High-energy-density Li-ion batteries use positive electrodes made of layered transition metal (TM) oxides. To further improve their energy density, massive efforts have been made in developing either Li-rich or Ni-rich layered oxides. Li-rich means that TM ions are partially substituted by a substantial number of Li ions leading to an increased Li:TM molar ratio; Ni-rich means a higher Ni content — often accompanied by a lower Co content — in ternary metal oxides than that in the commercialized counterparts. While these advanced oxides offer high-energy prospects, they suffer from issues such as cycling instability in battery applications. Now, Jean-Marie Tarascon and colleagues across Europe and Australia design a class of electrodes that are both Li- and Ni-rich by placing additional Li and Mo ions into LiNiO2. The oxide materials they produce are also Co-free and offer encouraging cycling stability while maintaining high capacities.
The Li- and Ni-rich oxides are synthesized via a two-step process: a solution-based fabrication approach for obtaining atomically-homogeneous precursors, which is followed by high-temperature annealing. This synthesis route leads to unique structural features. Instead of solid solutions, the oxides (characterized as Li1+xNi(3–5x)/3Mo2x/3O2) exhibit two intergrown domains — one that is LiNiO2-rich and one that is Li4MoO5-rich. The intergrown microstructure restrains structural transformation and enhances the mechanical stability during cycling. Meanwhile, reversible Li intercalation takes place largely in the LiNiO2 domain, which gives rise to high energy efficiency. Tarascon and team anticipate that this class of Li- and Ni-rich oxides could be further expanded by considering high-valence TM ions other than Mo6+.
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