A three-electrode dual-power-supply electrochemical pumping system for fast and energy efficient lithium extraction and recovery from solutions

The demand for Li-ion batteries (LIBs) for use in electric vehicles, which is key to realizing a decarbonized society, is accelerating. However, the supply of Li resources has recently become a major issue, thereby necessitating the development of economical and sustainable technologies of brine/seawater-based Li extraction and recycling Li from spent LIBs. This paper presents an innovative electrochemical pumping technology based on a new cell structure for Li extraction/recovery. This system can provide large electrochemical driving forces while preventing the occurrence of electronic conduction due to electrolyte reduction. This electrochemical pumping system allows extraction/recovery of Li ions from the anode side to the cathode side, rather than the diffusion of other ions, due to the ion-diffusion-bottleneck size of the electrolyte material. Using this system, high-purity Li can be collected with high energy efficiency and at least 464 times faster than that via conventional electrochemical pumping, even with a commercially available Li-ion electrolyte plate.

Reviewer #2 (Remarks to the Author): This reviewer finds the present manuscript not suitable for publication for the reasons explained below.Major revistion of the manuscript is required before publication.The authors present a new experimental set up with respect to their previous publication in which an LLTO solid lithium conducting membrane with two platinum electrodes deposited in both faces separates two LiOH solutions of different concentration, on the anode side 1 M LiOH and on the cathode side 0.001 M LiOh solution, with an applied voltaje 2.0 V accross the LLTO membrane.
In the new set up resented in the manuscript, a third Ni cathode and a second power supply are introduced.Upon applying a second voltage with the second power supply, the real impedance (Rct) of the LLTO decreases and the lithium collection rate increases.Accoridng to data in Figure 1 anode-side solution, 1.0 mol/L aqueous LiOH solution; cathode-side solution, 1 × 10−3 mol/L aqueous LiOH solution; distance between the second and third electrodes, 57 mm.A large ohmic drop in the catholyte is expected, which reduces by decreasing the distance to the third electrode and increasing the LiOH concentration in the catholyte, both of which reduce the liquid electrolyte resistance.It is clear that the effect of the second power supply results mainly in ohmic drop in the electrolyte (Joule effect) and some increase in the Ni electrolyte/liquid electrolyte interface which drives the hydrogen evolution reaction (Her) at the Ni third electrode.It is common practise in electrochemistry to measure the electrode/electrolyte potential with a reference electrode, i.e.Ag/AgCl reference electrode to separate the effects of ohmic drop and polarization.The authors should do this in order to understant why increasing the voltage of the second potential source, they find a larger lithium collection rate.Furtherr comments, which should be addressed by the authors 1. Inroduction: ion pumping methods based in lithium ion intercalation in battery cathode materials are not referred, they should.See for instance: Electrochemical methods for sustainable recovery of lithium from natural brines and battery recycling, Current Opinion in Electrochemistry, 15, (2019), 102-108.Direct Lithium Recovery from Aqueous Electrolytes with Electrochemical Ion Pumping and Lithium Intercalation, ACS Omega 2021, 6, 51, 35213-35220 Recent advances in reactor design and control for lithium recovery by means of electrochemical ion pumping, Current Opinion in Electrochemistry, 35, (2022), 101089.
2. Reaction 3 is limited by the Exchange of lithium ions at the liquid electrolyte/solid LLTO interface.These reactions are slow due to the high hydration energy of Li(OH2)4+ in the loss and recapture of hydration wáter molecules by the Li+ ion.
Subsdequent diffusion and migration of non hydrated lithium ions takes place at the LLTO Li ion conducting membrane.Diffusion is driven by a concentration gradient and electro-migration by the electric field, i.e. voltaje drop acrross the LLTO solid membrane.3.At the Pt/liquid electrolyte and Ni/liquid electrolyte interfaces the Faradaic reactions take place: These reactions are driven by the potential drop at the Pt(or Ni)/electrolyte interface and follow a Buttler-Volmer exponential dependence.The Exchange current density of the HER on Pt is much larger than the OER, so hydrogen evolution is not a slow reaction.The flux of lithium accross the LLTO membrane should equal the flux of O2 or HO-at the anode because of mass balance of lithium ions and charge balance in the electyrolyte.
4. The SHE refers to activity of proton 1, and in alkaline solutions the hydrogen electrode has a lower value.The authors should refeer to the REVERSIBLE HYDROGEN ELECTRODE RHE instead of SHE in page 6, lines 236-239: "In many cases, the potential of the cathode-side surface of LLTO does not drop 137 below the hydrogen-gas-generating potential (0 V vs. standard hydrogen electrode 138 [SHE]) and is always higher than the tetravalent-to-trivalent reduction potential of Ti 139 (0.0 V vs. SHE)."Lines 143-144 by significantly increasing the overpotential of the oxygen-gas-generating reaction at the anode should reade "decreasing the overpotential" 5. Increasing the secondary-power-supply voltage in the new electrochemical pumping system results in a larger current with a larger ohmic drop and hydrogen evolution.
6.The electrode geometric áreas of the Pt and Ni electrodes in the cathode compartment should be defined, as well as the Pt electrode área in the anode compartment, since the total current will be defined by the specific local current given by the Butler-Volmer eqn.and the electrode área.7. It is not clear if the lithium collection rate comes from chemical analysis of the catholyte or from current I1.The autors describe "The Li-ion concentration of the cathode-side solution after 1 h of electrochemical 366 pumping was determined by ICP-AES (SPECTROBLUE® FMX26, HITACHI, Japan" But in Fig. the lithium collection rate is ploted.8.By using reference electrodes at the anode and cathode the authors could separate the effects of interfacial electrode potential acting on the Faradaic reactions, and the electric field driving the lithium ion flux accress the LLTO membrane, and the ohmic drops in the electrolytes, particularly the large ohmic drop at the diluted LiOH in the catholyte.9.The authors should explain clearly the mechanism that leads to the improvement in the new experimental design. Reviewer #3 (Remarks to the Author): In this work, the authors designed a novel electrochemical pumping system using three electrodes and two power supplies.And, they demonstrated the performance of the newly designed pumping system in lithium extraction.But, the weakness of this work is the lack of experiments and insufficient discussion supporting the author's claims.Thus, I cannot agree to the publication of this current form of the manuscript.Before reconsideration of the publication, the manuscript should be improved with a major revision.
Specific comments: 1.In P3L68, the authors claimed the electrochemical pumping system exhibits a high selectivity for lithium ions compared to sodium and potassium ions.But, the selective extraction mechanism is ambiguous.Please explain it in detail.2. Although the authors addressed the electrochemical pumping system as an economic process, it remains unclear.Please carry out the techno-economic analysis of the pumping system compared to other technologies such as adsorption and ion exchange.In particular, recently, the battery system (ACS Omega 2021, 6, 51, 35213-35220;Processes 2022, 10(12), 2654) has been widely examined the lithiumion recovery.The battery system would recover the consumed energy during extraction, and thus the battery system is considered a very energy-efficient process.3.In this regard, the proposed pumping system seems to be more complicated than the battery system.Please clarify the pros and cons of the pumping system compared to the battery system.4. In this work, the information on the lithium source (anode side solution) was not well addressed.Please clarify and justify the rationale for why the authors selected the composition of the solution on the anode side.Is there any target application?5.The selectivity in lithium extraction could be a pivotal factor governing the system's performance.But, in this work, the selectivity was not well examined.Please show the selectivity results and discuss further them.

1
Reviewer #1 (Remarks to the Author): The article titled: "A three-electrode dual-power-supply electrochemical pumping system for ultrafast high-energy-efficiency lithium extraction and recovery" describes the development of a new electrochemical cell set-up allowing faster and improved lithium recovery.The design of the cell is innovative and the research field of great interest; however, additional experiments and clarifications are required before its publication in Communications Engineering.
Response: We would like to thank the reviewers and editors for their constructive comments to improve our manuscript.We have carefully considered the reviewers' comments and provided responses to all the points raised by the reviewers.We hope that the new manuscript will meet the standards of Nature Communications.
The point-by-point responses to the reviewers' comments are given below.
(Q#1-1) The authors claim that "the Li collection rate of this new system can be limitlessly increased."I doubt the LLTO solid electrolyte will allow an unlimited rate.The lithium diffusion coefficient in its structure is limited.The lithium must migrate through its structure to reach the cathode side, which will roll the rate.What is the value of the lithium diffusion coefficient in LLTO, and how would it affect the "limitlessly" rate proposed?(Q#1-2) The authors also mentioned an "ultrafast high-energy-efficiency."However, I have some doubts about the suitability of such terminology.Increasing the recovery rate by increasing the voltage applied must have a penalty in the energy consumption required to extract the lithium.

Response
There is no calculation regarding the energy needed to extract the lithium in the manuscript; such a value will have a tremendous impact on the potential applicability of this new set-up.These calculations must be included, and the energy consumption per mol of lithium must be included.
Response to Q#1-2: As reviewer #2 pointed out, the energy efficiency required for Li decreases upon increasing the collection rate by increasing the applied voltage.This relationship between recovery rate and energy efficiency is a tradeoff.Figure R1 shows the relationship between the collection rate and energy efficiency depending on the main powersupply voltage when the secondary power-supply voltage is fixed at 10 V.The dependence of energy efficiency on the collection rate in the conventional method, which is the system with one power supply and two electrodes, is also shown.In the conventional method, the energy efficiency significantly decreases upon increasing the Li collection rate due to the occurrence of electronic conduction.In contrast, the new system shows an improvement in collection rate and also an enhancement in energy efficiency upon increasing the applied voltage of the main power source.Therefore, the installation of a secondary power source not only increases the collection rate but also improves energy efficiency.In an electrochemical pumping system, the Li recovery amount from the anode side to the cathode side is

The orange line in
where F and t are the Faraday constant and operating time, respectively.Furthermore, the energy consumption ∆ of the recovery unit during electrochemical pumping is where is the applied voltage.From equations (1) and ( 2), the energy efficiency can be obtained as follows The Li collection rate (g/m 2 /h) is calculated using equation ( 4 Q#1-3) The manuscript shows the improvement obtained with this new set-up; however, the electrolyte used was pure lithium electrolyte.In real samples, lithium concentration is significantly smaller than other cations present in solution (Na, K, Mg…).The increment in the lithium rate could also affect the LLTO capacity to discriminate between these cations; therefore, the method's selectivity in the conditions described must be included, showing the concentration on the cathode-side of the different cations.If other co-cations are co-transported with the increment of the rate, the capacity of this methodology to extract lithium will be compromised; therefore, such measurements are critical for correctly analyzing the proposed method.(Q#1-4) On the other hand, increasing the rate will also increase the accumulation of cations on the LLTO surface since these co-cations will be blocked because of LLTO lithium selectivity.

Response to
How will such charge accumulation affect the recovery process?
Response to Q#1-4: The incorporation of Li ions from the solution into the LLTO solid electrolyte near the anode is not due to electrical potential difference but to chemical potential differences.In our system, the anode is the positive electrode; Na and K ions do not approach the anode electrode because of Coulombic repulsion, and thus, a blocking layer of cations is not expected to form.
(Q#1-5) How much is the final lithium concentration reached after applying the electrochemical method?Reviewer #3 (Remarks to the Author):

Response
In this work, the authors designed a novel electrochemical pumping system using three electrodes and two power supplies.And, they demonstrated the performance of the newly designed pumping system in lithium extraction.But, the weakness of this work is the lack of experiments and insufficient discussion supporting the author's claims.Thus, I cannot agree to the publication of this current form of the manuscript.Before reconsideration of the publication, the manuscript should be improved with a major revision.
Specific comments: (Q#3-1).In P3L68, the authors claimed the electrochemical pumping system exhibits a high selectivity for lithium ions compared to sodium and potassium ions.But, the selective extraction mechanism is ambiguous.Please explain it in detail.(Q#3-2).Although the authors addressed the electrochemical pumping system as an economic process, it remains unclear.Please carry out the techno-economic analysis of the pumping system compared to other technologies such as adsorption and ion exchange.In particular, recently, the battery system (ACS Omega 2021, 6, 51, 35213-35220;Processes 2022, 10(12), 2654) has been widely examined the lithium-ion recovery.The battery system would recover the consumed energy during extraction, and thus the battery system is considered a very energyefficient process.

Response
Response to Q#3-2: When comparing economic processes, it is necessary to discuss other factors in addition to energy consumption during the recovery process.In particular, this study uses a continuous process, which is different from batch processing methods for battery systems or adsorption systems.Furthermore, because of the high Li selectivity and purity, there is no need for further purification .Considering these points, it can be estimated that this method has better economic efficiency than other methods.
(Q#3-3).In this regard, the proposed pumping system seems to be more complicated than the battery system.Please clarify the pros and cons of the pumping system compared to the battery system.
Response to Q#3-3: The advantage of the electrochemical pumping system is that it can recover Li ions continuously without the replacement of absorbent materials/solutions or switching between charging and discharging cycles as in other lithium recovery methods.
Furthermore, from reaction equations (1) and (2), highly pure hydrogen and oxygen gases are generated from each electrode.These gases can also be recovered and used as fuel.This explanation was added on p. 4 L. 93-p. 4 L. 99 of the revised manuscript.
(Q#3-4).In this work, the information on the lithium source (anode side solution) was not well addressed.Please clarify and justify the rationale for why the authors selected the composition of the solution on the anode side.Is there any target application?
Response Q#3-4: As this paper focusses on a lithium recovery device with a new structure, it was decided that a lithium solution without other cations is better suited.It has already to Q#1-1: Thank you for this insightful comment.As indicated by Fick's law (see below), the diffusion constant, D, is the proportionality coefficient between the Li collection rate (flow rate, J) and the potential gradient (dc/dx), and increases monotonically with increasing potential difference, which is the electrochemical potential gradient.= − Prof. Inaguma has reported that the diffusion constant of LLTO is estimated to be 3 × 10 −8 cm 2 /s from the Nernst-Einstein equation, assuming that the interaction between diffusing Li ions is small based on electrical conductivity [R1].As long as the crystal structure of LLTO is maintained, it is theoretically possible to increase the recovery rate indefinitely.[R1]Y. Inaguma, A Review of Recent Research on Perovskite-Type Lithium Ion-Conducting Oxides.J. Cryst.Soc.Japan 5862-72 (2016).10.5940/jcrsj.58.62.
Fig. R1 corresponds to the equation relating the reaction rate to energy efficiency estimated as follows.
Figure R1 shows a log-log plot; the orange line has the form = − + (6).whereA and B are fitting parameters.The energy efficiency of the new system can be seen approaching the orange line as the main power-supply voltage increases.This means that we have developed a Li recovery system that achieves high energy efficiency by suppressing electronic conduction due to the reduction of LLTO.

Fig. R3 .
Fig. R3.LiOH concentration dependency of measured pH of LiOH solution and degree of ionization estimated from pH.