Molecular dynamics simulations of short-chain lithium polysulfides clustering in ionic liquids

doi:10.12034/j.issn.1009-606X.220221

Abstract

Abstract:

Ionic liquids have been widely used in lithium-sulfur battery electrolytes in recent years due to their excellent physicochemical properties and the ability to inhibit the dissolution of lithium polysulfides intermediates. Among those products during the battery cycling processes, insoluble Li₂S and Li₂S₂ are inclined to aggregate and deposit on the electrode surface, affecting the battery performance. However, there are few studies on the microscopic mechanism of their clustering behaviors and electrolyte properties. In this work, the microstructure of Li₂S/Li₂S₂ in ionic liquids and the formation of clusters were studied by DFT calculations and molecular dynamics simulations. From the optimized configurations using DFT methods, it can be seen that ionic liquids and Li₂S/Li₂S₂ always tended to form a "cation-short chain polysulfide-anion" sandwich-like structures. By analyzing the microstructures of the molecular dynamics simulation systems, it can be found that the methyl group in side chain of cation mainly interacted with S in Li₂S/Li₂S₂, and the Li-S interaction between short-chain polysulfides was much stronger than Li-O interaction in anions. The results of cluster size distribution showed that short-chain polysulfides were more likely to form large clusters in the [TFSI]-based ionic liquid, while the proportion of large clusters in Li₂S₂ system was higher than Li₂S systems. Moreover, the tendency of forming large clusters increased with the concentration of Li₂S/Li₂S₂. Additionally, stronger coordination ability of anions brought smaller proportion of large Li₂S clusters. However, the configuration characteristics and interaction forms of anions-Li₂S will also affected the sizes and structures of clusters. These understandings could provide theoretical guidance for future systematic studies on screening and designing ionic liquids electrolytes for lithium-sulfur batteries.

Key words: lithium-sulfur batteries, ionic liquids, short-chain lithium polysulfides, clusters, molecular dynamics simulations

摘要：

离子液体因其优异的物化性质、能抑制多硫化物溶解等特点，近年来被广泛应用于锂硫电池电解液中。在电池充放电产物中，难溶性Li₂S和Li₂S₂易聚集沉积在电极表面，影响电池性能，而目前关于其团聚行为与电解液性质的微观机理研究较少。本工作利用量化计算和分子动力学模拟分析了短链Li₂S和Li₂S₂在离子液体中的微观结构以及形成团簇的情况。通过分析体系的微观结构发现，阳离子中主要与S作用的是侧链甲基，短链多硫化物之间Li-S作用远强于与阴离子的Li-O作用。团簇尺寸分布的结果表明，短链多硫化物在[TFSI]型离子液体中易形成多分子的大团簇，Li₂S₂体系比Li₂S体系中的大团簇比例更高；离子液体阴离子配位能力越强，形成大的Li₂S团簇比例越少，但阴离子的构型特点和作用形式也会对团簇的尺寸结构造成影响。

关键词: 锂硫电池, 离子液体, 短链多硫化物, 团簇, 分子动力学模拟

CLC Number:

TQ460.6+4

Tianyuan HU, Yanlei WANG, Feng HUO, Hongyan HE. Molecular dynamics simulations of short-chain lithium polysulfides clustering in ionic liquids[J]. The Chinese Journal of Process Engineering, 2021, 21(7): 847-856.

胡天媛, 王艳磊, 霍锋, 何宏艳. 短链多硫化物在离子液体中聚集行为的分子动力学模拟[J]. 过程工程学报, 2021, 21(7): 847-856.

Figures/Tables 12

Fig.1 Ions and molecules investigated in this work

Table 1 Number of molecules in the simulation systems

System	Number of ILs	Concentration of Li₂S/Li₂S₂/(mol/L)
System	Number of ILs	0.2	0.4	0.6	0.8	1.0
[PP13][TFSI]-Li₂S₂	800	49	98	147	196	245
[PP13][OTf]-Li₂S₂	800	37	75	112	149	186
[PP13][TFSI]-Li₂S	800	49	98	147	196	245
[PP13][OTf]-Li₂S	800	37	75	112	149	186
[PP13][PF₆]-Li₂S	800					184
[PP13][DCA]-Li₂S	800					163
[PP13]Br-Li₂S	800					176

Table 2 Densities and self-diffusion coefficients of ILs at 298 K from MD simulations

RILs	Density/(g/cm³)	D₊/×10^-11 m²/s	D_-/×10^-11 m²/s
[PP13][TFSI]	1.38	1.843±0.019	1.339±0.013
[PP13][OTf]	1.25	1.177±0.012	0.734±0.010

Table 3 Angle parameters of Li2S and Li2S2 in this work ([a]for Li2S, [b] for Li2S2)

Angle	θ₀/°	K_θ /[kcal/(mol·rad²)]
Li-S-Li^[a]	138.00	55.00
S-Li-S^[b]	60.00	218.00
Li-S-Li^[b]	98.00	55.00

Table 4 Force field simulations for LJ parameters (compared with experimental densities of Li2S and S8)

Simulation	σ_Li/×10^-1 nm	σ_S/×10^-1 nm	ε_Li/(kcal/mol)	ε_S/(kcal/mol)	ρ_Li2S/(g/cm³)	ρ_S8/(g/cm³)
System-1^[20]	2.8700	3.5500	0.0005	0.2500	1.83	1.71
System-2^[27]	2.0261	3.5640	0.0183	0.2504	1.91	1.70
System-3^[28]	2.1300	3.5500	0.0765	0.2500	1.73	1.71
System-4^[20,29]	3.6000	3.3032	0.0005	0.2500	1.71	1.90
System-5^[20,29,30]	3.6000	3.3900	0.0005	0.4070	1.63	1.98
System-6^[27,29,30]	3.6000	3.3900	0.0183	0.4070	1.11	1.98

Fig.2 Interaction energy between ions/ion pairs and short-chain lithium polysulfides

Fig.3 Structures of ion pairs and Li2S/Li2S2 optimized at B3LYP/6-311+g(d,p) level (unit: 0.1 nm)

Fig.4 The site-site RDFs in ILs and short-chain polysulfides systems

Fig.5 SDFs of S around central cations (up panels) and Li around central anions (bottom panels) in ILs and short-chain polysulfides system

Fig.6 Cluster size distributions in different concentrations of Li2S/Li2S2

Fig.7 Partial properties of Li2S and ILs containing anions with different coordination ability

Table 5 The mean size of Li2S clusters in five ILs containing anions with different coordination ability

Anion	[PF₆]^-	[TFSI]^-	[OTf]^-	[DCA]^-	Br^-
$N ¯$	2.91	2.91	2.41	2.10	2.65

Table 5 The mean size of Li2S clusters in five ILs containing anions with different coordination ability

Anion	[PF₆]^-	[TFSI]^-	[OTf]^-	[DCA]^-	Br^-
$N ¯$	2.91	2.91	2.41	2.10	2.65

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