Mechanism of Phase Propagation During Lithiation in Carbon-Free Li₄Ti₅O₁₂ Battery Electrodes

Functional electrodes for batteries share a common design rule by which high electronic and ionic conductivity pathways must exist throughout the electrode in its pristine state. Notable amounts of conductive carbon additive in the composite electrode are usually included to form an electronically conductive matrix. However, excellent high rate cycling performance has been achieved in electrodes composed of the insulating Li₄Ti₅O₁₂ without any conductive additives. This behavior opens the possibility of a new paradigm for designing functional electrodes by which high electronic conductivity in the pristine electrode is not required. The mechanism of operation that enables such unexpected electrochemical behavior is evaluated and discussed. Electronically conductive pathways due to the reduction of Ti⁴⁺ to Ti³⁺ form and percolate throughout the Li₄Ti₅O₁₂ electrode in the early stage of Li insertion, eliminating the need for conductive additives. This work highlights the importance of the mass and charge transport properties of the intermediate states during cycling and of good interparticle ohmic contact in the electrode. This physical behavior can lead to novel system designs with improved battery utilization and energy density.