Engineering commercial polycrystalline precursors to single crystal Ni-rich cathodes with outstanding long-cycle performance

Abstract

Single-crystal LiNi_1−x−yCo_xMn_yO₂ (NCM) is a promising cathode material featuring improved structural integrity, suppressed particle cracking and enhanced capacity retention during cycling. Herein, well-defined single crystal NCM cathode materials are prepared from commercial polycrystalline precursors through a multi-step molten salt method by crystal construction, crystal purification, and crystal restoration for the first time. Li₂SO₄ acts as a novel molten agent and provides an excellent growth environment for the crystals. The resulting single crystal NCM622 cathode shows an octahedral structure with a diameter of around 2 μm and displays the first specific discharge capacity as high as 168.4 mA h g⁻¹ (25 °C, 1.0C) together with excellent capacity retention (92.2% after 200 cycles). Notably, NCM622 delivers a high-temperature performance with a specific discharge capacity up to 178.9 mA h g⁻¹ (50 °C, 1.0C) and an outstanding capacity retention (72.2%, 1000 cycles), exceeding those of reported single-crystal NCM622 in terms of long-term cycle performance. Besides, the cathode demonstrates prominent stability with an intact crystal structure after 1000 cycles which is the same as the original one. The formation mechanism and the excellent cycling performance are well illustrated by the first principles density functional theory-generalized gradient approximation (DFT-GGA). This work provides an effective strategy for fabricating single-crystal NCM cathode materials from commercial precursors in an economical manner with extra-long-cycle performance and paves the way for large-scale production in the vehicle industry.