NITRITE ADDITIVES FOR NONAQUEOUS ELECTROLYTE RECHARGEABLE CELLS

ADDITIFS A BASE DE NITRITE POUR CELLULES ELECTROCHIMIQUES RECHARGEABLES A ELECTROLYTE NON-AQUEUX

English Abstract

A lithium ion electrochemical cell having high
charge/discharge capacity, long cycle life and
exhibiting a reduced first cycle irreversible capacity,
is described. The stated benefits are realized by the
addition of at least one nitrite additive to an
electrolyte comprising an alkali metal salt dissolved in
a solvent mixture that includes ethylene carbonate,
dimethyl carbonate, ethylmethyl carbonate and diethyl
carbonate. The preferred additive is an alkyl nitrite
compound.

Claims

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What is claimed is:

1. An electrochemical cell, which comprises:
a) a negative electrode which intercalates with an
alkali metal;
b) a positive electrode comprising an electrode active
material which intercalates with the alkali metal;
c) a nonaqueous electrolyte activating the negative and
the positive electrodes; and
d) a nitrite additive provided in the electrolyte,
wherein the nitrite additive has the formula:
(RO)N(=O), wherein R is an organic croup of either a
saturated hydrocarbon or heteroatom group containing
1 to 10 carbon atoms or an unsaturated hydrocarbon
or heteroatom group containing 2 to 10 carbon atoms.

2. The electrochemical cell of claim 1 wherein the nitrite
additive is selected from the group consisting of methy
nitrite, ethyl nitrite, propyl nitrite, isopropyl nitrite,
butyl nitrite, isobutyl nitrite, t-butyl nitrite, benzyl
nitrite, phenyl nitrite, and mixtures thereof.

3. The electrochemical cell of claim 1 wherein the nitrite
additive is present in the electrolyte in a range of about
0.001M to about 0.20M.

4. The electrochemical cell of claim 1 wherein the nitrite
additive is t-butyl nitrite present in the electrolyte at a
concentration up to about 0.05M.

5. The electrochemical cell of claim 1 wherein the
electrolyte includes a quaternary, nonaqueous carbonate
solvent mixture.



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6. The electrochemical cell of claim 1 wherein the
electrolyte comprises at least one linear carbonate selected
from the group consisting of dimethyl carbonate, diethyl
carbonate, dipropyl carbonate, ethylmethyl carbonate,
methylpropyl carbonate, ethylpropyl carbonate, and mixtures
thereof.

7. The electrochemical cell of claim 6 wherein the
electrolyte comprises at least three of the linear carbonates.

8. The electrochemical cell of claim 1 wherein the
electrolyte comprises at least one cyclic carbonate selected
from the group consisting of ethylene carbonate, propylene
carbonate, butylene carbonate, vinylene carbonate, and
mixtures thereof.

9. The electrochemical cell of claim 1 wherein the
electrolyte comprises ethylene carbonate, dimethyl carbonate,
ethylmethyl carbonate and diethyl carbonate.

10. The electrochemical cell of claim 9 wherein the ethylene
carbonate is in the range of about 10% to about 50%, the
dimethyl carbonate is in the range of about 5% to about 75%,
the ethylmethyl carbonate is in the range of about 5% to about
50%, and the diethyl carbonate is in the range of about 3% to
about 45%, by volume.

11. The electrochemical cell of claim 1 wherein the
electrolyte includes an alkali metal salt selected from the
group consisting of LiPF6, LiBF4, LiAsF6, LiSbF6, LiClO4,
LiAlCl4, LiGaCl4, LiNO3, LiC(SO2CF3)3, LiN(SO2CF3)2, LiSCN,
LiO3SCF2CF3, LiC6F5SO3, LiO2CCF3, LiSO3F, LiB(C6H5)4, LiCF3SO3, and
mixtures thereof.



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12. The electrochemical cell of claim 1 wherein the alkali
metal is lithium.

13. The electrochemical cell of claim 1 wherein the negative
electrode comprises a negative electrode active material
selected from the group consisting of coke, carbon black,
graphite, acetylene black, carbon fibers, glassy carbon, and
mixtures thereof.

14. The electrochemical cell of claim 1 wherein the negative
electrode active material is mixed with a fluoro-resin binder.

15. The electrochemical cell of claim 1 wherein the positive
electrode comprises a positive electrode active material
selected from the group consisting of lithiated oxides,
lithiated sulfides, lithiated selenides and lithiated
tellurides of the group selected from vanadium, titanium,
chromium, copper, molybdenum, niobium, iron, nickel, cobalt,
manganese, and mixtures thereof.

16. The electrochemical cell of claim 15 wherein the positive
electrode active material is mixed with a fluoro-resin binder.

17. The electrochemical cell of claim 15 wherein the positive
electrode active material is mixed with a conductive additive
selected from the group consisting of acetylene black, carbon
black, graphite, nickel powder, aluminum powder, titanium
powder, stainless steel powder, and mixtures thereof.

18. An electrochemical cell, which comprises:
a) a negative electrode which intercalates with
lithium;
b) a positive electrode comprising an electrode active
material and which intercalates with lithium; and
c) an electrolyte solution activating the anode and the


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cathode, the electrolyte including an alkali metal
salt dissolved in a quaternary, nonaqueous carbonate
solvent mixture of ethylene carbonate, dimethyl
carbonate, ethylmethyl carbonate and diethyl
carbonate; and
d) a nitrite additive provided in the electrolyte.

19. The electrochemical cell of claim 18 wherein the nitrite
additive has the formula: (RO)N(=O), wherein R is an organic
group of either a saturated or unsaturated hydrocarbon or
heteroatom group containing 1 to 10 carbon atoms.

20. The electrochemical cell of claim 18 wherein the nitrite
additive is selected from the group consisting of methyl
nitrite, ethyl nitrite, propyl nitrite, isopropyl nitrite,
butyl nitrite, isobutyl nitrite, t-butyl nitrite, benzyl
nitrite, phenyl nitrite, and mixtures thereof.

21. The electrochemical cell. of claim 18 wherein the ethylene
carbonate is in the range of about 10% to about 50%, the
dimethyl carbonate is in the range of about 5% to about 75%,
the ethylmethyl carbonate is in the range of about 5% to about
50%, and the diethyl carbonate is in the range of about 3% to
about 45%, by volume.

22. The electrochemical cell of claim 18 wherein the
electrolyte includes an alkali metal salt selected from the
group consisting of LiPF6, LiBF4, LiAsF6, LiSbF6, LiClO4,
LiAlCl4 LiGaCl4, LiNO3, LiC(SO2CF3)3, LiN(SO2CF3)2, LiSCN,
LiO3SCF2CF3, LiC6F5SO3, LiO2CCF3, LiSO3F, LiB(C6H5)4, LiCF3SO3, and
mixtures thereof.

23. An electrochemical cell, which comprises:
a) an anode of a carbonaceous material capable of
intercalating lithium;


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b) a cathode comprising lithium cobalt oxide; and
c) a nonaqueous electrolyte activating the anode and
the cathode, the nonaqueous electrolyte comprising a
nitrite additive that provides lithium nitrite or
the lithium salt of a nitrite reduction product on a
surface of the lithium intercalated anode in contact
with the electrolyte.

24. A method for providing an electrochemical cell,
comprising the steps of:
a) providing a negative electrode which intercalates
with an alkali metal;
b) providing a positive electrode comprising an
electrode active material which intercalates with
the alkali metal;
c) activating the negative and positive electrodes with
a nonaqueous electrolyte; and
d) providing a nitrite additive in the electrolytes
wherein the nitrite additive has the formula:
(RO)N(=O), wherein R is an organic group of either a
saturated hydrocarbon or heteroatom group containing
1 to 10 carbon atoms or an unsaturated hydrocarbon
or heteroatom group containing 2 to 10 carbon atoms.

25. The method of claim 24 including selecting the nitrite
additive from the group consisting of methyl nitrite, ethyl
nitrite, propyl nitrite, isopropyl nitrite, butyl nitrite,
isobutyl nitrite, t-butyl nitrite, benzyl nitrite and phenyl
nitrite, and mixtures thereof.

26. The method of claim 24 wherein the nitrite additive is
present in the electrolyte in a range of about 0.001M to about
0.20M.

27. The method of claim 24 wherein the nitrite additive is


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t-butyl nitrite present in the electrolyte at a concentration
up to about 0.05M.

28. The method of claim 24 including providing the
electrolyte comprising a quaternary, nonaqueous carbonate
solvent mixture.

29. The method of claim 24 wherein the electrolyte comprises
at least one linear carbonate selected from the group
consisting of dimethyl carbonate, diethyl carbonate, dipropyl
carbonate, ethylmethyl carbonate, methylpropyl carbonate and
ethylpropyl carbonate, and mixtures thereof.

30. The method of claim 29 wherein the electrolyte comprises
at least three of the linear carbonates.

31. The method of claim 24 wherein the electrolyte comprises
at least one cyclic carbonate selected from the group
consisting of ethylene carbonate, propylene carbonate,
butylene carbonate, vinylene carbonate, and mixtures thereof.

32. The method of claim 24 wherein the electrolyte comprises
ethylene carbonate, dimethyl carbonate, ethylmethyl carbonate
and diethyl carbonate.

33. The method of claim 32 wherein the ethylene carbonate is
in the range of about 10% to about 50%, the dimethyl carbonate
is in the range of about 5% to about 75%, the ethylmethyl
carbonate is in the range of about 5% to about 50%, and the
diethyl carbonate is in the range of about 3% to about 45%, by
volume.

34. The method of claim 24 wherein the electrolyte includes
an alkali metal salt selected from the group consisting of
LiPF6, LiBF4, LiAsF6, LiSbF6, LiClO4, LiAlCl4, LiGaCl4, LiNO3,



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LiC(SO2CF3)3, LiN(SO2CF3)2, LiSCN, LiO3SCF2CF3, LiC6F5SO3, LiO2CCF3,
LiSO3F, LiB(C6H5)4, LiCF3SO3, and mixtures thereof.
35. The method of claim 24 wherein the alkali metal is
lithium.
36. The method of claim 24 including providing the positive
electrode comprising a positive electrode active material
selected from the group consisting of lithiated oxides,
lithiated sulfides, lithiated selenides and lithiated
tellurides of the group selected from vanadium, titanium,
chromium, copper, molybdenum, niobium, iron, nickel, cobalt,
manganese, and mixtures thereof.
37. The method of claim 24 including providing the negative
electrode comprising a negative electrode active material
selected from the group consisting of coke, carbon black,
graphite, acetylene black, carbon fibers, glassy carbon, and
mixtures thereof.
38. An electrochemical cell, which comprises:
a) a negative electrode which intercalates with an
alkali metal;
b) a positive electrode comprising an electrode active
material which intercalates with the alkali metal;
c) a nonaqueous electrolyte activating the negative and
the positive electrodes; nrd
d) a nitrite additive selected from the group
consisting of methy nitrite, ethyl nitrite, propyl
nitrite, isopropyl nitrite, butyl nitrite, isobutyl
nitrite, t-butyl nitrite, benzyl nitrite, phenyl
nitrite, and mixtures thereof provided in the
electrolyte.



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39. The electrochemical cell of claim 38 wherein the nitrite
additive is t-butyl nitrite present in the electrolyte at a
concentration up to about 0.05M.
40. An electrochemical cell, which comprises:
a) a negative electrode which intercalates with an
alkali metal;
b) a positive electrode comprising an electrode active
material which intercalates with the alkali metal;
c) a nonaqueous electrolyte including a quaternary,
nonaqueous carbonate solvent mixture activating the
negative and the positive electrodes; and
d) a nitrite additive provided in the electrolyte.
41. The electrochemical cell of claim 40 wherein the
electrolyte comprises at least one linear carbonate selected
from the group consisting of dimethyl carbonate, diethyl
carbonate, dipropyl carbonate, ethylmethyl carbonate,
methylpropyl carbonate, ethylpropyl carbonate, and mixtures
thereof.
42. The electrochemical cell of claim 41 wherein the
electrolyte comprises at least three of the linear carbonates.
43. The electrochemical cell of claim 40 wherein the
electrolyte comprises at least one cyclic carbonate selected
from the group consisting of ethylene carbonate, propylene
carbonate, butylene carbonate, vinylene carbonate, and
mixtures thereof.
44. The electrochemical cell of claim 40 wherein the
electrolyte comprises ethylene carbonate, dimethyl carbonate,
ethylmethyl carbonate and diethyl carbonate.



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45. The electrochemical cell of claim 44 wherein the ethylene
carbonate is in the range of about 10% to about 50%, the
dimethyl carbonate is in the range of about 5% to about 75%,
the ethylmethyl carbonate is in the range of about 5% to about
50%, and the diethyl carbonate is in the range of about 3% to
about 45%, by volume.
46. The electrochemical cell of claim 40 wherein the
electrolyte includes an alkali metal salt selected from the
group consisting of LiPF6, LiBF4, LiAsF6, LiSbF6, LiClO4,
LiAlCl4, LiGaCl4, LiNO3, LiC (SO2CF3)3, LiN(SO2CF3)2, LiSCN,
LiO3SCF2CF3, LiC6F5SO3, LiO2CCF3, LiSO3F, LiB (C6H5)4, LiCF3SO3, and
mixtures thereof.
47. The electrochemical cell of claim 40 wherein the alkali
metal is lithium.
48. An electrochemical cell, which comprises:
a) a negative electrode which intercalate with an
alkali metal, wherein the negative electrode
comprises a negative electrode active material
selected from the group consisting of coke, carbon
black, graphite, acetylene black, carbon fibers,
glassy carbon, and mixtures thereof;
b) a positive electrode comprising a positive electrode
active material which intercalates with the alkali
metal;
c) a nonaqueous electrolyte activating the negative and
the positive electrodes; and
d) a nitrite additive provided in the electrolyte.
49. The electrochemical cell of claim 48 wherein the negative
electrode active material is mixed with a fluoro-resin binder.



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50. An electrochemical cell, which comprises:
a) a negative electrode which intercalate with an
alkali metal;
b) a positive electrode comprising a positive electrode
active material which intercalates with the alkali
metal, wherein the positive electrode active
material is selected from the group consisting of
lithiated oxides, lithiated sulfides, lithiated
selenides and lithiated tellurides of any of the
group selected from vanadium, titanium, chromium,
copper, molybdenum, niobium, iron, nickel, cobalt,
manganese, and mixtures thereof;
c) a nonaqueous electrolyte activating the negative and
the positive electrodes; and
d) a nitrite additive provided in the electrolyte.
51. The electrochemical cell of claim 50 wherein the positive
electrode active material is mixed with a fluoro-resin binder.
52. The electrochemical cell of claim 50 wherein the positive
electrode active material is mixed with a conductive additive
selected from the group consisting of acetylene black, carbon
black, graphite, nickel powder, aluminum powder, titanium
powder, stainless steel powder, and mixtures thereof.
53. A method for providing an electrochemical cell,
comprising the steps of:
a) providing a negative electrode which intercalates
with an alkali metal;
b) providing a positive electrode comprising an
electrode active material which intercalates with
the alkali metal;
c) activating the negative and positive electrodes with
a nonaqueous electrolyte; and
d) providing a nitrite additive in the electrolyte,



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wherein the nitrite additive is selected from the
group consisting of methyl nitrite, ethyl nitrite,
propyl nitrite, isopropyl nitrite, butyl nitrite,
isobutyl nitrite, t-butyl nitrite, benzyl nitrite,
phenyl nitrite, and mixtures thereof.

54. The method of claim 53 wherein the nitrite additive is
t-butyl nitrite present in the electrolyte at a concentration
up to about 0.05M.

55. A method for providing an electrochemical cell,
comprising the steps of:
a) providing a negative electrode which intercalates
with an alkali metal;
b) providing a positive electrode comprising an
electrode active material which intercalates with
the alkali metal;
c) activating the negative and positive electrodes with
a nonaqueous electrolyte including a quaternary,
nonaqueous carbonate solvent mixture; and
d) providing a nitrite additive in the electrolyte.

56. The method of claim 55 wherein the electrolyte comprises
at least one linear carbonate selected from the group
consisting of dimethyl carbonate, diethyl carbonate, dipropyl
carbonate, ethylmethyl carbonate, methylpropyl carbonate,
ethylpropyl carbonate, and mixtures thereof.

57. The method of claim 56 wherein the electrolyte comprises
at least three of the linear carbonates.

58. The method of claim 55 wherein the electrolyte comprises
at least one cyclic carbonate selected from the group
consisting of ethylene carbonate, propylene carbonate,
butylene carbonate, vinylene carbonate, and mixtures thereof.


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59. The method of claim 55 wherein the electrolyte comprises
ethylene carbonate, dimethyl carbonate, ethylmethyl carbonate
and diethyl carbonate.
60. The method of claim 59 wherein the ethylene carbonate is
in the range of about 10% to about 50%, the dimethyl carbonate
is in the range of about 5% to about 75%, the ethylmethyl
carbonate is in the range of about 5% to about 50%, and the
diethyl carbonate is in the range of about 3% to about 45%, by
volume.
61. The method of claim 55 wherein the electrolyte includes
an alkali metal salt selected from the group consisting of
LiPF6, LiBF4, LiAsF6, LiSbF6, LiClO4, LiAlCl4, LiGaCl4, LiNO3,
LiC(SO2CF3)3, LiN(SO2CF3)2, LiSCN, LiO3SCF2CF3, LiC6F5SO3, LiO2CCF3,
LiSO3F, LiB(C6H5)4, LiCF3SO3, and mixtures thereof.
62. A method for providing an electrochemical cell,
comprising the steps of:
a) providing a negative electrode which intercalates
with an alkali metal;
b) providing a positive electrode comprising a positive
electrode active material which intercalates with
the alkali metal and including selecting the
positive electrode active material from the group
consisting of lithiated oxides, lithiated sulfides,
lithiated selenides and lithiated tellurides of any
of the group selected from vanadium, titanium,
chromium, copper, molybdenum, niobium, iron, nickel,
cobalt, manganese, and mixtures thereof;
c) activating the negative and positive electrodes with
a nonaqueous electrolyte; and
d) providing a nitrite additive in the electrolyte.


-34-
63. A method for providing an electrochemical cell,
comprising the steps of:
a) providing a negative electrode comprising a negative
electrode active material which intercalates with an
alkali metal, and including selecting the negative
electrode active material from the group consisting
of coke, carbon black, graphite, acetylene black,
carbon fibers, glassy carbon, and mixtures thereof;
b) providing a positive electrode comprising an
electrode active material which intercalates with
the alkali metal;
c) activating the negative and positive electrodes with
a nonaqueous electrolyte; and
d) providing a nitrite additive in the electrolyte.

Description

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NITRITE ADDITIVES FOR NONAQUEOUS
ELECTROLYTE RECHARGEABLE ELECTROCHEMICAL CELLS
BACKGROUND OF INVENTION
The present invention generally relates to an
electrochemical cell, and more particularly, to a
rechargeable lithium ion cell. Still more particularly,
the present invention relates to a lithium ion
electrochemical cell activated with an electrolyte
having an additive provided to achieve high
charge/discharge capacity, long cycle life and to
minimize the first cycle irreversible capacity.
According to the present invention, the preferred
additive to the activating electrolyte is a nitrite
compound.
Lithium ion rechargeable cells typically comprise a
carbonaceous anode electrode and a lithiated cathode
electrode. Due to the high potential of the cathode
material (up to 4.3V vs. Li/Li' for Lil_YCoOz) and the low
potential of the carbonaceous anode material (0.01V vs.
Li/Li' for graphite) in a fully charged lithium ion cell,
the choice of the electrolyte solvent system is limited.
Since carbonate solvents have high oxidative stability
toward typically used lithiated cathode materials and
good kinetic stability toward carbonaceous anode

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materials, they are generally used in lithium ion cell
electrolytes. To achieve optimum cell performance (high
rate capability and long cycle life), solvent systems
containing a mixture of a cyclic carbonate (high
dielectric constant solvent) and a linear carbonate (low
viscosity solvent) are typically used in commercial
secondary cells. Cells with carbonate based electrolytes
are known to deliver more than 1,000 charge/discharge
cycles at room temperature.
U.S. Patent No. 6,153,338 is directed to a
quarternary mixture of organic carbonate solvents in the
activating electrolyte for a lithium ion cell capable of
discharge at temperatures below -20°C and down to as low
as -40°C while exhibiting good cycling characteristics.
The quaternary solvent system includes ethylene carbonate
(EC), dimethyl carbonate (DMC), ethylmethyl carbonate
(EMC) and diethyl carbonate (DEC).
Lithium ion cell design generally involves a trade
off in one area for a necessary improvement in another,
depending on the targeted cell application. The
achievement of a lithium ion cell capable of low
temperature cycleability by use of the above quaternary
solvent electrolyte in place of a typically used binary
solvent electrolyte (such as 1.0M LiPFs/EC:DMC=30:70, v/v
which freezes at -11°C) is obtaiend at the expense of
increased first cycle irreversible capacity during the
initial charging (approximately 65 mAh/g graphite for
1.0M LiPFs/EC:DMC:EMC:DEC=45:22:24.8:8.2 vs. 35 mAh/g
graphite for 1.0M LiPFs/EC:DMC=30:70). Due to the
existence of this first cycle irreversible capacity,
lithium ion cells are generally cathode limited. Since

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all of the lithium ions, which shuttle between the anode
and the cathode during charging and discharging
originally come from the lithiated cathode, the larger
the first cycle irreversible capacity, the lower the
cell capacity in subsequent cycles and the lower the
cell efficiency. Thus, it is desirable to minimize or
even eliminate the first cycle irreversible capacity in
lithium ion cells while at the same time maintaining the
low temperature cycling capability of such cells.
According to the present invention, these
objectives are achieved by providing an organic nitrite
in the quaternary solvent electrolyte. Lithium ion
cells activated with these electrolytes exhibit lower
first cycle irreversible capacities relative to cells
activated with the same quaternary solvent electrolyte
devoid of the nitrite additive. As a result, cells
including the nitrite additive present higher subsequent
cycling capacity than control cells. The cycleability
of the present invention cells at room temperature, as
well as at low temperatures, i.e., down to about -40~C,
is as good as cells activated with the quaternary
electrolyte devoid of a nitrite additive.
SUMMARY OF THE INVENTION
It is commonly known that when an electrical
potential is initially applied to lithium ion cells
constructed with a carbon anode in a discharged
condition to charge the cell, some permanent capacity
loss occurs due to the anode surface passivation film
formation. This permanent capacity loss is called first
cycle irreversible capacity. The film formation
process, however, is highly dependent on the reactivity
of the electrolyte components at the cell charging

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04645.0615
potentials. The electrochemical properties of the
passivation film are also dependent on the chemical
composition of the surface film.
The formation of a surface film is unavoidable for
alkali metal systems, and in particular, lithium metal
anodes, and lithium intercalated carbon anodes due to
the relatively low potential and high reactivity of
lithium toward organic electrolytes. The ideal surface
film, known as the solid-electrolyte interphase (SEI),
should be electrically insulating and sonically
conducting. While most alkali metal, and in particular,
lithium electrochemical systems meet the first
requirement, the second requirement is difficult to
achieve. The resistance of these films is not
negligible, and as a result, impedance builds up inside
the cell due to this surface layer formation which
induces unacceptable polarization during the charge and
discharge of the lithium ion cell. On the other hand,
if the SEI film is electrically conductive, the
electrolyte decomposition reaction on the anode surface
does not stop due to the low potential of the lithiated
carbon electrode.
Hence, the composition of the electrolyte has a
significant influence on the discharge efficiency of
alkali metal systems, and particularly the permanent
capacity loss in secondary cells. For example, when
1. OM LiPF6/EC:DMC=30:70 is used to activate a secondary
lithium cell, the first cycle irreversible capacity is
approximately 35 mAh/g of graphite. However, under the
same cycling conditions, the first cycle irreversible
capacity is found to be approximately 65 mAh/g of
graphite when 1.0M LiPF6/EC:DMC:EMC:DEC=45:22:24.8:8.2 is
used as the electrolyte. Further, lithium ion cells

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activated with the binary solvent electrolyte of ethylene
carbonate and dimethyl carbonate cannot be cycled at
temperatures less than about -11°C. The quaternary
solvent electrolyte of U.S. Patent No. 6,153,338, which
enables lithium ion cells to cycle at much lower
temperatures, is a compromise in terms of providing a
wider temperature application with acceptable cycling
efficiencies. It would be highly desirable to retain the
benefits of a lithium ion cell capable of operating at
temperatures down to as low as about -40°C while
minimizing the first cycle irreversible capacity.
According to the present invention, this objective is
achieved by adding a nitrite additive in the above
described quaternary solvent electrolytes. In addition,
this invention may be generalized to other nonaqueous
organic electrolyte systems, such as binary solvent and
ternary solvent systems, as well as the electrolyte
systems containing solvents other than mixtures of linear
or cyclic carbonates. For example, linear or cyclic
ethers or esters may also be included as electrolyte
components. Although the exact reason for the observed
improvement is not clear, it is hypothesized that the
nitrite additive competes with the existing electrolyte
components to react on the carbon anode surface during
initial lithiation to form a beneficial SEI film. The
thusly formed SEI film is electrically more insulating
than the film formed without the nitrite additive and, as
a consequence, the lithiated carbon electrode is better
protected from reactions with other electrolyte
components. Therefore, lower first cycle irreversible
capacity is obtained.

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These and other objects of the present invention
will become increasingly more apparent to those skilled
in the art by reference to the following description and
to the appended drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a graph showing the averaged discharge
capacity through twenty cycles for two groups of lithium
ion cells, one group activated with a quaternary
carbonate solvent mixture devoid of a nitrite additive
in comparison to a similarly constructed cell group
having the nitrite electrolyte additive.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A secondary electrochemical cell constructed
according to the present invention includes an anode
active material selected from Groups IA, IIA, or IIIB of
the Periodic Table of Elements, including the alkali
metals lithium, sodium, potassium, etc. The preferred
anode active material comprises lithium.
In secondary electrochemical systems, the anode
electrode comprises a material capable of intercalating
and de-intercalating the alkali metal, and preferably
lithium. A carbonaceous anode comprising any of the
various forms of carbon (e. g., coke, graphite, acetylene
black, carbon black, glassy carbon, etc.) which are
capable of reversibly retaining the lithium species, is
preferred. Graphite is particularly preferred due to
its relatively high lithium-retention capacity.
Regardless of the form of the carbon, fibers of the
carbonaceous material are particularly advantageous
because the fibers have excellent mechanical properties
which permit them to be fabricated into rigid electrodes

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that are capable of withstanding degradation during
repeated charge/discharge cycling. Moreover, the high
surface area of carbon fibers allows for rapid
charge/discharge rates. A preferred carbonaceous material
for the anode of a secondary electrochemical cell is
described in U.S. Patent No. 5,443,928.
A typical secondary cell anode is fabricated by
mixing about 90 to 97 weight percent graphite with about 3
to 10 weight percent of a binder material which is
preferably a fluoro-resin powder such as
polytetafluoroethylene (PTFE), polyvinylidene fluoride
(PVDF), polyethylenetetrafluoroeth~rlene (ETFE), polyamides
and polyimides, and mixtures thereof. This electrode
active admixture is provided on a current collector such
as of a nickel, stainless steel, or copper foil or screen
by casting, pressing, rolling or otherwise contacting the
active admixture thereto.
The anode component further has an extended tab or
lead of the same material as the anode current collector,
i.e., preferably nickel, integrally formed therewith such
as by welding and contacted by a weld to a cell case of
conductive metal in a case-negative electrical
configuration. Alternatively, the carbonaceous anode may
be formed in some other geometry, such as a bobbin shape,
cylinder or pellet to allow an alternate low surface cell
design.
The cathode of a secondary cell preferably comprises
a lithiated material that is stable in air and readily
handled. Examples of such air-stable lithiated cathode
materials include oxides, sulfides, selenides,

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and tellurides of such metals as vanadium, titanium,
chromium, copper, molybdenum, niobium, iron, nickel,
cobalt and manganese. The more preferred oxides include
LiNiO~, LiMn~O~, LiCo02, LiCoo.~~Sno,oe0, and LiCol_::Ni.,O~.
Before fabrication into an electrode for
incorporation into an electrochemical cell, the
lithiated active material is preferably mixed with a
conductive additive. Suitable conductive additives
include acetylene black, carbon black and/or graphite.
Metals such as nickel, aluminum, titanium and stainless
steel in powder form are also useful as conductive
diluents when mixed with the above listed active
materials. The electrode further comprises a fluoro-
resin binder, preferably in a powder form, such as PTFE,
PVDF, ETFE, polyamides and polyimides, and mixtures
thereof.
To discharge such secondary cells, the lithium ion
comprising the cathode is intercalated into the
carbonaceous anode by applying an externally generated
electrical potential to recharge the cell. The applied
recharging electrical potential serves to draw the
alkali metal ions from the cathode material, through the
electrolyte and into the carbonaceous anode to saturate
the carbon comprising the anode. The resulting Li~C6
electrode can have an x ranging between 0.1 and 1Ø
The cell is then provided with an electrical potential
and is discharged in a normal manner.
An alternate secondary cell construction comprises
intercalating the carbonaceous material with the active
alkali material before the anode is incorporated into
the cell. In this case, the cathode body can be solid
and comprise, but not be limited to, such materials as
manganese dioxide, silver vanadium oxide, copper silver

CA 02298417 2000-02-14
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vanadium oxide, titanium disulfide, copper oxide, copper
sulfide, iron sulfide, iron disulfide and fluorinated
carbon. However, this approach is compromised by the
problems associated with handling lithiated carbon
outside of the cell. Lithiated carbon tends to react
when contacted by air.
The secondary cell of the present invention
includes a separator to provide physical segregation
between the anode and cathode active electrodes. The
separator is of an electrically insulative material to
prevent an internal electrical short circuit between the
electrodes, and the separator material also is
chemically unreactive with the anode and cathode active
materials and both chemically unreactive with and
insoluble in the electrolyte. In addition, the
separator material has a degree of porosity sufficient
to allow flow therethrough of the electrolyte during the
eiecLrochemicai reaction oz the cell. 'she iorm of the
separator typically is a sheet which is placed between
the anode and cathode electrodes. Such is the case when
the anode is folded in a serpentine-like structure with
a plurality of cathode plates disposed intermediate the
anode folds and received in a cell casing or when the
electrode combination is rolled or otherwise formed into
a cylindrical "jellyroll" configuration.
Illustrative separator materials include fabrics
woven from fluoropolymeric fibers of
polyethylenetetrafluoroethylene and
polyethylenechlorotrifluoroethylene used either alone or
laminated with a fluoropolymeric microporous film.
Other suitable separator materials include non-woven
glass, polypropylene, polyethylene, glass fiber
materials, ceramics, a polytetraflouroethylene membrane

CA 02298417 2003-09-12
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*
commercially available under the designation ZITEX
(Chemplast Inc.), a polypropylene membrane commercially
available under the designation CELGARD (Celanese
Plastic Company, Inc.) and a membrane commercially
S available under the designation DEXIGLAS (C. H. Dexter,
Div., Dexter Corp.).
The choice of an electrolyte solvent system for
activating an alkali metal electrochemical cell, and
particularly a fully charged lithium ion cell is very
limited due to the high potential of the cathode
material (up to 4.3V vs. Li/Li' for Lil__tCoO,) and the low
potential of the anode material (0.01V vs. Li/Li' for
graphite). According to the present invention, suitable
nonaqueous electrolytes are comprised of an inorganic
salt dissolved in a nonaqueous solvent and more
preferably an alkali metal salt dissolved in a
quaternary mixture of organic carbonate solvents
comprising dialkyl (non-cyclic) carbonates selected from
dimethyl carbonate (DMC), diethyl carbonate (DEC),
dipropyl carbonate (DPC), ethylmethyl carbonate (EMC),
methylpropyl carbonate (MPC) and ethylpropyl carbonate
(EPC), and mixtures thereof, and at least one cyclic
carbonate selected from propylene carbonate (PC),
ethylene carbonate (EC), butylene carbonate (BC) and
vinylene carbonate (VC), and mixtures thereof. Organic
carbonates are generally used in the electrolyte solvent
system for such battery chemistries because they exhibit
high oxidative stability toward cathode materials and
good kinetic stability toward anode materials.
Preferred electrolytes according to the present
invention comprise solvent mixtures of EC:DMC:EMC:DEC.
Most preferred volume percent ranges for the various
carbonate solvents include EC in the range of about l00
*Trade-mark

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to about 50°s; DMC in the range of about 5% to about 750;
EMC in the range of about So to about 500; and DEC in
the range of about 3~ to about 45°s. Electrolytes
containing this quaternary carbonate mixture exhibit
freezing points below -50oC, and lithium ion cells
activated with such mixtures have very good cycling
behavior at room temperature as well as very good
discharge and charge/discharge cycling behavior at
temperatures below -20~C.
Known lithium salts that are useful as a vehicle
for transport of alkali metal ions from the anode to the
cathode, and back again include LiPF6, LiBF4, LiAsF6,
LiSbF6, LiClO~, LiA1C14, LiGaCl;, LiC (SO,CF;) 3, LiN03,
LiN (SOZCF3) 2, LiSCN, Li03SCF,CF3, LiC6F;S0" LiO,CCF3,
LiS03F, LiB (C6H~ ) a and LiCF3S03, and mixtures thereof .
Suitable salt concentrations typically range between
about 0.8 to 1.5 molar.
i:i cW:~:J~i.~dTiv,c witCi ~iiE: ~r'C~CrW iiiJ~liW i~li, a~ icaW
one organic nitrite additive, preferably an alkyl
nitrite compound is provided as a co-solvent in the
electrolyte solution of the previously described alkali
metal ion or rechargeable electrochemical cell. The
nitrite additive is preferably an alkyl nitrite compound
having the general formula (RO)N(=O) wherein R is an
organic group of either a saturated or unsaturated
hydrocarbon or heteroatom substituted saturated or
unsaturated organic group containing 1 to 10 carbon
atoms. The greatest effect is found when methyl
nitrite, ethyl nitrite, propyl nitrite, isopropyl
nitrite, butyl nitrite, isobutyl nitrite, t-butyl
nitrite, benzyl nitrite and phenyl nitrite, and mixtures
thereof are used as additives in the electrolyte.

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The above described compounds are only intended to
be exemplary of those that are useful with the present
invention, and are not to be construed as limiting.
Those skilled in the art will readily recognize nitrite
compounds which come under the purview of the general
formula set forth above and which will be useful as
additives for the electrolyte to achieve high
charge/discharge capacity, long cycle life and to
minimize the first cycle irreversible capacity according
to the present invention.
While not intending to be bound by any particular
mechanism, it is believed that due to the presence of
the N=O bond in the nitrite functional group, the bond
between oxygen and the R group is severed and the
nitrite intermediate is able to compete effectively with
the other electrolyte solvents or solutes to react with
lithium and form a nitrite salt, i.e., lithium nitrite,
~r ehe lithium sail o= a nitrite reduction product on
the surface of the carbonaceous anode. The resulting
SEI layer is ionically more conductive than the SEI
layer which may form in the absence of the organic
nitrite additive. As a consequence, the chemical
composition and perhaps the morphology of the
carbonaceous anode surface protective layer is believed
to be changed with concomitant benefits to the cell's
cycling characteristics.
The assembly of the cell described herein is
preferably in the form of a wound element cell. That
is, the fabricated cathode, anode and separator are
wound together in a "jellyroll" type configuration or
"wound element cell stack" such that the anode is on the
outside of the roll to make electrical contact with the
cell case in a case-negative configuration. Using

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suitable top and bottom insulators, the wound cell stack
is inserted into a metallic case of a suitable size
dimension. The metallic case may comprise materials
such as stainless steel, mild steel, nickel-plated mild
steel, titanium or aluminum, but not limited thereto, so
long as the metallic material is compatible for use with
components of the cell.
The cell header comprises a,metallic disc-shaped
body with a first hole to accommodate a glass-to-metal
seal/terminal pin feedthrough and a second hole for
electrolyte filling. The glass used is of a corrosion
resistant type having up to about 50~ by weight silicon
* * * *
such as CABAL 12, TA 23, FUSITE 425 or FUSITE 435. The
positive terminal pin feedthrough preferably comprises
titanium although molybdenum, aluminum, nickel alloy, or
stainless steel can also be used. The cell header
comprises elements having compatibility with the other
components of the electrochemical cell and is resistant
to corrosion. The cathode lead is welded to the
positive terminal pin in the glass-to-metal seal and the
header is welded to the case containing the electr~,de
stack. The cell is thereafter filled with the
electrolyte solution comprising at least one of the
nitrite additives described hereinabove and hermetically
sealed such as by close-welding a stainless steel ball
over the fill hole, but not limited thereto.
The above assembly describes a case-negative cell,
which is the preferred construction of the exemplary
cell of the present invention. As is well known to
those skilled in the art, the exemplary electrochemical
system of the present invention can also be constructed
in a case-positive configuration.
*Trade-mark

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The following examples describe the manner and
process of an electrochemical cell according to the
present invention, and set forth the best mode
contemplated by the inventors of carrying out the
invention, but are not construed as limiting.
EXAMPLE I
Five lithium ion cells were constructed as test
vehicles. The cells were divided into two groups. One
group of three cells was activated with a quaternary
carbonate solvent system electrolyte devoid of a nitrite
additive while the remaining two cells had the same
electrolyte but including the nitrite additive. Except
for the electrolyte, the cells were the same. In
particular, the cathode was prepared by casting a LiCo02
cathode mix on aluminum foil. The cathode mix contained
f1 'I o r ' n , ~ 0 1. - i ~) _n : .v .-7 '~ o rvc m r-~ L ' ..t ~ t
i1'b LlvVOz, CJO l~iG~llilu.C d~..ul:.1'v~ QlllA J-O L VL1. iJ1111AC1, 1/y
weight. The anode was prepared by casting an anode mix
containing 91.7s graphite and 8.3a PVDF binder, by
weight, on a copper foil. An electrode assembly was
constructed by placing one layer of polyethylene
separator between the cathode and the anode and spirally
winding~the electrodes to fit into an AA sized
cylindrical stainless steel can. The cells were
activated with an electrolyte of
EC:DMC:EMC:DEC=45:22:24.8:8.2 having 1. OM LiPF6 dissolved
therein (group 1). The group 2 cells fabricated
according to the present invention further had 0.05M
t-butyl nitrite (TBNI) dissolved therein. Finally, the
cells were hermetically sealed.
All five cells were then cycled between 4.1V and
2.75V. The charge cycle was performed under a 100 mA

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04645.0615
constant current until the cells reach 4.1V. Then, the
charge cycle was continued at 4.1V until the current
dropped to 20 mA. After resting for 5 minutes, the
cells were discharged under a 100 mA constant current to
2.75 V. The cells were rested for another 5 minutes
before the next cycle.
The initial average charge and discharge capacities
of both groups of cells are summarized in Table 1. The
first cycle irreversible capacity was calculated as the
difference between the first charge capacity and the
first discharge capacity.
Table 1
First Cycle Capacities and Irreversible Capacities
1st Charge 1st Discharge Irreversible
Group (mAh) (mAh) (mAh)
1 641.4 ~ 1.5 520.2 ~ 9.5 121.2 ~ 10.7
2 621.7 ~ 5.3 547.0 ~ 2.4 74.7 ~ 7.7
The data in Table 1 clearly demonstrate that
both groups of cells had similar first cycle charge
capacities. However, the first cycle discharge
capacities are quite different. The group 2 cells
activated with the electrolyte containing the t-butyl
nitrite additive had significantly higher first cycle
discharge capacities than that of the group 1 cells
(approximately 5.2% higher). As a result, the group 2
cells also had about 38°s lower first cycle irreversible
capacity than that of the group 1 cells.

CA 02298417 2000-02-14
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EXAMPLE II
04645.0615
After the initial cycle, the cycling of the five
cells continued for a total of 10 times under the same
cycling conditions as described in Example I. The
discharge capacities and the capacity retention of each
cycle are summarized in Table 2. The capacity retention
is defined as the capacity percentage of each discharge
cycle relative to that of the first cycle discharge
capacity.
Table 2
Cycling Discharge Capacity and Capacity Retention
Group 1 Group 2


Capacity Retention Capacity Retention


Cycle (mAh) (%) (mAh) ($)



1 520.2 100.0 547.0 100.0


2 510.2 98.1 542.0 99.1


3 503.4 96.8 536.9 98.1


4 497.6 95.7 532.1 97.3


5 . 493.2 94.8 528.2 96.6


6 489.4 94.1 524.6 95.9


7 486.1 93.4 521.7 95.4


8 483.2 92.9 518.7 94.8


9 480.2 92.3 516.3 94.4


10 478.2 91.9 513.9 93.9


The data in Table 2 demonstrate that the group 2
cells with the t-butyl nitrite additive consistently
presented higher discharge capacities in all cycles. In
addition, this higher capacity was not realized at the

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04645.0615
expense of lower cycle life. The group 1 and 2 cells had
essentially the same cycling capacity retention
throughout the various cycles.
EXAMPLE III
After the above cycle testing described in
Example II, the cells were charged according to the
procedures described in Example I. Then, the cells were
discharged under a 1000 mA constant current to 2.75 V
then a five minute open circuit rest, followed by a 500
mA constant current discharge to 2.75 V then a five
minute open circuit rest, followed by a 250 mA constant
current discharge to 2.75 V then a five minute open
circuit rest and, finally, followed by a 100 mA constant
current discharge to 2.75 V then a five minute open
circuit rest. The averaged total capacities under each
discharge rate are summarized in Table 3 and the
comparison of averaged discharge efficiency (defined as
o capacity of a 100 mA constant current discharge) under
the various constant currents are summarized in Table 4.
In Table 3, the discharge capacities are cumulative from
. one discharge current to the next.
Table 3
Discharge Capacities (mAh) under Various Currents
Group 1000 mA 500 mA 250 mA 100 mA
1 277.8 439.8 459.8 465.9
2 262.2 479.0 499.9 505.8
____~_ _.__

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Table 4
Discharge Efficiency (s) under Various Currents
Group 1000 mA 500 mA 250 mA 100 mA
1 59.7 94.4 98.1 100.0
2 51.8 94.7 98.8 100.0
The data in Table 3 indicate that the group 2 cells
with the nitrite additive delivered increased discharge
capacity in comparison to the group 1 control cells
under a discharge rate equal to or less than 500 mA
(approximately a 1C rate). Under a higher discharge
rate (1000 mA, approximately a 2C rate), however, the
group 1 control cells delivered higher capacity than
that of tha nrnp 7 rcl 1 c _ '~['rP canna i-ranrlc arc al, cn,
shown in Table 4. Under a 500 mA or lower discharge
current, the group 2 cells presented similar discharge
efficiencies than that of the group 1 cells. Under a
higher discharge current (i.e. 1000 mA), the group 1
control cells afforded a higher discharge efficiency
than that of the group 2 cells.
EXAMPLE IV
After the above discharge rate capability
test, all the cells were fully charged according to the
procedure described in Example I. The five test cells
were then stored on open circuit voltage (OCV) at 37°C
for thirteen days. Finally, the cells were discharged
and cycled for eight more times. The % of self-

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discharge and the capacity retention were calculated and
are shown in Table 5.
Table 5
Rates of Self-Discharge and After Storage Capacity
Retention
Group Self-Discharge (o) Capacity Retention
1 12.6 93.4
2 12.6 93.4
The data in Table 5 demonstrate that both groups of
cells exhibited similar self-discharge rates and similar
after storage capacity retention rates. However, since
tl-,c ynt 7 ~el l c ~r~l h; r~hvr .-7; ~...h ,-... ..: ~~ L _
,. .... ~-ap ._ ~~ h.r ..~.yi .. .wrvvtu3r~Wrt:IZ.~.iWril.iC:J t.t:uli
that of the group 1 cells, the capacities of the group 2
cells were still higher than that of the group 1 cells,
even though they presented similar self-discharge and
capacity retention rates. A total of 20 cycles were
obtained and the results are summarized in Fig. 1. In
particular, curve 10 was constructed from the averaged
cycling data of the group 1 cells devoid of the nitrite
additive while curve 12 was constructed from the
averaged group 2 cells having the t-butyl nitrite
additive. The increased discharge capacity through the
twenty cycles is clearly evident.
In order to generate an electrically non-conductive
SEI layer containing the reduction product of a nitrite
additive according to the present invention, the
reduction reaction of the nitrite additive has to

CA 02298417 2003-09-12
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effectively compete with reactions of other electrolyte
components on the anode surface. In that regard, the R-0
bond in the nitrite additive having the general formula
(RO)N(=0) has to be weak or reactive.
This point has been demonstrated in U.S. Patent No.
6,027,827. In that application it is described that when
the nitrite additive has a relatively weak C-0 bond, such
as t-butyl nitrite, the beneficial effect is observed for
primary lithium/silver vanadium oxide cells in terms of
voltage delay reduction and reduced Rdc growth.
Based on similar reasoning, it is believed that the
same type of nitrite additives which benefit the discharge
performance of a primary lithium electrochemical cell will
also benefit first cycle irreversible capacity and cycling
efficiency of lithium ion cells due to the formation of a
good SEI film on the carbon anode surface. Therefore, for
lithium ion cells the R group in the nitrite additive
having the general formula (RO)N(=0) should be a saturated
or unsaturated organic group containing 1 to 10 carbon
atoms.
While not intended to be bound by any particular
theory, if the R group is activated (t-butyl for example),
the 0-R bond is relatively weak and it is believed that
due to the presence of the N=0 bond in the nitrite
functional group, [-0-N(=0)], the bond between oxygen and
the R group is severed. The nitrite intermediate is then
able to compete effectively with the other electrolyte
solvents or solutes to react with lithium and form a
nitrite salt, i.e., lithium nitrite, or the lithium salt
of a nitrite reduction product on

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the surface of the anode. The formation of (0=)N-(0-Li)
(n = 1 or 2) deposited on the anode surface is
responsible for the improved performance of the lithium
ion cells. The resulting salt is ionically more
conductive than lithium oxide which may form on the
anode in the absence of the organic nitrite additive.
As a consequence, the chemical composition and perhaps
the morphology of the anode surface protective layer is
believed to be changed with concomitant benefits to the
cell's discharge characteristics. This is believed to
be the reason for the observed improvements in the
lithium ion cells, as exemplified by those having the
TBNI additive.
The concentration limit for the nitrite additive is
preferably about O.OO1M to about 0.20M. The beneficial
effect of the nitrite additive will not be apparent if
the additive concentration is less than about O.OOlM.
On the other hand, if the additive concentration is
greater than about 0.20M, the beneficial effect of the
additive will be canceled by the detrimental effect of
higher internal cell resistance due to the thicker anode
surface film formation and lower electrolyte
conductivity. .
It is appreciated that various modifications to the
inventive concepts described herein may be apparent to
those of ordinary skill in the art without departing
from the spirit and scope of the present invention as
defined by the appended claims.