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无机材料学报, 2022, 37 (12): 1337, 网络出版: 2023-01-12  

金属铋纳米颗粒原位修饰碳纳米管促进锂均匀沉积

In-situ Modification of Carbon Nanotubes with Metallic Bismuth Nanoparticles for Uniform Lithium Deposition
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蔡佳 *黄高旭 金晓盼 魏驰 毛嘉毅 李永生 
作者单位
锂金属电池 锂枝晶 铋纳米颗粒 集流体 Li metal battery Li dendrite bismuth nanoparticle current collector 
摘要
锂金属具有高理论比容量和低电化学电位, 是发展高能量密度电池最有吸引力的负极材料之一。然而, 锂金属负极在反复的沉积/剥离过程中, 不可避免地会出现不规则的锂枝晶生长, 这将严重影响锂金属电池的循环寿命和使用安全性。本研究发展了一种简单温和的策略, 在碳纳米管上原位修饰铋纳米颗粒, 并涂覆在商业铜箔表面用作锂金属负极的集流体。研究表明, 原位修饰的铋纳米颗粒可显著促进锂均匀沉积, 抑制锂枝晶生长, 从而提高锂金属电池的电化学性能。在电流密度为1 mA·cm-2的条件下, 基于Bi@CNT/Cu集流体的锂铜电池循环300圈后库仑效率可稳定在98%。基于Li@Bi@CNT/Cu负极的对称电池可稳定循环1000 h。基于Bi@CNT/Cu集流体的磷酸铁锂(LFP)全电池也获得了优异的电化学性能, 在1C(170 mA·g-1)倍率下可稳定循环700圈。本研究为抑制锂金属负极枝晶生长提供了新的思路。
Abstract
Lithium (Li) metal is one of the most attractive anode materials for the development of high energy density batteries due to its high theoretical specific capacity and low electrochemical potential. However, during the repeated deposition/stripping of Li metal anode, irregular Li dendrite growth inevitably takes place, which seriously affects the cycle life and safety of Li metal batteries. In this study, a simple and mild strategy was developed to in-situ modify the carbon nanotubes with bismuth (Bi) nanoparticles, followed by coating the as-prepared materials on the surface of commercial copper foil as current collector for Li metal anode. It is demonstrated that the in-situ modified Bi nanoparticles promotes the uniform Li deposition, thereby inhibiting the growth of Li dendrites and improving the electrochemical performance of Li metal batteries. Under the current density of 1 mA·cm-2, Coulombic efficiency of Li|Cu cell based on the Bi@CNT/Cu current collector maintains 98% after 300 cycles. Meanwhile, the symmetric cell based on the Li@Bi@CNT/Cu anode can maintain the stable cycling for 1000 h. When it is applied in LiFePO4 (LFP) full cell, the Bi@CNT/Cu current collector also exhibits excellent electrochemical performance, which can retain the stable cycling for 700 cycles at the rate of 1C (170 mA·g-1). This study provides a new strategy for suppressing dendrite growth of Li metal anodes.

随着电动汽车和便携式电子产品在全世界快速普及, 开发高能量密度和长循环寿命的二次电池显得尤为重要。然而, 当前锂离子电池(LIBs)的能量密度几乎达到其理论极限[1]。因此, 开发具有更高比容量的新型电极材料迫在眉睫。锂金属负极由于具有高理论比容量(3860 mAh·g-1)和低电化学电位 (-3.04 V (vs SHE))等优势[2], 被誉为锂电池负极材料的“圣杯”, 是下一代锂硫电池[3]和锂空气电池[4]最理想的负极材料。然而, 锂金属负极在反复沉积/剥离的过程中不可避免地会形成锂枝晶和“死锂”[5-6], 这不仅会阻碍锂离子在电极/电解液界面处的快速传输, 导致电池内阻增加、容量快速衰减, 而且锂枝晶还会刺穿隔膜, 造成电池内部短路, 带来安全隐患。此外, 在充放电过程中, 锂金属表面会反复形成不稳定的固体电解质界面(Solid electrolyte interface, SEI)层[7-8], 导致锂离子通量不均匀, 进一步加剧锂枝晶生长, 并且损耗电解液。这些问题最终将导致锂金属电池(LMBs) 的库仑效率较低和循环稳定性较差, 使其实际应用受到严重阻碍。

近年来, 研究者们开发了多种策略来抑制锂枝晶, 以提高锂金属电池的电化学性能。这些策略主要包括: (1)人工修饰SEI膜[9-10]。SEI膜在锂金属电池中起着至关重要的作用, 它可以阻隔负极与电解液的直接接触, 避免发生副反应。因此, 在锂金属负极表面原位修饰杨氏模量高的人工SEI膜作为保护层, 是抑制枝晶生长和缓解锂金属体积膨胀的有效方法。(2)隔膜改性[11]。隔膜是影响电池性能和寿命的关键部件, 研究表明, 在商业隔膜表面修饰功能化材料可使锂离子通量更加均匀, 有效抑制锂枝晶的形成。(3)电解液设计[12]。电解液和锂金属负极发生电化学反应会生成SEI膜, 隔绝活泼的金属锂和电解液, 以免进一步发生副反应。电解液中引入硝酸锂(LiNO3)和氟化碳酸乙烯酯等添加剂, 可以在锂金属负极表面形成富含氮化锂或氟化锂的SEI膜, 起到抑制锂枝晶生长的作用。(4)集流体的功能化。设计表面亲锂性的三维集流体作为储存锂金属的宿主, 可以抑制锂金属负极枝晶生长和体积膨胀。三维集流体较大的比表面积可以有效降低电极的局部电流密度, 调节电场分布, 从而减缓锂枝晶生长[13]。但三维集流体还有很多问题有待解决, 如孔径大小不一、骨架表面电流分布不均等, 这些因素会导致锂不均匀沉积和锂枝晶生长。此外, 三维集流体相比于二维集流体具有更大的密度, 应用于锂金属电池中会严重降低电池整体的能量密度[14]。因此, 修饰密度更小的二维铜箔作为锂金属负极的集流体, 有望获得高能量密度的锂金属 电池。

商业铜箔的“疏锂”性质不利于锂离子的成核和均匀分布, 易形成锂枝晶, 从而阻碍其在锂金属电池的直接应用[15-16]。因此, 在铜箔表面修饰一层“亲锂”性材料可以使其具有亲锂特性, 降低锂的成核过电势, 实现锂离子通量的均匀分布[17-18]。但修饰过程目前已有的报道大多需要使用复杂的工艺和昂贵的设备, 如化学气相沉积[19]或磁控溅射设备[20]等。因此, 迫切需要开发一种简易的策略在铜箔表面构建亲锂层来促进锂的均匀沉积, 提高锂金属负极的电化学性能。

鉴于此, 本研究发展了一种简单温和的策略, 在碳纳米管表面修饰铋纳米颗粒, 并将所得材料涂覆在商业铜箔表面, 用作锂金属负极的集流体来抑制锂枝晶生长。金属铋和金属锂可以形成合金, 从而降低锂的成核过电势, 促进锂均匀沉积。同时, 相互交织的碳纳米管可为金属锂的沉积提供空间, 使得锂沉积更加均匀致密。最后, 本课题组通过组装锂铜电池和对称电池, 研究了金属锂在不同集流体上沉积/剥离的可逆性和稳定性; 并将预沉积金属锂的复合负极应用于全电池, 考察了改性铜集流体实际应用的可行性。

1 实验方法

1.1 材料合成

铋纳米颗粒原位修饰碳纳米管(Bi@CNT)的制备流程如图1所示。首先将0.1 g带有羧酸基团的多壁碳纳米管分散在50 mL去离子水中, 超声1 h。然后配置2 mL 0.25 mol·L-1硝酸铋(Bi(NO3)3)水溶液、加入4 mmol无水柠檬酸三钠(C6H5Na3O7)和150 mL去离子水, 搅拌30 min。将上述溶液加入到碳纳米管溶液中, 继续搅拌30 min。随后, 在搅拌下滴加60 mL 0.1 mol·L-1硼氢化钠(NaBH4)水溶液。悬浮液继续搅拌8 h, 过滤黑色沉淀并用去离子水洗涤数次。最后, 将得到的固体产物在50 ℃真空烘箱中干燥过夜。

图 1. Bi@CNT样品的制备示意图

Fig. 1. Schematic diagram for the synthesis of Bi@CNT

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1.2 结构和形貌表征

采用扫描电子显微镜(Scanning electron microscope, SEM, JEOL 4800, 日本)和配备有能量色散X射线(Energy-dispersive X-ray spectroscope, EDS)光谱仪的透射电子显微镜(Transmission electron microscope, TEM, JEOL JEM 2100F, 日本)观察样品的微观形貌和元素分布。采用X射线衍射仪(X-ray diffraction, XRD Bruker D8, 德国, 石墨单色CuKα辐射, λ= 0.15405 nm) 测试样品的晶体结构。采用X射线光电子能谱仪(X-ray photoelectron spectroscope, XPS, Thermo ESCALAB 250, 美国, 单色AlKα)测试样品的元素组成和价态, 以C1s的结合能284.8 eV进行校准。

1.3 电化学测试

电极的制备: 将Bi@CNT和黏结剂聚偏二氟乙烯(PVDF)按质量比9 : 1混合均匀。将适量溶剂N-甲基吡咯烷酮(NMP)加入到材料中, 匀速搅拌形成浆料后均匀刮涂在铜箔表面, 随后在60 ℃下真空干燥12 h。最后将铜箔压成ϕ12 mm的圆片。

锂铜电池组装和测试: 为了探究不同集流体的锂金属电池的库仑效率, 分别以Bi@CNT/Cu、CNT/Cu和Cu集流体为工作电极, 锂箔作为对电极和参比电极组装锂铜电池。隔膜的型号为Celgard 2400型, 电解液为含有质量分数2%硝酸锂添加剂的醚类电解液(1 mol·L-1双三氟甲基磺酰亚胺锂(LiTFSI)的二氧戊环/乙二醇二甲醚(体积比1 : 1)溶液)。锂铜电池测试条件为: 电流密度分别取1和3 mA·cm-2, 循环容量为1 mAh·cm-2。

对称电池的组装和测试: 使用对称电池研究不同集流体的锂沉积/剥离稳定性。将组装的锂铜电池以恒流充电的方式沉积6 mAh·cm-2的金属锂到集流体表面, 得到对称电池。对称电池测试条件: 电流密度分别为1和2 mA·cm-2, 循环容量为1 mAh·cm-2。

磷酸铁锂(LiFePO4, LFP)全电池组装和测试: 将商用的LFP粉末、PVDF黏结剂和导电剂超导炭黑(Super P)按质量比8 : 11 : 11混合, 加入适量的NMP, 匀速搅拌以形成浆料。然后将黏度适中的浆料涂覆到涂炭铝箔上, 110 ℃真空干燥后, 在冲片机下冲成ϕ9 mm的圆片作为全电池的正极。经过计算, LFP正极片上的活性物质平均面负载约为3 mg·cm-2。全电池的负极采用预先沉积6 mAh·cm-2金属锂的Li@Bi@CNT/Cu、Li@CNT/Cu和Li@Cu复合负极。电解液为1.0 mol·L-1六氟磷酸锂(LiPF6)的碳酸亚乙酯/碳酸二甲酯(体积比1 : 1)溶液。LFP全电池测试条件为: 在1C(170 mA·g-1)倍率和2.5~4.2 V的电压区间内进行恒流充放电测试以获得全电池的循环性能; 在0.5C~5C的电流密度下分别循环充放电10圈, 评估全电池的倍率性能。

在充满氩气的手套箱(H2O和O2含量均低于10-7)中组装CR2032型纽扣电池, 电解液用量为50 μL。采用电池测试系统(LAND CT2001A, 中国)测试所有电池的电化学性能。采用电化学工作站(Chenhua CHI600, 中国)测试电池的电化学阻抗谱(Electrochemical impedance spectroscopy, EIS), 频率范围为0.01 Hz~100 kHz。

2 结果与讨论

2.1 结构和形貌表征

图2(a)为所得样品的XRD图谱, 从图中可以看到, 除了归属于碳纳米管的较强衍射峰之外, Bi@CNT样品在2θ=27.2°、38°和39.6°处还出现了三个明显的特征峰, 分别归属于金属铋(JCPDS 44-1246)的(012)、(104)和(110)晶面, 说明金属铋单质成功修饰在碳纳米管上。进一步利用X射线光电子能谱(XPS)表征Bi@CNT材料中Bi的化学价态。图2(b)为Bi@CNT样品的XPS测试全谱, 从图中可以观察到四个特征峰, 分别对应Bi4f、C1s、N1s和O1s的信号。图2(c)为Bi@CNT的Bi4f XPS高分辨率图谱, 159.0 eV(Bi4f7/2)和164.5 eV(Bi4f5/2)处出现的两个特征峰归属于金属Bi0, 这表明在溶液反应中Bi3+被强还原性的NaBH4完全还原为金属铋单质, 与XRD测试的结果一致。Bi@CNT样品的TEM照片证明, Bi纳米颗粒均匀地生长在碳纳米管上(图2(d))。Bi@CNT的元素分布图(图2(e, f))则进一步证实了金属铋在碳纳米管上均匀生长。

图 2. 样品的微观结构表征

Fig. 2. Microstructure characterization of samples

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2.2 电化学性能测试

为了探究所得材料对锂枝晶生长的抑制作用和锂均匀沉积的促进作用, 将Bi@CNT与PVDF黏结剂混合均匀制备成浆料, 涂覆在Cu集流体上。以Bi@CNT/Cu为工作电极, 锂箔为对电极组装成锂铜电池, 并测试其库仑效率。对照组采用CNT/Cu和Cu作为工作电极。库仑效率(Coulombic efficiency, CE)是评估锂金属负极循环可逆性的重要参数, 定义为锂沉积量与剥离量的比值。当电流密度为 1 mA·cm-2, 面积容量为1 mAh·cm-2时, 以Bi@CNT/Cu为集流体的锂铜电池循环300圈后库仑效率依旧在98%左右, 库仑效率和循环稳定性明显优于以CNT/Cu和Cu为集流体的锂铜电池, 表明Bi@CNT材料可以提升锂金属沉积/剥离的可逆性与稳定性(图3(a))。这是因为金属锂和金属铋能形成锂铋合金, 为锂沉积过程提供了成核位点。并且碳纳米管能为沉积的锂提供空间, 促进锂沉积均匀致密。为了进一步研究高电流密度下的库仑效率, 将电流密度提高至 3 mA·cm-2。如图3(b)所示, 以Bi@CNT/Cu为集流体的锂铜电池循环100圈后库仑效率可以维持在96%左右, 但另外两组锂铜电池的库仑效率波动很大, 说明在较大电流密度下, CNT/Cu和Cu集流体表面都出现了严重的锂枝晶生长。图3(c~e)为基于3种集流体的锂铜电池在1 mA·cm-2电流密度和 1 mAh·cm-2面积容量下的容量-电压曲线, 其中以Bi@CNT/Cu为集流体的锂铜电池的电压极化更小, 可逆性更好, 说明Bi@CNT/Cu集流体的界面反应动力学以及锂沉积/剥离可逆性更好。

图 3. 基于Bi@CNT/Cu、CNT/Cu和Cu集流体的锂铜电池在(a)1 mA·cm-2, 1 mAh·cm-2和(b)3 mA·cm-2, 1 mAh·cm-2条件下的库仑效率, 基于(c)Bi@CNT/Cu、(d)CNT/Cu和(e)Cu集流体的锂铜电池在1 mA·cm-2, 1 mAh·cm-2条件下的容量-电压曲线

Fig. 3. Coulombic efficiencies of Li|Cu cells based on Bi@CNT/Cu, CNT/Cu and Cu current collectors at (a) 1 mA·cm-2, 1 mAh·cm-2 and (b) 3 mA·cm-2, 1 mAh·cm-2; Capacity-voltage curves of Li|Cu cells based on (c) Bi@CNT/Cu, (d) CNT/Cu, and (e) Cu current collectors at 1 mA·cm-2, 1 mAh·cm-2Colorful figures are available on website

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为了探究不同集流体的锂沉积情况, 将3种锂铜电池在1 mA·cm-2电流密度和1 mAh·cm-2面积容量下循环50次后, 取出集流体进行表面形貌观察。如图4(a)所示, Bi@CNT/Cu集流体表面比较平整, 没有形成明显的锂枝晶。而CNT/Cu(图4(b))和Cu集流体(图4(c))表面更加粗糙, 出现了不规则生长的锂枝晶。这些结果证明Bi@CNT材料可以有效促进锂均匀沉积, 抑制锂枝晶生长。图4(d)为基于3种集流体的锂铜电池的首圈电化学交流阻抗(EIS)图谱。从图中可以看出, 以Bi@CNT/Cu为集流体的锂铜电池的界面阻抗最小, 说明亲锂的铋纳米颗粒修饰的碳纳米管可以有效提高电池界面处的电化学反应动力学, 从而实现锂的均匀沉积。

图 4. 锂铜电池循环50次后(a) Bi@CNT/Cu、(b) CNT/Cu和(c) Cu集流体的SEM照片, (d)基于Bi@CNT/Cu、CNT/Cu和Cu集流体的锂铜电池的首圈EIS图谱, 基于Li@Bi@CNT/Cu、Li@CNT/Cu和Li@Cu负极的对称电池在(e)1 mA·cm-2, 1 mAh·cm-2和(f)2 mA·cm-2, 1 mAh·cm-2的电压-时间曲线

Fig. 4. SEM images of (a) Bi@CNT/Cu, (b) CNT/Cu, and (c) Cu current collectors in Li|Cu cells after 50 cycles; (d) First cyclic EIS plots of Li|Cu cells based on Bi@CNT/Cu, CNT/Cu and Cu current collectors, and voltage-time curves of symmetric cells based on Li@Bi@CNT/Cu, Li@CNT/Cu and Li@Cu anodes at (e) 1 mA·cm-2, 1 mAh·cm-2 and (f) 2 mA·cm-2, 1 mAh·cm-2

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为了探究不同集流体在长循环过程中对于锂枝晶的抑制作用, 将基于3种集流体的锂铜电池预沉积金属锂形成对称电池后, 进行恒流充放电测试以研究锂的沉积/剥离行为。如图4(e)所示, 在电流密度为1 mA·cm-2、面积容量为1 mAh·cm-2的条件下, 基于Li@Cu负极的对称电池在循环140 h后电压极化开始增大。这是随着循环次数增加, 电解液不断消耗, 电池的内部极化增大所导致的。当使用Li@CNT/Cu负极时, 对称电池可稳定循环500 h, 但随后也出现了电压极化增大的现象。相比较而言, 基于Li@Bi@CNT/Cu负极的对称电池具有更加优异的循环稳定性(1000 h)和最小的电压极化(13 mV), 其循环稳定性优于大多数文献的报道(表1)。当电流密度提升至2 mA·cm-2时, 基于Li@Bi@CNT/Cu负极的对称电池可以稳定循环260 h(图4(f)), 具有良好的循环稳定性。

表 1. 使用不同材料修饰铜箔后的电化学性能对比

Table 1. Comparison of electrochemical properties of copper foils modified by different materials

Symmetric cellLi|Cu cell
Current collectorCurrentdensity/(mA·cm-2)Planting/strippingcapacity/(mAh·cm-2)Cyclingtime/hCurrentdensity/(mA·cm-2)Planting capacity/(mAh·cm-2)Cycle number, nCoulombicefficiency/%Ref.
Bi@CNT1110001130098This work
212603110096
SMC-2112200.5121097[21]
PDA0.10.28001110096[22]
3D-CuZn114501115095[23]
Li-MMT317020.2510097.9[24]
LHCE117001120099.1[25]
NMPC0.50.54001120098[26]
Duplex Cu118801130097.3[27]
Ti3C2Tx115001125098.4[28]
q-PET311001110098[29]
SF333501120096[30]

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为了进一步探究Bi@CNT/Cu集流体实际应用的可能性, 将其组装成全电池进行电化学性能测试。首先将Bi@CNT/Cu集流体预沉积6 mAh·cm-2的金属锂得到复合负极, 再与LFP正极匹配组装成全电池。图5(a)为基于3种复合负极的LFP全电池在1C倍率下的循环性能对比。如图所示, 基于Li@Bi@CNT/Cu负极的LFP全电池表现出更好的循环寿命和容量保持率, 在循环700圈后仍然能维持146.8 mAh·g-1的放电比容量。相比之下, 基于Li@CNT@Cu和Li@Cu负极的LFP全电池不仅放电比容量更低, 而且在循环200圈后就出现了容量衰减, 这可能是因为Li@CNT@Cu和Li@Cu负极表面形成了锂枝晶和死锂, 导致循环稳定性下降。对比不同全电池的倍率性能(图5(b)), 基于Li@Bi@CNT/Cu负极的LFP全电池相比于其它两种全电池, 在不同倍率下都表现出更高的放电比容量。图5(c~e)为3种全电池在1C条件下不同循环圈数的容量-电压曲线, 图中基于Li@Bi@CNT/Cu负极的LFP全电池的充放电可逆性更好, 电压极化更小, 说明Li@Bi@CNT/Cu负极与电解液界面处的电化学反应动力学更迅速, 可以有效抑制锂枝晶生长, 从而提高了全电池的循环稳定性。以上测试结果说明Bi@CNT/Cu集流体应用于LFP全电池具有更加优异的电化学性能。

图 5. 基于Li@Bi@CNT/Cu, Li@CNT/Cu和Li@Cu负极的LFP全电池的(a)长循环性能, (b)倍率性能, 基于(c)Li@Bi@CNT/Cu, (d)Li@CNT/Cu, (e)Li@Cu负极的LFP全电池在1C下的容量-电压曲线

Fig. 5. (a) Cycling performances and (b) rate performances of LFP full cells based on Li@Bi@CNT/Cu, Li@CNT/Cu, and Li@Cu anodes, and (c-e) capacity-voltage profiles of LFP full cells based on (c) Li@Bi@CNT/Cu, (d) Li@CNT/Cu, and (e) Li@Cu anodes at 1C

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3 结论

本研究通过一种简单温和的方法制备了金属铋纳米颗粒原位修饰的碳纳米管, 并将其涂覆在商业铜箔表面用作锂金属负极的集流体。研究发现, 修饰在碳纳米管上的金属铋可以诱导锂均匀成核, 实现锂均匀沉积, 抑制锂枝晶生长。以Bi@CNT/Cu为集流体的锂铜电池在电流密度为1 mA·cm-2, 容量为 1 mAh·cm-2条件下, 循环300圈后库仑效率仍然在98%, 基于Li@Bi@CNT/Cu负极的对称电池可以稳定循环1000 h。在以磷酸铁锂作为正极材料的全电池中, 预沉积锂的Li@Bi@CNT/Cu负极也表现出优异的循环性能, 在1C条件下可以稳定循环700圈。本研究证明金属铋纳米颗粒原位修饰的碳纳米管可有效促进锂均匀沉积, 为二维铜集流体在锂金属负极中的应用提供了参考。

参考文献

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蔡佳, 黄高旭, 金晓盼, 魏驰, 毛嘉毅, 李永生. 金属铋纳米颗粒原位修饰碳纳米管促进锂均匀沉积[J]. 无机材料学报, 2022, 37(12): 1337. Jia CAI, Gaoxu HUANG, Xiaopan JIN, Chi WEI, Jiayi MAO, Yongsheng LI. In-situ Modification of Carbon Nanotubes with Metallic Bismuth Nanoparticles for Uniform Lithium Deposition[J]. Journal of Inorganic Materials, 2022, 37(12): 1337.

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