There and back again ‐ The journey of LiNiO₂ as cathode active material

LiNiO₂ (LNO) has been introduced as cathode active material (CAM) for Li‐ion batteries in 1990. After years of intensive research, it emerged that several instability issues plague the material, so that it was abandoned in favor of isostructural metal‐substituted compounds called NCA (for lithium nickel cobalt aluminum oxide) and NCM (lithium nickel cobalt manganese oxide). These sacrifice a certain amount of specific energy in exchange for stability, durability and safety. With few exceptions, NCA and NCM are nowadays the industrial standard when it comes to automotive applications; however, the continuous push towards electric cars with longer driving range is synonym, for these compounds, with increasing the nickel content (which is already beyond 80%), eventually leading back again to LiNiO₂. For this reason we provide here a comprehensive review of the material, almost 30 years after its introduction as CAM. We aim at highlighting its physicochemical peculiarities, which make LNO complex in every aspect. We specifically stress the effect of the Li off‐stoichiometry (Li1‐zNi1+zO2) on every property of LNO, especially the electrochemical ones. We then focus on the key instability issues that plague the compound and on the strategies implemented so far to overcome them. Finally, in the course of the review we point to open questions that still remain to be addressed by the scientific community, and to which research directions seem more promising to lead LNO to its full exploitation.

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