Lithium Storage in Carbon Cloth–Supported KNb3O8 Nanorods Toward a High‐Performance Lithium‐Ion Capacitor

Exploration of novel anodes with a high capacity and fast charge rate is crucial for developing high‐energy‐density lithium‐ion capacitors. Herein, high‐rate Li+ insertion into KNb3O8 nanorods grown on conductive carbon cloth (CC) by a facile electrodeposition technique is reported. The hierarchically porous network and the enhanced conductivity enable the CC‐KNb3O8 electrode to deliver a high discharge capacity of 271 mA h g−1 at 0.01 A g−1 with a low yet safe voltage (1.2 V versus Li/Li+) while exhibiting outstanding cycling stability (225 mA h g−1 after 100 cycles) and superior rate capability (159 mA h g−1 at 0.5 A g−1). Electrochemical results and X‐ray photoelectron spectroscopy (XPS) analysis show that accompanying mutielectron transfers involving the Nb5+/Nb4+ and Nb4+/Nb3+ redox reactions, KNb3O8 remains in the stable orthorhombic phase without notable volume expansion during the whole charging and discharging process. A lithium‐ion capacitor built with a CC‐KNb3O8 anode and active carbon cathode delivers a maximum energy density of 69 W h kg−1 at a power output of 346 W kg−1 and retains 88% capacity after 1000 cycles at 2.0 A g−1. This work opens up a new avenue for designing high‐capacity niobium‐based electrodes toward a high‐performance charge storage device.

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/sstr.202100029. DOI: 10.1002/sstr.202100029 Exploration of novel anodes with a high capacity and fast charge rate is crucial for developing high-energy-density lithium-ion capacitors. Herein, high-rate Li þ insertion into KNb 3 O 8 nanorods grown on conductive carbon cloth (CC) by a facile electrodeposition technique is reported. The hierarchically porous network and the enhanced conductivity enable the CC-KNb 3 O 8 electrode to deliver a high discharge capacity of 271 mA h g À1 at 0.01 A g À1 with a low yet safe voltage (1.2 V versus Li/Li þ ) while exhibiting outstanding cycling stability (225 mA h g À1 after 100 cycles) and superior rate capability (159 mA h g À1 at 0.5 A g À1 ).
Electrochemical results and X-ray photoelectron spectroscopy (XPS) analysis show that accompanying mutielectron transfers involving the Nb 5þ /Nb 4þ and Nb 4þ /Nb 3þ redox reactions, KNb 3 O 8 remains in the stable orthorhombic phase without notable volume expansion during the whole charging and discharging process. A lithium-ion capacitor built with a CC-KNb 3 O 8 anode and active carbon cathode delivers a maximum energy density of 69 W h kg À1 at a power output of 346 W kg À1 and retains 88% capacity after 1000 cycles at 2.0 A g À1 . This work opens up a new avenue for designing high-capacity niobium-based electrodes toward a high-performance charge storage device.
highly pseudocapacitive LiNbO 3 nanoparticles and highly conductive 3D graphene network, the GA-LiNbO 3 anode exhibits enhanced kinetics that match well with the fast charge-discharge property of the boron carbonitride nanotube cathode, leading to a high-energy and high-power LIC device. In contrast, orthorhombic NaNbO 3 also holds great promise as an anode of LICs as it could offer possible channels for Li þ insertion through the (101) and (141) crystal planes. [23] Among the various niobium-based compounds, KNb 3 O 8 represents a typical electrode that features a layered structure containing NbO 6 octahedral units and a K þ interlayer. Most recently, we found that KNb 3 O 8 could exhibit a good pseudocapacitive behavior and offer a reversible capacity of 470 F g À1 in aqueous alkaline media. [24] However, electrochemical and X-ray photoelectric spectroscopy (XPS) analysis reveals that %5.8% niobium was involved in the redox reaction of Nb 5þ /Nb 4þ , which contributes to a low pseudocapacitance.
Considering two-electron transfer (Nb 5þ /Nb 3þ ) for the KNb 3 O 8 electrode in the voltage range of 1.0À3.0 V, the theoretical capacity of KNb 3 O 8 reaches as high as 360 mA h g À1 . Inspired by the aforementioned merits, this work reports on lithium storage in KNb 3 O 8 nanorods that were electrochemically deposited on conductive carbon cloth (CC). CC-KNb 3 O 8 exhibits a discharge capacity of 271 mA h g À1 at 0.01 A g À1 and retains 159 mA h g À1 at 0.5 A g À1 . Electrochemical analysis reveals that Li þ storage in KNb 3 O 8 nanorods is achieved by a capacitivedominated process due to the large diffusion efficiency of Li þ in the hierarchically porous network composed of rod-like KNb 3 O 8 . XPS characterization further demonstrates that with the high-rate Li þ insertion, 98.4% of Nb 5þ was reduced into Nb 4þ in the potential range of 3.0À1.64 V and 4.3% of Nb 4þ was further reduced into Nb 3þ in 1.64À1.0 V. A LIC fabricated with CC-KNb 3 O 8 as the battery-type anode and activated carbon (AC) as the capacitive cathode delivers a maximal energy density of 69 W h kg À1 at a power density of 346 W kg À1 while preserving 88% capacity and nearly 100% Coulombic efficiency after cycling at 2.0 A g À1 for 1000 cycles.

Results and Discussion
Commercial CC has been widely used as a current collector because of its high conductivity and superior mechanical flexibility. The CC was first treated with mixed acids to enhance its surface wettability and ensure an uniform distribution of KNb 3 O 8 . Figure S1a, Supporting Information, compares an optical photograph of the CC before and after KNb 3 O 8 deposition. Clearly, the color change from black to white indicates the deposition of KNb 3 O 8 . A low-magnification scanning electron microscopy (SEM) image confirms the homogeneous deposition of KNb 3 O 8 on the CC substrate ( Figure S1b, Supporting Information). Note that by extending the number of cyclic voltammetry (CV) cycles, CC-KNb 3 O 8 with mass loading of 1.5, 2.8, and 4.0 mg cm À2 could be easily obtained, which was examined with SEM images as shown in Figure 1. The KNb 3 O 8 on the CC shows a uniform rod-like shape. A low mass loading of 1.5 mg cm À2 leaves behind a small portion of carbon fiber uncovered due to insufficient KNb 3 O 8 (Figure 1a,d). At a higher mass loading of 2.8 mg cm À2 (Figure 1b (Figure 2a). Nevertheless, the conductivity of CC-KNb 3 O 8 is several orders of magnitude higher than that of the previous Nb 2 O 5 [25] and other Nb-based electrodes. [20,26] The typical wide-angle X-ray powder diffraction (XRD) pattern of CC-KNb 3 O 8 -2.8 is shown in Figure 2b. The distinct diffraction peaks of CC-KNb 3 O 8 -2.8 are well consistent with the orthorhombic phase of KNb 3 O 8 (JCPDS 75-2182), [27] showing successful growth of KNb 3 O 8 on the CC substrate. The CC-KNb 3 O 8 electrodes with lower and higher mass loading of KNb 3 O 8 exhibit nearly identical diffraction patterns ( Figure S1c, Supporting Information). In addition, the Raman spectrum of CC-KNb 3 O 8 -2.8, as shown in Figure S1d, Supporting Information, displays prominent peaks characteristic of KNb 3 O 8 . [24,28,29] To detect the chemical composition and element valence states, XPS of CC-KNb 3 O 8 -2.8 was performed.
The O, C, Nb, and K elements appear in the full XPS spectrum (Figure 2c). In the Nb 3d spectrum, the Nb 3d peaks at 209.9 and 207.2 eV correspond to Nb 3d 3/2 and Nb 3d 5/2 , respectively, showing that Nb (V) is present in the KNb 3 O 8 nanorods. [30,31]  The self-supporting CC-KNb 3 O 8 electrodes were directly used as anodes for constructing a CR2032 coin-type cell, and their electrochemical properties for Li þ storage were systematically investigated. Figure S2, Supporting Information, shows the galvanostatic charge-discharge (GCD) curves in the voltage range of 1.0-3.0 V (vs. Li þ /Li) at 0.02 A g À1 . Compared with the CC-KNb 3 O 8 -1.5 and CC-KNb 3 O 8 -4.0 electrodes, the CC-KNb 3 O 8 -2.8 electrode delivers the highest reversible capacity, a relatively low overpotential, and a superior rate capability, indicating that the enhanced conductivity as well as the uniform arrangement of KNb 3 O 8 nanorods is more favorable for fast Li þ insertion/extraction. The electrochemical properties of Li þ storage in the CC-KNb 3 O 8 -2.8 electrode were systematically investigated. Figure 3a shows the first three GCD curves of the CC-KNb 3 O 8 -2.8 electrode at a low current density of 0.01 A g À1 . A little irreversible capacity in the first cycle is likely associated with the incomplete solid electrolyte interphase formation as well as the irreversible decomposition of the electrolyte. [32] From the second cycle, a reversible discharge capacity of 271 mA h g À1 could be achieved. Although this capacity is slightly lower than those of previous K 2 Nb 8 O 21 -nanotubes (NT) (335 mA h g À1 ) and K 2 Nb 8 O 21 -microtubes (MT) (285 mA h g À1 ) electrodes, [19] it is much higher than those of other niobium-based electrodes, including FeNbO 4 (125.5 mA h g À1 ), [33] Nb 2 O 5 (220 mA h g À1 ), [10] KNb 5 O 13 (130 mA h g À1 ), [34] K 6 Nb 10.8 O 30 (160 mA h g À1 ), [34] and VNb 9 O 25 (220 mA h g À1 ) [35] (Figure 3b). Meanwhile, according to the intersection point of the charge and discharge curve at 0.01 A g À1 , the discharge plateau of CC-KNb 3 O 8 -2.8 was determined to be 1.2 V, a safe working potential that is higher than that of a graphite electrode but much lower than those of other Nb-based oxides ( Figure 3b). The   Figure 3c). Moreover, a working voltage of 1.2 V with relatively low polarization is still maintained at a high current density of 0.5 A g À1 . Figure 3d shows the discharge capacity of CC-KNb 3 O 8 -2.8 at various current densities. The electrode not only exhibits a superior rate versus cycle performance, but also keeps nearly 100% Columbic efficiency and recovers most of its discharging capacity as the current rate returns to 0.02 A g À1 . Moreover, note that the CC-KNb 3 O 8 -2.8 electrode is capable of maintaing a continuously high capacity of 225 mA h g À1 during the 100 cycles at a low current density of 0.01 A g À1 (Figure 3e), and retains 82% capacity (132 mA h g À1 ) after 2500 cycles at a high current density of 0.5 A g À1 (Figure 3f ). Such results are much superior to previous KNb 3 O 8 nanowire electrodes, which only show a moderate cycling stability with 67.3% capacity retention after 150 cycles at 0.05 A g À1 and 64.1% capacity retention after 750 cycles at 0.5 A g À1 . [21] In addition, the Raman spectra of the CC-KNb 3 O 8 -2.8 before and after 2500 cycles were collected and compared in Figure S4, Supporting Information. The nearly identical bands suggest a stable crystal structure of KNb 3 O 8 even after repeated insersion/extraction of Li þ , in good agreement with its superior long-term cycling performance. The Li þ storage in CC-KNb 3 O 8 -2.8 was further investigated by CV in the scan rate range of 0.1-5.0 mV s À1 (Figure 4a). The peak currents (i) and scan rates (v) obey the following formula. [36,37] i ¼ av b where a and b are adjustable parameters, with b ¼ 1.0 and 0.5 meaning the capacitive-controlled and diffusion-controlled electrochemical process, respectively. From the linear fitting of log (i) and log (v) in the scan rate range of 0.1-5.0 mV s À1 . the b value of anodic the peak and cathodic peaks is determined to be 0.83 and 0.82, respectively (Figure 4b). This result indicates that insertion/ extraction of Li þ into/from the CC-KNb 3 O 8 -2.8 electrode is achieved by a capacitive-dominated electrochemical process. In contrast, we applied a method described by Dunn [38,39] to quantitatively distinguish the capacity contribution from the capacitive and diffusion-controlled process.
In this formula, the total current is considered as the current contribution from the capacitive and diffusion-controlled process. Using this current separation method, the capacitive capacity current can be separated from the total current at different scan rates. Figure 4c shows the CV curve of the CC-KNb 3 O 8 -2.8 electrode at 5.0 mV s À1 , with the shaded area corresponding to the current contribution from the capacitivecontrolled electrochemical process. It is found that %82% of the total capacity of CC-KNb 3 O 8 -2.8 is from capacitive contribution. In addition, the capacitive contributions at scan rates from 0.1 to 5.0 mV s À1 are shown in Figure 4d. This contribution is 27% at 0.1 mV s À1 and it continuously increases to 67% and 82% at scan rates of 1.0 and 5.0 mV s À1 , respectively. These results demonstrate that fast Li þ storage in CC-KNb 3 O 8 -2.8 is dominated by a capacitive process. Furthermore, by fitting the electrochemical impedance spectra (EIS) curve, the diffusion coefficient of Li þ (D Li ) in pristine CC-KNb 3 O 8 -2.8 is caclulated to be 1.45 Â 10 À14 cm 2 s À1 (Figure 4e), and this value is slighltly decreased to 1.09 Â 10 À14 cm 2 s À1 after discharging the electrode to 1.0 V (Figure 4f ). The comparison suggests that there are sufficient tunnels in the KNb 3 O 8 nanorods that are favorable for rapid Li þ diffusion. It is noted that the diffusion coefficients of Li þ in KNb 3 O 8 nanorods are much higher than in previous niobium-based anodes, including Ti 2 Nb 10 O 29   [20] thus suggesting that the rod-like KNb 3 O 8 with short ion diffusion lengths substantially benefits Li þ diffusion.
To further understand the charge storage mechanism and probe the variation of the valence state of Nb, the CC-KNb 3 O 8 -2.8 at various charging and discharging stages was characterized by XPS. Figure 5a shows the CV profiles of the first three cycles at a scan rate of 0.1 mV s À1 . The almost fully overlapped CV curves reveal a superior electrochemical reversibility. In the discharge curves from 3.0 to 1.0 V, the first peak appears at 1.64 V and the second peak occurs at 1.00 V. The Nd 3 d XPS spectra at these two stages are shown in Figure 5c,d, respectively. Clearly, when discharging the electrode to 1.64 V, the original peaks of pristine KNb 3 O 8 at 209.9 and 207.2 eV (Figure 5b) shift distinctly to the lower binding energy of 208.5 and 205.8 eV, respectively (Figure 5c), suggesting an efficient conversion of Nb 5þ to Nb 4þ . [19] XPS quantitative analysis reveals that %98.4% of Nb 5þ was reduced in this step (Table S1, Supporting Information). Upon further discharging to 1.00 V, a weak peak at 203.7 eV is ascribed to the formation of Nb 3þ (Figure 5d), [40] which only accounts for 4.3% of the total Nb 4þ according to XPS analysis (Table S1, Supporting Information). In the reverse charging process from 1.0 to 1.11 V, the peak at 203.8 eV attributed to Nd 3þ nearly disappears (Figure 5e), suggesting that Nd 3þ is fully oxidized into Nb 4þ in this step. Further charging the electrode to 1.73 V, the Nd 4þ is fully oxidized into Nd 5þ and the Nd 3d XPS spectrum returns to its pristine state (Figure 5f ). These findings suggest that the reversible Li þ insertion and extraction reactions might proceed following Equation (1) and (2). Figure 6 plots the valence variation of Nb and the corresponding XRD patterns of the electrode at different discharging/ charging stages. As shown in Figure 6a, insertion of 3 mol of Li þ into 1 mol of KNb 3 O 8 induces the simultaneous reduction of 98.4% Nb 5þ to Nb 4þ upon discharging the electrode from 3.00 to 1.64 V. Further discharging from 1.64 to 1.00 V only leads to reduction of %4.3% of Nb 4þ to Nb 3þ on the basis of XPS results. The ex situ XRD patterns of the CC-KNb 3 O 8 -2.8 electrode are provided in Figure 6b. Interestingly, insertion/extraction of Li þ into/from KNb 3 O 8 does not lead to the phase variation of the KNb 3 O 8 , in good accordance with the Raman spectrum as shown in Figure S4, Supporting Information. Moreover, the (020), (120), and (240) diffraction peaks remain intact during the discharging and charging stages, showing negligible volume expansion with the insertion of Li þ into the KNb 3 O 8 . The XRD patterns of the CC-KNb 3 O 8 -2.8 electrode in the second discharging and charging process is provided in Figure S5  Correlation between the peak currents and the scan rates. c) CV response at a scan rate of 5 mV s À1 , with the shaded area corresponding to the capacitive contribution. d) Capacity contribution from the capacitive and diffusion-controlled process at diverse scan rates. e,f ) Nyquist plot and correlation between Z 0 and ω À1/2 of pristine sample and discharge to 1.00 V, respectively.
www.advancedsciencenews.com www.small-structures.com To configure a LIC, commercial active carbon (AC) was applied as the cathode. SEM images show that the AC particles have irregular shapes and relatively rough surfaces ( Figure S6, Supporting Information). The N 2 adsorption/desorption isotherm reveals the specific surface area of AC reached 1947 m 2 g À1 , and the pore size distribution is in the range of 1.5À3.0 nm ( Figure S7, Supporting Information). The large specific surface area and suitable pore size benefit the adsorption/desorption of PF 6 À ions during the rapid chargedischarge process. [41] The charge-discharge behavior of the AC in a half-cell was also investigated. As shown in Figure S8a, Supporting Information, the AC electrode is dominated by capacitive behavior and delivers a discharge capacity of 62 mA h g À1 at 0.05 A g À1 . Moreover, it exhibits a high rate capability with 82%  capacity retention when the current density increases to 0.5 A g À1 ( Figure S8b, Supporting Information). About 83% capacity is maintained after 1000 continuous GCD cycles ( Figure S8c, Supporting Information), showing satisfactory cycling performance as a cathode for LICs. Using AC as a capacitive cathode and CC-KNb 3 O 8 -2.8 as the battery-type anode, three LICs with different mass ratios of AC to CC-KNb 3 O 8 -2.8 were assembled to gain optimized performance. Figure 7a shows the CV profiles of the three LICs. Both CC-KNb 3 O 8 -2.8//AC-3 and CC-KNb 3 O 8 -2.8//AC-3.5 show slightly distorted profiles in the voltage range 0À3.5 V, whereas an obvious polarization is found for CC-KNb 3 O 8 -2.8//AC-2, suggesting a capacity mismatch between the cathode and anode. The CV curves of CC-KNb 3 O 8 -2.8//AC-3 at various scan rates are shown in Figure 7b. All of them display slightly distorted rectangular profiles and show the current densities increasing with the scan rates, indicating a good electrochemical behavior benefiting from the combination of capacitive AC with the battery-type CC-KNb 3 O 8 -2.8. The GCD at various current densities shows quasi-triangular shapes with symmetric profiles over a wide voltage window (Figure 7c), indicating a good capacitive behavior with a superior electrode reversibility. Figure 7d plots the dependence of the device capacity on the current densities. Among the three devices tested, CC-KNb 3 O 8 -2.8//AC-3 displays a gravimetric capacitance of 41 F g À1 at 0.15 A g À1 and retains 26 F g À1 at 2.0 A g À1 . This performance is far superior to 31 and 35 F g À1 of CC-KNb 3 O 8 -2.8//AC-2 and CC-KNb 3 O 8 -2.8//AC-3.5, respectively. Meanwhile, the CC-KNb 3 O 8 -2.8//AC-3 cell also displays a satisfying cycling stability. About 88% capacity retention and almost 100% Coulombic efficiency are achieved after charging and discharging at 2.0 A g À1 for 1000 cycles (Figure 7e).
The energy densities of the device are plotted in Figure 7f. Specifically, CC-KNb 3 O 8 -2.8//AC-3 could deliver an energy density of 69-36 Wh kg À1 at power densities varying from 346 to 3812 W kg À1 . The maximum energy density of 69 W h kg À1 is much higher than those of previous LICs devices, including CuBi 2 O 4 //AC (24 W h kg À1 at 300 W kg À1 ), [42] T-Nb 2 O 5 @nitrogen-doped carbon tube (NC)//AC (49 W h kg À1 at 8750 W kg À1 ), [43] V 2 O 5 -carbon nanotubes (V 2 O 5 -CNT)//AC (40 W h kg À1 at 200 W kg À1 ), [44] ginger straw carbon//   [45] and Nb 2 O 5 core-cell//AC (63 W h kg À1 at 70 W kg À1 ). [46] Nevertheless, it is still lower than those of previously reported Li x MnO 2 //AC (88 W h kg À1 , 151 W kg À1 ), [47] Nb 2 O 5 film//AC (95 W h kg À1 , 191 W kg À1 ), [10] and T-Nb 2 O 5 @ TC//AC (86 Wh kg À1 , 6.0 kW kg À1 ). [17] To demonstrate the highenergy density and potential application, a coin-type cell was fabricated with 3.54 mg of AC and the CC-KNb 3 O 8 -2.8 electrode containing 1.21 mg KNb 3 O 8 . After charging at 0.2 A g À1 for 15 min, the cell can serve as an energy supply to power a fan to run for 6.5 min or drive a toy car (72 g) to move forward for 1.4 m (Figure 7g). In addition, this cell can also light up 12 LEDs for over 25 min (Figure 7h). These demonstration experiments further confirm the high energy density of the LIC and its potential application as a high-performance electrochemical energy storage device.

Conclusion
In summary,

Experimental Section
Material Preparation: Growth of KNb 3 O 8 nanorods on CC was conducted following a method reported recently except that \commercially available CC was applied as the flexible substrate. [24] Prior to electrochemical deposition, the CC was refluxed with mixed acids to gain hydrophilic surface nature. [48,49] Deposition of the KNb 3 O 8 precursor on the CC was conducted on a CHI660D electrochemical working station. The mass loading of KNb 3 O 8 on CC could be facilely controlled by extending the CV number at 0.5 mV s À1 . For example, repeating the CV for 2, 3, and 4 cycles yielded the final CC-KNb 3 O 8 with areal mass loading of 1.5, 2.8, and 4.0 mg cm À2 , respectively. The final products after thermal annealing were labeled as CC-KNb 3 O 8 -x, where x means the areal loading of KNb 3 O 8 , which was determined by measuring the weight of the CC before and after KNb 3 O 8 deposition.
Material Characterizations: The microscopic morphology and microstructure of the samples were characterized by field-emission scanning electron microscopy (FESEM, SU8020) and TEM (JEM-2010). The crystal phase of a sample was examined by DX-2700 XRD with Cu Kα radiation. Raman spectra were collected on a Renishaw Raman spectrometer with a 532 nm laser as the excitation source. The surface chemical compositions and valence states of electrodes were probed by XPS (Kratos Analytical). All the XPS peaks were calibrated with C 1s at a binding energy of 284.6 eV. The Brunauer-Emmett-Teller (BET) surface areas were obtained by an ASAP 2460 analyzer from standard N 2 adsorption-desorption isotherms. Sample conductivities were measured at room temperature using a standard four-probe technique on an RTS-9 measurement system.
Cell Fabrication: The electrochemical performance of CC-KNb 3 O 8 was tested as a working electrode in a CR2032-type coin cell with lithium foil as the counter electrode and glass microfiber filters (GF/A, Whatman) as the separator. LiPF 6 (1.0 M) in a mixture of dimethyl carbonate and ethylene carbonate (EC/DMC, 1:1 vol%) was used as the electrolyte. In assembly of an LIC device, AC coated on Al foil served as the cathode and selfsupporting CC-KNb 3 O 8 was used as the anode. The cathode was prepared by first mixing AC powder, acetylene black, and polyvinylidene fluoride (PVDF) (8:1:1 in mass ratio) with N-methylpyrrolidone. After being fully ground, the slurry was homogenously cast on an Al foil and then vacuum dried at 80 C for 8 h. Finally, LICs with different mass ratios of cathode to anode were assembled by separating the two electrodes with glass microfiber filters and filling several drops of the 1.0 M LiPF 6 electrolyte. The LIC devices were denoted as CC-KNb 3 O 8 //AC-x, with x representing the mass ratios of AC to CC-KNb 3 O 8.
Electrochemical Measurements: The electrochemical performance of the half-cells and LICs were analyzed by GCD, CV, and EIS. For the half-cells, GCD cycling was tested on a Land-CT2001A multi-channel battery test system (Wuhan Jinnuo, China). The cutoff voltage was set at 1.0À3.0 V (vs Li/Li þ ) and the current density was varied from 0.01 to 0.5 A g À1 . The CV and EIS were conducted on an Autolab electrochemical workstation (PGSTAT100N). The performance of the LIC device was evaluated on an Autolab electrochemical workstation and the voltage window was in the range of 0-3.5 V.
The diffusion coefficient of Li þ (D Li ) could be gained using the following formula. [19,20] Z 0 ¼ R s þ R f þ R ct þ σω À1=2 (5) D Li ¼ ðRTÞ 2 2ðAn 2 F 2 C Li σÞ 2 (6) where D Li is the Li þ diffusion coefficient and R, T, A, n, F, and C Li are the gas constant, absolute temperature, contact surface area between KNb 3 O 8 and the electrolyte, charge-transfer number, Faraday constant, and molar concentration of Li þ ions in KNb 3 O 8 , respectively. The specific capacitance (F g À1 ), energy density (W h kg À1 ) and corresponding power density (W kg À1 ) of the LIC device was calculated according to the following equations. [50][51][52] where I m is the discharge current density (A g À1 ), ∫Vdt is the integral current area of the discharge curves, V is the potential (V) varying from an initial (V i ) to a final value (V f ) and t (s) is the discharge time.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.