1 Introduction
Environmental pollution and resource scarcity are becoming more and more serious, which makes scientists all over the world realize the importance of developing green and clean energy, and promotes the development of a series of renewable energy, such as wind energy, solar energy and water energy
[1,2][3][4][5]. Due to the randomness and volatility of new energy generation, effective energy storage means are needed to support its grid connection. In recent years, electrochemical energy storage has become the second largest energy storage mode after pumped storage, and rechargeable batteries account for the largest proportion of electrochemical energy storage
[6,7].
Lithium-ion batteries have quickly occupied most of the market because of their high energy density, long cycle life, green environmental protection and other advantages
[8⇓~10][11⇓~13][14,15]. However, with the upgrading of electronic equipment and the development of electric vehicles, consumers have put forward higher requirements for the energy density of batteries, and the development of batteries with higher energy density has become a hot issue to be solved in the scientific research community
[16⇓~18].
Lithium-sulfur batteries (LSBs) are considered to be the most promising next generation rechargeable batteries due to their high theoretical specific discharge capacity (1675 mAh·g
-1) and theoretical energy density (2600 Wh·kg
-1), abundant sulfur in the earth's crust, low price and environmental friendliness
[19,20][21,22][23⇓~25]. However, LSBs can not be applied on a large scale due to some problems. These problems mainly include: (1) sulfur and its discharge products have poor conductivity, resulting in low utilization of active materials
[26,27]; (2) The polysulfides formed in the discharge process are easily dissolved in the electrolyte to form a shuttle effect, which results in irreversible loss of active materials
[28,29]; (3) The conversion rate of polysulfide is slow in the redox process, which aggravates the shuttle effect and is not conducive to the stable cycle of the battery
[30].
In order to solve the above problems, researchers have made great efforts in conductive matrix (carbon black, graphite, carbon nanotubes), electrolyte additives (
, indium iodide) and diaphragm modification for many years, and have achieved some positive results
[31,32][33,34][35,36][37,38][39][21,40]. Among them, the use of low-cost carbon-based materials to provide sufficient conductivity for the positive electrode and to suppress polysulfide shuttling has become the most common and practical means.
Meng et al. Prepared a new mesoporous carbon material (MCM) with excellent electronic conductivity and high specific surface area by carbonizing litchi shells at high temperature
[41]. The MCMs they produced exhibited electrical conductivity similar to that of the conductive carbon black Super P. Electrochemical test results showed that the specific discharge capacity of MCMs-S composite cathode prepared by using MCM as sulfur carrier was as high as 1667 mAh·g
-1 at 0. 2 C. This study shows that the use of conductive carbon materials has a positive effect on improving the problem of poor conductivity of sulfur. However, in Meng et al.'S work, the sulfur loading on the positive electrode was only 50 wt%, and the specific discharge capacity of the MCMs-S electrode decayed from 1520 mAh·g
-1 to 499 mAh·g
-1 after 200 cycles at 0.5 C, with a capacity retention of only 32.8%. To some extent, the rapid decay of capacity indicates that the physical confinement effect of non-polar porous carbon on polysulfides is very limited
[42]. Doping carbon materials with elements can enhance the interaction between the conductive carbon matrix and polysulfides, thus improving the inhibition effect of polysulfide shuttle effect
[43].
In this paper, the applications of element-doped carbon-based materials, such as porous carbon, carbon nanotubes, carbon nanofibers and graphene, in the cathode of LSBs are reviewed, including single-element doping (N, B, Co), double-element doping (NS, NB, NCo, NNi) and multi-element doping (NOCo, NBCo, NOP).The interaction mechanism between different doping elements and polysulfides was discussed, and the development direction of element-doped carbon-based materials in LSBs was prospected.
2 Rational Design of Element Doped Carbon-based Material
Traditional carbon-based materials, including porous carbon, carbon nanotubes and graphene, have been widely used in LSBs. However, the nonpolar carbon-based materials mentioned above have weak confinement effect on polar polysulfides, and their confinement effect on polysulfides must be improved by modification. Among them, the more effective strategy is to dope the traditional carbon-based materials with elements. The introduction of heteroatoms helps to enhance the adsorption capacity of non-polar carbon materials for polysulfides, effectively improve the utilization of active material sulfur and improve the electrochemical performance of LSBs.
Zhang et al. Conducted systematic density functional theory (DFT) calculations on carbon nanomaterials doped with various elements
[44]. The results show that, compared with B, F, S, P and Cl doping, N and O doping can effectively enhance the interaction between the carbon matrix and polysulfides, thus preventing the shuttle of polysulfides, resulting in high specific capacity, excellent coulombic efficiency and good cycle stability. The work of Zhang et al. Theoretically proved that element doping has a positive effect on improving the shuttle effect of polysulfide.
It is well known that the discharge process of LSBs consists of two discharge plateaus. The first discharge plateau corresponds to the ring-opening of solid cyclic S
8 molecules and the transformation into soluble long-chain Li
2S
n(4≤n≤6), and the second discharge plateau corresponds to the further reduction of long-chain Li
2S
n(4≤n≤6) to solid Li
2S
2/Li
2S
[45]. According to the electrochemical reaction process of LSBs and the characteristics of polar polysulfides, the element doping of carbon-based materials can be designed from the following three aspects: (1) introducing adsorption sites with "polar-polar" effect or Lewis acid-base effect on polysulfides into carbon materials,The carbon material cooperatively inhibits the polysulfide from shuttling to the negative electrode through the physical limitation of the structure and the adsorption effect of the site, thereby improving the cycle stability of the battery
[46]; (2) doping a transition metal element (M) with a catalytic function into a carbon matrix to accelerate the conversion of the long-chain polysulfide by increasing the redox current density, reducing the polarization and lowering the redox reaction potential barrier
[30,47]; And (3) some doping elements also have the advantage of improving the conductivity of the carbon material while improving the polysulfide adsorption capacity of the carbon matrix, so that the electron transmission can be accelerated in the charging and discharging process, and the battery has high specific capacity and excellent rate performance.
3 Single Element Doping
In element doping, single element doping is the easiest to achieve, and the doping of N, B and Co is the most common
[48]. This part respectively introduces the representative work of using N, B and Co elements to dope carbon and apply them to LSBs in recent years, and summarizes the electrochemical performance of LSBs prepared from carbon substrates such as porous carbon, carbon nanotubes and graphene after element doping in Table 1.
3.1 N-doped
N-doping has obvious advantages in improving the polysulfide shuttle effect
[49]. The N element and the C element are adjacent to each other in the periodic table of the elements, and the radiuses of the N element and the C element are similar, so that the carbon material can be easily doped. The results show that the N atoms introduced into carbon materials can form S
xLi…N bonds with polysulfides during the cycle of batteries, thus limiting the shuttle of polysulfides to the negative electrode through chemisorption and improving the utilization of active materials
[50]. In addition, N-doping can also improve the conductivity of carbon materials and catalyze the redox reaction of sulfur. The N atoms doped into the carbon material exist in four ways, namely pyridine N, pyrrole N, graphite N and amino N, as shown in fig. 1
[51]. Among them, pyridine N and pyrrole N are effective reaction sites, and the lone pair electrons on them can form bonds with Li
+ in lithium polysulfide (LiPSs), thus effectively improving the adsorption of carbon materials on polysulfides and inhibiting the shuttle of polysulfides
[52,53]. The main function of graphite N is to promote electron transport and improve the conductivity of the material.
The doping state and content of N atoms in carbon materials can be controlled by certain experimental conditions. The main control factors include the annealing temperature, the nitrogen source and the catalyst used in the preparation process. The nitrogen sources commonly used in the preparation of nitrogen-doped carbon-based materials mainly include urea, nitrile amine, dopamine, pyrrole, etc. In addition, the NH3 atmosphere used in the calcination process of materials is also a common nitrogen source, and the commonly used catalysts are Ni, Co, Fe, etc.
Using acetonitrile as nitrogen source, von Dommele et al studied the effect of annealing temperature on the doping state and amount of N atoms in N-doped carbon nanotubes (NCNT)
[54]. The results show that NCNT can be obtained in a wide temperature range (823 ~ 1023 K) using acetonitrile as nitrogen source. When the annealing temperature is 823 K, the main form of N element is pyridine N. However, when the annealing temperature increases from 1023 K to 1123 K, the main N element in NCNT changes into the more stable aminated N, and the N/C ratio decreases with the increase of temperature.
Chung et al. Prepared nitrogen-doped carbon nanotubes (N-CNTs) with high redox activity by using cyanamide (NCNH
2) as nitrogen source and Co(CH
3COO)
2·4H
2O, Fe(CH
3COO)
2 and Ni(CH
3COO)
2·4H
2O as catalyst precursors respectively
[55]. The experimental results show that compared with Co-catalyst and Fe-catalyst, the Ni-catalyst has the highest nitrogen content of 9. 2%.
In addition to carbon nanotubes, the above control factors are also applicable to graphene. Lai et al. Doped graphene oxide (GO) by adjusting the annealing temperature and reasonably selecting the nitrogen source, and achieved the effective control of N doping state and N doping amount
[56]. They used NH
3 as nitrogen source to anneal GO at 550, 850 and 1000 ℃ respectively to obtain nitrogen-doped reduced graphene oxide (N-RGO). The results showed that with the increase of annealing temperature, the content of pyridine N in graphene showed a downward trend, which they believed might be caused by the poor thermal stability of pyridine N. When polypyrrole (Ppy) was used as the nitrogen source, the N-doped state in the Ppy/RG-O composite was mainly pyrrole nitrogen, and the atomic percentage of pyrrole nitrogen in the composite was 3.66%. However, in the PANi/RG-O composite obtained by using aniline (PANi) as the nitrogen source, pyridine nitrogen is the main form of N (atomic fraction is 3.58%) (fig. 2).
3.1.1 N-doped porous carbon
Han et al. N-doped (N-IOP) inverse opal carbon with hierarchical porous structure, and explored the improvement of N-doping on battery performance
[57]. The electrochemical test results show that the prepared high-surface sulfur content cathode material (6.0 mg·cm
-2) has high initial specific discharge capacity of 1162 mAh·g
-1. The XPS results showed that the N atoms were mainly pyrrole N and pyridine N, and their atomic number ratio was close to 7 ∶ 3. The excellent electrochemical performance is mainly due to the fact that these two N atoms can effectively capture polysulfides, increase the utilization of active materials, and through the synergistic effect with conductive carbon, the performance of the battery can be optimized.
Xia et al prepared N-doped porous carbon microspheres (NPCs) using renewable microalgae as precursors
[58]. The porous structure of NPCMs has enough space to load more sulfur, and the doped N atoms in the microspheres can achieve strong chemisorption of polysulfides, thus improving the utilization of active material sulfur. Based on the above advantages, the NPCMs electrode shows excellent electrochemical performance, and still has 1030.7 mAh·g
-1 reversible capacity after 100 cycles at 0.1 A·g
-1, and the capacity retention rate is as high as 91%.
3.1.2 N-doped carbon nanotube
Wang et al. Prepared nitrogen-doped carbon nanotube microspheres (HNCM) by spray drying, and explored the effect of carbonization temperature on the N content of HNCM
[59]. XPS analysis showed that the N content decreased to 18. 28 at%, 12.43 at% and 7. 05 at% at the calcination temperatures of 700 ℃, 800 ℃ and 900 ℃, respectively, which was mainly due to the decomposition of pyridine N with poor thermal stability. As shown in fig. 3, when the calcination temperature is 800 ° C, the HNCM electrode loaded with active material sulfur (HNCM800) has the most excellent electrochemical performance, which still has a high specific capacity of 804 mAh·g
-1 after 1000 cycles at 0.5 C, and the capacity decay rate is only 0.011% per week. This is because the appropriate N-doping amount of the HNCM800 electrode not only has high conductivity to promote electron transport, but also ensures sufficient polysulfide confinement capability. This study has reference value for exploring the optimal doping amount of N in carbon materials in the follow-up study.
Zhao et al. Prepared nitrogen-doped carbon nanotube composites (N-CNT) as the cathode carrier of LSBs
[60]. N-doping not only improves the electronic conductivity of the carbon matrix, but also enhances its adsorption capacity for soluble polysulfides. The N-CNT carrier can limit the polysulfide on the positive electrode side, thereby improving the utilization rate of the active material sulfur. The initial specific discharge capacity of the S/N-CNT electrode was 1276 mAh·g
-1 at 0. 2 C, and the capacity retention rate was 73. 5% after 100 cycles.
3.1.3 N-doped graphene
Qiu et al prepared N-doped graphene sheets (NG) by thermal nitridation of graphene oxide, and carried out theoretical calculations on the basis of experiments to further study the role of N-doping in inhibiting polysulfides
[61]. The theoretical results show that N-doped graphene sheets have stronger binding to polysulfides, and this enhancement is caused by the interaction between the negatively charged N atoms and the Li
+, while there is no interaction with the S
2-. XPS analysis shows that the main types of N in the product are pyridine N and pyrrole N, which can provide additional lone pair electrons as electron-rich donors to form S
xLi…N with electrophilic Li
+ in polysulfides through dipole-dipole interaction, thus effectively alleviating the shuttle effect of polysulfides and improving the electrochemical performance of LSBs.
Cheng et al. prepared three-dimensional N-doped graphene-based porous carbon (3DG @ NPC) with high N content and used it as a flexible electrode for LSBs
[62]. The high N content of 18 at% makes 3DG @ NPC have strong adsorption capacity for polysulfide. Due to the porous structure of carbon materials and the synergistic effect of N-doping, 3D G @ NC shows certain advantages in promoting electron/ion transfer and immobilizing polysulfides. Electrochemical measurements showed that the 3DG @ NPC/S electrode had a 1280 mAh·g
-1 initial discharge capacity at 0. 2 C and a capacity retention of 81% after 100 cycles.
According to the above introduction of N-doped carbon materials, the following conclusions can be drawn: (1) The doping state of N in carbon matrix mainly includes pyridine N, pyrrole N and graphite N, and the doping state and content of N can be controlled by experimental conditions; (2) Pyridine N and pyrrole N with lone pair electrons have strong electronegativity, which can form SxLi…N interaction with polysulfides during the cycle of the battery, which is more conducive to inhibiting polysulfides from dissolving into the electrolyte and shuttling to the negative electrode.
3.2 B-doped
According to the periodic law of the periodic table of elements, the electronegativity of B element is smaller than that of C, so it is electropositive after doping into the carbon matrix, which is conducive to the formation of strong charge adsorption effect with polysulfide anions
[63]. Boric acid is commonly used as boron source, which can effectively realize the boron doping of carbon matrix.
3.2.1 B-doped porous carbon
Chen et al. Prepared B-doped microporous carbon/sulfur composite (BMC/S) and used it as the cathode of LSBs
[64]. The theoretical calculation confirmed that the B-doped microporous carbon has stronger interaction with polysulfides than the undoped microporous carbon: the binding energy of undoped carbon to Li
2S
3 molecule is 0. 0577 Ha, while the binding energies of different B-doped States to Li
2S
3 molecule are 0. 0796 Ha, 0.0801 Ha and 0. 0803 Ha, respectively. This may be due to the fact that the electronegativity of B is weaker than that of C, which makes the electron cloud move to the C atoms around B atoms.
3.2.2 B-doped carbon nanotube
The results show that the conductivity of B-doped carbon materials is higher than that of B-undoped carbon materials. Xu et al. prepared boron-doped carbon nanotubes/sulfur (BUCNTs/S-1) composites by a simple one-pot hydrothermal process
[65]. Through a series of characterization, they proved that B doping could increase the conductivity of BUCNTs support, promote the uniform dispersion of sulfur, and enhance the adsorption capacity of BUCNTs support for polysulfides. The initial discharge specific capacity of the prepared BUCNTs/S-1 electrode is as high as 1251 mAh·g
-1 at 0. 2 C, and the specific capacity is still about 750 mAh·g
-1 after 400 cycles, showing excellent cycle stability.
3.2.3 B-doped graphene
Han et al. Prepared 3D boron-doped graphene aerogel (BGA) by hydrothermal method, and used BGA as the cathode carrier of LSBs (BGA-S)
[66]. B is positively charged on the graphene skeleton, which is beneficial to the chemisorption of polysulfide anions by the doped matrix. Electrochemical test results showed that the BGA-S positive electrode still had a high specific capacity of 994 mAh·g
-1 after 100 cycles at 0. 2 C, indicating that boron-doped graphene is an ideal active material support material for LSBs.
Li et al. Prepared B-doped graphene and activated carbon (B-G/AC) modified LSBs functional membranes by hydrothermal method
[67]. B-G has good conductivity, and the modified separator can not only accelerate electron transport, but also further hinder the shuttle of polysulfides on the positive side, thus improving the utilization rate of sulfur. Compared with the battery using conventional separator, the battery using B-G/AC modified separator showed good cycle stability, and its capacity was still as high as 1062 mAh·g
-1 after 100 cycles at 0. 1 C, which proved that B doping had a good effect on limiting polysulfides.
3.3 Co doping
Researchers usually use physical confinement and chemisorption to inhibit the shuttle of polysulfides, but this method can only restrict the polysulfides produced during the discharge process, and can not activate the deactivated Li2S deposited on the substrate to participate in the redox process of polysulfide.
It is well known that three quarters of the theoretical capacity of LSBs results from the reduction of the Li
2S
4 to the Li
2S in the second discharge plateau, and that the uncontrolled deposition of poorly conducting S/Li
2S results in electrical passivation of the sulfur anode
[68]. To reduce the generation of inactive Li
2S, it is necessary to control the deposition process of Li
2S so that it can be uniformly deposited on the conductive substrate. At the same time, the decomposition barrier of the Li
2S is reduced, and the redox reaction kinetics of the polysulfide on the surface of the substrate is accelerated, so that the utilization rate of active material sulfur is improved, and the electrochemical performance of LSBs is improved.
The results show that the doping of transition metals with catalytic ability can enhance the binding force of the doped substrate to polysulfides, accelerate the redox reaction kinetics of polysulfides, and realize the controllable deposition of Li
2S on the substrate. At the same time, the element doping endows the Li
2S with a smaller decomposition barrier on the doped substrate, which is more conducive to the decomposition of the Li
2S in the charging process and realizes the efficient utilization of sulfides
[69].
In 1964, Dorothy Hodgkin, the winner of the Nobel Prize in Chemistry, discovered Co-N
4 coordination structure, which was imitated by the subsequent development of homogeneous and heterogeneous cobalt catalysts, laying the groundwork for Co to play an important role in the field of catalysis
[70]. Cobalt element has the characteristic of accelerating chemical reaction kinetics, and cobalt oxide is a redox catalyst with low overpotential and high current density, which can effectively promote the electrochemical reaction
[71,72]. The carbon material is doped with the Co element, so that the carbon material is endowed with excellent catalytic performance, and the binding force between the carbon material and the polysulfide is enhanced through polar interaction. At present, cobalt-doped carbon-based materials are mostly obtained by using inorganic cobalt salts such as Co(NO
3)
2·6H
2O as cobalt sources and combining with heat treatment
[72,73].
3.3.1 Co doped porous carbon
Huang et al. Doped Co atoms into mesoporous carbon by supramolecular self-template method, which enhanced the interaction between carbon matrix and polysulfide while maintaining the mesoporous structure of carbon matrix
[74]. At the same time, Co-doped atoms can also be used as active centers to improve the kinetics of sulfur redox reaction. It can be seen from the cyclic voltammogram of the electrode shown in fig. 4 that the electrode polarization current of the Co-doped mesoporous carbon (SC-Co) is four times that of the mesoporous carbon (SC), and the large polarization current makes the binding force between the doped matrix and the polysulfide stronger, which can better control the deposition of lithium sulfide. The LSBs with Co-doped mesoporous carbon as the sulfur support of the positive electrode can obtain a high specific capacity of 1130 mAh·g
-1 at 0. 5 C and a capacity retention of 74. 1% after 300 cycles.
3.3.2 Co-doped carbon nanofiber
Xie et al. Designed a Co-doped carbon nanofiber (Co-CNF) as a sulfur carrier. Electrochemical test results showed that the electrode prepared with Co-CNF had a specific discharge capacity of 970 mAh·g
-1(0.1 C) when the sulfur loading was 4.6 mg·cm
-2[75]. They believe that its excellent electrochemical performance is due to the fact that Co doping can promote the conversion of LiPSs to Li
2S, thus reducing the dissolution and shuttle of polysulfides in the electrolyte. In addition, the existence of Co element is beneficial to the uniform nucleation of Li
2S on the surface of carbon nanofibers, which reduces the surface polarizability of the electrode.
3.4 Other
In addition to the commonly used N, B and Co, there are also some less commonly used doping elements in single element doping. Palmqvist et al. Prepared S-doped ordered mesoporous carbon (OMC) and applied it to the cathode of LSBs
[76]. The experimental results show that the interaction between sulfur and S-doped mesoporous carbon is stronger than that between sulfur and undoped mesoporous carbon. It was also found that the capacity retention of the S-doped mesoporous carbon cathode was 8% higher than that of the undoped mesoporous carbon cathode after 50 cycles. This improvement is mainly due to the fact that the doped S can provide effective reaction sites and enhance the interaction between the doped matrix and the polysulfide.
Hu et al. prepared S-doped layered carbon nanocages (hSCNC) and used them as cathode supports for LSBs
[77]. The hSCNC has a higher reduction potential, a lower oxidation potential, and a smaller Tafel slope, which effectively improves the polysulfide conversion kinetics. The experimental results show that the sulfur cathode coated with hSCNC exhibits better performance than the sulfur cathode using undoped carbon nanocages, with a high discharge specific capacity of 579 mAh·g
-1 after 400 cycles at 2 A·g
-1 even at a high sulfur loading of 4.5 mg·cm
-2. This indicates that S doping plays a catalytic role in the conversion of polysulfides. Although S doping can improve the performance of doped carbon electrodes to some extent, the doping process is more difficult because of the larger size of S atoms compared with N and B atoms, so the doping amount of S is generally low, so it is rarely used
[76].
It can be seen from Table 1 that in the same element doping, with the increase of doping element content, the initial discharge capacity of the battery mostly shows a downward trend, while the capacity retention rate of the battery basically shows an upward trend, indicating that element doping helps to limit the shuttle of polysulfide in the discharge process and improve the electrochemical performance of the battery. Although the effect of single element doping is relatively single, it can choose the appropriate doping components to improve the existing problems of LSBs, and the doping source and doping process are relatively mature. At present, single element doping in carbon-based materials has been widely used, which has the following advantages: (1) the doping process and doping source are relatively mature, and has gradually become one of the basic means of modification; (2) The single element doped carbon-based materials are economical and effective, which is helpful to realize the commercial application of LSBs.
4 Double Element Doping
Dual-element doping can fully combine the advantages and characteristics of the two doping elements, and the functional synergy between the doping elements can be realized through a reasonable ratio design, thereby greatly improving the electrochemical performance of the doped carbon matrix. In general, two-element doping is based on one doping element, and the other doping element can cooperate in the same direction to further enhance the performance of the basic doping element, or cooperate in a different direction to achieve additional performance beyond the function of the basic doping element. Table 2 summarizes the results of some related studies on the application of porous carbon, carbon nanotubes and graphene to LSBs in recent years.
4.1 N and S co-doping
N-doping of carbon matrix can enhance the chemical bonding of nonpolar carbon materials to polysulfides, and this strategy has been widely used in LSBs. However, due to the influence of precursors and synthesis conditions, the amount of N doping is generally low (N atom content is 3% ~ 9%), which hinders the further improvement of the suppression effect of carbon matrix on polysulfide shuttle effect. Co-doping of N and S on the carbon matrix can further enhance the binding force of the support material to the LiPSs(Li
+ (and
) by changing the electron cloud density distribution of the conductive matrix and providing more active adsorption sites
[78].
4.1.1 N, S co-doped porous carbon
Pang et al. Synthesized N and S co-doped nanoporous carbon (NSC) by self-template method using polythiotannin @ cellulose-derived carbon nanocrystals as raw materials
[79]. XPS analysis shows that the N and S atoms on the porous carbon form N-Li and S-S interactions with LiPSs, respectively, which effectively enhances the chemisorption ability of NSC to LiPSs, and the conductivity of NSC is also greatly improved compared with that of undoped carbon. The sulfur-loaded NSC/S electrode has a high specific capacity of 1370 mAh·g
-1 at 0. 05 C, and still has a high specific capacity of 775 mAh·g
-1 after 500 cycles, while the capacity of the N-doped nanoporous carbon electrode decays to 420 mAh·g
-1 after 200 cycles. This is because the double-element doped carbon provides abundant active sites for the chemisorption of LiPSs, and the synergistic doping of N and S atoms greatly changes the electron density distribution of the carbon matrix and significantly enhances the conductivity of the cell, thus making the binding ability of the carbon matrix with polysulfides stronger than that of undoped carbon or single-element doped carbon.
Wu et al. Synthesized 3D coral-like nitrogen and sulfur co-doped mesoporous carbon (CNSMC) and applied it to LSBs as a sulfur carrier
[78]. Pyridine-N, pyrrole-N, thiophene-S and sulfonic acid-S exist in CNSMC to adsorb polysulfides simultaneously, and the interconnected mesoporous carbon network is conducive to the rapid transport of electrons. Based on the synergistic effect of N and S co-doping and mesoporous carbon conduction path, the prepared CNSMC/S cathode was cycled 250 times at 4 C, and the capacity decay rate per cycle was only 0. 085%, and the electrode still had a high discharge specific capacity of 693 mAh·g
-1 at 10 C. This study shows that fast ionic and electronic conduction is conducive to the realization of high-rate performance of batteries, and N and S co-doping can synergistically promote the suppression of polysulfide shuttle effect by doped matrix.
Wang et al. Prepared nitrogen-sulfur co-doped hierarchical porous biocarbon (NSHPC) by a simple one-step method and used it as a flexible intercalation of LSBs
[80]. The experimental results show that the charge transfer resistance (R
ct) of the battery with NSHPC flexible intercalation is much lower than that of the battery with Celgard membrane only, and the initial discharge capacity of the battery can reach 1549 mAh·g
-1 at 0. 1 C (the discharge capacity of the cell with Celgard membrane is 947 mAh·g
-1 at 0. 1 C). The high specific discharge capacity of the battery is mainly due to the honeycomb porous structure of the intercalation and the strong chemical binding effect of N and S atoms on the intercalation of LiPSs, which effectively inhibits the shuttle effect of polysulfides.
4.1.2 N, S co-doped carbon nanofiber
Yang et al. Prepared nitrogen-sulfur co-doped porous carbon nanofibers (SN-PCNF) and used them as carrier materials for sulfur in LSBs
[81]. The porous structure of SN-PCNF provides enough space for the volume expansion of sulfur, and the co-doping of N and S enhances the ability of the electrode to fix polysulfides during cycling. In addition, the DFT calculation also confirmed that the N, S co-doped carbon matrix has a stronger polysulfide binding ability than the undoped carbon matrix or the singly doped carbon matrix. The initial specific discharge capacity of the SN-PCNF @ S electrode with sulfur loading of 80 wt% was 1133 mAh·g
-1 at 0. 1 C.
4.1.3 N, S co-doped graphene
Zhou et al. Prepared a N, S co-doped three-dimensional graphene flexible electrode, and loaded soluble LiPSs as active material on the positive electrode
[82]. By comparing the electrochemical properties of S-doped, N-doped and N, S-co-doped graphene electrodes, they proved that the synergistic effect of N and S co-doping was more conducive to the adsorption of polysulfides and improved the cycle performance of batteries. Electrochemical results showed that the N and S-co-doped graphene electrodes showed excellent rate capability at different current densities, and the specific discharge capacity of the N and S-co-doped graphene electrodes remained 550 mAh·g
-1 after 500 cycles, while the specific discharge capacities of the S-doped and N-doped graphene electrodes were only 190 and 221 mAh·g
-1 after 500 cycles.
Liu et al. prepared nitrogen-sulfur co-doped carbon/reduced graphene oxide (N, S-CDs/rGO) composites by mixing hydrothermally reduced graphene oxide with egg white and then heat treating, in which egg white served as a nitrogen and sulfur source
[83]. The results show that the N, S-CDs/rGO composite has high electrical conductivity and abundant reactive sites. The sulfur loaded N, S-CDs/rGO @ S65 cathode has a discharge capacity of 1550 mAh·g
-1 at 0. 5 C, and also shows a high discharge capacity of 1038,861.1 and 783.6 mAh·g
-1 at 1 C, 3 C and 5 C, respectively. It is worth noting that when the current density reaches 7 C, the battery still has a 740.8 mAh·g
-1 discharge specific capacity. Because the materials used in the preparation of the cathode are cheap and environmentally friendly, this work provides an economical material preparation route for the design of LSBs.
Huang et al. designed a porous S, N co-doped graphene (SNGE) intercalation and applied it to LSBs (Fig. 5)
[84]. SNGE intercalation can not only maintain the integrity of the separator in the reaction process and play the role of "second current collector", but also inhibit the shuttle of polysulfides through the synergistic effect of physical limitation of polysulfides and adsorption sites, so that the battery has good cycle stability. The electrochemical test results showed that the electrode with SNGE intercalation had a high specific discharge capacity of 1460 mAh·g
-1 at 0. 25 C, and the capacity decay rate was only 0. 01% per week after 1000 cycles at 8 C.
4.2 N and B co-doping
Although N-doping can enhance the inhibition effect of carbon matrix on polysulfide shuttling, the electronegativity of N element is greater than that of C, so the conductivity of the matrix will be reduced when the N-doping content is high, which will have a negative impact on the performance of the battery. The electronegativity of element B is smaller than that of C, so it is easier to ensure the positive charge of the matrix, so it is easier to achieve the synergistic effect of N and B in theory by doping the matrix with N and B
[63].
4.2.1 N, B co-doped porous carbon
Zhu et al. prepared an amorphous nitrogen-boron co-doped porous carbon aerogel (NB-PPCA) using green natural biomass waste (pomelo peel), and used it as a membrane modification layer for LSBs
[85]. The initial discharge capacity of the battery with NB-PPCA modified separator was as high as 1463.6 mAh·g
-1 at 0. 1 C, and the specific discharge capacity was still 586.6 mAh·g
-1 after 500 cycles at 1 C. It is worth noting that the preparation material of NB-PPCA is green and sustainable, so it has broad application prospects in the field of energy storage.
4.2.2 N and B co-doped graphene
Wang et al. Synthesized a graphene-supported nitrogen-boron-rich carbon layer (G-NBCL) through a two-step annealing process and used it as a cathode carrier for LSBs
[86]. They point out that the electronegative N atoms in the carbon matrix can form S
x-Li…N interactions with polysulfides to inhibit polysulfide shuttle, while the B atoms can chemisorb polysulfide anions (
) because they are electropositive in the carbon framework. Therefore, when both N and B atoms are introduced into the carbon matrix, the chemisorption capacity of the carbon matrix to the LiPSs can be enhanced, thereby further improving the cycle stability of the C/S composite cathode. In addition, they proved through theoretical calculations that if the B atom is adjacent to the N atom, forming an N = B/N-B structure in the carbon skeleton, the polarization of the B and N atoms will be further enhanced, which can further improve the chemisorption of LiPSs and Li
2S, as shown in Figure 6.
Chen et al. Used urea and boric acid as N and B sources to prepare N and B co-doped bent graphene nanoribbons (NBCGNs) as sulfur carriers by hydrothermal method
[87]. The results show that compared with the single element doping, the co-doping of N and B can mutually promote the B and N atoms to enter the graphene lattice, thus significantly improving the doping amount and conductivity of graphene. XPS results also confirmed that the negatively charged N atoms can effectively bind to the positively charged Li
+ through Lewis acid-base interaction, while the positively charged B atoms can chemisorb on the
, thus producing a synergistic effect to effectively inhibit the shuttle of polysulfides. The battery based on NBCGNs as the cathode support showed excellent cycle stability, with a reversible capacity of 740 mAh·g
-1 after 300 cycles at 0. 2 C and a capacity retention of 76%.
To sum up, N and B co-doping can not only realize the chemisorption of positively charged Li+ and negatively charged , improve the cycle performance of the battery, but also improve the utilization rate of sulfur by improving the conductivity of carbon-based support, which shows great potential in LSBs batteries.
4.3 Co-doping of N and Co
The poor electrochemical performance of LSBs is mainly due to the low conversion rate of polysulfides in the redox process and the uneven deposition of Li2S during discharge. The N and Co co-doping on the conductive carbon matrix can not only provide abundant polysulfide adsorption sites for the carbon matrix in the process of repeated dissolution and deposition of sulfur, but also effectively promote the catalytic conversion between long-chain polysulfide and Li2S2/Li2S by doped Co atoms,To induce the uniform deposition of Li2S and improve the redox reaction kinetics of LSBs, the synergistic effect of N and Co elements is of great significance for the development of high-performance LSBs.
4.3.1 N, Co co-doped porous carbon
Luo et al. Used metal-organic framework (MOF) polyhedron ZIF-67 to prepare cobalt and nitrogen Co-doped nanoporous carbon as the cathode support of LSBs (S @ C-Co-N)
[88]. They believe that the 3D porous carbon matrix derived from MOF polyhedron (ZIF-67) can not only ensure enough space to accommodate sulfur nanoparticles, but also effectively inhibit the shuttle of polysulfides due to its rich microporous structure. In addition, the presence of Co and N in S @ C-Co-N composites effectively limits the migration of soluble polysulfides to the negative electrode, and the catalytic effect of Co nanoparticles also plays an important role in the cycling process. Electrochemical test results show that the initial discharge specific capacity of S @ C-Co-N composite is 916.6 mAh·g
-1 at 0. 5 C, and the reversible capacity still has 460.5 mAh·g
-1 after 500 cycles.
4.3.2 N, Co co-doped carbon nanomaterial
Qiu et al. Synthesized a kind of N, Co co-doped hollow carbon nanorods as cathode carrier (Co @ NHCRs), which has the advantages of both high conductivity and fixed polysulfide
[89]. Theoretical calculations show that the doping of N and Co atoms can adjust the electronic structure of the hollow carbon nanorods. As shown in Figure 7A, with the doping of N and Co in the potential distribution diagram, the electron cloud density on the surface of carbon nanorods is locally enhanced and is no longer uniformly distributed, which significantly improves the adsorption of carbon nanorods on LiPSs. As shown by the DFT calculation in Fig. 7B, the binding energy of Li-S molecules and the element-doped carbon nanorods increases, confirming that the doping of N and Co atoms can significantly promote the adsorption of LiPSs on the carbon nanorods, thereby preventing the precipitation of LiPSs on the surface of the carbon nanords and self-aggregation during discharge. The final composite cathode has a high specific capacity of 971 mAh·g
-1 at 0. 5 C, and the capacity remains 704 mAh·g
-1 after 100 cycles, showing good cycle stability.
Shao et al. prepared a honeycomb-shaped mesoporous Co and N Co-doped carbon nanosheet (MC-NS) with high specific surface area and a large number of defects, which simultaneously realized the immobilization of polysulfides and the efficient catalytic conversion of active substances
[90]. Theoretical calculations and experimental results show that the defects in the Co-N catalytic sites and the carbon skeleton of MC-NS contribute to the suppression of the polysulfide shuttle effect. In order to illustrate the problem of intermediate product transformation during charge/discharge, they carried out in-situ Raman testing. As shown in fig. 8, at the end of the discharge, the peak corresponding to the Li
2S
2 in the MC-NS becomes the main peak, indicating that the conversion degree of the long-chain polysulfide to the Li
2S
2 is high; At the end of charging, the intensity of S
8 in MC-NS is the highest, which proves that MC-NS electrode has good reversibility. Raman results further confirmed the enhanced adsorption capacity and conversion efficiency of polysulfides on the support.
The delayed redox reaction kinetics of LiPSs and its shuttle effect are the main reasons that hinder the practical application of LSBs. In order to solve these problems, Sun et al. Constructed Co-N-Co-doped carbon nanotube arrays (Co-N-CNTA) and used them as sulfur carriers
[91]. In addition to the structure of CNTA itself, CNTA doped with Co and N can also chemisorb polysulfides and accelerate their oxidation and reduction. By comparing the current density and overpotential from the Li
2S
6 of different carbon substrates to the reduction peak of Li
2S
4, they found that Co-N-CNTA had the highest current density and the lowest degree of polarization in the electrochemical reaction, indicating that the incorporation of Co and N reduced the reaction barrier of polysulfide and effectively accelerated its conversion kinetics. Electrochemical results show that the LSBs assembled by Co-N-CNTA have 882 mAh·g
-1 reversible capacity at 0. 5 C, and the capacity retention is close to 100% after 200 cycles. In contrast, the capacity of Co-CNTA electrode decayed from 827.3 mAh·g
-1 to 681.2 mAh·g
-1 after 200 cycles at 0. 5 C, showing a lower cycle stability. In addition, the soft-pack battery assembled by the material can be stably cycled for more than 150 weeks, which further proves the great potential of Co and N Co-doped LSBs in practical applications.
Sun et al. prepared monodisperse cobalt atom-catalyzed nitrogen-doped carbon nanosheets (CoSA-N-C) and used them as sulfur carriers
[92]. The experimental and theoretical results show that the Co atoms in the support provide sufficient reaction sites for the interaction with the
, and the negatively charged N-doping can bind to the terminal Li
+ in the LiPSs molecule, which contributes to the strong chemisorption of polysulfides on the CoSA-N-C support. XANES analysis shows that the Co-N
4 between Co and N atoms in CoSA-N-C not only promotes the transformation of polysulfides, but also reduces the decomposition barrier of Li
2S, thus promoting the kinetics of sulfur redox reaction in LSBs. As shown in fig. 9, the Co-N
4structure can adjust the controlled deposition of the Li
2S so that it is uniformly deposited on the surface of the electrode instead of forming a deactivated Li
2S passivation layer, which is beneficial for maintaining high conductivity and high sulfur utilization during cycling.
Wan et al. Prepared N-doped graphene (Co-N/G) with monodisperse Co atoms embedded
[93]. As shown in fig. 10, they proved by DFT calculation that the N and Co atoms doped in Co-N/G form Co-N
4 coordination centers, which can promote the formation and decomposition of Li
2S during discharge and charge. Fig. 10 (B) shows the Gibbs free energy of reduction of polysulfides on N-doped graphene (N/G) and Co-N/G substrates, respectively. The reduction of polysulfides is non-spontaneous except for the reduction of S
8 molecules, which is a spontaneous exothermic transformation. The Gibbs free energy of the reduction of Li
2S
2 to Li
2S is the largest, which indicates that this is the rate-limiting step in the whole discharge process. This value is closer to 0 on the Co-N/G substrate than on the N/G substrate, indicating that the polysulfide conversion rate is faster on the Co-doped substrate. In addition, it can be seen from the decomposition barriers of the Li
2S on the two substrates in figs. 10c and 10d that the oxidation of the Li
2S is more likely to occur on the Co-N/G substrate. This indicates that the Co-N/G matrix can accelerate the Li-S phase transition and improve the utilization of active material sulfur. The battery assembled with Co-N/G as sulfur carrier still had 681 mAh·g
-1 specific discharge capacity after 500 cycles, and the coulombic efficiency was 99.6%.
Chen et al. Prepared a lightweight multifunctional layer (Co-N-C/rGO) composed of reduced graphene oxide and Co, N Co-doped carbon nanofibers, and used it to modify the diaphragm
[94]. Co-N-C/rGO with abundant mesoporous structure has strong polysulfide immobilization ability and high electronic conductivity, which can capture and reuse polysulfides through the synergistic effect of physical confinement/chemisorption. The LSBs with Co-N-C/rGO/PP separator have good rate capability, the specific discharge capacity is 1344 mAh·g
-1 at 0. 1 C, and the specific discharge capacity is still 658 mAh·g
-1 at 5 C.
N and Co co-doped carbon-based materials can not only enhance the restriction of polysulfide shuttle by increasing adsorption sites, but also catalyze the transformation of sulfur and polysulfide redox reaction kinetics, which has a positive role in promoting the research and development of high-performance LSBs.
4.4 N and Ni co-doping
Ni is adjacent to Co in the periodic table of elements, and the properties of Ni and Co are similar, and Ni has lower cost than Co. Co-doping of N and Ni in the carbon matrix can restrict the shuttle of polysulfides to the negative electrode during cycling by forming Ni-S and Li-N interactions with polysulfides
[95]. In addition, Ni element also has the ability to catalyze the redox reaction of polysulfides
[96].
4.4.1 N and Ni co-doped porous carbon
Liu et al. Prepared a kind of nitrogen-doped multifunctional carbon (MFCH) hybrid with nickel nanoparticles as a membrane modification material for LSBs
[97]. N doping can enhance the chemisorption of polysulfides, while Ni doping can promote the reaction kinetics of sulfur. The battery assembled with the separator modified by the material shows excellent cycle stability, and when the sulfur loading is 65%, the capacity fading rate is only 0.115%/week after 200 cycles at 0.34 A·g
-1.
4.4.2 N and Ni co-doped graphene
Niu et al. Introduced nickel (Ni) atoms into nitrogen-doped graphene (Ni @ NG) and applied it to LSBs
[98]. Through theoretical calculation, they proved that the binding energy between Ni @ NG and LiPSs is stronger than NG (Fig. 11A), and from the reaction mechanism diagram of Fig. 11B, it can be seen that Ni atoms introduced into nitrogen-doped graphene can form Ni-N
4 with N atoms.Thus forming a
with the polysulfide, the Ni-N bond limits the shuttle effect, and the Ni atom can change the electron distribution of the polysulfide, so that the polysulfide has a lower decomposition energy barrier in the electrochemical process, thereby accelerating the kinetics of the electrochemical reaction. The LSBs based on the Ni @ NG-modified separator exhibited excellent cycle stability, with a high reversible capacity of 826.2 mAh·g
-1 after 500 cycles at 1 C and a high capacity retention of 70% after 500 cycles at 10 C.
4.5 Other
In addition to the above two elements co-doping, N element can also be combined with P, O and other elements to modify the carbon matrix. Yu et al. prepared nitrogen and phosphorus co-doped carbon (N, P-C) using polyaniline hydrogel doped with phytic acid as the active material carrier, and prepared binderless LSBs cathode using Li
2S as the active material
[99]. N and P co-doping can inhibit the shuttle of polysulfide by enhancing the interaction between sulfur and carbon skeleton, and P doping can also catalyze the redox reaction of sulfur and polysulfide. The electrochemical test results showed that the Li
2S/N,P-C composite electrode still had a capacity of 700 mAh·g
-1 and an average coulombic efficiency of 99.4% after 100 cycles at 0.1 C.
Zhou et al. Successfully prepared a new type of honeycomb N and O co-doped porous carbon (NOPC) by using biomass waste bean dregs as N and O sources, and the obtained NOPC has a special hierarchical porous structure
[100]. The initial discharge capacity of the NOPC/S composite cathode material is 1185.4 mAh·g
-1 at 0. 2 C. The excellent electrochemical performance of the NOPC/S composite cathode material is due to the synergistic effect of the hierarchical porous structure of NOPC and the co-doping of N and O, which restricts the shuttle of polysulfides through physical limitation and strong adsorption. Xiong et al. Prepared a nitrogen-oxygen in-situ co-doped nonporous carbonaceous material (NONPCM) and used it to load sulfur
[101]. N, O co-doping provides abundant active sites for the carbonaceous materials, in which the N atoms anchor the LiPSs by forming Li
2S
x-N bonds with the Li
+, and the electronegative O atoms also restrict the shuttle of the LiPSs by interacting with them. The sulfur content of the finally prepared composite electrode can reach 70 wt% to 90 wt%, and the composite electrode with the sulfur content of 70% prepared on a kilogram scale can still provide the specific discharge capacity of 540 mAh·g
-1 after being cycled for 300 times at 1600 mA·g
-1.
Song et al. Prepared a N, O co-doped mesoporous carbon/sulfur composite nanomaterial (MPNC) with high sulfur loading, and proved by X-ray near-edge absorption spectroscopy (XANES) that N, O co-doping of carbon materials can promote the formation of S — O bonds between sulfur and oxygen-containing functional groups on the carbon matrix
[102]; The results of DFT calculation also show that the interaction between S and O is stronger than that between S and N on pyridine N-doped — COOH and pyrrole N-doped — C = O, which is more inclined to form S — O bond. This is because the doped N has higher electronegativity than C, which polarizes the oxygen-containing groups (such as carbonyl and carboxyl) near the N atom and is more vulnerable to the attack of sulfur atoms at high temperature (155 ℃). Due to the strong physical confinement and adsorption of polysulfides on the N and O co-doped carbon matrix, the electrode exhibited a high capacity retention of 95% after 100 cycles at a current density of 0.7 mA·cm
-2 at a sulfur loading of 4.2 mg·cm
-2.
As discussed above, the design criteria for dual-element doped carbon-based materials generally have the following characteristics: (1) the doping elements each have unique advantages, and generally the realization of the performance of one element cannot weaken the performance of the other doping component too much; (2) Synergistic effect between doping elements, based on one well-known doping component, such as N-doping, whose doping amount is mostly controlled in the range of 3 wt% to 6 wt%, and the modification of another doping component is usually easy to characterize and meet the economic requirements as far as possible.
5 Multi-element doped carbon-based material
Because the chemical state and environment of the doping elements will affect the electronic structure of adjacent carbon atoms, the process of doping carbon-based materials with three or more elements is not very mature, and so far there are only a few reports on multi-element doping
[103].
Zhang et al. Prepared a graphitized mesoporous carbon composite Co-doped with N, O and Co, and used it as a support for LSBs with high sulfur loading (90%) (N-O-Co-C/S)
[104]. XPS analysis shows that O-S, N-S and Co-S bonds are formed in N-O-Co-C/S, which can synergistically inhibit the shuttle of polysulfides. The N-O-Co-C/S electrode showed excellent cycle performance at a sulfur loading of 2.45 mg·cm
-2, and the capacity was 610 mAh·g
-1 after 200 cycles at 0. 5 C. The EIS results also show that N, O and Co co-doped graphitized carbon can effectively promote the charge transfer in the carbon matrix. The work of Zhang et al. Achieved high specific capacity and stable cycle of LSBs under high sulfur loading, which provides a reference for the practical application of LSBs.
Huang et al. Synthesized a B-N Co-doped porous Co hollow graphite carbon composite (h-Co-BN-GC)
[105]. Carbon materials doped with Co, B and N can act as dual electrocatalysts, which is beneficial to promote the kinetics of polysulfide redox reaction. The results show that the h-Co-BN-GC composite with sulfur content of 80% has a discharge capacity of 1089 mAh·g
-1 in the first cycle at 0. 5 C, and still has a 877 mAh·g
-1 after 500 cycles, maintaining a coulombic efficiency of nearly 100%. This work demonstrates that multi-element co-doping can play a synergistic role in inhibiting polysulfide shuttling and accelerating the slow redox kinetics of polysulfides.
Piao et al. Prepared P, O and N co-doped hollow carbon (PONHC/G) on graphene using nano-scale LiFePO
4(LFP) as a hard template, and used it as a carrier for LSBs
[106]. The PONHC/G prepared by using LFP nanoparticles as a hard template has pores with various pore sizes, which can be used for sulfur storage, and the carbon structure is doped with P, O and N heteroatoms, which has a large number of active adsorption sites to inhibit the shuttle of polysulfides. The LSBs prepared with PONHC/G exhibited excellent cycling stability. After 500 cycles at 1 C, the specific capacity of the electrode was 607.7 mAh·g
-1, and the capacity decay rate was only 0.052% per week.
In multi-element doping, with the increase of doping atoms, it is more difficult to distinguish the improvement of electrochemical performance. Therefore, when modifying carbon-based materials, the selection of appropriate doping components and doping ratio is an important factor affecting the performance of the doping body, and the further study of the multi-doping mechanism is also the key to guide its application practice.
6 Conclusions and outlook
As a new type of secondary battery, LSBs have attracted wide attention in the field of energy, and the cheap and readily available carbon-based materials are the preferred materials to improve the poor conductivity of LSBs active material and promote its large-scale application. Unfortunately, carbon-based materials, such as porous carbon, carbon nanotubes and graphene, are weak in suppressing the polysulfide shuttle effect, especially when the sulfur content of the cathode is high (> 70 wt%). The results show that the stability and reversibility of LSBs in the cycling process can be greatly improved by doping elements into the conductive carbon matrix. The main functions of the element doping on the carbon-based material are as follows: by providing adsorption sites, the adsorption capacity of the doped carbon-based material on the Li+ or the in the polysulfide is increased, so that the inhibition of the doped matrix on the polysulfide shuttle effect is enhanced; By changing the local electron distribution of the carbon matrix, the conductivity of the doped carbon-based material is enhanced, and the utilization rate of active substances is improved; Doping carbon matrix with catalytic elements can catalyze the oxidation-reduction reaction of polysulfides and accelerate their transformation to Li2S/Li2S2.
In order to improve the application of element-doped carbon-based materials in LSBs, the following three suggestions are given in this paper: (1) Different doping elements have different functions, and the effective realization of the functions of doping elements is very important to improve the electrochemical performance of batteries. It is necessary to reasonably use auxiliary methods such as theoretical calculation and electronic structure analysis to judge the effectiveness of doping elements and select the optimal doping elements. (2) The content and existing form of the doping elements have a great influence on the properties of the doped materials. When doping elements, it is necessary to further clarify the doping mechanism and try to increase the force on polysulfides on the basis of maintaining the high conductivity of carbon matrix. (3) In multi-element doping, with the increase of doping elements, the difficulty of defining the mechanism of different doping elements will increase. When doping multiple elements, attention should be paid to whether there is interaction between elements, and the mass ratio of doping elements should be controlled, so as to weaken the impact on the mass energy density of the battery and ensure the excellent performance of the battery.
At present, the application of element-doped carbon-based materials in LSBs is still in the laboratory research stage. Future work will not only study the mechanism of the effect of doping elements on polysulfides and deeply analyze the electrochemical reaction process of LSBs,It is also necessary to simplify the process to increase the operability of the doping process, and to achieve the controllability and reproducibility of product performance, so as to make the production of doped carbon-based materials possible on a large scale. It is believed that through continuous exploration, doped carbon-based materials with reasonable cost and stable quality will be widely used in LSBs in the future.
National Natural Science Foundation of China(51802019)
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