METHOD FOR PREPARING LITHIUM METAL BY MOLTEN SALT ELECTROLYSIS

Abstract

A method for preparing lithium metal by molten salt electrolysis is provided. The method is carried out by using an electrolytic cell. The electrolytic cell is divided into an anode chamber and a cathode chamber. The anode chamber is filled with an anode molten salt electrolyte and inserted with an anode, and the cathode chamber is filled with a cathode molten salt electrolyte and inserted with a cathode. The bottom of the electrolytic cell is further filled with a liquid alloy. After the electrolytic cell is powered on, raw materials including lithium chloride, lithium carbonate, lithium hydroxide, lithium oxide, etc. are added into the anode chamber so as to obtain a lithium metal product in the cathode chamber. The method of the present invention has advantages such as continuous production, low requirements for a lithium chloride raw material, and high purity of a lithium metal product.

Description

CROSS REFERENCE TO THE RELATED APPLICATIONS

This application is the national phase entry of International Application No. PCT/CN2022/088931, filed on Apr. 25, 2022, which is based upon and claims priority to Chinese Patent Applications No. 202110499605.X, filed on May 8, 2021, and No. 202110499886.9, filed on May 8, 2021, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention belongs to the field of lithium metallurgy, and in particular, to a method for preparing lithium metal by molten salt electrolysis.

BACKGROUND

Lithium metal, known as “the energy metal in the 21st century”, is widely used in energy storage materials, nuclear industry, light alloys, and other fields. Especially the reviving of lithium metal batteries with high energy density further drives the demand for battery-grade lithium metal (Li≥99.90%).

SUMMARY

Technical Problem

Methods for producing lithium metal in industry are mainly divided into vacuum reduction method and molten salt electrolysis method. A raw material adopted in the vacuum reduction method is lithium carbonate, lithium hydroxide, or lithium oxide, and reacts with a reducing agent under high temperature and vacuum to obtain lithium vapor. The lithium vapor condenses to obtain lithium metal. The method has the advantages of low raw material cost, but also has the disadvantages of low production capacity and efficiency, working temperature as high as 1000° C., strict material requirements of the reduction tank, etc. In contrast, the molten salt electrolysis method has become the mainstream process for producing lithium metal due to the advantages of continuous production, good working performance of the electrolytic cell, mature process, etc. The principle lies in that refined anhydrous lithium chloride as the raw material is electrolyzed in the electrolytic cell containing LiCl—KCl molten salt electrolytes at 420-460° C., Cl⁻ is oxidized to precipitate chlorine gas on graphite anode, and Li⁺ is reduced to liquid lithium metal on steel rod cathode. However, the molten salt electrolysis method has the following two major disadvantages.

First, the raw material cost is high. Water brought in the raw material will have side reactions with lithium metal floating on the surface of molten salt, so as to cause a lithium loss. Also, impurity elements such as sodium, calcium, magnesium, and iron in the raw material will be preferentially reduced and enter lithium metal liquid, so as to reduce the purity of lithium product. Therefore, the raw material for electrolysis is required to be high-purity anhydrous lithium chloride. Usually, Li₂CO₃ or LiOH, used as the raw material, is subjected to recrystallization, dissolution by low sodium hydrochloric acid, solution purification, evaporation crystallization, drying and dehydration, and other processes, so as to obtain the high-purity anhydrous lithium chloride. Battery-grade anhydrous lithium chloride (YS/T744-2010) for preparing battery-grade lithium metal is more required to satisfy: LiCl≥99.5 wt %, water≤0.3 wt %, Na≤0.0015 wt %, and K≤0.05 wt %.

Second, the purity of a lithium metal product is not high. The lithium metal product obtained by electrolysis usually cannot meet the requirements of the battery-grade lithium metal. Na is the main impurity element. In order to obtain the battery-grade lithium metal, it is necessary to further distillate and purify industrial-grade lithium metal, which, however, greatly increases equipment cost and production cost.

In order to avoid the use of expensive high-purity anhydrous lithium chloride and the generation of chlorine, researchers have proposed many alternative raw materials, such as Li₂CO₃ and LiOH. However, the raw material Li₂CO₃ or LiOH in the molten salt in the conventional single-chamber electrolytic cell is very easy to react with lithium metal product, so as to cause low current efficiency.

In a word, the method for preparing lithium metal by electrolysis with lithium chloride as the raw material is still the mainstream industrial application choice at present. The method for preparing lithium metal by molten salt electrolysis with lithium carbonate, lithium hydroxide, and other raw materials can realize chlorine-free generation. A new molten salt electrolysis method with wide adaptability of raw materials, low purity requirements of raw materials, and high purity of lithium metal products is urgently needed at present.

Solution for the Problem

Technical Solution

An object of the present invention is to provide a method for preparing lithium metal by molten salt electrolysis with lithium chloride and lithium carbonate as the raw materials. The method has the advantages of low requirements for the raw materials, continuous operation, and directly available high-purity lithium metal.

To achieve the above object, the present invention adopts the following technical solution.

The method is carried out by using an electrolytic cell. The electrolytic cell is divided into an anode chamber and a cathode chamber. The anode chamber is filled with an anode molten salt electrolyte containing lithium ions and inserted with an anode. The cathode chamber is filled with a cathode molten salt electrolyte containing lithium ions and inserted with a cathode. The bottom of the electrolytic cell is further filled with a liquid alloy. The anode molten salt electrolyte and the cathode molten salt electrolyte are connected via the liquid alloy without contacting each other.

After the electrolytic cell is powered on, a lithium raw material is added into the anode chamber, oxidation reaction occurs on the anode surface. The lithium ions in the anode molten salt electrolyte are reduced to lithium atoms at the interface between the anode molten salt electrolyte and the liquid alloy and enter the liquid alloy. The lithium atoms in the liquid alloy are oxidized to lithium ions at the interface between the cathode molten salt electrolyte and the liquid alloy and enter the cathode molten salt electrolyte. The lithium ions in the cathode molten salt electrolyte are reduced to lithium atoms on the surface of the cathode. A lithium metal product is formed in the cathode chamber.

The lithium raw material includes at least one of lithium chloride, lithium carbonate, lithium hydroxide, and lithium oxide.

In the present invention, the lithium raw material is preferably lithium chloride and/or lithium carbonate.

In the present invention, the anode molten salt electrolyte is a lithium salt, or contains a lithium salt and an additive. The lithium salt is one or more of LiCl, LiF, and Li₂CO₃. The additive is one or more of KCl, KF, and BaCl₂.

In a preferred solution of the present invention, when the lithium raw material is lithium chloride, the anode molten salt electrolyte preferably consists of LiCl and one or more of KCl, LiF, and KF. And the mole percentage of LiCl in the anode molten salt electrolyte is preferably 40-85%. The decomposition voltage of KCl, LiF, or KF is higher than that of LiCl at working temperature, which can ensure the preferential decomposition of LiCl. Moreover, all the above halides have great solubility to LiCl, while fluoride is beneficial to improve the physical and chemical properties of the molten salt, such as surface tension and volatility.

In a preferred solution of the present invention, when the lithium raw material is lithium carbonate, the anode molten salt electrolyte is a lithium salt, or consists of a lithium salt and an additive. The lithium salt is one or more of LiCl, LiF, and Li₂CO₃, and the additive is one or more of KCl, KF, and BaCl₂. The decomposition voltage of LiCl, LiF, KCl, KF, or BaCl₂ is higher than that of Li₂CO₃, and Li₂CO₃ in the molten salt or the raw material is preferentially electrolyzed in the anode chamber. In addition, Li₂CO₃ has high solubility in the LiCl or LiF molten salt. For example, Li₂CO₃—LiF molten salts with the mole ratio of 0.5:0.5 can completely melt at 650° C., while Li₂CO₃—LiCl molten salts with the mole ratio of 0.3:0.7 can also completely melt at 550° C. The addition of KCl, KF, BaCl₂, and other additives is beneficial to improve the physical and chemical properties of the anode molten salt electrolyte, such as lowering the primary crystal temperature and adjusting the density of the molten salt.

In the present invention, the cathode molten salt electrolyte is a lithium salt, or contains a lithium salt and a modifier (or referred to as an additive in the present invention). The lithium salt is preferably one or more of LiF, LiCl, LiBr, and LiI. The modifier is preferably one or more of KF, KCl, KBr, and KI. When the cathode molten salt electrolyte contains a lithium salt and a modifier, the mole percentage of the lithium salt is not less than 40%.

In the present invention, when the lithium raw material is lithium chloride, the cathode molten salt electrolyte is a lithium salt, or consists of a lithium salt and a modifier. The lithium salt is one or more of LiF, LiCl, LiBr, and LiI, and the additive is one or more of KF, KCl, KBr, and KI. When the cathode molten salt electrolyte consists of the lithium salt and the modifier, the mole percentage of the lithium salt is not less than 40%.

In the cathode chamber, only Li⁺+e⁻↔Li reaction is involved, and oxidation reactions of halogen ions are not involved. Therefore, the cathode molten salt electrolyte may consist of one or more lithium salts (LiF, LiCl, LiBr, or LiI), or other halide salts (one or more of KF, KCl, KBr, and KI) may be added to adjust the melting point and other physical and chemical properties of the cathode molten salt electrolyte on this basis.

According to the method for preparing lithium metal by molten salt electrolysis in a specific embodiment of the present invention, the liquid alloy is a Li-M alloy. M is a metal element that is denser and less active than lithium metal. Preferably, M is one or more of Sn, Zn, Pb, Ag, In, Ga, Bi, and Sb. The content of lithium in the liquid alloy is preferably 5-90 at %. The density of the liquid alloy is greater than that of the anode molten salt electrolyte and that of the cathode molten salt electrolyte.

According to an alloy phase diagram, Sn, Zn, Pb, Ag, and other metals may form, with lithium, a liquid alloy having a melting point of less than 800° C. and further less than 650° C. and the lithium content of more than 5 at %. Moreover, the density of the metal elements is far greater than that of lithium metal, and therefore, the proportion of metals may be effectively controlled to form a liquid alloy having higher density than molten salt electrolytes.

The liquid alloy (Li-M alloy) may be prepared by melting lithium metal and metal or alloy M, or by electrolysis. For example, electrolysis is performed after powering on at 420-450° C. by using tin as a liquid cathode, graphite as an anode, and LiCl—KCl with the mole ratio of 3:2 as a molten salt electrolyte, so as to obtain a lithium-tin alloy.

As the electrolysis proceeds, the content of lithium in the liquid alloy may be reduced. Therefore, the lithium metal may be supplemented by fusion methods or electrolysis methods. When impurities are enriched to a certain extent, the liquid alloy is extracted and purified.

According to the method for preparing lithium metal by molten salt electrolysis in a specific embodiment of the present invention, the anode is carbon material, and the cathode is metal or alloy material that is difficult to alloy with lithium, such as steel, tungsten, and molybdenum. Preferably, the anode is graphite, and the cathode is low-cost steel. Preferably, the steel is stainless steel. The anode and the cathode may be made of the components. For example, the anode is made of a carbon material, and the cathode is made of stainless steel, tungsten, and molybdenum.

The carbon material is a widely used electrode material, which has good corrosion resistance to halide molten salts and is inert (non-consumable) to an anodic chlorine evolution reaction. The cathode materials are difficult to form an alloy with lithium metal within the working temperature, thus effectively preventing the pollution of the cathode materials to lithium metal and ensuring the purity of lithium metal products.

According to the method for preparing lithium metal by molten salt electrolysis in a specific embodiment of the present invention, the content of the main components in the lithium raw material is not less than 80 wt %. For example, the content of LiCl in the lithium chloride raw material is not less than 80 wt %. The lithium chloride raw material may be high-purity anhydrous lithium chloride used in industry at present, or ordinary anhydrous lithium chloride, or a mixture containing anhydrous lithium chloride and lithium chloride monohydrate. For another example, the lithium raw material is a lithium carbonate, and the purity of lithium carbonate is not less than 80%. The present invention has the advantages that the quality requirements of raw materials is relaxed, lithium metal with higher purity may be produced without very pure lithium carbonate, and impurities in the lithium carbonate are intercepted into the molten salt electrolyte of the anode chamber and the liquid alloy.

According to the method of the present invention, lithium raw materials such as lithium carbonate and lithium chloride are added into the anode molten salt electrolyte, and lithium ions are dissolved in the anode molten salt electrolyte. Based on an oxidation-reduction potential difference between metal and metal ions, the lithium ions in the anode molten salt electrolyte are reduced at the interface between the anode molten salt electrolyte and the lithium-containing liquid alloy and enter the lithium-containing liquid alloy. Meanwhile, lithium is oxidized at the interface between the lithium-containing liquid alloy and a cathode molten salt electrolyte to generate lithium ions which enter the cathode molten salt electrolyte. The lithium ions in the cathode molten salt electrolyte are reduced to lithium metal on the surface of the cathode and float on the cathode molten salt electrolyte.

In the electrolysis process, ions that are more difficult to reduce than the lithium ions remain in the anode molten salt electrolyte (such as K⁺) during the metal ion reduction at the interface between the anode molten salt electrolyte and the lithium-containing liquid alloy, while metals that are more difficult to oxidize than lithium remain in the lithium-containing liquid alloy (such as Na, Mg, and Fe) during the metal oxidation at the interface between the cathode molten salt electrolyte and the lithium-containing liquid alloy. Therefore, only the lithium ions enter the cathode molten salt electrolyte, the impurities in original lithium carbonate are removed, and the purity of reduced product lithium metal reaches the battery grade (purity>99.9%).

In the present invention, the anode molten salt electrolyte, the cathode molten salt electrolyte, and the liquid alloy are all in molten state at the selected electrolysis temperature in overall consideration of melting points of the anode molten salt electrolyte, the cathode molten salt electrolyte, and the liquid alloy.

Preferably, an electrolysis operation is performed in an inert atmosphere. The inert atmosphere is preferably an argon atmosphere.

In the present invention, when the electrolytic cell works normally, the electrolysis temperature is preferably 380-800° C.

In the present invention, when the electrolytic cell works normally, a current density of the cathode is 0.1-5.0 A/cm².

In a preferred solution of the present invention, when the lithium raw material is lithium chloride and the electrolytic cell works normally, the current density of the anode is preferably 0.1-2.0 A/cm² and the temperature is preferably 380-650° C.

In another preferred solution of the present invention, when the lithium raw material is lithium carbonate and the electrolytic cell works normally, the electrolysis temperature is 400-800° C., and the current density of the cathode is preferably 0.1-5.0 A/cm².

When the lithium raw material is lithium chloride, the principle of lithium chloride is as follows:

In the anode chamber, the added lithium chloride raw material may be dissolved in the anode molten salt electrolyte and dissociated into Cl⁻ and Li⁺. Cl⁻ in the anode molten salt electrolyte loses electrons on the surface of the anode and is converted into chlorine gas to evolution. Li⁺ obtains electrons at the interface between the anode molten salt electrolyte and the liquid alloy, is reduced to Li, and enters the liquid alloy. Meanwhile, Li in the liquid alloy is oxidized at the interface between the cathode molten salt electrolyte and the liquid alloy to form Li⁺ and enters the cathode molten salt electrolyte. Li⁺ in the cathode molten salt electrolyte obtains electrons at the cathode, is reduced to Li, and enters/forms a lithium metal product. The total electrolysis equation is:

LiCl=Li+0.5Cl₂↑

Since the lithium chloride raw material is added into the anode molten salt electrolyte in the anode chamber and the lithium metal product is produced from the cathode molten salt electrolyte in the cathode chamber, water (crystal water or free water) in the lithium chloride raw material cannot migrate to the cathode molten salt electrolyte, but undergoes a dehydration or evaporation reaction at high temperature, enters a gas phase and is discharged with gas flow. The lithium metal product is basically unaffected.

Based on the electrode potential differences between different elements, impurities in the lithium chloride raw material have different electrochemical behaviors. Elements more active than lithium (such as K and Ba) are enriched in the molten salt electrolyte, while elements more inert than lithium (such as Na, Mg, and Fe) are enriched in the liquid alloy. These elements are difficult to enter into the lithium metal product. Therefore, the method of the present invention not only ensures the purity of the prepared lithium metal product, but also appropriately eases the quality requirements of the lithium chloride raw material.

When the lithium raw material is lithium carbonate, the principle of lithium chloride is as follows:

Lithium carbonate, used as a raw material, is added into an anode molten salt electrolyte, and carbonate radical is dissolved or partially dissolved in the anode molten salt electrolyte. Based on the oxidation-reduction potential differences between metal and metal ions, lithium ions in the anode molten salt electrolyte are reduced at the interface between the anode molten salt electrolyte and the liquid alloy and enter the liquid alloy. Meanwhile, lithium is oxidized at the interface between the liquid alloy and the cathode molten salt electrolyte to generate lithium ions which enter the cathode molten salt electrolyte. The lithium ions in the cathode molten salt electrolyte are reduced to lithium metal on the surface of the cathode and float on the cathode molten salt electrolyte.

The total electrolysis equation is:

Li₂CO₃+0.5C=2Li(l)+1.5CO₂(g)

Beneficial Effects of the Invention

Beneficial Effects

(1) Continuous production is realized in the electrolysis process. In principle, only lithium raw materials such as LiCl and lithium carbonate are consumed in the electrolysis process. Therefore, continuous production can be realized by timely supplementing the lithium raw materials to the anode chamber and timely taking out liquid lithium metal products from the cathode chamber, and the production efficiency is high.

(2) The quality requirements of raw materials are eased. Lithium chloride raw materials with certain water and impurity content may be used for producing lithium metal by electrolysis, which reduces the raw material production cost caused by the need of high-purity anhydrous lithium chloride raw materials in the conventional electrolysis method. In addition, lithium carbonate, as a bulk industrial lithium salt, does not contain crystal water and will not deliquesce in the air, and may be used as a stable and easily obtained lithium electrolysis raw material. The electrolysis operation is continuous, the production efficiency is high, and it is possible to perform continuous charging in the anode chamber and continuous discharging in the cathode chamber. In the electrolysis process, chlorine production can be avoided, the control of impurity content can be eased, the cost of raw materials and equipment can be reduced, and the purity of the lithium metal product is high, which has obvious advantages over the conventional molten salt electrolysis method with lithium chloride as a raw material.

(3) The purity of products is guaranteed. Impurity ions with different electrochemical behaviors can be effectively controlled in the molten salt electrolyte or liquid alloy, and the purity of the lithium metal product can reach over 99.50%. Battery-grade lithium metal products can also be directly obtained under optimized conditions, thus improving product value and economic advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of the electrolytic cell of the method carried out in solution 1. In FIG. 1, 1—insulating partition; 2—cathode; 3—lithium metal product; 4—cathode molten salt electrolyte; 5—liquid alloy; 6—electrolytic cell; 7—anode molten salt electrolyte; 8—anode.

FIG. 2 is an electrolysis device diagram of the method for preparing lithium metal by molten salt electrolysis in solution 2. In FIG. 2, 1—liquid alloy; 2—anode chamber; 3—cathode chamber; 4—anode molten salt electrolyte; 5—cathode molten salt electrolyte; 6—anode; 7—cathode; 8—heating resistance wire; 9—feed inlet; 10—refractory ceramic; 11—lithium metal; 12—argon gas inlet; 13—argon gas outlet; 14—sealing flange.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In order to make the object, technical solution, and advantages of the present invention clearer, the technical solution of the present invention will be described in detail below. It is obvious that the described examples are only part of and not all of the examples of the present invention. Based on the examples in the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort fall within the scope of protection of the present invention.

1. Solution for Continuously Preparing Lithium Metal Using Lithium Chloride as the Raw Material (Solution 1):

A method for preparing lithium metal by electrolysis of lithium chloride in solution 1 is carried out by using an electrolytic cell shown in FIG. 1. The bottom of the electrolytic cell 6 is filled with a liquid alloy 5. An internal region of the electrolytic cell 6 above the liquid alloy 5 is divided into an anode chamber and a cathode chamber by an insulating partition 1. The anode chamber is filled with an anode molten salt electrolyte 7 and inserted with an anode 8. The cathode chamber is filled with a cathode molten salt electrolyte 4 and inserted with cathode 2. The anode molten salt electrolyte 7 and the cathode molten salt electrolyte 4 are connected via the liquid alloy 5 without contacting each other.

In solution 1, the method is carried out by using an electrolytic cell in FIG. 1. The electrolytic cell is divided into an anode chamber and a cathode chamber. The anode chamber is filled with an anode molten salt electrolyte containing lithium ions and inserted with an anode. The cathode chamber is filled with a cathode molten salt electrolyte containing lithium ions and has a cathode. The bottom of the electrolytic cell is further filled with a liquid alloy. The anode molten salt electrolyte and the cathode molten salt electrolyte are connected via the liquid alloy without contacting each other. After the electrolytic cell is powered on, a lithium chloride raw material is added into the anode chamber and oxidized on the surface of the anode. The lithium ions in the anode molten salt electrolyte are reduced to lithium atoms at the interface between the anode molten salt electrolyte and the liquid alloy and enter the liquid alloy. The lithium atoms in the liquid alloy are oxidized to lithium ions at the interface between the cathode molten salt electrolyte and the liquid alloy and enter the cathode molten salt electrolyte. The lithium ions in the cathode molten salt electrolyte are reduced to lithium atoms on the surface of the cathode. A lithium metal product is formed in the cathode chamber. The anode molten salt electrolyte consists of LiCl and one or more of KCl, LiF, and KF. The mole percentage of LiCl in the anode molten salt electrolyte is 40-85%. The cathode molten salt electrolyte is a lithium salt, or consists of a lithium salt and an additive. The lithium salt is one or more of LiF, LiCl, LiBr, and LiI, and the additive is one or more of KF, KCl, KBr, and KI. When the cathode molten salt electrolyte consists of a lithium salt and an additive, the mole percentage of the lithium salt is not less than 40%. The liquid alloy is a Li-M alloy. M is a metal element that is denser and less active than lithium metal. Preferably, M is one or more of Sn, Zn, Pb, Ag, In, Ga, Bi, and Sb. The density of the liquid alloy is greater than that of the anode molten salt electrolyte and that of the cathode molten salt electrolyte. The content of lithium in the liquid alloy is 5-90 at %. The anode is carbon material, preferably graphite. The cathode is metal or alloy material that is difficult to alloy with lithium, and is preferably one of steel, tungsten, and molybdenum. The content of LiCl in the lithium chloride raw material is not less than 80 wt %. When the electrolytic cell works normally, the current density of the anode is controlled at 0.1-2.0 A/cm², and the temperature is 380-650° C.

Typical cases of solution 1 include:

Example 1-1

The bottom of the electrolytic cell was filled with a pre-alloyed Li—Pb alloy with Li content of 40 at %. The anode was graphite, and the cathode was stainless steel. The anode molten salt electrolyte was LiCl—KCl with the mole ratio of 1:1, and the cathode molten salt electrolyte was LiCl—KCl with the mole ratio of 3:2. The electrolytic cell was placed in an atmosphere filled with dry argon gas, and the temperature was programmed to 500° C. and kept for 2 h. The current density of the anode was controlled at 1.0 A/cm² after powering on, and the electrolysis lasts for 10 h. During the electrolysis, a lithium chloride raw material (containing 98.4 wt % LiCl and 0.8 wt % water) was added regularly, and the content of Li in cathode lithium metal product was determined to be 99.92%.

Example 1-2

The bottom of the electrolytic cell was filled with a pre-alloyed Li—In alloy with Li content of 80 at %. The anode was modified graphite, and the cathode was a tungsten wire. The anode molten salt electrolyte was LiCl—KCl—KF with the mole ratio of 6:3.5:0.5, and the cathode molten salt electrolyte was LiF—LiCl with the mole ratio of 3:7. The electrolytic cell was placed in an atmosphere filled with dry argon gas, and the temperature was programmed to 550° C. and kept for 2 h. The current density of the anode was controlled at 2.0 A/cm² after powering on, and the electrolysis lasts for 10 h. During the electrolysis, a lithium chloride raw material (containing 89.4 wt % LiCl and 6.1 wt % water) was added regularly, and the content of Li in cathode lithium metal product was determined to be 99.83%.

Example 1-3

The bottom of the electrolytic cell was filled with a pre-alloyed Li—Ag alloy with Li content of 70 at %. The anode was graphite, and the cathode was a molybdenum wire. The anode molten salt electrolyte was LiCl—KCl—LiF with the mole ratio of 6:3.9:0.1, and the cathode molten salt electrolyte was LiCl—LiI with the mole ratio of 3.5:6.5. The electrolytic cell was placed in an atmosphere filled with dry argon gas, and the temperature was programmed to 450° C. and kept for 2 h. The current density of the anode was controlled at 0.8 A/cm² after powering on, and the electrolysis lasts for 10 h. During the electrolysis, a lithium chloride raw material (containing 93.8 wt % LiCl and 3.2 wt % water) was added regularly, and the content of Li in cathode lithium metal product was determined to be 99.90%.

Example 1-4

The bottom of the electrolytic cell was filled with a pre-alloyed Li—Sn alloy with Li content of 20 at %. The anode was graphite, and the cathode was stainless steel. The anode molten salt electrolyte was LiCl—KCl with the mole ratio of 3:2, and the cathode molten salt electrolyte was LiI—KI—KF with the mole ratio of 6:3.5:0.5. The electrolytic cell was placed in an atmosphere filled with dry argon gas, and the temperature was programmed to 385° C. and kept for 2 h. The current density of the anode was controlled at 0.5 A/cm² after powering on, and the electrolysis lasts for 10 h. During the electrolysis, a lithium chloride raw material (containing 99.1 wt % LiCl and 0.6 wt % water) was added regularly, and the content of Li in cathode lithium metal product was determined to be 99.97%.

Example 1-5

The bottom of the electrolytic cell was filled with a pre-alloyed Li—Pb alloy with Li content of 90 at %. The anode was graphite, and the cathode was carbon steel. The anode molten salt electrolyte was LiCl—KCl with the mole ratio of 8.5:1.5, and the cathode molten salt electrolyte was LiF—KF with the mole ratio of 1:1. The electrolytic cell was placed in an atmosphere filled with dry argon gas, and the temperature was programmed to 650° C. and kept for 2 h. The current density of the anode was controlled at 0.2 A/cm² after powering on, and the electrolysis lasts for 10 h. During the electrolysis, a lithium chloride raw material (containing 95.7 wt % LiCl and 1.5 wt % water) was added regularly, and the content of Li in cathode lithium metal product was determined to be 99.95%.

Example 1-6

The bottom of the electrolytic cell was filled with a pre-alloyed Li—Ga alloy with Li content of 5 at %. The anode was graphite, and the cathode was a tungsten wire. The anode molten salt electrolyte was LiCl—KCl with the mole ratio of 3:2, and the cathode molten salt electrolyte was LiBr—KBr with the mole ratio of 3:2. The electrolytic cell was placed in an atmosphere filled with dry argon gas, and the temperature was programmed to 420° ° C. and kept for 2 h. The current density of the anode was controlled at 0.1 A/cm² after powering on, and the electrolysis lasts for 10 h. During the electrolysis, a lithium chloride raw material (containing 90.5 wt % LiCl and 4.3 wt % water) was added regularly, and the content of Li in cathode lithium metal product was determined to be 99.89%.

Example 1-7

The bottom of the electrolytic cell was filled with a pre-alloyed Li—Bi alloy with Li content of 10 at %. The anode was modified graphite, and the cathode was a tungsten bar. The anode molten salt electrolyte was LiCl—KCl with the mole ratio of 2:3, and the cathode molten salt electrolyte was LiCl—KCl with the mole ratio of 2:3. The electrolytic cell was placed in an atmosphere filled with dry argon gas, and the temperature was programmed to 600° C. and kept for 2 h. The current density of the anode was controlled at 0.4 A/cm² after powering on, and the electrolysis lasts for 10 h. During the electrolysis, a lithium chloride raw material (containing 85.6 wt % LiCl and 8.9 wt % water) was added regularly, and the content of Li in cathode lithium metal product was determined to be 99.71%.

Example 1-8

The bottom of the electrolytic cell was filled with a pre-alloyed Li—Zn alloy with Li content of 60 at %. The anode was graphite, and the cathode was stainless steel. The anode molten salt electrolyte was LiF—LiCl with the mole ratio of 3:7, and the cathode molten salt electrolyte was LiF—LiI with the mole ratio of 1:4. The electrolytic cell was placed in an atmosphere filled with dry argon gas, and the temperature was programmed to 550° C. and kept for 2 h. The current density of the anode was controlled at 1.5 A/cm² after powering on, and the electrolysis lasts for 10 h. During the electrolysis, a lithium chloride raw material (containing 81.2 wt % LiCl and 13.1 wt % water) was added regularly, and the content of Li in cathode lithium metal product was determined to be 99.61%.

Comparative Example 1-1

The comparative example differs from Examples 1-6 in that the bottom of the electrolytic cell was not filled with the Li—Ag alloy, and the electrolyte was LiCl—KCl with the mole ratio of 3:2. Other conditions are the same. After electrolysis, the content of Li in cathode lithium metal product was determined to be 97.23%.

It can be seen that in the absence of the liquid alloy (the separation effect based on electrochemical reactions at the interface between the liquid alloy and the molten salt electrolyte are lost), the purity of lithium metal produced by electrolyzing lithium chloride with an ordinary partition electrolytic cell was low, and the content of impurities such as Na and Mg was high. Water in the lithium chloride raw material may react with lithium metal after entering the molten salt electrolyte, thus reducing the current efficiency.

2. Solution for Preparing Lithium Metal Using Lithium Carbonate (Solution 2):

Solution 2 is realized by an electrolysis device in FIG. 2. The electrolysis device in FIG. 2 includes an electrolytic cell 10 (refractory ceramic 10) with a jacket and a cover plate. The cover plate and the electrolytic cell are fixed by a flange 14. A heating resistance wire 8 is arranged outside a side wall of the electrolytic cell.

An inner chamber of the electrolytic cell is divided into an upper chamber and a lower chamber. The lower chamber is filled with a liquid alloy 1. The upper chamber is divided into an anode chamber 2 and a cathode chamber 3 arranged left and right by an insulating plate.

The anode chamber 2 includes an anode molten salt electrolyte 4 arranged at the bottom and floating on the surface of the liquid alloy in the lower chamber, and an anode 6 inserted in the anode molten salt electrolyte 4 and having the other end extending out of the anode chamber. And a top cover plate wall of the anode chamber is provided with a feed inlet 9, an argon gas inlet 12, and an argon gas outlet 13.

The cathode chamber 3 includes a cathode molten salt electrolyte 5 arranged at the bottom and floating on the surface of the liquid alloy in the lower chamber, a lithium metal product 11 floating on the surface of the cathode molten salt electrolyte, a cathode 7 inserted in the cathode molten salt electrolyte and having the other end extending out of the anode chamber 3, and a product extraction tube for extracting lithium metal. And a top cover plate wall of the cathode chamber 3 is provided with an argon gas inlet 12 and an argon gas outlet 13.

The method of solution 2 is carried out by using an electrolytic cell. The electrolytic cell is divided into an anode chamber and a cathode chamber. The anode chamber is filled with an anode molten salt electrolyte containing lithium ions and inserted with an anode. The cathode chamber is filled with a cathode molten salt electrolyte containing lithium ions and inserted with a cathode. The bottom of the electrolytic cell is further filled with a liquid alloy. The anode molten salt electrolyte and the cathode molten salt electrolyte are connected via the liquid alloy without contacting each other. After the electrolytic cell is powered on, lithium carbonate is added into the anode molten salt electrolyte. The lithium ions in the anode molten salt electrolyte are reduced to lithium atoms at the interface between the anode molten salt electrolyte and the liquid alloy and enter the liquid alloy. Meanwhile, the lithium atoms in the liquid alloy are oxidized to lithium ions at the interface between the liquid alloy and the cathode molten salt electrolyte and enter the cathode molten salt electrolyte. The lithium ions in the cathode molten salt electrolyte are reduced to lithium metal on the surface of the cathode. The anode is made of a carbon material, and the cathode is made of stainless steel, tungsten or molybdenum. Preferably, the cathode is made of stainless steel. The anode molten salt electrolyte is a lithium salt, or consists of a lithium salt and an additive. The lithium salt is one or more of LiCl, LiF, and Li₂CO₃, and the additive is one or more of KCl, KF, and BaCl₂. The purity of lithium carbonate is not less than 80%. The cathode molten salt electrolyte contains one or more of LiCl, LiF, LiBr, and LiI. The cathode molten salt electrolyte contains a lithium salt and a modifier. The lithium salt is one or more of LiCl, LiF, LiBr, and LiI, and the modifier is one or more of KCl, KF, KBr, and KI. The liquid alloy is an alloy formed by at least one of Zn, Ag, Sn, Pb, Sb, Bi, In, and Ga, and Li. The density of the liquid alloy is greater than that of the anode molten salt electrolyte or that of the cathode molten salt electrolyte. When the electrolytic cell works normally, the current density of the cathode is 0.1-5.0 A/cm². When the electrolytic cell works normally, the electrolysis temperature is 400-800° C. An electrolysis operation is performed in an inert atmosphere. The inert atmosphere is preferably an argon atmosphere.

Typical implementation cases of solution 2 are:

Example 2-1

This example provides a method for preparing lithium metal by molten salt electrolysis, including the following steps:

(1) As shown in FIG. 2, under the protection of an argon atmosphere, 800 g Sn—Li alloy (the mass percentage of lithium was 6%) was added to the bottom of the electrolytic cell, the alloy was heated to 550° C. and in a molten state to completely separate the anode chamber and the cathode chamber. 200 g anode chamber molten salt with the mass fraction of LiCl 44%, KCl 54%, and LiF 2% was added into the anode chamber, and 200 g cathode chamber molten salt with the mass fraction of LiCl 45% and KCl 55% was added into the cathode chamber. After the anode chamber molten salt and the cathode chamber molten salt were completely molten, a graphite anode and a tungsten bar cathode were inserted respectively.

(2) Grade-2 Li₂CO₃ in GB/T 11075-2003 was slowly added into the anode chamber, and electrolysis was performed after powering on. The current density of the cathode was 1.2 A/cm². The purity of lithium metal was 99.92% after 10 h of electrolysis under the inert condition.

Example 2-2

This example provides a method for preparing lithium metal by molten salt electrolysis, including the following steps:

(1) As shown in FIG. 2, under the protection of an argon atmosphere, 800 g Sn—Li alloy (the mass percentage of lithium was 1%) was added to the bottom of the electrolytic cell, heated to 400° ° C., and molten into a liquid alloy to completely separate the anode chamber and the cathode chamber. 200 g anode chamber molten salt with the mass fraction of LiCl 44%, KCl 54%, and LiF 2% was added into the anode chamber, and 200 g cathode chamber molten salt with the mass fraction of LiCl 45% and KCl 55% was added into the cathode chamber. After the anode chamber molten salt and the cathode chamber molten salt were completely molten, a graphite anode and a tungsten bar cathode were inserted respectively.

(2) Grade-2 Li₂CO₃ in GB/T 11075-2003 was slowly added into the anode chamber, and electrolysis was performed after powering on. The current density of the cathode was 0.1 A/cm². The purity of lithium metal was 99.92% after 10 h of electrolysis under the inert condition.

Example 2-3

This example provides a method for preparing lithium metal by molten salt electrolysis, including the following steps:

(1) As shown in FIG. 2, under the protection of an argon atmosphere, 800 g Ag—Li alloy (the mass percentage of lithium was 20%) was added to the bottom of the electrolytic cell, heated to 400° C., and molten into a liquid alloy to completely separate the anode chamber and the cathode chamber. 200 g anode chamber molten salt with the mass fraction of LiCl 45% and KCl 55% was added into the anode chamber, and 200 g cathode chamber molten salt with the mass fraction of LiCl 45% and KCl 55% was added into the cathode chamber. After the anode chamber molten salt and the cathode chamber molten salt were completely molten, a graphite anode and a tungsten rod cathode were inserted respectively.

(2) Grade-2 Li₂CO₃ in GB/T 11075-2003 was slowly added into the anode chamber, and electrolysis was performed after powering on. The current density of the cathode was 1.2 A/cm². The purity of lithium metal was 99.93% after 10 h of electrolysis under the inert condition.

Example 2-4

This example provides a method for preparing lithium metal by molten salt electrolysis, including the following steps:

(1) As shown in FIG. 2, under the protection of an argon atmosphere, 800 g lithium-containing liquid alloy, namely Ag—Li alloy (the mass percentage of lithium was 20%) was added to the bottom of the electrolytic cell, heated to 800° C., and molten into a liquid alloy to completely separate the anode chamber and the cathode chamber. 200 g anode chamber molten salt with the mass fraction of LiCl 45% and KCl 55% was added into the anode chamber, and 200 g cathode chamber molten salt with the mass fraction of LiCl 45% and KCl 55% was added into the cathode chamber. After the anode chamber molten salt and the cathode chamber molten salt were completely molten, a graphite anode and a tungsten rod cathode were inserted respectively.

(2) Li₂CO₃ with a purity of 80% was slowly added into the anode chamber, and electrolysis was performed after powering on. The current density of the cathode was 0.8 A/cm². The purity of lithium metal was 99.90% after 10 h of electrolysis under the inert condition.

Example 2-5

This example provides a method for preparing lithium metal by molten salt electrolysis, including the following steps:

(1) As shown in FIG. 2, under the protection of an argon atmosphere, 800 g Ag—Li—Sn alloy (the mass fraction of lithium was 20% and the mass fraction of Sn was 20%) was added to the bottom of the electrolytic cell, heated to 800° C., and molten into a liquid alloy to completely separate the anode chamber and the cathode chamber. 200 g anode chamber molten salt with the mass fraction of LiCl 40%, KCl 50%, and LiF 10% was added into the anode chamber, and 200 g cathode chamber molten salt with the mass fraction of LiCl 45% and KCl 55% was added into the cathode chamber. After the anode chamber molten salt and the cathode chamber molten salt were completely molten, a graphite anode and a tungsten rod cathode were inserted respectively.

(2) Li₂CO₃ with a purity of 80% was slowly added into the anode molten salt electrolyte, and electrolysis was performed after powering on. The current density of the cathode was 0.8 A/cm². The purity of lithium metal was 99.90% after 10 h of electrolysis under the inert condition.

Examples 2-6 to examples 2-12 below were compared with Example 2-1:

									Purity
			Anode	Cathode			Current		of
			molten	molten			density of		lithium
	Alloy	Temperature/	salt	salt			cathode/	Raw	metal/
Example	component	° C.	electrolyte	electrolyte	Anode	Cathode	A · cm−2	material	%
2-1	Sn-6	500	LiCl	LiCl	Graphite	Tungsten	1.2	Grade-2	99.92
	wt. % Li		44 wt. %	45 wt. %		bar		Li2CO3
			KCl	KCl				in GB/T
			54 wt. %	55 wt. %				11075-2003
			LIF
			2 wt. %
2-6	Sn-6	500	LiCl	LiCl	Graphite	Stainless	1.2	Grade-2	99.92
	wt. % Li		44 wt. %	45 wt. %		steel		Li2CO3
			KCl	KCl				in GB/T
			54 wt. %	55 wt. %				11075-2003
			LIF
			2 wt. %
2-7	Sn-10	500	LiCl	LiCl	Graphite	Tungsten	1.2	Grade-2	99.92
	wt. % Li		44 wt. %	45 wt. %		bar		Li2CO3
			KC1	KCl				in GB/T
			54 wt. %	55 wt. %				11075-2003
			LiF
			2 wt. %
2-8	Sn-6	600	LiCl	LiCl	Graphite	Stainless	1.2	Grade-2	99.92
	wt. % Li		44 wt. %	45 wt. %		steel		Li2CO3
			KCl	KCl				in GB/T
			54 wt. %	55 wt. %				11075-2003
			LiF
			2 wt %
2-9	Ag-10	800	LiCl	LiCl	Graphite	Stainless	0.1	Grade-2	99.92
	wt. % Li		50 wt. %	50 wt. %		steel		Li2CO3
			LiF	LIF				in GB/T
			50 wt. %	50 wt. %				11075-2003
2-10	Ag-20	400	LiCl	LiCl	Graphite	Stainless	0.1	Grade-2	99.93
	wt. % Li		45 wt. %	45 wt. %		steel		Li2CO3
			KCl	KCl				in GB/T
			55 wt. %	55 wt. %				11075-2003
2-11	Pb-5	800	LIF	LIF	Graphite	Tungsten	0.1	Grade-2	99.94
	wt. % Li		80 wt. %	60 wt. %		bar		Li2CO3
			Li2CO3	LiCl				in GB/T
			20 wt. %	40 wt. %				11075-2003
2-12	Ag-20	400	LiCl	LiCl	Graphite	Tungsten	0.1	Li2CO3	99.90
	wt. % Li		45 wt. %	45 wt. %		bar		with
			KCl	KCl				purity of
			55 wt. %	55 wt. %				85%

Comparative Example 2-1

1000 g molten salt with the mass fraction of 45% LiCl and 55% KCl was added into the electrolytic cell, heated to 450° C., and molten, and graphite as anode and a tungsten rod as cathode were immersed in the electrolyte respectively.

Grade-2 lithium carbonate (with a purity of 98.5%) in GB/T 11075-2003 was slowly added into the anode molten salt electrolyte in the argon atmosphere, and electrolysis was performed after powering on. The current density of the anode was 1.2 A/cm². The purity of lithium metal was 98% at the cathode after 10 h of electrolysis.

The above descriptions are only specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions which may be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be covered by the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope of protection of the claims.

Claims

1. A method for preparing a lithium metal by a molten salt electrolysis,

wherein the method is carried out by using an electrolytic cell, the electrolytic cell is divided into an anode chamber and a cathode chamber, the anode chamber is filled with an anode molten salt electrolyte containing lithium ions and inserted with an anode, the cathode chamber is filled with a cathode molten salt electrolyte containing lithium ions and inserted with a cathode, a bottom of the electrolytic cell is further filled with a liquid alloy, and the anode molten salt electrolyte and the cathode molten salt electrolyte are connected via the liquid alloy without contacting each other;

after the electrolytic cell is powered on, adding a lithium raw material into the anode chamber for an oxidation reaction occurring on an anode surface, wherein the lithium ions in the anode molten salt electrolyte are reduced to lithium atoms at an interface between the anode molten salt electrolyte and the liquid alloy and enter the liquid alloy, the lithium atoms in the liquid alloy are oxidized to lithium ions at an interface between the cathode molten salt electrolyte and the liquid alloy and enter the cathode molten salt electrolyte, the lithium ions in the cathode molten salt electrolyte are reduced to lithium atoms on a surface of the cathode, and a lithium metal product is formed in the cathode chamber; and

the lithium raw material comprises at least one of lithium chloride, lithium carbonate, lithium hydroxide, and lithium oxide.

2. The method for preparing the lithium metal by the molten salt electrolysis according to claim 1, wherein the anode molten salt electrolyte is a lithium salt, or contains a lithium salt and an additive;

the lithium salt is one or more of LiCl, LiF, and Li2CO3; and

the additive is one or more of KCl, KF, and BaCl2.

3. The method for preparing the lithium metal by the molten salt electrolysis according to claim 1, wherein when the lithium raw material is the lithium chloride, the anode molten salt electrolyte consists of LiCl and one or more of KCl, LiF, and KF.

4. The method for preparing the lithium metal by the molten salt electrolysis according to claim 3, wherein a mole percentage of LiCl in the anode molten salt electrolyte is 40-85%.

5. The method for preparing the lithium metal by the molten salt electrolysis according to claim 1, wherein the cathode molten salt electrolyte is a lithium salt, or contains a lithium salt and a modifier;

the lithium salt is one or more of LiF, LiCl, LiBr, and LiI; and

the modifier is one or more of KF, KCl, KBr, and KI.

6. The method for preparing the lithium metal by the molten salt electrolysis according to claim 5, wherein when the cathode molten salt electrolyte contains the lithium salt and the modifier, a mole percentage of the lithium salt is not less than 40%.

7. The method for preparing the lithium metal by the molten salt electrolysis according to claim 1, wherein the liquid alloy is a Li-M alloy,

M is a metal element denser and less active than the lithium metal; and

a density of the liquid alloy is greater than a density of the anode molten salt electrolyte and a density of the cathode molten salt electrolyte.

8. The method for preparing the lithium metal by the molten salt electrolysis according to claim 7, wherein a content of lithium in the liquid alloy is 5-90 at %.

9. The method for preparing the lithium metal by the molten salt electrolysis according to claim 1, wherein the anode is a carbon material;

the cathode is a metal material or an alloy material that is-difficult to alloy with lithium.

10. The method for preparing the lithium metal by the molten salt electrolysis according to claim 1, wherein the lithium raw material is a lithium chloride raw material, and a content of LiCl in the lithium chloride raw material is not less than 80 wt %; or

the lithium raw material is a lithium carbonate raw material, and a purity of lithium carbonate in the lithium carbonate raw material is not less than 80%.

11. The method for preparing the lithium metal by the molten salt electrolysis according to claim 1, wherein when the electrolytic cell works normally, an electrolysis temperature is 380-800° C. and a current density of the cathode is 0.1-5.0 A/cm2.

12. The method for preparing the lithium metal by the molten salt electrolysis according to claim 1, wherein the lithium raw material is the lithium chloride, and when the electrolytic cell works normally, a current density of the anode is controlled at 0.1-2.0 A/cm2 and a temperature is 380-650° C.

13. The method for preparing the lithium metal by the molten salt electrolysis according to claim 1, wherein the lithium raw material is the lithium carbonate, and when the electrolytic cell works normally, an electrolysis temperature is 400-800° ° C., and a current density of the cathode is 0.1-5.0 A/cm2.

14. The method for preparing the lithium metal by the molten salt electrolysis according to claim 5, wherein when the lithium raw material is lithium chloride, the cathode molten salt electrolyte is the lithium salt, or consists of the lithium salt and the modifier.

15. The method for preparing the lithium metal by the molten salt electrolysis according to claim 2, wherein the liquid alloy is a Li-M alloy,

M is a metal element denser and less active than the lithium metal; and

a density of the liquid alloy is greater than a density of the anode molten salt electrolyte and a density of the cathode molten salt electrolyte.

16. The method for preparing the lithium metal by the molten salt electrolysis according to claim 3, wherein the liquid alloy is a Li-M alloy,

M is a metal element denser and less active than the lithium metal; and

a density of the liquid alloy is greater than a density of the anode molten salt electrolyte and a density of the cathode molten salt electrolyte.

17. The method for preparing the lithium metal by the molten salt electrolysis according to claim 4, wherein the liquid alloy is a Li-M alloy,

M is a metal element denser and less active than the lithium metal; and

a density of the liquid alloy is greater than a density of the anode molten salt electrolyte and a density of the cathode molten salt electrolyte.

18. The method for preparing the lithium metal by the molten salt electrolysis according to claim 5, wherein the liquid alloy is a Li-M alloy,

M is a metal element denser and less active than the lithium metal; and

a density of the liquid alloy is greater than a density of the anode molten salt electrolyte and a density of the cathode molten salt electrolyte.

19. The method for preparing the lithium metal by the molten salt electrolysis according to claim 6, wherein the liquid alloy is a Li-M alloy,

M is a metal element denser and less active than the lithium metal; and

a density of the liquid alloy is greater than a density of the anode molten salt electrolyte and a density of the cathode molten salt electrolyte.

20. The method for preparing the lithium metal by the molten salt electrolysis according to claim 7, wherein M is one or more of Sn, Zn, Pb, Ag, In, Ga, Bi, and Sb.

21. The method for preparing the lithium metal by the molten salt electrolysis according to claim 9, wherein the anode is graphite; and

the cathode is one of steel, tungsten, and molybdenum.