Let thermodynamics do the interfacial engineering of batteries and solid electrolytes

This review/perspective article critically assesses recent efforts and emerging opportunities to utilize the equilibrium formation of 2D interfacial phases to tailor batteries and solid electrolytes. In contrast to traditional kinetically-controlled atomic layer deposition (ALD) and other interfacial engineering methods, these 2D interfacial phases form spontaneously at thermodynamic equilibria; they are also termed as “complexions” to differentiate them from thin layers of 3D bulk phases whose thicknesses are controlled by kinetic/processing parameters such as the number of ALD cycles or deposition time. Here, two classic examples are represented by the impurity (dopant) based surface amorphous films (SAFs) and intergranular films (IGFs), both of which possess thermodynamically-determined “equilibrium” thicknesses on the order of 1 nm. Recently, the spontaneous formation of nanometer-thick SAFs and other less-disordered 2D surface phases have been utilized to improve the rate capability and cycling stability of various electrode materials. Detrimental and beneficial IGFs and other prewetting-like 2D interfacial phases have also been found in solid electrolytes or the solid electrolyte-electrode interfaces. A variety of other complexions, e.g., ordered adsorbates, can also play important, yet unrecognized, roles in various battery systems and solid electrolytes. Opportunities of utilizing 2D interfacial phases to control or improve the processing, ionic conductivity, and interfacial stability in solid electrolytes as well as solid-state and other battery systems are discussed. A potentially transformative idea is to utilize 2D interfacial phases to achieve superior properties unattainable by conventional bulk phases since they can exhibit structures that are neither observed nor necessarily stable as 3D bulk phases.