Electrochemical cell

Details Electrochemical cell Electrochemical cell

2001-01-05

2003-12-23

Weiner, Laura (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Electrode

C429S231300, C429S224000, C429S231800, C429S231400, C429S330000, C429S331000, C429S332000, C429S337000, C429S338000

Reexamination Certificate

active

06667131

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to rechargeable electrochemical cells.
BACKGROUND OF THE INVENTION
The use of non-aqueous electrolytes has allowed the development of high voltage lithium-based electrochemical cells for energy storage. Such cells are further characterised in that their electrodes may be intercalation compounds. The positive electrode structures may be based on transition metal oxides operating at a potential close to 4V vs. Li/Li
+
. Negative electrode structures of carbons and graphites may be applied, which reversibly intercalate lithium at a potential close to the potential of metallic lithium. Such cells are referred to as lithium-ion cells, as the active lithium is always in its ionic form. Alternatively, alloy negative electrode structures like Li-Al and Li-Sn may be used. Such cells will be referred to as lithium-alloy cells. All of the above configurations provide voltages close to 4V.
For the cells referred to above, one of the limiting factors in terms of energy density has been a low initial capacity retention. In operation, a capacity loss during initial charging of the cells is observed, as is a fading capacity upon extended cycling or storage, which in combination define the initial capacity retention.
These capacity reduction phenomena can be ascribed to the instability of the electrolyte towards the electrodes. Instability towards the negative electrode leads to gassing and formation of a passivating film, whereas instability against the positive electrode leads to corrosion of the electrode structure. Both phenomena involve electrolyte decompositon and result in loss of active lithium and a fading capacity of the cell.
In lithium-ion cells, the losses from the anode reactions dominate the losses at the cathode. The magnitude of the losses merely depends on the type of carbon(s)/graphite(s), the electrolyte and their combination. Using carbon-based anodes, active lithium corresponding to 30-50% of the amount of active lithium in the cell may be lost during the first charge-discharge cycles of the cell, i.e. during the initial charging and the young life of the cell. The use of graphites permits somewhat lower losses in the range 5-30%, however, with poorer long term capacity retention.
In the lithium-ion cell active lithium is provided solely via the cathode. Although prelithiation of carbon/graphite anode structures has been investigated, traditionally lithium-free carbon/graphite structures are applied. Compared to cells based on pure metallic lithium, the loss of active material is quite detrimental. Whereas metallic lithium can be added at 3,800 mAh/g, the specific capacities of the cathode materials are significantly lower.
Currently, active cathode materials are generally selected from LiMn
2
O
4
, LiCoO
2
, LiNiO
2
, LiNi
x
Co
1−x
O
2
, 0<x<1, these materials having capacities in the range 120-160 mAh/g. Therefore, simply providing additional cathode material to compensate for any loss of active material is relatively inefficient and may reduce the lithium-ion cell capacity and energy density significantly.
Losses also occur in the lithium-alloy cells and in such cells the lithium alloys are generally formed in-situ as this prevents the difficult handling of low potential lithium compounds, e.g. under inert conditions. In such cells active lithium is provided solely via the cathode.
In one type of alloy cell the base material is provided as an oxide. In the case of tin, the reaction scheme is:
4Li+SnO
2
→2Li
2
O+Sn
4.4Li+Sn→Li
4.4
Sn
The scheme clearly shows the irreversible loss of lithium as lithium oxide which can be in the range of 48% of the total amount of active lithium.
In another type of lithium-alloy cell lithium is simply alloyed into the base metal, such as aluminium, or into a semi-metal, such as silicon, which is applied directly in the cell. In the case of aluminium, the reaction scheme is:
x
Li+Al→Li
x
Al
In this case a loss is observed as the diffusion of lithium in the &agr;-phase of the lithium-aluminium alloy is so slow, that lithium therefrom is for practical purposes not released during discharge of the cell. Further, the above instability phenomena still exist and cause additional loss of active lithium.
Accordingly, there is a need for an efficient way of providing additional active lithium to compensate for capacity losses in lithium ion cells as well as in lithium-alloy cells. Such active lithium is provided entirely via the cathode.
A number of patents describes approaches to compensate for the loss of active lithium:
U.S. Pat. No. 4,507,371 and U.S. Pat. No. 5,240,794, both to Technology Finance Corporation, describe lithium manganese oxides with excess lithium compared to LiMn
2
O
4
. '371 describes cathode structures of Li
1+x
Mn
2
O
4
, x>0, whereas '794 describes a range of compositions within the compositional area defined by its corner compositions Li
14
Mn
5
O
12
, Li
2
Mn
3
O
4
, LiMn
3
O
4
and Li
4
Mn
5
O
12
.
U.S. Pat. No. 5,370,710 to Sony describes an approach to alleviating the capacity loss by doping a LiMn
2
O
4
cathode material with an additional amount of lithium to obtain a compound Li
1+x
Mn
2
O
4
either by chemical or electrochemical means. A specific chemical doping method is described in U.S. Pat. No. 5,266,299 to Bell Communication Research, which claims doping of LiMn
2
O
4
or &lgr;-MnO
2
with LiI.
Although a number of approaches exists for the introduction of additional active lithium into rechargeable lithium cells, there is still a need for cathodes of such cells, which provide high capacity, safe and simple processing and which are low cost compounds.
More specifically, an objective of the present invention is to provide rechargeable lithium cells of the lithium-ion or the lithium-alloy type, in which the cathode material has a high capacity, and which can be used for alleviation of the consequences of the capacity loss as well as for subsequent charge-discharge cycling.
The cathode material should display a higher capacity than the capacities of the traditional cathode materials LiMn
2
O
4
, LiCoO
2
, LiNiO
2
and LiNi
x
Co
1−x
O
2
, 0<x<1. According to the invention, however, there is no need for the full capacity of the cathode material to be rechargeable. In fact, as the cathode capacity is used for loss compensation as well as for cycling, the fraction used for loss compensation might as well be non-rechargeable. Therefore, whereas the first charge capacity of the cathode material should be higher than for the above prior art materials, the rechargeable capacity does not need to be higher. On the other hand, high rechargeable capacity will not reduce the cell performance.
This objective is accomplished by electrochemical cells, the cathode structure of which comprises a lithium cobalt manganese oxide of a tetragonal structure.
Cobalt manganese oxides and related materials are described in/by:
U.S. Pat. No. 5,084,366 to Matsushita Electric Industrial Co., Ltd., describes a secondary cell, the cathode of which is selected from a group of transition metal oxides including lithium cobalt manganese oxide spinel structures. Such compounds, however, have in their discharged state a chemical composition of Li
x
Co
y
Mn
2−y
O
4
, 0.85≦x≦1.15 and 0.02≦y≦0.3. In their charged state, x reaches a value of 0.7, i.e. the patent does not describe of any compound Li
2
Co
y
Mn
2−y
O
4
, 0<y<0.6 or its use for compensation of capacity losses.
U.S. Pat. No. 5,506,078 to National Research Council of Canada describes a method of forming a spinel-related &lgr;-Li
2−x
Mn
2
O
4
upon electrochemical deintercalation of lithium from an orthorhombic LiMnO
2
of space group Pmnm and unit cell a=4.572 Å, b=5.757 Å and c=2.805 Å. Although the patent describes the use of spinel-related structures of composition Li
2
M
2
O
4
, M being a transition metal, it does not describe cobalt manganese oxides such as Li
2
Co
y
Mn
2−y
O
4

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