A bipolar membrane electrodialysis method and system for preparing lithium hydroxide from lithium nitrate
By combining a five-compartment bipolar membrane electrodialysis device with a specific type of membrane, the problems of large chemical reagent consumption and environmental pollution in the existing lithium hydroxide production have been solved. This has enabled efficient and low-cost recovery of lithium hydroxide and nitric acid, and is suitable for the recycling of waste lithium batteries and the production of inorganic acids and alkalis.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- TIANJIN POLYTECHNIC UNIV
- Filing Date
- 2024-12-30
- Publication Date
- 2026-06-30
AI Technical Summary
Existing lithium hydroxide production processes, such as precipitation and electrolysis, suffer from problems such as large consumption of chemical reagents and environmental pollution. Furthermore, the preparation of battery-grade lithium hydroxide by membrane electrolysis of lithium nitrate has not yet formed a mature industrial application method.
A five-compartment bipolar membrane electrodialysis device was used to prepare high-purity lithium hydroxide and nitric acid by introducing lithium nitrate solution into the salt compartment and deionized water into the acid and alkali compartments, and applying current to both sides of the anode and cathode plates. A specific model of bipolar membrane and ion exchange membrane produced by Hangzhou Huamo Technology Co., Ltd. was used, combined with a peristaltic pump and conductivity measuring device, to control the current density and voltage to optimize the process.
It achieves high recovery rates of lithium hydroxide and nitric acid, with recovery rates exceeding 99%, reducing energy consumption and material costs, simplifying the process, and is suitable for the recycling of waste lithium batteries and the production of inorganic acids and alkalis.
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Abstract
Description
Technical Field
[0001] This invention relates to a bipolar membrane electrodialysis system and a method for preparing lithium hydroxide, and particularly to a method for preparing lithium hydroxide from lithium nitrate solution via bipolar membrane electrodialysis. Background Technology
[0003] Currently, the most widely used lithium hydroxide production processes are precipitation and electrolysis. The precipitation method involves adding a lithium carbonate solution to limewater; the difference in solubility between lithium carbonate and calcium hydroxide allows them to react, producing calcium carbonate precipitate and lithium hydroxide. The calcium carbonate precipitate is then separated by filtration, and after multiple crystallizations, a high-purity lithium hydroxide product is obtained. However, this method requires large amounts of chemical reagents, and the resulting calcium carbonate precipitate is a significant environmental pollutant. Electrolysis is a novel method for preparing lithium hydroxide. High-purity lithium hydroxide products have been obtained from various lithium salts through electrolysis. Numerous studies have also explored the bipolar membrane method for preparing lithium hydroxide. In patent CN201510526884, a bipolar membrane method for recovering lithium hydroxide from solution involves treating a lithium sulfate solution with a bipolar membrane system. The resulting water after treatment yields lithium hydroxide and sulfuric acid. Bipolar membrane technology has also been used to prepare lithium hydroxide from salt lake brine (CN201710972445) and lithium carbonate (CN201410124102, CN202110235269). However, a mature method suitable for industrial application has not yet been developed for the membrane electrolysis of lithium nitrate to produce battery-grade lithium hydroxide. To address these issues, a bipolar membrane electrolysis process for preparing lithium hydroxide from lithium nitrate is proposed. Summary of the Invention
[0004] This invention provides a method for preparing lithium hydroxide with high recovery rate, comprising the following steps:
[0005] Using a five-compartment bipolar membrane electrodialysis device, lithium nitrate solution is introduced into the salt compartment, deionized water is introduced into the acid and alkali compartments respectively, and strong electrolyte solutions are introduced into the anode and cathode compartments. Then, current is applied to both sides of the anode and cathode plates to perform electrodialysis. High-purity lithium hydroxide is obtained in the alkali compartment, and high-purity nitric acid is obtained in the acid compartment.
[0006] The present invention also provides a small five-compartment bipolar membrane electrodialysis device, comprising a cathode plate, a membrane stack, and an anode plate arranged sequentially; the membrane stack includes a set of electrodialysis units and a second bipolar membrane; the electrodialysis unit includes a first bipolar membrane, a cation exchange membrane, an anion exchange membrane, and a second bipolar membrane arranged sequentially; the cation exchange membrane layers of the first and second bipolar membranes face the cathode plate, and the anion exchange membrane layers of the first and second bipolar membranes face the anode plate; the cathode plate and the first bipolar membrane form a cathode chamber; the first bipolar membrane and the cation exchange membrane in the electrodialysis unit form an alkali chamber; the cation exchange membrane and the anion exchange membrane in the electrodialysis unit form a salt chamber; the anion exchange membrane and the second bipolar membrane in the electrodialysis unit form an acid chamber; and the second bipolar membrane and the anode plate form an anode chamber.
[0007] The bipolar membrane is an HMBM-4012 type bipolar ion exchange membrane manufactured by Hangzhou Huamo Technology Co., Ltd.; the cation exchange membrane is a TCEM8040 type cation exchange membrane manufactured by Hangzhou Huamo Technology Co., Ltd.; the anion exchange membrane is a TAEM8040 type anion exchange membrane manufactured by Hangzhou Huamo Technology Co., Ltd.; sealing gaskets are provided between the cathode chamber and the alkali chamber, between the alkali chamber and the salt chamber, between the salt chamber and the acid chamber, and between the acid chamber and the anode chamber, and the sealing gaskets are all made of silicone. The electrodialysis device is connected in series with bolts in the order of "cathode chamber - alkali chamber - salt chamber - acid chamber - anode chamber"; the anode plate is connected to the positive terminal of the power supply; the cathode plate is connected to the negative terminal of the power supply; the anode plate and cathode plate are titanium-coated ruthenium electrode plates; the bipolar membrane electrodialysis device also includes a power supply; the bipolar membrane electrodialysis device preferably also includes a conductivity measuring device; the conductivity measuring device is used to test the conductivity of the liquid in the salt chamber; when the conductivity of the salt chamber is lower than 500 μs / cm or the voltage is raised back to the upper voltage limit, electrodialysis is stopped.
[0008] The electrodialysis apparatus includes a feed tank and a peristaltic pump; the cathode feed tank is connected to the cathode chamber via the peristaltic pump; the alkali chamber feed tank is connected to the alkali chamber via the peristaltic pump; the salt chamber feed tank is connected to the salt chamber via the peristaltic pump; the acid chamber feed tank is connected to the acid chamber via the peristaltic pump; and the anode feed tank is connected to the anode chamber via the peristaltic pump.
[0009] All membranes in the device have an effective area of 9 cm². 2 The thickness of each chamber is 12 mm; the thickness of each gasket is 1 mm; the thickness of each anode and cathode plate is 1.5 mm; the effective contact area between the anode and cathode plates is 9 cm². 2 The usable volume of the cathode chamber and anode chamber is 10.35 cm³. 3 The usable volume of the alkali chamber, acid chamber, and salt chamber is 12.6 cm³. 3The catholyte, salt chamber solution, acid chamber solution, alkali chamber solution, and anolyte are circulated separately at a flow rate of 25 mL / min; the operating pressure is atmospheric pressure, and the operating temperature is room temperature; the applied voltage for the entire device is 30-60V, and the operating current density is 20-60 mA / cm². 2 Before applying current, the solution needs to be circulated for more than 30 minutes under the action of a peristaltic pump to ensure that there are no air bubbles in the infusion pipeline.
[0010] Experimental results show that the lithium hydroxide solution prepared using the bipolar membrane electrodialysis device provided by this invention has a conductivity of 20-60 mA / cm². 2 The recovery rate of lithium hydroxide produced at various current densities can reach over 99%, and the recovery rate of nitric acid produced at a current density of 40 mA / cm² can reach 99%. The lithium hydroxide solution prepared by this invention can achieve a recovery rate of over 99%, and the nitric acid recovery rate can also reach 99%, maximizing the recovery of both acid and alkali. Furthermore, the method and equipment of this invention are simple, the materials are readily available, and the cost is low. A high-purity lithium hydroxide solution can be obtained through a single electrodialysis step, avoiding the energy consumption associated with subsequent lithium hydroxide purification. This invention has significant implications for the recycling of waste lithium batteries and the production of inorganic acids and alkalis. Attached Figure Description
[0011] Figure 1 This is a schematic diagram of the bipolar film device for preparing lithium hydroxide according to the present invention.
[0012] Figure 2 This is a schematic diagram of the bipolar membrane component of the bipolar membrane device for preparing lithium hydroxide according to the present invention.
[0013] Figure 3 This is a schematic diagram of voltage changes during the electrodialysis process in Examples 1 to 5 of the present invention.
[0014] Figure 4 This is a schematic diagram of the change in conductivity during the electrodialysis process in Examples 1 to 5 of the present invention.
[0015] Figure 5 The diagram shows the current efficiency and energy consumption of lithium hydroxide in Examples 1 to 5 of this invention.
[0016] Figure 6 The diagram shows the recovery rates of lithium hydroxide and nitric acid in Examples 1-5 of this invention.
[0017] Figure 7 This is a schematic diagram of voltage changes during the electrodialysis process in Examples 3 and 6-8 of the present invention.
[0018] Figure 8 This is a schematic diagram of the change in conductivity during the electrodialysis process in Examples 3 and 6-8 of the present invention.
[0019] Figure 9This is a schematic diagram showing the current efficiency and energy consumption of lithium hydroxide in Examples 3 and 6-8 of the present invention.
[0020] Figure 10 This is a schematic diagram showing the recovery rates of lithium hydroxide and nitric acid in Examples 3 and 6-8 of the present invention. Detailed Implementation
[0021] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.
[0022] Example 1.
[0023] This example uses, for example Figure 1 The bipolar membrane electrodialysis apparatus shown is used. 150 mL of 0.3 mol / L sodium sulfate solution is introduced as the strong electrolyte solution; 100 mL of 0.3 mol / L lithium nitrate solution (conductivity ~23 mS / cm) is introduced into the salt chamber; and 100 mL of deionized water is introduced into both the acid and alkali chambers. During the experiment, the flow rate of each solution in the apparatus is 25 mL / min, and the operation is constant current with a current density of 20 mA / cm². 2 The current is set to 0.18A.
[0024] The experiment continues until the conductivity of the salt chamber drops below 500 μS / cm or the voltage is increased back to the upper voltage limit. Figure 3 and Figure 4 The diagram shows the voltage and conductivity. The lithium-ion content in the alkali chamber was detected using inductively coupled plasma atomic emission spectrometry (ICP-AES). The lithium hydroxide concentration obtained in the alkali chamber was 0.2972 mol / L, and the sulfuric acid concentration obtained in the acid chamber was 0.2603 mol / L. The recovery rate of lithium hydroxide from alkali production was 99.07%, and the recovery rate of nitric acid from acid production was 86.76%. The energy consumption was 16.58 kWh / kgLiOH. The current efficiency, energy consumption, and recovery rate of lithium hydroxide are shown in the diagram below. Figure 5 and Figure 6 As shown, this indicates that the device can achieve a high lithium hydroxide recovery rate.
[0025] Example 2
[0026] The bipolar membrane electrodialysis device used in this example is the same as the device used in Example 1.
[0027] 150 mL of 0.3 mol / L sodium sulfate solution was introduced as a strong electrolyte solution; 100 mL of 0.3 mol / L lithium nitrate solution (conductivity ~23 mS / cm) was introduced into the salt chamber of the bipolar membrane electrodialysis device; 100 mL of deionized water was introduced into the acid and alkali chambers of the bipolar membrane electrodialysis device, respectively. During the experiment, the flow rate of each solution in the membrane stack was 25 mL / min, and the operation was constant current with a current density of 30 mA / cm². 2 The current is set to 0.27A.
[0028] The experiment continues until the conductivity of the salt chamber drops below 500 μS / cm or the voltage is increased back to the upper voltage limit. Figure 3 and Figure 4 The diagram shows the voltage and conductivity. The lithium-ion content in the alkali chamber was detected using inductively coupled plasma atomic emission spectrometry (ICP-AES). The lithium hydroxide concentration obtained in the alkali chamber was 0.2972 mol / L, and the sulfuric acid concentration obtained in the acid chamber was 0.28 mol / L. The recovery rate of lithium hydroxide from alkali production was 99.07%, and the recovery rate of nitric acid from acid production was 93.33%. The energy consumption was 20.44 kWh / kgLiOH. The current efficiency, energy consumption, and recovery rate of lithium hydroxide are shown in the diagram below. Figure 5 and Figure 6 As shown, this indicates that the device can achieve a high lithium hydroxide recovery rate.
[0029] Example 3
[0030] The bipolar membrane electrodialysis device used in this example is the same as the device used in Example 1.
[0031] 150 mL of 0.3 mol / L sodium sulfate solution was introduced as a strong electrolyte solution; 100 mL of 0.3 mol / L lithium nitrate solution (conductivity ~23 mS / cm) was introduced into the salt chamber of the bipolar membrane electrodialysis device; 100 mL of deionized water was introduced into the acid and alkali chambers of the bipolar membrane electrodialysis device, respectively. During the experiment, the flow rate of each solution in the membrane stack was 25 mL / min, and the operation was constant current with a current density of 40 mA / cm². 2 The current is set to 0.36A.
[0032] The experiment continues until the conductivity of the salt chamber drops below 500 μS / cm or the voltage is increased back to the upper voltage limit. Figure 3 ( Figure 7 )and Figure 4 ( Figure 8The diagram shows the voltage and conductivity. The lithium ion content in the alkali chamber was detected using inductively coupled plasma atomic emission spectrometry (ICP-AES). The lithium hydroxide concentration obtained in the alkali chamber was 0.2972 mol / L, and the sulfuric acid concentration obtained in the acid chamber was 0.28 mol / L. The recovery rate of lithium hydroxide from alkali production was 99.07%, and the recovery rate of nitric acid from acid production was 99%. The energy consumption was 24.32 kWh / kgLiOH. The current efficiency, energy consumption, and recovery rate of lithium hydroxide are shown in the diagram below. Figure 5 ( Figure 9 )and Figure 6 ( Figure 10 As shown in the figure, this indicates that the device can achieve high lithium hydroxide and high nitric acid recovery rates.
[0033] Example 4
[0034] The bipolar membrane electrodialysis device used in this example is the same as the device used in Example 1.
[0035] 150 mL of 0.3 mol / L sodium sulfate solution was introduced as a strong electrolyte solution; 100 mL of 0.3 mol / L lithium nitrate solution (conductivity ~23 mS / cm) was introduced into the salt chamber of the bipolar membrane electrodialysis device; 100 mL of deionized water was introduced into the acid and alkali chambers of the bipolar membrane electrodialysis device, respectively. During the experiment, the flow rate of each solution in the membrane stack was 25 mL / min, and the operation was constant current with a current density of 50 mA / cm². 2 The current is set to 0.45A.
[0036] The experiment continues until the conductivity of the salt chamber drops below 500 μS / cm or the voltage is increased back to the upper voltage limit. Figure 3 and Figure 4 The diagram illustrates the voltage and conductivity. The lithium-ion content in the alkali chamber was determined using inductively coupled plasma atomic emission spectrometry (ICP-AES). The lithium hydroxide concentration obtained in the alkali chamber was 0.2972 mol / L, and the sulfuric acid concentration obtained in the acid chamber was 0.273 mol / L. The recovery rate of lithium hydroxide from alkali production was 99.07%, and the recovery rate of nitric acid from acid production was 91%. The energy consumption was 29.71 kWh / kgLiOH. The current efficiency, energy consumption, and recovery rate of lithium hydroxide are illustrated in the diagram below. Figure 5 and Figure 6 As shown, this indicates that the device can achieve a high lithium hydroxide recovery rate.
[0037] Example 5
[0038] The bipolar membrane electrodialysis device used in this example is the same as the device used in Example 1.
[0039] 150 mL of 0.3 mol / L sodium sulfate solution was introduced as a strong electrolyte solution; 100 mL of 0.3 mol / L lithium nitrate solution (conductivity ~23 mS / cm) was introduced into the salt chamber of the bipolar membrane electrodialysis device; 100 mL of deionized water was introduced into the acid and alkali chambers of the bipolar membrane electrodialysis device, respectively. During the experiment, the flow rate of each solution in the membrane stack was 25 mL / min, and the operation was constant with a current density of 60 mA / cm². 2 The current is set to 0.54A.
[0040] The experiment continues until the conductivity of the salt chamber drops below 500 μS / cm or the voltage is increased back to the upper voltage limit. Figure 3 and Figure 4 The diagram illustrates the voltage and conductivity. The lithium-ion content in the alkali chamber was determined using inductively coupled plasma atomic emission spectrometry (ICP-AES). The lithium hydroxide concentration obtained in the alkali chamber was 0.2972 mol / L, and the sulfuric acid concentration obtained in the acid chamber was 0.2706 mol / L. The recovery rate of lithium hydroxide from alkali production was 99.07%, and the recovery rate of nitric acid from acid production was 90.2%. The energy consumption was 40.07 kWh / kgLiOH. A schematic diagram illustrating the current efficiency, energy consumption, and recovery rate of lithium hydroxide is shown below. Figure 5 and Figure 6 As shown, this indicates that the device can achieve a high lithium hydroxide recovery rate.
[0041] Example 6
[0042] The bipolar membrane electrodialysis device used in this example is the same as the device used in Example 1.
[0043] 150 mL of 0.3 mol / L sodium sulfate solution was introduced as a strong electrolyte solution; 100 mL of 0.1 mol / L lithium nitrate solution (conductivity ~9 mS / cm) was introduced into the salt chamber of the bipolar membrane electrodialysis device; 100 mL of deionized water was introduced into the acid and alkali chambers of the bipolar membrane electrodialysis device, respectively. During the experiment, the flow rate of each solution in the membrane stack was 25 mL / min, and the operation was constant current with a current density of 40 mA / cm². 2 The current is set to 0.36A.
[0044] The experiment continues until the conductivity of the salt chamber drops below 500 μS / cm or the voltage is increased back to the upper voltage limit. Figure 7 and Figure 8 The diagram illustrates the voltage and conductivity. The lithium-ion content in the alkali chamber was determined using inductively coupled plasma atomic emission spectrometry (ICP-AES). The lithium hydroxide concentration obtained in the alkali chamber was 0.297 mol / L, and the sulfuric acid concentration obtained in the acid chamber was 0.28 mol / L. The recovery rate of lithium hydroxide from alkali production was 99.03%, and the recovery rate of nitric acid from acid production was 99%. The energy consumption was 74.36 kWh / kgLiOH. The current efficiency, energy consumption, and recovery rate of lithium hydroxide are illustrated in the diagram below. Figure 9 and Figure 10 As shown, this indicates that the device can achieve high lithium hydroxide and high nitric acid recovery rates.
[0045] Example 7
[0046] The bipolar membrane electrodialysis device used in this example is the same as the device used in Example 1.
[0047] 150 mL of 0.3 mol / L sodium sulfate solution was introduced as a strong electrolyte solution; 100 mL of 0.2 mol / L lithium nitrate solution (conductivity ~16 mS / cm) was introduced into the salt chamber of the bipolar membrane electrodialysis device; 100 mL of deionized water was introduced into the acid and alkali chambers of the bipolar membrane electrodialysis device, respectively. During the experiment, the flow rate of each solution in the membrane stack was 25 mL / min, and the operation was constant current with a current density of 40 mA / cm². 2 The current is set to 0.36A.
[0048] The experiment continues until the conductivity of the salt chamber drops below 500 μS / cm or the voltage is increased back to the upper voltage limit. Figure 7 and Figure 8 The diagram shows the voltage and conductivity. The lithium-ion content in the alkali chamber was determined using inductively coupled plasma atomic emission spectrometry (ICP-AES). The lithium hydroxide concentration obtained in the alkali chamber was 0.2972 mol / L, and the sulfuric acid concentration obtained in the acid chamber was 0.28 mol / L. The recovery rate of lithium hydroxide from alkali production was 99.07%, and the recovery rate of nitric acid from acid production was 99%. The energy consumption was 30.91 kWh / kgLiOH. The current efficiency, energy consumption, and recovery rate of lithium hydroxide are shown in the diagram below. Figure 9 and Figure 10 As shown, this indicates that the device can achieve high lithium hydroxide and high nitric acid recovery rates.
[0049] Example 8
[0050] The bipolar membrane electrodialysis device used in this example is the same as the device used in Example 1.
[0051] 150 mL of 0.3 mol / L sodium sulfate solution was introduced as a strong electrolyte solution; 100 mL of 0.4 mol / L lithium nitrate solution (conductivity ~30 mS / cm) was introduced into the salt chamber of the bipolar membrane electrodialysis device; 100 mL of deionized water was introduced into the acid and alkali chambers of the bipolar membrane electrodialysis device, respectively. During the experiment, the flow rate of each solution in the membrane stack was 25 mL / min, and the operation was constant current with a current density of 40 mA / cm². 2 The current is set to 0.36A.
[0052] The experiment continues until the conductivity of the salt chamber drops below 500 μS / cm or the voltage is increased back to the upper voltage limit. Figure 7 and Figure 8The diagram shows the voltage and conductivity. The lithium-ion content in the alkali chamber was detected using inductively coupled plasma atomic emission spectrometry (ICP-AES). The lithium hydroxide concentration obtained in the alkali chamber was 0.2972 mol / L, and the sulfuric acid concentration obtained in the acid chamber was 0.266 mol / L. The recovery rate of lithium hydroxide from alkali production was 99.07%, and the recovery rate of nitric acid from acid production was 88.82%. The energy consumption was 22.99 kWh / kgLiOH. The current efficiency, energy consumption, and recovery rate of lithium hydroxide are shown in the diagram below. Figure 9 and Figure 10 As shown, this indicates that the device can achieve a high lithium hydroxide recovery rate.
[0053] The above description is merely a preferred embodiment of the present invention and is illustrative rather than restrictive. Those skilled in the art will understand that many changes, modifications, and even equivalent alterations can be made within the spirit and scope defined by the claims of the present invention, but all such changes will fall within the protection scope of the present invention.
Claims
1. A method for preparing lithium hydroxide with high recovery rate, characterized in that, Includes the following steps: A lithium nitrate solution is introduced into the salt chamber of a five-compartment bipolar membrane electrodialysis device; deionized water is introduced into the acid and alkali chambers respectively; and a strong electrolyte solution is introduced into the anode and cathode chambers. Then, an electric current is applied to both sides of the anode and cathode plates to perform electrodialysis. The alkali chamber produces high-purity lithium hydroxide, and the acid chamber produces high-purity nitric acid.
2. The five-compartment bipolar membrane electrodialysis device according to claim 1, comprising a cathode plate, a membrane stack, and an anode plate arranged sequentially, wherein the membrane stack comprises an electrodialysis unit and a second bipolar membrane, and the electrodialysis unit comprises a first bipolar membrane, a cation exchange membrane, an anion exchange membrane, and a second bipolar membrane arranged sequentially, wherein the cation exchange membrane layers of the first and second bipolar membranes face the cathode plate, and the anion exchange membrane layers of the first and second bipolar membranes face the anode plate; the cathode plate and the first bipolar membrane form a cathode chamber; the first bipolar membrane and the cation exchange membrane in the electrodialysis unit form an alkali chamber; the cation exchange membrane and the anion exchange membrane in the electrodialysis unit form a salt chamber; the anion exchange membrane and the second bipolar membrane in the electrodialysis unit form an acid chamber; and the second bipolar membrane and the anode plate form an anode chamber.
3. The five-compartment bipolar membrane electrodialysis device according to claim 2, characterized in that, Sealing gaskets are installed between the cathode chamber and the alkali chamber, between the alkali chamber and the salt chamber, between the salt chamber and the acid chamber, and between the acid chamber and the anode chamber. All sealing gaskets are made of silicone.
4. The five-compartment bipolar membrane electrodialysis device according to claim 2, characterized in that, It also includes a power supply, the anode plate is connected to the positive terminal of the power supply, the cathode plate is connected to the negative terminal of the power supply, and the anode plate and cathode plate are titanium-coated ruthenium electrode plates.
5. The five-compartment bipolar membrane electrodialysis device according to claim 2, characterized in that, It also includes a feed liquid storage tank and a peristaltic pump. The cathode feed liquid tank is connected to the cathode chamber via the peristaltic pump. The alkali chamber feed liquid tank is connected to the alkali chamber via the peristaltic pump. The salt chamber feed liquid tank is connected to the salt chamber via the peristaltic pump. The acid chamber feed liquid tank is connected to the acid chamber via the peristaltic pump. The anode feed liquid tank is connected to the anode chamber via the peristaltic pump.
6. The preparation method according to claim 1, characterized in that, The flow rates of the lithium nitrate solution, deionized water, and sodium sulfate solution are each independently 25 mL / min.
7. The preparation method according to claim 1, characterized in that, The applied current density is 20-60 mA / cm². 2 .