Structural Causes of Electrochemical Hysteresis and Voltage Fade in Lithium- and Manganese-Rich Composite Cathodes

Layered lithium- and manganese-rich, TM oxide (LMR-NMC) intercalation cathode structures have been widely investigated as the next generation cathodes for advanced lithium-ion batteries. This class of materials, denoted as xLi2MnO3•(1-x)LiMO2 (M=Mn, Ni, Co), possesses complex nanocomposite structures consisting of two distinctly different average local environments one rich in Li and Mn (e.g., Li2MnO3) and the other in the constituent TMs (e.g., LiMO2)¹. In order to achieve reversible capacities greater than 200mAh/g, electrochemical activation of the Li- and Mn-rich component above ~4.4V is necessary.  However, activation followed by cycling above ~4.0V results in continuous structural changes to the material. Hysteresis in the open-circuit voltage is one manifestation associated with high-voltage cycling after the activation process.  In addition, a continuous decrease in the average voltage of cells, known as voltage fade, occurs ^2,3. ⁶Li NMR spectroscopy is the only structural probe currently available that can quantitatively characterize local lithium environments that dominates the free energy for site occupation and can be used to monitor the evolution of local order and low concentration defect formation with the goal of correlating local structural changes with hysteresis and voltage fade phenomena. Here, we show direct evidence of path-dependent lithium site occupation, correlated to structural reorganization of the metal oxide and the electrochemical hysteresis, during lithium insertion and extraction.

1) Croy, J. R., Gallagher, K. G., Balasubramanian, M., Long, B. R. & Thackeray, M. M. Journal of the Electrochemical Society 2014 161, A318-A325

2) Bettge, M.; Li, Y.; Gallagher, K.; Zhu, Y.; Wu, Q.; Lu, W.; Bloom, I.; Abraham, D. P. Journal of the Electrochemical Society 2013, 160, A2046

3) Gallagher, K. G.; Croy, J. R.; Balasubramanian, M.; Bettge, M.; Abraham, D. P.; Burrell, A. K.; Thackeray, M. M. Electrochemistry Communications 2013, 33, 96.