Fig. 1. (a) Schematic illustration of the formation process for the NCMTi; (b) the XRD Rietveld refinement and (c, d) Ti 2p and Ni 2p3/2 XPS spectra for the NCM and the NCMTi with various Ti content.

Fig. 2. (a) SEM image and the particle size distribution (inset), (b) TEM image and the corresponding SAED pattern (inset), (c, d) high-magnification and high-resolution TEM images, and (e-j) cross-sectional SEM image and the corresponding element mappings of the NCMTi-1.

Fig. 3. (a) Initial Coulombic efficiency (ICE), (b) rate capabilities, and (c) cycle performances of the pristine NCM and the NCMTi with different Ti content; (d) voltage hysteresis, (e) charge transfer resistance during cycling, and (f) linear relationship between anodic/cathodic peak current (ip) and the square root of the scan rate (V1/2) for the NCMTi-1 and NCM.

Fig. 4. (a, b) Initial three CV curves at 0.2 mV s - 1 within 2.7-4.3 V and the locally enlarged curves (inset); (c, d) the calculated dQ/dV curves during 100 cycles of the NCM and NCMTi-1.

(c) O and C = O for the NCMTi-1 is 18.5%, much lower than that of NCM (23.8%), indicating reduced parasitic reactions during cycling. Additionally, the structure of the NCMTi-1 and NCM cycled at 1 C for 100 times was examined by XRD (Fig. 5). The crystal structure of the NCMTi-1 is well-maintained without any impurities even after 100 cycles. However, the NCM undergoes serious structural deterioration and harmful phase transition with the appearance of the inert rock salt phase (NiO). Besides, the SEM images of the cycled materials are displayed in Fig. S7. The NCMTi-1 exhibits excellent structural integrity with concurrently enhanced surface and bulk stability, whereas the pristine NCM appears evident cracking. As partial TM ions would dissolve in electrolyte and deposit on the anode side in the repeat charge-discharge processes, causing the loss of active materials and fast capacity fading [37]. Thus, the content of Ni element on the Li anode was quantitatively analyzed by ICP. As depicted in Fig. 5(d), the black area corresponding to the deposited TM elements is smaller for the NCMTi-1 compared with the NCM. Concretely, the mass of Ni deposition is 0.90 μg for the NCMTi-1, much lower than that of the NCM (4.25 μg), implying that the dual-modified tactic could effectively reduce the loss of active substances to prolong the cycle life.

Fig. 5. (a, b) F 1 s and C 1 s XPS spectra, (c) XRD patterns, and (d) the mass of Ni deposition on Li foil and the corresponding digital photos for the NCM and NCMTi-1.