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of information within a network. In this context of miniaturization, thin-film Li-ion microbatteries are a favorite option due to their superior energy density, but there is still a need to meet the high energy and power demands of IoT applications. The judicious choice of cathode materials in thin film configuration is thus an essential step in the development of Li-ion microbatteries.Chemical vapor deposition and sputtering have been the most frequently used techniques to synthesize mostly LiCoO 2 and LiMn 2 O 4 thin films, with electrode potentials of 3.8 and 4.1 V versus Li/Li + , respectively. [1,2] The partial replacement of manganese in LiMn 2 O 4 by various transition metals is known to endow this compound with the ability to deliver a substantial capacity at a high voltage near 4.5 V. [3] LiNi 0.5 Mn 1.5 O 4 (LNMO) is the substituted spinel exhibiting the best electrochemical properties relying entirely on the nickel redox system (Ni 2+ being oxidized in Ni 3+ and Ni 4+ ) at a high working potential of about 4.7 V versus Li/Li + . Furthermore, the substitution of 25% of manganese by nickel allows having manganese only in the +4 oxidation state in the structure, then preventing detrimental effects such as Jahn-Teller distortion as well as disproportionation of Mn 3+ ions. With the use of relatively abundant elements associated to an attractive theoretical capacity of 147 mAh g −1 (65.7 µAh cm -2 µm -1 assuming a bulk density Due to its great theoretical capacity (147 mAh g −1 ) and high operating potential (4.7 V vs Li + /Li), Co-free spinel LiNi 0.5 Mn 1.5 O 4 (LNMO) is one of the most promising thin film cathodes allowing designing Li-ion micro-batteries with a high specific energy. In this work, the Li extraction-insertion mechanism in sputtered LNMO thin films is investigated by X-ray diffraction and Raman spectroscopy during the first electrochemical cycle. A one-step phase transition involving two cubic phases is revealed, consisting of a wide solid solution region (0.3 ≤ x ≤ 1 in Li x NMO) and a narrow biphasic domain (0 < x ≤ 0.3). Remarkably, significant variations are observed in the Raman spectra, which are linked to the activity of the Ni redox system at 4.7 V. It is demonstrated that an appropriate analysis of the bands corresponding to pure Ni-O stretching modes leads to an accurate estimation of the electrode states of charge and depth of discharge, which opens the way for a reliable quantification of the self-discharge phenomenon. The mechanism of Li extraction insertion here pictured for the first time for LNMO thin layers is consistent with their disordered nature and accounts for their good electrochemical performance.
of information within a network. In this context of miniaturization, thin-film Li-ion microbatteries are a favorite option due to their superior energy density, but there is still a need to meet the high energy and power demands of IoT applications. The judicious choice of cathode materials in thin film configuration is thus an essential step in the development of Li-ion microbatteries.Chemical vapor deposition and sputtering have been the most frequently used techniques to synthesize mostly LiCoO 2 and LiMn 2 O 4 thin films, with electrode potentials of 3.8 and 4.1 V versus Li/Li + , respectively. [1,2] The partial replacement of manganese in LiMn 2 O 4 by various transition metals is known to endow this compound with the ability to deliver a substantial capacity at a high voltage near 4.5 V. [3] LiNi 0.5 Mn 1.5 O 4 (LNMO) is the substituted spinel exhibiting the best electrochemical properties relying entirely on the nickel redox system (Ni 2+ being oxidized in Ni 3+ and Ni 4+ ) at a high working potential of about 4.7 V versus Li/Li + . Furthermore, the substitution of 25% of manganese by nickel allows having manganese only in the +4 oxidation state in the structure, then preventing detrimental effects such as Jahn-Teller distortion as well as disproportionation of Mn 3+ ions. With the use of relatively abundant elements associated to an attractive theoretical capacity of 147 mAh g −1 (65.7 µAh cm -2 µm -1 assuming a bulk density Due to its great theoretical capacity (147 mAh g −1 ) and high operating potential (4.7 V vs Li + /Li), Co-free spinel LiNi 0.5 Mn 1.5 O 4 (LNMO) is one of the most promising thin film cathodes allowing designing Li-ion micro-batteries with a high specific energy. In this work, the Li extraction-insertion mechanism in sputtered LNMO thin films is investigated by X-ray diffraction and Raman spectroscopy during the first electrochemical cycle. A one-step phase transition involving two cubic phases is revealed, consisting of a wide solid solution region (0.3 ≤ x ≤ 1 in Li x NMO) and a narrow biphasic domain (0 < x ≤ 0.3). Remarkably, significant variations are observed in the Raman spectra, which are linked to the activity of the Ni redox system at 4.7 V. It is demonstrated that an appropriate analysis of the bands corresponding to pure Ni-O stretching modes leads to an accurate estimation of the electrode states of charge and depth of discharge, which opens the way for a reliable quantification of the self-discharge phenomenon. The mechanism of Li extraction insertion here pictured for the first time for LNMO thin layers is consistent with their disordered nature and accounts for their good electrochemical performance.
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