The role of metal substitutions in the development of Li batteries, part I: cathodes

Metal substitutions into known structures have served a pivotal role in developing two of the three main components in state-of-the-art Li-ion batteries: the cathode and electrolyte. In this first installment of this review, we will discuss the design principles established for cathodes, focusing on the benefits and limitations of substitutions in terms of the performance metrics of highest import for high-energy applications such as electric vehicles and grid storage. We emphasize the high number of metrics that must be simultaneously optimized and review the methods best suited to determine such metrics. In particular, partial metal substitutions have proven to be important in increasing energy density, but also in improving safety and cyclability. However, for many materials, cathode particle coatings have been found to be as effective as partial metal substitution in improving the lifetime of the battery since lifetime in state-of-the-art batteries is limited by reactions between the cathode particles and the liquid electrolyte, which can be addressed through surface coating. Though, the effects of substitutions and coatings can be additive to some degree. After a detailed overview of the main classes of cathodes used commercially and in development; we find a number of potentially interesting cathodes that operate at such high potentials that the use of liquid electrolytes becomes impractical that open the door to large leaps in Li-ion battery energy density. This motivates the development of solid electrolytes, the topic of part II of this review.