In following chapters 2 and 3, rational strategies for for 650 mAh/g and 1200 mAh/g Si-dominant anode will be introduced. Recent progress in investigating phase transformation on Si dominant anode (A) Lithiation extent in Si and phase transformation of Si during lithiation (a) Chemical element STEM-EELS mapping of Si anode after 10th lithiation. Representatives of the void space engineering strategy. a-h: silicon nanotubes and i-k: . yolk-shell structures). a) SEM images of Si nanotubes.
Introduction of anode materials for lithium ion batteries
Demands of energy storages
Principle of lithium ion batteries
The theoretical capacity of Si is 3579 mAh/g, while the theoretical capacity of graphite is 372 mAh/g. Li-ion diffusion coefficient of graphite: 26 Li-ion diffusion coefficient of silicon: 27 Surface capacity of graphite: 28. Surface capacity of silicon: 28 Surface capacity of Li metal: 29 Volumetric capacity of graphite: 28 Volumetric capacity of silicon: 28 Volumetric capacity of graphite: 30.
Which will be the next generation of anode material, Si vs. Li metal?
Therefore, the Li metal anode theoretically has the possibility of assembling lighter batteries compared to Si anode batteries. Therefore, the Si anode theoretically has the potential to build denser batteries compared to Li metal anode batteries. I have calculated the theoretical capacities assuming no deterioration of the Si and Li metal anode.
Critical phenomenon and analytical methods for Si anode
Massive volume changes and SEI formation are the origin of the fading mechanism at the Si dominant anode. The newly exposed surface causes fresh SEI formation and repeated cycling creates growth of the SEI layer (Figure 7A). Consequently, there are several methods for detecting the mechanical properties related to the volume change of the Si-based anode.
Previous Strategies for Si anode
Representatives of the surface coating strategy (a–d: silicon carbide-free graphene coating and e–h: graphene cage coating). a) TEM image of graphene-coated Si NP (Gr-Si). The line profiles from the two red fields indicate that the interlayer spacing between the graphene layers is ~3.4 Å. b) magnified TEM image of graphene layers visualizing the origin (red arrows) from which individual they grow. A schematic illustration showing the sliding process of the graphene coating layers that can buffer the volume expansion of Si. d) Half-cell cycle performance of Gr-Si depending on the weight ratio of graphene. e) SEM image of a graphene-encapsulated Si microparticle (SiMP@Gr). f) TEM image of an individual particle of SiMP@Gr. g) High-resolution TEM image of the layered structure of the graphene cage.
The yellow dotted curves indicate the boundaries between graphite, amorphous Si and carbon. e) Stress profiles of pristine graphite (PG), graphite embedded in Si nanolayers (SG), SGC and physically mixed nano-Si/graphite (B-Si/G) measured at 0.1 C. g) Coulombic efficiency of the samples depending on cycle number. j) Cross-sectional SEM image of SEAG. k) STEM images of an enlarged surface of SEAG with EDS mapping analysis. l) HR-TEM images at the interfacial region of SEAG. m) Voltage profiles of SEAG, SEAG with Ni-silicide and graphite in the first cycle (n) Half-cell cycle performance of SEAG, SEAG with Ni-silicide and graphite. o) Assess the performance of SEAG, compared to SEAG with Ni-silicide and graphite. p) Voltage profiles during the charging process of SEAG and graphite, measured at increasing current densities; the.
Scope and organization of this dissertation
C/Si nanolayer/macroporous C trilayer covered graphite: Silicon-graphite
Introduction
However, these Si–graphite composites suffer from limiting the Si content below about 6 wt%, otherwise excessive deposition of Si causes thickened Si nanolayers on graphite, leading to accelerating performance degradation with dampening of the benefits of nano-sizing. Herein, we introduce unique design of Si nanolayer-embedded macroporous carbon architecture on graphite for advanced Si-graphite composite anode using a simple SiO2 template and CVD methods. This design enables much Si content above 6 wt% in the Si-graphite composite anode without significant degradation.
Experimental detail
Results and discussion
The designed characteristics of final sample; C/Si/macroporous C trilayer coated graphite (CSMG) is summarized in figure 16b. The CSMG structure has three distinctive features; 1) MCL, 2) Ultrathin Si layer, and 3) outer C layer. They clearly showed components of CSMG; core graphite, Si nanolayer embedded MCL (SMCL), and outer C was wrapped and overlapped with SMCL structure. The curvature surface of MG and smoothed surface of CSMG were clearly observed via SEM images (figure 16e and f) and there was no significant change of particle distribution between pristine graphite and CSMG (figure 16g).
To understand the relationship between the thickness of the deposited Si layer and the surface area, we compared the calculated Si thickness ratio of CSMG/CSG based on the BET surface areas of the target Si coating materials (MG and graphite) and the measured thickness ratio How to CSMG/CSG (figure 22f) (Detailed calculation method is in additional information). The calculated and measured Si thickness ratios (internal and surface) of CSMG/CSG were 0.469 (calculated), 0.447 (internal measured) and 0.5 (surface measured). -ADF images of (d) pristine CSMG, (e) lithiated CSMG, (g) pristine CSG, and (h) lithiated CSG. a) First cycle voltage profiles of CSMG and CSG.
The lower volume expansion of the CSMG electrode was due to the ultrathin Si layer, void space, and buffer matrix induced by the MCL. As a result, we compared the IR drops and overpotentials of CSMG and CSG in the delithiation process and separated the lithium graphite and Si delithiations with voltage ranges of 0.01–0.3 V for lithiated graphite and 0.3–0.5 V for lithiated Si (Figures 25c and 27). The shape of the stress profiles of CSMG was almost maintained from cycle 1 to 50, but the stress plateaus of CSG rose and gradually disappeared as the cycle progressed. a) Voltage and current profiles of the CSMG during the GITT test.
In contrast, there was no significant change in magnified cross-sectional image of CSMG electrode.
Conclusion
There was a large crack between the particles in the CSG electrode, however there was no such crack in the CSMG electrode after the 50th cycle (figure 25e and h). In the enlarged cross-sectional images of the CSG electrode after the 50th cycle, the expanded Si layer detached from the graphite and the graphite plates were released from the tightly wrapped spherical state, causing large cracks between the plates.
C/Si nanolayer/C micro cage: Silicon-carbon composite for > 1000 mAh/g anode
Introduction
Experimental detail
The electrodes were dried at 80℃ for 1 hour and they were vacuum dried at 110℃ for 6 hours immediately before cell assembly. The composition of the LCO electrode is LCO, carbon black (Super P, TIMCAL), polyvinylidene fluoride (PVDF) with a mass ratio of 94:3:3. Modified cyclic voltammetry; Charged linear sweep voltammetry (C-LSV) was performed for SiNP, MC, CSMC600, CSMC1200 and CSMC2000 electrode.
In the C-LSV method, the charging process was carried out using constant current-constant voltage mode (cutoff: 0.01V and 0.01C) and the discharging process was linear sweep voltammetry with the sweep rate range of 0.04-1 mV/s (cut-off: 2.0 V). In situ chemical characterization was performed in a SEM with two nanomanipulators (MM4-EM, Kleindiek Nanotechnik). To accelerate the spontaneous bonding reaction, W tip bearing samples were contacted with lithium metal strained W tip119,120.
In situ SEM analysis was performed under an acceleration voltage and emission current of 5.00 or 10.00 kV and 0.40 nA, respectively.
Results and discussion
The weight ratio of C and Si was measured individually by elemental analyzer (EA) and inductively coupled plasma atomic emission spectroscopy (ICP-AES). There were inevitable errors measuring the exact ratio of C and Si due to SiO2 formation and small amount of insolubility of Si. Moreover, the expected reversible capacities of CSMC series based on C and Si ratio of EA and ICP-AES were different from the actual values.
To determine the redox mechanisms of the CSMC series, modified linear sweep voltammetry (C-LSV) was performed for each sample (the detailed method was shown in the Supplementary Information). As the Si nanolayer became thinner (from CSMC2000 to CSMC600), the 0.1 V charge plateau and 0.44 V discharge gradually disappeared. The reversible capacities of CSMC did not agree well with the calculated capacities from EA and ICP-AES results (Figure 43).
The real capacities were mostly lower than the calculated capacities, and the difference was larger as the thickness of the Si nanolayer decreased. Although the native SiO2 layer was not a significant amount that changed the gravimetric capacitances in the Si > 50 nm nanostructure, there was a significant amount of change in the trend of the gravimetric capacitances depending on the Si thickness of the CSMC structures (< 31 nm). The remarkable increase in ICE after C and Si coating was caused by the deactivated surface reaction of MC 124.
Lithium cobalt oxide (LCO) and CSMC1200 whole cell assay. A) LCO forming voltage profiles (B) LCO half cell 0.5C test cycle (C) LCO-CSMC1200 full cell forming voltage profiles (D) full cell 0.5C test cycle.
Conclusion
Placement was simply composed of two nanomanipulator tips; one was for sample and the other was for lithium metal. Contact was maintained until there was no sample volume change (detailed in situ method is described in the supplementary information). The void space of SMC1200 was nearly filled in the fully lithiated state; however there was no size change on the particle scale (Figure 46E-H).
Challenges facing anode materials for lithium ion batteries
Confronting issues of the practical implementation of Si anode in high-energy lithium-ion batteries. Green synthesis and stable Li-storage performance of FeSi2/Si@C Nanocomposite for lithium-ion batteries. Quantification of microstructural dynamics and electrochemical activity of graphite and silicon-graphite lithium ion battery anodes.
Detection of lithium-silicide phase transformations in nanostructured silicon-based lithium-ion batteries via in situ NMR spectroscopy. Fast-charging high-energy lithium-ion batteries via amorphous silicon nanolayer implantation on edge-plane activated graphite anodes. Recent developments and understanding of novel mixed transition metal oxides as anodes in lithium ion batteries.
Hollow core-shell structured silicon@carbon nanoparticles embedded in carbon nanofibers as binder-free anodes for lithium-ion batteries. Embedding nano-silicon into graphene nanosheets by plasma-assisted milling for high-capacity anode materials in lithium-ion batteries. Optimizing charging strategy by preventing lithium deposition on anodes in high-energy lithium-ion batteries - electrochemical experiments.
Mesoporous amorphous silicon: A simple synthesis of a high-rate and long-life anode material for lithium-ion batteries.
