As depicted in Fig. 1, the synthetic process of CoMoP2-MoP@NPC composites was performed via a simple and effective route. Specifically, glucose, urea, H₃PO₄, Co(NO3)2·6H2O and (NH4)6Mo7O24 were added in deionized water under continuous stirring to form a reaction solution. After evaporating the solvent, the resultant was transferred into tube furnace for annealing activation, and composite material could be successively achieved.
FESEM observation of precursors were conducted, as shown in Fig. S2, and it could be seen that the morphology of NPC precursor seems like a cheese. With the increase of urea addition contents, precursors of CMP@PC, CMP-2@NPC, CMP-4@NPC presented apparent porous composition with more particles.
It was observed that NPC exhibit a lamellated structure in Fig. 2a, and CMP-2@NPC show a sandwich structure fully loaded with nanoparticles insides in Fig. 2b. Similar structure of CMP-4@NPC could be seen in Fig. S3, inferring that the NPC could play a vital role in homogenizing distribution of nanoparticles, and as for CMP@PC, the nanoparticles formed uneven clusters. As depicted in Fig. 2c, the TEM image indicates that these nanoparticles uniformly distributed throughout the layered nanosheets, HRTEM results were also recorded and shown in Fig. 2d. It could be clearly seen that the interplanar distances of two contiguous lattice fringes labeled by the orange and blue squares are 0.210 and 0.232 nm, corresponding to the (101) planes of the CoMoP2 and (103) planes of the MoP, respectively, and interplanar distances of 0.334 nm are attributed to (002) planes of the graphitic matrix. These results turn out the existence of the heterostructures containing CoMoP2 and MoP nanoparticles loaded on the carbon sheets. The selected area electron diffraction (SAED) pattern of the CMP-2@NPC is shown in Fig. 2e, demonstrating the diffraction rings matched well with lattice planes of (103) and (104) for CoMoP2, (101) for MoP and (002) for graphitic matrix, respectively. The elemental mapping images were collected and presented in Fig. 2f-2k, manifesting the uniform distribution of Co, Mo N and P elements on the composite material. In order to learn the proportion of different ingredients in CMP-2@NPC, EDX measurement was performed, and the relative data was provided in Fig. S4. It is calculated that the mass ratio of CoMoP2/MoP is 23.97/34.52, and the carbon matrix accounts for 41.51 wt% of the total composite material. According to the ICP result in Table. S1, the mass ratio of Co/Mo is 37.68/62.32, and it was deduced that the mass ratio of CoMoP2/MoP is 1.52/1, which is close to EDX result.
Depicted in Fig. 3a, XRD patterns exhibit the crystalline structures of different samples. XRD pattern of the NPC shows a wide peak alone at about 26°, accorded with the (002) planes of graphitic material. Notably, with the change of urea contents, three samples display different compositions. Specifically, the phases of CMP@PC, CMP-2@NPC and CMP-4@NPC were identified with CoMoP-MoP, CoMoP2-MoP and CoMoP2, respectively, and all these peaks could be indexed by CoMoP2 (JCPDS, No. 33–0428), CoMoP (JCPDS, No. 32–0299), MoP (JCPDS, No. 24–0771). XRD results of NPC, CMP-2@NPC and CMP-4@NPC precursors in Fig. S5 demonstrate that their main crystal phase is urea, and the precursor of CMP@PC appears a wide peak, owing to the presence of urea.
To investigate the degree of defects and graphitization on carbon matrix, Raman spectra of as-synthesized samples were employed, as shown in Fig. 3b. Two characteristic peaks could be seen at about 1346 and 1590 cm− 1, assigned to the D band and the G band, respectively, verifying carbon materials existed in these composites. As it is well known that the D band is associated with the disorders and defects in the graphene, caused by the point defects like nitrogen, phosphorus doping, displacements and distortions in the crystal faces, while the G band is related to the in-plane stretching vibration of sp2-bonded carbon [36]. Consequently, the intensity ratio of the D band and the G band (ID/IG) reveals the degree of defects and disorders on the carbon matrix. It was traced that ID/IG of NPC, CMP@PC, CMP-2@NPC, CMP-4@NPC are 0.765, 0.639, 0.932 and 0.849 respectively, and the largest ID/IG value of CMP-2@NPC illustrate the formation of more active sites and defects on the carbon matrix, which could improve its electrochemical performance in a certain degree.
The specific surface area and pore size distribution of CMP-2@NPC was tested through the Brunauer-Emmett-Teller (BET) N2 isothermal absorption-desorption measurement. As can be seen in Fig. 3c, the isotherm curve is corresponding to type H3 hysteresis loop in the P/P0 range of 0.4-1.0, which shows a saturated adsorption platform and demonstrates the formation of uniform mesopores. It is calculated that the specific surface area of CMP-2@NPC is 75.368 m2 g− 1, and the pore volume is 0.069 cm3 g− 1. The pore size distribution curve analyzed by the BJH method inserted in Fig. 3c suggests a sharp peak located within 3–10 nm. This unique structure with sandwiched nanoparticles in the multilayered N, P co-doped carbon nanosheets is deem as a perfect accommodation to ease the volume expansion of CMP-2@NPC material during the lithiation/delithiation processes, shortening the lithium-ion transition distances and advancing the battery reversibility.
XPS measurement of CMP-2@NPC was conducted to investigate the surface elemental chemical compositions and the bonding configurations. The survey spectra in Fig. 3d overview the elements existing in CMP-2@NPC, and based on which, the N and C weight ratio on the NPC matrix could be roughly estimated to be 6.97%. The weight ratio of doped P and C can be affirmed to 3.43% according to the high-resolution XPS spectra of C 1s in Fig. 3e, and the peaks at binding energies of 284.8, 285.8 and 289.6 eV are matched well with C = C, C-N and C-P signals, implying the co-doped nature with N and P on carbon matrix [22]. As shown in high-resolution XPS spectra of Co 2p in Fig. 3f, doublets of Co 2p3/2 at 779.1 and 782 eV are ascribed to the Co-P and Co-O bonds, respectively, with a satellite peak located at 786.6 eV, and two peaks located at 794 and 798.3 eV are assigned to the Co 2p1/2 with a satellite peak at 803.1 eV [37–39]. In the Mo 3d region in Fig. 3g, two peaks seated at 229.2 and 231.5 eV indicate the Mo-P bond, turning out the intense incorporative interaction between Mo and P atoms. Moreover,peaks at 228.3, 232.6, 232.9, and 236 eV are indexed to Mo4+ and Mo6+ signals, mainly because of the surface oxidation of MoP originated from high surface energy [40–42]. As presented in P 2p region in Fig. 3h, the P-C bond settled at 133.7 eV as evidence, interpreting that P atoms successfully inserted in the carbon matrix. The peaks at binding energies at 130.1 and 131.2 eV could be assigned to P 2p3/2 and P 2p1/2 signals, and the P-O bond located at 134.3 could be attributed to phosphorus oxide species [24, 43–45]. It could also be found that the information of N atoms doped on the carbon matrix was offered in the high-resolution N1s spectrum in Fig. 3i. Concretely speaking, a distinct peak could be found at 398.9 eV, corresponding to N-C bond, and small peaks could be found at 397.9 and 400.6 eV, related to pyrrolic N and pyridinic N signals, respectively [22, 45, 46].
To elucidate the reactions at different potentials, CV curves of CMP-2@NPC anode was recorded within 0.01 to 3 V at 0.2 mV s− 1, as provided in Fig. 4a. It is clear that the negative peak centered at about 0.55 V in the initial cycle is distinguished from those after three cycles, and the differences is ascribed to the formation of solid electrolyte interface (SEI) films on the anode. The reaction routes were proposed as follows [13, 47, 48]:
CoMoP2 + xLi+ + xe− → LixCoMoP2 (1)
LixCoMoP2 + (6-x) Li+ + (6-x) e− → 2Li3P + Co + Mo (2)
MoP + xLi+ + xe− → LixMoP (3)
MoP + (3-x) Li+ + (3-x) e− → Li3P + Mo (4)
An oxidation peak raised at about 1.25 V in the first anodic sweep, which could be assigned to the transformation from Li3P to CoMoP2 and MoP. In the follow cycles, the SEI layer peak disappeared, and a series of redox pairs located at 0.65 and 1.28 V appeared, which can be attributed to the reversible phase transition of LixCoMoP2 and LixMoP in the lithium-ion insertion and disembedding processes. Additionally, the subsequent cycles display a similar characterization in the oxidation and reduction reactions, and the peaks overlapped and their positions almost unchanged, indicating an excellent reversible behavior and excellent stability.
Rate performance of the CMP-2@NPC anode for LIBs was researched, as given in Fig. 4b. It is revealed that at current densities of 100, 200, 400, 800 and 1500 mA g− 1, its specific capacities are 581, 524, 470, 407 and 360 mAh g− 1, respectively. Even at a high current density of 2000 mA g− 1, the corresponding reversible specific capacity still remained 344 mAh g− 1, and this specific capacity retention is 59.2% relative to that at 100 mA g− 1. When the current density returned back to 100 mA g− 1, the specific capacity recovered to 626 mAh g− 1, even higher than that of the initial cycles. This highly reversible specific capacity was stable in the following cycles, resulting in favorable retention capability of CMP-2@NPC anode.
Cycling stability of different anodes were measured. As shown in Fig. 4d, the initial discharge/charge specific capacities of CMP-4@NPC, CMP-2@NPC, CMP@PC and NPC anodes at 100 mA g− 1 are 692/433, 1001/680, 764/453 and 1108/584 mAh g− 1, with the corresponding initial coulombic efficiency (ICE) of 62.6%, 67.9%, 59.3% and 52.7%, respectively. The specific capacities of corresponding composite anodes increased to 556.6, 693.2, 621.7 mAh g− 1 after 150 cycles, revealing the superior capacity of the CMP-2@NPC anode. Figure 4c shows the selected discharge-charge profiles of CMP-2@NPC anode at 100 mAh g− 1 within a cutoff voltage window of 0.01-3 V. It could be seen that the initial discharge/charge specific capacities are 1001.9/620.4 mAh g− 1, revealing the ICE of approximately 62%. The relatively low ICE can be mainly ascribed to the formation of the SEI films, consistent with the CV results. From the second cycle, the ICE raised and reached almost 93%, and this value could be further increase through the process of activation [17, 49, 50]. After 150 cycles, the specific capacity of discharge/charge process come to 693/657 mAh g− 1, which is higher than that of the first several cycles.
Even at 500 mA g− 1 in Fig. 4e and Fig. S6, the CMP-4@NPC, CMP-2@NPC and CMP@PC anodes delivered the retentive specific capacity of 607.6, 689.3 and 514.3 mAh g− 1 after 700 cycles, respectively. Besides, the cycling performance of CMP-2@NPC anode at 1 A g− 1 was also included in Fig. S7, and it still exhibited about 530 mAh g− 1 after 900cycles with the similar phenomenon occurred. This enhanced specific capacity was mainly as a result of the activation processes, which has been widely reported in other conversion-type and transition metal phosphide anodes for LIBs [17, 47, 51].
In order to excavate the transport kinetics of the CMP-2@NPC anode, its EIS data were provided with the equivalent circuit model inserted. Fig. S8 shows the EIS profiles of CMP-2@NPC anode monitored after 200, 400 and 600 cycles. Normally, the semicircle seated at middle-frequency region is assigned to the charge transfer resistance (Rc) with the double-layer capacitance (Cd). It is worth noting that Rc after 200, 400 and 600 cycles gradually become smaller. These results are well consisted with the discharge/charge profiles showing an obvious specific capacity increase, and this phenomenon could also be attributed to the activation treatment, which greatly improved the cycling performance [17]