2.1.1. Na Iron Phosphate

Zhang et al. [104] fabricated a mesoporous Na₂FePO₄F@C composite from mesoporous FePO₄ as a SIB cathode material. The synthesis involved a two-step method to create mesoporous amorphous FePO₄ and Na₂FePO₄F@C (M-NFPF@C) composites. Initially, 0.02 mol of (NH₄)₂Fe (SO₄)₂ and ammonium dihydrogen phosphate (NH₄H₂PO₄) were dissolved in deionized water. The NH₄H₂PO₄ solution was gradually mixed with the (NH₄)₂Fe (SO₄)₂ solution with stirring. Next, 4 mL of 30 wt% H₂O₂ was added as an oxidizer, followed by 2 h of stirring, yielding FePO₄·3H₂O. The precursor was dehydrated at 400°C for 24 h to form FePO₄. In the second step, M-NFPF@C was synthesized by ball-milling CH₃COONa, NaF, and FePO₄ with citric acid, then sintering at 600°C [104]. The resulting electrode exhibited a high reversible capacity (114 mAh g⁻¹ at 0.1 C) and superior rate capability (42 mAh g⁻¹ at 10 C), showing promise for energy storage applications. In a different study by Kim et al. [28], maricite NaFePO₄, an initial cycle 142 mA h g⁻¹ capacity was dispensed. The findings of this study significantly contribute to the understanding of NaFePO₄ and its potential in SIBs, particularly its unique advantages in energy storage technologies, such as its amorphous structure and consistent voltage variation profiles. The outcome of the findings demonstrates highly impressive cycling stability, with negligible capacity deterioration observed even after 200 cycles [28].

A one-step solvothermal method was employed by Qin et al. [105] to create NVPF@C/CNTs nanocomposites of NVPF@C/CNTs. When comparing NVPF@C with NVPF@C/CNTs, it was noted that the latter displayed a reasonable performance rate and a larger specific discharge capacity. Specifically, NVPF@C/CNTs achieved a discharge capacity of 107 mA h g⁻¹ for 200 cycles at 0.2 C and 75 mA h g⁻¹ for 500 cycles at 5 C. The study focused on the impact of carbon coating, nanostructure, and a 3D conductive network on the performance of the electrode in SIBs (Figure 2). Wang et al. [106] conducted comparative scrutiny on electrode materials of amorphous and crystalline FePO₄. The results suggest that the amorphous material demonstrates outstanding electrochemical efficiency, such as improved longevity, rate capability, and capacity. The discharge capacities of amorphous FePO₄ were estimated as 179 and 168 mA h g⁻¹, respectively, when subjected to current densities of 20 and 50 mA g⁻¹. Two crucial assumptions support the claim that amorphous materials are more desirable to crystalline FePO₄. Amorphous materials have the potential to enhance the movement of Na ions and accommodate volume changes during the process of charging and discharging.

Additionally, the nanosized particles and porosity of amorphous materials promote the passage of ions and electrons, while the large surface area resulting from the nanoscale effect ensures an electronically active surface that remains unaffected by distribution regulations. In a separate study, Boyadzhieva et al. [107] reported that the oxidation of nanosized NaFePO₄ occurred at around 280°C in the presence of oxygen. The preparation involved the use of the phosphate-formate method, along with the inclusion of carbon modifications using ball milling. This technique is used to improve the electrochemical efficiency of the materials. Eventually, the maricite NaFePO₄ is not electrically active. However, pretreatments such as amorphization, doping, and oxidation can transform it into an electrically active cathode material [107]. The electronic conductivity and capacity of NaFePO₄ are not up to the theoretical valve and are strongly reliant on carbon combination and composite materials fabrication in nanoscale methods [108]. The olivine NaFePO₄ particles ~80 nm in size coated with carbon deliver around 120 mA h g⁻¹ at a specific capacity and retain nearly 90% after 100 cycles at 0.1 C [15]. In a different study, Zheng et al. [109] examined triphylite-NaFePO₄ and mari-cite-NaFePO₄ redox reactions. The investigations demonstrated that NaFePO₄ exhibits a noticeable charge reduction for oxygen ions during the charging process, but LiFePO₄ shows less loss for oxygen ions. Moreover, it is noted that the Bader charges are less than 0.1 e/oxygen, indicating that the oxygen exhibits inadequate electrochemical activation [109]. This section has thoroughly examined the investigation into NaFePO₄, encompassing a complete analysis of several synthesis techniques and their influence on electrochemical efficacy. Regardless of the obstacles encountered in attaining the theoretical capability, this assessment contributes to the continuous efforts to advance high-performing SIBs. The findings of this study present opportunities for addressing the need for sustainable energy storage solutions and furthering the practical implementation of NaFePO₄-based cathodes in the field of next-generation energy storage systems.

2.1.2. Sodium Vanadium Phosphate (NVP)

Research into the feasible usage of NVP for the cathode material in SIBs has been driven by the increasing need for effective and environmentally friendly energy storage alternatives. With the growing worries over the environmental effect and availability of resources, the need for dependable energy storage technologies has escalated. SIBs have emerged as a viable replacement for conventional LIBs, displaying significant potential. The compound NVP, characterized by its NASICON crystal structure, notable capacity, and desirable thermal stability, emerges as a convincing candidate for use as cathode materials in SIBs. The main focus of this investigation is to enhance our awareness of cathode materials based on NVP, optimize their electrochemical properties, and make significant contributions to the advancement of SIBs with superior performance. Researchers attempt to address the issues of reversibility, ion diffusion, and volumetric changes during cycling by investigating diverse synthesis processes and integrating novel techniques, including carbon coating and doping. The primary objective is to enable the seamless incorporation of cathodes based on NVP into actual conditions, hence advancing the adoption of sustainable energy storage alternatives.

Several cathode materials were previously examined for SIBs. Among them, NVP has received significant interest due to its NASICON structure, higher capacity of approximately 118 mA h g⁻¹, favorable thermal stability, and a high possible level of around 3.4 [98, 110]. Meligrana et al. [111] studied the production of NVP/CNF electrodes using electrospun carbon nanofibers. The researchers specifically focused on optimizing the stabilization and carbonization procedures to enhance the efficacy of the electrodes. NVP, a highly attractive material for cathodes for Na⁺ technology, has stimulated the exploration of an alternate strategy. The fabricated NVP/CNF electrodes had exceptional cycling performance and demonstrated high C-rate capability, surpassing 200 cycles with no degradation [111]. Zheng et al. [112] presented a two-step process for the development of the resulting products. Initially, NaOH, NH₄VO₃, and NH₄H₂PO₄ were precisely mixed in a 3 : 2 : 2 as part of the synthesis process. The resulting mixture was placed in a muffle furnace and reacted for 1 h at a temperature of 800°C. The reaction yielded an intermediate product with a green color, indicating the presence of V⁴⁺. Subsequently, a procedure known as ball milling was used for around 12 h to incorporate sucrose powder, the carbon source, into the intermediate outcome. The product of the ball milling process was then subjected to thermal treatment in an electric oven at a temperature of 80°C overnight. The specimen underwent sintering at a temperature of 350°C for 5 h to eliminate the presence of water and ammonia. Ultimately, calcination was conducted for 8 h at 750°C in an argon gas environment, forming the composite materials. On the other hand, a one-step method was used to synthesize the carbon-coated NVP materials using a solid-state reduction procedure for comparison. Sodium hydroxide, NH₄H₂PO₄, and ammonium metavanadate (NH₄VO₃) were added to the mixture, along with an appropriate quantity of sucrose powder. The precursors were homogeneously combined using ball milling for approximately 12 h. The resulting product from the ball milling process was subjected to drying in an oven at a temperature of 80°C for one night. The specimen underwent sintering at a temperature of 350°C for 5 h. Subsequently, the calcination process was commenced at a temperature of 750°C for 8 h in an atmosphere of argon to produce the G-NVP composite. Scheme 1 depicts the mechanism for the formation of X-NVP. The discharge capacity reaches 117 mA h g⁻¹ at 0.5C and then slightly degrades after 250 cycles at 2C. However, an 80 mA h g⁻¹ capacity was measured at 50C, and even after 3000 cycles, the reverse capacity retention remains at 78% (Figure 3). The X-NVP has excellent stability and rate performance when operating at a lower temperature of 20°C [112]. Duan et al. [113] adopted a hydrothermal and sol–gel process to synthesize NVP nanoparticles, yielding a capacity of 104.3 mA h g⁻¹ at 0.5 C and 94.9 mA h g⁻¹ at 5 C and maintaining 96% of its capacity after 700 cycles. The advancement of SIBs has encountered challenges including minimal reversibility, lethargic ion diffusion, and substantial fluctuations in volume. Hydrothermally assisted self-assembly, subsequent in situ crystallization, and N-doped carbon coating comprised the practical method that Cao et al. [114] utilized to produce novel hierarchical porous microspheres (NVP/C-MSs). This methodology guarantees strong structural integrity by employing Na oleate as a structure-directing agent and a Na source. It reduces the extent of volume deformation that may occur during recurrent Na⁺ extraction and insertion procedures. The half-cell exhibits noteworthy performance characteristics, including a capacity of 116.3 mA h g⁻¹ at a rate of 0.5 C, a capacity of 99.3 mA h g⁻¹ at a high rate of 100 C, and exceptional cyclic stability for more than cycles at 20 C [114]. Salehi et al. [115] studied synthesizing porous NVP/C composite materials employing a cost-effective and scalable sol–gel approach. The structure of the composite materials was modified by adjusting the concentrations of CTAB and glycine, which impacted their electrochemical performance. This modification was carried out in addition to the chemical processing of the materials. The discharge rate of the NVP/C composite materials decreased from 113 to 99 mA h g⁻¹ after 200 cycles at 1 C, while maintaining a capacity retention of 88.1%. Additionally, the composite materials demonstrated remarkable performance at high discharge rates, delivering a discharge capacity of 96 mA h g⁻¹ at a rate of 10 C. This can be attributed to their distinctive structure, conductivity, and morphology [115]. The synthesis of NVP/C through a solution-based technique, aiming at improving Na-ion diffusion, resulted in a capacity of 61.5 mA h g⁻¹ at 40C [116]. Gečė et al. [117] effectively produced and examined the electrochemical characteristics of mixed phosphate–pyrophosphate compounds, namely Na₇V₄(PO₄)(P₂O₇)₄, Na₄Mn₃(PO₄)₂P₂O₇, and Na₄Fe₃ (PO₄)₂P₂O₇. The compounds Na₄Fe₃(PO₄)₂P₂O₇ and Na₇V₄(PO₄)(P₂O₇)₄ exhibited outstanding performance in organic electrolytes but experienced degradation in “water-in-salt” aqueous electrolytes. The compound Na₄Mn₃(PO₄)₂P₂O₇ showed low reactivity in organic electrolytes and negligible reactivity in “water-in-salt” electrolytes, which can be because of breakdown mechanisms [117]. Using the ball milling method, Jian et al. [16] synthesized NVP carbon-coated composites by coating NVP materials with a 6 nm carbon layer. The composites exhibited initial charge and discharge capacities of 98.6 mA h g⁻¹ and 93 mA h g g⁻¹, respectively. Furthermore, these materials retained 99% of their capacity after performing 10 cycles. According to their study, improving the particle size to maximize surface area and modifying the carbon concentration in the composite materials can improve the performance of the cathode. In an alternative report [118], a two-step methodology involved a simple chemistry process and heated pretreatment to synthesize nanosized NVP particles coated with carbon embedded into a porous carbon framework as a cathode material. The composite materials of C/NVP/pC outperformed the cathodes of Li, demonstrating enhanced charge/discharge properties, allowing for a 6-second cycle at 22 A/g current density and 44 mA h g⁻¹ specific capacity (Figure 4).

It is reported that NVP/C composites were prepared via (i) ball milling, (ii) spray drying, and successive calcination process. The prepared composite materials displayed consistency when shared with stable counter electrodes and attained 1500 cycles with 80% retention at 100 mA/g [119]. Zheng et al. [34] investigated Li ion doping to enhance the intrinsic properties of NVP. It was observed that when the doping ratio is lower (x ≤ 0.1), NVP-Li ions tend Na₂, while at a higher ratio (i.e., x ≥ 0.1) upholding Na1 and Na2 sites due to repulsion (Figure 5) [34].

Chen et al. [52] performed a specific study describing their research results on improving Na_3+xV_2–xM^IIx (PO₄)₃/C composite cathode materials. They accomplished this by using a scalable sol–gel technique and incorporating various portions of Co- and Cu- (x—0, 0.01, 0.03, 0.05). The investigation concluded that doping elevated conductivity, which greatly enhanced the electrochemical performance of the doped materials. Moreover, composite materials like Na₃.₀₁V_1.99Co_0.01 (PO₄)₃/C and Na_3.03V_1.97Cu_0.03(PO₄)₃/C showed significant initial discharge capacities of 116 and 114 mA h g⁻¹ at 0.5°C, respectively [52]. Wang et al. [110] investigated the effect of Br and N doping on the electrochemical characteristics of NVP/C composite cathode materials. The presence of doping elements induces the production of amorphous carbon on the nanosize articles of NVP. The doped Br/N/a-C/NVP-2 nanocomposite materials demonstrated a specific capacity of 41 mA h g⁻¹.

The electrochemical efficiency of NVP can be further enhanced by cationic replacement, as demonstrated by the capacity and cyclic stability enhancement of Mg/Fe substitution [120, 121]. Li et al. [122] showed that the phase structure of Mn-HCF undergoes a transition from rhombohedral to cubic when Sn⁴⁺ is introduced. In addition, they noted higher capacity retention of 80.5% after 100 cycles at 240 mA g⁻¹, compared to 54.0% for the Mn-HCF without doping. In a related investigation, the impact of Mn in NVP/C cathode materials was the focus of a doping investigation by Park et al. [123]. Mn is known to strengthen the crystal structure, thereby proving its stability and electrochemical performance. The addition of Mn resulted in a decrease in the band gap and enhanced electron transport compared to NVP. The electrochemical analysis demonstrated that administering Mn to NVP/C enhanced the initial discharge capacity. Particularly, NVMP showed a discharge capacity of 95 mA h g⁻¹, while C/NVMP displayed a higher discharge capacity of 107.4 mA h g⁻¹. Additionally, NVMP composite structural materials, including carbon, showed an impressive discharge capacity of 78 mA h g⁻¹ after 50 cycles at 10C (Figure 6).

Feng et al. [124] used a solid-phase method to produce cathode materials for SIBs, specifically NVP/C composite. The synthesis of the materials was carried out in a two-step approach: (i) ~1.1 of sodium dihydrogen phosphate (NaH₂PO₄), NH₄VO₃ ~ 0.5, and 1.0 g agarose were mixed. (ii) In the next step at 400 rpm, the mixed product was ball-milled for 24 h, and in the final step (iii), two-step calcination treatment was provided at 650°C and 750°C for 6 h and 8 h, respectively, under the environment of Ar/H₂ (Scheme 2).

SEM and TEM studies disclosed the framework structure and numerous secondary particles with sizes ranging from 500 to 1000 nm (Figure 7). Additionally, Figure 7d shows the crystallinity of the composite materials, with NVP particles uniformly coated with carbon material measuring around 2–5 nm. The NVP/C composite materials exhibited a significant initial capacity of 116 mA h g⁻¹ following 500 cycles at 1 C, with a capacity retention of 97.3% of its capacity. Furthermore, they maintained 78% capacity retention at 20 C after 7000 cycles [124]. In a critical study, Ling et al. [125] presented a significant 3D hierarchical porous NVP/C structure, which shows enhanced electrochemical performance as a cathode material for SIBs. The cathode materials were produced using a hydrothermal method. The cathode materials were synthesized using hydrothermal methods, milling, and subsequent calcination. The nanocomposite cathode materials demonstrated exceptional electrochemical characteristics, achieving a capacity of 93.3 mA h g⁻¹ after 8000 cycles at 20 C and displaying outstanding performance at high rates [125].

The investigation of Na₃V₂ (PO₄)₃ (NVP) for SIBs provides critical insights into the optimization of NVP for energy storage. Carbon coating, particle size modification, and doping with elements such as Li, Co, Cu, and Mn have been employed by researchers to improve the conductivity, stability, and efficacy of NVP through a variety of synthesis methods. These improvements address critical challenges in SIB development by enhancing discharge capacity, cycle stability, and rate performance. This research significantly advances NVP-based cathodes, paving the way for their practical use in energy storage. However, it also underscores the need for continued refinement of these materials, as the field of energy storage is constantly evolving.

2.2.1. Fluorine-Based NaVPO₄F

Sodium vanadium-fluorophosphate (NVPF) is a unique polyanion molecule characterized by a NASICON structure, which offers a stable 3D framework that allows fast transit of Na⁺ [135]. Ling et al. [136] devised a carbonization-assisted solvent thermal approach to synthesize NaVPO₄F/C composite materials in a single step. The initial process involved the addition of citric acid and NH₄VO₃ in a 1 : 1 stoichiometric ratio. The mixture was dissolved in 200 mL of double distilled water while stirring at 85°C for 5 h. The gel obtained was subjected to vacuum drying at an oven temperature of 80°C for 12 h. Subsequently, the desiccated precursor transformed into a powdered form and was subjected to sintering at 350°C for 5 h. Later, the product was subjected to heat treatment at 750°C for 5 h in an environment containing argon gas, forming V₂O₃/C. For 6 h, carbon-coated materials were produced by exposing V₂O₃/C to ball milling treatment at 200 revolutions per minute. This process resulted in the formation of carbon-coated V₂O₃/C─B. During the subsequent stage, a mixture of various vanadium sources (V₂O₃/C, V₂O₃/C─B, VPO₄, and NH₄VO₃), combined with NaF and NH₄H₂PO₄, in a V/Na/F/P ratio of 1 : 1 : 1 : 1, was introduced into a container made of poly (tetrafluoroethylene). To avoid aggregating together, Na dodecylbenzene sulfonate was employed. The initial mixture was combined with a 1 : 1 of distilled water and ethanol. Then, 70 mL of the resulting mixture was transferred to stainless steel autoclave vessels and heated in an electric oven at 180°C for approximately 18 h. After repeated washings, the final product was stored in a vacuum oven for 12 h at 80°C. Postannealing treatment in the Ar environment was employed to achieve the monoclinic phase of NaVPO₄F at 750°C for 8 h. The NaVPO₄F/C composite materials demonstrated exemplary performance and cycling stability. The rate performance at 10 C was 75 mA h g⁻¹, whereas the initial capacity of around 133 mA h g⁻¹ reached 0.2 C. The composite materials’s energy density (i.e., 509 W h kg⁻¹) was attained, revealing its high performance. After 1000 cycles, the capacity holding of NaVPO₄F/C displayed 77% at 1 C (Figure 8) [136].

Cheng et al. [137] performed a study where they produced nano-NaVPO₄F particles with average particle sizes of around 100 nm attached to rGO sheets using the freeze-drying procedure. TEM examinations yielded valuable insights into the morphology of the composite materials. The uniform crystalline nanoparticles had a size range of 60–90 nm. However, in certain areas, the particle size was found to vary between 100 and 200 nm (Figure 9a), indicating the presence of monoclinic NVPF planes. The composites revealed exceptional electrochemical performance, specifically when used as cathode materials in SIBs. During 3000 cycles, the composite materials exhibited a capacity of 100.0 mAh g⁻¹ at a rate of 10 C and 86.5 mAh g⁻¹ at 100 C. The synthesis technique and exceptional performance of composite materials show great potential for their application as electrode materials in SIBs (Figure 9b). The research conducted on the utilization of fluorine-based NaVPO₄F as a cathode material has shown encouraging findings that display its efficacy in SIBs. Carbon coatings and refined synthesis techniques have enhanced electrochemical characteristics, including significant capacity preservation across several cycles. Consequently, fluorine-based NaVPO₄F emerged as highly desirable for advanced energy storage purposes. The advancement in comprehending and enhancing the efficacy of cathodes using fluorine plays a vital role in the advancement of battery technologies, which are both practical and sustainable. NVPF is a cathode material that is highly promising for SIBs. This is due to its exceptional electrochemical performance and reliable NASICON structure. Carbon coatings and advanced synthesis methods have further improved its capacity retention and cycling stability, making NVPF a desirable candidate for sustainable, high-performance energy storage applications.

2.2.2. Fluorine-Based Na₃V₂ (PO₄)₃F₃

Recent developments have shown that vanadium-based cathode materials for SIBs can be improved by replacing phosphate with fluorine, taking advantage of its strong electronegativity. Gao et al. [138] developed a method to create composite electrode materials by cross-linking a dual-carbon framework with NVPF, improving electric charge transport within the particles. The NVPF@C@CNTs composite demonstrated a remarkable capacity of 129.10 mA h g⁻¹, coupled with exceptional stability. It retained 93% of its capacity even after 1000 cycles at 20 C, as shown in Figure 10 [138]. A separate study addressed a research group’s development of electrode materials for the SIBs. They utilized an easy approach to produce carbon-coated NVPF composite electrodes, with the carbon sourced from polytetrafluoroethylene (PTFE) and fluorine as a substitution agent. The NVPF materials, which are composed of carbon, exhibited exceptional performance in terms of both capacity and stability [139]. In addition, another investigation investigated the synthesis of NVPF/NVP/C composite cathode materials using a carbon thermal reduction process. The hybrid cathode materials were improved in terms of their electrochemical properties, including capacity and stability. They achieved a specific capacity of 83 mA h g⁻¹ at a rate of 100 C and maintained 63.1% of their capacity after 3000 cycles [140]. Thamodaran et al. [141] introduced a single-step process to produce NVPF hierarchical microspheres (NVPF-HMS) as cathode materials for LIBs, which are suitable for use at low and high temperatures. The findings showed a reversible capacity of 119 mA h g⁻¹ at a rate of 0.1 C, along with a capacity of 85 mA h g⁻¹ at a rate of 1 C. Additionally, there was an 80% capacity retention after 250 cycles at a temperature of 25°C [141]. Separate research used the hydrothermal method to synthesize NVPF/C with Mn doping (Mn^x+x = 2, 3, 4). The performance of NVPF@C was improved by introducing Mn²⁺ doping, which resulted in a decrease in charge transfer resistance (Rct) and an increase in Na⁺ diffusion kinetics. The enhanced electrochemical performance of the improved composite material cathodes depends on structural stability, optimized Mn ions, and homogeneous carbon coatings. The cathode materials, NVM2PF/C, were altered to have optimized valence states. These modified materials showed excellent stability and enhanced rate performance. They had an initial discharge capacity of 116.2 mA h g⁻¹ at a rate of 0.2 C, which reduced to 93.2 mA h g⁻¹ after 100 cycles [142]. In addition, Li et al. [143] displayed effective methods of producing NVPF nanotube particles with a nitrogen-doped carbon layer using a solid-phase process. The thickness of the carbon layer was around 4 nm, as shown in Figure 11 [143]. Using a one-step carbothermal reduction method, carbon-coated NVPF composite cathode materials are synthesized incorporating varying amounts of glucose reagent to synthesize (NVPF/C) with different levels of excess carbon. The NVPF/C composites exhibited outstanding electrochemical characteristics and improved cycling stability, with an initial capacity of 112.3 mA h g⁻¹ at 1 C and retaining 89.7% of its capacity after 100 cycles [144]. Criado et al. [145] adopted the emulsion approach and created a micellar system by using oleic acid and Vaseline oil as surfactants and organic media during the synthesis of C@NVPF. NVPF and NVPF/CNT were fabricated by spray-drying and subjected to a 2-h heat treatment in an Ar environment. Further, a composite material consisting of NVPF and CNT was created by adding CNT to NVPF using a spray-drying technique. The CNT-based NVPF composite showed good electrochemical performance with lithium, reaching capacities of 125 mA h g⁻¹ at C/10 and 103 mA h g⁻¹ at 1 C [146].

Extensive research on fluorine-based NVPF as a SIBs cathode material shows promising results. Electrochemical performance, stability, and cycle retention have been substantially enhanced by implementing techniques such as carbon coating and composite designs. Charge transfer kinetics and overall battery efficiency are improved by including fluorine, which represents a significant advancement in the development of cathode materials. These findings contribute to the ongoing efforts to create high-performance, sustainable SIBs, bringing us closer to their practical application in future energy storage solutions.