Depth-Resolved Structure Change of LiNi0.8Co0.2O2 Cathode during Battery Operation
Authors: Shirasawa Tetsuroh , Kobayashi Shigeru , Voegeli Wolfgang , Arakawa Etsuo , Nishio Kazunori , Shimizu Ryota , Hitosugi Taro 
Published: December 2024

Ni-rich layered oxides, which are isostructural to LiCoO2, are regarded as promising cathode materials alternative to LiCoO2 for lithium-ion batteries, since partial substitution of Ni with Co leads to a higher capacity and lower materials cost. However, increasing the Ni content causes a relatively rapid capacity fading, which is a challenge to overcome. It is considered that many factors are involved in the instability, such as irreversible crystal structure evolutions, degradation of morphological structures, and interface instabilities. To clarify the underlying mechanism the structure changes have been studied by various analysis techniques [1,2]. Regarding the structure evolutions near interfaces, the analyses were limited to static observations and dynamic observation during battery operation is lacking.

In this study, we observed depth-resolved structure evolutions of LiNi0.8Co0.2O2 cathode near the interface to a solid electrolyte during battery operation using synchrotron X-ray crystal truncation rod (CTR) scattering. In order to realize the real-time atomic scale analysis, we utilized following two unique techniques. (i) A thin film battery with an epitaxial LiNi0.8Co0.2O2 cathode layer grown by pulsed-laser-deposition (PLD) was used as the specimen (Fig. 1(a)), which had a very sharp electrolyte/cathode interface suited to the atomic scale analysis [3,4]. (ii) A high-speed CTR measurement technique was used, which realized the operando observation of depth-resolved structure changes of thin films with a temporal resolution of 1 s [5,6].

Fig. 1(b) shows a change of specular reflection CTR scattering profile observed during a discharge-charge. Since the LiNi0.8Co0.2O2 cathode layer was (001)-orientated, the CTR profile represents the structure along the [001] direction (c-axis). Fig. 1(c) shows the distribution of c-axis length obtained by analyzing the CTR scattering profiles. The H1, H2, H3, and M represent the crystal phases assigned based on the c-axis lengths, referring to previous reports [1,7]. When the Li content x in Li
x
Ni0.8Co0.2O2was larger than 0.9, the hexagonal H1 phase was uniformly formed. With decreasing x the variation of c-axis length along the depth direction was observed, and eventually a spatially separated two phases were observed in the range 0.3 < x < 0.6: hexagonal H2 and H3 phases were formed at the surface side and bottom side, respectively. The structure distribution indicates that the thermodynamically unstable H3 phase [1] was nucleated at the bottom.

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Figure 1