AMC  >> Vol. 6 No. 3 (July 2018)

    导电聚合物PEDOT:PSS包覆对LiNi0.8Co0.15Al0.05O2正极材料性能的影响
    The Effect of PEDOT:PSS-Coated on the Electrochemical Performance of LiNi0.8Co0.15Al0.05O2 Cathode Materials

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作者:  

李 荐,刘良秦,胡乐善:中南大学材料科学与工程学院,湖南 长沙;
周宏明:湖南省正源储能材料与器件研究所,湖南 长沙

关键词:
LiNi0.8Co0.15Al0.05O2导电聚合物表面包覆电化学性能LiNi0.8Co0.15Al0.05O2 Conductive Polymer Surface Coating Electrochemical Performance

摘要:

本文采用湿法包覆法成功制备了聚(3,4-亚乙基二氧噻吩):聚(苯乙烯磺酸盐) (PEDOT:PSS)包覆的LiNi0.8Co0.15Al0.05O2正极材料,研究了包覆改性前后材料的微观结构及电化学性能。LiNi0.8Co0.15Al0.05O2颗粒表面的聚合物PEDOT:PSS包覆层厚度大约为14 nm,PEDOT:PSS包覆的LiNi0.8Co0.15Al0.05O2具有良好的电化学性能,在0.1 C倍率下首次放电比容量193.8 mAh/g,1 C循环100次后容量保持率为90.3%。PEDOT:PSS包覆层具有高电导率,可以提高材料的导电性,因此PEDOT:PSS包覆的LiNi0.8Co0.15Al0.05O2具有高放电比容量、好的循环稳定性和良好的倍率性能。

The poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) coated LiNi0.8Co0.15Al0.05O2 cathode material was successfully prepared by wet coating method, and their microstructure and electrochemical properties were studied. The conductive polymer films on the surface of the LiNi0.8Co0.15Al0.05O2 particles are about 14 nm thick, and the PEDOT:PSS-coated LiNi0.8Co0.15Al0.05O2 has good electrochemical performance. The first discharge specific capacity was 193.8 mAh/g at 0.1 C, and the capacity retention rate was 90.3% at 1C after 100 cycles. The PEDOT:PSS cladding layer has high electrical conductivity and can improve the electrical conductivity of the material. Therefore, the PEDOT:PSS-coated LiNi0.8Co0.15Al0.05O2 has high discharge specific capacity, good cycle stability, and good rate performance.

1. 前言

锂离子电池(LIB)由于具有高能量密度和长使用寿命而被广泛用于便携式电子设备,并且在电动汽车和能量存储系统等领域也有广泛的应用前景 [1] [2] [3] 。在传统的正极材料中,LiCoO2具有易于合成、循环性能好等优点,使其在商用锂离子电池中应用最为广泛。但是其高成本、毒性大等缺点限制了它在电动汽车领域的应用。因此,富镍层状LiNi0.8Co0.15Al0.05O2(NCA)和LiNixCoyMn1-x-yO2材料因其比容量高、低成本和低污染等优点而备受关注 [4] - [9] 。但是,富镍层状NCA和LiNixCoyMn1-x-yO2材料高温循环时,材料中的过渡金属会高电压下与电解质溶液发生副反应,造成过渡金属的溶解以及电解液的分解。因此,富镍层状正极材料展现出差的结构和热稳定性,过充电时其循环稳定性差。为了克服这些问题,研究者在无机氧化物和导电聚合物包覆正极材料方面作了大量的研究 [10] - [18] 。

研究表明导电聚合物对正极材料进行表面改性,有利于电荷的传递,提高正极材料的电化学活性 [19] [20] 。在各种导电聚合物中,聚(3,4-亚乙基二氧噻吩):聚(苯乙烯磺酸盐)(PEDOT:PSS)由于其高导电性和良好的电化学稳定性,备受研究人员的青睐 [21] 。根据先前的研究报导表明PEDOT:PSS包覆可以提高材料的导电性和稳定性。使用导电聚合物PEDOP:PSS改性的硫正极,锂硫电池中硫的利用率和容量保持率明显得到改善 [22] 。Dae等人用PEDOT:PSS作为空气电极中氧化还原反应基体和粘合剂,可以抑制碳和电解质之间的副反应并提高锂空气电池的循环性能 [23] 。PEDOT:PSS也被引入作为锂离子电池的粘结剂,PEDOT:PSS作为粘结剂可以提高电极材料的电导率和材料与集流体之间的附着力,以提高材料的循环性能 [24] 。Liu等人通过用PEDOT包覆层状LiNi1/3Co1/3Mn1/3O2材料,其倍率性能和循环性能得到提高 [25] 。Schougaard等人通过电化学聚合法制备PEDOT/LiFePO4复合材料,以提高材料的倍率性能 [26] 。鉴于先前的研究,本文采用高温固相法合成了高容量的NCA正极材料,用简单的湿法包覆制备了PEDOP:PSS包覆的NCA正极材料,通过PEDOP:PSS对LiNi0.8Co0.15Al0.05O2进行表面改性,改善材料的循环性能及倍率性能。

2. 实验部分

2.1. LiNi0.8Co0.15Al0.05O2的制备

Ni0.8Co0.15Al0.05 (OH)2前驱体从金驰能源材料有限公司购入,将前驱体Ni0.8Co0.15Al0.05 (OH)2和LiOH·H2O (分析纯 AR)按照摩尔比1:1.03的比例混合研磨,装入方形刚玉方舟中,然后将方舟推入管式炉的恒温区域。接着在流动的氧气气氛中升温至480℃预烧5 h,750℃烧结12 h得到NCA正极材料。

2.2. LiNi0.8Co0.15Al0.05O2的表面包覆改性

将11.5 g PEDOT:PSS溶液(Aldrich,1.3 wt%水分散液)分散到N-甲基吡咯烷酮(NMP)中,随后加入5 g NCA正极材料于分散液中,并将混合物加热到60℃搅拌4 h。之后将混合物过滤,在100℃下真空干燥24 h,得到3 wt% PEDOT:PSS包覆的NCA正极材料。

2.3. 电极的制备及电池的组装

将正极材料、炭黑、聚偏氟乙烯按照质量比8:1:1的比例进行研磨混合,然后加入适量的NMP溶剂进行混合,制成混合均匀的浆料。之后将浆料涂布到铝箔上,置于120℃的真空干燥12 h,经过裁片得到正极极片。以锂为负极、Celgard 2400聚丙烯微孔膜为隔膜和NCA极片为正极,以1.0 M LiPF6/EC + DMC + EMC (1:1:1,体积比)作为电解液,在充满氩气的手套箱中组装组成的CR2032型纽扣电池。

2.4. 表征和测试

未改性的NCA和包覆改性的NCA正极材料的晶体结构使用XRD (Rigaku, Rint-2000)进行分析;通过扫描电子显微镜(SEM, JEOL JSM-6300)和透射电子显微镜(TEM, JEOL JEM-3010)表征未包覆和改性的NCA颗粒的形态;能量色散X射线光谱(EDX)被用于表面改性的NCA颗粒上的形态评估和表面元素表征;热分析使用热重分析仪(NETZSCH STA 449C)进行表征:在氮气气氛中,以5℃/min的升温速率从30升至700℃;采用红外光谱(Nicolet 6700)表征NCA改性前后表面物相的结构变化,测试范围为400~4000 cm−1。充放电测试采用新威电化学仪,在2.8~4.3 V的电压范围内进行电池的充放电测试;电化学阻抗谱(EIS)测试在CHI 660A电化学工作站上进行,参数设定为:频率范围0.01 Hz~100 kHz,交流幅值5 mV,工作温度为25℃。

3. 结果与讨论

3.1. XRD结构分析

未改性的NCA和3 wt% PEDOT:PSS包覆的NCA正极材料的XRD图(如图1)所示。表明合成的样品均为α-NaFeO2结构,无杂相,特征峰(108)/(110)和(006)/(102)的分裂明显,I(003)/I(104)的强度比值大于1.2,说明材料具有有序的层状结构和良好的结晶性 [27] 。对于包覆的样品,XRD图表明所有衍射峰与原始样品一致,未发生明显的变化且没有杂质或第二相出现。通过结构精修得出材料的晶格常数NCA (a = 2.8621 Å, c = 14.1551 Å)和PEDOT:PSS包覆的NCA (a = 2.8623 Å, c = 14.1564 Å),表明NCA经包覆改性后材料的晶体结构没有发生明显变化。同时也说明XRD不能检测出含量少且薄的有机物包覆层。

3.2. FT-IR和TG分析

通过比较未改性的NCA和表面改性的NCA颗粒的FT-IR光谱,进一步证实了表面改性的NCA颗粒上存在聚合物PEDOT:PSS。如图2所示发现未改性的NCA和表面改性的NCA具有相似的光谱,同时

Figure 1. The XRD patterns of pristine and PEDOT:PSS-coated NCA

图1. 未改性NCA和PEDOT:PSS包覆NCA的XRD图

Figure 2. The FT-IR spectra of pristine NCA, PEDOT:PSS, and PEDOT:PSS-coated NCA: (a) full scale and (b) magnified scale

图2. 未改性NCA和PEDOT:PSS包覆NCA的FT-IR图:(a) 全图和(b) 局部放大图

在566 cm−1处出现了NCA颗粒中M-O键(M为金属元素Ni、Co、Al)的振动峰,表明在该温度下NCA和PEDOT:PSS之间没有发生化学反应 [28] 。PEDOT:PSS包覆的NCA具有典型的PEDOT:PSS吸收峰,在放大的图中,波数为1497,1357,1162,1119处的吸收峰分别对应PEDOT:PSS中的官能团C=C,C-C,S-O,S-苯键的振动峰。波数在1274,1044,1029,998处对应PEDOT:PSS中的官能团C-O-C的弯曲振动峰,因此从红外光谱可以证明PEDOT:PSS对NCA正极材料进行了表面包覆 [29] 。为了确定PEDOT:PSS的包覆质量,通过热重分析获得如图3所示,PEDOT:PSS包覆的NCA正极材料中实际的PEDOT:PSS含量估计为2.62 wt%,与理论质量3 wt%非常接近。

3.3. 形貌分析

未改性的NCA和3 wt% PEDOT:PSS包覆的NCA正极材料颗粒的形貌用SEM和TEM表征。显然,从图4(a),图4(b)看出这两种样品的形貌没有明显区别,都是一次颗粒紧密团聚而成的球形或类球形的

Figure 3. TGA curves of pristine and PEDOT:PSS-coated NCA

图3. 未改性NCA和PEDOT:PSS包覆NCA的热重图

Figure4. SEM images of (a) pristine NCA and (b) PEDOT:PSS-coated NCA particles and EDX mapping images of (c) Ni, (d) Al, (e) Co and (f) sulfur in the PEDOT:PSS-coated NCA particles

图4. 未改性NCA (a)和PEDOT:PSS包覆NCA (b)颗粒的SEM图以及PEDOT:PSS包覆NCA的EDX谱图:(c) Ni,(d) Al,(e) Co和(f) 硫元素

二次颗粒,表明包覆改性没有破坏NCA颗粒的形貌。通过EDX能谱图检测PEDOT:PSS包覆NCA颗粒表面Ni(c)、Co(e)、Al(d)、S(f)元素的分布,从图4(c)~(f)中可以观察到活性材料NCA中Ni、Co、Al元素的均匀分布,同时也可以看出导电聚合物PEDOT:PSS中S元素的均匀分布,这表明导电聚合物PEDOT:PSS均匀的包覆在NCA正极材料的表面。

图5显示了用聚合物PEDOT:PSS进行包覆改性之前和之后的NCA材料颗粒的高分辨率TEM图像。未改性的NCA颗粒表面光滑且没有明显的包覆层,如图5(a)所示。相反,图5(b)中所示的表面包覆改性的NCA颗粒边缘的放大图像,图像显示有均匀包覆的导电聚合物PEDOT:PSS薄层,其厚度约为14 nm。NCA颗粒表面均匀包覆的导电聚合物PEDOT:PSS,有利于材料在充放电过程中的电子转移。

Figure 5. TEM images of (a) pristine NCA and (b) PEDOT:PSS-coated NCA particles

图5. 未改性NCA (a)和PEDOT:PSS包覆NCA (b)颗粒的TEM图

3.4. 电化学性能分析

通过组装2032型电池来测试未改性NCA和PEDOT:PSS包覆的NCA正极材料的电化学性能。图6(a)为未改性NCA和PEDOT:PSS包覆的NCA正极材料的首次充放电曲线,从图可知在0.1 C (1 C = 180 mAh/g)倍率下,由PEDOT:PSS包覆的NCA正极材料的初始放电容量(193.8 mAh/g)比未改性的NCA正极材料的初始放电容量(191 mAh/g)稍高。这主要是导电聚合物PEDOT:PSS包覆层降低活性正极颗粒之间的接触电阻并促进材料中的电子传递,因此PEDOT:PSS提高了活性材料的可逆容量,所以由导电聚合物PEDOT:PSS包覆的NCA正极材料表现出较高的放电容量。

在低倍率0.1 C下循环2圈后,再用1 C倍率测试未改性NCA和PEDOT:PSS包覆的NCA正极材料的循环性能,并比较这两种材料的容量保持率如图6(b)。表面包覆PEDOT:PSS的NCA正极材料,其循环性能也明显得到改善。PEDOT:PSS表面改性的NCA正极材料在100次循环后的容量损失仅为9.7%,而未改性的材料容量损失为15%。表面包覆改性的NCA正极材料有良好的容量保留率主要原因为其颗粒表面上存在导电聚合物薄膜EDOT:PSS。表明导电聚合物膜用作保护层覆盖在活性材料颗粒的表面,阻碍了颗粒与电解质的直接接触,缓解电解质在高电压下的氧化分解,从而可以提高材料的结构稳定性。

图7为在同一温度下未改性NCA和PEDOT:PSS包覆的NCA正极材料的倍率性能。电池以0.5 C倍率充电至4.3 V,并以0.2至5.0 C的不同倍率进行放电。从图7可以看出未改性NCA和PEDOT:PSS包覆的NCA正极材料放电容量之间的关系。使用导电聚合物PEDOT:PSS包覆的NCA在高倍率下其放电容量明显高于未改性的NCA正极材料,如使用导电聚合物PEDOT:PSS包覆的NCA正极材料在5 C倍率下放电比容量为155.6 mAh/g,而未改性的NCA电极在相同的倍率下其容量为148.7 mAh/g。这表明通过在正极材料表面包覆导电聚合物,为正极材料提供了良好的导电网络,因此其电子导电性得到改善,这有利于电荷转移反应,降低了材料在高倍率下的极化,从而提高了材料倍率性能。

交流阻抗(EIS)提供有关电极电化学反应动力学及电极界面结构信息。为了解导电聚合物PEDOT:PSS包覆层NCA正极材料电池交流阻抗特性的影响,测试了电池循环前后的交流阻抗。图8(a)为在室温下,循环前未改性NCA和PEDOT:PSS包覆的NCA正极材料的交流阻抗图谱,主要由在高频区的半圆弧和低频区的斜率为45˚的直线组成,这分别反映的是电荷传递电阻和Li+在正极材料内部的迁移能力。可以从图8(a)可以得出导电聚合物PEDOT:PSS包覆的NCA的电极和电解质之间的电荷转移电阻(Rct)为42.4 Ω,而为改性的NCA材料的Rct为58.27 Ω,表明导电聚合物PEDOT:PSS的包覆使得材料的电荷转移电阻降低,从而提高了材料的电化学性能。图8(b)为在倍率1 C下,循环100圈后未改性NCA和PEDOT:PSS

Figure 6. (a) Initial charge and discharge cures of pristine NCA and PEDOT:PSS-coated NCA materials at 0.1 C rate; (b) Cycling behavior of pristine NCA and PEDOT:PSS-coated NCA materials at 1 C rate

图6. (a) 0.1 C倍率下未改性NCA和PEDOT:PSS包覆的NCA正极材料的首次充放电曲线;(b) 1 C倍率下未改性NCA和PEDOT:PSS包覆的NCA正极材料的循环性能曲线

Figure 7. The rate capability for pristine NCA and PEDOT:PSS-coated NCA materials

图7. 未改性NCA和PEDOT:PSS包覆的NCA正极材料的倍率性能

包覆的NCA正极材料的交流阻抗图谱。从图中可以看出两个明显的半圆,根据之前的交流阻抗研究,高频区域内的半圆为Li+离子通过电极上的表面膜(Rf)迁移而产生的电阻,而在中低频范围内的半圆为电极和电解质之间的电荷转移电阻(Rct) [30] [31] 。表面包覆PEDOT:PSS的NCA材料的电池的表面膜电阻(137.8 Ω)和电荷转移电阻(220.7 Ω)均低于未改性的NCA材料的表面膜电阻(202.4 Ω)和电荷转移电阻(493.3 Ω)。这说明导电聚合物包覆层有效的抑制了电解质的有效分解,阻碍了绝缘LiF的沉积。材料表面上的导电聚合物层PEDOT:PSS也将在较低导电性的NCA颗粒之间形成良好的导电网络,有助于电子转移并且保护了NCA颗粒免受HF的侵蚀。这些结果表明,通过导电聚合物PEDOT:PSS对NCA正极材料的表面包覆,能有效的降低材料循环过程中的界面电阻。

Figure 8. AC impedance spectra of pristine NCA and PEDOT:PSS-coated NCA materials: (a) before cycle; (b) after 100 cycles

图8. 未改性NCA和PEDOT:PSS包覆的NCA正极材料的交流阻抗图谱:(a) 循环前;(b) 循环100次之后

4. 结论

用高温固相法合成具有高放电比容量的NCA正极材料,并通过湿法包覆法合成了PEDOT:PSS包覆的NCA正极材料。表面包覆PEDOT:PSS的NCA正极材料与未改性的NCA材料相比,其具有更高的初始放电比容量和更稳定的循环性能。由于导电聚合物PEDOT:PSS具有高的电子电导率,包覆在NCA正极表面上形成的良好的导电网络,增强了其高倍率性能。因此,用导电聚合物PEDOT:PSS对NCA正极材料进行表面包覆,提高了材料的可逆容量、循环稳定性和倍率性能。

文章引用:
李荐, 刘良秦, 周宏明, 胡乐善. 导电聚合物PEDOT:PSS包覆对LiNi0.8Co0.15Al0.05O2正极材料性能的影响[J]. 材料化学前沿, 2018, 6(3): 80-89. https://doi.org/10.12677/AMC.2018.63010

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