Na Super Ionic Conductor Research Articles

Na3.5(MnVFeTi)0.5(PO4)3: A Multi‐Transition‐Metal‐Ion‐Engineered NASICON‐Type Cathodes for Sodium Ion Batteries

AbstractElectrochemically active Na‐superionic conductor (NASICON)‐type cathodes have the structural flexibility to include various transition elements, thus enabling high power outputs benefited by multi‐electron redox reactions. This study amalgamated multiple transition metal ions to construct a new NASICON‐type cathode i. e., carbon coated Na3.5(MnVFeTi)0.5(PO4)3 (NMVFTP/C) for Na‐ion batteries (NIBs). The NMVFTP/C cathode engineered in this study demonstrated stable Na+‐storage capacity, including long‐term cycling stability up to 4000 cycles at 3000 mA g−1 with 96 % capacity retention and a high‐rate output capacity of 85.16 mAh g−1 at 2500 mA g−1. To elucidate the ion transport process within the cathode, density functional theory modeling was employed. The low energy barrier for the diffusion of Na+ in the NMVFTP/C materials was proved to be a key factor supporting our material's superior electrochemical performances.

Cation/Anion Doping Strategy for Na4MnV(PO4)3 with High Energy Density and Long Cycling Life through Construction by Aspergillus niger.

Na4MnV(PO4)3 (NMVP) has gained attention for its high redox potential, good cycling stability, and competitive price but suffers from poor intrinsic electronic conductivity and Jahn-Teller effect from Mn3+. In this work, cation/anion doping strategy was used for Aspergillus niger-bioderived carbon-coated NMVP (NMVP/AN) to improve the structural stability and electrochemical performance, where Al3+ doping inhibited the dissolution of Mn and enhanced the Mn3+/Mn2+ redox pair activity; besides, F- doping not only weakens the Na2-O bond but also endows the hierarchical and porous structure of NMVP/AN, which led to a more rapid and fluid transfer of Na+. The elaborately designed Na3.9Mn0.9Al0.1V(PO4)3/AN (NMAVP/AN) exhibits 105.9 mA h g-1 at 0.5 C, and the as-prepared Na3.1MnV(PO3.7F0.3)3/AN (NMVPF/AN) delivers 104.1 mA h g-1 at 5 C. Further demonstration of the hard carbon//NMAVP/AN full cell manifests the good potential of Al3+-doped NMVP/AN for practical applications (100.6 mA h g-1 at 1 C). These findings open up the possibility of unlocking the high-performance Na superionic conductor (NASICON).

Fostering Li-ion conduction in Zr-Sn-Al-based mid-entropy NASICON electrolyte

NASICON (Na Super Ionic CONductor) type materials are an important class of solid-state electrolytes due to their high ionic conductivity along with decent chemical and electrochemical stability. In this study, a medium-entropy Li1.5Zr1.0Sn0.5Al0.5(PO4)3 (LZSAP) ceramic electrolyte was prepared via a solid-state synthesis method. Rietveld refinement confirmed the rhombohedral structure of the conventionally sintered sample (LZSAP-CS) at 1000 °C. The spark plasma sintering (SPS) technique was used for the densification of the pellet and resulted in ∼2.64 times higher room temperature conductivity (∼4.42 × 10−5 S cm−1) than that of the LZSAP-CS. The activation energy, calculated in the 30–100 °C range, decreased from 0.37 ± 0.01 eV for LZSAP-CS to 0.31 ± 0.01 eV for spark plasma sintered LZSAP pellet. The transference number of Li+ was ∼0.98, indicating that Li+ is the predominant charge carrier in this electrolyte. Further investigation of the lithium metal-electrolyte interface was conducted using a symmetric Li|LZSAP-SPS|Li cell configuration, demonstrating stability for over 500 h at 5 μA cm−2. LZSAP extends the list of suitable solid-state electrolytes for all-solid-state lithium batteries.

Enhanced ionic conductivity of NASICON-type NaTi2(PO4)3 solid electrolytes with sodium-ion conduction: based on NaF-assisted synthesis

The ceramic composite electrolytes in the NaTi2(PO4)3–NaF system were characterised by thermal analysis, X-ray diffraction, Fourier infrared absorption spectroscopy, scanning electron microscope, and impedance spectroscopy. X-ray diffraction confirmed that the ceramic composites have a Na Super Ionic Conductor (NASICON) structure. The scanning electron images showed that the addition of NaF significantly improved the microscopic morphology of the NaTi2(PO4)3 crystals and made the packing of crystal grains closer. The addition of NaF in a certain proportion is beneficial to the growth of crystals, making the crystal grains more compact and improving the density of the solid electrolytes. The ionic conductivity analysis was carried out by alternating current (AC) impedance spectroscopy. The conductivity can be effectively improved when a certain proportion of NaF is added to the ceramic composites. The maximum total conductivity is about 1.50 × 10−5 S·cm−1 obtained when the molar ratio of NaF to NaTi2(PO4)3 reaches 0.4.

Heterogeneous NASICON-Type Cathode With Reversible Multielectron Reaction for High-Performance Sodium-Ion Batteries.

Na superionic conductor (NASICON)-structured compounds demonstrate great application potential by their robust framework and compositional diversity. However, they are blamed for the mediocre energy density, and achieving both multielectron reaction and good cycling stability simultaneously is challenging. Herein, a novel heterogeneous Na4Fe3(PO4)2(P2O7)/Na2VTi(PO4)3 (NFPP/NVTP) material with stable multielectron reaction is constructed by spray drying technology. The mutual promotion effect of intergrowth structures effectively improves the purity and the crystallization of NFPP/NVTP during the fabrication process, which is beneficial to the high capacity and cycling stability. As a result, the optimized NFPP/NVTP demonstrates a high reversible capacity of 155.3 mAh g-1 at 20mA g-1 and outstanding cycling stability with 82.9% capacity retention over 2500 cycles at 1 A g-1, which are much superior to those of NFPP and NVTP individually. Even in full cell configuration, the energy density remains high at ≈380Wh kg-1 based on the cathode mass. The high capacity of NFPP/NVTP is also attributed to the successive reduction/oxidation mechanism involving the introduction of Ti3+ and interfacial charge redistribution effect between the heterogeneous phases, which greatly improve the electronic and ionic conductivity. Moreover, high reversible structural evolution during the multisodium storage process further guarantees excellent cycling stability.

Anion Substitution to Suppress the Voltage Hysteresis of Na3MnTi(PO4)3 as a Cathode Material for Sodium-Ion Batteries.

The Mn-based polyanion compound Na3MnTi(PO4)3 (NMTP) with a Na superionic conductor (NASICON) structure has attracted incremental attention as a potential cathode material for sodium-ion batteries. However, the occupation of Mn2+ on Na+ vacancies usually leads to severe voltage hysteresis, which in turn results in significant capacity loss, slow Na+ diffusion kinetics, and poor cycling stability. Herein, anion-substituted compounds Na3MnTi(PO4)3-x(SiO4)x (x = 0.1, 0.2, and 0.3) are synthesized. It reveals that the SiO44- substitution could induce partial oxidation of Mn2+ to Mn3+, and the latter has a lower occupancy preference on Na+ vacancies. By the proposed charge compensation strategy, the Mn2+ occupation on Na+ vacancies can be significantly suppressed. As a result, the voltage hysteresis is substantially inhibited, and greatly improved electrochemical performance is achieved. This study offers an alternative strategy to address the voltage hysteresis associated with NMTP and other Mn-based NASICON cathode materials.

Structure and crystallization behavior of Na2O–ZrO2–SiO2–P2O5 glass for solid-state electrolyte separation

Glass ceramics containing Na super-ionic conductor (NASICON) crystals are promising candidates for solid-state electrolyte separators in aqueous sodium ion batteries due to their high ion conductivity, water stability, and productivity. Our objective was to fabricate a glass-ceramic separator by crystallizing a NASICON phase within the glass via heat-treatment. We conducted a detailed investigation into the structural evolution of the glass and the crystallization morphology. Upon heat-treatment, Si units polymerized to form an island-like phase, while P contributed to increasing non-bridging O to form a sea-like phase during droplet phase separation; subsequently, the Na2Zr2SiP2O12 crystalline phase formed. The remaining glass comprised Q4 units of Si in the island-like phase, and Q1 and Q2 units of P in the sea-like phase. The ionic conductivity of the sample increased by crystallization up to 5.4 × 10–5 S⋅cm –1 at 300 K. Furthermore, the water leakage tests showed that both the glass and glass-ceramics exhibited low leakages.

A multicationic-substituted configurational entropy-enabled NASICON cathode for high-power sodium-ion batteries

Na-superionic-conductor (NASICON)-type Na3V2(PO4)3 is considered as one of the potential cathodes for sodium-ion batteries (SIBs), but its inherent low electronic conductivity and non-adjustable voltage plateau is an urgent issue hindrance to its practical application. Hereof, the concept of multicationic-substituted high-entropy doping is innovatively utilized to modify the fine structure within crystals, enhance electronic conductivity, and achieve excellent local diffusion kinetics and high redox activity according to theoretical calculations. The carbon-free Na3V1.8(CrMnFeZnAl)0.2(PO4)3 (HE-NVP-0.2) cathode exhibits a maximum specific capacity of 119.8 mA h g−1, achieved through V3+/V4+/V5+ redox reactions, while maintaining an impressive capacity retention rate of 80 % after 3000 cycles at 10 C. Moreover, the in-situ X-ray diffraction (XRD) analysis reveals that the incorporation of multi-(high entropy) elements induces a stable transition state, facilitating rapid phase transitions. Combined with in-situ digital image correlation (DIC), reversible structural evolution has been verified at both micro and macro scales. More competitively, coupling with a hard carbon (HC) anode, the HE-NVP-0.2//HC full cells exhibit exceptional energy density/power density characteristics while simultaneously maintaining excellent high-rate performance and prolonged cycle life (2000 stable cycles at 50 C). The regulation of configurational entropy enables the realization of an intermediate phase in the equilibrium state and makes the unlikely potential regulation possible, presenting innovative approaches to expedite the commercialization of high-power SIBs.

Phase-field determination of NaSICON materials in the quaternary system Na2O-P2O5-SiO2-ZrO2: II. Glass-ceramics and the phantom of excessive vacancy formation

This work focuses on a very narrow region in the quaternary system Na2O-P2O5-SiO2-ZrO2 to explore the occasionally proposed deficiency in zirconium and oxygen content of Na+ super-ionic conductor (NaSICON) materials. In addition, this region is known for the formation of glass-ceramics, but a systematic study of such materials has not been carried out yet. For this purpose, 2 series of compositions were defined and synthesized: Na3.4Zr2-3x/4Si2.4-x/4P0.6+x/4O12-11x/8 and Na3.4Zr2-3x/4Si2.4+x/4P0.6+1.5x/4O12-x/16. They only differ in the silicate and phosphate content. In the first series the molar content is constant, nSi+ nP = 3. The latter series allows an excess of the 2 cations to meet the composition Na3.1Zr1.55Si2.3P0.7O11 or alternatively re-written as Na3.4Zr1.7Si2.52P0.77Ol2, which was formerly regarded as a superior material to the frequently reported composition Na3Zr2Si2POl2.Several characterization techniques were applied to better understand the relationships between phase formation, processing, and properties of the obtained glass ceramics in the context of the quasi-quaternary phase diagram. The investigations gave clear evidence that a glass phase is progressively formed with increasing x. Therefore, compounds with x > 0.2 have to be regarded as glass-ceramic composites. The resulting NaSICON materials revealed a very limited Zr deficiency with charge compensation by Na ions and a non-detectable amount of oxygen vacancies verified by neutron scattering and atomistic simulations.Hence, this work is the first systematic investigation of pretended Zr-deficient NaSICON materials, which clearly show the chemistry of a 2-phase region. The 2 investigated series are directed toward a region that is orthogonal to the series Na3Zr3-ySi2PyO11.5+y/2 reported in the first part of this series of publications.

Ion Dynamics at the Intermediate Charging State of the Sodium Vanadium Fluorophosphate Cathode.

Na super ionic conductor (NASICON)-type polyanionic vanadium fluorophosphate Na3V2O2(PO4)2F (NVOPF) is a promising cathode material for high-energy sodium-ion batteries. The dynamic diffusion and exchange of sodium ions in the lattice of NVOPF are crucial for its electrochemical performance. However, standard characterizations are mostly focused on the as-synthesized material without cycling, which is different from the actual battery operation conditions. In this work, we investigated the hopping processes of sodium in NVOPF at the intermediate charging state with 23Na solid-state nuclear magnetic resonance (ssNMR) and density functional theory (DFT) calculations. Our experimental characterizations revealed six distinct sodium coordination sites in the intermediate structure and determined the exchange rates among these sites at variable temperatures. The theoretical calculations showed that these dynamic processes correspond to different ion transport pathways in the crystalline lattice. Our combined experimental and theoretical study uncovered the underlying mechanisms of the ion transport in cycled NVOPF and these understandings may help the optimization of cathode materials for sodium-ion batteries.

Machine-Learning-Driven High-Throughput Screening for High-Energy Density and Stable NASICON Cathodes.

The Na super ionic conductor (NASICON), which has outstanding structural stability and a high operating voltage, is an appealing material for overcoming the limits of low specific energy and larger volume distortion of sodium-ion batteries. In this study, to discover ideal NASICON cathode materials, a screening platform based on density functional theory (DFT) calculations and machine learning (ML) is developed. A training database was generated utilizing the previous 124 545 electrode databases, and a test set of 3126 potential NASICON structures [NaxMyM'1-y(PO4)3] with 27 dopants at the metal site and 6 dopants at the polyanion central site was constructed. The developed ML surrogate model identifies 796 materials that satisfy the following criteria: formation energy of <0.0 eV/atom, energy above hull of ≤0.025 eV/atom, volume change of ≤4%, and theoretical capacity of ≥50 mAh/g. The thermodynamically stable configurations of doped NASICON structures were then selected using machine learning interatomic potential (MLIP), enabling rapid consideration of various dopant site configurations. DFT calculations are followed on 796 screened materials to obtain energy density, average voltage, and volume change. Finally, 50 candidates with an average voltage of ≥3.5 V are identified. The suggested platform accelerates the exploration for optimal NASICON materials by narrowing the focus on materials with desired properties, saving considerable resources.

Cathode nanoarchitectonics with Na3VFe0.5Ti0.5(PO4)3: Overcoming the energy barriers of multielectron reactions for sodium‐ion batteries

AbstractHigh electrochemical stability and safety make Na+ superionic conductor (NASICON)‐class cathodes highly desirable for Na‐ion batteries (SIBs). However, their practical capacity is limited, leading to low specific energy. Furthermore, the low electrical conductivity combined with a decline in capacity upon prolonged cycling (&gt;1000 cycles) related to the loss of active material‐carbon conducting contact regions contributes to moderate rate performance and cycling stability. The need for high specific energy cathodes that meet practical electrochemical requirements has prompted a search for new materials. Herein, we introduce a new carbon‐coated Na3VFe0.5Ti0.5(PO4)3 (NVFTP/C) material as a promising candidate in the NASICON family of cathodes for SIBs. With a high specific energy of ∼457 Wh kg−1 and a high Na+ insertion voltage of 3.0 V versus Na+/Na, this cathode can undergo a reversible single‐phase solid‐solution and two‐phase (de)sodiation evolution at 28 C (1 C = 174.7 mAh g−1) for up to 10,000 cycles. This study highlights the potential of utilizing low‐cost and highly efficient cathodes made from Earth‐abundant and harmless materials (Fe and Ti) with enriched Na+‐storage properties in practical SIBs.

Optimizing the Na/NASCION electrolyte interface for stable solid-state Na metal batteries

Na super ionic conductor (NASICON)-type Na3Zr2Si2PO12 (NZSP) electrolytes, with their excellent safety and high energy density, are a promising candidate material for all-solid-state sodium-ion batteries. However, poor sintering performance and interfacial issues have limited its practical application. Herein, we propose the addition of sintering aids to densify grain boundaries and fine-tune the grain to achieve the densest microstructure, improve interfacial contact with sodium, and realize stable Na plating/stripping cycles at room temperature. (CuO)2-(B2O3)3 (CBO) is selected to reinforce grain boundary densification, inhibit dendrite growth, and ensure interfacial stability upon sintering at the lower temperature of 1000 °C. The optimized NZSP-0.2CBO demonstrates an ion conductivity reaching 6.83×10−4 S cm−1 at room temperature, exhibiting a lower activation energy of 0.27 eV compared to bare NZSP. The interface impedance of Na/NZSP-0.2CBO (34.48 Ω cm²) is significantly lower than that of bare NZSP (373.43 Ω cm²). The corresponding Na/NZSP-0.2CBO/Na exhibits stable deposition/stripping cycles over 850 h at 0.1 mA cm−2 and room temperature, with a low polarization voltage of 65 mV. These superior performances can be attributed to the densification effect that enhances conductivity, enabling a uniform and enhanced Na+ flux across the electrode/electrolyte interface without dendrite formation. A Na/NZSP-0.2CBO/Na3V2(PO4)3 battery is assembled accordingly, exhibiting a capacity of 87.4 mAh g−1for 100 cycles with a capacity retention rate of 84.9% and a high average coulombic efficiency of 97.6% at room temperature.

Boosting sodium-ion battery performance with binary metal-doped Na3V2(PO4)2F3 cathodes

Na3V2(PO4)2F3 (NVPF), recognized for its Na superionic conductor architecture, emerges as a promising candidate among polyanion-type cathodes for sodium ion batteries (SIBs). However, its adoption in practical applications faces obstacles due to its inherently low electronic conductivity. To address this challenge, we employ a binary co-doped strategy to design Na3.3K0.2V1.5Mg0.5(PO4)2F3 cathode with nitrogen-doped carbon (NC) coating layer. This configuration enhances electronic conductivity and reduces diffusion barriers for sodium ion (Na+). The strategy of incorporating nitrogen-doped carbon coating not only facilitates the formation of a porous structure but also introduces additional defects and active sites. Such modifications accelerate the reaction kinetics and augment electrolyte interaction through an expanded specific surface area, thus streamlining the electrochemical process. Concurrently, strategic heteroatom substitution leads to a more efficient engagement of Na+ in the electrochemical activities, thereby bolstering the cathode's structural integrity. The vanadium fluorophosphate Na3.3K0.2V1.5Mg0.5(PO4)2F3@NC exhibits an electrochemical performance, including a high discharge specific capacity of 124.3 mA h g−1 at 0.1C, a long lifespan of 1000 cycles with a capacity retention of 93.1 % at 10C, and a rate property of 73.2 mA h g−1 at 20C. This research provides a method for preparing binary doped NVPF for energy storage electrochemistry.

Boosting sodium-ion battery performance by anion doping in NASICON Na4MnCr(PO4)3 cathode

Na superionic conductor (NASICON)-structured Na4MnCr(PO4)3 (NMCP) possessing unique three-electron transfer process renders admirable energy density for sodium ion batteries (SIBs). However, the current issues like its sluggish Na+ diffusion kinetics, deficient intrinsic conductivity, and unsatisfactory structural stability, hinder its practical application. Herein, a selective replacement of O elements in PO4 group by Cl anions in the NMCP system was developed to significantly enhance its electrochemical performance. The results affirm that the enhanced performance of Cl doped samples can be attributed to the enlargement of cell size, the creation of Na vacancies and the weakness of Na2O bond after Cl doping. The as-prepared Na3.85□0.15MnCr(PO3.95Cl0.05)3/C (NMCPC − 15/C) cathode delivers a high capacity (128.0 mAh/g at 50 mA g−1) and excellent rate performance (73.0 mAh/g at 1000 mA g−1) in contrast to NMCP/C that merely provides 105.2 mAh/g at 50 mA g−1 and reduces to 47.4 mAh/g at 1000 mA g−1. Meanwhile, NMCPC − 15/C shows a capacity retention of 60.7 % at 1000 mA g−1 after 500 cycles, while only 37.1 % for NMCP/C in the same test conditions. Moreover, the satisfactory performance and energy density of NMCPC − 15/C||hard carbon (HC) full cell confirm the potential practicality of NMCPC − 15. Therefore, chloride ions doping into NMCP has practical application prospects in the preparation of high-performance cathode materials and our work also offers new inspiration to apply anion doping strategies in promoting the performance of the other NASICON-structured cathodes for SIBs.

Na3V2(PO4)3 Cathode for Room-Temperature Solid-State Sodium-Ion Batteries: Advanced In Situ Synchrotron X-ray Studies to Understand Intermediate Phase Evolution.

Sodium-ion batteries (NIBs) can use elements that are abundantly present in Earth's crust and are technologically feasible for replacing lithium-ion batteries (LIBs). Hence, NIBs are essential components for sustainable energy storage applications. All-solid-state sodium batteries are among the most capable substitutes to LIBs because of their potential to have low price, great energy density, and consistent safety. Nevertheless, more advancements are needed to improve the electrochemical performance of the Na3V2(PO4)3 (NVP) cathode for NIBs, especially with regard to rate performance and operational lifespan. Herein, a core-shell NVP/C structure is accomplished by adopting a solid-state method. The initial reversible capacity of the NVP/C cathode is 106.6 mAh/g (current rate of C/10), which approaches the theoretical value (117.6 mAh/g). It also exhibits outstanding electrochemical characteristics with a reversible capacity of 85.3 mAh/g at 10C and a cyclic retention of roughly 94.2% after 1100 cycles. Using synchrotron-based operando X-ray diffraction, we present a complete examination of phase transitions during sodium extraction and intercalation in NVP/C. To improve safety and given its excellent ionic conductivity and broad electrochemical window, a Na superionic conductor (NASICON) solid electrolyte (Na3.16Zr1.84Y0.16Si2PO12) has been integrated to obtain an all-solid-state NVP/C||Na battery, which provides an exceptional reversible capacity (95 mAh/g at C/10) and long-term cycling stability (retention of 78.3% after 1100 cycles).

Fluoride Doping Na3Al2/3V4/3(PO4)3 Microspheres As Cathode Materials for Sodium-Ion Batteries with Multielectron Redox.

Sodium-ion batteries (SIBs) are potential candidates for large energy storage usage because of the natural abundance and cheap sodium. Nevertheless, improving the energy density and cycling steadiness of SIB cathodes remains a challenge. In this work, F-doping Na3Al2/3V4/3(PO4)3(NAVP) microspheres (Na3Al2/3V4/3(PO4)2.9F0.3(NAVPF)) are synthesized via spray drying and investigated as SIB cathodes. XRD and Rietveld refinement reveal expanded lattice parameters for NAVPF compared to the undoped sample, and the successful cation doping into the Na superionic conductor (NASICON) framework improves Na+ diffusion channels. The NAVPF delivers an ultrahigh capacity of 148 mAh g-1 at 100mA g-1 with 90.8% retention after 200 cycles, enabled by the activation of V2+/V5+ multielectron reaction. Notably, NAVPF delivers an ultrahigh rate performance, with a discharge capacity of 83.6 mAh g-1 at 5000mA g-1. In situ XRD demonstrates solid-solution reactions occurred during charge-discharge of NAVPF without two-phase reactions, indicating enhanced structural stability after F-doped. The full cell with NAVPF cathode and Na+ preintercalated hard carbon anode shows a large discharge capacity of 100 mAh g-1 at 100mA g-1 with 80.2% retention after 100 cycles. This anion doping strategy creates a promising SIB cathode candidate for future high-energy-density energy storage applications.

Structural Evolution of Lithium-Exchanged Na3(VO)2(PO4)2F Cathode under Operation in Sodium Ion Batteries.

Na superionic conductor (NASICON)-type Na3(VO)2(PO4)2F (NVOPF) exhibits excellent cycling stability for high-voltage sodium ion batteries. Various strategies have been developed to form ion-exchanged NVOPF which can enhance the ionic and electronic conductivity. However, the underlying ion transport mechanism and complex structural transitions during battery operation remained uninvestigated. In this work, we prepared lithium-exchanged NVOPF (namely NLVOPF) which shows improved ionic conductivity and increased capacity at high discharging rates. Solid-state nuclear magnetic resonance (SSNMR) revealed the distinctive presence of two kinds of Li-exchanged sites in the NLVOPF, which are attributed to the occupied lithium ions at the Na1 and Na2 sites (namely Li1 and Li2, respectively). The Li1 site was metastably replaced in the first cycle, yet the Li2 site participated in ion insertion/extraction in the subsequent cycles. Our characterizations show that the dynamic doping of lithium in NLVOPF could contribute to the improved cycling stability and capacity retention.

Conduction/Densification Enhancement of Na1+X Zr2Si x P3-X O12 Nasicon Solid Electrolyte for Solid-State Na Batteries

Due to the better safety feature, replacing organic liquid electrolyte with ceramic solid electrolytes (SE) may effectively suppress the fire incidents caused by conventional rechargeable batteries. Recently, solid-state batteries using Na+ ion conductors tend to receive much attention because the abundance of Na in earth’s crust. However, solid-state batteries (SSBs) have been suffered by the relatively high interface polarization between electrode and SE. For higher Na+ conductivity, Na1+x Zr2Si x P3-x O12 with NASICON (Na Super Ionic Conductor) structure is selected for this study. Although solid state reaction is adopted, it is believed the selection of precursors may be critical for better crystallization and densification of Na1+x Zr2Si x P3-x O12. For instance, adequate amount of Na2CO3 may be used as the Na source as well as flux for densification. Thus, the objectives of this study is to (1) to identify the role of precursors such as Na2CO3 for conduction/densification of NASICON, (2) to search the appropriate temperature for synthesis/crystallization of NASICON, (3) to investigate the factors, such as crystallinity/density, affecting the ionic conductivity. During the different stages of processing, the structural, microstructural and electrical properties of decomposed precursors and resultant NASICON will be examined using XRD, SEM and DC/AC impedance measurements.

Aqueous Electrochemical Degradation of Phosphate Framework Materials: Mechanisms and Mitigation Strategies

Phosphate framework materials are attracting a lot of interest as very stable, structurally and electronically diverse materials, suitable for various electrochemical applications. For example, Na SuperIonic Conductor (NASICON) structure type AxMe2(PO4)3 (A = Li, Na, K; Me = Ti, V, Mn etc.) compounds, in addition to be being archetypical ionic conductors for solid state batteries are also potent electrode active materials for various devices such as aqueous batteries and Faradaic deionization cells. However, there is still a number of issues to be solved before their full potential could be utilized. This involves not only finding the right compositions of transition metals but also improving the stability of aqueous electrolyte/electrode interface. Aqueous stability and electrochemical degradation of electrode materials is related to complex acid-base equilibria, metal leaching, material dissolution, corrosion of cell components etc.Some of the recent contributions in terms of understanding and enabling of Ti-, Mn- and V-based phosphate framework materials for the use in aqueous electrochemical systems is reviewed. The effects of parasitic reactions such as oxygen reduction (ORR) and hydrogen evolution (HER) and their relation to local pH changes towards the stability of NaTi2(PO4)3 (NTP) are determined. The results show that it is the local alkalinity which is responsible for most of the NTP degradation which is only accelerated at high currents. Several mitigation strategies by the use of conformal atomic layer deposited (ALD) coatings and oxygen management by the use of strongly reducing additives are suggested. The dissolution of transition metals as the main aqueous degradation mechanism for Na1+2xMnTi2-x(PO4)3 and Na3-xV2-xTix(PO4)3 positive electrodes is established. However, the results suggest that in Mn- and V-based positive aqueous battery electrodes it is not the material degradation which is responsible for most of the observed capacity fade per se but rather the loss of contact between particles and the conductive phase. The latter comes from the dissolution at the interfacial points but most of the electrode material, is shown to stay intact after extended cycling. Various metal substitution strategies leading to superior stability in half- and full-cells are presented.[1] Plečkaitytė et al. J. Mater. Chem A, 9, 12670-12683 (2021).[2] Snarskis et al. Chem. Mater., 33, 8394-8403 (2021).[3] Tediashvili et al. Electrochim. Acta, 417, 140294 (2022).[4] Petrulevičienė et al. Electrochim. Acta, 424, 140580 (2022). Figure 1