A New Insight in the Redox Mechanism Involved during the Delithiation/Lithiation of Spinel Materials Provided By Raman Spectroscopy

Abstract

Lithium-ion batteries have been widely used as power source for portable devices and are available for applications to electric vehicles, which demand high power, high energy, inexpensive and safe batteries [1]. Their continuous improvement is tightly bound to the understanding of lithium (de)intercalation phenomena in electrode materials. Here we address for the first time the use of Raman spectroscopy to understand the mechanisms involved in high voltage spinels LiNixMn2-xO4 (x = 0; x = 0.5). Although the “classical” lithium-containing spinel LiMn2O4 (LMO) has been extensively studied, certain points remain unclear. For instance, very little is known on the nature of the intermediate composition Li0.5Mn2O4 postulated halfway during charge/discharge [2] for which attempts to model a lithium/vacancy ordering have been made [3, 4]. On the other hand, compositions within the solid solution LiNixMn2-xO4 (0 < x ≤ 0.5) are of interest as they exhibit increasing voltage upon lithium extraction. The best compromise is found for LiNi0.5Mn1.5O4 (LNMO): with a high specific capacity of 140 mAh g-1 available along a wide potential plateau at ≈ 4.7 V vs Li+/Li, LNMO is thoroughly studied as a next-generation positive electrode material for Li-ion batteries [5]. LNMO exists as two polymorphs, “disordered” (Fd-3m) where Mn/Ni occupy 16d sites randomly and “ordered” (P4332) where Ni and Mn sit on ordered 4a and 12d sites. So far, the disordered form has outperformed the ordered form from an electrochemical point of view [6]. The delithiation/lithiation mechanism of disordered LNMO has been previously investigated using mainly X-ray and neutron diffraction [7-10] and in situ synchrotron [11]. Although these methods provide a picture of the phase evolution during cycling, we still lack information about the atomic level variations as lithium ions deintercalate/intercalate in the crystal structure of LNMO. Raman spectroscopy (RS) on the other hand is a very appropriate tool to enrich the knowledge of the structure of Li intercalation compounds at the scale of the chemical bond. A wealth of structural information including symmetry changes, local disorder, changes in bond lengths, emergence of secondary phases, etc… can be extracted from the Raman study of various Li intercalated host lattices, providing therefore a better insight into the mechanisms governing the electrode performances [12]. In this work, RS is used to explore the short-range environment in LMO and "disordered" LNMO. We provide here a complete assignment of the LMO and LMNO Raman fingerprints as well as a detailed picture of structural and vibrational changes during the charge-discharge of these cathode materials. A careful RS investigation led on the LMO cathode during the charge process provides the first detailed experimental evidence of an ordering scheme at the scale of the chemical bond of the intermediate composition Li0.5Mn2O4. A combined approach using XRD and RS allows picturing changes in LNMO crystal structure and in the oxidation states of the nickel during stages of the charge-discharge process. Raman spectra collected between 3.5 and 4.9 V display rich and varying features (Figure 1). Appropriate analysis based upon Raman spectra fittings allow access to a quantitative estimation of the different Ni2+, Ni3+ and Ni4+ redox species in the LNMO spinel oxide at different oxidation-reduction states. This makes RS a powerful probe to evaluate the state of charge of the cathode material. Finally, we demonstrate the great efficiency of RS as an efficient and simple diagnostic tool of the self-discharge phenomenon occurring in the LNMO electrode.