Surface Of LiMn2O4 Research Articles

Enhancing High-Rate and Elevated-Temperature Performances of Nano-Sized and Micron-Sized LiMn2O4 in Lithium-Ion Batteries with Ultrathin Surface Coatings

LiMn2O4 suffers from severe capacity degradation when used as a cathode material in rechargeable lithium-ion batteries, especially when cycled at high rates and elevated temperatures. To enhance its high-rate electrochemical performance at elevated temperature (55 degrees C), we use atomic layer deposition (ALD) to deposit ultrathin and highly conformal Al2O3 coatings (as thin as 0.72 nm) onto micron-sized and nano-sized LiMn2O4 with precise thickness-control at atomic scale. All ALD-modified electrodes exhibit significantly improved capacities and cycling stability compared to bare electrodes. In particular, the effect of ALD coating to improve electrochemical performance of LiMn2O4 is more distinct for nano-sized LiMn2O4 than for micron-sized LiMn2O4, and more distinct for electrochemical cycling at higher charge/discharge rates. For example, nano-LiMn2O4 electrode coated with 6 Al2O3 ALD layers delivers higher initial capacity (124.7 mA h/g) and final capacity (106.7 mA h/g) after 100 cycles than bare electrode with an initial capacity of 112.3 mA h/g and a final capacity of only 95.5 mA h/g, when cycled at a very high rate of 5 C at 55 degrees C. In addition, nano-LiMn2O4 electrodes show much better rate performance than micron-LiMn2O4 electrodes at 5 C. The enhanced electrochemical performance of ALD-modified LiMn2O4 is ascribed to high-quality ALD oxide coatings that are highly conformal, dense, complete, and thus protect active material from severe dissolution, and to a formed robust glass layer on the surface of LiMn2O4 that suppress its crystallographic transformation during electrochemical cycling. Surface modifications of LiMn2O4 are also carried out by either ALD coating directly onto the entire LiMn2O4/carbon/PVDF composite electrode or coating only on LiMn2O4 particles in electrode. The former results in more significantly improved electrochemical performance of cathode, possibly because ALD coating onto entire electrode provides better mechanical integrity and preserves contact between LiMn2O4 particles and carbon/poly-vinylidenefluoride network.

Enhanced electrochemical properties of LiMn2O4 cathode material coated by 5 wt.% of nano-La2O3

Spinel LiMn2O4 was synthesized by solid state reaction. 5.0wt.% of La2O3 was coated on the surface of LiMn2O4 via ball-milling method, followed by calcination at 650°C for 5h in the air. The uncoated and La2O3-coated LiMn2O4 materials were physically characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Brunauer–Emmett–Teller surface area measurements. It was observed that Li/LiMn2O4 electrode obtained the capacity retention of 76.7% after 205cycles at 1C rate at 25°C while that of the coated sample was 90.1%. At 55°C, the bare and coated LiMn2O4 showed a capacity retention of 69.4% and 82.6% after 95cycles, respectively. Rate capability was also improved. The capacity retention of uncoated and La2O3-coated materials reduced to 46.9% and 80.3% at 10C-rate, respectively.

Synthesis, structure, and electrochemistry of Ag-modified LiMn2O4 cathode materials for lithium-ion batteries

The composite of silver-modified lithium manganese oxide were prepared using thermal decomposition method of different mole ratio. Structural characterization was carried out by X-ray diffraction (XRD). XRD analysis revealed different patterns as the content of the dopant in the spinel increases. Phase analysis shows that Ag particles were dispersed on the LiMn2O4 surface instead of entering the spinel structure. On the other hand, the electrochemical behavior of cathode powder was examined by using two-electrode test cells consisting of a cathode, metallic lithium as anode, and a solid polymer electrolyte of 0.87PEO-0.13LiCF3SO3-0.10DBP. According to the electrochemical tests results, the influence of the Ag additive content on the electrochemical properties of Ag/LiMn2O4 composites is clearly shown.

The effect of nanolayer AlF3 coating on LiMn2O4 cycle life in high temperature for lithium secondary batteries

The surface of the spinel LiMn2O4 was coated with AlF3 by a chemical process to improve its electrochemical performance at high temperatures. The morphology and structure of the original and AlF3-coated LiMn2O4 samples were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM). All the samples exhibited a pure cubic spinel structure without any impurities in the XRD patterns. It was found that the surfaces of the original LiMn2O4 samples were covered with a nanolayer AlF3 after the treatment. The charge/discharge of the materials were carried at 220 mA/g in the range of 3.0 and 4.4 V at 55°C. While the original LiMn2O4 showed 17.8% capacity loss in 50 cycles at 55°C, the AlF3-coated LiMn2O4 (118.1 mA h/g) showed only 3.4% loss of the initial capacity (122.3 mA h/g) at 55°C. It is obvious that the improvement in cycling performance of the coated-LiMn2O4 electrode at 55°C is attributed to the presence of AlF3 on the surface of LiMn2O4.

Preparation of TiO2-coated LiMn2O4 by carrier transfer method

The TiO2-coated LiMn2O4 has been prepared by a carrier transfer method and investigated. This novel synthetic method involved the transfer of TiO2 into the surface of LiMn2O4 with Vulcan XC-72 active carbon powders as a dispersant. The X-ray diffraction shows that spinel structure of materials does not change after the coating of TiO2. The electrochemical performance tests show that the initial discharge capacity of TiO2-modified LiMn2O4 is 111.5 mA h g−1, which is better than that of pristine LiMn2O4 (103.8 mA h g−1). The cyclic performance is significantly improved after surface modification. The TiO2-modified LiMn2O4 by a carrier transfer method exhibits better discharge capability and lower resistance.

Electrochemical characterization of surface-modified LiMn2O4 cathode materials for Li-ion batteries

To improve the performance, the surface of LiMn2O4 was coated with very fine MgO, Al2O3 and ZnO by sol-gel method, respectively. The structure and morphology of the coated materials were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The charge and discharge performance of uncoated and surface modified LiMn2O4 spinel at 25°C and 55°C were tested, using a voltage window of 3.0–4.35 V and a current density of 0.1 C rate. There is a slight decrease in the initial discharge capacity relative to that of uncoated LiMn2O4, but the cycle ability of Li LiMn2O4 coated by metal-oxide has remarkably been improved. The EIS measurements of uncoated and Al2O3-coated LiMn2O4 were carried out by a model 273 A potentiostat/galvanistat controlled by a computer using M270 software, and using a frequency response analyzer (Zsimpwin) combined with a potentiostate (PAR 273). Consequently, the reason for the improved cycle properties is that the surface modification reduces the dissolution of Mn, which results from the suppression of the electrolyte decomposition, and suppresses the formation of passivation film that acts as an electronic insulating layer. In conclusion, the use of surface modification is an effective way to improve the electrochemical performance of LiMn2O4 cathode material for lithium batteries.

Enhanced Electrochemical Properties of Li Secondary Batteries Using Polypyrrole/LiMn2O4 Composite Cathodes

Polypyrrole(PPy)/LiMn2O4 composites were prepared by chemical oxidation of pyrrole onto LiMn2O4 with dodecylbenzene sulfonate as a dopant. The polymerization occurred mainly on the surface of LiMn2O4 to yield the entirely coated LiMn2O4 particles. The capacity of the cell reached the maximum of 124 mAh/g at 4.1 wt% of PPy in the active material. The composite cell showed enhanced charge-discharge properties compared with a pure LiMn2O4 cell. The PPy may enhance the electron exchange ability and stabilize the structure of LiMn2O4.

Origin of self-discharge mechanism in LiMn2O4-based Li-ion cells: A chemical and electrochemical approach

LiMn2O4-based Li-ion cells suffer from a limited cycle-life and a poor storage performance at 55 °C, both in their charged and discharged states. To get some insight on the origin of the poor 55 °C storage performance, the voltage distribution through plastic Li-ion cells during electrochemical testing was monitored by means of 3-electrode type measurements. From these measurements, coupled with chemical analysis, X-ray diffraction and microscopy studies, one unambiguously concludes that the poor performance of LiMn2O4/C-cells at 55 °C in their discharged state is due to enhanced Mn dissolution that increases with increasing both the temperature and the electrolyte HF content. These results were confirmed by a chemical approach which consists in placing a fresh LiMn2O4 electrode into a 55 °C electrolyte solution. A mechanism, based on an ion-exchange reaction leading to the Mn dissolution is proposed to account for the poor storage performance of LiMn2O4/C Li-ion cells in their discharged state. In order to minimize the Mn dissolution, two surface treatments were performed. The first one consists in applying an inorganic borate glass composition to the LiMn2O4 surface, the second one in using an acetylacetone complexing agent.