By reshaping the primary particles of the cathode into radially aligned rod- or needle-like grains, the strain during cycling can be homogeneously distributed within the cathode particle to inhibit intergranular cracking and subsequent electrolyte penetration. However, recent studies on B-, W-, Ta-, and Sb-doped cathodes have offered promising countermeasures against degradation 23, 24, 25, 26. In this regard, the conventional stabilisation strategy of doping the crystal structure fails to adequately address the problematic origins of the degradation mechanism as these dopants do not alter the random orientation of the equiaxed grains 19, 20, 21, 22 they merely delay the onset of decay. Thus, engineering the geometry of cathode particle grains to dissipate strain build-up is vital to the cycling stability of Ni-rich layered cathodes. The microcracks originate from randomly oriented grains in cathode particles, which non-uniformly distribute the anisotropic internal strain caused by the abrupt collapse of the layered structure during phase transitions, particularly in highly delithiated states. These instabilities are caused by the formation of intergranular microcracks, which serve as channels for deleterious electrolyte infiltration of the cathode particle interior and expedite the degradation of interior primary particles by reacting with the formed unstable Ni 4+ ions 16, 17, 18. However, realising further improvements with this approach is challenging, as NCM and NCA cathodes with Ni contents exceeding 80% exhibit behaviours similar to those observed in LNO that undermine cycling and thermal stabilities. Tremendous efforts have been devoted to this approach, and much progress has been made in successfully stabilising these cathodes, with Ni contents of up to 80%, for practical applications 14, 15. Research on NCM and NCA layered cathodes has mainly focused on increasing the Ni content and, concomitantly, decreasing the Co content for the triple benefit of lower cost, higher specific capacity, and higher voltage, while maintaining cycling stability. LiO 2 (NCA) and LiO 2 (NCM) layered cathode materials were obtained from these efforts, and they are currently the two most widely used layered cathode materials in commercial batteries 13. Therefore, mixtures of LCO and LNO, supplemented with a wide variety of tertiary elements (LiO 2 (M = doping element)), have been explored to realise cathodes with adequate capacity and cycling performance 10, 11, 12. Metallic dopants (such as Al, Ga, Mn, Mg, and Ti) were added to LNO for stability however, they could not overcome the loss in capacity 5, 6, 7, 8, 9. Although LNO delivered much higher capacity at lower cost, it was unsuitable for commercial application owing to its inferior cycling and thermal stabilities, which were ascribed to the heightened surface chemical reactivity of Ni 3+/4+ and crystal structure destabilisation resulting from anisotropic internal strain caused by phase transitions in the deeply charged state 2, 3, 4. This material showed adequate electrochemical performance however, due to its high cost, toxicity, and mediocre capacity, LiNiO 2 (LNO) was suggested as an alternative. The first transition metal (TM) oxide to be applied as a LIB cathode was LiCoO 2 (LCO) 1. Among the various LIB components, the cathode is the most expensive and heaviest, and thus, it considerably influences the cost as well as the overall performance of a LIB hence, the development of cathodes is critical to the success of LIBs. Lithium-ion batteries (LIBs) have attracted significant attention as power sources for contemporary electric vehicles (EVs). 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 different battery performances and correlate with the oxidation states of their dopants. In particular, Li-ion pouch cells with Ta 5+- and Mo 6+-doped LiO 2 cathodes retain about 81.5% of their initial specific capacity after 3000 cycles at 200 mA g −1. Galvanostatic cycling measurements in pouch-type Li-ion full cells show that cathodes featuring dopants with high oxidation states significantly outperform their undoped counterparts and the dopants with low oxidation states. Here, we explore the impact of the oxidation states of various dopants (i.e., Mg 2+, Al 3+, Ti 4+, Ta 5+, and Mo 6+) on the electrochemical, morphological, and structural properties of a Ni-rich cathode material (i.e., LiO 2). 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. Doping is a well-known strategy to enhance the electrochemical energy storage performance of layered cathode materials.
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