Doping‐Induced Electronic/Ionic Engineering to Optimize the Redox Kinetics for Potassium Storage: A Case Study of Ni‐Doped CoSe2

Abstract Heteroatom doping effectively tunes the electronic conductivity of transition metal selenides (TMSs) with rapid K+ accessibility in potassium ion batteries (PIBs). Although considerable efforts are dedicated to investigating the relationship between the doping strategy and the resulting electrochemistry, the doping mechanisms, especially in view of the ion and electronic diffusion kinetics upon cycling, are seldom elucidated systematically. Herein, the crystal structure stability, charge/ion state, and bandgap of the active materials are found to be precisely modulated by favorable heteroatom doping, resulting in intrinsically fast kinetics of the electrode materials. Based on the combined mechanisms of intercalation and conversion reactions, electron and K+ ion transfer in Ni‐doped CoSe2 embedded in carbon nanocomposites (Ni‐CoSe2@NC) can be significantly enhanced via electronic engineering. Benefiting from the synthetic controlled Ni grains, the heterointerface formed by the intermediate products of electrochemical reactions in Ni‐CoSe2@NC strengthens the conversion kinetics and interdiffusion process, developing a low‐barrier mesophase with optimized potassium storage. Overall, an electronic tuning strategy can offer deeper atomic insights into the conversion reaction of TMSs in PIBs.

Theoretical calculation: The Spin-polarization density functional theory (DFT) calculations within the generalized gradient approximation (GGA) was performed using the Perdew-Burke-Ernzerhof (PBE) formulation. [1] The projected augmented wave (PAW) potentials were chosen to describe the ionic cores and valence electrons were taken into account using a plane wave basis set with a kinetic energy cutoff of 450eV. [2] Partial occupancies of the Kohn−Sham orbitals were allowed using the Gaussian smearing method and a width of 0.05 eV. The electronic energy was considered self-consistent when the energy change was smaller than 10−6 eV. A geometry optimization was considered convergent when the energy change was smaller than 0.03 eV Å − 1 . What's more, K ions migration barrier energies were evaluated using the climbing nudged elastic band (CI-NEB) methods.  The morphologies and structures of Ni-CoSe 2 @NC-I/II/III can be investigated at different magnification. As shown in Figure S2, the high-magnification SEM images display the surface of the nanospheres are covered with many fine particles and become coarse. By using ZIF-67 as the template, Ni-CoSe 2 @NC nanocomposites are successfully formed. Figure S3. Elemental mapping images of (Ni,Co)Se 2 @NC. Figure S4. Inverse FFT liner profiles of Ni-CoSe 2 @NC-II. s7 Figure S6. The dispersive X-ray spectroscopy (EDX) maps of Ni-CoSe 2 @NC-II. Figure S7. Raman spectra of CoSe 2 @NC, Ni-CoSe 2 @NC-II and (Ni,Co)Se 2 @NC.
The D-band reveals the structural defects, and the G-band corresponds to sp 2 -hybridized graphitic carbon. The high ID/IG ratio in the samples suggests the existence of defects in the carbon matrix. [3] 1200 1400 1600 1800 ( %) = 54.34% Figure S9. N 1s spectra of Ni-CoSe 2 @NC-II.
Using deconvolution method, the N 1s spectrum was fitted by the assumption of four species: pyridinc N (398.4), pyrrolic N (399.6 eV), graphitic N (400.9 eV) and oxidized N (402.1 eV). [4] Figure S10. The EXAFS fitting results of the Co K-edge for CoSe 2 @NC and Ni-CoSe 2 @NC-II as well as the Ni K-edge for Ni-CoSe 2 @NC-II.  Stage I (1.07V), reconversion: Stage II (1.94V), K + -deintercalation: Stage IV (0.44V), conversion: Figure S14. a) Log (i) versus Log (v) plots at peak currents corresponding to the CV curves. b) b-values calculated through cathodic scan for the Ni-CoSe 2 @NC-II electrode. c) The shaded region shows the CV profile of Ni-CoSe 2 @NC-II with pseudocapacitive contribution at a scan rate of 1.0 mV s −1 . d) Capacitive-controlled contribution at different scan rates and e) the relavent calculations of K + adsorption energy for CoSe 2 @NC, (Ni, Co)Se 2 @NC and Ni-CoSe 2 @NC-II.
Concerning the contribution based on the two major mechanisms of pseudo-capacitance and K + insertion/extraction, the peak current (i) and the scan rate (υ) follow the equations: [5] Figure S14a, which accordingly, both samples exhibit capacitive-dominant characteristics. [7] Taking the Ni-CoSe 2 @NC-II anode as an instance, b-values resulted from the voltage range of 2.2-0.01 V in the discharge process are shown in Figure S14b. Notably, all the b-values are nearly 0.75 during the whole cathodic scan, demonstrating an almost linear relationship between the scan rate and current. [8] Furthermore, it reveals a capacitive ratio of 72.5% at a specific scan rate of 1.0 mV s −1 ( Figure S14c). The high contribution of the surface-driven process can be ascribed to the well-controlled structure with an appropriate doping content of Ni ions. The detailed proportions of capacity dominated by the capacitive behavior are summarized in Figure S14d and Table S7. Additionally, the models of equilibrium states for the three typical samples with K atom are optimized ( Figure S15) and the corresponding adsorption energy of K for each model is illustrated in Figure S 14e, which is calculated to be -0.58, -1.73 and -3.95 eV. It further confirms that Ni-CoSe 2 @NC-II is constructive for the adsorption of K + , thereby boosting the K + intercalation kinetics. [9] Hence, the reaction s13 mechanisms on the K + intercalation/deintercalation should be investigated by regulating the potential window. Figure S15. The optimized models of equilibrium states with one K atom for a) CoSe 2 @NC, b) (Ni, Co)Se 2 @NC and c) Ni-CoSe 2 @NC-II.     Figure S20. More HRTEM analyses of the Ni-CoSe 2 @NC-II electrode when discharged to 0.75V. Figure S21. E vs. t curves of Ni-CoSe 2 @NC-II electrode in a single GITT. Figure S22. EIS spectra of Ni-CoSe 2 @NC-II and (Ni, Co)Se 2 @NC electrodes at 0.734 V in the 2 nd discharge process.
In the equivalent circuits, Rs is the total resistance of the electrolyte, electrode, current collector and separator. Rct is sum of the interface resistance from SEI and the charge transfer resistance. Ws is the Warburg impedance, and CPE stands for constant phase element, which is related to the capacitive behavior during the respective process. [10] Notably, the curves are composed of a nearly straight line in the low-frequency region and a depressed semicircle in the high frequency range, representing the Warburg impedance related to the solid-state diffusion and the charge-transfer impedance (Rct), respectively.  It can be seen that this heterointerface has continuous distributions around the Fermi level, indicating that the K 2 Se/Co(Ni 50%) also has the metallic properties. [1] Nevertheless, compared with that of K 2 Se/Co(Ni 10%), there were less electron accumulation (yellow color) in the K 2 Se/Co(Ni 50%) interface, which indicated that K 2 Se/Co(Ni 10%) has high electronic conductivity duo to the rational doping content.