High-performance lithium – selenium batteries enabled by nitrogen-doped porous carbon from peanut meal

: Biomass-derived porous carbon displays a great potential for lithium – selenium (Li – Se) batteries owing to its green resource and inherent structural advantages, which can e ﬀ ectively restrict the shuttle e ﬀ ect of Se cathode. Peanut meal, by-product of the extraction of peanut oil, is a promising precursor for N-doped porous carbon. However, peanut meal is di ﬃ cult to be activated in solution due to its high hydrophobicity. Thus, non-reports have been available for peanut meal-derived porous carbon used as Li – Se battery cathode host. In this work, we have innovatively proposed a very simple method of activating peanut meal by directly physically grinding the activator with the peanut meal and then annealing it to convert it into nitrogen-doped three-dimensional porous carbon (N-PC) with rich nanoscale pore size structures, which is then used as the Se host for Li – Se batteries. The N-PC shows a high speci ﬁ c surface area of 938.872 m 2 g − 1 . The Se/N-PC composite cathode delivers a speci ﬁ c capacity of 461.4 mA h g − 1 for 250 cycles at 0.2 C, corresponding to a high-capacity retention of 97.2%. Moreover, the Se/N-PC composite maintains a high capacity over 340.1 mA h g − 1 after 1,000 cycles at a high current density of 2 C. Our work e ﬀ ectively resolves the hydrophobic biomass activation problem and manufactures abundant and low-cost Se host for Li – Se batteries.


Introduction
The electric vehicles desire high energy density, long life cycle, and low-cost rechargeable batteries.Today's lithiumion batteries are becoming increasingly difficult to meet these demands.It is critical to develop novel electrode materials with high energy density [1][2][3][4][5][6][7][8][9][10].Selenium (Se) is considered to be a promising cathode material because of its higher-volume energy density (2,528 W h L −1 ) [11,12].However, its practical application is restricted by three major challenges.First of all, in ether-based electrolyte, the intermediate polyselenides (Li 2 Se x , x > 4) during cycling are easily dissolved into the electrolyte, which cause the shuttle effect, leading to side reactions and poor cycling performance [13].Furthermore, the enormous volume expansion of Se cathode during charging and discharging processes results in the electrode structure destruction and capacity loss [14].Finally, the electrical conductivity of Se is not high, which causes sluggish reaction kinetics of Se and poor rate performance [15].
To overcome the aforementioned issues, numerous strategies have been adopted.Porous carbon is the widely used Se host material due to the high specific surface area and electrical conductivity, which can effectively restrict the shuttle effect of Se cathode and enhance the electrical conductivity of Se [16][17][18][19].Various carbon materials prepared using nanotechnology, such as CMK-3 [20], graphene [21], and carbon spheres [22], are frequently used as good building blocks to architect fibrous carbon substrates.For instance, Yang et al. [20] prepared ordered mesoporous carbon (CMK-3) materials using the template method and attached selenium to CMK-3 materials by the melt-diffusion method.Stronger Se-C bonds confine the selenium molecules in the mesopore, thus improving the stability.The Se/CMK-3 cathode shows a specific capacity of 335 mA h g −1 at 0.1 C.However, the preparation of porous carbon substrates using the template method is complex, and the reagents used are environmentally hazardous and costly.In contrast, biomass carbon materials have significant advantages.Bio-inspired material, an abundant, renewable, and sustainable green resource, is the excellent precursor for porous carbon materials [23][24][25].The biomass-derived carbon has many advantages.The biomass has natural ordered porous structure, which enables the biomass-derived porous carbon to possess high specific surface area [26,27].Moreover, hierarchical biomass-derived porous carbon composed of hard carbon has stable structure and high strength, which shows excellent cycling performance [28].The inherent N, S, P, and O elements in biomass can be uniformly doped in biomass-derived carbon during the carbonization process, which significantly improves the electrochemical performance of biomass-derived porous carbon.
Up until now, many biomasses have been reported as precursors for porous carbons in energy storage application.For instance, Deng et al. [12] reported porous nitrogen-doped carbon (HPNC) from water hyacinth activated by KOH solution and calcined at 750°C.The Se/HPNC cathode maintains a high capacity of 410 mA h g −1 at 1.0 C after 500 cycles.Peanut meal is a type of biomass material that is by-product of peanut oil processing.In recent years, several million tons of peanut meal waste is created per annum [29], which is a promising precursor for N-doped porous carbon.However, peanut meal is difficult to be activated in solution due to its high hydrophobicity.Thus, non-reports have been available for peanut meal-derived porous carbon used as a cathode host in Li-Se batteries.
Herein, we develop a grinding activating/annealing method to converse peanut meal to an N-doped threedimensional porous carbon (N-PC), which is then used as the Se host for Li-Se batteries.We used environmentally friendly ZnCl 2 instead of KOH as an activation agent.In order to overcome the unmixed problem of peanut meal in the activation solution system, the peanut meal and ZnCl 2 powder were ground to obtain a uniform activated mixture.Then, the mixture was calcined at high temperature to obtain N-PC.The N not only enhances the electrical conductivity of porous carbon but also possesses an adsorption effect on Se.Thus, the Se/N-PC composite delivers a specific capacity of 461.4 mA h g −1 for 200 cycles at 0.2 C, corresponding to a high capacity retention of 97.2%.Moreover, the Se/N-PC composite maintains a high capacity of 340.1 mA h g −1 after 1,000 cycles at a high current density of 2 C. Our work may open a door to resolve the hydrophobic biomass activation problem and fabricate other biomass-derived porous carbon for various applications.

Materials
All chemicals were provided by Sigma-Aldrich.Conductive carbon black was purchased from Sinopharm Group.Aluminum foil was purchased from Shenzhen Kejing Co. Ltd.Lithium foil was purchased from Shenzhen Baker Energy Technology Co. Ltd.Diaphragm (Celgard 2400) was purchased from Celgard, USA.Peanut meal was purchased from www.taobao.com.

Preparation of N-PC and Se/N-PC
The peanut meal was dried in a vacuum oven at 70°C for 12 h.The dried peanut meal was mixed with the activator (ZnCl 2 ) in a ratio of 1:2; then, it was put into a mortar and ground for 15 min to make the peanut meal and the activator fully mixed.The mixed samples were placed in a tube furnace and heated to 800°C at a heating rate of 2°C min −1 under a nitrogen atmosphere for 2 h to obtain a black product.The black product was immersed in 1 M hydrochloric acid solution and stirred for 2 h, then washed with deionized water until the solution became neutral, and dried at 70°C for 12 h to finally obtain N-PC.As a comparison, N-doped carbon (N-C) was prepared from peanut meal activated by ZnCl 2 solution and calcined at 800°C for 2 h.
Se powder was loaded in N-PC by the melt infiltration method.The commercial Se powder and N-PC were fully ground in a mortar at a ratio of 1:1.The mixture was then kept at 270℃ for 12 h in an argon atmosphere in a tubular furnace, and finally, the Se/N-PC composite was obtained.The synthesis process of Se/N-PC is illustrated in Figure 1.

Structural characterization
X-ray diffraction (XRD, Rigaku-Rint-2000, Cu-Ka ray as radiation source, working voltage of 40 KV, scanning speed of 5°min −1 , step size of 0.02°) was used to study the phase composition and unit cell of the material parameters.The vibrational energy levels and functional groups of the molecules in the samples were investigated using Raman spectroscopy (WTTEC-Alpha-300M, wavelength 632 nm).X-ray spectroscopy (XPS, Thermal-VG Multilab-2000, Al-Ka radiation source) was used to analyze the elemental composition and chemical state of the sample surface.Scanning electron microscope (SEM, JEOL-JSM-7800F) and transmission electron microscope (TEM, JEOL-JEM-2100) were used to study the microscopic morphology and crystal structure of the samples.Thermogravimetric analysis (TG, Mettler-Toledo, temperature range from 25 to 800°C, heating rate of 10°C min −1 ) was used to determine the content of different components of the complex.The specific surface area and pore size distribution of the samples were tested by a specific surface area tester (Tristar-II-3020) based on Brunauer-Emmett-Teller (BET), Barrett-Joyner-Halenda (BJH), and Horvath-Kawazoe theories, respectively.

Electrochemical measurements
About 80 wt% active material, 10 wt% Super-P, and 10 wt% polyvinylidene difluoride (PVDF) binder were mixed in 1-methyl-2-pyrrolidone to form a slurry, which was then uniformly coated on foam nickel and dried at 80°C for 12 h as the cathode of the battery, and the Se loading of each pole piece is about 1.0 mg cm −2 .Celgard 2500 was used as the separator, 1 M LiPF 6 in ethylene carbonate and diethyl carbonate (1:1 by volume), containing 5% fluoroethylene carbonate additive as the electrolyte, and pure lithium foil as the counter electrode.CR2025 assembled button cell for electrochemical experiments.Cyclic voltammetry (CV, CHI 660E, Shanghai Chenhua Electrochemical Workstation) was used to measure the equipped cells with a scan rate of 0.02 mV s −1 and a voltage range of 1.0-3.0V.In the battery test system (LAND, CT2001A), the constant current chargedischarge and cycle performance of the coin-type battery in the potential range of 0.01-3 V were tested.Electrochemical impedance (EIS) measurements were performed in the frequency range 10 −2 -10 5 Hz with an amplitude of 5 mV.

Results and discussion
We used the ZnCl 2 solution immersion method and direct mixed grinding method to activate peanut meal (Figure 2a  and b).In order to prove the grinding activating validity, the morphologies of N-C and N-PC were studied by SEM.The peanut meal-derived carbon without activating shows an irregular particle structure (Figure S1) in which no pores were observed.The N-C shows a similar morphology to peanut meal-derived carbon without activating (Figure 2c and d).Peanut meal is a by-product of peanut oil processing, which enables peanut meal high hydrophobicity, so that peanut meal could not be activated in the activation solution system; thus, N-C shows a block morphology.Figure 2e shows the N 2 adsorption/desorption isotherm of N-C.The specific surface area of N-C is only 65.4 m 2 g −1 , confirming the particle structure.In contrast, the N-PC shows a three-dimensional interconnected porous structure (Figure 2f and g). Figure 2h shows the N 2 adsorption/ desorption isotherm of N-PC.It can be seen that the N 2 adsorption/desorption isotherm of N-PC belongs to a typical type Ⅳ isotherm, which indicates that the N-PC is mainly dominated by micropores.There is an obvious hysteresis loop in the pressure range of 0.3-1.0,which indicates that there are also mesopores in N-PC.The specific surface area of N-PC is calculated to be 938.872m 2 g −1 .The pore volume is 0.528 cm 3 g −1 .BJH analysis shows that the pore size of N-PC ranges from 1 to 10 nm.SEM and BET results prove that the grinding activating/annealing method is effective to converse the hydrophobic peanut meal to porous carbon.The three-dimensional amorphous interconnected porous structure of N-PC provides abundant Se storage sites, and the rich mesoporous structure facilitates the lithium ion/ electrolyte transport, which gives Se/N-PC cathode materials excellent electrochemical properties.
In view of the extremely high specific surface area and abundant pore structure of N-PC, we compounded commercial selenium powder into N-PC by the melt infiltration method to synthesize Se/N-PC.The Se/N-PC are characterized by XRD, and the results are shown in Figure 3a.N-PC shows a broad peak at 25°, indicating an amorphous phase [30,31].The XRD curve of Se/N-PC composite is similar to that of N-PC.No characteristic trigonal peaks of Se are observed because the Se is completely immersed in the pores of N-PC [32].The Raman spectra of N-PC and Se/N-PC are shown in Figure 3b.Both Raman spectra have two distinct peaks around 1,350 and 1,580 cm −1 , corresponding to typical D band (≈1,350 cm −1 ) and G band (≈1,580 cm −1 ), which belong to disordered carbon and graphitic carbon [ [33][34][35].The intensity ratio of D peak to G peak (I D /I G ) effectively characterizes the degree of graphitization [36].By calculation, the I D /I G ratio of N-PC and Se/N-PC is 0.993 and 1.011, respectively, indicating that Se/N-PC has a higher degree of graphitization, which is due to the extended sp 2 C-C bond after Se is bonded to carbon.No obvious lattice fringes can be seen in the high resolution transmission electron microscope (HRTEM) image of N-PC (Figure 3c), which indicates that N-PC is amorphous, consistent with XRD results.In addition, the HRTEM image clearly shows the microporous structure of N-PC, as shown in the area marked by the red dotted line in the figure.As shown in the SEM image of Se/N-PC (Figure 3d and e), no agglomeration of selenium was found on the surface of N-PC; however, the electronic differential system (EDS) image (Figure 3h) showed that elemental Se was uniformly dispersed in N-PC, and thus, selenium has been uniformly dispersed into the pore structure of N-PC.EDS images (Figure 3f and j) show the uniform distribution of C, Se, O, and N, further confirming the uniform distribution of Se and N in Se/N-PC.TG result (Figure S2) shows that the content of Se in Se/N-PC is 53.78%.Furthermore, we can see that the TG curves of Se/N-PC show two phases, 300-420 and 420-510°C, corresponding to the weight loss of selenium in the mesopores and selenium in the micropores, respectively, which is due to the difference in the energy required to evaporate selenium in the mesopores and selenium in the micropores.More selenium was lost in the higher temperature range, suggesting that selenium nanoparticles are mainly stored in the microporous structure of N-PC.The porous structure of N-PC provides abundant sites for Se, and thus, Se can be embedded in N-PC, which inhibits the aggregation of selenium and alleviates the volume expansion of selenium.
The elemental composition and the chemical state of N-PC and Se/N-PC were further analyzed by XPS.As shown in Figure 4a, C, N, and O are presented in both Se/N-PC and N-PC.For Se/N-PC, Se is also observed in the Se/N-PC spectrum.Figure 4b-e shows the high-resolution XPS patterns of C 1s, N 1s, Se 3d, and O 1s for Se/N-PC, respectively.As shown in the high-resolution XPS spectrum of C 1s (Figure 4b), the peak can be deconvoluted into three peaks that correspond to Sp 2 hybrid graphite (284.9 eV), C-O/C-N (286.2 eV),  [37], respectively.The N 1s spectrum (Figure 4c) shows the pyridine-N peak (398.43 eV), pyrrole-N peak (399.93 eV), and graphitized-N peak (401.03eV); pyridine nitrogen and pyrrole nitrogen increase the conductivity of carbon matrix and improve the affinity of nonpolar carbon atoms for polar Se and Li 2 Se, enhancing the stability and cycling performance significantly [38].In addition, the peak at 407.21 eV can be attributed to the N-Se bond [39].Figure 4d shows the high-resolution XPS spectrum of Se 3d for Se/N-PC.Two peaks at 55.33 and 56.13 eV correspond to Se 3d 5/2 and Se 3d 3/2 .It is worth noting that the obvious wide peaks at 58.33 and 59.08 eV are attributed to the Se-N bond and Se-C bond [40], respectively, indicating that there is a strong chemical bond between the N-PC host and Se, which improves the cycling stability of Se/N-PC [41].Oxygen (Figure 4e) content may arise from physical/chemical adsorption of oxygen/moisture from atmosphere during the synthesis and/or due to HCl wash [38].The presence of the Se-C bond is very effective in inhibiting the aggregation of selenium within the carbon matrix, improving the capacity utilization of selenium.
The electrochemical performance of Se/N-PC was tested by using a coin cell, which is composed of Se/N-PC as cathode and lithium foil as anode.CV tests were first performed to study the Li + storage mechanism.Figure 5a displays the CV curves of N-PC/Se electrode and the 0.1 mV s −1 scanning rate at 0.2-1.5 V.It can be seen that Se/N-PC has only one pair of redox peaks, which corresponds to the insertion of Li + and the formation of Li 2 Se as well as the extraction process of Li + In the first cycle, cathodic scan of Se cathode demonstrates two reduction peaks at 2.37 and 1.64 V, corresponding to the conversion of cyclic Se 8 into a chainlike (linear) Se n molecule that reduces further to form Li 2 Se [38,42,43].The corresponding anodic scan consists of one strong oxidation peak at 2.03 V that signifies the single-step conversion of Li 2 Se into a chainlike Se n molecule [44].The peak at 1.64 V in the first cycle increases to 1.78 V in the subsequent cycles, which may be due to the electrochemical activation process of the cathode [45].In the subsequent cycles, the curves show a high coincidence, indicating a high electrochemical reversibility for Se/N-PC.Figure 5b displays the cycling performance of Se/N-PC at a current density of 0.2 C, and the capacity calculation was made based on the amount of Se in the composite cathode (1 C rate = 675 mA g −1 ).A high stable specific capacity of 461.4 mA h g −1 after 250 cycles is obtained.At a current density of 0.2 C, the voltage profiles of Se/N-PC composites are representatively depicted in Figure 5c.Each discharging curve shows a stable voltage plateau that is attributed to the reduction of Se to Li 2 Se, which coincides well with the CV results.In addition, after 250 cycles, it can be seen that the charge-discharge curves overlap, and the capacity decay rate is only 2.8%.As shown in the SEM images of Se/N-PC electrode after 250 cycles (Figure 6), the surface of the electrode after cycling is still flat, and its porous structure has not been damaged, further verifying its excellent cycle stability.Figure 5d shows the rate performance of Se/N-PC.It can be seen that when the current densities are 0.1, 0.2, 0.5, 1, and 2 C, Se/N-PC delivers reversible specific capacities of 532.6, 434.7, 381, 334.4,and 283.1 mA h g −1 , respectively.When the current density returned to 0.2 C, the specific capacity recovered to 458.6 mA h g −1 , which indicating a good rate performance.Moreover, Se/N-PC composite possesses a high Coulombic efficiency at both low rate and high rate from the galvanostatic discharge-charge profiles of the Se/N-PC at different current densities (Figure S3).The excellent rate performance is attributed to the abundant pores in N-PC and the high electrical conductivity of N-PC. Figure 5e displays the long-term cycling performance of Se/N-PC at a high current density of 2 C. A high reversible specific capacity of 340.1 mA h g −1 is maintained after 1,000 cycles, corresponding to a high capacity retention of 98.49%.The coulombic efficiency is retaining around 100%, suggesting excellent stability and rate performance.However, the Se/N-C only possesses a reversible specific capacity of 60.7 mA h g −1 after 950 cycles, corresponding to a capacity retention of 45.68% (Figure 5e).In addition, the Coulombic efficiency of Se/N-PC has been maintained at 100%, while Se/N-C is very unstable, further proving that the ultra-  High-performance lithium-selenium batteries  7 high specific surface area and rich pore structure of Se/N-PC endow it with excellent cycle stability.The excellent electrochemical performance of N-PC makes it an advantage in materials that have been reported as selenium hosts (Table S1).
To deeply analyze Li + storage kinetics for Se/N-PC cathode, we performed CV measurements at different scan rates from 0.2 to 1.0 mV s −1 , as shown in Figure 7a.In all CV curves, one reduction peak (peak 1) and one oxidation peak (peak 2) were observed.Moreover, the shapes of these peaks were similar.As the scanning rate increases, the reduction peak shifts to lower voltage, while the oxidation peak shifts to higher voltage.The shift may be attributed to diffusion-controlled reactions and concentration polarization.According to the following equations [46], where i and v are the peak current (mA) and scan rate (mV s −1 ), respectively.And a and b are the adjustable coefficients.The b is between 0.5 and 1.0.When b is close to 0.5, the electrochemical reaction is controlled by intercalation process.When b is close to 1.0, the electrochemical reaction is dominated by pseudocapacitive contributions [47].The b values of peak 1 and peak 2 are 0.57 and 0.64 (Figure 7b), meaning that the lithium storage mechanism is dominated mainly by the diffusion behavior.The proportion of different capacity contributions can be analyzed according to the following equation [48]: By calculating both coefficients, k 1 and k 2 , the contributions of intercalation process and pseudocapacitive contributions can be distinguished.As shown in Figure 7c, the Se/N-PC electrode exhibits a 47% pseudocapacitive contribution at a scan rate of 0.2 mV s −1 .It increases from 47 to 67% with the scan rate increasing from 0.2 to 1.0 mV s −1 .The high proportion of pseudocapacitive contribution indicates the fast reaction kinetics of Se/N-PC in Li-Se batteries battery.In addition, we also performed EIS analysis, as shown in Figure 7d.After 50 cycles, the electrochemical transfer impedance (R ct ) is significantly decreased compared with that before cycling [42,49].Moreover, the impedance did not significantly change after 150 cycles, indicating that Se/N-PC cathode has excellent cycling stability.

Conclusion
In conclusion, to avoid that the peanut meal, which is rich in oils and fats, cannot be fully infiltrated with the activator in the aqueous solution of activator, we used a very simple physical grinding method to fully mix the activator directly with the peanut meal powder and obtained nitrogen-doped 3D porous carbon by high-temperature annealing in an inert atmosphere.Then, the Se/N-PC cathode composite was fabricated by the melt infiltration method.The three-dimensional interconnected nanoscale porous structure of N-PC provides abundant pores for Se.Moreover, the N doping enhances the adsorption of Se and alleviates the loss of Se, improving the utilization of Se.Therefore, the Se/N-PC composite cathode retains a specific capacity of 461.4 mA h g −1 for 250 cycles at 0.2 C with a high capacity retention of 97.2%.Even at a high current density of 2 C, the Se/N-PC composite maintains a high capacity over 340.1 mA h g −1 after 1,000 cycles.Our work provides some insights into the application of hydrophobic-biomass precursor-derived carbon materials for energy storage devices.

Figure 1 :
Figure 1: Schematic illustration of the synthesis process of Se/N-PC.

Figure 2 :
Figure 2: Synthesis diagram of N-C (a) and N-PC (b); morphological characterizations of N-C and N-PC: SEM images of N-C (c and d), N-PC (f and g); N 2 adsorption/desorption isotherms of the N-C (e), N-PC (h), and the inset of panel (e) and (h) shows the pore size distribution of N-C and N-PC, respectively.

Figure 3 :
Figure 3: Characterizations of N-PC and Se/N-PC: (a) XRD patterns, (b) Raman spectrum; (c) HRTEM image of N-PC; (d and e) SEM images of Se/N-PC; and SEM image (f) and elemental mapping (g-j) of Se/N-PC.

Figure 7 :
Figure 7: Electrochemical kinetics mechanism: (a) CV curves at different scan rates from 0.2 to 1.0 mV s −1 ; (b) log (scan rate) vs log (current) plot, as fitted from CV curves; (c) bar chart showing the contribution ratio of pseudocapacitive at different scan rates; and (d) AC impedance spectra of the Se/N-PC electrode after various discharge-charge cycles (frequency range of 10 −2 -10 5 Hz).