Advantageous electrochemical behaviour of new core–shell structured cathodes over nickel-rich ones for lithium-ion batteries

Abstract

Currently, layered Ni-rich cathodes of LiNi_xMn_yCo_zO₂ (x ≥ 0.8) have gained significant attention for high energy density Li-ion batteries (LIBs) owing to their high specific capacity of ∼200 mA h g⁻¹ within a limited voltage range. However, the large-scale use of these cathodes is severely limited by their poor structural stability, high surface reactivity, and severe capacity fading resulting from intergranular microcracks triggered by large volume changes and formation of rock salt in the highly de-lithiated state. Considering the demand for high specific capacity and high cycling stability cathodes, a core–shell oxide material 0.8LiNi_0.85Mn_0.10Co_0.05O₂–0.2Li_1.2Ni_0.16Mn_0.56Co_0.08O₂ (NR-CS) with a Ni-rich oxide core of LiNi_0.85Mn_0.10Co_0.05O₂ (NMC85) and an outer shell of a Mn-based Li-rich Li_1.2Ni_0.16Mn_0.56Co_0.08O₂ oxide is synthesized. The NR-CS material delivered an initial discharge capacity of 212 mA h g⁻¹ when cycled at 20 mA g⁻¹ (0.1C rate) in the potential range of 2.8–4.3 V vs. Li with 89% capacity retention after 120 cycles when tested in half-cells using Li anodes. Additionally, we compared its performance with Ni-rich oxide NMC85 alone and with integrated materials of both Ni-rich and Mn-based Li-rich oxides (NR–LR). It was found that the Ni-rich oxide NMC85 cathodes could deliver a specific capacity of 216 mA h g⁻¹ with around 67% capacity retention, and those comprising the Li and Mn-rich NMC material possessed a specific capacity of 140 mA h g⁻¹ with 71.4% capacity retention when cycled under similar experimental conditions at 0.1C in the potential range of 2.8–4.3 V vs. Li after 120 cycles. Thus, the core–shell structure benefits from the high capacity of the core (Ni-rich NMC85) and surface chemical stability of the Mn-based Li-rich oxide. Moreover, when tested in full Li-ion cells with pre-lithiated graphite anodes, graphite‖NR-CS cells could provide a discharge capacity of about 170 mA h g⁻¹ and an energy density of 274 W h kg⁻¹ during 200 cycles when cycled at 0.5C rate. Thus, this study highlights the importance of the core–shell structure of Ni-rich NMC oxide cathodes over Ni-rich oxide or Li- and Mn-rich NMC cathodes in providing high capacity, higher rate and better cycling stability when charged up to 4.3 V in LIBs.