Lithium metal batteries have achieved large-scale application, but still have limitations such as poor safety performance and high cost, and limited lithium resources limit the production of lithium batteries. The construction of these devices is also hampered by limited lithium supplies. Therefore, it is particularly important to find alternative metals for lithium replacement. Sodium has the properties of rich in content, low cost and ability to provide high voltage, which makes it an ideal substitute for lithium. Sulfur-based materials have attributes of high energy density, high theoretical specific capacity and are easily oxidized. They may be used as cathodes matched with sodium anodes to form a sodium-sulfur battery. Traditional sodium-sulfur batteries are used at a temperature of about 300 °C. In order to solve problems associated with flammability, explosiveness and energy loss caused by high-temperature use conditions, most research is now focused on the development of room temperature sodium-sulfur batteries. Regardless of safety performance or energy storage performance, room temperature sodium-sulfur batteries have great potential as next-generation secondary batteries. This article summarizes the working principle and existing problems for room temperature sodium-sulfur battery, and summarizes the methods necessary to solve key scientific problems to improve the comprehensive energy storage performance of sodium-sulfur battery from four aspects: cathode, anode, electrolyte and separator.
With the development of society and the depletion of natural resources, people have to start using renewable energy to develop low-cost and high-efficiency energy storage devices, such as secondary batteries. The ideal performance characteristics of energy storage devices are high energy density, high power density, long cycle life, low cost and high safety [1]. Among the existing secondary batteries, lithium-ion batteries have been industrialized, but their high cost, low practical energy density (100–200 Wh kg−1) and poor safety performance limit their application [2][3]. In order to meet the energy storage needs of current society, it is necessary to design and develop other batteries with lower cost, longer cycle life and higher energy density and power density.
Sodium is a low-cost alternative to lithium. The content of sodium in the Earth’s crust and water is 28,400 mg kg−1 and 1000 mg L−1, respectively, which far exceeds the content of lithium. The electrochemical reduction potential of sodium is −2.71 V, which is slightly higher than that of lithium (−3.02 V) [4], and is similar to the standard hydrogen electrode (SHE) potential [5]. When sodium is used as the anode, it can provide a battery voltage greater than 2 V when matched with an appropriate cathode. The high content, low cost and ability to provide high voltage make sodium an ideal choice for the anode materials of high-energy secondary batteries [6]. Sulfur has the advantages of strong oxidizing property, mature treatment technology, low cost, ready use [7], no toxicity and high capacity (when each atom transfers two electrons [8], the capacity of sulfur is 1.675 mAh g−1) [9], etc. Sulfur has an attractive advantage over lithium as a battery cathode. Compared with lithium-sulfur batteries, sodium-sulfur batteries are a better choice from the perspective of sustainable development and economy, or from the perspective of battery preset performance [10].
The earliest sodium-sulfur battery was constructed in the laboratory of Ford Motor Company, and Kummer and Weber confirmed its feasibility [11]. The battery uses sodium and sulfur as the active materials for the cathodes and anodes, and β-Al2O3 ceramics are used as both the electrolyte and the separator. In order to reduce the transmission resistance of sodium ions in the alumina solid electrolyte, it is necessary to ensure that the electrode material is in a molten state, so the working temperature is set at 250–300 °C. Due to the advantages of long service life, high charging efficiency and high energy density, high-temperature sodium-sulfur battery systems have been used in stationary energy storage systems [12]. However, in order to maintain the molten conductive state of the two poles, a high operating temperature is required. The high operating temperature not only causes a loss of electrical energy, but also may cause the failure of the solid electrolyte, which causes explosions and fires due to contact between the cathode and the anode. These problems limit the wide application of high-temperature sodium–sulfur batteries [13].
In order to obviate the above problems, research has been directed toward the development of room temperature sodium-sulfur batteries. The first room temperature sodium-sulfur battery developed showed a high initial discharge capacity of 489 mAh g−1 and two voltage platforms of 2.28 V and 1.28 V [14]. The sodium-sulfur battery has a theoretical specific energy of 954 Wh kg−1 at room temperature, which is much higher than that of a high-temperature sodium–sulfur battery. Although room temperature sodium-sulfur batteries solve the problems of explosion, energy consumption and corrosion of high-temperature sodium-sulfur batteries, their cycle life is much shorter than that associated with high-temperature sodium-sulfur batteries. For a wider range of applications, its cycle performance needs to be improved [13].
Room temperature sodium-sulfur batteries have the advantages of high safety performance, low cost, abundant resource and high energy density [15][16]. They not only solve the safety problem of high-temperature sodium-sulfur batteries, but also solve the problem of high cost of lithium-ion batteries, and have received widespread attention. Like the lithium-sulfur battery system, room temperature sodium-sulfur batteries also face many problems, such as:
(1) Low conductivity of sulfur (5 × 10−30 S·cm−1) and significant volume expansion (180%) [17]; (2) capacity attenuation caused by the dissolution of intermediate polysulfide in the electrolyte; (3) short circuit caused by sodium dendrites piercing the separator; (4) low utilization rate of the cathode; (5) poor reversibility, etc. [1]. This article will start with a description of the electrochemical reaction mechanism for the room temperature sodium-sulfur battery, and describe the development of room temperature sodium-sulfur battery in recent years in terms of its cathode, electrolyte, separator design and anode protection.
Compared with high-temperature sodium-sulfur batteries, room-temperature sodium-sulfur batteries have a higher capacity. However, most reported room-temperature sodium-sulfur batteries still fail to reach one third of the theoretical capacity of sulfur [18][19][20]. This may be due to the following theoretical and technological issues:
(1)Sulfur and sulfide have poor conductivity. During battery cycling, electronic conductivity is very important to the electrodes [21]. However, sulfur (5 × 10−30 S·cm−1) and its final recrystallized Na2S are both semiconductors, lacking inherent high electronic conductivity [22];
(2)The volume expansion of sulfur causes serious changes in structure and morphology [23]. During battery discharge, the volume expansion/contraction of sulfur is a key factor in determining battery capacity. When Na2S3 is generated during the discharge process, the volume expansion rate of the sulfur cathode is 36%, for Na2S2, the volume expansion rate is 67% and reaches 157% after Na2S is completely generated [24];
(3)Soluble polysulfide diffuses from cathode the to the anode [6][25]. The inevitable dissolution of Na2Sx (4 ≤ x ≤ 8) leads to a serious shuttle effect between the cathodes and anode, resulting in poor battery cycle stability and high self-discharge rate;
(4)The formation of needle-like sodium dendrites and deposits [15]. Due to the large difference in size between sodium atoms and sodium ions, sodium easily forms unstable electrodeposited layers and dendrites. Once the dendrites reach a certain length, short circuits will occur [1][2];
(5)Due to the large radius of Na+, the reaction activity between S and Na is slow, so the conversion of S to Na2S is incomplete, resulting in low sulfur utilization [26][27][28];
(6)Impedance increases caused by irreversible side reactions [5][29].
Room temperature sodium–sulfur batteries face safety problems caused by the anode sodium dendrites, the insulation problem of the cathode sulfur, the shuttle effect of the intermediate product polysulfide and the loss of active materials caused by its dissolution. Starting from its cathode, electrolyte, anode and separator (Table 1 shows part of the research progress of room temperature sodium–sulfur batteries in the past six years), many methods have been developed to solve these problems:
Table 1. The main research progress of room temperature sodium–sulfur batteries in the past six years.
Publish Date | Reference | Anode | Cathode | Electrolyte | Separator | 1st dis. Capacity/mAh/g (C-Rate) | 10th dis. Capacity/mAh/g (C-Rate) |
---|---|---|---|---|---|---|---|
2013 | Lee et al. [30] | Na-Sn-C comp. | 60 wt % S/Hollow C comp. (viz. 56 wt % S) 20 wt % CB 20 wt % PEO |
NaCF3SO3 In TEGDME (4:1 mol %) |
- | 1200 | 600 |
Hwang et al. [23] | Na | 70 wt % S/C–PAN comp. (viz. 32 wt % S) 15 wt % CB 15 wt % PVDF |
0.8 M NaClO4 in EC:DMC (1:1) | - | 364 | - | |
2013 | Xin et al. [31] | Na | 80 wt % S/(CNT@MPC) comp. (viz. 32 wt % S) 10 wt % CB 10 wt % PVDF |
1 M NaClO4 in PC:EC (1:1 v/v) | - | 1610 | 1100 |
2014 | Bauer et al. [32] | Na | 42.5 wt % S 42.5 wt % C 12 wt % PVDF 3 wt % PTFE (dry) |
1 M NaClO4 in TEGDME | Nafion coating on PP separator | 400 | 370 |
Zheng et al. [33] | Na | 80 wt % HSMC–Cu–S comp. (viz. 50 wt % S) 10 wt % CB 10 wt % CMC (in H2O) |
1 M NaClO4 in EC/DMC (1:1) | - | 1000 | 690 | |
Yu et al. [34] | Na | 60 wt % S 30 wt % CB 10 wt % PVDF |
1.5 M NaClO4 0.3 M NaNO3 in TEGDME | - | 900 | 600 | |
Yu et al. [35] | Na | MWCNT/Na2S6 | 1.5 M NaClO4 0.3 M NaNO3 in TEGDME | - | 945 | 535 | |
Nagata et al. [36] | Na-Sn (15:4.9 mol/mol) | 50 wt % S 40 wt % SE 10 wt % AC |
P2S5 (viz. 71 wt % S) | - | 1456 | - | |
NaPS3 (viz.75 wt % S) | 1522 | ||||||
Na3PS4 (viz.80 wt % S) | 1100 | ||||||
2015 | Kim et al. [37] | Na | 60 wt % S/C comp. (viz. 55 wt % S) 20 wt % PVDF 20 wt % Super-P |
β-Al2O3 (SE) | 855 | 674 | |
1M NaCF3SO3 in TEGDME (L) | PP (porous) | 350 | - | ||||
Kim et al. [38] | Na | SPAN comp. (viz. 41 wt % S) | 1M NaPF6 in EC/DEC (1:1 v/v) | GF | 342 | 260 | |
2016 | Wei et al. [25] | Na | MCPS1 (viz. 47 wt % S) | 1 M NaClO4 5 v% SiO2–IL–ClO4 in EC/PC |
GF | 1459 | 762 |
Fan et al. [39] | Na | 70 wt % CSCM comp. (viz.18 wt % S 1 g BDTD) 20 wt % AB 10 wt % CMC in C2H5OH/H2O (1:2.5 w/w) |
1 M NaClO4 in EC/DMC (6:4 v/v) | - | 1000 | 962 | |
Wang et al. [40] | Na | S/iMCHS comp. (viz.46 wt % S) | 1.0 M NaClO4 5 wt % FEC in PC/EC (1:1 v/v) | - | 1213 | 430 | |
Qiang et al. [41] | Na | N,S-HPC/S comp. (viz. 22 wt % S) | 1M NaClO4 in EC/PC (1:1 v/v) | GF/B | 400 | 399 | |
Yu et al. [42] | Na | Na2S/AC-CNF comp. (viz. 66 wt % Na2S) | 1.5 M NaClO4 0.2 M NaNO3 in TEGDME |
Na-Nafion membrane | 563 | 658 | |
2017 | Yue et al. [43] | Na-Sn-C | Na3PS4-Na2S-C (2:1:1 w/w) comp. | Na3PS(SE) | - | 869 | 704 |
2018 | Ye et al. [44] | Na | 70 wt % N/S-OMC-5 comp. (viz.20.32 at% N 0.82 at% S) 20 wt % Super-P 10 wt % PVDF |
1 M NaClO4 5 wt % FEC in EC/PC (1:1 v/v) | - | 1742 | 419 |
Lee et al. [45] | Na | S/C-PAA 9:1 | 1M NaClO4 in PC | GF | 623 | 558 | |
1M NaClO4 in PC/FEC | |||||||
Xu et al. [46] | Na | 80 wt % S@MPCF (6:4 w/w) 10 wt % Super-P 10 wt % CMCNa |
2M NaTFSI in PC/FEC (1:1 v/v) with 10 mM InI3 | GF/A | 1544 | 1032 | |
2019 | Zhang et al. [47] | Na | 70 wt % S@Fe-HC 10 wt % CB 20 wt % CMC |
1M NaClO4 in PC/EC (1:1 v/v) with 5 wt % FEC | GF | 945 | 630 |
Li et al. [48] | Na | 70 wt % Te0.04S0.96@pPAN 20 wt % SuperP 5 wt % SBR 5 wt % NaCMC |
1 M NaClO4 in EC/DMC (1:1 v/v) | - | 1816 | 1015 | |
1M NaClO4 in DOL/DME (1:1 v/v) with 10% FEC) | 1682 | 868 | |||||
Zhu et al. [49] | Na | 60 wt % S/CPAN comp. 40 wt % SE |
PEO−NaFSI−1% TiO2 comp. | - | 300 | 253 | |
Yan et al. [50] | Na | NiS2@NPCTs/S | - | - | 957 | 508 | |
2020 | Ma et al. [26] | Na | S@Co/C/rGO | - | - | 490 | 367 |
Aslam et al. [51] | Na | S@BPCS (hollow polar bipyramid prism catalytic CoS2/C as a sulfur carrier) |
- | GF | 1347 | 787 | |
Guo et al. [52] | Na | ACC-40S (the carbon–sulfur composite electrode with 40 wt % sulfur loading) | - | GF | 1492 | 1200 | |
Du et al. [16] | Na | 80 wt % rGO/VO2/S comp. 10 wt % AB 10 wt % PVDF |
1 M NaClO4 in TEGDME. | GF | 526.2 | 346 |
This article, the working principle of room temperature sodium–sulfur battery, the existing challenges and the research results of its cathode, anode, separator and electrolyte to cope with these problems are stated.
Cathode research mainly focuses on improving the conductivity of sulfur, effective sulfur fixation and sodium inhibiting dendrites. Although various carbon-based mesoporous cathode bodies can increase the sulfur utilization rate and thereby increase the initial capacity, the cycle stability is not ideal, especially for long-term cycles. Therefore, in future research, carbon-based materials integrated with metal compounds, such as MOF, metal nitrides and metal oxides, can be further studied to eliminate unnecessary capacity degradation. At the same time, the sulfur equivalent cathode material is also a good choice for sodium–sulfur batteries. In addition, an independent binder-free cathode is also a good research idea. After removing the influence of the binder on the conductivity of the cathode material, the conductivity of the cathode will be improved.
Electrolyte is a key factor affecting the performance of sodium–sulfur batteries. The performance of carbonate-based electrolytes shows a relatively high capacity and has a different voltage curve without a different platform. The combination of an ionic liquid-based electrolyte and carbonate-based electrolyte has been proved to be a better choice. Combining ionic liquids and gels can obtain electrolytes with high conductivity, independent mechanical properties and dimensional stability, which may replace carbonate electrolytes.
The membrane with ion selectivity and physical adsorption will have a good inhibitory effect on the shuttle of polysulfides. Ordinary diaphragms cannot meet these requirements. The surface of the diaphragm must be modified to allow only Na+ to pass through. The modified diaphragm has an excellent capacity retention rate. It is also possible to add an intercalation layer between the diaphragm and the cathode to reduce electrical resistance while suppressing sulfur shuttle. Due to its nanosized frame and pores, the carbon nanofoam sandwich cannot only achieve the above-mentioned purpose, but also can digest volume expansion.
Since sodium dendrites will be generated on the surface of the sodium anode, which will cause a short circuit in the battery, sodium surface protection is necessary. Depositing Al2O3 layer atomic layer or molecular layer on sodium effectively inhibits the formation of sodium dendrites and prolongs the service life of sodium. Among them, MLD Al2O3 coating has better performance. In the future, MLD can be considered as an anode coating technology. In addition, other substances can be combined with sodium to form a composite anode to improve battery performance. The Na2S produced by sodium and some metal sulfides cannot only be used as an artificial SEI membrane to inhibit the growth of sodium dendrites, it can also be used as a 3D host of sodium, digesting the volume changes of sodium and greatly improving the stability of the sodium anode. In addition, it is also an effective idea for composite anodes to manufacture layered nanoboxes and combine the advantages of various hybrid configurations to improve sodium storage performance.