With the rapid development of portable electronic equipment and electric vehicles, the demand for energy storage devices with high energy density and high safety is increasing. Among the large-scale energy storage devices, lithium-ion batteries (LIBs) have attracted extensive attention due to their high energy density, environmental friendliness, and high efficiency. Due to insufficient lithium resources, researchers turned their attention to resource-rich potassium-ion batteries (PIBs) (Hu et al., 2017; Ge et al., 2018). Compared to the LIBs, PIBs have similar energy storage mechanisms, moreover, have the advantages of low production cost and low reduction potential (relative to the standard hydrogen electrode ≈2.94 V) (Liu et al., 2020; Rajagopalan et al., 2020). Therefore, PIBs are expected to become an alternative to LIBs(Zhao et al., 2021). Obtaining the high energy density of PIBs mainly depends on technological breakthroughs in electrode materials. However, toward the PIBs, due to the large radius of potassium ions, the kinetics of K⁺ during the intercalation/delamination process is slow and the volume is prone to expansion, which will cause the battery’s capacity to decay rapidly and the decrease of coulomb efficiency (Zhang et al., 2021). The key to improving the performance of PIBs is to develop an anode material with excellent electrochemical performance (Zheng et al., 2021).

Among the various explored anode materials, transition metal compounds show great potential due to their excellent electrochemical performance. Compared to the transition metal sulfides and oxides, transition metal selenides have a narrower band gap and a higher volumetric capacity (Qiu et al., 2020). Moreover, the metal-Se bond in the metal selenide is weak and easy to break, which is beneficial to the conversion reaction (Zhu et al., 2017; Hou et al., 2018; Xie et al., 2019; Luo et al., 2020)_. NiCo₂Se₄ as a kind of bimetallic selenides is widely used as PIB anodes because of its rich electronic reaction processes, high conductivity, and excellent chemical stability. Nevertheless, NiCo₂Se₄ still has some problems, such as pulverization of the electrode material due to volume expansion during charging and discharging, which causes the battery’s capacity to rapidly decay. In view of these problems, how to solve the volume change during the continuous intercalation and delamination of K⁺ ions are the main challenge to improve the electrochemical performance and cycle stability of PIBs. In order to solve the above-mentioned problems of NiCo₂Se₄, Zhou et al. deposited the hierarchical NiCo₂Se₄ nanoneedles/nanosheets on the skeleton of N-doped three-dimensional porous graphene (NPG) as sodium-ion battery anode and found that the high conductivity and porous NPG improves the transport of electrons of the anode as well as ions (Zhou et al., 2021). Always, incorporating the carbonaceous materials is considered to be the competitive way to improve the cycling performance and rate capability of PIB anode materials. Huang et al. designed the hierarchical carbon-coated MoSe₂/MXene hybrid nanosheets (MoSe₂/MXene@C) as anode material for PIBs(Huang et al., 2019). The coated carbon layer reinforced the composite structure of MoSe₂/MXene, meantime enhancing the overall conductivity of the hybrid MoSe₂/MXene@C, which finally promoted the charge-transfer kinetics and improved the durability of PIB.

Herein, the nitrogen, phosphorus, and sulfur tri-doped carbon-coated NiCo₂Se₄ needle arrays grown on carbon cloth (NiCo₂Se₄⊂SPNC/CC) was prepared as a binder-free anode for PIBs. The NiCo₂Se₄⊂SPNC/CC owns an internally hierarchical stable structure, which is composed of CC as the base, NiCo₂Se₄ needle arrays as the middle active layer, and an outer layer of nitrogen, phosphorus, and sulfur tri-doped carbon layers (SPNC) which effectively alleviate the powdering and volume expansion of the anode material of NiCo₂Se₄ during the charging and discharging process. The coating of SPNC could improve the conductivity of the anode and the doping of S, P, and N provide abundant active sites for the storage of potassium ions. The NiCo₂Se₄⊂SPNC/CC anode exhibits high reversible capacity, excellent cycle stability, and good rate performance.

As shown in Figure 1A, the preparation process of NiCo₂Se₄⊂SPNC/CC was mainly divided into three steps. In the first step, the NiCo₂O₄/CC was synthesized via hydrothermal reaction. From Figure 2A,B, the surfaces of the carbon fibers of CC are clean and smooth. After the hydrothermal reaction, it is obvious that the NiCo₂O₄ nanoneedle arrays with the mean root width of ∼85 nm and length of ∼1.5 µm are grown on the carbon fibers of CC (Figure 2C). Secondly, the surfaces of NiCo₂O₄/CC have coated the polymer layers of the polyphosphazene (PSZ) via in-situ polycondensation of hexachlorotripolyphosphazene (HCCP) and 4,4′-dihydroxydiphenylsulfone (BPS) (Figure 1B), which is demonstrated by the Fourier transform infrared spectroscopy (FTIR) spectra (Supplementary Figure S1). After the following carbonization in an N₂ atmosphere at 600 °C for 2 h, the obtained NiCo₂O₄⊂SPNC/CC were further converted into NiCo₂Se₄⊂PSZ/CC by using solvothermal selenization at 200°C for 10 h. From Figure 2E,F, after selenization and carbonization, the NiCo₂Se₄ needles were connected by the layers of SPNC and the surfaces of NiCo₂O₄ nanoneedle arrays become very rough.

Figure 3 shows the XRD patterns for the CC, NiCo₂O₄/CC, and NiCo₂Se₄⊂SPNC/CC. For the XRD patterns of the pure CC and NiCo₂O₄/CC, there are evident diffraction peaks at 2θ = 26.2^o which are ascribed to the C (002) plane of graphite carbon. For the XRD pattern of NiCo₂Se₄⊂SPNC/CC, the diffraction peaks at 2θ = 33.7°, 45.3°, and 51.2° are attributed to the (101), (102) and (110) crystal planes of the monoclinic NiCo₂Se₄ phase standard card (JCPDS No. 08–4821), demonstrating the formation of NiCo₂Se₄(Zhang et al., 2020). It is worth noting that the diffraction peaks of the C (002) plane of NiCo₂Se₄⊂SPNC/CC moves to the relatively lower diffraction angles which are attributed to the increase of interplanar spacing after the introduction of nitrogen, phosphorus, and sulfur heteroatoms (Li et al., 2019). X-ray photoelectron spectroscopy (XPS) spectrum for the NiCo₂Se₄⊂SPNC/CC was further used to characterize the valence states and surface chemical composition. From the XPS survey spectrum, the Ni, Co., Se, N, S, P, C, and O elements exist in the NiCo₂Se₄⊂SPNC/CC (Figure 3B). In the Co. 2p spectrum (Figure 3C,D), there are six cobalt chemical states. The peaks at binding energies of 785.6 and 802.1 eV correspond to the satellite peaks (Yu et al., 2019). The spin-orbit peaks of Co²⁺ and Co³⁺ are located at 778.9, 793.8 eV, and 781.4, 797.1 eV, respectively (Guan et al., 2019). Similarly, toward the Ni 2p spectrum, there are also two satellite peaks which are appeared at 861.7 and 882.2 eV (Zeng et al., 2017). The two pair’s peaks at 853.8, 872.7 eV and 856.3, 877.5 eV are ascribed to the spin-orbit peaks of Ni²⁺ and Ni³⁺(Xu et al., 2020). In Figure 3E, the peaks at 55.6 and 54.7 eV correspond to the spin-orbit peaks of Se 3d_3/2 and Se 3d_5/2. A broad peak at 59.4 eV can be related to the SeO_x (Chong et al., 2021). The C 1s high-resolution XPS spectrum is depicted in Figure 3F, which can be fitted into four peaks including C-C/C=C (284.4 eV), C–N or C–S (285.9 eV), C-P (287.1 eV), and O=C-O (290.1 eV). As illustrated in Figure 3G, the peak at 161.2 eV is assigned to the C-S-C bonding in the PSZ-derived carbon framework (Shen et al., 2017). The peak at 166.7 eV is mainly attributed to the oxidation state of sulfur (C–SO_x). By fitting, the P 2p spectrum could split into the P-C and P-O peaks which are located at 133.5 and 137.7 eV (Qian et al., 2019). Besides the S and P elements, the N dopant in the PSZ-derived carbon framework was also confirmed by the N 1s of the XPS spectrum, as shown in Figure 3I, the peaks for the pyridinic N, pyrrolic N, and graphitic N are clearly observed at 398.6 400.4 and 402.1 eV (Xu et al., 2021). In addition, the peak of the oxidation state N is located at 403.23 eV (Cui et al., 2020). Based on the above analysis, the N, S, and P elements were successfully doped in the carbon framework, which could be acted as active sites for the storage of potassium ions. The atom percentages of N, S, and P elements are 2.66, 4.23, and 2.3%, respectively.

The K-ion storage behavior of the NiCo₂Se₄⊂SPNC/CC anode was first studied by cyclic voltammetry (CV). The CV curves for the first three circles are shown in Figure 4A at the scan rate of 0.1 mV·s-1 within the voltage window of 0.01–3 V. In the first cycle, the reduction peak around 0.31 V should be related to the formation of the solid electrolyte interface (SEI) film, and the large reduction band at 1.2 V should be related to the K⁺ ion insertion in the structure of NiCo₂Se₄ and the formation of NiCo₂Se₄(NiCo₂Se₄+xK⁺+xe⁻ = K_xNiCo₂Se₄) (Li et al., 2020). During the first cycle process, the oxidation peaks at 0.54, 1.73, 1.94 and 2.19 V are attributed to the reaction from K_xNiCo₂Se₄ to Co., Ni, and K₂Se(Chen et al., 2021)^. The peak around 0.01–0.03 V in the cathodic scan may be due to the intercalation of K⁺ ions in the SPNC layer of the NiCo₂Se₄⊂SPNC/CC. In the subsequent cycles, the large reduction band was shifted to 1.44 V which is related to the potassiumization process. The difference between the first and second cycles indicates that the reaction paths of the two cycles are inconsistent, which is the reason for the irreversible capacity in the first cycle. The CV curves almost overlap after the second cycle, indicating the excellent cycle performance of the NiCo₂Se₄⊂SPNC/CC electrode.

Figure 4B shows the galvanostatic charge-discharge (GCD) curves for the NiCo₂Se₄⊂SPNC/CC electrode in the first, second, third, fifth, and 10th circles at a current density of 0.1 A g⁻¹ and applied voltage window of 0.01–3 V. During the initial cycle, a high discharge specific capacity of 1549.2 mA h·g⁻¹ was obtained. The first charge specific capacity is 1076.9 mA h·g⁻¹, and the initial Coulombic efficiency is about 69.5%, which is related to the formation of the SEI film on the electrode surface. In addition, the discharge plateaus of 1.0–1.2 V and 0.2–0.4 V in the first circle are mainly attributed to the formation of K_xNiCo₂Se₄ and the conversion reaction from K_xNiCo₂Se₄ to Co., Ni, and K₂Se, which is consistent with the CV analysis. The NiCo₂Se₄⊂SPNC/CC electrode shows the reversible capacity of ∼890.6 mA h·g⁻¹ after the end of 10 cycles with a Coulombic efficiency of nearly 100%. Moreover, from the second to the 10th cycle, the GCD curves basically coincide, which also shows the excellent cycle stability of the NiCo₂Se₄⊂SPNC/CC electrode. The rate performances test of CC, NiCo₂O₄/CC, and NiCo₂Se₄⊂SPNC/CC are shown in Figure 4C. When the current densities are 0.1, 0.2, 0.5, 1, and 2 A g⁻¹, the reversible discharge capacities of NiCo₂Se₄⊂SPNC/CC are 880.9, 591.4, 356.5, 157.4, and 68.5 mA h·g⁻¹, respectively, indicating that the NiCo₂Se₄⊂SPNC/CC has good rate performance. Afterward, as the current density recovers to 0.1 A g⁻¹ after 50 cycles, the specific capacity recovers to 689.8 mA h g⁻¹, and after 90 cycles it recovers to 968.6 mA h·g⁻¹, which means that potassium ions at high current density the storage performance has not declined. For pure NiCo₂O₄/CC, when the current density is restored, the specific capacity cannot be restored to the capacity under the initial current density. The excellent rate of performance of the NiCo₂Se₄⊂SPNC/CC is due to the increase in selenization capacity and the heteroatoms doping of S, P, and N in the carbon layer originated from the polyphosphazene, which increases the active sites for K⁺ ions storage. More importantly, the coating of the SPNC layer on the surfaces of NiCo₂Se₄ improved the structure stability, largely alleviated the volume expansion of anode in the charge-discharge process, and finally improved the cycle performances of KIB. In addition, from the EIS spectra of the NiCo₂O₄/CC and NiCo₂Se₄⊂SPNC/CC, the radius of the semicircle in the EIS spectrum of the NiCo₂Se₄⊂SPNC/CC is significantly smaller than that of NiCo₂O₄/CC, implying the selenization and SPNC coating improved the conductivity and interfacial charge transport (Supplementary Figure S2). Figure 4D illustrates the cycle performance and Coulombic efficiency of CC and NiCo₂Se₄⊂SPNC/CC at the current density of 0.2 A g⁻¹. As a pure carbon material anode, although the CC has a relatively low specific capacity, it has a very good cycling stability electrode. For demonstrating good cycling stability, the cycle performances of CC and NiCo₂Se₄⊂SPNC/CC were compared. As shown in Figure 4E, the initial specific capacity of the NiCo₂Se₄⊂SPNC/CC is ∼1256.7 mA h·g⁻¹, and the Coulombic efficiency stabilizes at about 99.7% in the subsequent cycles. After 60 cycles, the specific capacity of the NiCo₂Se₄⊂SPNC/CC can still keep at 497.3 mA h·g⁻¹. Further, the long-cycle performance of the NiCo₂Se₄⊂SPNC/CC was also tested at a current density of 0.5 A g⁻¹. After 500 cycles, the NiCo₂Se₄⊂SPNC/CC anode still maintains a high reversible capacity of 268.1 mA h·g⁻¹, and the Coulombic efficiency is close to 100%.

In order to further understand the diffusion dynamics and charge storage of NiCo₂Se₄⊂SPNC/CC, the electrochemical kinetics of the NiCo₂Se₄⊂SPNC/CC was further studied by CV curves at different scan rates (Figure 5A). Increasing with the scan rate, the cathode peak moves to a high potential, and the anode peak moves to a low potential due to the polarization of the electrode at a high current density. The relationship between current and scan rate follows the following formula (Huang et al., 2020): i = a v b (1) log ( i ) = b log ( v ) + log ( a ) (2) where a is a constant, and b reflects an adjustable parameter that reflects the mechanism of the charge-storage process. When b = 0.5 or b = 1.0, the charging and discharging process is considered to be controlled by the diffusion control behavior or pseudocapacitance behavior. The value of b calculated from the selected two redox peak potentials is 0.68 and 0.73, respectively (Figure 5B), indicating that the electrochemical process of the NiCo₂Se₄⊂SPNC/CC anode has the evident diffusion behavior. Toward the structure and morphology of the NiCo₂Se₄⊂SPNC/CC anode, the needle-like NiCo₂Se₄ array with sufficient void spaces could facilitate the electrolyte penetration and alleviate the effect of the volume change of NiCo₂Se₄. And, the coating of the SPNC layer not only formed a conductive network which improves the conductivity and interfacial charge transport, but also provides the heteroatomic active sites for K⁺ ions storage. The contribution ratio of pseudocapacitance can be calculated by the following equation (Zhu et al., 2022): i = k 1 v + k 2 v 1 / 2 (3) i / v 1 / 2 = k 1 v 1 / 2 + k 2 (4) where k₁v and k₂v^1/2 refer to the currents of the capacitance and diffusion process, respectively. As shown in Figure 5C, the blue shaded part is corresponding to the capacitance current response, and the scan rate is 0.1 mV s⁻¹, where the capacitance contribution is calculated to be ≈41.2%. Figure 5D shows the contribution rate of pseudocapacitance and diffusion control at different scan rates. When the scan speed is increased from 0.1 to 1.2 mV s⁻¹, the contribution rate of pseudocapacitance gradually increases from 41.2 to 88.3%, demonstrating that the NiCo₂Se₄⊂SPNC/CC anode has a good potassium ion storage capacity during the charge and discharge process.

In summary, the NiCo₂Se₄⊂SPNC/CC were successfully fabricated by coating the polymer layer of polyphosphazene on the surfaces of NiCo₂O₄/CC followed by selenization and carbonization. The polyphosphazene-derived SPNC layer effectively alleviates the volume expansion of NiCo₂Se₄ and is beneficial to increase more active sites for the storage of K⁺ ions. Based on the effective combination of the SPNC and NiCo₂Se₄, the binder-free NiCo₂Se₄⊂SPNC/CC anode shows a high reversible capacity of 880.9 mA h·g⁻¹ at a current density of 0.1 A g⁻¹ as well as good rate performance. Also, the NiCo₂Se₄⊂SPNC/CC anode could maintain a high reversible capacity of 268.1 mA h·g⁻¹ after 500 cycles, at a current density of 0.5 A g⁻¹. We have reason to believe that the anode materials of multiheteroatoms co-doped bimetallic selenides will show great potential application in PIB batteries.