| In recent years, with the development of the electric vehicle industry, alkaline nickel base secondary battery become one of the frontier of the current energy system, obtained the rapid development. Also,research of the the improvement of high-temperature performance on alkaline nickel-based secondary battery has been a hot area which is concerned by domestic and foreign researchers. As a result of excellent electrochemical properties, spherical Ni(OH)2 has been widely used in commercial alkaline rechargeable batteries. However, it is imperative to inhance the high temperature performance of the spherical Ni(OH)2 in order to meet the demand in electric vehicle battery.As is known, the actual capacity of spherical Ni(OH)2 has reached its theoretical capacity, and it is difficult to reduce its material cost. But, α-Ni(OH)2 as a new cathode material, not only has a high theoretical capacity, and by replacing the aluminum can reduce the amount of nickel, further greatly reducing the cost of materials. So, research of α-Ni(OH)2 has attracted worldwide research interest.However, the preparation procedures of α-Ni(OH)2 are complex and the obtained product with a low tap-density can hardly meet the requirements of industrial mass production, which seriously hampered its development. With the development of nickel-based secondary battery, there is substantial scientific and technological interest in the study of α-Ni(OH)2 synthesis.1、 Improvement of high-temperature performance of alkaline Ni-MH batteries1) Sodium tungstate(Na2WO4) used as an electrolyte additive has been added into two types of binary electrolytes(KOH-Li OH and Na OH-Li OH) to improve the high-temperature properties of Ni(OH)2electrodes in nickel-metal hydride batteries in this paper. The effects of electrolyte composition on the electrochemical performance of nickel electrodes have been systematically investigated. It has been proved that the Na2WO4 can be used as an effective electrolyte additive to improve the high-temperature performance of Ni-MH cells with either KOH electrolyte or Na OH electrolyte. It is found that by adding(1.0 wt.%) Na2WO4, the performance of nickel electrodes is significantly improved in both Na OH and KOH electrolytes at 70 °C. Especially, Na2WO4 has different effects on the electrochemical behavior of the nickel electrode in KOH and Na OH electrolytes, which may be due to the different conductivities and viscosities of KOH and Na OH electrolytes. CV tests indicate that the charge acceptance of Ni-MH cell canbe improved due to the high overvoltage for oxygen evolution. The steady-state polarization measurements show that the use of Na2WO4 additive is beneficial to the controlling of OER in KOH electrolyte. This performance improvement is also ascribed to the lower charge transfer resistance, as indicated by EIS. The mechanism of the enhanced performance of Ni-MH cells has been investigated.Based on the results of XRD and SEM, it can be concluded that the improved performance can be attributed to the deposition of WO3·2H2O solid film on the surface of nickel electrode, which plays an important role to the increase in oxygen evolution overvoltage, the slow-down of oxygen evolution and the decrease in charge transfer resistance. Therefore, Na2WO4 is an effective electrolyte additive to improve the high temperature performance of nickel hydroxide cathode for Ni-MH batteries.2) Calcium metaborate(CMB) as a novel additive is proposed to improve the high-temperature characteristics of the nickel electrodes for nickel-metal hydride batteries. As a soluble calcium salt, CMB can easily and uniformly be dispersed in the nickel electrodes. The effects of CMB on the nickel electrode are investigated via a combination of cyclability, capacity retention, electrochemical impedance spectroscopy, scanning electron microscope and X-ray diffraction. The high-temperature charge/discharge performance of Ni-MH batteries was successfully improved by the introduction of CMB. Compared with conventional cells, the in-house prepared cells exhibit enhanced high-rate discharge ability, improved discharge capacity and good cycle stability at room temperature and an elevated temperature(70 °C). EDX studies show that a pasted nickel electrode with a uniform distribution of Ca can be easily prepared by the proposed method. EIS studies show that CMB has little negative effect on the electron transfer resistance of nickel electrode. CV and steady-state polarization tests indicate that CMB can efficiently enhance the oxygen evolution overvoltage and depress the oxygen evolution, leading to a higher charge acceptance and proton diffuse coefficient. Compared with insoluble calcium salts(Ca(OH)2, Ca CO3, and Ca F2), CMB is more effective to improve the performances of Ni-MH batteries. It is believed that this distinctive approach is promising and effective for high-temperature and high-rate electrochemical performance improvement of rechargeable alkaline Ni-MH batteries.2 、 Study on the design synthesis and electrochemical properties of high-density α-Ni(OH) 2cathode materials1) Positive electrode active materials, Al-substituted α-Ni(OH)2, with a high tap-density and highperformance for alkaline nickel-based rechargeable batteries have successfully been synthesized using a polyacrylamide(PAM) assisted two-step drying method and subsequent hydrothermal treatment at 140 °C for 2 h. The tap-density of the resulting powders reaches 1.84 g cm-3, which is significantly higher than that of α-Ni(OH)2 powders obtained by the conventional co-precipitation(CCP) and hydrothermal(HT)methods. Compared with commercial spherical β-Ni(OH)2, the resulting sample is electrochemically more active, providing discharge capacities of 315.0 and 255.2 m Ah g-1, and volume capacities of 579.6 and469.6 m Ah cm-3 at rates of 0.2 C and 5C, respectively. It is also found that although hydrothermal treatment has a significantly negative impact on the tap-density of the sample, hydrothermal treatment for a short time still plays a key role in improving the electrochemical performance of the powders. This performance improvement could be attributable to the anion exchange of NO3- by OH- and higher crystallinity, both resulting from the hydrothermal treatment process. The new method is also simple and facile for synthesizing Al-substituted α-Ni(OH)2 powders with high tap-density and high performance. Due to the lower cost and outstanding performance, the sample synthesized by the proposed method is a promising positive electrode active material for alkaline nickel-based rechargeable batteries.2) The Al-substituted α-type nickel hydroxides containing different interlayer anions(NO3-, SO42-, Cl-,CO32-, OH-) are synthesized via a polyacrylamide(PAM) assisted two-step drying method. The effects of interlayer anions on the microstructure, morphology and electrochemical performance of the prepared nickel hydroxides are investigated. The effects of interlayer anions on the physical and electrochemical performance of the prepared α-Ni(OH)2 were systematically investigated. It has been found that the intercalated anions have a critical effect on the basal spacing, the degree of crystallinity, and electrochemical properties of the resulting products. The basal spacing of α-Ni(OH)2 increases with the anionic radius in the order of intercalated anions Cl-, NO3-, OH-, CO32-and SO42-. Among the five samples,the α-Ni(OH)2-Cl- sample shows the highest crystallinity. As a result, the intercalated anions have shown a significant impact on the activation process of the nickel electrodes. The Cl- intercalated α-Ni(OH)2 sample with the highest tap-density shows the fastest activation in all studied electrodes, which basically meets the demands of commercialization. All the results clearly show that the anion exchange ability and the anion size play an important role in the diffusion rate of the proton, which directly affects the electrochemical properties of α-Ni(OH)2. Under the same testing conditions, the resulting Cl- intercalated α-Ni(OH)2 sampleexhibits a higher crystallinity, better high-rate discharge ability and better cycle stability than the other samples, which are attributable to the better anion exchange ability and the smaller anion size. Therefore,the Cl- intercalated α-Ni(OH)2 is promising for further development in terms of its fast formation rate and good electrochemical performance.3) In order to improve its electrochemical performance, Al-substituted α-Ni(OH)2 with interlayer NO3-is modified by a novel anion exchange method using Na Cl solution. It is demonstrated that the electrochemical performance of high-density Al-substituted α-Ni(OH)2 with interlayer NO3- can be improved greatly by a simple anion exchange method using Na Cl solution. The chloridion was successfully introduced into the interlayer of α-Ni(OH)2 by two ways. Anion exchange at room temperature can retain the high tap-density of the precursor, while the HT exchange method has a significantly negative effect on tap-density. Compared with NO3- intercalated α-Ni(OH)2 precursor, the resulting Cl- intercalated α-Ni(OH)2samples exhibit an enhanced activation rate, improved high-rate discharge ability and excellent cycle stability. CV and EIS results show that these performance improvements are ascribed to the higher proton diffusion coefficient and the lower charge transfer resistance. Importantly, the proposed method is simple and conducive to mass production. Therefore, it is believed that this distinctive approach is promising and effective for electrochemical performance improvement of α-Ni(OH)2 for alkaline rechargeable batteries. |
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