Managing Life Span of High-Energy LiNi0.88Co0.11Al0.01O2|C–Si Li-Ion Batteries

The life span of high-energy cells (3.5 Ah, 18 650, LiNi0.88Co0.11Al0.01O2 (NCA)|C/Si, cell type A) is investigated as a function of depth of discharges (DoD, between 20 and 100%) and cycling rates (between 1C and C/5). The most relevant degradation mechanism for this cell type is the cycling-induced fracturing of active material. This mechanical degradation of the anode is particularly damaging for the cell life span because it generates chain reactions, i.e., solid electrolyte interphase (SEI) formation. The impedance analysis indicates that electrolyte shortage occurs at the end of life (when the capacity loss exceeds 20%) of all cells, regardless of their cycling protocols. It is revealed that electrochemical activation of the Li0.75Si phase at around 3.0 V causes enormous mechanical stress. Therefore, all of the cells discharged down to 2.65 V show poor lifetime, regardless of their cycling rates and DoDs. The lifetime could be significantly prolonged by cycling the cells above 3.1 V. The scanning electron microscopy (SEM)–energy-dispersive spectrometry (EDX) reveals that some graphite particles are coated by the dense agglomeration of Si particles. The large volume changes of Si might also induce mechanical stress onto the topmost layer of graphite particles underneath the Si coatings, in addition to the mechanical degradation of the Si particle itself.