High-rate performance of a mixed olivine cathode with off-stoichiometric composition - C5CC04434K

Superior operating safety with long cycle life and lowmaterial cost makes lithium iron phosphate (LiFePO4) an important Li storage material. For this olivine compound, many efforts have been expended in order to achieve desirable electrochemical properties such as particle nanosizing and applying electrically conductive coating. However, these processes reduce tap density and therefore lower practical energy density, making the material lose much of its appeal toward commercialization as compared to current oxide-based cathodes. Thus, enhancing the energy density of nanosized and coated LiFePO4 is an important problem for battery scientists and engineers. Higher theoretical energy density for LiFePO4 can be achieved by mixing Mn with Fe, taking advantage of the Mn redox potential at 4.1 V over Fe at 3.4 V. It is also reported that Mn substitution can alter the delithiation mechanism from phase separation to a solid solution reaction. Compositions with large Mn content, however, tend to lack reasonable rate performance. In this communication, we present a simple and efficient method to enable high rate capability of the mixed olivine cathode, LiFe0.6Mn0.4PO4, by controlling off-stoichiometry to create an electrically conductive glassy coating. This concept is previously established in LiFePO4, 22 and the effectiveness to achieve high power density has also been demonstrated in other cathode materials. The molar ratio of the off-stoichiometric composition is 1 : 0.9 : 0.95 for Li : (Fe0.6 + Mn0.4) : P, as optimized previously, so that the nominal composition becomes LiFe0.54Mn0.36P0.95O4ˇd. The experimental details of synthesis, characterization, and electrochemistry are summarized in ESI.† Fig. 1a shows the X-ray diffraction (XRD) patterns of the as-synthesized samples with nominal compositions of LiFe0.54Mn0.36P0.95O4ˇd and LiFe0.6Mn0.4PO4. The peak positions and intensity ratios of LiFe0.54Mn0.36P0.95O4ˇd are indistinguishable from those of LiFe0.6Mn0.4PO4, suggesting that the crystalline olivine phase in both samples is the same with the offstoichiometry accommodated as an additional phase. Lattice parameters of LiFe0.54Mn0.36P0.95O4ˇd (a = 10.3648 Å, b = 6.0400 Å, and c = 4.7122 Å) calculated from Rietveld refinement using Pnma space group in Fig. 1b also match those of LiFe0.6Mn0.4PO4 (a = 10.3672 Å, b = 6.0407 Å, and c = 4.7138 Å) obtained in this study. The lattice parameters and Rietveld refinement details are summarized in ESI,† Table S1. Similar full width at half maximum for LiFe0.54Mn0.36P0.95O4ˇd and LiFe0.6Mn0.4PO4 shown in the inset of Fig. 1a implies a similar particle size for both compounds. Indeed, the particle size distribution of LiFe0.54Mn0.36P0.95O4ˇd observed by scanning electron microscopy (SEM) in Fig. 1c is similar to that of LiFe0.6Mn0.4PO4 in Fig. 1d with the average particle size being approximately 40 nm. Note that in both compounds, some particles form secondary agglomerates with the size ranging between 200 and 500 nm. Fig. 1e and f show high resolution transmission electron microscopy (HRTEM) images obtained from LiFe0.54Mn0.36P0.95O4ˇd and LiFe0.6Mn0.4PO4 particles, respectively. Clearly observable lattice fringes indicate wellcrystallized olivine phases in both particles. However, the surface morphology noticeably differs from each other: the off-stoichiometric particle is covered with a non-crystalline layer (average 4.5 nm) whereas the surface of the stoichiometric particle is crystalline, as similarly observed in off-stoichiometric LiFe0.9P0.95O4ˇd and LiMn0.9P0.95O4ˇd. 22–24 The formation of these amorphous films with self-limiting thickness has been discussed in detail in ref. 24. In order to analyze the composition of the non-crystalline surface phase, we performed scanning transmission electron microscopy (STEM) electron energy loss spectroscopy (EELS) a Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA b Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Republic of Korea c Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, CA, USA. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/c5cc04434k Received 30th May 2015, Accepted 13th July 2015