Kyushu Synchrotron Light Research Center（ Form: No.5）

Experiment Report for Prefectural Beamline
Proposal no. : 1604016P
Beamline no.: BL11/BL7
Report date: 02/08/2016

Study of Li + conduction pathway bottleneck size of M 4+ doped
Li 7 La 3 M x Zr 2-x O 12 solid electrolytes using X-ray absorption finestructure
Yanhua Zhang 1 , Lianmeng Zhang 1 , Jiajun Gu 2 , Di Zhang 2 , Katsuhiko Saito 3 , andQixin Guo 3
1Wuhan University of Technology, China
2Shanghai Jiaotong University, China
3Saga University, Japan

１ ． Summary（ Note: Please include conclusions）
The effect of M 4 + ionic radius on the Li + conduction pathway bottleneck size on
Li 7 La 3 M x Zr 2 - x O 1 2 solid electrolytes were studied by X -ray absorption fine structure. In thesubstituted system Li 7 La 3 M x Zr 2 - x O 1 2 (M= Ti 4 + , Te 4 + , Hf 4 + , Ge 4 + , Sn 4 + , Zr 4 + ), the length of M-O
for M-doped Li 7 La 3 Zr 2 O 1 2 at Zr sites have been measured via EXAFS . The relationshipbetween Li + conduction pathway size with various ionic radius and the Li + migration activeenergy can be established.

２．Purpose of experiment and backgroundThe structure of LLZO is reported to consist of the garnet framework structure and Li atoms. Theframework structure is composed of dodecahedral LaO8 and octahedral ZrO6 by polyhedron connection [1-4].
Li atoms fill in the interstices of the framework structure. The Li ionic conductivity can be improved by doping
at Zr or La sites which adjusting the Li+ conduction pathway bottleneck size [5-8]. So far, the optimized Li-ion
conductivity of LLZO by super-valent cation substitutions such as Ta5+ at Zr sites reached to 1×10-3 S cm-1
which is available for practical application [9]. Cation substitutions by different ionic radius at Zr sites canmodify the ZrO8 volume size which results in changing of the Li+ conducting bottleneck size [10]. However,
the relationship between the Li+ conduction pathway bottleneck size and the Li+ migration activation energy
has not been studied yet.
The purpose of this proposal is to use x-ray absorption fine structure (XAFS) to study the effect ofdifferent ionic radius (M element) doping at Zr sites on Li+ conducting bottleneck size. By measuring thelength of M-O bond via XAFS, one can obtain the corresponding lithium ion conducting bottleneck size. Thus
we can build the relationships between lithium ion transport bottlenecks size and the Li+ migration activationenergy.
３．Experimental（Note: Description of sample，method of experiment and analysis, etc.）
The doped Li7La3MxZr2-xO12 (M= Ti4+, Te4+, Hf4+, Ge4+, Sn4+, Zr4+) samples were prepared by fieldassisted sintering technology. Field assisted sintering technology (FAST) is a combination of a uniaxialpressing process and a direct passing of electric DC current through the graphite tool to sinter the material, if
the powder is not electrically conductive. The production of Joule heat within tool enables the densification ofvarious materials with higher heating rates and shorter dwell times. Firstly, the powders were ball milled using
zirconia balls in 2 - propanol for about 12 h and then the powders were dried in vacuum oven at 80 oC for 6 h.
Secondly, powders were loosely filled into the graphite die and then it was put into the FAST system. FAST
consolidation was conducted with a plasma activated sintering system (ED - PASIII, Elenix Ltd., Japan).
Before the sintering, there was an activation step running for 30 s with a pulse current of 30V/10A. Sintering
process was performed at the temperature range of 900 oC to 1200 oC under a constant uniaxial pressure of 10
MPa with a holding time of 3min. The subsequent heating rate was set as 300 oC /min. Temperature
measurement and control were conducted using pyrometer through the hole of the die. Ar was filled into thesystem as protect gas to balance the pressure in order to prevent the loss of lithium. After the dwell, sample
was cooled down to room temperature naturally.
Pellets with 1~2mm in thickness and 10mm in diameter were measure by BL11 and BL7 Beam line. The
corresponding absorption edge energy range: 2.0 KeV-22.0 KeV. The corresponding energy absorption edge ofdoping elements M are as following:
Ti K: 4966eV;
Te K:31814eV, L3:4341eV;
Hf K:65351eV, L3:9561eV;
Ge K:11103eV;
Sn K:29200eV, L3:3929eV.

４．Results and Discussions

Figures (a)~(e) are the fitted XAFS results in Rq space of system Li7La3MxZr2-xO12 (M= Ge4+, Hf4+, Sn4+,Ti4+, Zr4+). The lengths of M-O for M-doped Li7La3Zr2O12 at Zr sites have been obtained via EXAFS fitting inR space. The fitting M-O length for Ge4+, Hf4+, Sn4+, Ti4+, Zr4+ are 1.951 Å, 2.092 Å, 2.106 Å, 2.021 Å,2.094Å, respectively, which is very close to the value calculated by First-principles calculations method, asshown in Table 1. The M-O length increases with the increase of cation radius which indicates that thesubstituted large radius cation can stretch the lattice skeleton. The increase of M-O length might expand the Li+conduction pathway bottleneck size which would reduce Li+ migration active energy. These results will berelated to our measured and calculated active energy thus the relationship between the Li+ conduction pathway
size and the Li+ migration active energy will be established.
Table 1 The M-O length calculated and measured by First-principles calculations and XAFS, respectively.

Elements

Zr4+

Ge4+

Ti4+

Sn4+Hf4+

Te4+

Ionic radius (Å)

0.72

0.53

0.605

0.69

0.71

0.97

Ion radius difference

0-26% -16% -4.2% -1.4% +35%

Calculated average M-Olength

2.110 1.940 2.012 2.104

2.096

2.221

XAFS measured M-O length2.094 1.951 2.021 2.106

2.092

N/A

５．Future issues
The measured data have many noise signals and the data of Te-doped samples was not successfully obtained.
We would like to measure one more time if it is possible.
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[5] A. Logéat, T. Köhler, U. Eisele, B. Stiaszny, A. Harzer, M. Tovar, A. Senyshyn, H. Ehrenberg, B.
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[10] Y. Kihira, S. Ohta, H. Imagawa, T. Asaoka, ECS Electrochemistry Letters 2 (2013) (7) A56.
７．Publications, patents（Note: Typical deliverables related to this proposal．
）

８．Keywords（Note: 2-3 words about samples and experimental methods．）
Li7La3MxZr2-xO12，Solid electrolyte, XAFS
９．About the publication of research results
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described in ※2. Also, please fill in an expected date of publication of a refereed journal article or an
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