Correction for ‘ In situ investigation of Li permeation through grain boundaries in garnet-based solid electrolytes’ by Sung Heo et al. , J. Mater. Chem. A , 2026, 14 , 7388–7393, https://doi.org/10.1039/D5TA09003B.

The growing need for selective ion separation in water treatment and resource recovery has driven research beyond conventional desalination. Flow-electrode capacitive deionization (FCDI) exhibits the advantages of continuous operation and high capacity and is therefore well suited for these applications. This review focuses on the recent and rapid advances in achieving selective ion separation using FCDI. Initially, we discuss the fundamental mechanisms of achieving selectivity, broadly categorizing them into the engineering of flowelectrode materials (e.g., functionalization, intercalation hosts, nanostructured carbon), tailoring of ionexchange membranes (e.g., polyelectrolyte coatings, nanofiltration membranes, carrier-facilitated transport), and tuning of operational parameters and system design (e.g., voltage, flow rate, cell architecture). Subsequently, we survey state-of-the-art applications according to target ions, including the separation of monovalent cations from divalent ones for water softening (Ca/Mg removal) and the recovery of critical resources such as lithium and ammonia. Furthermore, we cover the selective removal of anions, including nutrients (nitrate, phosphate), contaminants (fluoride, chromate), and valuable organic acids. Distinct from prior reviews on CDI selectivity, this work specifically highlights FCDI's unique features including continuous slurry-electrode operation, feed channels separated by ion-exchange membranes, and diverse flow/rocking-chair/redox modes, thus providing a focused synthesis of strategies and potential applications. Finally, key challenges related to system stability, fouling, and cell design are summarized, and future research directions are highlighted. This review demonstrates that through the sophisticated combination of materials science and system engineering, FCDI is evolving into a robust and versatile technology critical for the future of sustainable water treatment and the circular economy.

Polymer electrolytes in lithium batteries typically suffer from low ionic conductivity and unstable interactions with lithium metal, limiting their applicability in high-energy-density systems. To address these challenges, a novel nanoporous filler (Li-IL@CuBTC) was synthesized by encapsulating a lithium-containing ion-conductive liquid within the CuBTC metal–organic framework (MOF), and this filler was incorporated into a polyethylene oxide (PEO) matrix to form a composite polymer electrolyte. The multifunctional filler enhances ion transport, suppresses PEO crystallinity, and improves electrolyte stability. Consequently, the composite polymer electrolyte has a broad electrochemical stability window (5.9 V), strong ionic conductivity (1.2 × 10⁻4 S cm⁻¹ at room temperature), and a high lithium-ion transference number (0.69), along with excellent compatibility with lithium metal. In LFP/PLLC/Li full cells operated at 60 °C, the electrolyte delivers outstanding cycling stability, maintaining reversible capacities of 160.5 mAh g⁻¹ after 200 cycles at 0.2 C and 151.5 mAh g⁻¹ after 250 cycles at 0.2 C and 2 C respectively. This study demonstrates an effective strategy for improving composite polymer electrolytes, offering promising potential for safe, durable, and high-energy-density energy storage systems.

This study presents a continuous flow-electrode capacitive deionization (FCDI) system that simultaneously enhances salt removal performance and enables tunable monovalent/divalent ion selectivity using sulfuric acid-functionalized activated carbon (F-AC) slurry electrodes. The electrode surface functionalization introduced negatively charged sulfur- and oxygen-containing groups, enabling ion selectivity to originate directly from the electrode without additional ion-exchange membrane modification. Under mixed Ca & sup2;(+)/ Na+ single-pass operation, F-AC exhibited a pronounced dual effect: the average salt removal rate (ASRR) increased from 1.70 to 4.45 mu mol/cm & sup2;& centerdot;min (2.6-fold compared with pristine activated carbon (P-AC)) and the Ca/Na selectivity index (rho(Ca/Na)) decreased from 3.78 to 2.65 (similar to 30% reduction). By tuning the operating parameters, ASRR reached up to 7.11 mu mol/cm & sup2;& centerdot;min and rho(Ca/Na) decreased to as low as 1.07, indicating a substantial mitigation of Ca & sup2;(+)-preferential uptake and a relative shift in competitive selectivity toward Na+. Operating parameters such as slurry flow rate, electrolyte flow rate, and feed salinity further allow tuning of the throughput-selectivity balance in continuous FCDI. Sulfuric acid functionalization markedly improved electrode wettability, promoting electrolyte accessibility and more efficient electrical double-layer charging in aqueous slurry-based flow electrodes. Electrochemical analyses provided a consistent mechanistic explanation for the observed behavior. Electrochemical impedance spectroscopy revealed reduced interfacial and transport resistances for F-AC, supporting faster ion transport and higher throughput. These results indicate that electrode-level surface functionalization can effectively decouple ion selectivity from bulk textural properties under continuous flow operation. Overall, F-AC offers a scalable and low-complexity electrode platform that mitigates the selectivity-throughput tradeoff in FCDI, enabling high-rate desalination with controllable monovalent/divalent ion fractionation under continuous operation.

Selective potassium (K) recovery from saline streams remains challenging because K+ is typically present as a minor component in the presence of excess sodium ions (Na+) and competing divalent cations. In this study, we developed a copper based potassium hexacyanoferrate (KCuFC)-based hybrid capacitive deionization (HCDI) system that achieved a maximum recovery selectivity of alpha(des)(K/Total(Na+Mg+Ca)) = 15.24 +/- 3.13 in simulated seawater reverse osmosis (SWRO) concentrate, highlighting its practical potential for K recovery from Na-rich brines. To clarify the origin of K selectivity, the KCuFC-based HCDI configuration was compared with conventional membrane capacitive deionization (MCDI) and asymmetric membrane capacitive deionization (ACDI). In 10 mM KCl, HCDI achieved a final K+ adsorption capacity of 36.89 +/- 0.66 mg/g, the lowest energy consumption of 0.49 +/- 0.01 Wh/g-KCl. In an equimolar mixed-cation solution, only HCDI exhibited clear K preference, with alpha(K/Na) = 3.01 +/- 0.16, alpha(K/Mg) = 2.50 +/- 0.39, and alpha(K/Ca) = 2.14 +/- 0.03, whereas the MCDI and ACDI showed selectivity values below unity. The most favorable operating window was identified at 1.2 V, 2 mL/min, and 5-10 mM KCl. Under binary K/Na competition, alpha(K/Na) = 3.66 +/- 0.18 at K:Na = 1:100 and 1.2 V, indicating robust K preference under severe Na excess. In addition, KCuFC-mid retained 84% of its initial electrochemical capacity after 400 cycles, while maintaining a coulombic efficiency >99%. Furthermore, small-data supervised learning analysis was used as a supplementary local consistency check for the held-out binary K/Na family, yielding a q(K) mean absolute error (MAE) of 2.315 mg/g. Overall, this study demonstrated that KCuFC-based single-membrane HCDI is a promising and practical tunable platform for selective K recovery from Na-rich brines.

Perfluoroalkyl substances (PFAS), which are used in various industries due to their excellent thermal stability and chemical durability, have strong C-F bonds that are not easily degraded over time, causing them to accumulate in the human body and in the environment. Therefore, the development of PFAS treatment technology is emerging as an urgent issue, and PFAS regulations are being strengthened internationally. This study proposes an optimized hybrid capacitive deionization (HCDI) system using activated carbon composite and Nafion-coated Prussian blue (PB) electrodes for the effective removal of PFAS with short fluoroalkyl chains, such as perfluorobutane sulfonic acid (PFBS). It was found that the HCDI system exhibited higher deionization capacities (26.9 mg/g and 0.60 mg/cm²), corresponding to increases of 11.6

Silicon is a highly attractive anode material for lithium-ion batteries because of its exceptional theoretical capacity; however, its application is limited by extreme volume expansion and unstable solid-electrolyte interphase (SEI) formation. In this study, a robust composite anode was developed by embedding silicon nanoparticles within a reduced graphene oxide nanoribbon (rGONRs) matrix via the freeze-drying technique and enhanced by applying a direct contact prelithiation strategy. The resulting Si@rGONRs composite exhibited a flexible, conductive core-shell structure with strong Si-C and Si-O-C interfacial bonding, enabling both mechanical resilience and fast charge transport. Prelithiation boosts the initial Coulombic efficiency to over 100 %, compensating for the lithium loss and stabilizing the SEI. The anode delivers a high reversible capacity of 1600 mAh g- 1 at 200 mA g- 1 and retains 1261 mAh g- 1 at 2000 mA g- 1, with outstanding cycling stability (95.5 % retention after 400 cycles at 500 mA g-1). Post-mortem analysis revealed only a 59 % thickness expansion, which is far lower than the 142 % observed in the pure Si anode. This study demonstrates a scalable strategy to integrate structural confinement and prelithiation for long-life, high-capacity Si-based anodes, offering a promising direction for next-generation lithium-ion batteries.

This study presents a rocking-chair electrochemical lithium recovery (RCELR) system that uses lambda-MnO2and LiMn2O4 electrodes, optimized for high-salinity brine conditions. A partial state-of-charge (SoC) control strategy was introduced to address the structural degradation and capacity fading of LiMn2O4 electrodes during deep cycling. Electrochemical tests confirmed that LiMn2O4 exhibits high selectivity for Li* over Na*, with minimal Na* intercalation. Compared to full charge/discharge operation (100 % SoC), partial (60 %) SoC control significantly enhanced the cycling stability (78.1 % vs. 55.1 % capacity retention after 100 cycles) while maintaining comparable lithium recovery rates. An X-ray diffraction analysis revealed reduced lattice distortion under SoC control operation, indicating improved structural integrity. Additionally, the results of lithium recovery tests show that 60 % SoC operation lowered energy consumption by 24 % and improved Li*/Na* selectivity. In conclusion, the SoC control operation in RCELR is an effective and practical design strategy to enhance the durability, energy efficiency, and separation performance of systems operating in high-Na* environments.

Garnet-type solid electrolytes, such as Li7La3Zr2O12 (LLZO), are promising candidates for next-generation solid-state batteries due to their high ionic conductivity, mechanical stability, and excellent compatibility with lithium metal anodes. However, a major safety concern remains: internal short-circuits caused by lithium dendrite penetration, a mechanism that is not yet fully understood. To address this, we employed a suite of in situ techniques-including conductive atomic force microscopy (C-AFM), scanning electron microscopy (SEM), and scanning transmission electron microscopy (STEM) to directly observe the mechanism of lithium plating and propagation in Ta-doped Li6.5La3Zr1.5Ta0.5O12 (LLZTO) solid electrolytes. Our findings reveal that non-uniform current distribution within the LLZTO is the primary driver for lithium dendrite formation. We observed that lithium crystals initially nucleate and grow as discrete islands along the grain boundaries where current is concentrated. These isolated crystals subsequently merge, forming continuous dendritic pathways that lead to short-circuiting. The growth of these lithium crystals was further confirmed by in situ electron beam induced current (EBIC) experiments. Based on these insights, we developed a novel C-AFM-based technique to artificially induce lithium dendrite growth from the LLZTO surface, which serves as a powerful diagnostic tool for identifying regions of non-uniform current flow. This work elucidates the fundamental mechanism of lithium dendrite formation and provides a valuable method for assessing the safety and performance of solid-state electrolytes.

Over the past two decades, capacitive deionization (CDI) has garnered significant attention in water treatment owing to its energy efficiency and cost-effectiveness. Flow-electrode CDI (FCDI) enables continuous operation using a flowable carbon slurry electrode; however, its performance is often limited by poor electronic percolation and low charge-transfer efficiency within the slurry. Herein, we incorporated para-phenylenediamine (pPD) as a soluble organic redox-active mediator into an activated carbon (AC) slurry flow electrode to overcome these limitations. The results demonstrate that pPD undergoes reversible proton-coupled redox reactions (two-proton and two-electron transfer) and facilitates electron transport within the slurry by acting as a molecular electron shuttle and conductive bridge between isolated carbon particles, thereby enhancing charge storage and interfacial charge transfer. The FCDI system, utilizing an optimized pPD concentration of 0.15 M under short-circuited closed-cycle (SCC) operation at 1.2 V with a 35 g/L NaCl feed, achieved a 46% increase in salt removal efficiency and a 48% increase in salt removal rate compared with the pristine AC electrode. Electrochemical characterization confirmed the pseudocapacitive contribution and reduced charge transfer resistance. Furthermore, system performance was systematically evaluated under various operating conditions, including electrode flow rate, feed concentration, and applied voltage. This study provides a cost-effective strategy for upgrading conventional carbon slurries for high-efficiency desalination in FCDI.

The escalating demand for high-purity lithium to support the global transition to electric vehicles necessitates the development of efficient technologies for extracting lithium from complex aqueous sources such as continental brines and battery leachates. However, effective lithium fractionation is often hindered by the presence of chemically similar monovalent sodium and divalent magnesium. Here, we present an integrated single-cell redox-flow desalination (RFD) system capable of direct lithium recovery as high-purity Li3PO4. The system utilizes a solubility-driven Li/Na phase-separation mechanism, complemented by an ethylenediaminetetraacetic acid-assisted chelation strategy to exclude Mg2+ interference. Optimization of the electrochemical parameters revealed that a low current (15 mA) is critical for maximizing precipitation kinetics while keeping the cell-level specific energy consumption at 0.4 kWh/mol of Li (57.6 kWh/kg Li). Li3PO4 formation was consistently achieved, even at a disadvantageous Li:Na ratio of 1:10. The addition of 3% H3PO4 to the FePO4 cathode further enhanced performance, achieving a peak precipitation efficiency of 56.1% by reducing internal cell resistance. The resulting Li3PO4 product exhibited high purity (98.6%). The proposed RFD configuration provides a single-cell pathway for the direct production of high-purity Li3PO4 from multi-ion feedstocks, significantly reducing the process complexity and chemical intensity associated with conventional hydrometallurgical methods.

Capacitive deionization (CDI) is a promising desalination technology but is often limited by the low ion removal capacity of carbon-based electrodes. In this study, a hybrid capacitive deionization (HCDI) system employing an asymmetric silver/activated carbon (Ag/AC) electrode configuration was investigated under constant-current operation and compared with a conventional membrane capacitive deionization (MCDI) system. Desalination performance was evaluated using CDI Ragone plot analysis, while galvanostatic charge–discharge measurements were used to examine the potential evolution of individual electrodes. The results show that the Faradaic Ag/AgCl reaction stabilizes the cell voltage and expands the usable potential window of the activated carbon electrode. As a result, the HCDI system consistently outperformed MCDI, achieving a maximum salt adsorption capacity of 36.5 mg g⁻¹ in a 10 mM NaCl solution, more than twice that of MCDI (17.0 mg g⁻¹), along with a faster ion removal rate. These findings demonstrate that integrating Faradaic electrodes effectively overcomes the intrinsic capacity limitations of conventional CDI systems.

The rapid growth of lithium-ion batteries (LIBs) has intensified the demand for sustainable recycling approaches to recover valuable metals and reduce environmental risks. Electrochemical separation technologies have emerged as attractive alternatives to conventional hydrometallurgy, but most existing strategies rely on single ion targeting or sequential, multi-stage processing, limiting their scalability. Here, we present a single-cell redoxflow desalination (RFD) system that simultaneously separates Li+, Ni2+, and Co2+ in one step. The design integrates two complementary mechanisms: a cation-exchange membrane (f-CEM) functionalized with PAH/PSS multilayers, which enables Li+ transport while repelling divalent cations, and EDTA-assisted chelation that converts Ni2+ into NiY2-for selective transport through an anion-exchange membrane (AEM), leaving Co2+ in the central chamber. Key operating parameters were systematically evaluated, including applied current (10-200 mA), electrolyte concentration (50-200 mM), and EDTA:Ni molar ratio (0-1.2). Under optimized conditions, recoveries reached 98.3 % for Li+, 78.0 % for Ni2+, and 77.3 % for Co2+, with low specific energy consumptions of 0.9 kWh mol-1 (Li+) and 1.2 kWh mol-1 (Ni2+). The system maintained performance with synthetic battery leachates, demonstrating tolerance to realistic ionic strength and composition. This compact and integrated RFD provides a scalable pathway for simultaneous multi-ion fractionation, reducing chemical consumption and process complexity, and supporting closed-loop LIB recycling in a circular economy.

Ammonium ion (NH4+) contamination in water has emerged as a serious environmental issue due to its ecological toxicity and its role in eutrophication and particulate matter formation. Although conventional methods such as biological treatment and ion exchange have been employed, these methods suffer from operational complexity and high energy demands. In this study, we propose a hybrid capacitive deionization (HCDI) system employing Prussian blue analogue, copper hexacyanoferrate (CuHCF), as a redox-active electrode material for energy-efficient NH4+ removal. Specifically, we evaluated the benefits in terms of energy consumption at low operating voltages by comparing the system with two representative CDI configurations (Membrane capacitive deionization (MCDI) and asymmetric MCDI (ACDI)). The HCDI system with CuHCF exhibited a high deionization capacity (13.4 +/- 0.1 mg/g) and charge efficiency (76 +/- 3 %) at 1.0 V, outperforming both MCDI and ACDI systems. Notably, even when operated at a reduced voltage of 0.8 V, the HCDI system with CuHCF not only exhibited superior deionization capacity compared with the MCDI and ACDI systems operated at 1.2 V but also demonstrated significantly lower energy consumption (0.46 +/- 0.02 Wh/g) because of its low-voltage operation. These results highlight the advantages of integrating redox-active materials such as CuHCF into capacitive deionization systems for low-voltage and energy-efficient NH4+ removal.

Ammonium ions in wastewater can induce toxicity in aquatic organisms and accelerate eutrophication. Therefore, the efficient removal of ammonium ions is necessary. The electrochemical ion separation method, based on an electrode with selectivity for ammonium ions, is a promising recovery technique that can efficiently adsorb and desorb ammonium ions. In this study, we propose an electrochemical ion exchange system (EIXS) that relies on copper hexacyanoferrate (CuHCF) and iron hexacyanoferrate (FeHCF) that is capable of recovering ammonium ions from source water containing various cations. This EIXS operates without expensive polymer electrolyte membranes, utilizing CuHCF electrodes to adsorb ammonium ions selectively while the FeHCF electrodes release sodium ions. The study analyzes the ion exchange behavior in solutions containing equal molar concentrations of sodium and ammonium ions, as well as simulated wastewater. Additionally, the cycling performance of the system was evaluated under various cationic conditions. The results demonstrate that EIXS effectively separates and recovers ammonium ions while maintaining lower energy consumption (6.9 Wh per mole of ammonium recovery from simulated wastewater) with high stability (capacity retention after 300 cycles: 96.8 %), making it a promising approach for practical applications.

Foreign metallic particles frequently contaminate lithium-ion battery (LIB) manufacturing processes, posing a serious risk of internal short circuits that compromise battery safety and performance. Voltage drop screening, conducted during the formation process by monitoring subtle changes in open circuit voltage (OCV), is used to identify such defective cells. However, this method is operationally demanding, and frequent OCV drops caused by internal defects can significantly reduce production efficiency and yield. To mitigate these issues, proactive contamination control through stringent technical cleanliness is critical for minimizing early-stage particle intrusion and reducing dependence on extensive screening. In this study, we analyze key aspects of the formation process and evaluate the effectiveness of voltage drop screening, with particular focus on the electrochemical behavior and risk profiles of various metallic contaminants. Our findings demonstrate how metallic particle contamination affects defect rates across different manufacturing stages. By integrating robust contamination control with optimized screening protocols, we propose practical strategies for enhancing LIB production efficiency, yield, and safety.

Defects in perovskite oxide solid electrolytes (SEs) impact Li-ion conductivity. However, the role of oxygen vacancies (Vo) in transport behavior has been less explored. Herein, our study elucidates the microscopic origin of the role of Vo in enhancing the total ionic conductivity of a prototype lithium lanthanum titanate while maintaining its insulating properties. Scanning transmission electron microscopy and theoretical calculations reveal that the presence of Vo significantly lowers the activation energy of Li-ion migration. The Vo is revealed to be preferentially aligned parallel to c-planes and causes modulated lattice expansion in an alternating manner, resulting in easy directional Li-ion transport. The effect of Vo-assisted Li-ion transport is optimized through the hierarchical rearrangement of structural features at multiple length scales close to the direction of the Vo arrays. Our results offer novel insights into the microscopic origins of superior ion conductivity facilitated by Vo, contributing to the design of high-performance SEs.

Prussian blue analogue (PBA) electrodes are widely used cation-selective electrodes for electrochemical desalination technologies due to their high specific capacity rates and fast kinetic properties. Despite the fact that previous studies of PBAs for electrochemical desalination have shown remarkable desalination capacity levels, they remain insufficient if used to desalt highly concentrated salt water such as seawater. Here, we applied highly crystalline sodium cobalt hexacyanoferrate (NaCoHCF) electrodes, a type of PBA that can utilize two redox active sites, to a rocking-chair capacitive deionization (RCDI) process. The specific capacity of the NaCoHCF electrode using two redox active sites was 88 mAh g-1 (active material: 110 mAh g-1), confirmed to be 1.5 times higher than that of PBA electrode that use one redox active site. As a result of desalination tests, this system achieved a high desalination capacity of 123 mg g-1 (active material: 154 mg g-1) with 88% ion removal in a 500 mM NaCl solution. The results of this study present a considerable increase in the desalination capacity through the introduction of NaCoHCF electrodes that utilize two redox active sites in the RCDI system.

Ammonium ions in wastewater can induce toxicity in aquatic organisms and accelerate eutrophication. Therefore, the efficient removal of ammonium ions is necessary. The electrochemical ion separation method, based on an electrode with selectivity for ammonium ions, is a promising recovery technique that can efficiently adsorb and desorb ammonium ions. In this system, the electrode is composed of copper hexacyanoferrate (CuHCF) and silver (Ag), and ammonium ions are captured by the CuHCF electrode. However, the stability of the CuHCF electrode is not sufficient for continuous ammonium ion removal. We aim to report an improvement in the stability of this system realized through the use of the state-of-charge (SoC) control method. The stability of the system can be enhanced by using SoC control at only 60 % of the maximum capacity. After 300 cycles, the discharge capacity for the 60 % SoC approach remained at 55.8 %, showing a 10 % improvement in stability compared to the 100 % SoC method (45.8 %). Furthermore, in concentrated solutions with a ratio similar to that of domestic wastewater, the 60 % SoC control strategy exhibited ammonium removal capacity and selectivity comparable to that with 100 % SoC control but with higher energy efficiency (77 % reduction in energy consumption). These results indicate that the state-of-charge (SoC) control method can enhance both the stability and the efficiency of the electrochemical ammonium separation system.

For efficient lithium recovery, the electrochemical lithium recovery (ELR) process that uses LiMn2O4 (LMO) electrodes with selectivity for lithium ions, has been introduced. The electrochemical system is environmentally friendly and allows for the recovery of lithium at a high yield, but the issue of manganese dissolution in LMO electrodes, decreasing their stability, remains to be solved. Herein, we suggest a solution to the existing problem through a rapid lithium recovery method that also enhances the stability of LMO electrodes through the state-of-charge (SoC) control approach. The retained discharge capacity of the system with a high current density (0.4 A/g) remains at 99.2% at 60% SoC after 300 cycles. Compared to the results under full charge/discharge operation (44.2% after 300 cycles), the proposed method demonstrates the state-of-charge (SoC) control adjustments at high current density levels to enhance the recovery rate and stability of the electrode. Additionally, high lithium-ion selectivity with a similar recovery rate is maintained at a high current density under 60% SoC operation compared to 100% SoC in lithium recovery tests. These results indicate that the SoC control strategy can increase the efficiency of ELR by improving the stability of the electrode under high-rate operational conditions.