Publication date: 10th April 2024
Experimental studies have revealed a kinetic plateau at 3.5 V in the composition-voltage curve of LiNiO2 during slow charge/discharge cycling, such as C/20. However, this plateau and its associated capacity diminish at higher rates. Interestingly, even slight temperature variations have a significant impact on this kinetic plateau: cycling LiNiO2 at 45 °C instead of 25 °C results in the recovery of a substantial capacity, primarily at 3.5 V. Additionally, slow discharge at low voltage (a potentiostatic process at 2 V for 1 month) can also restore the lost capacity.
Over the years, various explanations have been proposed to account for the hindered kinetics plateau and the corresponding pseudo-irreversible capacity. In our study, we demonstrate that the presence of excess Ni in LiNiO2 significantly slows down the material's kinetics. This effect can be attributed to two critical mechanisms.
Firstly, the excess Ni in the Li layer exerts an attractive force on Li vacancies, reducing their energy compared to defect-free regions and effectively transforming excess Ni into a sink for lithium vacancies. This attractive force arises from the relatively positive charge of Ni and the strain it induces in the material. It extends across a radius of two lattice sites, substantially reducing the barrier for Li vacancies to approach the defect. Similar arguments apply to divacancies, which are predominantly pinned to excess Ni in the form of split divacancies. At low temperatures, trapping leads to lower diffusion coefficients compared to values extrapolated from higher temperatures. This finding expands upon Delmas' popular hypothesis, which has been the prevailing explanation for the difficulty in reintercalating Li after the first cycle, but challenges the notion of excess Ni oxidation or any local lattice shrinkage of the Li layers surrounding the defect.
Secondly, the presence of excess Ni hinders Li migration due to its inherent immobility and inability to undergo site exchange with adjacent Li vacancies. These findings highlight the crucial role of excess Ni in influencing material kinetics by simultaneously reducing the number of charge carriers and increasing the tortuosity of diffusion paths.
This project was funded by BASF SE and the German Federal Ministry of Education and Research (BMBF) under the funding numbers 03XP0484A and 03XP0484C. Computing time provided at the NHR Centers NHR4CES at TU Darmstadt (project p0020076) is gratefully acknowledged. This was funded by the German Federal Ministry of Education and Research and the state governments participating on the basis of the resolutions of the GWK for national high-performance computing at universities (www.nhr-verein.de/unsere-partner).
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