Today, lithium-ion battery technology is quite mature and dominates in the portable electronics market. However, for many applications, its long-term cycling and storage behaviour needs to be improved. Especially for the stationary energy storage, the lifetime is an important and critical issue which directly enters into cost calculations. Here we show that mechanical damage as a result of the electrochemical cycling can build up and lead to severe degradation. The material under investigation is LiMn1.95Al0.05O4 which fractures upon extended cycling. SEM images of the same locations are made before and after cycling to study crack formation and growth. Crack growth is observed both during lithiation and delithiation. Larger particles are more susceptible to cracking than smaller ones. The number of cracks increases with the number of cycles and the damage exhibits a characteristic shape. Both can be explained by coupled electrochemical-mechanical fatigue. This damage mechanism is inherent to the observed material and is quite likely also present in other materials. It can be suppressed by limiting the particle size in the electrodes.