The present invention relates to an electrochemical device, in particular a lithiumair
type battery with an aqueous electrolyte, as well as a method for storing and releasing
electrical energy using a lithium-air battery according to the invention.
5
The energy density per unit mass (expressed in Wh/kg) of batteries is still the
main factor limiting their use in portable appliances, such as portable electronics or
electric vehicles. The limited energy density of these batteries is mainly due to the
performance of the materials from which they are made. The best negative-electrode
10 materials currently available generally have a specific capacity of between 300 and
350Ah/kg. The specific capacity is only about 100 to 150Ah/kg for positive-electrode
materials.
The advantage of lithium-air systems is that the positive electrode has an infinite
capacity. The oxygen consumed at the positive electrode does not need to be stored in
15 the electrode but can be obtained from the ambient air.
The air electrode requires a basic or acidic aqueous medium to operate optimally.
Unfortunately, the lithium metal used for the negative electrode reacts too strongly with
water, and it is impossible for it to form in the presence of water during recharging
because water reduces at voltages that are much too low, preventing lithium metal from
20 forming. A watertight physical barrier is therefore required between the negativeelectrode
compartment, which is based on lithium metal, and the positive-electrode
compartment containing an aqueous electrolyte. This watertight physical barrier must
nonetheless selectively allow metal cations to pass from the aqueous electrolyte to the
negative electrode and in the opposite direction.
25
Ceramic materials that meet these requirements, called “Li Super Ionic
Conductors” (LISICON), have been known for some time. These materials have
advantageously high conductivities ranging up to 10-4 or even 10-3 S/cm at 25 °C and
have a good chemical stability with respect to the aqueous electrolyte in the positive30
electrode compartment (air electrode). However, they react very strongly with the
lithium metal in the anode compartment and it is essential to isolate them, in a known
3
way, from the lithium metal using a protective coating, for example a coating based on a
lithium phosphorous oxynitride (LiPON) glass.
The first work done to develop a primary, i.e. nonrechargeable, Li-air battery dates
from the 1970s (US 4 057 675). These batteries suffered from high self-discharge and a
short lifetime due to corrosion (reaction of the lithium with water). A battery deliverin5 g
around 1.2 kW of power, composed of six modules, was nevertheless constructed (W.
R. Momyer et al. (1980), Proc. 15th Intersoc. Energy Convers. Eng. Conf., page 1480).
A rechargeable Li/O2 battery without an aqueous phase, employing a polymer
electrolyte containing a lithium salt, was also produced (K. M. Abraham et al. (1996), J.
10 Electrochem. Soc. 143(1), pages 1-5). Employing a porous carbon-based positive
electrode in this cell gave good results in terms of oxygen reduction, but this electrode
was not adapted to oxidation during recharging. It was possible to implement only three
cycles, and, to the knowledge of the Applicant, this work was complemented by two
publications: Ogasawara et al., Journal of the American Chemical Society (2006) 125(4)
15 1393 and Kumar et al., Journal of the Electrochemical Society (2010) 157 (1): A50–
A54.
During discharge of a lithium-air battery, the oxygen is reduced in the positiveelectrode
compartment (O2 + 4 e- + 2 H2O → 4 OH-), the alkali metal is oxidated in the
negative-electrode compartment (4 Li → 4 Li+ + 4 e-) and the alkali-metal ions thus
20 formed migrate to the positive-electrode compartment where they can precipitate if their
concentration reaches the solubility limit and form lithium hydroxide. The
concentration of lithium hydroxide therefore increases in the aqueous electrolyte during
discharge of the battery and decreases during charge of the battery when the alkali-metal
ions migrate again to the negative-electrode compartment where they are reduced there
25 and the hydroxyl ions are oxidated at the positive electrode.
The specific capacity of the battery thus depends solely on the capacity of the
negative electrode and on the capacity of the battery to store the product of the oxygen
reduction, i.e. the lithium hydroxide formed in the compartment of the positive electrode
during discharge of the battery.
30 So that the battery has the highest possible specific capacity, it is desirable to
strongly limit the volume of aqueous electrolyte and to use the most concentrated
solutions possible.
4
However, the inventors have shown that the presence of lithium ions in the
aqueous electrolyte has a blocking effect on oxygen release during recharge of the
battery. This blocking effect had already been observed in the past, but in a very
different context: lithium had been used in the aqueous electrolyte of nickel batteries as
an additive to prevent oxygen release when this reaction competes with the reaction o5 f
nickel electrodes (Constantin et al, “The influence of some additives on the
electrochemical behaviour of sintered nickel electrodes in alkaline electrolyte”, Journal
of Power Sources, 74 (1998), 188-197). On the contrary, in the context of the present
invention, and in particular in the context of lithium-air batteries, the blocking effect of
10 the lithium ions is detrimental because it necessitates the application of an additional
overvoltage across the terminals of the battery during recharge. The energy efficiency of
the battery, i.e. the ratio (electrical energy flowing out of the battery when discharging /
electrical energy consumed to recharge the battery), is thus decreased.
To improve the energy efficiency of the battery, it is therefore desirable to
15 decrease the concentration of lithium ions in the aqueous electrolyte.
From these observations it transpires that the improvement in the specific capacity
of the battery and in the energy efficiency of the battery seem to be two irreconcilable
goals. In spite of this, the inventors have succeeded in improving the energy efficiency
20 of a lithium-air type battery using an aqueous electrolyte without decreasing its specific
capacity.
The subject of the present invention is a lithium-air battery comprising:
- a negative-electrode compartment containing lithium metal;
- a positive-electrode compartment comprising at least one positive air electrode in
25 contact with an aqueous solution containing lithium hydroxide;
- a solid electrolyte separating, in a gas- and liquid-tight manner, the negativeelectrode
compartment from the positive-electrode compartment;
characterized in that the aqueous solution containing lithium hydroxide further
contains at least one additive decreasing the solubility of the lithium ions.
30
The documents US 4 684 584 and US 5 427 873, which refer to an additive
decreasing the solubility of lithium ions, relate to lithium-water cells and not lithium-air
5
batteries. These cells are not intended to be recharged. The problem of energy efficiency
does not therefore arise. Moreover, these cells do not comprise any positive air
electrode.
The solubility of a compound denotes the maximum concentration of thi5 s
compound that can be solubilized in a solvent. The additive present in the aqueous
solution has the effect of decreasing the solubility of the lithium ions, and of decreasing
the concentration of lithium Li+ ions in an aqueous solution saturated with lithium.
The solubility limit of lithium hydroxide in water is 5.2 mol/L at 20 °C (according
10 to D.R. Lide, CRC Handbook of Chemistry and Physics, New York, 2005). The
presence of an additive according to the invention advantageously makes it possible to
decrease this solubility limit of lithium hydroxide. The solubility limit of lithium
hydroxide in the aqueous solution containing the additive is preferably below 4 mol/L,
more preferably below 3 mol/L, and more preferably still below 2 mol/L. The solubility
15 limit is however preferably maintained above 1 mol/L, at lower values issue of resolubilization
of the lithium hydroxide when recharging the battery can occur.
Advantageously, the solubility limit of lithium hydroxide in the aqueous solution
containing the additive according to the invention can lie between 1 mol/L and 2 mol/L.
This additive can be chosen by those skilled in the art from among the known
20 chemical compounds having a solubility in water above the solubility of lithium and
forming a salt with the hydroxide ions. Preferably, the additive is an alkali metal
hydroxide. More preferably, the additive decreasing the solubility of the lithium ions is
chosen from the group formed by potassium hydroxide and sodium hydroxide. More
preferably still, the additive is potassium hydroxide.
25 The concentration of the additive decreasing the solubility of the lithium ions in
the aqueous solution is fixed by those skilled in the art, as a function of the nature of this
additive, so as to lower the solubility limit of lithium hydroxide down to the values
described above. When the additive is potassium, its concentration in the aqueous
solution containing lithium hydroxide can lie between 1 mol/L and 10 mol/L, more
30 preferably between 4 mol/L and 9 mol/L, and more preferably still between 7 mol/L and
8 mol/L.
6
The additive can be added to the aqueous solution in any form. In particular, the
additive can be added in the form of a hydroxide salt. If the additive is sodium, it is
possible to add to the aqueous solution a sodium hydroxide salt. If the additive is
potassium, it is possible to add to the aqueous solution a potassium hydroxide salt.
The inventors have observed that the presence of at least one additive according t5 o
the invention in the aqueous solution constituting the liquid electrolyte in a lithium-air
battery has the effect of significantly reducing the charging voltage of the battery.
Furthermore, the presence of the additive according to the invention has no impact on
the energy released during discharge of the battery. As a consequence, the energy
10 efficiency of the lithium-air battery is improved.
Another subject of the present invention is therefore the use of at least one
additive decreasing lithium ion solubility to improve the energy efficiency of a lithiumair
battery, the additive being contained in the aqueous solution containing lithium
hydroxide constituting the liquid electrolyte of the lithium-air battery.
15
Furthermore, the presence of at least one additive according to the invention in the
aqueous solution constituting the liquid electrolyte in a lithium-air battery has no
prohibitive impact on the specific capacity of the battery. Indeed, if the concentration of
the lithium hydroxide reaches and exceeds the saturation concentration, the alkali metal
20 hydroxide precipitates. The formation of a precipitate is not problematic because, when
the battery is being recharged, the precipitate can solubilise again and release the lithium
ions. The lithium hydroxide precipitate thus constitutes a lithium ions reservoir.
It has however been observed in the past that just as lithium hydroxide precipitates
in the aqueous electrolyte, a dense crystalline layer of lithium hydroxide can form at the
25 surface of the solid electrolyte membrane. The presence of this dense layer, which
cannot conduct cations, can provoke a very large increase in the cationic resistance of
the system at the interface between the solid electrolyte membrane and the aqueous
electrolyte.
Advantageously, the battery according to the invention has a means for preventing
30 the formation of a dense crystalline layer of lithium hydroxide at the surface of the solid
electrolyte.
7
The means for preventing the formation of a dense crystalline layer of lithium
hydroxide at the surface of the solid electrolyte can be a organic cation-conducting
polyelectrolyte layer. Such organic polyelectrolytes have for example been described in
the patent application WO 2011 051597.
According to an advantageous embodiment, the subject of the present invention i5 s
a lithium-air battery containing,
- as the solid electrolyte, a alkali-metal cation conducting ceramic membrane,
covered with an organic, insoluble and cation-conducting polyelectrolyte that is
chemically stable in water with a basic pH, and
10 - as the liquid electrolyte, a lithium hydroxide aqueous solution of, in contact with
said organic polymer, the aqueous solution containing lithium hydroxide, furthermore
containing at least one additive decreasing the solubility of the lithium ions.
The negative-electrode compartment can comprise any electrode able to form
15 lithium ions, for example a lithium metal electrode, an alloy electrode, for example
lithium/silicon or lithium/tin, or an electrode of a insertions material , for example
lithium/graphite. Preferably, the negative-electrode compartment comprises a lithium
metal electrode, because of the high energy density of this type of electrode.
Positive air electrodes are known to the prior art. Generally, an air electrode has a
20 solid porous structure, with a large specific surface area, in contact with the liquid
electrolyte. The interface between the air electrode and the liquid electrolyte is a socalled
“triple contact” interface at which the active material at the electrode, the gaseous
oxidant and the liquid electrolyte are simultaneously present.
The positive air electrode is preferably designed to allow the triple contact
25 between the electrolyte, the gaseous oxidant and the solid active material of the
electrode. In principle, it can be chosen from among all those usually used in the art and
described in particular in the article by Neburchilov et al. “A review on air cathodes for
zinc-air fuel cells”, Journal of Power Sources, 195 (2010), 1271-1291.
This preferably involves an electrode obtained by agglomeration of a carbon
30 powder composed of carbon grains with a high surface area, such as Vulcan® XC72,
marketed by Cabot. The specific surface area of carbon can be increased by reacting it
with a gas such as CO2, prior to its incorporation in the air electrode. Advantageously,
8
the specific surface area of the carbon grains is high. Indeed, the higher it is, the higher
the current density per unit of geometrical electrode surface area. The porous electrode
is fabricated preferably by agglomeration of the carbon grains using a binding agent,
which is preferably a hydrophobic fluoride polymer such as Teflon® FEP marketed by
Dupont. A detailed description of an air electrode for a metal-air accumulator can fo5 r
example be found in application WO 2000/036677.
Preferably, the positive air electrode further contains at least one oxygen reduction
catalyst. This oxygen reduction catalyst is preferably chosen from the group consisting
of manganese oxide and cobalt oxide.
10 The positive air electrode can further comprise an anion exchange polymer
membrane forming a separation between the electrode material and the aqueous solution
constituting the liquid electrolyte. Such membranes, which are for example described in
the patent application WO 2010/128242, advantageously make it possible to protect the
positive air electrode from deterioration due to the progressive carbonatation of the
15 electrolyte.
Recharging a lithium-air battery using an alkali electrolyte in contact with the air
electrode is carried out by reduction of Li+ ion into lithium metal at the negative
electrode (4 Li+ + 4 e- → 4 Li) and by oxidation of OH- ions at the positive electrode to
20 produce molecular oxygen and water (4 OH- → O2 + 4 e- + 2H2O).
The oxygen-releasing reaction can be carried out directly on an air electrode, but
the air electrode is designed and optimized for reducing an electrochemical reaction
with a gas (oxygen of the air) and a liquid (the electrolyte). For this reason, this
electrode is preferably porous, with the largest possible reaction surface area. This
25 structure makes it more fragile and less suited to a reaction with a liquid only to produce
a gas. Furthermore, the catalysts used in the air electrode for improving the reaction of
electrochemical reduction of oxygen in the alkaline electrolyte (manganese oxides or
cobalt-based compound) are not stable at the more positive potentials required for the
oxygen releasing reaction.
30 It is therefore preferable to use a second positive electrode which will be used
during the battery recharging phases only.
9
The battery according to the invention can therefore further advantageously
comprise a second positive oxygen releasing electrode, in contact with the aqueous
solution containing lithium hydroxide. The second positive oxygen releasing electrode
can, for example, be an electrode made of steel, preferably stainless steel, typically a
316L type steel , or a nickel electrode. In the battery, this electrode can typically have 5 a
grid-type or perforated plate-type structure, and it can be located between the negative
electrode and the air electrode. The perforated structure of this electrode is used to
ensure free passage of the components of the liquid electrolyte between the negative
electrode and the air electrode during discharge.
10 According to an embodiment using a battery equipped with a second positive
electrode, the first positive air electrode is decoupled during the phases of recharge of
the battery. Charge is then carried out on the second positive oxygen releasing electrode.
During the battery discharging phase, the second positive oxygen releasing electrode is
decoupled and discharge is carried out on the first positive air electrode. The battery can
15 be equipped with means for switching between electrodes, and possibly with a
controlling means making it possible to control the switching. Such means are described
for example in the patent application FR 11 54356.
Finally, one subject of the present invention is a method for storing and releasing
20 electrical energy using a lithium-air battery according to the invention, comprising the
following successive steps:
(a) a charging phase during which the lithium metal contained in the negativeelectrode
compartment oxidates and precipitates in the form of lithium hydroxide in the
aqueous solution of the positive-electrode compartment;
25 (b) a recharging phase during which the lithium hydroxide solubilizes to release
lithium ions which are reduced at the negative electrode .
The invention will be better understood in the light of the following non-limiting
and purely illustrative examples, accompanied by the appended figures among which:
30 - Figure 1 is a graph representing the saturation concentration of Li+ ions in
solution as a function of the concentration of K+ ions (theoretical concentrations and
concentrations actually measured by ICP-AES) ;
10
- Figure 2 is a graph representing the oxygen evolution reaction (OER) potential,
measured at 33 mA.cm-2, on an unactivated steel fabric as a function of the theoretical
concentration of K+
x ions in the electrolytic mixtures KxLiSat (T = 25°C ; Surface area =
3.14 cm2 ; measurements performed after 30 minutes of operation) ;
- Figure 3 is a graph representing the overvoltage of the OER, at 33 mA.cm-2, as 5 a
function of the actual concentration of Li+ ions in solution (T = 25°C ; Surface area =
3.14 cm2 ; the values were acquired after 30 minutes of operation).
Example
10
Preparation of the liquid electrolytes
Aqueous solutions saturated with lithium hydroxide and containing an additive
according to the invention (here potassium) with a concentration varying from 1 M to
8 M were prepared according to the following protocol:
15 The exact masses of LiOH powder and KOH powder theoretically contained in
the solutions were mixed. The mixtures were introduced into graduated flasks and water
was added to the graduation line. The solutions were then mixed by magnetic stirring
and ultrasounds. The solubilization of the compounds gave rise to an increase in the
temperature of the solutions, which were naturally cooled to the laboratory temperature,
20 T = 25°C, because of this. The water level was topped up a second time, then the
solutions were blended again by magnetic stirring. The solution/powder mixtures
obtained were then filtered and the level of the electrolyte was topped up.
The various solutions were named KxLiSat, where x is the theoretical concentration
of the solution in potassium ions; K0LiSat refers to a solution saturated with LiOH
25 without additive.
Effect of the additive on the decrease in lithium ions solubility
The electrolytes thus prepared were analyzed by ICP-AES; they were diluted
between 1 000 times, for the K0LiSat solution, and 10 000 times for the K8LiSat mixture.
30 Details on the ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry)
method of analysis can be found in the text “Handbook of Elemental Speciation:
Techniques and Methodology”, by Klaus G. Heumann.
11
Figure 1 shows the saturation concentrations of Li+ ions of the solutions as a
function of their concentration of K+ ions. Up to an approximate concentration of 6 M of
potassium ions in solution, the theoretical values are in agreement with experimental
values. Once this value of K+ ion concentration is exceeded, the concentration of Li+
ions falls drastically with respect to the theoretically defined value5 .
It is observed that the addition of potassium makes it possible to decrease the
concentration of lithium ions in the aqueous solution saturated with lithium hydroxide.
The influence of the modification of the electrolyte on 316L steel fabrics with
imposed current was measured. On the values shown below, the ohmic drop has been
10 compensated for.
Effect of the additive on the oxygen releasing reaction potential
The oxygen releasing reaction potentials were measured after 30 minutes of
operation at 33 mA.cm-2 on 316L steel fabrics. A new electrode was used on each run in
15 order to avoid being disturbed by changes due to the development of a catalytic layer at
their surface.
Figure 2 illustrates the evolution of the oxygen evolution reaction (OER)
potential, measured at 33 mA.cm-2, on an unactivated steel fabric as a function of the
theoretical concentration of K+
x ions in the electrolytic mixtures KxLiSat (T = 25°C ; Sgeo
20 = 3.14 cm2 ; measurements performed after 30 minutes of operation).
Figure 3 illustrates the evolution of the overvoltage of the OER, at 33 mA.cm-2,
as a function of the actual concentration of Li+ ions in solution. The standard potential
of the reaction was determined in aqueous solution saturated with lithium hydroxide at
270 mV vs. [Hg/HgO - 1 M KOH] (T = 25°C ; Surface area = 3.14 cm2 ; the values
25 were acquired after 30 minutes of operation).
It is noted that the presence of the additive according to the invention in the
aqueous electrolyte has the effect of decreasing the overvoltage at the terminals of the
electrode during the oxygen releasing reaction.
30 Effect of the additive on the energy losses of the oxygen releasing reaction
12
The measurements of the electrocatalytic performance of an electrode of 316L
steel have been recorded in quasi-stationary mode on a turning disk electrode (TDE) of a
geometric surface area 0.19 cm2 (T = 25°C ; vb = 0.1 mV s-1 ; Sgeo = 0.19 cm2).
It has been observed that the use of an additive in the electrolyte makes it possible
to reduce the OER potential, whatever the electrode current. The differences are all th5 e
more marked when the concentration of additive in the mixture is higher, but also when
the electrode current is higher.
The various kinetic parameters of the electrode of 316L steel are summarized in
table 1, according to the electrolytic medium used.
10
13
Electrode potential E
vs. [Hg/HgO – 1 M
KOH] at 33 mA cm-2
Electrode
overvoltage
ηO2
Reduction of
overvoltages
K0Lisat = aqueous
solution saturated
with LiOH
868 mV 598 mV
K1LiSat 820 mV 550 mV 8.0 %
K2LiSat 807 mV 537 mV 10.2 %
K3LiSat 793 mV 523 mV 12.5 %
K4LiSat 782 mV 512 mV 14.4 %
K5LiSat 766 mV 496 mV 17.1 %
K6LiSat 758 mV 488 mV 18.4 %
K7LiSat 743 mV 473 mV 20.9 %
Table 1
The overvoltages ηO2 (KxLiSat) were determined with respect to the standard
potential E°H2O/O2 in the aqueous solution saturated with LiOH without additive (ηO2 o5 f
K0LiSat). The standard potential E°H2O/O2 (in aqueous solution saturated with LiOH
without additive) was determined at 270 mV vs. [Hg/HgO – 1 M KOH].
The reduction of the overvoltages was computed in the following manner:
Value = ( (ηO2 of K0LiSat) – (ηO2 of KxLiSat) ) / ηO2 of K0LiSat
10
In conclusion, the use of an additive in the aqueous electrolyte significantly
reduces the overvoltage, and therefore the energy losses of the oxygen releasing
electrode under current of systems of the type involving lithium-air batteries with
aqueous electrolytes.
15
14
I/We Claim:
1. A lithium-air battery comprising :
- a negative-electrode compartment containing lithium metal ;
- a positive-electrode compartment comprising at least one positive air electrode5 ,
in contact with an aqueous solution containing lithium hydroxide ;
- a solid electrolyte separating, in a gas- and liquid-tight manner, the negativeelectrode
compartment from the positive-electrode compartment;
characterized in that the aqueous solution containing lithium hydroxide
10 furthermore contains at least one additive decreasing the solubility of the lithium ions.
2. The battery as claimed in claim 1, characterized in that the solubility
limit of lithium hydroxide in the aqueous solution containing the additive is below
4 mol/L, more preferably below 3 mol/L, and more preferably still between 1 mol/L and
15 2 mol/L.
3. The battery as claimed in either one of claims 1 or 2, characterized in that
the additive decreasing the solubility of the lithium ions is an alkali metal hydroxide.
20 4. The battery as claimed in claim 3, characterized in that the additive
decreasing the solubility of the lithium ions is chosen from the group made up of
potassium hydroxide and sodium hydroxide, the additive preferably being potassium
hydroxide.
25 5. The battery as claimed in any one of claims 1 to 4, characterized in that
it possesses a means for preventing the formation of a dense crystalline layer of lithium
hydroxide at the surface of the solid electrolyte.
6. The battery as claimed in claim 5, characterized in that the means for
30 preventing the formation of a dense crystalline layer of lithium hydroxide at the surface
of the solid electrolyte is a layer of an organic cation-conducting polyelectrolyte.
15
7. The battery as claimed in any one of claims 1 to 6, characterized in that
it further comprises a positive oxygen releasing electrode, in contact with the aqueous
solution containing lithium hydroxide.
8. The battery as claimed in any one of claims 1 to 7, characterized in tha5 t
the positive air electrode further comprises an anion exchange polymer membrane
forming a separation between the electrode material and the aqueous solution
constituting the liquid electrolyte.
10 9. A method for storing and releasing electrical energy using a lithium-air
battery as claimed in any one of claims 1 to 8, comprising the following successive
steps :
(a) a charging phase during which the lithium metal contained in the negativeelectrode
compartment oxidates and precipitates in the form of lithium hydroxide in the
15 aqueous solution of the positive-electrode compartment;
(b) a recharging phase during which the lithium hydroxide solubilizes to free
lithium ions which are reduced at the level of the negative electrode.
10. A use of at least one additive reducing the solubility of the lithium ions
20 to improve the energy efficiency of a lithium-air battery, the additive being contained in
the aqueous solution containing lithium hydroxide constituting the liquid electrolyte of
the lithium-air battery.