Red phosphorus (red P) is a promising anode material for lithium-ion batteries (LIBs) due to its high theoretical capacity of 2596 mAh·g−1, abundant resource and low cost. However, the application of P-based anode suffers from several crucial issues, including limited electronic conductivity and drastic volume variation during its electrochemical lithiation/delithiation processes. Here, we reported a red P/anthracite composite featuring red P embedded into micrometer-scale porous anthracite framework fabricated through a one-pot ball milling synthesis process. The micrometer-sized anthracite not only provided high electronic conductivity but also worked as buffer matrix to mitigate the volume change of the active materials during cycling. P–C and P–O–C bonds between P and anthracite enabled their close contact and thus stabilized the structure of red P/anthracite composite. Also, the one-pot synthesis operation was facile, and the raw materials of red P and anthracite were in low cost. As expected, the red P/anthracite composite showed stable electrochemical cycling and satisfied rate capability for LIBs. It delivered an overall high initial capacity of 810.1 mAh·g−1 (corresponding to 2025.3 mAh·g−1 on the mass of phosphorus) at 425 mA·g−1 and 627.2 mAh·g−1 after 300 cycles with capacity retention of 77.4%. Additionally, high capacity was realized at 3400 mA·g−1 with reversible capacity of 480 mAh·g−1. Also, the prepared red P/anthracite composite exhibited superior sodium storage properties for sodium-ion batteries.

Lithium-ion batteries (LIBs) are the dominating power sources in portable electronics and electric vehicles nowadays [1,2,3,4,5,6,7]. Graphite has been the choice of anode for LIBs since 1991 due to its stable electrochemical performance [8]. However, its low theoretical specific capacity (372 mAh·g−1) becomes a limiting factor for further increasing the energy density of LIBs [9], and its low working potential (~ 0.1 V vs. Li+/Li) makes graphite not ideal for use in fast-charging LIBs, since the potential would easily drop to 0 V (vs. Li+/Li) and induce the deposition of metallic lithium under fast-charging condition [10]. Red phosphorus (red P) has many virtues as battery anode, including high theoretical capacity of 2596 mAh·g−1, low cost, good chemical stability and reasonable lithiation potential (~ 0.7 V vs. Li+/Li) [11, 12], which could help to avoid lithium metal plating during fast charging. However, red P suffers from poor electronic conductivity (1 × 10−14 S·cm−1) and drastic volume variation (~ 300%) [13], which cause quick structural degradation during cycling and inferior electrochemical performance. Construction of carbon-based compositions could effectively improve electronic conductivity and buffer the volume change. Ball milling [14,15,16,17] and vaporization conversion [18] are two common methods for the fabrication of red P/carbon composites. Vaporization conversion approach takes advantage of low sublimation temperature of red P and red P can easily condense into the pore of carbon materials. On the other hand, using ball milling approach can avoid the formation of toxic white phosphorus and is facile for large-scale fabrication. Various carbon materials including carbon black (CB) [19, 20], graphene [21, 22], carbon nanotubes (CNT) [23, 24], biomass driven carbon [25], porous carbon [26], and heteroatom doped carbon [27, 28] have been combined with phosphorus, and the as-fabricated composite materials often deliver high electronic conductivity and enhanced electrochemical performance [29,30,31]. However, the involved preparation process of these carbon materials or P/carbon composites was often complex and the cost was often high. Therefore, it is desirable to explore advanced phosphorus/carbon composites with superior electrochemical performance using facile and scalable approach in consideration of commercial application.

Coal is an abundant resource with low cost, which is usually used as fossil fuel. Lignite, bitumite and anthracite are three representative coals with different metamorphic grades. Among them, anthracite possesses the highest mass ratio of carbon content (89 wt% or higher), the lowest volatile content, and high electronic conductivity. Coal has been reported as the raw materials to synthesize various carbon materials as anodes served for LIBs [32,33,34] and SIBs [35,36,37,38]. Typically, coal was used as precursor to fabricate needle cokes or mesophase asphalt microspheres [38,39,40], which can be further graphitized into artificial graphite. Carbon materials with different oxygen contents (0.3 at%–2.9 at%) and defect concentrations were fabricated with coal pitch as precursor annealed from 400 °C to 1550 °C. The sample achieved after annealing at 800 °C delivered 263 mAh·g−1 at 40 mA·g−1 served as anode for SIBs [36]. The pyrolyzed anthracite achieved after 1200 °C treatment displayed the best electrochemical performance among samples obtained under different annealing temperatures from 1000 to 1400 °C. Also, the 1200 °C-pyrolyzed anthracite delivered the highest reversible capacity of 222 mAh·g−1 at 60 mA·g−1 and the best rate capability for electrochemical sodium ion storage [37].

Herein, we showed a red P/anthracite composite featuring red P embedded into micrometer-scale porous anthracite framework, which was fabricated through a ball milling approach. Meanwhile, high specific surface area and pore volume of anthracite made it suitable to serve as an efficient host material to confine red P within the structure and buffer its volume change during cycling. Red P and anthracite were kneaded up together to produce uniform red P/anthracite composite after ball milling. The strong interactions of P–C and P–O–C bonds ensured tight combination between the red P and anthracite. The red P/anthracite composite with 40 wt% red P showed stable cycling performance and superior rate capability for LIBs. It delivered a high reversible capacity of 810.1 mAh·g−1 at 425 mA·g−1 and a high capacity retention of 77.4% was achieved after 300 cycles. As the current density increased to 3400 mA·g−1, a reversible capacity of 480 mAh·g−1 was obtained, showing good rate capability. Also, good cycling stability was achieved for SIBs.

Figure 1a shows the schematic illustration of fabrication process for red P/anthracite composite. Micro-sized red P/anthracite composite was obtained featured with red P uniformly embedded into the anthracite by ball milling operation. Figure 1b compares X-ray diffraction (XRD) results of commercial anthracite, commercial red P and red P/anthracite composite. The red P presented three broad peaks at about 15°, 33° and 55° [27]. The anthracite exhibited two sharp peaks at about 26.6° and 43.6°, suggesting high carbonization degree of the anthracite. In XRD pattern of the red P/anthracite, two weak graphitic peaks could be observed, while the original broad peaks of red P disappeared, indicating the successful fabrication of red P/anthracite composite [22, 41]. Raman spectra for the red P/anthracite and the bare anthracite are shown in Fig. 1c. The intensity ratio of D band to G band (ID/IG) for the two samples was calculated to compare the structure change of carbon. The increased ID/IG value from 0.93 for the bare anthracite to 1.02 for the red P/anthracite suggested that the amorphous degree was enhanced and defective sites were generated due to the permeation of red P into anthracite matrix [26]. The red shift of D band from 1442 cm−1 for the bare anthracite to 1334 cm−1 for the red P/anthracite could be attributed to the electron transfer from red P to anthracite, indicating the formation of strong interaction between red P and anthracite [42, 43].

Fig. 1
figure 1

a Schematic illustration of synthesis of red P/anthracite composite; b XRD patterns and c Raman spectroscopy of anthracite (after ball milling), red P and red P/anthracite composite; d XPS spectra of red P and red P/anthracite composite; e FTIR spectra of red P, anthracite and red P/anthracite composite; f N2 adsorption/desorption isotherms and g pore distribution curves of anthracite and red P/anthracite composite

Full size image

X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) were performed to investigate the chemical composition and functional groups of the obtained materials. Figure 1d displays high-resolution P 2p spectra of commercial P and red P/anthracite composite. In the spectra of pristine red P, two peaks centered at 130.1 and 131 eV were referred to P 2p3/2 and P 2p1/2, respectively. In comparison, two new peaks located at 131.6 and 133.9 eV were found in the spectrum of the red P/anthracite, which were referred to P–C and P–O–C bonds [26, 44], respectively. Different from anthracite and red P, an obvious adsorption peak centered at 1008 cm−1 of red P/anthracite emerged in FTIR spectra, which suggested the formation of P–O–C bond [21, 24, 45] (Fig. 1e). Brunauer–Emmett–Teller (BET) was conducted to analyze the change of specific surface area and pore distribution of anthracite before and after ball milling, as shown in Fig. 1f, g. According to the results of N2 adsorption/desorption measurement, the red P/anthracite composite showed much smaller specific surface area than bare anthracite (17.3 m2·g−1 for the P/anthracite vs. 544.9 m2·g−1 for the anthracite). The pore volume of the red P/anthracite composite was 0.14 cm3·g−1, which was also much smaller than that of anthracite (0.40 cm3·g−1). According to Barrett–Joyner–Halenda (BJH) pore size analysis of the anthracite, the average pore size of anthracite is ~ 4.1 nm. The significantly decreased specific surface area and pore volume indicated the successful filling of red P to the pores of anthracite [46]. All the results from Raman, FTIR, XPS and BET measurement confirmed the successful fabrication of red P/anthracite composite with chemical interaction between P and anthracite.

The morphology and microstructure of anthracite and P/anthracite were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In Fig. 2a, the anthracite particles showed irregular shape with micrometer-scale size. After ball milling, the red P and anthracite were kneaded together, and homogeneous particles of red P/anthracite in micrometer scale were obtained (Fig. 2b). TEM image and corresponding elemental mapping images of red P/anthracite are displayed in Fig. 2c–f. The results showed that P was filled into the interspace of anthracite and uniformly distributed over the entire composite. The anthracite with high specific surface area and pore volume not only can serve as efficient physical confinement for red P and buffer matrix for its volume change during the electrochemical reaction, but also can supply conductive pathway for electronic transport and enhanced the rate capability. Moreover, the close contact between red P and anthracite with P–C and P–O–C bonds can help to form a tight combination between red P and anthracite, which is beneficial for stable electrochemical cycling.

Fig. 2
figure 2

SEM images of a anthracite and b red P/anthracite composite; c TEM image of red P/anthracite composite and corresponding elemental mapping images of d carbon, e phosphorus and f carbon and phosphorus elements

Full size image

Figure 3 displays the electrochemical performance of red P/anthracite electrode for LIBs. Unless otherwise noted, the total weight of red P and anthracite was used to calculate the specific capacities. Figure 3a shows cyclic voltammetry (CV) curves of red P/anthracite composite at a scan rate of 0.1 mV·s−1 between 0.01 and 2.00 V (vs. Li/Li+). Cathodic peaks were located at 1.0 and 0.5 V during initial lithiation process. The weak peak at ~ 1.0 V was attributed to the formation of the solid electrolyte interface (SEI). This broad peak disappeared in the subsequent cycles, as a reason of forming stable SEI film after initial cycle [27]. Cathodic peaks of subsequent two cycles were located at around ~ 0.37 and ~ 0.62 V, corresponding to the lithiation of red P. The corresponding anodic peaks were located at 1.1 and 1.3 V, corresponding to the delithiation of red P. The curves of the subsequent two cycles coincided well and displayed identical characteristic peaks, suggesting good electrochemical reversibility.

Fig. 3
figure 3

Electrochemical performance of red P/anthracite composite for LIBs: a CV curves of red P/anthracite composite achieved at 0.1 mV·s−1; b galvanostatic cycling performance of red P/anthracite composite at 425 mA·g−1 and c voltage-capacity profiles of red P/anthracite composite for different cycles at 425 mA·g−1; d rate capability of red P/anthracite composite; e first-cycle voltage-capacity profiles of red P/anthracite composite electrodes without and with chemical prelithiation; f cross-sectional SEM image of red P/anthracite electrode before cycling; cross-sectional SEM images for 30th cycle at 425 mA·g−1 at g delithiation state and h lithiation state

Full size image

Figure 3b presents the cycling performance of red P/anthracite composite at 425 mA·g−1. After three activated cycles, the red P/anthracite electrode showed a high capacity of 810.1 mAh·g−1 (corresponding to 2025.3 mAh·g−1 for P). The red P/anthracite displayed capacity retention of 77.4% and average Coulombic efficiency of 99.77% for 300 cycles, presenting high electrochemical reversibility. The voltage-capacity profiles of the red P/anthracite composite at 1st, 10th, 100th and 200th cycles after three activation cycles are presented in Fig. 3c and these curves were highly overlapped, supporting the good electrochemical stability of the red P/anthracite.

The capacities of the red P/anthracite composite at various current densities are recorded in Fig. 3d. The composite electrode delivered high reversible capacities of 842.4, 742.4, 701.1, 653.9 and 580.4 mAh·g−1 at 425, 850, 1275, 1700, 2550 mA·g−1, respectively. Even at a high current density of 3400 mA·g−1, a high capacity of 480 mAh·g−1 was realized. Along with the current density went back to 425 mA·g−1 after tested at different current densities, a high capacity of 771.1 mAh·g−1 was still realized, which was close to the initial capacity achieved at 425 mA·g−1, showing good stability of the red P/anthracite electrode. It is noted that the capacity contributed by anthracite was negligible (Figs. S2, S3). The initial Coulombic efficiency of the red P/anthracite was moderately low, which would reduce the energy density of full batteries. To improve the initial Coulombic efficiency and compensate initial irreversible active lithium loss, the red P/anthracite electrode was chemically prelithiated using naphthalene solution [47,48,49] (see details in the experimental part). The initial Coulombic efficiency of the red P/anthracite was easily increased from 71.5% (the original electrode) to 88.2%, and 208 mAh·g−1 lithium ion capacity was compensated after prelithiation (Fig. 3e). What’s important, the prelithiated red P/anthracite electrode did not have negative effect on cycling stability, displaying a high capacity retention of 75.4% after 250 cycles (Fig. S5). Such result indicated that the prelithiation process had no negative effect on the electrochemical cycling stability of the P/anthracite electrode.

Figure S6 shows the top view SEM image of the red P/anthracite electrode before and after cycling. It was observed that the red P/anthracite electrode maintained an intact structure without obvious fractures after 300 cycles, which was ascribed to its rational structure design. The cross-sectional SEM images of the red P/anthracite electrode before cycling and after 30 cycles were further compared (Fig. 3f–h). The thickness of the electrode showed only slightly increase from 24.6 (delithiation state) to 25.2 μm (lithiation state), indicating that the drastic volume expansion of phosphorus during the lithiation was effectively restrained by the anthracite matrix. The embedding structure of red P/anthracite composite guaranteed the tight connection between phosphorus and the anthracite matrix, leading to small electrode thickness variation and long-term cycling stability.

Electrochemical sodium storage properties of red P/anthracite electrodes were also investigated. Figure 4a shows CV curves of the red P/anthracite composite at 0.1 mV·s−1 (0.01–1.50 V). Irreversible broad peak at 0.75 V in the cathodic scan corresponded to the formation of SEI layer [21]. In the anodic scan, two sharp peaks around 0.54 and 0.65 V were assigned to the desodiation process and overlapped well in the next two cycles, indicating good electrochemical reversibility of the red P/anthracite composite for sodium-ion storage. After three activated cycles at 85 mA·g−1, charge capacity of the red P/anthracite electrode reached 692.3 mAh·g−1 at 170 mA·g−1 (Fig. 4b). After 150 cycles, the red P/anthracite electrode still exhibited a high capacity of 541.5mAh·g−1 with capacity retention of 78.2% (Fig. 4b). Figure 4c shows the voltage-capacity plots of red P/anthracite for different cycles. Only slight increase in voltage hysteresis was observed for the red P/anthracite on cycling, showing its high electrochemical reversibility. The rate capability of the red P/anthracite electrode is shown in Fig. 4d. Reversible capacities of 676.4, 652.2, 600.1, 534.5 and 428.1 mAh·g−1 were achieved at 170, 425, 850, 1700, 3400 mA·g−1, respectively.

Fig. 4
figure 4

Electrochemical performance of red P/anthracite as anode material for SIBs: a CV curves of red P/anthracite at 0.1 mV·s−1; b galvanostatic cycling and c typical voltage-capacity profiles of red P/anthracite for different cycles at 425 mA·g−1; d rate capability and e typical voltage-capacity profiles of red P/anthracite at different current densities

Full size image

In summary, a novel red P/anthracite composite was prepared via a simple ball milling approach, where amorphous red P was embedded in a porous anthracite framework. Owing to the homogeneous distribution of P in porous anthracite matrix with chemical binding (P–C and P–O–C bonds), the volume change of active P during the electrochemical reaction was relieved, and the integrity of the red P/anthracite was realized after cycling. Also, the anthracite provided abundant pathways for fast electronic transport, which afforded good rate capability. As a result, remarkable electrochemical performance was achieved for the red P/anthracite composite for both lithium-ion storage and sodium-ion storage, including high overall capacity (810.1 mAh·g−1 at 425 mA·g−1 for LIBs), good cycling stability (capacity retention of 77.4% for 300 cycles for LIBs) and rate capability (480 mAh·g−1 at 3400 mA·g−1 for LIBs). The low cost and scalable synthesis of such a red P/anthracite composite make it a competitive candidate as anode material for practical battery applications.