Ni-rich cathodes can almost be cons

Ni-rich cathodes can almost be considered as another class of layered cathode material. While Ni-rich compounds were theoretically denser in energy and cheaper (lower Co content), they were usually problematic to implement due to safety and cycle stability concerns. The relationship of Ni content on thermal stability and oxygen gas generation is shown in Figure 10a. Following the convention: NMC 433 (4-Ni:3-Mn:3Co molar ratio), the ratio with the least amount of Ni exhibited the least amount of gas generation and at a higher decomposition initiation temperature. Every subsequent increase in Ni content noticeably decreased the onset temperature for phase change from the original layered to spinel structure and rock salt which sped up the overall degradation phase transformation process.[174,175] Additionally, the higher Ni content and reduced Co content promoted the irreversible migration of Ni2+ to Li+ sites[176] which severely hindered Li+ transfer throughout the structure of the NMC particle and increased the overall cell impedance.[177] Though the exact mechanism that caused these detrimental effects were not fully understood, it was clear that the combining effect of phase transition and impedance increase resulted in poorer cycle performance at higher Ni contents. A comprehensive relationship between Ni content, specific capacity, cycle stability and thermal stability is shown in Figure 10b. Very recently, based on density functional theory, researchers concluded that more Ni4+ are present in Ni-rich materials which readily oxidizes the electrolyte and oxygen ions due to its relatively low LUMO which promoted oxygen generation and electrolyte decomposition.[178]