Sodium sulfur battery is favored due to their high energy density, abundant resources, and low price, which are expected to be widely used in large-scale energy storage, power batteries, and other fields. Among them, sodium sulfide, the final discharge product of room temperature sodium sulfur battery, can be used as a positive electrode material, which not only has a high theoretical specific capacity (686 mAh/g), but also can be matched with non-sodium metal negative electrodes (such as hard carbon, tin metal) to avoid direct use.
The safety hazards and other advantages brought by sodium metal anodes have gradually become a research hotspot. However, the disadvantages of sodium sulfide cathode materials, such as low intrinsic conductivity, poor reactivity, and poor reversible cycle with polysulfides, limit its practical specific battery capacity and cycle life.
Characteristics of sodium sulfur battery
Sodium sulfur battery have attracted widespread attention because of their high theoretical energy density (1274 Wh/kg, and the final charge product is sodium sulfide Na2S), high abundance of positive (sulfur) and negative (sodium) pole elements in nature, and low price. For example, the price of metal sodium is about 2100 US dollars / ton, nearly 10 times cheaper than the price of lithium (lithium is about 25000 US dollars / ton), while the price of sulfur is lower, about 20 US dollars / ton, and the theoretical value of sulfur The specific capacity is 1672 mAh/g.
At present, the working temperature of commercial sodium sulfur battery is 300-350 ℃, and it uses sodium ion-conducting β"-Al2O3 (ionic conductivity is about 0.2 S/cm) as a solid electrolyte. At this time, both the negative electrode sodium and the positive electrode sulfur are in liquid state, and the basic reaction of the battery is: 2Na+xS picture Na2Sx (x=3～5). The final discharge product of high-temperature sodium sulfur battery is Na2S3, and its theoretical energy density is 760 Wh/kg.
However, the key technologies such as sealing materials and corrosion-resistant shells that high-temperature sodium sulfur battery rely on are monopolized by a few companies such as Japan's NGK company and the US GE company. The positive electrode of the room temperature sodium sulfur battery is also made of sulfur material, and the negative electrode is sodium metal.
However, at room temperature, the final discharge product is Na2S, so it has a higher theoretical energy density (1274 Wh/kg) than high-temperature sodium sulfur battery. And its operating environment at room temperature does not require an additional incubator, which not only reduces costs, but also avoids potential safety hazards caused by high temperatures.
Besides, room temperature sodium sulfur battery also face many challenges: for example, the final discharge product of the sulfur positive electrode generates Na2S, which expands by about 160% in volume, which is likely to cause the electrode material to fall off. The intermediate product polysulfide will dissolve in the electrolyte, and it will be irreversible when it is shuttled to the negative electrode.
The side reaction caused by the rapid decay of capacity. and the sodium dendrites generated by the sodium metal negative electrode during the cycle will pierce the separator and cause a short circuit. Therefore, the development of stable and safe electrode materials is crucial for room temperature sodium sulfur battery.
Working principle of Na2S cathode material
Based on this, using Na2S, the final discharge product of room temperature sodium sulfur battery, as the positive electrode can not only eliminate the volume expansion problem of the sulfur positive electrode, but also provide a sodium source to pair with other safe negative electrodes (such as hard carbon, tin metal, etc.), Avoid the potential safety hazards caused by the direct use of sodium metal anodes.
Therefore, the full-cell reactions of Na2S positive electrode-Sn negative electrode can be expressed as: Due to the larger particle size (about 1 mm) of the commercial Na2S cathode, the time required to complete the first charge is about twice that of the second cycle. At the same time, the Na2S positive electrode also needs to overcome a large overpotential during the first cycle of charging.
This is mainly due to the large particle size of commercial Na2S, which has a relatively large nucleation energy when it is sodiumized into polysulfides. After the initial post-barrier nucleation, highly localized and viscous sodium polysulfides are formed around unreacted Na2S, which reduces the diffusion rate of sodium ions. This kinetic barrier also explains the first long charging time.
Research status of Na2S cathode materials
The research on Na2S as the positive electrode of room temperature sodium sulfur battery is in its infancy. The key scientific issues such as the wide band gap (2.44 eV) of the semiconductor's energy band structure, poor conductivity, and slow conversion kinetics between Na2S and polysulfides need to be resolved urgently.
The current strategy to improve the conductivity of Na2S is to increase the interfacial charge transfer rate of Na2S by compounding with conductive substrate materials (such as carbon materials), improving the morphology and structure of Na2S, using catalysts to improve its reversible cycle, and designing battery structures. Improve its conductivity, promote the rapid conduction of electrons and shorten the diffusion path of sodium ions, and obtain high-performance sodium sulfur battery.
Na2S or C composite material
Carbon-based materials are often used as conductive substrate materials and composited with Na2S. Using the excellent electrical conductivity of carbon-based materials to increase the interface charge transfer rate can effectively improve the overall conductivity of Na2S/C composites. Scientists reported in 2015 that commercial Na2S and multi-walled carbon nanotubes (MWCNTs) with a diameter of 10–30 nm and a length of 100–150 mm were dissolved in tetraethylene glycol dimethyl ether (TEGDME), and then injected into on the self-woven, binder-free MWCNT fabric electrode, the as-prepared Na2S/MWCNT was used as the cathode material for room temperature sodium sulfur battery.
Although commercial Na2S can be directly used as the positive electrode material of room temperature sodium sulfur battery, it still needs to overcome a large overpotential during the first cycle of charging, which is mainly due to the large particle size (micron level) of commercial Na2S, which is difficult to convert into polysulfides. When its nucleation energy is relatively large, and the effect of direct compounding of commercial Na2S and carbon is limited. Therefore, the in-situ preparation of composite materials of Na2S and conductive materials will help to further improve the conductivity of the Na2S composite cathode, and at the same time reduce the overpotential during the first cycle of charging.
Na2S morphology control
Regulating the morphology and structure of Na2S can also promote the diffusion of sodium ions and the rapid conduction of electrons, thereby improving the reversible capacity and cycle life of room temperature sodium sulfur battery. Therefore, researchers control the morphology, size and other parameters of Na2S cathode materials through various strategies to improve the electrochemical performance of Na2S cathode materials, and then realize safe and high-performance room temperature sodium sulfur battery.
The large charge overpotential of commercial Na2S during the first cycle of charging is mainly due to its large particle size. When it is transformed into polysulfides, its nucleation energy is large. Therefore, reducing the particle size of Na2S can reduce its activation energy barrier. , and then realize the improvement of the reversible capacity of the sodium sulfur battery. When the current density is 50 mA/g, the reversible capacity of the Na2S/Na3PS4/C composite all-solid-state sodium-sulfur battery at 60 ℃ is as high as 800 mAh/g, and the capacity can be maintained at 650 mAh/g after 50 cycles.
Catalytic reversible cycle of Na2S
The low intrinsic reactivity of Na2S leads to slow conversion kinetics with polysulfides, which in turn aggravates the shuttling effect of polysulfides. Therefore, in the design of cathode materials, catalysts are introduced to improve the reactivity of Na2S, and the use of high-efficiency catalysts to catalyze the reversible cycle of Na2S and polysulfides can realize high-efficiency reversible room temperature sodium sulfur battery.
Effect of nitrides on sodium sulfur batteryIn addition to metal and metal sulfide catalysts, nitrides can also be used as electrocatalysts to promote the electrodeposition of Na2S, thereby significantly improving the efficiency of room temperature sodium sulfur battery. Using in situ synchrotron radiation XRD, it was found that Mo5N6 can improve the conversion kinetics of Na2S2 Na2S. At the same time, the deposition experiment of Na2S also showed that compared with MoN and Mo2N, Mo5N6 can significantly increase the current and capacity of Na2S electrodeposition.
Structural design of sodium-sulfur battery
In addition to the design of electrode materials, researchers also optimized the battery structure to increase the diffusion rate of sodium ions and the shuttle effect of polysulfides, thereby improving the electrochemical activity of Na2S cathode materials. Using Na2S cathode material can be developed into a sodium-metal-free room temperature sodium sulfur battery.
Compared with the traditional electrolyte-diaphragm sodium sulfur battery, the Na2S cathode material of the outer membrane electrode exhibits excellent room temperature sodium sulfur battery performance, and its specific capacity is about 800 mAh/g at a current density of C/10. After 100 cycles, its specific capacity remained at 600 mAh/g.
The challenges and future development direction of Na2S
The biggest advantage of the room temperature sodium sulfur battery constructed with Na2S cathode lies in the high theoretical energy density, abundant resources and low price of the Na2S cathode material. Therefore, in order to realize its large-scale application, it is necessary to maximize its advantages.
Design and preparation of high performance Na2S cathode material
The Na2S cathode material can be paired with other non-sodium metal anodes, such as hard carbon, Sn and its alloys, to realize safe and high energy density room temperature sodium sulfur battery. However, Na2S cathode materials also face key scientific problems such as high activation energy barrier for the first charge, poor intrinsic conductivity, poor reactivity, and poor reversible cycle with polysulfides.
Therefore, how to improve the electrochemical performance of Na2S cathode materials is an important development direction in the future. The introduction of electrocatalysts into the design of Na2S cathodes is expected not only to improve the intrinsic conductivity and reactivity of Na2S cathode materials, but also to improve the conversion kinetics of Na2S and polysulfides, thereby achieving high-performance room-temperature sodium sulfur battery performance.
Design optimization of electrolyte and electrolyte or electrode surface interface
A suitable electrolyte is crucial for a room-temperature sodium sulfur battery with high energy density and long cycle life. And also, electrolyte is significant for lithium ion battery and lithium ion battery electrolyte is different from that of sodium sulfur battery. Therefore, the development and design of a suitable electrolyte can not only ensure the high performance of the Na2S cathode material, but also optimize the electrolyte/electrode surface interface, Promote the mass transfer rate at the surface interface.
At present, there are two main types of electrolytes for room temperature sodium sulfur battery: ether electrolytes and carbonate electrolytes. Ether electrolytes usually have high polysulfide solubility, which can promote the conversion efficiency of polysulfide and Na2S, but its high polysulfide solubility will intensify the shuttle effect.
Mechanism exploration of Na2S cathode materialAt present, researchers have a certain understanding of the sodium storage mechanism of Na2S cathode materials, but have not used in-situ techniques to directly observe its charge-discharge process. Therefore, in situ characterization techniques can be used to explore the reaction mechanism of Na2S cathode materials in future research.
In view of the current development status of Na2S cathode materials, the key development directions in the future include: first, continue to explore new high-performance and stable Na2S cathode materials to achieve high energy density and long cycle life. Second, develop matching electrolyte systems, and deeply explore the film-forming mechanism of SEI and CEI at the interface between the electrode and the electrolyte, and deepen the understanding of the surface interface microenvironment on the capacity decay mechanism of sodium sulfur battery.
The third is to deeply understand the sodium storage mechanism of Na2S cathode materials, especially the use of advanced Combining in-situ characterization technology with theoretical calculations, the electrode sodium storage mechanism, charge transfer process and strengthening mechanism are explored, providing scientific and theoretical guidance for the structural design of high-performance sodium sulfur battery electrode materials.