Nanocrystalline lithium–manganese oxide spinels for Li-ion batteries — Sol-gel
synthesis and characterization of their structure and selected physical properties
M. Michalska
a,
⁎, L. Lipińska
a
, M. Mirkowska
a
, M. Aksienionek
a,b
, R. Diduszko
a
, M. Wasiucionek
b
a
Institute of Electronic Materials Technology, Wólczyńska 133, 01-919 Warsaw, Poland
b
Faculty of Physics, Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland
abstract article info
Article history:
Received 29 June 2010
Received in revised form 12 October 2010
Accepted 10 December 2010
Available online 14 January 2011
Keywords:
Nanocrystalline cathode
Lithium manganese oxide spinel
Sol-gel method
LiMn
2
O
4
Nanocrystalline lithium–manganese oxide spinels were synthesized by a modified sol-gel method. Simple
salts of lithium, manganese and iron were used as starting reagents and citric acid as a complexing agent. The
gelled materials turned into nanopowders after the calcination was carried out in air in the 450–700 °C
temperature range. The combined DSC-TGA measurements have shown important stages of the syntheses.
They also enabled to find optimum calcination temperatures. Microstructure of all synthesized materials, as
investigated by SEM, contains relatively large grains (over 100 nm), which are agglomerates of much smaller
crystallites. X-ray diffraction (XRD) has shown that the final powders are single-phased, with a lattice
parameter a varying from 8.16 to 8.25 Å, depending on the material. The average crystallite size, as estimated
from broadening of XRD reflections by a Scherrer formula, is about 30 nm. Electrical conductivity depends on
temperature according to an Arrhenius equation with the activation energy of 0.30 eV.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Spinel-structured lithium manganese oxide (LiMn
2
O
4
) belongs to
key cathode materials for rechargeable Li-ion batteries [1–4]. It has
important ecological and economic advantages — it is environment-
friendly and relatively inexpensive compared to lithium cobalt oxide
(LiCoO
2
) or lithium cobalt–nickel oxides — Li(Co,Ni)O
2
–used in
commercial Li-ion cells. On the other hand it has a satisfactory
capacity, high-energy density, low self-discharge and high thermal
stability [1,4,5]. Unfortunately, the mass-scale use of lithium manga-
nese spinel is hindered by an undesirable phenomenon — a gradual
destabilization of spinel structure of lithium manganese oxide [6,7].
The problem becomes especially important at temperatures above
ambient. This process is facilitated by a discernible structural
distortion known as the Jahn–Teller effect [3]. There are various
strategies to minimize that unwanted effect: (i) a partial substitution
of manganese ions by foreign ions such as e.g.: Fe, Co, Ni or Cu, (ii)
surface modification by oxide coating, (iii) using nonstoichiometric
lithium manganese spinels like: Li
1+ x
Mn
2 - x
O
4
— cation-mixed,
LiMn
2
O
4 - x
— oxygen-deficient, and Li
2
Mn
4
O
9
— oxygen-rich com-
pounds [8–13]. Commercial LiMn
2
O
4
powder is usually prepared by a
solid-state reaction, which has several disadvantages: it may lead to
inhomogeneities, irregular morphology and broad distribution of
particle sizes. Many soft chemical methods have been developed to
overcome the drawbacks: spray-drying [4], hydrothermal [9], sol-gel
[14], microwave-induced combustion [11,15], and micro-emulsion
[16].
In the present paper, nanocrystalline stoichiometric: LiMn
2
O
4
,
LiFe
0.5
Mn
1.5
O
4
and nonstoichiometric Li
1.4
Mn
1.7
O
4
, Li
1.27
Mn
1.73
O
4
spinels were prepared via a modified sol-gel method, using easily
accessible inexpensive chemicals and a straightforward processing.
The synthesized samples have been characterized by several
complementary methods: X-ray powder diffraction (XRD), scanning
electron microscopy (SEM), differential scanning calorimetry coupled
with thermogravimetry (DSC-TG) and impedance spectroscopy (IS).
2. Experimental
Stoichiometric: LiMn
2
O
4
, LiFe
0,5
Mn
1,5
O
4
and nonstoichiometric
Li
1.4
Mn
1.7
O
4
, Li
1.27
Mn
1.73
O
4
spinels were prepared by a modified sol-
gel method. The synthesis scheme of nanocrystalline stoichiometric
LiMn
2
O
4
spinel is shown in Fig. 1. The initial solution was obtained by
separately dissolving stoichiometric or nonstoichiometric ratios of:
manganese acetate tetrahydrate (C
2
H
3
O
2
)
2
Mn·4H
2
O (99%, CHEMPUR),
lithium acetate dihydrate (C
2
H
3
O
2
)Li·2H
2
O (97%, Fluka) or lithium
carbonate Li
2
CO
3
(99%, ITME), and iron citrate (C
6
H
5
O
7
)Fe·H
2
O (99%,
Fluka) in deionized water. Next that were added: a complexing reagent–
citric acid–C
6
H
5
O
7
·H
2
O, and successively: either ethylene glycol as a
polymerizing reagent or a second complexing reagent: acetic or glycolic
acid. The liquids were thoroughly mixed and stirred at a constant
temperature. Afterwards the product was left for drying, until the
solution had become more and more viscous. Further heating led to the
formation of a polymeric resin, which was heated at 150 °C for a few
hours in air and ground in an agate mortar. The process of calcination
Solid State Ionics 188 (2011) 160–164
⁎ Corresponding author. Tel.: + 48 22 835 3041; fax: + 48 22 864 5496.
E-mail address: monika.michalska83@gmail.com (M. Michalska).
0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ssi.2010.12.003
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