Rapeseed Meal-Derived Three-Dimensional Porous Carbon for High-Performance Lithium–Selenium Batteries

Abstract

Lithium–selenium batteries (LSeBs) have potential applications in mobile electronic devices and electric vehicles due to their high theoretical volume specific capacity (3253 mAh cm⁻³). However, their cycling performance is poor because of the serve shuttle effect. Porous carbon can restrict the shuttle effect. However, past porous carbon is cumbersome, expensive, and unsuitable for large-scale production. In this work, we develop an annealing/etching method to convert biowaste (Rapeseed meal) to a N, S co-doped three-dimensional porous carbon (NSPC) which is then used as the Se host for LSeBs. The Se/NSPC composite delivers a specific capacity of 496.5 mAh g⁻¹ for 200 cycles at 0.2 C, corresponding to a high-capacity retention of 91.8%. Moreover, the Se/NSPC composite maintains a high capacity over 200 mAh g⁻¹ after 1000 cycles at a high current density of 2 C. Our work provides an efficient approach to addressing biowaste issues while simultaneously facilitating the mass production of economical Se hosts for LSeBs.

1. Introduction

With the rapid development of society, the consumption of traditional fossil fuels such as coal, petroleum, and natural gas has been increasing, leading to a severe energy crisis. Moreover, the combustion of fossil fuels generates large amounts of CO₂, causing severe environmental issues. Therefore, it is crucial to develop novel, efficient, and environmentally friendly energy systems [1,2,3,4,5]. As a portable and recyclable energy storage device, lithium-ion batteries (LIBs) possess advantages of lightweight, high safety, excellent cycling performance, and environmental friendliness which make LIBs crucial energy storage systems [6,7,8,9,10]. They are widely used in various fields such as portable electronic devices (smartphones and laptops), electric vehicles (EVs), and energy storage systems.

In recent years, the demand for EV energy storage systems has been increasing, which imposes higher requirements (lightweight, thin, durable, and long cycle life) on energy storage technologies. However, the energy density of current LIBs remains relatively low due to the low theoretical specific capacity (<300 mAh g⁻¹) of cathode material such as LiCoO₂ and LiFePO₄. As a result, LIBs are gradually failing to meet the increasing energy demands of modern society, highlighting the urgent need to develop novel cathode materials with higher energy densities. Currently, lithium–sulfur batteries (LSBs) based on conversion reactions have attracted widespread attention due to their high theoretical specific capacity (1675 mAh g⁻¹) [11,12,13]. Sulfur is abundant, widely distributed, and easily accessible on earth. However, the practical application of LSBs is hindered by the poor conductivity of sulfur and the severe “shuttle effect” caused by polysulfides during charge–discharge processes. Compared to sulfur, selenium (Se) has a low theoretical specific capacity (675 mAh g⁻¹). However, Se has a higher melting point (~220 °C), enabling lithium–selenium batteries (LSeBs) to work at high temperature. Moreover, Se exhibits higher electrical conductivity (1 × 10⁻⁵ S cm⁻¹) compared to sulfur (5 × 10⁻³⁰ S cm⁻¹) [14]. During electrochemical reactions, S is highly reactive and prone to side reactions while Se demonstrates relatively lower reactivity which reduces the formation of byproducts. These advantages make Se a promising cathode material for secondary batteries. Nevertheless, improving the conductivity of Se remains necessary and challenges such as volume expansion, low coulombic efficiency, and poor cycling stability must be addressed.

Combining Se with carbon materials is considered to be a feasible strategy to enhance the performance of LSeBs. Carbon-based hosts include graphene, carbon nanotubes, mesoporous carbon, microporous carbon, and hierarchical porous carbon have been reported [15,16,17]. In the field of energy conversion and storage, porous carbon has been widely applied in LIBs, sodium-ion batteries (SIBs), lithium-air batteries, and supercapacitors owing to abundant frameworks, excellent conductivity, and tunable porosity [18]. However, conventional synthesis of porous carbon such as hard-templating and soft-templating often involves complex procedures and high cost, limiting their practical applications. Therefore, developing simple and low-cost methods to obtain porous carbon materials is a key research focus.

Many biomass materials possess natural hierarchical structures [19,20,21,22,23,24,25,26]. Biomass materials are often obtained from natural waste or industrial byproducts, offering cost-effectiveness and environmental friendliness [27,28,29,30,31,32]. Carbonization and activation treatment can convert them into valuable porous carbon materials. And the inherent N, S, P, and O elements in biomass can be uniformly doped in biomass-derived carbon [33,34]. However, different biomass materials have varying N, S, and P contents due to differences in biological origin, growth conditions, and processing history. Thus, it should address the problems of reproducibility of the composition of biomass-derived carbon. Rapeseed meal, a residue from rapeseed oil extraction, is inexpensive, carbon-rich, and biodegradable. However, rapeseed meal has limited applications. Most rapeseed meal is used as fertilizer, leading to resource wastage and environmental pollution. Given the increasing severity of the energy crisis and the growing emphasis on ecological protection, developing high-value downstream products from rapeseed meal aligns with the principles of resource conservation and environmental sustainability. In this work, we develop an etching/annealing method to convert a discarded biowaste material (rapeseed meal) to a N, S co-doped three-dimensional porous carbon (NSPC) which shows high electrical conductivity. Thus, the Se/NSPC composite delivers a specific capacity of 496.5 mAh g⁻¹ for 200 cycles at 0.2 C, corresponding to a high-capacity retention of 91.8%. Moreover, the Se/NSPC composite maintains a high capacity over 200 mAh g⁻¹ after 1000 cycles at a high current density of 2 C. Since NSPC has the advantages of a wide range of sources, cheap price, and simple synthesis, this NSPC we developed here may open a door to design and fabricate other carbons for various applications.

2. Experimental

2.1. Materials

All chemicals were supplied by Sigma-Aldrich (St. Louis, MO, USA). Rapeseed meal was purchased from www.taobao.com.

2.2. Preparation

The rapeseed meal was dried in a vacuum oven at 80 °C for 24 h. The dried rapeseed meal was then mixed with the activator (ZnCl₂) in a 1:2 mass ratio and ground in a mortar for 0.3 h to ensure thorough homogenization. The mixture was subsequently transferred to a tube furnace and carbonized at 800 °C (heating rate: 5 °C min⁻¹) under a nitrogen atmosphere for 4 h, yielding a black product. The obtained material was treated with 1 M HCl for 4 h under stirring, washed repeatedly with deionized water, and, finally, dried at 80 °C for 24 h to produce N, S co-doped three-dimensional porous carbon (NSPC).

Se was incorporated into NSPC via a melt-infiltration method. Briefly, commercial Se powder and NSPC were manually mixed in a 1:1 mass ratio and ground thoroughly in a mortar. The homogeneous mixture was then sealed in a stainless-steel autoclave and heated at 260 °C for 12 h under an argon atmosphere, resulting in the final Se/NSPC composite. The synthesis process is illustrated in Figure 1.

2.3. Characterization

The phase composition was identified by X-ray diffractometer (XRD, Rigaku-Rint-2000, working voltage 40 KV, scanning speed 5° min⁻¹, step size 0.02°) (Rigaku Corporation, Tokyo, Japan) with Cu Kα radiation (λ = 0.15406 nm) at a scanning angle (2θ) range of 10° to 70°. X-ray photoelectron spectra (XPS, Thermo Scientific K-Alpha) (Waltham, MA, USA) measurements were performed using an Al Kα X-ray source. The morphology and surface details of samples were analyzed by scanning electron microscopy (SEM, JEOL JSM-7800F Field Emission) (Manufacturer JEOL Ltd, Tokyo, Japan) and transmission electron microscopy (TEM, JEOL JEM, 1011) (Manufacturer JEOL Ltd, Tokyo, Japan). Using thermogravimetric analysis (TG, Mettler-Toledo, temperature range from 25 °C to 800 °C, heating rate of 10 °C min⁻¹) (Zurich, Switzerland) to study 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) (Anton Paar acquired Quantachrome Instruments, Inc., Boynton Beach, FL, USA) based on Brunauer–Emmett–Teller (BET), Barrett–Joyner–Halenda (BJH) and Horvath–Kawazoe (HK) theories, respectively.

2.4. Electrochemical Measurements

A slurry was prepared by mixing 80 wt% active material, 10 wt% SUPER-P conductive carbon, and 10 wt% PVDF binder in N-methyl-2-pyrrolidone (NMP). The slurry was uniformly coated onto aluminum foil and dried at 80 °C for 24 h to form the cathode, with a selenium loading of ~1.0 mg cm⁻². CR2025 coin cells were assembled using the prepared cathode, a Celgard 2500 separator (LLC-Headquarters & Charlotte Manufacturing Facility, Charlotte, NC, USA), and lithium foil as the counter electrode. The electrolyte consisted of 1 M LiPF₆ in a 1:1 (v/v) mixture of ethylene carbonate (EC) and diethyl carbonate (DEC), with 5% fluoroethylene carbonate (FEC) additive. Cyclic Voltammetry (CV): Conducted on a CHI 660E workstation (Shanghai Chenhua) at 0.1 mV s⁻¹ (1–3 V). Galvanostatic Cycling: Performed on a LAND CT2001A system (Wuhan Land Electronic Co. Ltd, Wuhan, China) (1–3 V) to evaluate charge/discharge behavior and cycling stability. The cycling performance at 2.0 C was tested before active 5 cycles at 0.05 C. The capacity is normalized to the mass of Se. Electrochemical Impedance Spectroscopy (EIS): Measured from 10⁻² to 10⁵ Hz with a 5 mV amplitude.

3. Results and Discussion

Figure 2a presents the SEM image of rapeseed meal-derived carbon synthesized without ZnCl₂, revealing an irregular particle structure with no observable pores. The addition of ZnCl₂ results in a three-dimensional interconnected porous carbon (NSPC) (Figure 2b). Figure 2c displays the TEM image of NSPC in which the absence of distinct lattice fringes confirms its amorphous nature. By melt infiltration method, most Se particles are uniformly dispersed into the porous framework of NSPC, forming Se/NSPC (Figure 2d). EDS mapping (Figure 2e–i) further verifies the uniform distribution of N, O, S, and Se, supporting the effective incorporation of Se within NSPC. The porous architecture provides abundant anchoring sites for Se, while N and S doping facilitates chemical bonding with Se. This structure can enhance the cycling performance.

The structure of NSPC and Se/NSPC were examined by XRD, with the results presented in Figure 3a. NSPC exhibits a broad peak at 25°, confirming its amorphous nature [35], in agreement with the TEM result. The XRD pattern of Se/NSPC closely resembles that of NSPC, with no detectable Se peaks (JCPDS #06-0362) [36], suggesting that Se is fully encapsulated within the pores of NSPC. Raman spectroscopy (Figure 3b) reveals two prominent peaks at approximately 1348 cm⁻¹ (D band, disordered carbon) and 1578 cm⁻¹ (G band, graphitic carbon) [37,38,39] for both NSPC and Se/NSPC. The intensity ratio (I_D/I_G) serves as an indicator of graphitization degree, with calculated values of 0.989 for NSPC and 1.009 for Se/NSPC. The slightly higher I_D/I_G ratio for Se/NSPC suggests weakened graphitization [40]. N₂ adsorption/desorption analysis (Figure 3c) reveals that NSPC follows a Type I isotherm, indicative of a microporous structure. A hysteresis loop in the 0.4–1.0 P/P₀ range confirms the coexistence of mesopores. BJH pore size distribution analysis (Figure 3d) indicates pore diameters between 2 and 10 nm, consistent with TEM findings. The specific surface area and pore volume of NSPC are determined to be 838 m² g⁻¹ and 0.51 cm³ g⁻¹, respectively. This three-dimensional amorphous porous framework offers abundant Se accommodation sites, ensuring high-capacity retention. Thermogravimetric analysis (TGA, Figure 3e) estimates the Se content in Se/NSPC to be 45 wt%.

XPS analysis was further conducted to characterize the NSPC and Se/NSPC composite. As illustrated in Figure 4a, elemental composition analysis confirms the coexistence of C, N, O, and S in both NSPC and Se/NSPC composite, with Se being exclusively detected in the Se/NSPC composite. The high-resolution XPS spectra presented in Figure 4b–e systematically investigates the chemical states of C 1s, N 1s, S 2p, and Se 3d in the Se/NSPC composite. The C 1s spectrum (Figure 4b) reveals three distinct components: graphitic carbon (285.1 eV), C-O (285.4 eV), and carbonyl groups (287.6 eV) [41,42]. The N 1s spectrum (Figure 4c) demonstrates multiple N configurations, including pyridinic (399.1 eV), pyrrolic (400.2 eV), and graphitic nitrogen (401.3 eV) [43,44]. In the S 2p spectrum (Figure 4d), characteristic peaks at 163.8 eV and 165.1 eV are assigned to S-S/S-C bonds, while higher binding energy peaks at 168.4 eV and 169.4 eV indicate the presence of oxidized sulfur (C-SOx-C) [45,46]. Moreover, the peaks at 156.2 eV and 238.4 eV are assigned to S 3p and S 3e. The incorporation of N and S heteroatoms into the carbon enhances Se chemisorption, thereby improving the electrochemical cycling performance of Se/NSPC. The Se 3d spectrum (Figure 4e) exhibits two primary peaks at 55.4 eV (3d_5/2) and 56.2 eV (3d_3/2), accompanied by a broad signal centered at 59.1 eV corresponding to Se-C bonding [47]. This pronounced chemical interaction between Se and the NSPC framework effectively mitigates Se dissolution during redox processes, ensuring structural stability.

To evaluate the electrochemical properties of Se/NSPC, CR2025 coin cells were assembled with Se/NSPC as the cathode and lithium foil as the anode. Cyclic voltammetry (CV) at a scan rate of 0.1 mV s⁻¹ (voltage range: 1.0–3.0 V) was conducted to elucidate the lithium-ion storage mechanism. As depicted in Figure 5a, the CV profiles exhibit a single redox couple, corresponding to Li⁺ insertion (forming Li₂Se) and subsequent extraction. Notably, no soluble lithium polyselenides (Li₂Se_x) were detected, confirming their absence in carbonate-based electrolytes. During the first cycle, the reduction and oxidation peaks were observed at 1.62 V and 2.01 V, respectively. A minor reduction shoulder at 2.35 V, attributed to SEI formation, vanished in subsequent cycles [48]. The primary reduction peak shifted to 1.76 V in later cycles, likely due to cathode activation [49,50]. The cycling performance was tested at 0.2 C (1 C = 675 mAh g⁻¹) (activation at 0.05 C for five cycles). The result is shown in Figure 5b. The initial reversible capacity is 541.0 mAh g⁻¹ (1983.3 mAh cm⁻³). After 200 cycles, the Se/NSPC retained a specific capacity of 496.5 mAh g⁻¹ (1820.2 mAh cm⁻³), corresponding to a high-capacity retention of 91.8%. We also tested the Se/NSPC electrode cross-sectional morphologies before cycling and after 200 cycles at 0.2 C by SEM (Figure S1). It was found that there was no obvious thickness variation in the Se/NSPC electrode after 200 cycles, suggesting stability of the Se/NSPC electrode. The GCD profiles for Se/NSPC cathode at various cycles are exhibited in Figure 5c. The result agrees with cycling performance. The long-term cycling performance was tested at 2 C (Figure 5d). The Se/NSPC electrode shows an initial reversible capacity of 351.6 mAh g⁻¹. After 100 cycles, the capacity fades to 309.4 mAh g⁻¹. The capacity decreases to about 229.8 mAh g⁻¹ at 500 cycles and 190.2 mAh g⁻¹ at 1000 cycles. The capacity retention is 54.1% after 1000 cycles. The coulombic efficiency is ~100% at 1000 cycles. The improved performance can be attributed from unique nanostructure [51,52,53,54,55,56]. The capacity fading may be due to polyselenide shuttle effect. There is no strong attachment between Se and NSPC. Thus, the Se particles are easily lost due to formation and dissolution of polyselenides (e.g., Li₂Se_n, where n = 4–8) during cycling, resulting in polyselenide shuttling and low cycling stability. Moreover, NSPC could not buffer the volume changes (~120%) of Se during cycling. This leads to particle pulverization, reducing cycle life. In addition, we also performed EIS analysis (Figure 5e). The semicircle in the high-frequency range of Nyquist plots is attributed to a charge-transfer phenomenon. It is noted that the diameters of the semicircle decrease from 109 (before cycling) to ~56 (1st cycle), ~51 (500th cycle), and ~48 Ω (1000th cycle), suggesting a decrease in charge transfer resistance (Rct). Figure 5f shows the rate performance of the Se/NSPC cathode at various current densities. At 0.2 C, an average reversible specific capacity of about 498.9 mAh g 1 was measured. At the current densities of 0.5, 1.0, 2.0, and 5.0 C, the Se/NSPC composite can still deliver reversible specific capacities of 395.5, 308.6, 215.1, and 172.2 mAh g⁻¹, respectively. After the current returns to 0.2 C, the reversible specific capacity is recovered to 421.9 mAh g⁻¹.

4. Conclusions

In conclusion, we successfully prepared a N, S co-doped three-dimensional porous carbon (NSPC) using low cost biowaste rapeseed meal as a precursor. Then, the LSeBs cathode was fabricated by melt infiltration method. The three-dimensional interconnected porous structure of NSPC provides abundant pores for Se. Moreover, the N and S doping enhances the adsorption of Se and alleviates the loss of Se, improving the utilization of Se. Therefore, the Se/NSPC composite cathode retains a specific capacity of 496.5 mAh g⁻¹ after 200 cycles at 0.2 C with a high-capacity retention of 91.8%. Even at a high current density of 2 C, the Se/NSPC composite maintains a high capacity over 200 mAh g⁻¹ after 1000 cycles. Our work offers valuable perspectives regarding the potential utilization of carbonaceous materials derived from diverse biomass precursors in next-generation energy storage systems.