Understanding Capacity Fade in Lithium-Ion Batteries: Causes and Solutions

Capacity fade is a significant challenge in the performance of lithium-ion batteries, particularly those utilizing spinel structures. This phenomenon is primarily driven by chemical interactions at the electrode-electrolyte interface, especially during high-voltage charging and at the end of discharge. While techniques like ball milling have been explored to address this issue, they have not proven effective in fully mitigating capacity fade.

One of the crucial aspects contributing to capacity fade is the charging voltage exceeding 4.2 V versus lithium. Under these conditions, the electrolyte can generate hydrofluoric acid (HF) when water is present, which leads to detrimental reactions at the battery's surface. These reactions cause charge disproportionation, resulting in the dissolution of certain phases and leaving a less effective rock-salt phase on the electrode surface.

Research has indicated that reducing the particle surface area can help alleviate capacity fade, although it does not eliminate the issue entirely. Notably, studies conducted by Amatucci et al. suggest that the majority of capacity loss occurs towards the end of discharge. The buildup of tetragonal phases at this point can be particularly vulnerable to HF attacks, exacerbating the degradation of battery performance.

Interestingly, variations in the lattice structure of spinel materials have been found to influence their stability against capacity loss. For instance, spinels with cubic lattice parameters show improved stability compared to their tetragonal counterparts. The hypothesis is that a smaller lattice parameter may require more energy to create larger ions, thus suppressing the disproportionation reaction.

To combat capacity fade, researchers have explored various strategies, including coating particles with substances that can "capture" HF before it induces damage. For example, the application of zeolites as HF getters has shown promise, as has the coating of spinel particles with zinc oxide (ZnO). These efforts aim to enhance the overall performance of lithium-ion batteries, particularly under elevated temperatures and high-voltage conditions.

Moreover, cation substitution in spinels has been investigated to minimize irreversible capacity loss. Substituting elements like magnesium, cobalt, and nickel not only influences voltage stability but also enhances discharge capacities at elevated voltages. By continuing to refine these materials and their compositions, researchers are paving the way for more reliable and efficient lithium-ion battery technologies.