High entropy lithium garnets – Testing the compositional flexibility of the lithium garnet system

There is a pressing need for higher energy density, safer batteries for electric vehicles and grid energy storage. The current consensus is that these batteries need to be replaced with fully solid equivalents, however the discovery of a suitable solid state electrolyte has proven troublesome. Of the promising materials, lithium garnets are popular due to their wide electrochemical window, good ionic mobility (>0.1 mS cm^-1) and compatibility with Li metal. However, numerous issues remain relating to interfacial resistance, time consuming and energy demanding synthesis and special handling requirements to ensure the best performing membranes. Of these challenges, little work has been done on the effect of entropy in the cubic lithium garnet systems, for example by addition of multiple elements on multiple sites. Such strategies have given interesting results in other areas (e.g. battery cathodes, metal alloys) and could well enable better performing membranes by increasing the, already high, disorder and take advantage of the numerous dopants reports which often singularly enable favourable properties (such as increased density, higher conductivity, lower interfacial resistance). Herein we test the compositional flexibility of the lithium garnet system with a 9 (Ga_0.2Li_5.75La_2.5Nd_0.5Nb_0.65Ce_0.1Zr₁Ti_0.25O₁₂) and 11 (Ga_0.2Li_5.75La_2.5Nd_0.5Nb_0.35Ta_0.3Ce_0.1Zr_0.75Hf_0.25Ti_0.25O₁₂) element system. Surprisingly, we did not find the limit of the garnet system, rather it was shown that (outside of the stoichiometric weighing) these systems were easy to synthesise, had high room temperature conductivity (0.2 mS cm^-1), and high density even when processed in air.