In lithium-ion batteries, ion insertion may result in lattice expansion and phase transitions, associated with slow charging. Nanostructured battery materials exhibit faster rate capability due to finite size effects, yet this mechanism remain anecdotal. This study explains those effects using potentiometric entropy measurements to examine connections among ion ordering, structural phase transitions, and kinetics. Strong ion ordering is demonstrated in bulk tunnel-structured Li x MoO 2 despite the presence of a single crystallographic site for lithium. The largest ion ordering event coincides with the onset of a first-order phase transition and a maximum cell overpotential. These observations suggest that the phase transition occurs to bypass the energetic minimum of the ion-ordered state slowing charge storage considerably. Then, in nanoporous MoO 2 , ion ordering signatures are lessened. Accordingly, the phase transition remains first-order, but with smaller lattice mismatch and composition range of two-phase coexistence with a much smaller overpotential. Finally, in MoO 2 nanocrystals, ion ordering nearly disappears and structural evolution occurs through a second-order solution process that does not increase the overpotential. Experimental open-circuit voltage and entropic potential observed for the three types of MoO 2 were recreated with thermodynamic modeling on a three-site system with different extents of ion ordering. Finally, operando calorimetry confirmed dramatic reduction in energy losses and heat generation in nanostructured materials. Overall, this work provides new mechanistic understanding of the role of ion ordering during cycling by highlighting that decreased ion ordering in nanostructured materials, even when a first-order phase transition remains, enables significantly enhanced rate performance.
Leport et al. (Fri,) studied this question.