Applications of silicon as a high-performance anode material has been impeded by its low intrinsic conductivity and huge volume expansion (> 300%) during lithiation. To address these problems, nano-Si particles along with conductive coatings and engineered voids are often employed, but this results in high cost anodes. Here, we report a scalable synthesis method that can realize high specific capacity (~800 mAh g-1), ultrafast charge/discharge (at 8 A g-1 Si) and high initial Coulombic efficiency (~90%) with long cycle life (1000 cycles) at the same time. To achieve 1000 cycle stability, micron-sized Si particles are subjected to high-energy ball milling to create nanostructured Si building blocks with nano-channel shaped voids encapsulated inside a nitrogen (N)-doped carbon shell (termed as Si micro-reactor). The nano-channel voids inside a Si micro-reactor not only offer the space to accommodate the volume expansion of Si, but also provide fast pathways for Li ion diffusion into the center of the nanostructured Si core and thus ultrafast charge/discharge capability. The porous N-doped carbon shell helps to improve the conductivity while allowing fast Li ion transport and confining the volume expansion within the Si micro-reactor. Submicron-sized Si micro-reactors with limited specific surface area (35 m2 g-1) afford sufficient electrode/electrolyte interfacial area for fast lithiation/delithiation, leading to the specific capacity ranging from ~800 to 420 mAh g-1 under ultrafast charging conditions (8 A g-1), but not too much interfacial area for surface side reactions and thus high initial coulombic efficiency (~90%). Since Si micro-reactors with superior electrochemical properties are synthesized via an industrially scalable and eco-friendly method, they have the potential for practical applications in the future.
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First principles and experimental studies of empty Si 46 as anode materials for Li-ion batteries
The objective of this investigation was to utilize the first-principles molecular dynamics computational approach to investigate the lithiation characteristics of empty silicon clathrates (Si 46 ) for applications as potential anode materials in lithium-ion batteries. The energy of formation, volume expansion, and theoretical capacity were computed for empty silicon clathrates as a function of Li. The theoretical results were compared against experimental data of long-term cyclic tests performed on half-cells using electrodes fabricated from Si 46 prepared using a Hofmann-type elimination–oxidation reaction. The comparison revealed that the theoretically predicted capacity (of 791.6 mAh/g) agreed with experimental data (809 mAh/g) that occurred after insertion of 48 Li atoms. The calculations showed that overlithiation beyond 66 Li atoms can cause large volume expansion with a volume strain as high as 120%, which may correlate to experimental observations of decreasing capacities from the maximum at 1030 mAh/g to 553 mA h/g during long-term cycling tests. The finding suggests that overlithiation beyond 66 Li atoms may have caused damage to the cage structure and led to lower reversible capacities.
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- Award ID(s):
- 1206795
- NSF-PAR ID:
- 10391228
- Date Published:
- Journal Name:
- Journal of Materials Research
- Volume:
- 31
- Issue:
- 23
- ISSN:
- 0884-2914
- Page Range / eLocation ID:
- 3657 to 3665
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Realizing more holistic electrification in society to disengage current dependence on nonrenewable fuels requires balancing between energy storage mechanisms and actual environmental benefits gained from the transition from traditional resources. Given that the majority of greenhouse gas emissions in battery value chains originate from material mining and production, silicon carbide (SiC) derived from the agricultural waste, rice hull ash (RHA), is introduced as an environmentally-benign alternate anode material. SiC with hard carbon (SiC/HC) exhibits capacity increases on long-term cycling, reaching capacities of >950 mAh g−1competitive with elemental Si with complementary porosity. Herein, a relatively low amount (<30 wt%) of graphite added to SiC/HC composites greatly promotes capacity increases while retaining sustainability. Comparison between graphite contents were optimal at ≈30 wt% graphite (SiC/HC/30G) boosted performance, doubling capacity increase rates and subsequently saving >70% time to reach target specific capacities at C/10. At 2C, SiC/HC/30G offers enhanced specific capacities at ≈220 mAh g−1. The positive effects from the coincidentally formed HC are demonstrated by oxidizing HC to form SiC/O, followed by graphite addition. Experimental post-mortem analyses support that SiC/graphite composites provide a promising solution for implementing agricultural waste-derived material for next-generation lithium storage.
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Abstract Alloy materials such as Si and Ge are attractive as high‐capacity anodes for rechargeable batteries, but such anodes undergo severe capacity degradation during discharge–charge processes. Compared to the over‐emphasized efforts on the electrode structure design to mitigate the volume changes, understanding and engineering of the solid‐electrolyte interphase (SEI) are significantly lacking. This work demonstrates that modifying the surface of alloy‐based anode materials by building an ultraconformal layer of Sb can significantly enhance their structural and interfacial stability during cycling. Combined experimental and theoretical studies consistently reveal that the ultraconformal Sb layer is dynamically converted to Li3Sb during cycling, which can selectively adsorb and catalytically decompose electrolyte additives to form a robust, thin, and dense LiF‐dominated SEI, and simultaneously restrain the decomposition of electrolyte solvents. Hence, the Sb‐coated porous Ge electrode delivers much higher initial Coulombic efficiency of 85% and higher reversible capacity of 1046 mAh g−1after 200 cycles at 500 mA g−1, compared to only 72% and 170 mAh g−1for bare porous Ge. The present finding has indicated that tailoring surface structures of electrode materials is an appealing approach to construct a robust SEI and achieve long‐term cycling stability for alloy‐based anode materials.
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null (Ed.)Silicon anodes are promising for high energy batteries because of their excellent theoretical gravimetric capacity (3579 mA h g −1 ). However, silicon's large volume expansion and poor conductivity hinder its practical application; thus, binders and conductive additives are added, effectively diluting the active silicon material. To address this issue, reports of 2D MXene nanosheets have emerged as additives for silicon anodes, but many of these reports use high MXene compositions of 22–66 wt%, still presenting the issue of diluting the active silicon material. Herein, this report examines the question of what minimal amount of MXene nanosheets is required to act as an effective additive while maximizing total silicon anode capacity. A minimal amount of only 4 wt% MXenes (with 16 wt% sodium alginate and no carbon added) yielded silicon anodes with a capacity of 900 mA h g Si −1 or 720 mA h g total −1 at the 200 th cycle at 0.5 C-rate. Further, this approach yielded the highest specific energy on a total electrode mass basis (3100 W h kg total −1 ) as comapared to other silicon-MXene constructs (∼115–2000 Wh kg total −1 ) at a corresponding specific power. The stable electrode performance even with a minimal MXene content is attributed to several factors: (1) highly uniform silicon electrodes due to the dispersibility of MXenes in water, (2) the high MXene aspect ratio that enables improved electrical connections, and (3) hydrogen bonding among MXenes, sodium alginate, and silicon particles. All together, a much higher silicon loading (80 wt%) is attained with a lower MXene loading, which then maximizes the capacity of the entire electrode.more » « less
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The guest‐free, type‐II Si clathrate (Si136) is an open cage polymorph of Si with structural features amenable to electrochemical Li storage. However, the detailed mechanism for reversible Li insertion and migration within the vacant cages of Si136is not established. Herein, X‐ray characterization and density functional theory (DFT) calculations are used to understand the structural origin of electrochemical Li insertion into the type‐II clathrate structure. At low Li content, instead of alloying with Si, topotactic Li insertion into the empty cages occurs at ≈0.3 V versus Li/Li+with a capacity of ≈231 mAh g−1(corresponding to composition Li32Si136). A synchrotron powder X‐ray diffraction analysis of electrodes after lithiation shows evidence of Li occupation within the Si20and Si28cages and a volume expansion of 0.22%, which is corroborated by DFT calculations. Nudged elastic band calculations suggest a low barrier (0.2 eV) for Li migration through interconnected Si28cages, whereas there is a higher barrier for Li migration into Si20cages (2.0 eV). However, if Li is present in a neighboring cage, a cooperative migration pathway with a barrier of 0.65 eV is possible. The results show that the type‐II Si clathrate displays unique electrochemical properties for potential applications as Li‐ion battery anodes.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1557/jmr.2016.408
