In situ Raman spectroscopic studies of LiNi_xMn_2-xO₄ thin film cathode materials for lithium ion secondary batteries

Chemical states and structural changes accompanying the electrochemical Li extraction and insertion of LiNi_xMn_2-xO₄ (0 < x < 0.5) thin films in LiBF₄-EC-DMC solutions, studied by in situ Raman spectroscopy, are reported for the first time. Ex situ Raman measurements for the virgin electrodes revealed that the oxidation state of Ni in the pristine thin films was Ni²⁺. In situ Raman spectra of the thin films collected in the organic electrolyte during Li ion extraction and insertion in the potential range 3.4-5.0 V vs. Li/Li⁺ showed a new Raman band at 540 cm^-1 appearing around 4.7 V, which is attributed to the Ni⁴⁺-O bond. In addition, from the in situ Raman spectral changes, it is suggested that Li ion extraction and insertion proceed as follows: the redox of Ni^2+/3+ takes place in the potential range 4.4-4.7 V, and Ni^3+/4+ in the 4.7-5.0 V range, while the redox at 3.8-4.4 V corresponds to Mn^3+/4+. Furthermore, it was confirmed that these changes in the Raman spectra were reversible upon changing the electrode potential, and the Li ion extraction and insertion proceed in a reversible manner.