A Thin-Layer Carbon Nanosphere Coating on Ni-Rich NMC811 Core-Shell Cathode Via Solvent-Free Mechanofusion Process: In Operando Investigation and Highly Stable 18650 Li-Ion Battery

Abstract

In the past decades, rechargeable Li-ion batteries (LIB) are of interest and become the most essential device for the storage of electric energy in many applications, especially for electric vehicles (EVs). To date, only Ni-rich cathode materials, especially the NMC811, could enhance the energy density of the LIB up to 250 Wh kg-1 suitable for long-range EVs. However, the practical application of the NMC811 has not yet been fully developed because of many issues, i.e. (i) the formation of high-valence state Ni4+ causes several parasitic reactions with continuous consumption of electrolyte, (ii) the formation of microcrack during the lattice changes leading to fast capacity decay, (iii) Li+/Ni2+ cation mixing causing the formation of inactive phase on the surface, and (iv) poor electron and ionic conductivities of the materials limiting its use at high rates. To solve these problems, the surface coating approach is utilized in which a thin layer of carbon nanosphere with a thickness of ca. 50-80 nm was coated on the surface of Ni-rich NMC811 cathode using a solvent-free mechanofusion process. It was found that carbon nanoparticles were mechanically fused into the NMC811 altering the surface property of the material. The improved performance was fundamentally investigated by several techniques, i.e. GITT, in operando XRD, and in situ DEMS. The NMC811cs exhibits faster Li-ion diffusivity, higher lattice parameter changes, and less gas formation at the high charging potential as expected from the uniformly coated carbon shell. In addition, to demonstrate the practical feasibility of this process, 18650 Li-ion batteries of NMC811cs cathode coupled with graphite anode were also fabricate in a dry room compared to the conventional NMC811 cell. The NMC811cs 18650 cell with a capacity of 2.5 Ah per cell exhibits excellent cycling retention after tested at a high rate of 1.0 C. This work provides a novel and economical strategy for highly stable practical 18650 Li-ion battery.