様式３R1402

Investigation of CeO2 nanoparticles' redox reaction in Li-O2 battery
Chunzhen Yanga, Misaki Katayamab, Yasuhiro Inadab, Hye Ryung Byona
理化学研究所 Byon 国際主幹研究ユニット, b 立命館大学
Byon Initiative Research Unit, RIKEN, Wako, Saitama 351-0198 Japanb
Department of Applied Chemistry, College of Life Sciences, Ritsumeikan University, Japanaa

The effect of ceria (CeO2) nanoparticle (NP) on carbon nanotube (CNT) in a lithium-oxygen (Li-O2)
battery is studied in this work. CeO2/CNT electrodes are collected at different discharge/recharge states fromLi-O2 batteries and investigated by Ce L3-edge XANES. The oxidation states of cerium are analyzed todetermine whether the Ce3+/Ce4+ redox reaction plays some catalytic role for the formation anddecomposition of Li2O2 in Li-O2 battery.
Keywords: Li-O2 battery, XANES, lithium peroxide, Cerium oxide nanoparticles.
背景と研究目的 (Introduction)：
The Li-O2 batteries have been widely
investigated thanks to high gravimetric energy
density. However, there are several criticalchallenges to realize commercialization, which
are high recharge overpotential and poor capacityretention. This is mainly due to poor conductivity
and large particle size of discharged product oflithium peroxide (Li2O2) that is deposited on thecarbonaceous electrode.1,2 We have explored theway to decrease in recharge overpotential using
metal oxide nanoparticle (NP), which can alter
Li2O2 structure and physical property.3 We haveparticularly focused on key mechanism forsuppression of recharge potential via distinct
nucleation and growth process of Li2O2 on themetal oxide from those on the carbon electrode.
Namely, the metal oxide has higher bindingenergy to reactive oxidative species (ROS), suchas oxygen and superoxide species, via surface
redox reaction of NPs or their physicochemicalaffinity. For a case study, we here employed ceria(CeO2) NPs to multi-walled carbon nanotube(CNT) as the positive electrode in the Li-O2 cell.
The CeO2 has strong binding affinity to the ROS
via charge transfer.4 The oxygen vacancy
associated with Ce3+ on the CeO2 is the active
site, which is abundant at smaller size (less 5 nm).
As the ROS reacts with the Ce3+ (Ce3+ + O2 ↔
Ce4+ + O2–) around the oxygen vacancy, thenucleation of Li2O2 can primarily occur on theCeO2. Therefore, the changed oxidation state on
the CeO2 may be important clue for nucleation ofLi2O2, which can be attributed to altered Li2O2structure and low recharge potential. On the otherhands, we can also consider the effect of physicaladsorption of ROS at defect site, without any
charge transfer.5 To corroborate the role of CeO 2NP for the Li-O2 battery, it is necessary toobserve oxidation-state change of cerium using
Ce L3-edge XANES.
実験 (Experimental)：i) Synthesis and cell fabrication
CeO2 NPs were synthesized by thermal
decomposition of cerium nitrate precursor in thepresence of protecting agent of oleylamine
according to the literature.6 Around 0.6 mg CeO2
NPs and CNT (1.8 – 2.0 mg) were well mixed byultra-sonication in hexane, then made binder-free
positive electrode via vacuum filtration. The
electrode consisting of 3-D framework of CNTs
has many voids (more than 100 nm of size),
which is appropriate for O2 transport in Li-O2battery. The positive electrode was annealed at
350 oC in Ar gas and rigorously dried undervacuum, which was then assembled with metallic
lithium as the negative electrode, glass fiber
separator and tetraethylene glycol dimethyl ether(tetraglyme) with 0.5 M LiClO4 electrolyte in an
Ar-filled glovebox. The complete cell was filledwith O2 gas at atmosphere.
ii) Electrochemical evaluation
Galvanostatic discharge/charge was performed
using a battery cycler (WBCS3000, WonATech,Korea) at 25 oC. The applied current (mA/g) isbased on the CNT mass. All potentials werereferred to Li+/Li.

11様式３iii) Characterization
The XANES spectra were acquired at thebeamline of BL-3 in the Synchrotron Radiation(SR) center of Ritsumeikan University. Theelectrode samples were enclosed by Al-laminatedpacks in the glove box. The XANES spectral
measurements were conducted with a Si (220)double crystal monochromator. The FY mode
was applied for different states of electrodes
while the transmission mode was used for
air-exposed reference samples of Ce(NO3)3 andCeO2 powder.

The 1st discharge (DC) and recharge (RC)potential profiles of Li-O2 battery withCeO2/CNT and CNT electrodes are shown inFigure 2. The DC potential shows a plateau of~2.63 V for both electrodes. On the contrary, theinitial RC potential for the CeO 2/CNT is lower
than that for the CNT. The RC potential
approaches to ~4.5 V at the end of RC for bothelectrodes. It is noted that the Ce 3+/Ce4+ redox
reactiontakes placearound 4.76 V(thermodynamic reversible potential), whichindicates negligible electrochemical redox
reaction of Ce3+/Ce4+ within the DC and RC
potential range.

結果、および、考察 (Results and Discussion)：
The characterization of CeO2 in Figure 1a shows
uniform size of CeO2 NPs (with an average
particle size of ~5.3 nm), which has fluorite
crystal (Figure 1c). These CeO2 NPs are wellincorporated onto the CNT without considerable
agglomeration (Figure 1b). The mass ratio ofCeO2 is around 25% to total electrode mass
(CeO2 + CNT).

Fig. 2. Normalized galvanostatic DC and RC
profile of Li-O2 cell with CeO2/CNT electrode inLiClO4/tetraglyme electrolyte. A current densityof 50 mA g-1-CNT and a cut-off discharge voltage
is 2.2 V.
Figure 3 exhibits Ce L3-edge XANES spectrawith FY mode for all electrodes. Different statesof DC and RC, such as as-prepared, 0.5DC(~50% of DC), 1DC (full DC with a cut-off
potential of 2.2 V), 0.25RC (~25% RC), 0.75RC(~75% RC) and 1RC (full RC) electrodes, reveal
the oxidation states of cerium in CeO2 NP during
the battery performance. The Ce(NO3)3 and CeO2
powders in the bottom indicate standard
references of Ce3+ and Ce4+, respectively, whichwere taken with transmission mode. All
CeO2/CNT electrodes show similar absorption
spectra with predominant doublet (as indicatedby dashed red lines) of Ce4+ at 5729 and 5736 eV.
In addition, a transition shoulder at 5725 eV isalso present, which arises from Ce3+. This result
reveals that the CeO 2 NPs are electrochemicallyinactive during the battery performance: there isno evidence of Ce3+ ↔ Ce4+ as the ROS is boundor unbound at the oxygen vacancy during DC andRC, which rules out the charge transfer effect.

Fig. 1. Characterization of CeO2 NPs andCeO2/CNT electrode: TEM images of (a) CeO2
NPs and (b) as-prepared CeO2/CNT. (c) XRD
pattern of fluorite CeO 2 on the CNT.
12

様式３
carbon nanotube for the formation ofnon-crystalline Li2O2 in Li-O2 batteries, 55thBattery Symposium Japan 2014, Electrochemical
Society of Japan, Kyoto, Japan[2] Chunzhen Yang, and Hye Ryung Byon, CeO2
Nanoparticle-GuidedFormation
andDecomposition of Amorphous Li 2O2 on Carbon
Nanotube Cathode in a Li-O 2 Cell, 227th ECS
Meeting 2015, Electrochemical Society, Chicago,USAFig. 3. The normalized Ce L3-edge XANES
spectra of CeO2/CNT electrodes with FY mode atdifferent DC and RC states in Li-O2 cells.
Commercial Ce(NO3)3 and CeO2 powders arecharacterized as standard references for Ce 3+ andCe4+, respectively, with transmission mode.
In summary, we investigated the electrochemical
performance of Li-O2 cells with CeO2 NPs
decorated on the CNT. The Ce L3-edge XANES
spectra demonstrate that CeO2 NPs are not
involved in reversible redox reaction during DCand RC. This result reveals that lower initial RCpotential on the CeO2/CNT electrode is not
attributed to the charge transfer effect of Ce3+ onthe CeO2. Presumably, other physicochemical
characteristics of CeO 2 can aid in decrease in theRC potential in Li-O2 cells.
文 献 (References)[1] J. Christensen et al., J. Electrochem. Soc.,
2012, 159, R1-R30.
[2] R. Black et al., Adv. Energy Mater., 2012, 2,801-815.
[3] E. Yilmaz et al., Nano Lett. 2013, 13,4679-4684.
[4] S. S. Lee et al., ACS Nano, 2013, 7,9693-9703.
[5] J. Xu et al., Chem. Comm., 2010, 46,1887-1889.
[6] S. Lee et al., ACS Nano, 2013, 7, 9693-9703.
論 文 ・ 学 会 等 発 表 （ 予 定 ） (Articles andmeetings)
Articles[1] C. Yang, R. A. Wong, K. Yamanaka, T. Ohta
and H.R Byon, Understanding the Role of
Structural Promoter in Lithium-Oxygen Battery
using CeO2 nanoparticle on CNT. (Inpreparation).
Meetings[1] Chunzhen Yang, and Hye Ryung Byon,
Structural effect of nano ceria incorporated to13