Electrochemical behavior of rutile phase TiO2-coated NCM materials for ASLBs operated at a high temperature

In order to improve the charge/discharge cycle characteristics of the all-solid lithium batteries (ASLBs) operated at high temperature (60–80 °C), the TiO2-coated NCM-424 (LiNi0.4Co0.2Mn0.2O2) by the sol-gel method was investigated. Two kinds of key parameter, coating amount of TiO2 on NCM-424 powder and calcination temperature, were controlled for synthesis of TiO2-coated NCM-424. Firstly, to investigate the effect of coating amount, NCM-424 powders were coated with 1–5 wt% TiO2 and calcined at 600 °C. Secondly, the calcination temperature was changed in the range of 525–800 °C on the precursor of 3 wt% TiO2-coated NCM-424. In the case of the former sample, it seems that although the number of fine TiO2 grains on the surface of NCM-424 particle was increased with increasing of coating amount, the main factors constituting the material, ie, lattice parameters are almost unchanged on all samples. For about 30 cycle on coin cells of a composite type ASLBs, the 3 wt%-coated sample calcined 600 °C showed the best discharge capacity (about 136 mAh g−1 at 70 °C) and good cycle behavior among the samples. In the latter case, it was found that when the calcination temperature was increased to 800 °C, side-reaction between TiO2 and NCM-424 powder was occurred to form a new interface, which had a good effect on cycling property of ASLB cell. That is, the initial capacity of cell with 3 wt% TiO2-coated NCM-424 was decreased slightly, but the capacity retention behavior with charge/discharge cycle was greatly improved when the calcination temperature was increased from 600 °C to 800 °C; the initial capacity of 3 wt% TiO2-coated NCM-424 calcined at 800 °C was about 127 mAh g−1 at 70 °C, and its capacity retention was about 81% at 100 cycle. On the other hand, the capacity retention of TiO2-coated NCM-424 materials calcined at 600 °C or less tend to decrease rapidly after 50 cycles. These results are presumed to be related to the structural change of TiO2, NCM-424 materials itself, and the difference of coating adhesion according to the calcination temperature.