Development of thin film cathodes for lithium-ion batteries in the material system
Li–Mn–O by r.f. magnetron sputtering
J. Fischer
a,
⁎, C. Adelhelm
a
, T. Bergfeldt
a
, K. Chang
b
, C. Ziebert
a
, H. Leiste
a
, M. Stüber
a
, S. Ulrich
a
,
D. Music
b
, B. Hallstedt
b
, H.J. Seifert
a
a
Karlsruhe Institute of Technology (KIT), Institute for Applied Materials, Applied Materials Physics (IAM-AWP), Hermann-von-Helmholtz-Platz 1,
76344 Eggenstein-Leopoldshafen, Germany
b
RWTH Aachen University, Materials Chemistry, Kopernikusstrasse 10, 46 52074 Aachen, Germany
abstract article info
Available online 6 November 2012
Keywords:
Lithium-ion battery
R.f. magnetron sputtering
Thin film cathode
All-solid-state battery
Lithium manganese oxide
Physical vapor deposition
Cubic spinel
Orthorhombic lithium manganese oxide
Today most commercially available lithium ion batteries are still based on the toxic and expensive LiCoO
2
as a
standard cathode material. However, lithium manganese based cathode materials are cheaper and environ-
mentally friendlier. In this work cubic-LiMn
2
O
4
spinel, monoclinic-Li
2
MnO
3
and orthorhombic-LiMnO
2
thin
films have been synthesized by non-reactive r.f. magnetron sputtering from two ceramic targets (LiMn
2
O
4
,
LiMnO
2
) in a pure argon discharge. The deposition parameters, namely target power and working gas
pressure, were optimized in a combination with a post deposition heat treatment with respect to
microstructure and electrochemical behavior. The chemical composition was determined using inductively
coupled plasma optical emission spectroscopy and carrier gas hot extraction. The films' crystal structure,
phase evolution and morphology were investigated by X-ray diffraction, micro Raman spectroscopy and
scanning electron microscopy. Due to the fact that these thin films consist of the pure active material without
any impurities, such as binders or conductive additives like carbon black, they are particularly well suited for
measurements of the intrinsic physical properties, which is essential for fundamental understanding. The
electrochemical behavior of the cubic and the orthorhombic films was investigated by galvanostatic cycling
in half cells against metallic lithium. The cubic spinel films exhibit a maximum specific capacity of
~ 82 mAh/g, while a specific capacity of nearly 150 mAh/g can be reached for the orthorhombic counterparts.
These films are promising candidates for future all solid state battery applications.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Thin film rechargeable lithium ion batteries (LIBs) are of great in-
terest for applications in stationary and portable power systems [1,2].
R.f. magnetron sputtering is an attractive technique to develop
nano-composite cathode materials for powerful thin film LIBs and to
reveal fundamental physical properties without influence of binder
phases. The large electrochemically active surface areas can in turn
give an increase in power density [3].
Nowadays, LiCoO
2
with the O3 structure is the most widely used
cathode material for thin film lithium ion batteries. Although LiCoO
2
possesses good reversibility and high capacity, the relatively high
cost and safety problems limit its application [4]. In recent years,
more and more attention has been paid to the lithium manganese ox-
ides owing to their high energy density, low cost and environmental
friendliness. Among them, LiMn
2
O
4
(cubic spinel phase with space
group of Fd
3m), LiMnO
2
(orthorhombic phase with space group of
Pmmn, o-LiMnO
2
or monoclinic phase with space group of C2/m,
m-LiMnO
2
) and Li
2
MnO
3
(monoclinic phase with space group of
C2/m) are promising candidates for the cathode materials in LIBs.
LiMn
2
O
4
is composed of three-dimensional frameworks of Mn
2
O
4
,
conducive to the intercalation and deintercalation of Li ions. In the
Li
x
Mn
2
O
4
cathode, lithium ions are reversibly intercalated into and
deintercalated out of the host spinel phase in two composition
ranges of 0 ≤ x ≤ 1 and 1 ≤ x ≤ 2, which produce two voltage plateaus
at 4 V and 3 V, respectively [5]. The plateau at 1 ≤ x ≤ 2 is related to
the Jahn–Teller distortion with a large volume change of the cathode,
which leads to capacity drop of the battery. Thus, the plateau at
0 ≤ x ≤ 1 is of more interest. In the case of LiMnO
2
, m-LiMnO
2
can
be synthesized by exchanging ions of Li and Na from the NaMnO
2
structure [6]. When synthesized under equilibrium conditions,
o-LiMnO
2
always forms [7]. In o-LiMnO
2
, oxygen ions are arranged
in nearly cubic-close-packing, and the octahedral interstices are
occupied by Li and Mn forming corrugated layers [8]. o-LiMnO
2
transforms into the spinel structure during electrochemical cycling,
which is an irreversible transformation [9]. Li
2
MnO
3
has an O3-type
structure and can also be described as a layered phase when the for-
mula is written as Li [Li
1/3
Mn
2/3
]O
2
. The interslab octahedral sites are
occupied only by Li ions and the slab octahedral sites are occupied by
Thin Solid Films 528 (2013) 217–223
⁎ Corresponding author.
E-mail address: julian.fischer@kit.edu (J. Fischer).
0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.tsf.2012.08.058
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Thin Solid Films
journal homepage: www.elsevier.com/locate/tsf