Intragranular cracking as a critical barrier for high-voltage usage of layer-structured cathode for lithium-ion batteries

Abstract

LiNi_1/3Mn_1/3Co_1/3O₂-layered cathode is often fabricated in the form of secondary particles, consisting of densely packed primary particles. This offers advantages for high energy density and alleviation of cathode side reactions/corrosions, but introduces drawbacks such as intergranular cracking. Here, we report unexpected observations on the nucleation and growth of intragranular cracks in a commercial LiNi_1/3Mn_1/3Co_1/3O₂ cathode by using advanced scanning transmission electron microscopy. We find the formation of the intragranular cracks is directly associated with high-voltage cycling, an electrochemically driven and diffusion-controlled process. The intragranular cracks are noticed to be characteristically initiated from the grain interior, a consequence of a dislocation-based crack incubation mechanism. This observation is in sharp contrast with general theoretical models, predicting the initiation of intragranular cracks from grain boundaries or particle surfaces. Our study emphasizes that maintaining structural stability is the key step towards high-voltage operation of layered-cathode materials.

Figure 1. Electrochemical performance and observations of fracture.

(a) Specific capacity as a function of cycle number, revealing the Li/LiNi_1/3Mn_1/3Co_1/3O₂ half cell's capacity fading has strong dependence on the high cutoff voltages, (b–d) charge/discharge profiles of Li/LiNi_1/3Mn_1/3Co_1/3O₂ half cells at different high cutoff voltages, and (e–g) low magnification HAADF images of LiNi_1/3Mn_1/3Co_1/3O₂ after 100 cycles at different high cutoff voltages. The red arrows indicate voids and the yellow arrows in g indicate intragranular cracks. Scale bars, 500 nm (e–g).

Figure 2. Intergranular and intragranular cracks.

Cross-sectional SEM images of secondary particles from (a) the pristine material and (b) the cycled one (100 cycles at the high cutoff voltage of 4.7 V). (c) and (d) are HAADF images from cycled LiNi_1/3Mn_1/3Co_1/3O₂ cathode particles, showing intragranular cracks along (001) plane. The yellow arrows indicate real cracks and the pink arrows indicate incubation cracks. Scale bars, 5 μm (a,b); 50 nm (c); and 10 nm (d).

Figure 3. Lattice images of premature cracks.

Each pair of HAADF and ABF images are taken simultaneously. (a,b) [010] axis. (c) The corresponding lattice model. (d,e) A crack tip; (f) The corresponding model. (g,h) [1–10] axis. (i) Strain map at Mode I crack tip, which matches the strain contrast in h. Scale bars, 2 nm.

Figure 4. Intragranular cracks in LiNi_1/3Mn_1/3Co_1/3O₂ (NMC333).

The NMC333 particles are cycled 100 times with the high cutoff voltage of 4.7 V. In a–d, the arrows highlight the crack tips that are terminated in grain interior, and (e) a schematic diagram showing crack formation in the grain interior due to tensile stress. Scale bars, 100 nm (a,b); 200 nm (c); and 50 nm (d).

Figure 5. Dislocations in both pristine and cycled LiNi_1/3Mn_1/3Co_1/3O₂ (NMC333).

High density of dislocations are shown in the bright-field images of (a) pristine and (b) cycled NMC333 (after 100 cycles at the high cutoff voltage of 4.7 V). (c) A HAADF image showing an end-on edge dislocation in pristine NMC333, (d) is the corresponding strain map by GPA and (e) shows the dislocation model of (c). Scale bars, 200 nm (a,b); 5 nm (c,d).

Figure 6. Dislocation associated with cracks in cycled LiNi_1/3Mn_1/3Co_1/3O₂.

(a,b) are the early incubation stages, showing vacancy condensation at dislocation core and (c) is the corresponding model. (d–f) show dislocations associated with cracks. Red arrows indicate crack tips. Scale bars, 2 nm; except f (5 nm).

Figure 7. Cycle voltage governed intragranular cracking and underlying dislocation-based mechanism.

(a) HAADF images overlaid diagram shows the apparent dependence of intragranular cracking on the cycle voltage; when cycled below 4.5 V, intragranular crack can be hardly generated, while above 4.7 V, intragranular density shows a drastic increase; and (b) schematic diagrams to illustrate the dislocation-assisted crack incubation, propagation and multiplication process.