Growing Intact Membrane by Tuning Carbon Down to Ultrasmall 0.37 nm Microporous Structure for Confining Dissolution of Polysulfides Toward High‐Performance Sodium–Sulfur Batteries

Room temperature sodium–sulfur (Na–S) batteries are severely hampered by dissolution of polysulfides into electrolytes. Herein, a facile approach is used to tune a biomass‐derived carbon down to an ultrasmall 0.37 nm microporous structure for the first time as a cathode in sodium–sulfur batteries. This produced an intact uniform Na2S membrane to greatly confine the dissolution of polysulfides while realizing a direct solid phase conversion for complete reduction of sulfur to Na2S, which delivers a sulfur loading of 1 mg cm−2 (50 wt.%), an excellent rate capacity (933 mAh g−1 @ 0.1 A g−1 and 410 mAh g−1 @ 2 A g−1), long cycle performance (0.036% per cycle decay at 1 A g−1 after 1500 cycles), and a high energy density for 373 Wh kg−1 (0.1 A g−1) based on whole electrode weight (active sulfur loading + carbon), ranking the best among all reported plain carbon cathode‐based room temperature sodium–sulfur batteries in terms of the cycle life and rate capacity. It is proposed that the solid Na2S produced in the ultrasmall pores (0.37 nm) can be squeezed out to grow an intact membrane on the electrode surface covering the outlet of the pores and greatly depressing the dissolution effect of polysulfides for the long cycle life. This work provides a green chemistry to recycle wastes for sustainable energies and sheds light on design of a unique pore structure to effectively block the dissolution of polysulfides for high‐performance sodium–sulfur batteries.


Introduction
Room temperature sodium-sulfur (RT Na-S) batteries with a high theoretical energy density of 1274 Wh kg −1 and low expense are very promising for large-scale energy storage systems. [1,2] The sulfur reduction at cathode in most of the reported RT Na/S batteries is a multistepped conversion as solid (S) ↔ liquid (polysulfides) ↔ solid (Na 2 S), which involves intermediates Na 2 S 8 , Na 2 S 6 , Na 2 S 5 , Na 2 S 4 , Na 2 S 2 , and then to the final product Na 2 S (S). The reaction pathways are varied depending on the kinds of intermediates generated upon discharge/ charge process on different cathode materials. [3,4] It is noted that the main reaction path for a high-temperature Na-S battery (HT Na-S) is a process from Na 2 S 8 to Na 2 S 4 , involving only long-chain polysulfides intermediates (Na 2 S n , when 4 ≤ n ≤ 8) [5,6] for electrochemical reactions as follows: It is reported that the RT Na-S batteries have the same long-chain intermediate-based reactions as that of HT Na-S batteries above, but they should continue two additional reductions of short-chain Room temperature sodium-sulfur (Na-S) batteries are severely hampered by dissolution of polysulfides into electrolytes. Herein, a facile approach is used to tune a biomass-derived carbon down to an ultrasmall 0.37 nm microporous structure for the first time as a cathode in sodium-sulfur batteries. This produced an intact uniform Na 2 S membrane to greatly confine the dissolution of polysulfides while realizing a direct solid phase conversion for complete reduction of sulfur to Na 2 S, which delivers a sulfur loading of 1 mg cm −2 (50 wt.%), an excellent rate capacity (933 mAh g −1 @ 0.1 A g −1 and 410 mAh g −1 @ 2 A g −1 ), long cycle performance (0.036% per cycle decay at 1 A g −1 after 1500 cycles), and a high energy density for 373 Wh kg −1 (0.1 A g −1 ) based on whole electrode weight (active sulfur loading + carbon), ranking the best among all reported plain carbon cathode-based room temperature sodium-sulfur batteries in terms of the cycle life and rate capacity. It is proposed that the solid Na 2 S produced in the ultrasmall pores (0.37 nm) can be squeezed out to grow an intact membrane on the electrode surface covering the outlet of the pores and greatly depressing the dissolution effect of polysulfides for the long cycle life. This work provides a green chemistry to recycle wastes for sustainable energies and sheds light on design of a unique pore structure to effectively block the dissolution of polysulfides for high-performance sodium-sulfur batteries.
intermediates (Na 2 S n , when 1 ≤ n < 4) until the formation of the final solid product Na 2 S: [7][8][9] Na 2 S 4 liquid ð The HT Na-S batteries have been well-industrialized. However, RT Na-S batteries suffer from the severe shuttle effect caused by the polysulfide dissolution, resulting in short cycle life and low sulfur utilization, which impedes their larger scale industrialization. [10,11] To date, intensive efforts have been devoted to designs of surface chemistry and cathode nanostructures for overcoming the shuttle effect. Catalysts including transition metal-single atoms, [12] -organic frameworks, [13] and -sulfides [14] synthesized by new chemistry methods, which possess strong interactions between catalytic active centers and polysulfide intermediates to ease the shuttle effect while accelerating the reduction rates of the mobile Na 2 S n (n = 4-8) to form the solid Na 2 S 2 or/and Na 2 S product. Unique nanostructures such as core-shell, [15] hollow sphere/tubes, [16,17] and conductive networks [18][19][20] made from nonpolar carbon-based materials [21] have been also developed, which can physically restrict the dissolution of polysulfides. Unfortunately, these approaches still cannot effectively block the dissolution of polysulfides for long cycle life and high rate performance.
Guo et al. have reported an interesting pioneering work, [22] which fabricates a microporeous carbon-sulfur composite cathode (sulfur loading 40 wt.%) with a pore size of~0.5 nm in Na-S batteries, which can accomplish a direct solid conversions of sulfur to solid Na 2 S 2 as 2S Solid ð Þþ2e À þ 2Na þ () charge=discharge Na 2 S 2 Solid ð Þ followed by a further reduction of Na 2 S 2 to Na 2 S by the sterically physical pore size restriction. [23,24] This work motivates us to explore even greater improvement of such a direct solid-phase reactions of all carbon cathode for high-performance Na-S batteries, while investigating the underlying mechanism of the pore size effect on the reaction process. In particular, we wonder what could happen if the micropores could be tailor down further smaller than 0.5 nm?
In this work, we delicately tailored pore structures of biomassderived plain carbon frameworks (BPCF) down to a range of 0.37-0.58 nm, and systematically investigated the pore size effect on the performance of sulfur/carbon cathods. Results indicate that different sizes can cause varied the dissolution of polysulfidess relying on the pores matching molecularly with differently sized polysulfides. For the first time, we tuned down the pore sizes of carbon down to 0.37 nm, and enabled formation of an intact Na 2 S membrane on the cathode surface to effectively prevent escape of sulfur/sulfindes into electrolyte, accomplishing a sulfur loading of 50 wt.%, long cycle performance (0.036% per cycle decay at 1 A g −1 after 1500 cycles) and a high specific capacity of 933 mAh g −1 and 373 Wh kg −1 in terms of sulfur alone and sulfur/ carbon composite, respectively, at 0.1 A g −1 , which is superior to the all reported plain carbon-based RT Na-S batteries as shown in Figure 4c,e. [9,12,17,25,26] 2. Results

Material Properties
The BPCF was prepared by our reported method but with modifications, in which pecan shell raw material was partially corrosioncalcinated by potassium hydroxide under various temperatures (600°C, 700°C, 800°C, and 900°C) for a host of RT Na-S batteries cathode. Since pecan shell cells are formed by gradual accumulation in physiological processes for composition of contains 32.9 wt.% cellulose and 22.5 wt.% hemicellulose, they have naturally strong tendency to generate abundant cavities and cracks (micropores) rather than mesopores during the pyrolysis process (>600°C). [27] Therefore, it is difficult to observe significant differences of BPCFs prepared at various calcination temperatures by field emission scanning electron microscope (FESEM) images ( Figure 1a and Figure S1, Supporting Information). The transmission electron microscope (TEM) images of BPCFs with 700°C treatment (BPCF-700) show rich micro-and mesopores on hierarchical ultrathin sheets (Figure 1b,c). The inset is the selected area electron diffraction (SEAD) that displays the typical ring of carbons for BPCF-700°C, thus confirming that the material is made up from amorphous carbon. To investigate morphology of the porous material, a measured typical bright-field TEM image ( Figure 1d) clearly illustrates that the ultrathin carbon sheets has a uniform micro-and mesoporous structure. The scanning TEM (STEM) image of BPCF-700 exhibits a carbon framework without distinct nitrogen or sulfur heterogeneous polar element doping except oxygen, hydrogen, and carbon (Figure 1e,f). Moreover, the thickness of hierarchically porous ultrathin sheets is approximately 5 nm determined by the inset of atomic force microscope (AFM) image, indicating that the carbon framework can offer fast access of ions or/and reactant to the electrode surface due to its channel size less than the diffusion layer thickness (Figure 1g). Furthermore, the structure and surface chemical state of BPCF were investigated by XRD pattern (Figure 1h), which shows that the shapes of carbon diffraction peaks at (002) and (101) are the same as those of the SAED pattern in Figure 1c. [28] Moreover, the concentrations of disorderedand graphitized-carbon can be verified by the intensities of D-(1350 cm −1 ) and G-band (1580 cm −1 ), respectively. The Raman spectrum in Figure 1i shows I G /I D values of 0.97, 0.98, 0.99, and 1 for the treated temperatures of 600°C, 700°C, 800°C, and 900°C, respectively, evidencing that the intensity of graphitized BPCF rises with increase in the treatment temperatures. [29] To explore the surface chemical state of the heat-treated samples, the X-ray photoelectron spectroscopy (XPS) result displays that BPCF samples have no heteroatom ( Figure S2a, Supporting Information), which is consistent with the STEM and XRD results discussed above. Furthermore, the FTIR spectrum shows that all primary function groups on carbon surface are carboxyl and hydroxyl ( Figure S2b, Supporting Information), indicating that all four host samples have similar electrolyte wettability.
The pore structures of BPCFs were studied by the nitrogen absorption-desorption isotherm method as shown in Figure 2a. When a P/P 0 <0.05, a sharp rise of the N 2 isotherm indicates typical type I of N 2 adsorption isotherm for BPCFs confirming a significant amount of micropores. For a P/P 0 range from 0.1 to 0.9, a feebly increase reveals scarce mesopores of BPCF with heat treatment at 600°C, 700°C, 800°C, and 900°C. In more detail, the micro-and mesopore size distributions are showed in Figure 2b,c, respectively, and the BPCF-700 has the smallest average micropore size of 0.37 nm in comparison with Energy Environ. Mater. 2023, 6, e12634 2 of 10 that of 600°C-0.41 nm (864 m 2 g −1 ), 800°C-0.37 nm (882 m 2 g −1 ), 800°C-0.48 nm (1197 m 2 g −1 ), and 900°C-0.57 nm (1754 m 2 g −1 )treated host carbon materials ( Figure 2b). The pore volumes of both BPCF-600 and BPCF-700 is much smaller than that of both BPCF-800 and BPCF-900, clearly indicating that the former two forms much less pores than the latter two simply due to the lower pyrolysis temperatures. This is why the latter two has higher specific surface area. Moreover, the pores are making by the gas evolution form the biomass pyrolysis, and the average pore size should be decided jointly by the molecule sizes and the amount of evolved gases from biomass. We believe that there is a rational temperature to generate the smallest average pores. It could be understandable when pyrolysis begins a large amount of gas produces at 600°C pyrolysis to form relatively large pores, and with increased pyrolysis temperature and time, no enough large amount of gas to form larger pores instead of smaller pores at 700°C. However, when pyrolysis temperature further increases, more cracks could produce to form relatively larger pores again. The results endorse that the BPCF samples can be tuned with different pore sizes distributions and experimentally confirm that 700°C is the optional temperature to make the smallest pores for this specific biomass.
The sulfur loading state in various sized pores was evaluated by the thermogravimetric analysis (TGA) (Figure 2d). Different BPCF samples loaded with 50 wt.%. sulfur at 155°C with high migration rates during 12 h to diffuse into micro-/mesopores of BPCF samples, [30] which exhibit typical two-temperature platforms for loss of sulfur at 250-350°C and 450-550°C, respectively, which are the typical TGA curves of micropore carbon material with loaded sulfur, proving that sulfur has two different states (S 8 and S 2-4 ) in BPCF. The two sulfur states are further evidenced by the XPS measured results as shown in Figure S3a-d, Supporting Information. Apparently, two states of sulfur, S 8 and S 2-4 , with different physical sizes can be loaded into macro-/mesopores and micropores, respectively. It is reported that in microporous structures, the presence of S 2-4 state can confirm successful loading of sulfur in micropores, while S 8 should be mainly ascribed to sulfur on the electrode surface. [29,30] Cyclic voltammetry (CV) was performed for differently pore-sized BPCFs at 0.1 mV s −1 in a range of 0.5-2.3 V (Figure 3a), in which R 1 and R 2 are used to denote the two reduction peaks. Figure 3a shows that the oxidation onset potentials for different temperaturetreated BPCFs are 1.33 V (700°C) < 1.41 V (600°C) < 1.45 V (800°C) < 1.48 V (900°C), respectively, while the reduction onset potentials for these materials are 2.06 V (700°C) > 2.05 V (600°C) > 1.98 V (800°C) > 1.94 V (900°C), respectively. Moreover, the potential difference between anodic and cathodic peaks are 0.78 Fundamentally, for an electrocatalyst, the more negative anodic onset potential, more positive cathodic onset potential and less potential difference between anodic and cathodic peaks have the higher electrocatalytic activity. Apparently, the smallest pore-sized carbon (0.37 nm) delivers the highest electrocatalytic activity among all pore-sized BPCFs.
The electrode kinetics behaviors of BPCFs were further investigated by the relationship of the peak current (i) versus scan rate (v) from the measured CVs according to equation where a and b are parameters to analyze the electrode process, in which b of 0.5 and 1 represents a diffusion-and surfacecontrolled reaction process, respectively. [31,32] A b value between 0.5 and 1.0 specifies a mixed diffusion-and surfacecontrolled process. Value b closer and closer to 1, the electrode process stronger and stronger to a surface-controlled process. Very surprisingly, the relationships of peak current versus scan rate in Figure 3b shows that except BPCF-900°C, all other BPCF electrodes have b higher than 0.5 for R 1 and R 2 reactions. Furthermore, the smaller porous electrode has larger b value, and thereby possesses stronger surface-controlled electrochemical reactions. The b value of BPCF-700°C with the smallest pores (0.37 nm) for either liquid/solid (R 1 ) or solid/ solid (R 2 ) conversion reactions are 0.7 and 0.8, respectively, achieves the largest among all BPCFs, indicating its mainly surfacecontrolled reaction process.
The charge/discharge potential gap (ΔE) measured from the median discharge/charge curves at a current density of 0.1 A g −1 (Figure 3c) can be used to gauge the polarization degrees on the four BPCF electrodes for comparison. The smaller the ΔE, the lower the polarization of the electrode. The magnitude order of ΔE is 374 mV (BPCF-700°C) < 415 mV (BPCF-600°C) < 464 mV (BPCF-800°C) < 589 mV (BPCF-900°C), respectively, and the smallest pore-structured carbon (0.37 nm)-composed C/S cathode has the lowest polarization, which well agrees with the electrocatalytic order of the tested BPCF electrodes determined by the CV results discussed above. As the results discussed above, BPCF samples have different average micropore sizes. R 1 is ascribed to liquid/solid conversion reaction, and R 2 should be the solid/solid reaction. These results indicate that R 2 reaction happens in the first few cycles for all BPCF samples. However, with consumption out of active materials in these less large pores or/and outside surface for the BPCF-700 sample, only solid/solid conversion reaction (R 2 ) exists for long cycles. Guo et al. [22] work in Figure 1b shows the same results with R 1 in the first few cycles, but no R 1 reaction anymore after longer cycles, symbolling that only solid-solid conversion reaction (R 2 ) occurs in very small pore structured electrodes. As shown in Figure 3d, the relationship between specific capacity and pore size of BPCF electrodes is a volcano curve, which is dependent on the pore size, and the highest specific capacity goes to the smallest microporous C/S composite electrode (0.37 nm).
Electrochemical impedance spectroscopy (EIS) was used to further investigate the electrocatalytic behavior of the four differently poresized electrodes (Figure 3e). The equivalent circuit model of all BPCFbased cathodes can be expressed as the inset in Figure 3e. For all four electrodes, the EIS spectra display two typical semicircles, which represent R int , the interphase contact resistance between the current collector and reaction sites ( Figure S5, Supporting Information) and R ct , the charge-transfer resistance at the interface of electrode/electrolyte, respectively. [31,32] The values of R int and R ct can be obtained from Figure 3e by simulation according to the equivalent circuit, and the results are listed in Table S1, Supporting Information, showing that the 700°C-treated BPCF has the lowest charge transfer resistance (247 Ω) than others (600°C @ 437.1 Ω, 800°C @ 310.6 Ω, and 900°C @ 316.2 Ω), possessing the fastest electron transfer among all BPCF-based cathodes, which is in good agreement with the results given by the CVs in Figure 3a,b. It is noted that the 600°C-treated host with the largest charge transfer resistance can be ascribed to its low graphitization with poor electronic conductivity as shown in the measured Raman spectra (Figure 1i). The relation of Z 0 versus ω −1/2 in Figure 3f clearly shows the 600°C-, 700°C-and 800°C-treated BPCFs have an almost same slope and are much smaller than that of 900°C-treated one. A smaller slope indicates a higher diffusion coefficient (D) in the relation of Z 0 versus ω −1/2 over a low-frequency region. [33] The results in Figure 3f identify that the former three electrodes with pore sizes than 5 nm possess diffusion rate much faster than that of the latter electrode with a pore size of 0.57 nm. These further confirm that the smaller microporous structures have a mainly surface-controlled electrochemical process rather than a diffusion-controlled process in contrast to the diffusion-controlled process occurred on the 900°C-BPCF electrodes.
The 600-, 700-, 800-, and 900-BPCF cathodes were all loaded with 1 mg cm −2 of sulfur to evaluate the rate performance of Na-S batteries by Galvanostatic charge-discharge (GCD) method. Figure 4a shows that the 700-BPCF cathode delivers the highest discharge capacity of 933 and 436 mAh g −1 at 0.1 A g −1 and at 2 A g −1 , respectively, while the 600-, 800-and 900-BPCF at 0.1 A g −1 /2 A g −1 achieve 610/229, 830/255 and 400/52 mAh g −1 , respectively. These have very consistent performance tendency for all BPCF cathodes with the CVs and EIS results discussed above. Figure 4b shows that galvanostatic dischargecharge profiles of 700-BPCF cathode at high cycling rates of 1 and 2 A g −1 accomplish discharge capacities of 528 and 439 mAh g −1 , respectively, while giving discharge plateaus at 1.1 and 1.05 V, respectively. To the best of our knowledge, this outstanding rate performance indeed ranks the best among all reported plain carbon-sulfur composite cathode-based Na-S batteries (Figure 4c). [9,12,17,25,26,34] The long cycle galvanostatic charge/discharge started at 0.05 A g −1 for the first cycle, which achieve 1873, 2408, 2061 and 1976 mAh g −1 for 0.37, 0.48, 0.41 and 0.57 nm-BPCF cathode, respectively. All capacities apparently exceed the theoretic value of a Na-S battery (1675 mAh g −1 ). It has been reported that carbon often makes capacity contributions above 0.8 V, [26,35] and we believe the biomass-derived carbon contains more electrochemically active species, which even contribute more to the capacities. This is why the discharge capacities for the first cycle are over the theoretic value. However, we can consider that the BPCF/S composite cathode-based Na/S batteries realize the theoretic values, especially the 0.37 nm-C/S-based one, which delivers the highest initial capacity. Then the long cycle-charge/discharge profiles continued at 1 A g −1 (Figure 4d), demonstrating that the initial-specific discharge capacities are 605, 426, 365 and 90 mAh g −1 for 0.37, 0.48, 0.41 and 0.57 nm-BPCF cathode, respectively, illustrating a magnitude order of 0.37 nm-BPCF > 0.48 nm-BPCF > 0.41 nm-BPCF > 0.57 nm-BPCF, b) The b value of oxide-and reduction-peak. c) The potential difference of various simples at 0.1 A g −1 . d) The relationship between specific capacity, treatment temperature and pore size. e) The A.C impedance (R e is the resistance of electrolyte, R int is the interphase contact resistance between the current collector and reaction sites, and R ct is the charge-transfer resistance among the interface between the conductive agent and the electrolyte. Z w is diffusion impedance to probably signify sodium ions diffusion process). f) The corresponding relationship between Z 0 and the square root of frequency (ω −1/2 ) in the low-frequency region, measured after 50 cycles at 1 A g −1 .
Energy Environ. Mater. 2023, 6, e12634 5 of 10 among which the 0.37 nm-BPCF one achieves the highest specific discharge capacity. Moreover, a plain 0.37 nm-BPCF cell (carbon loading of 1 mg cm −2 only without sulfur loading) shows an initial capacity of 267 mAh g −1 for the first cycle ( Figure S8a, Supporting Information) but sharply drops to 3-4 mAh g −1 for the following long cycles at 1 A g −1 ( Figure S8b, Supporting Information). This clearly indicates that carbon in the 0.37 nm-BPCF, carbon/sulfur cathode mainly plays a role of host or support for sulfur active material and has insignificant contribution to the high specific capacity. Furthermore, the 0.37 nm-BPCF cathode can still offer a discharge capacity of 250 mAh g −1 and 99.1% of columbic efficiency at 1 A g −1 after 1500 cycles, but the 600-, 800-, and 900-BPCF batteries drop their discharge capacities to 234, 263 and 80 mAh g −1 , respectively only after 226, 255 and 257 cycles. In the long-life experiments, the cell measurements were conducted at 25°C, but at 300, 450, and 1000 cycle 10°C was used for 5-10 h to gain the low-temperature performance and then 25°C was used again to continue the cycling. Apparently, the 0.37 nm-BPCF cathode demonstrates an outstanding long-cycle life @ 1 A g −1 performance with an average decay rate of 0.036% per cycle, which is much superior to the 0.41, 0.48, and 0.57 nm-BPCF cathodes, and also ranks the best among all reported carbon-sulfur composite cathode-based Na-S batteries (Figure 4e).
Field emission scanning electron microscope images of different pore-sized BPCF electrodes were measured after 50 cycles at 1 A g −1 as in Figure 5. They clearly display that less or more pieces of solid membranes on the BPCF surface. The chemical compositions of the membrane pieces after cycles were examined by XPS as shown in Figure 5c, f,i,l. The binding energies of the membrane after cycles are 169.2 and 168.0 eV, which are higher than that of the S 2-4 molecules attributing to the sulfur atoms located at the chain ends possessing higher binding energy than the middle ones. [22,36,37] In our ex-situ FESEM-EDS mapping scanning experiments, the electrode (after 50 cycles) was washed completely to avoid possible Na + left from electrolyte followed by drying at 120°C in the air. The oxygen is ascribed to the adsorbed oxygen during the air drying while F should be from the additive reagent (3 wt.% FEC) containing F element, possibly due to its strong adsorption with carbon. However, a sodium-to-sulfur element ratio of 2:1 is confirmed by the EDS analysis ( Figure S9, Supporting Information). Thus, we can conclude that the formed intact membrane on the BPCF surface is Na 2 S. It can be clearly seen that the 800°C and 900°C-BPCF electrode surface spectacles extremely nonuniform fish scale-like large chunks with voids but without intact membrane (thickness of 30-50 nm, Figure 5a,b,d,e), while the 600°C-BPCF cathode surface image mainly displays stacked loose micrometer-sized particles with a lot of voids (Figure 5j,k), which obviously results in poor electronic conductivity for poor electrochemical performance as shown above (Figure 3e, f). In contrast, the 700°C-BPCF exhibits an intact wrinkle Na 2 S membrane covered on BPCF surface without any interrupt, exposed carbon and void region (Figure 5g,h).
In our work, the derived 0.37, 0.41, 0.48, and 0.57 nm-BPCF based S/C composite cathodes succeed the theoretical value for the first cycle, which also verify a complete reduction process of sulfur during the discharge of the in Na-S batteries. However, these differently sized microporous BPCF/S composite cathodes realize distinguished battery performance, especially the cycle life. In particular, the 0.37 nmmicroporous structured C/S cathode in all tested Na-S batteries BPCF cathode. c) Rate performances of 700°C BPCF samples compared to references. [9,12,17,25,26] d) Long-term cycling stability of the BPCF sample with various processed temperature at a current density of 1 A g −1 . e) Comparison of cycle numbers and capacity retentions of recently reported RT Na-S batteries with the current work in which deeper color refers to greater current density. References for [S8] etc. are given in the Supporting Information.
Energy Environ. Mater. 2023, 6, e12634 6 of 10 provides the highest electrocatalytic activity, largest specific capacity and longest charge/discharge cycle life. As the FESEM images shown in Figure 5, only the 0.37 nm-microporous structure can grow an intact Na 2 S membrane. We can reasonably conclude that the intact Na 2 S membrane plays the key role in its superior long cycle life performance even at a high discharge rate of 1 A g −1 by effectively blocking the dissolution of polysulfides. The effect of micropore sizes of BPCFs on performance of Na-S batteries and a mechanism for the 0.37 nm microporous BPCFs to achieve the best performance is proposed as schematically illustrated in Figure 6. Guo et al. [22] experimentally and theoretically confirm that a 0.5 nm microporously structured C/S composite cathode in its RT Na-S battery strictly confines a solid phase reduction of S molecules to Na 2 S 2 (Equation 7) followed by further reduction to the final product of Na 2 S (Equation 6) for a complete reduction of sulfur. The simulation and calculation have been done by Guo's work for 0.40 nm work, [22] of which the conclusion is also applicable to our work and strongly support that our 0.37, 0.41, and 0.48 nm-BPCFs can also enable the same solid phase conversion process for a complete sulfur-reduction since their pore sizes are smaller than 0.5 nm. It is very interesting why a microporous structure down to an ultramall size of 0.37 nm leads to formation of an intact Na 2 S membrane. A squeezing mechanism to form the intact membrane is proposed as shown in Figure 6. The melt atomic sulfur can impregnated into the 0.37 nm pores due to its small size of 0.2 nm by 50% load in our work. During discharge, Na + with a size of 0.2 nm diffuses into the 0.37 nm inner pores reacting with S to produce Na 2 S 2 , followed by further reduction to Na 2 S. Since the sizes of S −1 and S −2 are 0.36 and 0.368 nm, respectively, and Na + locates at the two ends of S −2 indicated by its XPS data presented above, the 0.37 nm should sterically force Na 2 S crystals to tightly line up. Furthermore, due to the expansion pressure produced during discharge in the inner pores, we argue that well-organized Na 2 S crystal can be squeeze out to the electrode surface generating an intact membrane, which is poorly conductive and should be very hard to oxidize back to polysulfides or/and S 8 . This can well explain that such 0.37 nm-porous carbon can deliver long cycle performance. In contrast, the 0.41, 0.48, and 0.57 nm-porous carbons cannot form the intact membrane, it should be mainly due to their larger sizes, which cannot produce tightly stacked Na 2 S crystals while allowing the loose crystals lost into electrolyte by oxidation during the charge process. This why they cannot effectively inhibit the dissolution of polysulfides for long cycle life. It is worthy of note that the 0.37 nmmicroporous BPCF/S cathode also possesses the highest electrocatalytic activity and best rate performance among all BPCF/S-based Na-S batteries, which could be ascribed to the extreme proximity of S atoms to the inner reaction surface of the ultra-small pores for a mainly surface-controlled electrochemical process and fast electron transfer, which can be supported by the EIS and CV results discussed above. In addition, the poorly performed 600°C-BPCF cathode possibly due to its low conductivity resulting from its low graphitization by the low heattreatment temperature (600°C).

Conclusion
A biomass-derived carbon was tuned down to a ultrasmall 0.37 nm (80%) microporous structure as a cathode in RT Na-S batteries for the first time, which enables growth of an intact uniform Na 2 S membrane to effectively constrain the dissolution of polysulfides while realizing a fast direct solid phase conversion for a complete reduction of S to Na 2 S. The 0.37 nm-BPCF/S cathode delivers a sulfur loading of 1 mg cm −2 (50 wt.%), an excellent rate capacity (933 mAh g −1 @ 0.1 A g −1 and 410 mAh g −1 @ 2 A g −1 ), long cycle performance (0.036% per cycle decay at 1 A g −1 after 1500 cycles), and a highenergy density for 373 Wh kg −1 (0.1 A g −1 ) based on whole electrode weight (active sulfur loading + carbon), ranking the best among all reported plain carbon/S composite cathode-based RT Na-S batteries. A squeezing mechanism is proposed for formation of an intact membrane of Na 2 S, which is produced from a solid phase conversion under a restriction by the ultrasmall pores (0.37 nm), and could be partially squeezed out to grow an intact membrane on the electrode surface, while the extreme proximity of sulfur atoms to the inner reaction surface of the pores realizes a mainly surface-controlled electrochemical process and enpower a fast electron transfer. This work not only holds great potential of a green chemistry to recycle wastes for sustainable energies, but also demonstrates design of a ultra-small pore structure resulting in an intact membrane to effectively block the severe the dissolution of polysulfides toward highperformance RT C/S composite Na-S batteries.

Experimental Section
Preparation of biomass porous carbon framework with different pore structures: First, 5 g Pecan nut shell was directly carbonized at 450°C in a muffle furnace for 1 h. Then, the obtained pyrolytic carbons were mixed with KOH powder in a mass ratio of 1:2 and activated at 600°C, 700°C, 800°C, and 900°C, respectively, under Ar atmosphere for 2 h. The obtained samples were treated with HCl solution (10 wt.%) for 3 h and followed by washing with deionized water until the pH of filtrate arrived about~7.0. After being dried at 373 K for 6 h, a series of activated samples were obtained. Therefore, the obtained materials were denoted as BPCF-600, BPCF-700, BPCF-800, and BPCF-900, respectively.
Preparation of BPCF@S cathode material: The BPCF and S were mixed and grounded according to the weight ratio of 1:1, and then the mixture was annealed at 155°C for 12 h in raction kettle to obtian the BPCF@S cathode materials.
Electrochemical measurements: The working electrodes for sodium-sulfur cells were prepared by casting a N-methyl-2-pyrrolidone (NMP) slurry containing 80 wt.% S@BPCF (700-BPCF) or/and S@NC, 10 wt.% KB (Ketjen Black) carbon and 10 wt.% poly(vinylidene fluoride) binder were uniformly coated on carboncoated Al foil substrates and dried at 60°C for 8 h. The test cells were assembled with metallic sodium as the negative electrode, glass fiber separator (Whatman GF/F), and 1 M NaClO 4 in 1:1 ethylene carbonate/propylene carbonate, and 3 wt.% fluoroethylene carbonate additive (PC/EC + 3 wt.% FEC) electrolyte under an argon-filled glovebox, where water and oxygen concentrations were kept at <0.1 ppm. Electrochemical studies were performed using a LANHE testing system. Cells were operated in a voltage window of 0.5-2.3 V (vs Na/Na + ). Specific capacities were calculated with the mass of sulfur (the average electrode mass is~2 mg cm −2 and sulfur loading is~1 mg cm −2 ). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were measured with a CHI 760D electrochemical workstation over a frequency range of 100 kHz to 100 mHz.
Material characterization: The morphology and microstructure of the asobtained products were characterized by the field emission scanning electron microscopy (FESEM: JSM-7800F, Japan) and transmission electron microscopy (TEM: JEM-2100, Japan). The distribution of elements for as-prepared samples were observed by Energy-dispersive spectroscopy (EDS: JSM-7800F). The crystalline structures were examined by a powder X-ray diffraction (XRD-7000) with Cu Kalpha radiation (λ = 1.5406Å). The Raman spectrum was measured by Invia Refl (Renishaw, UK) with a 532 nm excitation laser. X-ray photoelectron spectroscopy (XPS) measurements were obtained with a spectrometer (Escalab 250xi; Thermo Scientific). The specific surface area and distribution of pore size were performed by JW-BK300, Beijing Jingwei Gao Bo Science and Technology Co., LTD. For this equipment, the application pore size range is from 0.35 to 40 nm by (HK mode for micropore analysis). [38,39]