《Table 1.The particle size distribution parameters, specific surface area and pH val-ues of NCA-P1 a

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《TiP_2O_7-coated LiNi_(0.8)Co_(0.15)Al_(0.05)O_2 cathode materials with improved thermal stability and superior cycle life》

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The XRD measurements were conducted to examine the crystal structure of both TiP2O7coated sample NCA-C1 and uncoated pristine sample NCA-P1，and the XRD results were shown in Fig.1（a）.As can be seen，both samples can be well indexed toα-NaFeO2layered structure with a space group of rhombohedral R3m.For NCA-C1 sample，no Ti P2O7phase can be found in the XRD pattern due to the low content.The values of lattice constants a and c were respectively calculated to be 2.8673?A and 14.1916?A for NCA-P1 sample，2.8668?A and 14.1909?A for NCA-C1 sample，which indicates that there are no obvious changes in the crystal structure of NCA after coated with Ti P2O7.The c/a ratios of NCA-P1 and NCA-C1 were calculated to be 4.97 and 4.95，respectively.The rather high c/a ratios and clear splits of（018）/（110）and（006）/（012）pairs indicate that the layered structure of both as-synthesized materials were well formed[17，26].The I003/I104value of Ni-rich cathode materials was usually used to represent the degree of cation mixing between lithium ions and metal ions，and the higher I003/I104value indicated better ordered arrangement.Serious cation mixing of NCA materials was supposed to result in structural degradation during cycling[5，7，8].The I003/I104values of NCA-P1 and NCA-C1 were calculated to be 1.508 and 1.641，respectively.Since the I003/I104ratio is corresponding to the intensity of cation mixing，the higher I003/I104ratio representing lower cation mixing and better layered structure of NCA-C1，the better structural crystallinity will finally benefit the electrochemical performances of NCA-C1.The particle size distribution curves of both samples were shown in Fig.1（b），and the main tested values were exhibited in Table 1.The particle size distributions of NCA-P1 and NCA-C1 were similar with each other，the D50 value of NCA-C1 was 12.0μm，which a little higher than 11.6μm of NCA-P1 sample，which indicates that TiP2O7coating makes no obvious difference in particle size.The tested results of specific surface area and pH of NCA-P1 and NCA-C1 were also displayed in Table 1.The BET values of NCA-P1 and NCA-C1 were 0.36 and 0.43 m2/g，respectively.The larger specific surface area of NCA-C1 was mainly brought by the residual TiP2O7layer which may allow more contact area between cathode and electrolytes.The pH value is an important parameter to judge the processability of Ni-rich materials[4，27].In this work，the TiP2O7coating was supposed to consume a part of redundant lithium on the surface of NCA particles to form LixTiP2O7which favors the conductivity of Li+.The p H values were 12.04 and 11.37 for NCA-P1and NCA-C1 respectively，revealing that coating a layer of TiP2O7on the surface of NCA particles was an efficient way to decrease the alkalinity and benefit the processability.