short-circuit the cell. Broadly, two mechanisms have been proposed to explain these observations, internal pressure build-up due to plating in sub-surface pores and cracks were proposed to result in fracture of solid electrolyte and dendrite growth [3, 4]; the second mechanism postulates metallic lithium deposition inside solid electrolyte due to inherent electronic conductivity of a solid electrolyte [5, 6]. Although, both models offer some insights into possible reasons for dendrites in solid electrolytes, quantitative understanding of dendrite formation that is consistent with high strength of thin lithium metal (with high aspect ratio) is still lacking and are a subject of this work. [7]

Critical current density (I_CCD) in LLZO estimated through commonly used cyclic protocols were found not to be a material or system parameter but dependent on the choice of protocol parameters even for samples with similar interfacial resistances and microstructures. Unidirectional plating experiments will be shown to be a cleaner way of estimating I_CCD and were found to result in consistently higher I_CCDs. More importantly, even pressures as high as 10 and 25 MPa during cyclic experiments were found not enough to recover unidirectional I_CCDs. Next, to understand the origin of I_CCD in solid electrolytes, a thermodynamically consistent electro-chemo-mechanical model capturing various non-equilibrium process involved in dendrite nucleation was developed. The energy change associated with Lithium ion (Li⁺) transfer from solid electrolyte to Li metal (Li⁰) in a prefilled surface flaw was found to be sufficient to break open a hard ceramic solid electrolyte and thus create a pathway for Li⁰ to plate inside the crack. A quantitative match without any scaling factors between the experimentally observed and theoretically predicted I_CCD was observed. Interestingly, I_CCD was found to be a range (even for samples with same interface resistance, test protocol and microstructure) rather than a single value due to the inherent geometry of the cells especially at low interface resistances (< 20 Ωcm²). Furthermore, through a combination of experiments and quantitative modelling, possible methods to increase I_CCD in solid electrolytes will be described.