Review on electrode-electrolyte solution interactions, related to cathode materials for Li-ion batteries

In this paper we review some critical aspects related to interactions between cathode materials and electrolyte solutions in lithium-ion batteries. Previous results are briefly summarized, together with the presentation of new results. This review deals with the basic anodic stability of commonly-used electrolyte solutions for Li-ion batteries (mostly based on alkyl carbonate solvents). We discuss herein the surface chemistry of the following cathode materials: LiCoO₂, V₂O₅, LiMn₂O₄, LiMn_1.5Ni_0.5O₄, LiMn_0.5Ni_0.5O₂, and LiFePO₄. The methods applied included solution studies by ICP, Raman, X-ray photoelectron and FTIR spectroscopies, and electron microscopy, all in conjunction with electrochemical techniques. General phenomena are the possible dissolution of transition metal ions from these materials, which leads to changes in the active mass and a retardation in the electrode kinetics due to the formation of blocking surface films. These phenomena are significant mostly at elevated temperatures and in electrolyte solutions containing acidic species. Water-contaminated LiPF₆ solutions can reach a high concentration of acidic species (e.g., HF), which is detrimental to the performance of materials such as LiCoO₂ and LiFePO₄. Both LiMn_1.5Ni_0.5O₄ and LiMn_0.5Ni_0.5O₂, even when used as nanomaterials, show a high stability in commonly-used electrolyte solutions at high temperatures. This stability is attributed to unique surface chemistry that is correlated to the presence of Ni ions in the lattice.