ARTICLE pubs.acs.org/JPCC
Influence of Cationic Substitutions on the Oxygen Loss and Reversible Capacity of Lithium-Rich Layered Oxide Cathodes Z. Q. Deng and A. Manthiram* Electrochemical Energy Laboratory, Materials Science and Engineering Program, University of Texas at Austin, Austin, Texas 78712, United States ABSTRACT: The influence of cationic substitutions on the ease with which oxygen loss occurs from lithium-rich layered oxide cathodes during first charge has been studied with three series of samples: Li[Li0.33Mn0.67xTix]O2 (0 e x e 0.67), Li[Li0.2Mn0.54xTixNi0.13Co0.13]O2 (0 e x e 0.27), and Li[Li0.2Mn0.60xTixNi0.20]O2 (0 e x e 0.40). The lattice parameters and cell volume increase monotonically with the substitution of a larger Ti4þ for Mn4þ, indicating the formation of solid solutions. It is found that substitution of Ti4þ for Mn4þ decreases the degree of oxygen loss and consequently the charge and discharge capacity values, while substitution of Co3þ for (Mn0.54þNi0.52þ) increases the oxygen loss and consequently the charge and discharge capacity values. While the former is attributed to a stronger binding of oxygen to Ti and a decrease in the metaloxygen covalence due to a larger charge transfer gap between the Ti3þ/4þ 3d and O2 2p bands, the latter is attributed to a weaker binding of oxygen to Co and an increase in the metaloxygen covalence due to an overlap of the Co3þ/4þ t2g band with the top of the O2 2p band.
’ INTRODUCTION As one of the simplest lithium-rich layered oxides with all Mn4þ, Li[Li0.33Mn0.67]O2 (commonly designated as Li2MnO3) has been found to exhibit unexpected chargedischarge capacities when charged to high potentials.13 Experimental evidence shows that the oxidation of O2 ions to O2 is the main source of electrons during the first charging (extraction of Liþ ions) process, which is consistent with the fact that Mn4þ cannot be oxidized further in an octahedral site coordinated with six oxide ions. Corresponding to the extraction of Liþ ions and oxidation of O2 ions to O2 (a net loss of Li2O from the layered lattice) during the first charge, a plateau region (denoted as P) characteristic of a two-phase reaction occurs at ∼4.5 V as seen in Figure 1. The irreversible loss of oxygen from the lattice during first charge facilitates the reduction of Mn4þ to lower oxidation states during first discharge, rendering rechargeable capacity during subsequent chargedischarge cycling.24 Thus, while the first charge profile is distinctly different, the dischargecharge profiles during the first discharge and subsequent chargedischarge cycling resemble each other. However, the rechargeable capacity of Li[Li0.33Mn0.67]O2 depends sensitively on the synthesis temperature, and the capacity is low (
