Understanding the Role of Ni in Stabilizing the Lithium Rich Cathode Material, Li[Nixli(1-2x)/3Mn(2-x)/3]O2 (0 ≤x≤0.5)

The class of lithium rich cathode material, Li[Ni_xLi_(1−2x)/3Mn_(2−x)/3]O₂ (0≤x≤0.5), shows great potential as a positive electrode for the next generation of high-energy materials for lithium ion batteries. These materials exhibit capacities exceeding 270 and over 300 mAh/g depending on the transition metal incorporated into the material structure[1]. However, over several cycles the material exhibits severe capacity deterioration and voltage decay. [2].  This is attributed to the instability of the material to change from layered to more spinel-like and the separation of the transition metals inducing migration of these transition metals to the bulk[3]. It has been suggested that Ni plays an important role in stabilizing this class of material [4].  In this study we try to elaborate further on the role of the secondary transition metal in stabilizing the structure and its relationship to oxygen by using soft and hard x-ray absorption spectroscopy to probe the electronic and local structural changes at different charged and discharged states for the Li[Ni_xLi_(1−2x)/3Mn_(2−x)/3]O₂ for x=0 ,0.2, and 0.5. The O K-edge, transition metal L _II,III- edges were performed both in partial electron yield and fluorescent yield modes that probe the surface and bulk, respectively. The transition metal K-edges were performed to observe the oxidation states. Observation of the O-K edge pre-edge intensity increase can be attributed to the Ni redox couple. Closer examination of the lower and higher photon energy bands reveals a decrease of the 3d versus 4sp intensity bands at the charged state for the oxygen activated stoichiometry, x=0 and x=0.2 indicating hybridization from the reductive coupling mechanism. Mn L _II,III-edges (figure) shows that with decreasing Ni content, the changes of the electronic structure from layered to more spinel-like occurs much earlier on and Mn redox couple participation. The results of the soft and hard x-ray absorption spectroscopy suggest the oxidation of oxygen will induce the reduction of Mn with an oxidation state between 3 – 4+ in order to stabilize the distorted structure and maintain the overall charge balance. When a secondary metal, nickel in this case, is incorporated, it will alleviate the changes by reducing in lieu of Mn thus maintaining the original non-distorted 4+ oxidation state.

[1] H. Yu, H. Zhou, The Journal of Physical Chemistry Letters, 4 (2013) 1268-1280.

[2] M. Gu, I. Belharouak, J. Zheng, H. Wu, J. Xiao, A. Genc, K. Amine, S. Thevuthasan, D.R. Baer, J.-G. Zhang, N.D. Browning, J. Liu, C. Wang, ACS Nano, 7 (2012) 760-767.

[3] S. Hy, W.-N. Su, J.-M. Chen, B.-J. Hwang, The Journal of Physical Chemistry C, 116 (2012) 25242-25247.

[4] D. Kim, J.R. Croy, M.M. Thackeray, Electrochemistry Communications, 36 (2013) 103-106.