The Effect of K-doping on Activation in Co-Free Li-Rich Cathodes.

The activation mechanism of Li-rich cathode has been discussed for many years, yet there is still debate on different theories. Potassium doping can assist the investigation on activation mechanism through its unique function in terms of blocking TM migration during activation.   K-doping works by occupying Li sites even after Li has been extracted, increasing stability by blocking transition metals from migrating into these sites, which can help us distinguish the pathway of activation.  We use in-situ XRD to show that K-doping significantly lowers the rate of transition metal migration during initial charging, and that this is correlated with less activation extent.  However, the ex-situ XAS results show that anionic redox is more reversible in the K-doped material.  These results cannot be easily explained by existing theories alone; therefore, we propose that K-doping hinders TM migration during activation and therefore favours a Reductive Coupling Mechanism over a dynamic TM migration mechanism.  These findings have significant practical and theoretical implications for the development of Lithium-Rich cathodes.