A High-Energy NASICON-Type Na3.2 MnTi0.8 V0.2 (PO4 )3 Cathode Material with Reversible 3.2-Electron Redox Reaction for Sodium-Ion Batteries.

Na superionic conductor (NASICON) structured cathode materials with robust structural stability and large Na+ diffusion channels have aroused great interest in sodium-ion batteries (SIBs). However, most of NASICON-type cathode materials exhibit redox reaction of no more than three electrons per formula, which strictly limits capacity and energy density. Herein, a series of NASICON-type Na3+x MnTi1-x Vx (PO4 )3 cathode materials are designed, which demonstrate not only a multi-electron reaction but also high voltage platform. With five redox couples from V5+/4+ (≈4.1 V), Mn4+/3+ (≈4.0 V), Mn3+/2+ (≈3.6 V), V4+/3+ (≈3.4 V), and Ti4+/3+ (≈2.1 V), the optimized material, Na3.2 MnTi0.8 V0.2 (PO4 )3 , realizes a reversible 3.2-electron redox reaction, enabling a high discharge capacity (172.5 mAh g-1 ) and an ultrahigh energy density (527.2 Wh kg-1 ). This work sheds light on the rational construction of NASICON-type cathode materials with multi-electron redox reaction for high-energy SIBs.

Ultrastable and High Energy Calcium Rechargeable Batteries Enabled by Calcium Intercalation in a NASICON Cathode.

Ca-ion batteries (CIBs) have been considered a promising candidate for the next-generation energy storage technology owing to the abundant calcium element and the low reduction potential of Ca2+ /Ca. However, the large size and divalent nature of Ca2+ induce significant volume change and sluggish ion mobility in intercalation cathodes, leading to poor reversibly and low energy/power densities for CIBs. Herein, a polyanionic Na superionic conduction (NASICON)-typed Na-vacant Na1 V2 (PO4 )2 F3 (N1 PVF3 ) with sufficient interstitial spaces is reported as ultra-stable and high-energy Ca ion cathodes. The N1 PVF3 delivers exceptionally high Ca storage capacities of 110 and 65 mAh g-1 at 10 and 500 mA g-1 , respectively, and a record-long cyclability of 2000 cycles. More interestingly, by tailoring the fluorine content in N1 PVFx (1 ≤ x ≤ 3), the high working potential of 3.5 V versus Ca2+ /Ca is achievable. In conjunction with Ca metal anode and a compatible electrolyte, Ca metal batteries with N1 VPF3 cathodes are constructed, which deliver an initial energy density of 342 W h kg-1 , representing one of the highest values thus far reported for CIBs. Origins of the uncommonly stable and high-power capabilities for N1 PVF3 are elucidated as the small volume changes and low cation diffusion barriers among the cathodes.

Full Activation of Mn4+ /Mn3+ Redox in Na4 MnCr(PO4 )3 as a High-Voltage and High-Rate Cathode Material for Sodium-Ion Batteries.

Developing high-voltage cathode materials is critical for sodium-ion batteries to boost energy density. NASICON (Na super-ionic conductor)-structured Nax MnM(PO4 )3 materials (M represents transition metal) have drawn increasing attention due to their features of robust crystal framework, low cost, as well as high voltage based on Mn4+ /Mn3+ and Mn3+ /Mn2+ redox couples. However, full activation of Mn4+ /Mn3+ redox couple within NASICON framework is still a great challenge. Herein, a novel NASICON-type Na4 MnCr(PO4 )3 material with highly reversible Mn4+ /Mn3+ redox reaction is discovered. It proceeds a two-step reaction with voltage platforms centered at 4.15 and 3.52 V versus Na+ /Na, delivering a capacity of 108.4 mA h g-1 . The Na4 MnCr(PO4 )3 cathode also exhibits long durability over 500 cycles and impressive rate capability up to 10 C. The galvanostatic intermittent titration technique (GITT) test shows fast Na diffusivity which is further verified by density functional theory calculations. The high electrochemical activity derives from the 3D robust framework structure, fast kinetics, and pseudocapacitive contribution. The sodium storage mechanism of the Na4 MnCr(PO4 )3 cathode is deeply studied by ex situ X-ray diffraction (XRD) and ex situ X-ray photoelectron spectroscopy (XPS), revealing that both solid-solution and two-phase reactions are involved in the Na+ ions extraction/insertion process.

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.

Improved Electrochemical Performance of NASICON Type Na3V2-xCox(PO4)3/C (x = 0-0.15) Cathode for High Rate and Stable Sodium-Ion Batteries.

In recent years, the Na-ion SuperIonic CONductor (NASICON) based polyanionics are considered pertinent cathode materials in sodium-ion batteries due to their 3D open framework, which can accommodate a wide range of Na content and can offer high ionic conductivity with great structural stability. However, owing to the inferior electronic conductivity, these materials suffer from unappealing rate capability and cyclic stability for practical applications. Therefore, in this work we investigate the effect of Co substitution at the V site on the electrochemical performance and diffusion kinetics of Na3V2-xCox(PO4)3/C (x = 0-0.15) cathodes. All the samples are characterized through Rietveld refinement of the X-ray diffraction patterns, Raman spectroscopy, transmission electron microscopy, etc. We demonstrate improved electrochemical performance for the x = 0.05 electrode with a reversible capacity of 105 mAh g-1 at 0.1 C. Interestingly, the specific capacity of 80 mAh g-1 is achieved at 10 C with retention of about 92% after 500 cycles and 79.5% after 1500 cycles and having nearly 100% Coulombic efficiency. The extracted diffusion coefficient values through the galvanostatic intermittent titration technique and cyclic voltammetry are found to be in the range of 10-9 to 10-11 cm2 s-1. The post-mortem studies show excellent structural and morphological stability after testing for 500 cycles at 10 C. Our study reveals the role of optimal dopant of Co3+ ions at the V site in improving the cyclic stability at a high current rate.

Reversible Multielectron Redox Chemistry in a NASICON-Type Cathode toward High-Energy-Density and Long-Life Sodium-Ion Full Batteries.

Na-superionic-conductor (NASICON)-type cathodes (e.g., Na3 V2 (PO4 )3 ) have attracted extensive attention due to their open and robust framework, fast Na+ mobility, and superior thermal stability. To commercialize sodium-ion batteries (SIBs), higher energy density and lower cost requirements are urgently needed for NASICON-type cathodes. Herein, Na3.5 V1.5 Fe0.5 (PO4 )3 (NVFP) is designed by an Fe-substitution strategy, which not only reduces the exorbitant cost of vanadium, but also realizes high-voltage multielectron reactions. The NVFP cathode can deliver extraordinary capacity (148.2 mAh g-1 ), and decent cycling durability up to 84% after 10 000 cycles at 100 C. In situ X-ray diffraction and ex situ X-ray photoelectron spectroscopy characterizations reveal reversible structural evolution and redox processes (Fe2+ /Fe3+ , V3+ /V4+ , and V4+ /V5+ ) during electrochemical reactions. The low ionic-migration energy barrier and ideal Na+ -diffusion kinetics are elucidated by density functional theory calculations. Combined with electron paramagnetic resonance spectroscopy, Fe with unpaired electrons in the 3d orbital is inseparable from the higher-valence redox activation. More competitively, coupling with a hard carbon (HC) anode, HC//NVFP full cells demonstrate high-rate capability and long-duration cycling lifespan (3000 stable cycles at 50 C), along with material-level energy density up to 304 Wh kg-1 . The present work can provide new perspectives to accelerate the commercialization of SIBs.

New Na1+xGe2(SiO4)x(PO4)3-x NASICON Series with Improved Grain and Grain Boundary Conductivities.

In this study, we synthesize glass-ceramics of the new Na1+xGe2(SiO4)x(PO4)3-x NASICON (Na super-ionic conductor) series to evaluate the effect of Si4+/P5+ substitution on the structural, microstructural, and electrical properties of the NaGe2(PO4)3 system. From X-ray diffraction, the presence of the NASICON phase is confirmed in all glass-ceramics. An expansion of the unit cell volume suggesting an increase in the bottleneck of the NASICON structure is also observed. Impedance spectroscopy allowed the separation of grain and grain boundary contributions. We observe that the grain conductivity is higher than the specific grain boundary conductivity in all of the investigated compositions (0 ≤ x ≤ 0.8). The Si4+/P5+ substitution causes an enhancement of about 2 and 3 orders of magnitude in the grain and specific grain boundary conductivities, respectively. This behavior is attributable to the introduction of new charge carriers (Na+) in the NASICON structure and a decrease in the activation energy. Finally, the lowest activation energy for grain (0.586 eV) is observed in the x = 0.6 sample, which indicates the easiest displacement of ions in the investigated series, suggesting that this composition presents the most suitable bottleneck size for (Na+) sodium ion conduction.

Symmetric Sodium-Ion Battery Based on Dual-Electron Reactions of NASICON-Structured Na3MnTi(PO4)3 Material.

Symmetric sodium-ion batteries possess promising features such as low cost, easy manufacturing process, and facile recycling post-process, which are suitable for the application of large-scale stationary energy storage. Herein, we proposed a symmetric sodium-ion battery based on dual-electron reactions of a NASICON-structured Na3MnTi(PO4)3 material. The Na3MnTi(PO4)3 electrode can deliver a stable capacity of up to 160 mAh g-1 with a Coulombic efficiency of 97% at 0.1 C by utilizing the redox reactions of Ti3+/4+, Mn2+/3+, and Mn3+/4+. This is the first time to investigate the symmetric sodium-ion full cell using Na3MnTi(PO4)3 as both cathode and anode in the organic electrolyte, demonstrating excellent reversibility and cycling performance with voltage plateaus of about 1.4 and 1.9 V. The full cell exhibits a reversible capacity of 75 mAh g-1 at 0.1 C and an energy density of 52 Wh kg-1. In addition, both ex situ X-ray diffraction (XRD) analysis and first-principles calculations are employed to investigate the sodiation mechanism and structural evolution. The current research provides a feasible strategy for the symmetric sodium-ion batteries to achieve high energy density.

Failure Mechanism and Interface Engineering for NASICON-Structured All-Solid-State Lithium Metal Batteries.

All-solid-state lithium metal batteries (ASSLiMB) have been considered as one of the most promising next-generation high-energy storage systems that replace liquid organic electrolytes by solid-state electrolytes (SSE). Among many different types of SSE, NASICON-structured Li1+ xAl xGe2- x(PO3)4 (LAGP) shows high a ionic conductivity, high stability against moisture, and wide working electrochemical windows. However, it is unstable when it is in contact with molten Li, hence largely limiting its applications in ASSLiMB. To solve this issue, we have studied reaction processes and mechanisms between LAGP and molten Li, based on which a failure mechanism is hence proposed. With better understanding the failure mechanism, a thin thermosetting Li salt polymer, P(AA- co-MA)Li, layer is coated on the bare LAGP pellet before contacting with molten Li. To further increase the ionic conductivity of P(AA- co-MA)Li, LiCl is added in P(AA- co-MA)Li. A symmetric cell of Li/interface/LAGP/interface/Li is prepared using molten Li-Sn alloy and galvanically cycled at current densities of 15, 30, and 70 µA cm-2 for 100 cycles, showing stable low overpotentials of 0.036, 0.105, and 0.257 V, respectively. These electrochemical results demonstrate that the interface coating of P(AA- co-MA)Li can be an effective method to avoid an interfacial reaction between the LAGP electrolyte and molten Li.

Elaboration, structural study and validation of a new NASICON-type structure, Na0.72(Cr0.48,Al1.52)(Mo2.77,Al0.23)O12.

The title compound, sodium chromium/aluminium molybdenum/aluminium dodeca-oxide, Na0.72Cr0.48Al1.74Mo2.77O12, was prepared by solid-state reaction. Its crystal structure is related to NaSICON-type compounds. The framework is built up from M1O6 (M1 = Cr/Al) octa-hedra and M2O4 (M2 = Mo/Al) tetra-hedra inter-connected by corners. The three-dimensional framework contains cavities in which sodium cations are located. Two validation models (BVS and CHARDI) were used to confirm the proposed structural model. The mobility of Na+ ions in the structure has been investigated by theoretical means.

Exploration of Ca0.5Ti2(PO4)3@carbon Nanocomposite as the High-Rate Negative Electrode for Na-Ion Batteries.

Exploring suitable electrode materials with high specific capacity and high-rate capability is a challenging goal for the development of Na-ion batteries. Here, we report a NASICON-structured compound, Ca0.5Ti2(PO4)3, with respect to its synthesis and electrochemical properties. The electrode is found to enable fast Na+ ion diffusion owing to the rich crystallographic vacancies, affording a reversible capacity of 264 mA h g-1 between 3.0 and 0.01 V. In particular, the hybrid Ca0.5Ti2(PO4)3@carbon exhibits remarkable rate performance with a discharge capacity of nearly 45 mA h g-1 at a current density of 20 A g-1, which is attributed to the pseudocapacitive effect.

Effect of carbon matrix dimensions on the electrochemical properties of Na3V2(PO4)3 nanograins for high-performance symmetric sodium-ion batteries.

Na3V2(PO4)3 nanograins dispersed in different carbon matrices are rationally synthesized and systematically characterized. The acetylene carbon matrix provides the best conductive networks for electrons and sodium ions, which endows Na3V2(PO4)3 stable cyclability and high rate performance. The Na3V2 (PO4)3 -based symmetric sodium-ion batteries show outstanding electrochemical performance, which is promising for large-scale and low-cost energy storage applications.