Inorganic electrolytes for solid state batteries with Li metallic anodes must combine properties such as high ionic conductivity, chemical stability, and resistance to failure due to propagation of Li filaments. While the ionic conductivity has been addressed well and there is a good selection of solid electrolytes with the conductivities comparable to those of commercial liquid electrolytes, resistance to propagation of metallic Li requires understanding of mechanical behavior in addition to ionic transport, and this area is somewhat lagging. Abundance of experimental evidence suggests that cracking of the solid electrolytes due to pressure exerted by lithium plating into the material defects is the primary source of failure [1]. In this regard, increasing resistance to cracking is one of the main goals of engineering robust solid ionic contactors for lithium metal batteries. Accommodation of stress via plastic deformation is hard to achieve in ceramics due to high energy barrier for creation of dislocations compared to the energy required to create fracture. Therefore, ceramic ion conductors are inherently brittle. Plastic deformation in glass can be achieved via shear and by densification. Using the example of lithium phosphorous oxynitride, Lipon, we demonstrate how these two mechanisms are capable of accommodating applied stress while avoiding creation of new surfaces by cracking. We investigate the resistance to fracture in Lipon and the underlying connection to its composition via instrumented nano-indentation and numerical simulations. We observe enhancement of isochoric shear in Lipon with increase of Li content, similarly to the reports of increased plasticity in sodium aluminoborate glases with high alkali content [2]. Nano-indentation demonstrates that Lipon is extremely resistant to fracture, compared to other inorganic solid electrolytes [3].

1. Porz, T. Swamy, B.W. Sheldon, D. Rettenwander, T. Fromling, H.L. Thaman, S. Berendts, R. Uecker, W.C. Carter, Y.-M. Chiang, Mechanism of lithium metal penetration through inorganic solid electrolytes. Adv. Energy Mater. 7, 1701003 (2017)
2. Sellappan, T. Rouxel, F. Celarie, E. Becker, P. Houizot, R. Conradt, Composition dependence of indentation deformation and indentation cracking in glass. Acta Mater. 61, 5949–5965 (2013)
3. Kalnaus, A. Westover, M. Kornbluth, E.J. Herbert, N.J. Dudney, Resistance to fracture in the glassy solid electrolyte Lipon. J. Mater. Research, 36, 787-795 (2021)