United States Patent 5
`Adachi
`
`[54] NON-AQUEOUS ELECTROLYTE
`SECONDARY CELL HAVING SHUTTLE
`AGENT
`
`[75]
`
`Inventor: Momoe Adachi. Tokyo. Japan
`
`[73] Assignee: Sony Corporation. Tokyo, Japan
`
`{21] Appl. No.: 638,181
`
`[22] Filed:
`
`Apr. 26, 1996
`
`[30]
`
`Foreign Application Priority Data
`
`Apr 28,1995
`Mar. 4, £996
`
`[FP]
`[JP]
`
`Japa oesssesssesesssseerensseeseseree 7-106401
`Japan...scssssesssssscorsesneaneseees $-046351
`
`[SL] Win. C18 acesccsesssssssasscasenssnvonsscesecssssenense H10M 10/40
`[52]
`TLS. C1. weecceeesesseneee 429/199. 429/198; 429/194.
`429/218
`[58] Field of Search...csscsseseseessestses 429/194, 198,
`429/218, 197, 199
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`8/1988 Kobayashi et al... 252/500
`4,762,644
`2/1989 Blomgrenet al... eee 429/194
`4,808,497
`FOREIGN PATENT DOCUMENTS
`
`A-O 319 182
`
`7/1989 European Pat. Off..
`OTHER PUBLICATIONS
`
`ee
`
`US005763119A
`
`(11] Patent Number:
`
`(45) Date of Patent:
`
`5,763,119
`Jun. 9, 1998
`
`Patent Abstracts of Japan. vol. 95, No. 3, Apr. 28, 1995 &
`JP-A-06 338347 (Sony Corp.). Dec. 6, 1994.
`
`Primary Examiner—Stephen Kalafut
`Assistant Examiner—Carol Chaney
`Attorney, Agent, or Firm—Hill & Simpson
`
`[57]
`
`ABSTRACT
`
`In a non-aqueouselectrolyte secondary cell according to the
`present invention, a particular redox shuttle is contained in
`an electrolyte. whereby an overcharge of the cell is effec-
`tively prevented. The non-aqueous electrolyte secondary
`cell comprising a negative electrode composed of a metal
`material containing lithium as a primary component or a
`carbonaceous material into which lithium can be doped and
`from which lithium can be dedoped. a positive electrode
`composed of a composite oxide of lithium and transition
`metal. and a non-aqueous electrolyte containing an organic
`compound of the general formula:
`
`—
`
`OCH;
`
`OCH3
`
`where X represents a halogen atom.
`
`Patent Abstracts of Japan, vol. 16, No.120 (B-1182), Mar.
`26, 1992 & JP-A-03 289062 (Furukawa Battery Co. Ltd).
`Patent Abstracts of Japan, vol. 17. No. 450 (E-1416) Aug.
`18, 1993 & JP-A-O05 101847 (Otsuka Chem. Co. Ltd).
`
`2 Claims, 3 Drawing Sheets
`
`
`
`APPLE-1030
`
`APPLE-1030
`
`1
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`

`

`U.S. Patent
`
`Jun. 9, 1998
`
`Sheet 1 of 3
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`5,763,119
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`VOLTAGE[V] 3.6
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`CELL
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`COMPARATIVE
`EXAMPLE1
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`DISCHARGE
`INTERRUPTION
`CHARGE
`ee ee
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`TIME [TIME]
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`FIG.2
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`2
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`U.S. Patent
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`Jun. 9, 1998
`
`Sheet 2 of 3
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`5,763,119
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`U.S. Patent
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`Jun.9, 1998
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`Sheet 3 of 3
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`5,763,119
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`

`1
`NON-AQUEOUS ELECTROLYTE
`SECONDARY CELL HAVING SHUTTLE
`AGENT
`
`5,763,119
`
`10
`
`2
`a high oxidation-reduction potential and a high electro-
`chemical stability and are therefore useful as a reduction-
`oxidation agents for the afore-mentioned 4 V-classlithium-
`ion secondary call. These agents has been proposed, for
`BACKGROUND OF THE INVENTION
`example, in Japanese patent application laid-open publica-
`1. Field of the Invention
`tion No. 6-338.347. These agents include a transition metal.
`as a central metal of the complexes, such as Fe, Ru or Ce.
`This invention relates to a non-aqueous electrolyte sec-
`These transition metals can be maintained in variousstable
`ondary cell capable of generating an electro-motive force by
`oxidation-reduction states depending upon conditions of (d)
`an action oflithium ion. and more particularly to a technique
`electron orbital or (f) electron orbital. so that an adequate
`for preventing an overcharging of the cell by using a
`so-called redox shuttle.
`ligand can be connected to the metal to form a solvated
`2. Prior Art
`molecule. The afore-mentioned agents which can form such
`a solvated molecule are useful for controlling the oxidation-
`In a lithium secondary cell (non-aqueous electrolyte sec-
`reduction potential and therefore suitable as redox shuttle.
`ondary cell), one of most important problems is to assure a
`However. the metal complexes or the cerium salts have
`safety thereof. Among them,
`the problem posed on the
`suchastructure that the large-size ligand is disposed around
`overcharge of the cell is significant.
`an electron orbital of the central metal. so that they show a
`For instance, it is known that a nickel-cadmium cell has
`large molecular weight and a large molecular volume.
`a mechanism for preventing the overcharging of cells in
`As a result, in case that these agents are dissolved in an
`which a charged energy is consumed by a chemical reaction
`electrolyte solution, its concentration and diffusion rate in
`of water occurring in association with increase in the
`charged voltage. On the other hand,
`in the case of the
`the electrolyte solution are limited to a particular range.
`which often leads to a problem that a sufficient overcharge-
`lithium-based secondary cell which is of a non-aqueous
`type, other types of mechanisms than those used in the
`preventing effect cannot be obtained.
`nickel-cadmium cell are required.
`25
`For instance, whenalithium ion is primarily concerned in
`Mechanismshitherto proposed to prevent the overcharge
`the reaction occuring under the overcharge condition, it is
`of the lithium-based secondary cell, include two methods;
`general that the lithium ion is dissolved in an electrolyte
`one method in which a chemical reaction is used and the
`solution at a concentration of about
`1 mole per liter.
`other method in which an electronic circuit
`is used.
`Therefore. it is desirable to dissolve the oxidation-reduction
`Practically, the latter method is predominantly employed.
`agent in the electrolyte solution at a molar concentration
`corresponding to the molar concentration ofthe lithium ion.
`However. such an overcharge-preventing method in
`which an electronic circuit is employed, is expensive. In
`If the metal complexor the cerium salt having such a large
`addition, such a method has a further defect that various
`molecule weight is dissolved in the electrolyte solution, the
`limitations are created in the course of product-designing for
`agent occupies a large volumein the electrolyte solution so
`the cell.
`that a viscosity or other dissolving properties of the elec-
`Under these circumstances, attempts have been made to
`trolyte solution are adversely affected. This results in dete-
`establish a technique for preventing the overcharge of the
`rioration in an ionic conductivity of the lithium ion. Thus,
`cell using a chemical reaction. One of such methods in
`there is a limitation concerning the concentration of the
`which the overcharge of the non-aqueouscell is prevented
`oxidation-reduction agent used.
`by using a chemical reaction,
`is to add an adequate
`In addition, some of the afore-mentioned metal com-
`reduction-oxidation agent to an electrolyte solution. By this,
`plexes have as large volume as one liter per one mole. If
`if the reduction-oxidation agent has a good reaction
`such metal complexes are used, it becomes impossible to
`reversibility, an effective overcharge-preventive mechanism
`prepare an electrolyte solution containing the reduction-
`is established because the agent can be freely moved
`oxidation agent at a molar concentration of one mole per
`between positive and negative electrodes of the cell to
`liter.
`thereby consume an overchargedelectrical current.
`Such a reduction-oxidation agent is referred to as “redox
`shuttle” or the like. The method in which the redox shuttle
`is used to simplify the safety mechanism of the lithium-
`based secondary cell, is less expensive than those using the
`electronic circuit. Such a method has a further advantage
`that the safety mechanism used there does not cause dete-
`rioration of an energy density of the cell.
`A possibility of applying the afore-mentioned redox
`shuttle to the lithium-based secondary cell has been already
`reported. For instance, In the case of 3 V-classcalls, it is
`suggested that ferrocenes are useful for the overcharge-
`preventing purpose.
`However, ferrocenes have a low oxidation-reduction
`potential of 3.1 V to 3.5 V relative to a lithium electrode.
`Therefore, ferrocenes are not applicable to cells having a
`higher cell voltage. For example, in the case of 4 V-Class
`cells such as carbon-LiCoO,-type lithium-ion cell,
`it
`is
`necessary to use compounds having an oxidation-reduction
`potential of about 4.0 v to about 4.5 V.
`As a result of further investigations, it has been revealed
`that, for instance, metal complexes of Fe. Ru or Ce as have
`
`Furthermore, in general, a large-volume molecule such as
`the metal complexor the cerium salt has a low diffusion rate
`in an electrolyte solution. If the large-volume oxidation-
`reduction agent having a lower diffusion rate than that of the
`lithium ion is used at a concentration lower than that of the
`lithium ion. it will be difficult to prevent an overcharge
`reaction of the lithium ion to a sufficient extent.
`A current status is such that there exist no oxidation-
`reduction agents which can fulfill all the requirements for
`the redox shuttle, except the afore-mentioned defective
`agents.
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`OBJECT AND SUMMARY OF THE INVENTION
`
`The present invention has been accomplished in view of
`the afore-mentioned problems encoutered in the prior art.
`It is therefore an object of the present invention to provide
`a non-aqueous electrolyte secondary cell having an excellent
`safety and a high energy density by using an oxidation-
`reduction agent (redox shuttle) capable of exhibiting an
`optimum oxidation-reduction potential and an improved
`dissolving ability to an electrolyte solution and generating
`
`5
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`

`5,763,119
`
`3
`chemically stable oxidation- and reduction-species without
`causing deterioration in performance of the cell due to
`undesired side reactions.
`
`In order to achieve the afore-mentioned objects, in accor-
`dance with the present invention, there is a non-aqueous
`electrolyte secondary cell comprising a negative electrode
`composed of a metal material containing lithium as a
`primary component or a carbonaceous material into which
`lithium can be doped and from which lithium can be
`dedoped. a positive electrode composed of a composite
`oxide of lithium and transition metal, and a non-aqueous
`electrolyte containing an organic compound of the general
`formula:
`
`OCH3
`
`OCH;
`
`where X represents a halogen atom.
`The compoundof the afore-mentioned general formula
`has a chemical structure in which two methoxy substituent
`groups and onehalogen group are bonded to a benzenering.
`The compoundacts as an oxidation-reduction agent because
`the methoxy substituent groups can function as an oxidation-
`reduction radical. Specifically,
`the compound has an
`oxidation-reduction potential suitable as a redox shuttle for
`4 V-class cells. In addition, oxidation-species and reduction-
`species derived from the compound are chemically stable.
`Furthermore, the compound has a benzene ring as a basic
`skeleton having a molecular weight of 78. Thus, the com-
`pound has a relatively small molecular weight and molecular
`volume as compared with those of metal complexes such as
`metallocenes and polypyrizine complexes or cerium salts.
`This means that these compounds have a small volume
`occupancy and a high diffusion rate in the electrolyte solu-
`tion. Therefore, properties of the electrolyte solution are not
`adversely affected by the addition of these compounds, so
`that a high mobility of the compounds in the electrolyte
`solution is maintained.
`Accordingly, when the compounds are contained in a
`non-aqueous electrolyte solution of the secondary cell, an
`overcharged current generated in the cell is effectively
`consumed sothat the cell can be protected from an excessive
`increase in cell voltage.
`Meanwhile,it has been found that the compoundin which
`the two methoxy substituent groups are bonded to the 1 and
`2 positions or the 1 and 4 positions of its benzene ring.
`exhibits a particularly higher reversibility in the oxidation-
`reduction reaction as compared with the compoundin which
`the substituents groups are bonded to the L and 3 positions
`of the benzene ring, though the latter compounds are also
`useful as a redox shuttle. The reason why the 1 and 3
`position-substituted compound shows a relatively low
`reversibility in the oxidation-reduction reaction, is as fol-
`lows.
`That is, the 1,3 and 5 positions of the benzene ring have
`a conjugated relation to each other. In consequence, if any
`electron located at the 1 position of the benzene ring is
`liberated therefrom, the lack of electron is compensated with
`those in an electron cloud located at the 3 or 5 position of the
`benzene ring. In this case, assuming that the two methoxy
`substituent groups are introduced to, for example, the 1 and
`3 positions of the benzene ring and any of these methoxy
`
`4
`substituent groups is concerned in the oxidation reaction of
`the compound, it will be easily suggested that. when the
`methoxy substituent group bonded to the | position of the
`benzene ring is oxidized,
`lack of electrons due to the
`oxidation is immediately compensated with those located at
`the 3 position whereby lack of electrons at the 3 position
`occurs, Such a condition in which both the methoxy sub-
`Stituent groups bonded to the | and 3 positions of the
`benzene ring are oxidized. is unstable. Accordingly,
`the
`compoundin which the two methoxy substituent groupsare
`bonded to the 1 and 2 positions or the 1 and 4 positions of
`the benzenering can exhibit a relatively high reversibility in
`the oxidation-reduction reaction as compared with the com-
`pound in which those methoxy substituent groups are in the
`1 and 3 positions.
`These and other objects, features and advantages of the
`present invention will become more apparent from the
`following detailed description when read in conjunction
`with the accompanying drawings and the appendedclaims.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a sectional view of a cell according to one
`embodimentof the present invention;
`FIG,2 is a graph showing a relation between cell voltage
`and elapsed time in a charge/discharge cycle. in which a cell
`employing an electrolyte containing 1, 4-di-methoxy-2-
`fluoro-benzene is compared with that employing anelectro-
`lyte without such a compound;
`FIG.3 is a graph showing a relation between cell voltage
`and elapsed time in a charge/discharge cycle, in which a cell
`employing an electrolyte containing 1, 2-di-methoxy-4-
`bromo-benzene, 2. 5-di-methoxy-1-bromo-benzene or 1.
`2-di-methoxy-4-fluoro-benzene is compared with that
`employing an electrolyte without such a compound;
`FIG.4 is a graph showinga relation between cell voltage
`and discharge capacity in a charge/discharge cycle, in which
`a cell employing an electrolyte containing 1, 2-di-methoxy-
`4-bromo-benzene, 2, 5-di-methoxy-1-bromo-benzene or 1,
`2-di-methoxy-4-fluoro-benzene is compared with that
`employing an electrolyte without such a compound;
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`The non-aqueous electrolyte secondary cell according to
`the present invention, includes a negative electrode made of
`4 metal material containing lithium as a primary component
`or a carbonaceous material into which lithium can be doped
`and from which lithium can be dedoped, and a positive
`electrode made of a composite material of lithium and
`transition metal. The use of the negative and positive elec-
`trodes provides as much high a cell voltage as 4 V or more.
`In the non-aqueouselectrolyte secondary cell according to
`the present invention, in addition to the use of the afore-
`mentioned negative and positive electrodes, there is used a
`non-aqueous electrolyte containing the compound of the
`general formula:
`
`OCH
`
`OCH3
`
`where X represents a halogen atom.
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`5,763,119
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`5
`The non-aqueous electrolyte containing such a compound
`chemically consumes, in its oxidation-reduction reaction, a
`current generated due to overcharge of the cell, wherebyit
`can serve as a redox shuttle. The specific properties of the
`compound are described in detail below.
`The compound of the afore-mentioned general formula
`contains a benzenering asa basic skeleton. When methoxy
`substituent groups are introduced to the benzene ring, the
`compound exhibits an oxidation-reduction potential suitable
`for a redox shuttle for 4 V-class cells. and generates oxidized
`species and reduced species both having a high stability to
`a chemical reaction.
`That is, when a pair of electrons are shared by adjacent
`two atoms of an organic compound, a covalent single bond
`is principally formed therebetween. Accordingly, if one
`electron is removed from or added to an electron system of
`the bond by oxidation or reduction of the organic compound,
`an unpaired electron is generated in the compound. This
`unpaired electron is stabilized only by decomposition of the
`compound or bonding thereof with another compound.
`Accordingly, the organic compound having such an unpaired
`electron is principally unstable.
`However, in case that the unpaired electron is present in
`a non-localized orbital such as 1 orbital of an aromatic
`compoundand shared by 2 or more atoms within its molecu-
`lar structure, a chemical stability of the organic compoundis
`not adversely affected by the existence of the unpaired
`electron.In this case, the oxidation-reduction potential of the
`compound is almost determined by a degree of the non-
`localization of the unpaired electron and a symmetry of the
`orbital for the unpaired electron. When the degree of the
`non-localization of the electron orbital is too large, an
`adequate level of the oxidation-reduction potential cannot be
`obtained. From this standpoint, organic compounds having,
`as a basic skeleton, an aromatic ring such as a benzenering,
`which has a relatively small molecular weight, are suitable
`for a redox. shuttle.
`Whenthe benzenering has not only methoxy substituent
`groups but a halogen substituent group, the following advan-
`tages are attained.
`Thatis, in general, the oxidation-reduction potential of the
`compound is almost determined by a basic skeleton of the
`molecular structure thereof as described above. When any
`substituent group is introduced to the basic skeleton, the
`oxidation-reduction potential of the compoundis influenced
`by the nature of the substituent group introduced. If the
`substituent group is an electron attractive group,
`the
`oxidation-reduction potential is increased. On the contrary,
`if the substituent proup is an electron donative group, the
`oxidation-reduction potential is decreased. In addition, when
`a plurality of substituent groups are introduced to the basic
`skeleton, it is known that the afore-mentioned effect on the
`oxidation-reduction potential
`is caused in a cumulative
`manner.
`
`The introduction of the halogen sustituent group to the
`benzene ring causes increase in the oxidation-reduction
`potential of the compound. In an actual cell system, the
`oxidation-reduction potential of the compoundis fluctuated
`by several hundred millivolts depending upon the kind of
`electrolyte used. However, the oxidation-reduction potential
`of the compound is finely adjusted by the effect of the
`halogen substituent group introduced to the benzenering. As
`atesult, the compound having the benzene ring to which the
`halogen substituent group is introduced shows an adequate
`oxidation-reduction potential irrespective of the kind of the
`electrolyte, so that it can suitably function as a redox shuttle.
`Incidentally,
`the number of the methoxy substituent
`groups to be introduced to the benzene ring should be two
`
`6
`per one molecule of the compound. If the benzene ring has
`only one methoxy substituent group, the compound cannot
`exhibit a sufficient oxidation-reduction effect so that an
`increased amount of the compound must be used to obtain
`a necessary Oxidation-reduction effect.
`As described above, the organic compound having the
`benzene ring to which two methoxy substituent groups and
`the halogen substituent group are introduced, can exhibit a
`sufficient oxidation-reduction potential suitable to be used as
`a redox shuttle for 4 V-class cells. and generates oxidized
`and reduced species having a high chemical stablility. In
`addition, such a compound does not cause undesired side
`reaction which adversely affects the performance ofthe cell.
`The benzene ring as the basic skeleton of the compound has
`a molecular weight of 78. Namely, not only the molecular
`weightbut also the molecular volume of the compound are
`smaller than those of metal complexes such as metallocene
`and polypyridine complexesor cerium salts. This means that
`the compound has a low volume occupancy and a high
`diffusion rate in the electrolyte, so that the solvent charac-
`teristic of the electrolyte is not largely influenced by the
`existence of the compound whereby a good mobility of the
`compoundin the electrolyte can be achieved.
`Specific examples of such organic compounds may
`include 1, 4-di-methoxy-2-fluoro-benzene, 1, 3-di-methoxy-
`5-chloro-benzene, 3, 5-di-methoxy-1-fluoro-benzene. 1.
`2-di-methoxy-4-fluoro-benzene, 1, 3-di-methoxy-4-bromo-
`benzene, 2, 5-di-methoxy-1-bromo-benzene, or the like.
`Amongthem, the preferred organic compoundssuitable
`for a redox shuttle are those having the two methoxy
`substituent groups which are bonded to the benzene ring at
`the 1 and 2 positions or the 1 and 4 positions.
`EXAMPLES
`
`Thepresent invention is described in detail below by way
`of examples with reference to the accompanying drawings.
`Example 1
`FIG. 1 is a sectional view of a coin-shaped cell having an
`outer diameter of 20 mm anda height of 2.5 mm according
`to the present invention.
`The coin-shaped cell was produced in the following
`manner.
`
`Metal lithium as a negative electrode active ingredient 1
`and LiCoO, as a positive electrode active ingredient 2 are
`filled into an upper and lower casings 4 and 5, respectively.
`The upper and lower casings 4 and 5 were mated with each
`other through a separator 3 formed from a porous polypro-
`pylenefilm. so as to form a laminate structure composed of
`layers of the negative electrode active ingredient 1 and the
`positive eléctrode active ingredient 2. and the separator 3
`interposed therebetween. Separately, propylene carbonate
`and dimethyl carbonate were mixed with each other at a
`mixing ratio of 1:1 to prepare a mixture solvent. Dissolved
`into the mixture solvent were 1.0 mole of LiPF, and 100 ml
`of 1, 4-di-methoxy-2-fluoro benzene to obtain an electrolyte.
`Theelectrolyte was charged into a space formed between the
`upper and lower casings. Successively, the upper and lower
`casings were caulked together at their peripheral mating
`edges through a sealing gasket to form a hermetically sealed
`coin-shaped cell.
`Incidentally, when 1, 4-di-methoxy-2-
`fluoro-benzene was used as the component of the
`electrolyte, cyclic voltammetry revealed that a reversible
`oxidation-reduction reaction of the compound was caused in
`proximity of 4.2 V and 4.45 V relative to lithium.
`Example 2
`The procedure of Example 1 was repeated in the same
`manner as described above to produce a coin-shaped cell
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`5,763,119
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`7
`except that 1, 2-di-methoxy-4-bromo-benzene was used in
`the electrolyte in place of 1, 4-di-methoxy-2-fluoro-benzene.
`
`Example 3
`
`The procedure of Example 1 was repeated in the same
`manner as described above to produce a coin-shaped cell
`except that 2, 5-di-methoxy-1-bromo-benzene was used in
`the electrolyte in place of 1, 4-di-methoxy-2-fluoro-benzene.
`
`Example 4
`
`The procedure of Example 1 was repeated in the same
`manner as described above to produce a coin-shaped cell
`except that 1, 2-di-methoxy-4-fluoro-benzene was used in
`the electrolyte in place of 1, 4-di-methoxy-2-fluoro-benzene.
`Comparative Example 1
`The procedure of Example 1 was repeated in the same
`manner as described above to produce a coin-shaped cell
`except that nothing was used in the electrolyte in place of 1,
`4-di-methoxy-2-fluoro-benzene.
`The thus-produced coin shaped cells were subjected to a
`charge/discharge cycle under an overcharged condition to
`examine change in voltage generated by the cells.
`Meanwhile, the charge/discharge cycle was performed as
`follows. First. a constant-current charge was conducted at a
`current of 150 pA for 100 hours while the cell voltage was
`controlled so as not to exceed 4.5 V, followed by 10 hour
`interruption of the cycle. Successively, a constant-current
`discharge was conductedat a current of 150 pA until the cell
`voltage reached 4.5 V decreased to 2.7 V.
`The change in the cell voltage obtained in the afore-
`mentioned charge/discharge cycle are shown in FIGS. 2 and
`3. FIG. 4 shows a relation betweenthe cell capacity and the
`voltage. FIG. 2 shows the measurement data obtained in
`Example 1 and Comparative Example 1. FIGS. 3 and 4
`comparatively show the measurement data obtained in
`Examples 2 to 4 and Comparative Example 1.
`In the afore-mentioned charge/discharge cycle.the cell of
`Comparative Example 1 exhibited considerable increase in
`the cell voltage during the charging step so that the cell
`voltage reached the upper limit of the cell voltage
`(overcharged condition). Whereas, in the cell of Example 1,
`the increase in the cell voltage was observed until it reached
`4.1 V but thereafter almost no increase in the cell voltage
`was observed. This was because the addition of 1, 4-di-
`methoxy-2-fluoro-benzene to the electrolyte caused con-
`sumption of a current generated due to the overcharge,
`whereby a further increase in the cell voltage was sup-
`pressed. Incidentally, the constantcell voltage thus achieved
`by the addition of the redox shuttle is called a shuttle voltage
`(relaxation voltage).
`On the other hand. in the discharge cycle, the cell of
`Example 1 showed rapid decreasein the cell voltage to the
`lower limit of 2.7 V as compared with that of Comparative
`Example 1. At this time, the discharge capacity was about
`110 mAh/g. This discharge capacity approximately corre-
`spondedto that obtained when the cell was charged to 4.05
`V relative to a standard cell.
`The cell of Comparative Example 1 showed slow
`decrease in the cell voltage to the lower limit of 2.7 V, so that
`the discharge capacity of the cell was larger than 140 mAh/g
`as a standard discharge capacity of the afore-mentioned
`standard cell. This was because the cell of Comparative
`Example 1 was overcharged in the charge cycle.
`As described above,
`it was ascertained that 1, 4-di-
`methoxy-2-fluoro-benzene was useful to prevent the over-
`charge of the cell. In addition, since the discharge capacity
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`30
`
`55
`
`65
`
`8
`of the cell to which 1, 4-di-methoxy-2-fluoro-benzene was
`added. was in consistent with the standard discharge
`capacity, it was confirmed that 1. 4-di-methoxy-2-fluoro-
`benzene did not adversely affect the discharge capacity of
`the cell at all.
`FIG. 3 showsthe change in cell voltage measured for the
`cells of Examples 2 to 4 and Comparative Example 1 in the
`charge cycle. As appreciated from FIG. 3, in the cells of
`Examples 2 to 4, the increase in cell voltage was observed
`until it was raised to the oxidation-reduction potential of the
`redox shuttle but no further increase of the cell voltage was
`subsequently recognized. Particularly, the cell voltage of the
`cell of Example 2 was maintained at an optimum constant
`level slightly higher than 4.2 V.
`Furthermore, FIG. 4 shows the relation between a dis-
`charge capacity and a cell voltage which was obtained in the
`charge cycle of the cells of Examples 2 to 4 and Comparative
`Example 1. FIG. 4 also shows the relation between a
`discharge capacity and a cell voltage which was obtained in
`the discharge cycleof the cells of Example 2 and Compara-
`tive Example 1. As appreciated from FIG. 4, the cells of
`Examples 2 to 4 whose cell voltages were maintained at the
`constant level in the charge cycle by the addition of the
`redox shuttle, had a discharge capacity almost identical to or
`extremely approximate to a theoretical value.
`Meanwhile.
`the shuttle voltages and the discharge
`capacities, which are plotted in FIGS. 2. 3 and 4. and
`theoretical values of the discharge capacities calculated
`based on the shuttle voltages, are shown in Table 1.
`
` TABLE 1
`
`Theoretical value
`Discharge
`of discharge
`capacity
`Shuttle
`capacity (mAh/g)
`(mAh/g)
`voltage (V)
`Example No.
`106.2
`92
`4.04
`Example |
`147.6
`146
`4.27
`Example 2
`131.2
`126
`4.17
`Example 3
`
`
`
`3.93 92Example 4 _
`
`As understood from Table 1, it was confirmed that 1,
`2-di-methoxy-4-bromo-benzene, 2, 5-di-methoxy-1-bromo-
`benzene and 1, 2-di-methoxy-4-fluoro-benzene provided an
`optimum mechanism for preventing the overcharge of the
`cell and did not adversely affect functions of electrodes of
`the cell, similar to 1, 4-di-methoxy-2-fluoro-benzene.
`Furthermore, 1, 3-di-methoxy-5-chloro-benzene, 3, 5-di-
`methoxy-1-fluoro-benzene and 1, 3-di-methoxy-4-bromo-
`benzene were dissolved in the electrolytes of the respective
`cells in place of 1, 4-di-methoxy-2-fiuoro-benzene. Thecells
`were subjected to the charge/dischargecycle test in the same
`manner as described above. As a result, the cell voltages
`were maintained at a constantlevel ranging from about4.05
`V to 4.4 V and the discharge capacities thereof were also
`maintained at an adequate level which correspondsto those
`obtained for the cell charged to the afore-mentioned voltages
`and ranges from 110 mAh/g to 140 mAh/g. This indicates
`that these compounds were also useful to provide an effec-
`tive mechanism for preventing the overcharge of thecell.
`Studies on Positions of Methoxy Substituent Groups in
`Redox Shuttle:
`Next, positions of methoxy substituent groups introduced
`to the benzene ring were examined from a standpoint of
`reversibility of the redox shuttle. 1, 2-di-methoxy-4-bromo-
`benzene (1, 2-diMe-4-BrB), 1, 2-di-methoxy-4-fluoro-
`benzene (1, 2-diMe-4-FB), 3, 5-di-methoxy-1-chloro-
`benzene (3. 5-diMe-1CIB), 3, 5-di-methoxy-I-fluoro-
`benzene (3. 5-diMe-1-FB). 2, 4-di-methoxy-l-bromo-
`
`8
`
`

`

`9
`benzene (2, 4-diMe-1-BrB), 2. 5-di-methoxy-1-bromo-
`benzene (2, 5-diMe-1-BrB) and 1, 4-di-methoxy-2-
`fluorobenzene (1, 4-diMe-2-FB) were respectively dissolved
`
`10
`reversibility. In general, in case that the compound has a
`good oxidation-reduction reversibility. “IE,—E,,.|”=0.0565/
`n(V) can be established.
`
`5,763,119
`
` TABLE 2
`Presence
`of reduction
`current
`YES
`
`Structural
`formula
`OCH;
`
`OCH;
`
`Br
`
`1,2-diMe-4-BrB
`
`OCH;
`
`CyOCH3
`
`F
`
`Threshold
`potential
`4.2V,
`4.5V
`
`E,-Eye
`Epo
`E,
`462V 457V 0.05 V
`
`4.15 V,
`445 V
`
`YES
`
`452V 446V 0.06 V
`
`43 V
`
`NO
`
`_—
`
`_—
`
`_
`
`44V
`
`NO
`
`—_—
`
`—
`
`—
`
`1,2-diMe-4-FB
`
`ci
`
`HjCO' C OCH;
`
`3,5-diMe-1-CIB
`F
`
`HgCO ° OCH;
`
`3,5-diMe-1-FB
`
`at a molar concentration of 100 mM in an electrolyte which
`was prepared by dissolving LiPF, at a molar concentration 45
`of 1.0M in a mixture solvent composed of propylene car-
`bonate and dimethy! carbonate at a mixing volumeratio of
`1:1. The thus-obtained electrolytes were subjected to a
`cyclic voltammetry using a three-pole cell.
`Meanwhile. platinum plates were used as a working 50
`electrode and a counter electrode while a stainless steel plate
`to which metal lithium was adhered, was used as a reference
`electrode. The sweep rate was 20 mV/s. Threshold potential,
`E,. E,2 and E,—E,,2. which were measured by a cyclic
`voltammograph, and presence of a reduction current corre- 55
`sponding to an oxidation current are shown in Tables 2 and
`3.
`Incidentally, the threshold potential means a potential at
`which a current starts to flow in case that the potential is
`swept. The threshold potential is a useful criterion to know 60
`an oxidation-reduction potential. In Tables 2 and 3, in the
`case where two threshold potentials appear in one row, this
`indicates that two oxidation currents were recognized. “E,,”
`represents a potential at which a peak value of the oxidation
`or reduction potential is obtained. “E,,.”, represents apoten- 65
`tial at which one-half of the current at “E,” is caused to flow.
`“E,~E,,2” can be used as an index of the oxidation-reduction
`
`TABLE 3
`
`
`Pre-
`sence
`ofre-
`Structural
`Threshold
`duction
`E, ~
`§
`:
`ia
`potential
`¢
`Ey
`Ep
`Ben
`
`44V
`~
`—
`~
`
`NO
`
`Br
`
`OCH,
`
`OCH;
`
` 2,4-diMe-1-BrB
`
`9
`
`

`

`5,763,119
`
`12
`nized that suitable oxidation-reduction agents (redox
`shuttle) were those having a halogen substituent group and
`two methoxy substituent groups introduced to the benzene
`ring at the 1, 2-positions or the 1, 4-positions.
`Whatis claimed is:
`1. A non-aqueouselectrolyte secondary cell comprising:
`a negative electrode composed of a metal material con-
`taining lithium as a primary component or a carbon-
`aceous material into which lithium can be doped and
`from which lithium can be dedoped;
`a positive electrode composed of a composite oxide of
`lithium and transition metal; and
`a non-aqueous electrolyte containing an