Lithium Extraction Performance of LiFePO₄ with Different Coating Density via Electrochemical De-Intercalation/Intercalation Method AITranslate

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Basic Information:

DOI：10.13373/j.cnki.cjrm.XY21040023

Chinese Library Classification Number:TF826.3

Citation Information:

Electrochemical de-intercalation/intercalation method is a new process for lithium extraction from salt lake brine，which has the advantages of strong selectivity for lithium，green and pollution-free，and low impurity content of lithium-rich anolyte. However，it owns a low current density. For electrochemical technology，current density is an important indicator to measure economic benefit. Therefore，high current density is conductive to promoting the industrial application of lithium extraction from salt lake brine via electrochemical de-intercalation/intercalation method. For the method，increasing the coating amount of active material LiFePO₄ on the electrode surface is one of the effective methods to improve the current density. But the increase of electrode coating density has a negative effect on the mass transfer performance and cycle performance of the electrode. Therefore，cyclic voltammetry （CV） was used to investigate the differences in the electrochemical performance of LiFePO₄ electrodes with different coating densities，the effects of electrode coating density on lithium adsorption capacity，current density and cycle performance were investigated by lithium de-intercalation and intercalation experiments using a battery test system，and the crystal structure of electrode material was characterized by X-ray diffraction （XRD）. Aimed at selecting a appropriate coating density of LiFePO₄ electrode with excellent lithium extraction performance and cycle performance. The electrochemical study results showed that with the increase of electrode coating density，on the one hand，the content of active material LiFePO₄ and the porosity of electrode material increased，which was conducive to improving the adsorption capacity of LiFePO₄ electrode for lithium ions and strengthening the mass transfer process of lithium ions，thus increasing the current density；on the other hand，the thickness of the electrode increased，which led to longer lithium ions transfer distance and greater mass transfer resistance，making it difficult for lithium ions to diffuse into the internal of LiFePO₄ electrode to participate in the reaction in a short period of time，resulting in more serious polarization of the electrochemical system and even hydrogen or oxygen evolution side reactions. This caused a deterioration of the electrode cycle performance. The results of the lithium de-intercalation and intercalation study showed that the lithium adsorption capacity of the electrode and lithium ions exchange rate gradually increased when the coating density increased from 40 to 55 mg·cm^-2，but as the coating density continued to increase，the whole electrolysis process time consumption increased significantly，the lithium adsorption capacity of the electrode and lithium ions exchange rate gradually decreased. XRD patterns of FePO₄ cathode after the completion of electrolysis showed that the active material FePO₄ which not involved in reaction of the electrode inner layer increased with the increase of electrode coating density，which explained why the lithium adsorption capacity of electrode with high coating density was lower. The results were consistent with CV test. Meanwhile，the amount of LiFePO₄ active material in the electrode increased with the increase of coating density，so the total electrolytic capacity of electrode in the lithium extraction process gradually increased. But the constant current electrolytic capacity ratio in the electrolysis process showed a trend of increasing and then decreasing with the increase of electrode coating density. At 55 mg·cm^-2，the constant current electrolytic capacity accounted for 48.7% of the total electrolytic capacity，and the maximum current density up to 13.83 A·m^-2，while the electrode showed excellent lithium adsorption capacity. The results of electrode surface morphology analysis showed that the surface of LiFePO₄ electrode was covered with cracks due to the volatilization of N-Methyl-2-pyrrolidone （NMP） during the drying process of electrode，which was conducive to the soakage of electrolyte into the interior of electrode and improved the mass transfer process of lithium ions. However，the electrode with higher coating density，the greater amount of NMP was evaporated at the beginning of the drying process，which increased the stress expansion between active material and made the cracks larger. The larger cracks reduced the adhesion of active material on the collector fluid，resulting in the active material fell off under the immersion and washing of solution in the process of long lithium extraction cycles，which reduced the cycle life of LiFePO₄ electrode. After 50 lithium extraction cycles，the mass loss rate of the electrode with 40 mg·cm^-2 coating density was only 2.75%，and the capacity retention rate was 94.44%，while the mass loss rate of the electrode with 100 mg·cm^-2 coating density was as high as 9.65%，and the capacity retention rate was decreased to 83.28%. Therefore，considering the influence of coating density on electrochemical performance，lithium extraction performance and cycle performance of LiFePO₄ electrode，55 mg·cm^-2 was determined as the optimal electrode coating density. In the simulated brine，the lithium adsorption capacity of LiFePO₄ electrode was 32.5 mg·g^-1，which showed excellent lithium extraction performance；and the mass ratio of magnesium to lithium in the lithium-rich anolyte reduced to 0.58，which fully met the requirements of Li₂CO₃ preparation process. At the same time，the capacity retention rate of LiFePO₄ electrode was 90.62% after 50 cycles，which showed great cycle performance