Beyond Conventional Reaction Mechanism of Ordered Mesoporous Nickel Oxide for Lithium Ion Batteries
Authors: Kim Taewhan , Kim Kyoung Ho , Yoon Jaesang , Kim Hyunwoo , Kim JI Man , Yoon Won-Sub 
Published: February 2020

In a recent days, as cutting-edge devices, such as cell phone, electric vehicle (EV) and energy storage system (ESS) become more sophisticated, the battery performance is required to be higher. As a result, metal oxides having a capacity several times larger than that of carbon have attracted attention as a candidate for a next-generation secondary battery. Although metal oxides show higher capacity than one of conventional theory, which is insertion, alloy and conversion reaction, but it has not been clearly determined.

Nickel oxide (NiO) is very attractive anode material due to its chemically stable, cheap, environmentally benign, easy to synthesize properties, and high theoretical capacity (718 mAh/g). However, commercialization of NiO is hampered by its poor cycle performance and rate capability due to inherently severe volume changes during cycling. One of strategies to overcome these limitations is to synthesize nanostructured NiO with various morphology, such as nanofibers, nanowalls, hollow spheres, hierarchical structures, and especially porous structure.

In this study ordered mesoporous NiO is synthesized by using KIT-6 as a hard template for the sake of higher performance. Low-angle X-ray diffraction (XRD) patterns of highly ordered mesoporous NiO is quite different from KIT-6 silica template. A new (110) peak appears before 2θ=1°, which indicates the meso-structure transformation from cubic to tetragonal or the lower space group after the silica template etching process. In wide angle XRD pattern, synthesized highly ordered mesoporous NiO is matched with cubic NiO (JCPDS 47-1049). N2 adsorption-desorption isotherms show that the obtained replicas possess high specific surface areas (92 m2/g), and large pore volumes (0.24 cm3/g). The SEM images prove their mesoporous structures, indicating that nanoreplication was uniformly performed without formation of other metal oxide phase. Its theoretical capacity is calculated through conversion reaction, in which NiO reacts with Li ion and is converted to metallic Ni nanoparticles and Li2O, but the conventional reaction mechanism cannot elucidate its high reversible capacity (~1000 mAh/g). So synchrotron radiation analyses are used to research detail mechanism study.

Conventional reaction mechanisms in anode, such as insertion, conversion, and alloy, have respective consistency as shown in operando SAXS. Operando SAXS spectra of mesoporous NiO shows different behavior during 1st discharge compared to one of other anode materials, which follow conversion reaction. There are no significant changes of SAXS until ~1 mol of Li ion is reacted. Thereafter, the pore net volume changes increase and peak intensity decrease gradually indicating the loss of mesostructure upon the conversion reaction to form Ni and Li2O. These observation is similar to in situ XRD, which shows constant peak intensity until ~1 mol of Li ion is reacted and decrease because of phase transition from ordered mesoporous structure to amorphous phase, result of conversion reaction. In these results, retained peak intensity might be attributed to electrolyte participation reaction, not associated with the NiO lattice. For further study, ex situ XAS analysis is used to study the local atomic structure of cycled NiO in detail. Ni K-edge XANES spectra show negligible changes in initial stage of 1st discharge. These changes are also clearly observed in Ni K-edge EXAFS spectra, which show pristine and discharged to 200 mAh/g samples are similar and peak intensity of discharged to 400 mAh/g sample is lower. It indicates that electrolyte participation reaction (oligomer layer formation) takes place first, and then the conversion reaction occurs. In the middle stage of 1st discharge, it goes to metallic Ni, product of conversion reaction. The amplitudes of Ni-O and Ni-Ni peaks in mesoporous NiO decrease significantly and the amplitudes of Ni-Ni peak in metallic Ni increase with the increase of depth of the discharge due to displacement of reacting species during conversion reaction. There are no significant changes due to SEI layer formation in final stage of 1st discharge. Note that little changes of Ni K-edge XANES spectra are observed until charged to 600 mAh/g sample, and it goes to NiO-like phase till fully charged state in the 1st charge. It indicates that reversible conversion reaction takes only some part of entire reversible capacity, and the reversible oligomer formation / decomposition attributes to the remained reversible capacity. A similar consistency is shown on 2nd discharge. The XPS and HRTEM experiments were carried out for research of the electrolyte participation reaction. More details will be discussed in the meeting.