Advantageous surface engineering to boost single-crystal quaternary cathodes for high-energy-density lithium-ion batteries

Single-crystalline Ni-rich cathode active materials (CAMs) are considered as promising candidates for high-energy-density lithium-ion batteries (LIBs) with favorable cycling stability and safety, due to their grain boundaryless characteristics efficiently alleviate the structural degradation of intergranular microcracks in poly-crystalline counterparts. However, their practical application not only suffers from sluggish Li diffusion kinetics, surface reconstruction and parasitic cathode/electrolyte interfacial reactions upon repeated cycling but also encounters chemical instability during storage and slurry processes. Herein, we constructed a uniform LiAlO₂/Li₃PO₄ protective layer with gradient Al doping (LAP modification) on the surface of single-crystalline LiNi_0.90Co_0.05Mn_0.04Al_0.01O₂ (SC[sbnd]NCMA) CAMs through an in situ modification process to relieve these intrinsic instability issues. This advantageous surface engineering significantly reduces Li⁺/Ni²⁺ mixing, inhibits parasitic side reactions and surface phase transformation, and notably improves Li⁺ diffusion kinetics. Therefore, LAP-modified SC[sbnd]NCMA exhibits superior cycling performance with a capacity retention of 74.4% at a high voltage of 4.5 V after 200 cycles at 1C compared to that of SC[sbnd]NCMA. Moreover, the enhancement of air storage properties after modification was further confirmed by the reduced surface residual lithium, improved rheological properties and well-maintained electrochemical performance. This work provides an effective strategy for the modification of single-crystal Ni-rich cathodes and further accelerates their practical application.