Early-stage latent thermal failure of single-crystal Ni-rich layered cathode

High nickel content worsens the thermal stability of layered cathodes for lithium-ion batteries, raising safety concerns for their applications. Thoroughly understanding the thermal failure process can offer valuable guidance for material optimization on thermal stability and new opportunities in monitoring battery thermal runaway (TR). Herein, this work comprehensively investigates the thermal failure process of a single-crystal nickel-rich layered cathode and finds that the latent thermal failure starts at ∼120 °C far below the TR temperature (225 °C). During this stage of heat accumulation, sequential structure transition is revealed by atomic resolution electron microscopy, which follows the layered → cation mixing layered → LiMn2O4-type spinel → disordered spinel → rock salt. This progression occurs as a result of the continuous migration and densification of transition metal cations. Phase transition generates gaseous oxygen, initially confined within the isolated closed pores, thereby not showing any thermal failure phenomena at the macro-level. Increasing temperature leads to pore growth and coalescence, and eventually to the formation of open pores, causing oxygen gas release and weight loss, which are the typical TR features. We highlight that latent thermal instability occurs before the macro-level TR, suggesting that suppressing phase transitions caused by early thermal instability is a crucial direction for material optimization. Our findings can also be used for early warning of battery thermal runaway.