Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-12-08
2003-04-22
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S218100, C429S221000, C429S224000, C429S231800
Reexamination Certificate
active
06551746
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to rechargeable electrochemical cells of the lithium-ion or lithium-alloy type.
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 for their energy density has been a low initial capacity retention. Upon 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.
The capacity reduction phenomena are 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 rather detrimental. Whereas metallic lithium can be added at 3,800 mAh/g, the specific capacities of the cathode materials are significantly lower.
Currently, LiMn
2
O
4
is one of the active cathode materials used in lithium-ion cells. The active lithium capacity thereof depends to some extent on the preparation method, but is generally of the order of 122 mAh/g.
Therefore, simply providing additional LiMn
2
O
4
to compensate for any loss of active material is somewhat inefficient and may reduce the lithium-ion cell capacity and energy density significantly.
Losses occur in the lithium-alloys cells, too. In the alloy cells with which the present invention is concerned, the lithium alloys are formed in-situ, as this obviates the need for 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:
4 Li+SnO
2
→2Li
2
O+Sn
4.4 Li+Sn→Li
4.4
Sn
This scheme clearly shows the irreversible loss of lithium in terms of lithium oxide, in this case being 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 silicon, which is applied directly in the cell. In the case of aluminium, the reaction scheme is:
xLi+Al→Li
x
Al
In such 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 practically not released during discharge of the cell. Further, the above instability phenomena still exist and cause additional loss of active lithium.
Therefore, there is a need for an efficient concept for 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. Nos. 5,429,890 and 5,561,007, both to Valence Technology, suggest the use of LiMO
2
additives ('890: M═Ni,Co and mixtures thereof, '007: Li
y
-&agr;-MnO
2
) to a LiMn
2
O
4
based cathode. As the additives mainly display rechargeable capacity, these patents are merely aiming at closing the voltage gap between the 3 V and the 4 V plateaus of the Li/LiMn
2
O
4
system.
U.S. Pat. No. 5,370,710 to Sony describes a different approach to alleviating the capacity loss, in particular doping of a LiMn
2
O
4
cathode material with an additional amount of lithium to obtain a compound Li
1+x
Mn
2
O
4
compound 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 involves doping of LiMn
2
O
4
or &lgr;-MnO
2
with LiI.
U.S. Pat. Nos. 4,507,371 and 5,240,794, both to Technology Finance Cooperation, 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 the 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
, including Li
1+x
Mn
2
O
4
where x≧0.25.
Although a number of approaches exists for the introduction of additional active lithium into rechargeable lithium cells, there is still a need for additives to cathodes of such cells, which provide high capacity, safe and simple processing and which are low cost compounds.
A number of patents describe the use of alkali metal transition metal oxide cathode materials.
U.S. Pat. No. 3,970,473 to General Electric Company discloses a solid state electrochemical cell, the cathode comprising a non-stoichiometric lithium compound of the composition Li
x
Mn
y
O
z
, 0<x<1 and 0<y≦3 and z has a value to obtain electrical neutrality. Although such compositions include LiMnO
2
structures, the patents does not suggest the use of such compounds as an additive to LiMn
2
O
4
-cathode structures.
U.S. Pat. No. 4,302,518 by Goodenough and Mizuchima describes an A
x
M
y
O
2
structure (A: Li, Na, K, M: transitions metal, x<1, y≈1) having the layers of &agr;-NaCrO
2
, which is monoclinic. The patent, however, does not disclose on the use of such compounds as an additive to LiMn
2
O
4
-cathode structures.
U.S. Pat. No. 4,668,595 to Asahi describes a secondary battery with a negative electrode of a carbonaceous material and a positive electrode of layered composite oxide of the formula A
x
M
y
N
z
O
2
, where A is an alkali metal, M is a transition metal, N is selected from the group of Al, In, and Sn, and 0.05≦x≦1.10, 0.85≦y≦1.00 and 0.001≦z≦0.10, respectively. The patent, however, does not suggest composite cathode structures.
U.S. Pat. No. 5,316,875 to Matsushita discloses a process for the lithiation of LiMn
2
O
4
, LiMnO
2
, LiCoO
2
, LiNiO
2
LiFeO
2
or &ggr;-V
2
O
5
by exposure to butyllithium, phenyllithium or naphtyllithium. The patent, however, does not suggest use of the cathode active materials as additives to LiMn
2
O
4
-cathode structures.
U.S. Pat. No. 5,352,548 to Sanyo describes the use of cathode materials selected from V
2
O
5
, TiS
2
, MoS
2
, LiCoO
2
, LiMnO
2
, LiNiO
2
, LiCrO
2
, LiMn
2
O
4
and LiFeO
2
in a secondary
Koksbang Rene
Vitins Girts
West Keld
Chaney Carol
Danionics A/S
Yuan Dah-Wei D.
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