FR2698488A1 - High power electrochemical cell functioning at high temperature - uses lithium@ anode, metallic sulphide cathode and eutectic electrolyte constituted by mixture of lithium salts, which is liquid above 485-550 deg.C - Google Patents

Abstract

The electrochemical cell includes an anode of pure or alloyed lithium, and a cathode constituted by a sulphide of e.g. iron, nickel, cobalt or titanium. The electrolyte, integrally accommodated in a retaining structure, is a eutectic mixture of lithium chloride (60-80%) and lithium fluoride (40-20%), having a melting point between 485 and 550 deg.C. Suitable retaining structures are oxides of silicon, magnesium or aluminium, or boron nitride. The compound selected forms an additive, between 10 and 50% by weight, preferably 20 to 40%, to the electrolyte, and is solid at the cell's operational temperature, increasing the viscosity of the electrolyte and stabilising it mechanically. Optionally a porous dielectric material may serve as the structure. ADVANTAGE - Higher energy per unit volume, lower internal resistance, and better voltage stability throughout discharge cycle, than with previous designs.

Description

High-performance lithium electrochemical generator
temperature
The present invention relates to an electrochemical lithium generator operating at high temperature intended for high power applications.

 Electrochemical generators with thermal release are sources of energy of great reliability and supporting long periods of storage at room temperature (of the order of 20 years) without notable deterioration of their performances.

 This type of generator includes an electrolyte which is in the solid state at a temperature below the operating temperature of the electrochemical generator.

At operating temperature, it must be immobilized to avoid any risk of short circuit; this is achieved by integrating the electrolyte into a reception structure. This structure may for example be a porous material in which the electrolyte is solidified, or a chemical compound, added to the electrolyte in the form of a powder mixture, capable of increasing the viscosity of the molten electrolyte-additive mixture with the generator operating temperature.

Most current electrochemical generators, operating at high temperature (of the order of 500C), use the electrochemical couple lithium / iron sulfide, with an anode of lithium alloy (lithium-aluminum
Li-Al, lithium-silicon Li-Si, or lithium-boron Li-B), in an electrolyte which is the eutectic LiCl-KCl (melting temperature: 352C).

 Today, it is the electrolyte that solidifies which limits the discharge of these electrochemical generators: the presence of potassium results in the formation of practically insoluble discharge products which block the reaction and increase the melting temperature of the electrolyte.

In some cases the eutectic LiCl-LiBr-LiF (melting temperature 435'C) is also used as an electrolyte. It has drawbacks linked to the presence of bromine:
- limitation of usable current densities,
- high hygroscopy requiring particularly harsh processing conditions,
- shaping made difficult by a small proportion of LiCl.

The object of the present invention is to produce an electrochemical generator operating at high temperature capable of supplying a power and a volume energy greater than existing systems.
The object of the present invention is an electrochemical lithium generator operating above 4850C comprising: - at least one anode chosen from lithium alloys, such as lithium-aluminum Li-Al, lithium-silicon
Li-Si, or lithium-boron Li-B, and lithium inserted in a metallic receiving structure, - at least one cathode containing a metallic sulfide, such as iron sulfide FeS2, nickel sulfide NiS2, sulfide of cobalt CoS2, or titanium sulfide TiS2, - and an electrolyte integrated in a host structure, characterized in that said electrolyte consists of an eutectic mixture of lithium chloride and lithium fluoride with melting point between 4850C and 5500C, the molar proportion of said lithium chloride being between 60% and 80%, and that of said lithium fluoride being between 40% and 20%.

 The proportions of LiCl and LiF correspond to the temperature range in which the generator is used; above 6000 C, the electrodes degrade.

 The presence of a high proportion of LiCl (greater than 60%) facilitates the shaping of the electrolyte.

 According to one embodiment, said reception structure is an additive chosen from silica SiO2, magnesia MgO, and alumina A1203, and boron nitride BN, in a proportion of between 10% and 50% by weight of said electrolyte, preferably 20% to 40%. The additive is solid at the operating temperature of the electrochemical generator; its role is to increase the viscosity of the molten electrolyte-additive mixture at this temperature, and to keep the electrolyte in place under the effect of the capillarity and surface wetting.

 For an additive content of less than 10%, the liquid electrolyte is not maintained enough and may cause short circuits. When the proportion exceeds 50%, the electrolyte becomes too resistant and the voltage drops.

 According to another embodiment, said reception structure is a porous dielectric material chosen from felts and fabrics of alumina, boron nitride, or even asbestos.

 Said anode is composed of a mixture of a lithium alloy chosen from lithium-aluminum Li-Al and lithium-silicon Li-Si, and said electrolyte in a proportion of between 5% and 30% by weight of said alloy, preferably 10% to 20%.

 For an electrolyte content of less than 5%, the wetting of the anode is not sufficiently homogeneous; beyond 30%, the anode cannot retain the excess electrolyte which escapes.

 Said cathode is composed of a mixture of iron sulfide, such as FeS or FeS2, and of said electrolyte in a proportion of between 15% and 50% by weight of iron sulfide, preferably 20% to 40%.

 For an electrolyte content of less than 15%, the wetting of the cathode is not sufficiently homogeneous and causes operating heterogeneities; beyond 50%, the cathode cannot retain the excess electrolyte which escapes.

 According to one embodiment, said electrochemical generator is rechargeable.

 The main advantages of the present invention are a reduction in the internal resistance in discharge, thanks to the higher ionic conductivity of the LiCl-LiF eutectic, and a better solubility of the products formed during the discharge, obtained by the suppression of potassium. . Higher discharge voltages are thus obtained which remain stable throughout the duration of the discharge, and the faradaic discharge yields are increased.

 Other advantages of the present invention are also the shaping of the electrolyte pellets by the powder compacting technique made easier by the higher content of LiCl, and better storage and less restrictive processing of these lozenges, because the LiCl-LiCF eutectic is much less hygroscopic than the LiCl-LiBr-LiF eutectic.

 Other characteristics and advantages of the present invention will appear on reading the following examples of embodiments, and in the appended drawing, given by way of illustration but in no way limitative.

In the attached drawing:
- Figure 1 shows the DC discharge curves of a monocouple according to the prior art and a monocouple according to the invention in which the anode comprises the lithium-aluminum alloy Li-Al, the discharge is carried out at 5000 C;
- Figure 2 shows the pulse current discharge curves of a monocouple according to the prior art and a monocouple according to the invention, identical to those of Figure 1, the discharge is carried out at 5000 C;
- Figure 3 gives a variant of Figure 2 where the discharge is carried out at 550qu.

- Figure 4 shows the DC discharge curves of a monocouple according to the prior art and a monocouple according to the invention in which the anode comprises the lithium-silicon alloy Li-Si, the discharge is carried out at 5000 C;
- Figure 5 shows the DC cycling curves of a rechargeable electrochemical generator according to the prior art and of a generator according to the invention, the cycling is carried out at 5000 C;
Figures 1 to 5 include on the ordinate the voltage U in volts and on the abscissa the time t in minutes.

EXAMPLE 1 Generator with lithium aluminum anode
In FIG. 1 are shown discharge curves produced with a direct current density of 0.1 A / cm 2, at a temperature of 500 @ C. In most applications of these electrochemical cells, the discharge is continued as long as the voltage remains greater than 75% of the voltage at the start of the discharge. The discharge stop voltage corresponds to this limit value, in this case 1.5 Volts (discharge start voltage of the order of 2 Volts).

 According to the prior art, an electrolyte composed of the eutectic LiCl-KCl was produced. The receiving structure of the electrolyte is constituted by the addition of an additive, such as magnesia MgO, in a proportion of 30% by weight of electrolyte. The liquid mixture is homogenized at 5000C under a neutral atmosphere for 30 minutes. After cooling, the solidified mixture is reduced to powder, then compacted to produce an electrolyte tablet 0.35 mm thick.

The anodic mixture is produced from a lithium-aluminum alloy Li-Al powder and the electrolyte
LiCl-KCl also in powder at 10% by weight of
Li-Al. The powder mixture is compacted to produce an anode 0.4 mm thick.

 The cathode mixture contains iron sulfide FeS2 and the electrolyte LiCl-KCl in an amount of 30% by weight of FeS2.

The liquid mixture is homogenized at 500ex for 30 minutes. After cooling, the solidified mixture is finely ground, then pelletized by compaction to produce a cathode 0.4 mm thick.

 An electrochemical test is then carried out on a monocouple comprising the Li-Al anode, the LiCl-RCl electrolyte tablet and the FeS2 cathode. The discharge curve 1 corresponding to the prior art is obtained: the voltage at mid-discharge is 1.76V, and the duration of discharge is 9.3 minutes until the stop voltage is 1.5V.

In the same way as that described above, an electrolyte is produced according to the invention which is the eutectic LiCl-LiF with a melting point of the order of 485 C, composed of approximately 70 mol% of lithium chloride
LiCl and about 30 mol% of lithium fluoride LiF. An additive, such as magnesia MgO, is added to it in a proportion of 30% by weight of electrolyte.

 The anode is produced as above from the powders of the lithium-aluminum alloy Li-Al and the electrolyte LiCl-LiF in an amount of 10% by weight of Li-Al.

Carried out in the same way as above, the cathode contains iron sulfide FeS2 and the electrolyte LiCl-LiF at a rate of 30% by weight of FeS2.

 An electrochemical test was carried out on a monocouple according to the invention, comprising the Li-Al anode, the LiCl-LiF electrolyte tablet and the FeS2 cathode. The discharge curve 2 according to the invention is shown in FIG. 1. A mid-discharge voltage of 1.79V is observed, and a discharge duration of 13.5 minutes up to the stop voltage of 1, 5V.

This result corresponds to an increase in the duration of discharge of the order of 45% compared to the prior art.

 Another electrochemical test was carried out with a pulse current, as shown in FIG. 2. The discharge is carried out at a temperature of 500 μl, with a pulse current density of lA / cm 2 imposed for 5 seconds followed by a period of rest 10 seconds.

 The discharge curve 3 is obtained with a monocouple of the prior art, identical to that described above.

A mid-discharge voltage of 1.60V is observed, and a discharge duration of 12 pulses up to the stop voltage of 1.5V.

 Under the same conditions, a monocouple according to the invention was tested, identical to that described above. The discharge curve 4 obtained is shown in FIG. 2. A mid-discharge voltage of 1.66V is observed, and a discharge duration of 22 pulses up to the stop voltage of 1.5V. This result corresponds to an increase in the duration of discharge of the order of 83% compared to the prior art.

 Yet another electrochemical test was carried out in which the discharge is carried out in pulse current under the same conditions as the test shown in FIG. 2, but at 550 "C.

 The discharge curve 5 is obtained on the monocouple of the prior art described above. A mid-discharge voltage of 1.62V is observed, and a discharge duration of 13 pulses up to the stop voltage of 1.5V.

 Under the same conditions, the monocouple according to the invention was tested, identical to that previously described. The discharge curve 6 obtained is shown in FIG. 3. A mid-discharge voltage of 1.74V is observed, and a discharge duration of 24 pulses up to the stop voltage of 1.5V. This result corresponds to an increase in the duration of discharge of the order of 85% compared to the prior art.

EXAMPLE 2 Generator with Lithium-Silicon Anode
FIG. 4 shows discharge curves carried out with a direct current density of 0.1 A / cm 2, at a temperature of 500ex. The stop voltage corresponds to 75% of the start of discharge voltage, in this case 1.60 Volts (start of discharge voltage of about 2.15 Volts).

 In the same manner as described in Example 1, an electrolyte according to the prior art composed of the eutectic LiCl-KCl was produced. The reception structure is constituted by the addition of an additive, such as magnesia MgO, in a proportion of 30% by weight of electrolyte.

 The anode is produced as in Example 1 from the powders of the lithium-silicon alloy Li-Si and the electrolyte LiCl-KCl in an amount of 10% by weight of Li-Si.

 Manufactured as described in Example 1, the cathode contains iron sulfide FeS2 and the electrolyte LiCl-KCl in an amount of 30% by weight of FeS2.

An electrochemical test is then carried out on a monocouple according to the prior art comprising the anode of
Li-Si, the electrolyte tablet LiCl-KCl and the cathode of
FeS2. On the discharge curve 7 given in FIG. 4, a mid-discharge voltage of 1.88V is observed, and a discharge duration of 8.2 minutes up to the stop voltage of 1.60V.

 According to the invention, an electrolyte composed of the eutectic LiCl-LiF is produced, to which an additive, such as MgO, is added in a proportion of 30% by weight of electrolyte.

 As above, the anode is produced by mixing the powders of the lithium-silicon alloy Li-Si and the electrolyte LiCl-LiF in an amount of 10% by weight of Li-Si.

Similarly, the cathode contains iron sulfide FeS2 and the electrolyte LiCl-LiF in an amount of 30% by weight of FeS2.

 An electrochemical test was carried out on a monocouple according to the invention, comprising the Li-Si anode, the LiCl-LiF electrolyte tablet and the FeS2 cathode. The discharge curve 8 according to the invention is shown in FIG. 4. A mid-discharge voltage of 1.92V is observed, and a discharge duration of 14 minutes up to the stop voltage of 1.60V.

This result corresponds to an increase in the duration of discharge of the order of 70% compared to the prior art.

EXAMPLE 3: Rechargeable generator
FIG. 5 shows the cycling curves of a rechargeable electrochemical generator according to the prior art and of a rechargeable electrochemical generator according to the invention. Cycling is carried out at a direct current density of 50mA / cm2 and a temperature of 500 C.

 In the same manner as described in Example 1, an electrolyte according to the prior art composed of the eutectic LiCl-KCl was produced. The receiving structure of the electrolyte is constituted by the addition of an additive, such as magnesia MgO, in a proportion of 30% by weight of electrolyte.

 The anode is produced as in Example 1 from the powders of the lithium-aluminum alloy Li-Al and the electrolyte LiCl-KCl in an amount of 10% by weight of Li-Al.

 Manufactured as described in Example 1, the cathode contains iron sulfide FeS2 and the electrolyte LiCl-KCl in an amount of 30% by weight of FeS2.

 The electrochemical test of the electrochemical generator comprising the Li-Al anode, the LiCl-KCl electrolyte tablet and the FeS2 cathode is then carried out.

 The initial discharge is carried out according to curve 9 in FIG. 5 up to a stop voltage of 1.5 volts; the mid-discharge voltage is 1.70 Volts for a total discharge time of 20 minutes. Recharging is then carried out to a stop voltage of 2.15 Volts (curve 10). A second discharge is obtained under the same conditions as the initial discharge which is given by curve 11; the mid-discharge voltage is 1.71 Volts and the duration of the discharge is 16 minutes.

In the same way as that described above, an electrolyte is produced according to the invention which is the eutectic LiCl-LiF with a melting point of the order of 485 C, composed of approximately 70 mol% of lithium chloride
LiCl and about 30 mol% of lithium fluoride LiF. An additive, such as magnesia MgO, is added to it as a reception structure in a proportion of 30% by weight of electrolyte.

 The anode is produced as above from the powders of the lithium-aluminum alloy Li-Al and the electrolyte LiCl-LiF in an amount of 10% by weight of Li-Al.

Carried out in the same way as above, the cathode contains iron sulfide FeS2 and the electrolyte LiCl-LiF at a rate of 30% by weight of FeS2.

The electrochemical test of the electrochemical generator according to the invention was carried out, comprising the anode of
Li-Al, the electrolyte tablet LiCl-LiF and the cathode of
FeS2.

 The initial discharge curve 12 of the generator according to the invention is shown in FIG. 5. A mid-discharge voltage of 1.73V is observed, and a discharge duration of 33 minutes until the stop voltage of 1 , 5V. This result corresponds to an increase in the duration of discharge of the order of 65% compared to the prior art.

 After recharging the generator to a voltage of 2.15 volts, a second discharge is carried out under the same conditions as the initial discharge. The mid-discharge voltage is 1.71 Volts for a discharge time of 30 minutes, an increase in the duration of the discharge by 87% compared to a generator of the prior art.

 Of course, the present invention is not limited to the embodiments described, but it is capable of numerous variants accessible to those skilled in the art without departing from the spirit of the invention. In particular, without going beyond the scope of the invention, any means can be replaced by equivalent means.

Claims (5)

1. / Electrochemical lithium generator operating above 4850C comprising: - at least one anode chosen from lithium alloys and lithium inserted in a metallic receiving structure, - at least one cathode containing a metallic sulfide, - and an electrolyte integrated into a reception structure, characterized in that said electrolyte consists of an eutectic mixture of lithium chloride and lithium fluoride with a melting point of between 4850C and 5500 C, the molar proportion of said lithium chloride being between 60% and 80%, and that of said lithium fluoride being between 40% and 20%. 2. / an electrochemical generator according to claim 1, characterized in that said receiving structure is an additive chosen from silica, magnesia, and alumina, and boron nitride, in a proportion of between 15% and 50% by weight of said electrolyte.  3. / electrochemical generator according to one of the preceding claims, characterized in that said anode is composed of a mixture of a lithium alloy chosen from lithium aluminum and lithium silicon, and said electrolyte in a proportion between 5% and 30% by weight of said alloy. 4. / electrochemical generator according to one of the preceding claims, characterized in that said cathode is composed of a mixture of iron sulfide and said electrolyte in a proportion between 15% and 50% by weight of iron sulfide. 5. / electrochemical generator according to one of the preceding claims, characterized in that it is rechargeable.