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无机材料学报, 2022, 37(5): 481-492 DOI: 10.15541/jim20210502

综述

水系锂离子电池负极材料LiTi2(PO4)3的研究进展

王禹桐,1, 张非凡1, 许乃才2, 王春霞1, 崔立山1, 黄国勇,1

1. 中国石油大学(北京) 新能源与材料学院, 重质油国家重点实验室, 北京 102249

2. 青海师范大学 化学化工学院, 西宁 810008

Research Progress of LiTi2(PO4)3 Anode for Aqueous Lithium-ion Batteries

WANG Yutong,1, ZHANG Feifan1, XU Naicai2, WANG Chunxia1, CUI Lishan1, HUANG Guoyong,1

1. State Key Laboratory of Heavy Oil, College of New Energy and Materials, China University of Petroleum-Beijing, Beijing 102249, China

2. School of Chemistry and Chemical Engineering, Qinghai Normal University, Xining 810008, China

通讯作者: 黄国勇, 教授. E-mail:huanggy@cup.edu.cn

收稿日期: 2021-08-13   修回日期: 2021-10-22  

基金资助: 国家自然科学基金(52022109)
国家自然科学基金(51834008)
北京市自然科学基金(2202047)
中国石油大学(北京)科研基金(2462018YJRC041)
中国石油大学(北京)科研基金(2462020YXZZ016)

Corresponding authors: HUANG Guoyong, professor. E-mail:huanggy@cup.edu.cn

Received: 2021-08-13   Revised: 2021-10-22  

Fund supported: National Natural Science Foundation of China(52022109)
National Natural Science Foundation of China(51834008)
Beijing Municipal Natural Science Foundation(2202047)
Science Foundation of China University of Petroleum, Beijing(2462018YJRC041)
Science Foundation of China University of Petroleum, Beijing(2462020YXZZ016)

摘要

锂离子电池作为一种绿色可充电电池, 具有较高的能量密度及功率密度, 但市售锂离子电池主要以有机物为电解液, 当电池过充或短路时存在一定的燃烧及爆炸风险。为应对此问题, 水系锂离子电池逐渐走进人们的视野, 它具有清洁环保、安全性能高等优点, 其工作电压为1.5~2.0 V, 主要应用于储能领域。考虑到水系电池的析氢析氧反应, 常规负极材料无法应用于水系锂离子电池, 因此水系锂离子电池的研发关键在于负极材料的选取。LiTi2(PO4)3具有开放的三维通道以及合适的嵌锂电位, 可以作为水系锂离子电池的负极材料。LiTi2(PO4)3的合成方法主要有高温固相法、溶胶-凝胶法和水热法等。为进一步提高LiTi2(PO4)3的电化学性能, 可以采用颗粒纳米化、形貌控制、元素掺杂及碳包覆等方式进行改性。本文从合成方法及改性手段的角度, 对近年来国内外水系锂离子电池负极材料LiTi2(PO4)3的研究进行综述, 并对LiTi2(PO4)3负极材料的发展前景做出展望。

关键词： 水系锂离子电池; 负极材料; LiTi2(PO4)3; 综述

Abstract

As green rechargeable batteries, lithium-ion batteries feature high energy and power density. However, commonly-used electrolytes, organic compounds, in commercially available lithium-ion batteries are flammable and toxic, which leaves them at the risk of combustion and explosion when being overcharged or short-circuited. In order to solve this problem, much attention has been paid to lithium-ion batteries with aqueous electrolytes, which take low-toxicity and high safety as the prominent advantages. The working voltage, 1.5-2.0 V, indicates their usage mainly in the field of energy storage. Considering the hydrogen and oxygen evolution, conventional anode materials used in commercially available lithium-ion batteries are inconformity for water-based lithium-ion batteries. Therefore, the key to the development of aqueous lithium-ion batteries lies in the selection of anodes. The anode material, LiTi2(PO4)3, has drawn the attention of researchers due to its advantages such as three-dimensional channel, and appropriate lithium-ion intercalation potential. The synthesis methods of LiTi2(PO4)3 mainly include high temperature solid-phase calcination, Sol-Gel methods and hydrothermal reaction, etc. To further improve the electrochemical performance of LiTi2(PO4)3, strategies can be used such as particle nanocrystallization, morphology control, element doping, and carbon-coating, etc. This review focuses on the synthesis and modification of LiTi2(PO4)3, as well as related research progress. At last, the future development of LiTi2(PO4)3 as anode material for lithium-ion battery is properly prospected.

Keywords： aqueous lithium-ion battery; anode material; LiTi2(PO4)3; review

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本文引用格式

王禹桐, 张非凡, 许乃才, 王春霞, 崔立山, 黄国勇. 水系锂离子电池负极材料LiTi2(PO4)3的研究进展. 无机材料学报, 2022, 37(5): 481-492 DOI:10.15541/jim20210502

WANG Yutong, ZHANG Feifan, XU Naicai, WANG Chunxia, CUI Lishan, HUANG Guoyong. Research Progress of LiTi2(PO4)3 Anode for Aqueous Lithium-ion Batteries. Journal of Inorganic Materials, 2022, 37(5): 481-492 DOI:10.15541/jim20210502

锂离子电池具有能量密度大、工作电压高、循环寿命长、无记忆效应等显著优势[1,2,3,4,5]。然而, 锂离子电池并非十全十美, 其安全问题亟待解决。近年来, 电池燃烧、爆炸事故频发, 事故的主要原因之一在于锂离子电池电解液所使用的溶剂(如碳酸乙烯酯(Ethylene carbonate, EC); 碳酸二甲酯(Dimethyl carbonate, DMC); 碳酸二乙酯(Diethyl carbonate, DEC)等)为易燃有机物, 当电池过充或温度过高时, 便存在爆炸的风险[6]。为应对当下锂离子电池的安全问题, 科研人员开始关注水系锂离子电池。

水系锂离子电池的概念最早由加拿大的Dahn团队[7]提出。如表1所示[8], 与有机系锂离子电池相比, 水系锂离子电池的主要特点在于其以非可燃性锂盐的水溶液为电解液, 可以从根源上有效避免热失控反应, 大大降低了安全隐患。此外, 水溶液的离子电导率通常较有机系电解液高出两个数量级, 有助于提高电池的倍率性能[9]。另一方面, 组装水系锂离子电池无需严格无水条件, 其装配复杂程度低于有机系锂离子电池, 有助于实现规模化生产。水系锂离子电池兼顾了安全与成本优势, 具有很高的开发潜力和应用价值[7,9-10]。

构建水系电池需要考虑以下问题: 电极材料与H2O及O2的副反应; 正极材料的质子共嵌入现象; 析氢析氧反应等[11]。其中, 电极材料的副反应及质子共嵌入现象可以通过消除O2、调整电解液的pH得到改善[12]。因此, 析氢析氧反应成为了构建水系锂离子电池所需考虑的首要问题, 由于体系以水为溶剂, 当电极材料的嵌锂电位超出水的分解电压窗口时, 电池会发生明显的析氢析氧现象[13], 影响正常使用, 因此电极材料应严格在该范围内选取。

表1   水系锂离子电池和有机系锂离子电池的特征比较[8]

Table 1  Comparison of the characteristics of aqeuous and organic lithium-ion batteries[8]

TypeOperating voltage/VSafetyElectrolyteSolventCost
Organic Li-ion battery3.6-4.2LowLiPF6, LiAsF6, etcEC, DMC, DEC, etcHigh
Aqeuous Li-ion battery1.5-2.0HighLi2SO4, LiNO3, etcH2OModerate

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由图1可知, 用于传统有机系电解液的正极材料(如LiCoO2、LiNiO2、LiMn2O4和LiFePO4等)的嵌锂电位低于氧气析出电位, 在水系锂离子电池中可以充当正极活性材料。但对于负极而言, 商用负极材料石墨的嵌锂电位过低, 无法应用于水系锂离子电池。因此, 构建水系锂离子电池的重点就是在稳定的电压窗口内选取合适的负极活性材料。只有高于氢气析出电位的材料[14](如LiTi2(PO4)3、TiP2O7、VO2和LiV3O8等)才能在水系锂离子电池体系中充当负极。

图1

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图1   电极材料的嵌锂电位-pH的关系图

Fig. 1   Potential-pH diagram of typical electrode materials


如表2所示[14], 在众多负极材料中, LiTi2(PO4)3具有嵌锂电位适宜、比容量较大及骨架稳定等优势。本文将对LiTi2(PO4)3的制备方法及改性手段等进行综述。

表2   部分水系锂离子电池的负极材料的参数[14]

Table 2  Parameters of some anode materials for aqeuous lithium-ion battery[14]

Anode materialSpecific capacity/
(mAh·g-1)		Potential/
V(vs. Li+/Li)		Potential/
V(vs. NHE)		Features
LiTi2(PO4)31382.5-0.5Moderate specific capacity, stable framework
TiP2O71212.6-0.4Low specific capacity, high Li-intercalation potential
VO22502.6-0.4High specific capacity, poor cycling performance
LiV3O82502.6-0.4Fragile during cycling

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1 磷酸钛锂的结构及嵌锂行为

LiTi2(PO4)3属于菱方晶系, 空间群为R-3c, 离子电导率为2.0×10-6 S·cm-1, 理论比容量为138 mAh·g-1。晶格中3个PO4四面体和2个TiO6八面体通过共用顶点O原子连接而形成[Ti2(PO4)3]-1菱面体[15,16], 如图2所示[17]。晶格中P-O共价键具有较大的键能, 使得O原子在充放电过程中难以析出, 从而确保了晶体结构的稳定性。此外, 引入P导致Ti-O键的离子键特性突出, 增大了反键轨道与锂的费米能级差, 从而使Ti4+/Ti3+电对具有较高的氧化还原电位(~2.5 V (vs Li/Li+))[18], 符合水系锂离子电池对负极材料的电位要求。

图2

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图2   LiTi2(PO4)3的晶体结构图[17]

Fig. 2   Crystal structure of LiTi2(PO4)3[17]


LiTi2(PO4)3三维刚性骨架中存在两种Li间隙位点, 分别记作M1和M2。M1位于沿z轴方向上两个相邻的TiO6八面体间, M2位点则处在O3TiO3M1O3TiO3M1···O3TiO3M1空腔与八个O原子键合形成的导电通道拐弯处。由于M1位点具有较低的势能, 因此在LiTi2(PO4)3晶格中, Li+占据M1位点, 而M2位点则处于空腔状态。在嵌锂过程中, 来自正极的Li+开始占据晶格中的M2位点, 与此同时, 处于M1位点的Li+也开始逐渐迁移至M2位点[15,19], 为满足电荷守恒, Ti的价态由+4降低至+3, 形成Li3Ti2(PO4)3相。

LiTi2(PO4)3 + 2e- + 2Li+→Li3Ti2(PO4)3

在Li3Ti2(PO4)3晶格中, 3个Li+均匀分散在M2位点中, 而M2位点可进一步划分为M3和M3’亚位点, 两个Li+占据M3位点(2/3), 另一个Li+占据M3’位点(1/3)[15]。由此可见, 充放电过程仅涉及Li+位点的重排, 而晶格体积不发生明显变化, 保证了其循环使用的稳定性和安全性。

2 合成方法

目前, 文献报道的LiTi2(PO4)3合成方法主要有高温固相法、溶胶-凝胶法、水/溶剂热法及液相共沉淀法等, 这些方法的特点如表3所示。

表3   常见LiTi2(PO4)3合成方法比较

Table 3  Comparison of common synthetic methods of LiTi2(PO4)3

MethodStarting materialsProduct characteristicFeaturesRef.
Li sourceTi sourceP sourceMorphology
Solid stateLiH2PO4TiO2NH4H2PO4Irregular particlesLong calcination time, high temperature[20]
Sol-GelCH3COOLiTi(C4H9O)4H3PO4ParticlesShort calcination time, low temperature[21]
Hydrothermal synthesisCH3COOLiTi(C4H9O)4NH4H2PO4Regular particlesRegular particle morphology, great crystallinity[22]
Co-precipitation methodLiOHTi(C4H9O)4H3PO4ParticlesRequiring precise control[23]
ElectrospinningCH3COOLiTi(C4H9O)4NH4H2PO4FiberIdeal electrochemical performance, difficult industrialization[24]

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2.1 高温固相法

高温固相法, 即将锂源、钛源和磷源按化学计量比混合均匀后直接置于加热炉中煅烧, 之后经冷却、研磨, 即可获得LiTi2(PO4)3粉末。高温固相法的最大优势在于工艺流程简单、易于工业化生产, 缺点在于煅烧温度高、时间长, 而长时间的高温煅烧以及较差的原料混合程度导致产物的粒径尺寸较大且均一度差, 较大的粒径尺寸使Li+的扩散路径延长, 且增加了阻力, 最终影响产物的电化学性能。此外, 高温固相法合成LiTi2(PO4)3的温度通常在900 ℃以上[20,25], 此温度下锂元素易挥发, 需要在物料中添加过量的锂盐, 从而使得配方难以控制, 产品中容易出现杂相。

中国科学院物理研究所陈立泉院士团队[20]研究了LiTi2(PO4)3作为水系锂电负极材料的性能。他们将化学计量比的LiH2PO4、TiO2和NH4H2PO4充分混合并球磨5 h, 然后升温至300 ℃并恒温4 h, 冷却至室温后再球磨2 h, 随后升温至600 ℃, 恒温24 h, 最后将样品升温至1000 ℃恒温48 h得到纯相LiTi2(PO4)3。该团队将纯相LiTi2(PO4)3作为负极材料, 并以LiMn2O4为正极材料, 5 mol·L-1 LiNO3溶液为电解液, 组装全电池, 其初始放电比容量约为45 mAh·g-1。该全电池的比容量衰减较快, 在0.1C倍率下, 经10次充放电循环后, 放电比容量保持率降至75%, 这可能是源于充放电过程中晶体结构分解和新物质生成[26,27]。郑威[28]采用相同的原料制备了纯相LiTi2(PO4)3, 但煅烧方案非常简单: 在950 ℃下恒温12 h, 并采用非原位X射线衍射(X-ray diffraction, XRD)和热重-差热分析(Thermogravimetric-differential thermal analysis, TG-DTA)探究了LiTi2(PO4)3合成机理, 涉及的化学反应如下:

2H3PO4+TiO2→TiP2O7+3H2O
LiH2PO4→LiPO3+H2O
TiP2O7+TiO2+LiPO3→LiTi2(PO4)3

Wang等[25]以Li2CO3、TiO2和NH4H2PO4为原料合成了LiTi2(PO4)3。首先在900 ℃下保温12 h以分解Li2CO3和NH4H2PO4, 冷却至室温后取出样品并研磨, 最后在1250 ℃下煅烧24 h, 煅烧过程中总化学反应如下:

0.5Li2CO3+2TiO2+3NH4H2PO4→ LiTi2(PO4)3+3NH3+4.5H2O+0.5CO2

冯传启等[29]对固相法稍加改进, 在原料Li2CO3、TiO2和NH4H2PO4中加入适量草酸和柠檬酸溶液, 将原料调制成流变态混合物的形式进行煅烧, 于600 ℃下恒温48 h获得目标产物。在流变相前驱体中各反应组分的混合更加均匀, 能量交换更容易进行[30]。在300 mA·g-1的电流密度下, 经120次循环后, 比容量为80 mAh·g-1。

2.2 溶胶-凝胶法

溶胶-凝胶法是目前实验室中使用频率最高的合成LiTi2(PO4)3的方法, 它可以实现分子级别的组分混匀, 因此与高温固相法相比, 该方法所需的煅烧温度更低, 时间更短, 产物粒径也更均一, 电化学性能通常优于固相法。但是, 溶胶-凝胶法的工序较固相法复杂, 不易于扩大生产。

复旦大学夏永姚课题组[31]通过溶胶-凝胶法合成了负极材料LiTi2(PO4)3。他们将化学计量比为1:2:3的Li2CO3、TiO2和NH4H2PO4投入到2%聚乙烯醇(PVA)溶液中, 在80 ℃下, 恒温搅拌混合物直至水分完全蒸发得到白色前驱体, 之后将其置于N2气氛下900 ℃煅烧12 h获得目标产物, 惰性气氛有助于提供氧空位, 从而提高Li+的扩散速率, 降低电荷转移阻抗[32,33]。该课题组对比了相同原料及煅烧条件下, 溶胶-凝胶法和高温固相法所得产物颗粒的微观差异, 实验结果表明, 溶胶-凝胶法产物的平均粒径为200~300 nm, 而高温固相法的颗粒尺寸较大且粒径分布不均匀。程承[34]将相同的原料加入至2%聚乙二醇(PEG)溶液中, 搅拌并蒸发溶剂后获得了乳白色凝胶, 最后在800 ℃煅烧20 h获得目标产物, 粒径约为1 μm。在0.1C倍率下, 其全电池体系LiTi2(PO4)3|1mol·L-1 Li2SO4|LiMn2O4经20次循环后放电容量的保持率为39.9%。一些学者在溶胶体系中引入柠檬酸和乙二醇, 利用其螯合及聚酯化反应定量生成树脂, 以使Ti原子固定在其中, 从而确保各组分的均匀分散程度, 使得产物粒径分布更加均匀。Mariappan等[35]以Li2CO3、NH4H2PO4和钛粉为原料, 通过此法合成了前驱体, 经850 ℃煅烧2 h得到目标产物, 粒径在50~125 nm, 但钛粉价格昂贵且分散性差。Wessells和崔屹等[36]以Ti(C3H7O)4代替Ti粉, 在800 ℃煅烧12 h获得了LiTi2(PO4)3, 探究目标产物在Li2SO4溶液中的电性能。在0.2C倍率下, 初始放电比容量为113 mAh·g-1, 经100次循环, 放电比容量约为101 mAh·g-1。他们认为容量保持率降低的原因可能在于LiTi2(PO4)3颗粒之间的电接触减弱, 而充放电循环后, 材料的结晶度未见变化。

Zhou等[37,38]以聚碳酸酯膜为模板, 将其置于LiTi2(PO4)3的前驱体溶胶中浸泡3 h, 随后将聚碳酸酯模板取出, 紧贴在集流体铂片上, 在空气气氛600 ℃恒温5 h, 获得了生长于集流体上的单晶LiTi2(PO4)3纳米线材料。所得材料可直接作为电池的组件使用, 避免了引入粘结剂和导电剂, 其工艺路线如图3所示。纳米尺度的线状结构优势明显, 在充放电过程中可以承受更大的应力, 防止电极材料发生破碎。但由于体系中不存在导电剂, 其电化学性能较差。

图3

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图3   LiTi2(PO4)3 纳米线(LTPNMs)的制备过程示意图[38]

Fig. 3   Schematical illustration of the fabrication process of lithium titranium phosphate nanowires (LTPNMs)[38]


2.3 水/溶剂热法

水/溶剂热法利用体系在高温下的自生压力实现晶粒生长, 其优势在于所制备的晶粒发育相对完整、粒径尺寸较小且分布均匀[39,40,41,42]。Yue等[43]利用水热法合成了LiTi2(PO4)3, 他们将Li2O、TiO2和P2O5置于反应釜中, 250 ℃保温5~7 d制备了立方形态的目标产物, 其粒径在40~60 μm。Liang等[44]

以LiOH、H3PO4和钛粉为原料, 钽片为基板, 在220 ℃恒温24 h获得了薄膜电极材料。但Yue等和Liang等均未对产物进行深入的电化学性能研究。Li等[45]采用水热法, 以CH3COOLi、Ti(C4H9O)4和NH4H2PO4为原料, 经180 ℃恒温12 h获得LiTi2(PO4)3。目标产物在LiPF6 (EC : DMC=1 : 1)电解液中展现出优异的电化学性能, 在10C倍率下, 其初始放电比容量为98.6 mAh·g-1, 经2000次循环后其容量保持率仍为85%。Yu等[22]对比了原料相同时, 溶剂热法和溶胶-凝胶法所得产物的形貌及电性能差异。溶剂热法所得产物呈纺锤状, 在1C倍率下, 首次放电比容量为133.8 mAh·g-1, 循环50次后放电比容量为107.6 mAh·g-1。而溶胶-凝胶法所得产物则无规则形状, 1C倍率下的初始放电比容量为123.7 mAh·g-1, 50次充放电循环后放电比容量为90.2 mAh·g-1, 其电性能略低于溶剂热法所得产物。

2.4 共沉淀法

共沉淀法[46,47,48]是利用几种阳离子相近的溶解度, 使之同时形成沉淀的一种方法。该方法兼具产物颗粒小和易工业化生产的优势, 缺点在于滴加原料时必须保持缓慢匀速, 否则将导致产物粒径不均一。盖利刚等[49]将Ti(C4H9O)4加入丁醇和乙醇(体积比1 : 1)的混合溶液中, 逐滴加入LiOH和植酸并持续搅拌获得悬浊液, 之后在室温下继续搅拌2 h, 对悬浊液进行抽滤、洗涤、干燥和煅烧, 获得目标产物LiTi2(PO4)3。其全电池体系LiTi2(PO4)3|1 mol·L-1 Li2SO4|LiMn2O4在1.37C倍率下, 经1000周循环, 容量保持率仍为71.5%。但Ti(C4H9O)4价格较高, 且遇空气极易水解变质, 贮存成本高, 工业化的难度增加。

2.5 微波加热法

微波加热法的主要优势在于加热均匀、加热速度快、无滞后效应和温度梯度可控, 因而结晶度好, 粒径均一, 理论上微波加热法可以有效抑制颗粒团聚现象[50,51,52,53]。郭晓伟等[54]采用溶胶-凝胶法合成LiTi2(PO4)3前驱体, 之后将其置于微波加热炉中, 在600 ℃下, 仅保温15 min便制得纯相LiTi2(PO4)3, 远短于其他所有方法。但微波加热法的相关文献报道较少, 因此需要积累其最佳工艺参数。此外, 微波加热设备价格昂贵, 一定程度上限制了其大规模应用。

3 改性手段

如前文所述, LiTi2(PO4)3本身具有骨架稳定、嵌锂电位适宜等优势, 但是在LiTi2(PO4)3晶格中TiO6八面体被PO4基团分隔开, 导致波函数重叠较小, 使得材料的电子电导率偏低。另一方面, 水系锂离子电池通常以Li2SO4或LiNO3为电解质, 无法参与形成SEI膜, 导致纯相电极材料裸露于电解液中, 加剧了电池比容量的衰减。为解决以上问题, 研究者采用了多种方法对LiTi2(PO4)3进行改性处理, 取得了一定进展。目前常见的改性手段可以归纳为颗粒纳米化、微观形貌控制、碳包覆和元素掺杂等。

3.1 颗粒纳米化及形貌控制

颗粒纳米化是提升电极材料电化学性能的重要思路[55,56,57,58,59]。当材料粒径达到纳米尺度时, 能够有效缩短Li+的扩散路径, 同时抑制缺陷的阻碍作用, 控制极化现象, 另一方面, 纳米尺度的材料具有较大的比表面积, 可以增大电极/电解液界面, 进而增加反应活性位点, 因此颗粒纳米化可以提高材料的倍率性能和可逆容量[60]。纳米尺度的LiTi2(PO4)3颗粒主要通过溶胶-凝胶法、水热法等液相方法合成。

颗粒形貌控制是提高电极材料电性能的另一重要思路。为获得具有特定形貌的颗粒往往需要采用水/溶剂热法、静电纺丝法和模板法。Liang等[44]采用水热法, 在220 ℃下保温48 h合成了具有规则立方形貌的LiTi2(PO4)3, 随着时间延长和温度升高, 颗粒的微观形貌逐渐呈现规则的方块形状, 最大颗粒接近50 μm。Deng等[61]采用溶剂热法合成了具有规则立方形貌的LiTi2(PO4)3, 其粒径尺寸约为200 nm。在10C倍率下, 其全电池体系LiTi2(PO4)3|1 mol·L-1 LiTFSI|LiMn2O4经1000次循环, 容量保持率约为80%。Liu等[24]将原料置于静电纺丝仪中, 获得了纤维状前驱体, 经煅烧后获得LiTi2(PO4)3纤维材料, 其流程如图4(a)所示。

图4

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图4   静电纺丝法示意图(a), LiTi2(PO4)3纤维与颗粒的倍率性能曲线(b)[24]

Fig. 4   Schematic diagram of electrospinning (a), comparison of rate performance between LiTi2(PO4)3 fibers and particles (b)[24]


与相应的颗粒材料相比, 纤维材料具有比表面积大和长径比高等优点, 能够有效缩短Li+的迁移距离, 增大材料和电解液的接触面积, 有利于电化学反应, 展现出更优秀的倍率性能, 由图4(b)可知, 纤维材料的倍率性能明显优于相应的颗粒材料。

3.2 碳包覆

碳包覆是提升电极材料电性能的最常用手段之一[62,63,64,65], 该方法的优势在于操作简便、原料来源丰富和提升效果显著。

3.2.1 碳包覆提升LiTi2(PO4)3电化学性能的机制

包覆碳层与电极材料之间为面与面的接触, 电子更容易在活性材料和集流体中穿梭。此外, 碳包覆可以有效抑制颗粒长大[66], 从而获得粒径较小的产物。另一方面, 碳层还可以增大材料的放电比容量, 在LiTi2(PO4)3/C界面, 存在C-O基团, 最外端悬空的O可以结合一部分Li, 从而增大比容量。相比之下, 未包覆碳层时, LiTi2(PO4)3表面不存在C-O- Ti/Li键, 表面的悬空原子如Ti、Li和O会发生自钝化, 因此悬空O不会与额外的Li成键[61]。除此之外, 表面的纳米级碳层阻隔了LiTi2(PO4)3与电解液的直接接触, 能够有效阻止放电态的Li3-xTi2(PO4)3被O2及H2O氧化成LiTi2(PO4)3和其他不纯相, 从而延缓电池容量的衰减。另一方面, 碳具有还原性, 在煅烧过程中可将痕量的Ti4+还原为Ti3+, 从而提高电子电导率[12, 67]。因此碳包覆提升LiTi2(PO4)3电化学性能的机制在于表面悬空键、石墨化碳层和痕量Ti3+的共同作用。

3.2.2 碳源对LiTi2(PO4)3电化学性能的影响

根据前驱体的种类, 含碳试剂可分为有机碳源和无机碳源。在高温裂解时, 有机碳源可形成同质化碳层, 其碳层厚度易于控制, 但石墨化程度通常难以控制, 无机碳源有助于形成3D导电网格结构, 但对单一颗粒的表面包覆可能效果不佳, 因此同时引入有机和无机碳源可以提供互补优势[68]。有机碳源主要包括柠檬酸、葡萄糖、蔗糖、淀粉和聚乙烯醇等, 无机碳源主要有乙炔黑、石墨烯和碳纳米管(Carbon nanotubes, CNTs)等。

叶嘉明等[69]对比了葡萄糖、蔗糖、聚乙烯醇和淀粉作为碳源对LiTi2(PO4)3电化学性能的影响。在0.1C倍率下经过50次循环后, 采用葡萄糖为碳源制备的LiTi2(PO4)3/C复合负极材料的放电比容量为121.6 mAh·g-1, 明显高于蔗糖、聚乙烯醇和淀粉(69.8、92.8和83.1 mAh·g-1)。其原因可能在于几种碳源的分子量和结构性质存在差异, 使得产物的碳化程度、碳化历程和积碳含量有所不同。葡萄糖对电性能的提高最为明显, 其原因可能在于实现了晶化程度和积碳含量的平衡。聚乙烯醇煅烧后的晶化程度通常较高[70], 因而其电化学性能优于蔗糖和淀粉。蔗糖包含环状结构, 其裂解历程更为复杂, 同时蔗糖分子量高于葡萄糖, 煅烧后碳含量过高[71], 增加了Li+脱嵌的阻力, 因而提升电化学性能的效果不佳。

张晨阳等[72]以1 mol·L-1 Li2SO4和2 mol·L-1 ZnSO4的混合溶液为电解液, 锌片为对电极和参比电极组成扣式电池, 探讨了聚多巴胺、酚醛树脂、聚丙烯腈和葡萄糖的包覆效果, 其倍率性能见图5(a, b)。酚醛树脂高温碳化后的结晶度更高, 导电性最佳, 其次为聚多巴胺, 但聚多巴胺的附着力超强, 可以在多种纳米材料上形成均匀包覆层[73], 因而复合材料的循环稳定性最佳, 在5C倍率下, 聚多巴胺包覆的LiTi2(PO4)3经1000次循环, 容量保持率达86%。

He等[74]以酚醛树脂为包覆碳源, 其水系全电池LiTi2(PO4)3|饱和Li2SO4|LiMn2O4在1C、10C和20C倍率下, 放比电容量分别为111.4、93.7、74.2 mAh·g-1。在5C倍率下循环1000次后, 容量保持率为84.3%。Sun等[75]以酚醛树脂为碳源, 通过水热法合成了目标材料。他们采用十二烷基苯磺酸钠对碳源和LiTi2(PO4)3前驱体进行分散, 降低了体系的表面能, 提高了碳源的分散程度, 使包覆层更加均匀。此外, 碳介孔有效缓解了颗粒在充放电过程中的体积膨胀, 增强了循环稳定性, 机制如图5(c)所示。其全电池LiTi2(PO4)3|Li2SO4|LiMn2O4在750 mA·g-1的电流密度下经5500圈循环, 容量保持率为82.7%。

图5

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图5   四种包覆碳源的倍率性能图

(a, b)[72]((a)蓝、黑分别为聚多巴胺和酚醛树脂; (b)蓝、黑分别为聚丙烯腈和葡萄糖), 具有均质碳层的介孔LiTi2(PO4)3中Li+插入的机制示意图(c)[75], rGO-LTP的合成步骤示意图(d)[78], LC和LCG在5C下循环1000次的循环性能曲线(e)[80]

Fig. 5   Comparison chart of rate performance of four coated carbon sources

(a, b)[72](Blue and black in (a) indicating polydopamine and phenolic resin; blue and black in (b) indicating polyacrylonitrile and glucose), schematic illustration of the tentative Li+ insertion mechanism in mesoporous LiTi2(PO4)3 with carbon coating layer (c)[75], schematic diagram of the synthesis steps of rGO-LTP (d)[78], and cyclic performance of LC and LCG anodes at 5C for 1000 cycles (e)[80] Colorful figures are available on website


Xu等[76]在合成体系中引入了柠檬酸和CNTs, 制备了LiTi2(PO4)3@C/CNTs复合材料。柠檬酸在单一LiTi2(PO4)3颗粒表面具有良好的包覆效果, CNTs搭建了三维导电网格, 使LiTi2(PO4)3颗粒分散于其中。与单一的柠檬酸包覆相比, 引入CNTs提高了材料的电子电导率, 抑制了极化现象。其水系全电池LiTi2(PO4)3|5 mol·L-1 LiNO3|LiMn2O4在3 A·g-1的充电倍率下, 经过500次循环, 容量保持率为72.9%。

Roh等[77]将锂源、钛源、磷源与氧化石墨烯的二乙二醇溶液混合, 在水热反应釜中进行反应, 获得前驱体, 随后于800 ℃恒温10 h获得复合材料, 探讨了其半电池的电化学性能。在1C倍率下, 经100次循环, 放电比容量为初始值的93.2%, 库仑效率接近100%。Lim等[78]在合成体系中引入蔗糖和氧化石墨烯(GO)双碳源, 石墨烯可在LiTi2(PO4)3周围形成导电网络, 而蔗糖包覆在单一LiTi2(PO4)3颗粒表面, 两种碳源形成优势互补, 其合成思路如图5(d)所示。该课题组首先将Ti(C4H9O)4与氧化石墨烯置于水-乙醇中80 ℃搅拌12 h, 混合过程中, Ti(C4H9O)4发生水解, 产物在氧化石墨烯表面生长, 得到GO-TiO2混合物, 随后加入锂源、磷源和蔗糖, 在惰性气氛下900 ℃恒温12 h, 获得了具有导电网格结构的复合材料(rGO-LTP)。该方法确保了LiTi2(PO4)3颗粒(~200 nm)在氧化石墨烯表面原位生长, 理论上防止了团聚。LiTi2(PO4)3通过Ti/P-O-C键与氧化石墨烯结合, 或通过范德华力与氧化石墨烯的芳环区域结合[79]。该复合材料的碳含量低至1.79%, 在未引入导电剂的情况下依然展现出了优异的电化学性能。在210 Wh·kg-1的能量密度下, 比功率为10000 W·kg-1。在10C倍率下, 100次循环后, 容量保持率为92%。Zhou等[80]采用同样的策略, 以酚醛树脂和氧化石墨烯为双碳源对LiTi2(PO4)3进行改性, 获得了LiTi2(PO4)3/C/石墨烯复合材料(LCG), 其碳的质量分数约为16.2%。在5C倍率下, 全电池体系LiTi2(PO4)3| 2mol·L-1 Li2SO4|LiMn2O4的初始放电比容量为101.1 mAh·g-1。在5C倍率下循环1000次后, 与单碳源包覆材料(LC)相比, 其容量保持率提升了19.1%, 如图5(e)所示。

3.2.3 包覆方法对LiTi2(PO4)3电化学性能的影响

根据包覆方式不同, 碳包覆可以分为原位和非原位包覆。原位包覆即在原料体系中引入含碳试剂, 待各组分充分混合均匀后置于高温加热设备中进行煅烧。而非原位包覆则是将纯相LiTi2(PO4)3与碳源进行混合并煅烧。表4对比了采用不同碳源及包覆方式所得到的LiTi2(PO4)3的电化学性能参数。由表可知, 非原位碳包覆对材料循环寿命的延长和容量保持率的提升效果通常低于原位包覆, 其主要原因在于含碳试剂分散性的差别。对于原位包覆而言, 在前驱体内, 含碳试剂与原料的接触面积更大、分散更均匀, 在煅烧过程中可以形成分散性更好的碳层, 对电性能的提高更明显。而非原位包覆则是基于固相混料, 但固相混料的均匀度较差, 因此碳包覆更多选择原位法。通过对以上结果的归纳可以初步判断, 在LiTi2(PO4)3表面进行碳包覆可以减小颗粒尺寸、阻止电解液与LiTi2(PO4)3的直接接触, 从而抑制副反应, 最终提高产物的倍率性能和容量保持率。适当的碳层厚度可以改善电子电导率, 较低的碳含量对电子电导率的提升有限, 而当碳含量过高时则会增加Li+嵌入-脱出时的阻力, 不利于改善电性能。

表4   溶胶-凝胶法不同碳源和包覆方式的电性能比较

Table 4  Comparison of electrochemical performance of different carbon sources and coating methods by Sol-Gel

Calcination parameterCoating methodCarbon sourceWeight percentage of carbon/%Current density/(mA·g-1)Specific capacity (cycles)/(mAh·g-1)Capacity retention/%Ref.
800 ℃-12 hIn-situCitric acid6.2138106.1(1)-89(1300)84[36]
900 ℃-12 hEx-situToluene12700100(1)-83(200)83[31]
800 ℃-12 hEx-situAcetylene Black18140106.3(1)-86.5(100)81[81]
850 ℃-12 hEx-situAcetylene Black-140091.3(1)-74.4(100)81[82]
700 ℃-12 hIn-situPitch17.51380107(1)-75.5(1000)70[83]
550 ℃-24 hIn-situSucrose3.51400110(1)-104(800)94[17]
750 ℃-5 hIn-situPolyaniline5.9276115.2(1)-94.6(1000)82[84]
750 ℃-5 hIn-situPolyacrylonitrile5.969095(1)-82.1(1000)86[85]
900 ℃-12 hIn-situGraphene oxide1.79~1380110(1)-100(100)91[78]
800 ℃-10 hIn-situGraphene oxide-~276105(1)-97.86(100)93.2[77]
700 ℃-5 hIn-situGraphene oxide, phenolic resin16.2~690101.1(1)-78(1000)77.2[80]
800 ℃-8 hEx-situβ-Cyclodextrin3.13~690120(1)-(200)111.388.7[86]

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3.3 元素掺杂

元素掺杂, 即在晶格内部引入一种或多种元素。根据掺杂元素的种类, 可分为金属元素掺杂和非金属元素掺杂。金属元素掺杂占据晶格中的Li或Ti位。目前, 常见的金属掺杂元素主要有Sn、K、Na等。非金属元素占据LiTi2(PO4)3晶格中的阴离子位。见诸报道的非金属掺杂元素较少, 仅有卤族元素。元素掺杂对电性能的提升主要归因于杂元素降低了电荷转移阻抗Rct, 并拓宽了晶格中Li+的传输通道, 导致Li+扩散系数增大。

3.3.1 金属元素掺杂

Bounar[87]、Liu[24]和He等[88]分别在LiTi2(PO4)3晶格中掺杂Sn元素, 但由于合成方法和原料不同, 最佳掺杂量和产物电化学性能也有所差别。Bounar等[87]主要探究了Sn元素对晶体结构的影响, 认为在晶格中, Ti与Sn的最佳摩尔比为7 : 3, 相应的离子电导率为2.17×10-4 S·cm-1, 较掺杂前提升了两个数量级, 适量的掺杂仅引起晶胞参数的线性变化, 而空间群的种类并未发生改变。

Liu等[24]的合成产物兼具优良的倍率性能和循环寿命。他们采用静电纺丝法制备了LiTi1.8Sn0.2(PO4)3/C复合材料, 掺杂后该复合材料展现出非常优异的大电流充放电性能, 掺杂前后的电化学性能对比如图6所示。在20 mA·g-1电流密度下, 初始放电比容量为116.2 mAh·g-1。在电流密度为1000、2000、4000、6000 mA·g-1的测试条件下, 比容量分别为104、102、100、94 mAh·g-1。在600C充电倍率下, 比容量为 68.9 mAh·g-1, 这是目前为止文献记录的最高充电倍率。He等[88]以Sn为掺杂元素, 选择了能耗较低、工艺相对简单的溶胶-凝胶法, 获得了颗粒状材料LiTi1.7Sn0.3(PO4)3/C。其水系锂离子全电池LiTi1.7Sn0.3(PO4)3/C|Li2SO4|LiMn2O4展现出优异的循环寿命及倍率性能。在0.2C、6C和20C倍率下, 放电比容量分别为118.9、115.3、104.2 mAh·g-1。即使在10C倍率下, 1000次循环后, 其容量保持率仍可达76.8%。

图6

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图6   LiTi2(PO4)3/C和LiTi1.8Sn0.2(PO4)3/C在不同电流密度下连续循环的放电容量(a), 电流密度为4和6 A·g-1时的长期循环性能(b)[24]

Fig. 6   Discharge capacity for successive cycling at different current densities (a), long-term cycling behavior at current densities of 4 and 6 A·g-1 (b) of LiTi2(PO4)3/C and LiTi1.8Sn0.2(PO4)3/C[24]


Na与K为同族元素, 占据晶格中的Li位。Na和K元素对电化学性能的提升主要归因于其增大了晶格体积, 拓宽了Li+传输通道。Liu等[89]采用溶胶-凝胶法制备了Li0.97Na0.03Ti2(PO4)3/C复合材料。掺杂后, Li+的扩散系数由1.6×10-8 cm2·s-1提升至8.9×10-8 cm2·s-1。Jiang等[21]采用相同的制备方法合成了K元素掺杂的Li1-xKxTi2(PO4)3/C纳米复合材料, 当x=0.03时电化学性能最佳, 0.2C倍率下的初始放电比容量为104.4 mAh·g-1, 在6C倍率下, 1000次循环后, 容量保持率为78.8%。

3.3.2 非金属元素掺杂

目前报道的非金属元素掺杂仅有卤族元素, 它们占据LiTi2(PO4)3的阴离子位。Wang和Zhang 等[90,91]通过溶胶-凝胶法合成了F掺杂碳包覆的LiTi2(PO4)3-xFx电极材料, 其粒径为20~50 nm, 碳层厚度为5 nm, 最佳掺杂量为x=0.12。其理论计算结果表明, 掺杂后禁带宽度由2.504 eV降低至1.931 eV, 表明掺杂后材料的本征电子电导率得到了提高。其水系全电池LiTi2(PO4)0.88F0.12|2 mol·L-1 Li2SO4|LiMn2O4的功率密度和能量密度达到了2794.2和43.7 Wh·kg-1。受到Wang等研究的启发, Luo等[92]在晶格中掺杂了氯元素。掺杂后, 在5C倍率下, 1000次循环后, 其容量保持率由掺杂前的35.3%升至61.3%。Jiang等[93]对LiTi2(PO4)3进行溴掺杂和碳包覆, 得到的LiTi2(PO4)2.82Br0.18/C复合材料在15C倍率下的初始放电比容量达到86.3 mAh·g-1。

4 结束语

LiTi2(PO4)3具有合适的嵌锂电位和稳定的骨架结构, 是一种有发展前景的水系锂离子电池负极用活性材料, 其主要合成方法有高温固相法、共沉淀法、溶胶-凝胶法和水/溶剂热法等。高温固相法最适宜工业化, 但产物颗粒较大, 电化学性能较差。当前的固相法主要以NH4H2PO4为磷源, 高温时会产生NH3, 腐蚀设备并污染环境。因此, 亟待开发绿色的新型工艺, 以解决当前存在的问题。共沉淀法的产物粒径小, 电化学性能优异, 同时具有易于工业化的优势。但目前关于该工艺路线的探究较少, 且主要以Ti(C4H9O)4为钛源, 该原料极易水解变质, 增加了工业化成本, 因而亟待开发以稳定的无机钛盐为原料的新工艺。溶胶-凝胶法是实验室最常用的合成方法之一, 其煅烧温度低、时间短, 但工业化难度稍大。水/溶剂热法可以获得具有一定特殊形貌的颗粒, 产物的电化学性能通常较好。水体系电解液环境通常较为复杂, 为使LiTi2(PO4)3更好地适应水系电解液, LiTi2(PO4)3需要进行改性方能更明显地展现自身优势。目前, 主要的思路有: 碳包覆、元素掺杂、颗粒纳米化及形貌控制。在目前的碳包覆研究中, 研究人员多以有机物为碳源, 但有机试剂优势单一, 若能在体系中同时引入有机和无机碳源, 将会实现更优异的包覆效果, 大大提高电化学性能。元素掺杂可以提高材料的本征电子电导率和离子扩散系数, 其作用机理及对嵌锂行为的影响值得深入探究。这些改性方法各具特色, 若单独使用, 则改善效果有限, 因此, 彼此间需要进行有机结合。理论计算作为一种较先进的研究手段, 未来可为LiTi2(PO4)3的掺杂改性和动力学研究提供理论指导。其中, 密度泛函理论可以计算LiTi2(PO4)3晶面生长的表面热力学稳定性和电子结构、探索各晶面的生长速率、分析改性机理、预测最佳反应条件、优选掺杂元素等。但理论计算并不能完全替代实验, 应当进一步研究如何实现两者的有机结合。

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