Surface enrichment and diffusion enabling gradient-doping and coating of Ni-rich cathode toward Li-ion batteries

Critical barriers to layered Ni-rich cathode commercialisation include their rapid capacity fading and thermal runaway from crystal disintegration and their interfacial instability. Structure combines surface modification is the ultimate choice to overcome these. Here, a synchronous gradient Al-doped and LiAlO2-coated LiNi0.9Co0.1O2 cathode is designed and prepared by using an oxalate-assisted deposition and subsequent thermally driven diffusion method. Theoretical calculations, in situ X-ray diffraction results and finite-element simulation verify that Al3+ moves to the tetrahedral interstices prior to Ni2+ that eliminates the Li/Ni disorder and internal structure stress. The Li+-conductive LiAlO2 skin prevents electrolyte penetration of the boundaries and reduces side reactions. These help the Ni-rich cathode maintain a 97.4% cycle performance after 100 cycles, and a rapid charging ability of 127.7 mAh g−1 at 20 C. A 3.5-Ah pouch cell with the cathode and graphite anode showed more than a 500-long cycle life with only a 5.6% capacity loss.

2 condition was sat as 0.001. The grain orientation was random (in the range of 0-360 °). The transformation matrix was used to represent the relationship between the local expansion/contraction and the global expansion/contraction: α ij L = tr ki tr lj α kl . The corresponding matrix was shown below.

Supplementary Note 2. The testing process and calculation equation of GITT measurement
Before the GITT measurement, the cells were firstly galvanostatic charge/discharged for 5 cycles. GITT measurements were performed by charging/discharging the fully activated cells where Vm is the molar volume of active materials, MB and mB are the molecular weight and mass of the host oxide, respectively, and A is the total contact area between the electrolyte and the electrode, L is the thickness of the electrode. If sufficiently small currents and short time program. In detail, the cell was first charged to 4.3 V at different current density from 0.5 C and 5 C. Then, the cell was CV charged at voltage of 4.3 V until the current reduces to 0.05 C.
Besides, the charge time also needs to set the upper limit according to the charging current.
Finally, the cells were discharged at the same current density of 1 C to compare the capacity.   Fig. 2). The Al(OH)3 coated Ni-rich precursors were prepared by the methods of oxalate-assisted deposition. Compared to the pristine precursors, the uniform coating layer of Al(OH)3 exists on the surface, which is demostrated by EDS mapping images.  it is noticed that overmuch Al element will generate excess electrochemical inert LiAlO2, which will dramatically reduce the specific capacity of the cathode. Therefore, synergistically considering the specific capacity and comprehensive electrochemical properties, it is logical to speculate that NCAl-LAO is the most suitable cathode in the series of materials for this work, which is selected to deeply scrutinize. which is attributed to the increased content of Ni 2+ with larger ionic radius. Impressively, the NCAl-LAO with more Ni 2+ still displays a lower value of Li/Ni disorder (1.3%) than that of NC91 (2.4%), which is also verified by the larger c/a. Conceivably, the NCAl-LAO with the larger lattice parameters and the lower Li/Ni disorder will display higher intrinsic Li + 9 conductivity and more stable crystal structure during lithiation/delithiation process. To further certify the Al 3+ doping throughout NCAl-LAO cathodes, EDS point analyses in a cross-section of single secondary microsphere were carried out to scrutinize the atomic ratio of Al 3+ at different areas, and the data was displayed in Supplementary Fig. 8c. It is noted that the content of Al 3+ on the surface of secondary particles is extraordinarily high, which is due to the coating of LiAlO2 on the surface. The similar atomic ratio of approximately 2% at other internal areas indicates that the Al 3+ distribute over all the area in secondary particles.  Fig. 13a, b). The fine linear relationship between the peak current (ip) and the square root of the scanning rate (v 1/2 ) indicates the Li + transfer in these cathodes exhibits a diffusion-controlled process, and the larger slopes of NCAl-LAO demonstrate higher diffusivity than that in NC91 (Supplementary Fig. 13c, d) 3,6 . The GITT analysis was also carried out to obtain the specific Li + diffusion coefficients at each stage of cell operation. Supplementary Fig. 13e shows the GITT curves of NCAl-LAO and NC91 in discharging process, while the testing process and calculation equation are presented in

Supplementary notes. The as-calculated Li + diffusion coefficients (DLi+) as a function of the
Li extraction content are shown in Supplementary Fig. 13f. The values of the two samples are both 10 -10 -10 -9 cm 2 s -1 , which is in accordance with the earlier reports 3,7 . Impressively, the  The charge/discharge profiles of both NCAl-LAO and NC91 over 100 cycles were depicted in Supplementary Fig. 14a, Fig. 14c, d). The XPS surface analysis was performed for NCAl-LAO and NC91 after 100 cycles at 1 C to certify the stability of interfacial chemistry, and the intensities of all the XPS spectra were normalized. As depicted in C 1s peaks (Supplementary Fig. 15a), the peaks at ca 285 eV represent the Super P (C-C), while the peaks of C-H and C-F located at ca 286 and ca 290 eV are both related to PVDF binder. Meanwhile, the peaks at ca 287 eV and 288 eV can be 16 designated to ether and carbonate, which are attributed from the electrolyte decomposition 8 . Supplementary Fig. 15b shows the F 1s spectra for both samples, the C-F bonds (687 eV) are assigned to the PVDF binder. Meanwhile, the LixPOyFz/LixPFy (685 eV) and LiF (684 eV) peaks are the components of CEI films derived from the parasitic reactions at the electrode/electrolyte interface 9 . It is noteworthy that the peak intensities of carbon-oxygen compounds, LixPOyFz/LixPFy and LiF in NCAl-LAO are all lower than that of NC91, suggesting less parasitic reactions and thinner cathode-electrolyte interphase (CEI) films.
Therefore, the analysis of the interface chemical composition has validated that the interface stability has indeed be improved after modification. One side, the LiAlO2 coating can refrain the direct contact between the cathode and the electrolyte, forming more stable and thinner CEI films. On the other hand, the improvement of structural stability can reduce the release of oxidizing species (O 2-, O -, etc.) during phase transformation, which can mitigate the decomposition of electrolyte to generate impurity (such as HF). Furthermore, the existence of Al-F signal in F 1s region (ca 686.5 eV in Supplementary Fig. 15b) and Al 2p region (ca 76.5 eV in Supplementary Fig. 16) of NCAl-LAO after cycling indicate that the LiAlO2 layer can be partly fluorinated to form AlF3 during electrochemical process 10 . A similar phenomenon was also observed when using another Al-containg coating 11,12 . According to previous studies 13 The electrochemical impedance measurements of NCAl-LAO and NC91 were performed after various cycles. The Nyquist plots and corresponding equivalent circuits are shown in Supplementary Fig. 17a, b. The first semicircle in the high-medium frequency range corresponds to the surface film resistances (Rsf), resulting from the CEI films, while the second semicircle at medium-low frequency is identified as the charge-transfer resistance (Rct) of the cathode 15 . The data extracted from the Nyquist plots with respect to the cycle number are displayed in Supplementary Fig. 17c, d and  Supplementary Fig. 19 The (003)  The simulation model was established according to the microstructure characteristics obtained from SEM and TEM images. As shown in Supplementary Fig. 20a, the sizes of primary particles were set as 200-500 nm and the constituent secondary particle displayed a diameter of 10 μm. It should be pointed out that the primary particles were considered as single crystals and the homologous crystallographic orientations were stochastically set. The mesh for this model 20 was displayed in Supplementary Fig. 20b and the simulations with disregarded any plastic deformation were performed by using mechanics and thermal modules in 2D finite element analysis.
Supplementary Fig. 21 The variations of a-axis lattice parameters as a function of the charging voltage.
Supplementary Fig. 22 The distribution of (a) volume deformation, (b) Von mises stress, (c) tensile and compressive stress thoughout the secondary particles for NC91 and NCAl-LAO when charging to 4.0 V.

Supplementary Tables
Supplementary