New insights into structural changes of LiNiO₂

Nowadays, the electric automotive market requires an increase in the capacity of Li-ion batteries to allow longer driving ranges, and also faster-charging capability. The requirement has fueled the development of Ni-rich LiNi_1-x-yCo_xAl_yO_2 (NCA) and LiNi_1-x-yCo_xMn_yO_2 (NCM) cathode materials, in which the Ni content is beyond 80% (with respect to the overall transition metal content). Academia and industry have the same interest in enriching these materials in Ni, pushing the cathode systems toward LiNiO_2. However, this effort is hindered by the trade-off between capacity and stability. Approaching LiNiO_2 (high Ni content) results in intrinsic stability issues from this classic layered material. Hence, there is huge interest in refocusing on LiNiO_2, which has been investigated for almost three decades. Much of our understanding of the structure and property is based on excellent but relatively old studies. With the improvements in advanced characterization methods, fresh perspectives on poorly understood behaviors can be provided, and some theoretical assumptions have chances to be checked. Therefore, in this Ph.D. work, some new insights are provided, highlighting the Li/vacancy ordering structures and thermal stabilities at low temperatures. ... mehrThese new perspectives are believed to facilitate the understanding of remaining challenges, in the hope that they can be overcome someday. The thesis on the LiNiO_2 is organized in two sections as described below:

(I) New insights into lithium hopping and ordering in LiNiO_2 during Li (de)intercalation.
\emph{Ex situ} ^6Li and \emph{in situ} ^7Li nuclear magnetic resonance (NMR) spectroscopy is applied to monitor lithium mobility of LiNiO_2 during Li-ion (de)intercalation. Experimental NMR shifts are compared with the calculated shifts based on the theoretical Li/vacancy ordering models. The observed shifts are somehow close to estimated values. A considerable line broadening is first observed at a state of charge (SoC) of 20% (in this thesis, 100% belongs to the hypothetical full delithiation of the material). The movement of Li hopping slows down just before the ordering structure Li_0.75NiO_2 appears. This first Li/vacancy ordering structure can strengthen the Jahn-Teller effect, thus distorting the layered structure. The distortion further hinders the Li diffusion and broadens the NMR signal. For the last three SoC states (75%, 80%, and 85%), the NMR peaks remain at higher ppm positions. This suggests that the remaining Ni^3+ is located preferentially in the vicinity of the remaining Li ions, fitting well with the calculation model of Li_0.25NiO_2: each Li ion is surrounded by six Ni^3+ with the 180° configuration (Li_A-O-Ni^3+-O-Li_B). Such a special Li-ordered and Ni^3+/ Ni^4+-ordered structure corresponds to the single H2 phase and appears to be the necessary preparation step for the H2 to the H3 phase transition. Galvanostatic intermittent titration (GITT) and \emph{in situ} X-ray absorption spectroscopy (XAS) were also conducted to monitor changes in Li mobility and the oxidation state of Ni.

(II) Investigation of structural and electronic changes induced by post-synthesis thermal treatment of LiNiO_2
LiNiO_2 is investigated with respect to the subtle structural and electronic changes induced by a thermal treatment in air after synthesis in pure oxygen. These structure evolutions can also be regarded as the reflection of thermal instabilities of LiNiO_2 at low temperatures. A combination of thermogravimetry (TG), synchrotron radiation diffraction (SRD)/ X-ray absorption spectroscopy (XAS), ^7Li magic-angle spinning (MAS) NMR spectroscopy, magnetic measurements, and transmission electron microscopy (TEM) were applied to identify such subtle changes. These measurements reveal that Ni migration to the Li layers already starts at 200°C, later followed by reactions between LiNiO_2 and CO_2. With the heat treatment temperature increasing, strongly off-stoichiometric Li_1-zNi_1+zO_2 phases and products of Li_2CO_3 have been obtained. At 700°C, bulk decomposition occurs at a sluggish rate, suppressing the reactions with CO_2. The decomposition corresponds to the increasing rate of Ni reduction and preference of Ni^2+ for occupying the Ni sites. The extent of both reactions may not be homogeneous through the material, a larger reaction rate is instead found at the surface. The changes in Ni oxidation state and local Li environments are also monitored during the whole process. In addition, mild thermal treatment at 360-380°C can improve the electrochemical performance of LiNiO_2 since a ferromagnetic cubic phase in the surface region can serve as a protection layer.