Electrolyte for lithium secondary battery

Details Electrolyte for lithium secondary battery Electrolyte for lithium secondary battery

2001-01-19

2003-04-08

Chaney, Carol (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Include electrolyte chemically specified and method

C429S345000, C252S062200

Reexamination Certificate

active

06544685

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATION
This application is based on application No. 2000-2946 filed in the Korean Industrial Property Office on Jan. 21, 2000, the content of which is incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to an electrolyte for a lithium secondary battery and a lithium secondary battery, and more particularly, to an electrolyte for a lithium secondary battery that undergoes almost no change in thickness when the battery is stored at a high temperature after charging, and a lithium secondary battery including the same.
(b) Description of the Related Art
The use of portable electronic instruments is increasing as electronic equipment gets smaller and lighter due to developments in high-tech electronic industries. Studies on lithium secondary batteries are actively being pursued in accordance with the increased need for a battery having a high energy density for use as a power source in these portable electronic instruments. Lithium-transition metal oxides are used as a positive active material of a lithium secondary battery, and lithium metals, lithium alloys, crystalline or amorphous carbons, or carbon complexes are used as a negative active material of a lithium secondary battery.
An average discharge voltage of a lithium secondary battery is about 3.6 to 3.7 V, which is higher than other alkali batteries, Ni—MH batteries, Ni—Cd batteries, etc. However, an electrolyte which is electrochemically stable in the charge and discharge voltage range of 0 to 4.2 V is required in order to generate such a high driving voltage. Because of this reason, a mixture of non-aqueous carbonate based solvents such as ethylene carbonate, dimethyl carbonate, diethyl carbonate, etc. is used as an electrolyte. However, such an electrolyte has a significantly lower ion conductivity than an aqueous electrolyte solution which is used in a Ni—MH battery or Ni—Cd battery, thereby resulting in the deterioration of battery characteristics during high rate charging and discharging.
During the initial charge of a lithium secondary battery, lithium ions, which are released from a lithium-transition metal oxides positive electrode of the battery, are transferred to a carbon negative electrode where the ions are intercalated into the carbon. Because of its high reactivity, lithium reacts with the carbon negative electrode to produce Li
2
CO
3
, LiO, LiOH, etc., thereby forming a thin film on a surface of the anode. This film is referred to as a solid electrolyte interface (SEI). The SEI film formed during the initial charging not only prevents the reaction between lithium ions and the carbon negative electrode or other materials during charging and discharging, but also acts as an ion tunnel, allowing the passage of only lithium ions. The ion tunnel prevents the disintegration of the structure of the carbon negative electrode, which is caused that organic solvents in an electrolyte with a high molecular weight make to solvate lithium ion and the solvent and the solvated lithium ion are co-intercalated into the carbon negative electrode.
Once the SEI film is formed, lithium ions are not side reacted with the carbon negative electrode or other materials such that an amount of lithium ions is maintained. That is, carbon of the negative electrode reacts with an electrolyte during the initial charging, thus forming a passivation layer such as a SEI film on the surface of the negative electrode such that the electrolyte is no longer decomposed, and stable charging and discharging are maintained (J. Power Sources, 51(1994), 79-104). Because of these reasons, in the lithium secondary battery, an irreversible formation reaction of the passivation layer dose not occur and a stable cycle life after the initial charging reaction is maintained.
In the case of a thin prismatic battery, there occurs a problem in which gases are generated inside the battery since a carbonate based organic solvent is decomposed during the SEI film forming reaction (J. Power Sources, 72(1998), 66-70). These gases include H
2
, CO, CO
2
, CH
4
, CH
2
, C
2
H
6
, C
3
H
8
, C
3
H
6
, etc. depending on the type of non-aqueous organic solvent and negative active material used. The thickness of the battery is expanded during charging due to the generation of gas inside the battery, and a passivation layer is slowly disintegrated by electrochemical energy and heat energy which increase with the passage of time when the battery is stored at high temperatures after it is charged. Accordingly, a side reaction in which an exposed surface of the negative electrode reacts with surrounding electrolyte occurs continuously. Furthermore, an internal pressure of the battery is increased with this generation of gas. The increase in the internal pressure induces the deformation of the prismatic battery and lithium polymer battery (PLI). As a result, regional differences in the cohesion between pole plates inside an electrode element (positive and negative electrodes, and separator) of the battery occur, thereby deteriorating the performance and stability of the battery, and making the mounting of the lithium secondary battery set itself difficult.
As a method for solving the internal pressure problem, there is disclosed a method in which the stability of a secondary battery including a non-aqueous electrolyte is improved by mounting a vent or a current breaker for ejecting an internal electrolyte when the internal pressure is increased above a certain level. However, a problem with this method is that mis-operation may be caused by an increase in internal pressure itself.
Furthermore, a method in which the SEI forming reaction is changed by injecting additives into an electrolyte so as to inhibit the increase in internal pressure is known. For example, Japanese Patent Laid-open Publication No. 97-73918A discloses a method in which high temperature storage characteristics of a battery are improved by adding a diphenyl picrylhydrazyl compound of 1% or less to the electrolyte. Japanese Patent Laid-open Publication No. 96-321312A discloses a method in which cycle life and long term storage characteristics are improved using a N-butyl amine group compound of 1 to 20% in an electrolyte. Japanese Patent Laid-open Publication No. 96-64238A discloses a method in which storage characteristics of a battery are improved by 3×10
−4
to 3×10
−3
M of calcium salt. Japanese Patent Laid-open Publication No. 94-333596A discloses a method in which storage characteristics of a battery are improved by adding an azo compound to inhibit the reaction between an electrolyte and a negative electrode of the battery.
Such methods as described above for inducing the formation of an appropriate film on a negative electrode surface such as a SEI film by adding a small amount of organic or inorganic materials are used in order to improve the storage characteristics and stability of a battery. However, there are various problems with these methods: the added compound is decomposed or forms an unstable film by interacting with the carbon negative electrode during initial charging and discharging according to inherent electrochemical characteristics, resulting in the deterioration of the ion mobility in an electron; and gas is generated inside the battery such that there is an increase in internal pressure, resulting in the significant worsening of the storage characteristics, stability, cycle life, and capacity of the battery.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an electrolyte for a lithium secondary battery including 1-alkylphosphonic acid cyclic anhydride which is capable of inhibiting the generation of gas inside the battery caused by the decomposition of a carbonate based organic solvent when the battery is stored at a high temperature after charging.
It is another object of the present invention to provide a lithium secondary battery that undergoes almost no variation in thickness and which obtains improved high temperature capacity

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