RIKEN Accel. Prog. Rep. 48 (2015)

Ⅲ-2. Atomic & Solid State Physics (Muon)

Li-ion diﬀusion in Lix FePO4 with x = 0, 0.25 and 0.5
I. Umegaki,∗1 M. Månsson,∗2 H. Nozaki,∗1 G. Kobayashi,∗3 R. Kanno,∗4 H. Guo,∗5 K. Ishida,∗5 I. Watanabe,∗5and J. Sugiyama∗1

∗1
∗2
∗3
∗4
∗5Toyota Central Research and Development Labs., Inc.
Material Physics, Royal Institute of Technology
Institute for Molecular Science
Department of Electronic Chemistry, Tokyo Institute ofTechnology
RIKEN Nishina Center

AP0 (t)(a)T = 125 K

ZF
LF=5 G

AP0 (t)(b)T = 175 K
TIME [µ s]ZF
LF=5 G

AP0 (t)(c)T = 250 K
TIME [µ s]ZF
LF=5 G

TIME [µs]Fig. 1. ZF- and LF-µ+ SR spectra on FePO4 at (a) 125 K,(b) 175 K, and (c) 250 K.
LixFePO4

6

Temperature [K]LixFePO46x=0x=0.25
x=0.5

ν [10 /s](a) ∆∆ [10 /s]

Lithium iron phosphate, LiFePO4 , is used as a cathode material in Li-ion batteries. A comparison withthe conventional cathode materials such as LiCoO2 ,LiNiO2 , and LiMn2 O4 showed that LiFePO4 has aspecial advantage over because of its high stability
during the lithium extraction/intercalation reaction atmedium temperatures (around 400 K).
Based on electrochemical1,2) and structural analyses,3)
the lithium extraction reaction from LiFePO4 is represented as follows: LiFePO4 - xLi → (1 − x)LiFePO4+ xFePO4 . Both LiFePO4 and FePO4 phases coexist
in Lix FePO4 , and single phase samples of Lix FePO4
have not been obtained so far. Following upon theµ+ SR work on LiFePO4 ,4) we have measured µ+ SR
spectra on Lix FePO4 (x = 0, 0.25, and 0.5) in order tounderstand the diﬀusive property shown in Lix FePO4
sample consisting of the two phases.
Powder samples of Lix FePO4 were prepared fromLiFePO4 by reacting it with NO2 BF2 in acetonitrile.
Then, the Li-deﬁcient powder sample was sealed intoa titanium cell with a gold o-ring. The window of thecell was made of a Kapton ﬁlm of 50 µm thickness.
The cell was mounted onto the Cu plate of a liquid-He
ﬂow-type cryostat in the temperature range between
10 and 400 K.
Figure 1 shows the temperature dependence of zeroﬁeld (ZF-) and longitudinal ﬁeld (LF-) µ+ SR spectra for FePO4 . Since there is no crucial change withtemperature, the implanted muons are static up to250 K. In fact, the spectra were ﬁtted by a combinationof a dynamic Kubo-Toyabe (KT) signal and a timeindependent background signal from a powder cell.
The ﬁeld distribution width (∆) and the ﬁeld ﬂuctuation (ν) were found to be independent of temperature;∆FePO4 = 7.1 × 104 s−1 and νFePO4 = 2.6 × 104 s−1 .
On the other hand, dynamic behavior was clearlyobserved for Li0.25 FePO4 and Li0.5 FePO4 . The µ+ SR
spectra were well ﬁtted by a combination of two KTsignals and a background signal. The two KT signals
came from the muons stopped in the LiFePO4 phaseand those in the FePO4 phase. Therefore, we used thesame values of ∆FePO4 and νFePO4 for the KT signal
from the FePO4 phase. Figure 2 shows the temperature dependence of ∆ and ν for the three samples. As
expected, the result is very consistent with that obtained for LiFePO4 . Assuming that the Li-ion jump to(b) νx=0x=0.25
x=0.5

Temperature [K]Fig. 2. Temperature dependence of (a) ∆ and (b) ν obtained for FePO4 , Li0.25 FePO4 , and Li0.5 FePO4 .

interstitial sites,4) we obtained the diﬀusion coeﬃcient
as: DLi = 5.8 × 10−11 cm2 /s at 200 K for Li0.5 FePO4
and DLi = 1.3 × 10−11 cm2 /s for Li0.25 FePO4 . These
values are smaller than DLi for LiFePO4 ,4) implying
the eﬀect of Li-ion diﬀusion between the LiFePO4 andFePO4 phases. Such interphase diﬀusion is believed tobe the most interesting process shown in the LiFePO4
cathode. In order to understand the interphase Li-ion
diﬀusion, however, we need to study the relationshipbetween DLi and x in Lix FePO4 in more detail.
References
1) A. K. Pahdi et al.: J. Electrochem. Soc. 144, 1188
(1997).
2) W. -J. Zhang.: J. Power Sources, 196, 2962 (2011).
3) A. Yamada et al.: J. Electrochem. Socl. 148, A224
(2001).
4) J. Sugiyama et al.: Phys. Rev. Lett. 103, 147601 (2009).

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