Passivation of Spinel Cathode Surface through Self-Segregation of Iron
The Manthiram Lab has developed an iron-doped, high-voltage cathode based on lithium nickel manganese oxide that results in extended cyclability and improved electrochemical performance. The cathode’s spinel structure enables 3-D Li+-ion diffusion and direct metal-metal interaction across the shared octahedral edges, which supports high power capability for HEV and PHEV applications at high operating voltages (> 4.3 V vs. Li/Li+). The iron-enrichment on the cathode surface prevents surface-electrolyte instability at these high operating voltages.
A LiMn1.5Ni0.42Fe 0.08O4 cathode containing a small amount of iron exhibits 136 mAh/g capacity with 100% capacity retention through 100 cycles, whereas the unsubstituted cathode (LiMn1.5Ni0.5O4) has a ~12% reduction in capacity after 50 cycles (Figure 1). Furthermore, the Fe-substituted cathodes exhibit superior rate capability and lower surface resistance (Rs) and charge transfer resistance (Rct) compared to the unsubstituted cathode.
X-ray photoelectron spectroscopy of the Fe-substituted cathodes reveals that the surface has a higher concentration of Fe and a lower concentration of Ni compared to the bulk. The iron presumably segregates to the surface during synthesis due to the preference of trivalent iron for lower coordination numbers (tetrahedral coordination), as the surface contains atoms bonded to fewer nearest neighbors than the bulk. The resultant Fe-rich layer passivates the surface, thereby preventing metal (e.g., manganese) dissolution and electrolyte decomposition at high voltage. Therefore, iron doping prevents the formation of a thick solid electrolyte interphase (SEI) layer on the cathode. The suppression of SEI formation results in lower Rs and Rct and superior performance.
Researchers often coat the cathode with other inert materials like Al2O3 or AlPO4 to suppress SEI formation. However, such chemical coating adds processing cost, and it may be difficult to obtain a uniform, robust coating. The surface self-segregation of iron, on the other hand, does not require additional processing as the segregation occurs during the regular solid-state synthesis process. The self-segregation can also provide a uniform, robust coating on the surface.
The Manthiram Lab is exploring surface self-segregation with other dopants such as Ga and Cr to establish a firm scientific understanding of the process. They have found that a common key contributor to the enhanced electrochemical performance of doped spinels is the reduction of Mn4+ to Mn3+. The presence of Mn3+ along with the dopant cations stabilizes the spinel structure with cation disorder in the octahedral sites. This disordered spinel has a higher electronic conductivity than its ordered counterpart.
This work is accompanied by advanced characterization methodologies such as X-ray diffraction, Fourier transform-infrared spectroscopy, and high-resolution transmission electron microscopy. The goal is to identify the best dopants for providing good electrochemical performance at high operating voltages.