Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2002-04-10
2004-08-03
Cantelmo, Gregg (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C423S351000
Reexamination Certificate
active
06770400
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an improvement of the oxidation and decomposition resistance property of negative electrode materials for providing nonaqueous electrolyte secondary batteries having large capacity. More particularly, the present invention relates to nonaqueous electrolyte secondary batteries excellent in recovery property from overdischarge.
In recent years, lithium ion batteries using LiCoO
2
as the positive electrode and a carbon material as the negative electrode have been widely used as power supplies for portable equipment. With advances in reduction in the size and weight of electronic equipment, these batteries have been requested to have higher energy density, and studies on various materials have been vigorously carried out.
As positive electrode materials for nonaqueous electrolyte secondary batteries, transition metal oxides and chalcogenide compounds such as LiMn
2
O
4
, LiFeO
2
, LiNiO
2
, V
2
O
5
, Cr
2
O
5
, MnO
2
, TiS
2
and MoS
2
, have been proposed so far in addition to LiCoO
2
.
As negative electrode materials, on the other hand, various materials have been examined and carbon materials have been commercialized and have come into widespread use. However, carbon materials have already achieved a capacity near their theoretical capacity (about 370 mAh/g) and, therefore, a novel negative electrode material having a large capacity is desired.
Here, in JP-A-9-102311, there is proposed the use of a nitride represented by the formula: A
x
M
y
N, wherein A is at least one selected from the group consisting of Na, K, Mg, Ca, Ag, Cu, Zn and Al, M is a transition metal element (M≠A), and 0.0<x and y≦3.0 are met, as a support for a negative electrode active material in order to enhance the energy density of secondary batteries. In accordance with this publication, the element represented by A in the formula can be intercalated/deintercalated into/from the nitride, and moreover lithium ions can be intercalated/deintercalated.
However, the above material does not contain lithium at the time of synthesis thereof. Therefore, to intercalate/deintercalate lithium as a negative electrode active material of a lithium ion battery, it is necessary to deintercalate the element A in the above formula in some way to secure the site for lithium or directly intercalate lithium into a crystal lattice.
It is therefore considered more appropriate to use a composite nitride containing lithium, as proposed in JP-A-7-78609, from the viewpoint of easiness in production of a lithium ion battery, constructional stability during charge/discharge, and reversibility of charge/discharge. The lithium-containing composite nitride proposed in this JP-A-7-78609 is represented by the formula: Li
a
M
b
N, wherein M is a transition metal and a is a variable indicating the content of lithium in the active material varying with charge/discharge, and contains lithium from the beginning which is to be intercalated/deintercalated with charge/discharge. The average operating electrode potential is around 0.8 V with reference to the lithium potential. In addition, the reversible capacity of a battery using this material as the negative electrode is significantly large compared with the case of carbon materials. In other words, this is a promising material for attaining high-capacity batteries.
The present inventors evaluated a battery produced using LiCoO
2
as the positive electrode material, in which lithium was electrochemically deintercalated in advance, and a lithium-containing composite nitride as the negative electrode material, which is represented by the formula: Li
3−x
M
x
N, wherein M is a transition metal and 0.2≦x≦0.8. As a result, although no problem occurs under normal use and ambient conditions, it was found that the battery swells when the battery is in the overdischarged state and generates a gas, and the subsequent recovery is not sufficient. It is noted, herein, that the reaction of the negative electrode active material (that is, the lithium-containing composite nitride) absorbing (intercalating) lithium is referred to as “charge”, while the reaction thereof desorbing (deintercalating) lithium is referred to as “discharge”.
The mechanism of the gas generation in the overdischarged state is presumed as follows. The lithium-containing composite nitride immediately after the synthesis is in the state of being filled with lithium, unlike carbon materials. This state corresponds to the charged state of a battery using the lithium-containing composite nitride as the negative electrode. Therefore, the lithium-containing composite nitride is lower in stability and thus higher in reactivity when it is in the discharged state with lithium deintercalated than when it is in the initial charged state. This is presumably the reason for the gas generation.
The present inventors analyzed the generated gas and found that nitrogen was the main component. From this fact, it is presumed that this gas is not a gas generated by decomposition of an electrolyte but a gas caused by the lithium-containing composite nitride, that is, a gas generated by decomposition of the lithium-containing composite nitride.
The charge of the lithium-containing composite nitride during discharge is compensated by the valence change of the contained transition metal element as the lithium is being deintercalated from the lithium-containing composite nitride. In this way, presumably, the electric neutrality of the lithium-containing composite nitride is maintained.
However, it is expected that as the amount of lithium to be deintercalated increases, the charge will no more be compensated only by the valence change of the transition metal, and a load will be applied to the nitrogen element. For example, if the composition of the lithium-containing composite nitride is represented by the formula: Li
2.6
Co
0.4
N, the composition presumably changes to Li
1.0
Co
0.4
N when lithium is deintercalated.
On the assumption that, in the initial state of the compound (Li
2.6
Co
0.4
N), the oxidation numbers of lithium (Li), cobalt (Co) and nitrogen (N) are formally +1, +1 and −3, respectively, and that Co is in charge of the entire charge compensation, the oxidation number of cobalt will become +5 in the discharged state (Li
1.0
Co
0.4
N). This oxidation number implies a high oxidation state normally inconceivable. In such a state, presumably, nitrogen also participates in the charge compensation by supplying electrons. Therefore, in the overdischarged state in which the nitride is further oxidized, it is presumed that the binding of the nitrogen in the lithium-containing composite nitride becomes very unstable, resulting in decomposition of the nitride.
An object of the present invention is to solve the above problem and to attain a highly reliable battery excellent in recovery property by avoiding the overdischarged state.
BRIEF SUMMARY OF THE INVENTION
To attain the above object, the present invention provides a negative electrode material for a nonaqueous electrolyte secondary battery, comprising a lithium-containing composite nitride represented by the formula (1): Li
3−x−y
A
y
M
x
N wherein A is at least one selected from the group consisting of alkaline metals except for lithium and alkaline earth metals, M is a transition metal, 0.2≦x≦0.8 and 0.0<y≦0.8.
Preferably, the negative electrode material comprises core particles made of a lithium-containing composite nitride represented by the formula (2): Li
3−x
M
x
N, wherein M is a transition metal and 0.2≦x≦0.8, and surface layers made of the lithium-containing composite nitride covering the respective core particles.
Further, the present invention provides a method for producing a negative electrode material for a nonaqueous electrolyte secondary battery, comprising the steps of:
(a
1
) synthesizing core particles made of a lithium-containing composite nitride represented by the formula (2): Li
3−x
M
x
N wherein M is a transition metal and 0.2≦x≦0.8;
(b
Hasegawa Masaki
Kasamatsu Shinji
Kayama Miho
Nitta Yoshiaki
Tsutsumi Shuji
Akin Gump Strauss Hauer & Feld L.L.P.
Cantelmo Gregg
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