Review ArticlePhase structures and electrochemical properties of La–Mg–Ni-based hydrogen storage alloys with superlattice structure
Introduction
Requirements for clean energy have become increasingly urgent with the depletion of fossil energy resources and growing environmental issues. Investigations show that nearly half of the oil resources are consumed by transportation vehicles which subsequently release one-quarter of total global CO2 [1], [2], [3]. The transformation of power sources from fossil fuels to clean energy is expected to relieve this situation. The nickel metal hydride (Ni/MH) secondary battery is considered to be an ideal clean energy source due to the advantages of high power output, high rate capability, long life-time, good reliability, non-toxicity, no memory effect and low cost [4], [5]. During the exploitation of Ni/MH batteries, novel negative electrode materials have always been the focus. In general, the development of the negative electrode materials for Ni/MH batteries has experienced three generations. The representative for the first generation is rare-earth-based AB5-type alloys. This alloy system with the advantages of easy activation and fast hydrogen desorption has been extensively commercialized. However, their discharge capacity is limited to 300–320 mAh g−1, which can hardly satisfy the requirements of high energy density for Ni/MH batteries [1], [6], [7], [8]. The second generation mainly refers to AB2-type Ti-, Zr-based Laves phase alloys. These alloys have high discharge capacity and good cycling stability, but poor activation and high-rate dischargeability due to their low Ni contents [6]. Recently the industry has put more focus on the third generation which is the La–Mg–Ni-based hydrogen storage alloy. Research found that La–Mg–Ni-based alloys whose structures are the combination of both [AB5] and [AB2] subunits can compensate for the disadvantages of the poor discharge capacity, activation and rate capability of AB5- and AB2-type alloys, thus exhibiting good overall electrochemical properties including easy activation, high discharge capacity and good high-rate dischargeability [9], [10].
It was reported that the maximum discharge capacity of a La–Mg–Ni-based La0.7Mg0.3Ni2.8Co0.5 alloy reached as high as 410 mAh g−1, about 30% higher than that of AB5-type alloys [11], [12]. Since then, numerous studies on the La–Mg–Ni-based alloy system have been conducted to develop them as a practical electrode material for high-energy and high-power Ni/MH batteries [9], [13], [14], [15], [16], [17]. And now, La–Mg–Ni-based alloy electrodes have been applied in commercial Ni/MH batteries with promising results: long cycle-life (>6000 cycles) consumer batteries have been produced by FDK Incorporation [13], and large-capacity, high-power (40 cell – 1500 Ah) stationary batteries have been produced by Kawasaki Heavy Industries for industrial applications [13], [18]. Meanwhile, driven by people's increasing needs for high-power, high-energy and long-cycle life of batteries, research on La–Mg–Ni-based alloys has never been stopped. It is widely recognized that phase compositions and structures have great impacts on the electrochemical performance of La–Mg–Ni-based alloys. Therefore, understanding the phase transformation process and its correlation with electrochemical properties of La–Mg–Ni-based alloys are critical for further improving their electrochemical performance. However, there are no sufficient systematic summarizations in this area. This review essentially focuses on the above mentioned issues, and is divided into five sections. In the first section, the structural characteristics of the superlattice phases in La–Mg–Ni-based alloys are discussed. In the second and third sections, the influential rules of preparation methods and elemental compositions on the phase transformation of La–Mg–Ni-based alloys are reviewed. In the fourth and fifth sections, starting from the electrochemical characteristics of single-phase La–Mg–Ni-based alloys, the effects alloy phases and microstructures on the electrochemical performance are analyzed. In the final section, challenges and outlooks of La–Mg–Ni-based alloys are referred.
Section snippets
Structural characteristics of La–Mg–Ni-based alloy phases
La–Mg–Ni-based alloys were originated from the corresponding binary La–Ni-based alloys [19], [20]. The phases that are commonly seen in La–Mg–Ni-based alloys are (La,Mg)Ni3, (La,Mg)2Ni7 and (La,Mg)5Ni19 phases with superlattice structures. Similar to La–Ni binary phases, these ternary phases are also composed of [A2B4] and [AB5] subunits alternatively stacking along c axis in different patterns [21], [22], as shown in Fig. 1 [23]. The chemical formula for this alloy system can be expressed as m
Phase diagram and peritectic reaction
Phase diagram offers significant guidance for alloy preparation. However, the phase diagram for La–Mg–Ni ternary phases has not been reported so far. But as we know, La–Mg–Ni ternary phases are derived from La–Ni binary phases; therefore, La–Ni binary phase diagram is the reference that is often used when preparing La–Mg–Ni-based alloys. So far, there are three versions of La–Ni binary phase diagrams. The initial La–Ni binary phase diagram was reported by Buschow and Mal of Dutch Philips in
Effect of element composition on the phase composition of La–Mg–Ni-based alloys
Elements are the basic composition unit of materials and determine their structure characteristics. In the case of La–Mg–Ni-based alloys, Mg generates the most obvious effect on the phase compositions. In addition, A-site elements such as Ce, Pr, Nd etc. are often used to partially substitute for La, while B-site elements such as Co, Mn Al etc. are used to substitute for Ni to improve the electrochemical performance of La–Mg–Ni-based alloys. Therefore, understanding the effect of the different
Electrochemical characteristics of single phase La–Mg–Ni-based alloys
The knowledge of the electrochemical characteristics of single phase La–Mg–Ni-based alloys offers the bases for analyzing the effects of different phases in this alloy system. Among ternary La–Mg–Ni-based alloys, AB3.0-type LaMg2Ni9 alloy with 3R-type (La,Mg)Ni3 phase was first reported by Kadir et al. [20], [28]. Its crystal structure is a stacking of a [LaNi5] subunit and two [MgNi2] subunits along c axis. LaMg2Ni9 alloy exhibited a rather small hydrogen storage capacity of 0.3 wt.% [28],
Influence of microstructure on the electrochemical properties of La–Mg–Ni-based alloys
In addition to phase composition, microstructure such as grain size and homogeneous degree is also very important factor influencing the electrochemical performance of La–Mg–Ni-based alloys.
Grain size of La–Mg–Ni-based alloys is often sensitive to constituent elements and processing routes. For examples, Zhang et al. found that the grains of La0.75−xPrxMg0.25Ni3.2Co0.2Al0.1 (x = 0, 0.1, 0.2, 0.3, 0.4) and ZrxMg0.25Ni3.2Co0.2Al0.1 (x = 0–0.2) alloys are refined by the addition of Pr and Zr
Outlook and challenges of La–Mg–Ni-based alloys as negative electrode materials for Ni/MH batteries
Driven by the competitive advantages, tremendous breakthroughs in the research of La–Mg–Ni-based alloys have been witnessed. Firstly, the composition and structure characteristics of the superlattice phases in La–Mg–Ni-based alloys have been clearly recognized. Secondly, the phase transformation mechanisms and conditions of different phases have been understood. Thirdly, the effect of different preparation methods and elemental compositions on the phase transformation has been summarized.
Acknowledgement
This work was financially supported by the National Natural Science Foundation of China (NOs. 51571173, 51171165 and 21303157) and the Natural Science Foundation of Hebei Province (NO B2014203114).
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