Engineering hierarchical pore network for Li-ion battery electrodes

doi:10.11949/0438-1157.20210813

Abstract

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

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

CLC Number:

TQ 028.8

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.

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.

Figures/Tables 12

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)

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

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

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)

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)

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

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)

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)

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)

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)

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

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

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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