Electrolytes and lithium ion battery using the same

Details Electrolytes and lithium ion battery using the same Electrolytes and lithium ion battery using the same

2001-09-07

2004-02-17

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Include electrolyte chemically specified and method

C429S330000, C429S331000, C429S332000

Reexamination Certificate

active

06692874

ABSTRACT:

This application claims priority of Korea patent Application No. 2000-000934, filed on Jan. 10, 2000.
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to a lithium ion battery, more particularly to a new electrolyte and a lithium ion battery which comprises the same, using an anode including graphitized carbon and a cathode including lithium-containing transition metal oxide.
(b) Description of the Related Art
Ever since the lithium ion liquid secondary battery was first commercialized by Sony Co., the lithium ion liquid battery has been used increasingly in portable computers and cellular phones etc., instead of the lithium ion secondary batteries of prior art, as it has a higher energy density. The lithium ion liquid secondary battery comprises an anode including carbonaceous material as an anode active material and a cathode including a metal oxide of LiCoO
2
etc. as a cathode active material, and is prepared by intercalating a porous polyolefin-based separator between the anode and the cathode, then by injecting a non-aqueous electrolyte having a lithium salt of LiPF
6
etc. When the battery charges, the lithium ions of the cathode active material are released and then are inserted into the carbon layer of the anode. When the battery discharges, the lithium ions of the carbon layer of the anode are released and then are inserted into the cathode active material. The non-aqueous electrolyte plays a mediating role moving the lithium ions between the anode and the cathode. The electrolyte should be stable within the scope of the operation voltage of the battery, and be able to transfer the ion with sufficiently fast velocity.
As an electrolyte, U.S. Pat. Nos. 5,521,027 and 5,525,443 disclose an admixture electrolyte of a linear carbonate and cyclic carbonate. The cyclic carbonate has a large polarity and thus is sufficiently capable of dissociating lithium, but has low ion conductivity due to the large viscosity. Therefore, in these patents, mixing linear carbonate with a low polarity and a low viscosity reduces the viscosity of the electrolyte comprising the cyclic carbonate.
The above cyclic carbonate includes carbonates such as ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate (VC), butylene carbonate (BC) etc. PC has a good low temperature performance due to a low freezing point of −49° C. However, if an anode uses graphitized carbon of a large capacity, there is the problem of a sudden reaction between PC and the anode when the battery is charging. Thus, EC is commonly used, as it forms the most stable protecting film among the cyclic carbonates in a battery comprising an anode using graphitized carbon. However, if EC is used in a large amount, the low temperature performance of the electrolyte is abruptly deteriorated due to the EC's high melting point of 37° C. To resolve this problem, it is common to use a two-component electrolyte by mixing in a linear carbonate having a low melting point and a low viscosity as a second component with the EC.
The above linear carbonates include carbonates such as dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) etc. If EMC, which has the lowest melting point of the listed carbonates, namely −55° C., is used, the battery exhibits excellent low temperature performance.
However, merely mixing cyclic carbonate and linear carbonate does not satisfy the need for a high capacity and low temperature performance in the lithium ion battery.
In fact, there has been research directed to improving the low temperature performance of an electrolyte comprising EC, by adding another electrolyte or new additives to the electrolyte of the prior art. The literature (J. Electrochem. Soc. 146(2), 485, 1999) discloses that an electrolyte of a three-component system prepared by mixing EC, diethyl carbonate (DEC), and dimethyl carbonate (DMC) has an excellent low temperature performance, better than the two-component system. Other literature (J. Fluorine Chem. 87 (1998) 221) discloses that low temperature performance is improved by adding CHF
2
COOCH
3
to an electrolyte comprising EC and DEC.
If additives are added to the electrolyte as in the above methods, ion conductivity can be improved due to a lower freezing point and lower viscosity at low temperature, as the molecules of the electrolyte are prevented from forming a regular configuration with each other. That is if an electrolyte of more than the three components is prepared, wherein a suitable organic material as the third component is further added to the electrolyte of the two-component system, there is the effect of a freezing point depression when the electrolyte has a suitable composition, and also the effect of improving the charge performance at low temperature due to a reduction of viscosity at low temperature.
In fact, the electrolyte must be shown to be electrochemically stable within the scope of the operation voltage of the battery, and must be shown to have low reactivity with the graphitized carbon, and must not shorten the charge-discharge cyclic life by forming a stable protecting film on the anode. However, there has heretofore been no electrolyte which is known to satisfy the above condition.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a new compound that is electrochemically stable within the scope of the operation voltage of the battery, as the compound has both a cyclic carbonate structure and a linear carbonate structure in the molecule. The compound has a low reactivity with graphitized carbon of high electric capacity, and does not shorten a charge-discharge cyclic life by forming a stable protective film on an anode and thus can be used as a component of electrolyte.
It is other object to provide an electrolyte comprising the above new compound.
It is another object of the present invention to provide a lithium ion battery having a large electric capacity and a superior low-temperature performance comprising an anode including a graphitized carbon and a cathode including a lithium-containing transition metal oxide, a porous separator, and an electrolyte including a lithium salt and the above new compound.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In order to achieve the above objects and others, the present invention provides a compound shown in Formula 1:
wherein, R is methyl or ethyl group.
The present invention also provides an electrolyte comprising:
a) a lithium salt; and
b) the following compound shown in Formula 1 comprising both a cyclic carbonate structure and a linear carbonate structure in molecule:
wherein, R is methyl or ethyl group.
The present invention also provides a lithium ion battery comprising an anode including graphitized carbon as an active material, capable of absorbing and releasing lithium ions reversibly, a cathode including a lithium-containing transition metal oxide as an active material, capable of absorbing and releasing lithium ions reversibly, a porous separator, and an electrolyte, the electrolyte comprising:
a) a lithium salt; and
b) the compound shown in Formula 1 comprising both a cyclic carbonate structure and a linear carbonate structure in molecule:
wherein, R is methyl or ethyl group.
The present invention is described in detail as follows.
The present invention relates to the compound shown in Formula 1 as an additive for an electrolyte of a lithium ion battery comprising an anode including a graphitized carbon, a cathode including a lithium-containing transition metal oxide, a porous separator, and a lithium salt-containing electrolyte.
Since the compound comprises both a cyclic carbonate structure and a linear carbonate structure in the molecule, the battery can possess a large capacity and improved low temperature performance when the compound is used for the lithium ion in a secondary battery including a lithium salt. In particular, the compound is electrochemically stable within scope of the operation of the battery, has low reactivity to graphitized carbon, and forms a stable

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