US005858573A
`5,858,573
`[11] Patent Number:
`(15
`United States Patent
`Abraham etal.
`[45] Date of Patent:
`Jan. 12, 1999
`
`
`[54] CHEMICAL OVERCHARGE PROTECTION
`OF LITHIUM AND LITHIUM-ION
`SECONDARYBATTERIES
`
`[75]
`
`Inventors: Kuzhikalail M. Abraham, Needham,
`Mass.; James F. Rohan, Cork City,
`Ireland; Conrad C. Foo, Dedham,
`Mass.; David M. Pasquariello
`Pawtucket. RL
`
`,
`
`,
`.
`[73] Assignee: EIC Laboratories, Inc., Norwood,
`Mass.
`
`[21] Appl. No.: 703,577
`[22]
`Filed:
`Aug.23, 1996
`,
`[51]
`Tint. C18 oc cccccecessseesssseeesssseeeeens HO1M 10/40
`[52] U.S. Che cece 429/198; 429/192; 29/623.1
`[58] Field of Search occccc 429/198, 192,
`429/105; 29/623.1
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`12/1984 Abraham et al. oo...eee 429/197
`
`.. 429/192
`7/1990 Casalbore-Miceliet al.
`6/1993 Abraham et al. oo... 429/192
`
`4,489,145
`4,943,499
`5,219,679
`
`10/1993 Alamgiret al... 429/192
`5,252,413
`11/1993 Armandetal. .....
`
`. 429/192 X
`5,260,145
`
`5,474,860 12/1995 Abraham etal.
`oe 429/192
`9/1996 Albers 00... eects 429/198 X
`5,556,524
`
`OTHER PUBLICATIONS
`.
`.
`:
`.
`Prema,S. and Srinivasan, M., “Preparation and Properties of
`Polyamides Containing Thianthrene Units,” Eur. Polym.J.,
`vol. 23, No. 11, pp. 897-903, 1987. (Month unknown).
`Primary Examiner—Stephen Kalafut
`Attorney, Agent, or Firm—Edward J. Kelly; Foley, Hoag &
`Eliot LLP
`
`ABSTRACT
`[57]
`This invention features the use of redox reagents, dissolved
`in non-aqueouselectrolytes, to provide overcharge protec-
`tion for cells having lithium metal or lithium-ion negative
`electrodes (anodes). In particular, the invention features the
`use of a class of compoundsconsisting of thianthrene andits
`derivatives as redox shuttle reagents to provide overcharge
`protection. Specific examples of this invention are thian-
`threne and 2,7-diacetyl
`thianthrene. One example of a
`rechargeable battery in which 2,7-diacetyl
`thianthrene is
`used has carbon negative electrode (anode) and spinet
` LiMn,O, positive electrode (cathode).
`
`21 Claims, 7 Drawing Sheets
`
`O|C
`
`CH3
`
`APPLE-1009
`
`APPLE-1009
`
`1
`
`

`

`U.S. Patent
`
`Jan. 12, 1999
`
`Sheet 1 of 7
`
`5,858,573
`
`FIG. 1.
`
`2
`
`

`

`U.S. Patent
`
`Jan. 12, 1999
`
`Sheet 2 of 7
`
`5,858,573
`
`CHsG
`
`T
`-~CCH3
`
`S
`
`S
`
`FIG. 2.
`
`3
`
`

`

`U.S. Patent
`
`Jan. 12, 1999
`
`Sheet 3 of 7
`
`5,858,573
`
`1.5
`
`a=
`
`1.0
`
`2
`0.5&
`
`0.0
`
`25
`
`aQ
`
`y
`
`5 -05
`
`-1.0
`
`3.0
`
`3.2
`
`3.4
`
`4.0
`3.8
`3.6
`E/V vs. Li*/Li
`
`4.2
`
`4.4
`
`4.6
`
`FIG. 3.
`
`4
`
`

`

`U.S. Patent
`
`Jan. 12, 1999
`
`Sheet 4 of 7
`
`5,858,573
`
`4.0
`
`3.0
`
`2.0
`
`1.0
`
`0.0
`
`
`
` CurrentDensity(mA/cm2)
`
`
`
`-1.0
`
`-2.0
`
`-3.0
`
`-4.0
`
`-5.0
`
`3.8 40 42 44 4.6
`30 3.2 34 3.6
`E/V vs Lit/Li
`
`FIG.4.
`
`5
`
`

`

`U.S. Patent
`
`Jan. 12, 1999
`
`Sheet 5 of 7
`
`5,858,573
`
`
`
`Capacity(mA/cm’)
`
`1.0
`
`0.8
`
`0.6
`
`0.4
`
`0.2
`
`0.0 +
`
`-0.2
`
`-0.4
`
`-0.6
`
`-0.8
`
`30
`
`32
`
`34
`
`40
`38
`36
`E/V vs. Li*/Li
`
`42
`
`44
`
`4.6
`
`FIG. 5.
`
`6
`
`

`

`U.S. Patent
`
`Jan. 12, 1999
`
`Sheet 6 of 7
`
`5,858,573
`
`AOLA©
`
`>o
`
`
`
`CellPotential(V) WowSoun
`
`
`
`Nin
`
`2.0
`
`0
`
`5.0
`
`50
`
`100
`150
`200
`Capacity (mAh/g-LiMn,0,)
`
`250
`
`FIG. 6A.
`
`wa 45
`co
`—_ 4.0
`
`.5
`
`3.0
`
`25
`
`2.0
`
`0
`
`50
`
`200
`150
`100
`Capacity (mAh/g-LiMn,0,)
`
`250
`
`FIG. 6B.
`
`= 3
`
`°e
`
`oY
`
`7
`
`

`

`U.S. Patent
`
`Jan. 12, 1999
`
`Sheet 7 of 7
`
`5,858,573
`
`anNoO
`
`o1S
`
`
`
`CurrentDensity(mA/cm2)©Sewvbo
`
`
`
`2os)
`
`SrN
`
`2.8
`
`3.0
`
`4,2
`4.0
`3.8
`3.6
`3.4
`3.2
`ECvs Li Reference (V)
`
`4.4
`
`46
`
`FIG. 7.
`
`8
`
`

`

`5,858,573
`
`1
`CHEMICAL OVERCHARGE PROTECTION
`OF LITHIUM AND LITHIUM-ION
`SECONDARYBATTERIES
`
`This invention was made with support from the Depart-
`ment of Energy under Contract DE-FC02-91CE50336
`administered by the United States Advanced Battery Con-
`sortium. The consortium and the U.S. Government have
`certain rights in this invention.
`PIELD OF INVENTION
`
`This invention relates to clectrochemical cells and more
`
`particularly to improved non-aqueous liquid and polymer
`electrolytes for cells incorporating alkali metal negative
`electrodes (anodes), and especially lithium anodes or
`lithium-ion anodes. The improvementfeatures the use of a
`new class of compounds as redox reagents, dissolved in
`non-aqueouselectrolytes, to provide overcharge protection.
`BACKGROUND OF THE INVENTION
`
`The electrolyte solution is a crucial component in an
`ambient temperature secondary lithium cell. A non-aqueous
`solvent or mixture of solvents which dissolves an appre-
`ciable amount of lithium salts to form highly conducting
`solutions is desirable. The electrolyte should afford high
`efficiency for cycling of the lithium or lithtum-ion electrode,
`and exhibit good thermal stability up to 70° C. (the usual
`upper temperature limit for operation of ambient tempera-
`ture batteries).
`A highly desirable liquid electrolyte solution established
`for ambient temperature Li secondary cells is described in
`US. Pat. No. 4,489,145. It comprises a solution of LiAsF,
`dissolved in a mixed solvent of tetrahydrofuran (THF),
`2-methyltetrahydrofuran (2-Me-THF), and 2-methylfuran
`(2-Me-F). Other aprotic electrolytes have contained cyclic
`carbonates such as propylene carbonate (PC) and ethylene
`carbonate (EC), and have been the subject of much study in
`terms of both basic electrochemistry and battery applications
`for many years.
`More recent developments have included the use of
`electrolytes based on polymers such as polyacrylonitrile
`(PAN), poly(tetraethylene glycol diacrylate) (PEGDA), poly
`(vinyl) pyrrolidinone (PVP), poly (vinyl chloride) (PVC), or
`poly(vinyl sulfone) (PVS). In these electrolytes the poly-
`mers are matrices to immobilize complexes (solvates)
`formed between Lisalts, such as LiAsF,, LiCF,S0,, LiPF,,
`LiN(CF,SO,), and LiBF,, and an aprotic organic solvent(or
`mixture of such solvents) to allow fabrication of free-
`standing electrolyte films to be used in solid-state Li and
`Li-ion batteries (K. M. Abraham et al., U.S. Pat. Nos.
`5,219,679; 5,252,413; 5,457,860).
`Unlike aqueous cells, non-aqueous electrolyte (organic
`electrolyte) cells may not be overcharged without causing
`irreversible electrolyte side-reactions which deteriorate cell
`performance. Cells are safeguarded during laboratory
`charge/dischargetests by careful control of the voltage limits
`by means of the electronic equipment used in the test.
`Electronic overcharge control comprises a sensing circuit
`which prevents current from flowing into the cell once it
`reaches the voltage corresponding to complete charge. The
`charge voltage limit is selected according to the electro-
`chemical couple in the cell. For example, Carbon/L.iMn,0,
`cclls have an upper charge limit of 4.3V vs. Li*/Li.
`Chemical overcharge protection of a battery consisting of
`cells connected in series is particularly important for two
`reasons. Firstly, it will replace electronic overcharge con-
`
`10
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`15
`
`30
`
`35
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`40
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`45
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`50
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`55
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`60
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`65
`
`2
`trollers in individual cells. Electronic controllers lower the
`energy density of the battery and increase battery cost.
`Secondly, it will provide capacity balance amongthe indi-
`vidual cells and prevent oxidative degradation of the elec-
`trolyte. The capacity balance amongthe cells in a battery
`may be lost, especially after repeated charge/discharge
`cycles. This meansthat the accessible capacity of individual
`cells may not remain equal. In this instance, the cathode of
`the cell with the lowest capacity will be pushed above the
`normal upper voltage limit. Oxidative degradation of the
`electrolyte will occur at these higher potentials, and this will
`degrade the cycle life of the battery at an accelerated rate.
`Evenif the electrolyte does not decompose, the weaker cell
`will contribute a larger fraction of the total cutoff voltage for
`the battery causing the capacity of the cells in the battery to
`becomeincreasingly out of balance at each additional cycle,
`since the stronger cells will not be charged to their full
`capacity. The result is a reduced cycle life for the battery as
`compared to its individualcells.
`A redoxshuttle offers the best approach to cell overcharge
`protection. In this scheme, a material with an appropriate
`oxidation potential is dissolved in the electrolyte where it
`remains unreactive until
`the cell
`is charged fully. At a
`potential slightly above the cell charge limit (upper cutoff
`voltage), the redox shuttle is activated by its electrochemical
`conversion. The cell potential during overchargeis fixed at
`the oxidation potential of the redox shuttle. This process is
`supported by diffusion of the oxidized products to the anode
`where they recombineto form the starting material. Once the
`reformed material diffuses back to the cathode, it is oxidized
`and the cathode potential is maintained indefinitely at the
`oxidation potential of the redox reagent, until the time that
`charging is terminated.
`Necessary properties of a redox shuttle include: high
`solubility in the electrolyte; an oxidation potential slightly
`higher than the normal chargelimit ofthe cell but lower than
`the oxidation potential of the electrolyte; the ability of the
`oxidized form to be reduced at
`the anode without side
`reactions; and chemical stability in the cell of both the
`oxidized and reduced forms of the shuttle reagent.
`Accordingly, an object of this invention is the use of redox
`reagents to provide a means of chemical overcharge protec-
`tion to secondary non-aqueousliquid and polymerelectro-
`lyte cells with lithium or lithium-ion anodes.
`
`SUMMARYOF THE INVENTION
`
`The inventionfeatures a rechargeable electrochemicalcell
`which includes an anode, a cathode, and an electrolyte. ‘The
`electrolyte is a non-aqueous solvent or a mixture of non-
`aqueous solvents which may or may not be immobilized in
`a polymer matrix, and in which one or more salts and the
`redox reagent are dissalved. The redox reagent is ideally
`present in an amount sufficicnt
`to maintain proper mass
`transport for the desired steady overcharge current for the
`cell.
`
`DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 showsthe structure of thianthrene.
`
`FIG. 2 showsthe structure of 2,7-diacetyl thianthrene.
`FIG. 3 shows the cyclic voltammogram obtained with a
`Li//organic electrolyte//glassy carbon cell in which the elec-
`trolyte was 50% EC:50% PC-1.0M LiPF; with 10 mM
`thianthrene. The scan rate was 100 mV/s.
`FIG. 4 showsthe reversible extraction and insertion of
`
`lithium for LiMn,O,. Data were collected using a cell with
`
`9
`
`

`

`5,858,573
`
`3
`the configuration Li//PAN-EC-PC-LiPF,//LiMn,O, and the
`scan rate was 20 “V/s.
`FIG. 5 showsthe shift in the redox potential obtained by
`acetylating thianthrene to form 2,7-diacetyl thianthrene. The
`electrolyte was 50% EC:50% PC-1.0M LiPF, with 10 mM
`2,7-diacetyl thianthrene, and the scan rate was 100 mV/s.
`FIGS. 6A and 6B show the voltage profile for charge/
`discharge of a Li//Solid Polymer Electrolyte//LiMn,O,cell
`at the first (a) and second (b) cycles. The charge limit was
`4.2V for the first cycle. The charge limit was raised to 4.3V
`for the second cycle but did not yield significantly more
`capacity.
`FIG. 7 is a cyclic voltammogram for a carbon//polymer
`electrolyte//LiMn,O, cell containing 2,7-diacetyl
`thian-
`threne in the electrolyte and cycled at 20 uV/s.
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`In a preferred embodiment of the invention, thianthrenes
`(R) are used for the protection of lithium and lithium-ion
`secondary cells from the effects of overcharge. Theiractivity
`is manifested through the redox shuttle reactions depicted in
`SchemeI.
`
`Scheme I: Redox Shuttle
`
`At the cathode:
`
`R— Rt+e-;
`
`#R> cathode
`
`[1]
`
`4
`although thianthrene exhibits the desired redox behavior, it
`is not suitable for use in the C/LiMn,O, cell since the
`activation of the redox shuttle would overlap with the
`removal of lithium from the cathode. This would interfere
`
`with the cathode utilization so that the cell would not charge
`fully.
`In order to shift the redox potential to more positive
`values,it is necessary to modify the thianthrene structure by
`replacing hydrogen atoms on the aromatic rings with elec-
`tron withdrawing groups. For this reason, thianthrene was
`derivatized by replacing hydrogen atomsin the 2,7 positions
`with acetyl groups, according to a previously published
`procedure (S. Prema and M. Srinivasan, Eur. Polym. J., 23,
`897 (1987)). When this compound was tested under the
`same conditions as thianthrene,
`the redox potential was
`found to have shifted to 4.3V for oxidation and 4.2V for the
`
`subsequent reduction (FIG. 5). The redox potential ranges
`for the thianthrene and the acetylized derivative compounds
`are given in Table 1.
`
`15
`
`TABLE1
`
`Redox Potential Ranges of Chemical Shuttle Reagents.
`
`Compound
`Thianthrene
`2,7-diacetyl Thianthrene
`
`Redox Potential Range
`(V vs. Li'/Li)
`4.06-4.12
`4.19-4,30
`
`At the anode: Rte- —> R
`
`[2]
`
`30
`
`Table 2 shows the current function (i*,/v""”, where i” is
`the peak current for the anodic peak andvis the scan rate)
`According to this scheme, R represents thianthrene (FIG.
`obtained for the oxidative and reductive peaks seen in the
`1), or one of its derivatives such as 2,7-diacetyl thianthrene
`cyclic voltammogram for 2,7-diacetyl thianthrene, along
`(FIG.2). R is added to the electrolyte where it is available
`with the voltages for the anodic and cathodic peaks (“E, and
`for a reversible oxidation-reduction (redox) shuttle reaction.
`“E,, respectively),
`the peak width (A’E,,.), and the peak
`The electrolyte may be a liquid solution with a single
`separation (A*E,).
`solvent, such as propylene carbonate-1.0M LiPF,, or a
`The peak width and the peak separation for a one-electron
`mixed-solvent solution such as 50% ethylene carbon-
`reaction can be predicted from the Nernst equation accord-
`ate:50% propylene carbonate-1.0M LiPF,. Other embodi-
`ing to equations [3] and [4] below:
`ments of this invention may have the liquid electrolyte
`immobilized into a polymer such as poly(acrylonitrile)
`(PAN), poly(tetraethylene glycol diacrylate)
`(PEGDA),
`polyvinyl pyrrolidinone (PVP), poly(vinyl chloride), poly
`(vinyl sulfone), poly(ethylene oxide) or poly(vinylidene
`fluoride) (PVdF) as described in the aforementioned US.
`Patents pertaining to polymerelectrolytes.
`The protective redox shuttle reactionis initiated when the
`cathode reaches the oxidation potential of the redox reagent
`as in [1]. It proceeds with the diffusion of the oxidized
`species (R*) to the anode whereit is reformed as in[2].'The ;
`reaction is sustained by diffusion of the reformed reagent
`back to the cathode. Judicious selection of the particular
`thianthrene will allow the overcharge protection agent to be
`tailored to the appropriate cell voltage.
`The redox potential of prospective shuttle candidates is
`determined readily from cyclic voltammetry of glassy car-
`bon microelectrodes in electrolyte containing the compound
`of interest. For example, a solution of 10 mM thianthrene, in
`50% EC:50% PC-1.0M LiPEF,, was tested at a scan rate of
`100 mV/s. FIG. 3 shows a symmetrical wave which is
`characteristic of a reversible reaction with the peak oxida-
`tion current at 4.12V and the corresponding reduction
`(regeneration of the starting material) at 4.06V vs. Lit/1i.
`The redox potential of the insertion cathode determines
`the suitability of the protective agent. FIG. 4 showsthat Li
`is removed from LiMn,O, in two stages, with peaks cen-
`tered at 4.05V and at 4.17V vs.. Li*/Li. Consequently,
`
`Peak Width=A°E,.="E,~°E,p=2-20RT/nF=0.0565/n 25°C.
`
`Peak Separation=A°E,="F,-E,=2.22RT/nF=0.058/n 25°C,
`
`[3]
`
`[4]
`
`45
`
`‘TABLE 2
`
`Electrochemical Data for Cyclic Voltammetry of
`2.7-Diacetyl Thianthrene at Different Sweep Rates.
`
`Current
`Sweep
`Function
`Rate
`Vv yi
`(Vis)
`AKVIs)¥2
`0.001
`1.897 x 10+
`0.005
`1.824 x 10+
`0.010
`1.820 x 10+
`0.020
`1.789 x 10+
`0.050
`1.766 x 10+
`
`Peak
`Cathodic
`Anodic
`Peak Voltage Peak Voltage Width
`‘ED
`Ep
`ME»
`(Vv)
`(v)
`(Vv)
`4.255
`4.205
`0.055
`4.258
`4.200
`0.053
`4.260
`4.200
`0.055
`4.260
`4.200
`0.055
`4.260
`4.200
`0.055
`
`Peak
`Separation
`A
`(Vv)
`0.050
`0.058
`0.060
`0.060
`0.060
`
`The data in Table 2 were obtained from the cyclic
`voltammetry of 2,7-diacetyl thianthrene at sweep rates of 1
`mV/s to 50 mV/s, and they showclose agreement between
`the experimental and theoretical values for peak width and
`peak separation. This indicates that the redox of 2,7-diacetyl
`thianthrene is a reversible reaction involving a one-electron
`transfer. The constancy of the current function indicates that
`the redox reactions are diffusion controlled.
`WhenLi/LiMn,O,cells are cycled under constant current
`conditions, voltage profiles such as those shown in FIGS. 6A
`
`10
`
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`
`

`

`5,858,573
`
`5
`and 6B are obtained. In this instance, the upper charge limit
`was 4.2V for the first cycle and 4.3V for the second cycle.
`The peaks observed in the cyclic voltammogram (FIG. 4)
`correspondto the plateaus in FIGS. 6A and 6B. Increasing
`the upper limit from 4.2V to 4.3V vs. Lit/ Li does not
`increase the available capacity significantly, nor does it
`jeopardize the integrity of the clectrolytc. Tests with similar
`cells for which the upper charge limit was increased pro-
`gressively to 5V vs. Li*/li showed the electrolyte to be
`stable toward oxidation over this potential range.
`Proper matching of the overcharge protection additive
`(shuttle) to the electrochemical couple requires sufficient
`separation between the completion of lithium extraction
`from the cathode reaction and the onset of the shuttle
`
`activation. For this reason, an ideal match for the LiMn,O,
`cathode would probably involve a shuttle activated at poten-
`tials between 4.2 and 4.5V. This would ensure complete
`cathode utilization, yet be well within the voltage stability
`windowof the electrolyte.
`In preferred embodiments, the redox shuttle is thianthrene
`(FIG. 1) or a derivative of thianthrene such as 2,7-diacetyl
`thianthrene (FIG. 2), and the rechargeable cell is carbon/
`spinel LiMn,O,containing a nonaqueous liquid electrolyte
`such as ethylene carbonate/propylene carbonate-lithium salt,
`or a nonaqueous polymer electrolyte as described in US.
`Pat. Nos. 4,857,423; 5,219,413; 5,252,413; and 5,474,860.
`The anode maybe either disordered carbon or graphite. The
`disordered carbon may be one obtained from the pyrolysis of
`petroleum coke, and known to those skilled in the ficld as
`“petroleum coke’ or ‘coke’. The graphite may have the usual
`flake morphology, but may also be formed as graphite fibers
`or microtubules. The primary purpose of the acetyl func-
`tional groups of diacetyl thianthrene is to shift the redox
`potential to a more positive value. In a secondary role,
`functional groups may be added to promote solubility of the
`derivatized compound in the electrolyte. Other functional
`groups can be attached to thianthrene to either increase or
`decrease its redox potential. Electron withdrawing substitu-
`ents such as acetyl, nitro and chloro groups are expected to
`increase the oxidation potential while electron releasing
`substituents such as alkyl groups will decreasethis potential.
`We have discovered that thianthrene and its derivatives
`such as 2,7-diacetyl thianthrene are useful as overcharge
`protection additives for lithium or lithium-ion cells. The
`choice of a particular redox reagent will vary with the
`cathode material used in a rechargeable Li or lithium-ion
`cell. Thus, although the carbon/spinel LiMn,O, couple is
`mentioned specifically, other cathodes such as LiCoQ, or
`LiNiO, might also be used.
`Other
`features, objects and advantages will become
`apparent from the following specification when read in
`connection with the accompanying drawings which show
`the principle involved and the practical considerations in
`matching the cathode with the appropriate redox material.
`Cyclic voltammetry was used to screen candidates for use as
`redox shuttle reagents for overcharge protection in recharge-
`able Li or lithtum-ion cells. Results of such experiments are
`given in Table 1.
`The requirementof the selection of a compoundfor use as
`a redox shuttle reagent for a given positive electrode is that
`the oxidation potential of the shuttle reagent be slightly
`higher than the full charge limit of the cell.
`In Li or
`lithium-ion cells with high voltage cathodes such as spinel
`LiMn,0,, LiCoO, or LiNiO., oxidation of the redox reagent
`should take place after the full capacity of the cathode has
`been accessed. For LiMn,O,
`this means that the shuttle
`should be activated at a potential above 4.2V vs. Li*/Li.
`
`10
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`
`6
`Substitution of acetyl groups for hydrogen atoms at the
`2,7 positions in thianthrene resulted in a shift of the redox
`potential to values more positive than those obtained with
`thianthrene. The preferred electrolytes are resistive to oxi-
`dation in this range. In particular, liquid electrolytes with
`ethylene carbonate (EC), propylene carbonate (PC), dipro-
`pyl carbonate (DPC), methyl cthyl carbonate (MEC) and
`similar solvents or mixtures of solvents are known to those
`skilled in the art as desirable electrolytes for use with high
`voltage cathodes. Likewise, these solvents or mixtures of
`solvents and the solvates which they form with lithium salts
`such as LiAsF,, LiPF,, LiClO,, LiN(SO,CF;),, LiC
`(SO,CF;)3, LiBF,, andlithium salts of organic acids such as
`trichioroacetic, trifluoromethane sulfonic, and formic acids
`can be immobilized according to the teachings of the patents
`by Abraham etal. cited above.
`While the specific goal of this invention is the prevention
`of overcharge for cells incorporating high voltage cathodes,
`the stability of the compound (thianthrene derivative) and
`the other electrolyte components at lower voltages allowsits
`use with lower voltage cathodes also. Recent developments
`in the area of lithium-ion cells have included work with
`
`two-phase mixtures, notably Li,,,Mn,0, which includes
`both spinel (4V) LiMN.,O, and tetragonal (3V) Li,.Mn,QO,.
`During the initial activation of the cell,
`the 3V cathode
`capacity is available to compensate for irreversible anode
`capacity. This low-voltage capacity is sacrificial, and is
`exhausted at the first charge. Accordingly, to demonstrate
`the appropriateness of our invention with this most recent
`cathode development, the example showsthe usefulness of
`2,7-diacetyl
`thianthrene in the presence of T1,;Mn,O,,
`whichis a mixture of 0.5 mole of LiMn,O, and 0.5 mole of
`Li,Mn,0O,.
`The cell used to demonstrate the invention contained a
`Li,;Mn,0, cathode, a solid polymer electrolyte, and a
`carbon counter electrode. The cathode composition was
`93.5% Li,.sMn,0,:4% carbon black:2.5% PAN containing
`the liquid electrolyte 50% EC:50% PC-1.0M LiPF,, with 10
`mM 2,7-diacetyl thianthrene. The carbon electrode com-
`prised 97.5% petroleum coke:2.55 PAN andthe sameliquid
`electrolyte. The solid polymer electrolyte film used as a
`separator between the two electrodes had the composition
`13.87% PAN:38.63% EC:38.63% PC:7.93% LiPF,:0.93%
`2,7-diacetyl
`thianthrene. Here % stands for percent by
`weight.
`
`EXAMPLE1
`
`thianthrene (DAcTI]) was made by the
`2,7-diacetyl
`method cited earlier, and used to prepare a solid polymer
`electrolyte with the composition 13.87% PAN:38.63%
`EC:38.63% PC:0.93% DAcTH:7.93% LiPF,. This clectro-
`lyte was passed betweenrollers to form a thin (5 mil) film
`and used as a separator between the anode and cathode in a
`carbon/LiMn,O,cell. The electrodes in this cell were pre-
`pared by solvent casting from N-methylpyrollidinone. The
`cathode electrode was 93.5% LiMn,0,:4% carbon
`black:2.5% PAN, while the anode electrode was 97.5%
`petroleum coke:2.5% PAN. The current collector was alu-
`minum foil for the cathode and copper foil for the anode.
`Both electrodes were dried under vacuum at room tempera-
`ture for 2 hours, then hot-pressed at 130° C. and 5,000 psig
`for 2 min. The electrodes were trimmedto size, and a liquid
`electrolyte comprising 50% EC:50% PC-1.0M LiPF,, 10
`mM 2,7-diacetyl thianthrene was added to each electrode,
`with the excess allowed to drain away. The cell was
`assembled in a metallized plastic bag which washeat-sealed
`at its periphery to prevent air and moisture ingression.
`
`11
`
`11
`
`

`

`5,858,573
`
`7
`This cell was tested under cyclic voltammetric conditions
`as per the cell described in FIG. 4 (20 nV/s, 4.5V upper
`limit). FIG. 7 shows clearly the presence of the two peaks
`expected for the lithium extraction from LiMn,O, at 4.05
`(la, and its inverse peak 1c; where a represents the anodic
`peak and c represents the cathodic peak) and 4.17V (2a, 2c)
`and an additional reversible peak at 4.4V (3a, 3c). These last
`peaks corresponds the oxidation and reduction of acetyl
`thianthrene.
`We claim:
`
`1. An electric current producing, rechargeable, electro-
`chemicalcell having an anode, a cathode andanelectrolyte
`in contact with said anode and cathode, said electrolyte
`containing thianthrene or a derivative of thianthrene as a
`redox shuttle reagent to provide overcharge protection to the
`cell.
`
`2. A rechargeable cell of claim 1 wherein said anode
`comprises Li and said cathode comprises a compound
`selected from LiMn,O0,, LiCoO,, LiNiO,, LiV.,0;, LiVS.,
`LiCry5Vo.59>5, and mixtures thereof.
`3. A rechargeable cell of claim 1 wherein said anode
`comprises a Li insertion anode selected from carbon, metal,
`alloys, metal oxides, metal sulfides and metal nitrides, and
`said cathode comprises a compound selected trom
`LiMn,0,, LiCoO,, LiNiO.,,, LiV,0., LiCry .S,VS, or mix-
`tures thereof.
`
`
`
`8
`LiN(SO.CF;)5, LiC(SO,CF3)3, and mixtures thereof; said
`polymer host is selected from polyacrylonitrile, poly(vinyl
`chloride), poly(vinyl pyrrolidinone), poly(vinyl sulfone),
`poly(vinylidine fluoride) or
`its copolymers, and poly
`(ethylene oxide), and said plasticizer solvent consists of an
`organic solvent selected from ethylene carbonate, dipropyl
`carbonate, dimethyl carbonate, diethyl carbonate,
`butyrolactone, N-methyl pyrrolidinone, sulfolane and mix-
`tures thereof.
`
`10. Arechargeable cell of claim 4 wherein the thianthrene
`derivative is 2,7-diacetyl thianthrene.
`11. Arechargeable cell of claim 5 wherein the thianthrene
`derivative is 2,7-diacetyl thianthrene.
`12. Arechargeable cell of claim 8 wherein the thianthrene
`derivative is 2,7-diacelyl thianthrene.
`13. A rechargeable cell of claim 9 wherein the thianthrene
`derivative is 2,7-diacetyl thianthrene.
`14. Arechargeable cell according to claim 1 wherein said
`electrolyte comprises a Li salt dissolved in an organic
`solvent or
`a solvent mixture selected from ethers,
`carbonates, esters, sulfones, ketones and lactones.
`15. Arechargeable cell according to claim 1 wherein said
`electrolyte comprises a polymer electrolyte composed of a
`Li salt, a polymer host and a plasticizer solvent.
`16. A rechargeable cell according to claim 1 wherein the
`thianthrene derivative comprises 2,7-diacetyl thianthrene.
`17. Arechargeable cell according to claim 1 wherein said
`cathode comprises an oxideor sulfide containing compound.
`18. A method for providing a rechargeable, electrochemi-
`cal cell, comprising the steps of
`providing an anode anda cathode,
`providing an electrolyte in contact with said anode and
`said cathode; and
`dissolving within said electrolyte a thianthrene based
`compound for acting as a redox shuttle reagent
`to
`provide overcharge protection to the cell.
`19. Amethod for providing a rechargeable, electrochemi-
`cal cell according to claim 18 wherein the step of providing
`an electrolyte includes the step of providing a Li salt
`dissolved in an organic solvent or a solvent mixture selected
`from ethers, carbonates esters, sulfones, ketones and lac-
`tones.
`
`20. A method for providing a rechargeable, electrochemi-
`cal cell according to claim 18 wherein the step of providing
`an electrolyte includes the step of providing a polymer
`electrolyte comprised of a Li salt, a polymer host and a
`plasticizer solvent.
`21. A method for providing a rechargeable, electrochemi-
`cal cell according to claim 18 wherein the step of providing
`a thianthrene containing compound includes the step of
`providing 2,7-diacctyl thianthrenc.
`*
`% eR Ok
`
`10
`
`15
`
`30
`
`35
`
`40
`
`45
`
`50
`
`4. Arechargeable cell of claim 2 wherein said electrolyte
`comprises Li salt dissolved in an organic solventor a solvent
`mixture sclected from cthers, carbonates, esters, sulfones,
`ketones and lactones.
`
`5. Arechargeable cell of claim 3 wherein said electrolyte
`comprises Li salt dissolved in an organic solventor a solvent
`mixture selected from ethers, carbonates, esters, sulfones,
`ketones and lactones.
`6. Arechargeable cell of claim 2 wherein said electrolyte
`comprises a polymer electrolyte composed of a Li salt, a
`polymer host and a plasticizer solvent.
`7. Arechargeable cell of claim 3 wherein said electrolyte
`comprises a polymer electrolyte composed of a Li salt, a
`polymer host and plasticizer solvent.
`8. A rechargeable cell of claim 6 wherein said Lisalt is
`selected from LiPF,, LiAsF,;, LiClO,, LiBF,,
`LiN(SO,CF;),, LiC(SO,CF,);, and mixtures thereof; said
`polymer host is selected from polyacrylonitrile, poly(vinyl
`chloride), poly(vinyl pyrrolidinone), poly(vinyl sulfone),
`poly(vinylidine fluoride) or
`its copolymers, and poly
`(ethylene oxide) and said plasticizer solvent includes an
`organic solvent selected from ethylene carbonate, dipropyl
`carbonate, dimethyl carbonate, diethyl carbonate,
`butyrolactone, N-methyl pyrrolidinone, sulfolane and mix-
`tures thercof.
`
`9. A rechargeable cell of claim 7 wherein said Li salt is
`ected from LiPF,, LiAsF,;, LiClO.,, LiBF,,
`
` se
`
`12
`
`12
`
`