Inducing Favorable Cation Antisite by Doping Halogen in Ni‐Rich Layered Cathode with Ultrahigh Stability

Abstract The cation antisite is the most recognizable intrinsic defect type in nickel‐rich layered and olivine‐type cathode materials for lithium‐ion batteries, and important for electrochemical/thermal performance. While how to generate the favorable antisite has not been put forward, herein, by combining first‐principles calculation with neutron powder diffraction (NPD) study, a defect inducing the favorable antisite mechanism is proposed to improve cathode stability, that is, halogen substitution facilitates the neighboring Li and Ni atoms to exchange their sites, forming a more stable local octahedron of halide (LOSH). According to the mechanism, it is demonstrated by NPD that F‐doping not only induces the antisite formation in layered LiNi0.85Co0.075Mn0.075O2 (LNCM), but also increases the antisite concentration linearly. F substitution (1%) induces 5.7% antisite, and it displays an excellent capacity retention of 94% at 1 C for 200 cycles under 25 °C, outstanding high temperature cyclability (153.4 mAh·g–1 at 1 C for 120 cycles under 55 °C). The onset decomposition temperature increases by 48 °C. The ultrahigh cycling/thermal stability is attributed to the stronger LOSH, and it keeps the structural integrity after long cycling and develops an electrostatic repulsion force between oxygen layers to increase the lattice parameter c, which benefits Li‐ion migration.

. Bond formation energies between X (O, F, Cl, Br, I, and S) elements and metals (Li, Ni, Co, and Mn)  neighboring each other shows the lowest total energy (Figure 1b). After that, on the basis of the LNCM parent model, the model oxygen atom substituted by fluorine atom can be made. halogen-doped LNCM can own thousands of possible variation due to micro-content of fluorine. To simplify the large amount of calculation, first halogen substitutions were made 36 times and the optimum structure can be selected; second halogen substitutions were carried on at sites adjacent to the first halogen and so on.
The formation energy ( f E) of F-doped is computed as follows.
Where f E O,x is the formation energy when a X atom is placed at an oxygen-site, The anti-site formation energy is computed as follows.
Where E(perfect) is the total lattice energy of the perfect halogen-doped and pristine LNCM, E(defective) is the total lattice energy of the halogen-doped and pristine LNCM including anti-site. The formation energies of anti-site were calculated by using DFT, shown in Table S3.    Table S2. Where V ave represents the average delithiation potential, E(MO 2 ) is the total energy of delithiated structures obtained by removing all Li atoms from the LiMO 2 structures. The V ave is an extremely important parameter for energy densities of lithium-ion battery that is equal to the product between average delithiation potentials and specific capacities. Table S2 shows the calculated average delithiation potentials at different F doping levels, the V ave increases as the F doping content increases, which derives from that F changes the electronic structure and total energy of LNCM, and consequently leads to the average delithiation potential is raised from 3.91V(the result is consistent with Ceder 1 ) to 3.96V. The doped F contributes to enhance V ave within a certain content limit, therefore F doping is a good way to raise voltage platform.

Equation S5
.    (1)  content have a significant impact on the morphology of LNCM particals. Which specific in two fronts, for secondary particle, the higher the content of introduced fluorine, the more incomplete the sphere is; and there are many chippings with increased fluorine. With regard to the primary particle, the primary particle size changed a little. When F content is 1%, there is no obvious change, however, when F content exceeds 2%, the average size and edge of the primary particles are becoming smaller and blur, respectively. In addition, many ~80nm fragments adhesion to the primary particle are discovered. Figure S4. The relationship between anti-site and first discharge capacity.   (Figure S5), the Li + diffusion coefficient in these three samples can be obtained from an analysis of the Warburg impedance.
Where is the angular frequency, and B is the Warburg coefficient (0.02837 for PRI, 0.01002 for 5.7% AS and 0.01031 for 8.2% AS). V m is molar volume of the samples (33.85cm 3 /mol for PRI, 33.89cm 3 /mol for 5.7% AS and 33.94cm 3 /mol for 8.2% AS, these values are obtained by NPD refinement), S is the apparent surface area of the electrode (~1.33cm 2 , it is smaller than real surface area, because the sample in our manuscript is not a film electrode), F is 96500 C/mol, dE/dx is the slope of the open-circuit potential vs. Li-ion concentration x at each x value(-0.0072 for PRI, -0.0037 for 5.7% AS and -0.0036 for 8.2% AS ). For pure PRI sample, the D Li+ is about 2.24×10 -9 cm 2 /S, and for the 5.7% AS and 8.2% AS samples, the D Li+ are estimated to be 4.26~4.75×10 -9 cm 2 /S. Because of the electrode surface area, the calculated D Li+ will be little bit larger than true values. But, the relative values of the 3 samples are accurate and meaningful, that indicate moderate anti-site promote the Li + diffusion, the possible reason is the developing electrostatic repulsion force bewteen oxygen layers increase the lattice parameter c.