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

摘要

We report here doping of Fe~(2+) and/or Mg~(2+) in LiMnPO4 cathode material to enhance its lithium storage performance and appraise the effect of doping. For this purpose, LiMn_(0.9)Fe_(0.1-x)Mg_XPO4/C (x = 0 and 0.05) and LiMn_(0.9)Mg_(0.05)PO4/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_(0.9)Fe_(0.05)Mg_(0.05)PO4/C sample shows better electrochemical performance compared to LiMn_(0.9)Fe_(0.1)PO4/C and LiMn_(0.95)Mg_(0.05)PO4/C samples. For instance, a lithium storage capacity of 159 mA h g~(-1) is obtained at 0.1 C for LiMn_(0.9)Fe_(0.05)Mg__(0.05)PO4/C material with a relatively low polarization of ~139 mV. This is in sharp contrast to LiMn_(0.9)Fe_(0.1)PO4/C and LiMn_(0.9)Fe_(0.05)PO4/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_(0.9)Fe_(0.05)Mg__(0.05)PO4/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_(0.9)Fe_(0.05)Mg__(0.05)PO4/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 Fe~(2+)/Fe~(3+) redox reaction (~3.45 V) favor enhanced electrochemical activity of the succeeding Mn~(2+)/Mn~(3+) redox couples. The fully charged state (4.6 V) contains a partially lithiated phase owing to the presence of electrochemically inactive Mg~(2+). 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 Fe~(2+) or Mg~(2+) 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.