Preoxidation of Ni0.8Co0.17Al0.03(OH)2 with LiClO4 to improve cycle stability of lithium-ion batteries
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摘要: 层状高镍正极材料(LiNi0.8Co0.17Al0.03O2)因为具有高的镍含量,相比于LiCoO2拥有更高的比容量和更低的成本,受到了大众的欢迎。然而,循环过程中容量的快速衰退阻碍了LiNi0.8Co0.17Al0.03O2的进一步商业化使用。其中,Li+/Ni2+混排现象是造成材料不良循环性能的主要原因之一。本文中,使用具有强氧化性的LiClO4对Ni0.8Co0.17Al0.03(OH)2前驱体进行预氧化处理。X射线衍射(XRD)测试和精修结果显示,LiClO4处理后的LiNi0.8Co0.17Al0.03O2(LiClO4-NCA)样品有着更低的Li+/Ni2+混排程度,这与X射线光电子能谱(XPS)测试得到的正极材料中Ni2+/Ni3+结果相一致。电化学测试结果显示,LiClO4-NCA相比于原始样品LiNi0.8Co0.17Al0.03O2(NCA)具有更优异的循环性能,1 C倍率循环100圈后,LiClO4-NCA的容量保持率(94.3%)明显高于NCA(82.4%)。LiClO4-NCA优异的电化学性能归因于LiClO4促进了材料中的Ni2+转化为Ni3+,减少了阳离子混排现象,保持了更完整的层状结构。因此,LiClO4对Ni0.8Co0.17Al0.03(OH)2前驱体进行预氧化处理可以改善材料中的Li+/Ni2+混排现象,优化层状高镍正极材料的循环稳定性。
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关键词:
- 层状高镍正极材料 /
- 预氧化改性 /
- Li+/Ni2+混排 /
- 循环稳定性
Abstract: Layered Ni-rich anode material (LiNi0.8Co0.17Al0.03O2) is welcomed by the public because of its high nickel content, higher specific capacity and lower cost than LiCoO2. However, the rapid capacity decline in the process of cycling prevents further commercial application of LiNi0.8Co0.17Al0.03O2. Among them, the mixing of Li+ /Ni2+ in Ni-rich anode materials is one of the main reasons for the poor cycle performance of materials. In this paper, LiNi0.8Co0.17Al0.03O2 precursor was preoxidized with highly oxidized LiClO4. The X-ray diffraction (XRD) and Rietveld refinement results show that the LiNi0.8Co0.17Al0.03O2 (LiClO4-NCA) sample after LiClO4 treatment has a lower Li+/Ni2+ mixing degree, which is consistent with the results of Ni2+/Ni3+ in the anode material tested by X-ray photoelectron spectroscopy (XPS). The electrochemical test results show that LiClO4-NCA has better cycling performance than the original sample LiNi0.8Co0.17Al0.03O2 (NCA). The capacity retention rate of LiClO4-NCA (94.3%) is significantly higher than that of NCA (82.4%) after 100 cycles of 1 C. The excellent electrochemical performance of LiClO4-NCA is attributed to the fact that LiClO4 promotes the conversion of Ni2+ into Ni3+ in the material, reducing cation mixing phenomenon and maintaining a more complete layered structure. Therefore, LiClO4 preoxidation of LiNi0.8Co0.17Al0.03O2 precursor can improve the mixing phenomenon of Li+/Ni2+ in the material and optimize the cycle stability of layered Ni-rich anode material. -
目前的锂离子电池正极材料主要集中在镍钴锰系,但这些正极材料都有着各自的缺陷. 商品化的LiCoO2正极材料由于钴资源稀有,价格昂贵并且LiCoO2的实际容量不高,高温条件下的热稳定性和安全性能差及钴离子的环境污染等问题阻碍了其进一步发展[1]. LiNiO2理论比容量为274 mAh/g,实际容量可达190~210 mAh/g,在容量和价格上比LiCoO2有优势,但是LiNiO2存在诸多缺点,如合成困难[2-3]、在充放电过程中因Jahn-Teller效应导致循环性能差[4]、首次充放电库仑效率较低、存在安全问题[5]等. 尖晶石型LiMn2O4的主要缺点在于Mn3+的溶解、Jahn-Teller效应和电解液分解引起的循环容量下降,高温条件下尤为明显[6-7]. 1999年,Liu等[8]最早用混合氧氧化物法制备LiNi1-x-yOxMnyO2三元系列材料,这种材料充分综合LiNiO2的高比容量、LiCoO2良好的倍率性能,以及LiMnO2的高安全稳定性和低成本等优点,综合性能优于单组分化合物. 王海燕等[9]采用碳酸盐共沉淀法制备LiNi1/3Co1/3Mn1/3O2正极材料,在2.8~4.3 V,1 C下,放电比容量为137.8 mAh/g,前40 次循环容量保持率为96 %. 马全新等[10]采用液相共沉淀法制备LiNi0.5Co0.2Mn0.3O2,正极材料,在20 mA/g电流密度,2.5~4.3 V下,首次放电比容量为175 mAh/g,首次库伦效率在89 %~90 %之间,30次循环未见容量衰减. Li等[11-12]采用固相法,制备LiNi0.5Co0.2Mn0.3O2,在40 mA/g电流密度,3.0~4.6 V下,首次放电比容量达到172 mAh/g,25次循环后,容量保持率为85 %. 钟盛文等[13]采用共沉淀法制备LiNi0.5Mn0.3Co0.2O2,并研究掺杂Al(OH)3对材料循环性能的影响,在2.75~4.2 V,1 C下,首次放电比容量达到157.2 mAh/g,循环50次容量保持率为94.8 %.少量Al(OH)3的掺杂对材料结晶性有所提高,但是放电比容量有所下降,前100次循环掺杂对材料循环性能无明显效果. 袁超群等[14]采用高温固相反应合成了LiNi0.75-xCoxMn0.25O2(x=0.05,0.1,0.15,0.2,0.25),结果表明:当x=0.2时,材料性能最好,在50 mA/g电流密度,3~4.3 V下,首次放电比容量为172.5 mAh/g,放电效率为90.9 %. 中南大学Li等[15]采用快速共沉淀法制备LiNi0.8Co0.1Mn0.1O2纳米材料. 在18 mA/g电流密度下,2.7~4.3 V,首次放电比容量达192.4 mAh/g,40次循环后容量保持率达91.56 %. Cho等[16]用AlPO4对LiNi0.8Co0.1Mn0.1O2进行表面包覆. 结果表明:包覆后,材料的容量几乎没有减少,首次放电比容量达188 mAh/g,热稳定性和循环稳定性都得到改善. 杨驰[17]通过溶胶-凝胶法制备出LiNi0.92Co0.04Mn0.04O2正极材料,在2.8~4.3 V,0.5 C下,首次放电比容量为211.3 mAh/g,50次循环后仍有174.5 mAh/g,容量保持率为82.6 %. 通过对文献的参阅研究发现,国内外对镍基尤其是镍基三元正极材料的研究时间不长,相关方面的研究也不是很多. 用资源丰富的Ni、Mn取代昂贵且是战略资源的Co,融合了3种元素的优势,因此制备出容量高并且循环稳定的镍基三元正极材料显得尤为重要. 而LiNi0.6Co0.1Mn0.3O2作为一种很有潜力的三元镍基正极材料,文中将采用共沉淀法合成镍钴锰氢氧化物前驱体,使其和LiOH·H2O混合均匀后,经高温焙烧合成正极材料LiNi0.6Co0.1Mn0.3O2,并研究不同焙烧温度和锂配量对材料综合性能的影响,以期制备出高容量和稳定的LiNi0.6Co0.1Mn0.3O2正极材料.
1 实 验
1.1 LiNi0.6Co0.1Mn0.3O2材料的合成
以NiSO4·6H2O、MnSO4·H2O、CoSO4·H2O为原料,按照6∶3∶1的摩尔比混合均匀,配制成2 mol/L的金属盐溶液,另称取一定量的NaOH配制成4 mol/L的碱溶液,通过恒流泵将混合盐溶液和碱溶液同时均匀的滴加到有N2保护的反应釜中,控制反应釜中的温度为55 ℃,反应pH值控制在11.35附近,保持共沉淀反应在20 h左右,保证制得形貌规则的类球形前驱体Ni0.6Co0.1Mn0.3(OH)2,反应完成后,将前驱体进行洗涤,过滤,干燥24 h后与LiOH·H2O按化学一定的计量比混合,放于石英管式炉中,在不同的温度和锂配量下进行焙烧,制得正极材料LiNi0.6Co0.1Mn0.3O2.
1.2 LiNi0.6Co0.1Mn0.3O2材料形貌及结构表征
实验采用日本理学公司生产的Miniflex X射线衍射仪(XRD)进行结构分析,测试条件为:铜阳极CuKα射线(波长Å=0.154 18 nm),电压为45 kV,电流为40 mA,扫描速度为5°/min,扫描步长为0.02°,扫描范围为10°~80°.采用荷兰飞利浦XL30W/TMP型扫描电镜对材料进行形貌分析.
1.3 LiNi0.6Co0.1Mn0.3O2材料的电化学性能测试
将称量好的聚偏氟乙烯(PVDF)溶于N-甲基吡咯烷酮(NMP)中,待PVDF完全溶解后,按照正极材料∶PVDF∶导电剂=90∶6∶4的配制,搅拌调成糊状料浆,将料浆均匀涂覆于铝箔上,放于120 ℃的恒温鼓风干燥箱中干燥,待干燥完全后,在双辊轧膜机上对辊,通过手动冲片机冲片制得极片,放于60 ℃的恒温干燥箱中干燥12 h. 选用的负极材料为锂片,以1 mol/L的LiPF6/DMC+EMC+EC(体积比为 1∶1∶1)为电解液,隔膜采用 Celgard2400,在充满氩气的手套箱中装配成2032型纽扣电池.采用NEWARE BTS型电池测试系统对电池进行电性能测试,充放电电压范围为2.75~4.2 V,测试温度为25 ℃.
2 结果分析与讨论
2.1 iNi0.6Co0.1Mn0.3O2材料的电化学性能
图 1所示为不同焙烧温度的材料在25 ℃,电压范围为2.75~4.2 V,1 C倍率下,50次循环后的循环曲线. 表 1为不同焙烧温度,材料在1 C倍率下50次循环后的放电比容量.
表 1 不同温度下LiNi0.6Co0.1Mn0.3O2 放电比容量表Table 1. Discharge specific capacity of LiNi0.6Co0.1Mn0.3O2 at different temperature温度/℃ 首次放电容量
/(mAh·g原员)第50 次放电容量
/(mAh·g-1)效率/% 750 120.3 108.4 90.10 800 133.2 121.0 90.84 850 137.1 121.2 88.40 900 146.0 130.9 89.66 结合图 1和表 1分析可知:焙烧温度对材料的电化学性能有较大的影响,在一定温度范围内,随着焙烧温度的提高,合成材料的循环放电比容量随着温度的升高而增大.这是因为焙烧温度的提高,材料的固相反应更加完全,晶体生长的更加完整,材料的结构完整性更好,在后期电池循环过程中,材料晶体的完整有利于锂离子的顺利脱嵌,保证了材料具有一个比较高的放电比容量以及循环稳定性,综上可知:900 ℃时材料性能较好,在2.75~4.2 V,1 C倍率下,首次放电比容量为146.0 mAh/g,经过50次循环后仍有130.9 mAh/g,容量保持率为89.66 %.
通过前期对正极材料焙烧温度的探讨,得到较好焙烧温度900 ℃,实验中发现在相同的焙烧温度下,不同的锂配量对正极材料的电化学性能有较大影响,后期将对材料的锂配量进行探讨,实验中选择在最佳焙烧温度900 ℃下,对正极材料的锂配量进行探讨. 图 2所示为900 ℃下,不同锂配量的材料在25 ℃,2.75~4.2 V,1 C倍率下的循环曲线. 表 2为不同锂配量下LiNi0.6Co0.1Mn0.3O2在50次循环后的放电比容量.
表 2 在不同锂配量下LiNi0.6Co0.1Mn0.3O2 的放电比容量表Table 2. Discharge specific capacity of LiNi0.6Co0.1Mn0.3O2 at different Li/M(Ni, Co, Mn)锂配量 首次放电容量
/(mAh·g-1)第50 次放电容量
/(mAh·g-1)效率/% 1.01 122.8 104.8 85.9 1.03 133.4 124.2 93.1 1.05 146.0 130.9 89.6 1.07 139.7 131.2 93.9 1.09 143.7 141.3 98.3 1.10 138.8 99.7 71.8 根据图 2和表 2分析可以得到:在最佳焙烧温度900 ℃下,正极材料的电化学性能随着锂配量的增加,放电比容量呈现先增大后减小的趋势,且当锂配量不足或过多时,正极材料的循环稳定性会受到较大的影响.在焙烧过程中,锂元素会挥发,导致正极材料出现缺锂现象,Ni2+占据Li+位置,使Li+和Ni2+混排严重,锂配量过多又会导致Li+析出吸附在正极材料表面,阻碍Li+的脱嵌,使材料性能下降,所以锂的适当过量将弥补烧结过程中锂的挥发,让材料的晶体生长完整,结构更加稳定以及良好的循环性能. 综上可知,当锂配量为1.09时,在2.75~4.2 V,1 C倍率下,正极材料的首次放电比容量为143.7 mAh/g,50次循环后仍有141.3 mAh/g,容量保持率为98.3 %,所以锂配量为1.09下正极材料的电化学性能最好.
2.2 iNi0.6Co0.1Mn0.3O2材料的晶体结构
通过对材料焙烧温度和锂配量的探讨,确定了较佳烧结温度和最佳锂配量,XRD测试中选取最佳条件下制备的正极材料进行结构分析,图 3所示为最佳条件下合成LiNi0.6Co0.1Mn0.3O2的XRD图谱.
由图 3分析可知,最佳条件下合成的材料在2θ为(003)、(101)、(006)、(102)(104)、(108)、(110)等特征晶面附近处都出现LiNiO2特征峰,表明材料具有典型的α-NaFeO2型层状结构,尤其在(006)和(102)以及(108)和(110)晶面处出现明显的层状LiNiO2的特征峰,并且两组特征峰分裂明显,强度相近,表明材料中合成了层状结构较好的正极材料,由此我们可以知道,在较佳温度900 ℃和较佳锂配量1.09下焙烧出的正极材料其固相反应完全,材料晶体发育完整,阳离子混排程度较小,二维层状有序度高,因此其电化学性能更好.
2.3 LiNi0.6Co0.1Mn0.3O2材料的表面形貌
SEM测试过程中同样选取较佳温度900 ℃和最佳锂配量1.09下烧制出的正极材料进行形貌分析,图 4所示为最佳条件下合成LiNi0.6Co0.1Mn0.3O2的扫描电镜图.
从图 4中可以看到,合成的正极材料颗粒均是由多个亚微米级的一次颗粒堆积形成的二次颗粒,一般在10 μm左右,且颗粒之间存在较多的空隙,颗粒表面光滑,球形度好,比表面积较大,有利于活性物质与电解液的充分接触,降低材料的内阻以及在充放电过程在层通道中的传递,从而提升了材料的电化学性能.
3 结 论
实验采用共沉淀法合成Ni0.6Co0.1Mn0.3(OH)2前驱体,采用高温固相法,在空气氛下,合成LiNi0.6Co0.1Mn0.3O2正极材料. 通过实验探究其合成的工艺条件,结果表明,相同条件下,900 ℃时材料的性能较优. 在25 ℃,2.75~4.2 V,1 C倍率下,材料的首次放电比容量为146.0 mAh/g,50次循环后放电比容量为130.9 mAh/g,容量保持率为89.66 %.较佳的焙烧温度900 ℃下,通过锂配量的优化对比得出,锂配量为1.09时放电比容量最好,在25 ℃,2.75~4.2 V,在1 C倍率循环下,其首次放电比容量为143.7 mAh/g,50次循环后放电比容量仍有141.3 mAh/g,容量保持率为98.3 %,由此可以看出制备出的LiNi0.6Co0.1Mn0.3O2正极材料具有较高的比容量并且循环性能优异,是一种很有潜力的锂离子电池正极材料.
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表 1 正极材料的晶格参数
Table 1 Lattice parameters of the cathode materials
表 2 正极材料的电化学数据
Table 2 The electrochemical data sheets of the cathode materials
表 3 2种材料的锂离子扩散系数
Table 3 Li+ diffusion coefficient of both materials
表 4 2种材料的阻抗值和锂离子扩散系数
Table 4 Impedance and Li+ diffusion coefficient of both materials
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