Influence of cerium (IV) oxide for reversible oxygen activity in Li2RuO3 and Li3RuO4
M. Nedyalkov, P. Tzvetkov and R. Stoyanova
Abstract: Introduction – ruthenates are group of inorganic compounds with variety of oxidation states of ruthenium ranging from plus (IV) to (VII). That rich variety of oxidation states combined with layerd structure makes them highly perspective for use of battery applications. A very high specific capacity for some of these compounds has been reported [1,2]. This is possible because two different forms of active redox reaction affecting both the transition metal and the oxygen are possible. Our main objective is to activate the oxygen redox activity which can significantly enhance the specific capacity of ruthenate based electrode materials to above 300 mAh/g. Achieving such a high specific capacity poses a challenge because the oxygen activity becomes irreversible shortly after few cycles and hence oxygen gas is relessed into the battery enclosure and a serious decrease of specific capacity is observed. In the present work we study the influence of the irreversible oxygen activity. We hypothesize that addition of cerium (IV) oxide to ruthenate based electrode materials can influence the irreversible oxygen activity by reversibly binding during battery charging and releasing oxygen during the discharge phase. The modification of ruthenates with CeO2 is motivated due to its exceptional capability to store oxygen reversibly.
Experimental – high temperature solid state reaction is chosen for preparing the Li2RuO3 and Li3RuO4 for pure samples. For the CeO2-modification, a combined method including co-precipitation of cerium (III) on the ruthenate surface followed by high temperature calcination in oxygen rich atmospehere is developed. The electrochemical characterization of oxides was carried out in half lithium-ion cell using lithium electrolyte containing 1M LiPF6 solution in EC:DMC.
Results and Discussion – the specific capacities of the ruthenates were determined using galvanostatic cycling with potential limitation (GCPL). Figure 1 presents the charge-discharge curves of CeO2-modified Li3RuO4. The data show that the modified oxide delivers a reversible capacity of around 250 mAh/g, which is close to the theoretical ones (i.e. 288 mAh/g corresponding to 2 mol Li+). After modification, the reversible capacity remains stable during cycling in a wide potential range (i.e. between 1.5 and 4.2 V). Structural changes of oxides during cycling was monitored by ex-situ X-ray diffraction (XRD), while the variation in the oxidation states of paramagnetic ions was accessed by ex-situ electron paramagnetic resonance spectroscopy (EPR).
References:
[1] H. Li, S. Ramakrishnan, J. W. Freeland, B. D. McCloskey, and J.Cabana JACS, 142 (18), (2020) p. 8160.
[2] Li, B., Shao, R., Yan, H., An, L., Zhang, B., Wei, H., Han, X. Advanced Functional Materials, 26(9), (2016) p. 1330.
Acknowledgment: The author’s thanks for the financial support of the project CARiM (NSP Vihren, КП-06-ДВ-6/2019).
The authors from CARiM’s Research Team are bolded.
VIHREN-CARiM 2019 