锂离子电池(LIBs)具有能量密度高、无记忆效应和工作温度范围宽等优点，已广泛应用在便携式电子设备和新能源汽车等领域[1]。目前应用的石墨类负极材料其理论比容量较低(372 mAh·g-1)，不能满足新一代锂离子电池的需求[2~4]。过渡金属氧化物(Mx O y，M=Fe、Ni、Mn、Cu等)有较高的理论比容量(600~1200 mAh·g-1)、低成本和环境友好等优点[5,6]。与过渡金属氧化物相比，过渡金属氢氧化物的制备成本更低，在储锂反应过程中生成的LiOH在低电位下还可与Li进一步可逆生成Li2O和LiH，提供更高的理论比容量，极有应用前景[7~9]。例如，Ni(OH)2的理论储锂比容量高达1735 mAh·g-1，约为NiO理论比容量(718 mAh·g-1)的2倍[10]。但是，过渡金属氢氧化物在充放电过程中的动力学缓慢和显著的体积效应使其倍率性能和循环性能降低[11]。将电极材料纳米化可缩短Li+在材料中的扩散路径、提高材料的比表面积和缓解材料的体积效应，进而改善电极材料的动力学性能和循环稳定性[12]。在各种纳米结构材料中，3D微/纳分级结构(由低维纳米结构单元构成的微米级二次结构)材料具有纳米材料优势的同时还具有协同效应(即有效避免低维纳米材料的自团聚、提高振实密度)[13,14]。Lim等[9]制备的花状微/纳分级结构Ni(OH)2Cl，用作锂离子电池负极材料，在0.2 A·g-1电流密度下经过150次循环其比容量为1236 mAh·g-1；Inamdar等[15]以Cu箔为基体用表面化学氧化法制备的由Cu(OH)2纳米棒构成的3D微/纳分级网状结构材料，在0.1 A·g-1电流密度下可逆比容量高达1472 mAh·g-1，在0.2 A·g-1电流密度下循环100次后比容量保持为506 mAh·g-1。
但是，以往制备微/纳分级结构电极材料使用的水热法、溶剂热法、模版法和溶胶凝胶法条件苛刻、过程冗长且成本高。本文以十二烷基硫酸钠(SDS)为辅助剂，用简便的均相沉淀法制备具有开放结构的3D微/纳分级结构的α-Ni(OH)2负极材料，研究SDS用量对其结构和储锂性能的影响。
1 实验方法
1.1 材料的制备
用均相沉淀法制备α-Ni(OH)2材料：先将1.745 g的Ni(NO3)2·6H2O(分析纯)溶解在90 mL蒸馏水中，然后分别按n(SDS)/n(Ni2+)为0∶10、1∶10、2∶10和3∶10的比例加入SDS，最后再加入尿素18.862 g，原料充分溶解后在90℃搅拌反应4.5 h。反应结束后，将生成的沉淀在60℃母液中陈化20 h，然后进行抽滤、洗涤和冷冻干燥。将n(SDS)/n(Ni2+)等于0∶10、1∶10、2∶10和3∶10的产物样品分别标记为NS-0、NS-10、NS-20和NS-30。
1.2 材料的表征
使用X'Pert3 Powder型多功能X射线衍射仪分析样品的物相结构(测试条件：Cu靶，加速电压为40 kV，加速电流为30 mA，扫描的速度为5 (°)·min-1)；用SU5000型场发射扫描电子显微镜观察样品的微观形貌；使用NEXUS 470型傅里叶变换红外光谱仪采集样品的红外光谱；用SDTQ 600型热重分析仪(TGA)表征样品的组分含量(测试条件：空气气氛，温度范围为25~900℃，升温速度为10℃/min)；用ESCALAB250Xi型X射线光电子能谱仪(XPS)分析样品中的元素组成和价态。
将制备出的样品与导电剂(Super P)和粘结剂(PVDF)按照6∶3∶1的质量比在N-甲基吡咯烷酮(NMP)溶剂中混合成均匀的浆料，然后将浆料均匀涂覆在铜箔上并在80℃真空条件下干燥12 h。将烘干的极片冲裁成直径为16 mm的圆形工作电极(极片上活性物质负载量约为0.4 mg/cm2)，分别以金属锂片和聚丙烯多孔膜(Celgard2500型)为对电极和隔膜，使用1 mol·L-1 LiPF6/EC+DMC+DEC (体积比1∶1∶1)为电解液，在氩气气氛下组装成CR2016型纽扣半电池。用BTS-5 V/10 mA型多通道电池充放电测试仪对电池进行恒电流充放电测试，电位范围为0.01~3.0 V (vs. Li+/Li)；使用CHI760E型电化学工作站对电池进行循环伏安(CV)和交流阻抗(EIS)测试，CV测试的电位窗口为0.01~3.0 V (vs. Li+/Li)，EIS测试的频率范围为105~10-2 Hz。
2 结果和讨论
2.1 微观结构
图1a给出了不同SDS用量样品的XRD谱。可以看出，四个样品的衍射峰均与α-Ni(OH)2 (JCPDS No. 22-0444)相符，没有出现杂质相的峰，表明制备出的样品都是α-Ni(OH)2。相对而言，无SDS辅助的NS-0样品其(001)衍射峰最强，说明其结晶性较好；而SDS辅助的3个样品其(001)衍射峰明显宽化且强度变低，表明样品的结晶性降低[16]。利用Scherrer公式计算出样品的晶格常数和平均晶粒尺寸，结果列于表1。可以看出，随着SDS用量的增多样品的层间距由NS-0的0.7243 nm增大到NS-30的0.7338 nm，平均晶粒尺寸由5.6 nm降低到3.0 nm。这表明，SDS的加入影响了α-Ni(OH)2结晶和生长过程，在α-Ni(OH)2内引入了更多的结构缺陷、细化了晶粒并拓宽了层间距。电极材料中的结构缺陷可为锂离子存储提供更多的活性位点，并改善材料的电子/离子传输性能，因此有利于提升材料的储锂活性和反应动力学[17]。
图1
图1
SDS用量不同的Ni(OH)2样品的XRD谱和TGA图、NS-0和NS-20样品的FT-IR对比、NS-0和NS-20的Ni 2p XPS谱以及NS-20的S 2p XPS谱
Fig.1
XRD patterns (a) and TGA plots (b) of the series of Ni(OH)2 samples prepared with different amounts of SDS; FT-IR spectra of the NS-0 and NS-20 samples (c); high resolution Ni 2p XPS spectra of NS-0 and NS-20 samples (d,e) and high resolution S 2p XPS spectrum of NS-20 sample (f)
表1 不同SDS添加量样品的晶格参数和平均晶粒尺寸
Table 1
| Samples | Crystal plane | 2θ / (o) | Interlayer distance / nm | Average grain size / nm |
|---|---|---|---|---|
| NS-0 | (001) | 12.21 | 0.7243 | 5.6 |
| NS-10 | (001) | 12.10 | 0.7324 | 3.2 |
| NS-20 | (001) | 12.06 | 0.7335 | 3.0 |
| NS-30 | (001) | 12.05 | 0.7338 | 3.0 |
图1b给出了不同SDS用量的α-Ni(OH)2的TGA图。可以看出，NS-0样品有3个失重区间，25~300℃区间的失重(~3.7%)对应吸附水和层间水的去除，300~340℃区间的失重(~23.0%)对应Ni(OH)2分解为NiO和H2O，340~600℃区间的失重对应样品吸附的阴离子(如NO
图1c给出了NS-0和NS-20两个样品的FT-IR谱。可以看出，两个样品在3450和1632 cm-1处的吸收峰分别对应层间水分子中-OH伸缩振动和吸附水分子的弯曲振动，表明两个峰都是α-Ni(OH)2的特征吸收峰[21]；在2230 cm-1处的强吸收峰对应尿素在均相反应过程中分解生成的NCO-/NC-的伸缩振动[22]；在1462和1370cm-1附近出现的是CO
图2给出了NS-0、NS-10、NS-20和NS-30样品的SEM照片。从图2a可见，NS-0样品由大小不一的微米级球形颗粒团聚而成；在高放大倍数下可见球形颗粒又是由紧密的花瓣状初级纳米结构(厚度10~20 nm)构成(图2b,c)。从图2a、d、g、j可见，随着SDS量的增多球形颗粒的尺寸出现增大的趋势。其原因可能是，与层间阴离子NO3-相比DS-有更大的体积和空间位阻效应，在晶体形成过程中使初晶颗粒增大、材料更加蓬松多孔，进而使颗粒的尺寸增大。从图2d~l可见，NS-10、NS-20和NS-30样品的表面形貌相似，都由微米级类球形颗粒组成；与NS-0相比，其类球形颗粒具有更好的分散性，组成类球形颗粒的花瓣状初级纳米结构表现出更好的空间开放性。从图2e、h、k可见，NS-10、NS-20和NS-30三种样品均呈类球形花簇状；仔细对比图2f、i、l可见，NS-20比NS-10和NS-30样品材料三维结构更加开放，有利于电解液的渗透进而提高材料的电化学性能。以上结果表明，适量的SDS有利于形成开放的三维纳米结构，但是过量的SDS使开放的三维纳米结构坍塌。
图2
图2
NS-0、NS-10、NS-20和NS-30样品的SEM照片
Fig.2
SEM images of NS-0 (a~c), NS-10 (d~f), NS-20 (g~i), and NS-30 (j~l) samples
2.2 电化学性能
图3a~d给出了NS-0、NS-10、NS-20和NS-30样品的CV曲线，扫描速度为0.1 mV·s-1。从图中可见，四个样品的CV曲线的特征相似。在首圈CV负向扫描过程中，在1.15 V附近出现的强还原峰对应Ni(OH)2还原为Ni0(Ni(OH)2+2Li++2e-→Ni0+2LiOH)和固体电解质界面(SEI)膜的生成，在0.75 V附近出现的较宽还原峰对应LiOH进一步还原为LiH(LiOH+2Li++2e-→Li2O+LiH)；在首圈CV正向扫描过程中，在0.9~1.8 V的宽氧化峰对应LiH可逆氧化为LiOH，在2.3 V附近出现的氧化峰对应Ni0氧化为Ni2+[30,31]。从第二圈CV开始，样品的在1.15 V附近的还原峰强度明显降低并正移，其主要原因是在首圈负扫过程中SEI膜的生成产生了不可逆Li+消耗。仔细对比可以发现，随着循环次数的增加NS-0样品的CV曲线面积明显变小，表明电化学活性降低；而NS-10、NS-20和NS-30样品的第三、四圈CV曲线几乎重叠，表明其较高的循环稳定性。图3e给出了NS-0、NS-10、NS-20和NS-30四个样品在电流密度为0.2 A·g-1时活化5圈后又在电流密度为2 A·g-1条件下循环35圈的循环性能比较。结果表明，NS-0的循环性能最差，40次循环后其比容量已衰减至248 mAh·g-1；使用SDS辅助制备的三个样品均表现出更好循环性能，尤其是NS-20的循环性能最好，40次循环后其比容量仍然保持在800 mAh·g-1。图3f，g分别给出了NS-0和NS-20样品不同循环圈数的放电/充电曲线。可以看出，在充放电曲线的前5圈活化过程中放电曲线有两段不同斜率的近直线容量衰减，分别对应材料的两个嵌锂反应过程。在0.9~1.3 V附近的放电平台对应Ni(OH)2还原为Ni0(Ni(OH)2+2Li++2e-→Ni0+2LiOH)和固体电解质界面(SEI)膜的生成，在0.6~0.8 V附近出现的放电平台对应LiOH进一步还原为LiH(LiOH+2Li++2e-→Li2O+LiH)[32]。同时，NS-0和NS-20的首圈放电/充电比容量分别为2059/1316和1967/1245 mAh·g-1，对应的库伦效率分别为63.9%和63.3%。其主要原因是，材料在首次放电过程中生成SEI膜消耗了部分锂离子，引起了不可逆的容量损失[33]，这与首圈CV测试结果(图3a~d)一致。随着循环次数的增加两样品的充电电位逐渐升高、放电电位逐渐降低和可逆比容量逐渐降低。NS-0样品经过10次循环后已没有明显的充放电平台，而NS-20样品即使经过40次循环后仍出现较好的充放电平台。以上结果表明，随着SDS用量的增加样品的的电化学性能先提高后降低，其中NS-20的性能最佳。NS-20样品突出的电化学性能，与其自身的微观结构密切相关：一方面NS-20样品的结晶性较差(图1a)，结构缺陷使其含有更多的配位不饱和原子，有利于提高材料的电化学反应活性[34]；另一方面，NS-20样品更加开放的三维结构有利于暴露更多的反应位点、缓解材料在充放电过程中的体积效应、促进电解液在电极中的渗透，进而改善电极的循环性能和动力学性能。表2对比了NS-20样品与文献报道的氢氧化镍基负极材料的循环性能。从表中可以看出，本文制备的NS-20样品其电化学性能较为优异。
图3
图3
NS-0、NS-10、NS-20和NS-30样品在0.1 mV·s-1扫描速度下的CV曲线以及四个样品循环性能的对比、NS-0和NS-20样品在不同循环次数下的充放电曲线
Fig.3
CV curves of NS-0 (a), NS-10 (b), NS-20 (c), and NS-30 (d) samples at the scan rate of 0.1 mV·s-1; comparison of cycling performance of the four samples (e); charge/discharge curves of NS-0 (f) and NS-20 (g) samples in selected cycles
表2 NS-20与用于LIBs的其他氢氧化镍基负极材料的循环性能的比较
Table 2
| Materials | Current density / A·g-1 | Specific discharge capacity / mAh·g-1 | Voltage window / V vs. Li/Li+ | Reference |
|---|---|---|---|---|
| NS-20 | 2.0 | 800 mAh·g-1 after 40 cycles | 0~3.0 | This work |
| α-Ni(OH)2 | 1.0 | 743 mAh·g-1 after 50 cycles | 0~3.0 | [35] |
| Co-Ni-LDH | 0.05 | 450.4 mAh·g-1 after 40 cycles | 0~3.0 | [36] |
| Ni-Co-LDH | 0.1 | 335.4 mAh·g-1 after 50 cycles | 0~3.0 | [37] |
| Ni(OH)2-CTAB | 0.5 | 952 mAh·g-1 after 25 cycles | 0~3.0 | [38] |
| Fe-Ni-LDH | 0.2 | 1080 mAh·g-1 after 30 cycles | 0~3.0 | [39] |
| Ni- Fe-OH | 0.85 | 540 mAh·g-1 after 50 cycles | 0~3.0 | [40] |
| Ni(OH)Cl | 0.2 | 1236 mAh·g-1 after 150 cycles | 0~3.0 | [9] |
图4a给出了NS-0、NS-10、NS-20和NS-30四个样品的倍率性能对比。可以看出，随着电流密度的增大样品的放电比容量逐渐降低。NS-0的倍率性能最差，在3 A·g-1电流密度下其比容量已降低至231 mAh·g-1。相比之下，SDS辅助制备的三个样品均表现出更好倍率性能；尤其是NS-20的倍率性能最佳，在3 A·g-1高电流密度下其比容量仍高达662 mAh·g-1，电流密度回到较低的0.5 A·g-1时其比容量恢复到980 mAh·g-1并保持良好的循环稳定性。图4b~d给出了NS-0、NS-20和NS-30三个样品在不同电流密度下的充放电曲线。可以看出，电流密度为0.2 A·g-1时三个样品的充放电平台均较为明显，随着电流密度的提高三个样品电极的充放电平台逐渐变短表明电极的极化逐渐变大。相比之下，NS-20样品的极化明显低于其他两个样品。
图4
图4
四个样品的倍率性能对比、NS-0、NS-20和NS-30样品在不同电流密度下的充放电曲线
Fig.4
Comparison of magnification performance of four samples (a);charge and discharge curves of NS-0 (b), NS-20 (c) and NS-30 (d) samples at different current densities
进行定量分析[42]，式中i(V)为特定电位下的CV电流值；k1v和k2v1/2分别为赝电容效应电流和扩散控制电流。根据
图5
图5
NS-0和NS-20样品在不同扫描速度下的CV曲线、NS-20样品在0.5 mV·s-1扫描速度下的CV曲线、NS-10、NS-20和NS-30样品在不同扫描速度下的赝电容贡献百分比
Fig.5
CV curves of NS-0 (a) and (b) NS-20 samples at various scan rates; CV curve of NS-20 at 0.5 mV·s-1 (the region filled with blue color corresponds to pseudocapacitive effect) (c); pseudocapacitance contribution percentage diagram of NS-10 (d), NS-20 (e) and NS-30 (f) samples at different scanning speeds
图6a、b给出了NS-0、NS-10、NS-20和NS-30四个样品经过20次循环后的Nyquist图和对应的Bode图。可以看出，NS-0样品的Nyquist图由高频区的半圆、中低频区半圆和低频区的斜线组成，高频区的半圆对应SEI膜电阻和电化学反应电阻容抗弧(Rsf+ct)，中低频区的半圆对应材料的体相电阻(Rb，包括活性材料电阻、复合电极中孔道内部电解液的离子电阻、材料充放电结构变化引起的电阻)[43]；低频区的直线对应Li+在材料中扩散引起的Warburg阻抗[44]。对等效电路(含虚线框部分)的拟合结果表明，NS-0样品的Rsf+ct和Rb值分别为48.3和73.8 Ω·cm2。较大的Rb值表明其电化学活性低，与上文的循环性能(图3e)是相对应的。NS-10、NS-20和NS-30样品的Nyquist图都由高中频区的半圆和低频区的斜线组成，高中频区半圆对应SEI膜电阻和电化学反应电阻容抗弧，低频区斜线对应Li+在材料中扩散引起的Warburg阻抗。三个样品的EIS图均未出现明显的体相电阻容抗弧，表明其电化学活性较高。对等效电路图6c(不含虚线框部分)的拟合结果表明，NS-10、NS-20和NS-30样品的Rsf+ct值分别为33.3、29.4和33.6 Ω·cm2，明显低于NS-0的Rsf+ct值(48.3 Ω·cm2)，表明其电化学反应更容易进行。其中NS-20的Rsf+ct值最低，表明其电化学反应活性最高，与其循环性能曲线(图3e)相对应。
图6
图6
NS-0、NS-10、NS-20和NS-30样品20次循环后的Nyquist图、Bode图和等效电路
Fig.6
Nyquist plots (a) and Bode plots (b) of NS-0, NS-10, NS-20 and NS-30 samples after 20 discharge/charge cycles,equivalent circuit (c)
3 结论
以十二烷基硫酸钠(SDS)为辅助剂，采用简便的均相沉淀法可制备具有微/纳分级结构的α-Ni(OH)2材料。SDS辅助剂可细化α-Ni(OH)2的晶粒，拓宽α-Ni(OH)2层间距并有助于形成空间更加开放的微/纳分级形貌。SDS辅助制备的α-Ni(OH)2均比不使用SDS制备的α-Ni(OH)2具有更好的电化学性能，在n(SDS)/n(Ni2+)为2∶10条件下制备的α-Ni(OH)2具有最佳的循环稳定性(在2 A·g-1电流密度下循环40次后比容量保持在800 mAh·g-1)、出色的倍率性能(在3 A·g-1大电流密度下仍能具有710 mAh·g-1的可逆比容量)和显著的赝电容效应(在0.9 mV·s-1时其赝电容贡献率高达84.2%)。无SDS辅助制备的α-Ni(OH)2循环使用后出现明显的体相电阻，使其电化学活性和反应可逆性快速衰减；而SDS辅助制备的α-Ni(OH)2循环使用后不出现体相电阻且其Rct值明显低于无SDS辅助制备的α-Ni(OH)2的Rct值，使其反应动力学和循环性能更好。
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