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

RIKEN Accel. Prog. Rep. 49 (2016)

Li-ion diﬀusion in Li-ion battery material LiFe1−x Mnx PO4I. Umegaki,∗1 H. Nozaki,∗1 G. Kobayashi,∗2 R. Kanno,∗3 M. Månsson,∗4 A. Berlie,∗5 I. Watanabe,∗5and J. Sugiyama∗1-102

DLi (10 cm /s)

For the development of on-board batteries, the Liion battery is required to operate at high voltages. For
arealizing
practical Li-ion
Li-ion battery,battery, aa solid
solid solution
realizing aa practicalof the olivine-type lithium iron/manganese phosphate(LiFePO4 and LiMnPO4 ) is used as the positive electrode material. Olivine lithium phosphate is superior
in terms of stability and is a low cost material. Thetetrahedron PO4 is so stable that oxygen desorption
hardly occurs. Compared to other positive electrodematerials such as LiCoO2 , olivine lithium phosphate
is produced at a low cost because it contains no transition metals. High charge/discharge voltage can besteadily obtained for the Li(Fe,Mn)PO4 solid solution.
It is noted that the LiFe1−x Mnx PO4 (x∼0.7) solid solution is used for the realization of a practical Li-ionbattery.
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100.0

0.2

0.4

0.6

0.8

1.0x in LiFe1-xMnxPO4
Fig. 1. x dependence of the DLi at 300 K. The data for
x = 0 is reported in Ref. 1 and the data for x =0.2, 0.4, 0.6, 0.9 and 1 have been obtained with EMU
spectrometer in ISIS.

6 -1∆, ν (10 s )LiFe0.2Mn0.8PO4

Temperature (K)Fig. 2. Temperature dependences of (a) ∆(open circle) and(b) ν (closed circle) obtained for LiFe0.2 Mn0.8 PO4 .

We investigated Li diﬀusion in LiFePO4 and∗1∗2∗3∗4∗5LiMnPO4 by µ+ SR1) . However, we could not obtain information about Li diﬀusion for LiMnPO4 , because µ+ SR spectra were strongly aﬀected by magnetic
moment of Mn2+ . Although the diﬀusive nature ofLithium is represented in the dynamic Kubo-Toyabe(KT) type relaxed signal in zero ﬁeld (ZF) µ+ SR measurements, the small changes in KT signal are hidden
by fast relaxed signal caused by the magnetic momentof Mn2+ . In order to avoid the magnetic eﬀect due toMn2+ , we meausred µ+ SR spectra on LiFe1−x Mnx PO4with smaller x to presume the Li diﬀusive nature in
LiFe1−x Mnx PO4 with larger x (Fig. 1).
In order to investigate Li-ion diﬀusion in the solidsolution similar to the one used in the practical battery, we measured µ+ SR spectra on LiFe0.2 Mn0.8 PO4 .
In x=0.8 samples, the ZF-µ+ SR spectrum is ﬁtted by
the sum of the exponentially relaxed static KT function and fast relaxation, asumming the volume fractiondetermined for smaller x as the initial value. Then,the ZF-µ+ SR spectrum exhibits dynamic KT function at temperatures above 150 K. By ﬁtting the ZFand LF-spectra with a dynamic KT function, the ﬁeld
ﬂuctuation rate (ν), relaxation rate (λ) and the ﬁeld
distribution width (∆) were obtained. For x=0.8, ν
rapidly starts to increase and ∆ starts to decrease at150 K (Fig. 2). This indicates that Li-ion diﬀusion occurs above 150 K. The diﬀusion coeﬃcient is estimatedas DLi = 2.1×10−10 (cm2 /s) at 300 K.
By utilizing ARGUS spectrometer to obtain a large
asymmetry for the KT signal, we obtained additional
data to the systematic results taken with EMU spectrometer for the solid solution, LiFe1−x Mnx PO4 with
x =0.2 - 1. There appers to be a peak in DLi between
x=0.6 and x=1 (Fig. 1). However, we cannot conclude
that DLi has a small peak around x=0.8 since we havemeasured only for x=0.6 and 0.9. We would like to
study in detail the region between x=0.6 - 0.9, since
the composition is very close to that used in a practical
Li-ion battery.
We also measured µ+ SR spectrum on x=0 sample,
however, the obtained data was unsatisfactory. It is reported in Ref. 1 that DLi =3.6×10−10 (cm2 /s) at 300 K.
Since DLi changes drastically between x=0 and 0.2, weneed to try to conﬁrm such large changes.
The activation energy Ea was estimated from the relation Ea =kB T ln(ν) to be 53.0 meV for x=0.8. Theremay be a peak in Ea between x=0.6 and 1 (not shown).
Reference1) J. Sugiyama et al., Phys. Rev. B 85, 054111(2012).

Toyota Central Research and Development Labs., Inc.
Institute for Molecular Science
Tokyo Institute of Technology
KTH Royal Institute of Technology
RIKEN Nishina Center- 225 -