Evaluation of Fe Substitution Effect on the Electrochemical Properties of LiMnPO4 By Single Particle Measurement

Abstract

Introduction Lithium ion batteries (LIB) have been widely used as power sources for portable electronic devices such as laptop computers and cellular phones due to their high energy density. A battery research has been become more important issue for large scale applications such as EVs and HEVs. LiMnPO4 has been focused as a promising cathode material to realize high performance of LIB. So far, many approaches have been done to improve the electrochemical properties of LiMnPO4 by carbon coating, cation doping and so on. An electric and Li+ conductivities of LiMnPO4 are intrinsically low. The Mn substitution with Fe has been done as one of effective approaches to improve the electrochemical properties of LiMnPO4. However, its substitution effect has not been fully understood. The conventional evaluation method using composite electrodes has some difficulties due to porous structure of composite electrode. Therefore, we have conducted a single particle measurement using one active material particle (mainly secondary particle) or a composite particle obtained by taking out a part of composite electrode. We have reported to reduce an influence of the electrode structure on the electrochemical response by using single particle measurement (1). A phosphate-based cathode materials have an excellent thermal stability. These materials are promising active materials for large batteries. However, a slow electrochemical reaction of LiMnPO4 should be improved to obtain high rate capability. A Fe substitution for LiMnPO4 has improved its rate capability (2). In this study, we synthesized carbon-coated LiFexMn1-xPO4. A single particle measurement was employed to understand the intrinsic effect of Mn substitution with Fe on the electrochemical properties of LiMnPO4. Experimental Li2SO4, MnSO4•5H2O, FeSO4•7H2O, (NH4)2HPO4, and carboxymethyl cellulose (CMC) were used as starting materials to synthesized carbon coated LiFexMn1-xPO4 by hydrothermal method. The hydrothermal treatment was performed at 200 ºC for 3 h. The reaction product was dried and then grounded with planetary ball mill (400 rpm, 1 hour × 10 times) to obtain fine powder. The obtained powder was then heated at 700 ºC for 1 h under 97% Ar + 3% H2 atmosphere for a graphitization of CMC on particle surface to obtain LiFex Mn1-xPO4/C. A small composite particle used for single particle measurement was obtained by stripping from the composite electrode with a weight ratio of LiFexMn1-xPO4/C: AB: PVdF = 85: 10: 5 on aluminum current collector. Au micro-electrode with 10 mm diameter was attached to a LiFexMn1-xPO4/C composite electrode particle in an electrolyte (1 mol dm-3 LiPF6 / EC : EMC = 3 : 7 in vol.) using a micromanipulator under optical microscope observation. All the experiments were carried out under Ar atmosphere to remove the effect of water and oxygen. Results and discussion Figure 1 shows the XRD patterns of LiFexMn1-xPO4 synthesized by hydrothermal method. All peaks are attributable to olivine structure of LiFexMn1-xPO4, suggesting that the synthesized LiFexMn1-xPO4 samples are single phase without impurity. The Fe substitution in LiMnPO4was also confirmed from Rietveld analysis. Figure 2 shows the rate capabilities of LiFexMn1-xPO4/C composite electrode particles by single particle measurement. Discharge capacities at 1.5 C and 15 C were plotted against x in LiFexMn1-xPO4/C. In single particle measurement, in order to normalize a variation of particle size, the capacities were indicated by depth of discharge (DOD). The capacity retention at 15 C was improved with increasing amount of Fe substitution. A comparison of discharge curves for each composite particle, a potential plateau corresponding to Mn2+/3+ appeared at more noble potential with increasing of Fe substitution amount. The improvement of rate capability may be due to the improved electronic conductivity and suppressed Jahn-Teller deformation of Mn by Fe substitution. Reference [1] H. Munakata, K. Annaka, K. Kanamura, The 81th Electrochemical Society of Japan, p.343(2Q01). [2] Seung-Min Oh et al , J. Power Sources 196 (2011) 6924-6928 Figure 1