Bio‐Derived Surface Layer Suitable for Long Term Cycling Ni‐Rich Cathode for Lithium‐Ion Batteries

Abstract Since Ni‐rich cathode material is very sensitive to moisture and easily forms residual lithium compounds that degrade cell performance, it is very important to pay attention to the selection of the surface modifying media. Accordingly, hydroxyapatite (Ca 5 (PO 4 ) 3 (OH)), a tooth‐derived material showing excellent mechanical and thermodynamic stabilities, is selected. To verify the availability of hydroxyapatite as a surface protection material, lithium‐doped hydroxyapatite, Ca 4.67 Li 0.33 (PO 4 ) 3 (OH), is formed with ≈10‐nm layer after reacting with residual lithium compounds on Li[Ni 0.8 Co 0.15 Al 0.05 ]O 2 , which spontaneously results in dramatic reduction of surface lithium residues to 2879 ppm from 22364 ppm. The Ca 4.67 Li 0.33 (PO 4 ) 3 (OH)‐modified Li[Ni 0.8 Co 0.15 Al 0.05 ]O 2 electrode provides ultra‐long term cycling stability, enabling 1000 cycles retaining 66.3% of its initial capacity. Also, morphological degradations such as micro‐cracking or amorphization of surface are significantly suppressed by the presence of Ca 4.67 Li 0.33 (PO 4 ) 3 (OH) layer on the Li[Ni 0.8 Co 0.15 Al 0.05 ]O 2 , of which the Ca 4.67 Li 0.33 (PO 4 ) 3 (OH) is transformed to CaF 2 via Ca 4.67 Li 0.33 (PO 4 ) 3 F during the long term cycles reacting with HF in electrolyte. In addition, the authors’ density function theory (DFT) results explain the reason of instability of NCA and why CaF 2 layers can delay the micro‐cracking during electrochemical reaction. Therefore, the stable Ca 4.67 Li 0.33 (PO 4 ) 3 F and CaF 2 layers play a pivotal role to protect the Li[Ni 0.8 Co 0.15 Al 0.05 ]O 2 with ultra‐long cycling stability.