Insights about the irreversible capacity of LiNi0.5Mn1.5O4 cathode materials in lithium batteries

The accumulation of irreversible capacity in the first cycle and upon cycling has been studied for LiNi0.5Mn1.5O4-based cathodes (LNMO), bare and coated with ZnO. Materials have been synthesized at 800 ◦C and characterized by X-ray diffraction and transmission electron microscopy (TEM). The precipitation of a continuous ZnO film on the surface of LNMO has been highlighted by TEM. Galvanostatic cycling at room temperature and at 60 ◦C, linear sweep voltammetry (LSV), impedance spectroscopy and TEM techniques have been used to investigate the materials and the irreversible capacity accumulation upon cycling. Our study confirms that continuous parasitic processes occur upon cycling beyond the first charge/discharge. Anodic LSV test shows that side oxidation processes start on the surface of a LNMO electrode at potential slightly above the Ni2+/Ni4+ redox couple. At the end of charge an uniform and continuous thin film (3–4 nm) forms of on the bare LNMO. This film likely modifies upon cycling and it is apparently unable to passivate the LNMO surface preventing further decompositions. On the contrary the material coated with ZnO shows rough surfaces without large morphological alteration upon charge and cycling. The ZnO coating confirms its ability to mitigate the irreversible charge consumption.