Improving the Electrochemical Stability of the High-Voltage Li-Ion Battery Cathode LiNi0.5Mn1.5O4 by Titanate-Based Surface Modification

A sol-gel method was applied to add a chemically protective lithium titanate (LTO) and/or anatase (TiO2) layer to LiNi0.5Mn1.5O4-delta (LNMO). By TEM imaging, the coating thickness was 5-10 nm or 20-30 nm, depending on the volume of precursor used. Cells were cycled at room temperature and at 55 degrees C. A thin-layer TiO2 coating gives LNMO with a discharge capacity of 113 mAh/g when cycled between 3.4-5.0 V vs. Li+/0 at room temperature (1 C rate), with no capacity fade over the first 100 cycles. At higher temperature, the capacity is similar, with no change in the polarization gap (similar to 50 mV) after 55 cycles. A slight increase in the lattice parameter after cycling hints at oxygen-vacancy formation or transition-metal dissolution. At 10 C rate, bare LNMO showed the highest capacity (87 mAh/g). However, the thinly coated materials showed comparable rate capability. All materialS show a similar lithium-ion diffusivity (D-Li similar to 10(-11) cm(2)/s, estimated by GITT measurements), but the 10% loss in capacity at 10 Crate for coated samples may be due to differences in electrical conductivity (sigma similar to 1.12 x 10(-5) S/cm for bare LNMO; 7.15 x 10(-6) S/cm for the thicker coated LNMO). (C) 2013 The Electrochemical Society. All rights reserved.