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.     

Activation behaviour of the Ni/MH batteries electrodic material Ni(OH)2 by single particle microelectrode technique

The activation process of Ni(OH)₂ used as the positive electrode active material of Ni/MH batteries was studied by a single particle microelectrode method thanks to an improved apparatus. The images of the Ni(OH)₂ particle during the charge process were collected. The electrochemical properties of Ni(OH)₂ were studied by cyclic voltammetry and galvanostatic charge/discharge of a single particle. The charge efficiency (η) of the single particle was as high as 94%. The normalized output rate (NOR) was proposed as a parameter to evaluate the output performance of the electrode material. The NOR value varied with the electrode potential value. But the NOR value remained constant at fixed electrode potential value during the activation process. This implies that the activation process did not improve the reaction rate of the particle, although the capacity kept increasing during the activation process. The intrinsic nature of the activation of Ni(OH)₂ was deduced as the formation of dispersed Ni(III) in the active mass. The Ni(III) phase was formed during the charge process and some remained unreduced during the discharge process. The remaining Ni(III) resulted in a much higher electronic conductivity of Ni(OH)₂.     

Characterization of the performance of commercial Ni/MH batteries

An experimental study is presented for the characterization of commercially available Ni/MH batteries. An experimental study of AA-size cells from Matsushita, Sanyo, and Toshiba was performed to determine the charge and discharge behavior at different rates. The self-discharge characteristics, the cycle-life characteristics, and the effect of temperature on the charge and discharge characteristics are also reported at various rates. Apart from the performance study, a couple of cells from each manufacturer were disassembled to gain insight into the design of the cell and to characterize the separator. The characterization results show that although the three cells were identical in nominal capacity, weight and size, their performance greatly differed in terms of charge and discharge behavior, high rate discharge capabilities, self-discharge losses, charging efficiency, and cycle life.     

Thermal behavior of overcharged nickel/metal hydride batteries

This work provides a two-dimensional thermal model for cylinder Ni/MH battery. Thermal model is developed to analyze the thermal behavior of the battery when charged and overcharged. Quantity of heat and heat generation rate of the battery during charge and overcharge period are studied by quartz frequency microcalorimeter. Heat generation curve is fitted into a function, and heat transport equation is solved. Analysis with the model and experiment show that temperature rise is about 3 °C and difference between the model and the experiment is no more than 0.1 °C.     

Thermal analysis of rapid charging nickel/metal hydride batteries

A two-dimensional thermal model was presented to predict the temperature distribution of cylindrical 8-Ah Ni/MH battery. Under the forced convection, the temperature rise of the battery is up to about 37, 42 and 51 °C, and the temperature profiles become non-uniformed at the end of 1C, 2C and 4C rate charge, respectively. It is indicated that the increase of the convection coefficient can decrease the battery temperature, however, lead seriously to the less uniform temperature profile across the battery. The numerical studies indicate that the increase of thermal conductivity can improve the uniformity of temperature profile to some extends. The battery temperature increases obviously when charged at higher rates. Overcharge can result in an increasingly higher temperature rise and a steeper temperature gradient within a battery.     

Electrochemical behaviour of Ni(OH)2 ultrafine powder

Ultrafine Ni(OH)₂ powder is prepared by converting a Ni₂C₂O₄ precipitate in NaOH solution which contained Tween-80. The sample prepared by this method is β(II)-type phase and its particle size is about 30 nm. The electrochemical behaviour of nickel foam electrodes using this ultrafine Ni(OH)₂ powder as active material is studied and compared with micron-sized spherical Ni(OH)₂ by means of a galvanostatic charge–discharge method, cyclic voltamperommetry (CV) and electrochemical impedance spectroscopy (EIS). It is found that ultrafine Ni(OH)₂ powder has superior electrochemical properties, such as lower polarization, better reversibility, and smaller reaction resistance.     

Effect of surface modification on the performance of negative electrodes in Ni/MH batteries

The performance of a Nickel/Metal Hydride (Ni/MH) battery closely depends on the characteristics of the negative MH electrode. Exchange current density, high-rate dischargeability, discharge potential and apparent activation energy of a MH electrode are very important properties, among which the high-rate dischargeability and discharge potential of a MH electrode determine the specific energy and specific power of electric vehicles (EVs) when Ni/MH batteries are applied to EVs. Significant improvements in exchange current density, high-rate dischargeability and discharge potential of a MH electrode have been observed for a $\text{9.0}\text{wt}$% copper coated LaNi_4.7Al_0.3 MH electrode. The high-rate dischargeabilities were determined to be 88.4% for the LaNi_4.7Al_0.3 electrode and 99.4% for Cu-coated LaNi_4.7Al_0.3 electrode. The discharge potential for the Cu-coated LaNi_4.7Al_0.3 electrode is lower (i.e. more negative) than that for the LaNi_4.7Al_0.3 electrode, especially at a large discharge current density (i.e. $\text{200}\text{mA}\text{g}{}^{\mathrm{-1}}$). The discharge potentials of the Cu-coated LaNi_4.7Al_0.3 electrode are almost the same value (i.e. $\text{−0.930}\text{V}$ vs. Hg/Hgo) at both 20 and $\text{200}\text{mA}\text{g}{}^{\mathrm{-1}}$ discharge current densities. There is no significant difference between the two apparent activation energies for the electrode reactions for the electrodes with and without the microencapsulation of the MH powders at the same hydrogen concentration.     

Self-discharge characteristics of a metal hydride electrode for Ni-MH rechargeable batteries

The factors that affect the self-discharge characteristics of a metal hydride (MH) anode are investigated, which include the state of charge (SOC), temperature, storage time and microencapsulation of the alloy with a thin copper (Cu) layer. It is found that the self-discharge rate of the MH electrode decreases with decreasing temperature and storage time. Microencapsulation of the alloy with a thin Cu layer is found to greatly decrease the self-discharge rate. Also, it is found that the self-discharge rate of the Ni-MH cells is not correlated with SOC. It is believed that the main mechanism for the self-discharge of the Ni-MH cells is related to the hydrogen release from the MH anode.     

Recent progress in hydrogen storage alloys for nickel/metal hydride secondary batteries

This paper reviews the development of hydrogen storage alloys prepared by an effective method of mechanical alloying and milling. It emphasizes alloys based on Mg or that contain Mg due to their low cost, low weight and high hydrogen storage capacity. Hydrogen absorption/desorption and electrochemical measurements are briefly discussed. The electrochemical properties of the alloys that contain Mg are covered in detail, emphasizing the effects of changes in alloy composition. The system of Ti–Ni-based alloys is also introduced. At present, composite hydrogen storage alloys may be the most effective materials for practical application in new nickel/metal hydride secondary batteries. The steps of hydrogen absorption/desorption such as charge-transfer and hydrogen diffusion for evaluating the electrochemical properties of hydrogen storage alloys are discussed. The relationship between alloy composition and electrochemical properties is noted and evaluated.     

Recent progress in rechargeable nickel/metal hydride and lithium-ion miniature rechargeable batteries

VARTA is searching for alternative battery solutions for memory back-up and bridging applications, and for this, it is developing nickel/metal hydride and lithium-ion button cells. Presented are the results on different sizes and forms of lithium-ion cells (621, 1216 and 2025) containing different electrode materials and shapes. Presently, the most favoured cathode material is lithiated manganese dioxide. The electrodes are made from both solid and porous materials and, together with an organic electrolyte, result in a cell system with a voltage level of approximately three. Included are results, both from these lithium-ion cells, and also from ones using the nickel/metal hydride system.     

Electrochemical characteristics of the interface between the metal hydride electrode and electrolyte for an advanced nickel/metal hydride battery

The characteristics of the negative electrode of a Ni/MH (metal hydride) battery are related to the charge transfer and mass transfer processes at the interface between the MH electrode and the electrolyte. With increasing number of charge/discharge cycles, the MH alloy powders micro-crack into particles that are several microns in diameter and this then influences the exchange current density. A polarization experiment was used to analyze the charge transfer and mass transfer processes. The exchange current densities of uncoated and Pd-coated Mm_0.95Ti_0.05Ni_3.85Co_0.45Mn_0.35Al_0.35 alloy electrodes increase with increasing number of charge/discharge cycles before reaching a constant value after 20–30 cycles.     

High-rate discharge properties of nickel hydroxide/carbon composite as positive electrode for Ni/MH batteries

A chemical co-precipitation method was attempted to synthesize nickel hydroxide/carbon composite material for high-power Ni/MH batteries. The XRD analysis showed that there were a large amount of defects among the crystal lattice of the Ni(OH)₂/C composite, and the SEM investigation revealed that the as-synthesized spherical particles were composed of hundreds of nanometer crystals with a unique three-dimensional petal shape. Compared with pure Ni(OH)₂, the Ni(OH)₂/C composite showed improved electrochemical properties such as superior cycling stability, higher discharge capacity and higher mean voltage of discharge under high-rate discharge conditions, the discharge capacity and the mean discharge voltage of the Ni(OH)₂/C composite were about 281 mAh g⁻¹ and 0.303 V (vs. Hg/HgO) at 1 C-rate, 273 mAh g⁻¹ and 0.296 V at 5 C-rate, 250 mAh g⁻¹ and 0.292 V at 10 C-rate, respectively. The cyclic voltammetry (CV) tests showed that the Ni(OH)₂/C composite exhibited good electrochemical reversibility and the formation of γ-NiOOH during the charge–discharge processes was prevented. The existence of carbon in the Ni(OH)₂/C composite contributed great effect on the improvement of high-rate discharge performance.     

Fuzzy logic modelling of state-of-charge and available capacity of nickel/metal hydride batteries

Ni/MH batteries are playing important roles in many applications such as power tools, and a dominant role in hybrid electric vehicles (HEVs). In the case of HEVs it is particularly important to be able to monitor the state-of-charge (SoC) of the Ni/MH batteries. We have previously reported on the use of a fuzzy logic (FL) methodology to estimate the SoC of various battery chemistries, including lead-acid and lithium sulfur dioxide.In the present work, we have measured electrochemical impedance spectroscopy (EIS) on 2.7 Ah Sanyo Ni/MH cells and two- and three-cell strings of these cells at different SoC’s and over 100 cycles. We have been able to select features in this data to develop fuzzy logic models for both available capacity and SoC estimation, simply by measuring the impedance at three frequencies. The fuzzy logic model estimates the SoC to within ±5%. In this paper we will present the details of the experimental measurements, the details of the fuzzy logic models themselves, and the resulting accuracies of the developed models.     

Performance and electrochemical characterization studies of advanced high-power bipolar nickel/metal hydride batteries

This paper addresses recent activities relating to energy storage systems capable of delivering very high power. Specific power in the range of 2.0–2.2 kW/kg and power density in the range of 5.0–6.5 kW/l were obtained at current densities of 0.43–0.86 A/cm² (400–800 A/ft²). The designs incorporate the use of thin, plastic bound electrodes laminated to nickel foil substrates. The units tested were nominal 28 V, 24 Ah batteries, as well as individual cells.     

High-rate dischargeability enhancement of Ni/MH rechargeable batteries by addition of nanoscale CoO to positive electrodes

Nanoscale cobalt oxide (CoO) particles were synthesized by analysis of CoCO₃ in vacuum. Four groups of sealed Ni/MH batteries with different ratio of nanoscale CoO in the positive electrodes were assembled. The overall characteristics of Ni/MH batteries were investigated at different discharge rates at room temperature. The high-rate discharge performance of the Ni/MH batteries was improved by the addition of nanoscale CoO in positive electrode as compared with the addition of normal CoO. Under high-rate discharge conditions, the batteries with sufficient nanoscale CoO in positive electrodes presented much better cycling stability, higher discharge mean voltage, lower internal resistance and higher high-rate capacity. The addition of 8 wt.% nanoscale CoO was proved a desired amount to modify the battery performance at high discharge rates. Too much nanoscale CoO contributed no effect on the improvement of overall performance of Ni/MH batteries.     

New nanoparticles of (Sm,Zn)-codoped spinel ferrite as negative electrode in Ni/MH batteries with long-term and enhanced electrochemical performance

In this investigation, the longevity performance of the electrochemical hydrogen storage characteristics, the kinetic properties and the structural characterization of the Sm_0.6Zn_0.4Fe₂O₄ compound were all studied.The Sm0.6Zn0.4Fe2O4 nanomaterial was prepared using sol-gel process. The structural study of the alloy through X-ray diffraction proved the good crystallization of this compound. The electrochemical performance of the Sm0.6Zn0.4Fe2O4 electrode at C/10 rate and at ambient temperature was studied by various electrochemical techniques, such as chronopotentiometry, chronoamperometry and cyclic voltammetry. This compound showed a good electrochemical behaviour with a high capacity (142 mA h.g-1) as well as a good discharge capacity preservation of 64.7% over 100 cycles.There was adequate agreement between the evolution of the different electrochemical parameters (exchange current density, Nernst potential, DH/a² ratio and the electrochemical discharge capacity) versus cycle number.     

Electrochemical behaviors of a Li3N modified Li metal electrode in secondary lithium batteries

A lithium conductive Li₃N film is successfully prepared on Li metal surface by the direct reaction between Li and N₂ gas at room temperature. X-ray diffraction (XRD), Auger electron spectroscopy (AES), cyclic voltammetry (CV), scanning electron microscopy (SEM), AC impedance, cathodic polarization and galvanostatic charge/discharge cycling tests are applied to characterize the film. The experimental results show that the Li₃N protective film is tight and dense with high stability in the electrolyte. Its thickness is more than 159.4 nm and much bigger than that of a native SEI film formed on the lithium surface as received. An exchange current as low as 3.244 × 10⁻⁷ A demonstrates the formation of a complete SEI film at the electrode|electrolyte interface with Li₃N modification. The SEI film is very effective in preventing the corrosion of the Li electrode in liquid electrolyte, leading to a decreased Li|electrolyte interface resistance and an average short distance of 3.16 × 10⁻³ cm for Li ion diffusion from electrolyte to Li surface. The Li cycling efficiency depends on N₂ exposing time and is obviously enhanced by the Li₃N (1 h) modification. After cycling, a dense and homogeneous Li layer deposits on the Li₃N (1 h) modified Li surface, instead of a loose and inhomogeneous layer on the Li surface as received.     

Electrochemical performance of ZnO nanoplates as anode materials for Ni/Zn secondary batteries

Plate-like ZnO with good crystallinity were prepared by a simple hydrothermal synthesis method in Zn(NO₃)₂·6H₂O and NaOH solution at 180 °C. The dimension of ZnO powder ranged from 200 to 500 nm and the average thickness was about 50 nm. The electrochemical performances of ZnO nanoplates as anode active materials for Ni/Zn cells were investigated by galvanostatic charge/discharge cycling and cyclic voltammogram (CV). The ZnO nanoplates showed better cycle stability than the conventional ZnO, and the discharge capacity maintained 420 mAh g⁻¹ throughout 80 cycling tests. At the same time, they also exhibited higher midpoint discharge voltage and lower midpoint charge voltage. The continual SEM examinations on the electrode found that the morphology of the plate-like ZnO active material did not change essentially and the zinc dendrite was suppressed effectively, which resulted in the improvement of cycle stability of Ni/Zn secondary cells.     

Electrochemical behaviour of Heusler alloy Co2MnSi for secondary lithium batteries

Electrochemical lithiation of Co₂MnSi with a Heusler structure is investigated as a candidate negative electrode (anode) material for secondary lithium batteries. The electrode maintains a reversible discharge capacity of 112 mAh g⁻¹ for 50 cycles when cycled between 0.01 and 3 V. It is proposed that the lithiation mechanism consists of two steps. Co₂MnSi transforms to Heusler-type Li₂MnSi during the first charge cycle and subsequent charge–discharge cycles involve the formation of a solid solution in Li_xMnSi. The latter compound maintains its structural integrity throughout cycling to provide steady cycling behaviour. Magnetic measurements are also employed to substantiate further the structural changes during electrochemical cycling.     

Effect of Ce on electrochemical properties of the TiVNi quasicrystal material as an anode for Ni/MH batteries

In order to investigate effect of Ce on electrochemical properties of the Ti_1.4V_0.6Ni quasicrystal, (Ti_1.4V_0.6Ni)_99.4Ce_0.6 alloy ribbon is prepared by arc melting and subsequent melt-spinning technique. The electrochemical properties of the Ti_1.4V_0.6Ni and (Ti_1.4V_0.6Ni)_99.4Ce_0.6 are studied as negative electrode for nickel–metal hydride batteries in aqueous KOH solution. The structures of the alloys were characterized by XRD and TEM. Phase structure investigations of the (Ti_1.4V_0.6Ni)_99.4Ce_0.6 show that alloy mainly consist of the icosahedral quasicrystal (I-phase) and face centered cubic (FCC) phase with Ti₂Ni-type structure. The electrochemical measurements demonstrated that the negative electrode made by (Ti_1.4V_0.6Ni)_99.4Ce_0.6 alloy showed an improved electrochemical reversibility and a considerably higher charge–discharge capacity when compared to the Ce-free base alloy. Its maximum discharge capacity is about 300 mAh/g, which is higher than that of Ti_1.4V_0.6Ni, and remains 270 mAh/g after 30 cycles at a current density of 30 mA/g. The discharge process is also characterized by electrochemical impedance spectroscopy.