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章志珍, 施思齐, 胡勇胜, 陈立泉. 溶胶-凝胶法制备钠离子固态电解质Na3Zr2Si2PO12及其电导性能研究 . 无机材料学报, 2013, 28(11): 1255-1260
ZHANG Zhi-Zhen, SHI Si-Qi, HU Yong-Sheng, CHEN Li-Quan. Sol-Gel Synthesis and Conductivity Properties of Sodium Ion Solid State Electrolytes Na3Zr2Si2PO12 . Journal of Inorganic Materials, 2013, 28(11): 1255-1260  
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溶胶-凝胶法制备钠离子固态电解质Na3Zr2Si2PO12及其电导性能研究
章志珍1,2, 施思齐1,3, 胡勇胜2, 陈立泉2
1. 浙江理工大学 材料与纺织学院, 杭州310018
2. 中国科学院 物理研究所, 北京100190
3. 上海大学 材料科学与工程学院, 上海 200444
施思齐, 教授. E-mail:sqshi@shu.edu.cn; 胡勇胜, 研究员. E-mail:yshu@aphy.iphy.ac.cn

章志珍(1986-), 女, 硕士研究生. E-mail:zhangzhizhen1@126.com

基金:国家高科技研究发展计划(2011AA11A235)
国家自然科学基金(51072183, 11005161, 11205249)
摘要

采用溶胶-凝胶法, 探索了合成纯相Na3Zr2Si2PO12的条件, XRD分析表明前驱体中Na和P 均过量10%时, 在1050℃下烧结得到了纯相的Na3Zr2Si2PO12, 而P不过量时, 得到的产物中含有少量的ZrO2杂质。电化学阻抗谱测试表明Na和P 均过量10%时, 烧结得到的陶瓷片室温离子电导率达到了5.4×10-4S/cm, 比P不过量时烧结得到样品的离子电导率(室温时为3.7×10-4S/cm)要高。进一步分析可知高温下P的挥发造成ZrO2杂质相的析出, 从而使得离子电导率降低。与固相法相比, 溶胶-凝胶法需要的烧结温度更低, 且制得的样品的离子传导性更好。

关键词: 固态电解质; 溶胶-凝胶法; 离子电导率; Na3Zr2Si2PO12
中图分类号:TM912   文献标志码:A    文章编号:1000-324X(2013)11-1255-06
Sol-Gel Synthesis and Conductivity Properties of Sodium Ion Solid State Electrolytes Na3Zr2Si2PO12
ZHANG Zhi-Zhen1,2, SHI Si-Qi1,3, HU Yong-Sheng2, CHEN Li-Quan2
1. College of Materials and Textiles, Zhejiang Sci-Tech University, Hangzhou 310018, China
2. Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
3. School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China
Fund:National High Technology Research and Development Program of China (2011AA11A235)
National Natural Science Foundation of China (51072183, 11005161, 11205249)
Abstract

NASICON-structured Na3Zr2Si2PO12 was synthesized by a Sol-Gel approach. Phase-pure samples were successfully sintered at 1050℃ when adding 10% excessive Na and P in the precursors, while a small amount of ZrO2 impurity was detected without adding excessive phosphorus. Electrochemical impedance spectrum tests indicate that the ionic conductivity of the former is as high as 5.4×10-4 S/cm at room temperature, which is higher than that of samples prepared from the precursors without adding excessive phosphorus (3.7×10-4S/cm). Further analysis reveals that the evaporation of phosphorus at high temperature would cause the formation of ZrO2 impurity in the samples, leading to a lower ionic conductivity. Compared with solid state reaction approach, samples with enhanced ionic conductivity can be obtained at a rather lower temperature by Sol-Gel synthesis.

Keyword: solid state electrolytes; Sol-Gel synthesis; ionic conductivity; Na3Zr2Si2PO12

锂离子二次电池具有比能量密度大、电池电压高、工作温度范围宽和充放电寿命长等优点, 在小型移动消费电子产品领域占据主导地位, 近年来在动力汽车和储能领域也表现出令人瞩目的发展前景[ 1, 2, 3]。然而, 锂资源的有限性及其较高的价格限制了其未来大规模的应用。由于Na资源丰富、成本低、钠离子与锂离子具有类似的嵌入机理, 近年来钠离子二次电池重新受到研究学者的关注[ 4, 5, 6, 7, 8, 9]。另外, 传统的二次电池多采用液态电解质, 容易出现漏液、电极腐蚀, 甚至爆炸等问题。使用无机固态电解质代替液态电解质可以解决漏液问题, 并且不存在液态电解质的分解问题[ 10], 使得全固态电池具有更高的安全性能[ 1, 11, 12]和更长的循环寿命[ 13]。发展全固态电池, 除了要解决界面之间的接触问题外还需要寻找离子电导率高的固态电解质[ 14]。

NASICON(Sodium Super Ion Conductors)型快离子导体是目前最具有前景的钠离子固态电解质之一, 由Goodenough[ 15]和Hong[ 16]等首先提出。它是由NaZr2(PO4)3和Na4Zr2(SiO4)3形成的连续固溶体, 通式为Na1+ xZr2Si xP3- xO12(0≤ x≤3), 当 x= 2时, 即Na3Zr2Si2PO12的离子电导率最高, 300℃时可达0.2 S/cm[ 15], 与β"-Al2O3相当[ 17, 18]。因此后人有很多工作或探索Na3Zr2Si2PO12的合成, 或直接对Na3Zr2Si2PO12进行掺杂改性[ 19, 20, 21, 22, 23]。

采用传统的高温固相法制备Na3Zr2Si2PO12, 通常需要在1200℃以上进行烧结, 容易造成原料中Na和P组分的挥发, 导致ZrO2或其它杂相的析出,很难获得纯相, 且组分不易控制[ 24, 25]。Schmid等[ 26, 27]发现杂质相聚集在晶界附近, 降低体系的离子电导率。可见探索纯相Na3Zr2Si2PO12的合成条件至关重要, 这不仅是制备离子导电性能优异的固态电解质的要求, 也为进一步的掺杂改性研究奠定基础。

由于溶胶-凝胶法可以使反应物达到分子级的均匀混合, 缩短传质过程, 在较低的温度下就能得到结晶性高的纯相产物, 很多学者相继尝试溶胶-凝胶法来合成NASICON材料[ 28, 29, 30, 31, 32, 33, 34]。但是为了获得致密度高的NASICON陶瓷片, 前驱体仍然需要在较高的温度下进行烧结[ 29], 因此同样存在Na和P的挥发问题而使得最终产物中含有ZrO2杂相[ 33, 34]。

最近本课题组发现通过在前驱体中适当加入过量的Na源和P源, 可以弥补高温烧结过程中Na和P组分的损失, 进而制得单相NASICON产物。本工作采用溶胶-凝胶法, 探索合成纯相Na3Zr2Si2PO12的条件。

1 实验方法

实验中所用试剂均为分析纯。合成过程为: 首先, 量取一定量的正硅酸乙酯, 溶于酒精与水的混合溶液中, 向其中滴加柠檬酸的水溶液, 控制pH值在1~1.5之间, 密封并在60℃下加热搅拌1 h, 使正硅酸乙酯充分水解。加入NaNO3, 缓慢滴加ZrO(NO3)2水溶液, 继续搅拌30 min, 最后逐滴加入(NH4)2HPO4溶液, 得到无色透明溶液, 升温至80℃搅拌6 h, 直到得到溶胶。将溶胶置于烘箱中80℃陈化24 h得到干凝胶, 转移至氧化铝坩埚中500℃下热处理3 h后, 再升温至850~950℃预烧结12 h, 得到的粉末研磨压片, 1050℃下再次烧结12 h, 得到目标产物。为了探索P缺失对体系离子电导率的影响, 做了两组实验。第一组: Na过量10%, P过量10%, 该组样品记为SG1, 最终得到的陶瓷片记为SG1-1050; 第二组: Na过量10%, P不过量, 该组样品记为SG2, 最终得到的陶瓷片记为SG2-1050。固相法作为对比实验, 以Na2CO3、(NH4)2HPO4、ZrO2和SiO2为原料, 其中Na和P均过量15%, 在1100℃下合成了纯相的Na3Zr2Si2PO12, 记为SS-1100。

材料物相分析所用设备为X射线粉末衍射仪(Philips XPERT-PRO)。测试条件: 辐射源为Cu K α (λ = 0.15406 nm), 工作电压为40 kV, 工作电流为40 mA, 扫描速率为1o/min, 扫描的角度范围为5o ~70o。

采用电感耦合等离子体原子发射光谱仪(Inductively Coupled Plasma Atomic Emission Spectrum, ICP-AES)(Thermo公司的IRIS intrepid Ⅱ)对样品中Na、Zr、P元素的重量百分比进行测定, 进而确定样品中这三种元素的比例。精确度可达1%。

采用场发射扫描电子显微镜(Hitachi S-4800)对样品的微观形貌、颗粒大小以及陶瓷片的横断面形貌进行观察, 工作电压为10 kV。

采用德国Zahner Zennium, IM6ex电化学工作站测试样品的电化学阻抗谱(Electrochemical Impedance Spectrum, EIS), 测试频率范围为4 MHz~100 mHz, 所加微扰电压为5 mV。电极的制备过程为: 在烧结得到的陶瓷片两面都涂上银浆, 550℃下热处理15 min。

2 结果与讨论

不同温度下烧结得到样品的X射线衍射图谱如图1所示, 可以看出, 850℃时NASICON的主相已经基本生成, 950℃烧结得到的材料结晶性好, 通过与标准PDF卡片库对比可知, 与三方相的Na3Zr2Si2PO12(ICSD 063567)衍射峰吻合。与950℃烧结得到的产物相比, 1050℃烧结产物的XRD衍射峰在22.85°, 27.52°, 30.69°发生劈裂, 说明材料由三方相转化成了单斜相, 与ICSD 202154衍射峰吻合。且随着烧结温度的升高, 衍射峰的强度增加, 峰也越尖锐, 说明烧结温度越高, 产物的结晶性越好。对比图1(c)和图1(d)还可以看出, 当前驱体中加入10%过量P时, 1050℃烧结后得到的样品是纯相, 没有ZrO2或其它杂质生成(图1(c)); 当前驱体中P不过量时, 相同条件下烧结得到的样品中含有少量的ZrO2杂质。固相法1100℃烧结的终产物也为纯相。

图1 不同温度下烧结产物的XRD图谱Fig.1 XRD patterns of NASICON samples sintered at different temperatures

对不同合成条件下得到的NASICON材料进行了化学元素组成分析测试(见表1)。溶胶-凝胶法P过量10%条件下合成的产物中 n(Na): n(Zr) =1.515, n(P): n(Zr)=0.520。基本上接近化学计量比的Na3Zr2Si2PO12。P不过量条件下合成的产物中 n(P)︰ n(Zr)= 0.445, 说明在烧结过程中P部分损失。原料中Na和P均过量15%, 固相法1100℃烧结得到的产物中各元素的摩尔比也接近化学计量的Na3Zr2Si2PO12。

表1 不同合成条件下制得的NASICON材料的ICP-AES结果 Table 1 Results of ICP-AES of samples sintered at different conditions

采用Rietveld方法[ 35], 以标准卡片ICSD202154(晶胞参数 a = 1.565 nm, b = 0.906 nm, c = 0.922 nm, α = 90.0°, β = 123.74°, γ = 90.0°)为模型, 对SG1-1050℃产物进行晶体结构精修, Rp= 11.0, Rwp= 12.1, Rexp= 4.59(见图2)。精修得到的晶胞参数为 a = 1.563(3) nm, b = 0.904(2) nm, c = 0.922(3) nm, α = 90.0°, β = 123.62(1)°, γ = 90.0°, 与标准卡片ICSD 202154具有很好的一致性。

图2 溶胶-凝胶法制备得到的Na3Zr2Si2PO12晶体结构精修Fig. 2 Crystal structure refinement of Na3Zr2Si2PO12 synthesised by Sol-Gel method

从图3的扫描电镜照片可以看出, 850℃预烧产物的颗粒约在200~500 nm之间, 有明显的团聚现象, 950℃预烧产物的结晶性较好, 颗粒尺寸主要在500 nm左右, 1050℃二次烧结后大部分颗粒仍为几百纳米, 小部分颗粒长大到微米级。图3(d)、3(e)和3(f) 的横断面形貌图显示, 二次烧结后所得到的陶瓷片晶粒与晶粒之间接触密实, 没有观察到明显的孔隙。

图3 (a)SG1-850℃, (b)SG1-950℃和(c)SG1-1050℃烧结产物的形貌; 及陶瓷片(d)SG1-1050(e)SG2-1050和(f)SS-1100的横断面SEM形貌Fig. 3 SEM images of presintered samples (a) SG1-850℃; (b) SG1-950℃; (c) SG1-1050℃ and cross section of ceramics samples (d) SG1-1050; (e) SG2-1050; (f) SS-1100

采用交流阻抗谱仪对不同合成条件下制备得到的陶瓷片进行电化学阻抗谱测试, 其Nyquist图如图4所示。图中半圆体现了晶界容抗和晶界阻抗的并联阻抗特性, 半圆在横坐标的跨度反映了晶界阻抗。半圆左侧与横坐标的交点代表晶粒的阻值。后面的斜线体现了阻塞电极/电解质界面的阻抗特性。对图4中的电化学阻抗谱进行拟合, 可以得到各陶瓷片的电阻值, 并计算出室温下的离子电导率, 见表 2。采用溶胶-凝胶法, 前驱体中P不过量时制得的样品其室温离子电导率为3.7×10-4 S/cm, P过量10%时得到的样品室温离子电导率为5.4×10-4 S/cm, 高于Noguchi等[ 36]报道的7.48×10-5 S/cm, Yadav等[ 32]报道的2.01×10-4 S/cm及Wang等[ 20]报道的3.30×10-4 S/cm。Porkodi等[ 37]报道了采用溶胶-凝胶法制备出来的Na3Zr2Si2PO12的室温离子电导率约为5.5×10-3 S/cm(但是根据文中提供的陶瓷片的尺寸及电阻值, 计算出来的离子电导率约为5.5×10-5 S/cm)。原料中P过量10%得到的产物的离子导电性明显优于P不过量相同条件下得到的产物, 结合XRD的结果可知, 高温下P的挥发会造成ZrO2杂质的析出,富集在晶界附近, 降低体系的离子电导率。固相法1100℃下烧结得到的陶瓷片其室温离子电导率为1.7×10-4 S/cm。可见溶胶-凝胶法可以降低烧结温度, 并且获得的产物离子电导率更高。

图4 不同合成条件下制得的Na3Zr2Si2PO12陶瓷片的电化学阻抗谱Fig. 4 EIS spectra of Na3Zr2Si2PO12 ceramics sintered at different conditions

表2 不同合成条件下制得的Na3Zr2Si2PO12的室温离子电导率 Table 2 Room temperature ionic conductivity of Na3Zr2Si2PO12sintered at different conditions

对不同方法合成的陶瓷片在0~60℃之间进行变温阻抗谱测试, 并根据阿累尼乌斯方程 σ= σ0/ T exp(- Ea/k T)对其活化能进行拟合(如图5), 得到陶瓷

片活化能分别为0.39、0.42和0.42 eV, 与Baur等[ 38]报道的接近。

图5 不同条件下合成的NASICON材料离子电导率随温度变化的Arrhenius曲线Fig. 5 Arrhenius plots of ionic conductivity of NASICON ceramics sintered at different conditions

3 结论

采用溶胶-凝胶法, 以柠檬酸为螯合剂探索了合成纯相Na3Zr2Si2PO12固态电解质的条件。XRD分析表明, P不过量时, 1050℃烧结得到的产物含有少量的ZrO2杂质; 而前驱体中P源过量10%时, 相同条件下得到了纯相Na3Zr2Si2PO12。EIS测试表明, P过量10%时, 烧结得到的陶瓷片离子导电性能优异, 室温下达到了5.4×10-4S/cm, 活化能为0.39 eV; 比P不过量时烧结得到的陶瓷片离子电导率要高。与固相法相比, 溶胶-凝胶法在较低的烧结温度下就可获得离子传导性能更好的目标产物。

致谢 感谢卢侠博士在实验设计以及结构精修等方面的帮助与讨论。

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