First author: Xinye Zhao Corresponding author: Ouyang Bin, Gerbrand Ceder Corresponding unit: University of California, Berkeley [Research background] Lithium-ion battery cathode materials are often accompanied by volume changes during the cycle, which has a negative impact on th

First author: Xinye Zhao

Corresponding author: Ouyang Bin, Gerbrand Ceder

Corresponding unit: University of California, Berkeley

[Research background]

Lithium-ion battery cathode materials are often accompanied by volume changes during the cycle, which is very important for solid-state The integrity of the cathode particles and the electrolyte/cathode interface in battery poses challenges. Therefore, in order to improve the capacity retention of lithium-ion batteries, it is crucial to design structurally stable materials during electrochemical cycling. In view of this, Professor Gerbrand Ceder and Ouyang Bin of the University of California, Berkeley, USA, systematically studied the chemical properties of transition metals, sorting of cations, site occupancy, redox inert substances, and Effects of anion substitution and cation migration on volume changes of cathode materials. Through in-depth theoretical simulation analysis of the Li+–V3+–Nb5+–O2––F– (LVNOF) system, it was found that the Li1.3V0.4Nb0.3O2 and Li1.25V0.55Nb0.2O1.9F0.1 cathode materials are The volume change in is almost zero. This work has general guiding significance for the design of low-strain or zero-strain cathode materials.

[Details]

To examine the impact of cation arrangement, the researchers calculated the delithiation molar changes in three structures with face-centered cubic (FCC)-like cation packing: namely O3 layered, lithiated spinel and γ-LiFeO2 structure. As shown in Figure 1, lithiated spinel has an LT-Li2Co2O4 structure, occupied only by octahedral cations. Due to overlithiation, tetrahedral occupation is absent in this structure, although its cations have the same ordering as LiM2O4 spinel. Figure 1B gives the negative molar volume change of Li ions in each structure. Among all three polycrystalline -type structures, the layered structure generally exhibits the largest delithiation molar volume change. In comparison, the spinel and γ-LiFeO2 structures show smaller volume changes due to delithiation. In Figure 1C, the left vertical axis corresponds to the delithiated molar volume change (circles), and the right vertical axis corresponds to the delithiated molar lattice parameters (squares). For all chemicals, structures with cation mixing (yellow) show smaller volume changes than layered structures (red).

Figure 1. The influence of structure and cation ordering on the change of cathode volume .

In the lithiated spinel structure, Li can occupy the 16c site in the octahedron and the 8a site in the tetrahedron. This would be beneficial to detect whether the changes in molar volume of delithiation are different at sites 8a and 16c. Figure 2A compares the volume changes in two delithiation cases when the Li0.5MO2 material occupies the spinel-like 16d site: In the first case (red dot in Figure 2A), Li ions occupy the tetrahedral 8a site . In another case (green dot in Figure 2A), Li ions occupy half of the 16c site of the octahedron, where the distribution of vacancies on Li and 16c will minimize the electrostatic Ewald energy. For most chemicals, the structure occupying the 8a site has a larger negative delithiation molar volume change than the structure occupying the 16c site , except for Ni3+ (which has a very similar negative delithiation molar volume change , -4.55 and -4.58 Å3). For example, when Li occupies the 8a site, the negative delithiation molar volume changes of and for Li0.5TiO2 and Li0.5CoO2 are -1.92 and -2.91 Å3, while when Li occupies the 16c site, the negative delithiation molar volume changes of is only -0.50 and -2.28 Å3.

Figure 2. The influence of Li in the cathode structure.

The staff then used cluster expansion models and Monte Carlo (MC) simulation methods to evaluate the volume changes associated with delithiation of the LVNOF composition using a ternary phase diagram with LiVO2, Li3NbO4, and LiF as three endpoints, as shown in Figure Shown in 3A. Since the solubility of LiF and is typically only 7%–10%, in order to improve the accuracy of the model, this calculation only considers components with an F content below 0.4 in each model unit, as shown by the pink line in Figure 3A. Furthermore, to ensure sufficient TM redox capacity, only composites with lithium content between 1.0 and 1.4 per model unit were considered, as shown by the blue line. The contours in the phase diagram in Figure 3A show compounds with the same LiF solubility temperature based on cluster expansion MC simulations.For each MC sampled lithiated structure, the volume change was evaluated under two different delithiation scenarios: (1) removal of a fixed ratio of 0.2 Li/anion, calculating the volume change at low charge levels and (2) removal of the Li amount , full charge compensation through V3+/V5+ redox, inhibiting oxygen redox, and making the volume change trend of the material more complex. Figures 3B and 3C show color plots of delithiated molar volume changes interpolated between the compounds studied. The dotted line indicates the volume change when the Nb content is between 0.2 and 0.3 and Li is between 0.25-0.3. Two main trends can be seen from the figure: first, as the Nb content increases, the molar volume change for delithiation generally becomes less negative. Secondly, in the LVNOF system, the molar volume change of delithiation is weakly dependent on the degree of fluorination of to .

Figure 3. calculation of the volume change of delithiation in an LVNOF system.

staff further used in situ XRD to evaluate the changes in lattice parameters of LVNO43 and LVNOF552 cathode materials with delithiation, as shown in Figures 4A and 4B. Figures 4C and 4D also show the absolute volumes of the two cathode materials at each state of charge. As shown in Figure 4C, the maximum volume change of LVNO43 is 1.2%, corresponding to a lattice parameter change of 1.5 Å3, which is consistent with the theoretical prediction of 1.3 Å3. The fluorinated compound (i.e., LVNOF552) exhibits a lower volume change of 0.7% at a capacity of ∼200 mAh/g, corresponding to a volume change of 0.9 Å3.

Figure 4. Structural changes of LVNO43 and LVNOF552 during the cycle .

[Conclusion and Outlook]

In short, this study established the universal principles for the design of low-strain cathode materials based on FCC anionic skeleton. Studies have shown that having non-bonding electronic configurations, isotropic structures, cationic disorder, inactive elements and transition metal redox centers reduces the volume changes associated with delithiation. The principles established in this article can be effectively applied to both compositions in the LVNOF DRX system. The experimental results verified the universal design principle. This work can be applied not only to the design of cathode materials based on DRX structures, but also to materials based on layered and spinel frameworks, providing new insights into minimizing the electrochemical cyclic strain of potential cathode materials.

[Literature information]

.org/10.1016/j.joule.2022.05.018