Synthesis and characterization of the electrode

To achieve the cG-encapsulated 3D bicontinuous framework Ni 3 S 2 electrode (Ni 3 S 2 @cG/Ni, here Ni 3 S 2 is served as a typical conversion reaction material due to their high theoretical capacities, good safety and cost effectiveness19). Our fabrication process involves three overall stages (schematics, Fig. 2a): (1) Synthesis of graphene oxide encapsulated sulphur particles (S@GO) through the well-controlled reaction of sodium polysulphides with hydrochloric acid. The synthesized sulphur can adhere to and self-assemble on the negatively charged graphene sheets when GO was dispersed in solution. (2) Fabrication of graphene oxide-encapsulated 3D bicontinuous framework electrode by a one-step hydrothermal process using pretreated 3D nickel framework and the as-prepared S@GO nanoparticles. (3) Reducing the GO shell to graphene by hydrazine solution and hydrogen plasma. To better investigate the effect of cG on electrochemical performance, bare Ni 3 S 2 3D electrode (Ni 3 S 2 /Ni, replacing S@GO by sulphur powder and other conditions were the same as synthesis of Ni 3 S 2 @cG/Ni) and cG-encapsulated 3D electrode with half content of graphene (Ni 3 S 2 @0.5 cG/Ni, reducing the graphene to half in the synthesis of graphene encapsulated sulphur and other conditions were same as synthesis of Ni 3 S 2 @cG/Ni) were fabricated as control experiments.

Figure 2: Synthesis and characterization of cG-encapsulated 3D Ni 3 S 2 electrode (Ni 3 S 2 @cG/Ni). (a) Fabrication process of Ni 3 S 2 @cG/Ni electrode, including preencapsulating sulphur with graphene oxide and one-step hydrothermal reaction with 3D nickel framework. (b) Low-magnification SEM image of Ni 3 S 2 @cG/Ni. (c) SEM image of S@GO. (d) Zoom-in SEM image of Ni 3 S 2 @cG/Ni, the cG with sharp ridges encapsulate the micrometre-sized particles. (e,f) XPS of S 2p, C 1s spectra of electrode surface. TEM images of (g) cG shell, (h,i) Ni 3 S 2 @cG which derive from the surface of Ni 3 S 2 @cG/Ni electrode. High-resolution TEM image (i) showed the bend of crystal lattice (marked with red line) of graphene, which indicates the crumple in graphene. Scale bars (b) 1 μm, (c,d) 500 nm, (g) 200 nm, (h) 50 nm, (i) 10 nm. Full size image

The GO used in first stage was characterized by Raman spectrum and atomic force microscopy (AFM). The Raman peak at 1,351 and 1,590 cm−1 can be indexed to the D and G vibrational bands of graphene (Supplementary Fig. 2). The thickness of GO is determined by AFM is ~1 nm and the width is up to a micrometre (Supplementary Fig. 3). Then, S@GO was characterized by SEM, X-ray diffraction (XRD) and energy diffraction spectroscopy (EDS). The SEM images (Fig. 2c and Supplementary Fig. 4b,c) shows that sulphur particles of ~500 nm are randomly covered by GO shells. The main phase detected in XRD of S@GO is sulphur with no obvious diffraction peak of GO (Supplementary Fig. 4a). The EDS mappings (Supplementary Fig. 4e,f) confirm the presence of sulphur as core and GO as shell. After hydrothermal and reduction process, Ni 3 S 2 @cG/Ni electrode was obtained and characterized by XRD, EDS, SEM, TEM and X-ray photoelectron spectroscopy (XPS). The XRD patterns of Ni 3 S 2 @cG/Ni shown in Supplementary Fig. 5a can be indexed to Ni 3 S 2 (mainphase, JCPDS-ICDD Card No. 00-044-1418), with small amounts of NiS (JCPDS-ICDD Card No. 00-001-1286, marked with ▪) and Ni (JCPDS-ICDD Card No. 03-065-2865, marked with ▴). As SEM images shown in Fig. 2b,d, the as-prepared particles with size of ~500 nm are encapsulated by deeply cG layers and compactly stacked on the nickel framework. Compared with the randomly covered S@GO (Fig. 2c), graphene in Ni 3 S 2 @cG/Ni is deeply crumpled into a pinecone-like structure, which has a pattern of ridges with wavelength of ~30 nm. TEM and high-resolution TEM images (Fig. 2g–i and Supplementary Fig. 6) captured from cG encapsulated nickel sulphides (Ni 3 S 2 @cG and Ni 3 S 2 @0.5cG) confirm the crumpled nature of graphene. In particular, it is clearly observed that graphene on the boundary of Ni 3 S 2 @cG particles was highly crumpled (Fig. 2g,h and Supplementary Fig. 6a,b) and the bent crystal lattice of multi-layered graphene (Fig. 2i) was also detected under the high-resolution TEM, which demonstrates the deeply cG encapsulated structure. In contrast, the crumpling degree of graphene in Ni 3 S 2 @0.5 cG is relatively low (Supplementary Fig. 6c,d) and even some non-cG or bare particles can be observed (Supplementary Fig. 6g). The chemical bonding interaction among Ni 3 S 2 , cG and nickel framework is characterized by XPS (Fig. 2e,f). The S 2p (Fig. 2e) peaks can be resolved into three components centered at about 157.8, 162.8 and 167.8 eV and represent sulphide, disulphide and RSO 3 −S group, corresponding to the NiS, Ni 3 S 2 and cG, respectively. Similarly, the bonding signals of Ni-C, graphite C can be detected from the C 1s spectrum (Fig. 2f). In particular, the RSO 3 −S, Ni-C signals can be assigned to the chemical bonding of cG-Ni 3 S 2 and cG-Ni, which can improve the interfacial stability.

Crumpling mechanism

The cG has been realized in previous studies and is thought to be driven by thermal expansion, capillary compression and pre-strain relaxation27,28,29,30. In particular, Huang’s group first demonstrated that capillary compression effectively cG into a ball-like structure in rapidly evaporating aerosol droplets process30. Distinguished from previous results, the deeply cG-encapsulated materials is synthesized in wet chemistry environment in our experiment that has not been reported previously. Therefore, the chemical bonding interaction and self-assembly need to be taken into full consideration. Here, we carried out TEM characterization on the samples at different reaction stages and MD simulation to demonstrate the crumpling mechanism of graphene. The TEM images of samples with different reaction time (Supplementary Fig. 7a–h) show the morphological evolution of Ni 3 S 2 @cG during the synthetic process, which indicates that crumpling degree is increasing with an extension of reaction time. Furthermore, a detailed crumpling process of graphene (Supplementary Fig. 7i–l, Movie 1) is revealed by MD simulation through considering the volume variation and chemical binding interaction between graphene and nanoparticle, which well conforms to the TEM observation (More detailed discussion is provided in Supplementary Information).

As shown in Supplementary Fig. 8, in the formation process of cG-encapsulated 3D framework electrode, the encapsulated solid sulphur (melting point: 115 °C) firstly melts to liquid state at 180 °C, further covering the GO inner surface by capillary force. Because of the high surface energy-induced thermodynamics instability, GO with encapsulated liquid sulphur tend to adhere to the surface of nickel framework. At the early stage of reaction between liquid sulphur and nickel, solid nickel sulphides (melting point: 797 °C) is formed accompanied with volume contraction. With the phase contact line of liquid and solid receding during volume contraction, the slight crumple firstly emerges and assembles that is driven by adhesion and friction forces between nickel sulphides and graphene layers31. Meanwhile, the reaction of nickel sulphides and GO takes place that can be confirmed by the above XPS results. The chemical bonding forces will induce more deep crumpling assembly, because they robustly anchor nickel sulphides particles to the GO and restrain the slide of GO at further volume contraction process.

Electrochemical properties

To evaluate the electrochemical performance and explore the influence of cG, lithium batteries were fabricated based on the Ni 3 S 2 @cG/Ni, Ni 3 S 2 @0.05 cG/Ni and Ni 3 S 2 /Ni electrodes. The cycling performance of Ni 3 S 2 @cG/Ni electrode was firstly tested at a lower current density of 0.1 A g−1 and 0.5 A g−1 from 1.0 V to 3.0 V for 100 charge/discharge cycles (Fig. 3a). This voltage window is selected similar to Li 4 Ti 5 O 12 for good safety4,19. An initial capacity of 1,036 mAh g−1 can be obtained and stabilized to over 900 mAh g−1 after 100 cycles at a current density of 0.1 A g−1, which is over five times improvement compared with the commercialized graphite (~200 mAh g−1) (ref. 2) and Li 4 Ti 5 O 12 (~170 mAh g−1) (ref. 32). When the current density increases to 0.5 Ag−1, the specific capacity can reach 682 mAh g−1 and maintain 654 mAh g−1 after 100 cycles, with capacity retention up to 96%, which shows over one order of magnitude improvement compared with Ni 3 S 2 /Ni electrode (~7% capacity retention at 0.5 A g−1 after 100 cycles). The additional reversible capacity beyond their theoretical capacity is mainly contributed by interspatial lithium storage of cG and conversion reaction materials8. Crumpling of graphene and newly formed interfaces of metal and lithium sulphides both enlarge the interspatial area and lead to additional capacity.

Figure 3: Electrochemical performance and morphology characterization before and after charge/discharge cycling. (a) Cycling performance at current density of 0.1, 0.5 A g−1, (b) Rate performance of Ni 3 S 2 @cG/Ni and Ni 3 S 2 /Ni electrode. (c) Cycling performance at current densities of 8, 10 and 20 A g−1 with voltage from 0.01 to 3 V. (d,e) SEM images of Ni 3 S 2 @cG/Ni electrode before cycling and after 100 cycles, respectively. (f) Nyquist diagram of Ni 3 S 2 @cG/Ni before cycling and after 100 cycles. (g,h) SEM images of Ni 3 S 2 /Ni electrode before cycling and after 100 cycles. (i) Nyquist diagram of Ni 3 S 2 /Ni before cycling and after 100 charge–discharge cycles. Insets in e,h are the kinetics calculations of Ni 3 S 2 @cG/Ni (e), Ni 3 S 2 /Ni (h) based on the frequency (ω) and Z' values at low frequency region. Scale bars (d,e,g,h), 500 nm. Full size image

A summary of the rate performance (Fig. 3b and Supplementary Figs 9–11) with various voltage windows further shows a superb rate capability, and even at higher rate, the Ni 3 S 2 @cG/Ni electrode has larger capacity than Ni 3 S 2 /Ni. In particular, the capacity of Ni 3 S 2 @cG/Ni can reach 2,165 mAh g−1 at a current density of 0.1 A g−1 and 884 mAh g−1 at a current density of 8 A g−1 with voltage from 0.01 to 3.0 V. Additional studies carried out at rates of 8, 10 and 20 A g−1 for extended number of charge–discharge cycles (Fig. 3c) yield no capacity fading in 640, 660 and 1,000 cycles, which highlight the substantial improvement afforded by cG compared with baseline Ni 3 S 2 /Ni electrode. In particular, at current density of 20 A g−1, the power density can reach 18.4 kW kg−1, five to ten times higher than typical high-power density storage device—supercapacitor33. To our knowledge, this maybe one of the best performances among conversion reaction batteries. On the contrary, the Ni 3 S 2 @0.5 cG/Ni and Ni 3 S 2 /Ni electrodes had lower capacity, poorer cycling stability and rate performance (Fig. 3a,c and Supplementary Figs 9–11) due to the lack of sufficient protection from cG.

The charge–discharge curves are shown in Supplementary Fig. 9a. The voltage hysteresis, as another important indicator for conversion reaction materials, calculated from the different value between charge and discharge voltage at the half reversible capacity, noted as ΔV(Q/2), is also investigated and we find that the hysteresis of Ni 3 S 2 @cG/Ni (0.3 V) is much lower than that of Ni 3 S 2 /Ni (0.6 V) at the current density of 0.5 A g−1.

In addition, the ex situ SEM and electrochemical impedance spectrum (EIS) characterizations of Ni 3 S 2 @cG/Ni and Ni 3 S 2 /Ni electrode before and after cycling were carried out to evaluate the structural stability and the kinetics performance of charge and mass transfer. As shown in Fig. 3d, for Ni 3 S 2 @cG/Ni electrode before cycling, the nickel sulphide cores are encapsulated with cG shells. After 100 charge–discharge cycles, although the crumpled GO shells tend to be a bit disordered, the morphology of nickel sulphide core is maintained very well (Fig. 3e). As Nyquist diagram shown in Fig. 3f, a low-charge transfer resistance (R ct ) value can be obtained for Ni 3 S 2 @cG/Ni before cycling, which is reflected by a small diameter of semicircles at the high–medium frequency region. After 100 cycles, the R ct value shows no obvious variation. To assess the lithium ion diffusion kinetics, the calculation based on the impedance data in low frequency region is carried out. According to Supplementary equations (1,2), the slope in fitting line of ω−1/2 (ω is frequency) and Z' can be defined as Warburg factor and its square values have an inverse relationship with lithium ion diffusion coefficient (D Li ). The slope value is 4.72 (k 1 ) at pristine state and increases to 8.57 (k 2 ) after 100 cycles, which indicates that D Li degrades to one-fourth of the pristine value.