Dual functioned BiOF-coated Li[Li0.1Al0.05Mn1.85]O4 for lithium batteries

Abstract

Spinel-type lithium manganese oxides have shown considerable promise for the positive electrode in lithium batteries but have suffered from poor performance at high temperatures (40–60 °C). The electrochemical properties of BiOF-coated spinel Li[Li_0.1Al_0.05Mn_1.85]O₄ at elevated temperatures (55 °C) were investigated. BiOF nanoparticles were well coated on spinel Li[Li_0.1Al_0.05Mn_1.85]O₄, as confirmed by scanning and transmission electron microscopy. The BiOF-coated spinel Li[Li_0.1Al_0.05Mn_1.85]O₄ electrode had excellent capacity retention at 55 °C, maintaining its initial discharge capacity of 96.1% after 100 cycles. This improved cycling performance was ascribed to the presence of the BiOF layer on the spinel particles, which prevented the infiltration of HF generated by the decomposition of electrolytic salt, LiPF₆, in the electrolyte. This property resulted in a considerable reduction of manganese dissolution of the BiOF-coated spinel Li[Li_0.1Al_0.05Mn_1.85]O₄. Transmission electron microscopy of extensively cycled particles confirmed that the particle surface of the BiOF-coated Li[Li_0.1Al_0.05Mn_1.85]O₄ remained almost completely undamaged, whereas pristine spinel particles were severely deteriorated. In the present paper, we have tried to understand the role of the BiOF coating layer and why such coatings resulted in better battery performances, using BiOF-coated Li[Li_0.1Al_0.05Mn_1.85]O₄. The BiOF coating layer on the surface of Li[Li_0.1Al_0.05Mn_1.85]O₄ particles can be defined as follows; the oxyfluoride layer provides a strong protecting layer against HF attack and scavenges HF, thus resulting in reducing HF from the electrolyte. Synergistically, these actions resulted in the significantly improved high temperature cycling performances exhibited by the BiOF-coated Li[Li_0.1Al_0.05Mn_1.85]O₄.