Recent Research Process of Carbon Engineering on Na 3 V 2 (PO 4 ) 3 for Sodium-Ion Battery Cathodes: A Mini Review

: Owing to the 3D open framework, excellent structural stability, and high ionic conductivity, NASICON-type compounds are extensively employed as promising cathode materials for sodium-ion batteries (SIBs). Being one of the representative NASICON-type compounds, the Na 3 V 2 (PO 4 ) 3 delivers high theoretical capacity with an operating voltage exceeding 3.3 V, enabling it to be a good candidate for SIBs. Unfortunately, the Na 3 V 2 (PO 4 ) 3 suffers from low electronic conductivity. In this work, we brieﬂy review the recent research progress on novel carbon engineering strategies to enhance the electronic conductivity of Na 3 V 2 (PO 4 ) 3 . Moreover, we will point out the issues relating to the development of NASICON cathode materials and put forward some suggestions.


Introduction
Lithium-ion batteries (LIBs) are widely used in high-energy storage fields, such as electric vehicles, communications, and smart electronics [1][2][3][4]. However, the development of LIBs is threatened by the limited lithium resources where the earth's crust content of lithium is only 0.0065% [5][6][7]. Therefore, it is crucial to develop alternatives to replace LIBs in the future. Compared to lithium, the sodium reserve is abundant in the earth's crust (2.8%) and sea (10.77g/kg) [8], and Na exhibits similar properties as Li since they all settle in group IA of the periodic tables. As realized in Table 1, Na also has a low standard electrode potential of −2.71 V, which is just slightly higher than Li (−3.04 V) [9][10][11][12][13][14]. As a result, SIBs have similar working principles to LIBs in that Na + can reversibly shuttle between cathode and anode, causing a voltage potential difference that leads to the mutual conversion of electrical and chemical energy. However, the large ionic radius (1.02 Å) and atomic mass (22.99 g/mol) of Na + result in low theoretical capacity, high resistance, and sluggish transport kinetics during the Na + insertion/de-insertion [15][16][17][18]. Thus, some traditional electrode materials applied for LIBs are not suitable in SIBs directly, such as the extensively used LiCoO 2 cathode and the graphite anode. Therefore, it is significant to explore SIB electrode materials with exceptional Na + insertion/de-insertion kinetics and Na + storage performance. Associating with electrochemical batteries, cathode materials as an essential component of batteries, make great contributions to strengthening electrochemical performance. This is because the inherent crystal structure of the cathode materials determines the electrochemical behavior of SIBs, including redox potential, the number of electron transfers, and structural stability. The main cathode materials adapted to SIBs can be classified into three types: (1) stratified material [19][20][21][22], (2) polyanionic compounds [23][24][25][26], and (3) macromolecule polymer material [27][28][29][30][31]. Further, Table 2 shows the performance of different cathode materials for SIBs. 3.6 V C/10, 94 mAh/g 10 C, 55 mAh/g [45] Polyanionic compounds have been regarded as one of the most attractive cathode materials due to their Na + superionic conductor (NASICON) structure and adjustable redox potential [46][47][48][49][50]. The general formula of NASICON-type compounds is described as A x M 2 (XO 4 ) 3 (A = Na, Li, Fe, etc., M = V, Ti, Fe, Tr or Nb, etc., X = P or S and x = 0-4). The NASICON-type A x M 2 (XO 4 ) 3 is constructed by corner-shared MO 6 and polyhedral XO 4 groups, which provide convenient Na + diffusion channels [51]. The open 3D framework and large pore channels make the polyanionic compounds possess high Na + conductivity and long-term structure stability [52][53][54][55][56][57]. Among various NASICON-type A x M 2 (XO 4 ) 3 , the Na 3 V 2 (PO 4 ) 3 (NVP) cathode offers a high Na + storage capacity of 117 mAh/g with high structural stability. However, the separation of TiO 6 octahedrons leads to poor electronic conductivity, which further influence rate capability and cyclic stability [58][59][60][61][62][63]. In order to overcome these problems, many advanced strategies have been proposed. The typical approaches involve particle nano-sizing, doping, and coating with carbons, including graphene, carbon black, and carbon nanotubes (CNTs), etc. [64][65][66][67][68][69].
In this review, we initially analyze the structure−performance relationship of NVP for SIBs and then briefly summarize the research process of the carbon coating approaches to modify the NVP. Significantly, the modifications that can improve the conductivity of NVP by carbon coating strategies are emphatically introduced. Moreover, the challenges and future development direction of the NASICON-type cathode have been prospected. It is expected that this work will be a meaningful reference to guide the design of highperformance NVP cathodes for SIBs.

The Structure and Properties of NVP
Vanadium has rich oxidation valence states, which contain V 5+ , V 4+ , V 3+ , and V 2+ . Theoretically, the NVP cathode reveals a high capacity of 117 mAh/g because of the conversion of V 3+ /V 4+ [70,71]. Meanwhile, its highly open framework enables the creation of 3D diffusion channels and large migration gaps for Na + , and the diffusion coefficient of Na + can reach 10 −10 cm 2 /s [72,73]. Figure 1 shows the crystal structure of Na 3 V 2 (PO 4 ) 3 generated by VESTA with the monoclinic lattice, and the space group is P2 1 /c. Besides, lattice parameters are a = 8.87 Å, b = 9.01 Å, c = 15.44 Å and α = 55.326 • , β = 90 • and γ = 90 • . The structure is composed of VO 6 octahedrons and PO 4 tetrahedrons by sharing an O atom at the vertex. At the same time, two free Na + occupy the two oxidation state channels in the lattice. This structure promises excellent structural stability for NVP. For example, Choi et al. [74] studied the thermal stability of the NVP cathode by using TGA-DSC and in situ XRD at various temperatures. Basically, the NVP exhibits no endothermic/exothermic peak and prominent thermal stability even at 450 • C under a deoxidation state, which is ascribed to the high stability of the P-O bond. Moreover, the NVP illuminates the typical redox potentials at 3.4 V vs. Na/Na +, which is higher than that of the majority of cathode materials [75][76][77]. When employed as a cathode, the cell reaction can be described as: In the charging process, two Na + are expelled from the cathode to the anode. Meanwhile, V 3+ is oxidized to V 4+ to form NaV 2 (PO 4 ) 3 , accompanied by the transfer of two electrons. On the contrary, V 4+ is reduced to V 3+, and Na + is inserted into the cathode to form NVP.

The Modification Methods to Improve the Conductivity of NVP
Despite the fact that the NVP demonstrates an advance as a cathode for SIBs, the conductivity of NVP is still concerning. Carbon coating engineering is often considered to be one of the most effective methods to improve the electronic conductivity of base materials. It has many advantages, such as low cost, simple operation, and flexibility.

Graphene
Graphene is a 2D carbon allotrope constituted by sp 2 hybridization. The remaining p orbital electrons in the plane form a large π bond to move freely over graphene, resulting in fast electron mobility and high electrical conductivity. The charge carrier mobility and thermal conductivity of graphene exceed 2 × 10 5 cm 2 /Vs and 3000 W/Mk at an electron density of 2 × 10 11 cm −2 , respectively. As a result, graphene is employed as a conductive component to composite with NVP by either coating or doping. For example, Rui et al. [78] proposed a 3D NVP-based hybrid cathode, which is composed of NVP@amorphous carbon wrapped by reduced graphene oxide nanosheets (NVP@C@rGO) to achieve efficient charge transfer, as shown in Figure 2a,b. Essentially, the aqueous NVP precursor is mixed with poly (vinylpyrrolidone) (PVP) to construct a nanoporous carbon matrix on NVP nanoparticles. Then, the GO suspension was added to the above solution and followed by performing freeze-drying and heating. In Figure 2c,d, the NVP@C@rGO revealed prominent rate performance; that is, the discharge capacity remained at about 115 mAh/g when the current density varies from 1 to 20 C. Figure 2e shows that the charge transfer resistance (R ct ) of NVP@C@rGO was about 258 Ω, which was much lower than that of NVP@C (639 Ω). Chang et al. [79] synthesized a graphene-bound, binder-free NVP film electrode by simple vacuum-assisted filtration of the NVP and graphene. Figure 2f shows the SEM image of the as-prepared graphene-bound NVP film. Obviously, the 100-300 nm NVP particles are evenly bound to the graphene. Compared with pristine NVP particles, the graphene not only increased the conductivity of the whole electrode but also suppressed the agglomeration of NVP particles. As a result, it exhibited excellent performance under a high rate (Figure 2g). Further, in Figure 2h, the R ct of graphene-bound NVP was significantly smaller than that of pristine NVP, which again confirms that the GO benefits alleviate the electronic conductivity of NVP. Besides, Xu et al. [80] prepared NVP/reduced graphene oxide hollow spheres (NVP/rGO HSs) by spray drying and subsequent pyrolysis for Na+ storage (Figure 3a). The SEM and TEM images of NVP/rGO HSs are displayed in Figure 3b,c. The graphene layer was surrounded by a large number of NVP particles, which together formed a hollow sphere (~6 µm). The spherical hollow structure can make full contact between the electrode and electrolyte, and the rGO layer acted as a high-speed electron transferring bridge. Therefore, NVP/rGO HSs exhibited excellent electrochemical performance as well as remarkable rate performance, i.e., the specific discharge capacity was 107.5 mAh/g at 10 C. Further, Xu et al. [81] designed a straightforward and self-assembly approach to fabricate layer-by-layer structured NVP@reduced graphene oxide (NVP@rGO) nanocomposites. In NVP@rGO, the rGO was responsible for offering rapid electron transport. Thus, the NVP@rGO cathode presents superior rate capability (73 mAh/g at 100 C and 41 mAh/g at 200 C).

Carbon Nanotubes
Besides graphene, CNTs are also chosen as conductive blocks to couple with NVP. CNTs can be viewed as coiled from graphene sheets, and the p orbital electrons on CNTs provide more π bonds to produce a strong conjugation effect. As a result, high electrical conductivity is associated with CNTs. For example, Shen et al. [82] developed a simple solgel approach to prepare double nano-carbon modified NVP (NVP@C+N@CNTs) to improve the electrochemical characteristic of SIBs, and the corresponding schematic structures of NVP@C+N@CNTs are drawn in Figure 4a. In Figure 4b,c, associating with the TEM image of NVP@C+N@CNTs, it is clear to observe that the NVP is perfectly covered by carbon coatings and simultaneously connected by CNTs to form a 3D network. Such a structure accelerated the electron transport between NVP nanoparticles. In Figure 4d,   Chen et al. [83] designed and synthesized a CNT-decorated NVP microsphere via spray drying and a carbothermal reduction approach. The NVP microspheres were embedded by CNTs. Meanwhile, the particles were coated by amorphous carbon layers. Figure 5a-d shows the SEM images of NVP doped with different amounts of CNTs. As the CNTs content increases, fewer NVP particles are observed. This is presumably ascribed to the fact that the CNTs limited the growth of NVP particles. Further, the CNTs and carbon layer can be apparently examined from the TEM images in Figure 5e,f. In this structure, the unique carbon network effectively promoted electron transmission. Most importantly, the incorporation of CNTs significantly enhanced the rate capability (Figure 5g). By comparison with NVP/C, the NVP/C10 illustrated a smaller R ct (225.5 Ω), which is also beneficial to improve the electrochemical properties of the composite electrode (Figure 5h). Beyond these, Sun et al. [84] prepared Na 3 V 2 (PO 4 ) 3 /C@CNTs-WC by a sol-gel method. The C@CNTs formed a conductive framework that accelerated electron transport. At the same time, the introduction of W 6+ created abundant cavities on the surface of the NVP. Therefore, the Na 3 V 2 (PO 4 ) 3 /C@CNTs-WC delivers a high capacity of 92.6 mAh/g at a super high rate of 50 C and remains 84.5 mAh/g over 400 cycles.

Other Carbon Coatings
In any case, the carbon coating layer (CCL) can inhibit the size of NVP particles during growth. Besides, the CCL acted as a buffer layer to avoid direct contact between the electrolyte and active materials, which helped to weaken the corrosion of the electrolyte over the electrode. Moreover, the CCL is usually amorphous carbon, which has good mechanical properties and stability. For instance, Duan et al. [85] employed a hydrothermal-assisted sol-gel method to synthesize the core-shelled NVP@C. Notably, there presents a large gap between the as-prepared nano NVP@C particles, which is conducive to urging complete contact between the electrolyte and the NVP@C and thus shortening the Na + diffusion path. When it was applied for SIBs, the nano NVP@C realized the high specific discharge capacity of 88 mAh/g at 10 C. In the work performed by Li et al. [86], citric acid was used as a carbon resource, reducing agent, and complexing agent. They proposed a method by rheological phase reaction to prepare double carbon-wrapped NVP where amorphous carbon served as a framework together with graphitized carbon, which acted as a conductive network. By this design, the thickness of amorphous carbon is about 5 nm; electrons are quickly transferred to amorphous carbon through graphitized carbon and then transferred to NVP from amorphous carbon, which effectively shortens the transmission path of electrons. Consequently, the NVP@C calcined at 800 • C (NVPC-800) illuminated the best electrochemical performance among all samples calcined at 600, 700, and 900 • C. In Figure 6a,b, relating to the TEM images, the thickness of the disordered carbon layer was~5 nm, and the graphitized carbon can be clearly observed as well. Figure 6c,d show the cycling performance and EIS spectra of NVPC-800, verifying the prominent cycling stability and low R ct . Even the current density increased to 1 C, and the specific discharge capacity of NVPC-800 reached 99.8 mAh/g with more than 98.9% capacity retention after 500 cycles (Figure 6e). Jiang et al. [87] synthesized carbon-coated nanosized NVP into mesoporous carbons (NVP@C@CMK-3) by a nano-coating technique, as drawn in Figure 7a. It schematically described the reversible electrochemical reactions of the NVP@C nanoconfined within a single carbon pore that the special structure ensured efficient diffusion of electrons and Na + . In Figure 7b, the NVP@C@CMK-3 reveals the high specific discharge capacity of 115 mAh/g at 110 mA/g, which is much higher than that of pure NVP (83 mAh/g). From Figure 7c, the R ct of the NVP@CMK-3 and NVP@C@CMK-3 is 272 and 207 Ω, respectively, highlighting that the double carbon coating structure facilitates the transportation of Na + and e − and meanwhile generates lower polarization [88,89].

Heteroatom Doping
Researchers have found that either anion or cation doping is in favor of improving the intrinsic conductivity of NVP, such as Ag + , N 4-, etc. Basically, ion doping leads to a greater carrier concentration of the doped system over the intrinsic system, which would significantly enhance conductivity. Hong et al. [90] prepared C-Ag co-coated NVP (NVP/C-Ag) and C-coated NVP (NVP/C) by an uncomplicated solid-state method in Figure 8a. From the XRD patterns of NVP/C-Ag in Figure 8b, the typical characteristic peak of silver was captured at 38 • . Meanwhile, the diffraction peak of NVP/C-Ag (at about 23.5 • ) was wider and lower than that of NVP/C, which is attributed to the confined growth of NVP particles due to the presence of the C-Ag layer. Figure 8c displays the TG curves of NVP/C-Ag. When the temperature exceeded 500 • C, the carbon species had completely burned out, and the residual carbon content was about 6%. Figure 8d depicts the charge/discharge curves of NVP/C-Ag at various rates. Upon the current density increases, small changes can be obtained in the specific discharge capacity, featuring excellent rate performance. In Figure 8e, Wang et al. [91] prepared NVP/N-doped carbon composites (NVP-NC) by taking nitrilotrimethylene triphosphonic acid as the multifunctional raw material. Figure 8f shows the SEM image of NVP-NC. It can be seen that NTP particles were encapsulated in an N-doped carbon matrix, which facilitates the rapid transport of Na + and electrons. Among all samples, the NVP-NC obtained under 850 • C showed the best Na + storage performance, and the corresponding specific discharge capacity was 94.0 mAh/g at 10 C after 600 cycles. Jiang et al. [92] designed an (N, S) co-doped graphene (3DPGFs-NS) to accommodate the active materials. Then, the Na 3 V 2 (PO 4 ) 3 @C@3DPGFs-NS were prepared by a sol-gel method, as presented in Figure 9a. From the SEM image of Figure 9b, the obvious 3D porous structure was observed. Further, the carbon layer, NVP, and graphene can be clearly differentiated from the TEM image in Figure 9c. The charge/discharge curves of NVP and Na 3 V 2 (PO 4 ) 3 @C@3DPGFs-NS at 110 mA/g are shown in Figure 9d. The Na 3 V 2 (PO 4 ) 3 @C@3DPGFs-NS displays a wider discharge platform, and the curve is smoother, suggesting the outstanding electrical conductivity of (N, S) codoped 3DPGFs. The specific discharge capacity of Na 3 V 2 (PO 4 ) 3 @C@3DPGFs-NS is 112 mAh/g at 1 C, which is higher than pure NVP (60 mAh/g). Further, the Na 3 V 2 (PO 4 ) 3 @C@3DPGFs-NS reveals excellent cycling ability at 20 C in Figure 9e. Above all, the electrochemical performance of NVP has been improved through the carbon coating and heteroatom doping strategy in Table 3.

Conclusions and Outlook
NVP has demonstrated great potential in SIBs as the cathode due to its high operating voltage, high ionic conductivity, and good structural and thermal stability. In this review, we briefly analyze various carbon coating strategies, such as graphene, carbon nanotubes, and other carbon allotropes, to improve the electronic conductivity of NVP towards highperformance SIBs. The results suggest that various carbons not only ameliorate the structure of NVP but also enhance the conductivity of the whole electrode. Meanwhile, the carbon matrix can store Na + by itself. Beyond that, heteroatom doping has also been employed to enhance the conductivity of pure NVP by increasing the carrier concentration. Despite the NVP cathode materials have developed rapidly in recent years, there are still some key issues that have to be resolved before the application of NVP on a large scale. Firstly, high-capacity NVP electrodes should be designed by involving multi-electron reactions. In this regard, the multi-scale computational models are necessarily put forward as a guide. Further, it is still a challenge to achieve high conductive NVP with excellent electrochemical properties. Fortunately, great efforts have been devoted to dope transition metal ions into NVP to improve the electronic conductivity from the inherent structure of the material, such as Mg 2+ and Mn 2+ , etc. On the other hand, the substitution of V sites by Hf, Zr, and W deserves to be explored as well. Finally, high voltage electrolyte is essential to achieve the multi-electron transfer of NVP cathodes and thus higher energy density.
Funding: This work was supported by the project of Ningxia's key R&D plan (2021BEE03006).

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
No new data were created or analyzed in this study. Data sharing does not apply to this article.

Conflicts of Interest:
The authors declare no conflict of interest.