Unraveling the Mechanism and Practical Implications of the Sol-Gel Synthesis of Spinel LiMn2O4 as a Cathode Material for Li-Ion Batteries: Critical Effects of Cation Distribution at the Matrix Level.

Spinel LiMn2O4 (LMO) is a state-of-the-art cathode material for Li-ion batteries. However, the operating voltage and battery life of spinel LMO needs to be improved for application in various modern technologies. Modifying the composition of the spinel LMO material alters its electronic structure, thereby increasing its operating voltage. Additionally, modifying the microstructure of the spinel LMO by controlling the size and distribution of the particles can improve its electrochemical properties. In this study, we elucidate the sol-gel synthesis mechanisms of two common types of sol-gels (modified and unmodified metal complexes)-chelate gel and organic polymeric gel-and investigate their structural and morphological properties and electrochemical performances. This study highlights that uniform distribution of cations during sol-gel formation is important for the growth of LMO crystals. Furthermore, a homogeneous multicomponent sol-gel, necessary to ensure that no conflicting morphologies and structures would degrade the electrochemical performances, can be obtained when the sol-gel has a polymer-like structure and uniformly bound ions; this can be achieved by using additional multifunctional reagents, namely cross-linkers.

Electrochemical Properties of Multilayered Sn/TiNi Shape-Memory-Alloy Thin-Film Electrodes for High-Performance Anodes in Li-Ion Batteries.

Sn is a promising candidate anode material with a high theoretical capacity (994 mAh/g). However, the drastic structural changes of Sn particles caused by their pulverization and aggregation during charge-discharge cycling reduce their capacity over time. To overcome this, a TiNi shape memory alloy (SMA) was introduced as a buffer matrix. Sn/TiNi SMA multilayer thin films were deposited on Cu foil using a DC magnetron sputtering system. When the TiNi alloy was employed at the bottom of a Sn thin film, it did not adequately buffer the volume changes and internal stress of Sn, and stress absorption was not evident. However, an electrode with an additional top layer of room-temperature-deposition TiNi (TiNi(RT)) lost capacity much more slowly than the Sn or Sn/TiNi electrodes, retaining 50% capacity up to 40 cycles. Moreover, the charge-transfer resistance decreased from 318.1 Ω after one cycle to 246.1 Ω after twenty. The improved cycle performance indicates that the TiNi(RT) and TiNi-alloy thin films overall held the Sn thin film. The structure was changed so that Li and Sn reacted well; the stress-absorption effect was observed in the TiNi SMA thin films.

High Electrochemical Performance Silicon Thin-Film Free-Standing Electrodes Based on Buckypaper for Flexible Lithium-Ion Batteries.

Recently, applications for lithium-ion batteries (LIBs) have expanded to include electric vehicles and electric energy storage systems, extending beyond power sources for portable electronic devices. The power sources of these flexible electronic devices require the creation of thin, light, and flexible power supply devices such as flexile electrolytes/insulators, electrode materials, current collectors, and batteries that play an important role in packaging. Demand will require the progress of modern electrode materials with high capacity, rate capability, cycle stability, electrical conductivity, and mechanical flexibility for the time to come. The integration of high electrical conductivity and flexible buckypaper (oxidized Multi-walled carbon nanotubes (MWCNTs) film) and high theoretical capacity silicon materials are effective for obtaining superior high-energy-density and flexible electrode materials. Therefore, this study focuses on improving the high-capacity, capability-cycling stability of the thin-film Si buckypaper free-standing electrodes for lightweight and flexible energy-supply devices. First, buckypaper (oxidized MWCNTs) was prepared by assembling a free stand-alone electrode, and electrical conductivity tests confirmed that the buckypaper has sufficient electrical conductivity (10-4(S m-1) in LIBs) to operate simultaneously with a current collector. Subsequently, silicon was deposited on the buckypaper via magnetron sputtering. Next, the thin-film Si buckypaper freestanding electrodes were heat-treated at 600 °C in a vacuum, which improved their electrochemical performance significantly. Electrochemical results demonstrated that the electrode capacity can be increased by 27/26 and 95/93 µAh in unheated and heated buckypaper current collectors, respectively. The measured discharge/charge capacities of the USi_HBP electrode were 108/106 µAh after 100 cycles, corresponding to a Coulombic efficiency of 98.1%, whereas the HSi_HBP electrode indicated a discharge/charge capacity of 193/192 µAh at the 100th cycle, corresponding to a capacity retention of 99.5%. In particular, the HSi_HBP electrode can decrease the capacity by less than 1.5% compared with the value of the first cycle after 100 cycles, demonstrating excellent electrochemical stability.

Compositionally graded Ti-Ni alloys prepared by diffusion bonding.

A Ti-Ni alloy compositionally graded along the thickness direction in order to obtain a shape change over a wide temperature range, which is beneficial to the actuator for precise position control, was prepared by spark plasma sintering (SPS) after stacking Ti-Ni alloy ribbons in the sequence of Ti-51Ni, Ti-50Ni, Ti-49Ni and Ti-48Ni (at%) followed by annealing. Then, the microstructure and martensitic transformation behavior were investigated by using FE-SEM, DSC and thermal cycling tests under a constant load. The inter-ribbon defects observed after SPS due to insufficient diffusional bonding between the ribbons were eliminated by post-SPS annealing at 1023 K for 36 ks. The compositionally graded sample showed compositional variation of 1.5 at% Ti along the thickness direction (- 120 µm) and a martensitic transformation temperature window as large as 91 K on cooling and 79 K on heating. A recoverable elongation of 0.9% was obtained under a stress of 80 MPa and the deformation rate, which is defined as the ratio of the recoverable elongation to the temperature range where the elongation occurred was 0.015%/K in the compositionally graded sample.

Electrochemical properties of Si film electrodes grown on current collectors with CuO nanostructures for thin-film microbatteries.

Si film electrodes were deposited onto Cu foil current collectors fabricated with well-formed CuO nanostructures. The structural and electrochemical properties of the Cu foils oxidized for 1, 3, and 6 h and of the Si film electrodes were investigated using field-emission scanning electron microscopy, X-ray diffraction (XRD), and charge/discharge tests. The morphologies and XRD profiles suggested that the oxidized Cu foils consisted of a top CuO layer and a bottom Cu2O layer. The surface roughness of the Cu foils decreased with increasing oxidation time since the flower-like CuO nanostructures weakly adhered to the surface were easily detached by ultrasonic cleaning. The cycle performance of the Si film electrode with the rougher CuO layer rapidly deteriorated, whereas the flat Cu2O layer showing a relatively high electric conductivity induced the formation of a dense Si film and improved the electrochemical performance of the Si film electrode.

Protection effect of ZrO2 coating layer on LiCoO2 thin film fabricated by DC magnetron sputtering.

Bare and ZrO2-coated LiCoO2 thin films were fabricated by direct current magnetron sputtering method on STS304 substrates. Deposited both films have a well-crystallized structure with (003) preferred orientation after annealing at 600 degrees C. The ZrO2-coated LiCoO2 thin film provide significantly improved cycling stability compared to bare LiCoO2 thin film at high cut-off potential (3.0-4.5 V). The improvement in electrochemical stability is attributed to the structural stability by ZrO2 coating layer.

The effects of substrate and annealing on structural and electrochemical properties in LiCoO2 thin films prepared by DC magnetron sputtering.

LiCoO2 thin films were fabricated by direct current magnetron sputtering method on STS304 and Ti substrates. The effects of substrate and annealing on their structural and electrochemical properties of LiCoO2 thin film cathode were studied. Crystal structures and surface morphologies of the deposited films were investigated by X-ray diffraction and field emission scanning electron microscopy. The as-deposited films on both substrates have amorphous structure. The (104) oriented perfect crystallization was obtained by annealing over 600 degrees C in STS304 substrate. The LiCoO2 thin film deposited on Ti substrate shows the (003) texture after annealing at 700 degrees C. The electrochemical properties were investigated by the cyclic voltammetry and charge-discharge measurement. The 600 degrees C-annealed LiCoO2 film deposited on STS304 substrate exhibits the inithial discharge capacity of 22 uAh/cm2 and the 96% capacity retention rate at 50th cycles. The electrochemical measurement on annealed films over 600 degrees C was impossible due to the formed TiO2 insulator layer using Ti substrate. As a result, it was found that the STS304 substrate seems to be more suitable material than the Ti substrate in fabricating LiCoO2 thin film cathode.

Influences of Ti film thickness on electrochemical properties of Si/Ti/Cu film electrodes.

Si and Si/Ti films were fabricated on a Cu current collector (substrate) using the DC sputtering system. The Ti film as a buffer layer was inserted between the Si film and the Cu current collector. Their structural and electrochemical properties were investigated with various Ti film thicknesses of 20-90 nm. The Si and Ti films deposited on a polycrystalline Cu substrate were amorphous. The Si/Ti/Cu film electrode exhibited better electrochemical properties than the Si/Cu electrode in terms of capacity, charge-discharge efficiency, and cycleability. In the Si/Ti/Cu electrode, the film electrode with a 55 nm Ti film thickness showed the best electrochemical properties: 367 microA h/cm2 initial capacity, 91% efficiency, and 50% capacity retention after 100 cycles. These good electrochemical properties are attributed to the enhanced adhesion between the Si and Ti films. Additionally, the modified surface morphology of Si film with a cluster structure could withstand the lateral volume change during the charge-discharge process.

Patterned Si thin film electrodes for enhancing structural stability.

A patterned film (electrode) with lozenge-shaped Si tiles could be successfully fabricated by masking with an expanded metal foil during film deposition. Its electrochemical properties and structural stability during the charge-discharge process were examined and compared with those of a continuous (conventional) film electrode. The patterned electrode exhibited a remarkably improved cycleability (75% capacity retention after 120 cycles) and an enhanced structural stability compared to the continuous electrode. The good electrochemical performance of the patterned electrode was attributed to the space between Si tiles that acted as a buffer against the volume change of the Si electrode.