U.S. patent application number 13/578110 was filed with the patent office on 2012-12-06 for rechargeable electrochemical cell.
This patent application is currently assigned to fortu Intellectual Property AG. Invention is credited to Claus Dambach, Guenther Hambitzer, Joachim Heitbaum, Martin Kampa, Christian Pszolla, Christiane Ripp.
Application Number | 20120308876 13/578110 |
Document ID | / |
Family ID | 42237019 |
Filed Date | 2012-12-06 |
United States Patent
Application |
20120308876 |
Kind Code |
A1 |
Hambitzer; Guenther ; et
al. |
December 6, 2012 |
RECHARGEABLE ELECTROCHEMICAL CELL
Abstract
The present invention relates to a rechargeable electrochemical
battery cell having a housing, a positive electrode, a negative
electrode, and an electrolyte, the electrolyte containing sulfur
dioxide and a conductive salt of the active metal of the cell. The
total quantity of oxygen-containing compounds contained in the cell
that are able to react with the sulfur dioxide, reducing the sulfur
dioxide, is not more than 10 mMol per Ah theoretical capacitance of
the cell.
Inventors: |
Hambitzer; Guenther; (Bonn,
DE) ; Heitbaum; Joachim; (Bonn, DE) ; Dambach;
Claus; (Pfinztal, DE) ; Kampa; Martin;
(Mannheim, DE) ; Pszolla; Christian; (Karlsruhe,
DE) ; Ripp; Christiane; (Pfinztal, DE) |
Assignee: |
fortu Intellectual Property
AG
Sarnen
CH
|
Family ID: |
42237019 |
Appl. No.: |
13/578110 |
Filed: |
February 4, 2011 |
PCT Filed: |
February 4, 2011 |
PCT NO: |
PCT/EP2011/000506 |
371 Date: |
August 9, 2012 |
Current U.S.
Class: |
429/163 |
Current CPC
Class: |
H01M 10/056 20130101;
H01M 4/58 20130101; H01M 10/0525 20130101; H01M 2300/002 20130101;
H01M 4/5825 20130101; H01M 4/1397 20130101; H01M 4/5835 20130101;
H01M 4/366 20130101; Y02E 60/10 20130101; H01M 4/0471 20130101;
H01M 4/808 20130101; H01M 4/0473 20130101; H01M 4/587 20130101;
H01M 4/623 20130101; H01M 2004/028 20130101; H01M 4/0435 20130101;
H01M 4/62 20130101; H01M 2004/021 20130101; H01M 10/0563 20130101;
H01M 4/0478 20130101; H01M 4/136 20130101; H01M 4/581 20130101;
H01M 4/0404 20130101 |
Class at
Publication: |
429/163 |
International
Class: |
H01M 10/02 20060101
H01M010/02; H01M 4/48 20100101 H01M004/48; H01M 4/58 20100101
H01M004/58; H01M 2/00 20060101 H01M002/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 12, 2010 |
EP |
10001458.8 |
Claims
1-19. (canceled)
20. Rechargeable electrochemical battery cell having a housing, a
positive electrode, a negative electrode, and an electrolyte which
contains sulfur dioxide and a conductive salt of the active metal
of the cell, characterized in that the total quantity of oxygen in
the cell that is contained in compounds which are able to react
with sulfur dioxide in a reaction by which the sulfur dioxide is
reduced, is not more than mMol per Ah theoretical charge
capacitance of the cell.
21. Battery cell according to claim 20, characterized in that the
total quantity of oxygen in the cell that is contained in compounds
which are able to react with the sulfur dioxide in a reaction by
which the sulfur dioxide is reduced, is not more than 5 mMol per Ah
theoretical charge capacitance of the cell.
22. Battery cell according to claim 20, characterized in that the
total quantity of oxygen in the cell that is contained in compounds
which are able to react with the sulfur dioxide in a reaction by
which the sulfur dioxide is reduced, is not more than than 1 mMol
per Ah theoretical charge capacitance of the cell.
23. Battery cell according to claim 20, characterized in that the
total quantity of oxygen in the cell that is contained in compounds
which are able to react with the sulfur dioxide in a reaction by
which the sulfur dioxide is reduced, is not more than 0.5 mMol per
Ah theoretical charge capacitance of the cell.
24. Battery cell according to claim 20, characterized in that the
total quantity of oxygen in the cell that is contained in compounds
which are able to react with the sulfur dioxide in a reaction by
which the sulfur dioxide is reduced, is not more than 0.1 mMol per
Ah theoretical charge capacitance of the cell.
25. Battery cell according to claim 20, characterized in that the
quantity of oxygen in the negative electrode that is contained in
compounds which are able to react with the sulfur dioxide in a
reaction by which the sulfur dioxide is reduced, is not more than 6
mMol per Ah theoretical charge capacitance of the cell.
26. Battery cell according to claim 20, characterized in that the
quantity of oxygen in the positive electrode that is contained in
compounds which are able to react with the sulfur dioxide in a
reaction by which the sulfur dioxide is reduced, is not more than 8
mMol per Ah theoretical charge capacitance of the cell.
27. Battery cell according to claim 20, characterized in that the
quantity of oxygen in the electrolyte that is contained in
compounds which are able to react with the sulfur dioxide in a
reaction by which the sulfur dioxide is reduced, is not more than
10 mMol per Ah theoretical charge capacitance of the cell.
28. Battery cell according to claim 20, characterized in that, as
the reaction product of the reaction by which the sulfur is
reduced, a sulfur-oxygen compound is formed in which the sulfur has
an oxidation level not exceeding +111, in particular a thiosulfate
of the active metal of the cell.
29. Battery cell according to claim 20, characterized in that, as
the reaction product of the reaction by which the sulfur is
reduced, in particularly when the cell is overcharged, a
sulfur-oxygen compound is formed, which compound additionally
contains a halogen and in which compound the oxidation level of the
sulfur is +VI, in particular a chlorosulfonate of the active metal
of the cell.
30. Battery cell according to claim 20, characterized in that the
electrolyte contains at least 1 mMol sulphur dioxide per Ah
theoretical charge capacitance of the cell.
31. Battery cell according to claim 20, characterized in that the
electrolyte contains at least 5 mMol sulphur dioxide per Ah
theoretical charge capacitance of the cell.
32. Battery cell according to claim 20, characterized in that the
electrolyte contains at least 10 mMol sulfur dioxide per Ah
theoretical charge capacitance of the cell.
33. Battery cell according to claim 20, characterized in that the
active metal is selected from the group consisting of the alkali
metals, alkaline earth metals, and metals of the second subgroup of
the periodic system.
34. Battery cell according to claim 33, characterized in that the
active metal is lithium, sodium, calcium, zinc, or aluminum.
35. Battery cell according to claim 20, characterized in that the
negative electrode is an insertion electrode.
36. Battery cell according to claim 35, characterized in that the
negative electrode contains carbon.
37. Battery cell according to claim 20, characterized in that the
positive electrode contains a metal oxide or a metal halide or a
metal phosphate, wherein the metal is preferably a transition metal
having an atomic number from 22 to 28, most preferably the metal is
nickel, manganese, or iron.
38. Battery cell according to claim 37, characterized in that the
positive electrode contains an intercalation compound.
39. Battery cell according to claim 20, characterized in that the
electrolyte contains, as the conductive salt, a halide, an oxalate,
a borate, a phosphate, an arsenate, or a gallate of the active
metal, preferably a tetrahalogenoaluminate of an alkali metal,
further preferably a tetrachloroaluminate of an alkali metal.
40. Battery cell according to claim 20, characterized in that the
concentration of conductive salt in the electrolyte is at least
0.01 mol/1.
41. Battery cell according to claim 20, characterized in that the
concentration of conductive salt in the electrolyte is at least 0.1
mol/1.
42. Battery cell according to claim 20, characterized in that the
concentration of conductive salt in the electrolyte is at least 1
mol/1.
43. Battery cell according to claim 20, characterized in that the
concentration of conductive salt in the electrolyte is at least 5
mol/1.
44. Battery cell according to claim 20, characterized in that the
quantity of electrolyte, per Ah theoretical charge capacitance of
the cell, is not more than 50 ml.
45. Battery cell according to claim 20, characterized in that the
quantity of electrolyte, per Ah theoretical charge capacitance of
the cell, is not more than 20 ml.
46. Battery cell according to claim 20, characterized in that the
quantity of electrolyte, per Ah theoretical charge capacitance of
the cell, is not more than 10 ml.
47. Battery cell according to claim 20, characterized in that it
contains a separator that separates the negative and positive
electrodes from one another, and prevents metallic lithium
deposited at the negative electrode from penetrating to the surface
of the positive electrode.
48. Battery cell according to claim 20, characterized in that it
contains an isolator that separates the negative and positive
electrodes from one another electrically, the isolator being formed
and arranged in such a manner that active metal deposited on the
negative electrode during charging of the cell may contact the
positive electrode in such a manner that locally limited
short-circuit reactions occur at the surface of the positive
electrode.
Description
[0001] The invention relates to a rechargeable electrochemical
battery cell having a positive electrode, a negative electrode, and
an electrolyte, the electrolyte containing sulfur dioxide
(SO.sub.2) and a conductive salt of the active metal of the
cell.
[0002] The charge transport that is necessary for charging and
discharging the cell is based on the mobility of the conductive
salt ions in the electrolyte. In particular the electrolyte may be
a liquid or a gel. In SO.sub.2-containing electrolytes (hereafter
designated SO.sub.2 electrolytes), the SO.sub.2 contributes to the
mobility of the ions of the conductive salt that carry out this
charge transport. Thus the SO.sub.2 serves as a solvent of the
conductive salt. The electrolyte may contain another solvent to
promote the mobility of the ions in the conductive salt, in
addition to the SO.sub.2. Such solvents may be inorganic solvents
(for example sulfuryl chloride, thionyl chloride), organic
solvents, and ionic liquids, which may be used individually or in a
mixture. For the purposes of the invention, an electrolyte solution
is preferred that not only contains a SO.sub.2 in low concentration
as an additive, but in which the mobility of the conductive salt
ions is provided mainly, preferably entirely, by the SO.sub.2.
[0003] It has long been known that SO.sub.2 electrolytes offer
considerable advantages (see "The Handbook of Batteries" by David
Linden, 1994, McGraw Hill). One of these advantages is that its
conductivity is 5 to 10 times better than with organic electrolytes
normally used in lithium ion battery cells. This enables higher
charge and discharge currents, which in turn result in a high power
density. Good conductivity of the electrolyte is also advantageous
with regard to the charge capacitance of the cell, because it
allows use of relatively thick electrodes with high storage
capacity for the active metal of the cell.
[0004] The various types of cells with SO.sub.2 electrolyte differ
from each other mainly with respect to the active metal and
conductive salt used, and also with respect to the materials used
for the positive and negative electrodes. The present invention is
directed in particular to battery cells having the following
features: [0005] a) The active metal is preferably an alkali metal,
particularly lithium or sodium. In this case, the conductive salt
is preferably a tetrahalogen aluminate, more preferably a
tetrachloroaluminate of the alkali metal, for example LiAlCl.sub.4.
In any case, for any anion of the conducting salt, the electrolyte
contains at least 1 mole of the conducting salt per 22 mole
SO.sub.2. A preferred upper limit of the molar ratio is 1 mole of
conducting salt per 0.5 mole SO.sub.2. In the context of the
invention, preferred active metals, besides the alkali metals, also
include the alkaline earth metals and the metals of the second
subgroup of the periodic system, in particular calcium and zinc.
Particularly suitable conductive salts are the halides, oxalates,
borates, phosphates, arsenates and gallates thereof. [0006] b) The
positive electrode preferably contains (for storing the active
metal) a metal oxide or metal phosphate in the form of an
intercalation compound. The metal in the oxide or phosphate is
preferably a transition metal with an atomic number from 22 to 28,
particular cobalt, manganese, nickel or iron. Binary or ternary
oxides or phosphates containing two or three different transition
metals in the matrix structure, such as lithium-nickel-cobalt
oxide, also have particular practical importance. However, the
invention may also be used in combination with other positive
electrodes, for example on the basis of a metal halide such as
CuCl.sub.2. [0007] c) The negative electrode is preferably an
insertion electrode. This term is used to generally designate
electrodes that contain the active metal within themselves in such
manner that it is available for exchange with the electrolyte when
the cell is charged and discharged. Negative electrodes containing
graphite, wherein the active metal is taken up within the graphite
when the cell is charged (lithium ion cells) are particularly
significant, both generally and for the purposes of the present
invention. When the cell is charged normally no active metal is
deposited on the electrode surface. [0008] However, the invention
also relates to cells in which the active metal is metallically
deposited on the negative electrode (lithium-metal cells). In this
context, it is particularly preferable if the negative electrode
has a porous deposition layer, whose pores take up the active mass
during charging. Such an electrode is also called an insertion
electrode (in a broader sense). For such a deposition layer, a
glass fabric with a pore diameter of at least 6 .mu.m may be used,
for example. [0009] d) The necessary electrical separation of the
positive and negative electrodes may be assured either by a
separator or by an isolator. [0010] A separator is a structure that
prevents metallic lithium deposited at the negative electrode from
penetrating to the surface of the positive electrode. For cells
with SO.sub.2 electrolyte, in particular ceramic materials having a
pore diameter of no more than 1 .mu.m, and inert polymer materials,
or also mixtures of the two materials, are suitable. Generally,
separators made from polyethylene (PE), polypropylene (PP) or
ethylene tetrafluoroethylene (ETFE) may also be used. However,
chargers for cells of such kind should be designed so as to prevent
overcharging in order to avoid undesirable reactions. [0011] In
this context, an isolator is understood to be a structure that
separates the positive and negative electrodes electrically, but
does not prevent metallic lithium from penetrating to the surface
of the positive electrode. WO 2005/031908 describes that locally
limited short-circuits occurring in this context are not
necessarily harmful, but may even have positive effects. The
isolator preferably serves simultaneously as a deposition
layer.
[0012] The following documents contain more detailed information
about battery cells with SO.sub.2 electrolytes: [0013] (1) U.S.
Pat. No. 5,213,914 [0014] (2) WO 00/79631 and U.S. Pat. No.
6,730,441 [0015] (3) WO 2005/031908 and US 2007/0065714 [0016] (4)
WO 2008/058685 and US 2010/0062341 [0017] (5) WO 2009/077140
[0018] It is an object of the invention is to provide battery cells
with an SO.sub.2 electrolyte that feature improved properties, in
particular with regard to long-term operation with many charging
and discharging cycles.
[0019] The object is solved by an electrochemical rechargeable
battery cell having a housing, a positive electrode, a negative
electrode, and an electrolyte, the electrolyte containing sulfur
dioxide and a conductive salt of the active metal of the cell,
which is characterized in that the total quantity of oxides
contained in the cell that are able to react with sulfur dioxide
and reduce the sulfur dioxide, is no more than 10 mMol for each Ah
theoretical capacitance of the cell.
[0020] In the context of the invention, it has been found that
certain oxygen-containing contaminants can severely impair the
long-term function of battery cells that include an electrolyte
containing SO.sub.2. These are oxygen-containing compounds that are
able to react with and reduce the sulfur dioxide, that is to say,
they are capable of such a reaction under the conditions that may
occur during operational use of the battery. Oxygen-containing
compounds of such kind will be designated hereafter as "disturbing
oxides", as a non-limiting abbreviation. The reactions at issue are
often highly inhibited, so that they only take place over a very
long period. However, in view of the fact that batteries must
remain functional for long periods (several years), even such slow
reactions still impair battery function.
[0021] This observation applies for both lithium ion cells and
lithium metal cells. In the context of the invention, it has been
determined that the presence of metallic lithium does not affect
the reactions of the disturbing oxides. This is true regardless of
whether local short-circuits may occur (isolator) or whether the
active metal is effectively kept separate from the positive
electrode (separator). In both cases, even a small quantity of
disturbing oxides is capable of very significantly reducing the
cell's capacitance.
[0022] It is particularly surprising that even very low
concentrations of disturbing oxides are capable of massively
impairing the cell's function. For example, a SubC size cell that
was tested in the context of the invention has a theoretical
discharge capacitance of 2.3 Ah. The function of such a cell may be
affected by as little as about 5 to 10 mMol disturbing oxide so
severely that during long-term use, with many charging and
discharging cycles, its capacitance goes down to zero.
[0023] The theoretical charge capacitance of the cell depends on
its electrodes. In the context of the invention the theoretical
capacitance of the positive electrode is taken to be the
theoretical capacitance of the cell. This is the maximum
theoretical charge of the active metal that can be stored in the
electrode, i.e. the amount of electrical charge corresponding to
the maximum quantity of active metal that the positive electrode
may contain according to stoichiometric calculation. This value is
always greater than the actually achievable maximum capacitance of
the electrode (and thus also of the cell), because the charge
amount that can theoretically be stored can never be fully drawn
from the electrode in practice.
[0024] If the content of disturbing oxides in the cell is below the
limit values determined in the context of the invention, the cell's
long-term function is significantly improved thereby. In
particular, the electrical charge capacitance remains essentially
stable over many charging and discharging cycles. Overcharging
resistance is also significantly improved, i.e. the cell's function
is not destroyed even by overcharging.
[0025] As was explained in the introduction, the invention
primarily relates to cells with an electrolyte whose SO.sub.2
content is so high that the mobility of the conductive salt ions is
based mainly, or even entirely, on the presence of the SO.sub.2.
The minimum quantity of SO.sub.2 per Ah theoretical capacitance of
the cell is preferably 1 mMol, wherein a minimum quantity of 5 mMol
is particularly preferred and a minimum quantity of 10 mMol is
especially preferred, relative to the theoretical capacitance of
the cell in each case.
[0026] Having the knowledge of the invention, it is possible to
explain its effect by a plurality of reactions, usually involving
several elements of the cell (electrodes, electrolyte). This will
be explained hereafter with reference to an electrochemical cell
having a carbon-based negative electrode and a metal oxide-based
positive electrode.
a) Reactions of Disturbing Oxides During Charging and
Discharging
[0027] When the cell is charged and discharged normally (without
overcharging), according to the inventors' present understanding,
the reactions by disturbing oxides described hereafter cause the
sulfur dioxide to be reduced and ultimately result in a loss of
charge capacitance.
a1) Direct Reaction by Disturbing Oxides Present at the Active
Material of the Negative Electrode
[0028] A typical oxygen-containing contamination at the active
material of a negative carbon electrode is C.sub.6O. It reacts with
the LiC.sub.6 of the electrode and the SO.sub.2 of the electrolyte,
consuming six electrons (e.sup.-) according to the following
reaction equation:
6LiC.sub.6+C.sub.6O+2S.sup.(+IV)O.sub.2.fwdarw.7C.sub.6+LiS.sup.(+II).su-
b.2O.sub.3+2Li.sub.2O; Charge consumption 6 e.sup.- (1)
[0029] The reaction products are carbon (C.sub.6), lithium
thiosulfate (Li.sub.2S.sub.2O.sub.3) and lithium oxide
(Li.sub.2O).
[0030] The superscripted Roman numerals in parentheses indicate the
oxidation number of the sulfur in the respective compound. The
oxidation number of the sulfur is lowered by 2 during the reaction.
The sulfur dioxide is thus reduced as the lithium thiosulfate is
formed.
a2) Indirect Reaction by Disturbing Oxide at the Active Material of
the Negative Electrode
[0031] The presence of the disturbing oxide C.sub.6O indirectly
causes the following reaction sequence, which is accompanied by a
reduction of the sulfur dioxide, ultimately forming lithium
thiosulfate:
2Li.sub.2O+2AlCl.sub.3.fwdarw.2AlOCl+4LiCl; (2)
4LiS.sup.(+III).sub.2O.sub.4+2AlOCl+2AlCl.sub.3+LiC.sub.6+.fwdarw.4Li.su-
b.2S.sup.(+II).sub.2O.sub.3+2Al.sub.2O.sub.3+8LiCl+8C.sub.6; Charge
consumption 8 e.sup.- (3a)
[0032] According to (2), the lithium oxide formed in reaction
equation (1) reacts with aluminum chloride (AlCl.sub.3), a Lewis
acid that is usually present in the electrolyte as a result of the
autodissociation of the conductive salt, to produce aluminum
oxychloride (AlOCl) and lithium chloride (LiCl). But also the
neutral form of the conductive salt (LiAlCl.sub.4) may result in
the dissolution of the Li.sub.2O and further reaction with
Li.sub.2S.sub.2O.sub.4:
2Li.sub.2O+2LiAlCl.sub.4.fwdarw.2LiAlOCl.sub.2+4LiCl (2')
4Li.sub.2S.sup.(+III).sub.2O.sub.4+2LiAlOCl.sub.2+2LiAlCl.sub.4+8LiC.sub-
.6.fwdarw.4Li.sub.2S.sup.(+II).sub.2O.sub.3+2Al.sub.2O.sub.3+12LiCl+8C.sub-
.6; Charge consumption 8 e.sup.- (3a')
[0033] Regarding the subsequent reaction (3a), it should be noted
that in this type of cells, a covering layer of lithium dithionite
(usually advantageous for the function of the cell, and therefore
desirable) is present. This covering layer has been formed
according to
8LiC.sub.6+8SO.sub.2.fwdarw.8C.sub.6+4Li.sub.2S.sub.2O.sub.4;
Charge consumption 8 e.sup.- (4a)
from the LiC.sub.6 of the electrode and the SO.sub.2 of the
electrolyte, with consumption of eight electrons. It consists of
lithium dithionite (Li.sub.2S.sub.2O.sub.4). This compound of the
covering layer reacts further according to (3a). SO.sub.2 in the
lithium dithionite is in turn reduced (the oxidation number of the
sulfur is lowered from +III to +III), and lithium thiosulfate is
formed.
[0034] The presence of one molecule of C.sub.6O results in the
consumption of a total of 10 molecules SO.sub.2 and 22 e.sup.-
(wherein 5 molecules of Li.sub.2S.sub.2O.sub.3 are formed).
Expressed in other terms, this means that 594 mAh charge are
irreversibly consumed for every mMol C.sub.6O.
[0035] In the preceding calculations, it is also necessary to
consider the solubility of the reaction products lithium
thiosulfate (Li.sub.2S.sub.2O.sub.3) and lithium oxide (Li.sub.2O)
produced by reaction (1). If the Li.sub.2O does not dissolve,
reaction (2) cannot take place, whereby subsequent reactions (3a)
and (4a) also do not occur. The charge consumption is consequently
only 6e.sup.- resulting from reaction (1). Expressed in other
terms, in this case 162 mAh charge are irreversibly consumed for
every mMol C.sub.6O.
[0036] If the Li.sub.2O dissolves but the Li.sub.2S.sub.2O.sub.3
does not, reaction (4a) does not take place. In this case, the
charge consumption is lowered to 14e.sup.-. This means that 378 mAh
charge are irreversibly consumed for every mMol C.sub.6O.
[0037] The capacitance loss due to disturbing oxides at the active
material of the negative electrode is thus between 162 and 564 mAh,
depending on the solubility of the mentioned components, (which is
also affected by temperature and SO.sub.2 concentration, among
other factors).
a3) Reactions Due to Disturbing Oxides at the Active Material of
the Positive Electrode
[0038] A typical oxygen-containing contamination at the active
material of the positive electrode is a hydroxide that forms on the
surface thereof. In the case of a lithium metal oxide electrode of
the general formula LiMeO.sub.2 it may, for example, be
LiMeO(OH).sub.2. It reacts with the AlCl.sub.3 in the electrolyte
according to the following reaction equation to form aluminum
oxychloride, a chloride of the metal Me, HCl, and LiCl:
LiMeO(OH).sub.2+3AlCl.sub.3.fwdarw.3AlCl+MeCl.sub.3+2HCl+LiCl
(5)
[0039] The hydroxide may also react with the neutral form of the
conductive salt in accordance with:
LiMeO(OH).sub.2+3LiAlCl.sub.4.fwdarw.4LiAlCl.sub.2+MeCl.sub.3+2HCl+LiCl
(5')
[0040] The aluminum oxychloride formed thereby reacts further with
the lithium dithionite covering layer as shown in the following
equations (3b) and (4b). These equations differ from (3a) and (4a)
only in the molar numbers, and therefore do not need to be fully
explained again:
6Li.sub.2S.sup.(+III).sub.2O.sub.4+3AlOCl+3AlCl.sub.3+12LiC.sub.6+.fwdar-
w.6Li.sub.2S.sup.(+II).sub.2O.sub.3+3Al.sub.2O.sub.3+12LiCl+12C.sub.6;
Charge consumption 12 e.sup.- (3b)
[0041] Of course, the reaction path via lithium aluminum
oxychloride (LiAlOCl.sub.2), similarly to (3a'), is also possible
here.
12LiCl.sub.6+12SO.sub.2.fwdarw.12C.sub.6+6Li.sub.2S.sub.2O.sub.4;
Charge consumption 12 e.sup.- (4b)
[0042] Overall, the presence of one molecule of LiMeO(OH).sub.2
results in the consumption of 12 SO.sub.2 molecules and 24 e.sup.-,
while 6 molecules of Li.sub.2S.sub.2O.sub.3 and 2 molecules of HCl
are formed (with 3 molecules of AlOCl formed as an intermediate
product). The charge consumption is equivalent to a charge quantity
of 702 mAh for every mMOl LiMeO(OH).sub.2. Moreover, the lithium in
one molecule of LiMeO(OH).sub.2 is no longer available as an active
metal. This represents a further capacitance loss of 27 mAh per
mMol LiMeO(OH).sub.2. Accordingly, if the reactions proceed to
completion, a capacitance of 729 mAh is lost per mMol
LiMeO(OH).sub.2. This corresponds to 243 mAh per mole oxygen
because every mole of the disturbing oxide contains three moles of
oxygen.
[0043] As mentioned before, reactions (3b) and (4b) only take place
if the lithium thiosulfate formed according to (3a) is dissolved
and consequently lithium dithionite is reproduced in accordance
with (4b). If this does not happen, the charge consumption is
reduced to 12 e.sup.-. This is equivalent to the irreversible loss
of a capacitance of 351 mAh for every mMol LiMeO(OH).sub.2. Again,
a capacitance loss of 27 mAh due to consumed active metal must be
added. In this case, the overall capacitance loss is 378 mAh, i.e.
126 mAh per mole oxygen.
a4) Reactions Due to Disturbing Oxides in the Electrolyte
Solution
[0044] Other oxygen-containing contaminations occur when the
electrolyte solution is manufactured or due to the reaction of the
electrolyte solution with other components of the cell (for example
the housing, the separator, the current collectors to and from the
electrodes). Typically these are acidic (AlOCl) or neutral
(LiAlOCl.sub.2) aluminum oxychlorides, leading to the following
reactions, wherein reaction equations (3c) and (4c) are again
equivalent to (3a) and (4a) except for the molar numbers.
2Li.sub.2S.sup.(+III).sub.2O.sub.4+AlOCl+AlCl.sub.3+4LiCl.sub.3.fwdarw.2-
Li.sub.2S.sup.(+III).sub.2O.sub.3+2Al.sub.2O.sub.3+4LiCl+4C.sub.6;
Charge consumption 4 e.sup.- (3c)
[0045] Again, the alternative reaction path similar to (3a') is
possible.
4LiC.sub.6+4SO.sub.2.fwdarw.4C.sub.6+2Li.sub.2S.sub.2O.sub.4;
Charge consumption 4 e.sup.- (4c)
[0046] The presence of one molecule of AlOCl results in the
consumption of 4 SO.sub.2 molecules and 8 e.sup.-, while 2
molecules of Li.sub.2S.sub.2O.sub.3 are formed. The corresponding
charge consumption is 216 mAh for each mMol AlOCl.
[0047] If the lithium thiosulfate is not dissolved, the charge
consumption is correspondingly reduced in this case as well, to 4
e.sup.-. This is equivalent to 108 mAh.
[0048] The current collectors to and from the electrodes are often
made from nickel or a nickel alloy. The surface of these components
may be oxidized, that is to say, it may contain nickel oxide. If
this nickel oxide has not (yet) been dissolved in the electrolyte
solution, a direct reaction similar to reaction equation (1) may
occur, producing Li.sub.2S.sub.2O.sub.3 when the cell is
charged.
b) Secondary Reactions Caused by Disturbing Oxides During
Overcharging
[0049] When the cell is overcharged, the charge voltage increases.
Above a certain voltage, which varies according to cell type, this
increase in charge voltage causes decomposition of the electrolyte
solution. If the solution contains for example a
tetrachloroaluminate as the conductive salt, chlorine (Cl.sub.2) is
formed on the positive electrode as an overcharging product. This
in turn leads to other reactions, in which the lithium thiosulfate,
formed according to the reactions described before, makes a series
of further reactions with the chlorine diffused to the negative
electrode and with the LiC.sub.6 of the negative electrode. In
these reactions the sulfur is disproportionated until it reaches
oxidation number -II in one final product, and oxidation number +VI
in another final product. Examples of final products with oxidation
number -II are metal sulfides (such as Li.sub.2S.sup.(-II) or
Al.sub.2S.sup.(-II).sub.3). An example of a final product with
oxidation number +VI is lithium chlorosulfonate (LiSO.sub.3Cl).
[0050] The net equation of this multistage reaction may be
expressed as follows:
6LiC.sub.6+Li.sub.2S.sub.2O.sub.3+3Cl.sub.2.fwdarw.6C.sub.6+LiS.sup.(+VI-
)O.sub.3Cl+LiS.sup.(-II)+5LiCl (6)
e.sup.- are required in order to reduce one molecule of
Li.sub.2S.sup.(+II).sub.2O.sub.3 to sulfur compounds in accordance
with equation (6). In other terms, this means that 162 mAh of
capacitance are irreversibly lost in the reaction of one mMol
lithium thiosulfate.
[0051] In summary, the preceding explanations show that the
presence of disturbing oxides leads to the formation of a
sulfur-oxygen compound when a cell is charged and discharged
normally (without overcharging), and the sulfur in this compound
may reach a maximum oxidation number of +111. One important example
is a thiosulfate of the active metal (in the case of a lithium cell
lithium thiosulfate). In the reaction sequence, sulfur dioxide in
the electrolyte is reduced. If the cell is overcharged, giving rise
to overcharge products, further reactions follow in which the
sulfur of the thiosulfate is disproportionated to form sulfur
compounds having oxidation numbers -II and +VI. A characteristic
reaction product that is formed if the cell is overcharged in the
presence of disturbing oxides is a chlorosulfonate of the active
metal. In a lithium cell, this is lithium chlorosulfonate. The
electrical charge quantity required for these reactions is provided
by the cell, and is no longer available as usable battery
capacitance.
[0052] The disturbing oxides named in the preceding are to be
understood as exemplary of all types of disturbing oxides (in the
sense of the definition provided above) that are present on or in
the cell materials.
[0053] For example, the positive electrode may contain disturbing
oxides in the form of various oxide-containing compounds present on
the surface of the electrode material. These include hydroxides
(including water), oxide dihydroxides, carbonates, borates, and
others.
[0054] Similarly, the formula C.sub.6O serves as a simplified
representation of an oxygen that is bound to a negative carbon
electrode. The graphite surface contains covalently bonded oxygen,
for example as ketones, aldehydes, carbonyls, alcohols, epoxides,
carboxylic acids, and others. The oxygen may also be present as
adsorbed water or adsorbed metal hydroxide or similar.
[0055] However, the inventors have observed that a common property
of all disturbing oxides is that they are able to react with the
sulfur dioxide in the electrolyte, reducing the sulfur dioxide.
When the cell is operating normally (without overcharging) a
sulfur-oxygen compound is typically formed in which the sulfur has
a maximum oxidation level of III, for example a thiosulfate of the
active metal. During overcharging, a sulfur-oxygen compound is
typically produced that also contains a halogen, and in which the
oxidation level of the sulfur is +VI. For example, a
chlorosulfonate of the cell's active metal is formed.
[0056] The following table summarizes the described relationship
between the concentration of oxygen in disturbing oxides (O in StO)
at the negative electrode (NE), the positive electrode (PE), and in
the electrolyte:
TABLE-US-00001 TABLE 1 Charging and discharging Overcharging Total
Cap. loss Cap. loss Cap. loss SO.sub.2 Li.sub.2S.sub.2O.sub.3
mAh/mMol mAh/mMol mAh/mMol Site consumed formed O in StO O in StO O
in StO NE 2-10 mMol 1-5 mMol 162-594 810 1404 PE 0-12 mMol 6 mMol
126-243 324 567 Electrolyte 0-4 mMol 2 mMol 108-216 324 540
[0057] Since the capacitance loss during charging and discharging
(as explained) depends on the solubility of the components in the
electrolyte, the corresponding ranges are shown in the table. The
same applies for the SO.sub.2 consumption, which is also variable
according to the solubility of the reaction components. Depending
on the initial concentration of the SO.sub.2, the function of the
cell may also be impaired or destroyed by the SO.sub.2 consumption,
because the charge transport in the electrolyte depends on the
SO.sub.2 concentration.
[0058] According to the inventors' observations, the reactions that
were previously inhibited by poor solubility take place during
overcharging. For this reason, for capacitance loss during
overcharging, only one value is indicated.
[0059] As the table shows, disturbing oxides contained in an
electrolyte are responsible for the relatively smallest capacitance
loss per mMol. The numerical value of about 100 mAh/mMol shows
that, if the disturbing oxide in the electrolyte reacts to
completion (without the overcharging reactions described), the cell
capacitance goes down to zero if the electrolyte contains more than
about 10 mMol disturbing oxide per Ah theoretical capacitance of
the cell, even if no other disturbing oxides are present in the
cell.
[0060] In practice, however, it should be borne in mind that
depending on the field of application of the battery, operating
conditions may exist in which the disturbing oxides only partially
react. This applies for instance with batteries designed for
low-temperature applications, or battery systems in which
appropriately designed chargers reliably prevent overcharging of
the battery cell. In these cases, higher concentrations of
disturbing oxides may be tolerable.
[0061] Conversely, where strict requirements regarding the
long-term stability of the cell and unfavorable operating
conditions exist (for example high temperature applications), the
cell should contain smaller overall quantities of disturbing
oxides. According to preferred embodiments, the cell therefore
contains not more than 5 mMol, preferably not more than 2 mMol,
especially not more than 1 mMol, particularly preferably not more
than 0.5 mMol, and most preferably not more than 0.1 mMol of
disturbing oxides per Ah theoretical capacitance of the cell.
[0062] In the context of the invention, experiments were conducted
in which typical materials for the positive and negative
electrodes, namely graphite and a trimetal oxide
[Ln(Ni.sub.0.77Co.sub.0.13Mn.sub.0.1)O.sub.2] were kept at
60.degree. C. for eighteen days in an electrolyte solution having
the formula LiAlCl.sub.4.times.1.5 SO.sub.2. The Karl Fischer
value, an indicator of the disturbing oxides contained in the
electrolyte, was recorded several times during this storage. The
results are shown in FIG. 1. This figure shows the O.sup.2-
concentration in ppm, as determined by the Karl Fischer method,
plotted against the time in days for three experiments, namely with
the pure electrolyte (circles), the electrolyte with trimetal oxide
(squares), and the electrolyte with graphite (triangles).
[0063] The following conclusions can be drawn therefrom: [0064] The
electrolyte contains very low quantities of O.sup.2-, the variation
of the curve being within the error margin [0065] In the experiment
with trimetal oxide, a rise followed by a plateau and then another
rise is apparent. This indicates that at least two disturbing
oxides were present, which were dissolved at different rates.
[0066] In the experiment with the graphite, the increase in
O.sup.2- is very small. At any rate it is less pronounced than with
the trimetal oxide. Based on the knowledge of the inventors, this
is due to the fact that only a small fraction of the disturbing
oxides present on the graphite is readily dissolved in the
electrolyte solution. According to the present knowledge of the
inventors, these are disturbing oxides that are not covalently
bonded. However, this does not mean that there are no larger
quantities of disturbing oxides present on the graphite. On the
contrary, further experimental investigations showed that
relatively large quantities of disturbing oxides are present on
normal graphite, but they are covalently bonded and therefore only
react when the cell is being charged.
[0067] The results of such an experiment are shown in FIG. 2. This
relates to electrochemical measurements with a cell having a three
electrode configuration. The figure plots the electrical voltage U
in Volts against the time t in hours for the first charge of two
different negative graphite electrodes. For the first electrode,
commercial graphite without further treatment was used. The second
electrode was treated for 90 hours at 1000.degree. C. in a kiln
containing an argon protective gas, in order to remove the
disturbing oxides.
[0068] The cell was charged with both electrodes at a constant
current of 11 mA. In the figure, the resulting voltage-time curves
are shown with A for the untreated electrode, and B for the
pretreated electrode.
[0069] When a negative electrode is charged for the first time,
covering layers form on the graphite, specifically a covering layer
of lithium dithionite, which is advantageous for the function of
the cell, and an undesirable covering layer of lithium thiosulfate,
which is formed because of the presence of disturbing oxides. Other
experiments have shown that formation of the covering layer is
completed when the voltage is at 0.2 Volt. FIG. 2 shows that this
voltage value was reached after 0.2 hours for the treated electrode
and after 0.3 hours for the untreated electrode. Since the current
flow was constant at 11 mA, it was possible to calculate a consumed
charge quantity of 2.2 mAh for the pretreated, essentially
oxide-free electrode, and 3.3 mAh for the untreated electrode. This
shows that normal graphite contains relatively large quantities of
disturbing oxides, which react when the cell is charged for the
first time.
[0070] Other experiments were conducted with various materials that
are suitable for use as positive electrodes. Commercially available
forms of each of the materials listed in the table below were
stored in electrolyte solution for 21 days at 60.degree. C. The
increase in the disturbing oxide content of the electrolyte was
measured using the Karl Fischer method. It corresponds to the
release of disturbing oxides by the electrode material. The results
are shown in table 2 below.
TABLE-US-00002 TABLE 2 Disturbing oxide Material Formula content
Lithium cobalt oxide LiCoO.sub.2 33,000 ppm Lithium iron phosphate
LiFePO.sub.4 1,600 ppm Trimetal oxide batch 1
Li/Ni.sub.0.77Co.sub.0.13Mn.sub.0.1)O.sub.2 2,800 ppm Trimetal
oxide batch 2 Li(Ni.sub.0.77Co.sub.0.13Mn.sub.0.1)O.sub.2 4,500
ppm
[0071] The disturbing oxide content in lithium cobalt oxide is
extremely high. Although the values for the other electrode
materials are approximately an order of magnitude lower, even these
values are so high that the function of the cell would be severely
impaired if the materials were used without prior treatment.
[0072] Similar experiments were also performed with other materials
used in cells according to the invention. For example, the
disturbing oxide content of nickel foam (commonly used as
conducting-off element for the electrodes) and glass fabric
(commonly used as a isolator between the electrodes) was
determined, in each case in the commercially available form. The
materials were kept in electrolyte solution for seven days at
60.degree. C. The resulting increase in the disturbing oxide
content of the electrolyte was measured using the Karl Fischer
method. The following disturbing oxide contents were
determined:
TABLE-US-00003 TABLE 3 Material Disturbing oxide content Nickel
foam 19.2 ppm Glass fabric 33.8 ppm
[0073] Of course, other methods of chemical analysis, besides the
Karl Fischer method, may also be used to determine the presence of
disturbing oxides or of reaction products formed by the reactions
of the disturbing oxides in the cell. For example, FIG. 3 shows
results of measurements performed using Fourier Transformation
Infrared (FTIR) spectroscopy. The figure shows the FTIR spectrum of
an electrolyte with composition LiAlCl.sub.4.times.1.5 SO.sub.2,
which was taken from a cell after about 550 charging and
discharging cycles (solid line A) compared with the same SO.sub.2
electrolyte before it had been cycled (thin line B with crosses).
Absorbance a was plotted in arbitrary units against the wave number
k in cm.sup.-1. The spectrum shows typical maxima for the
chlorosulfonate at about 1218 cm.sup.-1 and 1070 cm.sup.-1. The
pronounced peak at about 1160 cm.sup.-1 may be attributed to the
sulfur dioxide in the electrolyte solution. The formation of
chlorosulfonate as a result of the cycling is evident.
[0074] It is possible to perform a quantitative determination of
the characteristic reaction product by appropriate calibration.
FIG. 4 shows a measurement corresponding to FIG. 3 for the SO.sub.2
electrolyte of a cycled cell (plot A as in FIG. 3) and for 3
calibration solutions that contained one percent by weight of
chlorosulfonate (plot a), 3% chlorosulfonate (plot b), and 10% by
weight chlorosulfonate (plot c).
Production of Starter Materials Containing Little or No Disturbing
Oxides
[0075] With the knowledge of the invention, one skilled in the art
would recognize various options for reducing the disturbing oxide
content of the components in the cell taking into account the
following explanations.
a) Positive Electrode
[0076] The disturbing oxide content of the material of the positive
electrode may be reduced by heating to elevated temperatures,
wherein both the entire electrode (active material and current
collector) and the active material alone may be subjected to the
heat treatment. In general, a high temperature is advantageous but
it must not be so high that fresh disturbing oxides are created
(particularly due to reactions of a binding agent present in the
electrode material). In the context of the invention, it was
determined that if excessively high temperatures are used, carbon
formed by the reduction of the binding agent in turn reduces the
lithium metal oxide to a metal oxide, with the production of
lithium oxide and carbon dioxide:
4LiMeO.sub.2+C2Li.sub.2O+4MeO+CO.sub.2
[0077] With the knowledge of the invention, it is easily possible
to synchronize the binding agent and the temperature in such manner
that the content of disturbing oxides is reduced to non-critical
values.
b) Negative Electrode
[0078] Heat treatment can also be applied to reduce the disturbing
oxides content of the negative electrode to a value below the
critical limit. For this, the graphite material or the entire
electrode, including the current collector, is heated to above
1000.degree. C. in an inert gas atmosphere.
[0079] The resulting material is practically free from oxides, and
has a consistency that is completely different to that of normal
graphite. It flows like a liquid. Therefore, a special process is
required in order to manufacture the negative electrode from
oxide-free graphite. In order to bond the oxide-free graphite
particles, for example, another carbon material may be added (for
example graphite that has been heated to 500.degree. C., in a 15%
concentration). Such an addition is sufficient to modify the flow
behavior of the largely oxide-free graphite to such an extent that
an electrode may be produced.
c) Electrolyte Solution
[0080] One option for lowering the disturbing oxide content in an
SO.sub.2 electrolyte solution is to dry the lithium chloride (LiCl)
starter substance. To this end, the salt is heated for example to
450.degree. C. for 12 hours in an argon atmosphere.
[0081] The precise conditions of the treatment for reducing
disturbing oxides must be adapted individually to each material and
each electrode. In light of the present invention, various
processes may be experimented with, and the result of such
experimentation may be tested using the Karl Fischer method or some
electrochemical measurement method.
[0082] In general, it is also possible to eliminate disturbing
oxides from the cell by covering the components in question with a
barrier layer, which will prevent any oxide on the corresponding
component from reacting with the sulfur dioxide in the electrolyte,
and thereby reducing the sulfur dioxide. For example, the option
exists to coat a graphite electrode (or the active mass thereof)
with a thin layer of an inert ceramic material that is permeable to
the ions of the conductive salt, particularly (Al.sub.2O.sub.3). In
this case too, it may be determined experimentally whether the
oxide has been blocked sufficiently to prevent it from reacting
with the sulfur dioxide, reducing the sulfur dioxide, and is no
longer a disturbing oxide.
Comparison of Cells with Different Contents of Disturbing Oxide
[0083] FIG. 5 shows the behavior of two cells with different
disturbing oxide contents. The discharge capacitance C (shown as a
percentage of the initial capacitance) is plotted against cycle
number N.
[0084] Curve A shows the cycle behavior of a cell containing about
0.25 mMol of oxygen in disturbing oxides on the positive electrode,
about 0.25 mMol oxygen in disturbing oxides in the electrolyte, and
about 1.1 mMol of oxygen in disturbing oxides on the negative
electrode. Accordingly, the total content of oxygen in disturbing
oxides was 1.6 mMol (per Ah theoretical capacitance of the cell in
each case).
[0085] Curve B describes the cycle behavior of a cell with about 12
mMol oxygen contained in disturbing oxides per Ah theoretical
capacitance of the cell.
[0086] The cell with fewer disturbing oxides (curve A) still has a
usable capacitance of about 50% after a thousand cycles, whereas
the capacitance of the cell with the higher disturbing oxide
content drops to very low levels even after fewer than a hundred
cycles.
[0087] The cell with the lower disturbing oxide content had a
capacitance of 1125 mAh after the lithium dithionite covering layer
was formed. After 1000 cycles, it had a capacitance of
approximately 414 mAh, capacitance loss was thus in the order of
709 mAh. With due allowance for measurement accuracy, this value
corresponds well to the capacitance loss that may be calculated on
the basis of the considerations outlined above.
* * * * *