Electrocatalytic characteristics of the metal hydride electrode for advanced Ni/MH batteries

The electrocatalytic characteristics of a metal hydride (MH) electrode for advanced Ni/MH batteries include the hydrogen adsorption/desorption capability at the electrode/electrolyte interface. The hydrogen reactions at the MH electrode/electrolyte interface are also related to factors such as the surface area of the MH alloy powder and the nature of additives and binder materials. The high-rate discharge capability of the negative electrode in a Ni/MH battery is mainly determined by the mass transfer process in the bulk MH alloy powder and the charge transfer process at the interface between the MH alloy powder and the electrolyte. In this study, an AB₅-type hydrogen-absorbing alloy, Mm (Ni, Co, Al, Mn)_5.02 (where Mm denotes Mischmetal, comprising 43.1 wt.% La, 3.5 wt.% Ce, 13.3 wt.% Pr and 38.9 wt.% Nd), was used as the negative MH electrode material. The MH electrode was charged and discharged for up to 200 cycles. The specific discharge capacity of the alloy electrode decreases from a maximum value of 290–250 mAh g⁻¹ after 200 charge/discharge cycles. A cyclic voltammetry technique is used to analyze the charge transfer reactions at the electrode/electrolyte interface and the hydrogen surface coverage capacity.     

Mechanically alloyed Mg2Ni for metal-hydride–air secondary battery

Mechanically alloyed Mg₂Ni and a single air (oxygen) electrode are used as the anode and cathode, respectively, in a Mg₂Ni|6 M KOH|O₂ rechargeable metal-hydride–air (MH–air) battery. The battery is tested for self-discharge by measuring the open-circuit voltage (OCV) and cycling characteristics. Battery degradation after charge–discharge cycling is characterized by means of X-ray diffraction (XRD) and scanning electron microscopic (SEM) analyses.     

Effects of F-treatment on degradation of Mg2Ni electrode fabricated by mechanical alloying

The effects of surface fluorination on the electrochemical charge–discharge properties of a Mg₂Ni electrode, prepared by mechanical alloying in Ni-MH batteries are investigated. After 20 h milling, Mg and Ni powder form nanocrystalline Mg₂Ni. The discharge capacity of this alloy increases greatly on the initial cycle but, due to the formation of a Mg(OH)₂ passive layer, displays rapid degradation in alkaline solution within 10 cycles. In a 6 M KOH+x M KF electrolyte (x=0.5, 1, and 2), a continuous and stable fluorinated layer is formed and the durability of the Mg₂Ni electrode increases marketly and a high rate discharge capability is obtained (90–100 mAh/g). Addition of 2 M KF leads to the highest durability of all the electrodes tested. The improvement is due to a thin MgF₂—flourinated layer, which reduces the charge-transfer resistance and protects the Mg₂Ni electrode from forming a Mg(OH)₂ layer.     

Polymer Ni–MH battery based on PEO–PVA–KOH polymer electrolyte

An alkaline polymer electrolyte film has been prepared by a solvent-casting method. Poly(vinyl alcohol), PVA is added to improve the ionic conductivity of the electrolyte. The ionic conductivity increases from 10⁻⁷ to 10⁻² S cm⁻¹ at room temperature when the weight percent ratio of poly(ethylene oxide), PEO to PVA is increased from 10:0 to 5:5. The activation energy of the ionic conductivity for the PEO–PVA–KOH polymer electrolyte is 3–8 kJ mol⁻¹. The properties of the electrolyte film are characterized by a wide variety of techniques and it is found that the film exhibits good mechanical stability and high ionic conductivity at room temperature. The application of such electrolyte films to nickel–metal-hydride (Ni–MH) batteries is examined and the electrochemical characteristics of a polymer Ni–MH battery are obtained.     

Multilayered Sn–Zn–Cu alloy thin-film as negative electrodes for advanced lithium-ion batteries

Sn-based alloy compounds have been considered as possible alternatives for carbon in lithium-ion batteries and attract great attentions because of their large electrochemical capacity compared with that of carbon. In this work, a multilayered Sn–Zn/Zn/Cu alloy thin-film electrode has been prepared by electroplating method. The structure and performance of the electrode before and after heat treatment have been investigated. It is found that Cu₆Sn₅ phase and multilayered structure in electrode are formed after heat treatment. This optimized structure of the heat-treated electrode results in enhanced cycle life. The capacity of the electrode is over 320 mA h g⁻¹ after 100 cycles; though it is 83 mA h g⁻¹ after 20 cycles for as-plated electrode. The Sn–Cu and Zn–Cu alloy formed a network in the electrode is considered to strengthen the electrode and reduce the effect of volume expansion and phase transition during cycling. Experimental results also reveal that lower cut-off potential (0.05 V) for charging and higher one (1.2 V) for discharging result in long cycle life and high discharge capacity, respectively. The reason of capacity decay of the heat-treatment electrode during cycling has also been investigated. All these results show that the electroplated Sn–Zn-based alloy film on Cu foil would be a promising negative material with high capacity and low cost for Li secondary batteries.     

Nickel metal hydride batteries for high power applications

Nickel metal hydride (Ni/MH) is presently the most promising battery system for electric and hybrid vehicle propulsion in the short and mid-term. This paper presents the results obtained in the development of prismatic Ni/MH batteries for high power, mainly hybrid vehicle, applications.Valve regulated Ni/MH cells rated at 25 and 60 Ah have been designed and assembled using improved positive and negative electrodes. Both types of cells showed excellent high rate discharge capability and fast rechargeability and a satisfactory charge retention when stored and cycle life under deep cycling and hybrid vehicle working conditions. On the other hand, energy and power efficiency ratios were improved in the 60 Ah cells.     

Development of a lead-acid battery for a hybrid electric vehicle

In September 2000, a project reliable, highly optimized lead-acid battery (RHOLAB) started under the UK Foresight Vehicle Programme with the objective of developing an optimized lead-acid battery solution for hybrid electric vehicles. The work is based on a novel, individual, spirally-wound valve-regulated lead-acid 2 V cell optimized for HEV use and low variability. This cell is being used as a building block for the development of a complete battery pack that is managed at the cell level. Following bench testing, this battery pack is to be thoroughly evaluated by substituting it for the Ni−MH pack in a Honda Insight.The RHOLAB cell is based on the 8 Ah Hawker Cyclon cell which has been modified to have current take-off at both ends—the dual-tab design. In addition, a variant has been produced with modified cell chemistry to help deal with problems that can occur when these valve-regulated lead-acid battery (VRLA) cells operate in a partial-state-of-charge condition. The cells have been cycled to a specially formulated test cycle based on real vehicle data derived from testing the Honda Insight on the various test tracks at the Millbrook Proving Grounds in the UK. These cycling tests have shown that the lead-acid pack can be successfully cycled when subjected to the high current demands from the vehicle, which have been measured at up to 15 C on discharge and 8 C during regenerative recharging, and cycle life is looking very promising under this arduous test regime.Concurrent with this work, battery development has been taking place. It was decided early on to develop the 144 V battery as four 36 V modules. Data collection and control has been built-in and special steps taken to minimize the problems of interconnect in this complex system. Development of the battery modules is now at an advanced stage. The project plan then allows for extensive testing of the vehicle with its lead-acid battery at Millbrook so it can be compared with the benchmark tests which have already been carried out on the vehicle with its Ni−MH batteries.     

Effects of Pd substitution on the electrochemical properties of Mg0.9−xTi0.1PdxNi (x = 0.04–0.1) hydrogen storage alloys

Amorphous Mg_0.9−xTi_0.1Pd_xNi (x = 0.04–0.1) hydrogen storage alloys were prepared by mechanical alloying (MA). The effects of Pd substitution on the electrochemical properties of the Mg_0.9−xTi_0.1Pd_xNi (x = 0.04–0.1) electrode alloys were studied by cyclic charge–discharge, linear polarization, anodic polarization, electrochemical impedance spectroscopy (EIS), and hydrogen diffusion coefficient experiments. It was found that the cyclic capacity retention rate C₅₀/C₁ of the quaternary alloys was greatly improved due to the substitution of Pd for Mg. Mg_0.8Ti_0.1Pd_0.1Ni electrode alloy retained the discharge capacity above 200 mAh g⁻¹ even after 80 charge–discharge cycles, possessing the longest cycle life in the studied quaternary alloys. The improvement of cycle life was ascribed to the formation of passive film on the surface of these electrode alloys. X-ray photoelectron spectroscopy (XPS) analysis proved that the passive film was composed of Mg(OH)₂, TiO₂, NiO, and PdO, which synergistically protected the alloy from further oxidation. The Auger Electron Spectroscopy (AES) study revealed that the thickness of passive film increased with augmentation of the Pd content. The electrochemical impedance study of electrode alloys after different cycles demonstrated that the passive film became thicker during cycles and its thickness also increased with Pd content augmentation. It was also found that the augmentation of Pd content resulted in the decrease of exchange current density I₀ and the increase of the charge-transfer resistance R_ct. With increasing the Pd amount in the Mg_0.9−xTi_0.1Pd_xNi (x = 0.04–0.1) electrode alloys, hydrogen diffusion coefficient D was gradually enhanced at first. Then, it decreased with augmentation of cycle due to the growth of passive film on the surface of the alloys.     

Multi-step constant-current charging method for an electric vehicle nickel/metal hydride battery with high-energy efficiency and long cycle life

This study sought to investigate methods of charging nickel/metal hydride (Ni/MH) batteries for use in an electric vehicle(EV). The specific conditions for the multi-step constant-current charging method with regulation of voltage and dT/dt were varied in an attempt to improve high-energy efficiency, shorten charging time, and increase cycle life. A commercial battery system with 12 modules was subjected to a discharge/charge cycling test using patterns of dynamic stress test (DST) 120. Two-step charging with a first-step current of 0.5 CA (1 CA=95 A) was completed in less than 2.5 h after DST120-pattern discharging to 80% DOD. Further, three-step charging with the first step of 1.0 CA reduced charging time to about 1.5 h. Multi-step constant-current charging provided a high-energy efficiency of more than 80%. The battery system withstood over 1800 cycles in a cycling test with reduction DST120-pattern discharge of 20%, but reductions in constant-current discharge of only 7%, due to a gradual decrease of discharge power, independently of the value of the charging current.     

Studies on preparation and performances of carbon aerogel electrodes for the application of supercapacitor

Carbon aerogel was prepared by the polycondensation of resorcinol (R) with formaldehyde (F), and sodium carbonate was added as a catalyst (C). Physical properties of carbon aerogel were characterized by infrared spectrometer (IR), X-ray diffraction (XRD) and scanning electron microscopy (SEM). It is found that carbon aerogel is an amorphous material with a pearly network structure, and it consists of one or two diffuse X-ray peaks. The results of cyclic voltammetry indicated that the specific capacitance of a carbon aerogel electrode in 6 M KOH electrolyte was approximately 110.06 F g⁻¹. Through the galvanostatic charge/discharge measurement, it was found that the electrode is stable in KOH electrolyte, the maximum capacitance of the supercapacitor with carbon aerogel as the electrode active material was 28 F g⁻¹. Besides, the supercapacitor has long cycle life. Thus, it was thought that the carbon aerogel is an excellent electrode material for a supercapcitor.     

Cresol–formaldehyde based carbon aerogel as electrode material for electrochemical capacitor

Carbon aerogels have been prepared through a polycondensation of cresol (Cm) with formaldehyde (F) and an ambient pressure drying followed by carbonization at 900 °C. Modification of the porous structures of the carbon aerogel can be achieved by CO₂ activation at various temperatures (800, 850, 900 °C) for 1–3 h. This process could be considered as an alternative economic route to the classic RF gels synthesis. The obtained carbon aerogels have been attempted as electrode materials in electric double-layer capacitors. The relevant electrochemical behaviors were characterized by constant current charge–discharge experiments, cyclic voltammetry and electrochemical impedance spectroscopy in an electrolyte of 30% KOH aqueous solution. The results indicate that a mass specific capacitance of up to 78 F g⁻¹ for the non-activated aerogel can be obtained at current density 1 mA cm⁻². CO₂ activation can effectively improve the specific capacitance of the carbon aerogel. After CO₂ activation performed at 900 °C for 2 h, the specific capacitance increases to 146 F g⁻¹ at the same current. Only a slight decrease in capacitance, from 146 to 131 F g⁻¹, was observed when the current density increases from 1 to 20 mA cm⁻², indicating a stable electrochemical property of carbon aerogel electrodes in 30% KOH aqueous electrolyte with various currents.     

MmNi3.55Co0.75Mn0.4Al0.3B0.3 hydrogen storage alloys for high-power nickel/metal hydride batteries

Non-stoichiometric La-rich MmNi_3.55Co_0.75Mn_0.4Al_0.3B_0.3 hydrogen storage alloys using B–Ni or B–Fe alloy as additive and Ce-rich MmNi_3.55Co_0.75Mn_0.4Al_0.3B_0.3 one using pure B as additive have been prepared and their microstructure, thermodynamic, and electrochemical characteristics have been examined. It is found that all investigated alloys show good activation performance and high-rate dischargeability though there is a certain decrease in electrochemical capacities compared with the commercial MmNi_3.55Co_0.75Mn_0.4Al_0.3 alloy. MmNi_3.55Co_0.75Mn_0.4Al_0.3B_0.3 alloys using B–Ni alloy as additive or adopting Ce-rich mischmetal show excellent rate capability and can discharge capacity over 190 mAh/g even under 3000 mA/g current density, which display their promising use in the high-power type Ni/MH battery. The electrochemical performances of these MmNi_3.55Co_0.75Mn_0.4Al_0.3B_0.3 alloys are well correlated with their microstructure, thermodynamic, and kinetic characteristics.     

Electrochemical profile of nano-particle CoAl double hydroxide/active carbon supercapacitor using KOH electrolyte solution

A nano-structured CoAl double hydroxide with an average particle size of 60–70 nm was prepared by a chemical co-precipitation. It was used as a positive electrode for the asymmetric hybrid supercapacitor in combination with an active carbon negative electrode in KOH electrolyte solution. The electrochemical capacitance performance of this kind of hybrid supercapacitor was investigated by means of cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge–discharge tests. A specific capacitance of 77 F g⁻¹ with a specific energy density of 15.5 wh kg⁻¹ was obtained for the hybrid supercapacitor within the voltage range of 0.9–1.5 V. The supercapacitor also exhibits a good cycling performance and keep 90% of initial capacity over 1000 cycles.     

The effects of partial substitution of Cr for Ni on the electrochemical properties of Mg1.75Al0.25Ni1−xCrx (0 ≤ x ≤ 0.3) electrode alloys

In this paper, the substitution of different amounts of Cr for Ni in the hydrogen storage electrode alloy of Mg_1.75Al_0.25Ni has been carried out to form quaternary Mg_1.75Al_0.25Ni_1−xCr_x (0 ≤ x ≤ 0.3) alloys by means of solid diffusion method (DM). The XRD profiles exhibited that the quaternary alloys still kept the same main phase of Mg₃AlNi₂ (S.G. Fd3m) as that of ternary Mg_1.75Al_0.25Ni alloy. The electrochemical studies found that Cr substituted quaternary alloy reached its maximum discharge capacity (165 mAh g⁻¹) after 2 cycles, which was larger than that of the Mg_1.75Al_0.25Ni alloy (154 mAh g⁻¹). Among these quaternary alloys, the Mg_1.75Al_0.25Ni_0.9Cr_0.1 electrode alloy was found possessing the highest cycling capacity retention rate. Cyclic voltammetry (CV) results and anodic polarization curves demonstrated that appropriate content (x lower than 0.1) of Cr effectively improved the reaction activity of electrode and inhibited the cycling capacity degradation to some degree. Electrochemical impedance spectroscopy (EIS) analyses indicated that the increase of Cr content would raise the polarization resistance R_p on the particle surface of these quaternary alloys.     

Lead-acid batteries with foam grids

Conventional lead-acid batteries are relatively heavy and thus have a low specific energy. Therefore, to improve the energy density, a lighter grid has been proposed. In this work, a novel lead-acid battery with high specific surface area negative foam current collectors was designed and constructed. The collectors were studied by cyclic voltametery (CV) and electrochemical impedance spectroscopy (EIS). The foam collectors were designed and suitable paste composition and formation algorithm was obtained. The basic cells were manufactured and its performance was evaluated. The results showed that the foam grids resistance was lower than that for lead grids and the specific surface area of the foam grids was very greater than lead grids. The foam battery has good discharge characteristics compared with common lead-acid batteries. The discharge curve was flat and negative mass utilization efficiency was higher than 50% when the cell was discharged with C5/5 A (137 Ah kg⁻¹ negative active materials). The foam grids were used as negative electrode for various types of lead-acid batteries such as 60 Ah starter battery, 2, 3 and 10 Ah VRLA batteries. The batteries with foam grids were shown longer cycle life than conventional lead-acid batteries.     

Development of high-capacity nickel-metal hydride batteries using superlattice hydrogen-absorbing alloys

New R–Mg–Ni (R: rare earths) superlattice alloys with higher-capacity and higher-durability than the conventional Mm–Ni alloys with CaCu₅ structure have been developed. The oxidation resistibility of the superlattice alloys has been improved by optimizing the alloy composition by such as substituting aluminum for nickel and optimizing the magnesium content in order to prolong the battery life. High-capacity nickel-metal hydride batteries for the retail market, the Ni-MH2500/900 series (AA size type 2500 mAh, AAA size type 900 mAh), have been developed and commercialized by using an improved superlattice alloy for negative electrode material.     

High-power batteries for the new 36/42 V automotive systems

The new automotive architectures will require high-voltage, high-power batteries that are completely different from existing 12 V flooded lead–acid products. Battery requirements for these applications are reviewed first from the standpoint of the vehicle, and then from an electrochemical/thermal performance perspective. Design and performance characteristics are then critically evaluated for the three major candidates for the new 36/42 V systems, namely: valve-regulated lead–acid (VRLA), nickel–metal-hydride (Ni–MH), and lithium-ion (Li-ion). Design and manufacturing requirements, performance strengths and weaknesses, reliability issues, markets and pricing are then examined for the VRLA battery, which appears to be the leading candidate at this time.     

Activation of raw pitch coke with alkali hydroxide to prepare high performance carbon for electric double layer capacitor

Powder of raw pitch coke was activated with alkali hydroxides at 500–900 °C to prepare carbon electrode of high capacitance for electric double layer capacitor (EDLC). KOH provided very high surface area of 2320 m²/g at 800 °C, while NaOH did moderate surface area of 1000 m²/g at 650–750 °C. High surface area provided by KOH led to a high capacitance per weight of 39 F/g. However, its capacitance per volume was as low as 16 F/ml. Although the coke of moderate surface area activated with NaOH showed a similar capacitance per weight, its capacity per volume was as high as 28 F/ml because of its high density. Adequate porosity must be selectively introduced by NaOH activation to the coke to obtain moderate surface area. Much smaller expansion of layers in the present needle type coke activated by NaOH than that by KOH is indicative for the higher density of the former activated coke.     

Evaluation of different approaches for improving the cycle life of MgNi-based electrodes for Ni-MH batteries

Several methods have been investigated to enhance the cycle life of amorphous MgNi used as the negative electrode for Ni-MH batteries. The first approach involves modifying its surface composition in different ways, including the electroless deposition of a chromate conversion coating, the addition of chromate salt or NaF into the electrolyte and the mechanical coating of the particles with various compounds (e.g. TiO₂). Another approach consists of developing (MgNi + AB₅) composite materials. However, the cycle life of these modified MgNi electrodes remains unsatisfactory. On the other hand, the modification of the bulk composition of the MgNi alloy with elements such as Ti and Al appears to be more effective. For instance, a Mg_0.9Ti_0.1NiAl_0.05 electrode retains 67% of its initial discharge capacity (404 mAh g⁻¹) after 15 cycles compared to 29% for MgNi. The charging conditions also have a great influence on the electrode cycle life as demonstrated by the existence of a charge input threshold below which minor capacity decay occurs. In addition, the particle size has a major influence on the electrode performance. We have developed an optimized electrode constituted of Mg_0.9Ti_0.1NiAl_0.05 particles with the appropriate size (>150 μm) showing a capacity decay rate as low as ∼0.2% per cycle when charged at 300 mAh g⁻¹.     

Preparation and optimization of RuO2-impregnated SnO2 xerogel supercapacitor

A Sb (6 mol%)-doped SnO₂ xerogel impregnated with RuO₂ nanocrystallites is prepared via an incipient-wetness method and is optimized for its electrochemical capacitance in aqueous 1 M KOH electrolyte by adjusting the calcination temperature and the RuO₂ loading. The electrode capacitance does not increase monotonically with increasing RuO₂ loading. A maximum electrode capacitance of 15 F g⁻¹, which represents a three-fold increase compared with the blank xerogel and a specific RuO₂ capacitance of 710 F g⁻¹ RuO₂, is obtained with a RuO₂ loading of 1.4 wt.% and by calcination at 200 °C. Higher loadings presumably result in a homogeneous nucleation upon drying, which causes severe reduction in the total surface area of the RuO₂ crystallites.