Failure Modes of Flexible LiCoO2 Cathodes Incorporating Polyvinylidene Fluoride Binders with Different Molecular Weights.

Understanding the mechanical failure modes of lithium-ion battery [Li-ion batteries (LIBs)] electrodes is exceptionally important for enabling high specific energy and flexible LIB technologies. In this work, the failure modes of lithium cobalt oxide (LCO) cathodes under repeated bending and the role of the polymer binder in improving the mechanical durability of the LCO electrodes for use in flexible LIBs are investigated. Mechanical and electrochemical evaluations of LCO electrodes (areal capacity of ≥2.5 mA h cm-2) employing poly(vinylidene fluoride) (PVDF) binder were carried out, followed by extensive optical and electron microscopies. We find that the molecular weight (MW) of the PVDF significantly influenced the surface and bulk microstructure of the LCO electrodes, particularly the distribution of carbon additive and binder, which plays a crucial role in affecting the mechanical and electrochemical properties of the electrodes. Multiple mechanical failure modes (e.g., surface scratches and microcracks) observed in the LCO electrodes subjected to repeated bending originated from the use of low MW PVDF; these failure modes were successfully mitigated by using a high MW PVDF. Remarkably, the optimized flexible LCO electrode incorporating high MW PVDF showed comparable discharge capacity retention during galvanostatic cycling after repeated bending (7000 cycles at 50 mm bending diameter) to electrodes not subjected to the repeated bending. This study highlights the importance of carrying out a comprehensive investigation of the failure mechanisms in flexible electrodes, which identified the pivotal role of the PVDF MW in the electrode microstructure and its effects on the electrode resilience to failure during repeated bending.

Scalable Synthesis and Electrochemical Properties of Porous Si-CoSi2-C Composites as an Anode for Li-ion Batteries.

Si-based anodes for Li-ion batteries (LIBs) are considered to be an attractive alternative to graphite due to their higher capacity, but they have low electrical conductivity and degrade mechanically during cycling. In the current study, we report on a mass-producible porous Si-CoSi2-C composite as a high-capacity anode material for LIBs. The composite was synthesized with two-step milling followed by a simple chemical etching process. The material conversion and porous structure were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and electron microscopy. The electrochemical test results demonstrated that the Si-CoSi2-C composite electrode exhibits greatly improved cycle and rate performance compared with conventional Si-C composite electrodes. These results can be ascribed to the role of CoSi2 and inside pores. The CoSi2 synthesized in situ during high-energy mechanical milling can be well attached to the Si; its conductive phase can increase electrical connection with the carbon matrix and the Cu current collectors; and it can accommodate Si volume changes during cycling. The proposed synthesis strategy can provide a facile and cost-effective method to produce Si-based materials for commercial LIB anodes.

Mechanochemically Reduced SiO2 by Ti Incorporation as Lithium Storage Materials.

This study presents a simple and effective method of reducing amorphous silica (a-SiO2 ) with Ti metal through high-energy mechanical milling for improving its reactivity when used as an anode material in lithium-ion batteries. Through thermodynamic calculations, it is determined that Ti metal can easily take oxygen atoms from a-SiO2 by forming a thermodynamically stable SiO2-x /TiOx composite, meaning that electrochemically inactive a-SiO2 is partially reduced by the addition of Ti metal powder during milling. This mechanically reduced SiO2-x /TiOx composite anode exhibits a greatly improved electrochemical reactivity, with a reversible capacity of more than 700 mAh g(-1) and excellent cycle performance over 100 cycles. Furthermore, an enhancement in the mechanical and thermal stability of the composite during cycling can be mainly attributed to the in situ formation of the SiO2-x /TiOx phase. These findings provide new insight into the rational design of robust, high-capacity, Si-based anode materials, as well as their reaction mechanism.

Size-Controlled Synthesis of Copper Oxide Particles on Reduced Graphene Oxide for Lithium-Ion Battery Anode Applications.

Copper oxide/reduced graphene oxide (rGO) hybrids have been successfully synthesized by attaching copper ions onto the functional groups of GO by means of a solution process, which causes precipitation and agglomeration of copper oxides during subsequent thermal reduction of the GO. The resulting copper oxide/rGO hybrid exhibited improved electrochemical performance compared to monolithic CuO, which is presumed to be due to rGO acting as a mechanical support that buffers the volume change in copper oxides that occurs as a result of the conversion reaction during charge/discharge cycling. Furthermore, it was found that the size of the copper oxide particles can be optimized by adjusting the annealing time, with a hybrid annealed for 30 min achieving a reversible capacity of 544 mA h g(-1) and an initial coulombic efficiency of 62.7%.

Self-assembly of asymmetric fan-shaped dendrimers with different generation numbers at the air-water interface.

Monolayer formation of two dendrimers containing a hydrophilic core group (COOH) and hydrophobic peripheral groups (anthracene and aryl ether tail groups), 4-{10-[4-(3,5-bis-benzyloxy)-phenyl]-anthracen-9-yl}-benzoic acid (G1) and 4-(10-{4-[3,5-bis-(3,5-bis-benzyloxy)-benzyloxy]-phenyl}-anthracen-9-yl)-benzoic acid (G2), were studied. To understand the mechanism of the self-assembly of these molecules, we measured the surface pressure-surface area (Pi- A) isotherm and investigated the surface texture of Langmuir-Blodgett monolayers transferred onto hydrophilic silicon wafers. Both dendrimers form circular domains at the onset point of surface pressure as a result of the difference in hydrophobicity between the core group and the peripheral end group. The core group has a functional group at the end of dendrimer and can be anchored on the water surface. Upon further compression, monolayer of G1 shows a domain of molecules whereas a monolayer of G2 is aligned in the direction of compression at 10 mN/m. At higher surface pressure (20 mN/m), G1 molecules have several aggregates of domains, but G2 molecules maintain their ordering. These results were confirmed by the electron density profile of G1 and G2 monolayers transferred to silicon substrates, as measured by X-ray reflectivity.

Structure formation of hydrophobically end-capped poly(ethylene oxide) in the solid state.

The conformational transition of hydrophobically end-capped poly(ethylene oxide), HP-PEO-HP [hydrophobic-poly(ethylene oxide)-hydrophobic], was studied using X-ray diffraction (XRD), differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy (FTIR) methods. Conformational transitions of HP-PEO-HP from a planar zigzag to a 7/2 helical conformation were observed as the molecular weight of the PEO main chain increased. HP-PEO-HP 1(18), with a PEO molecular weight of 1000 and 18 hydrocarbons on each end, has mainly an alpha-helical structure in poor solvents, whereas alpha and beta conformations coexist in good solvents. This means that the alpha-helical structure caused by the hydrogen bonds between the urethane linkages was broken by the high chain mobility caused by the melted adjacent chains of PEO, and instead, the beta-sheet was formed by the interaction of multiple hydrogen bonds. Another indication of hydrogen bonds breaking at increasing temperature is the transition of the N-H stretching peak in the FTIR data. HP-PEO-HP 2(18) and 4(18), which have 18 hydrocarbons on each end and PEO molecular weights of 2000 and 4000, respectively, and consist mostly of PEO, showed spherulites. This result also suggests that the PEO molecule has a 7/2 zigzag helical conformation. In contrast, HP-PEO-HP 1(18), which is composed of less PEO than HP-PEO-HP 2(18) and 4(18), did not show a spherulite structure.

Probe diffusion from dilute to concentrated in polyelectrolyte solution: salt effect.

The adsorption behaviors between a positively charged poly(allyamine) hydrochloride (PAH) matrix and negatively charged sulfate polystyrene (PS) particle probe were investigated using dynamic light scattering (DLS) and fluorescence photobleaching recovery (FPR) with reference to the matrix and salt concentration. The system experienced a steep decrease of diffusion (flocculation) under dilute conditions and a gradual decrease above semidilute concentrations. The fluorescence photobleaching recovery and viscometry experiments revealed that the probe behaviors in the polyelectrolyte solution were strongly affected by the coil overlap concentration (0.5 g/L poly(allyamine) hydrochloride). Near the coil overlap concentration, the hydrodynamic radius representing the entanglement dimension of the matrix was approximately 30 nm; however, at higher concentrations the radius gradually decreased, suggesting a transition toward a network structure. In this system, the salt performed two roles: (1) reinforcing the electrostatic interaction, and (2) preventing electrostatic interaction between the probe and the matrix.