Electrochemical lithium recovery from brine with high Mg2+/Li+ ratio using mesoporous λ-MnO2/LiMn2O4 modified 3D graphite felt electrodes
Graphical abstract
Introduction
Lithium has become one of the most significant metal resources around the world due to the fast-growing lithium ion battery demand in the past three decades [[1], [2], [3], [4], [5]]. Except for the conventional lithium ores, the rising lithium demand requires alternative raw materials for the extraction of lithium, such as lithium-rich seawaters and salt lake brines [6,7]. The ratio of Mg2+/Li+ in 78% of the salt-lake brines in western China is extremely high, which can reach up to 500 [8]. According to the diagonal relationship between lithium and magnesium placed obliquely in adjacent periods and groups in the periodic table, they present quite similar ionic properties in solution phases. Hence, it is a challenge to extract lithium from salt-lake brines.
Presently, different technologies have already been employed to extract lithium from salt-lake brines, including precipitation [9], solvent extraction [10], adsorption [11] and membrane technology [[12], [13], [14]]. The conventional industrial method for lithium extraction from salt-lake brines involves a series of complicated procedures to remove several competitive metal ions before the formation of lithium carbonate precipitation. While solvent extraction has high extraction efficiency for the brine with high Mg2+/Li+ ratios, but the risk of dissolved loss of extractants and large usage of organic solvents. Adsorption is one of the most prospective technologies for lithium extracting from brines with high Mg2+/Li+ ratios, because of its cost-effectiveness and environmentally-friendly, and lithium-ion sieves are adsorbents for Li+ with effective ion-sieving performance, such as Li-Mn-O (LMO) and Li-Ti-O (LTO) adsorbents. However, the adsorption and desorption reactions of these adsorbents in aqueous solutions usually display slow reaction rates and long equilibration time, as well as the dissolved loss of adsorbents during the acidic stripping. Hence, it is essential to develop a well-designed cell to achieve selective lithium extraction with high recovery efficiency in low-quality resources, energy-efficient and environmentally- friendly.
Electrochemical ion-separation has emerged as a promising strategy utilized for the extraction of Li with the advantages of tunability, reversibility and selectivity. Lots of studies were conducted to investigate and optimize the performance of the electrochemical extraction of lithium. For example, Cao et al. [15] developed a polyaniline/LixMn2O4 extraction system using two electrodes to adsorb Li+ and Cl− simultaneously at the cathode and anode. Ji et al. [16,17] reported highly selective Li+ extraction systems using two LixMn2O4 electrodes in different Li+ intercalation states and an anion-exchange membrane (AEM) with monovalent cation selective. This extraction system can adsorb and desorb Li+ simultaneously at the anode and cathode, and save as much as half the time it would take using the traditional Li+ extraction system. Ryoo et al. [18] developed a flow-type cell using the λ-MnO2/LiMn2O4 electrode pair. Electrolyte flows through the electrode in the flow-type cell, which has the advantages of space-saving and high efficiency. Generally, these methods employed the external electric field driving force to enhance the adsorption kinetics of lithium ions. Meanwhile, to further improve the intercalation and deintercalation rates of lithium ions, the strategy to increase the surface the reaction interfaces by the synthesis of electrode-active materials with nanostructures and/or ultra-small particle sizes is also reported. For example, nanowire LMO adsorbents with large specific surface area by hydrothermal synthesis can shorten the equilibration time to several hours [[19], [20], [21]]. Hao et al. [22] also developed a lithium ion-imprinted composite material containing of nanoscale λ-MnO2/PPy/PSS core-shell rods to extract lithium selectively, which reaches 35.2 mg/g of the Li+ ion adsorption capacity with an equilibrium time of less than 2 h.
Mesoporous materials, with pore sizes between 2 nm and 50 nm, possess high specific surface areas, large pore volumes, and tunable pore sizes. These features enable them as the ideal candidate for electrochemical systems due to the abundant active sites and reinforced transport efficiency of reactants. Therefore, many elaborated manipulations and nanoarchitecture engineering strategies have been applied to construct mesoporous electrodes for outstanding electrochemical performance. In fact, LMOs with ordered mesoporous architectures have already been used as the cathode materials in the lithium-ion battery, which provide special channels and electrochemical interface for the rapid ions transport kinetics and charge transfer under the mesoporous nanoconfined space [[23], [24], [25]]. However, little study about the mesoporous LMOs with high specific surface areas has been reported for the extraction of lithium ions.
In this study, we synthesized LiMn2O4 adsorbents with ordered mesoporous structures, which were coated on three-dimensional graphite felt electrodes, and a flow-type membrane-separated electrolytic cell was employed for adsorbing and desorbing lithium simultaneously, using mesoporous λ-MnO2/LiMn2O4 modified 3D graphite felt electrodes in a closed-loop electrolytic cell. The graphite felt electrodes was selected as the support due to their excellent mass transfer performance and space-time yield [[26], [27], [28]]. The LiMn2O4@GF electrodes were characterized by their structure, morphology, and electroactivity for the Li+ intercalation and deintercalation. The extraction mechanism was studied by electrochemical analysis, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The operating parameters of the flow-type cell were optimized, and the energy consumption of lithium extraction was also estimated. This flow-type reactor with ordered mesoporous LiMn2O4 could increase the reaction areas, shorten the distance of mass transfer in the liquid phase, and provide an energy-efficient constant-potential electrochemical method for extraction of Li+. And the research is useful for the further engineering application of the lithium extraction electrolytic cell.
Section snippets
Material preparation
The mesoporous LiMn2O4 spinel was synthesized using nano-casting technique with ordered mesoporous silica (KIT-6, Nanjing Xianfeng Nano) as the hard template [29]. Specifically, 0.013 mol of LiNO3 (99%, Macklin) and 0.026 mol of Mn(NO3)2·4H2O (98%, Macklin) were dissolved in 10 mL distilled water. Then the solution was added slowly to a 160 mL n-hexane (GR, Kemiou) with 3.5 g of mesoporous SiO2 under a 400 r/min mechanical stirring. The resulted mixture was stirred for 1 h and treated by
Results and discussion
Mesoporous LiMn2O4 was prepared successfully via the nano-casting technique by the KIT-6 silica template. Fig. 2(a) shows the XRD patterns of the LiMn2O4 products calcined at different temperatures. The mesoporous LiMn2O4 calcined at 700 °C (Meso-700) is the most successfully synthesized material. As the temperature increases from 600 °C to 700 °C, a significant increase in the crystallinity of LiMn2O4 occurs, which is consistent with the standard pattern of LiMn2O4 (JCPDS No. 88-1026). At
Conclusion
This work prepared an ordered mesoporous LiMn2O4 adsorbent using nanocasting technique with hard mesoporous silica KIT-6 template, and then coated it on three-dimensional graphite felt (GF) electrodes for electrochemical extraction of Li+. The mesoporous LiMn2O4 only takes one-quarter of the time that bulk LiMn2O4 takes to reach Li+ adsorption equilibrium, due to the high specific surface area of mesoporous LiMn2O4, which is 60 times larger than that of bulk LiMn2O4. XPS and Raman spectra
CRediT authorship contribution statement
Yingxin Mu: Investigation, Methodology, Formal analysis, Writing - Original draft preparation. Chengyi Zhang: Investigation, Formal analysis. Wen Zhang: Conceptualization, Visualization, Validation, Writing - Reviewing and editing, Funding acquisition, Supervision. Yuxin Wang: Supervision, Project administration.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by the National Natural Science Foundation of China (22076137 and 11705126), and the Science and Technology Program of Tianjin (20JCQNJC01000).
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