Enhancing the electrochemical kinetics of high voltage olivine LiMnPO4 by isovalent co-doping

Abstract

We report here doping of Fe²⁺ and/or Mg²⁺ in LiMnPO₄ cathode material to enhance its lithium storage performance and appraise the effect of doping. For this purpose, LiMn_0.9Fe_(0.1−x)Mg_xPO₄/C (x = 0 and 0.05) and LiMn_0.95Mg_0.05PO₄/C have been prepared by a ball mill assisted soft template method. These materials were prepared with similar morphology, particle size and carbon content. Amongst them, the isovalent co-doped LiMn_0.9Fe_0.05Mg_0.05PO₄/C sample shows better electrochemical performance compared to LiMn_0.9Fe_0.1PO₄/C and LiMn_0.95Mg_0.05PO₄/C samples. For instance, a lithium storage capacity of 159 mA h g⁻¹ is obtained at 0.1 C for LiMn_0.9Fe_0.05Mg_0.05PO₄/C material with a relatively low polarization of ∼139 mV. This is in sharp contrast to LiMn_0.9Fe_0.1PO₄/C and LiMn_0.95Mg_0.05PO₄/C which show only 136.8 and 128.4 mA h g⁻¹ at 0.1 C with the polarization of ∼222 and 334 mV respectively. Further, the LiMn_0.9Fe_0.05Mg_0.05PO₄/C electrode delivers discharge capacities of 155.8, 141.4, 118.8, 104.6, 81.4 and 51.8 mA h g⁻¹ at 0.2, 0.5, 1, 2, 5 and 10 C respectively. This electrode material also retains a capacity of 116 mA h g⁻¹ at 1 C after 200 cycles, which is 96% of its initial capacity. Such improved cycling stability of LiMn_0.9Fe_0.05Mg_0.05PO₄/C is attributed to the suppressed Mn dissolution in the electrolyte compared to the other samples. Further, during the Li extraction process, delithiated phases created from the Fe²⁺/Fe³⁺ redox reaction (∼3.45 V) favor enhanced electrochemical activity of the succeeding Mn²⁺/Mn³⁺ redox couples. The fully charged state (4.6 V) contains a partially lithiated phase owing to the presence of electrochemically inactive Mg²⁺. The presence of such lithiated phase provides a favourable environment for the subsequent lithium insertion process. We also observe improved electronic conductivity and Li-ion diffusion for the co-doped sample compared to LiMnPO₄ doped with either Fe²⁺ or Mg²⁺ by impedance measurements. The improved storage performance of co-doped LiMnPO₄ is thus explained in terms of (i) favorable extraction and insertion reactions and (ii) enhanced transport properties.