When an active material such as silicon, which has a large volume change, is used, electrode performance and cycle life are affected by the density of the electrode. Also, the difference in the transition points and causes of the degradation mechanism along the density was confirmed. Based on these analyses, we could propose the design strategy of graphite and silicon anode to maximize the performance of the two active materials at high density.
A pore distribution existed in the electrode with different densities measured by mercury porosimetry along the density. a) Average adhesion force for GrSi electrodes with different densities and optical images of electrode and strip surfaces after T-peel test with (b) d1.2 (c) d1.35 (d) d1.5 (e) d1.65. a) Electronic conductivity of the electrodes (b). Scaled voltage profile and (b) scaled dQ/dV curve of first lithiation to form SEI with reduction peak corresponding to EC decomposition potential at 0.8 V ( ) (c) with reduction peak at potential ~ 1.3 V ( ) corresponding to potential reduction of FEC Figure 13. a). Decreasing capacity per cycle along the density. a) Lithiation voltage profile and (b) delithiation voltage profile of Gr (black line), Si (pink line) and GrSi mixed electrodes (blue line).
Density in gcm-3, expansion factors and weight fraction of Gr, Si, carbon black, CMC and SBR are used to calculate the porosity of the electrode before/after active material expansion using E1 Table 2. The cycle areas ①, ② , and ③ respectively breakdown and the slope of the linear trend line.
Introduction
The direction of the anode development
Calendering effect on the Gr and Si
Experimental
Electrode and cell fabrication
Density in gcm-3, expansion factors and mass fraction of Gr, Si, soot, CMC and SBR used to calculate electrode porosity before/after active material expansion with E1. Electrode resistance measurement system (HIOKI, RM2610) measured electronic conductivity and interfacial resistance at room temperature. The T-peel test was performed using a universal testing machine (AGX-100NX, SHIMADZU) to measure the adhesion force.
As shown in Fig. 6, the electrode was cut to a width of 15 mm and a length of 50 mm, and the tape of 3M was cut to the same size as the electrode and attached to them.
Electrochemical characterizations
The test was performed at 50 mm/min. a) Schematic of T-peel test specimen and (b) T-peel test (b). a) Basic structure of dilatometer27 and (b) Detailed schematic of cell body27.
Results and Discussions
Physical properties along the density
- Changes in porosity
- Changes in adhesion force
- Changes in electronic conductivity
A pore distribution existed in the electrode with different densities measured by mercury porosimetry along the density. There are two types of bonding forces in electrodes, one is the cohesion between the particles that make up the electrode, and the other is the adhesion that acts between the composite and the current collector. If the anode active material contains silicon, delamination between the composite and the current collector is the main mode of failure due to the volume expansion of Si.29 Thus, the bond strength acting on the electrode is one of the key factors when using a GrSi electrode.
Therefore, to confirm the difference in bond strength along the density, the T-peel test was performed. Although the measured average forces were higher as the density increased (Fig. 10a), the electrode and tape must be observed after the test to determine where the failure mode occurs. In d1.2, complete delamination occurred between the composite and current collector, and in d1.35 its failure mode was weaker than d1.2.
This means that the cohesion and adhesion are stronger than the force acting between the sample and the tape. Namely, the failure mode is different depending on the pore size distribution, even if the binder content in the electrode was the same. The volume fraction of a relatively large pore is reduced, and the adhesion between the composite and the actual collector is improved.
In addition, reducing the volume fraction of small pores smaller than 1 µm increases the contact area between particles and increases cohesion. We measured the electronic conductivity in the composite and the interfacial resistance between the composite and the Cu foil. At d1.65, the electronic conductivity increased rapidly, indicating that it is about three times higher than d1.2.
From the previous section it can be seen that when the pore volume decreases, the contact area between the particles increases compared to the low density. However, there was little difference in interfacial resistance depending on the degree of compression. Electrod tape Electrod tape Electrod tape Electrod tape. a) Average adhesion force for the GrSi electrode with different densities and the optical images of the electrode and tape surfaces after the T-peel test with (b) d1.2 (c) d1.35 (d) d1.5 (e) d1.65. a) Electronic conductivity of the electrodes (b).
Electrochemical performances difference along the density
- Formation test
- Cycle test
Magnified voltage profile and (b) magnified dQ/dV plot of first lithiation for SEI formation with a reduction peak, which corresponds to EC decomposition potential at 0.8V ( ) (c) with a reduction peak at a potential of ~ 1.3V ( ) corresponds to the FEC reduction potential. The biggest difference was the specific capacitance in the first cycle along the density (Fig 13a). First, the specific capacity order was changed due to a rapid capacity reduction of d1.2 after the 45th cycle, even though it had the highest capacity of 727 mAhg-1 in the first cycle.
However, the CE of d1.65 converged to 99.5% the fastest than other densities, even when d1.65 had the largest capacity reduction. Halfway through the cycle there is a slight capacity drop and after 45 cycles another rapid capacity drop occurred at only d1.2. Through cycle testing, it was confirmed that there are different cycle performance trends along the density.
Capacity reduction per cycle was therefore analyzed to identify the transition point between degradation mechanisms during the cycle. Also, separation analyzes of Gr and Si were performed to investigate each active material behavior along the density. Specific capacity of 1st and 50th cycle and capacity retention at 50th cycle. means the highest value among the densities. a).
The specific capacity (b) CE (c) Capacity conservation with cycle number as x-axis (d) CE with 1/time as x-axis, the steepness of the slope indicates how fast the CE converged.
Analysis of degradation mechanism
- Capacity reduction per cycle
- Separation analysis (1): Gr and Si capacity distribution
- Separation analysis (2): Gr and Si R ct separation
- The cause of ① degradation mechanism: Dilatometer
- The cause of ② degradation mechanism: Change in Si content
The first is Gr and Si capacitance distribution and the second is charge transfer resistance (Rct) separation through EIS measurement. During lithiation, it is difficult to distinguish Gr and Si capacity when they are mixed because their lithiation voltages overlap. During delithiation, Gr below 0.4V reacts completely and Si starts a dealloying reaction above 0.4V. Therefore, it is possible to separate the capacity contribution of Gr and Si based on 0.4V during de-lithiation.
To identify the density effect on the Gr and Si in the mixed electrode, each active material data was compiled into one graph. It was therefore confirmed that the cycle stability of Gr and Si is opposite along the density. Another thing to note here is that at d1.2 capacity of Gr and Si varied after 40 cycles and Si capacity increased rapidly after 45 cycles.
The total capacity is GrSi (blue line), Gr capacity contribution measured below 0.4V (black line), and Si capacity contribution measured above 0.4V (pink line). a) Gr capacity contribution and (b) Gr capacity preservation. a) Si capacity contribution and (b) Si capacity preservation. First of all, to determine the possibility of Rct separation between Gr and Si in the GrSi electrode, Gr-only and Si-only electrodes were fabricated, respectively, and the EIS was measured every 0.1V from 1.0V to 0 .05V when charging (lithiation) and 0.1V to 1.0V when discharging (de-lithiation). As shown in fig. 22 and fig. 23, the evolution of the resistive behavior that occurs as the cycle progresses is characterized via ohmic resistance (Rohm), SEI resistance (RSEI) and charge transfer resistance (Rct).
This means that the reason for ① degradation is due to the effect of the density on Rct_Si. Thus, the reason for the degradation of ② is not due to the density effect, because the increasing degree of Rct_Si was the same regardless of density. However, the ② decay is not the density effect because the decay rate was almost identical regardless of the density.
And the cause of degradation ③ observed only in d1.2 is the overpotential caused by RSEI. In the previous section, it was confirmed that ① degradation was affected by electrode density. To understand the effect of density on Si and electrode, a dilatometer was performed.
The dilatometer was run at two extremes of the d1.2 and d1.65 variables, and after forming step 5, cycles were run at 0.1°C. Therefore, to reduce this phenomenon at high density, the apparent expansion of the electrode must be controlled.
Conclusion
Intermediate properties of a-Si∕ Cu: an active-inactive thin-film anode system for lithium-ion batteries. Production of high-energy lithium-ion batteries containing silicon-containing anodes and embedded cathodes. Chemometric investigation of the influence of process parameters on the performance of mixed Si/C electrodes.
Effect of electrode density on cycle performance and irreversible capacity loss of natural graphite anode in lithium-ion batteries. Acrylic acid-based copolymers as functional binder for silicon/graphite composite electrode in lithium-ion batteries. The study of the binder poly (acrylic acid) and its role in simultaneous solid-electrolyte interphase formation on Si anodes.
Advances in polymer binders for silicon-based anodes in high energy density lithium-ion batteries. Nanosilicon electrodes for lithium-ion batteries: interfacial mechanisms studied by hard and soft X-ray photoelectron spectroscopy. Improvement of charge/discharge efficiency by incorporation of conductive carbons into the carbon anode of Li-ion batteries.
A low-cost, high-performance ball-milling Si-based negative electrode for high-energy Li-ion batteries. In addition to my advisor, I would like to thank my thesis committee, Professor Yunseok Choi and Professor Dong-Hwa Seo, who provided helpful comments and encouragement. Also, I want to thank the members of ECheSL who have been through a lot with me for two years.
