C.C. Wan

Review of gel-type polymer electrolytes for lithium-ion batteries

This review describes the advantages and characteristics of employing polymer electrolytes in solid-state lithium-ion batteries. Criteria for an ideal polymer electrolyte and the differences between polyelectrolytes are discussed. The emphasis of this article is on plasticized or gelled electrolyte systems. Hence, the review focuses on four plasticized systems which have received particular attention from a practical viewpoint, i.e., poly(ethylene oxide) (PEO)-, poly(acrylonitrile) (PAN)-, poly(methyl methacrylate) (PMMA)-, and poly(vinylidene fluoride) (PVdF)-based electrolytes. Some critical concepts and points associated with this emerging technology that still require attention are discussed in the final part of the review.

Two- and three-electrode impedance spectroscopy of lithium-ion batteries

Abstract The interfacial impedance between PVDF/HFP-based electrolytes and lithium metal continues to increase and attains a delicate kinetic equilibrium upon prolonged storage. The graphite anode, on the other hand, is found to remain inert towards the electrolytes. Its interfacial impedance does not vary with increasing storage time or in the presence of different lithium salts. In addition, it is found that the impedance of a Li–C half-cell consists of the impedances of two interfaces and is therefore often mistakenly used in the interpretation of the behaviour of a single carbon electrode. Thus, a three-electrode impedance study is required. It is found that an inductive loop appears in the low-frequency region of the impedance spectrum of a carbon electrode immediately after the first lithium-intercalation step, which probably implies that an adsorption–desorption phenomenon might exist at the interface. Moreover, another inductive effect, which arises from the connecting leads, also appears in the high-frequency region. Finally, the cathode is found to be the major source of cell impedance and increases with increasing cycle number.

Conductivity Study of Porous Plasticized Polymer Electrolytes Based on Poly(vinylidene fluoride) A Comparison with Polypropylene Separators

The novel porous plasticized poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF/HFP)‐based electrolytes, compared with conventional polypropylene (PP) separators (Celgard® 2400), were studied via electrochemical impedance spectroscopy, gas adsorption/desorption method, differential scanning calorimetry, and a simple wettability test. The obtained impedance spectra of the electrolytes and separators are extensively discussed, inclusive of the effect of poor wetting upon them. The average pore diameter and Brunauer‐Emmett‐Teller surface area of PVDF/HFP‐based electrolyte membranes are 16.4 nm and , respectively. Ionic conductivity and electrolyte retention characteristic of these electrolyte membranes are superior to conventional PP separators. Moreover, PVDF/HFP‐based electrolyte membranes are free from the problem of wetting whereas the poor wetting of PP separators in some electrolytes may cause its effective conductivity to decrease by at least one order of magnitude. The enhanced wettability may be achieved by virtue of the swelling phenomenon between the polymer and the electrolytes. However, the activation energy for the conduction of PVDF/HFP‐based electrolytes is still larger than that of their parent neat electrolytes , which may imply that the influence of PVDF/HFP upon ionic mobility still exists even if they have been made nanoporous.

Heat dissipation design for lithium-ion batteries

Abstract A two-dimensional, transient heat-transfer model for different methods of heat dissipation is used to simulate the temperature distribution in lithium-ion batteries. The experimental and simulation results show that cooling by natural convection is not an effective means for removing heat from the battery system. It is found that forced convection cooling can mitigate temperature rise in the battery. Nevertheless, a non-uniform distribution of temperature on the surface of the battery is inevitable and this makes thermal management difficult. As a better means of suppressing increases in temperature, a heat pipe has been used to effect heat dissipation. The connection between the heat pipe and the battery wall pays an important role in heat dissipation. Inserting the heat pipe in to an aluminum fin appears to be suitable for reducing the rise in temperature and maintaining a uniform temperature distribution on the surface of the battery.

Composition analysis of the passive film on the carbon electrode of a lithium-ion battery with an EC-based electrolyte

Abstract This work examines the formation of a passive film on the carbon electrode of lithium-ion batteries. With a single solvent of EC (ethylene carbonate), the structure of the passive film is found to be (CH2OCOOLi)2. In a DEC (diethyl carbonate) or DMC (dimethyl carbonate) system, C2H5OCOOLi and Li2CO3 are formed on the surface of the carbon electrode. According to results from mass spectra, CO2 gas is the main product when EC is decomposed. By contrast, DEC is decomposed into CO and C2H6, and DMC into CO and CH4. These findings suggest that the composition of the passive film depends on the chosen solvent. In a binary solvent system which contains EC, the passive film contains chiefly (CH2OCOOLi)2, which is identical to a single EC solvent system.

Surface treatment for hydrogen storage alloy of nickel/metal hydride battery

Abstract The electrochemical performance of AB 2 -type (Ti 0.35 Zr 0.65 Ni 1.2 V 0.6 Mn 0.2 Cr 0.2 ) and AB 5 -type (MmB 4.3 (Al 0.3 Mn 0.4 ) 0.5 ) hydrogen storage alloys modified by hot KOH etching and electroless nickel coating has been investigated. It is found that the alloy modified with hot KOH solution shows quick activation but at the expense of cycle-life stability. The alloy coated with nickel was effectively improved in both cycle-life stability and discharge capacity. Both the exchange and limiting current densities were increased by modifying the alloys by hot KOH solution dipping or electroless nickel coating as compared with untreated alloy electrode. The electrode with higher exchange current density and limiting current density leads to increased high-rate dischargeability. A duplex surface modified alloy (i.e., alloy first treated with hot KOH solution and then coated with nickel) has been developed, which performs satisfactorily with respect to both quick activation and long cycle life. In addition, the high-rate dischargeability for the electrode with duplex surface modification is superior to that of electrode solely treated with KOH etching or Ni plating.

Thermal Stability of the Solid Electrolyte Interface on Carbon Electrodes of Lithium Batteries

The type of lithium salts (LiPF 6 and LiClO 4 ) was found to have a strong impact on the thermal stability of the solid electrolyte interface (SEI) layer formed on graphite electrodes. According to Fourier transform infrared spectra, the dominant species of the SEI layer is the ethylene carbonate reduction product. Hence, the change of the impedance of the SEI layer is determined by its structure. The oscillating phenomena of the thickness of the SEI layer in the LiPF 6 system can be attributed to an alternating deterioration and reformation of SEI, illustrating its inherent thermal instability. As for the LiClO 4 system, because it is less reactive, the buildup of the SEI layer is stable and gradual. In addition. PF 5 , a decomposition product of LiPF 6 , was identified to be a chief source of the thermal instability of SE1 in the LiPF 6 system.

An innovative process for PEMFC electrodes using the expansion of Nafion film

Swelling of a Nafion film makes it very difficult to deposit thin catalyst layers on a membrane electrolyte. In this study, a process has been developed that utilizes the expansion of Nafion film in the deposition so as to mitigate this problem. When the film is fully expanded, catalyst ink can be applied on to the membrane directly by means of spraying or blade pasting without concerns over swelling. The resulting membrane electrode assembly shows good performance relative to commercial products.

Impedance spectroscopic study for the initiation of passive film on carbon electrodes in lithium ion batteries

The formation of passive film at the interface between the mesocarbon microbeads (MCMB) electrode and the organic electrolyte in a lithium-ion battery during the initial period of intercalation was investigated by a.c.-impedance spectroscopy. An equivalent-circuit model consisting of five parallel RC-circuits in series combination was adopted for the curve-fitting analysis of the obtained impedance spectra. The results indicated that both the total interfacial resistance and the passive film thickness increased with decreasing intercalation potential in the ethylene carbonate (EC) or dimethyl carbonate (DMC) single-solvent system, whereas an opposite trend was observed in the system containing diethyl carbonate (DEC) only. In addition, the total interfacial resistance was clearly affected by the porous structure of the passive film in a single-solvent system. In binary solvent systems such as EC/DEC and EC/DMC, on the other hand, the effect of the porous structure on the total interfacial resistance was negligible. The total interfacial resistance and the passive film thickness were also smaller in these systems than those in single-solvent systems. Finally, the variation of the total interfacial resistance and of the passive film thickness in the EC/DEC (or EC/DMC) system were also found to be similar to those in the parent DEC (or DMC) system during intercalation.

Conductive behaviour of lithium ions in polyacrylonitrile

The conductivity of lithium ions in polyacrylonitrile (PAN) is examined. Using the d.c. polarization method, the electric transport coefficient was observed to be less than 0.05. X-ray diffraction analysis confirms that PAN-LiClO4 has an amorphous structure. Fourier-transform infrared spectroscopy results indicate that Li+ ions form bonds not only with the C≡N groups of the PAN, but also with CO groups of the dimethylformamide. The relationship between conductivity and temperature suggests that the conduction of Li+ ions in PAN is achieved through the polymer backbones segmental motion. Finally, the activation energy of PAN(LiClO4)0.2 is estimated to be 86 kJ/mol.