Surface Characterization of the Spinel Li_xMn₂O₄ Cathode before and after Storage at Elevated Temperatures

Abstract

Surface chemistry of the capacity fading of the Li x Mn2O4 cathode was investigated using atomic force microscopy (AFM), energy-dispersive X-ray analysis (EDAX), and X-ray photoelectron spectroscopy (XPS). Measurements show a decrease in the cathode capacity from 124 mA h g-1 before storage to 102 mA h g-1 after storage in an electrolyte of 1 M LiPF6/EC + DMC + DEC at 70 °C for 5 days. Surface morphological changes of the Li x Mn2O4 cathode were monitored using contact and tapping AFM and lateral force microscopy in air. Nanoscale changes of the charged cathode before and after storage at 70 °C were observed. Before storage, homogeneous grains of approximately 100−200 nm are seen. After storage, fine and nearly round shaped structures of 10−30 nm in size are observed covering the larger grains on the surface of the cathode. This change in morphology suggests film deposition on the cathode's surface, which increases the resistance for Li+ ion transport in and out of the cathode. Results from EDAX show that compounds containing phosphorus and fluorine are also deposited on the surface of the cathode. Surface analysis of the cathode with XPS suggests the presence of MnF2. The conversion of the oxidation state of manganese on the surface of the cathode from MnO2 to MnO during storage at the elevated temperature was observed with XPS.

Improvement of the Thermal Stability of Li-Ion Batteries by Polymer Coating of LiMn2O4

“…We used these imaging techniques to provide a basis for composite electrode imaging that is important in determining the imaging mode that maximize the information on surface topography and binder distribution. Our previous AFM observations performed in air on the surface of the spinel Li x Mn 2 O 4 cathode before and after storage at 70 0 C suggest that a film deposition on the cathode surface is responsible for the decrease of Li + ion transport conductivity into and out of the cathode [29]. However, even though morphology changes were clearly observed, no change in the roughness of the surface could be detected.…”

Effect of propane sultone on elevated temperature performance of anode and cathode materials in lithium-ion batteries

“…However, commercial cells with LiPF 6 -based electrolyte have several problems, including loss of power and capacity upon storage or prolonged use, especially at elevated temperature [2,3], which complicates the use of LiPF 6 -based electrolytes in lithium-ion batteries for many large power applications, such as hybrid electric vehicles and satellites. Analyses of electrodes from lithium-ion cells that have undergone accelerated aging experiments suggest that electrolyte decomposition reactions result in the formation of surface films on both the anode and cathode [2,[4][5][6][7]. The surface layers on both anode and cathode protect the electrodes from further reaction with the electrolyte, but they also create a barrier for lithium-ion intercalation/de-intercalation which increases cell impedance and decreases cycling efficiency.…”

Effect of structure on the storage characteristics of manganese oxide electrode materials

“…), proton exchange [18], phase separation [20], film formation, and precipitation of MnO and MnF 2 on particle surfaces [21], all of which have an adverse impact on the electrochemical characteristics, may also occur. The behavior of the spinel in contact with electrolytic solutions at elevated temperatures has been covered extensively in the literature and is not further considered here.…”