Armoring Black Phosphorus Anode with Stable Metal–Organic-Framework Layer for Hybrid K-Ion Capacitors

Highlights The ultrathin metal–organic-framework (MOF) interphase layer with high mechanical/chemical stability was in situ grown on black phosphorus nanosheets (BPNSs). MOF interphase layers as an ordered porous and robust protective layer can facilitate K ion diffusion and accommodate the volume change of the electrode. Benefiting from the improved reaction kinetics and enhanced electrode stability, the BPNS@MOF anode for potassium-ion capacitors exhibits outstanding cycle performance. Electronic supplementary material The online version contains supplementary material available at (10.1007/s40820-020-00570-7) contains supplementary material, which is available to authorized users.


Supplementary Figures and Tables
, and the result shows that (112) plane has the highest Zn atom density among different planes of (011), (002), (112), (013) and (222), which are most commonly seen in the XRD pattern. The results demonstrate that the higher interaction between Zn and P caused by relatively high Zn density drives the orientation of the dominant plane from (011) to (112).  S10 (a-d) Comparison of electrode thickness and expansion ratio between the BPNS electrode (a,b) and the BPNS@MOF electrode (c,d) before cycling and after cycles in KIB. Insets in Figure S10 (a1-d2) are the top view of electrode with different magnifications.

Fig. S11
Electrochemical impedance spectra of the BPNS electrode and the BPNS@MOF electrodes with different MOF contents for KIBs before cycling. Inset shows the corresponding equivalent circuit.
Note: As shown in Fig. S11, the BPNS electrode and the BPNS@MOF electrodes with different MOF contents consists of Rs (represents the internal resistance of the test battery), Rf and CPE1 (assigned to the resistance and constant phase element of SEI film), Rct and CPE2 (associate with the charge-transfer resistance and constant phase element of the electrode/electrolyte interface), and ZW (ascribe to Warburg impedance corresponding to the K + diffusion process). From the fitted values shown in Table S3, it can be clearly seen that the values of Rf of the BPNS@MOF electrodes (with different MOF contents) are much lower than that of the BPNS electrode, indicating the MOF interphase layer can reduce the resistance at the interface between the SEI film and the electrolyte. Moreover, the BPNS@MOF electrode offers the lowest Rct value of 750 Ω compared to the others that accounts for the better charge transfer capability and electrochemical kinetics.        XPS measurements were performed to analyze the composition of SEI layers on the surface of BPNS anodes.
First of all, we analyzed XPS spectrum of C, O, S and F in the 1 st cycle ( Fig. S19) with cut off voltage of 0.01V. The C 1s spectrum can be split into four peaks with binding energies of 284.8 (C-C), 285.8 (C-O), 287.8 (C=O) and 289.5 eV (RO-COOK) [S19, S20]. C-C bonded carbon is ascribed to the conductive additive, and the C−O, C=O bonded carbon and RO-COOK species in the surface layer should originate from the decomposition and reduction of EC and DEC solvent. In O 1s spectra, the high intensity peak at 530.8 eV is attributed to C=O bonds while the peak at 532.3 eV is ascribed to S=O and the peak at 533eV should originate from RO-COOR [S21]. The S 2p spectra exhibit two peaks around 168.5 (K2SO4) and 170 (KHSO4), further demonstrate salt reduction in the KFSI-based electrolyte [S22]. It was discovered that the F 1s spectrum shows two different fluorine species, which can be assigned to C-F, O-F and S-F species at 687.7 eV, due to incomplete decomposition of the salts, and KF species at a lower binding energy of 682.9 eV, which may be from the reduction of S-F bonds in the FSI - [S23, S24].
In order to evaluate the composition of SEI layers, we calculated the integral areas of individual peaks and the normalized content of each component is summarized in Fig.  S20. It can be obviously seen that K2SO4 accounts for the largest percentage among the inorganic components of SEI.
When the cell charge to 2.0 V (Fig. S19), no new components are observed. Even after the BPNS electrode was cycled for 10 cycles (Fig. S21), the XPS spectra show almost negligible changes at the cut off voltage of both 0.01 V and 2.0 V compared with the 1 st cycle in Fig. S19. This means that a SEI film with a stable composition is formed on the surface of the BPNS electrode. What's more, the XPS spectra of the BPNS electrode cycled in KIC (Fig. S22) is similar with that cycled in KIB, which means there is no difference in the component of SEI formed in KIB and KIC. Fig. S23 (a) dQ/dV plot of the 3rd cycle during discharge process and charge process for ZIF-8 at 50 mAh g -1 . (b) Rate performance of ZIF-8 at different current densities Note: As shown in Fig. S23a, the dQ/dV plot of the BPNS@MOF electrode doesn't display visible peak, indicating that ZIF-8 is almost electrochemical inert to potassium. Moreover, as shown in Fig. S23b, the BPNS@MOF electrode delivers a capacity as low as 14 mAh g -1 at a low current density of 50 mAh g -1 , and the capacity is decreased nearly to 0 mAh g -1 when the current is increased to 200 mAh g -1 . The rate performance of the ZIF-8 electrode is in good agreement with the result from the dQ/dV plot, confirming ZIF-8 is almost electrochemical inactive in KIBs.

Fig. S24 (a) FTIR spectra and (b) XPS spectra (Zn 2p) of the BPNS@MOF electrode before cycling and after 100 cycles in KIB and KIC
On the other hand, FTIR and XPS measurements testify that the MI layers in the BPNS@MOF also exhibit high structural stability and can preserve in the process of electrochemical potassium storage. Figure S24a shows the FTIR spectra of the BPNS@MOF electrodes in KIB and KIC before cycling and after 100 cycles. The FTIR peak at 1580 cm −1 belonging to C=N stretching vibration [S25] still exists after cycled in both KIB and KIC, suggesting the structural preservation of the imidazole ring skeleton. Besides, there is the no peak appearing at 1843 cm −1 which is assigned to the resonance between the N-H … N bending "out of plane" and N-H stretching vibrations [S26, S27], indicating that protonation of N in imidazole ring is not occurred and the 2-methylimidazole still coordinates with Zn ions. In addition, the corresponding Zn 2p XPS spectra display that the binding energy of Zn 2p3/2, Zn 2p1/2 of the BP@MOF electrode keeps unchanged before and after cycling in both KIB and KIC (Fig. S24b), proving that Zn-N bonds in MOF is not broken during the chargedischarge process, similar to the case in lithium-ion batteries [S28]. All considered, MOF framework of ZIF-8, that is, MI layers can preserve in KIBs and KICs.  , d) GITT profiles and the corresponding K + diffusion coefficient at the discharge process (c) and the charge process (d). Both curves were tested of the sixth cycle in the potential window of 0.01−2.0 V vs K + /K (current density: 50 mA g −1 ; time interval: 20 min; relaxation period: 3 h).

Note:
The Galvanostatic intermittent titration technique (GITT) was first developed by Weppner and Huggins to determine the chemical diffusion coefficient of lithium in the Li3Sb electrode and had been widely applied to systems of lithium ion batteries [S29], sodium ion batteries [S30], and potassium ion batteries [S31, S32]. Here, GITT was employed to determine the reaction kinetics of the BPNS@MOF anodes. We evaluated the diffusion coefficient of K + (Dk) by Fick's second law equation as the following equation: (S1) Where τ is the duration of current pulse, mB represents the mass of active material on the electrode, MB and VM are the molar mass and molar volume of active material, respectively, S is the surface area of the electrode, MB/VM is the gravimetric density of active material, L is the thickness of active material on the electrode. If the voltage has linear relation with τ 1/2 (Fig. S25a), the Fick's second law equation above can be simplified to following equation: (S2) Wherein, ΔEs is the difference in equilibrium potential before and after the current pulse, and ΔEτ is the variation of cell voltage caused by the impulse current. Figure  S25b shows Transient voltage responses versus time profiles of the BPNS and BPNS@MOF electrodes after activation for 5 cycles, which is the original data to calculate diffusion coefficient D. All GITT curves were obtained in a potential window of 0.01-2V vs K + /K. More detailed test information can be found in E vs t profile for a single GITT (Fig. S25c). Specifically, current of 50 mA g -1 was applied for 20 min followed by 3 h relaxation. In addition, D ̅ K values of BPNS@MOF-2 and BPNS@MOF-3 are 9.86×10 −13 and 3.09×10 −13 cm 2 s −1 (Fig. S25d), respectively, which are lower than that of the typical BPNS@MOF. The difference confirms that the BPNS@MOF can achieve best reaction kinetics. This conclusion can also correspond to the previous electrical performance.   Fig. 6a. Diffusion energy profiles of above three pathways for K ions diffusing through MOF are shown in Fig. S28, energy barriers are 0.28, 0.33, and 0.39 eV for pathway 1, 2, and 3, respectively. The results show that pathway 1 is more favorable for K ions to pass through in the model of BP(020)-MOF(112). Pathway 1 is chosen as the optimum pathway of K ions across MOF in BPNS@MOF compared with K ion across K2SO4 in BPNS/K2SO4 in Fig. 6c.

Fig. S29 Side views of different sites of K ions in K2SO4
Note: The optimum pathway when K ions across K2SO4 in BPNS/K2SO4 is identified by the energetically favorable adsorption site. The adsorption energy (Eads) at different sites in the BPNS/ K2SO4 system is calculated by the following formula: where E(BPNS/K2SO4+K) is the total energy of K atom on the BPNS/K2SO4, E(BPNS/K2SO4) is the energy of the BPNS/K2SO4 and E(K) is the energy of a single K atom. On the basis of the definition, a larger negative value of Eads suggests a stronger affinity of K atoms bonded with K2SO4 and thus a more stable system. As shown in Fig. S29, there are three positions in K2SO4, where P1 is in the middle of four S atoms, P2 is in the middle of four O atoms, and P3 is in the middle of four K atoms and is under one S atom. The adsorption energy is -0.62eV, -0.34eV and -0.28 eV for P1, P2, P3 in K2SO4, respectively. Thus, P1 is the energetically favorable adsorption site for K ions, and the optimum pathway when K ions pass across K2SO4 is along the direction of P1 and its equivalent site (Fig. S26d).

Table S1
Zinc atom density of different crystal planes in ZIF-8 MOF Note: The calculation results based on a single MOF unit cell. The sectional area is calculated by the formula of S = absinθ, where a and b are two side length of the parallelogram cross section, θ is the acute angle between a and b.  Table S3 Fitting parameters of electrochemical impedance spectra for the BPNS electrode and the BPNS@MOF electrodes with different MOF contents for KIBs before cycling Table S4 Comparison of performance of our work and the state-of-the-art KIC full cells