Effect of Lithium in Transition Metal Layers of Ni-Rich Cathode Materials on Electrochemical Properties

Abstract

Li was incorporated into transition metal layers of Ni-rich Li[Ni0.95Co0.05]O-2 by formation of a solid solution with Li2MnO3 (layer notation: Li[Li(0.33)Mu(0.67)]O-2), which can be denoted as (1-x)Li[Ni0.95Co0.05]O-2 - xLi[Li(0.33)Mu(0.67)]O-2, to understand the effect of Li on the structure, electrochemistry and thermal characteristic of the cathode. Structural analysis data obtained by Rietveld refinement of X-ray diffraction data indicate that the additional Li can be found in the transition metal layers of (1-x)Li[Ni0.95Co0.05]O-2 - xLi[Li0.33Mn0.67]O-2. An interesting feature is that the average oxidation states of Ni and Mn are 3+ and 4+, respectively, for (1-x)Li[Ni0.95Co0.05]O-2 - xLi[Li0.33Mn0.67]O-2 as demonstrated by X-ray absorption study. The first discharge capacity was approximately 217 mAh g(-1), and the resulting retention was above 90% for 0.9Li[Ni0.95Co0.05]O-2 - 0.1Li[Li0.3Mn0.67]O-2 (Li[Li0.033Ni0.855Co0.045Mn0.067]O-2) in the range of 2.7 to 4.5 V for 100 cycles, which is a surprising result for Ni-rich compounds. Moreover, the thermal stability of (1-x)Li[Ni0.95Co0.05]O-2 - xLi[Li0.33Mn0.67]O-2 was significantly improved over the cathodes with identical Ni fraction. These results highlight the role of tetravalent Mn ions, even in small amounts, in stabilizing the electrochemical performances and thermal properties in the Ni-rich layer cathodes.