The transformation of graphite electrode materials in lithium-ion batteries after cycling

• The negative electrodes of cycled LFP/graphite cells are investigated.
• The PVDF binder at the graphite anode side does not degrade during cycling.
• The volume of graphite particles increase by 30% after 800 cycling at 1C-rate.
• The d-spacings of the graphite particles do not change after cycling.
• The particle size increase is due to the void formation within graphite particles.

To reveal how graphite electrodes change with cycling in lithium-ion batteries, electrochemical experiments involving charge–discharge cycling at different current density conditions are performed on commercial pouch cells and the graphite electrodes after cycling. This research shows that the polyvinylidene fluoride (PVDF) binder does not degrade, confirmed by the stable molecular weight after different cycling conditions. Particle size analysis results indicate that the diameter of graphite particles after cycling is ∼10% larger than that of the graphite before cycling, which results in a ∼30% volume expansion after cycling. For the cells cycled at the same current density, the graphite particle size increases with cycle time. For the cells with the same cycle numbers, the graphite particle size is larger in the cells cycled at lower current density. X-ray diffraction characterization shows that the d-spacings of the graphite particles in all the cells at different cycling conditions are identical. These results suggest that the graphite particle size increase associated with cycling may arise specifically when primary particles inflate the secondary particle size, leading to dramatic volume expansion of graphite secondary particles.