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稀有金属2015年第6期

Sn替代Ni对R-Mg-Ni基合金结构和电化学性能的影响

王北平 赵丽敏 王兴蔚 侯春平

宁夏共享新能源材料有限公司研发部

摘 要：

采用元素替代的方法,研究了Sn元素部分替代Ni元素对La0.72Nd0.08Mg0.2Ni3.4-xSnxAl0.1(x=0~0.14)电极合金结构和电化学性能的影响。通过感应熔炼、退火处理、粉碎过筛后得到合金样品。X射线衍射(XRD)分析表明该合金为多相结构,包括(La,Mg)2Ni7(Gd2Co7型和Ce2Ni7型)、(La,Mg)5Ni19(Pr5Co19型)、(La,Mg)Ni3(Pu Ni3型)和La Ni5相(Ca Cu5型)。结构精修显示合金主相由Gd2Co7型(La,Mg)2Ni7相依次变化为(La,Mg)5Ni19,La Ni5相。恒电流充放电测试表明,合金放电容量最大值为387.4 m Ah·g-1。加入Sn后合金电极的放电容量下降,这与合金中相含量的变化是有关系的。Sn的加入导致合金中高吸氢相(La,Mg)2Ni7相的减少,而吸氢能力相对小的(La,Mg)5Ni19相和La Ni5相含量不断增加。高倍率放电测试表明随着Sn元素加入,高倍率放电性能下降。电化学循环稳定性测试表明随着Sn元素含量的增加,合金电极循环寿命先增加后下降。当Sn含量x=0.06时,在100次电化学循环后放电容量保持率达到最高水平83.8%。

关键词：

元素替代;储氢合金;相结构;电化学性能;

中图分类号： TG139.7

收稿日期：2014-01-11

Structure and Electrochemical Properties of R-Mg-Ni-Based Alloys with Sn Substitution for Ni

Wang Beiping Zhao Limin Wang Xingwei Hou Chunping

Research & Development Department,Kocel New Energy Material Company

Abstract：

With the method of element substitution,the effects of Sn substitution for Ni on the structure and electrochemical property of La0. 72Nd0. 08Mg0. 2Ni3. 4-xSnxAl0. 1( x = 0 ~ 0. 14) electrode alloys were investigated. Alloy samples were prepared by induction melting,annealing,pulverization and sieving. X-ray diffraction( XRD) analysis indicated that the alloys consisted of multi-phases,including( La,Mg)2Ni7( Gd2Co7,Ce2Ni7),( La,Mg)5Ni19( Pr5Co19),( La,Mg) Ni3( Pu Ni3) and La Ni5( Ca Cu5). Structural refinement indicated that the main phase varied from Gd2Co7-type( La,Mg)2Ni7to( La,Mg)5Ni19and La Ni5. The galvanostatic chargedischarge test indicated the maximum discharge capacity reached 387. 4 m Ah·g-1,and the discharge capacity decreased with the addition of Sn,which was related with the change of phase contents. In fact,the amount of( La,Mg)2Ni7phase with good hydrogen storage ability decreased,and that of( La,Mg)5Ni19and La Ni5 phases with relatively worse hydrogen storage ability increased when Sn was introduced. High-rate discharge measurements indicated that the high-rate discharge ability decreased with Sn increasing. The cyclic measurements indicated that cycle life firstly increased then decreased with the increase of Sn. When Sn content reached x = 0. 06,the cyclic retention over 100 cycles was up to 83. 8%.

Keyword：

element substitution; hydrogen storage alloy; phase structure; electrochemical properties;

Received： 2014-01-11

镍氢电池自20世纪90年代初实现商业化应用以来,其市场规模始终难以持续扩大,一定程度上受到了几乎与其同时实现商业化的锂离子电池的影响。以传统La Ni₅型储氢电极合金为负极材料的镍氢电池不能满足快速发展的市场需求。为进一步提高镍氢电池性能,寻求新型负极材料成为研究热点。

自从Kadir等^[1]报道了六方Pu Ni₃型RMg₂Ni₉系列合金(R=La,Ce,Pr,Nd,Sm和Gd)具有储氢性能以来,研究人员对R-Mg-Ni基储氢合金开展了大量的研究,认为该类合金作为镍氢电池负极材料具有很好的前景。Kohno等^[2]发现具有超晶格结构的La_0.7Mg_0.3Ni_2.8Co_0.5合金的放电容量达到410 m Ah·g^-1,是AB₅型合金的1.3倍。Pan等^[3]发现La_0.7Mg_0.3(Ni_0.85Co_0.15)_3.5的最大放电容量为398.4 m Ah·g^-1。Zhang等^[4]通过对La_0.67Mg_0.33Ni_2.5Co_0.5退火处理,将合金放电容量提高到402.5m Ah·g^-1。尽管R-Mg-Ni基合金展示出了很高的放电容量,但循环性能差的问题限制了该类合金的实际应用。

元素替代是提高储氢电极合金循环性能的有效方法之一。研究人员对Pr,Nd,Ce等元素部分替代La;Al,Cu,Mn,Fe,Co,Cr和Si等元素部分替代Ni或Co进行了大量的研究^{[5,6,7,8,9,10,11,12]},结果表明合适的Pr+Nd含量可以提高合金的放电容量、循环性能和动力学性能,Al,Cu,Mn和Co等元素替代Ni或Co后,合金电极的放电容量和循环性能得到了一定程度的提高。其他元素如Fe,Cr和Si等元素替代后会降低合金电极的放电容量,但会提高循环寿命。Kohno等^[13]研究了Ti,V,Nb,Sn和Ta等元素部分替代Ni对La(Ni_0.9M_0.1)₃储氢电极合金性能的影响,发现合金的放电容量按照Sn>V>Ta>Nb>Ti的顺序逐渐减小。目前用相关元素替代Ni的研究已有很多,但R-Mg-Ni基合金电化学循环性能差的问题仍然限制着其实际应用。为进一步研究元素替代Ni对合金结构和电化学性能的影响,前期考察了La_0.72Nd_0.08Mg_0.2Ni_3.38M_0.02Al_0.1(M=Ag,Sn,Si和Mn)合金的电化学性能,结果表明合金的放电容量按照Sn>Ag>Mn>Si的顺序减小,100次电化学循环寿命按照Mn<Ag<Si<Sn的顺序逐渐提高。

本文在前期实验的基础上,选取Sn部分替代Ni,设计合金成分体系为La_0.72Nd_0.08Mg_0.2Ni_3.4-xSn_xAl_0.1(x=0,0.02,0.06,0.10和0.14),系统研究了Sn部分替代Ni对合金的相结构与电化学性能的影响。

1 实验

在氩气气氛下通过感应熔炼制备得到La_0.72Nd_0.08Mg_0.2Ni_3.4-xSn_xAl_0.1(x=0,0.02,0.06,0.10和0.14)合金样品。为保证成分的均匀性,熔炼时应重熔两次。得到的合金样品在氩气气氛下于管式炉中在1173 K保温6 h。所有初始原料的纯度为99%。为补偿Mg的蒸发损失,应保证Mg适当过量。退火后的合金锭机械粉碎后过200目和300目筛,分别用于电化学测试和X射线衍射(XRD)分析。

XRD测试在日本Rigaku D/max-2400型X射线衍射仪上进行,采用Cu Kα射线和石墨单色器,工作电压、电流分别为40 k V和150 m A,扫描速度为4(°)·min^-1,2θ=20°~80°。衍射数据用Fullprof软件进行Rietveld全谱拟合分析。

合金电化学性能测试在开放式两电极测试电池中进行。阳极制作过程为:按照合金粉与羰基镍粉1∶4的质量比在玛瑙研钵中混合均匀后,称取含0.5 g活性物质的混合粉在20 MPa下于自制模具中压制成Φ16 mm×2 mm的圆片。将其放入两片泡沫镍中间,钳紧边缘后在8 MPa下再次压制,以使极片与泡沫镍之间保持良好的电接触。阴极为Ni(OH)₂,其容量远高于合金电极的容量。电解液为6 mol·L^-1KOH,室温下测试。以100 m A·g^-1电流密度充电5 h,静置10 min后以相同的电流密度放电至0.9 V测试活化性能和最大放电容量。300m A·g^-1充电1.5 h,静置10 min后以相同电流密度放电至1 V测试循环性能。通过测定合金电极在不同放电电流密度(300,600,900和1200 m A·g^-1)下的放电容量以研究合金的高倍率放电性能(HRD)。

2 结果与讨论

2.1 合金微观组织

图1为La_0.72Nd_0.08Mg_0.2Ni_3.4-xSn_xAl_0.1(x=0~0.14)合金的XRD图谱。通过XRD物相检索分析表明,该体系合金是多相组织,包括(La,Mg)₂Ni₇(Gd₂Co₇型和Ce₂Ni₇型)、(La,Mg)₅Ni₁₉(Pr₅Co₁₉型)、(La,Mg)Ni₃(Pu Ni₃型)和La Ni₅相(Ca Cu₅型)。经过Rietveld全谱拟合分析得到合金的组成相含量,结果列于表1中。由表1可以看出,当合金中不含Sn时,Gd₂Co₇型(La,Mg)₂Ni₇相为合金主相。随着Sn的加入,Gd₂Co₇型相逐渐减少,(La,Mg)₅Ni₁₉相先增加后减少,La Ni₅相逐渐增加,其他两相Ce₂Ni₇型(La,Mg)₂Ni₇和(La,Mg)Ni₃相含量较少且变化不大,合金主相由Gd₂Co₇型(La,Mg)₂Ni₇相依次变化为(La,Mg)₅Ni₁₉,LaNi₅相。相组成分析表明,Sn的加入促进了合金中La Ni₅相的形成,显著抑制Gd₂Co₇型(La,Mg)₂Ni₇相的形成。

2.2 活化性能与最大放电容量

图2为La_0.72Nd_0.08Mg_0.2Ni_3.4-xSn_xAl_0.1(x=0~0.14)合金电极的活化曲线。合金电极在2~4个电化学循环内即可完全活化,表现出良好的活化性能。图3为La_0.72Nd_0.08Mg_0.2Ni_3.4-xSn_xAl_0.1(x=0~0.14)合金电极的最大放电容量曲线。未加Sn之前,合金放电容量为最大值387.4 m Ah·g^-1。加入Sn后合金电极的放电容量下降。这与合金中相含量的变化是有关系的。Sn的加入导致合金中高吸氢相(La,Mg)₂Ni₇相的减少,而(La,Mg)₅Ni₁₉相和La Ni₅相的含量不断增加。Buschow,Busch和Maeland等^[14,15,16]研究发现对于La-Ni系合金来说,随着Ni含量的降低,合金吸氢量会增加,即(La,Mg)₅Ni₁₉和La Ni₅相的储氢能力要小于(La,Mg)₂Ni₇。因此随着Sn的增加,合金电极的放电容量不断下降。

图1 La_0.72Nd_0.08Mg_0.2Ni_3.4-xSn_xAl_0.1(x=0~0.14)合金的XRD图谱Fig.1 XRD patterns of La_0.72Nd_0.08Mg_0.2Ni_3.4-xSn_xAl_0.1(x=0~0.14)alloys

表1 La_0.72Nd_0.08Mg_0.2Ni_3.4-xSn_xAl_0.1(x=0~0.14)合金的相组成与含量Table 1Phase compositions and contents of La_0.72Nd_0.08Mg_0.2Ni_3.4-xSn_xAl_0.1(x=0~0.14)alloys

图2 La_0.72Nd_0.08Mg_0.2Ni_3.4-xSn_xAl_0.1(x=0~0.14)合金的活化曲线Fig.2 Activation curves of La_0.72Nd_0.08Mg_0.2Ni_3.4-xSn_xAl_0.1(x=0~0.14)alloys

2.3 循环稳定性

图4为La_0.72Nd_0.08Mg_0.2Ni_3.4-xSn_xAl_0.1(x=0~0.14)合金电极的电化学循环性能曲线。表2为该系列合金的基本电化学性能。其中,N为合金电极活化次数;C_max为充放电电流密度100 m A·g^-1时的最大放电容量;C'_max为充放电电流密度为300m A·g^-1时的最大放电容量;S₁₀₀为电极在100次循环后的放电容量保持率;HRD₉₀₀为放电电流密度为900 m A·g^-1时的倍率放电性能。由图4和表2可以看出,随着Sn的增加,合金电极的循环容量保持率先增加然后减小。当Sn含量x=0.06时合金电极的循环容量保持率为83.8%,达到最好水平。Ren等^[17]研究认为,Sn的加入可以抑制合金颗粒的粉化现象,同时在电化学循环过程中合金电极表面会形成Sn O₂,提高了合金抗碱液腐蚀能力。因此Sn的加入提高了合金电极的循环稳定性。但同时也应看到,增加Sn的含量,合金电极的循环稳定性反而下降,这是由于Sn含量的增加会导致电极表面形成更多的Sn O₂,阻碍了电极反应过程中氢的扩散,造成合金电极充、放电容量的损失。因此合金成分体系设计时应合理控制Sn的含量。

图3 La_0.72Nd_0.08Mg_0.2Ni_3.4-xSn_xAl_0.1(x=0~0.14)合金的最大放电容量曲线Fig.3Maximum discharge capacity curves of La_0.72Nd_0.08Mg_0.2Ni_3.4-xSn_xAl_0.1(x=0~0.14)alloys

图4 退火态La_0.72Nd_0.08Mg_0.2Ni_3.4-xSn_xAl_0.1(x=0~0.14)合金的循环性能曲线Fig.4 Cyclic performance curves of La_0.72Nd_0.08Mg_0.2Ni_3.4-xSn_xAl_0.1(x=0~0.14)alloys

表2 La_0.72Nd_0.08Mg_0.2Ni_3.4-xSn_xAl_0.1(x=0~0.14)合金的电化学性能Table 2Electrochemical properties of La_0.72Nd_0.08Mg_0.2Ni_3.4-xSn_xAl_0.1(x=0~0.14)alloys

2.4 高倍率放电性能

合金的高倍率放电能力以下式计算:

HRD_d为放电电流为I_d时电极的高倍率放电能力;C_d为放电电流为I_d时的放电容量;C₁₀₀为以I_d的电流密度放电至1 V后,静置10 min后,再以100m A·g^-1的电流密度放电至1 V时所放出的容量;I_d为300,600,900和1200 m A·g^-1。图5为La_0.72Nd_0.08Mg_0.2Ni_3.4-xSn_xAl_0.1(x=0~0.14)合金电极的高倍率放电性能曲线。结合表2可以看出,随着Sn的加入,合金电极的高倍率放电性能逐渐降低。这与Zhang等^[18]在AB₅型合金La_0.9Mg_0.1Ni_5-xSn_x(x=0~0.4)中得到的结果是不同的。这是由于:一方面Sn的加入降低了合金电极的电化学催化性能,另一方面由于颗粒表面Sn O₂的存在,增加了合金颗粒之间的接触阻抗,阻碍了电极电荷的传递与转移,造成电极动力学性能的下降。

图5 La_0.72Nd_0.08Mg_0.2Ni_3.4-xSn_xAl_0.1(x=0~0.14)合金的高倍率放电性能曲线Fig.5 High rate dischargeability curves of La_0.72Nd_0.08Mg_0.2Ni_3.4-xSn_xAl_0.1(x=0~0.14)alloys

3 结论

La_0.72Nd_0.08Mg_0.2Ni_3.4-xSn_xAl_0.1(x=0~0.14)合金由(La,Mg)₂Ni₇(Gd₂Co₇型和Ce₂Ni₇型)、(La,Mg)₅Ni₁₉,(La,Mg)Ni₃和La Ni₅相组成。Sn的加入可以促进(La,Mg)₅Ni₁₉和La Ni₅相形成,抑制Gd₂Co₇型(La,Mg)₂Ni₇相的形成。电化学性能研究表明,随着Sn的加入,合金电极放电容量和高倍率放电性能下降,当x=0.06时,在100次电化学循环后容量保持率为83.8%。

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