The role of metal substitutions in the development of Li batteries, part II: solid electrolytes
Of the three main components in state-of-the-art Li-ion batteries (cathode, anode and electrolyte), metal substitutions into known structures have primarily served a pivotal role in developing the cathode and solid electrolytes only. Previously, we reviewed the role of substitutions in optimizing cathodes, and these included multiple classes of materials that can only be commercialized with improved electrolytes able to operate at higher potentials than liquid carbonate-based electrolytes. Solid electrolytes are considered by many to be the path forward in this respect. In this review, we will discuss the design principles established for solid electrolytes, as well as both the successes and limitations of using substitutions to improve the performance metrics of highest import for critical applications such as electric vehicles and grid storage. A massive research push is underway in order to bring all-solid-state Li batteries to widespread use, particularly for electric vehicles, and metal substitutions are playing a critical role in developing the solid electrolytes to the point where they will, hopefully, out-perform the established Li-ion batteries with liquid electrolytes. This review further emphasizes metrics that are badly needed in further studies in order to systematically optimize solid electrolytes in a way that correlates well to better battery performance. Although the focus of this review article is on metal substitutions, we also discuss a few cases where non-metallic substitutions have been needed to bring a class of materials into viability where further metal substitutions become significant.