Functionally gradient materials for sustainable and high-energy rechargeable lithium batteries: Design principles, progress, and perspectives

Rechargeable lithium batteries with high-capacity cathodes/anodes promise high energy densities for next-generation electrochemical energy storage. However, the associated limitations at various scales greatly hinder their practical applications. Functional gradient material (FGM) design endows the electrode materials with property gradient, thus providing great opportunities to address the kinetics and stability obstacles. To date, still no review or perspective has covered recent advancements in gradient design at multiple scales for boosting lithium battery performances. To fill this void, this work provides a timely and comprehensive overview of this exciting and sustainable research field. We begin by overviewing the fundamental features of FGM and the rationales of gradient design for improved electrochemical performance. Then, we comprehensively review FGM design for rechargeable lithium batteries at various scales, including natural or artificial solid electrolyte interphase (SEI) at the nanoscale, micrometer-scale electrode particles, and macroscale electrode films. The link between gradient structure design and improved electrochemical performance is particularly highlighted. The most recent research into constructing novel functional gradients, such as valence and temperature gradients, has also been explored. Finally, we discussed the current constraints and future scope of FGM in rechargeable lithium batteries, aiming to inspire the development of novel FGM for next-generation high-performance lithium batteries.