Transition metal-doped Ni-rich layered cathode materials for durable Li-ion batteries

H Hohyun Sun,Chong S Yoon,Un-Hyuck Kim,Sang-Wook Park,Jeong-Hyeon Park,Dong-Hwa Seo,Adam Heller,Yang-Kook Sun,C Buddie Mullins

doi:10.1038/s41467-021-26815-6

H Hohyun Sun, Chong S Yoon + Show 7 more

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https://doi.org/10.1038/s41467-021-26815-6

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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[Ni0.91Co0.09]O2). 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. In particular, Li-ion pouch cells with Ta5+- and Mo6+-doped Li[Ni0.91Co0.09]O2 cathodes retain about 81.5% of their initial specific capacity after 3000 cycles at 200 mA 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 different battery performances and correlate with the oxidation states of their dopants.

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Doping is a well-known strategy to enhance the electrochemical energy storage performance of layered cathode materials
We investigated the crystal structures of the cathode materials; their Rietveldrefined, X-ray diffraction (XRD) patterns and determined structural characteristics are presented in Supplementary Fig. 14 and Supplementary Table 3, respectively
In previous studies[25,31], we demonstrated that their high-angle annular dark-field (HAADF) images display a superlattice structure formed by the ordering of anti-site defects where Li+ ions alternately occupy the transition metal (TM) layer and vice versa

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Doping is a well-known strategy to enhance the electrochemical energy storage performance of layered cathode materials. 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 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 Ni4+ ions[16,17,18]. Engineering the geometry of cathode particle grains to dissipate strain buildup is vital to the cycling stability of Ni-rich layered cathodes 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. We investigate how the oxidation states of the dopants alter the geometry and crystal structure of the cathode grains to influence the cycling performance of LIBs, such that they can last the lifetime of an EV and subsequently be repurposed in energy storage systems (ESSs)

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