2.1 Material preparation

N-doped C@NCM811 was prepared by a simple and low-cost rheological phase method. Typically, NCM811 (20 g, 19.9 g, 19.85 g, 19.8 g), a fixed amount of dopamine hydrochloride (0 g, 0.1 g, 0.15 g, 0.2 g) and 5 mL of ethanol were fully mixed, where the mass percents of dopamine hydrochloride ranged from 0 to 1 wt.%. And then, the mixed material was evenly ground by ball milling at 150 r/min under argon atmosphere for 30 min. After that, the rheological body was obtained and dried under vacuum at 110 ℃ for 2 h to form the precursor. Subsequently, the precursor was ground in a mortar and sintered in a tubular furnace with nitrogen as the protective atmosphere, sintering at a heating rate of 1 ℃/min for 3 h at 400 ℃ to produce N-doped C@NCM811 powders. According to the mass percents of dopamine hydrochloride, the prepared materials were recorded as P-NCM811, NCM811-CN0.5, NCM811-CN0.75 and NCM811-CN1.

2.2 Physico-chemical characterization

The phases of the as-obtained samples were identified by XRD (X'pert PRO, Panalytical) with Cu Kα in the 2θ range from 10° to 90° at a scanning rate of 2(°)/min. SEM (EVO18, Zeiss) with EDS (X-maxN, Oxford Instrument) and HR-TEM (Talos, FEI) were used to discern their morphologies and chemical compositions, respectively. Their elemental compositions and chemical valences were analyzed using X-ray photoelectron spectroscopy (XPS, PHI-5000 Versaprobe II).

2.3 Calculation methods

The electronic structure with spin polarization of P-NCM811 and the N-doped C coated NMC811 were calculated by DFT using the Perdew-Burke-Ernzerhof (PBE) rule in the generalized gradient approximation (GGA). Projected augmented wave (PAW) pseudo-potentials were used to construct the ionic core structure with a 500 eV kinetic energy cut-off for the plane-wave. The Kohn-Sham orbitals were computed using Gaussian measurements. A convergence of the geometry optimization was considered achieved when the energy change was less than 10^-5 eV and the maximal force was less than 0.01 eV per atom. According to lattice parameters a (14.568 Å), b (8.536 Å), c (25.195 Å), a grid of point K in Brillouin region is constructed and set as 2, 3 or 1. Moreover, the lithium-ion migration barrier energies were determined using the climbing image nudged elastic band (CI-NEB) methodology.

2.4 Electrochemical measurements

The coil type CR2025 half cells were assembled in a nitrogen protected glove box. 1 mol/L LiPF₆ was dissolved in a 1:1:1 volume ratio mixture of methyl carbonate (EMC), ethylene carbonate (EC), and dimethyl carbonate (DMC) solution, which was used as the electrolyte. Electrochemical impedance spectroscopy (EIS) was performed using an electrochemical workstation (AUTOLAB-PGSTAT 302N) with an amplitude voltage of 5 mV and a frequency range of 100 kHz to 0.01 Hz. NEWARE-CT-4008Tn battery testers were adopted to conduct galvanostatic charge and discharge (GCD) measurements at different C rates (1C=220 mA/g) and galvanostatic intermittent titration (GITT) at 0.1C from 2.7 V to 4.3 V. The lithium-ion diffusion coefficient (D_Li+) in the cathode could be calculated from GITT curve according to the following formula [34]:

(1)

The formula uses the following notations: V_M denotes the molar volume of the active materials (cm³/mol); S denotes the active surface of the cathodic electrode (cm²); M_B and m_B stand for the relative molecular mass (g/mol) and mass (g) of the active material, respectively; τ denotes the duration time of the current pulse (s); ΔE_s stands for the voltage change in two successive relaxation periods (V); and ΔE_τ stands for the voltage change during the constant current pulse (V). The cyclic voltammetry (CV) test is performed in the voltage range of 2.7-4.3 V at scan rates of 0.1, 0.25, 0.5, 0.75 and 1 mV/s. In this study, all electrochemical tests were conducted under 25 ℃, and all the coin cells were cycled three times at 0.1C for activation before testing.

3.1 Structure prediction by DFT calculation

To anticipate the effect of N-doped C coating on the structure of NCM811 cathode, DFT has been employed to calculate the structure of P-NCM811 and N-doped C@NCM811 for comparison as shown in Figure 1. The system energy of NCM811-CN was -824.94 eV that is much more negative than -446.77 eV of P-NCM, manifesting that the N-doped C coating would make the system more stable. It is worth noting that the doped-N atoms were inclined to interlock the Ni of NCM811 on the surface matrix, which would facilitate to promote the uniform distribution of coating. Moreover, the bond lengths of Li—O (2.271 Å) after coating became larger than those (2.224 Å) before coating, which would enlarge the Li⁺ diffusion channels to boot Li⁺ diffusion rate. The differential charge density of P-NCM811 and N-doped C@NCM811 was shown in Figures 1(b) and (d). For P-NCM811, it can be seen that the electron cloud was evenly distributed in the whole supercell model. After the N-doped C introduction, the electron cloud was redistributed and accumulated around the carbon layer, which could strengthen the interaction between the coating layer and NCM811 matrix. The density of states was assessed to investigate the effect of the N-doped C coating on electrical conductivity. As shown in Figure 1(e), the total DOS intensity of NCM-CN obviously enhanced compared with P-NCM, especially at the Fermi level reached around 33.0 eV much higher than 9.2 eV of P-NCM, manifesting that the N-doped C coating could obviously promote the electrical conductivity of the NCM811 matrix, which would accelerate the charge transfer during charge and discharge process. The rate performance of NCM811 was mainly determined by the Li ions diffusion rate in the matrix where the Li ions migrate through TSH pathway [35]. As shown in Figure 1(f), the Li ions first migrate from ① octahedral interstitial site to ② adjacent tetrahedral interstitial site and then to ③ another octahedral interstitial site during charge process. For exploring the influence of the coating on Li ions migration, the barrier energies of the Li ions migration were calculated by CI-NEB methodology as shown in Figure 1(g). The maximal battier energy of Li ions migration was reduced from 0.76 eV to 0.56 eV after coating, which was ascribed to the formation of Ni—N bond on the boundary that could expand Li ions migration channels. Nevertheless, a large number of electron clouds gathered around the Ni atoms near the carbon coating as demonstrated in Figure 1(d), indicating that the Ni atoms tend to be reduced to Ni²⁺ after coating due to more easily capturing electrons, which would exacerbate the Ni²⁺/Li⁺ mixing so as to disturb structural orderliness of NCM811, thus deteriorating its structural stability during charge/discharge process. Therefore, optimizing the amount of N-doped C coating is a prerequisite for obtaining prolonged cyclable NCM811 cathode.

Figure 1全屏查看

Optimized crystal model of (a) P-NCM811 and (c) NCM811-CN, differential charge density of (b) P-NCM811 and (d) NCM811-CN, (e) DOS of P-NCM811 and NCM811-CN, (f) schematic diagram of Li⁺ diffusion pathway, and (g) Li⁺ diffusion energy barrier of P-NCM811 and NCM811-CN

3.2 Crystallographic structure evolution after N-doped C coating

The XRD evolution of the four as-prepared samples with different amount of N-doped C coating were shown in Figure 2(a). The XRD pattern of P-NCM811 was well matched the JCPDS No. 74-0919, exhibiting a typical α-NaFeO₂ (R-3m) layered structure [36]. After coating, all diffraction peaks of the modified samples did not alter, manifesting that the N-doped C coating did not affect the structure of NCM811. And no phase of carbon or impurity was detected, indicating that the N-doped C coating was amorphous. While, their relative intensity gradually declined with the increase of the N-doped C amount, which was attributed to the absorption of X-ray by the coating. And the I(003)/I(104) ratio that reflects the degree of Li⁺/Ni²⁺ mixing gradually increased and then decreased with the increase of the N-doped C amount, manifesting that appropriate N-doped C coating could facilitate to inhibit the Li⁺/Ni²⁺ mixing, which was due to the fact that the Ni atoms on NCM811 surface could be anchored by the introduced N atoms as demonstrate in Figures 1(c) and (d). However, when the N-doped C content reached 1.0 wt.%, the (108)/(110) doublets of NCM811-CN1 merged together and the I(003)/I(104) ratio (R) sharply dropped to 1.49, indicating that excessive N-doped C coating conversely aggravated Li⁺/Ni²⁺ cation mixing, which was originated from more Ni³⁺ being reduced to Ni²⁺. Figure 2(b) shows an enlarged view of the (108)/(110) doublets that shifted towards a lower angle as the amount of N-doped C coating increased, indicating that the crystalline interplanar spacing d(108) and d(110) were gradually enlarged, which was consistent with the DFT calculation. Such enlargement in the interplanar spacing would expand the Li-ions channels in the bulk so as to accelerate the Li-ions diffusion rate. The phase evolution with different coating amounts was further verified by XRD Rietveld refinements as shown in Figures 2(c)-(f) and its corresponding refinement parameters were listed in Table S1 (see Supporting information). The volume of unit cell was slightly expanded from 101.188 ų to 103.097 ų with the increase of the coating amount, which was attributed to the formation of more N—Ni bond that would enlarge the bond lengths of the neighboring Ni—O. Additionally, the Li⁺/Ni²⁺ mixing degree was first reduced and then increased sharply with the increase of the coating amount, which well agreed with the I(003)/I(104) ratio, further confirming that appropriate N doping could confine Ni atom migration, but excessive carbon coating would reduce the valent of Ni³⁺. The morphological evolution of the four samples with different amount of coating was observed by SEM. As shown in Figure S1, the P-NCM811 presented regular secondary microspheres that were constituted by primary nano-polyhedrons. After coating, the three coated samples kept the same morphology as P-NCM811 unless their surfaces were obviously wrapped by gauzy substances, manifesting that the adopted rheological phase method did not damage the shape of the micro spherical matrix.

Figure 2全屏查看

(a) XRD patterns of the as-obtained samples, (b) (108)/(110) peak magnified image and (c-f) their corresponding XRD Rietveld refinement patterns

In order to closely observe the surface changes of the samples, the corresponding high-resolution SEM images were presented in Figures 3(a)-(d). It is evident that the smooth surfaces became rough and were coated by gauzy substances after coating. These gauzy substances gradually became homogenous and thick as the amount of dopamine hydrochloride increased, indicating that the amount of dopamine hydrochloride was the key factor for adjusting the N-doped C coating content. To identify the chemical component of the coated gauzy substances, EDS mapping of NCM811-CN was investigated and presented in Figure 3(e). The Ni, Co, Mn elements were well overlapped the secondary microsphere, which were derived from NCM811 matrix. And the C and N elements were also dispersed evenly on the whole microsphere, confirming that the gauzy substances were made of C and N elements that originated from the conversion of the dopamine hydrochloride during calcination. TEM image in Figure 3(f) shows that the N-doped C layer around 3 nm was continuously wrapped the whole matrix in the NCM811-CN0.75 sample, which would not only enhance the stability of particle surface structure, but also reduce direct contact different coating amount on the valence between active material and electrolyte.

Figure 3全屏查看

(a-d) High-magnification SEM images of the as-obtained samples, (e) low-magnification SEM image of NCM811-CN0.75 along with the EDS mappings and (f) its HRTEM

In order to investigate the influence of states of C, N and Ni in the as-obtained samples, XPS tests were conducted and their related results were presented in Figure 4. As shown in Figure 4(a), the typical peaks of Ni 2p, Co 2p, Mn 2p, O 1s, C 1s and Li 1s could be detected in the full spectra of the four samples, where the Ni, Co, Mn, O and Li elements were derived from the NCM811 matrix. While, the intensity of C 1s was gradually enhanced with the increase of the coating amount. Contrast to P-NCM 811, the peak of N 1s at around 399.3 eV appeared and gradually increased after coating. To accurately trace the source of C element, the high-resolution XPS spectra of C 1s were measured and presented in Figure 4(b). In the C 1s spectrum of P-NCM811, two peaks located 289.9 eV at and 284.8 eV corresponded to C=O and C—C respectively, where the C—C derived from the carbon tape during testing and the C=O originated from residual Li₂CO₃ on the P-NCM811 surface [37]. After coating, the intensity of C=O peak reduced obviously with the increase of the coating amount, signifying that the undesirable residues could be efficaciously eliminate by the N-dope C coating, which would facilitate to block the reaction of NCM811 matrix with air to extend its shelf-life. Different from the C—C peak of P-NCM811, the peak around 284.8 eV of the coated samples became asymmetric and could be deconvoluted into a N—C peak at 286.1 eV and a C—C peak at 284.8 eV that were both gradually enhanced with the increase of the coating amount, further confirming that the C element in the coated samples was originated from the coating rather than from the carbon tape. The existence of N—C peak implied that the N—C covalent bonds were formed after the pyrolysis of the dopamine hydrochloride, which would further boost the electronic conductivity of the carbon coating. As shown in Figure 4(c), the high-resolution XPS spectra of N 1s in the coating samples exhibited a broad peak that could be fitted into four peaks at 401.0, 399.4, 398.4 and 397.3 eV, which were allocated to graphite nitrogen, pyridine nitrogen, pyrrole nitrogen and Ni—N bond, respectively [38]. It is worth mentioning that N doping in carbon frameworks affected the spatial charge distribution and further enhanced the electron delocalization effect, which is beneficial to boosting its electrical conductivity. Furthermore, the existence of the Ni—N bond indicated a strong connection between the coating layer and NCM811 matrix, thus stabilizing the coating structure and restraining Li⁺/Ni²⁺ mixing during the de-lithiation process. It is well known that the Li⁺/Ni²⁺ mixing was closely related to the Ni²⁺ content in NCM811 matrix where the higher Ni²⁺ content tends to trigger more serious Li⁺/Ni²⁺ mixing, which would severely attenuate the cyclability. The effect of the N-doped C amount on Ni²⁺ content was analyzed by the high-resolution XPS spectra of Ni 2p. As shown in Figure 4(d), there were two pairs of peaks observed in Ni 2p spectra of each sample. The first pair of peaks ascribed to the satellite peaks, and the second pair of peaks corresponded to Ni 2p_1/2 at 872.8 eV and Ni 2p_3/2 at 855.5 eV that could be deconvoluted into Ni³⁺ and Ni²⁺ [28]. The content of Ni²⁺ and Ni³⁺ in P-NMC811 was 33.9% and 66.1%, respectively. After coating, the content ratio of Ni²⁺/Ni³⁺ almost kept constant, presenting 34.1% vs. 65.9% for NCM811-CN0.5 and 34.2% vs. 65.8% for NCM811-CN0.75 respectively, verifying that the Ni³⁺ in the matrix almost did not be reduced to Ni²⁺ when the coating amount was less than 0.75 wt.%. However, this ratio sharply increased when the coating amount was further elevated, delivering 40.6% vs. 59.4% for NCM811-CN1, which was due to the Ni³⁺ reduction reaction caused by the introduced C coating. Such increase in the Ni²⁺ content would aggravate the Li⁺/Ni²⁺ mixing so as to deteriorate cycling stability and capacity fading. From the above DFT calculation and characterization, an appropriate amount of N-doped C coating could ensure that the NCM811 matrix is uniformly covered to form a protective layer against the erosion of the electrolyte. Moreover, the introduced N could anchor Ni atoms to inhibit the Li⁺/Ni²⁺ mixing. Although excessive amount of N-doped C coating could make its layer thicker, the pyrolytic carbon would reduce the Ni³⁺ to Ni²⁺ so as to aggravate Li⁺/Ni²⁺ mixing, conversely worsening the stability of the crystal structure.

Figure 4全屏查看

XPS of the (a) full spectrum, (b) C 1s, (c) N 1s and (d) Ni 2p of the four samples

3.3 Electrochemical performance evaluation

Figure 5(a) exhibits the charge/discharge curves measured at 0.1C rate after activating 3 cycles. The charging/discharging curves of all samples showed the same profile, indicating that the introduction of the N-doped C coating did not induce any other redox reactions during the charging/discharging process. At a current density of 0.1C (1C=220 mA/g), the discharge capacity exhibited 215.1 mA·h/g for P-NCM811, 214.0 mA·h/g for NCM811-CN0.5, 213.2 mA·h/g for NCM811-CN0.75 and 212.0 mA·h/g for NCM811-CN1, respectively. The discharge capacity slightly reduced with the increase of the coating amount, which was ascribed to the increasing proportion of the non-active N-doped C layer in the active cathode materials. While, the Coulombic efficiency first increased and then decreased with the increase of the coating amount, delivering 93.9%, 96.6%, 98.0% and 97.1% respectively. Such increase in Coulombic efficiency was ascribed to the decline of the charge capacity triggered by the introduction of the coating, and its subsequent decrease was attributed to the obvious drop of the discharge capacity caused by Li⁺/Ni²⁺ mixing. The rate performances of the four samples at various C-rate from 0.1C to 10C were presented in Figure 5(b). It was observed that all the discharge capacities declined with the C-rate elevating, which was related to the sluggish kinetics at high current density. Among these four samples, NCM811-CN0.75 exhibited the highest discharge capacity at every C-rate. Especially at high C-rate the discharge capacity gap became more obviously from 1C to 10C. Even at 10C rate it still reached 151.6 mA·h/g superior than 128.4 mA·h/g of P-NCM811, indicating that the N-doped C coating could ameliorate the high C-rate performance due to the improvement of charge transfer and Li⁺ diffusion after coating. Figure S2 shows the charge/discharge curves of P-NCM811 and NCM811-CN0.75 at different rates. The discharge specific capacity and Coulomb efficiency of NCM811-CN0.75 are both much higher than that of P-NCM811 from 1C to 10C, proving that CN coating can improve the high C-rates performance of NCM811, which was attributed to the excellent electronic conductivity of the CN coating. However, the discharge capacity of NCM811-CN1 was conversely much lower than that of P-NCM811 at low C-rate (from 0.1C to 1C), which was related to the difficulty of Li⁺ re-insertion during discharge induced by the severe Li⁺/Ni²⁺ mixing degree. For further exploring the difference of D_Li+ before/after coating, the GITT tests of P-NCM811 and NCM811-CN0.75 at 0.1C were compared and the related curves were shown in Figures 5(c) and (d). It was obvious that the D_Li+ of NCM811-CN0.75 sample (10^-8.5 cm²/s) was significantly higher than that of P-NCM811 (10^-9 cm²/s) during the whole charge/discharge process, manifesting that the Li⁺ diffusion rate was accelerated after the introduction of the N-doped C coating. The D_Li+ increase was ascribed to the reduction of the Li⁺ diffusion energy barrier supported by the CI-NEB calculation, which was the main reason for the superior rate performance of NCM811-CN0.75. To evaluate the cycling stability at high C-rate after coating, the four samples were subjected to 300 cycles at 5C under 25 ℃ and their related performance was shown in Figure 5(e). In the whole cycling period, the NCM811-CN0.75 electrode demonstrated the most excellent cycling stability and capacity retention, delivering a discharge capacity of 137.9 mA·h/g with a capacity retention of 82.2% after 300 cycles. This was attributed to the combined effect that the introduced carbon coating could act as a stable protective layer to suppress the adverse additional reaction with the electrolyte and the introduced N atoms could anchor the Ni atoms to inhibit the Ni atom transportation to Li sites, resulting in the splendid cycling stability.

Figure 5全屏查看

(a) The charge/discharge curves of the four samples at 0.1C; (b) Rate capability from 0.1C to 10C; (c) GITT and (d) D_Li+ of P-NCM811 and NCM811-CN0.75; (e) Cycling performance at 5C for 300 cycles

To further excavate the effect of the coating on the electrochemical performance, CV at various scanning rates were tested to distinguish the impact of surface-pseudo capacitance on lithium storage kinetics after coating as shown in Figures 6(a)-(d). At all scanning rates, the coated samples exhibited better redox reversibility than the pristine sample, and the intensity of the oxidation and reduction peaks were higher than that of the pristine sample, further confirming that the introduction of the N-doped C coating could reduce the electrochemical polarization and promote the ions kinetics. A logarithmic regression analysis is conducted on the function of i=aυb [40], where i represents the peak current and υ represents the scanning rate in CV plots. When the value of b approaches 1, it signifies that the lithium storage process is dominated by pseudo capacitance behaviors. As shown in Figure 6(e), the b value of the coated samples (b=0.883 for NCM811-CN0.5, b=0.900 for NCM811-CN0.75, b=0.861 for NCM811-CN1) was slightly higher than that of P-NCM811 (b=0.823) but they almost stayed at the same level, manifesting that the lithium storage process dominated by pseudo- capacitance behaviors was unchanged after coating. Furthermore, their contribution ratio of pseudo- capacitance (k₁υ) and diffusion (k₂υ^1/2) were calculated by the equation i=k₁υ+k₂υ^1/2 [41] and illustrated in Figures 6(f) and S2. As demonstrated, their pseudo capacitance ratio exhibited an increase with the scanning rate (from 55.8% to 92.8% for P-NCM811, from 66.9% to 94.3% for NCM811-CN0.5, from 69.8% to 94.4% for NCM811-CN0.75 and from 62.3% to 93.6% for NCM811-CN1), indicating that the pseudo-capacitance played an important role at high C-rate. It was worth noting that the pseudo capacitance increased at every scanning rate after coating, which was attributed to the formation of more active sites after the introduction of the N-doped C coating, thereby ameliorating the high-rate performance, which was the reason for the discharge capacity of the NCM811-CN1 sample at high C-rate (from 2C to 10C) higher than that of P-NCM811.

Figure 6全屏查看

(a-d) CV curves of the four samples at different scan rates, (e) the relationship between the numerical anodic peak current and the logarithmic scan rate and (f) the capacitance contribution ratios for NCM811-CN0.75 at different scan rates