材料工程  2019, Vol. 47 Issue (3): 30-41   PDF    
http://dx.doi.org/10.11868/j.issn.1001-4381.2017.001563
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文章信息

胡安俊, 龙剑平, 舒朝著
HU An-jun, LONG Jian-ping, SHU Chao-zhu
设计稳定和可逆的锂-空气电池阴极催化剂的研究进展
Research progress on designing stable and reversible cathodes catalysts for lithium-air batteries
材料工程, 2019, 47(3): 30-41
Journal of Materials Engineering, 2019, 47(3): 30-41.
http://dx.doi.org/10.11868/j.issn.1001-4381.2017.001563

文章历史

收稿日期: 2017-12-18
修订日期: 2018-01-29
设计稳定和可逆的锂-空气电池阴极催化剂的研究进展
胡安俊 , 龙剑平 , 舒朝著     
成都理工大学 材料与化学化工学院, 成都 610059
摘要: 锂-空气电池具有极高的理论能量密度,成为下一代最有希望的电化学能量储存技术之一。锂-空气电池的性能主要取决于空气阴极表面发生的电化学反应,因此,合理设计具有高稳定性和可逆性的阴极是实现商业化可行的锂-空气电池的关键所在。然而,传统碳基电极的不稳定性导致的副反应会限制电池容量及其循环性能,因此,需要寻找能够替代碳基电极的新型电极。本文首先结合锂-空气电池的结构和阴极反应原理,提出了目前锂-空气电池面临的挑战,然后基于碳基阴极的不稳定性分析总结了设计稳定和可逆的锂-空气电池阴极的方法,最后提出了阴极催化剂的合理设计和催化机理的深入理解对锂-空气电池阴极的性能改善起着决定性作用的观点。
关键词: 锂-空气电池    阴极催化剂    化学稳定性    可逆性    碳的表面改性    无碳阴极   
Research progress on designing stable and reversible cathodes catalysts for lithium-air batteries
HU An-jun, LONG Jian-ping , SHU Chao-zhu    
College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu 610059, China
Abstract: Lithium-air batteries(LABs) are among the most promising electrochemical energy storage technologies in the future due to extremely high theoretical energy density. The performance of LABs is mainly governed by the electrochemical reactions that occur on the surface of the cathode. Therefore, the rational design of a cathode with high stability and reversibility is the key to commercially viable LABs. However, the side reactions are caused by the instability of traditional carbon-based electrodes can limit capacity and cycle performance of LABs, therefore, alternative carbon-based electrode materials are required. Based on the structure and the principle of cathode reaction of LABs, the current challenges of LABs were presented. Then based on the analysis of the instability of carbon-based cathode, the design of a stable and reversible cathode for LABs was summarized. Finally, a perspective that the rational design of the cathode catalyst and the in-depth understanding of the catalytic mechanism play a decisive role in the performance improvement of LABS was provided.
Key words: lithium-air battery    cathode catalyst    chemical stability    reversibility    surface modification of carbon    carbon-free cathode   

由于不可再生的化石燃料的过度消耗和日益严重的全球变暖,大力发展绿色和可持续能源(如风能、太阳能、潮汐能等)具有重要意义[1-5]。二次电池是一种有希望的电化学储存系统,其可通过可逆的电化学氧化还原反应直接将化学能转化为电能[6-8]。其中,锂离子电池(lithium-ion batteries, LIBs)是目前最受欢迎和最高效的储能系统,几乎实现了其潜在的最大性能(即能量密度250Wh·kg-1)[9-11],然而低的能量密度限制了其在需要高能量电池系统的广泛使用,如电池供电的电动汽车(electric vehicles, EVs)。在正在开发的各种电池系统中,锂-空气电池(lithium-air battery, LABs)的最高理论能量密度约为3608Wh·kg-1(基于可逆反应2Li+O2↔Li2O2, E°=2.96V)[12],被认为是最有希望的下一代电池技术之一。

尽管LABs具有广阔的发展前景,但许多科学技术限制阻碍了其进一步的发展,包括低的能量效率、差的倍率性能以及差的循环稳定性等技术挑战[13-14]。通常这些挑战与LABs的基本化学反应机理相关,包括氧化还原反应动力学、化学稳定性等[15-16]。研究表明,电化学过程中的能量转换通常受到高的活化势垒的限制,而电催化剂通常用来修饰电极以降低活化能并提高转化率,进而改善电池的电化学性能[17]。因此,开发对氧还原反应(oxygen reduction reaction, ORR)和氧析出反应(oxygen evolution reaction, OER)具有高催化活性的阴极催化剂是LABs的关键所在。目前在LABs中应用最为广泛的催化剂是碳基材料[17-22],但其不稳定性、对Li2O2差的催化活性和促进电解质分解等问题导致LABs的循环性能显著降低[17, 23],这使得碳基材料不是LABs的理想阴极的选择。因此,开发更稳定和可逆的阴极催化剂是目前亟待解决的首要问题。

在本综述中,首先结合LABs的结构和反应机理,介绍了目前LABs发展面临的主要技术挑战及影响因素,接着分析了碳基阴极的不稳定性及其原因,最后提出了实现稳定和可逆的LABs阴极的几种方法。

1 锂-空气电池的结构与工作原理 1.1 锂-空气电池的结构

与LIBs相比,LABs具有开放式电池结构[19, 24]。如图 1所示,典型的LABs通常由金属Li阳极、多孔阴极和其间的电解质组成[25-28]。现有的LABs技术根据其电解质的类型,分为非质子、水系、混合和全固态电池体系。由于基于非质子LABs的理论能量密度是最高的[20, 29-31],因此,目前大多数的研发工作一直集中非质子LABs的设计。本综述以下所有讨论的LABs均指非质子体系。

图 1 锂-空气电池的结构[24] Fig. 1 Schematic configuration of lithium-air batteries[24]
1.2 锂-空气电池的工作原理

LABs的工作原理是基于金属Li和氧气的可逆的氧化还原反应,如(1)式,即在放电过程中生成固体Li2O2的ORR过程和在充电过程中析出氧气的OER过程。

(1)

然而,通过原位技术表明,LABs的充放电过程并不是简单的Li2O2形成和分解的一步反应,而是涉及多步反应,包括在电化学反应中可逆形成/分解的亚稳态中间体(O2-,LiO2等)的过程[32-33]

1.2.1 氧还原反应(ORR)机理

放电时发生的ORR包含两个步骤[34]:(1)O2经历一个电子还原成O2-,并与电解质中的Li+结合形成LiO2中间体。(2)LiO2发生还原或者歧化反应,产生Li2O2。而在放电时,LiO2的溶解度在O2还原模型中起着决定性作用,如图 2所示,在弱Li+溶剂化溶液中,Li+与O2反应形成吸附在电极表面的LiO2中间体(LiO2*),这称为表面介导的机理,如式(2)~(4):

(2)
(3)
(4)

在强Li+溶剂化溶液中,生成的LiO2中间体溶解在电解质溶液中,这称为溶液介导的机理,如式(5)~(7):

(5)
(6)
(7)
图 2 锂-空气电池的ORR反应机理[34] Fig. 2 ORR reaction mechanism in lithium-air batteries[34]
1.2.2 氧析出反应(OER)机理

充电时Li2O2氧化成O2有关的OER包括3种类型的反应机理[12, 35]

(1) 放电产物Li2O2分解为Li+和LiO2,Li+迁移继续释放O2,如式(8),(9):

(8)
(9)

(2) 非晶态Li2O2相被氧化,在界面处产生一些Li+空位,形成Li+-缺陷的Li2-xO2相,之后Li2-xO2通过固溶反应驱动Li2O2氧化析出O2,如式(10),(11):

(10)
(11)

(3) 在Li2O2/电解质界面发生Li2O2的氧化,从Li2O2表面产生O2和Li+,而没有LiO2中间体的形成,如式(12):

(12)
2 锂-空气电池面临的主要挑战

基于前面叙述的机理,反应生成的放电产物Li2O2是具有约4~5eV宽带隙的固态绝缘体[26, 36-38],一般来说,LABs的放电电压约为2.7V,比平衡电压(E0=2.96V)低0.26V。然而,其充电过电位一般高于0.5V,这导致低于70%的能量效率(如图 3(a)所示)。此外,LABs还存在倍率性能差以及循环稳定性不佳等挑战(如图 3(b)(c)所示)[15]。通常这些挑战与LABs的基本化学反应机理相关,包括化学不稳定性(如O2-与碳酸盐溶剂,Li2O2与碳电极)、ORR与OER反应动力学缓慢以及传输动力学(如离子和电子传输)缓慢等因素[27]

图 3 锂-空气电池的主要挑战[15]  (a)能量效率;(b)倍率性能;(c)循环寿命 Fig. 3 Major challenges of lithium-air batteries[15]  (a)energy efficiency; (b)rate performance; (c)cycle life
3 传统的碳阴极及其不稳定性

众所周知,碳材料具有优异的导电性和大的表面积,在许多能量存储系统中广泛用作催化剂载体、导电添加剂和电极材料。碳材料(如碳纳米管[39],石墨烯[40-43]和分级多孔碳[44-47])也广泛用于LABs的阴极材料,并在放电过程中作为沉积Li2O2的基底。这些碳纳米材料具有独特的结构优势,通常满足对于LABs的阴极材料的要求,包括用于大量Li2O2沉积的高比表面积,用于电子传输的高电导率,用于O2和Li+扩散的多孔结构以及用于高比能量密度的轻质量。因此,碳纳米材料阴极通常具有大的放电容量和高的倍率性能,加上低成本和环境友好等特点,碳似乎应该是LABs的理想阴极材料。

然而,碳的不稳定性会导致阴极形成碳酸盐等副产物,由碳酸盐分解引起的高充电过电位会进一步分解造成更严重的碳腐蚀,最终导致活性位点的堵塞和电池循环性能的下降。McCloskey等[48]用XPS和差分电化学质谱(DEMS),研究了在高氧化环境下碳阴极的氧化(如图 4(a)(b))。当使用13C-炭黑阴极时,在高电位峰中析出约50%的13CO2,这表明碳阴极存在副反应,而充电产物13CO2主要来自Li213CO3,这是由于Li2O213C(Li2O2+C+1/2O2→Li2CO3和2Li2O2+C→Li2O+Li2CO3)的热化学反应。此外,在Li2O2存在下碳阴极的稳定性差,并对电解质的分解有负面影响。Bruce等[49]表明在充放电时,碳阴极在3.5V以下是相对稳定的,但在充电至3.5V以上(在Li2O2存在下)时是不稳定的,会氧化分解成Li2CO3(如图 4(c))。碳还促进了充放电过程中电解质的分解,在充电至约4V时,产生了不能完全氧化的Li2CO3和Li羧酸盐,Li2CO3在循环中累积,导致电极钝化和容量衰减,严重影响电池性能。

图 4 碳阴极的不稳定性分析[48-49] (a)13C阴极的Li-O2电池在200μA电流下恒流充放电的电位(U)与放电容量(Q)的关系;(b)13C阴极的Li-O2电池(1mol·L-1 LiTFSI in DME)在18O2下放电(≈1mAh·cm-2)后的线性扫描伏安图(0.5mV·s-1)和CO2析出图;(c)碳阴极在高电位下的不稳定性示意图 Fig. 4 Analysis of instability of carbon cathode[48-49] (a)potential (U) versus discharge capacity (Q) for Li-O2 galvanostatic discharge and charge at 200μA of current on a 13C cathode; (b)linear sweep voltammogram (0.5mV·s-1) and CO2 evolution from a Li-O2 cell with a 13C cathode discharged (≈1mAh·cm-2) under 18O2 (1mol·L-1 LiTFSI in DME); (c)schematic of the instability of carbon cathode under high potential

上述研究表明,碳阴极的不稳定性引起的腐蚀、较差的Li2O2催化活性以及促进电解质分解等问题使得碳基材料不是理想的阴极选择。

4 实现稳定和可逆的锂-空气电池阴极的方法

近年来,为了解决上节提到的涉及碳阴极的诸多问题,从而获得高性能的LABs,构建稳定和可逆的碳阴极或者寻找替代阴极来避免涉及碳基材料的副反应是首要任务。目前有两种策略得到了广泛的应用:(1)提高碳阴极的化学稳定性,即进行碳的表面改性;(2)设计“无碳”阴极。两种策略均能有效地减轻由碳的不稳定性引起副反应,从而避免或减少副产物的形成。下面的章节将具体讨论。

4.1 提高碳阴极的化学稳定性——碳的表面改性

改性sp2碳表面的一种常见方法是杂原子掺杂,在碳表面上产生的官能团和缺陷边缘可以提供用于形成Li2CO3的反应位点并抑制电解质分解,进而提高体系中的ORR性能[50-55]。例如,Shui等[56]设计了一种垂直排列的氮掺杂珊瑚状碳纳米纤维(VA-NCCF)电极,该电极在充放电期间表现出高达90%的能量效率,充放电过电位低至0.3V,如此低的过电位使得电解质分解最小化,从而提供高达1000mAh·g-1的比容量,并能可逆循环150次以上。这些优异的性能是多种因素共同决定的,如N掺杂诱导的催化活性可以降低充电过电位,使电解质分解最小化并促进Li2O2的沉积,独特的垂直排列的珊瑚状纤维结构以提供用于高效Li2O2沉积和增强的电子/电解质/反应物传输的自由空间。其他N掺杂碳阴极,如N掺杂石墨烯气凝胶也表现出优异的性能[43]。因此,通过合理设计具有明确分级结构和杂原子诱导催化活性的阴极,可以显著提高LABs的性能。最近,Wong等[57]设计了具有氧官能团的多壁碳纳米管(Ox-MCNT)和具有缺陷边缘的多壁碳纳米管(Ox-MCNT-900)。放电后在Ox-MCNT和Ox-MCNT-900阴极上生成的Li2O2膜能在随后的充电中很容易的分解,进而提高了电池的循环稳定性和可逆性。这些研究结果表明通过控制氧官能团和缺陷边缘的碳表面特性对改善LABs的性能起着关键作用。

另一种有效的改性碳表面的方法是通过金属或金属氧化物纳米粒子的表面修饰。许多负载有金属或金属氧化物的碳复合物通常可以提高LABs阴极ORR和OER性能。Lu等[58]报道了一种在碳表面上涂覆氧化铝(Al2O3)以钝化碳缺陷位点,并用Pd纳米催化剂沉积的表面改性碳阴极(如图 5),该阴极在100mA·g-1的电流密度下具有极低的充电过电位(约0.2V)。Al2O3涂层有效地防止了碳氧化和电解质分解,同时,Pd纳米催化剂颗粒促进了Li2O2的生长和形成。

图 5 表面改性碳阴极的结果[58]  (a)阴极结构示意图;(b)充放电曲线(电流密度100mA·g-1) Fig. 5 Carbon cathode with modified surfaces[58]  (a)schematic of the cathode architecture; (b)discharge-charge profiles(100mA·g-1)

用均匀的涂层完成碳钝化有助于完全阻断寄生反应的活性位点。Jian等[59]报道了核-壳结构的CNT@RuO2复合材料作为LABs的阴极。通过简单的溶胶-凝胶法,在阴极的CNT表面均匀地形成厚度约为4nm的RuO2壳。RuO2壳不仅可以提高CNT的化学稳定性,对ORR和OER表现出良好的催化活性,而且不会降低CNT的高电导率。与纯CNT阴极相比,CNT@RuO2阴极显示出高的比容量和显著降低的充电电位,以及优异的循环性(在500mA·g-1的高电流密度下循环100次以上)。这种核-壳结构良好地抑制了CNT与电解质和放电产物之间的接触。Wu等[60]报道了Pt包覆的中空石墨烯纳米笼阴极(Pt/HGNs),在100mA·g-1的电流密度下充电电压可以降至3.2V,即使在高达500mA·g-1的电流密度时,电压也可以保持在3.5 V以下。这种独特的结构不仅可以提供许多三相区域作为有效氧还原的活性位点,而且还提供大量用于O2快速扩散的介孔。此外,Pt还作为Li2O2生长的成核位点。同时,由于石墨烯-金属界面的相互作用,导电中空石墨烯基底可以提高贵金属Pt催化剂的催化活性。受益于中空石墨烯纳米笼和纳米级Pt催化剂之间的协同效应,Pt修饰的石墨烯纳米笼阴极表现出增强的电化学性能。Zhou等[61]开发了Co3O4功能化的多孔碳纳米管(p-CNT/Co3O4)作为LABs的高效阴极催化剂。p-CNT的多孔结构可有效促进Li+和O2扩散,Co3O4在p-CNT表面的官能化可显著增强阴极表面上的O2吸附,并且通过表面生长模式形成薄膜Li2O2,从而实现低的充电过电位[28]。在100mA·g-1的电流密度下,p-CNT/Co3O4的初始放电容量为4331mAh·g-1,过电位降至0.95V,在定容500mAh·g-1,电流密度为200mA·g-1下循环116次。

其他表面改性碳阴极如N掺杂石墨烯气凝胶(NPGAs)[43]、Pd修饰的涂覆FeOx的介孔碳(Pd/FeOx/C)[62]、NiCo2O4碳布复合(NiCo2O4/CC)[63]、Co3O4与石墨烯复合(Co3O4/G)[64]、δ-MnO2与石墨烯复合(δ-MnO2/G)[65]、CoSe2和CoO负载在Super P(CoSe2/CoO/SP)[66]、聚酰亚胺包裹的碳纳米管(PI/CNTs)[67]、Mo2C与碳纳米管复合(Mo2C/CNTs)[68]、碳布上生长TiO2(TiO2/CT)[69]等总结在表 1中。

表 1 典型的表面改性碳阴极 Table 1 Typical carbon cathodes with modified surfaces
RefCatalystCurrent densityFirst discharge capacity/(mAh·g-1)Cycling rateLimited capacity/(mAh·g-1)N/cycle
[56]VA-NCCF100mA·g-11000250mA·g-1500200
[43]NPGAs200mA·g-1100811000mA·g-1100060
[62]Pd/FeOx/C100mA·g-150050068
[58]Pd/C[Al2O3]100mA·g-12750100mA·g-150015
[63]NiCo2O4/CC18mA·g-198018mA·g-150013
[64]Co3O4/G200mA·g-110500200mA·g-1100080
[59]RuO2@CNT385mA·g-14350500mA·g-1300100
[60]Pt/HGNs100mA·g-15600100mA·g-1100010
[61]p-CNT/Co3O4100mA·g-14331200mA·g-1500116
[65]δ-MnO2/G0.083mA·cm-236600.333mA·cm-2492132
[66]CoSe2/CoO/SP0.1mA·cm-215000.1mA·cm-230
[67]PI/CNTs500mA·g-111000500mA·g-11500137
[68]Mo2C/CNTs100mA·g-1100mA·g-1500150
[69]TiO2/CT100mA·g-13000100mA·g-11000356
4.2 设计无碳阴极 4.2.1 常规无碳阴极

虽然构建了具有各种稳定且可逆的碳改性阴极,但是来自碳阴极的化学不稳定性问题仅仅得到了部分解决。因此,为了彻底摆脱与含碳相关的腐蚀问题,开发具有惰性的无碳材料引起了研究者的极大兴趣。这些催化剂包括Au[70-71],Ru[72],RuO2[73],TiN[74],Ti4O7[75-76]和TiO2[77]等。

通常,贵金属具有高导电性和高的ORR和OER催化活性,许多研究者将其作为LABs无碳阴极的首选。例如,Peng等[71]使用基于二甲基亚砜电解质的无碳纳米多孔金(NPG)阴极,尽管在500mA·g-1的电流密度下可逆容量仅为300mAh·g-1,但在100次循环后,电池容量可以达到初始容量的95%,随后通过FTIR和DEMS验证了该电池的稳定性和可逆性。Chen等[73]提出了一种具有核-壳结构的纳米多孔金/氧化钌复合材料(RuO2-NPG)作为LABs的无碳阴极,此阴极显示出更高的比容量、更好的电催化活性和更低的过电位,这些优异的性能归因于由金属氧化物层修饰的具有高导电性的纳米多孔金构成的独特结构,其多孔结构为Li2O2的沉积提供大量的存储空间,并可以提供大量的活性反应位点。

研究表明,双金属合金比单金属材料具有更好的催化活性[78-80],Luo等[80]利用电化学置换反应合成了一维多孔AgPd-Pd纳米管作为LABs阴极的双功能催化剂,与纯Ag纳米线相比,这种多孔的AgPd-Pd纳米管结构促进了O2和电解质的扩散,从而使LABs具有良好的可逆性和优异的能量效率。另外,Lu等[78]设计的PtAu纳米颗粒也表现出优异的电池性能,然而,由于这些纳米结构的贵金属稀缺,价格昂贵,质量大,难以实现大规模生产和使用。

金属碳氮化物也因在催化剂领域表现出优异的导电性而受到广泛的关注[74, 81-82]。Kim等[74]以嵌段共聚物作为模板,设计了具有二维六角结构和大孔(430nm)的介孔氮化钛(m-TiN)的无碳阴极(如图 6)。该阴极具有超过100次循环的稳定循环性能。m-TiN的微结构孔隙结构可能会限制Li2O2的生长,防止Li2O2从阴极脱落,这对于LABs的长期可循环性是非常重要的。此外,他们还使用聚氨酯隔膜来保护Li金属免受腐蚀,并通过加入LiI溶液作为氧化还原介体,电池在定容430mAh·g-1下,可稳定循环280次以上。

图 6 m-TiN阴极的制备和电化学性能测试结果[74]   (a)制备过程示意图;(b)首次充放电曲线(电流密度50mA·g-1);(c)不同催化剂的首次充放电曲线;(d)循环性能 Fig. 6 Synthesis and electrochemical performance of m-TiN cathode[74]   (a)schematic of processes for synthesis; (b)first discharge-charge profiles (50mA·g-1); (c)first discharge-charge profiles of batteries with five catalysts; (d)cycling performances of the same five catalysts

其他无碳复合阴极如过渡金属碳化物如碳化钛(TiC)[81]、二氧化钌(RuO2)[83]、金属氧化物如钌负载氧化铟锡(Ru/ITO)[84]、钌负载的锑掺杂氧化锡(Ru/STO)[85]等总结在表 2中。

表 2 典型的常规无碳阴极 Table 2 Typical conventional carbon-free cathodes
RefCatalystCurrent densityFirst discharge capacity/(mAh·g-1)Cycling rateLimited capacityN/cycle
[71]NPG500mA·g-1323500mA·g-1500mAh·g-1100
[73]RuO2-NPG50mA·g-1300mAh·g-150
[80]AgPd-Pd0.2mA·cm-226500.2mA·cm-21000mAh·g-1100
[78]PtAu100mA·g-1140015
[74]m-TiN70mA·g-139070mA·g-1800mAh·g-1100
[81]TiC1mA·cm-23501mA·cm-2100
[83]RuO2500mA·g-1654500mA·g-1100
[84]Ru/ITO0.15mA·cm-21.81mAh·cm-250
[85]Ru/STO0.05mA·cm-23750.1mA·cm-2750mAh·g-150
4.2.2 自支撑阴极设计

典型的LABs阴极是使用聚合物黏合剂将活性碳基材料涂覆在集电体上构成的[86-90]。然而,在放电期间会受到O2-自由基的亲核攻击,黏合剂的降解会形成不期望的副产物[91-93]。这些副产物往往在阴极表面上形成薄膜,抑制其催化活性,从而导致LABs的容量严重衰减[91]。因此,构建无任何聚合物黏结剂的自支撑阴极可以有效地解决因黏结剂降解带来的问题,这有助于实现更稳定和可逆的LABs。

已经有大量研究是使用具有高电导率和催化活性的金属及其合金直接生长在多孔导电金属(Ni)基底上作为无碳的阴极。Liu等[94]制备了一个在多孔Ni泡沫上生长Ru纳米颗粒的无碳氧化物阴极(UNF@Ru)(如图 7),避免了因碳和黏结剂反应导致的副反应,该阴极在100次循环中显示出稳定的循环而没有电压衰减。并通过DEMS未检测到CO2的析出,这表明UNF@Ru阴极可以有效地催化LABs的ORR和OER。

图 7 UNF@Ru阴极的制备和电化学性能测试结果[94]   (a)制备过程示意图;(b)充放电曲线(电流密度500mA·g-1);(c)循环性能(定容1500mAh·g-1,电流密度500mA·g-1) Fig. 7 Synthesis and electrochemical performance of UNF@Ru cathode[94]   (a)schematic of processes for synthesis; (b)discharge-charge profiles (500mA·g-1); (c)cycling performances at 500mA·g-1 with a limited capacity of 1500mAh·g-1

Kim等[95]提出了一种Au纳米颗粒修饰的自支撑Ni纳米线阴极(Au/Ni),采用氧化铝(AAO)模板通过电镀法合成Ni纳米线阵列,然后将Au纳米颗粒直接沉积在Ni纳米线作为空气阴极(见图 8)。该电极在高达500mA·g-1的电流密度下显示出高达590mAh·g-1的比容量,并且在100次循环内具有稳定的循环性能。

图 8 Au/Ni阴极的制备和电化学测试结果[95]   (a)制备过程示意图;(b)充放电曲线;(c)循环性能(电流密度500mA·g-1) Fig. 8 Synthesis and electrochemical performance of Au/Ni cathode[95]    (a)processes for synthesis; (b)first discharge-charge profiles; (c)cycling performances at 500mA·g-1 of Au/Ni electrode

金属氧化物也可以直接通过不同的方法沉积在集流体上,如电化学沉积、水热法、化学气相沉积等。Tan等[96]报道了一种由RuO2纳米颗粒修饰的NiO纳米片组成的阴极(RuO2/NiO/Ni)。此阴极能够在空气中以500mAh·g-1的电流密度循环200次,且具有稳定的库仑效率(100%)和高的能量效率(约75%)。这些优异的性能归功于RuO2纳米颗粒和NiO纳米片的协同作用,不仅催化ORR和OER,还促进了在空气中放电时副产物(如LiOH、碳酸盐等)的分解。Kang等[97]在镍网上制备了具有有序宏观介孔结构的氧缺陷TiO2-x(HOP-bTiO2)用作LABs的自支撑阴极。由于氧空位或Ti3+的高电导率以及提供用于存储Li2O2的多孔结构,该LABs在300次循环中表现出优异的能量效率。

其他典型的自支撑无碳阴极如Pt-Gd/Ni[98],Co3O4/Ni[99],ε-MnO2/Ni[100],Pt修饰的TiO2纳米自支撑阵列(Pt/TNT/Ni)[101],Pd/Co3O4/Ni[102]总结于表 3

表 3 典型的自支撑无碳阴极 Table 3 Typical free-standing-type cathodes
RefCatalystCurrent densityFirst discharge capacity/ (mAh·g-1)Cycling rateLimited capacity/ (mAh·g-1)N/cycle
[94]UNF@Ru150mA·g-12410500mA·g-1100
[95]Au/Ni300mA·g-1921500mA·g-1110
[98]Pt-Gd/Ni0.05mA·cm-237000.05mA·cm-215
[96]RuO2/NiO/Ni0.0125mA·cm-23240250mA·g-1500260
[97]HOP-bTiO2/Ni500mA·g-17500500mA·g-11000260
[99]Co3O4/Ni0.05mA·g-14118200mA·g-1100035
[100]ε-MnO2/Ni500mA·g-16300500mA·g-11000120
[101]Pt/TNT/Ni1000mA·g-11000140
[102]Pd/Co3O4/Ni0.05mA·cm-218430.1mA·cm-230070
5 结束语

随着人们对高效储能系统日益增长的需求,具有极高的理论能量密度的LABs被认为是储能领域的最有希望的技术之一。尽管LABs有许多优点,但其差的循环稳定性以及高的充电过电位导致的低能量效率阻碍了该技术的实际发展。因此,目前最重要的挑战之一是提高阴极的稳定性和可逆性,以降低充电过电位,从而提高LABs的性能。传统的碳电极腐蚀仍然是危害LABs性能的主要问题之一,为了解决这个问题,碳的表面改性和无碳阴极设计是两种十分有前景的解决方案,前者需要在碳表面均匀地分散保护材料,而后者需要更稳定和低成本的高导电材料。阴极结构对LABs的比容量和容纳更多的放电产物等方面起着决定性的作用,随着纳米技术的高速发展,催化活性更高、孔隙率更高、比表面积更大、质量更轻的材料也将大量问世。此外,进一步加强对LABs基本反应机理的理解将为开发高性能的LABs阴极提供实际的指导。总之,LABs性能的进一步提高需要多个领域的技术突破,实验和理论的有效结合最终将实现LABs的实际应用。

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