储能科学与技术, 2021, 10(2): 462-469 doi: 10.19799/j.cnki.2095-4239.2020.0361

储能材料与器件

锂离子电池正极材料循环稳定性的基因规律

杨民安,, 陈宁,, 王博, 张乾, 陈敬沛, 赵海雷, 李福燊

北京科技大学材料科学与工程学院,北京 100083

Gene law about cycle stability of cathode material for lithium-ion batteries

YANG Min'an,, CHEN Ning,, WANG Bo, ZHANG Qian, CHEN Jingpei, ZHAO Hailei, LI Fushen

School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China

通讯作者: 陈宁,副教授,主要从事新能源材料研究,E-mail:nchen@sina.com

收稿日期: 2020-11-06   修回日期: 2020-11-29   网络出版日期: 2021-03-05

基金资助: 科技部重点专项计划项目.  2016YFB0700503-7

Received: 2020-11-06   Revised: 2020-11-29   Online: 2021-03-05

作者简介 About authors

杨民安(1997—),男,硕士研究生,主要从事新能源材料研究,E-mail:yangminan666@163.com; E-mail:yangminan666@163.com

摘要

锂离子电池正极材料需要有较大的能量密度和稳定的循环寿命,循环寿命与其脱锂前后的结构变化有直接关系。但是,影响循环寿命的原子层次因素是什么,还没有明确的答案。探究和优化正极材料的核心工作就是要寻找微观结构与性能之间的关系,这里不仅需要用到大数据统计,也需要对比分析脱锂前后的结构变化特征参数。通过分子动力学方法计算得到了18种典型正极体系的体积和弹性模量的变化,分析发现不同正极材料体系都对应了一个反映收缩能力的压强值,它主要由体积变化率与弹性模量的乘积决定,体现了不同材料脱锂后的稳定性差异。对于含Co/Ni/Mn/Fe等过渡族金属的正极材料,这个参数与循环稳定性呈一定的线性关系,收缩压强大的体系有更优秀的循环性能。同时,电子结构层次中的电荷密度也是影响锂离子电池循环性能的本征因素之一。本研究探索也表明,大数据配合理论计算是寻找材料规律的有效方法,得到的基本规律对于优化和改善循环寿命有一定的理论指导意义。

关键词: 锂离子电池 ; 正极材料 ; 循环寿命 ; 大数据 ; 分子动力学 ; 电荷密度

Abstract

The cathode material of lithium-ion batteries must have a large energy density and stable cycle life. Cycle life is directly related to structural changes before and after lithium loss. However, the atomic-level factor affecting the cycle life has not been clearly determined. The core work of exploring and optimizing cathode materials is finding the relationship between microstructures and performance, which requires big data statistics and comparing and analyzing the characteristic parameters of structural changes before and after lithium loss. The volume and elastic modulus of 18 typical positive materials were calculated using the molecular dynamics method. We found that different cathode materials corresponding to different pressures can reflect the shrinkage ability, mainly decided by the product of the volume change rate and elastic modulus. The pressure reflects the system's adaptability during contraction after lithium loss and the difference in the stability of different materials before and after lithium loss. For cathode materials containing transition metals such as Co, Ni, Mn, and Fe, this parameter has a certain linear relationship with the cycling stability; that is, the system with the higher pressure has the better cyclic performance. At the same time, the charge density in the electronic structure layer is also one of the intrinsic factors affecting the cycle performance of lithium-ion batteries. This research also shows that using big data combined with theoretical calculations is an effective method to find the laws of materials. The basic laws obtained have certain theoretical guiding significance for optimizing and improving the cycle life.

Keywords: lithium-ion battery ; cathode materials ; cycle life ; big data ; molecular dynamics ; charge density

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本文引用格式

杨民安, 陈宁, 王博, 张乾, 陈敬沛, 赵海雷, 李福燊. 锂离子电池正极材料循环稳定性的基因规律[J]. 储能科学与技术, 2021, 10(2): 462-469

YANG Min'an. Gene law about cycle stability of cathode material for lithium-ion batteries[J]. Energy Storage Science and Technology, 2021, 10(2): 462-469

锂离子电池作为新能源汽车产业链中的核心环节,具有非常广阔的应用前景。文献统计[1]表明,锂电池主要性能中,容量是大家最为关注的,但是,当提升容量时循环寿命往往会明显降低,很难满足高能量密度和较好的循环寿命的综合实际要求[2]。目前,锂离子电池容量有很多提升手段和方法,但循环寿命影响因素非常复杂,已经成为锂离子电池设计的难点和瓶颈之一。

影响锂离子电池循环寿命的因素是多方面的。其中外部因素包括电池的结构设计[3]、使用温度[4-5]、充放电深度[6]、充放电倍率[7]等,这些影响因素非常重要,并且通过实验方法可以得到明确的结论。内部影响因素也很复杂,锂离子电池的正负极材料、电解液、隔膜、集流体都会对循环寿命有一定的影响。例如不同电极材料微观结构的差别,正负极之间的匹配,副反应,SEI膜等[8-11]。这其中涉及充放电过程中材料复杂的多层次的微观结构变化问题,而且很难通过实验方法对其中的机理进行研究[12],特别是关于循环寿命的微观结构层次研究中,理论上虽然能够计算得到很多微观变化参数,但是究竟哪些变化因素影响着循环寿命,并没有明确答案。

锂离子电池的循环寿命与材料脱锂前后的稳定性是有关系的,材料越稳定,循环寿命越长[13]。稳定性与微观结构下某些结构参数的变化有关。因此,可以在原子和电子层次的材料因素中,寻找一种能够反映材料稳定性的参数。本文利用大数据并结合材料计算对正极材料循环寿命与材料本征参数之间的关系进行了系统的研究。从数据库中得到了正极材料循环寿命的性能参数,计算了18种典型的正极材料体系脱锂前后的体积和弹性模量。根据弹性理论的近似分析,讨论体系中脱锂前后压强变化与循环稳定性(容量保持率)之间的关系并得到了明确的规律。

1 研究方法

通过专业的锂离子电池材料数据库(https://www.mgedata.cn/search/#/149742/86)获得了关键材料的容量保持率的数据。此数据库是在覆盖范围广且收录专业资源数量较多的Web of Science的基础上建立的,数据内容涉及很多,包括成分、晶体结构、合成条件、测试条件及关键性质等。因此,数据的质量和数量均有一定的保证。该数据库包括6872条实验性的文章数据,包含关键材料的主要性能数据近18000条,测试条件数据量近7500条。利用这个数据库,筛选出18种正极材料以及同一测试条件下其循环性能信息。

同时,分子动力学可以对研究体系的晶体模型进行几何优化和力学性能的计算,最终得到了脱嵌前后稳定状态下结构变化的参数。已知锂离子电池的循环寿命与正极材料的体积变化有密切关系,体积变化时,弹性模量也会发生变化,但是,这两个参数不能直接反映不同体系循环稳定性差异的原因,这些参数变化产生的结果也不清楚。因此需要从体积和弹性模量来了解脱锂前后体系的收缩能力,利用它与容量保持率的关系,最终揭示电极材料的循环稳定性的本质。

2 研究结果

2.1 大数据统计结果

采用的锂离子电池数据库涉及大量的材料体系。为了方便研究,首先需要对数据库中的数据进行统计。图1是基于该锂离子电池数据库所做的材料分类以及正极材料研究体系的统计结果,它对锂离子电池相关材料的研究热度有了更加清晰的认识,并且能够更加准确地选择当前研究者们最为关注的正极材料体系。从图1(a)可以看出,锂离子电池数据库中,无机非金属材料比例最高,占到了60%。图1(a)右侧是将无机非金属材料按照正极材料、负极材料、电解液等进行分类的结果。锂离子电池的正极材料种类很多,为了更直接地选择要研究的对象,只讨论纯元素体系材料,将基于同一母体材料的体系进行合并,得到锂离子电池典型正极材料的分布,如图1(b)所示。

图1

图1   (a)锂离子电池数据库中的材料分类和无机非金属材料分类;(b)典型正极材料分类:LiMO2是三元材料以及多元材料的统称,是在LiCoO2LiNiO2LiMnO2的基础上掺杂得到的材料,LiM2O4与此类似;Li2MSiO4(M=Fe, Mn, Ni)LiMBO3(M=Fe, Mn, Co)由几个相似的材料组成

Fig. 1   (a) Material classification in lithium ion battery database and classification of inorganic non-metallic materials; (b) Classification of typical cathode materials: LiMO2 is a general term for ternary materials and multi-component materials. It is a material doped based on LiCoO2, LiNiO2 and LiMnO2. LiM2O4 is similar; Li2MSiO4(M=Fe, Mn, Ni) and LiMBO3(M=Fe, Mn, Co) are composed of several similar materials


此外,该数据库中的性能数据主要包括容量、容量保持率、库仑效率、能量密度、电导率、电压、锂离子扩散系数等。统计表明,人们最密切关注的性能是容量,占到了约45%,其次是容量保持率,占到了约27%。所以,对于锂离子电池材料来说,容量保持率是一个很重要的性能。

为了讨论正极材料相关性能,选取无机非金属材料中的18种最典型的正极材料作为研究对象,统计显示,这些材料能够代表整个正极材料的84.1%。同时,对应收集了循环性能数据。由于,循环性能数据受到许多宏观因素的影响,为了找到影响循环性能的材料参数,应该使用统一的筛选标准,消除外部和宏观因素对循环性能的影响。当然,同一正极材料在不同文献中的循环性能数据存在波动,因此选择这些数据的平均值作为最终数据。具体数据和筛选标准在表1中有详细展示。其中,常用体系如LiCoO2、LiFePO4、LiMn2O4等的统计参数与大家的共识基本一致,而同一类型体系(如LiMPO4)与相关文献研究结果也一致[14]

表1   18种正极材料的容量保持率、脱锂前后的晶胞体积,弹性模量,脱锂系数和收缩压强

Table 1  Capacity retention rate, cell volume and elastic modulus before and after lithium loss, coefficient of lithium loss and compaction pressure of 18 cathode materials

体系化学式容量保持率/%V13V23B1/GPaB2/GPa脱锂系数p/GPa参考文献
层状LiCoO290.64240.01180.04154.08205.620.514.96[15-17]
LiNiO282.85225.53166.25175.43238.470.517.43[18-21]
LiMnO279.14167.95134.50158.68198.640.513.32[22-23]
橄榄石LiFePO491.95366.38321.03300.3458.73117.72[24-27]
LiCoPO49.40253.87239.93103.89258.150.750.83[28-31]
LiNiPO430.00320.77317.45394.6196.360.759.88[32]
LiMnPO448.19217.56210.61296.86359.570.7513.11[33-34]
尖晶石LiMn2O478.78251.95238.13266.86255.570.7515.96[35-37]
硅酸盐Li2FeSiO481.16135.82126.3481.33196.250.56.88[38]
Li2MnSiO417.61367.10288.2355.70108.390.54.45[39-40]
Li2NiSiO486.76195.59159.8881.6867.120.58.34[38]
硼酸盐LiFeBO383.19363.76287.80168.1984.580.7513.60[41]
LiMnBO338.07341.00308.66177.15126.610.336.50[42]
LiCoBO348.98346.20312.96205.00137.440.255.29[43-44]
其他Li3V2(PO4)369.70322.00292.21188.04162.170.6672.24[45-48]
Li2FeP2O778.38785.47772.58162.50155.070.55.20[49]
LiVOPO493.19242.10228.20165.67127.690.752.58[50-53]
Li4V3O877.74327.57320.9448.11157.410.753.90[54-55]

注:(1)容量保持率筛选标准,正极材料为颗粒状微米级粉末;测试所用负极为锂金属;电解质为LiPF6/EC/DMC;活性物质、导电剂(乙炔黑或super P)、黏结剂(聚偏氟乙烯PVDF)溶于N-甲基吡咯烷酮(NMP)溶液中制成浆料,并组装成半电池;循环次数为50次;充放电倍率0.1C;温度25 ℃;(2)计算参数采用Matrials Studio中的Forcit模块,力场为Universal;(3)1 Å=10-10 m。

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2.2 分子动力学计算结果

正极材料的循环寿命和安全性与脱锂前后的变化有很大关系,研究角度不同,看到的规律也不同,现在的研究工作主要是在原子层次进行观察。

表1列出了这些体系的容量保持率以及理论计算得到的体积和弹性模量。从计算结果看,脱锂前体积较大,完全脱锂后体积减小,这是锂离子脱嵌空间变化引起的,不同体系体积变化存在差异。但是,弹性模量的变化方向有所不同,脱锂后有的增加,有的减少。

图2所示,如果不考虑脱锂前后弹性模量的变化,体积变化(∆V)就是真实的体积变化。但脱去锂离子之后,弹性模量发生改变,那么体积变化就需要修正。

图2

图2   计算的原理:脱锂前后的体积变化V造成的压强变化,但是弹性模量发生变化,体积变化需要进行修正V'

Fig. 2   Schematic of calculation: volume change (V) before and after lithium loss leads to pressure change, but elasticity modulus change, volume change need to be revised (V')


脱去锂离子之后的弹性模量由B1变为B2V1V2为相应的体积变化,根据弹性力学理论来推算,利用这两个变化因素,可以得到较准确的压强极限值(plimit)。plimit=(∆V+∆V')B1;其中∆V=(V1-V2)/V1为主要因素,∆V'=(V1-V2)/V2×(B1-B2)/(2B1)为修正因素。很明显,当B2=B1,不用修正;当B1>B2,实际的压强极限修正变大;当B1<B2,实际的压强极限修正减小。

plimit这个参数是为了讨论循环稳定性而引入的,其单位是压强,但它不是指材料所受到的外部压力,也不是材料本身产生的内部压力,而是反映了脱锂后不同材料能够提供的体积收缩能力,收缩能力由体积和弹性模量共同决定,此值越大表示体系体积收缩能力越大。但是,plimit理论值并不考虑脱锂的深度,所以,实际体系能够提供的能力还需要再乘以实际脱锂的比率后,才能近似得到。因此,按照每种材料常见的实际脱锂深度将压强极限值进行了修正,得到了实际脱锂情况下的收缩压强(p),计算结果见表1

图3所示,将得到的收缩压强和容量保持率进行分析,得到了两者之间的关系。可以看出,对于一些比较常用的过渡族金属元素的体系,循环性能与收缩压强有很好的关系,容量保持率高的体系具有较大的收缩压强,容量保持率低的体系具有较小的收缩压强。单独看不同的类型结构体系,这种变化更加清晰,如磷酸盐体系,Fe、Mn、Ni、Co收缩压强越小循环稳定性越差。所以,收缩压强的规律对于含有Co、Ni、Mn和Fe的体系都非常适合,因此,综合体积和体积模量得到的收缩压强,可以作为一个通过原子层次计算得到的材料宏观参数,反映锂离子电池脱锂前后结构的稳定性,这对优化和认识锂离子电池循环寿命的问题有重要的理论价值。但是,从图3中还可以看见,V偏差较大,其原因并不只是材料收缩压强,将在下面进一步讨论。

图3

图3   18种正极材料的容量保持率(%)与收缩压强(GPa)关系图,黑线代表了Co/Ni/Mn/Fe体系的关系趋势

Fig. 3   Relationship between capacity retention rate (%) and compaction pressure(GPa) of 18 cathode materials. The black line represents trend of relationship between Co/Ni/Mn/Fe system


3 讨 论

影响锂离子电池正极材料的循环寿命的影响因素非常复杂,不仅有外在的,也有本征的。本文从原子层次入手,分析脱锂前后体积和弹性模量的变化与循环寿命的关系,计算参数实际上是通过体系压强的变化特征表现出来的,并不都是能够适应压强变化大的循环稳定性好,通过分析其内部的“电荷密度”(相当于费米面或内部化学势变化,可以讨论电子流入或流出时系统的变化)可知,循环稳定性好的体系一般是脱锂后收缩能力大的,特别是对于Mn、Ni、Co过渡族金属的纯相层状体系,实际脱锂量并不是100%,每脱一个锂离子,相当于平均只失去半个电子,而体系本身可以通过较大的压力变化来适应,导致体系反映内部化学势的电荷密度不变,与脱锂前体系电荷密度基本一致,见图4(a)。反之,对于循环稳定性不好的体系,不能通过压强变化,缩小体积,体系则不能保持电荷密度基本不变,于是,体系的循环稳定性比前一种情况差,见图4(b)。但是,对于V体系,从图3中可以看出,它并不是有很大的脱锂收缩能力,而是利用变价提供足够的电荷补充,见图4(c)。

图4

图4   (a)体积变化不大,电荷密度不变;(b)体积变小,电荷密度不变;(c)体积变小,电荷密度降低

Fig. 4   (a) Little change in volume; The electron density does not change; (b) Reduction in volume; The electron density doesn't change. (c) Reduction in volume; Electron density reduction


经过上文的讨论可以看出,通过原子层次计算得到的参数对锂离子电池循环寿命有一定的影响,但是,这些变化参数是通过脱锂收缩压强体现出来的,而V体系规律实际上还可以从电子结构层次中反映出来,我们发现,电荷密度才是影响电极材料循环寿命的“基因”参数,不管是什么体系,只要保持电荷密度不变,这些体系的循环稳定性就非常好。总之,对于锂离子电池循环性能来说,影响因素复杂,但是,可以从原子和电子层次,利用不同的参数对影响电极材料循环稳定性的本质因素进行分析。研究电极材料的结构稳定性对性能的影响规律是一件必要且重要的事情,本文得到的原子和电子层次的规律,将有助于从根本上认识锂离子电池循环寿命的问题,但是,很显然,即使从本征因素角度看,材料的颗粒形貌和尺寸角度也会影响脱锂后的压强变化,因此也会对循环稳定性有一定影响,有限元的计算方法可能更适合这类问题的细节研究。

4 结论

利用大数据和理论计算的研究方法,本文对锂离子电池正极材料的循环性能与脱锂前后的体积变化和体积模量进行综合分析,确定了锂离子电池的循环寿命与电极材料的微观结构变化有密切关系,并发现循环寿命与电极材料脱锂前后的收缩压强因素有关,得到的规律表明,正极材料的体积和体积模量参数可以推算出收缩压强变化,它可作为一个影响锂离子电池循环寿命的基本参数,压强越大,循环稳定性越好。但是,从本质上讲,其原因来自电荷密度,这是电子结构层次影响锂离子电池循环性能的真正“基因”。

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