(Invited) Li Rich FCC Materials As High Capacity Cathodes

Abstract

In 2014, we developed a new type of compound based on lithium-rich metal oxyfluoride of Li2MO2F.[1] The novel material exhibits an outstandingly high capacity of 420 mAh g-1 with small lattice expansion and shrinkage of only 3% upon cycling owing to its stable rock salt host structure. Lithium, occupying 2/3 of the cation sites, is highly mobile. The structure stabilizes well after nearly two Li+ depletion. The flexibility of the host structure means a paradigm change in the Li+ storage, which opens new possibilities for obtaining higher storage capabilities both gravimetrically and volumetrically. Particularly, high volumetric energy densities up to 4600 Wh L-1were achieved. The first generation of materials (M=V) [2] has recently been expanded by Cr as an alternative transition metal.[3] This presentation will address the first and the second generation of materials, where pure V and Cr compounds as well as systems with mixtures of the two metals have been synthesized and tested. It will be shown that Cr leads to higher discharge potentials, which are about 0.5 V above those of the pure V compounds. Moreover, the smaller cation enables higher packing densities of Li so that the theoretical gravimetric and volumetric energy densities of the respective Li rich fcc compound are high at average 2.8 V.