Surface Modification And Electrochemical Properties Of Gradient LiNi_0.73Co_0.12Mn_0.15O₂ Cathode Material For Lithium Ion Batteries

Rechargeable lithium ion batteries（LIBs） have attracted a great deal of attention as power sources for portable electronic devices, electric vehicles（EVs） or hybrid electric vehicles（HEVs）, due to their high energy density, high power density and long cycling life. Nickel-rich Li[Nix Coy Mn1-x-y]O₂（x≥0.6） layered oxides are among the most promising cathode materials because of their relatively low cost, low toxicity, high rate capability and excellent reversible capacity, especially for EV and HEV applications. However, highly delithiated nickel-rich Li[Ni_xCo_yMn_1-x-y]O₂ materials become unstable due to the appearance of abundant Ni4+ ions during charge/discharge, which react with organic electrolyte, increasing the impedance and lowering the cyclability of LIBs. Currently, to overcome these major drawbacks of the nickel-rich cathode materials for EVs and HEVs applications which are most notably poor cycle performance and security, the researchers developed a novel concept of concentration gradient core-shell Li[Ni_xCo_yMn_1-x-y]O₂ materials. Nevertheless, the surface of the concentration gradient material is still conventional Li[Ni_xCo_yMn_1-x-y]O₂ material, and the unstable Ni⁴⁺ in the charging process is present in the outermost surface of the material, which reacts easily with the electrolyte to release oxygen species and simultaneously forms the more stable NiO-like phase（cubic rock structure）.In order to enhance the electrochemical performance of the high capacity layered oxide cathode with a Ni-rich core and a concentration-gradient shell（NRC-CGS）, we use a freeze drying method to coat Al₂O₃ layer onto the surface of NRC-CGS Li[Ni_0.73Co_0.12Mn_0.15]O₂ material. It is revealed that an Al₂O₃ layer of about 5 nm in thickness is uniformly formed on the surface of NRC-CGS Li[Ni_0.73Co_0.12Mn_0.15]O₂ material by the freeze drying procedure. The freeze drying Al₂O₃-coated（FD-Al₂O₃-coated） sample demonstrates similar discharge capacity and significantly enhanced cycling performances, in comparison to the pristine and conventional heating drying Al₂O₃-coated（HD-Al₂O₃-coated） samples. The capacity decay rate of FD-Al₂O₃-coated Li[Ni_0.73Co_0.12Mn_0.15]O₂ material is 1.7 % after 150 cycles at 55 °C, which is 9 and 12 times lower than that of the pristine and HD-Al₂O₃-coated samples.The role of the fluorine modification on the electrochemical performance of the nickel-rich concentration gradient lithium transition-metal oxide Li[Ni_0.73Co_0.12Mn_0.15]O₂ has been investigated as well. It is found that the fluorine surface modification induces a partial phase transformation from a layered structure to a cubic rock structure（Ni O-like phase） on the surface region. Meanwhile, the lithium residues on the surface of the pristine material are remarkably reduced and transformed into fluorides after the fluorine modification. Novel fluorine modified cathode material exhibits not only improved cycling performance at ambient temperature, but also a stable layer at an elevated temperature of 55 oC, providing an interface protection and a stable structure for cathode material of lithium ion batteries. The pristine material showed a gradual decrease in capacity, leading to a capacity retention of 87.4 % at ambient temperature and 77.8 % at 55 oC after 200 cycles. However, the F-modified material has remarkably enhanced capacity retention of 97.5 % at ambient temperature and 92 % at 55 oC after same cycling periods. The superior electrochemical stability of the F modified Li[Ni_0.73Co_0.12Mn_0.15]O₂-x Fx samples is attributed to the synergistic protection of the Ni O-like phase and the surface fluoride layer, which can effectively restrain the side reactions between the active material and electrolyte.A facile high-temperature solid phase approach is employed to coat lithium phosphorus oxynitride（Li PON） layer on the concentration gradient Li[Ni_0.73Co_0.12Mn_0.15]O₂ material with Ni-rich core and Ni-poor surface. X-ray diffraction, scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy demonstrate that LiPON forms an even coating with ⁴ nm of thickness on the surface without destroying the bulk structure of Li[Ni_0.73Co_0.12Mn_0.15]O₂ material. The Li PON-coated concentration gradient Li[Ni_0.73Co_0.12Mn_0.15]O₂ material exhibits a capacity retention of 87.3 % at 25 oC after 1000 cycles and 89.9 % at 55 oC after 250 cycles, which is 36.1 % and 17.2 % higher than that of pristine Li[Ni_0.73Co_0.12Mn_0.15]O₂ material, respectively, under the same operating conditions. This significantly improved cycling performance is ascribed to the noncrystalline Li PON layer on the surface, which provides a reliable interface protection. The LiPON coating is an effective approach to enhancing the electrochemical performance of electrode materials for lithium ion batteries.