The role of niobium in layered oxide cathodes for conventional lithium-ion and solid-state batteries

Layered transition metal oxides (LTMOs), such as the LiNixCoyMn1-x-yO2 family, are the primary class of cathode active materials (CAMs) commercialized and studied for conventional lithium-ion (LIB) and solid-state battery (SSB) application. Despite nearly three decades of progress in improving stability, capacity, and cost, research has intensified to match global demand for high-performance materials. Nevertheless, (de)lithiation leads to irreversible degradation and subsequent capacity fading due to (chemo)mechanical particle disintegration and (electro)chemical side reactions. In this regard, surface and bulk modifications of CAMs by coating and doping/substitution are common strategies to enhance and support the electrochemical performance. Niobium has been featured in many studies exhibiting its advantages as a bulk dopant, where its ionic radius and unique valence character with respect to the metals used in LTMOs help prevent different degradation phenomena and therefore enhance performance. In addition, several niobium-based oxides (LiNbO3, Li3NbO4, Nb2O5, etc.) have been employed as a coating to increase cycling stability and rate capability through reduced surface degradation. Herein we illustrate how niobium serves as a coating constituent and a dopant, and discuss current understanding of underlying mechanisms, gaps in knowledge, and considerations for its use in a coating and/or as dopant in LTMO cathodes. This review article highlights the advantages of niobium as a dopant and a coating constituent for improving cycling performance of layered Ni-rich oxide cathodes in liquid- and solid-electrolyte-based Li-ion batteries.