Analysis of the Cycle Life Characteristics of Lithium-Ion Batteries according to their Anode

This work was investigated using various methods such as XRD, XPS and SEM after cycle life test. Also, variations of charging protocol have often been made to avoid mechanical stress conditions when Li plating is possible by reducing heat generation. In addition, superficial film construction was not even after cycling in a high rate charge and 1C discharge.

This non-uniform modification more closely followed the delamination of the active material from the current collector due to changes in the charging protocol and the density of the anode electrode. Scheme of Li deposition (a) at the microscopic level and (b) at the macroscopic level Figure 4. Sand's time, which means the time of dendrite initiation. Cross-sectional image of (a) pristine graphite, (b) graphite with good retention (3C), (c) graphite with poor retention (3C), and (d) graphite with 5C filling.

Surface image of pristine graphite, graphite with good retention (3C), graphite with poor retention (3C), and graphite with 5C loading.

Introduction

Lithium-ion batteries

Energy density = (cell voltage) x (specific capacitance) / (mass load) / (thickness). Cell voltage - Overvoltage) x (Specific capacity) / (Mass load) / (Thickness). As such, the increasing demand for fast charging is important in the EV market. Furthermore, energy density per volume is a crucial factor in the battery sector of the future, especially energy density per volume, an important metric for the electric vehicle market.

By the above Equation 1, the energy density required for an electric vehicle can be achieved by reducing the overvoltage and reducing the thickness while increasing the cell voltage. As rechargeable batteries for electric vehicles must have a short charging time, light weight and excellent performance, competition for technology development is expected to be fierce, with the focus on lithium-ion batteries.

Figure 2b demonstrates a theoretical scheme of charging time and charging rate as a function of charging power

Research about Lithium-Deposition

Hint of Li deposition
Sand’s time
Control the current density

This not only associates with the transfer of Li-ions in the anode openings [4,5], but also the transfer of electrons from the active materials to the current collector. Massive Li deposition Macroscopic, cellular level Fire during reaction with H2O Li reactivity depends on its. Among many hints of Li deposition, the first of them, Coulombic efficiency, was noted.

If the capacity is more charged than discharged, the difference in charge value affects the reaction separately, not the process of Li intercalation and de-intercalation [6,7]. The SEI layer affects the mobility of Li-ions to play a title role in the Age of Sand. As the mobility of Li-ion becomes larger and the effective current density (J) becomes smaller and smaller, the true time (τ) becomes larger.

Then, people should make efforts to reduce the local current density when the large bulk current flows to the Li deposition sites, which ensures the fast charging speed of cells [9]. Schematic representation of the effect of current density on the distribution of Li nuclei and Lithium nuclei deposited at different current densities [11,14]. As a result of the higher current density, the nuclei are spread further, the size of the nuclei is reduced and the shape of the nuclei changes into a dendritic structure.

Due to the lower current density, the nuclei grow and are deposited hemispherically and the deposition changes to a state of controlled charge transfer. 12,13] To suppress the formation of Li dendrites, an appropriate value of the current density must be present in the charge transfer controlled growth model. The nucleation of Li metal on Cu and the dependence of the range, shape and areal current density of lithium nuclei may interact with the current rate.

Table 1. Methods which give hints on Li deposition [8]

Research about fast charging

Among different charging protocols, multi-stage constant current (MCC) protocol and boost charging protocol were selected. This protocol is often decided by avoiding the mechanical stress environment when Li plating is done by dropping heat development.[16]. Higher current levels are generally chosen for the preliminary CC stage, since the anode potential grows less negative when charging occurs.

Boost charging features a CC-CV section with a higher average current at the start of charging and a more moderate current thereafter. The first boost charging step is simply a CC profile, a CV profile where the cell immediately reaches the set maximum voltage via a high initial current (CV-CC-CV), or a full CC-CV profile (CC-CV-CC - CV). In any case, the boost charging phase should allow a higher current or a higher maximum voltage compared to the following CC-CV section to reduce the total charging time[16].

Figure 7. Depiction of ordinary types of charging protocols proposed for fast charging

Experimental Method

Electrode and cell fabrication
Physical characterization
Chemical characterization
Electrochemical test

The anode and cathode before the cycle and the anode and cathode after the cycle were observed with a Scanning Electron Microscope (SEM,Nova NanoSEM,FEI). Before taking samples in the SEM holder, the full cell bag was disassembled in the dry room and the anode and cathode were washed with DMC solution to remove the residual salt. Charging and discharging of the rechargeable battery was performed with a cycler (PESC 05-0.1, PNE SOLUTION).

The voltage range is between 2.8V and 4.2V and the molding process was carried out with 0.1C charge and 0.1C discharge at 25 degrees. High-speed charging was performed by dividing single-stage charging and two-stage charging, starting from 3C charging to 5C charging. In the single-stage charging method, after 10 minutes rest in a cell discharged to 2.8 V, CC mode charging occurred up to 4.20 V at 3C or 5C and then continued CV mode so that the cut-off current drops to 1 / 20C.

In the dual-phase charging method, after 10 minutes of rest in the cell discharge to 2.8V, CC mode charge to 4.19V with 3C or 5C, and then continue with CV mode so that the cut-off current has dropped to 1/30C in and then CC mode recharges by 1/30 C to 4.20V as shown in figure 8.

Result and discussion

Electrochemical tests

Formation and standard process
Cycle life test on high rate charging
Compare charging time between single stage and double stage charging

In the forming process, the average charge capacity and discharge capacity of the anode samples whose electrode density is 1.5 g/cc were 16.63 mAh and 13.59 mAh, respectively, while the average charge capacity and capacity of the discharge of the anode sample whose electrode density is 1.7 g/cc was 14.56 mAh. and 12.35 mAh. The average ICE of the anode samples whose electrode density is 1.5 g/cc was 81% while that of the anode sample whose electrode density is 1.7 g/cc was 85%. The discharge capacity of the anode sample whose electrode density is 1.5 g/cc shows on average up to 1.16 mAh than that of 1.7 g/cc.

In the standard procedure, the average charge capacity and discharge capacity of the anode samples with an electrode density of 1.5 g/cc were separately 13.75 mAh and 13.08 mAh, while the average charge capacity and discharge capacity of the anode sample with an electrode density of 1, 7 g/cc 12.38. mAh and 11.87 mAh. The discharge capacity of the anode sample with an electrode density of 1.5 g/cc shows an average of 1.46 mAh rather than 1.7 g/cc. In the case of the 1.5 g/cc anode cell, the cell was discharged at 1 C and charged at 3 C in a double step.

In the case of a cell with a 1.7g/cc anode, the cell is discharged at 1C and charged at 3C in a double step. In the case of a cell with 1.5 g/cc anode and discharged at 1C charged 3C in a single step, the discharge retention was more than 89.2% in the 25th cycle and the value of coulombic efficiency was between 97.0 % and 98.2% after 5 cycles. In the case of a cell with a 1.5 g/cc anode, the cell is discharged at 1C and charged at 5C in a double step.

In the case of a cell with 1.7g/cc anode and discharged at 1C and charged 5C in double steps. In the case of a cell with a 1.5 g/cc anode, the cell is discharged at 1C and charged at 5C in a single step. The discharge retention was more than 72.1% in the 25th cycle, and the value of coulombic efficiency was between 96.0% and 97.0% after 10 cycles.

Figure 11 . Discharging capacity and retention of 3C charging 1C discharging Cycle life test

SEM/EDX/XRD/XPS analysis and wettability test

SEM and EDX analysis
Wettability test
XRD analysis
XPS analysis

Since the wettability of the electrolyte solution is high, the contact angle of the image taken at the moment it fell was measured. The lower the contact angle, the better the impregnation, and it turned out that the impregnation was slightly different depending on the density of the anode. The development of this spectrum is due to the deposition of a significant amount of LiF, which is a significant component of the outer part of the SEI layer.

It can be inferred that the LiF layer, an important element of the SEI layer, is maintained by the tendency of the LiF amount to be found more during the depth profiling process. The signal of the higher binding energy of 533.8 eV was assigned to organic carbonate (RCO3), and the peak of 532.1 eV was assigned to lithium carbonate, and the peak of 528.3 eV was assigned to lithium oxide. The binding energy point, 284.5 eV, is assigned to graphite, while the other components come from the first layer formed on the surface, mainly by rapidly charging the electrode.

The peak at 286.8 eV may be due to a carbon atom bonded to one oxygen in lithium alkyl carbonate (R–CH2–OCO2Li). Change of anode-electrode density occurs ranging from 1.5g/cc to 1.7g/cc of the whole NCM811/graphite lithium-ion battery on high-speed charging. Moreover, variations of charging protocol as the boost charging protocol where only a fraction of the battery capacity is charged with a high charging current still showed a large drop in imbalance due to the high current boosting interval.

On the other hand, the 1.7 g/cc graphite sample showed a Li-dendritic pattern taken in the charged state after 50 cycles. Therefore, the 1.5 g/cc graphite subjected to two-stage charging not only showed good cycle life characteristics, but also did not show lamination structure on the graphite surface. 34; Mathematical Modeling of the Lithium Deposition Overcharge Reaction in Lithium-Ion Batteries Using Carbon-Based Negative Electrodes.” Journal of The Electrochemical Society.

34;Effect of current density on nucleation and morphology of lithium during electrodeposition in ionic liquids." ECS Meeting Abstracts. 34;Surface film formation on a graphite electrode in Li-ion batteries: AFM and XPS study." Surface and Interface Analysis: An International Journal devoted to the development and application of techniques for the analysis of surfaces, interfaces and thin films.