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 LiNi0.90Co0.05Mn0.04Al0.01O2 (SCNCMA) 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 SCNCMA 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 SCNCMA. 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.