Structure and Electrochemistry of Cation Substituted "Layered-Layered Composite" Cathode Materials for Li-Ion Batteries

Recently, Li- and Mn-rich ‘layered-layered’ composite materials have drawn much attention as possible high-energy cathode materials for next generation electric vehicles. However, continuous voltage fade (VF) during cycling has been observed with virtually all Ni-Mn-Co based compositions and understanding and arresting the VF is urgently required for its commercial application.  The voltage fade is thought to be related to the instability of the composite ‘layered-layered’ structure. Such structural relaxation could be induced by oxygen loss during initial high voltage charging process and/or cation migration during cycling.

Cation substitution may be a way to retard VF of Li-rich layered cathode materials if the substituent can alter the path of structural degradation during the first charge in the parent phase.  Our hypotheses for substitution study contain three criteria:  (1) strong bonding with oxygen is required, (2) the oxidation state of the dopant can go higher than +4 and is stable (mitigate oxygen loss), and (3) it should form reversible dumbbell configuration with Li_tet during deep charge in order to maintain structural stability.  Each substitution tried can fit into one of these classifications.  This presentation will show the results from such substitutions and will describe the chemistry of these materials.