Improving the Electrochemical Stability of the High-Voltage Li-Ion Battery Cathode LiNi_0.5Mn_1.5O₄by Titanate-Based Surface Modification

A sol-gel method was applied to add a chemically protective lithium titanate (LTO) and/or anatase (TiO 2 ) layer to LiNi 0.5 Mn 1.5 O 4- δ (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°C. A thin-layer TiO 2 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 (∼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 ∼ 10 −11 cm 2 /s, estimated by GITT measurements), but the 10% loss in capacity at 10 C rate for coated samples may be due to differences in electrical conductivity (σ ∼ 1.12 × 10 −5 S/cm for bare LNMO; 7.15 × 10 −6 S/cm for the thicker coated LNMO).