Doping is a well-known strategy to enhance the electrochemical energy storage performance of layered cathode materials. Many studies on various dopants have been reported; however, a general relationship between the dopants and their effect on the stability of the positive electrode upon prolonged cell cycling has yet to be established. Here, we explore the impact of the oxidation states of various dopants (i.e., Mg²⁺, Al³⁺, Ti⁴⁺, Ta⁵⁺, and Mo⁶⁺) on the electrochemical, morphological, and structural properties of a Ni-rich cathode material (i.e., Li[Ni_0.91Co_0.09]O₂). Galvanostatic cycling measurements in pouch-type full Li-ion cells show that cathodes featuring dopants with high oxidation states significantly outperform their undoped counterparts and the dopants with low oxidation states. In particular, Li-ion pouch cells with Ta⁵⁺- and Mo⁶⁺-doped Li[Ni_0.91Co_0.09]O₂ cathodes retain about 81.5% of their initial specific capacity after 3,000 cycles at 200 mAh g^-1. Furthermore, physicochemical measurements and analyses suggest substantial differences in the grain geometries and crystal lattice structures of the various cathode materials, which contribute to their widely

• Myung et al. ACS Energy Lett. 2017, 2, 196-223.
• Kim et al. Energy Environ. Sci. 2018, 11, 1271-1279.
• Kim et al. energy. 2020, 5, 860-869.
• Kim et al. ACS Energy Lett. 2017, 2, 1848-1854.