Two types of NiO–Li2CO3 nanocomposite electrodes have been prepared for the electrochemical decomposition studies. The thin film electrode with a thickness of 225 nm and grain size around 5–8 nm is prepared by a pulsed laser deposition method. The powder sample is prepared by a solution evaporation and calcination method with primary particle size in the range of 20–50 nm. Using ex situ TEM, Raman and FTIR spectroscopy and synchrotron based in situ XRD, the electrochemical decomposition of Li2CO3 phase in both types of the NiO–Li2CO3 nanocomposite electrodes after charging up to about 4.1 V vs Li+/Li at room temperature is clearly confirmed, but not in the electrode containing only Li2CO3. The NiO phase does not change significantly after charging process and may act as catalyst for the Li2CO3 decomposition. The potential of using NiO–Li2CO3 nanocomposite material as additional lithium source in cathode additive in lithium ion batteries has been demonstrated, which could compensate the initial irreversible capacity loss at the anode side.