Sodium Superionic Conductor Research Articles

Sodium Super Ionic Conductor-Type Hybrid Electrolytes for High Performance Lithium Metal Batteries.

Composite solid electrolytes (CSEs), composed of sodium superionic conductor (NASICON)-type Li1+xAlxTi2-x(PO4)3 (LATP), poly (vinylidene fluoride-hexafluoro propylene) (PVDF-HFP), and lithium bis (trifluoromethanesulfonyl)imide (LiTFSI) salt, are designed and fabricated for lithium-metal batteries. The effects of the key design parameters (i.e., LiTFSI/LATP ratio, CSE thickness, and carbon content) on the specific capacity, coulombic efficiency, and cyclic stability were systematically investigated. The optimal CSE configuration, superior specific capacity (~160 mAh g-1), low electrode polarization (~0.12 V), and remarkable cyclic stability (a capacity retention of 86.8%) were achieved during extended cycling (>200 cycles). In addition, with the optimal CSE structure, a high ionic conductivity (~2.83 × 10-4 S cm-1) was demonstrated at an ambient temperature. The CSE configuration demonstrated in this work can be employed for designing highly durable CSEs with enhanced ionic conductivity and significantly reduced interfacial electrolyte/electrode resistance.

One-Step Synthesis of Three-Dimensional Na3V2(PO4)3/Carbon Frameworks as Promising Sodium-Ion Battery Cathode.

Sodium-ion batteries (SIBs) are essential for large-scale energy storage attributed to the high abundance of sodium. Polyanion Na3V2(PO4)3 (NVP) is a dominant cathode candidate for SIBs because of its high-voltage and sodium superionic conductor (NASICON) framework. However, the electrochemical performance of NVP is hindered by the inherently poor electronic conductivity, especially for extreme fast charging and long-duration cycling. Herein, we develop a facile one-step in-situ polycondensation method to synthesize the three-dimensional (3D) Na3V2(PO4)3/holey-carbon frameworks (NVP@C) by using melamine as carbon source. In this architecture, NVP crystals intergrown with the 3D holey-carbon frameworks provide rapid transport pathways for ion/electron transmission to increase the ultrahigh rate ability and cycle capability. Consequently, the NVP@C cathode possesses a high reversible capacity of 113.9 mAh g-1 at 100 mA g-1 and delivers an outstanding high-rate capability of 75.3 mAh g-1 at 6000 mA g-1. Moreover, it shows that the NVP@C cathode is able to display a volumetric energy density of 54 Wh L-1 at 6000 mA g-1 (31 Wh L-1 for NVP bulk), as well as excellent cycling performance of 65.4 mAh g-1 after 1000 cycles at 2000 mA g-1. Furthermore, the NVP@C exhibits remarkable reversible capabilities of 81.9 mAh g-1 at a current density of 100 mA g-1 and 60.2 mAh g-1 at 1000 mA g-1 even at a low temperature of -15 °C. The structure of porous carbon frameworks combined with single crystal materials by in-situ polycondensation offers general guidelines for the design of sodium, lithium and potassium energy storage materials.

Unveiling a high capacity multi-redox (Nb5+/Nb4+/Nb3+) NASICON-Nb2(PO4)3 anode for Li- and Na-ion batteries

Sodium superionic conductor (NASICON)-type materials are widely explored as Li- and Na-ion cathodes and solid-state electrolytes but are largely ignored as anodes due to their lower capacities and higher intercalation voltages, which reduce the overall energy densities of Li- and Na-ion batteries (LIBs and SIBs).

Initiating High-Voltage Multielectron Reactions in NASICON Cathodes for Aqueous Zinc/Sodium Batteries

Sodium superionic conductor (NASICON) is a class of compounds with robust polyanionic frameworks and high thermal stability, which are regarded as prospective cathodes candidates for secondary batteries. However, NASICON cathodes typically have low discharge plateaus and low practical capacities in aqueous electrolytes. Here, Na 3 V 1.75 Fe 0.25 (PO 4 ) 2 F 3 is investigated as a cathode material for the aqueous zinc/sodium batteries. While the addition of F helps with the improvement of NASICON structural stability, the low-cost Fe substitution has a positive impact on the capacity increment, reaction voltage increases, and cycling stability improvement. Because the Fe 3+ substitution could induce a change in the spin magnetic moments of the 3d orbitals of the VO 4 F 2 and FeO 4 F 2 octahedra, the 2-electron reaction of V is activated, which are V 4+ /V 3+ and V 5+ /V 4+ redox couples. As a result, the novel Na 3 V 1.75 Fe 0.25 (PO 4 ) 2 F 3 cathode delivers a high operating voltage of 1.7 V, a high energy density of 209 W·h·kg −1 and stable lifespan (83.5% capacity retention after 6,000 cycles at 1 A·g −1 ) in the aqueous zinc/sodium batteries. This research demonstrates the practicality of activating multielectron reactions to optimize the electrochemical properties of NASICON cathodes for aqueous secondary batteries.

Novel High-Pressure Nanocomposites for Cathode Materials in Sodium Batteries.

A new nanocomposite material was prepared by high pressure processing of starting glass of nominal composition NaFePO4. Thermal, structural, electrical and dielectric properties of the prepared samples were studied by differential thermal analysis (DTA), X-ray diffraction (XRD) and broadband dielectric spectroscopy (BDS). It was demonstrated that high-pressure-high-temperature treatment (HPHT) led to an increase in the electrical conductivity of the initial glasses by two orders of magnitude. It was also shown that the observed effect was stronger than for the lithium analogue of this material studied by us earlier. The observed enhancement of conductivity was explained by Mott's theory of electron hopping, which is more frequent in samples after pressure treatment. The final composite consisted of nanocrystalline NASICON (sodium (Na) Super Ionic CONductor) and alluaudite phases, which are electrochemically active in potential cathode materials for Na batteries. Average dimensions of crystallites estimated from XRD studies were between 40 and 90 nm, depending on the phase. Some new aspects of local dielectric relaxations in studied materials were also discussed. It was shown that a combination of high pressures and BDS method is a powerful method to study relaxation processes and molecular movements in solids. It was also pointed out that high-pressure cathode materials may exhibit higher volumetric capacities compared with commercially used cathodes with carbon additions.

High-entropy mechanism to boost ionic conductivity.

Advances in solid-state batteries have primarily been driven by the discovery of superionic conducting structural frameworks that function as solid electrolytes. We demonstrate the ability of high-entropy metal cation mixes to improve ionic conductivity in a compound, which leads to less reliance on specific chemistries and enhanced synthesizability. The local distortions introduced into high-entropy materials give rise to an overlapping distribution of site energies for the alkali ions so that they can percolate with low activation energy. Experiments verify that high entropy leads to orders-of-magnitude higher ionic conductivities in lithium (Li)-sodium (Na) superionic conductor (Li-NASICON), sodium NASICON (Na-NASICON), and Li-garnet structures, even at fixed alkali content. We provide insight into selecting the optimal distortion and designing high-entropy superionic conductors across the vast compositional space.

Exploration of NaSICON Frameworks as Calcium-Ion Battery Electrodes

The development of energy storage technologies that are alternative to state-of-the-art lithium-ion batteries but exhibit similar energy densities, lower cost, and better safety is an important step in ensuring a sustainable energy future. Electrochemical systems based on calcium (Ca)-intercalation or deintercalation form such an alternative energy storage technology but require the development of intercalation electrode materials that exhibit reversible Ca-exchange with reasonable energy density and power density performance. To address this issue, we use first-principles calculations to screen over the wide chemical space of sodium superionic conductor (NaSICON) frameworks, with a chemical formula of CaxM2(ZO4)3 (where M = Ti, V, Cr, Mn, Fe, Co, or Ni and Z = Si, P, or S) for Ca electrode materials. We calculate the average Ca2+ intercalation voltage, the thermodynamic stability (at 0 K) of charged and discharged Ca-NaSICON, and the migration barriers of (meta)stable Ca-NaSICON compositions. Importantly, our calculations indicate CaxV2(PO4)3, CaxMn2(SO4)3, and CaxFe2(SO4)3 Ca-NaSICONs to be promising as Ca-cathodes. We find all silicate Ca-NaSICONs to be thermodynamically unstable and hence unsuitable as Ca-cathodes. We report the overall trends in the ground state Ca-vacancy configurations, besides voltages, stabilities, and migration barriers. Our work contributes to unearthing strategies for developing practical calcium-ion batteries, involving polyanionic intercalation frameworks.

Electrochemical Performance of High-Valence Mo6+ and Low-Valence Mn2+ Doped- Na3 V2 (PO4 )3 @C Cathode for Sodium-Ion Batteries.

The sodium superionic conductor (NASICON)-Na3 V2 (PO4 )3 (NVP) is an attractive cathode for sodium-ion batteries, which is still confronted with limited rate performance due to its low electronic conductivity. In this paper, a chemical strategy is adopted to partially replace V3+ of the NVP framework by low-valence Mn2+ and high-valence Mo6+ substitution. The crystal structure, sodium-ion diffusion coefficient and electrochemical performance of Mn-Mo-doped [Na3.94 V0.98 Mo0.02 Mn(PO4 )3 @C] cathode were investigated. X-ray diffraction confirmed the NASICON-type structure and XPS analysis confirmed the oxidation state of Mn and Mo in doped NVP cathode. The Na ion diffusion processes were inferred from Cyclic Voltammetry (CV), Galvanostatic intermittent titration technique (GITT) and Electrochemical Impedance Spectroscopy (EIS) measurement, which clearly show rapid Na-ion diffusion in NASICON-type cathode materials. The Mn-Mo-substituted NVP shows smoother charge-discharge profiles, improved rate performance (64.80 mAh/g at 1 C rate), better energy density (308.61 mWh/g) and superior Na-ion kinetics than that of unsubstituted NVP@C cathode. Their enhanced performance is attributed to large interstitial volume mainly created by high valence Mo6+ and enhanced capacity is attributed to the low valence Mn2+ doping. These results demonstrate that Mn-Mo-doped NVP cathode is strongly promising cathode material for sodium-ion batteries.

Effect of Lithium Salt Concentration on Materials Characteristics and Electrochemical Performance of Hybrid Inorganic/Polymer Solid Electrolyte for Solid-State Lithium-Ion Batteries

Lithium-ion batteries are popular energy storage devices due to their high energy density. Solid electrolytes appear to be a potential replacement for flammable liquid electrolytes in lithium batteries. This inorganic/hybrid solid electrolyte is a composite of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt, (poly(vinylidene fluoride-hexafluoro propylene) (PVDF-HFP) polymer and sodium superionic conductor (NASICON)-type Li1+xAlxTi2−x(PO4)3 (LATP) ceramic powder. The structure, morphology, mechanical behavior, and electrochemical performance of this composite solid electrolyte, based on various amounts of LiTFSI, were investigated. The lithium-ion transfer and conductivity increased as the LiTFSI lithium salt concentration increased. However, the mechanical strength apparently decreased once the percentage of LITFSI was over 60%. The hybrid electrolyte with 60% LiTFSI content showed high ionic conductivity of 2.14 × 10−4 S cm−1, a wide electrochemical stability window (3–6 V) and good electrochemical stability. The capacity of the Li|60% LiTFSI/PVDF-HFP/LATP| LiFePO4 solid-state lithium-metal battery was 103.8 mA h g−1 at 0.1 C, with a high-capacity retention of 98% after 50 cycles.

Understanding the Structural Phase Transitions in Na3V2(PO4)3 Symmetrical Sodium-Ion Batteries Using Synchrotron Based X-Ray Techniques

Sodium ion batteries (SIBs) are the promising candidate for large scale applications due to their high abundance and low cost.The symmetrical SIBs (similar cathode and anode configuration) are highly promising as they exhibit negligible volume changes during the charge/discharge process and making SIBs highly stable. Several NASICON (sodium superionic conductor) have high potential towards symmetrical SIBs due to presence of multi-redox reactions in both anodic and cathodic regions. Na3V2(PO4)3 (NVP) has been found to be appeal for symmetrical SIB systems due to presence of flat voltage profile, multi-redox with a single transition metal, and a large working voltage difference between anodic and cathodic regions can greatly improve the output of the NVP symmetrical cell. However, the phase transitions associated during the electrochemical reaction in the symmetrical NVP cells are not widely studied. The synchrotron X-ray studies in NVP symmetrical cells confirms the redox reaction involving ≈2 moles of Na+ reaction instead of 1 mole of Na+ reaction. The in-situ X-ray diffraction and X-ray absorption spectroscopy during the electrochemical cycling confirms the reversible formation of Na5V2(PO4)3 phase in the anode and NaV2(PO4)3 phase in the cathode. The charge storage process in NVP cathode region is through V3+/V4+ redox, and charge storage process in NVP anode region is through V3+/V2+ redox. The 2 moles sodium-ion storage process can make a discharge capacity of ≈99 mAh g−1, increasing the energy output of the symmetrical NVP.

(Invited) Zero-Thermal-Quenching in a Nasicon-Type Phosphor By Na+ Ion Migration

Among the many potential Eu2+-activated sodium superionic conductor (NASICON)-based host materials, the Sc3+-based NASICON phosphor (Na3Sc2(PO4)3:Eu2+) is a promising phosphor material for high-power lighting applications owing to its unusual thermal stability at elevated temperatures. It has previously been shown that negative thermal quenching (TQ) can be tailored to zero TQ depending on the Eu2+ concentration. However, the obtained zero-TQ composition has low photoluminescent quantum yields, which hinders its applicability to high-power lighting. Herein, we report a holistic study of the tuning of thermal stability from negative TQ to zero-TQ while preserving the original emission efficiency by introducing Lu3+ ions in Na3Sc2(PO4)3:Eu2+. Furthermore, we fabricated a high-power white light emitting diode (WLED) using optimized Lu3+-doped Na3Sc2(PO4)3:Eu2+ as the blue component, delivering a high color rendering index value of 90 with a high luminous efficacy value of 25 lm/W obtained at a flux current of 1000 mA. Therefore, the findings of this work provide novel scientific insight into the importance of structure-property relationships in designing highly efficient thermally stable phosphors for high-power lighting applications.

Evaluating the Stability and Performance of Nasicon in Low-Cost High Charge Density Redox Flow Battery Electrolytes

Solid superionic conductor membranes are being considered as alternatives to polymer-based membranes for use in redox flow batteries (RFBs) due to their superior abilities to mitigate reactant crossover,1 and enable the deployment of aqueous electrolytes comprising small, inorganic earth-abundant reactants.2,3 Much however remains to be understood about the evolution of the electrochemical performance and microstructural stability of these membranes while they are immersed in aqueous electrolytes.In this work, we evaluate the suitability of von Alpen sodium superionic conductor (NaSICON) as a prospective RFB membrane material by examining its resistance, permeability and interfacial morphology over time as a function of electrolyte pH and composition, as well as temperature. NaSICON is found to have a stable resistance profile for several weeks while immersed in neutral to strongly alkaline ([OH-] = 3 M) aqueous electrolytes. Its permeability to polysulfide-based and permanganate-based small-molecule RFB reactants is negligible compared to that of Nafion. Its area-specific resistance falls dramatically with increasing temperature and decreasing membrane thickness; we project that a membrane with a thickness of 100 μm or lower, if operated slightly above ambient temperature (~ 40 °C), can enable power densities comparable to or better than those of conventional polymer membrane-containing RFBs. Nevertheless, the glassy phase of the NaSICON microstructure at the membrane-electrolyte interface was found to undergo small amounts of etching while in contact with aqueous electrolytes containing sodium ions; this etching became significantly more extensive when potassium ions were present in the electrolyte, leading in certain instances to complete disintegration of the membrane.The extraordinary high stability of NaSICON in strongly alkaline electrolyte permits the construction of flow cells containing a positive electrolyte based on permanganate, a high-potential, inexpensive reactant with high volumetric capacity (> 100 Ah/L). The flow cells had open-circuit voltages 1.2 V and greater, along with negligible reactant crossover and very low capacity fade (< 0.02 %/day). This work highlights the promise of ceramic membranes as effective separators in RFBs operating under neutral pH to strongly alkaline pH conditions. It also points to the need for further research on the long-term stability of the membrane and its interface with the electrolyte in solid-state membranes under investigation as separators in aqueous RFBs. (1) Yu, X.; Gross, M. M.; Wang, S.; Manthiram, A. Aqueous Electrochemical Energy Storage with a Mediator-Ion Solid Electrolyte. Advanced Energy Materials 2017, 7 (11), 1602454, https://doi.org/10.1002/aenm.201602454. DOI: https://doi.org/10.1002/aenm.201602454 (acccessed 2021/03/12).(2) Wei, X.; Xia, G.-G.; Kirby, B.; Thomsen, E.; Li, B.; Nie, Z.; Graff, G. G.; Liu, J.; Sprenkle, V.; Wang, W. An aqueous redox flow battery based on neutral alkali metal ferri/ferrocyanide and polysulfide electrolytes. J. Electrochem. Soc. 2016, 163 (1), A5150-A5153. DOI: 10.1149/2.0221601jes].(3) Colli, A. N.; Peljo, P.; Girault, H. H. High energy density MnO4-/MnO42- redox couple for alkaline redox flow batteries. Chem Commun (Camb) 2016. DOI: 10.1039/c6cc08070g.

Ligand‐Substitution Chemistry Enabling Wide‐Voltage Aqueous Hybrid Electrolyte for Ultrafast‐Charging Batteries

AbstractThe further development of sodium‐zinc hybrid batteries (SZBs) is seriously impeded by the narrow electrochemical stability window (ESW) of aqueous electrolytes. Exploring appropriate electrolytes with both wide ESW and high ionic conductivity is of great importance to achieve high‐performance SZBs yet remain challenging. Here, a rationally designed Na+/Zn2+ hybrid electrolyte is developed via a ligand‐substitution strategy, which effectively extends the ESW up to 2.9 V and combines with high ionic conductivity of 19.6 mS cm−1. The ligand exchange process reconfigures the cation solvation structure and optimizes the carrier mobility environment. Furthermore, Na+/Zn2+ hybrid cells are assembled by pairing Zn anode with two different kinds of sodium superionic conductor (NASICON) type cathodes, achieving a promising rate performance and long cycle life (3 A g−1 over 1000 cycles). Meanwhile, the high electrochemical reactivity of water molecules promotes the formation of the high‐quality NaF/ZnF2‐rich cathode electrolyte interphases, inhibiting the uncontrolled decomposition of the electrolyte on the cathode interface. This work provides guidance for designing aqueous hybrid electrolytes with wide ESW and high carrier mobility.

N-Doped Carbon-Layer-Modified Na3V2(PO4)3 as a High-Performance Cathode Material for Sodium-Ion Batteries

Sodium super ion conductor (NASICON)-type Na3V2(PO4)3 (NVP) has been seen as an extremely potential cathode material in sodium-ion batteries (SIBs) because it owns many prominent merits, like an open three-dimensional channel, high-voltage platform, structural stability, etc. Nevertheless, NVP is difficult to obtain excellent electrochemical performance at high rates with the defect of low electronic conductivity, which leads to the restriction of practical application. In this paper, a nitrogen-doped carbon layer-coated Na3V2(PO4)3 composite material (NVP/NC) was synthesized by a simple sol–gel method using urea as a nitrogen source. The further test proved that NVP/NC has a better rate performance compared to NVP/C. The initial reversible capacity of NVP/NC can reach 109.18 mAh g–1 at 1 C, and the discharge specific capacity can reach 88.3 mAh g–1, even when the ultrahigh current density is 50 C. In addition, NVP/NC has excellent long cycle stability (the capacity retention rate reaches 72.89% at 50 C after 8000 cycles, and the capacity reduction rate per revolution is only about 0.0034%). Because the N-doped carbon layer provides a surface channel for electron transmission of NVP, the electronic conductivity is greatly enhanced, making NVP/NC a better composite material for SIBs. Hence, this work offers a practical process to solve the poor electronic conductivity issue of NVP.

Hybrid solid state electrolytes blending NASICON-type Li1+xAlxTi2–x(PO4)3 with poly(vinylidene fluoride-co-hexafluoropropene) for lithium metal batteries

Lithium-ion batteries (LIBs) with Li metal as the anode electrode are very promising energy storage systems for high energy/power density applications. However, Li dendrite formation and growth on the Li anode due to non-uniform Li metal deposition is one of the major issues yet to be addressed. In this work, novel hybrid solid state electrolytes (SSEs) were developed through blending poly(vinylidene fluoride-hexafluoro propylene) (PVDF-HFP), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt, and sodium superionic conductor (NASICON)-type Li1+xAlxTi2‒x(PO4)3 (LATP) powders. The LATP powders were synthesized by the sol-gel technique followed by calcination at 850 °C. The as-prepared LATP particles formed a well-defined crystalline structure with a rhombohedral lattice. The LATP content within the hybrid SSEs plays a crucial role in improving the mechanical durability, ionic conductivity, rate capability, discharge capacity, and cyclic stability. Analyzing various SSE configurations, an optimal ratio of LATP was determined (i.e., 10–15 wt.%) enabling an ultra-high ionic conductivity of ∼1.11 × 10–4 S cm–1. Employing the LATP-based SSE developed in this work, the dissolution of iron from the LiFePO4 cathode was mitigated and a stable solid electrolyte interphase layer was formed resulting in uniform Li metal deposition along with exceptional cyclic durability during long-duration charge/discharge cycling.

Hierarchical Na3V2(PO4)2F3 Microsphere Cathodes for High-Temperature Li-Ion Battery Application.

Sodium superionic conductor (NASICON)-structured Na3V2(PO4)2F3 cathode materials have received vast attention in the high-temperature storage performance due to their structural and thermal stability. Herein, hierarchical Na3V2(PO4)2F3 microspheres (NVPF-HMSs) consisting of nanocubes were designed by a one-pot facial solvothermal method. The hierarchical Na3V2(PO4)2F3 microsphere size is 2–3 μm, which is corroborated by FE-SEM and HR-TEM analyses. The NVPF-HMSs have been demonstrated as a cathode in Li-ion batteries at both low and elevated temperatures (25 and 55 °C, respectively). The NVPF-HMS cathode in a Li-ion cell exhibits reversible capacities of 119 mA h g–1 at 0.1 C and 85 mA h g–1 at 1 C with an 82% retention after 250 cycles at 25 °C. At elevated temperatures, the NVPF-HMS cathode exhibits a superior capacity of 110 mA h g–1 at 1 C along with a retention of 90% after 150 cycles at 55 °C. Excellent capacity and cyclability were achieved at 55 °C due to its hierarchical morphology with a robust crystal structure, low charge-transfer resistance, and improved ionic diffusivity. The Li-ion storage performance of the NVPF-HMS cathode material at elevated temperatures was analyzed for the first time to understand the high-temperature storage property of the material, and it was found to be a promising candidate for elevated-temperature energy storage applications.

Unravelling capacity fading mechanisms in sodium vanadyl phosphate for aqueous sodium-ion batteries

Na3V2(PO4)3 (NVP), which is known as a sodium superionic conductor (NASICON), has been successfully developed as an excellent cathode material for sodium-ion batteries (SIBs). However, the capacity of NVP quickly fades when used in an aqueous electrolyte. Herein, the charge storage and capacity attenuation mechanisms of carbon-coated NVP (NVP@C) were carefully investigated by systematic material characterization and density functional theory (DFT) calculations. According to the results, protons in the aqueous electrolyte diffuse into the surface of NVP@C to occupy the sodium site and attack the nearby phosphates during the charge–discharge cycles, leading to the deformation and breakage of the POV bond. The distorted phosphates on the surface of NVP@C gradually dissolve into the electrolyte, causing a decrease in capacity. To stabilize the phosphates on the surface of NVP, DFT calculations suggest that iron doping of NVP can effectively relieve the deformation of the POV bond and suppress the capacity decay. The as-prepared Na3V1.5Fe0.5(PO4)3@C (NV1.5Fe0.5P@C) has a capacity retention of 95% in the first ten cycles, while NVP@C retains only 55% of the initial capacity in the same number of cycles.

(Digital Presentation) Evaluating Stability and Performance of Nasicon Membranes for Crossover Mitigation in Aqueous Redox-Flow Batteries

Most polymer-based membranes that are used in prototypical aqueous redox-flow batteries (RFBs) do not adequately prevent crossover of small-molecule reactants, causing high rates of capacity fade. Ceramic superionic conductor membranes are an attractive alternative due to their superior abilities to mitigate crossover;1 they can thus enable the deployment of electrolytes containing earth-abundant, small-molecule reactants.2 3 We test the performance and stability of a von Alpen sodium superionic conductor Na3.1Zr1.55Si2.3P0.7O11 (NaSICON) as an RFB membrane by examining its resistance, permeability and interfacial morphology as a function of electrolyte composition and temperature. The resistance of NaSICON is stable for several weeks while immersed in neutral to strongly alkaline ([OH-] = 3 M) aqueous electrolytes, and its permeability to polysulfide-based and permanganate-based small-molecule RFB reactants is negligible compared to that of Nafion. The glassy phase of the NaSICON microstructure at the membrane-electrolyte interface undergoes small amounts of etching while in contact with aqueous electrolytes containing sodium ions, which becomes more extensive when potassium ions are present in the electrolyte, leading in certain instances to complete disintegration of the membrane. We report a ferrocyanide-permanganate flow cell at a pH of 14.5 with a ~ 700 μm-thin NaSICON membrane supporting weeks of cycling with apparently negligible reactant crossover and very low capacity fade (< 0.04 %/day). Area-specific resistance of NaSICON falls dramatically with increasing temperature and decreasing membrane thickness, and a membrane that is 100 µm thick or thinner can enable power densities at above-ambient temperatures that are comparable to power densities of polymer membrane-containing flow cells.(1) Yu, X.; Gross, M. M.; Wang, S.; Manthiram, A. Aqueous Electrochemical Energy Storage with a Mediator-Ion Solid Electrolyte. Advanced Energy Materials 2017, 7 (11), 1602454, https://doi.org/10.1002/aenm.201602454. DOI: https://doi.org/10.1002/aenm.201602454 (acccessed 2021/03/12).(2) Wei, X.; Xia, G.-G.; Kirby, B.; Thomsen, E.; Li, B.; Nie, Z.; Graff, G. G.; Liu, J.; Sprenkle, V.; Wang, W. An aqueous redox flow battery based on neutral alkali metal ferri/ferrocyanide and polysulfide electrolytes. J. Electrochem. Soc. 2016, 163 (1), A5150-A5153. DOI: 10.1149/2.0221601jes].(3) Colli, A. N.; Peljo, P.; Girault, H. H. High energy density MnO4-/MnO42- redox couple for alkaline redox flow batteries. Chem Commun (Camb) 2016. DOI: 10.1039/c6cc08070g.

Efficient, Selective Sodium and Lithium Removal by Faradaic Deionization Using Symmetric Sodium Titanium Vanadium Phosphate Intercalation Electrodes.

NASICON (sodium superionic conductor) materials are promising host compounds for the reversible capture of Na+ ions, finding prior application in batteries as solid-state electrolytes and cathodes/anodes. Given their affinity for Na+ ions, these materials can be used in Faradaic deionization (FDI) for the selective removal of sodium over other competing ions. Here, we investigate the selective removal of sodium over other alkali and alkaline-earth metal cations from aqueous electrolytes when using a NASICON-based mixed Ti-V phase as an intercalation electrode, namely, sodium titanium vanadium phosphate (NTVP). Galvanostatic cycling experiments in three-electrode cells with electrolytes containing Na+, K+, Mg2+, Ca2+, and Li+ reveal that only Na+ and Li+ can intercalate into the NTVP crystal structure, while other cations show capacitive response, leading to a material-intrinsic selectivity factor of 56 for Na+ over K+, Mg2+, and Ca2+. Furthermore, electrochemical titration experiments together with modeling show that an intercalation mechanism with a limited miscibility gap for Na+ in NTVP mitigates the state-of-charge gradients to which phase-separating intercalation electrodes are prone when operated under electrolyte flow. NTVP electrodes are then incorporated into an FDI cell with automated fluid recirculation to demonstrate up to 94% removal of sodium in streams with competing alkali/alkaline-earth cations with 10-fold higher concentration, showing process selectivity factors of 3-6 for Na+ over cations other than Li+. Decreasing the current density can improve selectivity up to 25% and reduce energy consumption by as much as ∼50%, depending on the competing ion. The results also indicate the utility of NTVP for selective lithium recovery.

Unfolding the structural features of NASICON materials for sodium‐ion full cells

AbstractSodium‐ion batteries (SIBs) are potential candidates for the replacement of lithium‐ion batteries to meet the increasing demands of electrical storage systems due to the low cost and high abundance of sodium. Sodium superionic conductor (NASICON) structured materials have attracted enormous interest in recent years as electrode materials for safer and long‐term performance of SIBs for electric energy storage smart grids. These materials have a three‐dimensional robust framework, high redox potential, thermal stability, and a fast Na+‐ion diffusion mechanism. However, NASICON has low intrinsic electronic conductivity, which limits the electrochemical performance. This review describes the structural features of NASICONs to illustrate the ion storage mechanism and electrochemical performance of SIBs. Details of the NASICON crystal structure, the affiliated Na+‐ion diffusion mechanism, morphology, and electrochemical performance of these materials in sodium‐ion half‐cells as well as full cells are described. In addition to the application as electrode materials, the use of NASICONs as solid electrolytes is also elaborated in solid‐state SIBs. Based on these aspects, we have provided more perspectives in terms of the commercialization of SIBs and strategies to overcome the limitations of NASICONs. Hence, this review is expected to provide the researchers of energy storage with an in‐depth understanding of NASICON materials with the knowledge of structural features, which will provide a new avenue on the practicality of SIBs.