The Effect of Al and Fe Substitution on the Structure and the Electrochemical Properties of NMC Type Positive Electrode Materials for Lithium Ion Batteries

Positive electrode materials combining the advantages of nickel, manganese and cobalt such as higher reversible capacity, lower cost, and less toxicity [1–4] are currently in the focus of research for lithium ion batteries. The relevant electrochemical performance of NMC type cathode materials (LiNi_xMn_yCo_1−x−yO₂) has been widely discussed in literature. Al, Mg and Cr seem to be promising candidates for the partial substitution of Co in these materials [5-9] in order to further improve the electrochemical performance by decreasing the cation mixing in the structure.

The current study discusses three NMC type materials obtained by cationic substitution of cobalt with aluminum and/or iron in the starting material LiNi_0.6Mn_0.2Co_0.2O₂. The three compounds with the following compositions LiNi_0.6Mn_0.2Co_0.15Al_0.5O₂ (NMCA), LiNi_0.6Mn_0.2Co_0.15Fe_0.5O₂ (NMCF) and LiNi_0.6Mn_0.2Co_0.15Al_0.025Fe_0.025O₂ (NMCAF) were synthesized by the self-combustion method using sucrose as fuel. These materials belong to α-NaFeO₂-type structure (space group R-3m) with hexagonal ordering. Rietveld refinement analysis of the XRD patterns revealed a very low cationic mixing for the double substituted material NMCAF (5%) compared to the non-substituted material (13%) suggesting a higher structural stabilization for the double substituted compound. Galvanostatic cycling measurements indicate improved electrochemical performance, good cycling stability and higher reversible capacity preferentially for NMCAF (190 mAh.g^-1), NMCF (167 mAh.g^-1), NMCA (145 mAh.g^-1) then NMC (140 mAh.g^-1).

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