锂离子电池电极中多级孔道结构设计

doi:10.11949/0438-1157.20210813

摘要/Abstract

摘要：

在较高充放电速率下，锂电池电极中Li⁺扩散受限严重，致使电池性能显著降低。为了减弱扩散限制，设计电极的孔结构是一种行之有效的方法。本工作以LiCoO₂正极为模型电极，利用建立的二维模型，优化了电极中含低曲折因子孔道的多级孔道结构。这些低曲折因子孔道可作为Li⁺传输的“高速公路”，优化其孔隙率和孔径，可大幅度提升高放电倍率下的能量密度。低曲折因子孔道的最佳孔隙率高度依赖于其孔径，其直径小于10 μm较优。当电极厚度为200 μm且总孔隙率为0.36时，优化后的多级孔电极相比于传统电极，能量密度可提高45.9%~91.4%。此外，当Li⁺的扩散限制较弱时（例如，电极厚度≤50 μm和总孔隙率≥0.48），优化电极的多级孔结构并不会显著提升电极性能。本工作可为锂电池电极中多级孔道结构的设计提供一定的指导。

关键词: 锂离子电池, 电极, 扩散限制, 多级孔道, 优化

Abstract:

At higher charging and discharging rates, Li⁺ diffusion in the electrodes of lithium batteries is severely restricted, resulting in a significant decrease in battery performance. To reduce the diffusion limitations, engineering pore structure of electrodes can be an efficient approach. In this work, a LiCoO₂ cathode is taken as a model electrode, the hierarchical pore network of the cathode with low-tortuosity pores is engineered with the aid of a two-dimensional model. The low-tortuosity pores act as "highways" for the transport of Li⁺, and their porosity and diameter are optimized to achieve the maximum energy density at high discharge rates. The optimal porosity of low-tortuosity pores is highly dependent on the diameter of low-tortuosity pores, and a diameter of low-tortuosity pores less than 10 μm is preferable. The preferable hierarchically structured electrode can be 45.9%—91.4% higher in energy density when the electrode thickness is 200 μm and the total porosity is 0.36. Besides, optimizing hierarchically structured electrodes is unnecessary when the diffusion limitation of Li⁺ is weak (e.g., electrode thickness ≤50 μm and total porosity ≥0.48). This work should provide some guidance to engineer the hierarchical pore network for high-performance Li-ion battery electrodes.

Key words: lithium-ion battery, electrode, diffusion limitation, hierarchical pore network, optimization

中图分类号:

TQ 028.8

汪宇, 张禹, 童微雯, 叶光华, 周兴贵, 袁渭康. 锂离子电池电极中多级孔道结构设计[J]. 化工学报, 2021, 72(12): 6340-6350.

Yu WANG, Yu ZHANG, Weiwen TONG, Guanghua YE, Xinggui ZHOU, Weikang YUAN. Engineering hierarchical pore network for Li-ion battery electrodes[J]. CIESC Journal, 2021, 72(12): 6340-6350.

图/表 12

图1 多级孔电极的结构示意图(a)；三维结构到二维重复单元的简化(b)；以钴酸锂多级孔电极为正极和锂金属为负极的半电池的二维结构示意图(c)

Fig.1 A schematic of the electrode with the hierarchical pore network (a); Simplification of a three-dimensional repeating unit into a two-dimensional one (b); A two-dimensional schematic of the half-cell with the hierarchically structured electrode as LiCoO2 the cathode and the lithium metal as the anode (c)

表1 本工作数值模拟中所用到的参数

Table 1 Parameters for the numerical simulations in this work

参数	单位	数值
初始电解质盐浓度 (c₀)	mol/m³	1200 ^①
最大嵌锂浓度 (c_s,max)	mol/m³	20434
初始嵌锂浓度 (c_s0)	mol/m³	9604
锂在电极材料内的扩散系数 (D_s)	m²/s	10^{-11 ①}
锂离子在电解液中的扩散系数 (D_l)	m²/s	2.6×10^{-10 ①}
电导率 (σ)	S/m	10 ^①
膈膜厚度 (h_s)	μm	20
集流体的总厚度 (h_c)	μm	40
阳极锂金属厚度 (h_a)	μm	50
钴酸锂材料颗粒半径 (R_s)	μm	4 ^①
阳极电荷转移系数 (α_a)	—	0.5 ^①
阴极电荷转移系数 (α_c)	—	0.5 ^①
添加剂所占孔隙率 (ε_add)	—	0.007
锂离子传递系数 ( $t + 0$ )	—	0.363 ^①
温度 (T)	K	298
Bruggeman系数 (m)	—	2.5
1C时电流密度 (I)	A/m²	38.2
钴酸锂阴极厚度 (h)	μm	50~150
钴酸锂阴极总孔隙率 (ε_t)	—	0.28~0.48
分层结构中孔的孔径 (d)	μm	10~90
分层孔道结构的孔隙率 (d²/D²)	—	0.04~0.25

表1 本工作数值模拟中所用到的参数

Table 1 Parameters for the numerical simulations in this work

参数	单位	数值
初始电解质盐浓度 (c₀)	mol/m³	1200 ^①
最大嵌锂浓度 (c_s,max)	mol/m³	20434
初始嵌锂浓度 (c_s0)	mol/m³	9604
锂在电极材料内的扩散系数 (D_s)	m²/s	10^{-11 ①}
锂离子在电解液中的扩散系数 (D_l)	m²/s	2.6×10^{-10 ①}
电导率 (σ)	S/m	10 ^①
膈膜厚度 (h_s)	μm	20
集流体的总厚度 (h_c)	μm	40
阳极锂金属厚度 (h_a)	μm	50
钴酸锂材料颗粒半径 (R_s)	μm	4 ^①
阳极电荷转移系数 (α_a)	—	0.5 ^①
阴极电荷转移系数 (α_c)	—	0.5 ^①
添加剂所占孔隙率 (ε_add)	—	0.007
锂离子传递系数 ( $t + 0$ )	—	0.363 ^①
温度 (T)	K	298
Bruggeman系数 (m)	—	2.5
1C时电流密度 (I)	A/m²	38.2
钴酸锂阴极厚度 (h)	μm	50~150
钴酸锂阴极总孔隙率 (ε_t)	—	0.28~0.48
分层结构中孔的孔径 (d)	μm	10~90
分层孔道结构的孔隙率 (d²/D²)	—	0.04~0.25

图2 普通电极(a)和多级孔电极(b)对应的半电池在0.5C、1C、2C和3C放电倍率下的放电特性曲线；由图2(a)、(b)计算得到的Ragone图(c)；当放电倍率为2C时，普通电极和多级孔电极在放电120 s时的Li+浓度分布图(d)（模型参数：电池厚度h=200 μm，总孔隙率εt=0.36，低曲折因子孔道的直径d=20 μm，低曲折因子孔道所占的孔隙率d2/D2=0.0625，其他参数见表1）

Fig.2 Time-dependent cell potentials under the discharge rates of 0.5C, 1C, 2C and 3C for a conventional electrode (a) and a hierarchically structured electrode (b); The Ragone plots obtained from Figs. 2(a) and 2(b) (c); Typical concentration profiles of Li+ at t=120 s under the discharge rate of 2C for the conventional electrode and the hierarchically structured electrode (d) (Simulation parameters: h=200 μm, εt=0.36, d=20 μm, and d2/D2=0.0625. The other parameters are given in Table 1)

图3 普通电极与具有不同低曲折因子孔道孔隙率（d2/D2=0.04、0.09、0.16和0.25）多级孔电极对应的Ragone图：(a) d = 30 μm； (b) d=60 μm；(c) d=90 μm(模型参数：h=200 μm，εt=0.36，其他参数列于表1)

Fig.3 Ragone plots for the conventional electrode and the hierarchically structured electrodes with d2/D2=0.04, 0.09, 0.16, and 0.25. (a) d=30 μm; (b) d=60 μm; (c) d=90 μm (Simulation parameters: h=200 μm, εt=0.36. The other parameters are given in Table 1)

表2 3C放电倍率下普通电极与具有不同低曲折因子孔道孔隙率多级孔电极对应的能量密度

Table 2 Energy densities at the 3C discharge rate for the conventional electrode and the hierarchically structured electrodes with different porosities

d²/D²	能量密度/(Wh/L)
d²/D²	0 μm	30 μm	60 μm	90 μm
0	220^①	—	—	—
0.04	—	328	310	295
0.09	—	370	343	308
0.16	—	408	377	321
0.25	—	421	363	234

图4 多级孔电极以2C的放电倍率下放电120 s时的浓度分布：(a) d=30 μm，d2/D2=0.04；(b) d=30 μm，d2/D2=0.09；(c) d=30 μm，d2/D2=0.16；(d) d=30 μm，d2/D2=0.25；(e) d=90 μm，d2/D2=0.04；(f) d=90 μm，d2/D2=0.09；(g) d=90 μm，d2/D2=0.16；(h) d=90 μm，d2/D2=0.25(模型参数：h=200 μm，εt=0.36，其他参数列于表1)

Fig.4 Concentration profiles of Li+ in the hierarchically structured electrodes at t=120 s under the discharge rate of 2C. (a) d=30 μm, d2/D2=0.04; (b) d=30 μm, d2/D2=0.09; (c) d=30 μm, d2/D2=0.16; (d) d=30 μm, d2/D2=0.25; (e) d=90 μm, d2/D2=0.04; (f) d=90 μm, d2/D2=0.09; (g) d=90 μm, d2/D2=0.16; (h) d=90 μm, d2/D2=0.25 (Simulation parameters: h=200 μm, εt=0.36. The other parameters are given in Table 1)

图5 普通电极与具有不同低曲折因子孔道直径（d=10、20和40 μm）多级孔电极对应的Ragone图：(a) d2/D2=0.04；(b) d2/D2=0.09；(c) d2/D2=0.16(模型参数：h=200 μm，εt=0.36，其他参数列于表1)

Fig.5 Ragone plots for the conventional electrode and the hierarchically structured electrodes with d=10, 20, and 40 μm. (a) d2/D2=0.04; (b) d2/D2=0.09; (c) d2/D2=0.16 (Simulation parameters: h=200 μm, εt=0.36. The other parameters are given in Table 1)

图6 普通电极与多级孔电极在不同的电极厚度下的Ragone图：(a) h=50 μm；(b) h=100 μm；(c) h=150 μm(模型参数：d=25 μm，d2/D2=0.0625，εt=0.36，其他参数列于表1)

Fig.6 Ragone plots for the conventional electrode and the hierarchically structured electrode under different electrode thicknesses. (a) h=50 μm; (b) h=100 μm; (c) h=150 μm (simulation parameters: d=25 μm, d2/D2=0.0625, εt=0.36. The other parameters are given in Table 1)

图7 普通电极与多级孔电极在不同电极总孔隙率下的Ragone图：(a) εt=0.28；(b) εt=0.38；(c) εt=0.48(模型参数：d=30 μm，D=100 μm，h=200 μm，其他参数列于表1)

Fig.7 Ragone plots for the conventional electrode and the hierarchically structured electrode under different total porosities. (a) εt=0.28; (b) εt=0.38; (c) εt=0.48 (simulation parameters: d=30 μm, D=100 μm, h=200 μm. The other parameters are given in Table 1)

表3 3C放电倍率下普通电极与具有不同低曲折因子孔道直径多级孔电极对应的能量密度

Table 3 Energy densities at the 3C discharge rate for the conventional electrode and the hierarchically structured electrodes with different pore diameters

d/μm	能量密度/(Wh/L)
d/μm	d²/D²=0	d²/D²=0.04	d²/D²=0.09	d²/D²=0.16
0	220^①	—	—	—
10	—	336	378	416
20	—	322	371	410
40	—	309	359	400

表4 3C放电倍率下普通电极与多级孔电极在不同电极厚度下的能量密度

Table 4 Energy densities at the 3C discharge rate for the conventional electrode and the hierarchically structured electrodes with different electrode thicknesses

电极	能量密度/(Wh/L)
电极	50 μm	100 μm	150 μm
传统电极	182	212	217
多级孔电极	187	296	337

表5 3C放电倍率下普通电极与多级孔电极在不同电极总孔隙率下的能量密度

Table 5 Energy densities at the 3C discharge rate for the conventional electrode and the hierarchically structured electrodes with different total porosities

电极	能量密度/(Wh/L)
电极	0.28	0.38	0.48
传统电极	252	239	295
多级孔电极	368	302	302

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