Dual Confinement of CoSe2 Nanorods with Polyphosphazene-Derived Heteroatom-Doped Carbon and Reduced Graphene Oxide for Potassium-Ion Batteries.

High-capacity and highly stable anode materials are some of the keys to the realization of the application of potassium-ion batteries (PIBs). Cobalt diselenide (CoSe2) has been regarded as a high-potential anode material for PIBs. However, solving the problems of sluggish kinetics and large volumetric expansion during intercalation/deintercalation of K+ ions is always very challenging in terms of cobalt diselenide-based anode materials. Herein, reduced graphene oxide-encapsulated polyphosphazene-derived S, P, and N codoped carbon (SPNC)-coated CoSe2 nanorods (CoSe2⊂SPNC⊂rGO) were designed as PIB anode materials. CoSe2⊂SPNC⊂rGO delivers an excellent reversible capacity of 287.2 mAh g-1 at 100 mA g-1. Benefiting from the coating of heteroatom-doped carbon and encapsulation of rGO, the CoSe2⊂SPNC⊂rGO anodes exhibit a remarkable rate capability (100-1500 mA g-1 current density) and high stability (208.8 mAh g-1 after 500 cycles at 500 mA g-1). The results demonstrate that S, P, and N codoping in carbon layers provides active sites for K+ ion storage and increases the electrical conductivity. More importantly, the dual confinement of CoSe2 nanorods with carbon layers and rGO significantly reduced the volume expansion and kept the electrode structural integrity with repeating intercalation/deintercalation of K+ ions.

Introduction

Lithium-ion batteries (LIBs) have been massively used in portable electronic devices, electric vehicles, and aerospace because of their high-energy density and long-cycle life.[1−3] However, due to the low crust reserves of lithium resources and uneven geographical distribution, the cost of lithium has increased year by year, which has seriously hindered the development of LIBs.[4−6] In this case, potassium-ion batteries (PIBs) have been seen as a viable energy storage system and are expected to replace the LIBs due to their unique characteristics of abundant potassium resources and their electrochemical mechanism similarity with LIBs.[7−11]

Regarding the PIBs, they have the advantages of high energy density and high voltage with low cost, which results from the redox potential of K/K+ (−2.93 vs SHE), similar to that of Li/Li+ (−3.04 vs SHE).[12−14] Unfortunately, the radius of lithium ions (0.76 Å) is much smaller than that of potassium ions (1.38 Å), which leads to serious stress changes or even pulverization and slow reaction kinetics of electrode materials during the potassiation/depotassiation process. These problems will eventually cause low Coulombic efficiency and capacity attenuation.[15−20] Therefore, there are still many problems that researchers confront to explore high-electrochemical property electrode materials for PIBs.

Transition metal selenides based on a conversion-reaction type, including MSe (M = Fe, Zn, Co, and Mo) have been recently studied in PIBs due to large spacing, higher electronic conductivity, and environmental friendliness compared with transition metal sulfides.[21−27] Among the various MSe species, CoSe2 has been widely used for hydrogen evolution, alkali-ion batteries, and solar cells owing to its highly conductive characteristic.[28−31] However, CoSe2 as an anode material for PIBs is still up against severe problems, including the electrode material pulverization caused by the huge volume expansion and the deterioration of electrical conductivity in the charge/discharge processes, leading to severe capacity attenuation and poor rate performance. For the abovementioned reasons, how to relieve stress changes during the continuous insertion and extraction process of K+ ion is the main challenge for the research of high-electrochemical property and stable PIB conversion-based anode materials.[32] To address the above issues, coating conductive materials, mostly with carbonaceous materials, is a competitive way to boost cycling performance and rate capability of MSe-related anode materials toward PIBs.[33] Lu et al.[34] reported carbon-coated FeSe2 clusters as PIB anodes, which alleviated the volume expansion and improved the electron conductivity. Xu et al.[35] synthesized ZnSe@NC, which displayed an excellent electrochemical performance due to the carbon layer offering both a conductive substrate and structure support. The coating of a carbon layer on the surface of the conversion-based anode materials can effectively decrease the interface resistance and improve the conductivity of the material. Additionally, for further enhancing the performances of the MSe anode materials, nitrogen doping not only enriches the electrons at the doped site to obtain a high conductivity of the materials but also enhances the reaction kinetics by forming defects and active sites, thereby achieving a high reversible capacity and cycle stability.[36] Ci et al.[37] designed SnS2 in N and S codoped carbon nanofibers, and they confirmed that doping can availably promote the reaction kinetics and absorption of K+ ions, resulting in excellent electrochemical performance. However, although the single carbon-coated structure eases volume expansion to a certain extent, there are still problems. Yu et al.[38] synthesized MoSe2/N–C, which displayed an outstanding rate performance, but its long-cycle performance is not satisfactory, which may be caused by the powdering of the electrode material during the cycle. Therefore, there is an urgent need to further reduce the performance degradation caused by volume expansion.

Large volume expansion will still cause the carbon layer to crack, finally resulting in the degradation of the electrochemical performance. Thus, many efforts have been focused on further overcoming volume expansion.[39−41] Graphene nanosheets have been considered as a 2D matrix for electron/ion transport, which can relieve volume expansion to a certain extent and inhibit the aggregation and accumulation of nanosized anode materials.[42−44] Nevertheless, structural collapse of the electrode material is still inevitable because of the phase transition.[45] Then, the further agglomeration of the pulverized nanostructures leads to a decrease in capacity.

Based on the above analysis, herein, S, P, and N-doped carbon-coated CoSe2 nanorods encapsulated in a reduced graphene oxide (rGO) matrix (CoSe2⊂SPNC⊂rGO) were developed for PIB anodes. The conversion-based CoSe2 anode materials are first coated with a S, P, and N-doped carbon layer derived from polyphosphazene and then confined into the rGO matrix. The doping of S, P, and N in the carbon layer enriched the active defect sites, and the formation of N–C, P–C, and S–C bonds provided more active sites for K+ storage. More importantly, benefiting from dual confinement effects with carbon coating and rGO encapsulation, CoSe2⊂SPNC⊂rGO reveals remarkable rate capability and cycling stability. The optimized CoSe2⊂SPNC⊂rGO exhibits a high specific capacity of 208.8 mAh g–1 after 500 cycles at 500 mA g–1. A capacity of 144.9 mAh g–1 can be retained even at 1 A g–1 after 800 cycles, demonstrating promising cyclability and rate properties.

Results and Discussion

Preparation of CoSe2⊂SPNC⊂rGO

Figure illustrates the synthetic process of CoSe2 ⊂SPNC⊂rGO hybrid materials. First, the CoSe2 nanorods were generated via a two-step approach including the synthesis of a precursor of rod-like Co–OH–urea (Figure S1) and selenization. The CoSe2 sample exhibits an obvious rod-shaped structure, wherein the length:diameter ratio is about ∼4:1 (Figure a and Figure S2a). By means of in situ polycondensation of hexachlorocyclotriphosphazene (HCCP) and 4,4-sulfonyldiphenol (BPS), polyphosphazene was coated on the surfaces of CoSe2 nanorods (CoSe2⊂PSZ). As observed in Figure b and Figures S2b and S3, the thickness of the polyphosphazene (PSZ)-coated layer is about ∼27.2 nm. Through the π–π conjugation and electrostatic interaction between graphene oxide (GO) and the surface’s PSZ layer of CoSe2⊂PSZ, CoSe2⊂PSZ⊂rGO was obtained with CoSe2⊂PSZ encapsulated in the reduced graphene oxide (rGO) by a hydrothermal assembly strategy. From Figure c, the rGO nanosheets were assembled into three-dimensional rGO networks with CoSe2⊂PSZ intercalated inside. The ultrathin rGO sheets were coated on the surfaces of CoSe2⊂PSZ in the structures of CoSe2⊂PSZ⊂ rGO. After carbonization, the PSZ were transformed into S, P, and N codoped carbon (SPNC) with the comonomers of HCCP and BPS providing the N, P, and S atoms. It can be clearly seen that the PSZ polymer has been carbonized into an S, P, and N-doped carbon layer, with a thickness of approximately ∼27.2 nm, tightly wrapped on surfaces of CoSe2 nanorods (Figure d and Figure S4). The morphology and microstructure of CoSe2⊂rGO reveal the uniform coating of rGO nanosheets on the CoSe2 nanorods (Figures S2c and S5). CoSe2⊂SPNC⊂rGO exhibits a sandwich-like structure with CoSe2 nanorods in the innermost layer, SPNC in the middle layer, and rGO in the outermost layer (Figure e and Figure S2d). From the HRTEM image, the lattice spacing corresponding to the CoSe2 (111) crystal plane is 0.26 nm, and the lattice spacing relative to the d-spacing of C (002) in rGO nanosheets is 0.34 nm (Figure f). As shown in the HAADF-STEM and X-ray (EDX) elemental mapping images (Figure g), the elements of N, S, and P are well distributed in the hybrids of CoSe2⊂SPNC⊂rGO. Benefiting from the dual confinement of CoSe2 nanorods with SPNC and rGO, CoSe2⊂SPNC⊂rGO shows high structural stability and the volume expansion in the repeating intercalation/deintercalation of K+ ions is significantly alleviated.

Figure 1

Schematic illustration for the preparation of the CoSe2⊂SPNC⊂rGO hybrids.

Figure 2

Morphological and compositional characterizations of electrode materials. TEM images of (a) CoSe2, (b) CoSe2⊂PSZ, (c) CoSe2⊂PSZ⊂ rGO, (d) CoSe2⊂SPNC, and (e) CoSe2⊂SPNC⊂rGO. (f) HRTEM image of CoSe2⊂SPNC⊂rGO. (g) HAADF-STEM and elemental mapping images for CoSe2⊂SPNC⊂rGO.

The structure of CoSe2⊂SPNC⊂rGO was characterized through XRD. In XRD analysis of CoSe2⊂SPNC⊂rGO hybrid materials (Figure a), it is clearly confirmed that all the characteristic peaks are a good fit with the CoSe2 phase (JCPDS no. 53-0449) standard. The different peaks located at 30.8, 34.5, 36.0, 47.7, 50.2, and 53.5° correspond to the (101), (111), (120), (211), (002), and (031) planes of the orthorhombic CoSe2, respectively, which confirms the successful preparation of CoSe2. Raman spectra in Figure b show the characteristic peaks located at 1337.73 and 1598.55 cm–1, corresponding to the typical D-band and G-band derived from sp3- and sp2-hybridized carbons, and the ratio of the D-band intensity (ID) to the G-band intensity (IG) of the Raman spectrum could illustrate the number of defects. Therefore, compared with the ID/IG ratio of the CoSe2⊂SPNC (ID/IG = 1.06) and CoSe2⊂rGO (ID/IG = 1.07), the ID/IG ratio of CoSe2⊂SPNC⊂rGO (ID/IG = 1.1) reveals more defects and distortions in the graphite layers.[46] For all samples, in addition to the peaks of the D-band and G-band, there is another peak at about ∼188 cm–1, which is a characteristic peak of CoSe2.[47] FTIR spectra were examined to further characterize the structure of CoSe2, CoSe2⊂PSZ, and CoSe2⊂SPNC. As shown in Figure c, the absorption peaks located at 1470 and 1554 cm–1 correspond to the phenylene of BPS, and the typical absorption peaks located at 1281 and 1126 cm–1 belong to the O=S=O group. The characteristic absorptions of P–N and P=N groups are at 864 and 1173 cm–1, and the absorbance peak of Ar–O–P is at 914 cm–1. For CoSe2⊂PSZ and CoSe2⊂SPNC, there is a unique absorption located at 1049 cm–1 corresponding to the Co–O bonds of CoSe2⊂PSZ and CoSe2⊂SPNC, which indirectly indicates that the CoSe2 nanorods are coated with PSZ.[48]

Figure 3

(a) XRD patterns of CoSe2, CoSe2⊂SPNC, CoSe2⊂rGO, and CoSe2⊂SPNC⊂rGO. (b) Raman spectra of CoSe2⊂PSZ, CoSe2⊂SPNC, CoSe2⊂rGO, and CoSe2⊂SPNC⊂rGO. (c) FTIR spectra of CoSe2, CoSe2⊂PSZ, and CoSe2⊂SPNC.

The XPS was further carried out to analyze and study the electronic structure and chemical constituents of the CoSe2⊂SPNC⊂rGO sample to confirm the interface interaction. Co, Se, C, N, S, and P elements are all present in the CoSe2⊂SPNC⊂rGO sample, which is proven by the X-ray photoelectron survey spectrum (Figure S6). The atomic percentages of P, S, and N elements are 3.18, 2.06, and 1.79%, respectively. The existence of O elements in the sample may originate from the adsorbed air (Figure S13). The analysis of the XPS spectra of C 1s could be divided into two main peaks. Besides the peaks located at 284.8 eV that could be attributed to the graphitic carbon (sp2-hybridized C), another peak located at 286 eV relative to those from C–S/C–N/C–O bonds is also noticed (Figure a). The XPS curve of Co 2p (Figure b) could be deconvoluted into two spin-orbit doublets and two shake-up satellites (marked “sat.”). The dominant peaks of Co 2p3/2 at 782 eV and Co 2p1/2 at 798 eV, relative to Co2+ cations in CoSe2, with a binding energy of 782 eV, can be attributed to Co2+ cations coordinated with Se ions.[49]Figure c shows the XPS spectrum of Se 3d consisting of two peaks located at 55.8 and 56.7 eV, which can be put down to Se 3d5/2 and Se 3d3/2 of Co–Se bonding, respectively,[50] and the peak at 61.2 eV indicates the presence of SeO, which is consistent with previous reports.[51,52] The X-ray photoelectron survey scan spectrum in Figure d shows an obvious P 2p signal, and the high-resolution P 2p spectrum is divided into two peaks at 133.9 and 135 eV, which are ascribed to P–O and P–Se bonding, respectively.[53] The two strong peaks at 161.5 and 162.5 eV are the peaks of S 2p3/2 and S 2p1/2 of Co–S bonding, and those at 163.8 and 164.8 eV could be consistent with S 2p3/2 and S 2p1/2 spin orbits from the C–S species (Figure e).[54] The analysis of the N 1s spectrum is presented in Figure f, where three peaks are located at 401.9, 400.2, and 398.4 eV, corresponding to graphitized nitrogen, pyrrole nitrogen, and pyridinic nitrogen, respectively.[55] The XPS results illustrate that sulfur, phosphorus, and nitrogen have been successfully doped in the carbon layer. With the heteroatom doping, the carbon layer coupled with the rGO nanosheet not only avoids the aggregation of nanoparticles but also enhances the electronic conductivity and promotes the fast transportation of K+ ions, indicating a high electrochemical performance.

Figure 4

(a–f) High-resolution XPS spectra of C 1s, Co 2p, Se 3d, P 2p, S 2p, and N 1s for CoSe2⊂SPNC⊂rGO.

Electrochemical Performance of CoSe2⊂SPNC⊂rGO

The performances of the as-prepared CoSe2, CoSe2⊂SPNC, CoSe2⊂rGO, and CoSe2⊂SPNC⊂rGO anode materials were estimated by assembling the CR2016 coin cells using a potassium flake as the cathode electrode and 1 M potassium bis(fluorosulfonyl)imide (KFSI) in ethylene carbonate (EC): propylene carbonate (PC) = 1:1 as the electrolyte. It has been demonstrated that KFSI could boost the electrochemical behavior of the anode.[56] The electrochemical processes of K+ ion insertion and extraction behaviors via the CoSe2⊂SPNC⊂rGO hybrid were investigated by cyclic voltammetry (CV). The initial three CV cycles of the CoSe2⊂SPNC⊂rGO electrode were investigated in an applied potential range of 0.01–3.0 V vs K/K+ at 0.1 mV s–1 (Figure a). In the first cathode, the reduction peak located at about 0.97 V is ascribed to the K+ ion insertion in the CoSe2 crystal and the formation of KCoSe2 (CoSe2 + xK+ + xe–=KCoSe2). A peak evident at 0.2–0.4 V is possibly in relation to the formation of a solid electrolyte interphase (SEI) film on the anode surface, which vanishes in the following cycles. A broad and strong peak is found near 0.28 V, which corresponds to the potassiation reaction from KCoSe2 to Co and K2Se (KCoSe2 + (4 – x)K+ + (4 – x)e–=- Co + 2K2Se). The peak around 0.01–0.04 V may be put down to the intercalation of K+ ions through the SPNC layer of the CoSe2⊂SPNC⊂rGO hybrids.[57] During the first anodic scanning process, the oxidation peak located at 1.65 V relates to the depotassiation process that forms CoSe2 from Co and K2Se: Co + 2K2Se =CoSe2 + 4K+ + 4e–. The two peaks found at 1.15 and 0.45 V relate to the insertion of K+ ions into CoSe2 and the conversion reaction with more K+ ions in the following cycle. Moreover, the CV curve illustrates that the CoSe2⊂SPNC⊂rGO anode has significant reversibility and stability during the cycle because it almost coincides in the subsequent second and third cycles.[58]

Figure 5

Electrochemical performance of the CoSe2⊂SPNC⊂rGO electrode. (a) CV curves at 0.1 mV s–1 of the CoSe2⊂SPNC⊂rGO electrode. (b) GCD curves of the CoSe2⊂SPNC⊂rGO anode for the 1st, 2nd, 3rd, 5th, and 10th cycles. (c) Rate capabilities of CoSe2⊂SPNC, CoSe2⊂rGO, and CoSe2⊂SPNC⊂rGO electrodes. (d) Cycling performances of the CoSe2, CoSe2⊂SPNC, CoSe2⊂rGO, and CoSe2⊂SPNC⊂rGO electrodes at 100 mA g–1. (e) Long-term cycling performances of the CoSe2⊂SPNC⊂rGO electrode at 0.5 A g–1. (f) EIS spectra for the CoSe2⊂SPNC⊂rGO electrode before and after cycling. (g) Electrochemical performance comparison of anode materials for PIBs.

The galvanostatic charge–discharge (GCD) curves of CoSe2⊂SPNC⊂rGO hybrid materials were measured at 100 mA g–1 (Figure b). In the initial cycle, the GCD curves of CoSe2⊂SPNC⊂rGO exhibit ∼324.4 and ∼601.8 mAh g–1 of charge/discharge process, which corresponds to an initial Coulombic efficiency of 53.9%. The large capacity difference between the first charge and discharge processes can be put down to irreversible processes, corresponding to the formation of the SEI film. CoSe2⊂SPNC⊂rGO exhibits a reversible capacity of ∼253 mAh g–1 after the end of 10 cycles with a Coulombic efficiency of nearly 100%. There are two slopes in the initial discharge process, corresponding to the formation of KCoSe2 and the conversion reaction from KCoSe2 to Co and K2Se, which confirm the CV analysis. The plateaus at 1.6–1.7 V can be put down to the potassiation process in the charging curve. The GCD curves almost overlap in the subsequent cycle, which also confirms the CV. These results demonstrate that the CoSe2⊂SPNC⊂rGO electrode has high reversibility and good cycle stability. The GCD curves of CoSe2, CoSe2⊂SPNC, and CoSe2⊂rGO, within the first 10 cycles, are exhibited in Figure S7 (Supporting Information). For the CoSe2, CoSe2⊂rGO, and CoSe2⊂SPNC electrodes, the specific capacity is obviously attenuated from the second cycle. The situation of capacity decay was even worse for CoSe2, whose capacity decayed rapidly from the second cycle. At the end of the 10th cycle, the reversible capacities of the CoSe2, CoSe2⊂rGO, and CoSe2⊂SPNC electrodes are ∼19, 158, and 163 mAh g–1, respectively, which are inferior to that of the CoSe2⊂SPNC⊂rGO electrode. Therefore, the SPNC coating coupled with encapsulation of rGO nanosheets can significantly accommodate the volume expansion to improve the cycling stability.

The CoSe2⊂SPNC⊂rGO anode was further cycled at different current densities (Figure c). The CoSe2⊂SPNC⊂rGO anodes exhibited a significant rate performance with reversible capacities relative to the other anodes. Average capacities of 287.2, 246.7, 202.5, 144.1, and 115.4 mA h g–1 were obtained at 0.1, 0.2, 0.5, 1, and 1.5 A g–1, respectively. Even at 1.5 A g–1, CoSe2⊂SPNC⊂rGO can still achieve an excellent reversible capacity of 115.4 mA h g–1, which is markedly better than CoSe2⊂rGO and CoSe2⊂SPNC. More importantly, when it returned to 0.1 A g–1, CoSe2⊂SPNC⊂rGO returned to a high specific capacity of 268.2 mA h g–1. Upon a continuous long-term cycling test at 0.1 A g–1, CoSe2⊂SPNC⊂rGO maintained a remarkable capacity of ∼261 mA h g–1 after 80 cycles. Under the same conditions, the specific capacity of CoSe2⊂ rGO decayed rapidly when the current density was restored to 0.1 A g–1. Moreover, there was almost no capacity of CoSe2⊂ rGO at 0.5, 1, and 1.5 A g–1. Under different current densities, the specific capacity of CoSe2 has a significant decay (Figure S14). The above results clearly show that the CoSe2⊂ SPNC⊂rGO hybrid has outstanding cycle stability and rate performance, which are mainly attributed to its unique structure of SPNC coating and rGO nanosheet confinement.

Figure d compares the cycling performance of all anodes at 100 mA g–1. The CoSe2⊂SPNC⊂rGO anode showed a superior capacity of ∼270 mA h g–1 with the Coulombic efficiency approaching 100% over 60 cycles. Moreover, it had an excellent initial Coulombic efficiency of 51.2%. In contrast to this, the CoSe2, CoSe2⊂SPNC, and CoSe2⊂rGO electrodes delivered lower initial Coulombic efficiencies of 21.6, 30.2, and 37.8%, respectively (Figure S8, Supporting Information). Only the Coulombic efficiency of CoSe2⊂SPNC⊂rGO electrode was stable and close to 100% after 10 cycles. This caused the specific capacity of CoSe2, CoSe2⊂rGO, and CoSe2⊂SPNC electrodes to decrease significantly after several initial cycles. In particular, the CoSe2 electrode almost failed after 10 cycles, and the CoSe2⊂rGO electrode did the same after 30 cycles, while the CoSe2⊂ SPNC⊂rGO electrode kept a stable capacity without decaying for more than 20 cycles at 0.1 A g–1. Finally, the CoSe2, CoSe2⊂rGO, and CoSe2⊂SPNC electrodes delivered relatively lower specific capacities of 4, 28, and 147 mAh g–1 after 60 cycles in comparison with CoSe2⊂SPNC⊂rGO.

In order to explore the long-term cycle stability of the CoSe2⊂SPNC⊂rGO anode, a long-cycle performance measurement was performed at 500 mA g–1, as shown in Figure e. After 500 cycles, CoSe2⊂SPNC⊂rGO maintained a high reversible capacity of 208.8 mA h g–1, and the Coulombic efficiency was about 99.7%. Even at 1 A g–1, it still had a specific capacity of 144.9 mAh g–1 as an anode material after 800 cycles (Figure S9, Supporting Information). To research the structural stability of the CoSe2, CoSe2⊂SPNC, CoSe2⊂rGO, and CoSe2⊂SPNC⊂rGO electrodes after different cycles, the electrochemical impedance spectroscopy (EIS) spectra were measured within 0.1 Hz–10 kHz (Figure f and Figure S10). The impedance spectrum is formed by two parts, a semicircle and an oblique. The semicircle is caused by the charge transfer process and can be expressed by Rct at a high frequency, and the oblique line at a low frequency corresponds to the Warburg impedance of K+ diffusion in the equivalent circuit. The radius of all the semicircles becomes larger because the electrode surface forms an SEI film during the first charging and discharging process. Among them, the Rct of the CoSe2⊂SPNC⊂rGO electrode is significantly smaller compared with other similar materials. Through the modification of carbon coating and rGO nanosheets, the impedance of the hybrid material and the electrolyte interface has changed significantly. Benefiting from the carbon material as an ion–electron mixed conductor, the impedance of the interface charge transfer is reduced, and chemical reactions are easier to proceed. The semicircle of the CoSe2 electrode increased significantly from fresh to 40 cycles and then after 80 cycles due to the pulverization of the structure of the anode material. After 40 cycles, the semicircle change of the CoSe2 anode was much more serious than those of the CoSe2⊂rGO and CoSe2⊂SPNC anodes because the rGO nanosheets and SPNC coating have a certain effect in alleviating volume expansion. The CoSe2⊂SPNC⊂rGO electrode, especially, maintained an almost changeless radius of the semicircle over 80 cycles, proving the stable existence of the SEI film and the integrity of the nanohybrid structure. It powerfully demonstrates that the combination of the SPNC coating and rGO nanosheet can effectively avoid the crushing of the electrodes. As observed in Figure g and Table S1, the CoSe2⊂SPNC⊂rGO electrode shows an excellent K+ ion storage performance, which is better than most recently reported PIB anode materials.

In order to further inquire into the diffusion dynamics and charge storage of the CoSe2⊂SPNC⊂rGO hybrid anode, the electrochemical kinetics was explored by recording the CV curves under various scan rates. The CV curves of the CoSe2⊂SPNC⊂rGO anode at 0.1, 0.2, 0.4, 0.6, 0.8, and 1 mV s–1 are shown in Figure a, which present similar shapes with the increasing scan rates, indicating the low electrochemical polarization of the CoSe2⊂SPNC⊂rGO anode in the process of capacitance and diffusion K+ ion storage behaviors. Generally, the measured current (i) at different scan rates (v) is analyzed to characterize the capacitance contribution of the cell according to the following equations:[59,60]where the values of a and b are two variable constants; the plotting of log(i)–log(v) curves can calculate the b value. The value of b can reflect whether its capacity contribution comes from a diffusion-controlled or capacitance-controlled process. If it is a completely diffusion-controlled electrochemical behavior, then its b value is close to 0.5, and if it is a completely capacitance-controlled electrochemical behavior, then its b value is approaching 1.0. The b values of the reduction and oxidation peaks of CoSe2⊂SPNC⊂rGO are 0.78 and 0.84 and 0.73 and 0.77, respectively, indicating that the capacitive-controlled process for K+ ion storage of the CoSe2⊂SPNC⊂rGO anode accounts for a large proportion (Figure b). In addition, the capacitance contribution ratio at different scan rates can be further calculated by the following formulae:[61]where k1 and k2 are arguments determined by the slope and intercept, respectively. The k1v and k2v1/2 denote the capacitive contribution process and the diffusion-controlled process, respectively. As representatively displayed in Figure c, the orange color-shaded section corresponds to the capacitive current response in comparison with the total at 1.0 mV s–1, in which the orange area was found to be ∼54.8%, representing the capacitance control process. In addition, as shown in Figure d, the capacitive contribution ratio rises with the increasing rates from 0.2 to 1 mV s–1. This illustrates that the capacitance-controlled process is dominant at high scan rates, which means that the improvement of cycle stability and rate capability is in the fast K+ interaction/extraction process. The electrochemical performances of the CoSe2⊂SPNC⊂rGO hybrid can be explained as the synergistic effect of the structure with a P and S-doped carbon layer, dual carbon protection, and stabilized CoSe2 nanostructure. First, the confinement of dual carbon can effectively relieve the volume expansion upon the charge/discharge process. In addition, the S, P, and N-doped carbon layer and rGO nanosheets can not only largely strengthen the stability of the structure, which is well demonstrated by the morphological observation of the products after cycling (Figure S11), but also facilitate the electron transfer due to the better electric conductivity, as revealed by the EIS measurement. Second, nanosized particles strongly coupled with the carbon layer can shorten the diffusion path of K+ ions. Third, CoSe2 nanoparticles encapsulated in carbon coating and confined between rGO nanosheets can alleviate the agglomeration of CoSe2 nanorods and the rupture of the anode materials during the cycling process. Finally, the hybrid structure that incorporated partially capacitive contribution is also a non-negligible factor for the enhanced performance.

Figure 6

Kinetic analysis of the CoSe2⊂SPNC⊂rGO anode electrode. (a) CV curves at various scan rates of the CoSe2⊂ SPNC⊂rGO electrode. (b) Plots for b-value determination. (c) Sketch view of the capacitive behavior of the CoSe2⊂SPNC⊂rGO electrode at 1.0 mV s–1. (d) Capacitive contribution ratios at various scan rates.

The volume expansion of electrode materials is another factor that restricts the performance of PIBs, thereby hindering their commercialization. The structure and thickness changes of the anode materials before and after cycling were characterized by the ex situ SEM observations. All anodes were cycled under 100 mA g–1 with a similar mass loading (≈ 0.9 mg cm–2) of active materials. Figure a shows the cross-sectional SEM images of CoSe2⊂SPNC⊂rGO electrodes as fresh, and after the 1st, 40th, and 80th cycles, respectively. Figure S12 (Supporting Information) shows the thickness change of the CoSe2⊂SPNC and CoSe2⊂rGO electrodes. The percentages of anode thickness increase are shown in Figure b. All anodes exhibited an obvious increase in the thickness after the 40th cycle (K+ ion insertion/extraction) due to the SEI film on the surface and the volume expansion of the anode material. The thicknesses of the CoSe2⊂rGO, CoSe2⊂SPNC, and CoSe2⊂SPNC⊂rGO electrodes increased by 142.6, 138.8, and 118.5%, respectively. After the 80th cycle, the thickness of the CoSe2⊂rGO, CoSe2⊂SPNC, and CoSe2⊂SPNC⊂rGO electrodes increased by 200.6, 173.7, and 131.9%, respectively. The CoSe2⊂SPNC⊂rGO electrode maintained a low thickness increase (∼20%) within 40 cycles, while the thicknesses of the CoSe2⊂SPNC and CoSe2⊂rGO electrodes were ∼1.7 and ∼2.0 times thicker than those of uncycled anode materials. This indicates that the volume expansion of the anode material is alleviated, and the integrity of the nanostructure is maintained under the combined effect of SPNC coating and rGO nanosheets. The SEM images of the top view displayed in Figure S11 directly illustrate the morphology changes of the electrode materials after 40 cycles. It can be clearly seen that both CoSe2, CoSe2⊂rGO, and CoSe2⊂SPNC electrode materials were broken to different degrees, and the CoSe2 electrode material even has multiple cracks. Only the CoSe2⊂SPNC⊂rGO electrode material has no significant rupture or cracks, which basically retains the original structure. It strongly demonstrates that the combination of SPNC coating and rGO nanosheets maintains the structure of the internal material of the electrode, which efficiently buffers the volume expansion and improves the electrochemical performance.

Figure 7

Electrochemical reaction of CoSe2⊂SPNC⊂rGO as an anode material. (a) Cross-sectional SEM images of CoSe2⊂SPNC⊂rGO electrodes before and after cycling. (b) Electrode thickness increases of the CoSe2⊂SPNC, CoSe2⊂rGO, and CoSe2⊂SPNC⊂rGO electrodes after cycling. (c) Schematics of the electrochemical process that configured the conversion anode. (d) In situ Raman spectra of CoSe2⊂SPNC⊂rGO collected at the first cycle and charge/discharge curves at 100 mA g–1.

The above electrochemical performance characterization shows that the CoSe2⊂SPNC⊂rGO anode has better cycling stability, which can be explained by the mechanism diagram (Figure c). The electrochemical reaction process was summarized to inquire into and study the potassium storage mechanism of the CoSe2⊂SPNC⊂rGO electrode. In the discharge process, K+ ions are first intercalated in orthorhombic CoSe2 to form KCoSe2, which is then converted to cubic K2Se and Co. The charge process is just the opposite. During the cycle of insertion/extraction of K+ ions, the effective combination of SPNC coating and rGO prevents the volumetric expansion of the anode material to a certain extent, thereby avoiding cracking, and even the pulverization of the electrode material, thus improving the cycling stability. Furthermore, in situ Raman analysis was carried out to study the potassiation/depotassiation process of CoSe2 in the first cycle. As shown in the Raman spectra of Figure d, the characteristic peak intensities of CoSe2 show a gradual and continuous weakening trend during the discharge process. The peaks of CoSe2 almost totally disappear in the case of complete discharge. After full depotassiation (charged to 3.0 V), the Raman characteristic peaks appear again, indicating the reversible transformation of CoSe2.[62,63] This significant electrochemical performance can be put down to the following advantages: (i) The effective combination of the carbon layer and rGO nanosheet powerfully alleviates the stress changes resulting from volumetric expansion and avoids the aggregation of CoSe2 nanorods during the charging and discharging process. (ii) Heteroatom (S, P, and N) doping will enrich defects in carbon and generate more active sites, thereby enhancing the adsorption of K+ ions and the electronic conductivity. (iii) The electronic conductivity of the electrode could be enhanced by the continuous graphene network.

To further verify the commercial application of the PIBs, soft-packaged batteries were assembled. In a typical procedure, the soft-packaged PIBs were assembled by sandwiching the separator and electrolyte between the CoSe2⊂SPNC⊂rGO hybrid anode and K flake and sealed with an aluminum-plastic film, as depicted in Figure a. It demonstrates the potential application of this anode material in soft-packaged batteries because it can light up an LED. As shown in Figure b, it delivers an initial capacity of 507.6 mAh g–1 at 0.5 A g–1, and the soft-packaged battery can still be stabilized at 224.2 mAh g–1 after 200 cycles.

Figure 8

Assessment on the applicable prospect with the soft-packaged battery. (a) Demonstrations on practical use by lighting LEDs. Photograph courtesy of Zhongshu Zhao. Copyright 2020. (b) Cycling stability at 500 mA g–1.

Conclusions

In summary, reduced graphene oxide-encapsulated polyphosphazene-derived S, P, and N codoped carbon-coated CoSe2 nanorods (CoSe2⊂SPNC⊂rGO) were designed as a PIB anode material. CoSe2⊂SPNC⊂rGO delivers an excellent reversible capacity of 287.2 mAh g–1 at 100 mA g–1. Benefiting from the coating of heteroatom-doped carbon and encapsulation of rGO, the CoSe2⊂SPNC⊂rGO anodes exhibit a remarkable rate capability (100–1500 mA g–1 current density) and high stability (208.8 mAh g–1 after 500 cycles at 500 mA g–1). The results demonstrate that the S, P, and N codoping in carbon layers gives active sites for K+ ion storage and increases the electrical conductivity. More importantly, the dual confinement of CoSe2 nanorods with the SPNC layer and rGO significantly reduces the volume expansion and keeps the structural stability with repeating intercalation/deintercalation of K+ ions.

Experimental Section

Chemicals and Materials

Hexachlorocyclotriphosphazene (HCCP), 4,4-sulfonyldiphenol (BPS), urea, and triethylamine (TEA) were all obtained from Shanghai Aladdin Biochemical Technology Co., Ltd. Cobaltous nitrate hexahydrate (Co(NO3)2·6H2O), selenium (Se) powder, hydrazine hydrate (N2H4·H2O, 80% in water), anhydrous acetonitrile, and ethanol were all purchased from Sinopharm Chemical Reagent Co., Ltd. The chemicals in this work were used without any purification. A modified Hummers’ method was used to synthesize the GO and configured dispersion.

Synthesis of CoSe2 Nanorods

In a typical procedure, cobaltous nitrate hexahydrate (291 mg) and urea (450 mg) were first dissolved into 30 mL of deionized (DI) water. After intensely stirring for 15 min, the obtained pink solution was poured into a 50 mL Teflon-lined autoclave and reacted at 120 °C for 6 h. After cooling to ambient temperature, pink precipitates of Co–OH–urea were collected through centrifuge separation and rinsing with DI water and ethanol and then dried overnight at 60 °C under vacuum. The CoSe2 nanorods were synthesized by selenization of the precursor of Co–OH–urea. Fifty milligrams of Co–OH–urea was redispersed in 20 mL of DI water via sonication to obtain the dispersion. Sixty milligrams of Se powder was slowly added into 5 mL of N2H4·H2O to form a clear solution, which was then added dropwise into the Co–OH–urea dispersion under intense stirring. After that, the solution mixture was treated through a hydrothermal method at 200 °C for 10 h in the Teflon-lined autoclave. The obtained black sediments of CoSe2 nanorods were collected by centrifugation and washed several times using ethanol and DI water. Subsequently, the CoSe2 nanorods were obtained after drying at 60 °C under vacuum for 12 h.

Preparation of CoSe2⊂PSZ

In the beginning, the as-prepared 100 mg of CoSe2 nanorods was dispersed in 30 mL of anhydrous acetonitrile. Then, HCCP (25 mg) and BPS (55.7 mg) were added into the above dispersion with continuous stirring. Next, 10 μL of triethylamine was added into the above dispersion under quick stirring. After 6 h of stirring, the products of CoSe2⊂PSZ were washed and then vacuum dried at 60 °C for 12 h.

Fabrication of CoSe2⊂SPNC⊂rGO

Two hundred milligrams of CoSe2⊂PSZ was first dispersed into 20 mL of DI water and then added dropwise into 20 mL of 5 mg mL–1 GO dispersion followed by ultrasonication for 2 h. Subsequently, the above solution mixture was transferred into a 50 mL Teflon-lined autoclave and reacted at 180 °C for 12 h. Afterward, the black-gray product of CoSe2⊂PSZ encapsulated in reduced graphene oxide (rGO) (CoSe2⊂PSZ⊂rGO) was separated by filtration and lyophilized. Finally, the obtained CoSe2⊂PSZ⊂rGO was carbonized at 600 °C under N2 for 2 h at 2 °C min–1 to get CoSe2⊂SPNC⊂rGO. CoSe2⊂SPNC and CoSe2⊂rGO were prepared by following the same procedure as CoSe2⊂SPNC⊂rGO and CoSe2⊂SPNC⊂rGO, respectively.

Instruments and Characterizations

SEM (JEOL, JEM-7800F) and TEM (JEOL, JEM-2100F) were done to study the morpn>hology and microstructure characteristics of the samples. The crystal structure of the samples was analyzed by XRD using a Bruker D8 Advance under Cu Kα radiation (λ = 1.5418 Å). The chemical composition of the samples was analyzed by XPS under Al Kα X-ray (1486.6 eV) radiation on a Kratos Axis UltraDLD. The Raman spectrum was obtained on a Horiba Jobin Yvon LabRAM with a laser excitation wavelength of 532 nm, and the FTIR spectrum was obtained using a Shimadzu FTIR-8400s.

Electrochemical Characterization

The electrochemical performance of the sample was studied by assembling and testing a CR2016 button battery. The button cell was assembled in a glove box filled with Ar. A uniform slurry of 80 wt % of active material, 10 wt % of acetylene black, and 10 wt % of PVdF in N-methyl-2-pyrrolidone (NMP) was pasted on copper foil to make the anode with a mass loading of about 0.9 mg cm–2. Then, the copper foil was vacuum dried at 80 °C overnight. Using a potassium metallic sheet as a counter electrode, Whatman glass fiber (GF/D) as a separator, and 1 M potassium bis(fluorosulfonyl)imide (KFSI) in potassium carbonate (EC):propylene carbonate (PC) = 1:1 as the electrolyte, the button battery was assembled in the glove box filled with Ar. The electrochemical performances of the samples were observed on a multichannel land-based battery test system (LAND CT2001), and the potential range relative to K/K+ was 0.01–3.0 V. A CHI 660E electrochemical workstation was used to perform CV at various scan rates from 0.01 to 3.0 V. EIS was recorded in a frequency range of 100 kHz–0.1 Hz.