The Effect of Electrode Thickness on the High-Current Discharge and Long-Term Cycle Performance of a Lithium-Ion Battery

: Six groups of electrodes with different thickness are prepared in the current study by using Li[Ni 1/3 Co 1/3 MN 1/3 ]O 2 as the active substance; the electrode thicknesses are 71.8, 65.4, 52.6, 39.3, 32.9, and 26.2 µ m, respectively, with similar internal microstructures. The effect of electrode thickness on the discharge rate, pulse discharge, internal resistance, and long-term cycle life of a pouch cell are investigated. The results show that, with the decrease in the electrode thickness from 71.8 µ m to 26.2 µ m, the high-current-discharge performance of the cell gradually improves, the pulse-discharge power density under 50% SOC increases from 1561 W/Kg to 2691 W/Kg, the R dis decreases from 8.70 m Ω to 3.34 m Ω , and the internal resistance decreases from 3.36 m Ω to 1.21 m Ω . In the long-term cycle-life test, the thinner the electrode thickness, the less the capacity fading of the cell; the internal resistance of the cell is observed with the increase in the cycle index.


Introduction
In recent years, with the development of intelligent transportation and the promotion of clean energy, the application of lithium-ion batteries in the field of new-energy vehicles and electrochemical energy storage has become a research hotspot for many scientists and engineers [1][2][3][4]. Lithium-ion batteries have excellent performance characteristics, such as high-energy density and high-power density, but different application scenarios have different requirements [5,6]. For example, a pure electric vehicle (BEV) requires the high-energy density of a lithium-ion battery, and a plug-in hybrid electric vehicle (PHEV) requires both high-energy and power densities. In addition, hybrid electric vehicles (HEVs) and the 48V micro-hybrid system (MHEV) require a high-current charge-discharge performance and the high-power density of lithium-ion batteries [7]. The demand for long-term energy-storage technology in the power grid requires high-energy density and a long service life for electrochemical energy-storage technology, such as lithium-ion batteries [8,9], while short-term high-frequency energy-storage technology requires a high-current chargedischarge performance and high-power density of a lithium-ion battery [10]. Therefore, it is important to develop appropriate energy-power performances for different application scenarios [11][12][13][14].
In order to further improve the performance of lithium-ion batteries, many researchers have conducted a lot of research on material preparation and modification [15][16][17][18][19][20][21][22], formula optimization [23,24], and electrode-structure design [25][26][27][28]. Zhang et al. [29] prepared Li[Ni 1/3 Co 1/3 MN 1/3 ]O 2 and LiFePO 4 electrodes with different thicknesses. The research results show that the thicker the electrode, the higher its energy density, but its capacity declines more rapidly and its power density is lower. Denis et al. [30] studied the electrode parameters of an LiFePO 4 battery, and the study showed that the influence of an increasing electrode thickness on a lithium-ion battery was mainly reflected in increasing the electrode impedance and decreasing the conduction rate of lithium ions in electrolytes. LiMn 2 O 4 cells with different electrode thicknesses were also assembled to optimize the active material load, thus designing high-performance batteries [31]. Thunmana et al. [32] studied the effect of the electrode thickness of an Li 4 Ti 5 O 12 /LiMn 2 O 4 battery on discharge performance, and the results show that when the discharge current density is increased, the discharge capacity related to the polarization in the electrolyte and electrode.
The examples of literature stated above show that the electrode structure parameters, such as electrode thickness and porosity, have important effects on the performance of a lithium-ion battery. In the current paper, the Li[Ni 1/3 Co 1/3 Mn 1/3 ]O 2 material is used to prepare a series of electrodes with the same porosity and different thicknesses. Meanwhile, a pouch cell is prepared. Combined with the high-current discharge and long-term cycle tests, the influence of electrode thickness on cell performance is analyzed. The research focuses on the rate performance of cells with electrode designs of different thicknesses and the cycle-life performance of cells with electrode designs of different thicknesses. It provides reference for the electrode-structure design of a lithium-ion battery with different performance requirements.

Materials
The cathode material used was SN1(Li[Ni 1/3 Co 1/3 Mn 1/3 ]O 2 , Soudon New Energy Technology Co., Ltd., Xiangtan, China); we used secondary particles, the average particle size was 8 µm, and the BET specific surface area was 0.28 m 2 /g. The composite conductive agents, Super P-Li and CNT, built a rich conductive network; the binder used was PVDF (polyvinylidene fluoride). The negative active material used was artificial graphite (A8-5, BTR New Material Group Co., Ltd., Shenzhen, China), the average particle size was 15 µm, BET was 1.8 m 2 /g, the binder was SBR (styrene butadiene rubber), and the dispersant was CMC (carboxymethyl cellulose). The particle morphology of the cathode and anode materials are presented in Figure 1. The separator was PE coated with Al 2 O 3 ; the thicknesses of the PE and ceramic coating were 16 and 4 µm, respectively. The electrolyte used was LiPF 6 (1.2 mol/L) in EC/DEC/EMC (volume ratio: 1:1:1).
In order to further improve the performance of lithium-ion batteries, many researchers have conducted a lot of research on material preparation and modification [15][16][17][18][19][20][21][22], formula optimization [23,24], and electrode-structure design [25][26][27][28]. Zhang et al. [29] prepared Li[Ni1/3Co1/3MN1/3]O2 and LiFePO4 electrodes with different thicknesses. The research results show that the thicker the electrode, the higher its energy density, but its capacity declines more rapidly and its power density is lower. Denis et al. [30] studied the electrode parameters of an LiFePO4 battery, and the study showed that the influence of an increasing electrode thickness on a lithium-ion battery was mainly reflected in increasing the electrode impedance and decreasing the conduction rate of lithium ions in electrolytes. LiMn2O4 cells with different electrode thicknesses were also assembled to optimize the active material load, thus designing high-performance batteries [31]. Thunmana et al. [32] studied the effect of the electrode thickness of an Li4Ti5O12/LiMn2O4 battery on discharge performance, and the results show that when the discharge current density is increased, the discharge capacity related to the polarization in the electrolyte and electrode.
The examples of literature stated above show that the electrode structure parameters, such as electrode thickness and porosity, have important effects on the performance of a lithium-ion battery. In the current paper, the Li[Ni1/3Co1/3Mn1/3]O2 material is used to prepare a series of electrodes with the same porosity and different thicknesses. Meanwhile, a pouch cell is prepared. Combined with the high-current discharge and long-term cycle tests, the influence of electrode thickness on cell performance is analyzed. The research focuses on the rate performance of cells with electrode designs of different thicknesses and the cycle-life performance of cells with electrode designs of different thicknesses. It provides reference for the electrode-structure design of a lithium-ion battery with different performance requirements.

Materials
The cathode material used was SN1(Li[Ni1/3Co1/3Mn1/3]O2, Soudon New Energy Technology Co., Ltd. Xiangtan, China); we used secondary particles, the average particle size was 8 μm, and the BET specific surface area was 0.28 m 2 /g. The composite conductive agents, Super P-Li and CNT, built a rich conductive network; the binder used was PVDF (polyvinylidene fluoride). The negative active material used was artificial graphite (A8-5, BTR New Material Group Co., Ltd. Shenzhen, China), the average particle size was 15 μm, BET was 1.8 m 2 /g, the binder was SBR (styrene butadiene rubber), and the dispersant was CMC (carboxymethyl cellulose). The particle morphology of the cathode and anode materials are presented in Figure 1. The separator was PE coated with Al2O3; the thicknesses of the PE and ceramic coating were 16 and 4 μm, respectively. The electrolyte used was LiPF6(1.2 mol/L) in EC/DEC/EMC (volume ratio: 1:1:1).

Preparation of Electrode
The cathode slurry was composed of 93 wt% active material SN1, 3 wt% PVDF binder dissolved in moderate NMP (N-methyl-2-pyrrolidone), and 4 wt% conductive agents (3% Super P-Li and 1%CNT). The slurry was spread onto cleaned Al foil using the transfercoating technique to prepare the cathode plate followed by drying. Analogously, the well-

Preparation of Electrode
The cathode slurry was composed of 93 wt% active material SN1, 3 wt% PVDF binder dissolved in moderate NMP (N-methyl-2-pyrrolidone), and 4 wt% conductive agents (3% Super P-Li and 1%CNT). The slurry was spread onto cleaned Al foil using the transfercoating technique to prepare the cathode plate followed by drying. Analogously, the wellproportioned anode slurry was manufactured with 94.8 wt% graphite, 2 wt% conductive agents, 1.2 wt% CMC, and 2 wt% SBR mixed in DI (deionized) water in the planetary mixer, which was spread onto cleaned Cu foil with the transfer-coating technique, and then dried. By adjusting the parameters of the coating machine, six groups of positive and negative electrode pieces with different coating weights were prepared. The roll squeezer was used to pressurize the cathode to increase the compaction density. Table 1 presents the true density of the material in the cathode. The electrode consisted of a current collector and a uniformly mixed active material, conductive agent, and binder. The true density and weight ratio of each material were known, and the actual thickness of the electrode was measured. The porosity of the positive electrode was calculated according to Formula (1), as presented in Table 2; the porosity and compaction density of the six groups with different thicknesses were almost the same [33][34][35].
ε is the porosity; ω is the amount of the material coated; ρ is the true density of the material, where AM represents the active material; B represents the binders (PVDF, SBR, and CMC); and CA represents the conductive agent.

Preparation of Cell
The electrodes were die-cut into small electrode pieces; the effective areas were 77 × 176 cm 2 and 79 × 179 cm 2 for the cathode and anode plates, respectively. The capacity-balancing factor of the cathode and anode was 1:1.18. The anode piece, separator, and cathode piece were assembled into an electrode group by a laminating machine. Aluminum and nickel sheets were ultrasonically soldered to the negative and positive poles and controlled at 60 • C for 48 h in a vacuum. The electrolyte of the laminated cells sealed under vacuum within a dry chamber was 40 g (dew point temperature ≤ −50 • C). The pouch cells were sustained overnight at ambient temperature before formation and testing. The capacity of the pouch cell was about 8.5-9.0 Ah; the length and width of the pouch cell were 190 mm and 88 mm, respectively. Table 3 summarizes the size and relevant electrochemical performance data of the pouch cell. A physical view of the pouch cell is presented in Figure 2.

Characterization
The microscopic features of the cathode material were investigated using scanning electron microscopy (SEM, 250 FEG, FEI). The electrochemical characteristics of the discharge rate, pulse discharge, internal resistance, and long-term cycle-life measurements of the cells were obtained by a BTS05200C8-HP (Shenzhen Sinexcel Electric Co.,Ltd. Shenzhen, China) cell test instrument under ambient temperature. Three charge-discharge cycles were performed under the current of 1 C as the initial activation. For the rate discharge/charge capability experiment, the cell was charged at a constant current of 1 C and then discharged under various currents varying from 1 C, 3 C, 5 C,7 C, 10 C, 15 C, and 20 C. In the long-term cycle-life test, for electrode thicknesses of 71.8, 65.4, and 52.6 μm, a 1 C constant current was used for the charge-discharge cycle in the 2.8-4.2 V voltage window. For the electrode thicknesses of 39.3, 32.9, and 26.2 μm, a 3 C constant current was used for the charge-discharge cycle in the 2.8-4.1 V voltage window, and tested the internal resistance of the cell every 500 cycles.
The power performance experiment was employed with the HPPC technique within a 2.8-4.2 V range for the pouch cell. The characterization tis presentation in Figure 3, and the calculation formulas for he discharge pulse-power capability and internal resistance of the battery are as follows:

Characterization
The microscopic features of the cathode material were investigated using scanning electron microscopy (SEM, 250 FEG, FEI). The electrochemical characteristics of the discharge rate, pulse discharge, internal resistance, and long-term cycle-life measurements of the cells were obtained by a BTS05200C8-HP (Shenzhen Sinexcel Electric Co., Ltd., Shenzhen, China) cell test instrument under ambient temperature. Three charge-discharge cycles were performed under the current of 1 C as the initial activation. For the rate discharge/charge capability experiment, the cell was charged at a constant current of 1 C and then discharged under various currents varying from 1 C, 3 C, 5 C, 7 C, 10 C, 15 C, and 20 C. In the long-term cycle-life test, for electrode thicknesses of 71.8, 65.4, and 52.6 µm, a 1 C constant current was used for the charge-discharge cycle in the 2.8-4.2 V voltage window. For the electrode thicknesses of 39.3, 32.9, and 26.2 µm, a 3 C constant current was used for the charge-discharge cycle in the 2.8-4.1 V voltage window, and tested the internal resistance of the cell every 500 cycles.
The power performance experiment was employed with the HPPC technique within a 2.8-4.2 V range for the pouch cell. The characterization tis presentation in Figure 3, and the calculation formulas for he discharge pulse-power capability and internal resistance of the battery are as follows: The internal resistance of the battery during discharge is expressed by R dis . The time at the beginning of discharge is represented by t 0 , and t 1 is represented by the time after 10 s of discharge. The discharge pulse-power capability is abbreviated as DPPC. V MIN in the formula represents the upper and lower limits of the cell voltage, and OCV is the open-circuit voltage corresponding to SOC before discharge.
The internal resistance of the battery during discharge is expressed by Rdis. The time at the beginning of discharge is represented by t0, and t1 is represented by the time after 10 s of discharge. The discharge pulse-power capability is abbreviated as DPPC. VMIN in the formula represents the upper and lower limits of the cell voltage, and OCV is the opencircuit voltage corresponding to SOC before discharge.

Energy Density
As presented in Table 3, the capacity of the cell is similar, and the thickness of the cell decreases with the increase in the electrode thickness. As presented in Figure 4, the energy density of the cell increases with the increase in electrode thickness, but the increased amplitude gradually decreases. The cells with different thicknesses have the same capacity and contain the same amount of active substance. The same active substance is evenly coated on the current collector. The smaller the unit-coating amount, the larger the corresponding coating area. The smaller the unit-coating amount, the smaller the electrode's thickness. The thinner the electrode, the more electrode layers there are in the cell, as presented in Table 3, and the more current-collector layers are needed. As a result, the volume (thickness) and weight of the cell increase, so both the volume and mass energy densities decrease.

Energy Density
As presented in Table 3, the capacity of the cell is similar, and the thickness of the cell decreases with the increase in the electrode thickness. As presented in Figure 4, the energy density of the cell increases with the increase in electrode thickness, but the increased amplitude gradually decreases. The cells with different thicknesses have the same capacity and contain the same amount of active substance. The same active substance is evenly coated on the current collector. The smaller the unit-coating amount, the larger the corresponding coating area. The smaller the unit-coating amount, the smaller the electrode's thickness. The thinner the electrode, the more electrode layers there are in the cell, as presented in Table 3, and the more current-collector layers are needed. As a result, the volume (thickness) and weight of the cell increase, so both the volume and mass energy densities decrease.
The internal resistance of the battery during discharge is expressed by Rdis. The time at the beginning of discharge is represented by t0, and t1 is represented by the time after 10 s of discharge. The discharge pulse-power capability is abbreviated as DPPC. VMIN in the formula represents the upper and lower limits of the cell voltage, and OCV is the opencircuit voltage corresponding to SOC before discharge.

Energy Density
As presented in Table 3, the capacity of the cell is similar, and the thickness of the cell decreases with the increase in the electrode thickness. As presented in Figure 4, the energy density of the cell increases with the increase in electrode thickness, but the increased amplitude gradually decreases. The cells with different thicknesses have the same capacity and contain the same amount of active substance. The same active substance is evenly coated on the current collector. The smaller the unit-coating amount, the larger the corresponding coating area. The smaller the unit-coating amount, the smaller the electrode's thickness. The thinner the electrode, the more electrode layers there are in the cell, as presented in Table 3, and the more current-collector layers are needed. As a result, the volume (thickness) and weight of the cell increase, so both the volume and mass energy densities decrease.   Figure 5 presents the capacities of the cells with different electrode thicknesses obtained from various discharge rates between 0.5 C and 30 C. The cell capacities at 0.5-5 C (except 71.8 µm) obeyed the Peukert law Q = i k × t, where Q is the capacity, k is the Peukert coefficient, and t is the nominal discharge time (in hours) for a specific C rate. The k values range from 1.020 to 1.005 for cells with different electrode thicknesses. Thinner electrodes have a K value close to 1, meaning the accessible capacity of the electrode is independent to the discharge rate [36,37]. Figure 5 presents the capacities of the cells with different electrode thicknesses obtained from various discharge rates between 0.5 C and 30 C. The cell capacities at 0.5-5 C (except 71.8 μm) obeyed the Peukert law Q = i k × t, where Q is the capacity, k is the Peukert coefficient, and t is the nominal discharge time (in hours) for a specific C rate. The k values range from 1.020 to 1.005 for cells with different electrode thicknesses. Thinner electrodes have a K value close to 1, meaning the accessible capacity of the electrode is independent to the discharge rate [36,37].   [29]. When the discharge ratio is small, the electrode active layer thickness is less than the characteristic diffusion length of lithium ions, and the discharge capacity of the cell is consistent with the structure of the electrode. With the increase in the discharge rate, the characteristic diffusion length of the lithium ion increases. When the characteristic diffusion length is equal to the electrode thickness, the electrode can release almost all of the structure design. The discharge rate increases further, the characteristic diffusion length is less than the electrode's thickness, and the discharge capacity of the cell is less than the design capacity. In Figure 6a, the thickness of the positive electrode of the cell is 26.2 μm and the discharge rate reaches 20 C, but almost all of the designed capacity can be released. In Figure 6b, the electrode thickness is 71.8 μm. When the discharge rate exceeds 3 C, part of the lithium ions cannot be completely released from the active particles, and the cell cannot release the designed capacity. It is worth mentioning that when the discharge rate is greater than a certain value, the discharge curve drops sharply, as presented in Figure 6b at 7 C discharge, which is caused by internal polarization of the cell. When there is a high concentration difference of lithium ions in the active substance particles during the discharge process, the solidstate diffusion of Li + is the rate-controlling factor; the electrode potential instantly decreases to the cut-off voltage. Similar conclusions exist within the literature [38,39].  Figure 6 presents the discharge curves of the cells prepared by electrodes with thicknesses of 71.8 µm and 26.2 µm at different discharge rates. Some researchers proposed the concept of the characteristic diffusion length of lithium ions [29]. When the discharge ratio is small, the electrode active layer thickness is less than the characteristic diffusion length of lithium ions, and the discharge capacity of the cell is consistent with the structure of the electrode. With the increase in the discharge rate, the characteristic diffusion length of the lithium ion increases. When the characteristic diffusion length is equal to the electrode thickness, the electrode can release almost all of the structure design. The discharge rate increases further, the characteristic diffusion length is less than the electrode's thickness, and the discharge capacity of the cell is less than the design capacity. In Figure 6a, the thickness of the positive electrode of the cell is 26.2 µm and the discharge rate reaches 20 C, but almost all of the designed capacity can be released. In Figure 6b, the electrode thickness is 71.8 µm. When the discharge rate exceeds 3 C, part of the lithium ions cannot be completely released from the active particles, and the cell cannot release the designed capacity. It is worth mentioning that when the discharge rate is greater than a certain value, the discharge curve drops sharply, as presented in Figure 6b at 7 C discharge, which is caused by internal polarization of the cell. When there is a high concentration difference of lithium ions in the active substance particles during the discharge process, the solid-state diffusion of Li + is the rate-controlling factor; the electrode potential instantly decreases to the cut-off voltage. Similar conclusions exist within the literature [38,39].

Discharge Performance (Continuous)
The transport of lithium ions in the electrode is related to the position of active substances in the electrode, electrode potential, diffusion length, and ion-diffusion coefficient, etc. The kinetics of any active point in the electrode is dependent on time and space [40][41][42]. Figure 7 presents the discharge curves of cells prepared with electrodes of different thicknesses discharged at a 5 C rate. The electrode thickness had a considerable influence on the discharge capacity of the cell. As the electrode thickness increased from 26.2 to 71.8 µm, the cell polarization (IR drop) increased obviously, and the initial discharge voltage changed from 4033 to 3880 mV. The platform of the discharge curve also decreased, which resulted from the increase in the polarization of the cell with the increase in the electrode's thickness. The discharge curves with electrode thicknesses of 65.4 and 71.8 µm presented a significant capacity loss, which was mainly caused by the diffusion of lithium ions in the electrode. At the same discharge rate, the electrode thickness increased and exceeded the characteristic diffusion length of the lithium ions, resulting in the capacity not being fully released. In the discharge process, the greater the electrode's thickness, the greater the lithium-ion's transmission resistance, leading to a decrease in the discharge capacity of the cell [29]. The transport of lithium ions in the electrode is related to the position of active substances in the electrode, electrode potential, diffusion length, and ion-diffusion coefficient, etc. The kinetics of any active point in the electrode is dependent on time and space [40][41][42]. Figure 7 presents the discharge curves of cells prepared with electrodes of different thicknesses discharged at a 5 C rate. The electrode thickness had a considerable influence on the discharge capacity of the cell. As the electrode thickness increased from 26.2 to 71.8 μm, the cell polarization (IR drop) increased obviously, and the initial discharge voltage changed from 4033 to 3880 mV. The platform of the discharge curve also decreased, which resulted from the increase in the polarization of the cell with the increase in the electrode's thickness. The discharge curves with electrode thicknesses of 65.4 and 71.8 μm presented a significant capacity loss, which was mainly caused by the diffusion of lithium ions in the electrode. At the same discharge rate, the electrode thickness increased and exceeded the characteristic diffusion length of the lithium ions, resulting in the capacity not being fully released. In the discharge process, the greater the electrode's thickness, the greater the lithium-ion's transmission resistance, leading to a decrease in the discharge capacity of the cell [29].

Discharge Performance (Pulse)
As we all know, the performance of lithium-ion batteries depends on the transport of electrons and ions through the electrodes, which is composed of a porous solid phase and an electrolyte that fills the pores [43]. The transport of electrons and ions is closely related to the electrode's microstructure. ZHENG H. et al. [36] divided the transport of electrons and ions into several parts, and the transmission of each part had corresponding resistance values: (1) Re: electronic resistance of the electrode; (2) Rs: transport of Li ions in the electrolyte to the active material's particle surface; (3) RSEI: diffusion of Li ions through the solid electrolyte interphase (SEI) film; (4) Rct: Li-ion charge transfers at the electrode/electrolyte interface; and (5) Rdiff: Li ion diffused within the bulk electrode.
From the perspective of polarization, scholars [44][45][46] decomposed the charging and discharging processes of lithium-ion batteries into inadequate contact between the materials in the electrodes, mass transport of species in the solid phase, and, in the electrolyte, the activation of electrochemical reactions.
According to the continuous-rate-discharge performance of the cell, 5 C was used for a 10 S pulse discharge of the cell with electrode thicknesses of 71.8, 65.4, and 52.6 μm, respectively, and 15 C was used for a 10 S pulse discharge of the cell with electrode thicknesses of 39.3, 32.9, and 26.2 μm, respectively, and to test the power and DC impedance of the cell. The voltage and current changes with time during the pulse-discharge test at

Discharge Performance (Pulse)
As we all know, the performance of lithium-ion batteries depends on the transport of electrons and ions through the electrodes, which is composed of a porous solid phase and an electrolyte that fills the pores [43]. The transport of electrons and ions is closely related to the electrode's microstructure. ZHENG H. et al. [36] divided the transport of electrons and ions into several parts, and the transmission of each part had corresponding resistance values: (1) Re: electronic resistance of the electrode; (2) Rs: transport of Li ions in the electrolyte to the active material's particle surface; (3) RSEI: diffusion of Li ions through the solid electrolyte interphase (SEI) film; (4) Rct: Li-ion charge transfers at the electrode/electrolyte interface; and (5) Rdiff: Li ion diffused within the bulk electrode.
From the perspective of polarization, scholars [44][45][46] decomposed the charging and discharging processes of lithium-ion batteries into inadequate contact between the materials in the electrodes, mass transport of species in the solid phase, and, in the electrolyte, the activation of electrochemical reactions.
According to the continuous-rate-discharge performance of the cell, 5 C was used for a 10 S pulse discharge of the cell with electrode thicknesses of 71.8, 65.4, and 52.6 µm, respectively, and 15 C was used for a 10 S pulse discharge of the cell with electrode thicknesses of 39.3, 32.9, and 26.2 µm, respectively, and to test the power and DC impedance of the cell. The voltage and current changes with time during the pulse-discharge test at 50% SOC are presented in Figure 7. The initial discharge voltage (OCV), instantaneous discharge voltage (V t0 ), and discharge terminal voltage (V t1 ) are presented in Table 4. During the initial discharge, the capacity of the cell was almost the same, and the content of active substances was the same, so the electrochemical reaction's overpotential can be considered unchanged at the same rate of discharge. The voltage sharply decreased due to the contact impedance and ohmic drop of lithium ions in the electrolyte. When the electrode formula, porosity, and microstructure were consistent, the electrode's thickness increased. This was equivalent to series resistance, resulting in the contact impedance and transmission distance of lithium ions in the increased electrolyte. The ohmic drop of lithium ions in the electrolyte also increased [42]. As the electrode thickness in the cell increased, V t0 decreased more dramatically. The same conclusion is presented in Figure 8a,b.  Table 1 that under the same discharge rate, Vt0-Vt1 increase with the increase in thickness [47].
It is worth mentioning that the reason why the electrode thickness of the cell presented in Figure 8b is small and the voltage change is greater than that presented in Figure  8a is that the pulse discharge rate increased three times.  The power density and DC internal resistance Rdis calculated according to Formulas (2) and (3) are presented in Table 5. The power density of the cell increased from 1813.2 Wh/Kg to 3204.3 Wh/Kg; DC internal resistance Rdis decreased from 7.50 mΩ to 3.34 mΩ as the thickness of the electrode decreased.  As presented in Figure 8, in the discharge process, on the surface of the active substance, Li ions in the electrolyte on the surface of positive active substance, close to the current collector, are more likely to be consumed without effective replenishment. Li ions in the electrolyte on the surface of the active substance, close to the membrane, can be rapidly supplied by the diffusion of Li ions from the counter electrode. This difference led to a concentration gradient of Li ions in the electrode, and with the increase in the electrode's thickness, the concentration gradient of the Li ions in the electrode also increased, resulting in a increased polarization. The final expression was the inhomogeneity of SOC distribution in the direction of the thickness inside the electrode. It can be observed from Table 1 that under the same discharge rate, V t0 -V t1 increase with the increase in thickness [47].
It is worth mentioning that the reason why the electrode thickness of the cell presented in Figure 8b is small and the voltage change is greater than that presented in Figure 8a is that the pulse discharge rate increased three times.
The power density and DC internal resistance R dis calculated according to Formulas (2) and (3) are presented in Table 5. The power density of the cell increased from 1813.2 Wh/Kg to 3204.3 Wh/Kg; DC internal resistance R dis decreased from 7.50 mΩ to 3.34 mΩ as the thickness of the electrode decreased.

Long-Term Cycle Performance
The long-term cycling behavior of the full pouch cell with different electrode thicknesses was tested under different conditions. The 1 C charge and 1C discharge rates for the cell with electrode thicknesses of 71.8, 65.4, and 52.6 µm, with voltage windows of 2.8 V to 4.2 V, and 3 C charge and 3 C discharge rates for the cell with electrode thicknesses of 39.3, 32.9, and 26.2 µm, with voltage windows of 2.8 to 4.1 V were presented. Figure 9 presents the capacity fading of the cell during the long-term cycle. Following the long-term cycles, the capacity retention values of the cell, with electrode thicknesses of 71.8, 65.4, and 52.6 µm, were 85, 90, and 92%, respectively. The capacity retention values of the cell, with electrode thicknesses of 39.3, 32.9, and 26.2 µm, were 92, 93, and 94%, respectively. The cycle performance of the cell decreased with the increase in the electrode's thickness.

Long-Term Cycle Performance
The long-term cycling behavior of the full pouch cell with different electrode thicknesses was tested under different conditions. The 1 C charge and 1C discharge rates for the cell with electrode thicknesses of 71.8, 65.4, and 52.6 μm, with voltage windows of 2.8 V to 4.2 V, and 3 C charge and 3 C discharge rates for the cell with electrode thicknesses of 39.3, 32.9, and 26.2 μm, with voltage windows of 2.8 to 4.1 V were presented. Figure 9 presents the capacity fading of the cell during the long-term cycle. Following the longterm cycles, the capacity retention values of the cell, with electrode thicknesses of 71.8, 65.4, and 52.6 μm, were 85, 90, and 92%, respectively. The capacity retention values of the cell, with electrode thicknesses of 39.3, 32.9, and 26.2 μm, were 92, 93, and 94%, respectively. The cycle performance of the cell decreased with the increase in the electrode's thickness. The internal resistance of the cell was measured every 500 cycles. As shown in Figure  10, the internal resistance of the cell is a function of the cycle number. The internal resistance of the cell with difference electrode thicknesses displayed a similar trend, increas- The internal resistance of the cell was measured every 500 cycles. As shown in Figure 10, the internal resistance of the cell is a function of the cycle number. The internal resistance of the cell with difference electrode thicknesses displayed a similar trend, increasing with the number of cycles. For the active substance in the electrode undergoing a volume expansion and contraction during the cycle, the volume change brought about a connection between the active layer and the current collector, and the particles in between the active layer became worse, and the contact impedance increased during the cycle [48]. After repeated charging and discharging, an inactive layer appeared on the surface of the active substance particles, resulting in a loss of capacity and an increase in the diffusion polarization of Li ion in the solid phase [49]. The CEI layer was generated on the surface of the active substance, which blocked the pores in the electrode plate, hindered the transport of Li ions, and increased the transport impedance of Li ions [29].
The SOC in the thickness direction of the electrode was not uniform, which was due to the diffusion polarization of the Li ion in the charging and discharging processes, and the non-uniformity expanded with the increase in the electrode's thickness. Therefore, the increase in the electrode's thickness led to a further deterioration of the above-mentioned situation, resulting in impedance increasing and the deterioration of the cycle performance of the cell [50].

Conclusions
The pouch cells with electrode thicknesses of 71.8, 65.4, 52.6, 39.3, 32.9, and 26.2 μm, with the same microstructure, were prepared and investigated in the current study. The electrode thickness had a significant effect on energy density, rate performance (continuous and pulse), and long-term cycle performance of the cell. The energy density of the cell was improved by increasing the electrode's thickness. With the increase in the electrode's thickness, the decrease in the rate capacity and voltage mainly occurred due to the diffusion of the lithium ions inside the electrode. The long-term cycling performance of the cell presented a higher capacity-fading rate for thicker electrodes. High-capacity fading is ascribed to the mechanical issue of the electrode and severe polarization at larger thickness,. considering that all of the above factors, the rate performance (continuous and pulse), and the cycle-life performance of the cell had a strong correlation with the thickness of the For the active substance in the electrode undergoing a volume expansion and contraction during the cycle, the volume change brought about a connection between the active layer and the current collector, and the particles in between the active layer became worse, and the contact impedance increased during the cycle [48]. After repeated charging and discharging, an inactive layer appeared on the surface of the active substance particles, resulting in a loss of capacity and an increase in the diffusion polarization of Li ion in the solid phase [49]. The CEI layer was generated on the surface of the active substance, which blocked the pores in the electrode plate, hindered the transport of Li ions, and increased the transport impedance of Li ions [29].
The SOC in the thickness direction of the electrode was not uniform, which was due to the diffusion polarization of the Li ion in the charging and discharging processes, and the non-uniformity expanded with the increase in the electrode's thickness. Therefore, the increase in the electrode's thickness led to a further deterioration of the above-mentioned situation, resulting in impedance increasing and the deterioration of the cycle performance of the cell [50].

Conclusions
The pouch cells with electrode thicknesses of 71.8, 65.4, 52.6, 39.3, 32.9, and 26.2 µm, with the same microstructure, were prepared and investigated in the current study. The electrode thickness had a significant effect on energy density, rate performance (continuous and pulse), and long-term cycle performance of the cell. The energy density of the cell was improved by increasing the electrode's thickness. With the increase in the electrode's thickness, the decrease in the rate capacity and voltage mainly occurred due to the diffusion of the lithium ions inside the electrode. The long-term cycling performance of the cell presented a higher capacity-fading rate for thicker electrodes. High-capacity fading is ascribed to the mechanical issue of the electrode and severe polarization at larger thickness, considering that all of the above factors, the rate performance (continuous and pulse), and the cycle-life performance of the cell had a strong correlation with the thickness of the electrode. The electrode structure design of the lithium-ion battery was critical for the different performance requirements, such as for batteries used for HEV, PHEV and EV.