Studies on rechargeable NiMH batteries

Nickel-metal hydride (NiMH) batteries offer some advantages in the aspects of power, cycle life and environment. However, they are encountering challenge from other rechargeable batteries such as Li-ion batteries. In this paper, the possibility of performance improvements and cost reduction of NiMH batteries were analyzed. Some approaches to improve energy densities and reduce the battery cost were discussed. An attempt to increase the energy density of NiMH batteries was made through improvements in the specific capacity of rare earth-based AB₅${}_5$-type hydride alloys in the negative electrodes and capacity enhancement of nickel hydroxide electrode. A relatively lower cost small cylindrical NiMH cell was constructed and its properties were evaluated.     

Valve-regulated lead-acid batteries

Valve-regulated lead-acid (VRLA) batteries with gelled electrolyte appeared as a niche market during the 1950s. During the 1970s, when glass-fiber felts became available as a further method to immobilize the electrolyte, the market for VRLA batteries expanded rapidly. The immobilized electrolyte offers a number of obvious advantages including the internal oxygen cycle which accommodates the overcharging current without chemical change within the cell. It also suppresses acid stratification and thus opens new fields of application. VRLA batteries, however, cannot be made completely sealed, but require a valve for gas escape, since hydrogen evolution and grid corrosion are unavoidable secondary reactions. These reactions result in water loss, and also must be balanced in order to ensure proper charging of both electrodes. Both secondary reactions have significant activation energies, and can reduce the service life of VRLA batteries, operated at elevated temperature. This effect can be aggravated by the comparatively high heat generation caused by the internal oxygen cycle during overcharging. Temperature control of VRLA batteries, therefore, is important in many applications.     

HEV battery heating using AC currents

A unique method has been developed for internally heating hybrid electric vehicle (HEV) batteries at cold temperatures using alternating current (AC). The poor performance of these batteries in cold climates is of major concern because they suffer a huge loss in capacity. Another symptom of this low performance is a dramatic increase in the series resistance of the battery, R_B, as the temperature drops. Experiments were performed with both low and high frequency heaters, and several tests were conducted on both lead acid and nickel metal hydride (NiMH) batteries at different AC amplitudes, states of charge (SOCs) and cold temperatures. Low frequency 60 Hz heating was first tested on several different 38 Ah lead acid batteries. The feasibility of using high frequency heating was then tested using a 10–20 kHz inverter on a pack of 6.5 Ah nickel metal hydride (NiMH) batteries. A technique also was developed to estimate the internal battery temperature, T_bat, by measuring the battery source resistance, R_B.     

Effects of surface treatments of MlNi4.0Co0.6Al0.4 hydrogen storage alloy on the activation,charge/discharge cycle and degradation of Ni/MH batteries

The effects of the surface treatment of the hydrogen storage alloy on the activation property and cycle life of nickel/metal-hydride (Ni/MH) batteries were investigated by means of the electrochemical impedance spectra. It was found that the oxide layer on the alloy surface affected its electrochemical properties and catalysis for the oxygen combination. Therefore, Ni/MH battery employed the untreated alloy as negative electrode material exhibited bad activation property, short cycle life and high internal pressure. Because of the improvement in the metal hydride electrode electrochemical characteristics and catalysis for oxygen recombination by the surface treatment of the alloy in 0.02 M KBH₄+6 M KOH or 6 M KOH solution, the battery used the treated alloy as negative exhibited good activation, long cycle life and low internal pressure. The composition and dissolution of the alloy surface were analyzed by an electron probe microanalysis (EPMA) and induced coupled plasma spectroscopy (ICP). It was found that the Ni-rich surface layer was an important factor to improve the activation and cycle life of battery.     

Progress in high-power nickel–metal hydride batteries

High demands to power performance, high cycle and calendar life as well can be met by NiMH batteries, making this battery system very suitable for HEV applications. The hydrogen storage alloy plays an important role with respect to power performance and life duration. Power performance and cycle life behaviour are related to each other by the electrochemical and mechanical properties of the alloy, via a more or less reciprocal relationship. In terms of power performance at medium-discharge rates, the charge transfer reaction at the hydrogen storage alloy interface was found to be crucial for the temperature-dependent behaviour of the cell, whereas at discharge rates above about 15C diffusion limitation was found especially at the negative electrode. The alloy corrosion is taking place in alkaline media, leading to the formation of surface films and a change of the chemical composition, especially in near surface regions of the alloy particles. Consecutive electrochemical cycles lead to mechanical stress and finally cracking of the alloy particles. Stability against corrosion and pulverisation on one hand and good electrochemical performance on the other hand both depend on the chemical composition of the alloy, its morphological properties and the cycling regime used.     

Hydrogen storage material based on LaNi5 alloy produced by mechanical alloying

The electrochemical characteristics of the La_0.8Ce_0.2Ni_2.5Co_1.8Mn_0.4Al_0.3 compound, produced by mechanical alloying, are investigated for hydrogen storage in nickel-metal hydride (NiMH) batteries by discharging tests at constant current and by calculating equilibrium pressure of hydrogen from the equilibrium potentials. It is shown that the alloy produced by mechanical alloying, followed by annealing and activation exhibits high specific capacity at the stable potential plateau, even at the high discharge rate (10 mA cm⁻²), and low hydrogen equilibrium pressure. The alloy of such composition gives low capacity loss during cycling, which enables its application for metal hydride battery production.     

Structural,hydrogen storage,and electrochemical properties of Laves phase-related body-centered-cubic solid solution metal hydride alloys

Structure, gaseous phase hydrogen storage, and electrochemical properties of a series of Laves phase-related BCC solid solution metal hydride alloys with BCC/C14 ratios ranging from 0.09 to 8.52 were studied. Some properties are correlated to the phase abundance and V-content in the alloy with monotonic evolutions, for example, lattice constant, phase abundance, and hydrogen storage pressure. Other properties such as gaseous phase capacities, PCT hysteresis, high-rate dischargeability, and bulk hydrogen diffusion correlate better with the C14 phase crystallite size, which are considered to be more related to the synergetic effect between main and secondary phases. In contrast with conventional metal hydride alloys used in NiMH batteries, the electrochemical discharge capacities of these alloys are not between the maximum and the reversible hydrogen storage measured in the gaseous phase. The current study's alloys have electrochemical capacities that are insensitive to composition but have much room for improvement, with high-rate dischargeabilities that are superior compared to other commercially available alloys. With further research, these alloys show potential for high-rate battery applications.     

High performance nickel-metal hydride and lithium-ion batteries

In comparison to pure electric vehicles (EV) the opportunities for hybrid electric vehicles (HEV) are much better, since range restrictions no longer apply and the interaction of the internal combustion engine and electrical drive bring increased energy efficiency and environmental friendliness. The batteries used in such applications must meet very high standards in terms of performance and service life. Although the battery capacity is smaller than for a purely EV, it needs to be able to generate far higher levels of power. The technical challenges of hybrid applications have led to the development of high-performance batteries. At the forefront of these is the nickel-metal hydride system (NiMH). With specific power and energy data in the range from 300 to 900 W/kg, 55 to 37 Wh/kg, respectively (based on cell weight), excellent charge efficiency and energy throughput levels of more than 10,000 times the nominal energy, the NiMH system comes very close to satisfying the needs of the HEV. Parallel developments with the lithium-ion system based on manganese spinel as cathode material show that, with specific power and energy levels above 1000 W/kg, 50 Wh/kg, respectively, this technology will also be able to play an important role in the future. Service life figures in terms of calendar life have been improved tremendously to about three years, but there is still a need for further improvement in order to meet the specifications of car manufacturers. For this reason, an increase of life span is the subject of intensive development work.     

Enhancement of the high-temperature performance of advanced nickel–metal hydride batteries with NaOH electrolyte containing NaBO2

In this paper, an alternative approach to improve the high-temperature performance of nickel–metal hydride (Ni–MH) batteries is proposed by introducing NaOH electrolyte with sodium metaborate (NaBO₂) additives. Compared with conventional batteries using KOH electrolyte, the in-house prepared batteries with proposed electrolytes exhibit an enhanced discharge capacity, improved high-rate discharge ability, increased cycle stability and reduced self-discharge rate at an elevated temperature (70 °C). The charge acceptance of these Ni–MH batteries at 70 °C is over 96% at a charge/discharge rate of 1 C. These performance improvements are ascribed to the increased oxygen evolution overpotential, slower oxygen evolution rate and lower electrochemical impedance, as indicated by CV, steady-state polarization measurements and EIS. The results suggest that the proposed approach be an effective way to improve the high-temperature performance of Ni–MH batteries.     

An ultracapacitor circuit for reducing sulfation in lead acid batteries for Mild Hybrid Electric Vehicles

The nickel metal hydride (NiMH) batteries used in most hybrid electric vehicles (HEVs) provide satisfactory performance, but are quite expensive. In spite of their lower energy density, lead acid batteries would be much more economical except they are prone to sulfation in HEV applications. However, sulfation can be greatly reduced by a circuit that uses an ultracapacitor in conjunction with the battery. The resulting system will provide much cheaper energy storage if ultracapacitor prices can be reduced to levels predicted by some manufacturers.     

Effect of molybdenum content on structural, gaseous storage, and electrochemical properties of C14-predominant AB2 metal hydride alloys

The structure, gaseous storage, and electrochemical properties of Mo-modified C14-predominant AB₂ metal hydride alloys were studied. The addition of Mo expands the unit cell volume and stabilizes the metal hydride. This increased metal-to-hydrogen bond strength reduces the equilibrium plateau pressure, reversible hydrogen storage, and the high-rate dischargeability in the flooded cell configuration, but not the high-rate dischargeability in the sealed cell configuration. The low-temperature performance was improved by the addition of Mo through increases in bulk diffusion rate, surface area, and surface catalytic ability. The increase in bulk diffusion is the result of smaller crystallites and larger AB₂-AB₂ grain boundary densities. The increase in surface area is due to the high solubility of Mo in alkaline solution. Even with a higher leaching rate, the Mo-containing alloys still have strong corrosion resistance which contributes positively to both the charge retention and the cycle life performances. As the Mo-content in the alloy increases, the low temperature performance improves at the expense of a lower capacity.     

Spent NiMH batteries—The role of selective precipitation in the recovery of valuable metals

The production of electronic equipment, such as computers and cell phones, and, consequently, batteries, has increased dramatically. One of the types of batteries whose production and consumption has increased in recent times is the nickel metal hydride (NiMH) battery. This study evaluated a hydrometallurgical method of recovery of rare earths and a simple method to obtain a solution rich in Ni-Co from spent NiMH batteries. The active materials from both electrodes were manually removed from the accumulators and leached. Several acid and basic solutions for the recovery of rare earths were evaluated. Results showed that more than 98 wt.% of the rare earths were recovered as sulfate salts by dissolution with sulfuric acid, followed by selective precipitation at pH 1.2 using sodium hydroxide. The complete process, precipitation at pH 1.2 followed by precipitation at pH 7, removed about 100 wt.% of iron and 70 wt.% of zinc from the leaching solution. Results were similar to those found in studies that used solvent extraction. This method is easy, economic, and does not pose environmental threats of solvent extraction.     

Solar photovoltaic charging of high voltage nickel metal hydride batteries using DC power conversion

There are an increasing number of vehicle choices available that utilize batteries and electric motors to reduce tailpipe emissions and increase fuel economy. The eventual production of electricity and hydrogen in a renewable fashion, such as using solar energy, can achieve the long-term vision of having no tailpipe environmental impact, as well as eliminating the dependence of the transportation sector on dwindling supplies of petroleum for its energy. In this report we will demonstrate the solar-powered charging of the high-voltage nickel-metal hydride (NiMH) battery used in the GM 2-mode hybrid system. In previous studies we have used low-voltage solar modules to produce hydrogen via the electrolysis of water and to directly charge lithium-ion battery modules. Our strategy in the present work was to boost low-voltage PV voltage to over 300 V using DC-DC converters in order to charge the high-voltage NiMH battery, and to regulate the battery charging using software to program the electronic control unit supplied with the battery pack. A protocol for high-voltage battery charging was developed, and the solar to battery charging efficiency was measured under a variety of conditions. We believe this is the first time such high-voltage batteries have been charged using solar energy in order to prove the concept of efficient, solar-powered charging for battery-electric vehicles.     

Assessment and reuse of secondary batteries cells

The popularity of portable electronic devices and the ever-growing production of the same have led to an increase in the use of rechargeable batteries. These are often discarded even before the end of their useful life. This, in turn, leads to great waste in material and natural resources and to contamination of the environment. The objective of this study was thus to develop a methodology to assess and reuse NiMH battery cells that have been disposed of before the end of their life cycle, when they can still be used. For such, the capacity of these cells, which were still in good operating conditions when the batteries were discarded, was assessed, and the percentage was estimated. The results reveal that at the end of the assessment process, a considerable number of these cells still had reuse potential, with approximately 37% of all discarded and tested cells being approved for reuse. The methodology introduced in this study showed it is possible to establish an environmentally correct alternative to reduce the amount of this sort of electronic trash.     

Electrochemical performance of sealed NiMH batteries using nanocrystalline TiNi-type hydride electrodes

Mechanical alloying (MA) process was used to synthesize nanocrystalline TiNi-type hydrogen storage materials. The electrochemical behavior of these compounds in conditions of their performance in sealed rechargeable nickel–metal hydride batteries are presented and compared with those of Ni–Cd battery. The cells were constructed by the pressing negative and positive electrodes, polyamide separator and KOH (ρ=1.20 g cm⁻³) as electrolyte solution. The effects of the chemical composition of the nanocrystalline TiNi-type alloy with alkaline solutions were investigated. It was found that the respective replacement of nickel in TiNi by Fe or by Zr improved not only the discharge capacity, but also the cycle life of these electrodes. The results show that the sealed battery using the nanocrystalline TiNi_0.75Fe_0.25 alloy has a capacity about two times that with of TiNi.     

High performance nickel–metal hydride battery in bipolar stack design

The consumption of fuel in cars can be reduced by using hybrid concepts. Even for fuel cell vehicles, a high power battery may cut costs and allow the recovery of energy during retarding. Alkaline batteries, such as nickel–metal hydride batteries, have displayed long cycle life combined with high power ability. In order to improve the power/energy ratio of Ni/MH to even higher values, the cells may be arranged in a bipolar stack design.     

High energy density strategies: from hydride-forming materials research to battery integration

Two different strategies are outlined to develop both high energy density and space-efficient batteries, including the most widely applied rechargeable nickel-metal hydride (NiMH) and Li-ion batteries. The hydrogen storage capacity of fluorite-structured Mg-containing compounds are shown to have a reversible electrochemical storage capacity of more than four times that of the commonly used MischMetal-based AB₅ compounds in NiMH, i.e. 1500 mAh/g (5.6 wt.%). The formation of octahedral sites within the crystal lattice is argued to be very crucial for the improved kinetics of hydrogen absorption and desorption. It is shown that the fluorite-structure can be conserved with both precious Sc and the less expensive Ti up to a Mg content of 80 at.%. Both thermodynamic and kinetic data are presented in relation to the materials composition. In addition, the development of preshaped batteries, as the first step to battery integration, has contributed to a much higher level of design freedom for portable electronic equipment. The manufacturing process of preshaped batteries will be described together with their electrochemical characteristics. Advantageously, the mechanical stability is provided locally by polymer rivets, allowing to get rid of heavy metallic casings and to make use of a much wider range of battery shapes.     

Hydrometallurgical separation of rare earth elements, cobalt and nickel from spent nickel-metal-hydride batteries

The separation of rare earth elements, cobalt and nickel from NiMH battery residues is evaluated in this paper. Analysis of the internal content of the NiMH batteries shows that nickel is the main metal present in the residue (around 50% in weight), as well as potassium (2.2-10.9%), cobalt (5.1-5.5%), rare earth elements (15.3-29.0%) and cadmium (2.8%). The presence of cadmium reveals that some Ni-Cd batteries are possibly labeled as NiMH ones. The leaching of nickel and cobalt from the NiMH battery powder with sulfuric acid is efficient; operating variables temperature and concentration of H₂O₂ has no significant effect for the conditions studied. A mixture of rare earth elements is separated by precipitation with NaOH. Finally, solvent extraction with D2EHPA (di-2-ethylhexyl phosphoric acid) followed by Cyanex 272 (bis-2,4,4-trimethylpentyl phosphinic acid) can separate cadmium, cobalt and nickel from the leach liquor. The effect of the main operating variables of both leaching and solvent extraction steps are discussed aiming to maximize metal separation for recycling purposes.     

锂离子电池容量衰减机理研究进展

容量衰减与电池循环寿命直接相关。导致锂离子电池容量衰减的原因主要包括：固体电解质界面膜（SEI）的增长、电解液的分解、电极材料结构破坏、活性物质的溶解和相转变等。过充过放电、不良的储存或使用温度等外部因素也会导致电池容量衰减。本文综述了近年来锂离子电池容量衰减机理的研究进展。     

Metal hydride batteries research using nanostructured additives

We describe here, our recent research efforts to improve the capacity of metal hydride batteries using nanostructured additives. Nanostructured additives of palladium, copper and nickel were incorporated separately into the negative electrode of the metal hydride batteries. The nanomaterials were synthesized by template-based methods and characterized by scanning electron microscopy. These nanomaterials were incorporated in the negative electrode of the metal hydride battery and the electrochemical performance at 2 C rate was studied. The nanomaterial-incorporated negative electrodes all showed increased cell voltage and negative electrode potential compared to that of a pristine cell. The increase in discharge capacity for a cut-off voltage of 1 V depends on the nanomaterial incorporated and a comparative analysis of the performance of the different batteries is presented.