Structural evolution of layered Li1.2Ni0.2Mn0.6O2 upon electrochemical cycling in a Li rechargeable battery

Abstract

one of the most promising cathode candidates for next generation Li rechargeable batteries, has been consistently investigated especially because of its high lithium storage capacity, which exceeds beyond the theoretical capacity based on conventional chemical concepts. Yet the mechanism and the origin of the overcapacity have not been clearly understood. Previous reports on simultaneous oxygen evolution during the first delithiation may only explain the high capacity of the first charge process, and not of the subsequent cycles. In this work, we report a clarified interpretation of the structural evolution of Li 1.2 Ni 0.2 Mn 0.6 O 2 upon the electrochemical cycling, which is the key element in understanding its anomalously high capacity, through careful study of electrochemical profiles, ex situ X-ray diffraction, HR-TEM, Raman spectroscopy, and first principles calculation. Moreover, we successfully resolved the intermediate states of structural evolution upon electrochemical cycles by intentionally synthesizing sample with large particle size. All observations made through various tools lead to the result that spinel-like cation arrangement and lithium environment are gradually created and locally embedded in layered framework during repeated electrochemical cycling. Moreover, through analyzing the intermediate states of the structural transformation, this gradual structural evolution could explain the mechanism of the continuous development of the electrochemical activity below 3.5 V and over 4.25 V.

Origin of voltage decay in high-capacity layered oxide electrodes

“…Nevertheless, it remains that these materials have poor electrode kinetics and undergo large voltage decays (that is, reduction in average cell voltage) on cycling, both of which are plaguing their implementation in practical cells [14][15][16] . Present scenarios, although not fully conclusive, tend to relate this voltage decay to structural effects and more specifically to the formation of spinel-like domains [17][18][19][20][21][22][23] . We recently found that the replacement of Mn 4+ in Li 2 Ru 1−y Mn y O 3 by the larger and less electropositive cation Sn 4+ reduces voltage fading 12,24 .…”

Failure mechanisms of LiNi0.5Mn1.5O4 electrode at elevated temperature

“…One of the spotlighted materials is the lithium-excess layered oxide, xLi 2 MnO 3 À(1Àx)LiMO 2 (M ¼ Co, Ni, Mn), which has a higher working voltage and larger specific capacity than those of LiCoO 2 and LiMn 2 O 4 [1e3]. The intrinsic problem for this material is, however, the structural change upon cycling from the layered to spinel phase, which is undesirable since it lowers the working voltage [2,3] Moreover, this material is unstable as the Mn 3þ ions are prone to the disproportionation reaction and JahnTeller distortion [4].…”

High-performance lithium-rich layered oxide materials: Effects of chelating agents on microstructure and electrochemical properties

“…All the profiles show the characteristics of the lithium-rich layered oxide, comprising a sloping curve below 4.5 V and a long plateau around 4.5 V in the first charge process and continuous sloping curve in the discharge process[33][34][35][36][37]. During the initial charge, the sloping curve below 4.5 V corresponds to lithium extraction from LiNi 0.5 Co 0.2 Mn 0.3 O 2 component, while the plateau around 4.5 V relates to the removal of lthium from the crystal structure accompanied by oxygen evolution [6,9-11, 22, 23].…”