Abstract
For improving the electrochemical performance and thermal stability, magnesium was chosen as the doping element in Li(NiCoMn)02 cathode materials. LiNi 0.6-x Mg x Co 0.25 Mn 0.15 O 2 (x = 0 and 0.03) were successfully synthesized via the mixing hydroxide method. These materials exhibited α-NaFeO 2 structure as indicated by the XRD patterns. The intensity ratio of (0 0 3) to ( 1 0 4) showed that the Mg substitution could reduce the cation mixing. The pristine material exhibited the initial discharge of capacity 199 mAh g -1 and remained retention of 79% after 20 cycles in the voltage range of 3-4.5 V. When magnesium ions were substituted, the initial capacity was reduced due to the less active ions. However, the capacity retention was increased to 95%. Not only cycleability, but also the thermal stability was improved by Mg substitution at every delithiated state of electrodes with electrolytes. The in situ synchrotron X-ray diffraction patterns showed that the boundary of phase transition for H1 to H2 was much clearer in Mg-doped sample, indicating that the LiNi 0.57 Mg 0.03 Co 0.25 Mn 0.15 O 2 material exhibited higher structural integrity. The improvements of both electrochemical retention and thermal stability were possibly attributed to the reduced cation mixing and complete structural changes. © 2008 Elsevier B.V. All rights reserved.