Intrinsic Electrochemical Properties of LiNi0.5Mn1.5O4 Synthesized By Flux Method for High Energy Density Li-Ion Batteries

Lithium-ion batteries (LIBs) with high gravimetric and volumetric energy density are very important key devices for the establishment of the sustainable energy system which is consisted of solar cells, wind power generations, smart grid, and batteries. In addition, the LIBs are extensively expected for a power supply of plug-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs). Much higher energy density is needed to increase the mileage per charge. Current LIBs use the composite electrodes which are composed of active materials, binders, conductive agents, and current collector. The charging and discharging performance of the composite electrode must be affected by the “composite” state (kinds of materials, mixing ratio, thickness, and tap density). In addition to that, the materials which have large volume change during the charging and discharging are strongly affected by the state of the composite electrode. It is difficult to distinguish the electrochemical characteristics of only active materials from that of the composite electrode. However, the intrinsic properties of active materials are very important to evaluate the electrode performance and understand the mechanism of the charging and discharging reaction. In this study, the single particle measurement technique was used to study the intrinsic characteristics of LiNi_0.5Mn_1.5O₄ (LNMO) synthesized by the flux methods. This LNMO particles were coated by NbO_x nano-sheets for preventing the oxidation of electrolytes. Fig.1 shows the schematic diagram of the single particle measurement equipment. The two-electrode electrochemical cell was placed on the stage of an optical microscope. Glass coating Au wire (φ=10 μm) was used as the micro-probe for the measurement. The electrolytes were 1M LiPF₆-EC:PC=1:1, 1M LiBF₄-EC:PC=1:1, 1M LiPF₆-FEC, and 1M LiBF₄-FEC. The electrochemical measurement was conducted in the range from 3.0 V to 4.9V vs. Li metal counter electrode. After the electrochemical measurement, the measured particle was picked up by micro-tweezers and transferred to SEM, FIB-SEM and TEM. This transfer procedure was done by using transfer vessel in order to prevent the atmosphere exposure.

Fig.2 shows the SEM image and EDS mapping images of the LNMO coated by NbO_x nanosheet. Small crystallines coagulate to form large secondary particles. The secondary particle is about 5 ~ 20 μm, and adequate for single particle measurement. Nb was also detected due to the coverage by NbO_x nanosheets. Further characterization by XRD and Raman spectroscopy was also conducted. The detailed results will be shown in the presentation.

Fig. 3 demonstrates the charging and discharging curves of one NbO_x coated LNMO particle in LiBF₄-FEC electrolyte solution. The applied current is 1 nA. There is voltage plateau at about 4.75V because of Ni²⁺/Ni⁴⁺ redox, and the Mn³⁺/Mn⁴⁺ redox plateau is not appeared. The 2nd charging and discharging curve is almost same with 1^st curve. Therefore, this NbO_x coated LNMO particle has very good electrochemical performance as high voltage cathode material. However, the coulomb efficiency is 88% and 90% for 1^st and 2^nd cycle respectively. This irreversible capacity would be induced by the surface irreversible reaction at not only LNMO particle surface but also the Au micro-probe surface. In order to reveal the degradation mechanism of LNMO, the measured particle is picked up by micro-tweezers to analyze the particle by Raman-Spectroscopy, SEM, and TEM without atmosphere exposure. Based on those knowledge, we would like to discuss the detailed charging and discharging mechanism of LNMO.