A Study on Electrochemical Reaction Mechanism of RuO2 using Synchrotron Based X-Ray Techniques
Authors: Kim Yunok , Kim Hyunchul , Palanisamy Kowsalya , Oh Woong , Lee Wontae , Jeong Mihee , Yoon Won-Sub 
Published: February 2020

Recently, various transition metal oxides (M= Fe, Ni, Co, Cu) have been studied as alternative anode materials for lithium ion batteries. Transition metal oxides as anode follow conversion and alloying reaction otherwise lithium intercalation reaction in the graphite anode. Transition metal oxides (CoO, NiO, FeO, CuO) which involve the formation of Li2O with reduction of metal by conversion reaction have a high capacity (700mAh/g) and long cycle life. Accordingly, transition metal oxides have become candidate as anode materials in Li-ion batteries. But these materials show reversible additional capacity over theoretical one, which couldn’t explain insertion, alloying, or conversion reaction. For understanding these systems, it is necessary to investigate electrochemical reaction mechanism in the Li-ion battery observantly.
Ruthenium oxide is a well known anode material for its high capacity (1130mAh/g) and high coulombic efficiency (98%, at the first cycle). RuO2 stored 5.6M Li during the first discharge. Li up to 4mol per RuO2 is stored by the insertion and conversion reaction. Generally, suggestions of additional capacity of RuO2 are considerable debates. In this work, we have tried to explain the electrochemical reaction mechanism of RuO2 during first discharge.
This electrochemical reaction mechanism is studied by in situ XRD measurements. Appearance of new peak could be attributed to formation of LiRuO2 phase and at the same time, RuO2 phase disappears during first plateau. The change of XRD pattern is reflected in the results of insertion reaction. The appearance of broad peak around 2Θ of 43° in the in situ XRD pattern suggests formation of Li2O and nano-sized Ru metal by conversion reaction. We tried to combine in situ XAS for getting closer insight into the reaction mechanism. Oxidation state decrease and Ru metal peak appears by the insertion and conversion reaction until the second plateau region. This study suggests that, Li storage in RuO2 takes place via phase transformations. RuO2 transformed into LiRuO2 by Li insertion, intermediate LiRuO2 turns to Li2O and Ru metal by conversion reaction. But last sloped region can’t be described by X-ray techniques. After the conversion reaction, notable change wasn’t shown from XRD pattern and XAS data. Finally, additional capacity of RuO2 at the last sloped region needs to investigate observantly.
For observing formation of SEI layers, TEM images got on different state of discharge. In general, formation of SEI films for anode materials occurs at the low voltage range. But compared to pristine powder and discharged electrode, it can be observed to formation of SEI films during the first plateau region in rather high voltage, 2.0V. Although SEI films mainly grow below 0.8V, it is not related to the additional capacity directly. In the previous report, SEI films are mainly formed during the last sloped region, however, it is decomposed at the high voltage region during charge. When voltage region was limited in 0.05 to 2.1V after first discharge, reversible Li storage behavior was observed without formation or decomposition of SEI films during cycling. The additional capacity at last sloped region was caused by interfacial reaction which was accommodated to excess Li in the grain boundary between nano-sized Ru metal and Li2O.
In recent research, formation of LiOH related to additional capacity of RuO2 at last sloped region. Because some OH groups are present on the external and internal surfaces of RuO2, formed LiOH during discharge decomposed to LiH and Li2O at the last sloped region reversibly.
Many chance of storage mechanism are debated for understanding additional capacity. The more detailed mechanism of RuO2 during discharge will be presented at the time of meeting.