Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1999-09-23
2001-07-24
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S330000, C429S338000, C429S332000, C429S217000, C429S231100
Reexamination Certificate
active
06265110
ABSTRACT:
The present invention relates to a lithium secondary battery, in particular a lithium secondary battery having a graphite negative electrode structure and comprising a non-aqueous electrolyte, and to a method for the production thereof.
The use of lithium intercalation electrode structures and non-aqueous electrolytes has allowed the development of electrochemical systems based on carbon for the negative electrode and transition metal oxides for the positive electrode. Such batteries, which are referred to as “rocking chair batteries” since the lithium ions are “rocked back and forth” between the electrodes, or as “lithium-ion batteries” since the active lithium is always in its ionic form, display high energy density and high cyclability, i.e. the system can be discharged and recharged a large number of times.
For the negative electrode structure of such electrochemical systems, the electrode capacity, rate capability and cyclability are related to the physico-chemical characteristics of the constituent carbons (cf. Ebner, W. et al.: Solid State Ionics, Vol. 69 (1994) pp. 238-56). Ordered carbon structures like graphite are usually referred to as offering a higher reversible intercalation capacity, compared to the more disordered structures like coke. Graphite-based lithium-ion batteries also have a higher operational voltage and they display a flat discharge profile highly adapted for a large number of electronic applications.
Graphite is a highly ordered carbon with a well defined structure. The carbon atoms are arranged in hexagonal rings in a two-dimensional array. The length of this array is defined as L
a
. The hexagonal ring-layers are stacked on top of each other, either in an ABAB-sequence, or, less usually, in an ABCABC-sequence. The distance between two adjacent layers, defined at d
002
, is 3.354 Å, and the length of the stacked layers along the stacking direction is defined as L
c
. Graphite has a density of 2.26 g/cm
3
.
There are two types of graphite; natural graphite and artificial graphite. Natural graphite is found in the earth's crust whereas artificial graphite is produced through heating of e.g. cokes or carbonaceous gases up to 2500-3400° C.
Most commonly, the graphite used for negative electrodes in lithium ion batteries is an artificial graphite. The L
a
and L
c
values thereof are usually larger than 100 nm and L
a
/L
c
ratio less than 2. Compared to artificial graphite, natural graphites, and in particular natural flake graphites, display crystallites, which are longer and thinner. For the natural flake graphites, the L
a
-value is typically 2 to 10 times higher than the L
c
-value. A typical natural flake graphite has an L
a
-value of 300 nm and an L
c
-value of 50-100 nm.
A key feature for a negative electrode based on carbon is the initial irreversible loss which occurs during the first charge of the battery. This is due to an electrochemical reaction between the carbon negative electrode—or merely the lithium content thereof—and the lithium salt-containing organic electrolyte. During the reaction a passivating film is formed on the anode, preventing it from further reacting with the electrolyte.
Negative electrodes based on coke structures can function perfectly with most non-aqueous solvents, e.g. a propylene carbonate (PC)-based electrolyte. In contrast, this solvent decomposes on the surface of a graphite-based negative electrode. This reaction, due to the instability between PC and graphite, causes an exfoliation of the graphite-layered structure, which may destroy the electrode structure before any lithium intercalation can take place. Further, the reaction products (“reduced PC”) may react with the lithium salt of the electrolyte, causing further loss of active material. These phenomena have been described by many scientists, see e.g. Z. Jiang, M. Almagir and K. M. Abraham, J. Electrochem. Soc., Vol. 142, No.2, 333-340 (1995).
Therefore, the unfortunate drawback of most known lithium-ion batteries with graphite-based negative electrode structures is their poor compatibility with many electrolyte solvents.
A few organic solvent are stable with respect to graphite, including ethylene carbonate (EC) and dimethyl carbonate (DMC). These solvents, however, suffer from handling difficulties. Since DMC is a volatile solvent with a low boiling point of 90° C., and since EC has a melting point of 38° C. and therefore is solid at room temperature, their handling is difficult. During the addition of the electrolyte to the battery a large amount of DMC may evaporate, whereas EC may solidify. In terms of handling, solvents with high boiling points, which also have low melting points, are preferred in these batteries which operate between −20° C. and 60° C. Such solvents, including the abovementioned propylene carbonate, have traditionally been unstable with respect to graphite.
In the literature, few examples are given on graphite materials which can work with a PC-containing electrolyte.
U.S. Pat. No. 5,643,695 to Valence describes a battery, the first electrode of which is a graphite based electrode, characterised in that the interlayer spacing of the graphite (d
002
) is in the range 3.35-3.36 Å, the crystallite size in the direction of the c-axis (L
c
) is in the range 100-200 nm, the BET surface area is in the range 6-16 m
2
/g and at least 90% of the graphite particles have a size less than 16 &mgr;m. The electrolyte of the battery configuration is a mixture of EC, PC and optionally one other solvent, the EC being present in an amount not less than the amount of PC. The preferred graphite of this invention is SFG-15 from Lonza, which is an artificial graphite.
In most cases, however, PC-graphite combinations suffer from high losses of active material due to solvent decomposition and passivating film formation. In addition, for the above technology based on artificial graphite, only a rather poor density of the negative electrode structure can be obtained. The stacking of the “cubic” crystallites of the artificial graphites does not allow densities greater than 0.6 g/cm
3
. Such low gravimetric density of the negative electrode leads to a rather low energy density of the complete battery.
Therefore, there is a need for new lithium secondary battery configurations of applicable solvents and graphite electrodes in order to facilitate handling, achieve a broad working temperature range, and provide good chemical and electrochemical stabilities. Such configurations should have high capacity, high energy density, low initial loss and flat discharge voltage profile.
It is thus an object of the invention to provide a new type of lithium secondary battery of high capacity, energy density and flat discharge profile, based on a graphite negative electrode structures, which can work with a broader selection of PC-containing electrolytes.
Surprisingly it has been found, that a group of electrodes based on natural flake graphite is compatible with PC-containing electrolytes, thereby forming stable battery configurations. This new type of electrode material shows a low initial capacity loss of less than 100 mAh/g, i.e. 22% of the initial capacity, a high reversible capacity of more than 340 mAh/g, good cyciability and flat discharge voltage profile. The natural flake graphites allow construction of electrode structures of densities higher than 0.8 g/cm
3
, compared to the artificial graphites of an electrode density lower than 0.7 g/cm
3
with the same configuration. The higher gravimetric density of the negative electrode allows a higher energy density of the complete battery.
The use of natural graphite is described in the Japanese patent application JP 08,298,117 A to Kansai Coke & Chem. Co. Ltd. This patent application describes obtaining improved charge/discharge capacity and efficiency by using scalelike natural graphite pulverised by a jet mill. The application, however, does not describe the advantageous combination of natural graphite and propylene carbonate.
The present invention provides a lithium secondary battery comprising
Rao Ningling
Yde-Andersen Steen
Danionics A/S
Darb & Darby
Weiner Laura
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