(436g) Lithium Ion Sieves Vs. Li+ Intercalation Electrodes As Effective and Energy-Efficient Materials for Li+ Mining from Aqueous Resources
For adsorption, highly selective LIS such as Li1.6Mn1.6O4, Li1.33Mn1.67O4, LiMn2O4, Li4Ti5O12 and Li2TiO3 were employed due to their high theoretical Li+ capacities (38 â 127 mg g-1) and acceptable Li+ selectivities. For electrochemical Li+ recovery, Li+ intercalating materials such as l-MnO2, LiNi1/3Co1/3Mn1/3O2 and LiNi0.5Mn1.5O4 were prepared as electrodes and paired with Ag as Cl--selective counter electrode. These active materials (i.e. Li+ insertion capacity of 39 â 74 mg g-1) were chosen for their stability in water environment and selectivity towards Li+.
The LIS were electrospun into nanofiber (NF) with polymer support matrix (i.e. LIS NF) and were employed as membrane adsorption filters. Upon saturation of the LIS NF, Li+ was desorbed and recovered in a small volume of mild acid solution. This step also regenerated the membrane for the next cycle of Li+ adsorption. Using the same mild acid solution repeatedly for Li+ recovery eventually produced an LiCl-rich solution. Meanwhile, Li+-capturing electrodes (i.e. the active materials were coated on Pt foil, which also served as current collector) paired with Ag, captured Li+ from the feed source by applying a positive current in the cell. At reverse current, LiCl is released and recovered in a receiving electrolyte solution. Similar with membrane adsorption, the electrochemical process was repeatedly carried by âdischargingâ and âchargingâ the cell to capture and release the LiCl, respectively, in the same receiving solution until an enriched LiCl stream was produced.
Both processes were evaluated in simulated streams containing low to high Li+ concentrations which are typical to the following aqueous sources: < 10 mg Li+ L-1 in desalination retentate and coal ash leachates, 100 â 1000 mg Li+ L-1 in brine pools and > 1000 mg Li+ L-1 in certain industrial wastewaters.
Overall results indicate that LIS NF in membrane configuration features the following benefits: (1) Simplicity of operation and provision for full automation considering the adaptable state-of-the-art technology of existing membrane separation processes at industrial scale, (2) convective delivery of the feed allows faster mass transport of Li+ from the feed to the LIS surface (3) small dimensional property of the nanofibers significantly reduces the diffusion path lengths for Li+ to reach the LIS adsorption sites thereby shortening the down-time for LIS saturation and (4) convenient control of feed residence time either by appropriate membrane design (i.e. membrane thickness control) or changing the operating conditions (5) well-controlled cycling adsorption/desorption time (i.e. 24 h/cycle) depending on desired Li+ production rate and (6) the highly porous LIS NF membrane required minimal applied pressure, even lower than those applied in microfilters hence the process is highly energy-efficient and (7) superior LiCl quality due to high Li+ selectivity and Li+ capacity of the LIS, particularly the Li2TiO3 or H2TiO3 (active LIS form).
On the other hand, electrochemical Li+ recovery system offers the benefit of fast Li+ capture and release rates, as short as 1 h/cycle. However, the system suffers from the following drawbacks: (1) Even at high Li+ feed concentration, the Li+ capturing electrodes cannot be induced (i.e. charge rate increase, total charge flow increase) to achieve their maximum Li+ intercalation capacity due to current efficiency reduction and severe co-intercalation of competing ions, (2) aside from the energy requirement of the electrode system, existing batch design needs additional mechanical steps for back-and-forth transfer of the electrodes between the feed source and receiving solution, making the process more tedious to operate, (3) long-term stability of the electrodes is difficult to predict but surely will not exceed the lifetime of a typical aqueous Li+ battery system.
Both processes are not the best options to recover Li+ from low concentration streams. But between the two, LIS NF membrane adsorption is more competitive over electrochemical due to the high energy consumption and relatively lower Li+ capacity of the latter system. For feed streams with high Li+ concentrations, electrochemical system requires much lower energy to capture and release Li+. Thus, it can somehow compete with the LIS NF membrane adsorption in terms of Li+ production rate. Thus, for high throughput production of low quality Li, electrochemical system is the completive option. However, LIS have superior selectivity over the Li+ capturing electrode materials. Thus, if high product purity is desired, LIS NF membrane adsorption remains superior over the electrochemical process albeit at lower production rates.
This research was supported by the National Research Foundation of Korea (NRF) under the Ministry of Science and ICT (No. 2016R1A2B1009221) and the Ministry of Education (No. 22A20130012051(BK21Plus)).