A robust carbon coating of Na3V2(PO4)3 cathode material for high performance sodium-ion batteries

Optimization Strategies of Na3V2(PO4)3 Cathode Materials for Sodium-Ion Batteries

Na3V2(PO4)3 (NVP) has garnered great attentions as a prospective cathode material for sodium-ion batteries (SIBs) by virtue of its decent theoretical capacity, superior ion conductivity and high structural stability. However, the inherently poor electronic conductivity and sluggish sodium-ion diffusion kinetics of NVP material give rise to inferior rate performance and unsatisfactory energy density, which strictly confine its further application in SIBs. Thus, it is of significance to boost the sodium storage performance of NVP cathode material. Up to now, many methods have been developed to optimize the electrochemical performance of NVP cathode material. In this review, the latest advances in optimization strategies for improving the electrochemical performance of NVP cathode material are well summarized and discussed, including carbon coating or modification, foreign-ion doping or substitution and nanostructure and morphology design. The foreign-ion doping or substitution is highlighted, involving Na, V, and PO43- sites, which include single-site doping, multiple-site doping, single-ion doping, multiple-ion doping and so on. Furthermore, the challenges and prospects of high-performance NVP cathode material are also put forward. It is believed that this review can provide a useful reference for designing and developing high-performance NVP cathode material toward the large-scale application in SIBs.

Recent progress in high-voltage P2-Na x TMO2 materials and their future perspectives

P2-type layered materials (Na x TMO2) have become attractive cathode electrodes owing to their high theoretical energy density and simple preparation. However, they still face severe phase transition and low conductivity. Current research on Na x TMO2 is mostly focused on the modification of bulk materials, and the application performances have been infrequently addressed. This review summarizes the information on current common P2-Na x TMO2 materials and discusses their sodium-storage mechanisms. Furthermore, modification strategies to improve their performance are addressed for practical applications based on a range of key parameters (output voltage, specific capacity, and lifespan). We also discuss the future development trends and application prospects for P2 cathode materials.

Compositing pine pollen derived carbon matrix with Na4FeV(PO4)3 nanoparticle for cost-effective sodium-ion batteries cathode

Na super-ion conductor type material Na3V2(PO4)3 has been widely researched as the cathode of sodium-ion batteries (SIBs) in recent years, but the unsatisfying cost of Na3V2(PO4)3 impedes its wide application in SIBs. In this study, iron element is used to replace part of vanadium in Na3V2(PO4)3 to reduce its expense, and pine pollen is applied for the first time as a very effective carbon source to improve the performance of Na4FeV(PO4)3. The fabricated composite material achieves a capacity of 105 mA h g-1 under 0.2 C and fascinating cycling stability over 94 % under 2 C for 500 cycles and 98 % under 10 C for 1000 cycles. The excellent cycle performance is caused by the involvement of pine pollen that acts as a carbon matrix to enhance the electron conductivity and block the agglomeration of active material effectively, thus the well-dispersed nano sized Na4FeV(PO4)3 shortens the diffusion path of sodium ion and gains a remarkable rate capability. Moreover, the distinguished reversibility during the charge and discharge procedures is ascribed also to the robust structure of Na4FeV(PO4)3. This work provides an efficient route to realize the economic cathode material of SIBs with good performance.

A Comprehensive Review on Strategies for Enhancing the Performance of Polyanionic-Based Sodium-Ion Battery Cathodes

The significant consumption of fossil fuels and the increasing pollution have spurred the development of energy-storage devices like batteries. Due to their high cost and limited resources, widely used lithium-ion batteries have become unsuitable for large-scale energy production. Sodium is considered to be one of the most promising substitutes for lithium due to its wide availability and similar physiochemical properties. Designing a suitable cathode material for sodium-ion batteries is essential, as the overall electrochemical performance and the cost of battery depend on the cathode material. Among different types of cathode materials, polyanionic material has emerged as a great option due to its higher redox potential, stable crystal structure, and open three-dimensional framework. However, the poor electronic and ionic conductivity limits their applicability. This review briefly discusses the strategies to deal with the challenges of transition-metal oxides and Prussian blue analogue, recent developments in polyanionic compounds, and strategies to improve electrochemical performance of polyanionic material by nanostructuring, surface coating, morphology control, and heteroatom doping, which is expected to accelerate the future design of sodium-ion battery cathodes.

Entropy-Driven Enhancement of the Conductivity and Phase Purity of Na4Fe3(PO4)2P2O7 as the Superior Cathode in Sodium-Ion Batteries

Na4Fe3(PO4)2(P2O7) (NFPP) is regarded as a promising cathode material for sodium-ion batteries (SIBs) owing to its low cost, easy manufacture, environmental purity, high structural stability, unique three-dimensional Na-ion diffusion channels, and appropriate working voltage. However, for NFPP, the low conductivity of electrons and ions limits their capacity and power density. The generation of NaFeP2O7 and NaFePO4 inhibits the diffusion of sodium ions and reduces reversible capacity and rate performance during the manufacturing process in synthesis methods. Herein, we report an entropy-driven approach to enhance the electronic conductivity and, concurrently, phase purity of NFPP as the superior cathode in sodium-ion batteries. This approach was realized via Ti ions substituting different ratios of Fe-occupied sites in the NFPP lattice (denoted as NTFPP-X, T is the Ti in the lattice, X is the ratio of Ti-substitution) with the configurational entropic increment of the lattice structures from 0.68 R to 0.79 R. Specifically, 5% Ti-substituted lattice (NTFPP-0.05) inducing entropic augmentation not only improves the electronic conductivity from 7.1 × 10-2 S/m to 8.6 × 10-2 S/m but also generates the pure-phase of NFPP (suppressing the impure phases of the NaFeP2O7 and NaFePO4) of the lattice structure, which is validated by a series of characterizations, including powder X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). Benefiting from the Ti replacement in the lattice, the optimal NTFPP-0.05 composite shows a high first discharge capacity (118.5 mAh g-1 at 0.1 C), superior rate performance (70.5 mAh g-1 at 10 C), and excellent long cycling life (1200 cycles at 10 C with capacity retention of 86.9%). This research proposes a new entropy-driven approach to improve the electrochemical performance of NFPP and reports a low-cost, ultrastable, and high-rate cathode material of NTFPP-0.05 for SIBs.

Boosted Redox Kinetics Enabling Na3V2(PO4)3 with Excellent Performance at Low Temperature through Cation Substitution and Multiwalled Carbon Nanotube Cross-Linking

The open NASICON framework and high reversible capacity enable Na3V2(PO4)3 (NVP) to be a highly promising cathode candidate for sodium-ion batteries (SIBs). Nevertheless, the unsatisfied cyclic stability and degraded rate capability at low temperatures due to sluggish ionic migration and poor conductivity become the main challenges. Herein, excellent sodium storage performance for the NVP cathode can be received by partial potassium (K) substitution and multiwalled carbon nanotube (MWCNT) cross-linking to modify the ionic diffusion and electronic conductivity. Consequently, the as-fabricated Na3-xKxV2(PO4)3@C/MWCNT can maintain a capacity retention of 79.4% after 2000 cycles at 20 C. Moreover, the electrochemical tests at -20 °C manifest that the designed electrode can deliver 89.7, 73.5, and 64.8% charge of states, respectively, at 1, 2, and 3 C, accompanied with a capacity retention of 84.3% after 500 cycles at 20 C. Generally, the improved electronic conductivity and modified ionic diffusion kinetics resulting from K doping and MWCNT interconnecting endows the resultant Na3-xKxV2(PO4)3@C/MWCNT with modified electrochemical polarization and improved redox reversibility, contributing to superior performance at low temperatures. Generally, this study highlights the potential of alien substitution and carbon hybridization to improve the NASICON-type cathodes toward high-performance SIBs, especially at low temperatures.

Chemically bonding inorganic fillers with polymer to achieve ultra-stable solid-state sodium batteries

Inorganic/organic composite solid electrolytes (CSEs) have attracted ever-increasing attentions due to their outstanding mechanical stability and processibility. However, the inferior inorganic/organic interface compatibility limits their ionic conductivity and electrochemical stability, which hinders their application in solid-state batteries. Herein, we report a homogeneously distributed inorganic fillers in polymer by in-situ anchoring SiO₂ particles in polyethylene oxide (PEO) matrix (I-PEO-SiO₂). Compared with ex-situ CSEs (E-PEO-SiO₂), SiO₂ particles and PEO chains in I-PEO-SiO₂ CSEs are closely welded by strong chemical bonds, thus addressing the issue of interfacial compatibility and realizing excellent dendrite-suppression ability. In addition, the Lewis acid-base interactions between SiO₂ and salts facilitate the dissociation of sodium salts and increase the concentration of free Na⁺. Consequently, the I-PEO-SiO₂ electrolyte demonstrates an improved Na⁺ conductivity (2.3 × 10^-4 S cm^-1 at 60 °C) and Na⁺ transference number (0.46). The as constructed Na₃V₂(PO₄)₃ ‖ I-PEO-SiO₂ ‖ Na full-cell demonstrates a high specific capacity of 90.5 mAh g^-1 at 3C and an ultra-long cycling stability (>4000 cycles at 1C), outperforming the state-of-the-art literatures. This work provides an effective way to solve the issue of interfacial compatibility, which can enlighten other CSEs to overcome their interior compatibility.

Stabilized Multi-Electron Reactions in a High-Energy Na4 Mn0.9 CrMg0.1 (PO4 )3 Sodium-Storage Cathode Enabled by the Pinning Effect

Na superionic conductor (NASICON)-type Na₄ MnCr(PO₄ )₃ has attracted extensive attention among the phosphate sodium-storage cathodes due to its ultra-high energy density originating from three-electron reactions but it suffers from severe structural degradation upon repeated sodiation/desodiation processes. Herein, Mg is used for partial substitution of Mn in Na₄ MnCr(PO₄ )₃ to alleviate Jahn-Teller distortions and to prolong the cathode cycling life by virtue of the pinning effect induced by implanting inert MgO₆ octahedra into the NASICON framework. The as-prepared Na₄ Mn_0.9 CrMg_0.1 (PO₄ )₃ /C cathode delivers high capacity retention of 92.7% after 500 cycles at 5 C and fascinating rate capability of 154.6 and 70.4 mAh g^-1 at 0.1 and 15 C, respectively. Meanwhile, it can provide an admirable energy density of ≈558.48 Wh kg^-1 based on ≈2.8-electron reactions of Mn²⁺ /Mn³⁺ , Mn³⁺ /Mn⁴⁺ , and Cr³⁺ /Cr⁴⁺ redox couples. In situ X-ray diffraction reveals the highly reversible single-phase and bi-phase structural evolution of such cathode materials with a volume change of only 6.3% during the whole electrochemical reaction. The galvanostatic intermittent titration technique and density functional theory computations jointly demonstrate the superior electrode process kinetics and enhanced electronic conductivity after Mg doping. This work offers a new route to improve the cycling stability of the high-energy NASICON-cathodes for sodium-ion batteries.

Simultaneous defect regulation by p-n type co-substitution in a Na3V2(PO4)3/C cathode for high performance sodium ion batteries

The Na₃V₂(PO₄)₃ (NVP) cathode is deemed to be a promising candidate for sodium ion batteries due to its strong structural stability and high theoretical capacity. Nevertheless, its poor intrinsic conductivity restricts further development. To overcome these shortcomings, a dual modification strategy of Mn²⁺/Ti⁴⁺ co-substitution is proposed for the first time. Significantly, Mn doping can efficiently accelerate the transmission speed of electrons by introducing beneficial holes derived from the low valence state of +2, presenting the classical p-type doping modification. Moreover, the presence of Mn²⁺ with a larger ionic radius can support the crystal to stabilize the Na superionic conductor (NASICON) framework of the NVP system. Ti⁴⁺ is introduced for perfect charge compensation. Accordingly, the addition of Ti⁴⁺ can generate excess electrons due to the n-type substitution, which contributes to the favorable electronic conductivity. In addition, conductive carbon nanotubes (CNTs) are utilized to construct an efficient network to improve the rate capability of the NVP composite. Meanwhile, CNTs can inhibit particle growth and thus reduce particle size, shortening the transport path of Na⁺ and promoting the diffusion of Na⁺. Comprehensively, the optimized Na₃V_2-xMn_xTi_x(PO₄)₃/C@CNTs (x = 0.15) deliver high capacities of 70.3 and 68.2 mA h g^-1 at 90C and 180C, maintaining 58 and 53.8 mA h g^-1 after 1000 cycles with high capacity retention of 82.5% and 78.9%.

Facile synthesis of SnNb2O6@C composite with ultrathin carbon layer as anode materials for high-performance sodium-ion batteries

Niobium-based oxides have attracted a lot of attention as anode materials for sodium-ion batteries (SIBs) due to their high theoretical specific capacity, excellent rate capability and exceptional safety. However, their poor intrinsic electronic conductivity and sluggish sodium ions diffusion kinetics severely hinder their practical applicability. Here, SnNb 2 O 6 @C was successfully prepared by a simple solid-state reaction technique coupled with carbon coating. HRTEM images show that the SnNb 2 O 6 @C particles are covered by a uniformly ultrathin amorphous carbon layer of about 1.8 nm, thus improving the electronic conductivity and diffusion coefficient of sodium ions. As anode for SIBs, the as-obtained SnNb 2 O 6 @C material exhibits excellent specific capacity (369 mAh g -1 at a current density of 50 mA g -1 ) and remarkable rate performance (177 mAh g -1 at 1000 mA g -1 ), which indicates its good prospect in practical application.