Exceeding 50 000 Cycle Durability of Layered Hydroxide‐Based Hybrid Supercapacitor Through Scandium Doping‐Induced Superlong Activation Process

Layered double hydroxides (LDHs) are a class of promising cathode materials for supercapacitors. However, the bad cycling performance has always been the Achilles’ heel of LDHs‐based supercapacitors. In this contribution, a nonelectrochemical active element Scandium (Sc) is doped into NiCo‐LDHs to greatly improve the intrinsic cycling performance of active materials. The trimetallic NiCoSc‐LDHs exhibit an ultralong cycle lifespan with 40 000 charge–discharge cycles (exceeding 50 days) and a high specific capacity of 196 mAh g−1 (1695 F g−1). Sc doping greatly changes the degradation mode of NiCo‐LDHs from rapid decay in thousands of cycles to a two‐stage performance evolution, which consists of a superlong activation process of about 10 000 cycles and then extremely slow degradation. Moreover, Sc doping enhances the electrochemical activity of Ni3+, so as to not only avoid its Jahn–Teller distortion, but also perform as a structural stabilizer to alleviate the local strain of host layer. The assembled asymmetric supercapacitor delivers an ultralong cycling lifespan with 101% capacity retention even after 50 000 cycles. This work presents a new pathway to significantly improve the electrochemical performance, especially the cycling stability of LDH‐based electrodes for high‐performance supercapacitors.


Introduction
The issues of energy shortage, climate change, and environmental aggravation have drawn great attention of the academic, industrial, and society in the past few decades, which call for advanced and high-performance energy storage devices.3][4][5] Electrochemical double-layer capacitance (EDLC) materials, such as active carbon (AC), graphene, and carbon nanotubes, are the most widely-used materials in the supercapacitor industry, since they deliver excellent lifespan of hundreds of thousands of cycles. [6,7]However, the low theoretical specific capacitance less than 300 F g À1 determines the upper limit of EDLC-based devices.Among all the capacitive-type materials, layered double hydroxides (LDHs) are one of the most promising materials due to their highly flexible and controllable composition and structure, high theoretical specific capacitance (≈3000 F g À1 ), easy preparation, environmental friendliness, and so forth. [8,9]LDHs, with a general formula as [M 2þ 1Àx M 3þ x (OH) 2 ] xþ A nÀ x/n •yH 2 O] are a family of lamellar intercalation material comprising positively charged brucite-like host layers and interlayer anions.Due to a bunch of advantages, the poor cycling stability is its Achille's heel. [10]The unremitting redox reaction of transition metal ions, [11] Jahn-Teller distortion, [12] phase change, [13] weak interaction between LDHs, and current collector [14] could cause serious volume change, agglomeration, exfoliation, and performance degradation of LDHs after hundreds of cycles, preventing it from practical applications.
Many strategies have been developed to improve LDHs' cycling stability by researchers, such as modifying the surface of current collector, doping various elements, controlling the intercalated anions, [15] designing nanostructures, [16] and combining LDHs with other materials. [17,18]Among these, the first two strategies have been widely employed.Since LDHs deliver poor conductivity and weak interaction with current collectors like nickel foam or carbon cloth, the electrode's performance degrades rapidly once LDHs partially exfoliate or separate from the current collector.Li et al. developed a "nanoglue" strategy to use a nitrogen-doped carbon layer as the interface between NiCo-LDHs and carbon cloth, realizing a long lifespan with 88% capacitance retention after 10 000 cycles. [14]In our previous work, we demonstrated an interfacial method to access the intrinsic cycling stability of LDHs using polyaniline (PANI) as the interface, which could prevent LDHs from exfoliation. [19]In that case, the performance degradation was ascribed to the decay of active material itself rather than the interface exfoliation.The other strategy mainly adopted is doping various elements into the host layer of LDHs.Some monovalent metal elements, Zn, [19,20] Mg, [21,22] Al, [23,24] and Ga, [25] have been doped into NiCo-LDHs, which indeed enhance the cycling stability of LDHs.Although some of them have achieved a cycling life over 10 000 cycles, most of the presented could only exhibit a cycling life of about 10 4 cycles, which is much inferior to EDLC materials with 10 5 -10 6 cycling lifespans.Note that these doping monovalent elements are electrochemically inactive and do not take part in the energy storage process.Despite the fact that they can accommodate the detrimental Jahn-Teller distortion, alleviate the local strain, and perform as the structural stabilizer in the host layer, they do not influence the electronic structure and the electrochemical activity of NiCo-LDHs itself.To achieve superlong lifespan (>20 000 cycles) electrodes, it is necessary to find new doping elements that not only serve as a structural stabilizer in the host layer, but also regulate the electronic structure and electrochemical activity of the active substance.
In this work, a monovalent metal element, scandium (Sc), was doped into NiCo-LDHs to construct novel trimetallic NiCoSc-LDHs using hydrothermal methods on a carbon cloth (CC) coated by a PANI nanolayer; the obtained were named as composite electrode of LDHs and PANI (LDH@P).As a nonelectrochemical active element, Sc does not participate in the energy storage process and could act as a structural stabilizer of LDH's host layer.More importantly, compared with the Mg 2þ , Zn 2þ , Al 3þ , and Ga 2þ reported above, [19][20][21][22][23][24][25] Sc 3þ ion has fully empty 3d orbits, which are not only spared from Jahn-Teller distortion, but may also regulate the electronic structure of NiCo-LDHs' host layer and change the electrochemical activity of Ni or Co ions.Using various doping levels of Sc in Ni 2 Co 1 Sc x -LDHs and PANI composite electrodes (denoted as Ni 2 Co 1 Sc x -LDH@P, in which 2:1:x represents the feeding molar ratio of precursors, x = 0.25, 0.5, 0.75, for simplicity, Ni 2 Co 1 Sc x -LDH@P is reduced to NiCoSc x -LDH@P), a positive correlation between Sc stoichiometry and cycling performance is observed.Interestingly, the composite electrode undergoes a long capacity enhancement process at the beginning stage of cycles.The specific capacity of NiCoSc 0.75 -LDH@P increases by 27% in the first 10 000 cycles and then slowly decays to the initial value after 40 000 cycles.The more content of Sc there is, the longer the activation process is.To prove the specialty of Sc, a series of elements, including Zn, Mg, Cu, Ga, Al, V, are used to prepare doping NiCo-LDHs, while none of them have shown such a long activation process and cycling lifespan.The enhancement mechanism and its advantage are carefully discussed.Moreover, the asymmetric supercapacitor (ASC) assembled by NiCoSc 0.75 -LDH@P and AC exhibits an outstanding cycling stability with 101% capacity retention even after 50 000 cycles.

Results and Discussion
Figure 1 and S1-S4, Supporting Information, give the morphology and structure of the composite of NiCo-LDHs and PANI (NiCo-LDH@P) and NiCoSc x -LDH@P, characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).All the electrodes exhibit a similar nanostructure as nanorod arrays with a diameter of 20-30 nm and a length of about 1 μm.Notably, Sc doping improves the orderly growth of LDHs.NiCo-LDH@P shows weak orientation, while NiCoSc 0.75 -LDH@P exhibits a vertically growing orientation.The phenomenon is ascribed to the lower growing rate of LDHs with Sc doping.Energy-dispersive X-ray spectroscopy (EDS) mapping (Figure 1d) shows that Ni, Co, and Sc distribute homogeneously in NiCoSc 0.75 -LDH@P.As shown in Table 1, the contents of Sc among the three elements in NiCoSc 0.25 -LDH@P, NiCoSc 0.5 -LDH@P, and NiCoSc 0.75 -LDH@P are 3.7%, 6.9%, 10.3%, respectively, while the content ratio between Ni and Co is close to 2:1 in every electrode.The TEM in Figure 1e-f shows that the NiCoSc 0.75 -LDH exhibits a 1D nanorod morphology with a diameter of about 20 nm.The crystal phases of NiCoSc x -LDH and NiCo-LDHs are investigated by X-ray diffraction (XRD) as shown in Figure 2a.All the electrodes show similar peaks at 11.6°, 23.4°, 33.6°, 38.7°, 46.1°, 58.8°, and 60.6°, which could be indexed to the (0 0 3), (0 0 6), (0 1 2), (0 1 5), (0 1 8), (1 1 0), and (1 1 3) plane reflection of nickel hydroxide hydrate with representative hydrotalcite-like structure (Ni(OH) 2 •0.75H 2 O, JCPDS.38-0715), [23] indicating that the doping of Sc 3þ does not influence the crystal phase of NiCo-LDHs.The chemical states of different elements are studied by ray photoelectron spectroscopy (XPS) in Figure 2c-f.In the high-resolution Ni 2p spectra (Figure 2b), the peaks located at 874.4 and 856.2 eV are attributed to Ni 2p 1/2 and Ni 2p 3/2 signals of Ni 2þ , while the others at 880.0 and 861.9 eV are the shake-up satellite peaks.In the highresolution Co 2p spectra (Figure 2c), the peaks located at 797.3 and 781.2 eV are ascribed to Co 2p 1/2 and Co 2p 3/2 signals of Co 2þ , while the peaks at 803.1 and 786.2 eV are the shake-up satellite peaks.The content of Sc exerts little influence on Ni and Co, as their binding energy does not shift.In the high-resolution Sc 2p spectra (Figure 2d), the peaks at 407.1 and 402.6 eV are attributed to Sc 2p 1/2 and Sc 2p 3/2 of Sc 3þ .Due to a low Sc concentration in NiCoSc 0.25 -LDH@P, the signal of Sc 3þ is a little weak.With the increasing of Sc, the strength of the peaks gradually becomes stronger.
Ni 3þ is not thought to be transformed to Ni 4þ due to its high redox potential in NiCo-LDHs.However, the doping of Sc seems to lead to the emergence of the redox peak at high potential (i.e., P An,1 /P Ca,1 ), suggesting that Sc changes the electrochemical activity of Ni 3þ .With the scan rates increasing from 1 to 20 mV s À1 , the profiles of CV curves show little deformation with the peak's characteristics maintained (Figure 3a).It indicates that NiCoSc 0.75 -LDH@P exhibits good rate capability.Based on the galvanostatic charge-discharge (GCD) curves (Figure 3b), the specific capacity of NiCoSc 0.75 -LDH@P is 196 mA h g À1 at 1 A g À1 .Its specific capacitance is 1695 F g À1 , which is also provided as a reference.Since LDHs are now regarded as a battery-type material, specific capacity is used in the subsequent pages.When the current density increases to 20 A g À1 , the electrode exhibits a specific capacity of 128 mA h g À1 with 65% capacity retention.Moreover, the kinetics of redox reaction are investigated based on the logarithm relation between the peak current (i) and scan rate (v), as shown below. [26]¼ aυ b (4) Figure 3. a) CV curves of NiCoSc 0.75 -LDH@P at various scan rates from 1 to 20 mV s À1 ; b) GCD curves of NiCoSc 0.75 -LDH@P at various current densities from 1 to 20 A g À1 ; c) logarithm relation between the peak current of NiCoSc 0.75 -LDH@P and scan rate.Comparison of electrochemical performance between NiCo-LDH@P, NiCoSc 0.25 -LDH@P, NiCoSc 0.5 -LDH@P, and NiCoSc 0.75 -LDH@P; d) CV curves at 1 mV s À1 ; e) rate capability; f ) cycling performance at a current density of 10 A g À1 ; g) cycling performance of NiCoSc 0.75 -LDH@P at 10 and 20 A g À1 .The insets show the GCD curves at the times of beginning, end, and after the activation process.
It is known that the value of b indicates the charge storage mechanism of the reaction.When b equals to 1, the reaction shows a capacitive and surface-controlled electrochemical behavior; when b equals to 0.5, the reaction shows a battery-type and diffusion-controlled electrochemical behavior. [16]The kinetic characteristics of the redox reactions could be obtained by calculating slope of the fitting line of logarithmic peak current and scan rate.As shown in Figure 3c, P An,2 and P Ca,2 show a b value of 0.89 and 0.83 (b2 and b4 in Figure 3c), suggesting a surfacecontrolled kinetics, while P An,1 and P Ca,1 with a b value of 0.65 and 0.63 (b1 and b3 in Figure 3c) show a diffusion-controlled kinetics.An explanation could be made by combining the structure and redox process of LDHs.The host layer of LDHs consists of [M(OH) 6 ] octahedra (M = Ni, Co, Sc).When Ni(OH) 2 is oxidized to NiO(OH), the [Ni(OH) 6 ] lose an equivalent hydrogen, which combines with hydroxide ions to form water and diffuses to the electrolyte.The surface hydrogens exposed to the electrolyte tend to be taken in the transition process of Ni 2þ to Ni 3þ and Co 2þ to Co 3þ .Therefore, the redox reaction of P An,2 and P Ca,2 shows a surface-controlled kinetics.When NiO(OH) is further oxidized to NiO 2 , the other equivalent hydrogens located inside the interlayer are taken and diffuse through the interlayer spacing.Therefore, the redox reaction of P An,1 and P Ca,1 at high potential shows a diffusion-controlled kinetics.Since the diffusion-controlled kinetics of Ni 3þ /Ni 4þ is much slower than the surface-controlled kinetics of Ni 2þ /Ni 3þ , only part of Ni 3þ could be oxidized to Ni 4þ .To conclude, the emergent redox peaks in the high potential region (P An,1 /P Ca,1 ) should be ascribed to the transition of Ni 3þ /Ni 4þ .
To clearly verify the effects of Sc on the electrochemical performance of NiCo-LDHs, the comparison of CV curves of NiCo-LDHs and NiCoSc x -LDH@P is shown in Figure 3d.NiCo-LDHs exhibit one pair of redox peaks at 0.25/0.4V, while NiCoSc x -LDH@P exhibits a distinct cathodic peak near 0.45 V. Furthermore, the relative intensity of the cathodic peak becomes stronger with the increase of Sc content.The CV and GCD curves of NiCo-LDH@P and NiCoSc x -LDH@P at various scan rates are shown in Figure S8-S11, Supporting Information.In Figure 2e, the specific capacities of NiCo-LDH@P, NiCoSc 0.25 -LDH@P, NiCoSc 0.5 -LDH@P, and NiCoSc 0.75 -LDH@P are 250, 246, 225, and 196 mAh g À1 , respectively.The specific capacity of LDHs gradually decreases with the Sc content enhancing.As Sc is a nonelectrochemical active element, a high doping content could cause a decay of capacity.The capacity retention of NiCo-LDH@P, NiCoSc 0.25 -LDH@P, NiCoSc 0.5 -LDH@P, and NiCoSc 0.75 -LDH@P is 59%, 66%, 64%, and 65% at 20 A g À1 , respectively.Obviously, the rate capability of NiCoSc x @P is better than that of NiCo-LDHs.The electrochemical impedance spectroscopy (EIS) plots in Figure S12-S15, Supporting Information, show that the charge transfer resistances of NiCo-LDH@P, NiCoSc 0.25 -LDH@P, NiCoSc 0.5 -LDH@P, and NiCoSc 0.75 -LDH@P are 3.4, 1.3, 1.4, and 1.2, respectively, suggesting that Sc doping facilitates the charge transfer process.The cycling performances of NiCo-LDH@P and NiCoSc x @P are shown in Figure 3f-g).The doping content of Sc exhibits a positive relation with the cycling stability of NiCoSc x @P.When operating at 10 A g À1 , NiCoSc 0.75 -LDH@P delivers a 100% capacity retention after 40 000 cycles, as long as about 50 days, which is remarkably better than NiCoSc 0.5 -LDH@P (80%, 30 000 cycles), NiCoSc 0.25 -LDH@P (80%, 15 000 cycles), and NiCo-LDH@P (63%, 3000 cycles).In addition, when increasing the current density of cycling test to 20 A g À1 , NiCoSc 0.75 -LDH@P also delivers an ultralong lifespan with 105% capacity retention after 40 000 cycles.There is a phenomenon that all the NiCoSc x -LDH@P undergo a two-stage performance evolution during cycling, including an activation stage, that is, capacity enhancing process, and an extremely slow fading stage.The NiCoSc 0.25 -LDH@P, NiCoSc 0.5 -LDH@P, and NiCoSc 0.75 -LDH@P achieve their highest specific capacity at 2500, 5000, and 10 000 cycles, respectively.The more the content of Sc, the longer the activation process, and the slower the fading rate.
To explore the change of electrochemical behavior of NiCoSc 0.75 -LDH@P during the activation process, a consecutive 40-cycle CV test with a scan rate of 2 mV s À1 is employed.As shown in Figure 4a, the cathodic peak near 0.45 V is slightly reduced, while the broad redox peaks at 0.26/0.35V gradually increase during cycling.This evolution indicates that the contribution of Ni 3þ /Ni 4þ remains stable, while the contributions of Ni 2þ /Ni 3þ and Co 3þ /Co 4þ gradually enhance during the activation process.It may result from the infiltration of the electrolyte that slightly dissolves Sc(OH) 3 and exposes some new electrochemical active sites.The SEM images in Figure 4b,c show that NiCoSc 0.75 -LDH@P still maintains its original nanorod-array structure (Figure 4d) even after 40 000 cycles.No obvious agglomeration or deformation occurred, while the LDHs slightly exfoliate from the current collector (Figure S16, Supporting Information) due to the weak interfacial interaction that may account for the capacity degradation.Moreover, the XRD pattern of the NiCoSc 0.75 -LDH@P underwent 40 000 cycles and still shows strong intensity of the representative (0 0 3) and (0 1 2) plane reflection in Figure 4d, despite the interference of carbon cloth.In addition, XPS is conducted in NiCoSc 0.75 -LDH@P after cycling tests.As shown in Figure S35, Supporting Information, the chemical environment of Ni and Co hardly changed, while the signal of Sc became weaker compared with the pristine state.Since Sc is an amphoteric hydroxide, it would gradually dissolve in the alkaline electrolyte, leading to a relatively lower concentration of Sc at the surface of LDH.It should be noted that XPS is a technology that characterizes the surface property with a depth of 10 nm.Combining XPS with SEM and XRD could give us a full picture about the state of NiCoSc 0.75 -LDH@P after long cycles.The result indicates that NiCoSc 0.75 -LDHs keep a stable crystalline phase and nanostructure throughout the cycling process, while Sc 3þ at the surface partially dissolves.In addition, the charge transfer resistance of NiCo-LDH@P significantly increases from 3.4 to 10.0 Ω, while that of NiCoSc 0.75 -LDH@P slightly increases from 1.2 to 2.2 Ω (Figure 4e). [27]These results indicate that Sc doping makes NiCoSc 0.75 -LDH a low-strain cathode material.Density functional theory (DFT) methods are used to explain the effect of Sc on the electronic structure of NiCo-LDHs.Calculated density of states (DOS) in Figure 4f-g  To investigate whether the enhancement effects of Sc on the cycling performance of LDHs are special or not, a series of elements, including Zn, Mg, Cu, Ga, Al, V, are used to prepare doping NiCo-LDHs.In Yan's work, [28] DFTs were used to calculate the distortion angles of varieties of octahedral hexahydrated metal cations as model formula [M(OH 2 ) 6 ] nþ .Mg 2þ , Zn 2þ , Al 3þ , Ga 3þ , Sc 3þ , V 3þ exhibited a canonical structure with a distortion angle less than 1°, while Cu 2þ exhibited a slight distortion (the distortion angle equals to 5.3°) due to the Jahn-Teller effect. [28]The physical characterization, electrochemical performance, and DOS plots of these electrodes are exhibited in Figure S17-S34, Supporting Information.For the sake of simplicity, their electrochemical performance is concluded in the radar charts to better illustrate differences of their performance, as shown in Figure 4. Herein, the cycle lifespan is defined as the cycle number when the specific capacity degrades to 80% of the initial value.Based on the definition, NiCo-LDH@P, NiCoAl-LDH@P, NiCoZn-LDH@P, NiCoMg-LDH@P, NiCoGa-LDH@P, NiCoV-LDH@P, and NiCoCu-LDH@P exhibit cycle lifespan of 1800, 11 000, 10 000, 10 000, 8500, 6300, 1000, respectively.Al, Zn, Mg, and Ga show good improvement effects on cycling performance of NiCo-LDHs.The four elements are single-valence elements without electrochemical activity.The DOS plots show that these elements exert little influence on the electronic structure of LDHs (Figure S29-S32, Supporting Information).For NiCoV-LDH@P and NiCoCu-LDH@P, V 3þ and Cu 2þ could participate in the charge storage process by the Faradaic reaction between V 3þ /V 4þ and Cu þ /Cu 2þ .The DOS results also show that the peaks of V and Cu are close to the E f (Figure S33, S34, Supporting Information), indicating an activity of them.It is worth noting that NiCoCu-LDH@P exhibits a short lifespan of 1000 cycles, which is even inferior to NiCo-LDH@P.The serious Jahn-Teller distortion of Cu 2þ and redox reaction of Cu þ / Cu 2þ ruin the stability of the layered structure.Based on the electrochemical performance, the elements that are free from the Jahn-Teller distortion and without electrochemical activity could help to achieve a long lifespan of LDHs about 10 000 cycles.However, neither of these electrodes shows another pair of redox peaks at 0.45/0.55V belonging to Ni 3þ /Ni 4þ or a similar long activation process during cycling (Figure S18, S20, S22, S24, S26, S28, Supporting Information).It is believed that the specialty of Sc results from the effects of Sc on electronic structure of NiCo-LDHs, especially the activity of Ni 3þ .
To conclude, the improvement effect of Sc doping on the cycling performance could be ascribed to three aspects.First and the foremost, Sc doping changes the electronic structure of NiCo-LDHs and leads to a higher electrochemical activity of Ni 3þ and further oxidation to Ni 4þ .The host layer of LDHs comprises [Co(OH) 6 ] and [Ni(OH) 6 ] octahedra.Ni 3þ , suffered from Jahn-Teller distortion, could cause a great local stress and strain in the host layer during cycling, which further lead to the deformation and collapse of LDH crystal.Sc 3þ , with totally empty 3d orbits, could regulate the electronic structure of NiCo-LDHs and improve the electrochemical activity of Ni.By enhancing the activity of Ni 3þ , Sc doping helps to accommodate the detrimental Jahn-Teller distortion of Ni 3þ so as to stabilize the lattice. [12]Second, Sc, as a nonelectrochemical active element that does not participate in the energy storage process, could exert a pillar effect to stabilize the host layer of LDHs. [29]ased on the crystal field theory, Sc 3þ (t 2g 0 eg 0) would not suffer from Jahn-Teller distortion.In addition, the diameter of Sc 3þ (73 pm) is similar to Ni 3þ (69 pm) and Co 2þ (74 pm).The singlevalence ions' octahedron with highly stable and undistorted structure could act as a stable structural element to alleviate the local strain of host layer. [30]Last but not least, Sc(OH) 3 is an amphoteric hydroxide that could gradually dissolve in the strong alkaline solution.Based on Zhou's research, Sc(OH) 3 is more difficult to dissolve in alkaline than Al(OH) 3 , although both of them are amphoteric hydroxides. [31]The slow Sc 3þ leaching could slowly but constantly expose new electrochemical active sites and provide capacity, which is responsible for the superlong activation process to some extent.Although Al(OH) 3 is also an amphoteric hydroxide, NiCoAl-LDH@P does not exhibit such a long cycling lifespan because Al doping cannot influence the electronic structure of Ni.The easy dissolving of Al causes a much quicker surface degradation and structural collapse of NiCoAl-LDH.Thus, the change of the electronic structure of Ni is the key factor that may arouse the superlong activation process.It should be noted that an optimal doping ratio exists to balance the trade-off between capacitance and cycling stability, as Sc contributes little to the total capacitance but only plays a role as a structural stabilized unit.In addition to the effect of Sc, PANI nanolayer also delivers a synergistic effect with the Sc doping induced structural stability.The synergistic effect of PANI nanolayer has been carefully discussed in our previous work and is not a key point of this work. [19]In short, the interface modification strategy by introducing PANI nanolayer could prevent active materials exfoliation from the current collector.Table 2 shows the comprehensive electrochemical performance NiCoAl-LDH@CC 1137 F g À1 58% (20 A g À1 ) 97% (10 000 cycles) 1.5 [22]   NiCoMg-LDH 800 F g À1 61% (20 A g À1 ) 85% (30 000 cycles) - Mg-NiCo-LDH@NF 1931 F g À1 77% (20 A g À1 ) 95% (10 000 cycles) 2.3 [20]   NiCoZn-LDH@C 1928 F g À1 73% (50 A g À1 ) 96% (10 000 cycles) 1.8 [19]   NiCoZn-LDH@PANI 1749 F g À1 62% (20 A g À1 ) 89% (40 000 cycles) 1.6 [18]   NiMnCr-LDH@CS 569 C g À1 73% (20 A g À1 ) 76% (10 000 cycles) - This work 1695 F g À1 65% (20 A g À1 ) 100% (40 000 cycles) 1.8 Figure 5. Radar charts of electrochemical performance comparison between NiCo-LDH with a) NiCoAl-LDH@P; b) NiCoZn-LDH@P; c) NiCoMg-LDH@P; d) NiCoGa-LDH@P; e) NiCoV-LDH@P; and f ) NiCoCu-LDH@P.Cycle lifespan: the cycle number when specific capacity degrades to 80% of the initial value.Rate capability: capacity retention at 10 A g À1 .
of this work with some recent publications about LDH-based electrodes with long cycling lifespan.
To explore the application potential of the composite electrode, a simple ASC was assembled with NiCoSc 0.75 -LDH@P used as positive electrode and AC used as negative electrode.The electrochemical performance of the device is illustrated in Figure 5.The device shows a capacitive property with a pseudosymmetric charge-discharge curve.Based on the GCD curves at different current densities, its specific capacitance values are calculated as 106, 95, 82, 73, 60, 47, 38 F g À1 , corresponding to current densities of 0.5, 1, 2, 3, 5, 10 A g À1 .The energy density and power density of the supercapacitor device are also calculated and illustrated in the Ragone plot (Figure 5c).The device delivers a high energy density of 37.7 Wh kg À1 at a power density of 400 W kg À1 and an energy density of 13.5 Wh kg À1 at a high power density of 7915 W kg À1 .Such a performance is superior than many recent reported LDH-based hybrid supercapacitors, such as NiCoMg-LDH//AC (29 Wh kg À1 at 750 W kg À1 ), [32] NiCo-LDH@Ni(OH) 2 //AC (30 Wh kg À1 at 800 W kg À1 ), [33] NiCoAl-LDH//AC (27 Wh kg À1 at 400 W kg À1 ), [24] NiCo-LDH@ PANI//AC (26 Wh kg À1 at 728 W kg À1 ), [34] NiZnFe-LDH//AC (15 Wh kg À1 at 1077 W kg À1 ), [35] and NiCoAl-LDH@MC//AC (10 Wh kg À1 at 7347 W kg À1 ). [36]Furthermore, the supercapacitor gives a remarkable cycling durability with 101% capacitance retention after 50 000 cycles at a current density of 3 A g À1 .Similar to NiCoSc 0.75 -LDH@P, the device undergoes a long activation process with the specific capacitance increased by 18% in the first 10 000 cycles and then slowly decays.Moreover, the shapes of the last ten GCD curves are maintained well, as shown in the insets of Figure 5d, indicating the excellent cycling stability of the supercapacitor.Therefore, the results prove that the NiCoSc 0.75 -LDH@P//AC device harvests high power density with excellent cycling durability, corroborating that it has promising potential in practical application in an ASC (Figure 6).

Conclusion
In this contribution, we demonstrate a facile method to prepare NiCoSc-LDHs and PANI composite electrodes with a varying doping level of Sc (NiCoSc x -LDH@P, x = 0.25, 0.5, 0.75).The NiCoSc 0.75 -LDH@P exhibits a high specific capacity of 196 mAh g À1 (1695 F g À1 ), good rate capability (76% capacity retention at 10 A g À1 ), and an ultralong cycle lifespan with 40 000 charge-discharge cycles (exceeding 50 days).The electrode undergoes a two-stage performance evolution during cycling, which consists of a superlong activation process about 10 000 cycles, and then a slow degradation.Moreover, the introduction of Sc leads to the emergence of a new pair of redox peaks at high potential (0.4/0.45 V) in CV curve, which belongs to the transition between Ni 3þ /Ni 4þ .Sc could not only change the electrochemical activity of Ni 3þ so as to avoid the Jahn-Teller distortion of it, but also perform as a stable structural element to alleviate the local strain of host layer.In addition, the ASC also exhibits an excellent cycling stability with ≈100% capacity retention after 50 000 cycles and a high energy density of 37.7 Wh kg À1 .Furthermore, different trimetallic LDHs were also constructed by doping Mg, Zn, Al, Ga, V, and Cu into NiCo-LDHs.Although some of them could achieve a cycling lifespan of 10 000 cycles, none of them are found to possess an ultralong cycle life, a similar long activation process, and changes in CV curves like NiCoSc x -LDH@P.
This work provides a novel insight for the compositional design and preparation of LDH-based electrodes for supercapacitors with ultralong lifespan.Nevertheless, it should be noted that the cost of Sc is high, which would constrain its application.For one thing, the size scale and nanostructure of NiCoSc-LDH could be carefully designed to improve the utilization rate of Sc and decrease the optimal doping content of Sc.Besides, introducing other metal ions to construct a high-entropy layered hydroxides containing Sc may be a possible way to maintain its effect with a relatively low doping content.Furthermore, since there is a trade-off between capacity and cycling lifespan controlled by the content of Sc, one could also adjust the doping content of it for specific application scenario to decrease the cost.Researchers could also use NiCoSc-LDH as a cycling-stable framework to combine with other active materials with higher specific capacitance but poor cycling stability.In this way, both the cycling performance and capacity could be enhanced.Preparation of PANI-Coated CC: PANI nanolayer was electrodeposited on the surface of CC by a typical CV process in an aqueous solution of 0.1 M aniline and 1 M sulfuric acid with a voltage potential range from À0.2 to 0.8 V and a scan rate of 100 mV s À1 for six cycles.Platinum film was used as a counter electrode, with Hg/Hg 2 SO 4 used as reference electrode.The as-obtained film was rinsed with distilled water several times and dried in a vacuum at 60 °C for 4 h.The mass loading of PANI was about 0.1 mg cm À2 .
The autoclaves were heated at 110 °C for 10 h before being cooled to room temperature.Finally, the as-obtained electrodes were rinsed by distilled water several times and dried in a vacuum at 60 °C for 6 h.Based on the value of x (x = 0, 0.25, 0.5, 0.75), the as-obtained electrodes were named as NiCo-LDH@P, NiCoSc 0.25 -LDH@P, NiCoSc 0.5 -LDH@P, and NiCoSc 0.75 -LDH@P, respectively.The total loading mass (count in PANI) of them was about 2, 2.1, 1.9, and 1.8 mg cm À2 .
Preparation of NiCoAl-LDH@P, NiCoV-LDH@P, and NiCoCu-LDH@P: The precursor solutions were prepared by dissolving 3 mmol precursor salts and 24 mmol urea into 50 mL deionized water to form a homogeneous solution.The precursor salts included NiSO 4 , CoSO 4 , and X (X refers to Al 2 (SO 4 ) 3 , VCl 3 and CuSO 4 •5H 2 O), while the feeding ratios of them were all 2:1:1.Then, the precursor solutions were transferred to 100 mL stainless Teflon-lined autoclaves, with PANI-coated CC immersed.The autoclaves were heated at 110 °C for 10 h before being cooled to room temperature.Finally, the as-obtained electrodes were rinsed by distilled water several times and dried in a vacuum at 60 °C for 6 h.Based on the doping element, the as-obtained electrodes were named as NiCoAl-LDH@P, NiCoV-LDH@P, and NiCoCu-LDH@P, respectively.The total loading mass (count in PANI) of them was about 2.2, 2.2, 1.5 mg cm À2 .
Preparation of NiCoZn-LDH@P, NiCoMg-LDH@P, and NiCoGa-LDH@P: The precursor solutions were prepared by dissolving 3 mmol precursor salts and 24 mmol urea into 50 mL deionized water to form a homogeneous solution.The precursor salts included Ni(NO 3 ) 2 , Co(NO 3 ) 2 , and X (X refers to Zn(NO 3 ) 2 , Mg(NO 3 ) 2 , and Ga 2 (SO 4 ) 3 ), while the feeding ratios of them were 2:1:1.Then, the precursor solutions were transferred to 100 mL stainless Teflon-lined autoclaves, with PANI-coated CC immersed.The autoclaves were heated at 110 °C for 10 h before being cooled to room temperature.Finally, the as-obtained electrodes were rinsed by distilled water several times and dried in vacuum at 60 °C for 6 h.Based on the doping element, the as-obtained electrodes were named as NiCoZn-LDH@P, NiCoMg-LDH@P, and NiCoGa-LDH@P, respectively.The total loading mass (count in PANI) of them was about 2.5, 2.5, 2 mg cm À2 .
Materials' Characterization: SEM (Hitachi S-4800, Japan) and TEM (Hitachi-7700, Japan) with EDS were conducted to obtain the morphology and structure of samples.XRD (PAN analytical X'Pert PRO) was performed to obtain samples' crystallization structure under the following condition: 30 kV, 30 mA, and Cu Kα radiation (λ = 0.1542 nm).XPS was conducted on a spectrometer (AXIS SUPRA, Ktatos, British) with Al Kα excitation radiation (1486.6 eV).
Electrochemical Characterization: The electrochemical behaviors of all the electrodes were investigated in a three-electrode system with 3 mol L À1 KOH solution used as electrolyte, while Pt foil and Hg/HgO electrode were used as counter electrode and reference electrode.The CV, GCD, and EIS were operated on a CHI660E electrochemical workstation (Shanghai Chen Hua Instruments Co. Ltd., China).The long-term cycling test was performed on a Land Battery workstation (Wuhan Land Instrument Company, China).The specific capacity, C (mAh g À1 ), was calculated by the following equation.
where i m (A g À1 ) is the current density, and Δt (s) is the time of discharging process.The specific capacitance, C (F g À1 ) was calculated by the following equation.
where i m (A cm À2 ) is the current density, ∫ Vdt (V s) is the integral current area of the discharge curve, and V(V) is the potential with initial and final values of V i and V f , respectively.The ASC was assembled using AC as anode, NiCoSc 0.75 -LDH@P as cathode, and 3 mol L À1 KOH solution as electrolyte.The AC anode was prepared by coating the slurry of AC, Ketjen black, and
indicate that Sc doping makes the down-spin peak of Ni shift to Fermi level (E f ) and increases the charge density around E f , indicating that the redox activity of Ni is enhanced.Calculated crystal orbital Hamilton population (COHP) in Figure 4h shows that Sc doping enhances the stability of Ni─O bond, since the antibonding states of Ni─O below E f disappear after introducing Sc into NiCo-LDHs.The DFT results support the view that Sc doping improves the redox activity of Ni and stability of Ni─O bond.

Figure 4 .
Figure 4. a) The first 40 CV cycles of NiCoSc 0.75 -LDH@P at a scan rate of 2 mV s À1 ; b,c) SEM images of NiCoSc 0.75 -LDH@P after 40 000 cycles; d) XRD patterns of NiCoSc 0.75 -LDH before and after cycling test; e) Nyquist plots of NiCo-LDH@P and NiCoSc 0.75 -LDH@P before and after cycles; DOS calculated for f ) NiCo-LDH and g) NiCoSc x -LDHs systems; h) COHP plots of Ni─O and Co─O bond in NiCo-LDHs (left) and Ni─O and Sc─O bond in NiCoSc-LDHs (right).

Figure 6 .
Figure 6.The electrochemical performance of the NiCoSc 0.75 -LDH@P//AC ASC: a) CV curves at different scan rates; b) GCD curves at different current densities; c) Ragone plot; d) cycling performance at current density of 3 A g À1 .The insets show the GCD curves at the times of beginning, end, and after the activation process.

Table 2 .
Comparison of capacitive performances between recently reported metallic compound-based materials and this work.