Chemistry: electrical current producing apparatus – product – and – Sealed cell having gas prevention or elimation means – Prevention or elimination means is one of the cell...
Reexamination Certificate
2000-06-27
2002-02-12
Brouillette, Gabrielle (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Sealed cell having gas prevention or elimation means
Prevention or elimination means is one of the cell...
C429S231950
Reexamination Certificate
active
06346343
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a secondary lithium battery comprising a negative electrode having negative electrode material with a porous structure of electroconductive material, a positive electrode having positive electrode material capable of intercalating lithium, and a non-aqueous ion-conductive medium between the negative and the positive electrode.
The need for high-energy density secondary (i.e. rechargeable) batteries is increasing, due to a growing market for lightweight, portable cordless consumer products, e.g. CD-players, mobile telephones, laptop computers and video cameras. For acceptable portability, these batteries should contain the necessary amount of energy at the smallest possible weight and volume. The present rechargeable batteries on the market, e.g. nickel-cadmium (NiCd) and nickel-metalhydride (NiMH), do not meet all these requirements. Moreover, the use of cadmium as the negative electrode material should be avoided for environmental reasons.
A very interesting material for use in batteries is lithium. Lithium is the lightest of all metals, which promises an extremely high theoretical energy density of metallic lithium. Lithium is a leading contender in the field of a battery negative electrode materials, since it has a large negative thermodynamic potential. The use of lithium has no negative environmental consequences. Therefore, rechargeable lithium batteries are very promising, especially when weight is an important factor.
A rechargeable lithium battery consists of a positive (cathode) and a negative electrode (anode) separated by, for example, a porous polymeric film to prevent direct electrical contact in an organic electrolyte. A lithium transition metal oxide can be used as the positive electrode, and metallic lithium as the negative electrode. The electrolyte is for example, a lithium salt in a non-aqueous organic solvent with good ionic conductivity and negligible electronic conductivity. During charging, lithium ions are transported from the positive electrode towards the negative (lithium) electrode. During discharging, the lithium ions are transported in the reverse direction and inserted back into the positive electrode.
A battery using lithium metal foil or sheet for the negative electrode is called a lithium metal battery. Such a battery encounters the problem of short-circuits in the battery caused by a repetition of the charge/discharge cycles. Repetition of charge/discharge cycles leads to a repetition of the dissolution and precipitation of lithium metal, and dendrites of lithium metal can grow on the surface of the negative electrode. The growing dendrite will penetrate through the separator between the negative and the positive electrode and come into contact with the positive electrode, resulting in a short-circuit.
An alternative approach for rechargeable batteries consists in replacing a negative electrode foil or sheet of lithium metal with a lithium intercalating compound. Such a battery is known from U.S. Pat. No. 5,552,239. Another lithium intercalating compound is used as a positive electrode, which leads to a rechargeable battery free from lithium metal; a so called Li-ion battery. In commercial Li-ion batteries, the positive electrode is generally composed of Li
x
CoO
2
, in which x varies between 0.5 and 1, and the negative electrode is made of graphite (Li
y
C
6
), wherein y ranges between 0 and 1. During charging, lithium ions deintercalate from the positive electrode and move into the non-aqueous electrolyte. The negative electrode then intercalates these ions. During discharging the process is reversed. Both electrodes exhibit the so-called intercalation reaction, also known as the host-guest reaction. It does not involve an electrolyte concentration change, nor any dissolution of the active materials into the electrolyte. Therefore, Li-ion batteries sometimes bear the name “rocking-chair batteries”. Carbon materials are good hosts for use as a negative electrode because they are able to intercalate and deintercalate lithium ions during charging and discharging of the battery, respectively. The growth of dendrites is prevented in such a negative electrode of carbon, and the problem of a short-circuit in the battery is solved.
A disadvantage of the use of host materials is the significant reduction of the energy density of these batteries. At the upper limit, 1 lithium atom per 6 carbon atoms can be intercalated, which amounts to a theoretical capacity of only 372 mAh/(g carbon), whereas a lithium metal electrode has a capacity of 3860 mAh/g.
SUMMARY OF THE INVENTION
It is an object of the invention to provide, inter alia, a lithium metal secondary battery in which the growth of lithium dendrites on the negative electrode is inhibited during charging of t he battery.
According to the invention, this object is achieved in a battery as specified in the opening paragraph, characterized in that the intercalating capacity for lithium of the negative electrode is at most 25 per cent of the capacity of the positive electrode material. The invention is based on the insight that the formation of lithium dendrites during charging is inhibited by the use of low current density. This can be achieved by offering a negative electrode material which has a high specific surface area (in m
2
/g), such as in the cited U.S. Pat. No. 5,552,239. But at the same time the volume or mass of the negative electrode material should be low, so that the amount of lithium that is intercalated during charging is as low as possible. Surprisingly, it has been found that the amount of lithium that cannot be intercalated in the negative electrode material having a high specific surface area is deposited on that material as a smooth lithium metal layer, without the formation of dendrites. As a result, the battery obtained is a lithium metal battery.
In practical batteries the capacity C (in mAh) of the negative electrode is equal to the capacity of the positive electrode (which is also the capacity of the battery). The capacity of the negative electrode can be expressed as:
C
neg
=C
Li intercal
+C
Li metal
=C
pos
, (1)
or in other words: the total capacity of the negative electrode is equal to the sum of the amount of lithium that is intercalated (intercalating capacity) and the amount of lithium that is deposited as metal layer. According to the invention, the intercalating capacity for lithium C
Li intercal
is at most 25 per cent of the capacity of the positive electrode:
C
Li intercal
≦0.25
C
pos
(2)
As mentioned above, the current density at the negative electrode during charging should be as low as possible in order to prevent the growth of lithium dendrites. As the current I
neg
through the negative electrode is equal to the current I
pos
through the positive electrode during (dis)charging, the current densities i (in A/m
2
) are preferable chosen such that the following relationship applies:
i
neg
≦i
pos
(3)
A high surface area of the negative electrode can be achieved by using a porous structure of, for example, small particles of electroconductive material for the negative electrode material. If spherical particles are used, the diameter of the particles to be used depends on the densities and masses of the negative and positive electrode materials according to the relationship:
d
neg
&dgr;
neg
m
pos
≦d
pos
&dgr;
pos
m
neg,
(4)
wherein m is the mass of the electrode materials (in g), &dgr; is the density of the electrode materials (in g/m
3
), and d is the diameter of the particles (in m) of the electrode materials.
Preferably, the intercalation capacity for lithium in the negative electrode material is at most 10 percent of the capacity of the positive electrode, or more preferably, intercalation is even absent.
If particles are used for the negative electrode material, electroconductive particles can be chosen, such as particles of metal or carbonaceous material. Carbonaceous materials include petroleum coke, coal coke, acetylene black,
Feil Hans
Notten Petrus H. L.
Orsini Francois P. R.
Tarascon Jean M.
Brouillette Gabrielle
Halajian Dicran
Tsang Susy
U.S. Philips Corporation
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