Adsorption Of Sodium Ions Research Articles

Core‐Shell Structured CoP@C Cubes as a Superior Anode for High‐Rate and Stable Sodium Storage

AbstractTransition metal phosphides have emerged as a class of promising anode materials of sodium‐ion batteries owing to their excellent sodium storage capacity. However, the limited electronic conductivity and significant volume expansion have impeded their further advancement. In this work, we propose a rational design of cube‐like CoP @C composites with unique core‐shell structure via in situ phosphating and subsequent carbon coating processes. The uniform carbon coating serves as a physical buffering layer that effectively mitigates volume changes during charge/discharge processes, and prevents particle agglomeration and fragmentation, thereby enhancing the structural stability of electrode. Moreover, the nitrogen‐rich carbon layer not only provides additional active sites for sodium ion adsorption but also improves the electrode conductivity and accelerates charge transport dynamics. Consequently, the as‐synthesized CoP@C exhibits a remarkable capacity retention rate of 94.8 % after 100 cycles at 0.1 A g−1 and achieves a high reversible capacity of 146.7 mAh g−1 even under a high current density of 4.0 A g−1.

Liquid nitrogen quenching-induced grain boundary-rich intermetallic compound/metal/carbon composites for efficient capacitive deionization

In this work, a novel liquid nitrogen quenching treatment is introduced for the first time to design and prepare a grain boundary-rich intermetallic compound/metal/carbon composite, serving as an efficient electrode material for sodium ion removal from simulated seawater. The liquid nitrogen quenching method effectively enhances the grain boundary density within the composite due to the rapid cooling-induced forces, thereby creating abundant active sites. Consequently, Co7Fe3/Co/C-700 achieves optimal performance in sodium ion embedding and de-embedding, with a capacity of 207.5 mg/g. Furthermore, sodium ion adsorption/desorption mechanisms are substantiated through ex-situ techniques, revealing dynamic atomic and electronic structure evolutions under operational conditions. Density Functional Theory (DFT) calculations validate the pivotal role of grain boundary sites in enhancing activity, elucidating the correlation between sodium ion adsorption and grain boundaries. Overall, this work elucidates the significant correlation between adsorption behavior and grain boundaries, proposing an innovative strategy for developing electrode materials for capacitive deionization.

Seed strategy synthesis of porous NASICON structured NaTi2(PO4)3/carbon aerogel nanocomposites for high-performance hybrid capacitive deionization

The limited capacity of conventional carbon electrode materials hinders the practical application of capacitive deionization (CDI). Hence, it is imperative to develop electrode materials with high sodium ion adsorption and reliable stability to facilitate the construction of hybrid CDI (HCDI) systems. Notably, NaTi2(PO4)3 (NTP) has received significant attention due to its high capacity and rapid transportation ability of sodium ions. However, the poor electrical conductivity and limited electrolyte contact area of NTP have prevented further improvement in its deionization capacity. Herein, we report a seed strategy to synthesize NTP/carbon aerogel (CA). P25 was used as a seed crystal to prepare TiO2/CA via the sol–gel method and supercritical CO2 drying. NTP/CA was then obtained from TiO2/CA by hydrothermal synthesis and calcination. This strategy preserved the rational network skeleton of CA and endowed NTP/CA with a high specific surface area of 420 m2/g. The enhanced electrical conductivity provided by CA yielded a remarkable salt adsorption capacity of 43.3 mg/g in a 500 mg/L NaCl solution and a stable cycle desalination capability. This work provides a new perspective for constructing three-dimensional, highly efficient and stable HCDI electrode materials.

Structure–Activity Mechanism of Sodium Ion Adsorption and Release Behaviors in Biochar

Structure–Activity Mechanism of Sodium Ion Adsorption and Release Behaviors in Biochar

Encapsulation hierarchical bimetallic oxide with flexible electrospinning carbon nanofibers for efficient capacitive deionization

Water scarcity, an intensifying issue, has spurred extensive research into the development of sophisticated freestanding pseudocapacitive electrode materials. Characterized by their precise morphology, intricate mechanics, and superior desalination capabilities, these materials are revolutionizing water treatment technologies. However, their broad adoption is hampered by complex fabrication processes and the necessity for various unique components. Herein, the paper introduces a streamlined method combining electrospinning and carbonization to produce a freestanding nitrogen-doped carbon nanofiber encapsulating manganese cobalt oxide (MCO) nanoparticles for desalination purposes. The approach significantly enhances interfacial interactions, mitigates volume shifts during sodium ion adsorption and desorption, and strengthens interfacial stability. Benefiting from its structural and compositional merits, the optimal MCO-1.0 electrode showcases exceptional electrochemical attributes. It achieves an impressive desalination efficiency (34.68 mg g−1), swift capacitive deionization rate (0.58 mg g−1 min−1), and superior cyclic durability. The study enhances our understanding of freestanding carbon fibers encapsulated with metal oxide nanoparticles, a key step toward developing robust electrodes for industrial applications.

Work-function-prompted interfacial charge kinetics in hierarchical heterojunction flexible electrode for efficient capacitive deionization

The growing challenge of water scarcity has spurred extensive research into the development of advanced freestanding pseudocapacitive electrode materials. These electrodes, characterized by their controlled morphology, mechanical complexity, and enhanced desalination capabilities, are advancing water treatment technologies. Additionally, electrodes with interfacially influenced heterostructures demonstrate substantial potential to enhance electrochemical kinetics. However, their widespread adoption is hindered by intricate synthesis procedures and the necessity for multiple discrete components. Herein, the report introduces a cohesive approach that utilizes electrospinning and carbonization to create a freestanding, hierarchically porous nitrogen-doped carbon nanofiber network encapsulating FeNi3 alloy (FeNi3@CNFs) for desalination applications. The method enhances the interfacial effect, mitigates volume changes during sodium ion adsorption and desorption, and improves interfacial stability. By capitalizing on structural and compositional advantages, the novel FeNi3@CNFs electrode delivers outstanding electrochemical properties, including a high desalination capacity (46.47 mg g−1 at 1.2 V), rapid desalination rate (0.77 mg g−1 min−1), and enhanced cyclic durability. Computational analysis via Density Functional Theory shows that the work function prompts a surface charge redistribution at the FeNi3@CNFs heterojunction, optimizing the surface electronic structure and reducing the energy barrier for adsorption. The modification significantly boosts the diffusion kinetics of sodium adsorption. The study delineates a comprehensive methodology for fabricating high-performance heterostructured electrode materials that are effective for capacitive deionization.

Boron enriched edge-nitrogen defective carbon network toward high-capacity capacitive deionization

Nitrogen doping is an effective strategy to enhance the salt adsorption capacity (SAC) of carbon electrodes for capacitive deionization (CDI) via generating abundant active adsorption sites. However, the uncontrolled interior doping and the undesired defective sites usually restrict the further enhancement in SAC. Herein, the additional boron element is introduced into the carbon matrix to not only create the porous architectures but also modulate the types of doping species. It is observed that the introduction of B selectively converts the inert graphitic N into active pyrrolic N species with enhanced adsorption capacity in the carbon skeleton. Specifically, the optimized B enriched edge-N defective carbon network (ENC-X) exhibits a high edged N doping content of 85.8 %. Benefitting from the integrative modulation in surface functionalities, the optimized samples demonstrate a significantly higher SAC value of 84.6 mg g−1 in 500 mg L−1 NaCl solution at 1.2 V, comparable to that of the most carbonaceous materials reported so far. Additionally, density functional theory calculations uncover the synergistic effect between B and N doping, further facilitating the capacitive adsorption of sodium ions. Overall, this study highlights the advantages of regulating heteroatom species and co-doping, offering a new perspective for the application of heteroatom-doped carbon in CDI field.

Experimental investigation of sodium ion adsorption on polyacrylic acid grafted graphene oxide polymeric adsorbent: Kinetics, isotherms, and performance analyses

In this study, the sodium ion adsorption, as a desalination process of saline water, on polyacrylic acid grafted on graphene oxide (PAA-g-GO) was investigated. According to the results of GPC and FTIR analyses, it was confirmed that the synthesized adsorbent has a very effective function for Na+ adsorption. The remarkable finding about the synthesized adsorbent was that in all the tested pH range (8–10) and sodium ion concentrations (0.1–1 M), the measured adsorption capacities were reported to be more than 1750 mg/g. However, it is interesting that the maximum adsorption capacity was obtained a significant amount of 7462.31 mg/g at pH =10. According to the adsorption kinetics analysis results, the equilibrium time was obtained about 90 min. Also, by examining the effect of pH on the amount of sodium ion adsorption, it was found that it increased by increasing pH value. Adsorption data were also modeled with the Redlich-Peterson isotherm with R2 greater than 0.99 for all range of examined pHs. By examining the effect of adsorbent dosage on Na+ adsorption, it was found that the effect of the adsorbent dosage was very high at low initial concentrations (0.1 to 0.2 M), but this effect decreased by increasing the dosage.

Biomimetic Mineralization Synthesis of Flower-Like Cobalt Selenide/Reduced Graphene Oxide for Improved Electrochemical Deionization.

Rationally and precisely tuning the composition and structure of materials is a viable strategy to improve electrochemical deionization (EDI) performances, which yet faces enormous challenges. Herein, an eco-friendly biomimetic mineralization synthetic strategy is developed to synthesize the flower-like cobalt selenide/reduced graphene oxide (Bio-CoSe2/rGO) composites and used as advanced sodium ion adsorption electrodes. Benefiting from the slow and controllable reaction kinetics provided by the biomimetic mineralization process, the flower-like CoSe2 is uniformly constructed in the rGO, which is endowed with robust architecture, substantial adsorption sites and rapid charge/ion transport. The Bio-CoSe2/rGO electrode yields the maximum salt adsorption capacity and salt adsorption rate of 56.3mg g-1 and 5.6mg g-1 min-1 respectively, and 92.5% capacity retention after 60 cycles. These results overmatch the pristine CoSe2 and irregular granular CoSe2/rGO synthesized by a hydrothermal method, proving the structural superiority of the Bio-CoSe2/rGO composites. Furthermore, the in-depth adsorption kinetics study indicates the chemisorption nature of sodium ion adsorption. The structures of the Bio-CoSe2/rGO composites after long term EDI cycles are intensively studied to unveil the mechanism behind such superior EDI performances. This study offers one effective method for constructing advanced EDI electrodes, and enriches the application of the biomimetic mineralization synthetic strategy.

Fluorinated and safety-reinforced copolymer solid electrolytes for high-voltage all-solid-state sodium metal batteries

Poly (ethylene oxide)-based all-solid-state electrolytes are extensively regarded as highly promising contenders for the upcoming era of sodium metal batteries. Nevertheless, the intrinsic elevated highest occupied molecular orbital (HOMO) and the strong sodium-ion adsorption energy lead to a narrow electrochemical stability window (ESW) and a sluggish ion transport kinetics, thus retarding its widespread application in high-voltage batteries. About these shortcomings, we propose a strategy of fluorine substitution through a ready UV-initiated free-radical copolymerization to regulate the orbital level sodium-ion adsorption energy of poly (ethylene oxide) electrolytes. The characterization confirmed the success in the introduction of fluorine into the polymer (referred to as P-co-F). DFT calculations demonstrated a lower HOMO (-7.061 eV) in fluorine-substituted polymer electrolyte, which enabled to expand the ESW up to 5.55 V. Besides, the sodium-ion adsorption energy on P-co-F was decreased from −2.78 eV to −3.43 eV, facilitating the rapid transport of sodium ion. In addition, Polyethylene terephthalate (PET) was adopted as a support frame, which would reinforce the mechanical properties of polymer electrolytes. The electrochemical tests suggested P-co-F exhibited a profound interfacial stability in the Na//Na symmetrical system throughout an operational duration of 4690 h at 0.1 mA cm−2. The assembled all-solid-state battery manifested remarkable stability during continuous cycling at room temperature for 200 cycles, retaining an impressive 96.59 % of its initial capacity at 0.1C. The intriguing fluorine-induced effect, coupled with a cost-effective safety substrate, will provide valuable insights for the rational molecular design of innovative electrolytes and easy production scalability for all-solid-state Na metal batteries.

Metal-injection and interface density engineering induced nickel diselenide with rapid kinetics for high-energy sodium storage

The key to the innovation of sodium-ion batteries (SIBs) is to find efficient sodium-storage electrode. Here, metal Mo doping of NiSe2 is proposed by modified electrospinning strategy followed by in situ conversion process. The Mo-NiSe2 anchoring on hollow carbon nanofibers (HCNFs) would make full use of the multi-channel HCNFs in the inner layer and the active sites of Mo-NiSe2 in the outer layer, which plays an important role in buffering the volume stress of Na+ (de)insertion and reducing the adsorption energy barrier of Na+. Innovatively, it is proposed to jointly regulate the SIBs performance of NiSe2 by both metal atom doping and interface effects, thereby adjusting the sodium ion adsorption barrier of NiSe2. The Mo-NiSe2@HCNFs exhibits remarkable performance in SIBs, demonstrating a high specific capacity of 396 mAh/g after 100 cycles at 1 A/g. Moreover, it maintains outstanding cycling stability, retaining 77.6 % of its capacity (211 mAh/g) even after 1000 cycles at 10 A/g. This comprehensive electrochemical performances are due to the structural stability and outstanding electronic conductance of the Mo-NiSe2@HCNFs, as evidenced by the diffusion analysis and ex situ charge–discharge process characterization. Furthermore, coupled with the Na3V2(PO4)2O2F cathodes, the full cell also achieves a high energy density of 123 Wh kg−1. The theoretical calculation of the hypervalent Mo doing further proves the benefit of its Na+ adsorption and denser conduction band distribution. This study provides a reference for the construction of transition metal selenide via doping and interface engineering in sodium storage.

CdSe-nanoparticles-regulated synthesis of ZnCo-MOFs derived conductive porous carbon nanoflakes on carbon cloth for flexible sodium-ion supercapacitors

Porous carbon derived from metal-organic frameworks (MOFs) represents a novel class of carbonaceous materials with unique structural characteristics, conductivity, and chemical stability, rendering them suitable for electrodes in supercapacitors. Herein, a three-dimensional structure of conductive porous carbon nanoflakes (CPCN) is synthesized based on MOFs composed of organic ligands containing zinc and cobalt. Nanoscale CdSe particles serving as energy-storage materials are incorporated into CPCN (CdSe/CPCN@CC) to form the flexible electrode for sodium-ion supercapacitors. The electrochemical properties of CdSe/CC, CdSe/ZnCo-MOF@CC, Cd/CPCN@CC, and CdSe/CPCN@CC are studied in 1 M Na2SO4. The CdSe/CPCN@CC electrode shows an impressive specific capacitance of 893.52 F g-1 at a current density of 0.125 mA cm-2 and excellent cycling stability with 80.68% retention after 10,000 cycles in 1.0 M Na2SO4. The asymmetric supercapacitor constructed with CdSe/CPCN@CC and 1 M NaPF6 electrolyte shows a capacitance of 97.8 F g-1, an energy density of 139.10 Wh kg-1, and a power density of 589.82 W kg-1 at a current density of 1 mA cm-2. In addition, the capacitance retention is 88.45% after 10,000 cycles. The electrochemical properties of CdSe/CPCN@CC are improved due to the incorporation of more active materials as well as the synergistic effects of the CdSe nanoparticles and CPCN. Density-functional theory (DFT) calculations reveal a sodium ion adsorption energy of -1.0692 eV and large DOS near the Fermi level. More importantly, in spite of severe mechanical bending, the device continues to power an LED array boding well for flexible and wearable applications.

Architectural engineering of MoS2/Fe2O3@carbon fiber heterostructures to obtain ultra-stable anodes for sodium-ion batteries

Metal sulfides based on a conversion mechanism possess high theoretical specific capacity, making them suitable for high-energy-density sodium-ion batteries (SIBs). However, they suffer from rapid capacity decay when assembled into full batteries due to dynamic hysteresis and large volume expansion. In this paper, the architectural engineering approach to construct MoS2/Fe2O3 @ carbon fiber heterostructures is proposed to overcome the above issues. The heterojunction formed by MoS2 and Fe2O3 would promote ion/electron transfer, while the carbon nano-fibers serve as a holder to maintain structural stability and ensure the rapid transport of electrons. The full battery in which the as-prepared MoS2/Fe2O3 @ carbon fiber anode is paired with a Na3V2(PO4)3/C cathode delivers a reversible capacity of 61.1 mAh g−1 at 500 mA g−1 for 1600 cycles without attenuation. Experimental characterization and theoretical calculation results show that this structural engineering approach improves the ion diffusion efficiency, enhances the conductivity, provides a more appropriate sodium ion adsorption energy, and thus extends the cycling life of the SIB. This work demonstrates the efficiency of architectural engineering and paves the way to design ultra-long-cycling Na-ion batteries.

α-MnO2 composite with gold nanoparticles on carbon cloth modified with MOFs-derived porous carbon for flexible and activity-enhanced sodium-ion supercapacitors

Improving the catalytic activity of electrodes in a limited space is crucial to energy storage devices such as supercapacitors. Herein, the surface activity of the flexible electrode (Au-MnO2/CPCN@CC) is enhanced by combining gold particles (AuNPs) modified α-MnO2 with conductive porous carbon nanoflakes (CPCN) derived from the metal-organic framework (MOF) on carbon cloth (CC). The Au-MnO2/CPCN@CC electrode exhibits a higher specific capacity of 503.7 F/g at 0.125 mA/cm2 and retains 87.68% capacity after 10,000 cycles in 1 M Na2SO4, while the structure without AuNPs achieves 242.6 F/g and 61.07% capacity retention after 10,000 cycles. The high stability of Au-MnO2/CPCN@CC stems from the good pseudocapacitance ratio of 83.80% at 1 mV/s compared to 78.43% of MnO2/CPCN@CC and 40.88% of Au/CPCN@CC. The supercapacitor assembled with Au-MnO2/CPCN@CC as the positive electrode and activated carbon (AC) as the negative electrode in 1 M NaPF6 shows an excellent power density of 79.96 W/kg at 71.48 Wh/kg as well as 81.93% capacitance retention after 10,000 cycles. Density-functional theory (DFT) calculations of Au-MnO2/CPCN composite reveal a sodium-ion adsorption energy of 3.744 eV and a large DOS near Fermi level. Hence, Au nanoparticles (AuNPs) and MOFs-derived CPCN lead to enhanced surface electron transport of MnO2 and ultimately superior performance.

(Invited) Lessons in Electrode Architecture for Faradaic Desalination

Potable water is an essential supply for humanitarian missions worldwide, yet delivering pre-purified water to remote locations is challenging, dangerous, and expensive. Therefore, it is desirable to develop and deploy on-site and portable desalination/water-purification equipment that draws on local water sources. Present reverse-osmosis systems effectively desalinate seawater, but are energy and time-intensive, require regular maintenance due to membrane fouling, and suffer from poor scalability. Capacitive deionization (CDI) technology provides energy-efficient desalination of brackish waters, but its use remains limited by low-capacity carbon-only electrodes that rely on double-layer ion storage. Transitioning to electrodes that also incorporate Faradaic ion-capturing materials with substantively higher storage capacity expands the efficacy and applicability of CDI. As one example, NRL-developed porous carbon nanofoam (CNF) architectures infiltrated with electrolessly deposited nanometric MnO2 demonstrate 6-fold increase in sodium-ion adsorption capacity compared to bare-carbon CNFs, while solvothermally deposited BiOCl renders a high-capacity chloride-ion adsorption electrode. The tunability of CNFs as an electrode scaffolding affords the opportunity to balance ion-storage capacity and electrolyte transport for optimized desalination performance. Lessons of electrode architecture extend to NRL silver “sponge” electrodes that provide high capacity for chloride, fast uptake dynamics, and opportunities for various flow configurations. Continued progress in bench-top level flow-cells with high-capacity faradaic materials will demonstrate the promise of this technology en route to development of larger-scale prototype desalination devices.

Mechanistic insight and optimisation of hydrothermally pre-treated biowaste-derived biochar for saline water treatment

The valorisation of oil palm empty fruit bunch is challenging due to its poor surface functionalities, which require a comprehensive pre-treatment process. To ensure efficient valorisation of the agricultural waste, the biochar derived from empty fruit bunch is subjected to hydrothermal nitric acid pre-treatment to act as an adsorbent for sodium ions removal from the saline solution. For this newly developed adsorbent, the adsorption study provides important information on the adsorption behaviour of sodium ions and the optimum conditions for sodium ions removal, which is crucial for effective process design and operation control during practical applications. Physicochemical characterisation revealed the successful adsorption of sodium ions by Hydrothermal Nitric acid Pre-treated EFB Biochar (HNO3 EFB-BC. The highest sodium ions removal efficiency of HNO3 EFB-BC (92.04%) was achieved under the optimum reaction conditions: 0.39 M initial concentration of the saline solution, 4.96 g of HNO3 EFB-BC, contact time of 17.4 h and solution pH of 7.46. Upon process optimisation, the adsorption capacity of HNO3 EFB-BC towards sodium ions improved remarkably (p < 0.05) from 78.34 mg g-1 to 166.45 mg g-1. The adsorption isotherm and kinetic study are consistent with the Langmuir and pseudo-second-order models, implying a monolayer chemisorption-dominated adsorption process. The promising sodium ions adsorption capacity of HNO3 EFB-BC can be attributed to the enhanced surface functionalities of the adsorbent. The molecular modelling using the density functional theory approach has successfully identified the nitro group as the most favourable functional group in producing the charges sites for sodium ions adsorption, with the most stable reaction route between HNO3 EFB-BC and sodium ions being identified. This study highlighted the density functional theory approach as a tool for identifying the specific functional groups with enhanced adsorption capability for saline water treatment and provides a reference value for removing sodium ions in the presence of other cationic pollutants.

Effects of C-S-H gel surface structure on sodium chloride evaporation crystallization in C-S-H gel nanopores with molecular dynamics analysis

The evaporation crystallization of salt solutions in cement-based material pores accelerates durability degradation. It is necessary to investigate the mechanisms and process of evaporation crystallization in the pore channel of C-S-H gels on a nanoscale level. With molecular dynamics simulation, this paper explored the influence of C-S-H gel surface structure on the evaporation crystallization process of NaCl solution based on a new solid–liquid model. NaCl crystallization in the channel space of the C-S-H gel nanopore followed a two-step crystallization mechanism (nucleation and crystallization). The adsorption of sodium and chloride ions on the C-S-H gel surface shows significant differences. The surface of C-S-H gel is electronegative, so the C-S-H gel surface adsorption capacity of cations is stronger than that of anions and the C-S-H gel surface adsorption of sodium ions is more stable. The difference of silica-oxygen tetrahedron density of C-S-H gel surfaces caused the difference of charge characteristic of C-S-H gel surfaces. The C-S-H gel surface structure has an effect on the sodium chloride crystallization position, but has not obvious impact on the two-step evaporation crystallization process. These findings provide a theoretical basis for addressing the nanoscale characteristics of C-S-H gel surface and the macroscale durability of cement-based materials.

Functionalisation of biowaste-derived biochar via accelerated hydrothermal-assisted post-treatment for enhanced sodium ion adsorption

Functionalisation of biowaste-derived biochar via accelerated hydrothermal-assisted post-treatment for enhanced sodium ion adsorption

Constructing bimetallic heterostructure as anodes for sodium storage with superior stability and high capacity

Heterostructure materials hold promise as future-generation anodes for sodium-based energy storage. However, the complexity and time-intensity of heterojunction fabrication processes limit their large-scale application. This study utilizes an ultrafast, one-step microwave process to successfully design and fabricate bimetallic selenides heterojunction nanoparticles, anchored on a three-dimensional porous carbon substrate (NiSe2–SnSe2/C). The in-situ formation of NiSe2–SnSe2 heterostructure optimizes sodium ion adsorption and transfer, which facilitates reaction kinetics and subsequently improves both the rate capability and cycle life. The NiSe2–SnSe2/C electrodes demonstrate a high-rate capability (719.8 mAh g−1 at 0.1 A g−1, and 667.9 mAh g−1 after a 50-fold increase in current density to 5 A g−1), maintaining high stability even after 10,000 cycles at 5 A g−1. The impact of the NiSe2–SnSe2 heterostructure on reaction kinetics was examined using density functional theory. The NiSe2–SnSe2/C anode, when coupled with the Na3V2(PO4)3 cathode in a full battery, demonstrated an impressive energy density of 155 Wh kg−1. This work exemplifies an ultrafast synthetic method for constructing heterostructure materials having great rate, high capacity and strong stability for sodium storage, making them promising candidates for large-scale energy storage applications.

Hydrochemical Characteristics of Mine Water and Their Significance for the Site Selection of an Underground Reservoir in the Shendong Coal Mining Area

Underground reservoir technology can mitigate water shortage and pollution problems in water shortage coal mining areas and has a good application prospect. While still a new technology, the theory and method of underground reservoirs need to be improved. This research focused on the hydrochemical characteristics of mine water and their significance for the site selection of underground reservoirs. With the Shendong coal mining area as a case study, the hydrochemical major ions, toxicological indexes, and stable isotopes of hydrogen and oxygen were tested for the mine water samples, and the water quality was quantitatively evaluated and the origins of over-limit variables were investigated by hydrogeochemical numerical simulation and ionic ratio analysis. The influencing factors of water quality were analyzed and the significance of mine water quality for the site selection of underground reservoirs was discussed. The results show that the main over-standard variables are Na+, F−, SO42−, TDS, and sodium ion adsorption ratio (SAR), and a strong positive correlation exists between F− and SAR and a negative correlation exists between F− and Ca+. Na+ in mine water originates from the dissolution of halite and silicate rocks, as well as reverse cation exchange. F− originates from reverse cation exchange and the displacement between OH− in alkaline water and F− adsorbed on the surface of minerals. On the whole, the mine water quality is better on the east than on the west of the WL River. The water–rock interactions in goaf increase the concentrations of F− and Ca2+ and SAR. The areas where the mine water samples have low concentrations of Na+, F−, and low SAR values, such as the shallow coal seams at the SGT, DLT, and WL mines, are favorable sites for the underground reservoir. The outcomes may benefit the reasonable site selection of underground reservoirs in similar coal mining areas with water shortage.