Cathode Materials of Sodium-Ion Batteries: Comparison
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Please note this is a comparison between Version 1 by Thang Phan Nguyen and Version 3 by Lindsay Dong.

Emerging energy storage systems have received significant attention along with the development of renewable energy, thereby creating a green energy platform for humans. Lithium-ion batteries (LIBs) are commonly used, such as in smartphones, tablets, earphones, and electric vehicles. However, lithium has certain limitations including safety, cost-effectiveness, and environmental issues. Sodium is believed to be an ideal replacement for lithium owing to its infinite abundance, safety, low cost, environmental friendliness, and energy storage behavior similar to that of lithium. Inhered in the achievement in the development of LIBs, sodium-ion batteries (SIBs) have rapidly evolved to be commercialized. Among the cathode, anode, and electrolyte, the cathode remains a significant challenge for achieving a stable, high-rate, and high-capacity device. 

  • sodium-ion batteries
  • cathode materials
  • inorganic cathodes
  • organic cathodes

1. Introduction

The invention of batteries has played a key role in the development of miniaturized electrical devices. In particular, the use of lithium-ion batteries (LIBs) allows portable devices to continuously operate with no, or rarely occurring, disruptions [1]. LIBs are currently used in smartphones, tablets, notebooks, and vehicles. The significant achievement of LIBs is owing to the strong activity of lithium-ion insertion and desertion in storage materials with a high specific capacity (approximately 3860 mAh g−1) [2][3][4][5][2,3,4,5]. However, with an increase in capacity, various issues associated with LIBs need to be overcome, including safety, toxicity, and cost-effectiveness [6][7][8][9][10][6,7,8,9,10]. Meanwhile, sodium is abundantly available on Earth and has similar properties to lithium in storage devices, which is why it is receiving notable attention [11]. The use of sodium-ion batteries (SIBs) reduces the danger of lithium owing to its strong activation; furthermore, the cost and environmental issues can also be resolved [9][12][13][14][15][16][17][18][19][20][21][22][9,12,13,14,15,16,17,18,19,20,21,22]. Considering the development of LIBs, SIBs have become a promising alternative to LIBs. The working mechanisms of LIBs and SIBs are based on the storage of Li and Na ions in two materials with different potentials separated by an electrolyte, as shown in Figure 1. The insertion and desertion of Na ions in the anode and cathode through the electrolyte create and reduce the potential between the two electrodes, corresponding to charge and discharge processes, respectively. Anode materials can also undergo conversion reactions that react with Na ions, forming alloy states that allow high capacities, such as in expanded graphite (284 mAh g−1), TiO2-based anodes (200–300 mAh g−1), antimony sulfides (Sb2S3) (730 mAh g−1), Sn4P3 (>1100 mAh g−1), and phosphorous with a theoretical capacity of ~2596 mAh g−1, among others [23][24][25][26][27][28][29][30][23,24,25,26,27,28,29,30]. However, the development of a sodium cathode continues to present limitations such as an unstable and low capacity of 100–200 mAh g−1. SIB cathode materials include a variety of inorganic compounds (metal oxides, phosphates, pyrophosphates, etc.) and organic or organometallic materials [31][32][31,32]. Although achievements have been reported for SIBs and they are being commercialized, the current cathode material has been significantly improved and developed to have better electrochemical properties [33][34][35][33,34,35].
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Figure 1.
Schematic of the simple operation of a sodium-ion battery employing a layered cathode and graphene anode.

2. SIB Cathode Materials

2.1. Inorganic Compounds

2.1.1. Layered Oxide Materials (Na

x

MO

2

)

The layered oxide materials used for SIBs mostly consist of transition-metal oxides [36]. There are two common phases of NaMO
2, which are the O3 and P2 phases, classified based on the different stacking of the oxygen ion frameworks as ABCABCABC (O3) or ABBAABBA (P2) [37][38]. In addition, the O2 phase and birnessite are the layered structures with the tightest and loosest packing, respectively [39][40][41]. Among these phases, O3 phase can provide a high Na content and high specific capacity, which enables its application in full cells. However, the degradation of structure during cycling limits its application. To maintain structure, foreign metals with a large ionic radius such as Fe, Cr, Ti, and V can be used introduced [42]. On the other hand, P2 phase has a lower Na content but a wider layer spacing, which leads to faster diffusion of Na
, which are the O3 and P2 phases, classified based on the different stacking of the oxygen ion frameworks as ABCABCABC (O3) or ABBAABBA (P2) [37,38]. In addition, the O2 phase and birnessite are the layered structures with the tightest and loosest packing, respectively [39,40,41]. Among these phases, O3 phase can provide a high Na content and high specific capacity, which enables its application in full cells. However, the degradation of structure during cycling limits its application. To maintain structure, foreign metals with a large ionic radius such as Fe, Cr, Ti, and V can be used introduced [42]. On the other hand, P2 phase has a lower Na content but a wider layer spacing, which leads to faster diffusion of Na
+
ions and improves structural stability during cycling. Similar to LIBs, compounds of Na with Co, Ni, and Mn oxides have layered structures, such as Na
x
CoO
2
, Na
x
NiO
2
, and Na
x
MnO
2 [43][44][45][46]. However, owing to the large size of Na ions, the behavior of CoO
[43,44,45,46]. However, owing to the large size of Na ions, the behavior of CoO
6
or NiO
6
in the lattice with the intercalation of Na varies from that of Li [47]. Na
x
CoO
2
and Na
x
NiO
2
compounds have exhibited low capacities below or near 100 mAh g
−1 [48][49]. Reddy et al. fabricated P2-Na
[48,49]. Reddy et al. fabricated P2-Na
x
CoO
2
using the sol–gel method, capable of delivering a capacity of approximately 121 mAh g
−1
at a rate of 0.1 C [50]. Similarly, NaNiO
2
exhibits a capacity of only approximately 80 mAh g
−1
[51]. Meanwhile, Na
x
MnO
2
is a more promising cathode material owing to the multiple oxidation states of the Mn ions in the zigzag layers of the edge-sharing MnO
6
; therefore, this cathode exhibits a high theoretical capacity of approximately 240 mAh g
−1 [52][53][54]. Na
[52,53,54]. Na
x
MnO
2
can be synthesized from either NaOH and Mn salt or MnO
2
. Ma et al. used monoclinic NaMnO
2
as a cathode for SIBs and demonstrated a high first discharge capacity of approximately 185 mAh g
−1 in the 2–3.8 V range [55].
The drawback of layered materials is their unstable structure in air storage and during cycling; therefore, their capacities can be rapidly or irreversibly degraded [56][57]. Due to its hygroscopic nature, NaMO
in the 2–3.8 V range [55].
The drawback of layered materials is their unstable structure in air storage and during cycling; therefore, their capacities can be rapidly or irreversibly degraded [58,59]. Due to its hygroscopic nature, NaMO
2
is unstable in air and in moist environments; therefore, its applications are limited. To improve the performance of NaMnO
2, the partial replacement of Mn with other metals, such as Li, Ni, Co, Al, Fe, and Zn, has been investigated [58][59][60][61][62][63][64]. Kwon et al. proposed the use of a P2-NaLiMnO
, the partial replacement of Mn with other metals, such as Li, Ni, Co, Al, Fe, and Zn, has been investigated [60,61,62,63,64,65,66]. Kwon et al. proposed the use of a P2-NaLiMnO
2
cathode material that exhibited a high reversible capacity of approximately 160 mAh g
−1 [58]. The insertion of Li ions as dopants led to an inhomogeneous electrostatic repulsion between the Mn and Na ions, thereby enhancing the stability of β-Na
[60]. The insertion of Li ions as dopants led to an inhomogeneous electrostatic repulsion between the Mn and Na ions, thereby enhancing the stability of β-Na
0.7
[Mn
1-x
Li
x
]O
2+y
, which exhibited a stable cycling capacity for over 120 cycles without a faded capacity. Liu et al. investigated the use of P2-Na
2/3
Ni
1/3
Mn
2/3
O
2
as a cathode material for SIBs simply synthesized via a novel sol–gel method (NSG) by employing polystyrene as an additive. The main active metal is Ni with Ni
2+
/Ni
4+
states that contribute to the redox-pair peaks at a voltage between 3.0–4.0 V and a minor Mn
3+
/Mn
4+
redox potential between at 2.0–3.0 V. Meanwhile, Mn
4+
effectively maintains the structure of NaNiMnO
2
, thereby significantly improving its stability. At voltages below 2.0 V, the Mn
4+
ions were activated and reduced to Mn
3+
, suffered a disproportional reaction, and dispersed into the electrolyte (Mn
3+
solid → Mn
4+
solid → Mn
2+
electrolyte), and the redox at ~4.0–4.5 V was related to the phase transition from P2 to O2 phase due to the stacking faults. Therefore, the material can be rapidly degraded below 2 V. The NSG Na
2/3
Ni
1/3
Mn
2/3
O
2
cathode exhibited a reversible capacity of approximately 100 mAh g
−1
and an excellent rate performance even at rates of 5 C and 10 C.
A combination of more than three metals was also investigated, including NaLiNiMnCoO
2
, NaLiNiMnO
2
, NaFeMnTiVO
2
, and NaMnNiCuMgTiO
2 [65][66][67][68]. Kataoka et al. prepared a multi-metal complex of NaLiNiMnCoO
[72,73,74,75]. Kataoka et al. prepared a multi-metal complex of NaLiNiMnCoO
2 via co-precipitation and electrochemical ion-exchange methods [65]. The produced Na
via co-precipitation and electrochemical ion-exchange methods [72]. The produced Na
0.95
Li
0.15
(Ni
0.15
Mn
0.55
Co)O
2
was then employed as a highly stable cathode which delivered a capacity of greater than 200 mAh g
−1
for over 40 cycles. Xu et al. investigated the effect of Li ions on NaLiNiMnO
2 cathodes in SIBs and determined the importance of each element as follows [67]: The Ni metal was fully oxidized to Ni
cathodes in SIBs and determined the importance of each element as follows [74]: The Ni metal was fully oxidized to Ni
4+
to balance the overall charge of the cell, which also prevented the Jahn–Teller distortion owing to the active Mn
3+
. Moreover, Ni ions also contributed to the high-voltage redox state of the cathode, widening the range of the working potential from 2.0 to 4.4 V. Li ions were found surrounding Ni
4+
through NMR resonance methods, which indicated that Li could easily migrate to this material. The remaining Li during cycling enhanced the capacity retention; therefore, this cathode delivered a high reversible capacity of 140 mAh g
−1 in the 2.0–4.4 V range.
In addition to Co-, Ni-, and Mn-based metal oxide cathodes, Cr-, Cu-, and Fe-based oxides have also received significant attention [69][70][71][72][73][74]. Yu et al. developed carbon-coated NaCrO
in the 2.0–4.4 V range.
In addition to Co-, Ni-, and Mn-based metal oxide cathodes, Cr-, Cu-, and Fe-based oxides have also received significant attention [79,80,81,82,83,84]. Yu et al. developed carbon-coated NaCrO
2
as a SIB cathode via an emulsion-drying method that exhibited an excellent performance at a high rate of 50 C with a capacity of approximately 100 mAh g
−1 [69]. The NaCrO
[79]. The NaCrO
2
cathode also demonstrated significant thermal stability up to 400 °C. At temperatures above 290 °C, instead of oxygen evolution owing to the thermal decomposition, NaCrO2 decomposed to Na
0.5
CrO
2
and CrO
2
phases. Moreover, Na
0.5
CrO
2
continued to exhibit a stable layered structure from the insertion and desertion of the Na ions. Na
x
CuO
2
and Na
x
FeO
2
also have layered structures and deliver a capacity of approximately 100–200 mAh g
−1 [70][71][75][76]. 
[80,81,85,86]. 

2.1.2. Tunnel Oxides

The Na
x
MO
2
tunnel oxide consists of M
4+
and M
3+
ions at the MO
6
and MO
5 sites, respectively [77][78][79][80]. The mixing of MO
sites, respectively [87,88,89,90]. The mixing of MO
6
and MO
5
creates a tunnel structure that allows Na
+
ions to easily diffuse along the tunnels. This structure was first discovered by Parant et al. (1971) for Na
x
MnO
2 (x < 1) [81]. It is worth noting that this structure was simply synthesized using various approaches, such as sol–gel, hydrothermal, spray pyrolysis, and microwave-assisted methods [82][83][84][85]. Na
(x < 1) [91]. It is worth noting that this structure was simply synthesized using various approaches, such as sol–gel, hydrothermal, spray pyrolysis, and microwave-assisted methods [92,93,94,95]. Na
0.44
MnO
2
is the most noteworthy tunnel oxide owing to its large tunnels, high theoretical capacity of approximately 121 mAh g
−1, and high stability [86][87]. He et al. used a polymer-pyrolysis method to fabricate Na
, and high stability [96,97]. He et al. used a polymer-pyrolysis method to fabricate Na
0.44
MnO
2
nanoplates, which exhibited an outstanding capacity of approximately 96 mAh g
−1 at a rate of 10 C [88]. However, the capacity of this material could not be improved owing to the fully charged and discharged states of the Na
at a rate of 10 C [98]. However, the capacity of this material could not be improved owing to the fully charged and discharged states of the Na
0.22
MnO
2
and Na
0.66
MnO
2 phases, respectively [86]. Therefore, methods were developed to solve this problem, including cation/anion substitution and surface coating. In cation substitution, Mn
phases, respectively [96]. Therefore, methods were developed to solve this problem, including cation/anion substitution and surface coating. In cation substitution, Mn
4+ can be replaced by Ti, Fe, or Zr or by the partial replacement of Na with Li ions [89][90]. Shi et al. doped Zr ions in Na
can be replaced by Ti, Fe, or Zr or by the partial replacement of Na with Li ions [99,100]. Shi et al. doped Zr ions in Na
0.44
MNO
2
as a high-performance SIB cathode, which exhibited a high capacity of approximately 117 mAh g
−1
; at a high rate of 5 C, the capacity was reversible at approximately 97 mAh g
−1
.

2.1.3. Polyanionic Compounds

Phosphate-Based Compound

Polyanionic compounds are generally constructed by a tetrahedral XO4 group with Na and Me (Fe, V, Co, or Mn) or MeOx [91][92][93][94][105,106,107,108]. Basically, olivine NaFePO4 consists of tetrahedral PO4 and octahedral FeO6 sites, forming a framework that holds Na ions in the lattice or allows the diffusion of Na ions [95][109]. NaFePO4 is a cost-effective material owing to its abundance of elements and high theoretical capacity of approximately 154 mAh g−1. NaFePO4 exists in two phases: maricite and olivine. The maricite phase is a stable structure with cavities that trap Na ions, preventing their diffusion [91][96][105,110]. Meanwhile, the less stable olivine phase has a one-dimensional channel, allowing the diffusion of Na ions through this pathway. Therefore, the olivine phase is more attractive, and improving the stability of this structure with various types of doping has also been investigated [56][58]. Wang et al. used the DFT simulation method to predict the effect of doping Li into NaFePO4 in both maricite and olivine phases [96][110]. The results demonstrated that when the Li:Na ratio was above 25%, the olivine phase was more stable than maricite, whereas the presence of Li destabilized the maricite structure.

NASICON

A Na super-ionic conductor (NASICON) can be used as an electrolyte and electrode material owing to its 3D-open framework of NaxM2(PO4)3 (M = V, Fe, Ti, Nb, Zr) [97][115]. NASICON comprises MO6 and PO4 polyhedral sites in a framework that creates large channels for Na diffusion. This structure was first proposed by Hong and Goodenough in a Na1+xZr2P3-xSixO12 compound (P can be replaced by Si, S, Mo, and As) [98][99][116,117]. Owing to its high stability, high Na conductivity, and wide electrochemical windows (1.85–4.9 V vs. Na/Na+), NASICON is also applied as a solid electrolyte in SIBs [100][118]. The ion exchange of Zr4+ with Li+, K+, and Ag+ was first performed, while Si4+ was stabilized in the structure. As a complete NASICON with three full Na ions, Na3V2(PO4)3 (NVP) quickly received significant attention as a promising candidate material for providing a high probability of sodium insertion and desertion [101][102][119,120]. NVP has a theoretical capacity of ~117.6 mAh g−1 and a high redox voltage range of 3.3–3.4 V [103][121]. Therefore, with the modification process including the addition of conductive carbonaceous materials, NVP conductivity can be enhanced, exhibiting a notable rate performance [104][122].

2.1.4. Pyrophosphates

Pyrophosphate NaxMP2O7 consists of MO6 (M = V, Fe, Mn, Co, Ni) sites and a P2O7 group (interconnected PO4–PO4) that forms a framework with Na ions [105][106][107][108][109][110][130,131,132,133,134,135]. This framework allows the diffusion of Na ions; therefore, it is also a stable cathode material for SIBs. Barpanda et al. revealed that Na2FeP2O7 was constructed by corner-sharing FeO6–FeO6 to form Fe2O11, which combines with the P2O7 group to form a triclinic structure [111][136]. After calcination at temperatures above 560 °C, the triclinic Na2PeP2O7 transformed into a monoclinic phase, which improved the stability of this material during cycling. Kim et al. used the defect engineering of Na in Na2CoP2O7 to produce a high-voltage cathode for SIBs [112][137]. The deficiency of the Na-stabilized structure of Na2-xCoP2O7 (x > 0.2) was also found in Fe, Ni, and Mg pyrophosphates, such as Na1.66Fe1.17P2O7, Na1.82Ni1.09P2O7, and Na1.82Mg1.09P2O7 [113][114][115][116][138,139,140,141]. Specifically, Na2-xCo2P2O7 (x > 0.2) achieved a high average voltage of approximately 4.3 V versus Na/Na+ with a specific capacity of approximately 80 mAh g−1. Owing to the similar roles of the V, Fe, Mn, Co, and Ni transition metals in the structure, the replacement of a cheaper metal such as Fe and the improvement of the voltage by using Co and Ni in other pyrophosphate materials were investigated.

2.1.5. Silicates

Silicate compounds, such as lithium orthosilicate Li2FeSiO4 with a theoretical capacity of approximately 300 mAh g−1, generally have a higher theoretical capacity than other polyanions owing to their low molecular weight [117][146]. Similar to Li2FeSiO4, the sodium silicate Na2MSiO4 compound consists of MO4 (M = Fe, Ni, Mn, Co) and SiO4 sites, forming a framework that allows the diffusion of Na ions [118][119][120][147,148,149]. Silicates were previously popular in the glass industry owing to their high thermal and physical stabilities [121][150]. Co/Fe-compound sodium silicates were predicted to exhibit anti-site-exchange behavior, promising to be stable electrode materials [122][123][151,152]. Na2FeSiO4 is the most promising silicate compound, having a high theoretical capacity of approximately 276 mAh g−1 [124][153].

2.2. Organic Compounds

The development of flexible devices and environmentally friendly materials has encouraged the application of organic compounds as cathode materials in energy storage systems, such as LIBs and SIBs [125][158]. Ranging from small molecules to high-molecular polymers, organic materials are promising for applications in green renewable energy in the future. For example, the molecular structure of Na4C8H2O6 (2,5-dihydroxyterephthalic acid, NaDTA) was investigated as a SIB cathode material at working potential windows of approximately 1.6–2.8 V versus Na/Na+ and delivered a high capacity of approximately 180 mAh g−1 [126][159]. NaDTA can also be used as an anode material with a capacity greater than 200 mAh g−1 owing to it binding up to six Na ions [127][160]. Kim et al. demonstrated the use of C6Cl4O2 (tetrachloro-1,4-benzoquinone) in a porous carbon template as a cathode of SIBs. The carbon skeleton-supported C6Cl4O2 cathode exhibited a high initial capacity of approximately 160 mAh g−1 and an average voltage of approximately 2.72 V. Wang et al. produced a polymer from perylene 3,4,9,10-tetracarboxylic dianhydride, pyromellitic dianhydride (PMDA), and 1,4,5,8-naphthalenetetracarboxylic dianhydride, which contained C=O bindings, providing interactions with Na+ ions as a cathode for SIBs. This polymer demonstrated a high reversible capacity of approximately 150 mAh g−1 at a working voltage of 1.5–3.5 V and a long lifetime of over 5000 cycles, retaining 87.5% of the capacity in comparison to the initial cycle.

2.3. Metal–Organic Compounds: Prussian Blue Analogs

The combination of inorganic and organic structures has received considerable attention owing to the advantages of both material types [128][166]. Inorganic materials have a stable structure and high conductivity, whereas organic materials are eco-friendly, easy to process, and safe to use. Recently, the development of organometallic materials in framework structures has introduced an advanced technique for material design, enabling the discovery of new composite properties for metals and organics. Metal–organic frameworks (MOF) can form a tremendous structure from various metal–organic compounds, providing large channels that allow the capture of ions or molecules; therefore, they have been used in various applications, including drug delivery, catalysis, and energy storage [129][130][131][167,168,169]. Simple and famous MOFs used for energy storage are Prussian blue analogs (PBAs), which are alkaline metal ferrocyanides AxMFe(CN)6 (A = Na, K; M = Fe, Mn, Co, Ni, Cu) [132][170]. The CN, Fe, and M matrices create a cage-like structure, holding the Na and K ions. PBAs generally exhibit a face-centered cubic structure (Fm3-m) [133][134][135][171,172,173]. The performance of PBAs in SIBs is based on the redox reactions of Fe2+/Fe3+ and the metal M, believed to have a high theoretical capacity of approximately 170 mAh g−1 for SIBs [136][174]. The basic PBA, which is Na4Fe(CN)6, contains the highest number of Na ions; however, it is a soluble compound that is easily degraded during cycling [137][138][175,176]. Therefore, Yang et al. demonstrated a solid solution of Na4Fe(CN)6/NaCl in a SIB that exhibited a capacity of approximately 75 mAh g−1 [139][177].

3. Summary

LIBs have become popular in portable devices, vehicles, and energy storage systems for renewable energy. Owing to the abundance of Na, SIBs are believed to be an ideal replacement for LIBs. As shown in Figure 29, each type of cathode material has its advantages and disadvantages. For instance, layered metal oxides have a high capacity and low cost but are sensitive to moisture and structural degradation. Prussian blue is more stable but the effect of water molecules in the structure affects its performance. Organic cathode materials have a good flexibility and stable redox potential but their lower conductivity, thermal stability, and dissolvability in the electrolyte should be resolved. Therefore, the advantages and disadvantages of each practical condition should be carefully considered. To improve their performance, the approach methods were also varied for each type of material. Due to an instability in structure of layered metal oxide cathodes, they were fast degraded during cycling. To stabilize structural stability, inactive metals such as V, Mg, Zn, and Ca can be doped to the lattice, or anions like F can be added [140][141][197,198]. Considering a tunnel metal oxide, control of the tunnel size optimizes its capacity. Meanwhile, for polyanionic compounds such as NASICON or other phosphate-based compounds, defect engineering can be considered, including metal- and F-doping methods [142][199]. Silicate compounds are low-cost and eco-friendly metal sources, and their high capacity needs to improve the structural stability before commercialization [143][154]. The surfaces of inorganic compounds can be passivated using a carbon-coating method that not only enhances their conductivity but also protects against the effects of humidity or expansion during the insertion of Na ions. The stability of Prussian blue and other organometallic compounds can be enhanced by using a host material such as Ni foam or a porous carbon skeleton [144][200]. Organic materials can be designed to have a good structure to enhance capacity and conductivity but they remain in the activation group with C=O, C=C, or C=N. Sulfurization and other cross-linking methods can also be considered to yield better combinations [145][201]. In addition, the use of additives in the electrolyte is another approach to enhance stability, in which the solid electrolyte interface from cycling can be used as a protective layer [146][202]. Along with the development of electrode materials and electrolytes, SIBs have been commercialized with layered oxides, polyanions, and Prussian blue types [32]. These materials are simple to manufacture (hydrothermal, co-precipitation method, etc.) and inexpensive, and they mainly use Mn and Fe metals and add Ni, Zn, or Mg, to increase stability, and conductive carbon is introduced for air stability and structural protection. Organic materials with low thermal stability and conductivity are utilized for some specific purposes that require biocompatible and/or specified applications. Therefore, it is considered that most of the developed materials have the potential to be commercialized if SIBs can solve current issues such as cost-effectiveness, high capacity, high stability, and high rate performance.
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Figure 29. Comparison of SIB cathode materials’ (a) specific capacity and working potential; (b) specific capacity, cost-effectiveness, potential, stability, and safety issues.
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