Electronic and Electrochemical Properties of Li_1–xMn_1.5Ni_0.5O₄ Spinel Cathodes As a Function of Lithium Content and Cation Ordering

Abstract

The electronic and electrochemical properties of the high-voltage spinel LiMn1.5Ni0.5O4 as a function of cation ordering and lithium content have been investigated. Conductivity and activation energy measurements confirm that charge transfer occurs by small polaron hopping, and the charge carrier conduction is easier in the Ni:3d band than in the in Mn:3d band. Seebeck coefficient data reveal that the Ni 2+/3+ and Ni 3+/4+ redox couples are combined in a single 3d band, and that maximum charge carrier concentration occurs where the average Ni oxidation state is close to 3+, corresponding to x = 0.5 in Li Li1-xMn1.5Ni0.5O4. Accordingly, maximum electronic conductivity is found at x = 0.5, regardless of cation ordering. The thermodynamically stable phases formed during cycling were investigated by recording the X-ray diffraction (XRD) of chemically delithiated powders. The more ordered spinels maintained two separate two-phase regions upon lithium extraction, while the more disordered samples exhibited a solid-solubility region from LiMn1.5Ni0.5O4 to Li0.5Mn1.5Ni0.5O4. The conductivity and phase-transformation data of four samples with varying degrees of cation ordering were compared to the electrochemical data collected with lithium cells. Only the most ordered spinel showed inferior rate performance, while the sample annealed for a shorter time performed comparable to the unannealed or disordered samples. The results presented here challenge the most common beliefs about high-voltage spinel: (i) low Mn 3+ content is responsible for poor rate performance and (ii) thermodynamically stable solid-solubility is critical for fast kinetics.

Electrochemical and Electronic Charge Transport Properties of Ni-Doped LiMn2O4 Spinel Obtained from Polyol-Mediated Synthesis

“…When the C-rate was reduced to C/20, the as-prepared LNMO exhibited a discharge capacity retention of 98%, indicating that the irreversible damage to the material caused by the fast discharge was negligible. The result is comparable to the results of disordered LNMO materials with various particle sizes and morphologies reported in the literature [19,20]. Similar to cycling stability, the rate capability of LNMO materials with octahedral shape and {111} family of planes on the surface was also reported to be superior to that of materials with other particle morphologies [59,61,80].…”

“…Samples synthesized at temperatures higher than 700 °C exhibit the disordered phase, whereas prolonged annealing at 700 °C favors the formation of the ordered phase [16,20,59]. Kunduraci et al [24] showed that the ordered/disordered phase transition is driven by the oxygen deficiency in the spinel and is accompanied by the formation of a rock salt secondary phase.…”

“…Previous works have revealed that significant amount of short-range Ni/Mn ordering may occur in LNMO materials and the obtained LNMO materials are typically composed of both disordered and ordered phases [20,21,22]. The disordered spinel has been reported to show superior rate performance to the ordered phase [17,18,23].…”

“…Moreover, during the bulk measurements, it is challenging to discriminate between bulk conductivity and grain boundary conductivity. Therefore, it is desirable to directly investigate the electrical properties of individual micro- and nanometer-scaled particles with well-defined morphology, as already pointed out by Moorhead-Rosenberg et al [20], to interrelate these measurements to first principles calculations, and to correlate the electronic charge transport properties with the electrochemical performance.…”

Effect of local structural changes on rate capability of LiNi 0.5 Mn 1.5 O 4−δ cathode material for lithium ion batteries

“…The disordered structure had an extensive solid-solution region during charge M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 4 and discharge processes and delivered an enhanced rate capability compared to the ordered structure. It can be explained that the enlarged two-phase regions in the ordered spinel have many phase boundaries, imposing the limitation of Li + transport during charge and discharge, whereas the extended solid-solution region in the disordered spinel is beneficial to Li + transport due to the reduced number of phase boundaries that Li + has to overcome [12,[17][18][19].…”