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Journal of Power Sources 74 1998 228–233
Cycle life improvement of LiMn O cathode in rechargeable lithium
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batteries
Zhaolin Liu
a,)
, Aishui Yu
a
, J.Y. Lee
b
a
Institute of Materials Research and Engineering, National UniÕersity of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore
b
Department of Chemical Engineering, National UniÕersity of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore
Received 30 October 1997; accepted 6 March 1998
Abstract
Spinel LiMn O and LiCo Mn O were synthesized by solid state reactions and were used as the positive electrode in liquid cells
24 0.1 1.94
with Li as the negative electrode. Through extensive ball milling of the spinel with carbon black during the preparation of the cathodic
mix, significant decrease in contact resistance between the oxide phase and carbon particles was achieved. The spinel LiCo Mn O
0.1 1.9 4
cathode so prepared showed good cyclability and rate capability. q1998 Elsevier Science S.A. All rights reserved.
Keywords: Rechargeable lithium battery; LiMn O cathode; Solid state reaction
24
1. Introduction
Spinel LiMn O has been investigated extensively as a
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cathode material for rechargeable lithium batteries because
of its low cost, environmental benignity and its relative
wx
ease of preparation from solid state reactions 1–3 . It has
electrochemical properties that satisfy the basic requisites
of a 4 V cathode material. However, LiMn O has a lower
24
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specific capacity compared to LiCoO and most undesir-
2
ably, suffers from a progressive capacity loss upon cy-
cling. The origin for this capacity loss has not been clearly
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identified, but several possibilities exist: 1 reactions be-
tween the electrolyte and the electrode in the charged state,
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2 a structural transformation due to Jahn–Teller distor-
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tions of the spinel in the discharged state, 3 Mn dissolu-
wx
tion through a disproportionate reaction 4 .
Much work has been directed to the stabilization of the
spinel structure to obtain lithiated manganese oxide cath-
odes with improved cycle life and application performance
wx
5,6 . Considerable success has been obtained by the par-
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tial substitution of Mn III with M III admetals, as pro-
wx
posed by Li et al. 7 . The capacity fade can be slowed
down but not completely eliminated by such approach.
Since spinel LiMn O has low electronic conductivity,
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it has to be mixed with a conducting additive such as
)
Corresponding author. Fax: q 65-8720785; e-mail:
zl-liu@imre.org.sg.
carbon powder to make a working cathode. When the
carbon content in the cathode is low, Mn dissolution is
wx
inhibited, but the capacity loss is severe 8 . Nishizawa et
wx
al. 9 reported recently that composites of manganese
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oxide particles and polypyrrole PPy can be prepared by
adding pyrrole into an acidic aqueous suspension of the
oxide. The resulting composites can have a higher specific
capacity than oxides mixed with carbon power. Our results
here will show that an excess of carbon can be advanta-
geous to cyclability, although the energy and power densi-
ties of the resulting batteries are necessarily lower.
In this paper, we report the preparation and electro-
chemical behaviour of spinel LiMn O and
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LiCo Mn O . Electrochemical impedance spectroscopy
0.1 1.9 4
was used to determine the dominant factor for the capacity
loss on repeated cycling. It was found that the contact
areas between oxide and carbon particles could be in-
creased through prolonged ball milling. The contact resis-
tance and electrode reaction resistance were decreased by
such mechanical means, resulting in cathodes of improved
cyclability and rate capability.
2. Experimental
Spinel LiMn O and quaternary spinel LiCo Mn O
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were prepared by heating stoichiometric amounts of
Li CO and MnCO or Li CO , MnCO and CoCO in
23 3 23 3 3
0378-7753r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved.
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PII S0378-7753 98 00071-8